131-1 Reassessing metamorphic core complexes in the North American Cordillera

Session: Going with the Shear - New Insights into Lithospheric Extensional and Strike-Slip Systems

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Abstract:

The North American Cordillera (NAC) has been the site of seminal research on extensional tectonics, including low-angle normal faults and metamorphic core complexes (MCCs). Despite serving as an exemplar of extensional processes for >50 years, the geology of the western US exhibits a complicated and still-debated geodynamic history. Most modern interpretations of MCCs emphasize their generation to result from progressive, and often protracted (>40 Myr), lithospheric extension, with brittle detachment faults exhuming a kinematically linked ductile shear zone. However, this paradigm fails to reconcile the diversity of geochemical and structural observations from NAC MCCs. As a historical example, the recognition of a mylonitic front dipping away from the brittle normal fault in several MCCs necessitated reinterpretations to honor brittle structures exhuming preexisting mid-crust shear zones. Here, we highlight some case-study examples that emphasize the complexities of MCC development. First, we show how careful evaluation of the timing of brittle versus ductile structures is imperative to accurately interpret the causal drivers of MCC systems. For many MCCs, especially in the central NAC, there is a pronounced temporal decoupling between Eocene-Oligocene ductile shearing and Miocene brittle faulting. Across the NAC, ductile shear zone ages young toward central latitudes, broadly paralleling sweeps of post-Laramide foundering-induced magmatism. Conversely, Miocene brittle structures and syn-kinematic basins young monotonically northward. We interpret these observations to suggest that a phase of mid-crust diapirism followed crustal heating during rollback magmatism, which was later captured and exhumed by Miocene brittle extension that tracks northward triple-junction migration. Furthermore, there is growing evidence that MCCs sometimes exhume older Late Cretaceous Laramide-aged structures, including our newly obtained apatite U-Pb deformation dates from the Saddle Island and Chemehuevi MCCs, NV and CA. We also show that some mapped low-angle normal fault geometries simply reflect reactivation of a preexisting structural anisotropies, including the backlimb of a Mesozoic thrust or the brittle slip along a dome/nappe structure. The Cenozoic MCC history overprints an earlier deformed structural architecture that is relevant to palinspastic reconstructions and interpretations of presumed low-angle fault mechanics. We suggest that a careful temporal, geochemical, and structural reappraisal of MCCs throughout the NAC offer valuable insights into extensional processes and their drivers.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-8130

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