CRUSTAL DEFORMATION MODEL: OBLIQUE CONVERGENCE IN THE INNER CALIFORNIA BORDERLANDS ACCOMMODATED BY ACTIVE STRIKE-SLIP AND REVERSE FAULTS August 2013
SONGS SSC SSHAC Workshop # 2 (August 12-14, 2013) Crustal Deformation Model: Oblique convergence in the Inner California Borderlands accommodated by active strike-slip and reverse faults
SCEC Community Fault Model (CFM) John Shaw Conclusions Both strike-slip and reverse faults accommodate active crustal deformation in the Inner California Borderlands. Both classes of faults should be considered as seismic sources. The interactions of these two fault types produce geometrically complex, segmented sources. A range of single and multi-segment rupture scenarios appears viable. Challenges are to define the recent activity, slip rates, and seismogeneic potential of these offshore faults, including the reverse fault systems.
The Oceanside and Thirtymile Bank faults originated as Miocene-age detachments, associated with the clockwise rotation of the Transverse Ranges and opening of the Inner California Borderlands. (Crouch and Suppe,1993; Nicholson et al., 1993; Bohannon and Geist, 1998; Ingersoll and Rumelhart, 1999; Rivero et al., 2000; Rivero and Shaw, 2011) John Shaw Crouch & Suppe, (1993) Rivero et al., (2000); Rivero & Shaw (2011)
Large portions of these detachments were reactivated as thrust and reverse faults in the late Pliocene to early Pleistocene. John Shaw Rivero et al., (2000); Rivero & Shaw (2011)
Reprocessed 1979 Chevron datasets (Geopentech, GeoTrace, UNR, 2013) LINE 4522-R (Chevron 79) Reverse faulting & folding Extensional rollover Oceanside detachment
Rivero & Shaw (2011) The Oceanside and Thirtymile Bank faults are large, laterally continuous structures constrained by thousands of kilometers on industry data.
Seafloor deformation along the Oceanside fault, however, is distributed over a complex set of reverse faults and fault-related folds that lie in their hanging walls.
Deformation in the hanging wall of the Oceanside thrust Shallow contractional folding and faulting produces seafloor scarps. John Shaw Folding is limited to the hanging g wall of the Oceanside thrust, which serves as a detachment for the shallow contractional deformation.
John Shaw Deformation in the hanging wall of the Oceanside thrust LINE JSC 101B Reverse faulting & folding Oceanside detachment
Deformation in the hanging wall of the Oceanside thrust Some forethrust fault splays, with contractional foldingin their hanging walls, appear to extend to the seafloor.
Deformation in the hanging wall of the Oceanside thrust km John Shaw Backthrust with contactional folds in their hanging wall are common beneath the shelf. Strike-slip and thrust faults clearly interact within the brittle crust.
John Shaw Deformation in the hanging wall of the Oceanside thrust LINE 4524 (Chevron 79) Reverse faulting & folding Oceanside detachment Strike-slip faulting
Four general styles are considered viable for the interactions of these dip- and strike-slip fault systems (Newport-Inglewood-Rose Canyon/Oceanside & San Diego Trough/Thirtymile Bank). All should be considered in comprehensive PSHA.
Model A is viable if offsets of detachments are small. We favor models C & D because: 1) Tectonic history: Oceanside detachment was active in the Miocene, prior to the development of the modern NIRC right-lateral strike-slip system. 2) Pliocene and younger thrust separation on the OBT is about 2.7 km, more than can be reasonably explained as a restraining bend on the NIRC fault. 3) Folding and rotation of the NI fault. 4) By analogy to the Thirtymile Bank-San Diego Trough fault system interaction, which is constrained by seismic reflection data and the 1986 Oceanside earthquake sequence.
We favor models C & D because: 4) By analogy to the Thirtymile Bank-San Diego Trough fault system interaction,, which is constrained by seismic reflection data and the 1986 Oceanside earthquake sequence. - The 1986 Oceanside earthquake sequence ruptured a moderately east-dipping fault plane, consistent with the down projection of the Thirtymile Bank fault. The event had a dominant component of dip slip motion. - The event confirms the present and activity of a moderately dipping thrust east of the San Diego Trough fault (we interpret this to be the Thirtymile Bank detachment). By analogy, this would suggest that the OBT is present east of the NI fault. John Shaw Rivero & Shaw (2011); seismicity from Astiz & Shearer (2000)
Activity of the Oceanside and Thirtymile Bank faults We lack unequivocal evidence oflate Pleistocene to Holocene activity and a slip rate on the OST. However, there are several lines of evidence to suggest recent activity: it discrete seafloor scarps along thrust splays that sole into the OST. where the OST projects onshore, there is active contractional folding (San Joaquin Hills). 1986 Oceanside earthquake sequence implies activity of the Thirtymile Bank thrust. San Joaquin Hills John Shaw Rivero & Shaw (2011); seismicity from Astiz & Shearer (2000)
John Shaw Activity of the Oceanside and Thirtymile Bank faults seafloor scarps along the OST are suggestive of recent fault activity. Reprocessed d 1979 Chevron h ddatasets ((Geopentech, h GeoTrace, UNR, 2013)) Contractional folding LINE 4514 (Chevron 79) Oceanside detachment
Activity of the Oceanside and Thirtymile Bank faults seafloor scarps along the OST are suggestive of recent fault activity. Reprocessed 1979 Chevron datasets (Geopentech, GeoTrace, UNR, 2013) LINE 4538R (Chevron 79) Reverse fault & contractional fold Oceanside detachment
Constraints on slip rates of the Oceanside and Thirtymile Bank faults Grant et al., (1999) Shortening above the OBT since the Pliocene (2.4 to 1.8 ma) yields a dip-slip rate of 0.57 to 1.5 mm/yr. Uplift rates in the San Joaquin Hills resolved onto the OBT yield a minimum dip-slip rate of 0.27 to 0.41 mm/yr. No direct measure of slip rate on the TMB fault is available. Geodetic constraints are poor, but probably limit the maximum slip rates to no more than a few mm/yr. Rivero & Shaw (2011); Rivero (2004)
We lack critical information about these offshore thrusts systems that compromises our ability to assess the hazards that they pose. we do not have unequivocal evidence of late Pleistocene to Holocene activity it and slip rate on some of these faults. there are several possible modes of interaction between thrust and strike-slip systems that will influence source geometries at depth. we lack direct knowledge about slip styles and magnitudes in past earthquakes. A series of targeted marine geophysical data collection (high resolution bathymetry, side-scan sonar, 3D reflection surveys) y) and sampling projects are needed to constrain the activity, slip rate, and paleoearthquake history of these fault systems. These constraints will inform our assessment of fault linkages at depth and earthquake hazards.
Bottom line recommendation for SONGS Include both offshore strike-slip and reverse fault sources. Consider a range of low to moderate fault slip rates for the reverse faults. Consider 4 possible modes of interaction with strike-slip systems.