Late Cenozoic fault patterns and stress fields in the Great Basin and westward displacement of the Sierra Nevada block

The patterns of late Cenozoic faulting in the Great Basin apparently delimit two deformational fields, each extensional but contrasting in magnitude and style of extension. The field of smaller magnitude, which shows about 10 percent extension, occupies the northern and most of the central part of the Great Basin. It is characterized by steeply dipping normal faults and gently tilted blocks, with a preferred north to northeast trend. Evidence of greater extension occurs in the other deformational field, which lies between Walker Lane and the Sierra Nevada and extends across the narrow southern end of the Great Basin. This field contains most of the complementary strike-slip faults (northwest-striking right-lateral and northeast-striking left-lateral faults), long recognized as major components of the structural framework. It also contains abundant normal faults, most of which strike north to northeast. In certain areas, extension of 50 percent or more is indicated in the association of strike-slip and normal faults and in the palinspastic restoration of fault blocks that have been steeply tilted along gently dipping normal faults. These contrasts in structural pattern and apparent percentage of extension may be related to westward movement of the Sierra Nevada block and southward narrowing of the Great Basin. The faults along which strike-slip displacement occurred in late Cenozoic time appear to have functioned as conjugate shears, the shears and associated normal faults being first-order extensional features. The fault pattern also invites a simplistic interpretation that is based on the orientation of three mutually perpendicular directions of stress. The major normal faults, which strike north-northeast in most parts of the Great Basin, suggest a pervasive horizontal minimum compressive (maximum tensional) stress that is oriented west-northwest. Maximum compressive stress would be oriented perpendicularly where the horst and graben structure predominates and horizontally to the east-northeast where the strike-slip faults are abundant.