Tectonic evolution of the northern Sierra Nevada batholith [electronic resource]
Keywords: Geological and Environmental Sciences.

About a third of the Mesozoic Sierra Nevada Batholith actually extends northward out of the Sierra Nevada proper into the Basin and Range Province of northwest Nevada, where it is obscured by late Cenozoic volcanic rocks, extensional faulting, and sediment-filled grabens. This segment differs from the main Sierra Nevada batholith because it is intruded into marginal terranes that are not underlain by old continental crust. The region is also characterized by a widespread unconformity overlain by Eocene to Miocene volcanic and sedimentary strata, which are not present above the southern Sierra Nevada Batholith. Although Basin and Range normal faulting has disrupted the structural continuity of the northernmost Sierra Nevada Batholith, breaking it into tilted fault blocks, this has also exposed depth sections into the upper crust, supplying 3-D structural information on the upper crust of the batholith. Although the Sierra Nevada Batholith serves as a worldwide model for arc tectonics, the northern segment of this arc has received little study in the past. The basal Tertiary unconformity provides an important datum for reconstructing the pre-extensional geology of the area. Restored for Neogene extension, the geology beneath this unconformity was compiled to construct an early Tertiary paleogeologic map. This map outlines the extent of granitic rocks of the Sierra Nevada Batholith and other basement rocks that were eroding in the early Tertiary in the NW Basin and Range. To understand the structural and petrological architecture and history of this portion of the arc, eight 7.5-[minute] quadrangles covering a distinct intrusive suite in the Sahwave and Nightingale Ranges were mapped at 1:24,000 scale. The basement rocks are composed of concentrically arranged granodiorite intrusions with an area of ~1000 km^2, referred to as the Sahwave Intrusive Suite, intruded into older plutonic and metasedimentary rocks. The Sahwave Intrusive Suite youngs inward from equigranular hornblende biotite granodiorite to more felsic, K-feldspar megacrystic granodiorite. Zircon U-Pb dating by SHRIMP shows that the Sahwave Intrusive Suite spans ~93--88.5 Ma, and intrudes older plutonic rocks crystallized at 97--110 Ma. Sample transects analyzed for modal mineralogy, trace and major element geochemistry show variation similar to that measured in coeval large zoned intrusions along the Sierra Nevada crest, such as the Tuolumne Intrusive Suite, which represent the last and biggest pulse of magmatism in the Mesozoic arc. The similarity of the Sahwave Intrusive Suite to its southern counterparts, despite differences in crustal setting, suggests arc flare-up events, as represented by the ca. 90 Ma intrusive suites along the Sierra Nevada crest, are controlled by a subcrustal process. Therefore, high magmatic flux might be caused by fluid release from the downgoing slab and/or an increase in subducted sediment rather than by backarc crustal thickening. To constrain the detailed history of exhumation of the northern Sierra Nevada Batholith, nine transects of samples were collected for thermochronologic study. Across the NW Basin and Range, normal faults tilt the batholithic rocks, exposing material that once lay up to 7 km depth beneath the early Tertiary unconformity. Apatite and zircon U-Th/He, apatite fission track, biotite 40Ar/39Ar, and K-feldspar 40Ar/39Ar multi-diffusion-domain modeling were used together to understand the thermal history of the northern Sierra Nevada Batholith. Cooling and exhumation began promptly after intrusion in the Late Cretaceous, and continued at an exponentially decreasing rate into the mid-Tertiary. The timing of exhumation is correlated with the local intrusive age of the batholith, continuing until later in the younger, eastern part of the arc, suggesting that residual arc heat acted to focus topographic relief and denudation.