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Large vertical motions and basin evolution in the Outer Continental Borderland off Southern California associated with plate boundary development and continental rifting

Craig Nicholson, Christopher C. Sorlien, & Catherine S. Schindler

Published 2011, SCEC Contribution #1738

The Continental Borderland offshore southern California occupies a strategic position along the continental margin. It was the locus of ~75% of Pacific-North America displacement history, it helped accommodate the large-scale (>90°) tectonic rotation of the Western Transverse Ranges province, and is still accommodating potentially 20% of PAC-NAM plate motion today. As such, it represents an ideal natural laboratory to investigate plate boundary evolution and basin development associated with transform initiation, oblique continental rifting, transrotation and transpression. We have been using newly released grids of high-quality industry multichannel seismic (MCS) reflection data, combined with multibeam bathymetry and offshore well data to map and construct digital 3D fault surfaces and stratigraphic reference horizons over large parts of the Outer Continental Borderland. These 3D surfaces of structure and stratigraphy can be used to better understand and evaluate regional patterns of uplift, subsidence, fault interaction and other aspects of plate boundary deformation.
In the northern Outer Borderland, mapping in Santa Cruz basin, and across both Santa Rosa and Santa Cruz-Catalina ridges reveals a pattern of interacting high-and low-angle faults, fault reactivation, basin subsidence, folding, and basin inversion. Subsidence since early-Miocene time is significant (up to 4 km) and is much larger than predicted by simple thermal cooling models of continental rifting. This requires additional tectonic components to drive this regional subsidence and subsequent basin inversion. Farther south, a more en echelon pattern of ridges and basins suggests a distributed component of right-lateral shear also contributed to much of the modern Borderland seafloor topography, including major Borderland basins.
Vertical motions of uplift and subsidence can be estimated from a prominent early-Miocene unconformity that likely represents a regional, paleo-horizontal, near-paleo-sea-level erosional surface. As such, this paleo-reference datum can be used to reconstruct Borderland forearc basin geometry prior to rifting, subsidence and subsequent basin inversion. Although not well resolved, the age of the regional unconformity appears to be time transgressive, and tends to young to the east and south. This progression may thus correlate with the oblique subduction of the Pacific-Arguello spreading ridge, rather than the onset of later continental rifting, as rifting in the Borderland typically progressed to the north and west following each jump in the triple junction farther south. This sequence of 1) a regional unconformity requiring uplift, 2) followed by subsidence, and 3) later basin inversion to form ridges thus documents an unusual and unexpected pattern of vertical motion reversal associated with the initiation of a predominantly strike-slip PAC-NAM plate boundary.

Nicholson, C., Sorlien, C. C., & Schindler, C. S. (2011). Large vertical motions and basin evolution in the Outer Continental Borderland off Southern California associated with plate boundary development and continental rifting. Eos, Transactions, American Geophysical Union, 92(52), T31F-04.