KINEMATICS AND TIMING OF TERTIARY EXTENSION IN THE WESTERN LAKE MEAD REGION, NEVADA

Explanation of the origin of the complex array of structures in some e xtensional terranes (including folds and normal, strike-slip, and reve rse faults) includes many models that implicitly assume kinematic comp atibility between and contemporaneous operation of these structures. W e present new stratigraphic and age data from the highly extended west ern Lake Mead region, Nevada, together with an analysis of fault kinem atics (technique of Marrett and Allmendinger, 1990) to test the assump tions of kinematic compatibility and contemporaneity of structures in an area of excellent exposure and superb stratigraphic control. Our an alysis indicates an overlapping but clearly distinct chronology of def ormation. Early regional extension (> 18?-13.5 Ma) is marked by develo pment of a basin into which the middle Miocene lower Horse Spring Form ation was deposited. In the western Lake Mead region, this basin was d isrupted by more areally restricted, post-13 Ma normal and kinematical ly coupled right-slip faulting along the Las Vegas Valley shear zone. Kinematic analysis of faults indicates an average regional extension d irection of nearly due west for the middle and late Miocene. Extension and right-slip faulting was followed by development of dominantly sou th-vergent contractional structures including tight, east-plunging fol ds 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 L as Vegas Valley shear zone, reactivated as south-vergent reverse fault s. Northeast-striking left-slip faults cut folds and reverse faults. T hese observations show that north-south shortening and left-slip fault ing postdate the major phase of extension and right-slip faulting in t he western Lake Mead area. Dynamic models that invoke either a single stress field or rotating stress fields to explain development of struc tures in the western Lake Mead area are inconsistent with the kinemati c and age data. Similarly, kinematic models that view all structures i n the context of a single strain field are precluded by systematic cro ss-cutting relationships that demonstrate at least partial diachroneit y of deformational styles. Large-magnitude extension south of the Las Vegas Valley and Lake Mead fault zones appears to have been followed b y north-south contraction that was highly localized near the region of greatest extension. We suggest that lateral pressure gradients arisin g from differential crustal thinning at the northern end of the Colora do River extensional corridor may have provided the driving mechanism for localized contractional deformation.