Paleoseismic history of the Needles Fault, AZ: preliminary results from tectonogeomorphic analyses

Anneke Avery, & Christine Regalla

Published September 8, 2024, SCEC Contribution #13947, 2024 SCEC Annual Meeting Poster #108

The Needles fault on the Arizona-California border is a distributed active fault network in the Colorado River Extensional Corridor of the Basin and Range. It is one of few known active faults in the corridor and may pose seismic hazards to nearby communities. However, the timing, sip rates, kinematics, and magnitude of deformation in the Needles fault zone are under constrained. Here we report preliminary results of the first detailed tectonogeomorphic investigation of the paleoseismic history of the Needles fault. The fault system consists of a set of 0.1 to 1.5 km strands that extend 20 km, with a 4 km long central graben on the upthrown side of a 7 km long large fold. We focused our tectonogeomorphic analyses on a ~2 km segment of the fault bounding the northeastern margin of the Needles graben, a location where a semi-continuous fault strand offsets multiple generations of alluvium that may be used to reconstruct its earthquake history. We completed 1:6k scale mapping in the field and from an existing 1m DSM, developed a local Pleistocene to Holocene alluvial fan chronology based on weathering, desert pavement development, and pedogenic carbonate accumulation, and measured vertical separation of offset alluvium using high resolution topographic field surveys acquired by a Total Station and drone-based structure from motion digital surface models (SFM DSMs).

Detailed mapping of the study area shows a nearly continuous fault strand along the 2 km segment, with underlapping to overlapping fault step overs coincident with mapped alluvial unit boundaries. We identify four distinct middle to late Pleistocene fan units and four Holocene units in the study area. Pleistocene deposits are offset by the Needles fault whereas Holocene units are not offset. Our topographic profile data show that older Pleistocene units have progressively larger fault offset than younger units, with 0.55, 1.3, 3.3, and 5.2 m of vertical separation in units Qi3, Qi2b, Qi2a, and Qo, respectively. If we assume the minimum offset of 0.55 m is representative of slip in a single event, this implies that ~9 earthquakes have occurred since Qo was deposited. Existing alluvial fan ages are limited, but regional correlations and cosmogenic nuclide dating suggest a Qo age of ~800-1100 ka. Future work will refine alluvial fan ages and earthquake timing estimates with absolute ages obtained from quartz OSL and feldspar IRSL.

Key Words
Colorado River Extensional Corridor, tectonogeomorphology, drone SFM DSM, active fault, fault mapping

Citation
Avery, A., & Regalla, C. (2024, 09). Paleoseismic history of the Needles Fault, AZ: preliminary results from tectonogeomorphic analyses. Poster Presentation at 2024 SCEC Annual Meeting.

Related Projects & Working Groups
Earthquake Geology