New Perspectives on the Geology and Origin of the Northern Nevada Rift

ABSTRACT
The northern Nevada rift is a term applied to a prominent,
north-northwest-trending aeromagnetic anomaly that extends about
500 km from near the Nevada-Oregon border to southeast Nevada.
Its surface trace is marked in places by an alignment of middle
Miocene volcanic and hypabyssal rocks and epithermal gold-silver
and mercury deposits that formed during west-southwest-east-northeast
directed extension. On regional aeromagnetic maps, the rift
appears as a narrow positive anomaly that probably reflects the presence
of abundant intrusive mafic rocks emplaced mostly between
about 16.5 and 15 Ma. The rift may correspond in part to a pre-
Cenozoic deep crustal structure that was reactivated during the late
Cenozoic. Thermal bulging and shallow intrusion of mafic magmas
related to the Yellowstone hot spot near McDermitt led to the inception
of the rift at about 16.5 Ma and rapid propagation of the rift to
the south. The northern Nevada rift evolved in a manner similar to
the Mid-Continent rift of North America and dissimilar to many
intracontinental rifts, such as the long-lived Rio Grande rift.
However, duration of rifting along the northern Nevada rift was
short (mostly between about 16.5 to 15 Ma), the total amount of
extension of the crust along the rift was generally less than a few
kilometers, and the amount of subsidence was generally less than 1
kilometer. Interbedded sedimentary deposits generally are uncommon
along the rift, and the total volume of igneous rocks erupted
along the rift also was small.
Three distinct groups of middle Miocene igneous rocks are
associated with the rift: an early group of mafic (basalt to andesite)
compositions and later groups of trachydacite and rhyolite compositions.
The early mafic magmas probably were related to the
Yellowstone hot spot and were derived from upper mantle magmas.
The more silicic compositions probably represent lower crustal melts
that may have formed as a result of hot-spot-related deep crustal heating.
All magmas had low oxygen fugacities and low water contents.