New constraints on the tectonic evolution of the Salton Trough
Abstract
The Salton Trough is a critical structure where two very different styles of deformation meet; spreading-center dominated deformation to the south in the Gulf of California and dextral strike-slip deformation along the San Andreas fault system(SAF) to the north. Seismic CHIRP data acquired in the Salton Sea provide new constraints on the interaction between the San Andreas, San Jacinto and Imperial fault systems and reveal distinct changes in deformational style from north to south. Based on the stratal geometry observed in CHIRP profiles, we propose three distinct phases of tectonic deformation: (1) Late- Pleistocene transpression north of the Extra Fault Zone (EFZ) replaced by (2) late-Holocene differential subsidence south of the EFZ and (3) recent formation of the Brawley Seismic Zone (BSZ), a through-going crustal shear zone. An angular unconformity is observed to separate the folded and faulted (late?) Pleistocene strata of the Brawley Formation from the overlying Holocene Cahuilla Formation (CF). North of the EFZ reflectors in the CF suggest little to no active deformation. Conversely, south of the EFZ reflectors exhibit marked divergence with their dip systematically increasing with depth. Such a pattern of divergence indicates that the rate of sedimentation has kept pace with the rate of tectonically-induced accommodation. As such, it appears that the EFZ is a tectonic hinge zone delineating the northern limit of active subsidence, high heat flow, and volcanism. Furthermore, given the observed subsidence pattern, we predict the existence of a NE trending basin-bounding normal fault, or series of normal faults, near the southern shoreline of the Salton Sea. In our conceptual model, the early distributed faulting and transrotation between the San Andreas and San Jacinto faults accounts for the compressional folding observed in the Brawley Formation, but later gave way to extension-dominated deformation as significant slip became focused along the Imperial Fault. Reactivation of Miocene extensional structures by the Imperial - San Andreas releasing bend may account for the differential subsidence in the southern sea. In addition, we propose that the BSZ is a young feature similar to through-going shear zones formed in pull-apart basin analog models. We suggest that seismicity patterns in the BSZ do not reflect the dominant tectonic processes or kinematics occurring beneath the Salton Sea, but rather short-term, low magnitude stress release. Finally, we have established a chronostratigraphic framework for the Salton Sea by correlating seismic horizons in the CHIRP data with Lake Cahuilla stratigraphy observed in onshore paleoseismic excavations. The EFZ shows evidence for multiple events during the last ~1,500 years, and the most recent event occurred ~1700 AD, suggesting the potential for coincident ruptures along the SAF and EFZ. Initial estimates suggest that the vertical subsidence rate across the EFZ has been ~ 6 mm/yr for the last 800 years.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.T11A1840B
- Keywords:
-
- 3000 MARINE GEOLOGY AND GEOPHYSICS;
- 8105 Continental margins: divergent (1212;
- 8124);
- 8111 Continental tectonics: strike-slip and transform;
- 8150 Plate boundary: general (3040)