LATE CENOZOIC STRUCTURAL EVOLUTION AND DISPLACEMENT PARTITIONING WITHIN DEEP SPRINGS VALLEY, EASTERN CALIFORNIA

The northern segment of the Eastern California Shear Zone (ECSZ) is characterized by right-lateral transtension accommodated on a network of north-northwest and northwest striking dextral transcurrent faults connected by north-northeast striking normal faults. North-northeast faults bounding Deep Springs Valley transfer displacement from the Saline Valley and Owens Valley fault systems in eastern California eastward to the Fish Lake Valley (FLV) fault. The FLV fault is the northern segment of the most active structure in the ECSZ and marks the eastern boundary of the tectonic belt. Deep Springs Valley is 22 km long and 5 km wide and is bounded by a prominent north-northeast striking fault on the southeast that, to the northeast, emanates into six east-northeast splays transferring displacement east, across the range. The northwestern boundary of the basin is marked by a more subdued north-northeast trending fault zone stretching the length of the basin. We conducted a gravity survey within the basin along three transects collecting 381 stations at a nominal 300 m spacing. We used two Scintrex CG-5 gravimeters and Leica dual frequency GNSS receivers that provided positioning at 2.5 cm or better. Our data were combined with 32 existing gravity measurements, terrain corrected, and reduced, using a reference density of 2.67 g/cm³, to a complete Bouguer anomaly. We computed a residual complete Bouguer anomaly (RCBA) and inverted the gravity for depth using Geosoft Oasis Montaj GM-SYS 3D software with a basin-fill density of 2.4 g/cm³. The RCBA shows a -17 mGal gravity low in the southeastern part of the basin that gradually increases to -7 mGal to the northeast. The RCBA values correspond to depths of about 1.4 and 0.6 km, respectively. Restoration to a pre-extensional datum yields a vertical displacement of ~2.2 km. Using fault dips of 60° to 45° in N65°W extension, a horizontal elongation of ~1.3 to 2.2 km across the basin yields an extension rate of 0.3 to 0.6 mm/yr since 4 Ma or 0.8 to 1.3 mm/yr if displacement was initiated at 1.7 Ma. The southwest to northeast change in basin geometry, with a decrease in net vertical displacement of ~0.9 km along the basin axis, is accommodated by redistribution of slip on both sides of the northern basin and by transfer of displacement east on an array of east-northeast left-oblique faults, to the FLV fault.