REASSESSING THE TECTONIC EVOLUTION OF THE SOUTHERN INYO MOUNTAINS: A PERSPECTIVE FROM THE CERRO GORDO AREA

In the southern Inyo Mountains, the Late Permian to Triassic has been considered a period of tectonic quiescence. This interval is recorded in the southern Inyo Mountains by the Union Wash Formation. However, in this work we present evidence for two distinct phases of deformation of the Union Wash Fm in the Cerro Gordo area.

Our results show that there is a pronounced unconformity between the Middle and Upper members of this formation, previously considered to be concordant. The rocks underlying the unconformity are penetratively folded and recrystallized into mylonites, and represent deep water facies. The rocks overlying the unconformity have no pronounced penetrative strain and preserve sedimentary structures such as burrowing, and represent shallow water facies. Besides representing a sharp break in metamorphic grade and facies, the unconformity itself truncates the folding of the underlying Middle member of the Union Wash, shows no evidence of shear and is occasionally mantled by a basal conglomerate, preserved locally in channels. The unconformity is best observed mid-way between the Cerro Gordo road and Swansea canyon. It is sometimes observed to be locally sheared, as is the case along the Cerro Gordo road. The age of the Lower and Middle Union Wash is constrained to be Early Triassic based on conodonts, but is poorly constrained in the Upper member, although it has also been proposed to be late Early Triassic to early Middle Triassic based on ammonites. This timing implies that the development of the unconformity could be related to the Sonoma orogeny, which an event that has not previously been described in this area and would make this its southernmost expression.

The deformation of the Upper Union Wash represents a different phase of shortening where strata were imbricated and then folded into upright, tight folds. A felsic dyke mapped in the area and dated at ~140 Ma cuts across the cleavage related to the imbricate thrusts, propagates locally along the thrusts, but is folded into the upright folds thus providing a temporal constraint on the deformation. Overall these field relations describe a complex tectonic history with successive periods of deformation with distinctive styles. Ongoing work includes extending these field relations along strike, and obtaining new ages to constrain the timing of these events.