Abstract
Deformation observed along the San Mateo (SMT) and San Onofre trends (SOT) in southern California has been explained by two opposing structural models, which have very different hazard implications for the coastal region. One model predicts that the deformation is transpressional in a predominantly right lateral fault system with left lateral step-overs. Conversely in the alternative model, the deformation is predicted to be compressional associated with a regional blind thrust that reactivated detachment faults along the continental margin. State-of-the-art 3D P-Cable seismic data were acquired to characterize the geometry and linkage of faults in the SMT and SOT. The new observations provide evidence that deformation along the slope is more consistent with step-over geometry than a regional blind thrust model. For example, regions in the SOT exhibit small scale compressional structures that deflect canyons along jogs in the fault segments across the slope. The deformation observed in the SMT along northwesterly trending faults has a mounded, bulbous character in the swath bathymetry data with steep slopes ( ∼ 25°) separating the toe of the slope and the basin floor. The faulting and folding in the SMT are very localized and occur where the faults trend more northwesterly (average trend ∼ 285°) with the deformation dying away both towards the north and east. In comparison, the SOT faults trend more northerly (average trend ∼ 345°). The boundary between these fault systems is abrupt and characterized by shorter faults that appear to be recording right lateral displacement and possibly accommodating the deformation between the two larger fault systems. Onlapping undeformed turbidite layers reveal that the deformation associated with both major fault systems may be inactive and radiocarbon dating suggests deformation ceased in the middle to late Pleistocene (between 184 and 368 kyr). In summary, our preferred conceptual model for tectonic deformation along the SMT and SOT is best explained by left lateral step-overs along the predominantly right lateral strike-slip fault systems.
Highlights
Southern California’s complex tectonic evolution is recorded by the structural and stratigraphic deformation of the Inner California Borderland (ICB)
Southwest of the broad shelf offshore San Onofre, the slope exhibits a steep gradient at the juncture with the basin floor, with dips up to 25° (Figures 2, 5)
Previous studies have shown that the wide shelf offshore San Onofre (Figure 2) is spatially coincident with a large anticline related to a left lateral jog in the NIRC and suggest that this anticline may act as a barrier to sediment traversing the inner shelf up to the basin floor in this area (Sahakian et al, 2017; Wei et al, 2020; Holmes et al, 2021)
Summary
Southern California’s complex tectonic evolution is recorded by the structural and stratigraphic deformation of the Inner California Borderland (ICB). Since the late Miocene to early Pliocene the margin has been overprinted by right-lateral strikeslip motion, often along reactivated Mesozoic structures (Grant and Shearer, 2004; Hill, 1971). The majority of the slip budget between North America and Pacific plates is accommodated by onshore faulting (primarily east of the Elsinore fault systems, EF; Figure 1); approximately 5–8 mm/yr, or nearly 15% of relative plate motion, is partitioned offshore between a network of northwest-trending strike-slip fault systems ranging from just east of San Clemente Island to the coastline (Bennett et al, 1996; Ryan et al, 2012; Sahakian et al, 2017).
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