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1. Connecting subduction, extension and shear localization across the Aegean Sea and Anatolia

   https://doi.org/10.1093/gji/ggab078  

   Barbot, S; Weiss, J R (May 2021, Geophysical Journal International)

   null (Ed.)

   SUMMARY The Eastern Mediterranean is the most seismically active region in Europe due to the complex interactions of the Arabian, African, and Eurasian tectonic plates. Deformation is achieved by faulting in the brittle crust, distributed flow in the viscoelastic lower-crust and mantle, and Hellenic subduction, but the long-term partitioning of these mechanisms is still unknown. We exploit an extensive suite of geodetic observations to build a kinematic model connecting strike-slip deformation, extension, subduction, and shear localization across Anatolia and the Aegean Sea by mapping the distribution of slip and strain accumulation on major active geological structures. We find that tectonic escape is facilitated by a plate-boundary-like, trans-lithospheric shear zone extending from the Gulf of Evia to the Turkish-Iranian Plateau that underlies the surface trace of the North Anatolian Fault. Additional deformation in Anatolia is taken up by a series of smaller-scale conjugate shear zones that reach the upper mantle, the largest of which is located beneath the East Anatolian Fault. Rapid north–south extension in the western part of the system, driven primarily by Hellenic Trench retreat, is accommodated by rotation and broadening of the North Anatolian mantle shear zone from the Sea of Marmara across the north Aegean Sea, and by a system of distributed transform faults and rifts including the rapidly extending Gulf of Corinth in central Greece and the active grabens of western Turkey. Africa–Eurasia convergence along the Hellenic Arc occurs at a median rate of 49.8 mm yr–1 in a largely trench-normal direction except near eastern Crete where variably oriented slip on the megathrust coincides with mixed-mode and strike-slip deformation in the overlying accretionary wedge near the Ptolemy–Pliny–Strabo trenches. Our kinematic model illustrates the competing roles the North Anatolian mantle shear zone, Hellenic Trench, overlying mantle wedge, and active crustal faults play in accommodating tectonic indentation, slab rollback and associated Aegean extension. Viscoelastic flow in the lower crust and upper mantle dominate the surface velocity field across much of Anatolia and a clear transition to megathrust-related slab pull occurs in western Turkey, the Aegean Sea and Greece. Crustal scale faults and the Hellenic wedge contribute only a minor amount to the large-scale, regional pattern of Eastern Mediterranean interseismic surface deformation. 

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2. Refining the Extent and Depth of the Shear Zone Surrounding the Alpine Fault Using Receiver Function Harmonics

   https://doi.org/10.1029/2024GL112092  

   Mark, Hannah_F (November 2024, Geophysical Research Letters)

   Abstract Large continental transform faults are thought to cross‐cut the crust and extend into the lithospheric mantle. However, the strain distribution associated with these faults at mantle depths is not well understood. At the heart of this question is the rheology of the lithosphere: when stressed by displacement on a trans‐crustal fault, does the uppermost mantle localize strain in a continuing narrow fault zone, or is that strain instead distributed in a shear zone that widens with increasing depth? This study uses harmonic decomposition of receiver functions to measure the spatial and depth distribution of seismic anisotropy, a proxy for viscous deformation, around the Alpine Fault in Aotearoa New Zealand. Anisotropy aligned with the fault is present in the lithosphere at least 100 km away from the fault trace, suggesting that the Alpine Fault shear zone widens at depth. 

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3. Impact of Material Strength on Releasing Bend Evolution [Earth, Geographic and Climate Sciences, University of Massachusetts Amherst, USA]

   https://doi.org/10.55575/tektonika2025.3.1.81  

   Gabriel, Alana; Elston, Hanna; Cooke, Michele; Ramos_Sanchez, Christ (January 2025, Tektonika)

   Releasing bends along active strike-slip faults display a range of fault patterns that may depend on crustal strength. Scaled physical experiments allow us to directly document the evolution of established releasing bend systems under differing strength conditions. Here, we use a split-box apparatus filled with wet clay of differing strengths to run and analyze releasing bend evolution. Precut vertical discontinuities within the clay slip with right-lateral displacement of the basal plate followed by the development of oblique-slip secondary faults. In contrast to the weaker clay experiment, which produces left-lateral cross faults that facilitate major reorganization of the primary slip pathway, the stronger clay experiment produces negligible cross faults and has a persistent primary slip pathway. Within both experiments, the dip of initially vertical faults shallows due to lateral flow at depth and left-lateral slip develops along normal fault segments that have highly oblique strike. The experiments show that fault systems within weaker strength materials produce greater delocalization of faulting, with both greater number of faults and greater off-fault deformation that can impact hazard. For example, the hot, thin and weak crust hosting the Brawley Seismic Zone accommodates slip along many distributed faults, which is in sharp contrast to the more localized fault network of the Southern Gar Basin in cooler, thicker and stronger crust. The fault patterns observed in the experiments match patterns of crustal examples and may guide future models of fault evolution within relatively strong and weak crust that have differing heat flux and thickness. 

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4. Tectonic Inheritance With Dipping Faults and Deformation Fabric in the Brittle and Ductile Southern California Crust

   https://doi.org/10.1029/2020JB019525  

   Schulte‐Pelkum, Vera; Ross, Zachary E.; Mueller, Karl; Ben‐Zion, Yehuda (July 2020, Journal of Geophysical Research: Solid Earth)

   Abstract Plate motions in Southern California have undergone a transition from compressional and extensional regimes to a dominantly strike‐slip regime in the Miocene. Strike‐slip motion is most easily accommodated on vertical faults, and major transform fault strands in the region are typically mapped as near vertical on the surface. However, some previous work suggests that these faults have a dipping geometry at depth. We analyze receiver function arrivals that vary harmonically with back azimuth at all available broadband stations in the region. The results show a dominant signal from contrasts in dipping foliation as well as dipping isotropic velocity contrasts from all crustal depths, including from the ductile middle to lower crust. We interpret these receiver function observations as a dipping fault‐parallel structural fabric that is pervasive throughout the region. The strike of these structures and fabrics is parallel to that of nearby fault surface traces. We also plot microseismicity on depth profiles perpendicular to major strike‐slip faults and find consistently NE dipping features in seismicity changing from near vertical (80–85°) on the Elsinore Fault in the Peninsular Ranges to 60–65° slightly further inland on the San Jacinto Fault to 50–55° on the San Andreas Fault. Taken together, the dipping features in seismicity and in rock fabric suggest that preexisting fabrics and faults may have acted as strain guides in the modern slip regime, with reactivation and growth of strike‐slip faults along northeast dipping fabrics both above and below the brittle‐ductile transition. 

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5. Azimuthal Anisotropy of the Crust and Uppermost Mantle Beneath Alaska

   https://doi.org/10.1029/2020JB020076  

   Feng, L.; Liu, Chuanming; Ritzwoller, M. H. (December 2020, Journal of Geophysical Research: Solid Earth)

   Abstract This study presents an azimuthally anisotropic shear wave velocity model of the crust and uppermost mantle beneath Alaska, based on Rayleigh wave phase speed observations from 10 to 80 s period recorded at more than 500 broadband stations. We test the hypothesis that a model composed of two homogeneous layers of anisotropy can explain these measurements. This “Two‐Layer Model” confines azimuthal anisotropy to the brittle upper crust along with the uppermost mantle from the Moho to 200 km depth. This model passes the hypothesis test for most of the region of study, from which we draw two conclusions. (a) The data are consistent with crustal azimuthal anisotropy being dominantly controlled by deformationally aligned cracks and fractures in the upper crust undergoing brittle deformation. (b) The data are also consistent with the uppermost mantle beneath Alaska and surroundings experiencing vertically coherent deformation. The model resolves several prominent features. (1) In the upper crust, fast directions are principally aligned with the orientation of major faults. (2) In the upper mantle, fast directions are aligned with the compressional direction in compressional tectonic domains and with the tensional direction in tensional domains. (3) The mantle fast directions located near the Alaska‐Aleutian subduction zone and the surrounding back‐arc area form a toroidal pattern that is consistent with mantle flow directions predicted by recent geodynamical models. Finally, the mantle anisotropy is remarkably consistent with SKS fast directions, but to fit SKS split times, anisotropy must extend below 200 km depth across most of the study region. 

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