Distribution and provenance of the middle Miocene Eagle Mountain Formation, and implications for regional kinematic analysis of the Basin and Range province

Conglomeratic strata from middle Miocene sections in the central Resting Sp ring Range and nearby Eagle Mountain, California, contain a clast assemblag e including marble, orthoquartzite, fusulinid grain stone, and coarse (simi lar to1 m) monzogabbro, interstratified with tephras yielding laser-fusion Ar-40/Ar-39 ages of 11.6, 13.4, and 15.0 Ma. Petrographic and geochronologi c evidence ties the clast assemblage to a source area in the southern Cotto nwood Mountains, California, >100 km west-northwest of their present locati on, In the upper 100 m of the Resting Spring Range section, conglomerates a re derived almost exclusively from the southern Cottonwoods source, and san dstone modes are as much as 50% an gular plagioclase derived from the monzo gabbro, The lack of dilution of this detritus by other sources and sediment ary features in both sections indicate (1) that deposition occurred on an a lluvial fan with a north-northeast paleoslope and (2) that transport of the gravels by sedimentary processes was probably <20 km north-northeast, in a direction normal to the present azimuth to their source. Therefore, we int erpret most or all of the net east-southeast transport as a result of exten sional and strike-slip faulting between the Cottonwood Mountains and the Re sting Spring Range since 11-12 Ma, Restoration of these deposits to a posit ion 10-20 km north-northeast of the eastern margin of the monzogabbro sourc e (east margin of the Hunter Mountain batholith) yields a net tectonic disp lacement of the Cottonwood Mountains relative to the Resting Spring Range o f 104 km N67<degrees>W, This result confirms previous reconstructions based on the restoration of isopachs in the Cordilleran miogeocline, pre-Cenozoi c structural features, and other proximal Tertiary deposits in the region.