3D Vp-Gravity-Density Modeling Of Irregular High-Vp Diabase Intrusive Bodies And Shallow 3D Very High-Res Reflection Imaging Of Faulted Marine Terraces Of The Southern Irish Hills, California

Abstract

Active source seismic data collected in the Irish Hills of central coastal California in 2011-2012 provided 22 million first-break picks that were jointly inverted with gravity and density to create 3D acoustic wave velocity (Vp) and reflection volumes. 2011 data were acquired across the Irish Hills; 2012 acquisition focused on shallow faulting and velocity structure along the southern coast. Higher-res Vp models were made for marine terrace area with 2ft vertical and 10ft horizontal cells, and the Diablo Canyon Power Plant (DCPP) area with 5ft vertical spacing to 300 ft depth. Surface mapping, gravity, helimagnetic, and nearshore Multibeam bathymetry data are evaluated for faulting, Mesozoic basement structure of the inverted Pismo Synclinorium, and mapping subsurface diabase in the study area. Vp and S-wave 3D tomography volumes provided reflection processing inputs for foundation velocity analyses to inform ground motion modeling for DCPP. The Miocene Obispo Fm. in the DCPP area is intruded by diabase bodies with uniquely high Vp values = 6 km/s); typical Mesozoic basement velocities in this area range from ∼4 to 5 km/s. The diabase is well-imaged in Vp; no significant offset of diabase-Vp structure was observed. The marine terrace Vp model southeast of DCPP was evaluated for possible splay faults that may hard-link the San Luis Bay fault system (e.g., Rattlesnake fault) and the offshore Shoreline fault zone. Terrace deposits 20 to 50 feet thick overlie a platform of subvertical Cretaceous sandstone (Ks); the top of Ks was mapped by creating a depth-to-first-strong-reflector map in which three lineaments were identified. No consistent bedrock step is seen across the previously mapped Rattlesnake fault. An unnamed fault in Irish Canyon, coincident with a strong magnetic-anomaly lineament, is observed in 2011 reflection data as prominent reflector truncations along strike; we propose the name Irish Canyon fault, and hypothesize it as an en-echelon splay of the Tertiary San Miguelito-Shoreline fault system. Vertical faults cross-cut transpressional folds north and south of the basin imaged in 2D/3D depth-migrated data, similar to offshore faulting observed in 3D reflection and bathymetry data. The Shoreline fault may left step onshore onto the steeply-north-dipping Irish Canyon fault west of Pt. San Luis. 3D viscoelastic deformation modeling of long-term Hosgri oblique slip reproduces observed coastal terrace uplift rates along the Irish Hills.

AAPG Datapages/Search and Discovery Article #90215 © 2015 Pacific Section AAPG Convention, Oxnard, California, May 3-6, 2015