ACTIVE TECTONICS OF THE GARLOCK FAULT IN THE SOUTHERN SLATE RANGE OF THE NORTHERN MOJAVE DESERT, CALIFORNIA

We report here results from geologic mapping along a segment of the Garlock fault done in the southern Slate Range, northern Pilot Knob Valley, and southern Searles Valley in eastern California. The Garlock fault in this region crosses but is not cut by faults of the eastern California shear zone. Our new mapping confirms that the dominantly sinistral motion of the Garlock fault is accompanied by minor dip-slip (north side up) on the Garlock itself in the Pilot Knob area. From the Garlock northward to the Mesozoic bedrock of the Slate Range is a 3-km wide zone of deformed Pleistocene Christmas Canyon Formation. Numerous east-west-striking chevron folds and tight anticline-syncline fold trains deform the weak lacustrine deposits and indicate north-south shortening orthogonal to the Garlock fault. Multiple cut-and-fill terrace treads have been mapped on an overlying alluvial fan north of the fault. These tread surfaces will be considered for future cosmogenic exposure-age dating to constrain timing of strain and stream incision with the purpose of determining slip rates on the Garlock fault and north-south shortening rates.

The Christmas Canyon exposures are separated from the bedrock of the Slate Range to the north by another fault. This structure continues westward from the Slate Range into Searles Valley where it apparently forms a positive flower structure in the Christmas Canyon formation and disrupts all but the youngest alluvial/lacustrine units. It probably cores the eastern the “anomalous folds” along the Garlock of Smith (1991). Hence, this northern fault appears to uplift the Slate Range and possibly form a strike-slip duplex north of the Garlock.

Preliminary interpretations of the available data indicate that either the central Garlock fault acts as a weak fault with σ1 oriented north-south or east-west striking folds in the lacustrine beds have been rotated counter-clockwise through progressive slip. We also speculate that the rheology of the Garlock fault somehow prevents the necessary localization of dextral shear strain in the Eastern California shear zone from penetrating and offsetting it.

Smith, G.I., 1991, Anomalous folds associated with the east-central part of the Garlock fault, southeast California: Geological Society of America Bulletin, v. 103, p. 615–624.