Gondwana Research - Department of Ocean Engineering · India–Madagascar paleo-ﬁtbasedonﬂexural isostasy of their rifted margins R.T. Ratheesh-Kumara,⁎, C. Ishwar-Kumara,B.F.Windleyb,

India–Madagascar paleo-fit based on flexural isostasy of their rifted margins

R.T. Ratheesh-Kumar a,⁎, C. Ishwar-Kumar a, B.F. Windley b, T. Razakamanana c, Rajesh R. Nair d, K. Sajeev a

a Centre for Earth Sciences, Indian Institute of Science, Bangalore 560012, Indiab Department of Geology, The University of Leicester, Leicester LE1 7RH, UKc Département de Sciences Naturelles, Université de Toliara, Toliara, Madagascard Department of Ocean Engineering, Indian Institute of Technology Madras, Chennai 600 036, India

a b s t r a c ta r t i c l e i n f o

Article history:Received 24 February 2014Received in revised form 2 June 2014Accepted 10 June 2014Available online xxxx

Handling Editor: A.R.A. Aitken

Keywords:Continental marginIsostasyEffective elastic thicknessMohoLithosphere

The present study contributes new constraints on, and definitions of, the reconstructed platemargins of India andMadagascar based on flexural isostasy along the Western Continental Margin of India (WCMI) and the EasternContinental Margin of Madagascar (ECMM). We have estimated the nature of isostasy and crustal geometryalong the two margins, and have examined their possible conjugate structure. Here we utilize elastic thickness(Te) andMoho depth data as the primary basis for the correlation of these passive margins. We employ the flex-ure inversion technique that operates in spatial domain in order to estimate the spatial variation of effective elas-tic thickness. Gravity inversion and flexure inversion techniques are used to estimate the configuration of theMoho/Crust–Mantle Interface that reveals regional correlations with the elastic thickness variations. These re-sults correlate well with the continental and oceanic segments of the Indian and African plates. The presentstudy has found a linear zone of anomalously low-Te (1–5 km) along the WCMI (~1680 km), which correlateswell with the low-Te patterns obtained all along the ECMM. We suggest that the low-Te zones along theWCMI and ECMM represent paleo-rift inception points of lithosphere thermally and mechanically weakenedby the combined effects of the Marion hotspot and lithospheric extension due to rifting. We have produced anIndia–Madagascar paleo-fit representing the initial phase of separation based on the Te estimates of the riftedconjugate margins, which is confirmed by a close-fit correlation of Moho geometry and bathymetry of theshelf margins. The matching of tectonic lineaments, lithologies and geochronological belts between India andMadagascar provide an additional support for the present plate reconstruction.

© 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

1. Introduction

The temporal evolution and spatial configuration of continents canbe analyzed through their response to long-term forces, as a functionof the elastic property of the lithosphere, which is parameterized aseffective elastic thickness (Te). The Te method has been widelyused as a key proxy to examine the long-term strength/rigidity struc-ture of the lithosphere. It can be parameterized through flexural rigidity,D ≡ E · Te3 / 12(1− ν2), which is a measure of the resistance of the lith-osphere to flexure in response to loading (Watts, 2001), where Young'smodulus, E (1011 Pa), and Poisson's ratio, ν (0.25), are the materialproperties. The elastic thickness in oceanic regions has values between0 and 65 km that approximately correspond to the depth of the450 °C isotherm (Watts, 1992). In contrast, the continents exhibit a Terange as high as 80+ km in stable regions (Watts and Burov, 2003),and as low as ~5 km in young and tectonically rejuvenated regions(Watts, 2001). A possible correlation between Te and the age of the lith-osphere was studied in Europe (Pérez-Gussinyé and Watts, 2005) and

Australia (Simons and van der Hilst, 2002), but most studies concludedthat the age of the lithosphere is not the only controlling parameter fordetermining its mechanical strength. Many studies correlated elasticthickness variations with different factors including “sandwich” defor-mation (decoupling) when a weak ductile layer in the lower crustdoes not allow bending stresses to be transferred between strong brittlelayers (Burov and Diament, 1995; Ratheesh-Kumar et al., 2014), “fro-zen” deformation controlled by lattice-preferred orientation of olivineas result of increased melt production within the upper mantle(Simons et al., 2003; Pérez-Gussinyé et al., 2009), localized brittle failureof crustal rocks under deviatoric stress (Lowry and Smith, 1995; Tassaraet al., 2007; Ratheesh Kumar et al., 2010; Nair et al., 2011; RatheeshKumar et al., 2013), and surface and subsurface loading by large-scaletectonic features such as topographic masses and regional-scale faults(Audet and Mareschal, 2004).

In the present study, we aim to appraise spatial variations ofelastic thickness along the conjugate passive margins of India andMadagascar (Fig. 1) (the Western Continental Margin of India (WCMI)and the Eastern Continental Margin of Madagascar (ECMM)). Themajor objective is to understand how the nature of isostasy variesalong these margins, and to find any possible conjugate correlation

Gondwana Research xxx (2014) xxx–xxx

⁎ Corresponding author.E-mail address: [email protected] (R.T. Ratheesh-Kumar).

GR-01283; No of Pages 20

http://dx.doi.org/10.1016/j.gr.2014.06.0081342-937X/© 2014 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Contents lists available at ScienceDirect

Gondwana Research

j ourna l homepage: www.e lsev ie r .com/ locate /gr

Please cite this article as: Ratheesh-Kumar, R.T., et al., India–Madagascar paleo-fit based on flexural isostasy of their rifted margins, GondwanaResearch (2014), http://dx.doi.org/10.1016/j.gr.2014.06.008

http://dx.doi.org/10.1016/j.gr.2014.06.008

mailto:[email protected]


http://www.sciencedirect.com/science/journal/1342937X


PGM7

Highlight

between them. The present study can be regarded as a significant up-grade of the previous approach of Chand and Subrahmanyam (2003),which used Te estimates to examine the conjugate nature of India–Madagascar passive margins. The significance of our study relies onthe fact that for the first time it brings together the spatial variationsof elastic thickness and the Moho configuration in the WCMI and theECMM. In contrast to other geophysical investigations that used seismic,gravity and bathymetry data to constrain the geometry/structure of thepassive margins, by using Te variations the present study effectivelymaps the lithospheric deformations in the major tectonic structuresalong the WCMI and ECMM.

Previous studies of Te of passive margins in the world have shownvariable results. Stern and Brink (1989) estimated a Te of ~19 km in theRoss Sea where rifting occurred at about 60 Ma, whereas in the Valenciatrough where there is a comparatively young rift age of 20 Ma, elasticthickness estimates are ~5 km (Watts and Torné, 1992). Daly et al.(2004) computed the elastic thickness of the Irish Atlantic margin usingamultitaper coherencemethod between scaled bathymetry and Bouguergravity and obtained Te values of ~6–18 km. Wyer and Watts (2006)

appliedflexural back-stripping and gravitymodeling techniques to calcu-late the gravity anomaly associated with rifting and sedimentation alongthe eastern continental margin of the USA. They iteratively comparedthe calculated gravity anomaly to the observed free-air gravityanomaly to derive a best-fit Te structure that shows a significant variationof 0 b Te b 40 km,which they attributed to strength variation in the riftedlithosphere. Several studies revealed crustal thinning and depth of neck-ing as appropriate parameters to predict the flexural response of litho-spheric stretching (Braun and Beaumont, 1989; Fourno and Roussel,1994; Ratheesh Kumar et al., 2011). Ratheesh Kumar et al. (2011) usedthe orthonormalized Hermite multitaper method to estimate Te alongthe northeastern passive margin of North America, and suggested thatlow-Te values were indicative of the passive nature of the margin whenloads were emplaced during the continental break-up process at high-temperature gradients. Chand et al. (2001) examined the cross-spectralcorrelation between gravity and bathymetry along 1D profiles acrossthe Eastern Continental Margin of India (ECMI) and its conjugateEast Antarctica margin. They obtained Te ~ 10–25 km and Te b 5 kmover the northern and southern segments of the ECMI, and suggested

Fig. 1. Tectonic setting of the western Indian Ocean superimposed on a GEBCO (1 × 1min grid) bathymetrymap. The windows (a) and (b) represent the selected areas over theWesternContinental Margin of India and Eastern Continental Margin ofMadagascar respectively. Acronyms: KuB— Kutch Basin, SaB— Saurashtra Basin, BB— Bombay Basin, KoB— Konkan Basin,KeB — Kerala Basin.

2 R.T. Ratheesh-Kumar et al. / Gondwana Research xxx (2014) xxx–xxx



their possible match with the Te data of the corresponding congruentsegments of the East Antarctica margin. Subrahmanyam and Chand(2006) re-examined gravity and topography/bathymetry data overIndia and the adjoining oceans, and suggested that ECMI evolved in ashear tectonic setting, and was similar to its conjugate half in EastAntarctica.

There have been few noted studies on the continental margins ofIndia and Madagascar using different techniques of Te in spectral do-main. Chand and Subrahmanyam (2003) estimated Te of the WCMIand the ECMM through cross-spectral analysis of gravity and bathyme-try data by comparing the Te results of 8–15 km for the WCMI and 10–13 km for ECMM of the conjugate margins. Sheena et al. (2007)employed rectangular blocks for a coherence analysis along the KonkanandKerala basins of theWCMI,which revealed a variation of lithospher-ic strength of 5 to 10 km. Chaubey et al. (2008) derived Te using admit-tance (cross-spectral) analysis between 12 gravity and bathymetryprofiles across the Laccadive Ridge, and obtained low-Te (2–3 km)values, which they attributed to the local compensation of stretchedcontinental lithosphere. Ratheesh-Kumar et al. (2014) derived spatialvariations of the elastic thickness structure of the Indian Shield and ad-joining regions using a fan wavelet-based Bouguer coherence tech-nique. Their Te map showed zones of significantly low-Te along thewestern margin of the Indian Shield, which they attributed to riftingof the margin.

In the present study, we have employed a thin plate flexuremodel (Braitenberg et al., 2002, 2006), which is an alternative to thewidely used calculation of admittance/coherence of topography andgravity. Our analysis is based on the convolution method that modelssurface and subsurface loads with the point load response functionof an elastic plate in spatial domain. We have used Bouguer gravityand bathymetry/topography to estimate the spatial variation of the ef-fective elastic thickness and the Moho configuration in the WesternContinental Margin of India and the Eastern Continental Margin ofMadagascar.

2. Study region: tectonic setting

The past position and morphological relationship of India relative toMadagascar is one of the most debated, current problems of the tecton-ics of eastern Gondwana (e.g., Katz and Premoli, 1979; Collins andWindley, 2002; Braun and Kriegsman, 2003; Ghosh et al., 2004;Collins and Pisarevsky, 2005; Collins, 2006; Ashwal et al., 2013;Gibbons et al., 2013; Ishwar-Kumar et al., 2013; Rekha et al., 2013).The breakup of East Gondwana started with the simultaneous riftingof Madagascar, Seychelles, India, Antarctica and Australia from Africaat ca. 150Ma. Then,Madagascar, Seychelles and India separated togeth-er from Antarctica and Australia at about 128–130 Ma (Biswas, 1999).At ca. 90 Ma India and Seychelles further rifted from Madagascar, andat about 65 Ma India separated from Seychelles (Pande et al., 2001).The history of the position and movement of India after rifting fromMadagascar was long and eventful. Geophysical data suggest that theIndian plate continually drifted northwards after its separation fromAfrica in the Late Jurassic to finally collide with Eurasia at about 55 Ma(Yin and Harrison, 2000). The oldest seafloor anomaly recognized is34 (83 Ma), which means that the onset of seafloor spreading occurredsometime during the Cretaceous Quiet Zone (120–83 Ma). The frag-mentation took place in three stages: doming, rifting and drifting. Ac-cording to Storey et al. (1995) the Marion hotspot initiated thebreakup of India and Madagascar at about 88 Ma, and resulted in theformation of the Western Continental Margin of India and the EasternContinental Margin of Madagascar (Fig. 1).

2.1. The Western Continental Margin of India

The Western Continental Margin of India (WCMI) is charac-terized by awide continental shelf (N300 km) and thick shelf sediments

(7–8 km) of Indus fan origin (Zutshi et al., 1995). South of the Vengurlaarch (15°46′13″N, 73°40′48″E) (Fig. 1) the shelf is narrow (b100 km)and characterized by 3–4 km-thick sediments that are mainly derivedfrom denudation of theWestern Ghats and concentrated in small, local-ized depressions (Zutshi et al., 1995). The offshore shelf basins can beregionally classified into three: northern Kutch and Saurashtrabasins, central Bombay basin, and southern Konkan and Kerala basins(Fig. 1). To the west of the shelf margin the transitional crust isrestricted by the Kori–Comorin ridge, a typical longitudinal ridge identi-fied close to the foot of the continental slope along the westerncontinental margin, which could possibly be the ocean–continentboundary (Biswas, 1987, 1988). The western margin of India isgeomorphologically similar to other rifted continental margins likeParana of Brazil, Karoo in southeast Africa, and Etendeka in southwestAfrica (Widdowson, 1997).

The northern segment of theWCMI is occupied by plume-generatedflood basalts of the Deccan Traps (Beane et al., 1986) that have a maxi-mum thickness of N3 km, and which migrated southwards during theplume activity (Jay andWiddowson, 2008). Trace element geochemicaldata indicate increasing degrees of partial melting from north to south(Peng and Mahoney, 1995); the shallower and higher degrees of melt-ing in the southwere explained by Anil Kumar et al. (2001) as the resultof lithospheric thinning, which would be consistent with the progres-sive southward opening of the India–Madagascar rift. The DeccanTraps started to erupt at about 65 Ma ago from the Réunion hotspot(Courtillot et al., 2003).

2.2. The Eastern Continental Margin of Madagascar

The Eastern Continental Margin of Madagascar (ECMM) has a nar-row coastal plain marked by NNE/SSW-striking Cenozoic normal faultsthat impart a remarkable, strong linearity to the coastline. Cretaceousbasalts and minor rhyolites (ca. 88 Ma) are prominent all along theECMM (Storey et al., 1997); these coastal rift volcanic rocks and centralflood basalts formed within a short period between 92 and 84 Ma(Melluso et al., 2001). Swarms of coast-parallel dolerite dykes thathave K–Ar ages ranging from 97 to 89 Ma (Storetvedt et al., 1992) in-truded during Early Cretaceous rifting along the NE coast ofMadagascar (Bauer et al., 2011). Apatite fission track data suggest thatsome escarpments on the eastern coast date from the time of riftingwith India (Seward et al., 1999).

The geochemical signatures of the eastern coast basalts reflect theirdifferentmantle source regions as the rift with India opened from northto south (Storey et al., 1997; Melluso et al., 2001, 2002). On the north-eastern coast low-Ti basalts are similar to the low-Ti flood basalts ofthe Deccan Traps on the opposite northwestern coast of India. To thesouth along the central coast of Madagascar basalt geochemistry isdominated by an Indian Ocean-type MOR-source mixed with a compo-nent of old continental mantle lithosphere (Mahoney et al., 2008). Onthe southeast coast, high Fe–Ti basalts are similar to those on the EastGreenland volcanic rift margin, and Nd, Pb and Sr isotopic data indicatea significant Marion hotspot plume component (de Wit, 2003). Paleo-magnetic data from the basalts combined with magnetic anomaliesand fracture zones of the Indian Ocean provide strong evidencethat the Marion hotspot was situated within 100 km of southernMadagascar when it separated from the Seychelles–India continent atabout 90–88 Ma (Storey et al., 1997; Torsvik et al., 1998; Reeves andde Wit, 2000; Torsvik et al., 2000).

3. Data and method

Our study areas cover most of the Western Continental Margin ofIndia and the Eastern Continental Margin of Madagascar. The databaseused for this study comprises gravity, bathymetry/topography and sed-iment thickness. The bathymetry data (Figs. 2a and 3a) were obtainedfrom GEBCO Digital 1-minute bathymetry data (National Oceanic and

3R.T. Ratheesh-Kumar et al. / Gondwana Research xxx (2014) xxx–xxx



Atmospheric Administration, 2003). We merged the gravity data forland and ocean by using the land ocean deconvolution technique(Kirby and Swain, 2008). The free-air gravity data were derived from

the global marine gravity field from ERS-1 and GEOSAT geodetic mis-sion altimetry of Anderson and Knudsen (1998) and Anderson et al.(2008). The free-air gravity anomaly data (ΔGf) was converted to

Fig. 2. (a) Bathymetry. (b) Bouguer gravity anomaly. (c) Sediment thickness. (d) Gravity effect of the sediments. (e) Sediment-corrected bathymetry (basement depth). (f) Sediment-corrected gravity of the Western Continental Margin of India.




Bouguer gravity anomalies (ΔGb) (Figs. 2b and 3b) using the slab for-mula of Parker (1973):

ΔGb ¼ ΔGf þ 2πΔρGH ð1Þ

where Δρ=1670 kg·m−3 is the density contrast between surface rockand water, H is the bathymetry (in meters) and G is the gravitationalconstant.

We use the sediment thickness model (Figs. 2c and 3c) ofDivins (2003), which was compiled by the National Geophysical Data

Fig. 3. (a) Bathymetry. (b) Bouguer gravity anomaly. (c) Sediment thickness. (d) Gravity effect of the sediments. (e) Sediment-corrected bathymetry (basement depth). (f) Sediment-corrected gravity of the Eastern Continental Margin of Madagascar.




Centre (NGDC) of NOAA (National Oceanic and Atmospheric Adminis-tration), and has a resolution of 5 × 5 arc min.

3.1. Flexure modeling in spatial domain (convolution method)

We adopt a methodology that operates in the spatial domain intro-duced by Braitenberg et al. (2006), which they successfully used intheir analysis of the South China Sea Ridge. In thismethod,Moho depthsare first estimated from forward modeling of gravity anomalies; then,the lithosphere rigidity is inverted in order to retrieve isostatic Mohodepth undulations compatible with those previously obtained.

In this method, we first model the Crust Mantle Interface/Mohodepth undulations, which contribute to the long wavelength part ofthe observed gravity field, whereas the short wavelength part is gener-ated by superficial masses viz., sediment layers or intra-crustal densityinhomogeneities. However, sometimes sedimentary basins can alsoproduce long-wavelength signals. Hence, it is essential to estimate thegravity effect of sediments in isostatic flexure modeling. Furthermore,on a passive continentalmargin, large amounts of sedimentswill simplyerase any signal of load-induced topography (i.e. flat topography is un-related to flexure). For these reasonswe isolated the effect of sedimentsfrom the observed gravity and bathymetry to recover the basementstructure. As an initial step, the base of the sediments was generatedby subtracting the sediment thickness from the bathymetry. The obtain-ed sediment corrected basement (Figs. 2e and 3e) will now representthe actual bathymetric features that are previously masked by thethick sediment cover. A linear density variation with depth, ρ(z) is cal-culated from the following expression.

ρ zð Þ ¼ ρtop þ ρlow−ρtop� �

hsed= hlow−htop

� �ð2Þ

where ρtop and ρlow represents the density values corresponding to thetop (2.25 Mg/m3) and bottom (2.7 mg/m3) layers, hsed is the sedimentthickness, and hlow and htop represents the depth to the top and bottomlayers respectively. The gravity effect is then calculated by subtractingthe density of the referencemodel from ρ(z). The obtained gravity effectof sediments (Figs. 2d and 3d) is then subtracted from the observedgravity to obtain the sediment-corrected gravity (Figs. 2f and 3f),which is used for the flexural modeling. In order to filter the input grav-ity field, we defined a cut-off wavelength that suppresses all wave-lengths smaller than 100 km. This sediment-corrected Bouguer gravityfield is then inverted by applying an iterative algorithm that alternatesdownward continuation with direct forward modeling (Braitenberget al., 1997). Thus, we obtained Moho undulations inverted from theBouguer gravity data.

The next modeling step is the flexure inversion, an independentmeans to determine the physical flexural model of Moho undulations,and it allows the gravity-deduced Moho undulations to be checked forcompatibility. The flexure is calculated by the convolution of the crustalload (i.e., topographic and subsurface loads)with the point–loadflexureresponse curves (Braitenberg et al., 2002, 2003). In order to avoid sepa-rate analyses and inversions on land and ocean areas, we scaled thesediment-corrected ocean bathymetry (h) to equivalent topography(h′) using the equation, h′ = (ρc − ρw)h / ρc, where ρc and ρw are thedensities of crust and water, respectively. The equivalent topographyrepresents the bathymetry that one would assume if there were nowater present under isostatic conditions (Daly et al., 2004; Kirby andSwain, 2008). Accordingly the derived equivalent topography for theWCMI (Fig. 4a) and ECMM (Fig. 4b) are used in the present convolutionscheme. A series of flexural response functions are used in the convolu-tion tomodel the crust–mantle interface undulations, each correspond-ing to a Te value between 0 and 20 km. The spatial variation of Te is

Fig. 4. Equivalent topography map of the conjugate continental margins and adjoining oceanic terranes of India (a) and Madagascar (b) derived from sediment-corrected bathymetry.Acronyms are given in Table 1.




calculated on sliding squarewindows of side length 100 km that shiftedevery 20 km. The obtained Te value for a specific window is the one thatminimizes the root mean square (rms) difference between the flexureMoho and the observed Moho derived from the gravity inversion. Theelastic model parameters used in the flexure analysis are given inTable 2.

3.2. Advantages and limitations

We now discuss possible concerns regarding the validity of thepresent method and sensitivity of input parameters for inferring Te.We assumed a continuous-plate rather than broken-plate model forthe present analyses. Braitenberg et al. (2002) tested the convolutionmethod in a syntheticmodel situation, and successfully used the contin-uous plate model to recover the spatial variation of elastic thicknessover the Eastern Alps. However, they observed some discrepant de-crease in Te values in themain Alpine range, and explained it as the pos-sible result of recent tectonic forces acting at the border of twomergingplates. Braitenberg et al. (2006) assumed a continuous plate model anddemonstrated the use of the convolutionmethod for the estimated spa-tial variation of Te in a mixed land–ocean setting in and around theSouth China Sea. They derived the spatial variation of Te for the oceaniclithosphere, and the included terrestrial parts are blanked in their re-sults. We follow the approach of Braitenberg et al. (2002, 2006) by

assuming that the present study regions are paleo-rift margins wherethe continental and oceanic plates are expected to be very well coupledand hence can be considered as a continuous plate, in which case theformalism assumes that we have only vertical loads with no horizontalstresses. In other words, the continuous-platemodel assumes that thereis no edge of chaos in the passive margin setting in contrast to an activerift or subduction zone setting, where two different plates will bepushed/pulled from the side and a broken-plate model is likely moreapplicable. Furthermore, we used equivalent topography (Fig. 4a andb) rather than simple bathymetry, and that allows the land-loadingequations to be applied for a whole land–ocean setting (Pérez-Gussinyé et al., 2004). Kirby and Swain (2008) used scaling in their syn-thetic modeling and demonstrated its use in recovering Te in a mixedland–sea setting with a negligibly small bias. Recently, Jiménez-Díazaet al. (2014) by using bothmultitaper andwavelet (Bouguer coherence)methods in and around Central America successfully demonstrated thatTe can be better recovered in a mixed land–ocean setting when usingthe scaling (equivalent topography) and land-loading equations.

The sensitivity of the model to input parameters (e.g. density con-trast within a plate) would be another complicating factor in the flexuremodeling of a mixed land–ocean setting. In the absence of constrainingdata,we set a constant density contrast (Δρ) across the crust–mantle in-terface (CMI). However, we tested the model sensitivity with differentcombinations of density contrast (Δρ ~ 350–600 kg/m3) and referencedepth (d ~ 20–35 km) standard ranges of the CMI, and a best-fit Te isdeduced from the minimum of root mean square (rms) error betweenthe observed and computed CMI. The best results (i.e., minimum rms)were obtained for the set of parameters Δρ ~ 450 kg/m3 and d ~ 30 km.

Earlier Te estimates of comparable passivemargins, such as the flex-ural analyses of Chand and Subrahmanyam (2003), Sheena et al. (2007,2012), Tiwari et al. (2007), andChaubey et al. (2008)were carried out inspectral domain along a 1D profile or using some discrete windows ofvariable size, but they could not produce the spatial variations necessaryfor the effective elastic thickness (Te), which we consider a seriousshortcoming. In contrast to the earlier studies, the thin plate flexuremodel applied in the present study operates in the spatial domain (con-volutionmethod),whichhas such a significant advantage over the spec-tral methods that it overcomes the numerical instabilities in theadmittance/coherence calculations. The spatial variation is achieved bydividing the analysis area into overlapping windows of size 100 km2,where Te is calculated and inverted for each window, and then movingthe center of each window by 20 km in order to cover the entire inves-tigated area for each new estimate. This provides spatial variations ofthe flexural properties with higher resolution than any of the spectralapproaches. Another significant advantage is that this analysis can bemade over an area that is not necessarily rectangular, as required forthe spectral analysis. Recently, Ratheesh Kumar and Windley (2013)used flexure inversion technique in combination with a spectral tech-nique (Morelet wavelet-based Bouguer coherence) to derive the Testructure of theNinetyeast Ridge in the IndianOcean, anddemonstratedthat both the spatial and spectral estimates provide spatial variationsthat are mutually complementary.

Table 1Acronyms of the litho-tectonic and structural units in the study area, in and around Indiaand Madagascar.

Abbreviation Litho-tectonic and structural units

IndiaWCMI Western Continental Margin of IndiaASZ Achankoil Shear ZoneBB Bombay BasinCB Coorg BlockCSZ Chitradurga Shear ZoneDVP Deccan Volcanic ProvinceEDC Eastern Dharwar CratonKB Karwar BlockKeB Kerala BasinKoB Konkan BasinKSZ Kumta Shear ZoneLR Laxmi RidgeMB Madurai BlockMBR Midshelf Basement RidgeMcSZ Mercara Shear ZoneMeSZ Mettur Shear ZoneMSZ Moyar Shear ZoneNB Nilgiri BlockNSZ Nallamalai Shear ZonePCSZ Palghat Cauvery Shear ZonePR Prathap RidgeSASZ Salem Attur Shear ZoneSC Sedimentary CoverTB Trivandrum BlockWDC Western Dharwar Craton

MadagascarECMM Eastern Continental Margin of MadagascarAB Antananarivo BlockAC Antongil CratonAmSZ Amphanihy Shear ZoneAnD Anoshyan DomainAnSZ Angavo Shear ZoneAS Amboropotsy SheetBD Bemarivo DomainBeSZ Betroka Shear ZoneBSZ Betsimisaraka Shear ZoneIG Itremo GroupMC Masora CratonRSZ Ranotsara Shear ZoneSC Sedimentary CoverSG Sahantaha GroupTG Tsaratanana GroupTSZ Tranomaro Shear Zone

Table 2Input parameters used in the flexural modeling.

Parameter Symbol Value

Mean crustal density ρc 2800 kg/m3

Mantle density ρm 3300 kg/m3

Density contrast in crust–mantle interface Δρ 450 kg/m3

Sea water density ρw 1030 kg/m3

Sediment density ρs 2250 kg/m3

Reference depth of crust–mantle interface d 30 kmYoung's modulus E 1011 N/m2

Poisson's ratio σ 0.25Grid spacing dx, dy 20 kmGrid size used for WCMI Lx, Ly 18° × 18°Grid size used for ECMM Lx, Ly 17° × 17°




4. Results

The effective elastic thickness (Te) maps estimated for the data win-dows (a and b in Fig. 1) over the WCMI and ECMM are presented inFigs. 5 and 6 respectively. We also present the Moho models derivedfrom gravity inversion and flexure inversion analysis of the WCMI(Fig. 7a and b) and ECMM(Fig. 7c and d). The gravity inversion andflex-ure inversion results are in good agreement, because the residual Moho(Fig. 7e and f) (mismatch between the gravity inversion-derived Mohoand flexure inversion-derived Moho) has a very low range (average of±3 km). In Fig. 7e and f, it is clear that the marginal segments of Indiaand Madagascar show an rms range of ±1 km. The model of Moho un-dulation on each window is determined for a specific Te, and hence thelow-range of the root mean square error is yet another quality check ofthe present Te results.

Table 3 shows a comparison between Te values obtained in thepresent study andTe estimates fromprevious studies in various tectonicprovinces in and around the WCMI and ECMM. Our new Te maps(Figs. 5 and 6) correlate well with the morphological features in thestudy regions, and resolve regional-scale structures. We pay particularattention in this study to a narrow linear patch of anomalously low-Te

on the western Indian shelf region (Fig. 5). Away from the shelf margin,the Laccadive Ridge exhibits a significantly low-Te signature, whereasthe adjacent areas to its east and west are distinguished by higher Tevalues that separate the ridge from the shelf margin and Arabianbasin, respectively. To the north, the Laxmi ridge (Fig. 5) exhibits a sim-ilar low-Te range with higher values on its sides. Over most of the Ara-bian basin the elastic thickness is significantly low (Te b 3 km). Differenttectonic provinces included in the study areawithin the southern Indianshield exhibits significant Te variations. Within the continental regimeof India included in the data window, significant Te variations are ob-served over the Deccan Volcanic Province (DVP), Dharwar Craton, andsouthern granulite terrain (SGT) (Fig. 5).

In the case of Madagascar (Fig. 6), its Archaean cratonic interior ex-hibits high Te values (~20 km), whereas, themarginal zones are charac-terized by a significantly low-Te range (1–10 km). We find that theentire stretch of the ECMM including the narrow shelf zone and the ad-jacent ocean basin exhibits uniformly low-Te values, and matches wellwith those obtained from the WCMI. Towards the southern end of theeastern margin, the low-Te estimates in the shelf adjacent to theMadagascar basin, and the fossil ridge segment together are indicativeof an extensive weak lithosphere. Farther away from this margin, the

Fig. 5. Effective elastic thickness of the Western Continental Margin of India. Topography shaded relief is superimposed. Red dotted line represents the Reunion hotspot track. Acronymsare given in Table 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)After Torsvik et al. (2013).




Reunion (also called La Réunion) and Mauritius chains exhibit high Te(~18–20 km), whereas in its northward extension the Nazareth Bankregion has low-Te values (2–8 km) (Fig. 6).

The Moho models clearly depict the transition from thick continen-tal to thin oceanic crusts, and exhibit significant undulations that corre-late with regional-scale features. The continental interiors of India andMadagascar show a high crustal thickness (N35 km), which decreasestowards the margins. The Moho undulations beneath the continentalshelves of India and Madagascar correlate well with each other, andboth are in the range of 25 to 30 km. In the WCMI, the Laccadive ridgeis underlain by a 20–25 km thick crust, whereas in the Laxmi ridgeand its surroundings, it is in the range of 15–20 km. In the Arabianbasin the crustal thickness decreases progressively from north(b15 km) to south (~8 km). In the ECMM, the ocean basin adjacent tothe narrow shelf exhibits a uniformly low crustal thickness (average~10 km) from north to south, with a significant and extensive thincrust observed in the southernmost regimes. The Reunion–Mauritius–Nazareth Bank chain in the Moho map is distinguished by a highercrustal thickness than its surrounding oceanic lithosphere. A progres-sively increased crustal thickness is particularly evident from the Re-union (~20 km), Mauritius (~25 km), and Nazareth Bank (~30 km)

regimes. The present Moho depth values are in good agreement withthe published seismic and gravity-constrained estimates (Table 4).

5. Discussion

The spatial variations of elastic thickness and Moho depth in thecontinental–oceanic realms of India and Madagascar reveal some im-portant insights into the evolution and deformation of the different lith-ologic units. An interesting observation in the present study is the NW–

SE trending linear zone of significantly low-Te (0–5 km) along theWCMI (particularly in the shelf region) (Fig. 5) and its equivalent Tezone along the ECMM (Fig. 6). The present low-Te in the WCMI is con-sistent with the spectral estimates of Sheena et al. (2012), who inferredlow-Te variations over the Konkan Basin (Te ~ 5 km) and the KeralaBasin (Te ~ 10 km) according to the lithospheric neckingmodel. Severalstudies support the rift-related lithospheric structures along the WCMIand its congruent ECMM (Fourno and Roussel, 1994; Chandand Subrahmanyam, 2003; Minshull et al., 2008; Yatheesh et al.,2009). Chand and Subrahmanyam (2003) obtained an elastic thicknessof 8–15 km for the WCMI and 10–13 km for the ECMM using a one-dimensional free air admittance function, and suggested that these

Fig. 6. Effective elastic thickness of the Eastern Continental Margin ofMadagascar. Red dotted line represents the Reunion hotspot track. Acronyms are given in Table 1. (For interpretationof the references to color in this figure legend, the reader is referred to the web version of this article.)After Torsvik et al. (2013).




low-Te values are signatures of the rifting between India andMadagascar. TheMoho topography derived fromBouguer gravity inver-sion by Fourno and Roussel (1994) revealed a NE-trending zone of

substantially thinned crust in the Precambrian basement of easternand central Madagascar, which they attributed to the separation ofIndia during the Cretaceous. Windley and Razakamanana (1996)

Fig. 7.Moho undulations obtained from constrained gravity inversion and flexural inversion techniques for the Western Continental Margin of India (a and b) and Eastern ContinentalMargin of Madagascar (c and d) respectively. Panels (e) and (f) show the Residual Moho (the mismatch between gravity inversion-derived Moho and flexural inversion-derivedMoho) respectively for the WCMI and ECMM.




suggested that the Moho topography and zone of thinned crustof Fourno and Roussel (1994) coincided with a zone of extensionalstructures in the basement related to extensional collapse of theNeoproterozoic orogen. Moreover, Kusky et al. (2010) pointed outthat the thinned crustal zone is expressed by a post-Miocene grabensystem along the center of Madagascar, which may be an incipient ex-pression of a diffusive plate boundary of the East African Rift System.

5.1. The low-Te zones along the passive margins—a rift related signature?

Although we obtained comparable Te results along the conjugatemargins of India and Madagascar, there is a need to clarify why onewould expect the Te to be similar on both margins. By considering thewell-documented tectonic history of the WCMI, two major episodes oflithospheric deformation can be taken into account for the anomalouslylow-Te signature: 1. The deformation as a result of the rifting processesincluding lithospheric stretching, crustal necking and emplacement,and volcanic loading during the early phases of India–Madagascar sep-aration dates back to ca. 90 Ma BP; and 2. The lithospheric deformationcaused by the Reunion hotspot during the northward drift of India at ca.65Ma ago. Thus, there is a point of potential confusion regardingwhichparameter (rift or plume) played the predominant role in formulatingthe anomalously low elastic thickness along these passive margins.Chand and Subrahmanyam (2003) and Chaubey et al. (2008) ruledout the prevalent role of plume, based on the moving rate of Indianplate. According to their idea, the Indian Plate keeps a fast moving rateof 13.5 cm/year over the Reunion hotspot between 66 and 48 Ma andthat the thermal rejuvenation during this time may have been insuffi-cient to change the plate strength.

The traces of Reunion hotspot can be observed in both ECMM andWCMI (Tiwari et al., 2007; Chaubey et al., 2008). Themajor bathymetricfeatures in the ocean to the east of Madagascar such as Nazareth Bank,

Mauritius and Reunion Island, which are considered to be the remnantsof the Reunion hotspot, exhibit anomalously thick crust and significantTe variations. The crustal thicknesses beneath Reunion (~20 km),Mauritius (~25 km), and the Nazareth Bank (~30 km) obtained in thepresent study are consistent with the estimates of Torsvik et al.(2013). Tiwari et al. (2007), using a free-air admittance technique to es-timate Te along the Deccan–Reunion hotspot track, obtained a decreasein Te from 30 km over Reunion and Mauritius to 13 km over theNazareth Bank. They suggested that the higher Te regions resultedfrom intraplate emplacement on an old lithosphere, whereas thelower Te estimates in the Nazareth Bank were due to emplacement onthe flank of the Central Indian Ridge, where both plume and mid-ocean ridge basalts were emplaced on young lithosphere. Our presentTe results in Reunion and Mauritius with their high values (Te ~ 18–20 km), and Nazareth Bank with its low value (Te ~ 5 km) support theconcept of multiple emplacement mechanism, as proposed by Tiwariet al. (2007).

The Laccadive–Chagos ridge in the WCMI is considered to be thetrack of the Indian plate over the Reunion hotspot (see Fig. 5 for thehotspot track). Chaubey et al. (2008) analyzed the isostatic compensa-tion mechanism beneath the Laccadive Ridge using free air admittance,and they obtained a significantly low-Te value of ~2.5 km. Their resultsfavor a model of Airy isostatic compensation beneath the Laccadiveridge that resulted when stretched continental lithosphere was loadedduring an initial stage of rifting. The present study obtained a signifi-cantly low-Te (1–3 km) over the Laccadive Ridge (Fig. 5) with a crustalthickness estimate of ~20–25 km, which may support the idea that thecrustal loads in this ridge segment were isostatically compensated as aresult of thermal rejuvenation of the lithosphere and subsequent volca-nic loading by hotspot magmatism in the Late Cretaceous–Early Tertia-ry. The effect of the Reunion volcanism is also apparent in thecontinental part of the Indian plate. The regional-scale Te map of the

Table 3A comparison of the present Te results (obtained from flexure inversion analysis) with earlier estimates over different structural features in the study area.

Location Previous studies Model method Te (km)

Previous study Present study

Deccan Volcanic Province Tiwari and Mishra (1999) Bouguer coherence 10–15 1–5Jordan and Watts (2005) Forward and non-spectral inverse gravity modeling b5Ratheesh-Kumar et al. (2014) Bouguer coherence 0–5

Western Indian Margin Chand and Subrahmanyam (2003) Free-air admittance 8–15 1–5Sheena et al. (2007, 2012) Bouguer coherence 5–10

Laccadive Ridge Tiwari et al. (2007) Free-air admittance 2 1–3Chaubey et al. (2008) Free-air admittance 2–3Sheena et al. (2007) Bouguer coherence 5Ratheesh-Kumar et al. (2014) Bouguer coherence 0–3

Eastern Madagascar Margin Chand and Subrahmanyam (2003) Free-air admittance 10–13 1–5Réunion–Mauritius Tiwari et al. (2007) Free-air admittance 30 18–20

Trivedi et al. (2012) Flexure inversion 18–20Nazareth Bank Tiwari et al. (2007) Free-air admittance 13 ~5

Trivedi et al. (2012) Flexure inversion 14–18

Table 4A comparison of the present Moho results (obtained from flexure inversion analysis) with earlier estimates over different structural features in the study area.

Location Previous studies Model method Moho depth (km)

Previous study Present study

Western Indian Shelf Chaubey et al. (2002) Multichannel seismic reflection, gravity ~25 25–30Krishna et al. (2002) Forward model of gravity ~25 kmTorsvik et al. (2013) Gravity inversion 25–30

Laccadive Ridge Chaubey et al. (2002) Multichannel seismic reflection ~20 20–25Krishna et al. (2002) Forward model of gravity ~20Torsvik et al. (2013) Gravity inversion ~25

Laxmi Ridge Krishna et al. (2002) Forward model of gravity ~17 ~17Torsvik et al. (2013) Gravity inversion ~15

Eastern Madagascar Shelf Torsvik et al. (2013) Gravity inversion 25–30 25–30Réunion–Mauritius Torsvik et al. (2013) Gravity inversion 20–25 20–25Nazareth Bank Torsvik et al. (2013) Gravity inversion N30 N30




Indian Shield clearly demarcated the Deccan volcanic province affectedby the Reunion hotspot volcanism (Ratheesh-Kumar et al., 2014). In thepresent study, an anomalous low-Te (1–5 km) zone to the north of theDharwar Craton, supports the idea of long lithosphere thermal interac-tion with the Reunion plume centre (Ratheesh-Kumar et al., 2014).

The linear low-Te pattern of the Deccan Volcanic Province coincideswith the Reunion hotspot track (Fig. 5). In contrast, the anomalouslylow-Te zone observed parallel to the shelf region (Fig. 5) shows amark-edly different trend. These two contrasting Te patterns may imply twopossibly different possible tectonic events that resulted in lithosphericdeformation. The Te map of Ratheesh-Kumar et al. (2014) showszones of anomalously low-Te in the western margin of the WesternDharwar Craton and in the adjacent shelf region, which they inferredas thermally and mechanically weakened lithosphere caused by thecombined action of the Marion hotspot and rift-related lithospheric ex-tensional processes. Most importantly, the present study finds that theNNW/SSE trend of anomalously low-Te zone is coinciding with theprominent linear bathymetric features along the shelf basement includ-ing themid-shelf basement ridge, inner-shelf graben, shelfmargin basinand the Prathap Ridge complex (Fig. 5). Subrahmanyam et al. (1995)suggested that the mid-shelf basement ridge and the Prathap Ridgecomplex are rift-related ridges formed during the separation of Indiafrom Madagascar around 84 Ma, and that they followed the pre-existing trends of the Precambrian basement fabric. According toChaubey et al. (2002), the presence of rotated fault blocks at the shelfmargin basin, and the emplacement of the volcanic Prathap Ridge com-plex indicate a failed rift and volcanism of the stretched continental re-gime of the basin.We now suggest that the anomalously low-Te zone isthe sumeffect of the flexural response of the rift-related surface/subsur-face structural features and the volcanic emplacements along theWCMI. Support for our interpretation comes from available geochrono-logical data related to both passive margins. The rifting of India fromMadagascarwas accompanied by the formation of voluminousflood ba-salt flows and dolerite dykes with subordinate rhyolite flows along theriftedmargin of India (Pande et al., 2001). The rhyolites and rhyodacitesfrom St. Mary's island off thewestern coast of southern India have K–Arages in the range of 97–80Ma (Valsangkar et al., 1981), and a 40Ar–39Arage of ca. 86 Ma (Pande et al., 2001) related to rifting of India fromMadagascar. Torsvik et al. (2000) obtained a U–Pb zircon age of ca.91Ma from St.Mary's island, which they linkedwith the late Cretaceousmagmatic province in Madagascar (ca. 84–92 Ma) that contains theAnalalava gabbro pluton (ca. 91 Ma). Also related to the rifting areENE/WSW-striking dykes in Karnataka, western India that have a40Ar–39Ar age of about 88–90Ma (Kumar et al., 2001), and leucogabbroand felsite dykes from southwestern India that have a K–Ar age of ca.85 Ma (Pande et al., 2001). The eastern coast of Madagascar containsseveral mafic–ultramafic complexes, which are remnant signatures ofrifting that is dated at 92–84 Ma (Storey et al., 1995; Melluso et al.,1997; Torsvik et al., 1998; Melluso et al., 2001, 2002, 2005; Mahoneyet al., 2008; Cucciniello et al., 2011a,b). The Antampombato–Ambatovycomplex in the east-central part of the Cretaceous flood basalt provinceof Madagascar has a 40Ar/39Ar incremental heating age of ca. 90 Ma andU–Pb age of ca. 90± 2Ma (Melluso et al., 2005). Mahoney et al. (2008)suggested high-level, pre-breakup lithospheric extension between IndiaandMadagascar, inferred from the great concentration of rhyolite dykesand significant crustal contamination of basalt on the central easterncoast of Madagascar. These lines of evidence clearly suggest that theMarion hotspot and associated rifting processes contributed to theweak strength of both the WCMI and ECMM.

Previous geophysical studies in the present passive margin setting(Todal and Edholm, 1998; Minshull et al., 2008; Yatheesh et al., 2009;Torsvik et al., 2013) used wide-angle seismic data and/or gravity anom-aly data to explain the geometry/structure of the lithosphere and its cor-relation with rift-related structures. The present study of passivemargins based on a Te model is completely independent of other tradi-tional geophysical approaches as it reveals tectonic deformational

variations within the lithosphere. Therefore, Te variations should beinterpreted in such a way that they can help determine which tectonicscenario is suitable to explain the causes of strength variations in a par-ticular tectonic setting. In the present study, the anomalously low Tezones along the continental shelf and adjacent oceanic regimes indicatethe deformational variations within the passive margin lithosphere,which can be best explained by rift-related processes including litho-spheric thinning/necking and hotspot interactions. An outcome of thisdiscussion is that the mirrored deformational signatures (anomalouslylinear low-Te zones) obtained for these passive margins are geneticallyrelated and hence can be correlated to examine their possible conjugatenature.

5.2. Fit of conjugate margins reconstructed from Te correlation

Fig. 5 demonstrates that the zone of lithospheric deformation alongthe WCMI, indicated by an anomalously low-Te pattern, extends in aNW–SE direction for a total length of ~1680 km. This long low-Tezone ismirrored by a similar, uniformly low-Te zone along the completelength of the ECMM (Fig. 6). By matching these characteristic linearlow-Te zones of the two conjugatemargins, we obtain a best-fit positionofMadagascar against India,which is presented in Fig. 10.We examinedthe Moho models derived from flexure inversion analysis to find anypossible match of the conjugate margins with the best-fit position de-duced from our Te model. The present Moho models show a crustalthinning towards the continental margins of India (Fig. 8) andMadagascar (Fig. 9); these define the actual extent of the continentalmargins. Here, the oceanward extent of a continental margin/shelf canbe defined by a rectilinear zone of Moho configuration (~25 km deep).Clearly, Madagascar is characterized by a narrow shelf zone, while theIndian shelf is comparatively broader and its width increases towardsthe north. We matched the two shelf zones with similar Moho configu-rations to the best-fit position derived from the Tematch, and obtaineda close geometrical Moho fit between the continental margins of Indiaand Madagascar (Fig. 11). We then superimposed the bathymetry con-tours onto this best-fit position (Fig. 11), and found that the 1000 misobaths of the two shelf margins also show a reasonably good close-fit. Thus, the ‘best-fit position’ of the continental margins deducedfrom the Te correlation is well confirmed by the Moho and bathymetryconfigurations. Together these produce a unique paleo-continental con-figuration of India against Madagascar.

5.3. Perplexity on India–Madagascar correlation and new interpretations

The exact position of India against Madagascar has long been a mat-ter of debate, and clearly it is important to re-examine previous data inthe light of our current results to better understand, or even resolve, thisproblem.Most of the earlier reconstructions had problems by proposingcorrelations based on single factor such as structure, age, rock type or ageophysical parameter, and also inconsistencies exist in many paleo-fitmodels regarding the exact shape and size of the continents defined forthe reconstructions.

The time of breakup of the facing margins was determined by K–Ardating (Valsangkar et al., 1981) and apatite fission track analysis (Chandand Subrahmanyam, 2003; Emmel et al., 2006).Marks and Tikku (2001)used free-air gravity and topography in combination with magneticanomaly data to reconstruct the gravity and topography fields in theCretaceous in order to determine the correct fit of Africa, Antarcticaand Madagascar. Many published plate reconstructions used thematching of structural lineaments to establish the original form and co-herence of the WCMI and the ECMM (Katz and Premoli, 1979; Storeyet al., 1995; Braun and Kriegsman, 2003; Ghosh et al., 2004;Ishwar-Kumar et al., 2013). Katz and Premoli (1979) defined two possi-ble positions of Madagascar relative to India based on the matching oftectonic lineaments, namely the Bhavani lineament of southern Indiaand the Itremo and Ranotsara lineaments in Madagascar. Braun and




Kriegsman (2003) proposed amodel inwhich the Ranotsara shear zoneof Madagascar is correlated with the Achankovil shear zone of southernIndia, and the Betsimisaraka shear zone with the Moyar shear zone ofsouthern India. In contrast, Tucker et al. (2011) correlated the Angavoshear zone of Madagascar with the Moyar shear zone of southernIndia. Ishwar-Kumar et al. (2013) proposed a plate reconstructionbased on a possible extension of the Betsimisaraka suture inMadagascar, the Kumta suture, which re-enters southern India as theca. 1200 Ma Mercara (Coorg) suture (Santosh et al., 2014). Accordingto Powell et al. (1997), the southern tip of India was over 1000 kmsouth of Madagascar during its initial stage of separation from India,whichwas at around ca. 100Ma across the Albian–Cenomanian bound-ary (Schettino and Scotese, 2005; Gaina et al., 2007). But according toGibbons et al. (2013) the southern tip of India lay at that time~250 km north of the southern edge of Madagascar; to explain thisthey proposed a dextral–transtensional motion between the twocontinents.

Torsvik et al. (2000) postulated an India–Madagascar fit during theearly phase of separation of Madagascar in the Late Cretaceous (after~100 Ma) by correlating the breakup-related paleomagnetic anomaly,which extends sub-parallel with that in SW India. Recently, based onmicroscopic and mesoscopic structures and Th–U–Pb monazite ages,Rekha et al. (2013) proposed a correlation between crustal blocks in

western India and NE Madagascar. Ashwal et al. (2013) proposedIndia–Madagascar position in a late pan-African age Gondwana assem-bly. Their reconstructionwas based on the geochemical andpetrologicalcorrelations of the Mt. Abu granitic pluton (northwestern India) withMalani Igneous Suite and Praslin group granitoids of the Seychellesand northern Madagascar. Mishra et al. (2014) schematically showedthe position of the India–Seychelles bank and Madagascar in a 65–70 Ma plate reconstruction, primarily based on paleostress trends de-duced from field and remote sensing analysis in the western DeccanLarge Igneous Province. They reported a predominantly N–S zone of ex-tensional deformation in the western Deccan region, which theymatched with faults interpreted from seismic data to postulate strike-slip rifting between India and Seychelles.

From geophysical–geochronological studies, Torsvik et al. (2013)proposed that a Paleoproterozoic microcontinental fragment, Mauritia,was situated between southern India and Madagascar at 750 Maconfiguration. However, this model was heavily dependent only on 8spot U–Pb analyses of detrital zircons separated from two basalticbeach sand samples collected from the coast of Mauritius. It is also im-portant to note that the 8 analytical results consists only one concordantage analysis of ~790 Ma. Therefore we consider it premature to becertain on the existence of microcontinental fragments betweenMadagascar and India, and thus we have not included them in our

Fig. 8.Moho configuration of the WCMI (derived from flexure inversion method) super imposed by the topography/bathymetry shaded relief. Acronyms are given in Table 1.




modeling or correlation results. Further, Seychelles microcontinent isplaced far to the north of India–Madagascar in a Late Cretaceous platereconstruction (Torsvik et al., 2000; Gibbons et al., 2013) and thus outof the research focus in the present study.

We now verify the present fit-position of India–Madagascar pas-sive margins in the light of available geological and geophysicaldatasets. A plate tectonic reconstruction of the India–Madagascarpaleo-fit deduced from our Te correlation model (Fig. 10) is present-ed in Fig. 12. It shows how shear/suture zones and lithological unitsin the two continents closely correlate in the proposed ‘best-fit posi-tion’. The present India–Madagascar paleo-position is in closestagreement with the recently published plate tectonic reconstructionbased on shear/suture zones, geochronology and lithology (Ishwar-Kumar et al., 2013). In spite of recent controversial interpretations(e.g., Tucker et al., 2011; Brandt et al., 2014), in the present recon-struction the Betsimisaraka suture zone of northeasternMadagascar (Kröner et al., 2000; Collins and Windley, 2002;Collins, 2006) does correlate well with the recently proposedKumta suture zone (Ishwar-Kumar et al., 2013) and Mercara suture(Santosh et al., 2014) of southwestern India (Fig. 12). On the otherhand, the Te model-based paleo-position is not in complete agree-ment with some other recent shear zone-based models (Collinsand Windley, 2002; Braun and Kriegsman, 2003; Collins et al.,

2007; Tucker et al., 2011). Thus, the regional-scale structural linea-ments and shear-zones proposed by previous studies (e.g., Windleyet al., 1994; Windley and Razakamanana, 1996; Collins andWindley, 2002; Goncalves et al., 2003; Ghosh et al., 2004;Raharimahefa and Kusky, 2006; Chardon et al., 2008;Ishwar-Kumar et al., 2013) provide a better understanding of theTe model and its comparison with traditional continental correla-tions. A major outcome of the present contribution is that the corre-lation of continents proposed using Te data allows the integration ofapparently different data sets including Moho configuration, ba-thymetry, structures, age, rock type and metamorphic condition ina precise way that bring into a best-fit model of India–Madagascarpassive margins (Fig. 12). Accordingly, in this discussion we empha-size the importance of an integrated approach in paleo-continentalcorrelation studies whether using the Te method or other traditionaltools such as geochronology, regional structure and geology.

6. Conclusions

This contribution mapped the rigidity structure along the WesternContinental Margin of India and the Eastern Continental Margin ofMadagascar that reveals zones of significant rift-related lithospheric de-formation and their possible conjugate nature. A high resolution

Fig. 9. Moho configuration of the ECMM (derived from flexure inversion method) super imposed by the topography/bathymetry shaded relief. Acronyms are given in Table 1.




database of undulations on the Moho/Crust–Mantle Interface, derivedfrom gravity and flexure inversion analyses, and their regional correla-tionswith the Te variations, adds a newperspective to thepresent inter-pretations. We demonstrate that elastic thickness is a useful diagnostictool, which can be corroborated and integrated with crustal geometry,bathymetry, structure, lithology and geochronological datasets inorder to evaluate the evolution and deformation of the lithosphere.The following conclusions can be drawn from the present study.

1. The Te andMoho results exhibit significant variations of elastic thick-ness over the continental–oceanic margins of India and Madagascar,which reveal important insights into the evolution and deformationof different lithological units. TheMohodata demonstrate geotecton-ic segmentation with a transition from thick crust (N35 km) beneaththe continents to thin crust (8–15 km) beneath the oceans with atransitional crust (thickness ~ 25 km) beneath the continentalshelves. We observe that the cold and stable segments of the

Fig. 10.A correlation of elastic thicknessmaps of thewesternmargin of India (Fig. 5) and easternmargin of Madagascar (Fig. 6) shows the fit of anomalously low-Te zones (in blue, whichare about 1600 km long) along their conjugate margins. Based on this correlation, the present study deduces a best paleo-fit position of India against Madagascar. Acronyms are given inTable 1. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)




continental lithosphere exhibit higher Te values, while thermally ormechanically rejuvenated lithospheric segments are mechanicallyweak. Most of the oceanic parts of the Indian and African plates in-cluded in our study generally exhibit thinned crust and low-Teranges, whereas the hotspot fossil ridges exhibit variable Te that cor-relate with their emplacement setting.

2. We conclude that the significantly low-Te zones along the WesternContinental Margin of India and Eastern Continental Margin of

Madagascar represent their paleo-rift inception points, affected bysignificant lithospheric extension due to rifting combined with theeffect of Marion hotspot volcanism. The low-Te zone along the west-ernmargin of India is attributed to the presence of a failed rift and ofthe volcanism in the stretched continental lithosphere, which ismanifested by coincident linear structural features along the shelfbasement. The correlation of these mirrored low-Te zones enablesa best possible fit of India against Madagascar. This is confirmed by

Fig. 11. The close-geometrical fit between the Moho configurations of the India–Madagascar shelf margins, reconstructed at the ‘fit position’ deduced from the Te correlation (Fig. 10). Italso shows a close-fit of 1000 m bathymetry representing the shelf margins. Acronyms are given in Table 1.




an excellent geometrical fit between the bathymetry and the Mohoconfiguration of both shelf margins. The derived paleo-fit of the con-tinents is consistent with and supported by geological constraintssuch as the matching of shear zones, lithologies and geochronologi-cal belts.

3. Based on our present results, we assume a close-fit (inset map ofFig. 12) between India and Madagascar before rifting (Lawver et al.,1997). The rift-related stretching and subsequent thinning of the

congruent lithospheric margins led to the formation of a continuousshelf on both margins of India and Madagascar. The increasing de-grees of partial melting demonstrated from north to south by PengandMahoney (1995) reveal that theMarion plume activity off south-ern Madagascar during the time of rifting (Torsvik et al., 2000) mayhave had an integrated effect on the reduced mechanical strength ofthe lithosphere beneath both continental margins. Ultimately, physi-cal separation of the continents possibly resulted in two individual

Fig. 12. Plate tectonic reconstruction map of India–Madagascar paleo-fit deduced from elastic thickness (Fig. 10), Moho configuration and bathymetry (Fig. 11) correlations, depictingmatching of shear zones, lithologies and geochronological belts of mutual tectonic provinces. The inset map demonstrates a possible close-fit of India and Madagascar and the fit of theshear zones before rifting. Acronyms are as in Table 1.Shear zones based on correlations of Collins andWindley (2002) and Ishwar-Kumar et al. (2013). Paleo-coordinates after O'Neill et al., 2003. The inset ismodified after Ishwar-Kumar et al.(2013).




shelves on either side of amid-oceanic ridge (Fig. 12). The separationof India by rifting and then drifting from the relatively stationary con-tinental mass of Madagascar may have been a major reason for thepersistence of their asymmetric conjugate margins. The failed rift andvolcanism of the stretched lithosphere possibly created regional-scale features such as ridges, grabens and faults, and sub-crustalloads such as magmatic underplating along the congruent margin ofIndia,where theflexural response is frozen into the lithosphere and re-sulted in a low-Te anomaly.

4. The present study concludes that passive margins can retain theiroriginal structural and mechanical attributes developed duringrifting, if they are not influenced by later major tectonic events, andhence the effective elastic thickness can be used in such a tectonicsetting as a powerful proxy to examine the conjugate nature of pas-sive margins. Such an approach may facilitate further studies partic-ularly on the analogous occurrence of hydrocarbon deposits in therifted margins of India and Madagascar.

Acknowledgments

We thank Alan Aitken for editorial handling and two anonymousreviewers for constructive comments and suggestions that improvedthe manuscript significantly. R.T. Ratheesh Kumar gratefully acknowl-edges Centenary Research Associate Fellowship of IISc BangaloreR(IA)(RKRT/CEaS)/2014-8808, and the Best Paper Award contingencygrant of Kerala State Council for Science, Technology & Environment(Order No.1272/2014/KSCSTE). We utilized the laboratory facilities de-veloped through Ministry of Earth Sciences, Government of India pro-ject MoES/ATMOS/PP-IX/09. This study is a contribution to ISRO-IIScSpace Technology Cell project ISTC/CEAS/SJK/291.

References

Anderson, O.B., Knudsen, P.O., 1998. Global marine gravity field from ERS-1 and GEOSATgeodetic mission altimetry. Journal of Geophysical Research 103, 8129–8137.

Anderson, O.B., Knudsen, P., Berry, P., Freeman, J., Pavlis, N., Kenyon, S., 2008. The DNSC08Ocean wide altimetry derived gravity field. Presented EGU (2008), session G1,General assembly, Vienna, Austria, April 14–18.

Ashwal, L.D., Solanki, A.M., Pandit, M.K., Corfu, F., Hendriks, B.W.H., Burke, K., Torsvik, T.H.,2013. Geochronology and geochemistry of Neoproterozoic Mt. Abu granitoids, NWIndia: regional correlation and implications for Rodinia paleogeography. PrecambrianResearch 236, 265–281.

Audet, P., Mareschal, J.C., 2004. Variation in elastic thickness in the Canadian Shield. Earthand Planetary Science Letters 226, 17–31.

Bauer,W.,Walsh, G.J., DeWaele, B., Thomas, R.J., Horstwood,M.S.A., Bracciali, L., Schofield,D.I., Wollenberg, U., Lidke, D.J., Rasaona, I.T., Rabarimanana, M.H., 2011. Cover se-quences at the northern margin of the Antongil Craton, NE Madagascar. PrecambrianResearch 189, 292–312.

Beane, J.E., Turner, C.A., Hooper, P.R., Subbarao, K.V., Walsh, J.N., 1986. Stratigraphy,composition and form of the Deccan basalts, Western Ghats, India. BulletinVolcanologique 48, 61–83.

Biswas, S.K., 1987. Regional tectonic framework, structure and evolution of the westernmarginal basins of India. Tectonophysics 135, 307–327.

Biswas, S.K., 1988. Structure of the western continental margin of India and related igne-ous activity. Geological Society of India Memoirs 10, 371–390.

Biswas, S.K., 1999. A review on the evolution of rift basins in India during Gondwana withspecial reference to Western India basin and their hydrocarbon prospects. PINSA 65(3), 261–283.

Braitenberg, C., Pettenati, F., Zadro, M., 1997. Spectral and classical methods in the evalu-ation of Moho undulations from gravity data: the NE Italian Alps and isostasy. Journalof Geodynamics 23, 5–22.

Braitenberg, C., Ebbing, J., Gotze, H.J., 2002. Inverse modelling of elastic thickness by con-volution method: the Eastern Alps as a case example. Earth and Planetary ScienceLetters 202, 387–404.

Braitenberg, C., Wang, Y., Fang, J., Hsu, H.T., 2003. Spatial variations of flexure parametersover the Tibet–Quinghai plateau. Earth and Planetary Science Letters 205, 211–224.

Braitenberg, C., Wienecke, S., Wang, Y., 2006. Basement structures from satellite-derivedgravity field: South China Sea ridge. Journal of Geophysical Research 111, B05407.

Brandt, S., Raith, M.M., Schenk, V., Sengupta, P., Srikantappa, C., Gerdes, A., 2014. Crustalevolution of the Southern Granulite Terrane, south India: new geochronologicaland geochemical data for felsic orthogneissses and granites. Precambrian Research246, 91–122.

Braun, J., Beaumont, C., 1989. A physical explanation of the relationship between flank-uplifts and the breakup unconformity at rifted continental margins. Geology 17,760–764.

Braun, I., Kriegsman, L.M., 2003. Proterozoic crustal evolution of southernmost India andSri Lanka. In: Yoshida, M., Windley, B.F., Dasgupta, S. (Eds.), Proterozoic EastGondwana: Supercontinent Assembly and Breakup. Geological Society of London,Special Publications, 206, pp. 169–202.

Burov, E.B., Diament, M., 1995. The effective elastic thickness of continental lithosphere:what does it really mean? Journal of Geophysical Research 100, 3905–3927.

Chand, S., Subrahmanyam, C., 2003. Rifting between India and Madagascar mechanismand isostasy. Earth and Planetary Science Letters 210, 317–332.

Chand, S., Radhakrishna, M., Subrahmanyam, C., 2001. India–East Antarctica conju-gate margins: gravity and isostasy. Earth and Planetary Science Letters 185,225–237.

Chardon, D., Jayananda,M., Chetty, T.R.K., Peucat, J.J., 2008. Precambrian continental strainand shear zone patterns: South Indian case. Journal of Geophysical Research 113,B08402. http://dx.doi.org/10.1029/2007JB005299.

Chaubey, A.K., Gopala Rao, D., Srinivas, K., Ramprasad, T., Ramana, M.V., Subrahmanyam,V., 2002. Analyses of multichannel seismic reflection, gravity andmagnetic data alonga regional profile across the central-western continental margin of India. Marine Ge-ology 182, 303–323.

Chaubey, A.K., Srinivas, K., Ashalatha, B., Gopala Rao, D., 2008. Isostatic response of theLaccadive Ridge from admittance analysis of gravity and bathymetry data. Journalof Geodynamics 46, 10–20.

Collins, A.S., 2006. Madagascar and the amalgamation of Central Gondwana. GondwanaResearch 9, 3–16.

Collins, A.S., Pisarevsky, S.A., 2005. Amalgamating eastern Gondwana: the evolution of thecircum-Indian orogens. Earth Science Reviews 71, 229–270.

Collins, A.S., Windley, B.F., 2002. Tectonic evolution of central and northern Madagascarand its place in the final assembly of Gondwana. Journal of Geology 110, 325–339.

Collins, A.S., Clark, C., Sajeev, K., Santosh, M., Kelsey, D.E., Hand, M., 2007. Passage throughIndia: the Mozambique ocean suture, high-pressure granulites and the Palghat–Cauvery shear zone system. Terra Nova 19, 141–147.

Courtillot, V., Davaille, A., Besse, J., Stock, J., 2003. Three distinct types of hotspots in theEarth's mantle. Earth and Planetary Science Letters 205, 295–308.

Cucciniello, C., Conrad, J., Grifa, C.,Melluso, L.,Mercurio,M.,Morra, V., Tucker, R.D., Vincent,M., 2011a. Petrology and geochemistry of Cretaceous mafic and silicic dykes and spa-tially associated lavas in central-eastern coastal Madagascar. In: Srivastava, R.K. (Ed.),Dyke Swarms: Keys for Geodynamic Interpretation, pp. 345–375.

Cucciniello, C., Melluso, L., Morra, V., 2011b. New 40Ar–39Ar ages and petrogenesis of theMassif d'Ambre volcano, northern Madagascar. Geological Society of America SpecialPapers 478, 257–281.

Daly, E., Brown, C., Stark, C.P., Ebinger, C.J., 2004. Wavelet and multitaper coherencemethods for assessing the elastic thickness of the Irish Atlantic margin. GeophysicalJournal International 159, 445–459.

deWit, M.J., 2003. Madagascar: heads it's a continent, tails it's an island. Annual Review ofEarth and Planetary Sciences 31, 3–48.

Divins, D.L., 2003. Total Sediment Thickness of the World's Oceans & Marginal SeasNOAANational Geophysical Data Center, Boulder, CO (http://www.ngdc.noaa.gov/mgg/sedthick/sedthick.html).

Emmel, B., Jacobs, J., Kastowski, M., Graser, G., 2006. Phanerozoic upper crustal tectono-thermal development of basement rocks from of central Madagascar: an integratedstructural and fission-track study. Tectonophysics 412, 61–86.

Fourno, J.-P., Roussel, J., 1994. Imaging of the Moho depth in Madagascar through the in-version of gravity data: geodynamic implications. Terra Nova 6, 512–519.

Gaina, C., Müller, R.D., Brown, B., Ishihara, T., Ivanov, S., 2007. Breakup and earlyseafloor spreading between India and Antarctica. Geophysical Journal International.http://dx.doi.org/10.1111/j.1365-246X.2007.03450.x.

Ghosh, J.G., de Wit, M.J., Zartman, R.E., 2004. Age and tectonic evolution ofNeoproterozoic ductile shear zones in the Southern Granulite terrane of India,with implications for Gondwana studies. Tectonics 23, TC3006. http://dx.doi.org/10.1029/2002TC001444.

Gibbons, A.D., Whittaker, J.M., Muller, R.D., 2013. The breakup of East Gondwana: assim-ilating constraints from Cretaceous ocean basins around India into a best-fit tectonicmodel. Journal of Geophysical Research 118, 808–822.

Goncalves, P., Nicollet, C., Lardeaux, J.-M., 2003. Finite strain pattern in Andriamena unit(north-central Madagascar): evidence for late Neoproterozoic–Cambrian thrustingduring continental convergence. Precambrian Research 123, 135–157.

Ishwar-Kumar, C., Windley, B.F., Horie, K., Kato, T., Hokada, T., Itaya, T., Yagi, K., Gouzu, C.,Sajeev, K., 2013. A Rodinian suture in western India: new insights on India–Madagascar correlations. Precambrian Research 236, 227–251.

Jay, A.E., Widdowson, M., 2008. Stratigraphy, structure and volcanology of the SE Deccancontinental flood basalt province: implications for eruptive extent and volumes.Journal of the Geological Society of London 165, 177–188.

Jiménez-Díaza, A., Ruiza, J., Pérez-Gussinyéc, M., Kirby, J.F., Álvarez-Gómez, J.A., Tejero, R.,Capote, R., 2014. Spatial variations of effective elastic thickness of the lithosphere inCentral America and surrounding regions. Earth and Planetary Science Letters 391,55–66.

Jordan, T.A., Watts, A.B., 2005. Gravity anomalies, flexure and the elastic thickness struc-ture of the India–Eurasia collisional system. Earth and Planetary Science Letters 236,732–750.

Katz, M.B., Premoli, C., 1979. India and Madagascar in Gondwana land based on matchingPrecambrian lineaments. Nature 279, 312–315.

Kirby, J.F., Swain, C.J., 2008. An accuracy assessment of the fan wavelet coherencemethod for elastic cthickness estimation. Geochemistry, Geophysics, Geosystems 9(3), Q03022 (Correction: Geochemistry Geophysics Geosystems 9(5) (2008) Q05021).

Krishna, M.R., Verma, R.K., Purushotham, A.K., 2002. Lithospheric structure below theeastern Arabian Sea and adjoining West Coast of India based on integrated analysisof gravity and seismic data. Marine Geophysical Researches 23, 25–42.



http://refhub.elsevier.com/S1342-937X(14)00220-2/rf0010



















































http://dx.doi.org/10.1029/2007JB005299































http://www.ngdc.noaa.gov/mgg/sedthick/sedthick.html

http://www.ngdc.noaa.gov/mgg/sedthick/sedthick.html






http://dx.doi.org/10.1111/j.1365-246X.2007.03450.x

http://dx.doi.org/10.1029/2002TC001444



























PGM7

Highlight

Kröner, A., Hegner, E., Collins, A.S., Windley, B.F., Brewer, T.S., Razakamanana, T., Pidgeon,R.T., 2000. Age and magmatic history of the Antananarivo block, Central Madagascar,as derived from zircon geochronology and Nd isotopic systematics. American Journalof Science 300, 251–288.

Kumar, Anil, Pande, K., Venkatesan, T.R., Bhaskar Rao, Y.J., 2001. The Karnataka Late Cre-taceous dykes as products of the Marion hotspot at the Madagascar–India break-upevent: evidence from 40Ar/39Ar geochronology and geochemistry. Geophysical Re-search Letters 28, 2715–2718.

Kusky, T.M., Toraman, E., Raharimahefa, T., Rasoazanamparany, C., 2010. Active tectonicsof the Alaotra–Ankay graben system, Madagascar: possible extension of Somalian–African diffuse plate boundary. Gondwana Research 18, 274–294.

Lawver, L.A., Gahagan, L.M., Dalziel, I.W.D., 1997. A tight fit-early Mesozoic Gondwana, aplate reconstruction perspective. Memoir, 53. National Institute of Polar Research,Tokyo, pp. 214–229.

Lowry, A.R., Smith, R.B., 1995. Strength and rheology of the western US Cordillera. Journalof Geophysical Research 100, 17947–17963.

Mahoney, J.J., Saunders, A.D., Storey, M., Randriamanantenasoa, A., 2008. Geochemistry ofthe Volcan de l'Androy basalt–rhyolite complex, Madagascar Cretaceous igneousprovince. Journal of Petrology 49, 1069–1096.

Marks, K.M., Tikku, A.A., 2001. Cretaceous reconstructions of East Antarctica, Africa andMadagascar. Earth and Planetary Science Letters 186, 479–495.

Melluso, L., Morra, V., Brotzu, P., Razafiniparany, A., Ratrimo, V., Razafimahatratra, D.,1997. Geochemistry and Sr-isotopic composition of the late Cretaceous flood basaltsequence of northern Madagascar: petrogenetic and geodynamic implications.Journal of African Earth Sciences 34, 371–390.

Melluso, L., Morra, V., Brotzu, P., Mahoney, J.J., 2001. The Cretaceous igneous province ofMadagascar: geochemistry and petrogenesis of lavas and dykes from the central-western sector. Journal of Petrology 42, 1249–1278.

Melluso, L., Morra, V., Brotsu, P., D'antonio, M., Bennio, L., 2002. Petrogenesis of the LateCretaceous tholeiitic magmatism in the passive margins of northeasternMadagascar. Geological Society of America Special Papers 362, 81–85.

Melluso, L., Morra, V., Brotzu, P., Tommasini, S., Renna, M.R., Duncan, R.A., Franciosi, L.,d'Amelio, F., 2005. Geochronology and petrogenesis of the CretaceousAntampombato–Ambatovy complex and associated dyke swarm, Madagascar. Jour-nal of Petrology 46, 1963–1996.

Minshull, T.A., Lane, C.I., Collier, J.S., Whitmarsh, R.B., 2008. The relationship betweenrifting and magmatism in the northeastern Arabian Sea. Nature Geoscience 1,463–467.

Mishra, A.A., Bhattacharya, G., Mukherjee, S., Bose, N., 2014. Near N–S paleo-extension inthe western Deccan region, India: does it link strike-slip tectonics with India–Seychelles rifting? International Journal of Earth Science (Geologische Rundschau).http://dx.doi.org/10.1007/s00531-014-1021-x (in press).

Nair, R.R., Maji, T.K., Maiti, T., Kandpal, S.C., Ratheesh Kumar, R.T., Shekhar, S., 2011.Multitaper coherencemethodfor appraising the elastic thickness of the Indonesian ac-tive continental margin. Journal of Asian Earth Sciences 40, 326–333.

National Oceanic and Atmospheric Administration, 2003. General bathymetric chart ofoceans. http://www.ngdc.noaa.gov/mgg/gebco/grid.

O'Neill, C., Müller, D., Steinberger, B., 2003. Geodynamic implications of moving IndianOcean hotspots. Earth and Planetary Science Letters 205, 151–168.

Pande, K., Sheth, H.C., Bhutani, R., 2001. 40Ar–39Ar age of the St. Mary's Islands volcanics,southern India: record of India–Madagascar break-up on the Indian subcontinent.Earth and Planetary Science Letters 193, 39–46.

Parker, R.L., 1973. The rapid calculation of potential anomalies. Geophysical Journal of theRoyal Astronomical Society 31, 447–455.

Peng, Z.X., Mahoney, J.J., 1995. Drill hole lavas from the northwestern Deccan Traps, andthe evolution of Réunion hotspot mantle. Earth and Planetary Science Letters 134,169–185.

Pérez-Gussinyé, M., Watts, A.B., 2005. The long-term strength of Europe and its implica-tions for plate-forming processes. Nature 436, 381–384.

Pérez-Gussinyé, M., Lowry, A.R., Watts, A.B., Velicogna, I., 2004. On the recovery of the ef-fective elastic thickness using spectral methods: examples from synthetic data andfrom the Fennoscandian Shield. Journal of Geophysical Research 109.

Pérez-Gussinyé, M., Metois, M., Fernández, M., Vergés, J., Fullea, J., Lowry, A.R., 2009. Effec-tive elastic thickness of Africa and its relationship to other proxies for lithosphericstructure and surface tectonics. Earth and Planetary Science Letters 287 (1–2),152–167.

Powell, C., McA, Li, Z.X., Muller, D., Watkeys, M.K., 1997. The India–Madagascar transformboundary: implications for Jurassic and Cretaceous Gondwanaland breakup. In: Cox,R., Ashwal, L.D. (Eds.), Proceedings of the UNESCO-IUGS-IGCP-348/368 InternationalField Workshop on the Proterozoic Geology of Madagascar. Miscellaneous Publica-tion, 5. Gondwana Research Group, Osaka City University, pp. 73–74.

Raharimahefa, T., Kusky, T.M., 2006. Structural and remote sensing studies of thesouthern Betsimisaraka Suture, Madagascar. Gondwana Research 10 (1–2),186–197.

Ratheesh Kumar, R.T., Maji, T.K., Nair, R.R., 2010. Assessment of flexural analysis appliedto the Sumatra–Java subduction zone. Journal of Earth System Sciences 119 (5),717–730.

Ratheesh Kumar, R.T., Windley, B.F., 2013. Spatial variations of effective elastic thicknessover the Ninetyeast Ridge and implications for its structure and tectonic evolution.Tectonophysics 608, 847–856.

Ratheesh Kumar, R.T., Tanmay, K.M., Kandpal, S.C., Sengupta, D., Nair, R.R., 2011. Elasticthickness estimates at north-east passive margin of North America and its implica-tions. Journal of Earth System Science 120 (3), 1–12.

Ratheesh Kumar, R.T., Windley, B.F., Rajesh, V.J., Santosh, M., 2013. Elastic thickness struc-ture of the Andaman subduction zone: implications for convergence of theNinetyeastRidge. Journal of Asian Earth Sciences 78, 291–300.

Ratheesh-Kumar, R.T., Windley, B.F., Sajeev, K., 2014. Tectonic inheritance of the IndianShield: new insights from its elastic thickness structure. Tectonophysics 615–616,40–52.

Reeves, C., de Wit, M., 2000. Making ends meet in Gondwana: retracing the trans-forms of the Indian Ocean and reconnecting continental shear zones. TerraNova 12, 272–280.

Rekha, S., Viswanath, T.A., Bhattacharya, A., Prabhakar, N., 2013. Meso/Neoarchean crustaldomains along the north Konkan coast, western India: the Western Dharwar Cratonand the Antongil–Masora Block (NE Madagascar) connection. Precambrian Research233, 316–336.

Santosh, M., Yang, Q.Y., Shaji, E., Tsunogae, M., RamMohan, M., Satyanarayanan, M., 2014.An exotic Mesoarchean microcontinent: the Coorg, Block, southern India. GondwanaResearch. http://dx.doi.org/10.1016/j.gr.2013.10.005 (in press).

Schettino, A., Scotese, C.R., 2005. Apparent polar wander paths for the major continents(200 Ma to the present day): a palaeomagnetic reference frame for global plate tec-tonic reconstructions. Geophysical Journal International 163, 727–759.

Seward, D., Grujic, D., Schreurs, G., 1999. Exhumation history of the East Madagascar con-tinental margin: inferences from apatite fission track analysis. Journal of African EarthSciences 27, 75–78.

Sheena, V.D., Radhakishna, M., Subrahmanyam, C., 2007. Estimates of effective elasticthickness along the southwest continental margin of India using coherence analysisof gravity and bathymetry data — geodynamic implications. Journal of the GeologicalSociety of India 70, 475–487.

Sheena, V.D., Radhakrishna, M., Chand, S., Subrahmanyam, C., 2012. Gravity anomalies,crustal structure and rift tectonics at the Konkan and Kerala basins, western conti-nental margin of India. Journal of Earth System Science 121 (3), 813–822.

Simons, F.J., Van Der Hilst, R.D., 2002. Age-dependent seismic thickness and mechanicalstrength of the Australian lithosphere. Geophysical Research Letters 29 (11), 1529.http://dx.doi.org/10.1029/2002GL014962.

Simons, F.J., Van Der Hilst, R.D., Zuber, M.T., 2003. Spatio-spectral localization of iso-static coherence anisotropy in Australia and its relation to seismic anisotropy:implication for lithospheric deformation. Journal of Geophysical Research 108(ETG 1–8).

Stern, T., Brink, U.S.T., 1989. Flexural uplift of the Transantarctic mountains. Journal ofGeophysical Research 94, 10315–10330.

Storetvedt, K.M., Mitchell, J.G., Abranches, M.C., Maaloe, S., Robin, G., 1992. The coast-parallel dolerite dykes of East Madagascar: age of intrusion of intrusion,remagnetization and tectonic aspects. Journal of African Earth Sciences 15, 237–249.

Storey, M., Mahoney, J.J., Saunders, A.D., Duncan, R.A., Kelley, S.P., Coffin, M.F., 1995.Timing of hotspot-related volcanism and the breakup of Madagascar and India.Science 267, 852–855.

Storey, M., Mahoney, J.J., Saunders, A.D., 1997. Cretaceous basalts in Madagascar and thetransition between plume and continental lithosphere mantle sources. In:Mahoney, J.J., Coffin, M.F. (Eds.), Large Igneous Provinces. American GeophysicalUnion, Monograph, 100, pp. 95–122.

Subrahmanyam, C., Chand, S., 2006. Evolution of the passive continental margins ofIndia—a geophysical appraisal. Gondwana Research 10, 167–178.

Subrahmanyam, V., Gopala Rao, D., Ramana, M.V., Krishna, K.S., Murthy, G.P.S.,Gangadhara Rao, M., 1995. Structure and tectonics of the Southwestern ContinentalMargin of India. Tectonophysics 249, 267–282.

Tassara, A., Swain, C., Hackney, R., Kirby, J., 2007. Elastic thickness structure of SouthAmerica estimated using wavelets and satellite-derived gravity data. Earth and Plan-etary Science Letters 253, 17–36.

Tiwari, V.M., Mishra, D.C., 1999. Estimation of effective elastic thickness from gravityand topography data under the Deccan Volcanic province, India. Earth and PlanetaryScience Letters 171 (2), 289–299.

Tiwari, V.M., Grevemeyer, I., Singh, B., Phipps Morgan, J., 2007. Variation of effective elas-tic thickness andmelt production along the Deccan–Reunion hotspot track. Earth andPlanetary Science Letters 264, 9–21.

Todal, A., Edholm, O., 1998. Continental margin off Western India and Deccan LargeIgneous Province. Marine Geophysical Researches 20, 273–291.

Torsvik, T.H., Tucker, R.D., Ashwal, L.D., Eide, E.A., Rakotolosofo, N.A., de Wit, M.J., 1998.Late Cretaceous magmatism in Madagascar. Paleomagnetic evidence for a stationaryMarion hotspot. Earth and Planetary Science Letters 164, 221–232.

Torsvik, T.H., Tucker, R.D., Ashwal, L.D., Carter, L.M., Jamtveit, B., Vidyadharan, K.T.,Venkataramana, P., 2000. Late Cretaceous India–Madagascar fit and timing ofbreakup-related magmatism. Terra Nova 12, 220–225.

Torsvik, T.H., Amundsen, H., Hartz, E.H., Corfu, F., Kusznir, N., Gaina, C., Doubrovine, P.V.,Steinberger, B., Ashwal, L.D., Jamtveit, B., 2013. A Precambrian microcontinent inthe Indian Ocean. Nature Geoscience 6, 223–227.

Trivedi, D., Tanmay, K.M., Sengupta, D., Nair, R.R., 2012. Reappraisal of effective elasticthickness in the southwest Indian Ocean, and its possible implications. Annals ofGeophysics 55, 2. http://dx.doi.org/10.4401/ag-5171.

Tucker, R.D., Roig, J.Y., Delor, C., Amerlin, Y., Goncalves, P., Rabarimanana, M.H., Ralison, A.V.,Belcvher, R.W., 2011. Neoproterozoic extension in the Greater Dharwar Craton: areevaluation of the “Betsimisaraka suture” in Madagascar. Canadian Journal of EarthSciences 48, 389–417.

Valsangkar, A.B., Radhakrishnamurthy, C., Subbarao, K.V., Beckinsale, R.D., 1981. Paleo-magnetism and potassium–argon age studies of acid igneous rocks from St. MaryIslands. Geological Society of India 3, 265–276.

Watts, A.B., 1992. The effective Te of the lithosphere and the evolution of foreland basins.Basin Research 4, 169–178.

Watts, A.B., 2001. Isostasy and Flexure of the LithosphereCambridge University Press,Cambridge, UK.

Watts, A.B., Burov, E., 2003. Lithospheric strength and its relationship to the elastic andseismogenic layer thickness. Earth and Planetary Science Letters 213, 113–131.








































http://dx.doi.org/10.1007/s00531-014-1021-x



http://www.ngdc.noaa.gov/mgg/gebco/grid


































































http://dx.doi.org/10.1029/2002GL014962





































http://dx.doi.org/10.4401/ag-5171














Watts, A.B., Torné, M., 1992. Subsidence history, crustal structure, and thermal evolutionof the Valencia Trough: a young extensional basin in the western Mediterranean.Journal of Geophysical Research B13, 20,021–20,041.

Widdowson, M., 1997. Tertiary palaeosurfaces of the SW Deccan western India: implica-tions for passive margin uplift. In: Widdowson, M. (Ed.), Palaeosurfaces: Recognition,Reconstruction and Palaeoenvironmental Interpretation. Geological Society ofLondon, Special Publications, 120, pp. 221–248.

Windley, B.F., Razakamanana, T., 1996. The Madagascar–India connection in a Gondwanaframework. In: Santosh, M., Yoshida, M. (Eds.), The Archaean and Proterozoic Ter-rains in Southern India Within East Gondwana. Gondwana Research Group Memoir,3, pp. 25–37.

Windley, B.F., Razafiniparany, A., Razakamanana, T., Ackermand, D., 1994. Tectonic frame-work of the Precambrian of Madagascar and its Gondwana connections: a review andreappraisal. Geologische Rundschau 83, 642–659.

Wyer, P., Watts, A.B., 2006. Gravity anomalies and segmentation at the East Coast, USAcontinental margin. Geophysical Journal International 166, 1015–1038.

Yatheesh, et al., 2009. Early oceanic opening off Western India–Pakistan margin: the GopBasin revisited. Earth and Planetary Science Letters 284 (3–4), 399–408.

Yin, A., Harrison, T.M., 2000. Geological evolution of the Himalayan–Tibetan orogen.Annual Review of Earth and Planetary Sciences 28, 211–280.

Zutshi, P.L., Jain, M.M., Shah, T., Malhotra, S., 1995. Geology and hydrocarbon resources ofthe Indian offshore region. Oil Asia 1–21.


























Editorial

Developing unconventional oil and gas resources in South Asia

This is a special issue on the proceedings of the conference on‘‘Developing unconventional oil and gas resource in South Asia’’-named ‘‘DUOG 2013’’ heldMarch 1 toMarch 3, 2013 at Indian Insti-tute of Technology Madras (IIT Madras). The DUOG conferencewitnessed 300 participants and happened because of the vibrantefforts of Society of Petroleum Engineering student chapter IITMadras. The purpose of the conference was to highlight the currentproblems faced by the oil and gas industry and to find new ways tomitigate them.

The oil and gas industry faces numerous problems which can bebroadly divided into five categories involving activities from explo-ration to production. Each of the issues needs to be addressed. Thechallenges facing the industry related to

1. Increasing the recovery factor (RF)2. In-situ molecular manipulation3. Carbon-capture and sequestration4. Produced water management5. High resolution subsurface imaging

The typical recovery factor could be 35–45%. The key controls toincrease RF are sweep efficiency (volume swept by injected fluid/total volume of reservoir) and displacement efficiency (oil recov-ered/oil in place in the swept volume). A few methods which arein research or in prototype stage or could be developed as uncon-ventional methods in the future, wherever further improvementare possible include thermal and miscible gas flood methods(mature/high displacement efficiency/low sweep efficiency),co-injection of gases; water; nanoparticles and surfactants andEnhanced Oil Recovery (EOR) with surfactants and polymers. Onechallenge is the in-site molecular manipulation of the context of(mainly) extra-heavy oil (bitumen) and oil shale/kerogen (severaltrillion barrels of hydrocarbon are too deep to be accessed usingconventional recovery methods and would involve difficult surfaceprocessing). In such cases, down-hole processing, especially forkerogen, could be used. Going further, heat, catalysts, and microbescould be applied. In short, it is important to understand processesin the presence of reservoir heterogeneity as well as processeswhich are energetically efficient and fast at reservoir conditions.For example, for gas reservoirs with hydrogen sulphide, newmethods could be developed wherein only sweet gases could beproduced leaving the sour gas in-situ reservoir.

Oil and gas industry technology could also be used to addressenvironmental concerns. CO2 produced from burning fossil fuels/other industrial operations is widely regarded as provokingsignificant climate change. Carbon capture and sequestration(CCS) in geological formations is one application which could beexplored more extensively in the future. The present issuespertaining to such methods are dissolution of CO2 in a monoetha-nolamine solvent (MEA) which is still a very expensive idea. New

cost-effective technology is needed here. Also, there is a need toprove the long-term safety for sequestration, which is difficult be-cause of geologic variability. Currently, CO2 sequestration is donein reservoirs by injecting captured CO2 into oil reservoirs to en-hance oil recovery.

There is also the challenge of reusing the water used in welloperation and maintenance, of which there are significantly largequantities particularly in well construction and water flooding (bil-lions of barrels) in unconventional reservoirs (2–9 million gallons/well = 7–35 Km3). The challenge is that this water cannot be di-rectly discharged to estuaries or aquifers as it is chemically intox-icated and unfit for drinking or agricultural purposes. Along withoil and gas, reservoirs also produce large amounts of water fromreservoir pore space, from hydraulic fracturing (flows back to sur-face), and from dissolved species. The cost of treatment before dis-charge of such water is approximately USD 40 billion. In suchsituations, it becomes imperative to develop water managementpractices to yield clean, solid-free brines, monitor chemicals usedin fracturing, and recycle fracture flowback brines and producedwater in an environmentally safe contaminant disposal format.Development of new ideas in the area of cost-effective techniquesfor removing particles, chemicals, and oil, and reusing the recycledwater as potable drinking water is the need of hour.

However, all these paradigms and their cost-effective develop-ment are related to one important aspect higher resolution ofsub-surface imaging. Getting higher resolution sub-surface imagesis not a new challenge; instead, it is an old problem which contin-ues to plague the industry. Though some progress has been madeextensive research is still necessary to increase sub-surface imag-ing resolution. The various techniques deployed, include seismicimaging, well coring (cutting and collecting samples of rock whiledrilling), logging (measuring near-wellbore rock properties duringand after drilling) and well testing (analysing pressure and rate vs.time during production). Although, in the past decade, larger com-puting systems, better sources and detectors, and multiple datasensing (4D) have enhanced resolution, some limitations remainin terms of restrictions on the location, cost of installing seismicsources and receivers, seismic wavelengths that can be transmittedthrough the rock,coring and logging only sample a very small vol-ume of reservoirs, and so on. There is the need to increase both theresolution of images and volume imaged from wells as well asthose beneath massive salt layers (which substantially degradecurrent seismic images).

The works which attempt to address these issues are high-lighted here.

Today, we know of over 230 gas hydrate fields in the worldwhich hold potential as a future energy source. Potential reservesof gas hydrates are over 1.5 � 1016 m3. It is understood that about97% of the gas hydrate deposits are offshore and the rest 3% are

2213-3976/$ - see front matter � 2014 Published by Elsevier Ltd.http://dx.doi.org/10.1016/j.juogr.2013.12.003

Journal of Unconventional Oil and Gas Resources 6 (2014) 1–3

Contents lists available at ScienceDirect

Journal of Unconventional Oil and Gas Resources

journal homepage: www.elsevier .com/locate / juogr

http://crossmark.crossref.org/dialog/?doi=10.1016/j.juogr.2013.12.003&domain=pdf

http://dx.doi.org/10.1016/j.juogr.2013.12.003

http://dx.doi.org/10.1016/j.juogr.2013.12.003

http://www.sciencedirect.com/science/journal/22133976

http://www.elsevier.com/locate/juogr

PGM7

Highlight

PGM7

Highlight

onshore, highlighting the need to develop methods to profitablyand sustainably extract these deposits from inhospitable condi-tions and make them viable for energy generation.

Similar to gas hydrates, another unconventional resource whichcould be developed extensively in the future is Coal Bed Methane(CBM). CBM is the trapped methane in the coal seams and blocks,and given the energy content of methane and vast reserves of coal,it has emerged as one of the favorite sources of unconventional en-ergy in the current scenario. India’s energy needs are growing rap-idly and the total energy requirement of the country is likely toincrease from the current level of 500 MTOe to 1536-1887 MTOeby 2031-32 and given the fact that coal is the primary energysource of India, CBM could be an important player.The coal sectorcontributes about 8% of the total anthropogenic methane emission(IEA) globally. Considering the impact of methane emissions asso-ciated with coal mining on climate change and its importance as anadditional clean energy resource for an energy-starved country, thedevelopment of Coal-bed Methane on a commercial scale is one ofthe priority areas of the Government of India. Thus, there is im-mense potential for the development of CBM resources in thecountry to achieve the goal of sustainable development.

There is great potential for energy development in the oil andgas sector in terms of the production of shale gas or tight gas res-ervoirs after the boom in gas market. However, the spurt in hydrofracking, injection methods and so on in various reservoirs havealso helped in developing advanced stimulation technologies forsecondary and subsequent stages of production. Stimulation in-volves re-energizing the reservoir after it has lost its primary en-ergy drive which means that it is no longer able to produce onits natural energy drive. Artificial methods are then employed toreopen and reproduce such reservoirs. With the advancement oftechnologies, this has become more economically viable than be-fore. Mr. Iman Oraki Kohshour has done a study on Integrated Geo-logic Modeling and Reservoir Simulation of Umiat: A FrozenShallow Oil Accumulation in National Petroleum Reserve of Alaska,to highlight an example. A static model was built using the originaldata and an anisotropic model was subsequently incorporated.Using Monte Carlo simulations, uncertainties in OOIP estimatewere predicted. Since the study area was shallow-frozenlandscape, various temperature-related parameters were also eval-uated. Their study contributes to the understanding of uncertain-ties in resource estimates and evaluating ranges of oil recoveryin reservoir modeling.

There are risks associated with the production and explorationof unconventional resources. Coalbed methane is one of the provenand most accepted unconventional energy resource. Along withmodeling of reservoir injection and stimulation patterns, coalbedmethane studies should focus on understanding the various param-eters associated with drilling for extraction of gas. The process ofexploring or drilling for gas reservoirs, itself involves risks pertain-ing to over-pressure or pockets of low pressure. These conditionscould materialize into blow-out or sudden spurge of oil/gas, result-ing in huge losses monetarily, environmentally, and personally. Thecommercial viability of coalbed methane play depends on mini-mum investment during the exploration as well as the develop-ment and production phases in which directional drilling isusually employed. However, the development of a coalbedmethanefield would require a large number of wells. Therefore, during thedevelopment phase, one of the important aspects is to bring downthe drilling time by optimizing rate of penetration and other relateddrilling parameters without compromising on wellbore stability.Mr. Pallab Kumar Mazumdar has conducted a case study on opti-mizing drilling parameters in Raniganj Formation, Essar CoalbedMethane Block. This paper offers a direction to understand the

drilling parameters to achieve optimized ROP (Rate of Penetration)based on real-time drilling data gathered from directional wells.

Moving over, new stimulation strategies are further enhancedin various conventional reservoirs as well. Due to the ever-risingdemand for petroleum across the globe and substantial decreasein the rate of discovery of new fields, it has become necessary torecover as much hydrocarbon as possible from the available re-sources. The abundance of multiple vertically distributed discretereservoirs or resources contained in long productive intervals isrelatively high. Thus, it is inevitable that technology should bedeveloped to ensure economical recovery of maximum possiblehydrocarbons from this type of reservoir. Khush D. Desai’s studyholds that multi-zone stimulation technology is an importantmethod to increase the recovery factor for such reservoirs. Multi-zone stimulation technology, which involves of stimulating allzones individually, is based the idea thatoperations taking placein one zone do not affect those in the other zones. Such intelligentstimulation technologies seem to show that this technology holdsgreat promise for sustainable and economical field developmentand that it will help considerably in overcoming the hydrocarbondemand-supply gap.

It is necessary to check out prospects and potential reservoirsbefore exploration. A study by Mr. R Raajiv Menon in the contextof exploration and production issues in South Asia throws lighton the possible problems during prospecting and the challengesfaced subsequently during production. As the development ofany unconventional source of energy requires more than just sci-ence, the entire geopolitical landscape and the local environmentbecome important stakeholders in the process. He takes the partic-ular case of South Asia and shows the myriad problems faced withregard to the development of unconventional resources. He showshow unconventional resources, though discovered about a centuryago, never gained momentum due to the complexities of the tech-nologies involved and more primarily due to the readily availableconventional sources. South Asian countries in particular are heav-ily dependent on oil imports from the Middle East nations. Due tothe increasing instability in oil prices coupled with regional insta-bility, nations in South Asia need to invest in the production ofalternate resources to meet their future energy requirements. Inthe present scenario, unconventional resources are being increas-ingly considered the bridging option between rapidly depletingconventional resources and the nascent upcoming renewable andthorium (nuclear) based energy sources. He also highlights someissues that need to be addressed by the governments of such coun-tries and the efforts needed to attain energy self-sufficiency fordevelopment. He points out the need to develop unconventionalforms of energy.

Mr. Sarthak Shailesh Shah further investigates the CBM re-source prospect in the Indian scenario. He compares the Indianand the American CBM scenario and geopolitical context. Indiahas cbm potential of around 70tcf gas which can supplement thedeclining conventional gas production and also cater to the grow-ing demands of the country. Indian gas production is around 30%short of gas it requires. In the USA, around 100 tcf of cbm are eco-nomically recoverable and the current production level is approx-imately 3 tcf of cbm per year. In comparison, in India we have asuccessful pilot project at Raniganj which produces 22,000 scm/D. So, he shows the challenges that India faces in developingCBM fields which requires a phased and structured evaluation pro-gramme right from the fairway identification stage to full scaledevelopment for maximizing ROI (Return on Investment). CBMhas a very bright future in South Asia if proper steps are taken inthis direction The reasons why India has failed in tapping this vastsource of unconventional energy are discussed.

2 Editorial / Journal of Unconventional Oil and Gas Resources 6 (2014) 1–3

The study of gas hydrate dissociation and kinetics could paveway for more viable extraction and exploitation of gas hydrates.According to Mr. Vikash Kumar Saw, this study of kinetics couldgreatly improve our understanding of gas hydrates. He studiedkinetics of methane hydrate formation and its dissociation in thepresence of non-ionic surfactant tergitol and found that sub cool-ing is required to initiate hydrate formation. Following extensiveinvestigation of phase equilibrium pressure and temperature, he

came to conclusion that the phase equilibrium curves of methanehydrate are not affected on varying the concentration of Tergitol.

Dr.Rajesh R. NairAssociate Professor of Petroleum Engineering,

Department of Ocean Engineering, Indian Institute of TechnologyMadras,Chennai, India

E-mail address: [email protected]

Available online 31 December 2013

Editorial / Journal of Unconventional Oil and Gas Resources 6 (2014) 1–3 3

mailto:[email protected]

PGM7

Highlight

Gondwana Research - Department of Ocean Engineering · India–Madagascar paleo-ﬁtbasedonﬂexural isostasy of their rifted margins R.T. Ratheesh-Kumara,⁎, C. Ishwar-Kumara,B.F.Windleyb,

Documents