The timing and evolution of Cenozoic extensional normal faulting in the southern Tobin Range, Pershing County, Nevada

Graduation date: 2006The Tobin Range of central Nevada lies in the Basin and Range extensional\ud province near the transition between more extended terrane (>50%) to the south and\ud east, and generally less extended terrane to the north and west. Geologic mapping,\ud 40Ar/39Ar dating and whole-rock geochemical analysis were employed to establish the\ud Cenozoic stratigraphy, geometry and timing of normal faults, and magnitude of\ud extension in the vicinity of Golconda Canyon in the southern Tobin Range.\ud The Golconda Canyon area lies near the westernmost extent of a major eastwest\ud trending paleo-valley that likely predates Basin and Range extension in that\ud region. In-filling and over-topping the paleo-valley are a series of Oligocene to\ud Miocene volcanic rocks. Confined to the paleo-valley is a series of thin basaltic lava\ud flows above which lies the rhyolitic Caetano Tuff (~34 Ma). Units also present in the\ud Golconda Canyon area, but not confined to the paleo-valley include, from bottom to\ud top, a ~700m-thick sequence of andesite lavas, lahars, and ignimbrites (~33 Ma); four\ud late Oligocene-early Miocene rhyolite ignimbrites; a unit of syn-tectonic landslide\ud breccia and sedimentary rock; and a sequence of fluvial and lacustrine tuffaceous\ud sedimentary rocks intercalated in the center of the section with a 60m-thick series of\ud basaltic lavas (~14 Ma).\ud Three phases of extensional normal faulting have been identified in the\ud Golconda Canyon area. The earliest phase of normal faulting took place during the\ud early Oligocene in the form of northwest- and northeast-striking, west dipping faults,\ud which resulted in ~20º of tilting in the western part of the Tobin Range and occurred\ud coeval with andesitic volcanism. The next phase of faulting consisted of a major NW\ud striking fault and associated NE striking faults, both of which dip west. These faults\ud moved during the late Oligocene to Middle Miocene and produced syntectonic basins\ud filled with landslide breccia and sedimentary rocks. The last phase of faulting began\ud during the middle Miocene and continues today. This phase of faulting includes\ud north-striking, west-dipping faults localized to the eastern part of the range, where\ud they accommodated ~25º – 30º of tilting. Also present are north-striking faults that\ud dip steeply east. Active faults bound the eastern and western margins of the Tobin\ud Range. The Pleasant Valley fault bounds the western margin of the range, dips to the\ud west, and last ruptured in 1915. The east side of the range is bounded by an eastdipping\ud fault with large displacement in Buffalo Valley but the amount of offset\ud quickly diminishes southward as it enters Jersey Valley. These three phases of normal\ud faulting have resulted in an overall ~25º – 30º eastward tilt of the Tertiary rocks in the\ud Golconda Canyon area. The timing of this faulting and tilting varies, with the western\ud part of the range being dominated by early Oligocene to early Miocene tectonism, and\ud the eastern part of the range dominated by middle Miocene and younger tectonism.\ud Palinspastic restorations of cross sections and calculations based on fault and\ud rock unit dips suggest the southern Tobin Range in central Nevada has undergone\ud ~50% east-west crustal extension since 34 Ma. This amount of extension and the\ud initiation of normal faulting at ~33 Ma in the Tobin Range are consistent with the\ud westward decrease in the age and magnitude of extension at this latitude in the Basin\ud and Range Province