High-resolution lithospheric structure in the Gibraltar arc using P and S receiver functions

Resumen

Plate margin between Africa and Eurasia in westernmost Mediterranean is distributed as an irregular boundary that configures a tight orogenic belt, the Gibraltar arc. The region exhibits tectonic processes which may involve both oceanic and continental subduction and delamination (1). Thought the geodynamics of the zone has mainly been interpreted in terms of east dipping subduction rollback (2), the relationship between slab dynamics and surface tectonics is still under considerable debate (1). In this thesis we shed some light on the complex geodynamics of the Ibero-Maghrebian region by imaging precisely the crustal and upper mantle structure using the S and P receiver function approaches (3). The seismic imaging with S receiver functions all along the Gibraltar arc reveals the connection through an oceanic corridor between the Alboran slab under the Alboran Sea and the Atlantic oceanic crust. Previous P receiver function interpretations showed significant lateral heterogeneities in the lithosphere and suggest processes of underthrusting and continental delamination occurring mainly in Southeastern Spain (4). We analyze two high-resolution passive seismic profiles to probe the crustal structure in the central and eastern Betic orogen, implementing different and complementary receiver function interpretation techniques: (a) the conventional Common Conversion Point stacking method based on the simplifying flat-layered assumption that enhances the main crustal structure along the profiles; (b) the reverse time migration technique, which exploits the whole complexity of the wavefield, produces clearer images of the Moho topography; (c) backazimuth harmonic decompositions of receiver functions to constrain the geometry and strength of lateral heterogeneities that also allows to obtain an only sensitive to the flat-layered structure receiver function to be (d) jointly inverted with surface wave dispersion curves and therefore obtain the average 1D S-wave velocity depth profile. All these methods give coherent and complementary results. A sharp and prominent crustal step (∼15-17 km) is observed in both profiles, coinciding with the boundary between the Iberian and Alboran domains and interpreted as a near-vertical STEP fault that propagates to the surface as a positive flower fault structure. This STEP fault accommodated the differences in the subduction rollback velocity along the strike, at the northern edge of the Western Mediterranean system, when the thinned Iberian continental lithosphere started subducting under the Alboran domain. The changes in the topography of the Moho discontinuity, and the geometry of the STEP fault in Central Betics is driven by inherited weaknesses in the Iberian paleomargin. No crustal roots are observed under the highest altitudes of both profiles suggesting that its high topography is due to a combination of the uplift produced by the positive flower structure and the push up of the asthenosphere after the lithospheric removal of the underthrusting Iberia along the STEP fault. Using the harmonic decomposition of radial and transverse receiver functions we obtain a continuous representation of the lateral heterogeneity with depth along the crust and the uppermost mantle, determining the orientation of anisotropy and dipping interfaces. This analysis distinguishes several zones along the region: (a) the Alboran domain located in the SE section nearby the coastline and mainly characterized by a dipping NNW Moho discontinuity and a regional anisotropy governs by stress-induced shear anisotropy and structural anisotropy nearby the main faults of the Eastern Betics shear zone, (b) the transition zone between the Alboran domain and the Iberian Massif coinciding with the STEP fault and marked by the presence of significant dipping and/or anisotropy at crustal and subcrustal depths and (c) a stable region without considerable lateral heterogeneity. Bibliography: 1. Platt, John P. et al. (2013). “The Betic-Rif Arc and Its Orogenic Hinterland: AReview”. en. In:Annual Review of Earth and Planetary Sciences41.1, pp. 313–357.ISSN: 0084-6597.DOI:10.1146/annurev-earth-050212-123951.URL:http://www.annualreviews.org/doi/full/10.1146/annurev-earth-050212-123951. 2. Rosenbaum, Gideon, Gordon Lister, and Cecile Duboz (2002). “Reconstruc-tion of the tectonic evolution of the Western Mediterranean since the Oligocene”.In:Journal of the Virtual Explorer8, pp. 107–130.DOI:10.3809/jvirtex.2002.00053. 3. Kind, Rainer, Xiaohui Yuan, and Prakash Kumar (2012). “Seismic receiver func-tions and the lithosphere–asthenosphere boundary”. In:Tectonophysics536-537, pp. 25–43.ISSN: 00401951.DOI:10.1016/j.tecto.2012.03.005. URL:http://linkinghub.elsevier.com/retrieve/pii/S0040195112001333. 4. Mancilla, Flor de Lis et al. (2015b). “Slab rupture and delamination under theBetics and Rif constrained from receiver functions”. In:Tectonophysics.ISSN:00401951.DOI:10.1016/j.tecto.2015.06.028. URL:http://linkinghub.elsevier.com/retrieve/pii/S0040195115003595.