Unraveling a tectonic knot: structural domains, voluminous fault zones, creep, and dispersed strain between the San Andreas Fault and Brawley Seismic Zone

Clara Thomann, Susanne U. Jänecke, Daniel Markowski, James P. Evans, & Robert Quinn

Submitted August 15, 2018, SCEC Contribution #8673, 2018 SCEC Annual Meeting Poster #274

We report preliminary results from an ongoing analysis of the southern tip of the San Andreas fault (SAF) and northern Brawley seismic zone (BSZ). The SAF consists of the compressional Durmid ladder structure (DLS) along the northeast shore of the Salton Sea, and it progressively transitions into the only exposed part of the extensional BSZ north of Bombay Beach (Jänecke et al. 2018). We name this region between two dispersed and voluminous fault zones the Pope transition zone (PTZ) for a railroad stop within it. The PTZ has well exposed faults and folds, illustrating the complex boundary between major fault zones along the edge of the North America plate. The geometric pattern emerging from our field- and image-based research shows that the single main strand of the SAF loses its identity southward, and structural domains replace one another along strike and laterally. From NW to SE, the main strand of the SAF changes from a single, NW-striking trace with intense damage to a southward-widening zone of small-offset faults that span >1-km near Bombay Beach. A western fault zone abuts the DLS southeast-plunging anticline with folded 0.76 Ma Bishop Ash, and trends NNW overall with right-stepping mini-ladders and faults. On the NE side, an eastern fault zone separates the mostly NW-striking structures and significant Holocene cover of the PTZ from the exhuming structures of the Hidden Springs ladder structure. Deformation between the east and west belts is variable, including many N-, NE-, ENE-, and ESE-striking cross faults. Subsidence and Holocene sediment within the PTZ progressively increase moving southward, and therefore we identify active fault traces using creep-related fractures, sinks and erosional features. NNE- to NNW-striking normal-oblique faults in the PTZ cut modern sediment, a pattern consistent with the transtensional BSZ to the south. Plate boundary strain appears to be distributed across the entire volume of the dispersed PTZ and adjacent DLS regions. Long trenches will be necessary to assess the latest Holocene earthquakes given creep and recent fracturing is distributed across the PTZ. Trenching by Lindvall (1989) showed that 123-m long trenches were too short to capture all the active faults in the fault zone. These findings have clear implications for models and paleoseismic studies in a region overdue for a large earthquake, and whether large ruptures have passed through or been inhibited by the knot of structures within the PTZ.

Key Words
voluminous fault zones, ladder structures, geology, creep, San Andreas Fault, Brawley Seismic Zone, Hidden Springs Fault Zone

Citation
Thomann, C., Jänecke, S. U., Markowski, D., Evans, J. P., & Quinn, R. (2018, 08). Unraveling a tectonic knot: structural domains, voluminous fault zones, creep, and dispersed strain between the San Andreas Fault and Brawley Seismic Zone. Poster Presentation at 2018 SCEC Annual Meeting.


Related Projects & Working Groups
San Andreas Fault System (SAFS)