MIDDLE TERTIARY EXTENSION RECORDED BY LACUSTRINE FAN-DELTA DEPOSITS, PLUSH RANCH BASIN, WESTERN TRANSVERSE RANGES, CALIFORNIA

The Plush Ranch Formation (upper Oligocene and lower Miocene) consists of more than 1800 m of nonmarine sedimentary and volcanic rocks that record the history of an extensional basin referred to here as the Plu sh Ranch basin, Distinctive depositional facies, provenance, and sedim ent transport directions along each basin margin suggest an asymmetric basin shape that is consistent with a half-graben origin. The norther n basin margin consists of sandstone-dominated al luvial-plain deposit s (0.1-1.5 m thick, normally graded, lenticular sandstone beds), Small deltaic sequences 1-2 m thick were formed where these alluvial system s flowed southward into a lake, Lenses of massive, boulder-rich granit ic breccia that represent rockslide deposits derived from a nearby nor thern granitic provenance interfinger with the alluvial-plain facies, In contrast to the northern margin, the southern basin margin is repre sented by coarse grained fan-delta deposits, Matrix- and clast-support ed lenticular conglomerate beds 0.2-5 m thick with interbedded trough- cross-bedded pebbly sandstone represent braided-stream and flood flow and/or noncohesive debris-flow deposits of alluvial fans that drained a highland area to the south. The alluvial fan deposits interfinger to the north with several types of subaqueous sediment-gravity flow faci es including turbidite sandstone beds and matrix supported debris-flow conglomerate. Each of the basin-margin depositional systems grades ba sinward and to the east into lacustrine deposits that include organic rich dark shale, evaporite, and limestone, The lacustrine deposits rep resent the central and eastern parts of the Plush Ranch basin, which r eceived little coarse siliciclastic sediment, Basalt deposits that are at least 50 m thick in the west and thicken eastward are interbedded mainly with the lacustrine facies. The southern margin of the Plush Ra nch basin formed along a north dipping, normal-slip fault along which dip separation increased toward the southwest; the northern margin dev eloped on the tilted hanging wall block of this fault, This fault was later reactivated in post-middle Miocene time as the present left late ral strike slip Big Pine fault, The Plush Ranch is one of several exte nsional and transtensional basins that formed in southern California a nd western Arizona about 25-20 Ma as a response to the change from a c onvergent to a strike-slip tectonic regime along western North America .