Evidence for active subduction beneath Gibraltar M.-A. Gutscher Institut Universitaire Europeen de la Mer, UMR 6538, Domaines Oceaniques, Plouzane, Brittany F-29280, J. Malod J.-P. Rehault France I. Contrucci* F. Klingelhoefer* Bullard Laboratories, Cambridge University, Madingley Road, Cambridge CB3 0EZ, UK L. Mendes-Victor Institute of Earth Sciences, University of Lisbon, Rua de Escola Politecnica 58, Lisbon 1269-102, Portugal W. Spakman Faculty of Earth Sciences, Utrecht University, Budapestlaan 4, Utrecht NL-3508, Netherlands ABSTRACT We report on a marine seismic survey that images an active accretionary wedge west of Gibraltar. Ramp thrusts offset the seafloor and sole out to an east-dipping de´collement, indicating ongoing westward-vergent tectonic shortening. New traveltime tomographic results image a slab of oceanic lithosphere descending from the Atlantic domain of the Gulf of Cadiz, passing through intermediate-depth (60–120 km) seismicity beneath the westernmost Alboran Sea, and merging with a region of deep-focus earthquakes 600–660 km below Granada, Spain. Together, these new data provide compelling evidence for an active east-dipping subduction zone. Keywords: subduction, Africa, Iberia, accretionary wedge. INTRODUCTION The Gibraltar region features the arcuate Betic-Rif mountain belt, with outwarddirected thrusting (Maldonado et al., 1999), surrounding a zone of strong Neogene subsidence and crustal thinning in the Western Alboran Sea (Docherty and Banda, 1995). Until now its geodynamic interpretation has been controversial (Lonergan and White, 1997; Calvert et al., 2000). The Gibraltar Arc is located at the eastern end of the Azores-Gibraltar transform, a diffuse transpressional plate boundary between the Iberian and African plates (Sartori et al., 1994) (Fig. 1A, inset). Relative convergence between the two plates here is slow, only 4 mm/yr in a northwestsoutheast direction (Argus et al., 1989). However, attention has recently been focused on this plate boundary, while seeking the likely source of the destructive Lisbon great earthquake (M . 8.5) and tsunami of 1755 (Zitellini et al., 2001). Northward-vergent thrusting in the Internal Betics and southward-vergent thrusting in the Rif is coeval with subsidence in the Alboran domain. Farther west in the Gulf of Cadiz, a chaotic sedimentary melange, interpreted as an olistostrome, shows signs of intense deformation and westward transport, attributed primarily to gravity sliding (Maldonado et al., 1999; Torelli et al., 1997). Intermediate-depth seismicity (60–120 km depth) is observed beneath the Gibraltar Arc and westernmost Alboran Sea (Casado et al., 2001) and deepfocus earthquakes (600–660 km) occur beneath southern Spain, near Granada (Figs. 1A, 1B) (Buforn et al., 1991). Early tomographic studies indicated a low-velocity Pwave anomaly beneath the Alboran Sea un*Present address: Ifremer Centre de Brest, DROGM, Plouzane, Brittany F-29280, France.
derlain by a high-velocity body below ;150 km depth (Blanco and Spakman, 1993; Seber et al., 1996) (Fig. 1B). A variety of tectonic models has been proposed to explain these different observations, including past or present subduction or delamination of overthickened continental lithosphere (Platt and Vissers, 1989; Seber et al., 1996; Calvert et al., 2000). Subduction models have been proposed with southward (Sanz de Galdeano, 1990; Morales et al., 1999), northward (Torres-Roldan et al., 1986), eastward (Royden, 1993; Lonergan and White, 1997), and westward dips (Docherty and Banda, 1995; Zeck, 1997). The SISMAR (Sismique Maroc) marine seismic survey conducted in April 2001 acquired .3000 km of 360-channel seismic data with a 4.5-km-long streamer and 1000 km of wide-angle data recorded by ocean bottom seismometers (OBS), completely spanning the actively deforming region between the margins of Portugal and northwest Morocco. Survey results, together with other data presented here, offer compelling evidence of active eastward-dipping subduction from the Atlantic oceanic domain between Iberia and Africa to below the Alboran Sea. EVIDENCE SUPPORTING EASTDIPPING SUBDUCTION Newly acquired multichannel seismic (MCS) data image the olistostrome in the Gulf of Cadiz as an eastward-thickening wedge of deformed sediments overlying gently eastward dipping undeformed sediments and basement (Figs. 2 and 3). The steplike seafloor morphology and east-dipping reflectors deforming the seafloor indicate active ramp thrusts (Figs. 2A, 2B). Additional seismic profiles 20 and 22 (Figs. 2C, 2D) show the arcuate shape of the deformation front in map view, and con-
firm the consistent west-vergent deformation. We interpret these thrusts as bounding imbricate slices within an active accretionary complex we tentatively name the Atlantis accretionary wedge for geographical reasons. West-vergent thrust faults sole out to an east-dipping de´collement and indicate that deformation cannot be caused by gravity sliding along a west-dipping surface, but rather is tectonically driven. An overall eastward dip of the de´collement as well as the basement at depth is indicated by wide-angle seismic data recorded by OBS along profile 16 (Fig. 3). Ray-tracing software (Zelt and Smith, 1992) was used to construct the P-wave velocity model. Where ray coverage was insufficient, gravity modeling was used to determine the Moho geometry. Strong evidence for subduction is provided by new cross sections from a global tomographic model (Bijwaard and Spakman, 2000) that image an east-dipping, high P-wave– velocity body, extending continuously from the surface in the Atlantic domain west of the Gibraltar Arc to 660 km depth beneath the Western Alboran Sea (Fig. 4). These results are in excellent agreement with the east-dipping high-velocity body revealed by a tomographic inversion of locally and teleseismically recorded earthquake traveltimes (Calvert et al., 2000). This narrow, curved, north-south– trending slab is also imaged by horizontal sections of the tomographic model of the Mediterranean region (Wortel and Spakman, 2000). An east-dipping slab of oceanic lithosphere is supported by the distribution of intermediate- and deep-focus earthquakes (Buforn et al., 1991). Intermediate-depth seismicity is concentrated at 60–120 km depth in a north-south line beneath the westernmost Alboran Sea (Fig. 1). Such seismicity is common within subducting slabs where plate curvature increases abruptly, and is likely caused by pressure- and temperature-dependent dehydration reactions in the oceanic crust and uppermost oceanic mantle (Peacock, 2001). Seismicity at such depths is inconsistent with models of continental subduction (collision) or delamination of continental lithosphere. The highvelocity body imaged tomographically coincides with the locus of deep earthquakes (600–660 km) beneath southern Spain, where a strong (Mb 7.1) deep-focus earthquake with
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[email protected]. Geology; December 2002; v. 30; no. 12; p. 1071–1074; 4 figures.
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ceeding 908 (Platzman, 1992), consistent with rotation along the limbs of a retreating arc domain.
Figure 1. A: Location map with shaded hill relief (Smith and Sandwell, 1997) and seismicity (Engdahl et al., 1998; Preliminary Determination of Earthquakes [PDE] Catalog, 1973–present; Centroid Moment Tensor [CMT] Catalog, 1976– present) (M $ 3). Red thrust teeth symbols indicate Gibraltar Arc; green thrust teeth symbols indicate active Atlantis accretionary wedge. Seismicity sampling box (for B) is given. Newly acquired SISMAR seismic profiles are shown as orange lines and positions of ocean bottom seismometer are as shown as black triangles, with red fill. MAR is Mid-Atlantic Ridge. B: Lithospheric cross section showing earthquake hypocenters and principal tectonic domains. Inset shows schematic evolution since 10 Ma, with trench rollback and associated backarc extension of formerly thickened Internal Domain, leading to development of Western Alboran Sea basin. C: Simplified kinematic reconstruction of western Mediterranean region since 35 Ma (after Rehault et al., 1985; Lonergan and White, 1997); red triangles show arc volcanism. Note two narrow oceanic domains subducting beneath Calabria (slab rollback to southeast) and Gibraltar (slab rollback to west-southwest), last remnants of larger north-northwest– dipping Maghreb subduction system.
a steeply east dipping focal plane (Chung and Kanamori, 1976) occurred in 1954. An active accretionary complex is further supported by recent observations of active mud volcanoes in the Gulf of Cadiz between 500 and 1200 m water depth (Gardner, 2001). Mud volcanoes are common features in regions where sediments are overpressured and subjected to compressive stresses, as demonstrated in the Mediterranean Ridge (Kopf et al., 2001) and Barbados Ridge (Brown and Westbrook, 1988) accretionary complexes. 1072
Miocene arc volcanism and paleomagnetic data offer additional proof of east-dipping subduction beneath the Alboran Sea and slab retreat toward the west (Lonergan and White, 1997). A horseshoe-shaped belt of 5–15 Ma calc-alkaline and mixed potassium-alkaline volcanic rocks is located subparallel to the Betic-Rif belt and somewhat farther east, indicating subduction-related volcanism during the Miocene (Fig. 1C). Paleomagnetic data indicate clockwise rotations in the Betics and counterclockwise rotations in the Rif, both ex-
GEODYNAMICS OF THE WESTERN MEDITERRANEAN Coeval thrusting on three sides in an external belt and extension in an internal zone are consistent with westward slab rollback and associated backarc extension (Fig. 1B, inset). The continental crust of western Alboran Sea has been thinned to 10–15 km (Docherty and Banda, 1995). At water depths of 1000 m the basin is underlain by at least 4–7 km of Neogene sediments. Available centroid moment tensor focal mechanisms for the western Alboran Sea show normal faulting mechanisms consistent with east-northeast–west-southwest extension (Fig. 1A). Both the Calabrian and Hellenic Arcs, which are associated with backarc extension in the Tyrrhenian and Aegean Seas, respectively, have thick frontal wedges of deforming sediments. The Calabrian Arc in particular offers many structural similarities to the Gibraltar Arc. On the basis of tomographic images and analog modeling, the Calabrian Arc is concluded to have reached its current configuration by the rollback of a narrow (200–300 km wide) slab toward the southeast (Faccenna et al., 2001). We agree with the geodynamic analysis of Lonergan and White (1997), whereby the Gibraltar subduction is an analogue of the Calabrian arc system. Gibraltar is located at the western termination of the northwest-dipping paleo-Maghreb subduction system (Fig. 1C), whereas the Calabrian system is at its eastern end. In both cases the current subduction direction is nearly perpendicular to the overall north-south convergence between Africa and Eurasia and is controlled by rollback of a narrow corridor of remnant Tethyan oceanic lithosphere. Horizontal tomographic cross sections of the Mediterranean region image these two horseshoe-shaped dipping slabs at 200 km depth, as well as the deeper Tethyan lithosphere from the paleo-Maghreb subduction zone at 600 km depth (Wortel and Spakman, 2000). Lonergan and White (1997) proposed that east-dipping subduction had ceased recently at the level of the Gibraltar Arc, as all the oceanic lithosphere between Iberia and Africa may have been exhausted. Our marine seismic data showing thrust faults cutting the seafloor demonstrate that subduction remains active today. The presence of a narrow (,200 km) corridor of oceanic lithosphere between Iberia and Africa is suggested by the north-south tomographic cross section just west of Gibraltar (Fig. 4, C-C9), and this width corresponds well to the north-south line of intermediate depth seismicity in the Western Alboran Sea (Fig. 1A). GEOLOGY, December 2002
Figure 2. Time-migrated seismic profiles showing west-vergent thrusting above series of subhorizontal, undeformed reflectors. A: Close-up of profile 16. B: Line drawing of profile 16. East-dipping thrust faults locally offset seafloor (indicated by arrows). C: Line drawing of profile 20. D: Line drawing of profile 22 (see Fig. 1 for profile locations). MCS—multichannel seismic data. DF—deformation front.
Figure 3. Ocean bottom seismometer (OBS) profile 16 (MSC—multichannel seismic). A: P-wave velocity model obtained from wide-angle data using raytracing software (Zelt and Smith, 1992). B: Ray paths corresponding to traveltime picks; densities used for gravity modeling are given (g/cm3). C: Observed traveltime picks and calculated traveltimes. D: Gravity anomaly calculated from velocity model. GEOLOGY, December 2002
DISCUSSION The presence of a zone of low seismic velocities between 50 and 100 km beneath the Western Alboran Sea, overlying a highvelocity zone between 150 and 400 km depth, had been advanced as an argument in favor of delamination of continental lithosphere (Seber et al., 1996). Furthermore, peridotites and metamorphic rocks in the Rif-Betic belt, showing pressure-temperature-time paths indicating rapid exhumation from upper mantle depths (50 km) to the surface, have been interpreted in terms of extension of a thickened orogenic domain with accompanying delamination (Platt and Vissers, 1989; Vissers et al., 1995). Both observations are entirely consistent with subduction-related mechanisms. The cold, dense body at 150–400 km depth was later shown to be east dipping and to extend to 600 km depth (Calvert et al., 2000). A subducting slab induces corner flow in the asthenosphere and thus a hot zone of anomalously low seismic velocities in the backarc region, which is subject to extension and subsidence in response to slab rollback (Fig. 1B, inset). Delamination of a dense root of continental lithosphere would result in an isostatic response of uplift, in contradiction with the recent subsidence observed in the Western Alboran Sea. Such is the case for delamination beneath the Central Andean Puna-Altiplano plateau, with a mean elevation of 4000 m (Yuan et al., 2000). Previously, rapid exhumation of rocks from mantle depths was commonly attributed to extensional mechanisms in the crust (Platt and Vissers, 1989). However, in the Gibraltar Arc peridotites are located in the region of greatest modern crustal thickness (Banda and Ansorge, 1980). Analog modeling has demonstrated that during the transition from oceanic subduction to continental subduction, slices of continental material carried down to substantial depth (50–150 km) can become detached and exhumed rapidly during continued convergence, bringing bits of upper mantle material up to the surface (Chemenda et al., 1995). Such a geodynamic situation exists in the Betic and Rif orogens, where the Iberian and African continental margins, respectively, are currently underthrust beneath the Internal Zones. CONCLUSION Eastward subduction beneath Gibraltar offers a single, simple explanation for the seemingly contradictory observations from the complex Rif-Betic region. Slab rollback toward the west causes extension and subsidence in the Alboran Sea, while the associated westward advance of the Gibraltar Arc drives compressional deformation in the Atlantis accretionary wedge. Active subduction in the Gulf of Cadiz can be expected to have a strong impact on the natural hazard assessment of this region and must be considered as a possible source of the great M . 8.5 earthquake and tsunami that ravaged Lisbon and the Gulf of Cadiz in 1755. 1073
Figure 4. Cross sections from nonlinear inversion of global earthquake traveltime data (Bijwaard and Spakman, 2000). A-A9 and B-B9: East-west cross sections clearly show continuous high-velocity P-wave anomaly descending from Atlantic domain in Gulf of Cadiz to merge with region of deep-focus earthquakes below 600 km depth beneath Granada, Spain. We interpret this cold, dense body as slab of Mesozoic oceanic lithosphere. Above slab is lowvelocity anomaly consistent with induced corner flow in asthenospheric wedge. C-C9 and D-D9: North-south cross sections show narrow slab (
