A deep seismic reflection survey across the Betic Chain (southern Spain): first results

TECTONOPHYSICS

ELSEVIER Tectonophysics 232 (1994) 77-89

A deep seismic reflection survey across the Betic Chain (southern Spain) : first results

V. Garcia-Dueiias a~*, E. Banda b, M. Torn6 b, D. C6rdoba ‘, ESCI-BCticas Working Group **

n Departamento de Geodincimica, ZAGM, Uniuersidad de Granada-CSIC, 18701 Granada, Spain b Institute of Earth Sciences (Jaume Almera), CSZC, Marti i Franqub s /n, 08028 Barcelona, Spain

’ Departamento de Geof%ca, Universidad Complutense de Madrid, 28040 Madrid, Spain

(Received February 2, 1993; revised version accepted August 24, 1993)

Abstract

Two land seismic reflection profiles across the Betic Chain have imaged the deep structure of the crust belonging to two different crustal domains. To the north, one profile samples the crust of the Variscan Iberian Massif that underlies the sedimentary cover of both the Guadalquivir foreland basin and the South Iberian crustal domain. The upper crust is non-reflective along the profile, but the lower crust is reflective in the northern half of the profile between 7 and 12 s two-way travel time. In this segment of the profile the Moho is also well imaged. The quality of the data diminishes significantly as the profile enters the Neogene Guadix-Baza basin. A second profile crossing the Alpine metamorphic complexes of the Betics reveals a reflective lower crust and a conspicuous reflection in the upper crust that is continuous for about 10 km. At 11 s, a distinct reflection has been interpreted as the reflection Moho. Deeper reflections are also seen in the central part of the profile in a segment about 10 km long. Comparison with available refraction and wide-angle reflection data shows some differences in both the crustal configuration and in the depth to the Moho.

* Corresponding author. Fax: 34-58-243352. ** Universidad de Granada, Consejo Superior de Investiga- ciones Cientificas (CSIC), Universidad de1 Pais Vasco, Uni- versitat de Barcelona, Universidad Complutense de Madrid. The ESCI-Beticas Working Group is coordinated by V. Garcia-Dueiias and comprises teams represented by: E. Banda, C. Comas, D. Cbrdoba, F. GonzLlez-Lodeiro, A. Mal- donado, M. Muf~oz, M. Orozco, C. Sanz de Galdeano, E. Suriiiach, J.M. Tubia, and R. Vegas.

1. Introduction

The ESCI-BCticas project, which consists of a number of reflection lines onshore and offshore in southern Spain, was partially carried out in August-September 1991 and February 1992. This project is part of the Spanish programme ESCI (Deep seismic reflection studies of the Iberian crust) launched to study the deep crustal struc- ture of the Iberian Peninsula by seismic near- vertical reflection methods. The overall aim of the project is to seismically image the crust of the Betic Chain and Alboran basin, focusing on the

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V: Garcia-Due&u et al. / Tectonophysics 232 (1994) 77-89 19

following general subjects: nature and structure of the crust; development of collisional struc- tures; response to extensional stresses in regions of recently thinned crust; correlation between crustal structure and distribution of seismicity in a tectonically active area.

The first part of the project resulted in the collection of two land reflection profiles. The geographical location of these profiles is given in Fig. 1. Line ESCI-BCticas 1 (Profile 1 hereafter) is a 90 km long profile that extends from the Guadalquivir foreland basin to the Guadix-Baza basin crossing the cover of the South Iberian domain. Line ESCI-BCticas 2 (Profile 2 hereafter) with a total length of about 106 km cuts across the Betic Alpine metamorphic complexes of the Betics. Based on initiaI industrial processing of the data, we present the main seismic features observed on the stacked sections and compare them with previous results from refraction and wide-angle reflection surveys.

2. Tectonic setting

The Betic and Rif mountain chains, north and south of the Alboran basin respectively, and the Gibraltar Arc are the westernmost end of the Alpine erogenic belt (Fig. 1). These chains are located at a boundary between two large plates whose kinematic regime has changed significantly during the Mesozoic and Tertiary. The relative positions between them have been restored by different authors and compiled in various general syntheses (Biju-Duval et al., 1978; Smith and Woodcock, 1982; Olivet et al., 1984; Dercourt et al., 1986; Dewey et al., 1989; Srivastava et al., 1990). From the proposed relative movements between plates, it is inferred that during the Mesozoic sinistral strike-slip motion between the European and African plates took place, with presumably oblique extension during the late episodes. Later, an approximately N-S contrac- tion occurred in a convergent regime. However, the kinematics of co-existing crustal thickening and thinning in the Betic and Rif chains does not agree with the proposed plate-motion trajectories

(e.g. Dewey et al., 1989; Garcia-Dueiias et al., 1992).

In this context, the Gibraltar Arc resulted from ~ntinental collision and j~ta~sition of tectonic complexes and large paleogeographic elements belonging to pre-Miocene crustal domains. The Alboran domain corresponds to the internal zones of the Gibraltar Arc, including the Alboran basin, and the South Iberian and Maghrebian domains correspond to the external zones of this arc (Fig. 1). The cover units of the South Iberian and Maghrebian domains in the external part of the Gibraltar Arc were deposited on margins related to a wide transform-like zone that linked the extension in the Central Atlantic with that of the ~gurian-Tethys zone (Dercourt et al., 1986).

The syn- and post-metamorphic pre-Miocene Alpine nappes comprised the Betic and Rif com- plexes of the so-called Alboran domain and were separated from those of the South Iberian and Maghrebian margins by troughs. The southern Flysch trough was underlain by oceanic or very thin continental crust related to the transform zone (Dercourt et al., 1986), while the northern trough was probably a transform basin.

During the early and middle Miocene thinning of the Alboran domain, the effective hanging wall of the Gibraltar Arc, was coeval with its emplace- ment by overthrusting on both the South Iberian and Maghrebian domains (Sanz de Galdeano, 1990; Garcia-Duefias et al., 1992). At present, the units of the Alboran domain belonging to the Betic and Rif chains have a N-S continuity form- ing the basement of the Alboran Sea.

The seismic reflection profiles acquired within the ESCI programme in southern Spain are lo- cated in the South Iberian domain, whose base- ment is the Variscan crust of the Iberian Massif (Profile 11, and across the Alpine metamo~hic complexes of the Alboran domain (Profile 2; Fig. 1).

3. Data acquisition and processing

Both seismic reflection profiles (Profiles 1 and 2) were recorded in August-September 1991.

80 V. Garcia-Duetias et al. / Tectonophyk~ 232 (1994) 77-W

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Some segments of these profiles crossed rugged including static corrections, filtering and dynamic topography with altitudes ranging from 2500 m to equalization. For further details see ‘I’ables 1 and about sea level near the shore line. Both profiles 2; which summarize survey parameters and tech- have been processed by a commercial contractor. nical details of the processing applied to the The field data were sorted into CDP gathers stack. The southern end of Profile 1 and thtt. every 30 m. Standard techniques were applied to northern part of Profile 2 are not shown due to obtain the stacked sections presented in this study, the poor quality of the stacked section across the

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crust; iw = M&o: ?W’TT = two-way travel time. Dashed square indicates location of Fig. 4. No vertical exagg~r~ti~n at 6.0 km/s. See Fig. 1 for location.

Guadix-Baza basin <Fig. 11. Likewise, the south- ern end of Profile 2 has been omitted due to the presence of artifacts that need further processing.

4. Results

4.1 The Iberian Ivfassif and South Iberian domain (Profile 1)

A stacked seismic section from part of Profile 1 (Fig. 1) is dispfayed in Fig. 2. In Fig. 3, a fine drawing shows the most conspicuous reflections of the stacked section. The seismic image of the crust is good in the northern part, but the data quality decreases as the profile enters the Neo- gene Guadix-Baza basin, probably due to the effect of the sediments. In the upper part (O-2 s) of the seismic section the few scattered reflec- tions can be associated with the sedimentary cover. From this data set alone it is difficult to

distinguish the sediment-basement boundary. A combined interpretation of refraction, wide-angle reflection and near-verticat reflection data sug- gests that this boundary occurs at about 5 km depth in the central part of the profile, which coincides roughly with the reflection seen at 2 s from CDPs 950 to 1250 (Fig. 2).

The upper crust is characterized by a transpar- ent layer that thickens southward. At about 7 s, an abrupt development of subhorizontal reflec- tors marks the top of a reflective layer that ex- tends to 12-13 s from CDP 10 to 700 (Figs, 2 and 4). This layer, which may be associated with the lower crust, thins southward and is not observed at the south end of the profile. Thinning may be related to the geodynamic evolution of the study area. Alternatively, the absence of reflections at lower-crustai levels in the southernmost part of the profile could be due to poor signal-to-noise ratio as the profile enters the Guadix-Baza basin (Figs. 2 and 3). The reflective pattern of the

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83

84 V. Garcia-Dueiias et al. / Tectonophysics 232 (1994) 77-89

lower crust is seen better on Fig. 4. Reflections are continuous for a few kilometers, although some of them seem to be longer.

between CDP 400 and 650 at 13 s is thought to be an upper mantle reflection.

In the northern part of the profile, at about 12 s, the reflective lower crust gives way to a series of discontinuous gently dipping reflections, visible between CDPs 50 and 1750. We interpret the 12 s reflection at the beginning of the seismic line and the 14 s one at CDP 1750 as the reflection Moho (Figs. 2 and 4). From this interpretation the up- per mantle is not completely transparent, show- ing scattered short reflections at various loca- tions. A series of dipping reflections in the man- tle can be followed from CDP 700 to 1250, reach- ing a maximum depth corresponding to about 16 s. In particular, a strong and continuous event

4.2 The Alboran domain (Profile 2)

Profile 2 crosses the South Iberian and Albo- ran domains in a NE-SW direction, reaching the Mediterranean coast of southern Spain (Fig. 5). This profile shows a highly reflective character in the middle and lower parts where it crosses the Alboran domain (Fig. 1). Fig. 6 shows an inter- preted section in the form of a line drawing.

The upper crust along the profile is fairly transparent except for a conspicuous reflection band, 0.3 s thick, between CDPs 1000 and 1400 at 4.3 and 3.0 s, respectively (Figs. 5-7; U.C.R. in

INE

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Upper Crust

Lower Crust --

1 6-O km I ” I ” ” I ” ” I ” I ” ” I ” ” ( ” ” I ” ” I ” ”

loo0 1250 1500 1750 2000 2250 2500 2750 3000 CDP #

Fig. 6. Interpreted line drawing section corresponding to the stacked seismic section of Profile 2 shown in Fig. 5. U.C.R. = upper- crustal reflector; M = Moho; GB.B. = Guadiz-Baza basin; A.C. = Alpujarride Complex; U.B. = Ugijar Basin; TWTT = two-way

travel time. Dashed lines outline location of Figs. 7 and 8. See Fig. 1 for location.

V. Garcia-Dueiias et al. / Tectonophysics 232 (1994) 77-89 85

Table 1 Acquisition parameters for the Betic Chain deep seismic reflection profiles

Source We ~namite-single hole Hole depths 10-24 m (Profile l)/

IO-34 m (Profile 2) Charge lo-30 kg Spacing 240 m

Spread Geophone groups 240 Geophone pattern 18 geophones/array Geophone array Linear Geometry Split spread Spacing 60 m Spread length 14.46 km CDP interval 30 m

Recording Recording instrument SERCEL SN 348 Sampling 4 ms Record length 25 s Low-cut filter out High-cut filter 62.5 Hz, 12 db/oct

Fig. 6). This reflection cannot be traced to the surface. A reflective band at about 5.5-7.0 s can be followed along the entire profile with shorter

Table 2 Data processing sequence applied to the Betic Chain deep seismic reflection profiles

(1) DemultipIex and reformat (2) (3)

(4) (5)

Amplitude recovery Dynamic trace equalization Window: O-20 s; operator length: 5 s Edit Band pass filter

(6)

(7) (8)

Window: O-20 s; frequency: 2-8-35-45 Slalom line Common Mid Point CDP interval 30 m (nominal) Antialias filter and resample to 8 ms Statics-Field statics applied to near-surface

floating datum (FDP) (9) Mute (10) NM0 (11) Stack 3000% coverage (nominal) (12) F/X domain random noise attenuation. (13) Bandpass filter 6-18-35-48; O-20 s (14) Statics-Correction from FDP to survey datum (1000 m)

reflections above and below. The lower part of the crust shows moderate reflectivity with dipping reflections between CDPs 1850 and 2350 (Figs. 5-7). The reflectivity of the Moho varies from single reflections to reflective bands along the

PROFILE 2 a800 900 1000 1100 1200 1300 1400 1500

--I W =I

600 ”

Fig. 7. Enlarged portion of the stacked section from Profile 2 to illustrate the reflectivity of the upper and middle crust. See Fig. 6 for location’ of the window.

86

NNE COP11600 1700

V. Garcia-Duetias et al. / Tectonophysics 232 (1994) 77-89

PROFILE 2 ssw 1800 1900 2000 2100 2200 2300 2400

Fig. 8. Window of a portion of the stacked seismic section from Profile 2 to illustrate the reflectivity of the lower crust and upper

mantle. See Fig. 6 for location of the window.

profile, but it can be followed, with some undula- tions, at about 11 s from the beginning of the line to about CDPs 3000. Sub-Moho reflections be- tween CDPs 1850 and 2450 are also observed beneath this profile (Figs. 5, 6 and 8).

5. Discussion

The lower crust of the Iberian Massif has been studied seismically by a number of refraction and wide-angle reflection lines (Banda et al., 1981; Suriiiach and Vegas, 1988; ILIHA DSS Group, 1993; Tellez et al., 1993). In general, it appears as a well-differentiated layer with a distinct top and bottom (Moho) inferred from wide-angle reflec- tions. It has a velocity close to 7.0 km/s deduced from asymptotic travel-time curves, matching of critical distances and amplitude variations. The top of the lower crust is observed at about 22 km and it has a total thickness of around 10 km. Profile 1 confirms these findings assuming that

the top of the lower crust is marked by the high-reflectivity zone detected at about 7 s (22 km depth). The Moho beneath the beginning of Pro- file 1, however, seems to be deeper than expected from available seismic refraction data (Banda et al., 19931, making the lower crust there thicker than that found elsewhere in the Iberian Massif. Further south the Moho is found at even greater travel-times (13-14 s). We argue that the in- creased travel-time is due to the “velocity pull down effect” of the thick sedimentary cover of the South Iberian domain. This is confirmed by recent seismic and gravity results (Torn6 and Banda, 1992; Banda et al., 19931, which reveal an approximately 5 km thick layer of sediments in the central part of our profile, where the reflec- tion Moho is detected at 13-14 s. Because the sedimentary cover is much thinner at the begin- ning of the profile the differential pull down effect may amount to more than 1 s, which would result in a Moho depth similar to that found at the beginning of Profile 1.

V. Garcia-Duerim et al. / Tectonophysics 232 (1994) 77-89 87

The results described above are comparable to those reported for different parts of Hercynian Europe in terms of lower-crustal thickness and reflectivity pattern (e.g. Bois et al., 1988; Meiss- ner and DEKORP Research Group, 1991) and those from the Iberian Peninsula in the northern part of the Iberian Massif (Perez-Estatin et al., 1994-this volume) and the Pyrenees, Ebro basin and northeast margin of Iberia (EcoRs-Pyrenees Team, 1988; TornC et al., 1992; Surifiach et al., 1993; Gallart et al., 1994-this volume).

The southward thinning of the lower crust, if not due to observational problems, may be com- pared with a portion of wide-angle reflection line recorded close to it (Fig. 9a). Here, Banda et al. (1993) found the lower crust to be interrupted at about CDP 1600 (when projected). Furthermore, they suggested the lower crust might thin and disappear completely, as apparently imaged in

Profile 1. Such thinning may be attributed either to Neogene extension or to thickness variations related to older tectonic events.

The segment of Profile 2 shown in Figs. 5 and 6 lies entirely within complexes of the Alboran domain. The upper crust is almost transparent except beneath the northeastern part of the pro- file where a conspicuous band of reflections, la- beled U.C.R. in Fig. 6, is observed. This band of reflections is interpreted tentatively as a NNE- dipping mylonitic zone, the movement sense of which is unknown. The limit between the upper and lower crust is observed clearly at about 5.5- 7.0 s. This reflection band might be coincident with a widespread reflector observed in refraction and wide-angle reflection data, although it seems to be a few kilometers deeper in the near-vertical incidence reflection data. This deepening may be real, since we have no strictly coincident near-

PR2 PRl ENE

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I 20-r 0’ --. g 30_ &2km/s %. ./.

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40- b I I I I I I 1 7

-150 -100 -50 0 DISTANCE (km)

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Fig. 9. Cross-sections along profiles PRl (a) and PR2 (b), outlining the main results obtained from seismic refraction data together with geological interpretation. Shown for comparison with near-vertical reflection profiles (Figs. 3 and 6). Arrows indicate the transport sense along extensional detachments. See Fig. 1 for location (modified from Banda et al., 1993).

88 V. Garcia-Dueiias et al. / Tectonilphysics 232 (1994) 77-89

vertical and refraction/wide-Anglo reflection pro- files. A refraction profile, which crosses our re- flection profile (Fig. 9b) at about CDP 2100 in a perpendicular sense gives a depth to this disconti- nuity of 11 km. This reflection was tentatively interpreted by Banda et al. (1993) as a detach- ment fault. Our reflection data also favour such an interpretation

The presence of a distinct reflective pattern at lower-crustal levels is in apparent contradiction with the seismic refraction and wide-angle reflec- tion results of Banda et al. (1993), who stated that the lower crust is poorly differentiated. The differences between the results of the various surveys are likely a function of their different frequencies, acquisition geometries and line ori- entations. Suriiiach et al. (1993) observed, in a study of crustal seismic images of the Pyrenean range based on strictly coincident wide-angle and deep near-vertical incidence seismic profiles, completely different behaviours of lower-crust re- flectivity depended on the direction of the wave- front. Refraction and wide-angle reflection data along strike lines show no evidence of reflections in the lower crust. In contrast, seismic wide-angle and near-vertical incidence reflection profiles across the strike of the Pyrenees display high reflectivity at lower-crustal levels. Carbonell and Smithson (19911 using finite difference modelling concluded that the strikingly different seismic signatures obtained in perpendicular profiles across the Basin and Range province can be explained by mid- and/or lower-crustal hetero- geneities.

Profile 2 (Figs. 5, 6 and 8) shows a wide variety of reflections at lower-crustal levels that may be related to the presence of major shear zones associated with extensional features recognized at the surface. However, crustal units in the lower part of the crust, as imaged beneath Profile 2, and their configuration may not be recognized easily in wide-angle reflection data.

The interpreted Moho has a rather constant depth of around 11 s (about 35 km using the average crustal velocity of 6.3 km/s obtained from refraction surveys), which is shallower than the expected 38 km depth (Fig. 9) based on previous combined seismic and gravity studies

(Torn6 and Banda, 1992; Banda et al., 19931. However, the area sampled by wide-angle reflec- tion profiles close to Profile 2 falls around CDP 2400, where a zone of upper-mantle reflectivity is observed (Figs. 5, 6 and 8). In this case, we may argue that either the reflection Moho is not coin- cident with the refraction Moho, or that the reflections at about 11 s are not associated with the crust-mantle boundary. At this point we are unable to answer this question. A more thorough study of both data sets in this particular area is required.

Crustal thinning from the Betics towards the Alboran sea is not detected in Profile 2. This was to be expected based on the results of TorntS and Banda (19921, who found, from seismic and grav- ity modelling, that thinning is accommodated very abruptly under the shore-line in that area. Such thinning may be examined using the onshore recordings of offshore shooting during the marine part of the ESCI-B&as project. These data have not been processed yet.

In summary, Moho depths deduced from a first interpretation of Profiles 1 and 2 seem to be at variance with those obtained by refraction and wide-angle reflection techniques. However, these results have to be taken with caution until more accurate depth conversions are computed. Alter- natively, a refined interpretation of the refraction data may be appropriate. The crustal geometry obtained from Profiles 1 and 2 are thought to represent the actual crustal configuration, al- though detailed geological interpretation is still lacking.

The data acquisition of the ESCI-BCticas pro- ject has been funded by CICYT project GE0090- 0617. Additional funding comes from DGICYT project CE91-0013. John Platt, Fransoise Thou- venot and an anonymous reviewer made numer- ous comments and have helped to improve the manuscript. Melchor Gamarro is acknowledged for his constant technical support during data acquisition. The Compagnie G&r&ale de GCo- physique (C.G.G.) acquired and processed the near-vertical reflection data.

V. Garcia-Dueiius et al. / Tectonophysics 232 (199#) 77-89 89

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