Montserrat Torné

The transition from linear to diffuse plate boundary in the Azores–Gibraltar region: results from a thin-sheet model

Abstract We use the thin-sheet plane-stress approach to study the present-day dynamic behavior of the plate boundary between Eurasia and Africa along the Azores–Gibraltar region. This plate boundary, which extends from the Azores triple junction to the Gibraltar strait, shows a tectonic regime that changes from transtension in the west to transpression in the east, with a strike–slip motion in its central segment. Seismological data reveal that the western and central segments are currently marked by a linear series of earthquakes indicating that the plate boundary is located in a narrow zone. In contrast, the eastern segment is not so well defined and deformation spreads over a much broader area. To apply the thin-sheet approach, we combined heat flow, elevation and crustal thickness data to calculate the steady-state geotherm and the total strength of the lithosphere. Several models with different fault friction coefficients and geometries at the eastern segment of the plate boundary were tested. Results are compared with the maximum compressive stress directions from the World Stress Map, and the calculated seismic strain rates and slip vectors from earthquake data. The best fitting models are consistent with the rotation pole of Argus et al. [D.F. Argus et al., J. Geophys. Res. 94 (1989) 5585–5602], and show that the rheological behavior of the plate boundary must necessarily change from the western and central segments to the eastern segment. The diffuse character of the plate boundary east of the Gorringe Bank is dominated by the transition from oceanic to continental lithosphere, the weakness of the Alboran domain, and the convergence between the African and the Eurasian plates. The displacement of the Alboran domain relative to the African plate may play a major role in stress propagation through the Iberian Peninsula and its Atlantic margin.

Subduction‐related structures in the North Iberian Margin

The oblique convergence of Eurasia and Iberia since the Early Cretaceous, caused the formation of the Pyrenean intracontinental collisional orogen in the east, and progressed to continent-ocean collision with subduction of the Bay of Biscay oceanic crust beneath the North Iberian Margin in the west. Two deep multichannel seismic profiles (IAM-12 and ESCIN-4), integrated with gravity modeling and other geological and geophysical data, provide the crustal-scale architecture of this margin and its tectonic evolution during the convergence. The North Iberian Margin is modeled with a South or south-southeast dipping oceanic crust beneath the outer part of the continental shelf. Mesozoic basins on the shelf were inverted during the Tertiary, and compressional activity continued until recent times in the ESCIN-4 section, while a shallower, probably Neogene age basin is subjected to active recent erosion in the IAM-12 section. In the oceanic areas, a marginal trough deepens and widens toward the east as a result of the regional east dip of the oceanic basement. The accretionary prism increases in size from west to east (18–56 km), and its internal structure and morphology varies along strike. The prism is buried by postcorivergence sediments in both sections and in the IAM-12 section appears to have been active at least during Lutetian to Burdigalian times. The crustal-scale structure of the North Iberian Margin is that of an arrested subduction zone in which a remnant oceanic basin was being consumed near two continental plates that collided obliquely.

Geodynamic Evolution of the Eastern Segment of the Azores-Gibraltar Zone: The Gorringe Bank and the Gulf of Cadiz Region

Detailed structural interpretation of the recently acquired deep seismic multichannel profiles along the Iberian Atlantic Margins (IAM Project) provides new results on the geodynamic evolution of the eastern part of the Azores-Gibraltar plate boundary. Thrusting and folding of the oceanic basement and of Mesozoic and Cenozoic sedimentary cover of the Gorringe Bank region are consistent with the N–S convergence of Iberia and Africa. Compressive structures in the Gorringe Bank region are spread over a wide area. Deformation under compression took place mainly in Tertiary times, as is evidenced by a basal unconformity and several discontinuities in Tertiary sediments, although some deformation has also been recorded in Quaternary sediments. The compressive structures in the Gulf of Cadiz are E–W oriented thrusts, folds and related diapiric structures. N–S oriented transpressive deformation is likely to occur in the western Portuguese platform. There is no continuity of structures from the oceanic to the continental domain, suggesting that deformation transfers from one side to the other through a transcurrent fault zone. The fault contact between the two domains is located in the ocean-continent transition zone.

Pliocene uplift of the eastern Iberian margin: Inferences from quantitative modelling of the Valencia Trough

We present the results of quantitative modelling of subsidence in the Valencia Trough and associated uplift in the Catalan Coastal Ranges and the Balearic Islands. Basement subsidence, Moho depth and gravity anomalies are calculated for different effective elastic thicknesses (eet) and durations of rifting, along two profiles constrained by recently collected multichannel seismic and gravity data. The tectonic evolution of the Catalan Coastal Ranges cannot be explained with traditional two-layer lithospheric stretching models for the Valencia Trough, but is consistent with a thermomechanical model invoking lithospheric necking, combined with a late-stage uplift event. The modelling of the topography of the Balearic Islands suggests an interaction of extension in the Valencia Trough with extension in the South Balearic Basin. Our study supports a rifting model with low values for the flexural rigidity, an intermediate level of necking and one finite duration stretching event propagating from the northeast towards the southwestern part of the Valencia Trough. The modelling provides constraints on the magnitude of a large-scale Pliocene uplift of the eastern margin of the Iberian Peninsula.

Numerical modeling of foreland basin formation: a program relating thrusting, flexure, sediment geometry and lithosphere rheology

Abstract An algorithm has been developed which allows the flexural deflection of the lithosphere to be calculated under thrust loading and the geometry of the sedimentary infill in the adjacent foreland basin. To that purpose we have considered arbitrarily shaped thrust-load systems moving towards the foreland, and surface processes denudating the orogen and filling the basin. The regional compensation of topographic loads is based on the assumption that the lithosphere behaves as a thin plate with either homogeneous (elastic, viscoelastic) behavior or more realistic depth-dependent elastic–plastic rheology. When using heterogeneous rheology, the program calculates the flexural behavior as a function of crustal geometry and thermal regime of the lithosphere, thus relating the basin infill geometry with the deep lithosphere properties. We show some examples where the geometry of the basin and sedimentary infill (e.g. onlap/toplap patterns) are generated assuming different lithosphere rheologies and synthetic tectonic contexts which support the applicability of the model to study the formation and evolution of foreland basins.

Mantle-lithosphere bodies in the Alboran crustal domain (Ronda peridotites, Betic-Rif orogenic belt)

Abstract Geological and geophysical data are used to demonstrate the existence of intracrustal high-density/high P-wave velocity bodies in the western Betics. These bodies appear to correspond to buried peridotites similar to those that outcrop in the Ronda area. A gravity study shows how the gravity field is mainly the result of a combined effect of crustal thinning and the presence of ultramafic bodies. The size of the buried high-density body, as interpreted from gravity and seismic results, shows maximum dimensions of about 40 km in length (NNW-SSE), about 8 km in thickness, and a lateral extension (ENE-WSW) of about 70 km. The thinning of the crust from 32–35 km to 20–22 km takes place in a narrow area less than 35 km wide. Our results are compatible with an interpretation in terms of an unrooted peridotite slab. Dismembering of an initial slab of ultramafic rocks is a possible consequence of the extensional regime that originated the Alboran basin.

Crustal thinning in the Southwestern Iberia Margin

The mode of crustal thinning in the southwestern margin of the Iberian Peninsula is investigated along a transect that extends from onshore Iberia to the eastern end of the Horseshoe Abyssal Plain. On onshore areas, the crustal structure has been deduced using wide-angle seismic reflection data, whereas offshore we have used coincident steep and wide-angle reflection data along a NE-SW oriented seismic profile that extends from Cape San Vicente to the Horseshoe Abyssal Plain. In addition, 2D gravity modelling has been performed to validate the crustal structure deduced from seismic data. Our model results reveal that the crust undergoes a strong but continuous thinning from 31 km onshore Iberia to less than 15 km in the Horseshoe Abyssal Plain and that thinning occurs over horizontal distances of about 120 km.

Gravity and multichannel seismic reflection constraints on the lithospheric structure of the Canary Swell

Deep penetrating multichannel seismic reflection and gravity data have been used to study the lithospheric structure of the Canary Swell. The seismic reflection data show the transition from undisturbed Jurassic oceanic crust, away from the Canary Islands, to an area of ocean crust strongly modified by the Canary volcanism (ACV). Outside the ACV the seismic records image a well layered sedimentary cover, underlined by a bright reflection from the top of the igneous basement and also relatively continuous reflections from the base of the crust. In the ACV the definition of the boundary between sedimentary cover and igneous basement and the crust-mantle boundary remains very loose. Two-dimensional gravity modelling in the area outside the influence of the Canary volcanism, where the reflection data constrain the structure of the ocean crust, suggests a thinning of the lithosphere. The base of the lithosphere rises from 100 km, about 400 km west of the ACV, to 80 km at the outer limit of the ACV. In addition, depth conversion of the seismic reflection data and unloading of the sediments indicate the presence of a regional depth anomaly of an extension similar to the lithospheric thinning inferred from gravity modelling. The depth anomaly associated with the swell, after correction for sediment weight, is about 500 m. We interpret the lithospheric thinning as an indication of reheating of old Mesozoic lithosphere beneath the Canary Basin and along with the depth anomaly as indicating a thermal rejuvenation of the lithosphere. We suggest that the most likely origin for the Canary Islands is a hot spot.

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

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 Mono 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.

Deep structure of the Vøring Margin: the transition from a continental shield to a young oceanic lithosphere

The present-day lithospheric structure across the Norwegian Margin in the Voring region is presented. The Voring Margin is characterized by the presence of large volume of magmatic underplating, thick Mesozoic basins, and prominent crustal thinning. Results from recent deep seismic experiments in the Voring Basin and Voring Marginal High and 3-D gravity modelling have been implemented in a regional 2-D lithospheric transect that runs from the Norwegian Caledonian Belt to the oceanic domain, crossing the transition between the Precambrian Fennoscandian/Baltic shield and the Cenozoic northern North Atlantic oceanic lithosphere. The modelling approach integrates elevation, gravity, geoid and heat flow data under the assumptions of thermal steady-state and local isostasy. The results confirm and refine the major trends of the crustal geometry which is characterized by a Moho depth varying from about 45 km beneath the Caledonian thrusts, to 30 km beneath the Trondelag Platform, to 20–14 km beneath the Voring Basin and to 13 km in the oceanic domain. The lithosphere thins from the Norwegian Caledonian Belt (190 km thick) to the oceanic domain (<60 km) in a stepwise manner reflecting the progressive seawards migration of lithospheric deformation since the Paleozoic. Our lithospheric thickness results are different from those predicted by post-rift lithospheric cooling models. In the oceanic domain, observed residual bathymetry (∼600 m) is interpreted as the remaining effect of a deep seated thermal perturbation during the upper Cretaceous which would also produce a heat flow anomaly of ∼15 mW m−2. Alternatively, residual bathymetry can be interpreted in the absence of thermal anomalies as produced by depletion of the sublithospheric mantle extending down to 130–230 km depth depending on the density contrast (10–30 kg m−3).