Temporal changes in fault strike (to 90°) and extension directions during multiple episodes of extension: An example from eastern Nevada
In areas with prolonged extensional histories or multiple episodes of extension, temporally distinct extensional fault systems, including dip-slip-normal, oblique-slip, and/or strike-slip faults, may form. These fault systems may be spatially distinct, overlapped, or completely superimposed, and different fault systems may have different fault strikes and extension directions. Although fault strike is not a very sensitive indicator of principal stress directions, changes through time in fault strike of as much as 90° exceed the uncertainty. Overprinted extensional fault systems of different ages with significantly different (to 90°) fault strikes and extension directions provide strong evidence of changes in the regional stress field with time. Tens of millions of years of extension occurred in the Basin and Range Province of the United States and many temporally distinct fault sets formed through that time. We document at least four extensional episodes and the fault kinematics of each, where possible, in the central Hiko Range, eastern Nevada. The four Cenozoic extensional events are prevolcanic, synvolcanic, postvolcanic Miocene–Pliocene (?), and postvolcanic Pliocene (?)–Quaternary. The prevolcanic faults strike north-south and are interpreted as footwall faults to the regional Eocene–Oligocene Snake-Stampede extensional system, which caused approximately east-west extension. The synvolcanic faults are late Oligocene–Miocene age and strike east-west. Although they are rare and have minor offset, they accommodated north-south–directed extension. Postvolcanic faults, part of the Timpahute lineament, are approximately east-west– striking normal and northeast-striking oblique-slip faults. They also accommodated approximately north-south extension during Miocene time. The northeast-striking left-normal oblique-slip fault set formed as new faults and records a change between north-south and east-west extension in Miocene time. Later east-west extension occurred along the Pliocene (?) to Quaternary Hiko fault zone. These four overprinted extensional systems show two separate changes in normal fault strike of ∼90°, from north-south to east- west and from east-west back to north-south. Consequently, the extension direction similarly changed through time. We suggest that causes for the changes in direction of upper crustal extension include a change from stresses created by (1) plate interactions, which generated approximately east-west extension, to (2) mantle motions, which generated approximately north-south extension, and (3) back again. The extension related to mantle motions is associated with a band of volcanism that passed across the northern Basin and Range Province, moving generally from north to south, during the protracted period of extension. Therefore, a change from plate boundary– or interaction-derived stresses to mantle- or volcanism-related stresses and back again is possible.