Origin of the Betic-Rif mountain belt - Imperial College … AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT 507 $trik•sllp Alkali I Pllocene m shortening Basalts $-- Messlnlan

TECTONICS, VOL. 16, NO. 3, PAGES 504-522, JUNE 1997

Origin of the Betic-Rif mountain belt

Lidia Lonergan Department of Geology, Imperial College, London

Nicky White Bu]laxd Laboratories, University of Cambridge, Cambridge,

Abstract. In recent years, the origin of the Betic- Rif orocline has been the subject of considerable debate. Mudi of this debate has, focused on mechanisms

required to generate rapid late-orogenic extension with coeval shortening. Here we summarize the principal geological and geophysical observations and propose a model for the Miocene evolution of the Betic-lq. if moun-

tain belts, which is compatible with the evolution of the rest of the western Mediterranean. We regard palaeomagnetic data, which indicate that there have been large rotations about vertical axes, and earthquake data, which show that deep seismicity occurs beneath the Alboran Sea, to be the most significant data sets. Neither data set is satisfactorily accounted for by models which invoke convective removal or delamination of

lithospheric mantle. Existing geological and geophysical observations are, however, entirely consistent with the existence of a subduction zone which rolled or peeled back until it collided with North Africa. We suggest that this ancient subducting slab consequently split into two fragments, one of which has continued to roll back, generating the Tyrrhenian Sea and forming the present- day Calabnan Arc The other slab fragment rolled back to the west, generating the Alboran Sea and the Betic- Rif oroctine during the early to middle Miocene.

Introduction

Extensional basins associated with convergent plate boundaries are common in the western Pacific Ocean and in the Mediterranean Sea [e.g., Vine and Smith, 1981]. Whilst many of the Pacific Ocean basins have clearly formed as a result of back arc extension caused by subduction zone rollback, the origin of geometri- cally similar basins in the western Mediterranean Sea has proved more controversial. Much of the current debate is focused on the origin of the Alboran Sea together with the surrounding Betic-Pfif mountain belts, the most westerly of the Alpine mountain chains of southern Europe (Figure 1). This orocline developed

Copyright 1997 by the American Geophysical Union. Paper mlmber 96T003937. 0278-7407/97/96T0-03937512.00

504

during, and partly in response to, Late Mesozoic to Ter- tiary convergence between Africa and Iberia. In recent years, three principal tectonic models have been proposed to account for its geometry and evolution: (1) irapid westward motion of a rigid Alboran microplate, (2) subductmn zone rollback, and (3) radial extensional collapse caused by rapid convective removal or delann- nation of hthosphenc mantle.

Here we first summarize the important geological and geophysical observations that must be accounted for by any model for the evolution of the western end of the Mediterranean Sea. The limitations of models which rely on the rapid, convective removal of lithospheric mantle shall then be discussed We argue, m- stead, that the most important observations are consistent with a short, steep east dipping subduction zone which peeled or rolled back at a rate of between 50 and 100 mm/yr to the west during the early Miocene un•tl subduetable oceanic lithosphere was exhausted in the vicinity of Gibraltar.

Traditionally, rocks of the Betic-lq.if orogen have been divided into External Zones, Internal Zones, and Flysch nappes (Figure 2). In the Betic Cordillera, the External Zone consists of Mesozoic and Tertiary sedimentary rocks which were deposited in basinal (Subber- ics) and shelf (Prebetics) environments on the Iberian margin of the Tethyan Ocean. These rocks were deformed by northwest directed, thin skinned thrusting and folding during the early to late Miocene [Garcia- Hernandez et al., 1980; Banks and Warburton, 1991; Allerton et al., 1993]. Shortening was accompanied by development of the Guadalquivir foreland basin. The Internal Zone, located farther south, consists of mountain ranges of metamorphosed Palaeozoic and Mesozoic rocks separated by Neogene intermontane basins. These metamorphic rocks were affected by Palaeogene-early Miocene penetrative deformation and regional meta- morphism.

A threefold division of the lq.if belt of North Africa

is generally accepted [Wild•, 1983] (Figure 2). Its Infer- nal Zone contains metamorphic rocks broadly similar to those of the Betic Cordillera, although the nomen- clature differs. The intermediate Flysch Zone, infor- mally known as the Flysch nappes, consists mainly of Early Cretaceous to early Miocene deep marine clas-

LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT 505

Neogene thrust belts [

I• Major thr• f••, ,••• • Alboran Se•

5 ø lo ø 1õ' 2O ø 25

. ,s,øø,km.. •1 Figure 1. Map of Mediterranean Sea showing principal thrust belts and Neogene extensional basins as referred to in text. Area known as western Mediterranean includes: Tyrrhenian Sea, Valencia Trough, Alboran Sea, and surrounding Alpine mountain chains. Thrust belts have been simplified by division into two parts: "Internal Zones" deformed mainly in Palaengene, including metamorphic rocks, and "External Zones" which are thin-skinned and formed in Neogene. Regions that have undergone Neogene extension (western Mediterranean, Aegean Sea, and Pannonian Basin) are all surrounded by thrust belts. Note marked curwtture of the Betic-Pdf mountain chains and the Calabrian Arc at each end of the western

Mediterranean.

tic deposits [ Wildi, 1983]. The External Zone of the Rif is made up of Mesozoic and Tertiary sedimentary rocks deposited on the North African margin. Dur- ing the early Miocene, its Internal Zone was thrust onto the Flysch nappes. Continued convergence during the Miocene deformed the External Zones into a thin-

skinned fold and thrust belt [Andrieux, 1971, Frizon de Lametie, 1987, Morley, 1988]. Continental crust beneath the Alboran Sea itself consists of metamorphic rocks which are similar to those of the adjacent Inter- hal Zones [P!atl ei a!., 1996].

Key Observations

Coeval Extension and Shortening

The Alboran domain and adjacent Internal Zones underwent considerable extension at the same time as

northwest, south, and southwest vergent thrusting occurred in the External Zones of the Betic and Rif moun-

tains, respectively (Figure 3) [Frizon de Lamolte, 1987, Garcla-Dae•ias el aL, 1988, P!all and Vissers, 1989, Banks and Warburton, 1991, Fayre el a!., 1991, Garcia- Due•as el a!., 1992, Comas el a!., 1992, Lonergan el a!., 1994, Jabaloy el aL, 1993, Walls el a!., 1993, P!alzman el a!., 1993]. Close to Gibraltar, thrusting is directed westward [Balanyd and Garcia-Due6as, 1987, œ1atzman e• a!., 1993, P!all el a!., 1995]. The ENE striking Inter- hal/External Zone boundary and the Crevillcnte fault of the Betics together with the NE striking Jebha and Nekor faults of the Rifwere originally interpreted as the northern and southern boundaries of a westward mov-

ing Alboran nficroplate [Andrieuz el a!., 1971, œeblanc and O!ivier, 1984]. However, recent structural studies have shown that oblique convergence occurred along the Internal/External Zone boundary during early Miocene times [Lonergan. el a!., 1994, A!!erton el a!., 1993].

5O6 LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT

Iberian foreland .-: "'-:. ß - -' ? :'" ' '- ". -> '• Foreland Basi•ss " '• L'' "' 2:•;"'z: '•"f•VF •E ...... 1 Zones

7• Flysch Nappes

Neogene Volcanism

Calc-alkaline

(Shosm•te, S)

ß " 10Ma .':" ( :;.' . .. : ' • Strike-slip fault •J. JF 12 NF - 9-5 Ma ::" ....

- s-•s-s/v• African foreland NE Nekor Fault ß 60• v ---,••1 x•'"'- •o •o •o •o 34•1N ' iF JebhaFault

•'igure 2. Map of Alboran Sea and adjacent mountain belts. Main tectonic subdivisions of Betic and Pdf mountains are shown together with locations and compositions of Neogene volcanic rocks.

Wide-angle seismic data modeling shows that the crust varies from 40 km thickness at the Inter-

hal/External Zone bonudary in the Betics to about 15 km thickness in the center of the Alboran Sea [Banda and Artsot[e, 1980, Torn[ and Banda, 1992]. Setsuric reflection profiles which traverse the Aiboran Sea also show evidence of large-scale normal faulting [Watts et al., 1993, Comas et al., 1992, Mauffret et al., 1992]. Analysis of Neogene subsidence using well-log informa- tion from the northern margin of the basin gives stretching factors of 1.3-1.6 [Watts et al., 1993].

There is also good evidence for extension both within the metamorphic rocks of the Internal zones [e.g., Garcfa-Due•as et al., 1992, Jabaloy ctal., 1993, Crespo- Blanc et al., 199't] and within the well-exposed Neo- gene basius which occur onshore Spain [e.g., Montenat et al., 1987, Mora-Gluckstadt, 1993]. The existence of large amounts of crustal extension within the metamorphosed Internal Zones is inferred from the structural juxtaposition of rocks of signilictmtly different metamorphic grade along major extensional shear zones such as the Boric movement zone [Platt and Vissers, 1989]

and the Alpnjarride/Malaguide contact [Aldaya e! at, 1991, Lonergan and Platt, 1995].

Rapid exhumation of metamorphic rocks in the In- ternal Zones of the Betics has been documented using both radiometric dating and stratigraphic provenance studies [Zeck et al., 1992, Moni[ et al., 1991, 1994; Loner[an and Mange-Rajetzky, 1994]. Exhumation and rapid cooling of metamorphic rocks occurred from earliest Miocene times to ,-, 15 Ma, coeval with extension.

Rotations

Palaeomagnetic studies confirm that thrust sheets •vithin the External Zones of the Bette mountains have

rotated clockwise about vertical axes by up to 130 • post-Oligocene, probably in the Miocene [Oscte et al., 1988, 1989; Platzman, 1992, Allerton et al., 1993, Platt ctal., 1995]. Even larger rotations (,-, 200 •) have been documented in Malaguide rocks on the northern margin of the Iuternal Zone in the eastern Betics JAilerton et al., 1993]. In the Rif, counterclockwise rotatious of ,-, 100 • have been measured [Plat-'man et al., 1993].

Most of the published palacomagnetic data for the

LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT 507

$trik•sllp Alkali I Pllocene m shortening Basalts

$-- Messlnlan I K'enrich •ed- 1-

15- Langhian --,,, o Burdlgalian • • 4qu#anlan

Oligacene

35 Albaran Internal External Valcanlsm

Sea Zane Zane

Figure 3. Titrang of Neogene events in Bette Cmdillera and Alboran Sea

Behc and Rlf mountruns were collected from Am-

momt•co rosso and capas ro3as facies limestones of Up- per Jurassic and Upper Cretaceous ages, respectively. Teetome rotstrans must have occurred after deposition and are likely to have been associated with folding and thrurang during the early to middle Miocene tAllerton, 1994] The Alletlon et al. [1993] study was directed at hmmg palaeomagnetic rotations in a suite of rocks ranging in age from Triassic to upper Miocene located at the northern margin of the Internal Zones m the eastern Betks. Triassic, Jurassic, and upper Oligocene rocks w•thm a thrust stack all show similar amounts of ro-

tabon (,.. 200ø), but Oligo-Miocene marls from the associated foreland basin are only rotated by 140 ø. This result led Allerton et al. [1993] to conclude that rotation started in the upper Oligocene and continued while foreland basin sediments were being deposited. Uncleformed upper Miocene marls from the adjacent lntermontane basin are not rotated, providing an np- per time constraint on the rotations. If rotation started In the late Oligocene, the maximum time interval for the observed 200 ø rotation is -. 30 Ms, giving a mini- mum rotation rate of ~ 7ø/Ms. However, if we assume that rotation was contemporaneous w•th contractional deformation along the Internal/External Zone boundary, then rotation must have ceased by Langhmn times (• 15 Ma [Lonergan et al., 1994]), suggesting a rotation rate of ~ 13ø/Ma In the Subbetm of the eastern Beiges, the maximum rotatmns are 1300 Here shortening commenced in the early Miocene (~ 22 Ma), and ff we assume rotations took place between the early Mmcene and the present day, the minirnum rotation ra•e •s 6ø/Ma. If rotational deformation is accommo- dated along thrusts active in the Subbetic during the early-middle Miocene (22-15 Ma), then the rotation rates could have been as h•gh as 18ø/Ma. Rotation

rates of 6ø-18ø/Ma are the same order of magnitude as those proposed by Kissel and La• [1988] for parts of the Aegean region

Volcanism

Volcanism has both accompanied and postdated Neogene extension: calc-alkaline, potassic, and basaltic volcanism is scattered across the eastern sector of Alb-

oran Sea and Betic-R.if systems (Figures 2 and 3) [Bel- Ion, 1981, Hernandez and Bellon, 1985, Hernandez et al., 1987]. The earliest igneous activity is a basalttc dyke swarm at • 22 Ma (K/Ar dating) located in the central and western Internal Zones of the Betics [Tor- res Rolddn et al., 1986]. The earliest volcanic rocks are mostly calc-alkaline and in Spain are restricted to the S•erra de Gata-Carboneras area. In the eastern Rlf, calc-alkaline volcanic rocks are more widespread, occurring in Ras Taraf, Tro•s Fourches, and in the western Tell near Oran. In southern Spain, the dates obtained for this volcanic state range from 15 to 7 Ma, whereas in the Rif-Tell mountains, dates range from 13-8 Ma [Bel- Ion, 1981, Hernandez et al., 1987] Offshore, Alboran Is- land itsellis a calc-alkallnic volcanic edifice. Early work, quoted by Bellon [1981], suggested an age of 24•5 Ma, but more recent work by Apamco et al. [1991] dates the volcanism at 18-7 Ma, which is in more general agreement with the ages obtained for onshore suites of similar composition in both Spain and North Africa (Figure 2). Relatively large amounts of volcanism are inferred offshore from both magnetic data and dredging, these volcanic rocks are also assumed to be calc-alkaline m

colnposition [Hernandez et al., 1987]. A second broad state of dominantly potassium-

enriched rocks with a wide variety of compositions (shoshouitic to lampro•tlc) were erupted in Spain between 8 and 5 Ma and in North Africa between 9 and

4 Ma. The lamproitic volcanic rocks are very widely scattered (Figure 2) In southern Spain, the youngest lavas are Plio-Quarternary alkaline basalts which were erupted near Cartagena. The youngest lavas in North Atnca m'e also alkaline basalts, ranging in age from Messinian (5.9 Ms) to Quarternary.

In the past, both north and south dipping sub- ductran zones have been postulated between Spmn and Africa [e.g., Torres-Rolddn et al., 1985, Sanz de Galdeano, 1990]. However, it is clear from F•gure 2 that such subduction zones are not compatible with the spatial distribution of volcanism. More recently, French workers [Hernandez et al., 1987, de Larouz•re et al., 1988, Montenat et at., 1987], noting the spatial relationship between volcanic rocks and late Neogene strike- shp fault systems in SE Spain (Carboneras-Palomares faults), have attempted to relate the volcanism to a "trans-Alboran" shear zone. Seismic reflection data in

the Alboran Sea [e.g., Watts el al., 1993] provide no

508 LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT

344' 348' 352' 356' O' 4' 8' 12' 16 ø 20' _

ß - 19 " ß ß

44' .. .. _,,.,_.•,,• '. •"•' '• 44'

' ' ' i '• 40' • e•O"• , •'• 8•1 • -• / • % -

• ß . 851027 36'

L'-': 32'

344' 348' 352" 356' O' 4' 8' 12' 16' 20'

Figure 4. General seismicity and focal mechanism solutions of large (Mb >_ 5.5) earthquakes throughout western Mediterranean region. Solutions from the period 1977-1995 were taken from Harvard Centroid Moment Tensor (CMT) catalogue. For the region west of 10 ø E, older solutions were taken from review by Anderson [1985]. When possible, CMT solutions have been verified by comparison with solutions determined by conventional waveform inversion. The date of each event (year, month, and day) is given above each beach ball, the compressional quadrants of which are shaded.

evidence for a continuous shear zone offshore. The pattern of early calc-alkaline volcanism in the Alboran Sea is not controlled by the location of strike-slip faults. Existing offshore data suggest that the trend of calc- alkaline volcanic rocks crudely follows the strike of the orocline.

Selsnticity

Baforn et aL [1990] summarize data for three deep earthquakes which occurred beneath Grinreda this cen- tury. These earthquakes testify to the existence of material which is cold and rigid enough to produce sudden releases of strain energy at considerable depths beneath the Alboran Sea. The best known and largest of these earthquakes occurred in 1954 (Mb = 7.1, Figure 4) at a depth of ~ 630 4- 4 kin. The P axes for all three earthquakes dip steeply east [Chang and Kanamori, 1976, Grimison and Caen, 1986, Buforn et al., 1988, 1990]. Good station distribution throughout Europe mean8 that tile nodal plane strike of the 1954 event is well determined (2•4- 5 * E). Intermediate-depth and shal- low seismicity occurs throughout the region, the biggest events indicating oblique convergence between Africa and Europe (Figure 4). In the Albortm Sea, interme-

diate events do not occur below 150 kin, and there is also a smaller gap between 60 and 100 km [Sebcr el al., 19961.

Grimson and Caen [1986] and Buforn ei al. [1990] point out that the presence of a steep, east dipping, slab is very difficult to reconcile with the Neogene and Re- cent plate motion vectors. These vectors are oriented north-south to northwest-southeast and have been used

to invoke the existence of a north or south dipping subduction zone between North Africa and Europe. The above authors also argue that accepted plate recon- structions preclude the existence of 600 km of oceanic crust between Africa and Iberia during the Tertiary [e.g., Dewey et al., 1989].

This geometric problem has fueled more recent spec- ulation that the deep earthquakes are evidence for a block of lithospheric mantle which has been convectively removed [Grimson and caen, 1986]. $eber ei al. [1996] suggest that the distribution of relocated intermediate- depth seismicity is consistent with active delamination. Unfortunately, their analysis ignores the existence of the deep 1954 event, which dominates the total seismic moment release within the region. Furthermore, their inference that a large zone of upwelled astheno-

LONERGAN AND WHITE ORIGIN

spheric material occurs between 20 and 60 km depth is incompatible with models of melt generation by adiabatic decompression of the asthenosphere.

The presence of an east dipping P axis within an eastward dipping Wadati-Benioff zone is entirely consistent with the existence of a N-S striking subducted slab. The large seismic gap between 150 and 600 km suggests that this cold subducted slab has now detached, in agreement with tectonic evidence that 12-15 Ma have elapsed since subduelion terminated. We also note that, in fact, there is little difficulty m subducting 600 km of old oceanic lithosphere if the subduelion zone rolled back from a position west of the current eastern margin of Iberia toward Gibraltar.

Timing and Plate Motions

Foreland-directed thrusting, coeval with extension, occurs on all three sides of the orocline (Figure 3). On the Spanish mainland, typical foreland-basin style subsidence occurs within the Guadalquivir basin. This basin can be traced from the Prebetic arc in the east, through Gibraltar and onto the Moroccan margin where it is called the Rharb basin. Seismic reflection data

show that deformed units (the Flysch/External Zones) occur up to 250 km west of Gibraltar, probably rep- resenting the most westerly manifestation of shorten- lug [œajal et al., 1975]. Using bathymetric data, œa3at e! al. [1975] also propose that shortening, manifest as ohsostromes, extends up to 500 km west of Gibraltar and that Tortonian sedm•entary rocks are shortened.

Since latest Tortonian times (8-9 Ma), the Africa- Eurasia plate convergence vector has changed from north-south to northwest-southeast [Dewey el al, 1989, Mazzoh and Helman, 1994]. As a consequence, dex- tral oblique shortening is now occurring between North Africa and Iberia. This overall motion is manifest by strike-slip faulting, local folding, thrust faulting, and structural inversion of previous normal faults which affected sediments within both the Alboran Sea and

within onshore Neogene basins [Woodside and Maldon ado, 1992, Walls el al., 1993, de Larouz•re et al., 1988]. Since the Pliocene, the onshore Neogene basins in southern Spain have been rapidy uplifted, attesting to continued convergence between Africa and Eurasia and/or slab break-off.

Models for Generating Late Orogenic Extension

The paradox of coeval shortening and extension led Dewey [1988], Platl and Vzssers [1989], and Doblas and Oyarzun [1989] to propose that the Alboran Sea and surrounding mountain chains formed by extensional collapse of a previously thickened crust and lithospheric

OF THE BETIC-RIF MOUNTAIN BELT 509

mantle. Their work follows the earlier suggestion of Gr•m•son and Chen [1986] who used teleseismic earthquake data to infer the existence of a detached fragment of lithospheric mantle. Several other workers have elaborated these ideas [Platzman, 1992, Plat! and Eng- land, 1994, V•ssers et al., 1995]. Along with the TI~ betan plateau, the Betic-Rif mountain belt is now regarded as an important example of extensional collapse driven by rapid, convective removal or delamination of thickened lithospheric mantle [Houseman et ai., 1981, England and Houseman, 1989]. Here we briefly review two principal mechanisms which have been used to account for the main observations in the western Mediter- ranean: convective removal or delamination of the htho-

spheric mantle and subduelion zone rollback. We shall not consider models which invoke westward motion of

an Alboran "microplate" within a zone of overall convergence between Africa and Europe [Andr•eux et al., 1971, Leblanc and Ohwer, 1984]. Although this is an appealing mechanism for producing the dramatic oro- elihal bend about the Straits of Gibraltar, it is evident that the Alboran domain did not behave rigidly during the Miocene.

Convective Removal/Delamination Models

Using fluid dynamical arguments, Parsons and McKenzie [1978] showed that the vigorously convecting, adiabatic interior beneath the oceanic plate is separated from the mechanical boundary layer of the lithosphere (~ 80 km thick) by a thermal boundary layer (~ 50 km thick). This thermal boundary layer has a lower viscosity than the mechanical bounday layer and acts to maintain the thickness of the lithosphere at 120 km by periodically overturning [England, 1993]. Subsequently, Houseman et al. [1981] carried out a range of numer- ical experiments which showed that if an equilibrated lithospheric plate is instantaneously thickened, then the thermal boundary layer becomes unstable and drops off on timescales of about 10 Ma for geologically realistic Raleigh numbers. In their calculations, they systemati- cally varied the thickness of the thermal boundary layer by 1 order of magnitude.

The principal difficulties with convective removal/delamination models are twofold First, the time taken for delamination to occur is sensitive to the vis-

cosity structure of the lithosphere. Buck and Toksdz [1983] have carried out a much smaller number of nu- merical experiments, and they conclude that only a thin one at the base of the thermal boundary layer is re-

moved during convective overturn. This disagreement is a consequence of uncertainties in the theological prop- erties of the upper mantle. Furthermore, removal of lithospheric mantle is an inefficient means for generating substantial potential energy differences because the density contrast between lithospheric mantle and as-

$10 LONERGAN AND WHITE: ORIGIN OF THE BETIO-RIF MOUNTAIN BELT

thenosphere is small (,-, 0.13 Mg m-a). Second, House- man ei al. [1981] assumed that shortening of the lithosphere was both uniform and instantaneous. If, however, shortening occurs at geologically realistic strain rates, the thermal boundary layer will probably convectively overturn on a continual basis.

If convective instability of a thickened thermal boundary layer can take place sufficiently rapidly and on a large enough scale, then this model for the Betic- Rff area is appealing. However, there are two important sets of observations which are not easily explained by a combination of lithospheric thickening and radial extensional collapse. First, shortening has only been taken up on three sides of the orogen and is in no sense radial (Figures 1 and 2). It is unclear how the free eastern edge east of the Alboran Sea can be accounted for, except by inferring that extensional collapse generated the whole of the western Mediterranean region. This inference requires evidence for coeval outward directed thrusting all around the western Mediterranean Sea. Second, no serious attempt has been made to account for the large, clockwise palaeomagnetic declinations observed in the External Zones of the Betic mountains and for the comparable counterclockwise declinations within the Rif. Platzman [1992] suggests that these rotations could be incorporated into an extensional collapse model, although she concludes that the palaeomagnetic data require some westward motion of the Alboran region. In summary, large vertical-axis rotations are the crucial observation which cannot easily be accounted for by convective removal or delamination. McKenzze and Jackson [1983] showed that the spreading of either a circular or elliptical blob does not generate any rigid body rotation.

Potential Energy Calculations

If significant convective removal occurs, it will generate large amounts of gravitational p otentlal energy. It has long been recognized [e.g., Jeffreys, 1952, Bott and Dean, 1972] that whilst the lithosphere is in approx- m•ate isostatic equilibrium, considerable lateral varia- tion in density means that the stress balance is not complete: horizontal devlatoric stresses can be of the order of 100 MPa. If the gravitational potential energy of a region of continental lithosphere exceeds that of its surroundings, then that region will be subjected to extensional deviatoric stresses. Regions with greater potential energy are usually at greater surface elevation, but since potential energy depends upon the density distribution within the lithosphere, elevation alone cannot be used to calculate potential energy differences. It has been shown [hat differences in the depth-averaged horizontal deviatoric stresses for two lithospheric columns in isostatic equilibrium are proportional to differences between their gravitational potential energies [Molnar and

Lyon-Caen, 1988]. Lithospheric columns are balanced with reference to mld-oceamc ridges where it is reason. able to assume that horizontal and vertical stresses are equal (i.e., a• --=

Extensional collapse of the continental lithosphere thought to be triggered by a rapid increase in the poten. tial energy of a lithospheric column [Pla• and England, 1994]. This inference is based upon simple calculations which show that if the bottom part of thickened htho- spheric mantle is suddenly removed, a rapid increase in potential energy occurs (Figures 5a and 5b and appendix). This increase partly manifests itself as an increase in surface elevation. It is generally assumed that rap•d increases in potential energy (• 5 x 101-" Nm-1) will give rise to extensional collapse

The difficulty in using potential energy calculations to argue in favor of extensional collapse models is that the continental lithosphere is clearly able to sustain large differences in potential energy without generating extension. For example, the potential energy difference between southern Africa and the surrounding old ocean basins is as large as that expected by convective removal calculations (compare Figures 5b and 5d) In fact, differences m potential energy may have no par- ticular significance: it is more important to develop an understanding of mechanisms which allow stored potential energy to be released rather than interpret poten- hal energy differences per se. Even if large dtfferences in potential energy did lead to extensional collapse, •t is not clear why extension should continue after thmkened continental crust has been thinned and •ts upper surface restored to sea level, unless subsidence below sea level is entirely caused by thermal contraction

Subduerion Zone Rollback Models

At many island arcs, the subducting slab retreats away from the arc in the hotspot frame of reference [Molnar and Aiwaler, 1978, Chase, 1978, Dewey, 1980]. This peeling or rolling back of a slab generates and maintains back arc extension. At continent-ocean su}> duction zones, rollback is probably the principal means by which potential energy stored in thickened continental lithosphere is released.

Rollback is a natural consequence of subducting old oceanic lithosphere which is colder, and therefore denser, than the mantle through which it sinks [Le P•- chon and Anteher, 1981]. Within [he subducting slab, the vertical negative buoyancy force F can be resolved into two components: one component at right angles to the dipping slab R and a second component along the slab P (Figure 6a) If the slab is in static equilibrium, P is balanced by viscous forces on the two surfaces of the slab and by resistive forces at the slab tip. The only way in which /• can be supported is if the pressure within the asthenospheric mantle beneath the dipping slab •s

LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT $11

a)RI [• U.n•forrp.

Lithosphere •m • •

c)

Reference

b) • Continent- Continent

i ......... •. • ...... •o• ' • o . -

d) Ocean - Continent

• o .• ..... i .

Figure 5. Potential energy calculations for different columns of lithosphere. (a) cartoon illustrating two end-members for which potential energy difference is calculated. Reference column of continental lithosphere with 30 km thick crust is in isostatic equilibrium with standard mid-ocean ridge column. (b) Calculated potential energy difference (x 10 -•3 Nm -•) as a function of thickening for two end-members shown in Figure 5a. Upper curve is labeled "convective removalS: lithosphere which has been thickened by factor f but lower part of lithosphere has been removed to maintain lithospheric thickness at 125 km. Lower curve is labeled "uniform thickening": lithosphere allowed to thicken uniformly. If convective removal of thickened lithospheric mantle occurs at f = 1.8, then the path taken is shown by solid arrowheads. A value of +0.5 is maximum level of potential energy difference likely to be sustainable on Earth. (c) and (d) Potential energy calculations for southern Africa compared with surrounding ocean basins. Crustal thickness taken from Qiu [1995].

greater than the pressure above the slab. This pressure difference is superimposed upon the hydrostatic pres~ sure gradient and so will tend to drive mantle mate- ria} from high to low pressure thus facilitating rollback (Figures 6b and 6c). The buoyancy forces generated by slabs greatly exceed other convective forces withiu the mmitle, aud so motion will largely be controlled by the density excess of the slab. The component of velocity that is normal to the slab is determined by the ease with which material can flow around the slab. Flow

cmi easily occur in the strike direction (i.e., around the edges of the slab), •'ut the increase of viscosity with depth means that flow around the tip of the slab is more difficult. Hence the normal component of velocity is greatest at subduction zones that have a short strike length (< 1000 km).

When subduction zone rollback is initiated at a

continent-ocean margin, it is unlikely that the exten-

sional deviatetic stresses within thickened continental

lithosphere can be trmismitted through rigid oceanic plate to the mid-ocean ridge. As a result, continental lithosphere will collapse by rapid extension into the space provided.

Estimates for rollback velocity vary considerably but. are generally 100-200 nun/yr [Dyerkin et al., 1993]. In the Aegean, œe Pichoa and Anõeljer [1981] used the relative motions of Africa, Europe, and Turkey to infer that the 11ellenic Trench rolled back at -• 40 mm/yr. Vine and $'m•th [1981] quote 50 mm/yr for the Scotia Arc. Modern estimates based upon Global Position- ing System (GPS) data aud moment tensor inversiou of earthquake data yield higher values. Bilhris el al. [1991] obtained 110-t-28 mm/yr fro•n an analysis of GPS data in the Aegem•. The highest rollback veloci- ties are from the qbnga-Kermadec Treuch where Betas et al. [1995] carried out a series of GPS campaigns


SLibducfior'• ZoP•

/ • Roll-back / F / Extension

Figure 6. Cartoons which illustrate idealized kinematic evolution of subduction zone rollback: (a) cross section before rollback commences, (b) cross section after rollback, and (c) plan view of orocline showing juxtaposition of shortening and extension together with systematic vertical-axis rotations.

and obtained 160 mm/yr for the opening of Lau basin with convergence rates of 240 mm/yr across the Tonga •rencll.

Once rollback commences, it should continue until the system completely runs out of dense oceanic lithosphere. Thus there are two principal ways by which rollback ceases. First, it ceases when the retreating subduction zone collides with mid-oceanic ridge. Thns it is likely that the Scotia Arc will continue to roll back until it meets the mid-Atlantic ridge. Second, when the arc collides with continental lithosphere (i.e., dense oceanic lithosphere is completely consumed) rollback ceases. This more interesting case is of especial impor- tance within continent-ocean-continent collisions where

the original passive continental margins form irregular boundaries (e.g., during the closure of the Tethyan Ocean). When an arc collides with an irregular con- tincntal boundary, the subducting slab will probably split into two parts with each segment continuing to roll back.

As well as generating rapid back arc extension within a zone of overall convergence, rollback is also an excellent means for generating rapid rotations about vertical axes. The classic examples come from southeast Asia where palaeomagnetic declination anomalies are associated with the opening of many marginal seas

(see Jarrard and $asajima [1980] for summary). Palaeo- nmgnetic studies from many of these island arcs clearly demonstrate the role of rapid block rotations about vertical axes in back arc basins. The largest rotations occur in the Japan, Ryukyu, and Marianas basins where rotations of up to 80 ø have often been observed and where major diffcrences in the amount of rotation experienced by different segments of the same arc occur. For example, palaeomagnetic data from southwest Japan is plentiful and clearly demonstrates clockwise rotations of 380-47 ø . Most of this rotation occurred between 16

and 14 Ma [e.g., Olofuji el al., 1991]. In contrast, north- east Japan rotated counterclockwise by ~ 32 ø. These estimates yield an angular velocity of 20ø/Ma. It is interesting to note that the physiography of the Japmi Sea is a mirror image of that of the western Mediter- ranean (Figure 7).

Subduction Rollback in the Alboran Sea

We contend that the Neogene structure and evolution of the Betic-Rif mountain belts are best explained by the westward rollback of a short east dipping snl> duction zone [Frizon de Lamolle el al. 1991, Royden, 1993]. Since late Oligocene times, the African plate has

LONERGAN AND WHITE: ORIGIN OF THE BETIC~RIF MOUNTAIN BELT

A • 5OO Km

Bathymetry

Figm'e 7. Bathymetry of (a) western Mediterranean, (b) South China Sea, and (c) Japan Sea. Note similar phys- iographies.

converged with Iberia by about 300 km [Dewey et ai., 1989].

An iinportant argument in favor of a subduction rollback model iu the western Mediterranean is that the

boundary separating extension from shortening follows the oroclinal bend around the Straits of Gibraltar. The

Flysch nappes and External Zone "olistostrornes" west of Gibraltar may be analogotis to the imbricated trench sediments which are associated with arcs. If so, their location and strike can be used to pinpoint the final position of the inferred north-south striking subduction zone. Continental lithosphere at the Straits of Gibraltar has probably prevented rollback from proceeding any farther. In contrast, Royden [1983] suggests that rollback may have continued as far west ,as the Horseshoe Seamounts.

As described above, volcanism is relatively young when compared with deformation of Internal Zones. It is coeval with and younger than extension and is dif-

fusely located within the Alboran Sea, onshore North Africa, and in southeast Spain. Such a pattern is dit•i- chit to attribute to either a north or south dipping subduction zone, bul it is compatible with an east dipping subduction zone whid• rapidly rolled back to the west. The range of cronposition of volcanic rocks from calk- alkaline to alkaline and basaltic is typical of small back arc basins where both arc and extensional volcanism oc-

cur. In the Alboran Sea, no calk-alkaline volcanic rocks have been reported between the Straits of Gibraltar and ,.0 40 west (Figure 2). Assuming that the final surface position of the Alboran subduction zone was somewhere between the Straits of Gibraltar m•d the westerly limit of deformation (the "olistostromes") and that the calk- alkaline volcanic edifices in the Alboran Sea represent the dismembered rernains of a small arc, then the final arc-trench gap was of the order of 200 kin. This distance is similar to the arc-trench gap between the Aeolian Islands and the Calabrian trench. the lack of

a clearly defined volcanic arc in the Alboran Sea may be attributed to one of two causes: (1) late Miocene to Recent strike-slip faulting postdating the cessation of subduction and thus disrnembering the arc, or (2) an originally narrow snbduction zone that moved rapidly, preventing a large, stationary arc edifice from forming.

In summary, the geology of the Betic Rif orocline and formation of the Alboran Sea can be accounted for

by a north-south striking subduction zone which originally dipped east. Rocks that now make up the basement of the Alboran Sea and the Internal Zones of the

Betic and Rif mountains originally lay to the east of the subduction zone and formed a collisional wedge. As the subduction zone rolled back, thickened crust behind the subduction zone extended rapidly to fill the space generated by the retreating subduction zone. Rollback of the subduction zone and outward displacement of the extending Internal Zones-Alboran Sea impinged on the passive margins of both the Iberian and African plates, causing the onset of oblique thrusting and rotations in the External Zones of the Betms and Rif mountains in

the early Miocene.

Shortening in the External Zones and extension in the Alboran Sea terminated by the late Tortonian (Fig- ures 3 and 8), suggesting that the subducting slab had rolled back as far as the Straits of Gibraltar by this tirnc and that the Alboran Sea basin had achieved

its present-day dimensions. As the narrow subduction zone peeled back, rotational deformatim• on the margins was taken up by oblique-slip thrust faulting in the External Zones. Large rotations are probably also associated with major extensional faults in the Internal Zones and Alboran Sea, similar to those observed in the Aegean. The only published palaeomagnctic data for the Internal Zones come from the Cabo de Gata volcanic province where rotations are associated with


Neogene ocea•c crust Neogene

Neogene mrust •1• age of N•g•e shorting Figure 8. Map of Western Mediterranean showing the timing of Neogene extension and shortening and the tinling of formation of Neogene oceanic crust. Inset in bottom right illustrates plate motion vector for Africa with respect to stable Europe from 38 Ma to present [Dewey et al., 1989]. Data are compiled from sources referenced in text.

late Miocene strike-slip delbrmation in the Carboneras fault zone [Cairo el ai., 1994]. Pervasive west directed stretching lineations measured in metamorphic rocks of the western and central Betics [e.g., Garcia-Due•as el ai., 1988] are consistent with this part of the orogen hav- ing undergone the largest amount of westward transla- tion behind the retreating slab.

On tile basis of the tinling of onset and termination of both shortening in the External Zones and extension in the Internal Zones and Alboran Sea, we estimate that rollback occurred in the Alboran Sea hetween 23 and l0 Ma. Depending upon where the postulated subduction zone started cast of lberia, it travelled between 500 km (i.e., the length of the Betic orogen) and 900 kin, yielding a rollback rate of 42-75 llllU/yr with associated rotation rates of betwccn 6ø-18ø]Ma.

Evolution of the Western Mediterranean Sea

By elaborating npon tile ideas of Ah•arez el ai. [1974], Rchauit el al. [1985], D .... y el al. [1989], Roy-

den [1993], and T•'icar! e! at. [1994], we propose that the now extinct subduction zone beneath the Alboran Sea

was originally contiguous with tile Calabrian Arc, cur- rently located just north of Sicily. This original arc split into two shorter arcs, probably in the nliddle Miocene when the central segmeut of an original northward dipping subducting slab collided with the North African continental margin to form tile Kabylies.

The logical step is to link the Tyrrhenian subduction system to the subduction zone responsible for formation of the Betic-Rif orocline. In the Oligoccne, prior to the opening of tile western Mediterranean, the rocks now making up the metamorphic belts in Corsica, Sar- dinia, Calabria, Sicily, the Kabylies of Norlh Africa, and the l•if and Betic Internal Zones nmst have been

grouped behind a north dipping subduction zone as shown schenlatically in Figure 9a. The similarity hi the geology of these now dispersed metamorphic belt., has been noted by many previous authors (compare the AIKaPeCa terrmie of Bouilin et ai. [1986]). Once the subduction zone began to roll back, extension com- •nenced and Corsica and Sardina rotated couutcrclock-

wise, reaching their current position by 19:E 1 Ma [Mon-

LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT

c. 35 Ma

---;•E:. -_.. _.E: '_--%E E'-:-• - --• Iberian & African

Passive Margins

a --] Internal zones • Shortening fAirira Plate mollon vector (wrt Europe)

E•] Flysch basins •'• Block rolations • • r• Back arc extending • Subduction zone Subductlon Zone Thrust Front region rollback direction

E• External Zones

Fi•ur• 9.

516 LONERGAN AND WHITE' ORIGIN OF THE BETIC-RIF MOUNTAIN BELT

t•gny etal., 1981, V•ghott• and Langenhe•m, 1995] (Fig- ure 9a)

There is general agreement that the the Balearic and north Tyrrhenian basins initiated in a back arc setting behind the north dipping subduction zone in the early Miocene [Cohen, 1980, t•ehault etal., 1985, Mal- mvevno and t•yan, 1986, Kastens etal., 1988, Dewey e! al., 1989]. As a consequence of this extension, Corsica and Sardinia rotated counterclockwise, collided with continental Adria, and initiated the formation of the Apennines [Alvarez etal, 1974]. Continued south and southeast directed rollback of the subduction zone led

to the formation of the southern Tyrrhenian Sea and the Cainbrian Arc (see Dewey etal., [1989] for further details). As shown in Figure 8, the oldest documented extension m the western Mediterranean area occurs onshore in southern France where the narrow

Ohgocene graben of Camargue and Provence have an east-west strike [Rehault etal., 1985]. The extensional graben m western Sardinia which extends from Sassari to Cagliari now has a north-south str•ke, but during the late Oligocene-early Miocene, th•s basin was parallel to the Gulf of Lions graben [t•ehault etal., 1985]. By early Miocene times (23-20 Ma), extension started in the Va- lencia Trough, Balearic basin, Algero-Provenqal basin, and Alboran domain, suggesting that the same moch- arosin is responsible for the formation of these basins. The onset of rifting in the Valencia Trough gave rise to clockwise rotation of the Balearic Islands [Pargs et al, 1992]. The dispersion of the other units ultimately led to the formation of new oceanic crust in the Algero- Provencal basin (21-18 Ma [Rehault etal., 1985]).

We suggest that later collision of the Kabylies continental fragments with North Africa forced this subduction zone to divide into two segments: a short segment oriented north-south which subsequently tore around to form the present-day Alboran Sea and a longer segment which continued to evolve until the present day, forming the Tyrrhenian Sea and the Cainbrian Arc (Figure 9b). By 18 Ma, the rotation of Corsica and Sardina

was complete, shortening had commenced in the northern Apennines, and the collision of the Kabylies with Africa had commenced [Cohen, 1980, Wildi, 1983, carl ei al., 1994]. Shortening was also underway m the External Zones of the Betics and Rif. Hence the subduction zone must have been approximately located as sketched in Figure 10b. Continued convergence led to the evolution of two increasingly looped branches of the original subduction zone in the Alboran and Tyrrenh•aa seas, respectively. By Tortonian times, the Alboran Sea had evolved to its current size and shape. At the Cal- abrian Arc, rollback has continued throughout the late M•ocene until the present day (Figure 9b) [Anderson and Jackson, 1987]. The current position and asymme- try of the fiysch basins from S•cily around to the •n the western Mediterranean supports this subductmn zone geometry, provided that the Flysch nappes represent the deformed deposits of a trench-forearc basin.

Volcanism

Figure 10b summarizes the age and distribution of Neogene volcanic rocks in the western Mediterranean A similar pattern to that already described for the Betic-Rif mountain chain is evident. Volcanism accom-

panied and postdated the extension episode, the earliest volcamsm tending to be calk-alkaline followed by more alkaline (potassium-enriched) volcanism (compare F•g- ures 8 and 10).

The earliest documented Neogene volcanism in the western Mediterranean area is calk-alkaline and occurs

on both the western side of Sardinia (24-13 Ma), on the margins of the Ligurmn Basin (34-20 Ma) [Bellon, 1981, ]•ehault etal., 1985], and in the Valencm Trough These volcanic rocks are related to the earliest phases of back arc evolution which began as the Gulf of L•ons, the Ligurian, and the Valencia Trough basins opened, and as Sardinia and Corsica began their counterclockwise rotation. In the Valencm Trough, considerable quanm ties of volcanic rock have been encountered in wells and

can be inferred from seismic data [Maillard ei ak, 1992,

Figure 9. Reconstructruns filustr•ting Neogene evolution of western Mediterranean. (a) Late Oligocene, prior to opening of western Medlterranea• Sea by b•ck arc extension. Predominantly metamorphic rocks that make up Internal Zones of Betic-Rif orocline, Kabylies of North Africa, Calabria, Corsica, and Sardinia were located behind north to NW dipping subduction zone. Flysch deposits (Cretaceous-early Miocene) that can be tr•ced from Sicily alo•g the North African margin to the western Betics may have been deposited in the trench-foreaxc basin associated with this subduction zone. (b) Subduction zone rolled back, Corsica and Saxdlnia rotated counterclockwise, and Balearic Islands rotated clockwise as both Ligurian and Valenci• basins formed. By middle Miocene times (.-, 18 Ma), Kabylies had collided with the northern maxgin of Armco, and the subductlon zone now formed two branches. The narrow westerly segment retreated rapidly to form the Alboran Sea, causing shortening and rotation in surrounding mount,fin belts. Oceanic crust formed in Algero-Proven•al basin. The eastern branch of the western Mediterr•ean subduction system continued to evolve until the present day, forming the Tyrrhenian Sea and Calabrian Arc. Reconstructed position of subduction zone for Tyrrheniaaa Sea and plate vectors are from Dewey etal. [1989].

LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT

Neogene oceanic

[E• Assumed Tethyian

i ..... nic cmst 4øø

Figur• 10.

518 LONERGAN AND WHITE ORIGIN

Marllard and Mauffret, 1993]. The majority of the volcanic activity was associated with the opening of the basin in the early Miocene and, where sampled, these voltanits are dominantly calk-alkaline in composition

A second phase of alkaline volcanism occurred in the latest Miocene and Plio-Quarternary both in the Valen- cia Trough (8-1 Ma) and on Sardima (4-2 Ma). On the northern margin of Africa in the Kabylie and Tell ranges of Algeria and Tunisia, volcanic rocks are mainly calk- alkaline in composition and span the middle Miocene (i.e., both during and after collision of the Kabyhes with North Africa [Bellon, 1981]. A small amount of late Miocene alkaline volcanism also occurs.

In the Cainbrian Arc-Tyrrheman Sea-Apennines system, volcanism is notably younger but again clearly linked to the back arc evolution of the Tyrrhenian Sea. The earliest volcanism occurs on the eastern edge of Corsica associated with the oldest northern end of the

Tyrrhenian Sea. Subsequently, volcanism in the northern Apennines became younger from east to west and is spatially associated with the eastward migration of extension [$erw• et al., 1993]. Apennine volcanic rocks are heterogeneous, but they all generally show an en- richment in potassium. The Aeohan Islands and associated seamounts comprise the volcanic arc associated with the Cainbrian subduction zone tEllam et al., 1989]. This arc has been active for the last 1.3 Ma, and the volcanic rocks exhibit a range of compositions from tholeiitic to calkalkahne and shoshonitic. A modest K-

enriched trend w•th time can be observed [Beccaluva et al, 1981].

The Tyrrenhian Sea has two basalt-floored sub- basins, the Vaslov and Marsili basins (Figure 10b). Both formed very rapidly between the late Miocene and the P•ecent. Basaltic crust formation was widespread during the Pliocene in the Vaslov basin, and in the Marsill basin, basaltic crust formation began at -- 2 Ma [Kastens et al., 1988, Kastens and Muscle, 1990] Hence there is clear evidence that the Tyrrhenian basin is younger toward the southeast, as predicted by subduction rollback models. Two tholeiitic basaltic seamounts

occur in the southern Tyrrhenian Sea in the area pre- sumed to be underlain by oceanic crust. The composition of basalts both drilled on the seafloor and collected

OF THE BETIC-RIF MOUNTAIN BELT

from the seamounts in the Tyrrhenian Sea have a range of chemistries from alkali to talc-alkaline affinities.

The range of composition of Neogene volcanic rocks throughout the western Mediterranean is typical of small back arc basins where arc and extensional •01- canism overlap in a diffuse manner and where voltanit arcs sensu strlcto have a limited spatial distribution Striking similarities exist at the eastern and western extremities of the western Mediterranean Sea The Ae- olian volcanic arc is located -• 300 km northwest of the current position of the Cainbrian subduction zone.

Ro•ations

Conmderable palaeomagnetic data have been letted around the wesi, ern Mediterranean, and rotations about verticM axes on a variety of scales have been umented. A generahzed summary of the main trends and sense of rotations is shown in Figure 10a Earl• work m the region concentrated on using palaeomagnetic data to construct polar wandering curves for rigid microplates within the Tethyan suture realm (e g., the Adna microplate), but it rapidly became apparent that superimposed on the movement of microplates there were local small-scale relative rotations of fault blocks

w•thin the deformed regions [e g., Charmell, 1986]. At the largest scale, the onset of the south-southeastefiy mlgratmn of the subductmn zone in late Ohgocene and early Miocene led to the opening of the Ligurian basin and counterclockwise rotation of Corisca and Sardmla

[Mont•gny et al., 1981]. The asymmetric opening of the Valencia Trough presumably generated clockwise rotation of the Baleuric Islands A recent synthesis of palaeomagnetic data from the Baleuric Islands and adjacent areas onshore Iberia by Pargs et al. [1992] iden- tify three rotational components. The youngest rotations (20 ø) on Mallorca and Menorca occur in upper Miocene rocks and are attributed to rotations on listnc faults that occurred m the late Miocene and Pliocene

After subtracting the effects of late Miocene-Pliocene rotations, the magnetic directions in the Balearm Is- lands have been rotated clockwise by variable amounts when compared to the Spanish margin. Par•s et [1992] attribute these rotations to differential rotation thrust sheets in the early Miocene, including a compo-

Figure 10. (a) Generalized summary of sense and magnitude of Neogene-recent rotations about ve• tic• axes h'om palaeomagnetic data. T•ming of rotation given nt italics beneath amount of rotation. Dotted line shows original positions of Sardinig, Corsica, and Balegnc Islands. Except for Corsica, pMaeomag- netic analyses record variable rotations associgted with shortening structures. Note that in many cases, dgt• from numerous thrust sheets are summarized at one point. Data are compiled from references discussed m text. Ornament for mountain belts is given in Figure 1 (b) Age grid dlstributlon of volcarat rocks in western Mediterranegn: calk-alkaline volcanic rocks are distinguished from alkaline volcanic rocks (compiled from Bellon [1981], Hernandez and Bellon [1985], Hernandez et al. [1987], Rehault el al. [1985], Maillard and Mauffret [1993], Ellam et al. [1989], Beccaluva et al. [1981], $err• et al [1993], and Kastens et al. [1988]).

LONERGAN AND WHITE ORIGIN OF THE BETIC-RIF MOUNTAIN BELT $19

nent of microplate rotation of the whole of the Balearic Islands.

The Betic and Rif External zones and the Apen- tone and Sicilian thrust belts all have variable magnitude rotations associated with thrusting [Pla•zmaa ei d., 1993, All•rtoa e• al., 1993, Jackson, 1990, Oldow e• d., 1990, $cheepevs et al., 1993, AlleyIon, 1994]. Sense of rotation is reversed on the opposing sides of both the Gibraltar and Cainbrian Arcs. In both Sicily and in the Betifs, a decrease in the amount of rotation is observed toward the distal parts of the thrust belts tAller- ton e• al., 1993, Oldow e• al., 1990]. We note in pass- mg that detailed palaeomagnetic data froin the central Apennines nlay support the existence in some locations of small Miocene-Recent clockwise rotations [Ma•ei et al, 1995]. Rotations associated with thrusting indicate oblique convergence and so the edges of the retreating subduction zone are not defined by large-scale wrench faults A combination of transcurrent motion caused

by rollback and shortening caused by continmng plate covergence is clearly an efficient mechanism for generating oblique convergence, especially at the edges of an arcuate subduction zone.

Over the last 10 years, a similar pattern of rotations has been documented in other parts of the Mediter- ranean Sea. In the Aegean, K•sse! and La.• [1988] and $peraaza ei a! [1995] have measured rotations associated with both thrust and normal faulting. A testable prediction of the rollback model arises from the fact that the collision of the Kabylies with the African margin was orthogonal and that large rotations of thrust sheets should not have occurred within the Tell moun-

tains The largest rotations are to be expected in the most azcuate zones of the western Mediterranean

Conclusions

The general model for the western Mediterranean sea that we have discussed here is implicit in the work of Rehauli et al. [1985] We have extended their ideas to include the Alboran Sea and shown that new geophysical, structural• and palaeomagnetic data are consistent with subduction zone rollback rather than with convec-

tive removal or delamination of thickened lithospheric mantle. Thus late-orogenic extension, as documented m the Alboran Sea, is best regarded as a logical conse- qnence of a rollback mechanism whereby thickened con- tmental crust extends rapidly as the subduction zone retreats.

Neogene mountain belts throughout the Mediter- ranean region are characterized by coeval shortening and extension during the late stages of orogenesis. We argue, nl agreement with some previous workers to.g, Roy(lea, 1993], that rapid extension of previously shortened crust within a convergent setting is an in-

evitable consequence of the well-known subduction rollback mechanism. Convergence between the African and European plates commenced in the Upper Cretaceous and continued throughout the Tertiary. This shortening produced many Palneogene collisional mountain belts (e.g., the Alboran domain, Kabylies, Calabria, and Corsica-Sardinia) At the same time, continuing northward subduction of remants of the Tethyan Ocean, with associated subduction rollback, led to the onset of extension throughout these collisional edifices and caused their dispersion to the margins of the current western Mediterranean back arc basin in the Neogene. We see no reason to believe that the Alboran Sea at one end

of the Mediterranean has evolved in a fundamentally different way from the Aegean Sea at the other end.

Further work is needed to understand how a sub-

duction zone starts to roll back. It is also unclear why some subduction zones should remain static over long periods of geological time, forming relatively simple linear mountain chains, and others such as those of the Tethyan belt and the western Pacific are very mobde, become arcuate, and lead to the formation of complex orogens (e g., Figure 7). Short, highly mobile subduction zones that have generated arcuate mountain systems may be difficult to detect in the geological record because they may not form well-developed volcanic arcs. Once the back arc basin closes in the continuing cycle of convergence, there may be little evidence of the former existence of such a subduction zone.

Appendix

The gravitational potential energy calculations shown in Figure 5 were carried out using the approach described by Englaad and Housemaa [1988]. We briefly describe a corrected version of their formulation which

we have used to calculate Figure 5. All symbols and their values are identical to those used by England and Houseman [1988].

Crust and lithospheric mantle are assumed to have uniform compositions, and the density in each case is given by

• = •o(1 - ,•') (1)

where T is the temperature in degrees Celsius and a is the coefficient of thermal expansion. The variable Po is the density of either crust or mantle at 0øC. We assume that the geotherm •s linear through a lithosphere of original thickness a and that the temperature beneath the hthosphere is T•.

Let us consider the case of thickened continental

lithosphere in isostatic equilibrium with a standard column of continental lithosphere (Figure 5). It' the crust and lithr•sphere are thickened by factors f and 7, respectively, then isostatic balance between the thickened column and the reference column means that Ae , the

$20 LONERGAN AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT

elevation difference between the two columns, is given termined by integrating over the stress distribution as a function of depth:

Since er -- pgz, the density profiles, which are determined using Equation (1), have to be integrated.

Acknowledgments. We are very grateful to John Platt for organizing an excellent field trip to the Bette Mountains dunn[ Easter 1994. We also thank R. Edwards, D. Lyness, D. McKen-

(2) zie, and X. Qiu for their help. J. Dewey, J Platt, A. Robertson, and an anonymous referee provided thoughtful reviews but the

the difference in gravitational potential energy be- usual disclaimer applies. This is Cambridge Earth Sciences con- tween the thickened reference lithospheres, can be de- tribution 4846

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N White, Bullard Laborstoiles, Mad- ingley Road, Cambridge, CB3 0EZ, UK (email: [email protected]).

(Received Mamh 29,1996; revised December 3,1996; accepted December 14,1996 )

Origin of the Betic-Rif mountain belt - Imperial College … AND WHITE: ORIGIN OF THE BETIC-RIF MOUNTAIN BELT 507 $trik•sllp Alkali I Pllocene m shortening Basalts $-- Messlnlan

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