iPlex Lunch spring-2015
Large vertical motions and basin evolution in the Outer California Borderland
May 20, 2015,
noon - 12:50 p.m.
Geology 1707
Presented By:
Craig Nicholson
UCSB
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.