Timing and evolution of Cenozoic extensional normal faulting and magmatism in the southern Tobin Range, Nevada

摘要

Geologic mapping, 40Ar/39Ar dating, and whole-rock geochemical analyses were employed to establish the Cenozoic stratigraphy, geometry and timing of normal faults, and the magnitude of extension in the vicinity of Golconda Canyon in the southern Tobin Range. The Golconda Canyon area is near the westernmost extent of a major east-westa€“trending paleovalley that likely predates Basin and Range extension in that region. Latest Eocenea€“Miocene volcanic rocks infill and overtop the paleovalley.Four phases of extensional normal faulting have been identified in the Golconda Canyon area. The earliest phase of normal faulting was minor and took place during the early Oligocene in the form of northwesta€“ and northeasta€“striking, west-dipping faults. This faulting resulted in as much as ~5?°a€“10?° of tilting in the western part of the Tobin Range, and occurred coeval with andesitic volcanism. A second phase of faulting consisted of a major northwest-striking fault and associated northeast-striking faults, both of which dip west. These faults may have begun movement as early as the late Oligocene(?), with much displacement in the middle Miocene (prea€“14.1 Ma). The faults produced syntectonic basins filled with landslide breccia and megabreccia as well as coarse clastic sedimentary rocks. The next phase may represent an eastward progressive younging of the earlier phase, because it is documented largely in the middle and late Miocene record of the eastern part of the range. These north-striking faults are mostly west dipping, but some are east dipping, and they accommodated ~25?°, and locally 30?°, of eastward tilting. Younger moderately to steeply dipping active faults bound the eastern and western margins of the Tobin Range, and include the western fault that ruptured during the 1915 Pleasant Valley earthquake.Cumulative normal faulting has resulted in an overall ~25?°a€“30?° eastward tilt of the Tertiary stratified rocks in the Golconda Canyon area. The timing and magnitude of this faulting and tilting vary, with all areas showing the greatest magnitude of faulting and higher extension rates associated with the two sets of Miocene faults. The western part of the range displays evidence of early Oligocene faulting associated with andesitic magmatism, whereas these faults are not exposed in the eastern part of the range, where middle Miocene and younger tectonism produced slightly greater total stratal tilts.Palinspastic restorations of cross sections and calculations based on fault and rock unit dips suggest that the southern Tobin Range in central Nevada has undergone a minimum of ~50% east-west crustal extension since 34 Ma. This amount of extension and the initiation of normal faulting ca. 33 Ma in the Tobin Range are consistent with the westward decrease in the age and magnitude of extension at this latitude in the Basin and Range Province.