PRELIMINARY WORK ON MESOZOIC AND CENOZOIC DEFORMATIONAL FEATURES, CENTRAL NOPAH RANGE, CALIFORNIA

The Nopah Peak fault in the Nopah Range of eastern California has variously been interpreted as a thrust fault, a normal fault, a thrust fault reactivated by extension, or a gravity slide structure, and the interpretation has direct bearing on its correlation to regional strain markers used to quantify Cenozoic extensional strain. Fieldwork along the eastern portion of the Nopah Peak fault shows it puts early Paleozoic formations on top of younger Paleozoic formations, and is therefore fundamentally a thrust fault. Estimates indicate that the fault dips to the SW, but this and the map pattern suggest that the surface may be broadly folded about a shallowly SE plunging axis. In some exposures, the main fault seems to be the roof of a small-scale duplex with a ductiley deformed zone sandwiched in the footwall. Vergence of folds in the zone suggests top to the NE transport. A spaced, SW-dipping solution cleavage is variably developed within carbonate rocks in both the hanging wall and footwall, and is generally axial planar to, but not always associated with, folds. Steeply SW-dipping Cenozoic normal faults that cut across the Nopah Peak fault are also coincident in strike and location with it, thus obscuring outcrop relationships. Late Tertiary E-dipping sedimentary beds include weathered bedrock-paleosol zones and fanglomerate breccias that grade up into calcareous sandstones and interbedded conglomerate beds. The intimate relationship of the basin deposits to Cenozoic normal faults is demonstrated by a buttress unconformity along a Cenozoic fault scarp. Restoration of all features to the paleohorizontal of the Tertiary beds indicates that at that time the Nopah Peak fault was shallowly SW-dipping, and that Cenozoic normal faults initiated as vertical and steeply E-dipping structures prior to the tilting of the crustal block now known as the Nopah Range. The Nopah Peak fault is a Mesozoic contractional structure that was deformed by Cenozoic extension but was not reactivated by it. Ongoing work will elucidate kinematic relationships, carry correlations to the west and south and delineate paleobasin characteristics.