Paper No. 96-14
Presentation Time: 9:00 AM-1:00 PM
DECIPHERING THE POLYPHASE HISTORY OF THE RUBY MOUNTAINS-EAST HUMBOLDT RANGE METAMORPHIC CORE COMPLEX: A DYNAMIC COMPETITION BETWEEN PLATE-BOUNDARY FORCES, GRAVITATIONAL POTENTIAL ENERGY, AND THERMAL INSTABILITIES
The Ruby Mountains-East Humboldt Range (REHR) metamorphic core complex, northeast Nevada, records a polyphase Mesozoic-Cenozoic metamorphic, igneous, and deformational history. The geology broadly reflects the intracontinental response to competing plate-boundary conditions, gravitational potential energy (GPE), and thermal instabilities during Mesozoic contraction in the Cordilleran retroarc, Mesozoic-Cenozoic pulsed intrusion and anatexis, Oligocene mylonitic shearing, Miocene detachment faulting, and recent high-angle normal faulting. To better resolve this complex history, and refine our understanding of how these competing drivers of intraplate deformation are manifested in the geologic record, we have conducted 1:24k-scale mapping in the REHR with integrated (micro)structural, geochemical, geochronological (U-Pb; 40Ar/39Ar; cosmogenic), and carbon isotope analyses. Combined with previous published work, we support the following model. Rapid Farallon-North America plate convergence and/or terrane collision led to Late Jurassic and Late Cretaceous regional shortening and crustal thickening; Cretaceous metamorphism and anatexis was associated with an exceptionally hot retroarc. The region was an elevated orogenic plateau by the Cretaceous, in which through-going paleovalleys were incised. In the Eocene, mantle-derived mafic intrusions locally heated and weakened the crust, and volcanic rocks infilled the existing paleovalleys. Local normal faulting and subsidence resulted from a weak crust with elevated GPE and relatively fixed plate-boundary conditions. In the Oligocene, a warm thermal gradient and protracted crustal melting led to voluminous intrusions that initiated buoyancy-driven thermal doming, and generated a general-shear mylonite zone with >90% pure-shear stratigraphic attenuation. Mylonitic shear zones were cut by undeformed ca. 17 Ma basalt dikes, which were later cut by Miocene detachment faults that placed undeformed upper Paleozoic strata on highly strained lower Paleozoic rocks. Late Miocene-present high-angle faulting (0.2 mm/yr vertical slip rates) tilted the REHR eastward to its present configuration. Today, a positive feedback exists with extension-related exhumation fast enough to elevate the range (>3k km) where substantial glaciation and erosion drives continued isostatic uplift, thus exposing the core of North American Cordillera as a result of modern mantle and Earth-surface processes.