Abstract
In Southern California, plate boundary motion between the North American and Pacific plates is distributed across several sub-parallel fault systems. The offshore faults of the California Continental Borderland (CCB) are thought to accommodate ∼10–15% of the total plate boundary motion, but the exact distribution of slip and the mechanics of slip partitioning remain uncertain. The Newport-Inglewood-Rose Canyon fault is the easternmost fault within the CCB whose southern segment splays out into a complex network of faults beneath San Diego Bay. A pull-apart basin model between the Rose Canyon and the offshore Descanso fault has been used to explain prominent fault orientations and subsidence beneath San Diego Bay; however, this model does not account for faults in the southern portion of the bay or faulting east of the bay. To investigate the characteristics of faulting and stratigraphic architecture beneath San Diego Bay, we combined a suite of reprocessed legacy airgun multi-channel seismic profiles and high-resolution Chirp data, with age and lithology controls from geotechnical boreholes and shallow sub-surface vibracores. This combined dataset is used to create gridded horizon surfaces, fault maps, and perform a kinematic fault analysis. The structure beneath San Diego Bay is dominated by down-to-the-east motion on normal faults that can be separated into two distinct groups. The strikes of these two fault groups can be explained with a double pull-apart basin model for San Diego Bay. In our conceptual model, the western portion of San Diego Bay is controlled by a right-step between the Rose Canyon and Descanso faults, which matches both observations and predictions from laboratory models. The eastern portion of San Diego Bay appears to be controlled by an inferred step-over between the Rose Canyon and San Miguel-Vallecitos faults and displays distinct fault strike orientations, which kinematic analysis indicates should have a significant component of strike-slip partitioning that is not detectable in the seismic data. The potential of a Rose Canyon-San Miguel-Vallecitos fault connection would effectively cut the stepover distance in half and have important implications for the seismic hazard of the San Diego-Tijuana metropolitan area (population ∼3 million people).
Highlights
Pull-apart basins form in extensional step-overs between strikeslip fault sections and are common features of strike-slip fault systems around the word (Mann, 2007 and references therein)
As a result of the continuum of development exhibited by pull-apart basins (Mann, 2007), a variety of structural conditions have been observed ranging from relatively simple geometries, i.e., two parallel fault strands experiencing pure strike-slip motion with a central depocenter [e.g., the Dead Sea (Garfunkel and Ben-Avraham, 1996)], to more complex fault structures found in basins undergoing transtension [e.g., the Gulf of CaliforniaSalton Trough system (Lonsdale, 1995; Brothers et al, 2009)]
Reprocessing the legacy multi-channel seismic (MCS) data resulted in good-quality, interpretable data down to ∼450–500 ms two-way travel time and imaged many of the fault segments previously mapped in San Diego Bay
Summary
Pull-apart basins form in extensional step-overs between strikeslip fault sections and are common features of strike-slip fault systems around the word (Mann, 2007 and references therein) These structures receive considerable attention in academic studies due to their abundance, and their high potential in resource extraction, and their role in earthquake rupture processes (e.g., Mann et al, 1983; Oglesby, 2005; Mann, 2007; Wesnousky, 2008; Brothers et al, 2009; 2011; Wu et al, 2009; Watt et al, 2016; van Wijk et al, 2017). In the classical model of a pull-apart basin experiencing pure strike-slip deformation, a central basin is bounded by terraced normal faults striking ∼30◦ to the master strike-slip fault sections; in contrast, basins undergoing transtension appear to develop en-echelon sidewall faults and more complex central basins with multiple depositional centers (Wu et al, 2009)
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