Anomalous Uplift at Pitas Point: Implications from Onshore & Offshore 3D Fault & Fold Geometry and Observed Fault Slip

Craig Nicholson, Christopher C. Sorlien, & Tom E. Hopps

Published August 15, 2016, SCEC Contribution #6741, 2016 SCEC Annual Meeting Poster #006

Based on four Recent ~8-m uplift events of coastal marine terraces at Pitas Point, many believe these represent earthquakes near M8 on the N-dipping Pitas Point-Ventura fault (PPVF), part of the larger primarily offshore North Channel-Pitas Point-Red Mountain fault system. However, this model of multiple Holocene M8 events on the PPVF has major problems, not the least of which are: failure of the 2D fold model used to properly infer subsurface fault-fold geometry, an implied Holocene slip rate for the blind PPVF that is inconsistent with offshore observations, and the marked lack of near-surface fault rupture or widespread tsunami deposits expected from such shallow (<15 km depth) M8 events that would extend 10's of km offshore. The reason for these discrepancies may be that uplift at Pitas Point is driven primarily by slip on the S-dipping listric Padre Juan fault (PJF), not solely by the PPVF. The PJF juxtaposes the strongly N-verging San Miguelito anticline in its hanging wall above the more symmetric Ventura Avenue-Rincon anticline in its footwall. Fault and fold geometry is well determined by industry wells that produce from the distinctly different upper San Miguelito and lower Rincon oilfields, and by imaging offshore with seismic reflection data. In the upper 3 km, the PJF exhibits up to 2.6 km of dip separation, in contrast to ~200 m of inferred separation on the Ventura fault at similar depths. Much of this PJF slip is syntectonic with growth of the Rincon anticline as PJF splays are folded by this lower fold. The timing and slip involved for San Miguelito fold growth and its emplacement against and deformation of the lower Rincon anticline, the specifics of infered Ventura fault propagation, and the geometry of the PJF and PPVF requires that much of this fault slip occurred while the PJF acted independently—not as a backthrust to the PPVF. A still active listric PJF can help account for the observed uplift at Pitas Point without recourse to M8 earthquakes. Regardless, its presence helps explain why the uplift at Pitas Point is so anomalous, and not necessarily indicative of the expected slip at depth either along strike of the PPVF or the average slip during large earthquakes. Rather, this uplift at Pitas Point is probably localized to where slip on the PJF predominates, or where the PJF and PPVF strongly interact, which limits the length & depth of possible seismic ruptures and the geohazard & tsunami potential of the active fault(s) involved.

Key Words
Pitas Point uplift, fault geometry, fault slip, Padre Juan fault, Pitas Point-Ventura fault

Citation
Nicholson, C., Sorlien, C. C., & Hopps, T. E. (2016, 08). Anomalous Uplift at Pitas Point: Implications from Onshore & Offshore 3D Fault & Fold Geometry and Observed Fault Slip. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Unified Structural Representation (USR)