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