PLIO-PLEISTOCENE EXTENSION OF A LATE MIOCENE-EARLY PLIOCENE EROSION SURFACE IN THE SOUTHERN INYO, ARGUS AND COSO RANGES, EASTERN CALIFORNIA SHEAR ZONE

Subhorizontal to very gently inclined remnants of an erosion surface are found at elevations of 8000’ to 10000’ with more steeply inclined remnants step-faulted and/or tilted towards valley floors. These erosional surfaces along with overlying flood basalts provide a regional paleohorizontal datum that constrains the magnitude of tilting, the direction and scale of monoclinal flexures above listric normal faults, and the extensional component of Plio-Pleistocene strain within the mountain blocks. Analogs to the late Miocene landscape may be found in the present landscape of southeastern Oregon, north of the Steens Mountain Range, at the northwestern-most tip of the active Basin and Range which presently lies at 5000’-6000’ elevation.

Early Pliocene and late Miocene flood basalts are widely distributed in the southernmost Inyo Range, Darwin Plateau, Argus and Coso Ranges. Many of these flows have previously been radiometrically dated and locally correlated. The erosion surface which underlies the basalts forms a mappable geomorphic horizon that also is widely distributed and can be used in combination with the flood basalts to constrain the deformation of the late Miocene altiplano or peneplain that is locally preserved in horst blocks throughout the Eastern California shear zone. The peneplain is locally deeply dissected and characterized by broad flats with low relief that occur within mountain highlands. The absence of a significant paleosol and sparse clastic deposition along large segments of the disconformity suggest broad low-relief areas which were largely denuded prior to eruption of the basalts. However, thick (50 m – 350 m) late Miocene to early Pliocene deposits locally underlie the basalts commonly associated with clastic wedges deposited along northeast-trending faults that lie within or cut across the ranges. These structures are oblique to the main active range-front bounding structures, although they locally appear to be accommodating a small component of active northwest trending slip. Slip vectors determined from block-bounding normal faults defined by the step-faulted paleo-erosion surface and/or overlying basalts within the ranges indicate an ~285^o extension direction consistent with simple shear in ~325^o trending transtensive zone.