Overriding Plate Thickness as a Controlling Factor for Trench Retreat Rates in Narrow Subduction Zones

Slab width is a significant factor in controlling subduction zone dynamics, particularly the retreat velocities, which tend to decrease with wider slabs. However, observations of natural narrow subduction zones reveal no correlation between slab width and trench velocities. This suggests that other factors may exert a greater influence. In this study, we employ 3D numerical subduction models to systematically assess the impact of slab width, strength of slab coupling to the lateral plate (LP), and overriding plate (OP) thickness on trench kinematics and geometry. Our models focus on narrow slabs (400–1,200 km), and the results demonstrate that, in the case of narrow subduction zones, the slab width has little effect on trench migration rates and the viscous coupling at the lateral slab edge is only important for very narrow subduction zones (≤800 km). Conversely, the OP thickness emerges as a crucial factor, with increasing plate thickness leading to a strong decrease in trench velocities. These findings provide an explanation for the observed trench velocities in natural narrow subduction zones, where an inverse relationship with OP thickness is evident. Furthermore, our study reveals that not only slab width, but also the OP thickness and the slab coupling to the LP, significantly influence trench geometry. Strong lateral coupling promotes the formation of concave trench geometries, while thick overriding plates favor the development of “w”‐shaped geometries. Overall, a comprehensive understanding of subduction processes necessitates considering the interplay between slab width, OP thickness, and slab coupling to the LP

Slab Tearing Underneath the Bransfield Strait, Antarctica

We conduct a P-wave receiver function analysis of the Bransfield Strait (West Antarctica) to determine the lithospheric structure of this back-arc basin, thanks to 31 temporary and permanent stations. Our main finding is a 15 km tear of the Phoenix slab, coinciding with the location of the 2020–2021 Orca earthquake swarm's epicenters. Teleseismic wave modeling reveals that the two major earthquakes occurred at the base of the crust, suggesting that the swarm could have been triggered by active underplating driven by mantle flow through the slab tear. There is evidence for such an underplating layer at least under Deception Island and for a widespread low velocity zone in the mantle wedge probably undergoing partial melting. We found average crustal thickness (30.5 ± 1.0 km) and Vp/Vs (1.81 ± 0.04) values close to average extended continental crust, although results in the South Shetland Islands are significantly more heterogeneous than in the Antarctic Peninsula.Spanish national projects PID2019-109608GB-100/ SRA/10.13039/501100011033CMT2016-77315-R, the Andalusian regional project A-RNM-421-UGR18FPI Grant PRE2020-092556 (funded by MCIN/AEI/10.13039/501100011033 and the European Social Fund

Connection between the Jurassic oceanic lithosphere of the Gulf of Cádiz and the Alboran slab imaged by Sp receiver functions

We investigate the lithospheric structure beneath the Gibraltar arc (western Mediterranean) using S-wave receiver functions (SRFs). From a dense network deployed in the Ibero-Maghrebian region during different seismic surveys, we calculated ~11,000 SRFs that sample the upper mantle detecting the lithosphere-asthenosphere boundary (LAB). The observed seismic LAB belongs to different lithospheric domains: Iberian and African forelands, Alboran domain, and Atlantic Ocean. Common conversion point (CCP) migrated profiles show the geometrical relation among them. Under the Strait of Gibraltar, we observe a deep LAB (~150 km). It can be associated with Jurassic-age lithosphere of ~120 km thickness, one of the thickest ever reported in oceanic environments. There is an abrupt offset between the oceanic LAB and the shallow (80-km-deep) continental LAB of the Iberian foreland, suggesting displacement along a former transform fault. The northwestern African continental LAB is 90–100 km deep. The oceanic LAB under the Gibraltar arc continues to ~180 km depth beneath the Alboran Sea, showing the connection between the Alboran slab and the oceanic lithosphere in the central Gulf of Cádiz. This geometry agrees with an ~200-km-wide corridor of oceanic lithosphere between the central Atlantic and the Alpine Tethys, developed during the Middle–Late Jurassic. Our results support the proposed westward rollback of an oceanic east-dipping slab, which has continuity at least to the central Gulf of Cádiz.This work was supported by the Spanish national projects CGL2015-67130-C2-2-R/FEDER and CGL2012-31472

Slip Partitioning in the 2016 Alboran Sea Earthquake Sequence (Western Mediterranean)

This study was supported by FEDER/MINECO projects CGL2015-67130-C2-2-R and PID2019-109608GB-I00, FEDER/Junta de Andalucia project A-RNM-421-UGR18, and is part of the research group RNM104 of the Junta de Andalucia. JA has also received funding from the European Union's Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 754446 and UGR Research and Knowledge Transfer Found-Athenea3i; and by project 407141557 of the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation).A MW = 5.1 earthquake on January 21st, 2016 marked the beginning of a significant seismic sequence in the southern Alboran Sea, culminating in a MW = 6.3 earthquake on January 25th, and continuing with further moderate magnitude earthquakes until March. We use data from 35 seismic broadband stations in Spain, Morocco and Portugal to relocate the seismicity, estimate seismic moment tensors, and isolate regional apparent source time functions for the main earthquake. Relocation and regional moment tensor inversion consistently yield very shallow depths for the majority of events. We obtain 50 moment tensors for the sequence, showing a mixture of strike-slip faulting for the foreshock and the main event and reverse faulting for the major aftershocks. The leading role of reverse focal mechanisms among the aftershocks may be explained by the geometry of the fault network. The mainshock nucleates at a bend along the left-lateral Al-Idrisi fault, introducing local transpression within the transtensional Alboran Basin. The shallow depths of the 2016 Alboran Sea earthquakes may favor slip-partitioning on the involved faults. Apparent source durations for the main event suggest a ∼21 km long, asymmetric rupture that propagates primarily toward NE into the restraining fault segment, with fast rupture speed of ∼3.0 km/s. Consistently, the inversion for laterally variable fault displacement situates the main slip in the restraining segment. The partitioning into strike-slip rupture and dip-slip aftershocks confirms a non-optimal orientation of this segment, and suggests that the 2016 event settled a slip deficit from previous ruptures that could not propagate into the stronger restraining segment.FEDER/MINECO CGL2015-67130-C2-2-R PID2019-109608GB-I00FEDER/Junta de Andalucia project A-RNM-421-UGR18Junta de Andalucía RNM104European Union (EU) 754446German Research Foundation (DFG) 40714155

The contribution of the seismic component of Topo-Iberia to the imaging of the deep structure of the Iberian Peninsula and North Morocco

Topo-Iberia has been a large-scale Spanish project running from 2007 to 2013 that integrated more than 150 researchers on Earth Sciences. One of its key assets was the management of an observatory platform, named IberArray, aimed to provide new geophysical datasets (seismic, GPS, MT) to constrain the structure of Iberia with unprecedented resolution. The IberArray seismic pool was composed by 70+ BB stations, covering the study area in 3 deployments with a site-density of 60km x 60km. The data base holds ~300 sites, including the permanent networks in the area. Hence it forms a unique seismic database in Europe that allows for multiple analyses to constrain the complex geodinamics of the Western Mediterranean. A summary of new results coming from different techniques is presented here. The SKS splitting analysis has provided a spectacular image of the rotation of the fast velocity direction along the Gibraltar Arc. In central and northern Iberia, the fast polarization directions are close to EW, consistently with global mantle flow models considering contributions of surface plate motion, density variations and net lithosphere rotation. Those results suggest an asthenospheric origin of the observed anisotropy related to present-day mantle flow. Receiver functions have revealed the crustal thickness variations beneath the Atlas, Rif and southern Iberia, evidencing a relevant crustal root beneath the Rif, in agreement with recent, high- density active seismic experiments. The Variscan Iberian massif shows a flat Moho discontinuity, while the areas reworked in the Alpine orogeny show a slightly thicker crust. Beneath N Iberia, the imbrication of the Iberian and Eurasian crusts results in complex receiver functions. Depths exceeding 45 km are observed along the Pyrenean range, while the crust thins to values of 26-28 km close to the Atlantic coasts. The geometry of the 410-km and 660-km discontinuities has been investigated using novel cross-correlation/stacking techniques. Ambient noise tomography allows to identify the main sedimentary basins and to discriminate between the Variscan and the Alpine reworked areas. Local body-wave tomography in North Morocco has improved the location of the small magnitude events on the area and the details of the crustal structure. Teleseismic tomography has confirmed, using an independent data set, the presence of a high-velocity slab beneath the Gibraltar Arc.Peer Reviewe

Focal Mechanisms for Subcrustal Earthquakes Beneath the Gibraltar Arc

Earthquakes below 50‐km depth are usually associated with active subduction, and the direction of faulting is aligned with the orientation of the subduction zone. Faulting in 42 earthquakes beneath the Gibraltar Arc and Alboran Sea shows different characteristics. The most abundant solutions show horizontal slip, in agreement with relative plate motion between Africa and Europe. Further solutions are associated with shortening and suggest compression from the basal drag of the Earth's mantle on the moving plates. In turn, no signature of active subduction was found. Images of the Earth's interior from teleseismic waves suggest a relation between the earthquakes and a stalled remnant of ~150‐Ma‐old oceanic material that once formed the connection between two oceans and later has been buried beneath the Gibraltar Arc.We received financial support through Mineco/Feder Project CGL2015‐67130‐C2‐2‐R and Junta de Andalucía research group RNM 10

The Crust beneath Morocco: From the surface topography to the upper mantle a 700 km long seismic section across Morocco.

The most characteristic topographic features of Morocco are the Atlas Mountains and the Rif Coordillera. These two orogenic belts are the response of different geodynamic processes acting at lithospheric scale caused by a complex plate interaction. Both are located within the diffuse plate boundary zone separating Africa and Europe. The boundary zone is characterized by a relatively broad zone of deformation that includes mountain chains in southern Iberia, the Betics and in Morocco, the Rif Cordillera, separated by the Alboran basin. The zone delineates an arcuate arc system known as the Gibraltar arc. The area is characterized by a relatively large amount of earthquake activity at various depths and with a broad spectra of focal mechanisms. Within the last decade a large international effort have been devoted to the area. The topic has fostered a strong collaborations between Spanish and international research teams form Europe and USA. Key multi-seismic projects have been developed that aim to constrain the structure, composition and tectonic scenario from south of the Atlas to the Betics, across the Rif cordillera and the Alboran basin. The multidisciplinary research program included: natural source (earthquakes) recording with temporal deployments of broad band (BB) instrumentation and, controlled source seismic acquisition experiments where, spatially dense recording of wide-angle seismic reflection shot gathers were acquired. The natural source experiments consisted on a transect from Merzouga across the Gibraltar Arc and into the Iberian Peninsula (until south of Toledo) and, a nearly regular grid of BB. The controlled source data-sets were able to constrain the crustal structure and provide seismic P-wave propagation velocity models from the coast across the Rif and the Atlas. From south to north the crust features a relatively moderate crustal root beneath the Middle Atlas which can reach 40 km clearly differing from the 35 km thickness value observed at both sides of this root. Travel time inversion results position the crustal root just south of the High Atlas defining a thrusted mantle wedge and, also a limited crustal imbrication is suggested in the Middle Atlas. The most surprising feature is a prominent and unexpected crustal root (over 50 km) located beneath the external Rif and identified by both the wide-angle data and receiver function studies. To the east of this feature the crust thins rapidly by 20 km across the Nekkor fault zone, suggested to be related to the sharp change in crustal thickness. On shore-offshore recording of marine shots reveal further complexities in the transition to the Alboran basin. The low values of the Bouguer gravity anomalies beneath the Rif Cordillera are consistent with the crustal models derived from the new seismic data. The detailed knowledge on the crustal structure achieved by this high resolution imaging geophysical techniques is an asset to evaluate the earthquake and potential tsunami hazard for the coasts of North Africa and western Europe.This work has been primarily funded by the Spanish MEC project CGL2007–63889. Additional funding was provided by projects CGL2010–15416, CSD2006-00041, and CGL2009–09727 (Spain), CGL2008–03474-E, 07- TOPO_EUROPE_FP-006 (ESF Eurocores) and EAR-0808939 (US, NSF).Peer Reviewe

Preservation of the Iberian Tethys paleomargin beneath the eastern Betic mountain range

We are grateful to the staff involved in the TransCorBe project. The Geophysical Instrument Pool at GFZ-Potsdam provided most of the seismic equipment. We are grateful to Christian Haberland for his support. We want to thank two anonymous reviewers for the careful reading of the manuscript and the interesting and constructive criticism they provided. This work was funded by the Spanish State Research Agency (SRA) under the grant PID2019-109608GB I00/SRA/10.13039/501100011033, FEDER/MINECO project CGL2015-67130-C2-2-R, FEDER/Junta de Andalucia project A-RNM-421-UGR18 and research group RNM104 of the Junta de Andalucia. The Granada University/CBUA funding for open access charge.We obtain P-wave receiver functions from teleseismic earthquake recordings at a dense seismic broadband transect, deployed along 170 km across the Betic mountain range in southeastern Spain. Migrated images show the crustal structure of the orogen in detail. In particular, they reveal the situation of the subducted Iberian paleomargin, with full preservation of the proximal domain and the 50 km wide necking domain. Crustal thinning across the necking domain affects mainly the lower continental crust. The Variscan crust of the Tethys margin is bending downward beneath the Betics, reaching 45 km depth, and terminates abruptly at a major slab tear. The distal domain of the paleomargin cannot be reconstructed, but the migrated section suggests that material has been exhumed through the subduction channel and integrated into the Betic orogen. This supports an origin of the HP-LT Nevado-Filabride units from subducted, hyperextended Variscan crust. According to our profile, the present-day eastern Betics appear to have a much more significant contribution from metamorphic Iberian crust than previously thought.Geophysical Instrument Pool at GFZ-Potsdam - Spanish State Research Agency (SRA) PID2019-109608GB I00/SRA/10.13039/501100011033Spanish Government CGL2015-67130-C2-2-RFEDER/Junta de Andalucia project A-RNM-421-UGR18 RNM104Junta de Andaluci

A STEP fault in Central Betics, associated with lateral lithospheric tearing at the northern edge of the Gibraltar arc subduction system

We study the crustal and lithospheric mantle structure under central Betics in the westernmost Mediterranean region by migrating P-receiver functions along a dense seismic profile (∼2 km interstation distance). The profile, North–South oriented, probes the crustal structure of different geological units, from the Alboran domain in the south with metamorphic rocks, through the External Zones with sedimentary rocks to the Variscan terrains of the Iberian Massif in the north. From north to south, the Moho depth increases from ∼30 km to ∼46 km underneath the Guadix basin, due to the underthrusting of the Iberian crust below the Alboran crust, and suddenly shallows to ∼30 km underneath the Internal Zones with a step of 17 km. This sharp Moho step correlates well with a lithospheric step of ∼40 km, where the thickness of the lithosphere changes abruptly from ∼100 km in the north to ∼50 km in the south. We interpret this sharp and prominent lithospheric step as the termination of the Iberian lithosphere caused by a near-vertical STEP (Subduction-Transform-Edge-Propagator) fault that continues towards the surface as a positive flower tectonic structure of crustal scale. This STEP fault is located at the northern edge of the narrow Westernmost Mediterranean subduction system facilitating the slab rollback motion towards the west. The sharp termination of the Iberian lithosphere occurs under the contact between the Alpujarride and the Nevado-Filabride complexes of the Alboran domain in an ENE-WSW right-lateral transpressive shear zone. The thickest crust and lithosphere do not correlate with the highest topography along the profile suggesting that this high topography is a combined effect of the positive flower structure, and the push up of the asthenosphere produced by the removal of the Iberian lithosphere.This work was sup-ported by the projects: CGL2015-67130-C2-2-R, GCL2012-31472 (TRANSCORBE), HIRE (GFZ Potsdam) and PP2012-PIJD003 (Granada University). We acknowledge work on free softwares SAC and GMT

Leyes de escalado del movimiento sísmico de suelo en el Sur de España

Tesis Univ. Granada. Departamento de Física Teórica y del Cosmos. Leída el 23 de junio de 200