Kinematics and timing of Tertiary extension in the western Lake Mead region, Nevada

Abstract

Explanation of the origin of the complex array of structures in some extensional terranes (including folds and normal, strike-slip, and reverse faults) includes many models that implicitly assume kinematic compatibility between and contemporaneous operation of these structures. We present new stratigraphic and age data from the highly extended western Lake Mead region, Nevada, together with an analysis of fault kinematics (technique of Marrett and Allmendinger, 1990) to test the assumptions of kinematic compatibility and contemporaneity of structures in an area of excellent exposure and superb stratigraphic control. Our analysis indicates an overlapping but clearly distinct chronology of deformation. Early regional extension (>18?-13.5 Ma) is marked by development of a basin into which the middle Miocene lower Horse Spring Formation was deposited. In the western Lake Mead region, this basin was disrupted by more areally restricted, post-13 Ma normal and kinematically coupled right-slip faulting along the Las Vegas Valley shear zone. Kinematic analysis of faults indicates an average regional extension direction of nearly due west for the middle and late Miocene. Extension and right-slip faulting was followed by development of dominantly southvergent contractional structures including tight, east-plunging folds and east-striking reverse faults. These structures deform the post 8.5 Ma Muddy Creek Formation; the Muddy Creek Formation is not cut by the Las Vegas Valley shear zone. Older faults, including the eastern Las Vegas Valley shear zone, reactivated as south-vergent reverse faults. Northeast-striking left-slip faults cut folds and reverse faults. These observations show that north-south shortening and left-slip faulting postdate the major phase of extension and right-slip faulting in the western Lake Mead area. Dynamic models that invoke either a single stress field or rotating stress fields to explain development of structures in the western Lake Mead area are inconsistent with the kinematic and age data. Similarly, kinematic models that view all structures in the context of a single strain field are precluded by systematic cross-cutting relationships that demonstrate at least partial diachroneity of deformational styles. Large-magnitude extension south of the Las Vegas Valley and Lake Mead fault zones appears to have been followed by north-south contraction that was highly localized near the region of greatest extension. We suggest that lateral pressure gradients arising from differential crustal thinning at the northern end of the Colorado River extensional corridor may have provided the driving mechanism for localized contractional deformation.