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1. Cretaceous to Miocene magmatism, sedimentation, and exhumation within the Alaska Range suture zone: A polyphase reactivated terrane boundary

   https://doi.org/10.1130/GES02014.1  

   Trop, J.; Benowitz, J. (June 2019, Geosphere)

   The Alaska Range suture zone exposes Cretaceous to Quaternary marine and nonmarine sedimentary and volcanic rocks sandwiched between oceanic rocks of the accreted Wrangellia composite terrane to the south and older continental terranes to the north. New U-Pb zircon ages, 40Ar/39Ar, ZHe, and AFT cooling ages, geochemical compositions, and geological field observations from these rocks provide improved constraints on the timing of Cretaceous to Miocene magmatism, sedimentation, and deformation within the collisional suture zone. Our results bear on the unclear displacement history of the seismically active Denali fault, which bisects the suture zone. Newly identified tuffs north of the Denali fault in sedimentary strata of the Cantwell Formation yield ca. 72 to ca. 68 Ma U-Pb zircon ages. Lavas sampled south of the Denali fault yield ca. 69 Ma 40Ar/39Ar ages and geochemical compositions typical of arc assemblages, ranging from basalt-andesite-trachyte, relatively high-K, and high concentrations of incompatible elements attributed to slab contribution (e.g., high Cs, Ba, and Th). The Late Cretaceous lavas and bentonites, together with regionally extensive coeval calc-alkaline plutons, record arc magmatism during contractional deformation and metamorphism within the suture zone. Latest Cretaceous volcanic and sedimentary strata are locally overlain by Eocene Teklanika Formation volcanic rocks with geochemical compositions transitional between arc and intraplate affinity. New detrital-zircon data from the modern Teklanika River indicate peak Teklanika volcanism at ca. 57 Ma, which is also reflected in zircon Pb loss in Cantwell Formation bentonites. Teklanika Formation volcanism may reflect hypothesized slab break-off and a Paleocene–Eocene period of a transform margin configuration. Mafic dike swarms were emplaced along the Denali fault from ca. 38 to ca. 25 Ma based on new 40Ar/39Ar ages. Diking along the Denali fault may have been localized by strike-slip extension following a change in direction of the subducting oceanic plate beneath southern Alaska from N-NE to NW at ca. 46–40 Ma. Diking represents the last recorded episode of significant magmatism in the central and eastern Alaska Range, including along the Denali fault. Two tectonic models may explain emplacement of more primitive and less extensive Eocene–Oligocene magmas: delamination of the Late Cretaceous–Paleocene arc root and/or thickened suture zone lithosphere, or a slab window created during possible Paleocene slab break-off. Fluvial strata exposed just south of the Denali fault in the central Alaska Range record synorogenic sedimentation coeval with diking and inferred strike-slip displacement. Deposition occurred ca. 29 Ma based on palynomorphs and the youngest detrital zircons. U-Pb detrital-zircon geochronology and clast compositional data indicate the fluvial strata were derived from sedimentary and igneous bedrock presently exposed within the Alaska Range, including Cretaceous sources presently exposed on the opposite (north) side of the fault. The provenance data may indicate ~150 km or more of dextral offset of the ca. 29 Ma strata from inferred sediment sources, but different amounts of slip are feasible. Together, the dike swarms and fluvial strata are interpreted to record Oligocene strike-slip movement along the Denali fault system, coeval with strike-slip basin development along other segments of the fault. Diking and sedimentation occurred just prior to the onset of rapid and persistent exhumation ca. 25 Ma across the Alaska Range. This phase of reactivation of the suture zone is interpreted to reflect the translation along and convergence of southern Alaska across the Denali fault driven by highly coupled flat-slab subduction of the Yakutat microplate, which continues to accrete to the southern margin of Alaska. Furthermore, a change in Pacific plate direction and velocity at ca. 25 Ma created a more convergent regime along the apex of the Denali fault curve, likely contributing to the shutting off of near-fault extension- facilitated arc magmatism along this section of the fault system and increased exhumation rates. 

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2. Decoupled Oligocene mylonitic shearing and Miocene detachment faulting in the East Humboldt Range metamorphic core complex, northeast Nevada, USA

   https://doi.org/10.1130/GES02619.1  

   Zuza, Andrew V.; Dee, Seth (July 2023, Geosphere)

   The relationships between brittle detachment faulting and ductile shear zones in metamorphic core complexes are often ambiguous. Although it is commonly assumed that these two structures are kinematically linked and genetically related, direct observations of this coupling are rare. Here, we conducted a detailed field investigation to probe the connection between a detachment fault and mylonitic shear zone in the Ruby Mountain–East Humboldt Range metamorphic core complex, northeast Nevada. Field observations, along with new and published geochronology, demonstrate that Oligocene top-to-the-west mylonitic shear zones are crosscut by ca. 17 Ma subvertical basalt dikes, and these dikes are in turn truncated by middle Miocene detachment faults. The detachment faults appear to focus in preexisting weak zones in shaley strata and Mesozoic thrust faults. We interpret that the Oligocene mylonitic shear zones were generated in response to domal upwelling during voluminous plutonism and partial melting, which significantly predated the middle Miocene onset of regional extension and detachment slip. Our model simplifies mechanical issues with low-angle detachment faulting because there was an initial dip to the weak zones exploited by the future detachment-fault zone. This mechanism may be important for many apparent low-angle normal faults in the eastern Great Basin. We suggest that the temporal decoupling of mylonitic shearing and detachment faulting may be significant and underappreciated for many of the metamorphic core complexes in the North American Cordillera. In this case, earlier Eocene–Oligocene buoyant doming may have preconditioned the crust to be reactivated by Miocene extension thus explaining the spatial relationship between structures. 

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3. 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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4. Latest Cretaceous to Cenozoic Exhumation Patterns in the Northern Andes From the Sedimentary Provenance Record on the Broken Retro‐Foreland Putumayo Basin

   https://doi.org/10.1111/bre.70041  

   Nova, Giovanny; Parra, Mauricio; Cardona, Agustín; Horton, Brian_K; Valencia, Victor_A; Mora, Andrés; Soares, Cleber (June 2025, Basin Research)

   ABSTRACT The topographic growth of the Eastern Cordillera in the northern Andes of Colombia is a critical event in the tectonic and paleogeographic evolution of the western Amazon Basin. Documentation of early orogenic growth is enabled through multi‐proxy provenance signatures recorded in the adjacent retro‐foreland basin. In broken foreland basins, basement highs interrupt the lateral continuity of facies belts and potentially mask provenance signals. The Putumayo Basin is a broken foreland basin in western Amazonia at ~1°–3° N, where the Florencia, Macarena, and El Melón‐Vaupes basement highs have compartmentalised discrete depocentres during basin development. This study presents new evidence from stratigraphic, conglomerate clast count, sandstone petrography, detrital zircon U–Pb geochronology and novel apatite detrital U–Pb age trace element geochemistry analyses. The results show that the southern Eastern Cordillera (i.e., Garzon Massif) and Putumayo Basin basement highs were initially uplifted during the Late Cretaceous coeval with the Central Cordillera, most likely associated with the collision of the Caribbean Large Igneous Province (CLIP). Distinctive facies distributions and provenance changes characterise the Putumayo Basin over a ~300 km distance from south to north, in the Rumiyaco Formation and Neme Sandstone. Detrital zircon U–Pb ages record a sharp reversal from easterly derived Proterozoic to westerly sourced late Mesozoic–Cenozoic Andean zircons derived principally from the Central Cordillera. Provenance signatures of the synorogenic Eocene Pepino Formation demonstrate the continued exhumation of the Eastern Cordillera as a second‐order source area. However, the emergence of the northern intraplate highs modulated the provenance signature due to the rapid unroofing of relatively thinner marine sedimentary cover strata that overlie the Putumayo basement, in comparison to the thicker sequences of the southern basin. The provenance data and facies distributions of the Oligocene–Miocene Orito Group were more heterogeneous due to strike‐slip deformation, associated with major plate tectonic reorganisation as the Nazca Plate subducted under the South American margin. 

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5. Investigating Post-Taconic Deformation in the Pine Hill Thrust, Southern Vermont

   Khadka, S; Webb, L; Karabinos, P; Long, M; Espinal, K (December 2024, Transactions American Geophysical Union)

   The Pine Hill thrust, a western frontal thrust of the Green Mountain massif in southern Vermont, is characterized by reverse faults that place Precambrian basement rocks over mid-Ordovician rocks. Based on cross-cutting relationships, it has been considered a late-stage Taconic thrust. However, recent investigations in the western front of the Sutton Mountains, Green Mountain massif, and Berkshire massif of southern Quebec, Vermont, and Massachusetts, respectively, suggest fault displacement at 420 Ma and younger. Therefore, motion on these faults may instead be associated with the late Salinic or early Acadian orogeny. This study investigates the hypothesis that the Pine Hill thrust records deformational events associated with the late Salinic and/or Acadian orogenies. Preliminary studies from fieldwork and microstructural analysis of slabbed samples from transects across the Pine Hill thrust, where the lower Cambrian Dalton Formation is mapped as thrust over the Upper Ordovician Ira Formation, reveal at least four generations of foliation. The oldest tectonic foliation, S1, is parallel to primary compositional layering (S0) and is associated with isoclinal F1 folds. Moving from the Dalton Formation in the hanging wall towards the fault zone, S1 becomes progressively transposed into S2, marked by metamorphic compositional layering. Closer to the fault, S2 is crenulated, and S3 emerges as the dominant foliation, becoming the only foliation exhibited by the phyllonites in the fault zone. Finally, the youngest foliation, S4, is a localized crenulation cleavage developed in more pelitic material. These preliminary results suggest a complex deformation history, possibly involving multiple phases of post-Taconic motion on the fault during subsequent orogeneses. Further microstructural analysis and geochronology of these deformation fabrics will help establish the timing of deformation and its tectonic significance, helping to correlate surface geology with results from New England Seismic Transect (NEST) imaging of crustal and mantle lithospheric structure in the northern New England Appalachians. 

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