Abstract

Conglomeratic strata from middle Miocene sections in the central Resting Spring Range and nearby Eagle Mountain, California, contain a clast assemblage including marble, orthoquartzite, fusulinid grainstone, and coarse (∼1 m) monzogabbro, interstratified with tephras yielding laser-fusion 40Ar/39Ar ages of 11.6, 13.4, and 15.0 Ma. Petrographic and geochronologic evidence ties the clast assemblage to a source area in the southern Cottonwood Mountains, California, >100 km west-northwest of their present location. In the upper 100 m of the Resting Spring Range section, conglomerates are derived almost exclusively from the southern Cottonwoods source, and sandstone modes are as much as 50% angular plagioclase derived from the monzogabbro. The lack of dilution of this detritus by other sources and sedimentary features in both sections indicate (1) that deposition occurred on an alluvial 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 interpret most or all of the net east-southeast transport as a result of extensional and strike-slip faulting between the Cottonwood Mountains and the Resting Spring Range since 11–12 Ma. Restoration of these deposits to a position 10–20 km north-northeast of the eastern margin of the monzogabbro source (east margin of the Hunter Mountain batholith) yields a net tectonic displacement of the Cottonwood Mountains relative to the Resting Spring Range of 104 km N67°W. This result confirms previous reconstructions based on the restoration of isopachs in the Cordilleran miogeocline, pre-Cenozoic structural features, and other proximal Tertiary deposits in the region.

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