ROLE OF LATE QUATERNARY DETACHMENT FAULTING IN THE NORTHERN GULF EXTENSIONAL PROVINCE, SIERRAS EL MAYOR AND CUCAPA, BAJA CALIFORNIA, MEXICO

The Sierras Cucapa (SC) and El Mayor (SEM) are two uplifted blocks of Mesozoic basement located in the middle zone of shearing between the North America and Pacific plates. Internally both blocks are cut by an intense array of low-angle and high-angle faults that largely accommodate NE-SW extension perpendicular to the margins of the GEP. In the northern SC, two main low-angle normal fault systems have been documented with opposing senses of shear and the SW-directed detachment is structurally higher than the newly discovered NE-directed detachment. In general, high-angle faults cut low-angle faults but even the NE- and SW-directed detachments cut Plio-quaternary gravels, suggesting that they were active contemporaneously as conjugate systems. Two contrasting range bounding faults are found along the western margin of the uplifted blocks. The Laguna Salada Fault (LSF) in the north is a high-angle normal-dextral fault that bounds the SC. The Canada David detachment (CDD) in the south is a low angle normal fault that extends ~60 km along the western margin of the SEM. The presence of fault scarps, some over 7 m high, in non-consolidated alluvial fans and piedmont deposits along the western margin of the Sierras, reflect the activity of both master fault systems during the Quaternary. However, fault scarps arrays associated with each system differ significantly in terms of their structure and morphological features. As noted by Axen (1999) the LSF is characterized by a single synthetic scarp which itself defines the trace of the LSF along its strike. Conversely, the SEM scarps are typically found as broad arrays of both synthetic (range-side up) and antithetic scarps that follow the mullions of the CDD. The argument for Quaternary slip across the detachment is further supported by the fact that relative terrace height increases dramatically around antiformal mullions which indicates that the folds of the detatchment have continued to amplify in the late Quaternary. Quantitative morphologic analysis of 61 fault scarps adjacent to the detachment shows that the diffusion ages (kt) decrease systematically northward, which is consistent with the location of important historic seismic events in the north-western SEM and south-western SC. Multi-modal distribution of diffusion ages suggests 4-5 main scarp forming events along the CDD.