New Seismic CHIRP evidence for Transpression and Transtension Beneath the Salton Sea, California

The Salton Trough is a critical structure that separates spreading center dominated deformation in the Gulf of California and dextral strike-slip deformation along the San Andreas Fault (SAF) System. Geological and geophysical data suggest there is a transition within the Salton Trough near the town of Bombay Beach that separates transpression to the north from transtension in the south. To date, however, this transition remains poorly understood, in large part, due to a lack of geophysical subsurface data in the Salton Sea. We present preliminary analysis of > 350 line-km of high-resolution seismic CHIRP data acquired in 2006 and 2007 that imaged the different deformational styles beneath the Salton Sea, including several previously unidentified tectonic structures. The Extra Fault Zone (EFZ), which has been mapped onshore, is imaged in CHIRP profiles as an ~5 km wide deformation zone and can be traced offshore > 15 km along strike. Along the northern extent of the EFZ, ramp-flat deformation is observed with southward vergence. A marked angular unconformity between the Brawley and Cahuilla formations records fault-bend folding often predicted for ramp-flat thrust systems. Uplift and truncation of Pleistocene sediments along the northern edge of the EFZ is observed across the entire sea and appears to systematically increase towards the west. Onlapping sediments and growth folds in the Holocene Lake Cahuilla section to the north and south of the fault zone record ongoing uplift. Compression is also manifested in the bathymetry, with a mid-lake bathymetric high trending parallel the EFZ and separating southern and northern lake basins. The EFZ is interpreted to accommodate sinistral transpression related to clockwise- rotating crustal blocks. In contrast, at Bombay Beach the SAF takes an ~15 km releasing step towards the Imperial Fault, producing transtensional deformation through the Brawley Seismic Zone (BSZ). CHIRP profiles across the western margin of the BSZ image several down-to-the southeast, en echelon normal faults. Vertical offset of Lake Cahuilla sediments extends to within .5 m of the lake floor, suggesting recent deformation (possibly post-1905), and increases down-section to a maximum of ~6-8 m, thus providing evidence for several earthquakes during the past 1,300 - 1,500 years. The trend of these normal faults is approximately northeast, an orientation that is not aligned with the "ladder and rung" patterns in the relocated seismicity. Facies changes documented in ~15 m sediment cores collected by United Research Services (URS) allow us to identify at least four lake sequences. Semblance between coarse-grained materials in core logs and high-amplitude reflections in CHIRP data allow us to correlate strata throughout the basin and assign tentative ages. With an improved chronology for Lake Cahuilla sequences, the CHIRP data will allow us to constrain slip rates, paloeseismic event timing across the EFZ and faults in the BSZ, and to investigate relationships between events along the EFZ, SAF and San Jacinto Fault (SJF). The overall structure observed in CHIRP profiles might be a result of transrotation caused by strain interactions between the SAF and SJF and overprinted by regional transpression and transtension. Doming and uplift of LC sediments above the thrust ramp, as well as en echelon normal faults along the western edge of the BSZ suggest transpression and transtension are currently active and play an important role in the late Pleistocene- Holocene tectonic evolution of the Salton Trough.