Poster #121, Stress and Deformation Over Time (SDOT)

Restoring California, part I: 0-18 Ma

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Poster Presentation

2020 SCEC Annual Meeting, Poster #121, SCEC Contribution #10628
We are computing incremental stepwise restorations of maps of faults and outcrops in the western United States and northern Mexico using kinematic F-E program RESTORE (v.4) and datasets of: fault traces, fault offsets, timing constraints, paleomagnetic rotations (including a major contribution by Bruce Luyendyk), restored sections, and paleostress directions. The eastern edge of the model is fixed to stable North America, and the western edge is free. Three model parameters were tuned to match GPS velocities in the first (neotectonic) timestep. This report covers only the southern California part of the model in timesteps back to 18 Ma. Early results include:
(1) The model Garlock...
fault has neotectonic rates of 3.7 (W), 5.8 (central), and 4.1 (E) mm/a, and total sinistral offsets since initiation at 11 Ma of: 44 (W), 52 (central), and 39 (E) km, somewhat less than the geologic estimate of 64 km (central) from Andrew et al. [2015]. Under-fitting of geologic offsets is common in this model, except where offsets have small uncertainties.
(2) The restraining left-step of the San Andreas fault at San Gorgonio Pass began to form as soon as the southeastern San Andreas initiated at 6 Ma. Left-slip of 15 km on the Pinto Mountain fault (associated with general clockwise rotation of the region to its S) contributed to this. In our model, post-6 Ma shortening of 8 km on the Dillon thrust fault was another cause.
(3) We are able to nearly close the Gulf of California, and restore the Pelona Schist antiform opposite to the Orocopia Schist antiform at 6 Ma by about 240 km of San Andreas offset, as many authors have advocated. A remaining problem is that faults in the Orocopia area only back-rotate ~25 degrees--much less than the ~90 degrees that would be needed to align them with faults in the Pelona area.
(4) The Western Transverse Ranges (Santa Ynez, Santa Susana, & Santa Monica Mountains) have rotated clockwise since 18 Ma, as shown by many paleomagnetic studies. However, our model rotation is only ~70 degrees [as in Hornafius et al., 1986; also similar to Wilson et al., 2005] rather than ~120 degrees [as in Nicholson et al., 1994]. This implies that the sinistral faults now defining the N and S margins of the WTR formed at azimuths of ~020, consistent with stress directions just slightly counterclockwise from present.