Structural Architecture and Attenuation of the Ductile Lower Plate of the Ruby Mountain‐East Humboldt Range Metamorphic Core Complex, Northeast Nevada

Abstract

AbstractStrongly deformed footwall rocks exposed in metamorphic core complexes (MCC) of the North American Cordillera were exhumed via ductile attenuation, mylonitic shearing, and detachment faulting. Whether these structures accommodated diapiric upwelling or regional extension via low‐angle normal faulting is debated. The Ruby Mountains‐East Humboldt Range MCC, northeast Nevada, records top‐west normal‐sense exhumation of deformed Proterozoic‐Paleozoic stratigraphy and older basement. We conducted 1:24,000‐scale mapping of the southwestern East Humboldt Range, with integrated structural, geochemical, and geochronological analyses to characterize the geometry and kinematics of extension and exhumation of the mylonitized footwall. Bedrock stratigraphy is pervasively intruded by Cretaceous, Eocene, and Oligocene intrusions, but observations of a coherent stratigraphic section show >80% vertical attenuation of Neoproterozoic to Ordovician rocks. These rocks are penetratively sheared with top‐west kinematics. The shear zone thus experienced combined pure‐ and simple‐shear (i.e., general shear) strain. We argue that this shear zone was syn‐/post‐kinematic with respect to Oligocene plutonism because: (a) mylonitic shearing spatially corresponds with preceding Oligocene intrusions; (b) thermochronology reveals that the shear zone experienced substantial cooling and exhumation after Oligocene plutonism; and (c) the mylonites are crosscut by undated, but likely late Oligocene, leucogranite. We propose that Eocene mantle‐derived magmatism and thermal incubation led to Oligocene diapiric upwelling of the middle crust, with ductile stretching focused on the flanks of this upwarp. Regional Basin and Range extension initiated later in the middle Miocene. Therefore, the development of the East Humboldt Range shear zone was not driven by regional extension and coupled detachment faulting.