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1. Imaging the Tectonic Grain of the Northern Cordillera Orogen Using Transportable Array Receiver Functions

   https://doi.org/10.1785/0220200182  

   Schulte-Pelkum, Vera; Caine, Jonathan Saul; Jones, James V.; Becker, Thorsten W. (September 2020, Seismological Research Letters)

   null (Ed.)

   Abstract Azimuthal variations in receiver function conversions can image lithospheric structural contrasts and anisotropic fabrics that together compose tectonic grain. We apply this method to data from EarthScope Transportable Array in Alaska and additional stations across the northern Cordillera. The best-resolved quantities are the strike and depth of dipping fabric contrasts or interfaces. We find a strong geographic gradient in such anomalies, with large amplitudes extending inboard from the present-day subduction margin, the Aleutian arc, and an influence of flat-slab subduction of the Yakutat microplate north of the Denali fault. An east–west band across interior Alaska shows low-amplitude crustal anomalies. Anomaly amplitudes correlate with structural intensity (density of aligned geological elements), but are the highest in areas of strong Cenozoic deformation, raising the question of an influence of current stress state. Imaged subsurface strikes show alignment with surface structures. We see concentric strikes around arc volcanoes implying dipping magmatic structures and fabric into the middle crust. Regions with present-day weaker deformation show lower anomaly amplitudes but structurally aligned strikes, suggesting pre-Cenozoic fabrics may have been overprinted or otherwise modified. We observe general coherence of the signal across the brittle-plastic transition. Imaged crustal fabrics are aligned with major faults and shear zones, whereas intrafault blocks show imaged strikes both parallel to and at high angles to major block-bounding faults. High-angle strikes are subparallel to neotectonic deformation, seismicity, fault lineaments, and prominent metallogenic belts, possibly due to overprinting and/or co-evolution with fault-parallel fabrics. We suggest that the underlying tectonic grain in the northern Cordillera is broadly distributed rather than strongly localized. Receiver functions thus reveal key information about the nature and continuity of tectonic fabrics at depth and can provide unique insights into the deformation history and distribution of regional strain in complex orogenic belts. 

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2. 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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3. Inherited Crustal Features and Southern Alaska Tectonic History Constrained by Sp Receiver Functions

   Mann, Michael Everett; Fischer, Karen M; Benowitz, Jeffrey A (November 2024, Wiley)

   Ruppert, Natalia A; Jadamec, Margarete A; Freymueller, Jeffrey T (Ed.)

   Southern Alaska is a collage of accreted terranes. The deformation history of accreted terranes and the geometric history of their bounding faults reflect both inherited features and associated convergent margin events. We employ S-to-P receiver functions on multiple dense (<20km spacing) arrays of broadband seismometers across southern Alaska to investigate signals of dynamic tectonic activity. An inboard-dipping (∼15∘) boundary is imaged aligning with the trace of the Border Ranges Fault, which is interpreted as an unrotated inboard-dipping paleo-subduction (Mesozoic) interface. This observation, along with previous seismic imaging of the Border Ranges Fault and the next outboard terrane-bounding fault, the Contact Fault, buttresses a known history of convergent tectonics that varies along the margin. Large (>10 km) crustal thickness offsets imaged across both the Denali Fault system and the Eureka Creek Fault support a Mesozoic-to-Present inboard-dipping (east and northward) subduction polarity in the region. Additionally, our imaging reveals a significant velocity increase with depth at ∼25km beneath the Copper River Basin, which we interpret as the top of a region of active underplating and/or intrusion of basaltic magmatism. This feature may be related to the generation of a newWrangell Volcanic Field volcano, resulting from the underlying tear in the subducting slab. 

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4. Relict Back‐Arc Basin Crustal Structure in the Western Greater Caucasus, Georgia

   https://doi.org/10.1029/2024GC012036  

   Vasey, Dylan A; Cowgill, Eric; VanTongeren, Jill A; Anderson, Catherine O (May 2025, Geochemistry, Geophysics, Geosystems)

   Abstract Back‐arc basins frequently form within subduction zones, creating sources of lithospheric weakness that can accommodate subsequent compressional deformation. The crustal structure of these basins, including whether they contain extended preexisting crust and/or new crust formed by seafloor spreading, can thus exert a major influence on strain partitioning in orogenic belts. Here, we present field observations, petrographic analyses, and major/trace element geochemical data from the Caucasus Basin, a back‐arc basin that initiated in continental lithosphere in the Jurassic and subsequently localized deformation in the present‐day Greater Caucasus during the latter stages of Cenozoic Arabia‐Eurasia continent‐continent collision. Our results reveal distinct lithologic and geochemical domains separated by south‐vergent thrust faults within the North Georgia fault system (NGFS) in the Republic of Georgia. Along the Enguri River, shallow intrusive and volcanic rocks are thrust over dominantly volcaniclastic cover, whereas along the Tskhenistskali River, intrusions into metasedimentary rocks are juxtaposed against volcanic flows. The presence of a minor depleted mantle geochemical signature in intrusive rocks from the Tskhenistskali traverse supports an episode of Jurassic seafloor spreading in the Caucasus Basin, with the resulting lithosphere facilitating Cenozoic basin closure by north‐dipping subduction during Arabia‐Eurasia collision. The Khaishi fault along the Enguri River and the Lentekhi fault along the Tskhenistskali river mark major juxtapositions in back‐arc crustal structure and may be components of the terminal suture indicating Caucasus Basin closure. Our results highlight how magmatic rocks in relict basin rocks can yield key insights into basin structure and orogenesis, even when no ophiolite is present. 

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5. Mesozoic Fabric Influence on the Formation and Geometry of the Quaternary Cucamonga Fault

   Robles, F.; Schwartz, J.; Miranda, E.; Klepeis, K.; and Mora-Klepeis, G. (October 2022, Meeting of the Southern California Earthquake Center (SCEC), LAX, Sept., 2022)

   Robles, F.; Schwartz, J.; Miranda, E.; Klepeis, K.; and Mora-Klepeis, G. (Ed.)

   Ancient basement rocks in Southern California contain mechanical anisotropies that may influence the architecture of Quaternary faulting. We study exposed basement rocks found within the southeastern San Gabriel lithotectonic block with the intention of reconciling the relationship between inherited ductile fabrics and the geometry of Quaternary faults that are part of the San Andreas Fault system. By focusing our study on the southeastern corner of the San Gabriel block we can study the exposed lower- to middle crustal shear zone fabrics near where the Cucamonga Fault and the San Jacinto Fault intersect. The brittle Quaternary Cucamonga Thrust Fault strikes E-W and dips to the north-northeast (35-25°) and is localized at the range front and cuts these older fabrics, however there is also brittle deformation distal from the fault that also affects the sequence of lower- to middle crustal (6-8 kbar) granulite- to upper amphibolite facies mylonite and granulite-facies metasedimentary rocks. Near the Cucamonga Fault, mylonitic fabrics strike E-W and dip northeast (40-50°). Quaternary brittle faults that strike E-W and dip northeast (30-40°) reactivate the mvlonites and slickenlines and record a sinistral, top-to-the-west sense of shear. Investigation of host rocks indicates that they formed in the roots of a continental arc which was active from the Middle Jurassic to Late Cretaceous (172-86 Ma) at 740-800°C. Ductile deformation was associated with granulite-facies metamorphism at approximately 30 km depth during the Late Cretaceous (88-74 Ma) at 730-800 °C. Our work shows that the exhumed Late Cretaceous mylonitic fabrics may have operated as stress guides during Quaternary faulting in the Cucamonga Fault zone. We conclude that these lower crustal fabrics influence the geometry and kinematics of late Cenozoic faulting of the Cucamonga and San Jacinto fault zones. 

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