PLIOCENE REORGANIZATION OF EASTERN CALIFORNIA SHEAR ZONE KINEMATICS DOCUMENTED IN THE CUCOMUNGO CANYON RESTRAINNG BEND OF THE FURNACE CREEK – FISH LAKE VALLEY FAULT ZONE, NORTHERN DEATH VALLEY AND SOUTHERN FISH LAKE VALLEY, CALIFORNIA

The northwest-striking, right-oblique Furnace Creek – Fish Lake Valley fault zone (FC-FLVFZ) is the longest active structure in the Eastern California Shear Zone (ECSZ) and has a trace length of over 250 km. At the latitude of the central Sierra Nevada the FC-FLVFZ exits to the north from Death Valley into Fish Lake Valley via a 15 km left-step in the Sylvania Mountains and Last Chance Range, forming the Cucomungo Canyon Restraining Bend (CCRB). Uplift of at least 1.2 km and erosion in the last 0.76 Ma exposes the internal architecture of the CCRB and reveals the relation between synextensional volcanic-sedimentary cover and structures within the pre-Cenozoic basement. These exposures provide important constraints on the timing and kinematics of this portion of the ECSZ since the mid-Miocene. Distributed right-lateral displacement in basement rocks was accommodated within a fault zone 2-5 km wide that is characterized by cataclastic shear zones from tens to hundreds of meters wide that separate structural panels composed of granitoids locally overlain by rhyolite tuff and basalt. Within the CCRB, a 15.27 ± 0.03 Ma rhyolite tuff, dated by ⁴⁰Ar/³⁹Ar, rests nonconformably on cataclastically deformed Mesozoic granitoids within strands of the FC-FLVFZ. This tuff experienced right-lateral offset of ~28 ± 1 km along the FC-FLVFZ prior to burial by post-4 Ma volcanic and sedimentary rocks deposited in lacustrine and fluvial settings. The volcaniclastic rocks have a thickness of over 800 m and were localized in a transtensional basin. Internally, the volcanic-sedimentary succession is cut by and locally seals faults active during basin formation. The sedimentary succession is underlain by an >3.4 Ma tuff, and in the northern Fish Lake Valley by a 3.76 ± 0.04 Ma basalt, and marks the transition from strike-slip dominated displacement in a broad fault zone to more localized displacement on faults during transtensional deformation. Transtensional deformation is responsible for the opening of northern Death Valley and southern Fish Lake Valley, and continues to this day as documented by GNSS and field-based geologic studies.