Slip rate of the Calico fault: Implications for geologic versus geodetic rate discrepancy in the Eastern California Shear Zone

Abstract

Long‐term (105 years) fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the Eastern California Shear Zone (ECSZ). The Calico fault is one of a family of dextral faults that traverse the Mojave Desert portion of the ECSZ. Its slip rate is determined from matching and dating incised Pleistocene alluvial fan deposits and surfaces displaced by fault slip. A high‐resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features. The oldest geomorphically preserved alluvial fan, unit B, is displaced 900 ± 200 m from its source at Sheep Springs Wash in the northern Rodman Mountains. This fan deposit contains the first preserved occurrence of basalt clasts derived from the Pipkin lava field and overlies Quaternary conglomerate deposits lacking these clasts. The 40Ar/39Ar dating of two flows from this field yields consistent ages of 770 ± 40 ka and 735 ± 9 ka. An age of 650 ± 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic 3He exposure date of 653 ± 20 ka on a basalt boulder from the surface of unit B. This assigned age and offset together yield a mid‐Pleistocene to present average slip rate of 1.4 ± 0.4 mm/yr. A younger fan surface, unit K, records 100 ± 10 m of dextral displacement and preserves original depositional morphology of its surface. Granitic boulders and pavement samples from this surface yield an average age of 56.4 ± 7.7 ka after taking into account minimal cosmogenic inheritance of granitic clasts. The displaced and dated K fans yield a slip rate of 1.8 ± 0.3 mm/yr. Distributed deformation of the region surrounding the fault trace, if active, could increase the overall displacement rate to 2.1 ± 0.5 mm/yr. Acceleration of slip rate from an average of 1.4 mm/yr prior to ∼50 ka to 1.8 mm/yr since ∼50 ka is possible, though a single time‐averaged slip rate of 1.6 ± 0.2 mm/yr satisfies the data. These rates are faster than any other paleoseismic or long‐term slip rate yet determined for other dextral faults in the Mojave Desert and imply that fault slip rates and earthquake productivity are heterogeneous across this portion of the ECSZ. Total displacement across the Calico fault diminishes northward as shear is distributed into folding and sinistral faults in the Calico Mountains. This pattern is consistent with an approximately threefold drop in geologic slip rate as the Calico fault steps over onto the Blackwater fault and demonstrates the significance of fault interaction for understanding the pattern of present‐day strain accumulation in the ECSZ.