MID-CRETACEOUS HIGH-VOLUME PLUTONISM IN NW NEVADA, BUT NO NEVADAPLANO

It has often been supposed that the thickened crust of the Nevadaplano stretched all the way from the Sevier thrust belt, in the east, to the magmatic arc, in the west. However, this simplified view does not account for the northward divergence of these two features. Geochronology supports the existence of a high-flux mid-Cretaceous arc in NW Nevada, and thermochronology suggests that this arc may have been a relatively highstanding feature, but there is little evidence to support Cretaceous crustal thicknesses exceeding 40 km near this arc.

Although Cenozoic volcanism and Basin and Range normal faulting have buried much of the Mesozoic arc in NW Nevada, Cretaceous plutonic rocks in fact make up about 75% of the pre-Cenozoic outcrop along the axis of the arc. SHRIMP U-Pb zircon analyses indicate that the age of highest-flux magmatism decreases from north to south, from ca. 105 Ma near the Oregon border, to ca. 95 Ma east of Gerlach, to ca. 90 Ma west of Lovelock in the Sahwave intrusive suite. Based on modeling of low-temperature thermochronology data, in each of these areas, erosional exhumation of the batholith proceeded rapidly for about 30 million years after the local intrusion age, suggesting that magmatic input likely caused local, transient topographic uplift along the arc.

Nevertheless, NW Nevada does not seem to have the sort of thickened crust that is hypothesized to support the Nevadaplano further east. Evidence against elevated, extra-thick crust includes: 1) the presence of a deep marine basin from the Triassic into the Early Jurassic, apparently underlain by thin, oceanic or transitional crust, based on primitive isotope ratios of Cretaceous intrusions; 2) the lack of metamorphic rocks above greenschist facies, with basically unmetamorphosed early Mesozoic sedimentary rocks preserved east of the arc; and 3) the modest present-day crustal thickness, combined with evidence for no more than 25% Cenozoic extension in this region.