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

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

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

The Gibraltar slab dynamics and its influence on past and present-day Alboran domain deformation: Insights from thermo-mechanical numerical modelling

The origin and tectonic evolution of the Gibraltar Arc system is the result of a complex geodynamic evolution involving the convergence of the Eurasian and African plates and the dynamic impact of the Gibraltar slab. Although geologic and geophysical data collected in the last few years have increased our knowledge of the Gibraltar Arc region, it is still unclear which are the mechanical links between the Gibraltar slab and the past deformation of the overriding Alboran lithosphere, as well as to which degree this subduction system is presently active. In this study, we use 2D numerical modelling to investigate the impact of the Gibraltar slab dynamics on the deformation of the overriding Alboran lithosphere. Our model simulates a WE generic vertical section at an approximate latitude of 36°N and considers an initial setup at about Burdigalian times (∼20 Ma), when the subduction front position is relatively well constrained by recent tectonic reconstructions. Our modelling shows a switch in the overriding plate (OP) stress state from extensional stresses during the slab rollback to compressional stresses near the trench when the rollback velocity decreases, caused by the change in slab-induced mantle flow. We also find that much of the crustal and lithospheric deformation occur during fast slab rollback and OP extension in the first 10 Myr of evolution, while after that only moderate deformation associated with subduction is predicted. Finally, we find that despite the subduction rollback ceases, the ongoing motion of the deeper portion of the slab induces a mantle flow that causes some amount of west-directed basal drag of the Alboran lithosphere. This basal drag generates interplate compresional stresses compatible with the distribution of intermediate-depth earthquakes in western Alboran.Spanish Ministry of Science and Innovation projects PID 2019-109608GB-I00 PGC 2018-095154-B-I00 and PID 2020-114854GB-C22FEDERJunta de Andalucia-Conserjeria de Economia y Conocimiento/B-RNM-528-UGR20European Social Fun

Tectonic and seismic implications of an intersegment rupture The damaging May 11th 2011 Mw 5.2 Lorca, Spain, earthquake

On May 11th 2011, a Mw 5.2 earthquake stroke the city of Lorca in the SE Spain. This event caused 9 fatalities, 300 injuries and serious damage on the city and the surrounding areas. The Lorca earthquake occurred in the vicinity of a region bounding two well-known segments of a large active fault, the Alhama de Murcia fault (AMF). The Lorca earthquake offers a unique opportunity to study how strain is accommodated in an intersegment region of a large strike slip fault. We map recent tectonic structures in the epicentral region and we use radar interferometry to analyze the coseismic deformation. Combining these data with seismological observations of Lorca seismic sequence we first model the source of the earthquake. Then we analyze the influence of our preferred model in the adjacent segments by Coulomb failure stress modeling. The proposed earthquake source model suggests that this event ruptured an area of ~4×3 km within the complex structure that limits the Goñar–Lorca and Lorca–Totana segments of the AMF. The induced static stress change on the adjacent segments of the fault represents a seismic cycle advance equivalent to 200 to 1000 years of tectonic loading

Crustal thickness and images of the lithospheric discontinuities in the Gibraltar arc and surrounding areas

The Gibraltar arc and surrounding areas are a complex tectonic region and its tectonic evolution.since Miocene is still under debate. Knowledge of its lithospheric structure will help to.understand the mechanisms that produced extension and westward motion of the Alboran domain,.simultaneously withNW–SE compression driven by Africa–Europe plates convergence..We perform a P-wave receiver function analysis in which we analyse new data recorded at.83 permanent and temporary seismic broad-band stations located in the South of the Iberian.peninsula. These data are stacked and combined with data from a previous study in northern.Morocco to build maps of thickness and average vP/vS ratio for the crust, and cross-sections.to image the lithospheric discontinuities beneath the Gibraltar arc, the Betic and Rif Ranges.and their Iberian and Moroccan forelands. Crustal thickness values show strong lateral variations.in the southern Iberia peninsula, ranging from ∼19 to ∼46 km. The Variscan foreland is.characterized by a relatively flat Moho at ∼31 km depth, and an average vP/vS ratio of ∼1.72,.similar to other Variscan terranes, which may indicate that part of the lower crustal orogenic.root was lost. The thickest crust is found at the contact between the Alboran domain and the.External Zones of the Betic Range, while crustal thinning is observed southeastern Iberia.(down to 19 km) and in the Guadalquivir basin where the thinning at the Iberian paleomargin.could be still preserved. In the cross-sections, we see a strong change between the eastern.Betics, where the Iberian crust underthrusts and couples to the Alboran crust, and the western.Betics, where the underthrusting Iberian crust becomes partially delaminated and enters into.the mantle. The structures largely mirror those on the Moroccan side where a similar detachment.was observed in northern Morocco. We attribute a relatively shallow strong negativepolarity.discontinuity to the lithosphere-asthenosphere boundary. This means relatively thin.lithosphere ranging from ∼50 km thickness in southeastern Iberia and northeastern Morocco.to ∼90–100 km beneath the western Betics and the Rif, with abrupt changes of ∼30 km under.the central Betics and northern Morocco. Our observations support a geodynamic scenario.where in western Betics oceanic subduction has developed into ongoing continental subduction/delamination while in eastern Betics this process is inactive