Abstract

Since earliest Oligocene time, ∼15 km of extension has occurred along a major gently dipping normal fault system and a younger more steeply dipping system of normal faults in the northern Toiyabe Range in central Nevada. The Bernd Canyon fault is the structurally lowest of the gently dipping faults in this area. It dips 12°-28° west and is similar to well-known detachment faults. Paleozoic rocks in the hanging wall are cut by subsidary gently dipping normal faults and are tilted 40° to more than 90° to the east. The early Oligocene intracaldera tuff of Hall Creek is the oldest Cenozoic unit present in the hanging wall and is tilted 28°-58° eastward. Successively younger units have systematically decreasing dips and record a history of progressive, possibly episodic, extension. Bedding in greenschist-grade lower Paleozoic rocks in the footwall of the Bernd Canyon fault intersects the fault at angles of 5° to 25° and is arched across the range. Unlike many other detachment faults, the Bernd Canyon fault contains early Oligocene and younger volcanic and sedimentary units in the footwall that may be correlated to units in the hanging wall, and they record a similar tilting history. Sequential untilting of successively older Tertiary units suggests that the Bernd Canyon fault was initiated at a dip of 50°-60° and was rotated to a dip of ∼25°-30° during progressive extension before being cut and further rotated by younger faults. Restoration of Tertiary units also suggests that the doming of Paleozoic units across the range is a Tertiary feature and that associated low bedding-to-fault angles in the footwall of the Bernd Canyon fault are the result of penetrative inhomogeneous simple shear deformation of the footwall.

Including faults outside the area of detailed study, ∼20 km of east-west extension occurred across a domain that is now ∼45 km long (north-south) by ∼35 km wide (east-west). Extension apparently began in earliest Oligocene time, before or perhaps synchronously with, the onset of volcanism, and it continued into the Pliocene epoch. Tilt directions of Oligocene volcanic rocks, calcite veining near the detachment, and elongate fossils in footwall units indicate that the extension direction on the detachment and related faults may have been nearly east-west to possibly east-northeast-west-southwest. Changing tilt directions in younger rocks suggest that the extension direction switched to west-northwest-east-southeast at some time. Palinspastic restoration of normal faults also suggests that Oligocene basins were initially bounded by high-angle faults and had topographically elevated footwalls. The restoration implies that the styles of Miocene to Recent "basin and range" faulting and pre-Miocene "pre-basin and range" faulting are closely similar.

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