Paleomagnetic and Seismological Investigation of Rotations in Transpressional Folds, San Andreas Fault System, California

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AbstractTranspressional folds can be either wrench folds or slip-partitioned folds. The two can produce structures similar enough that folds adjacent and subparallel to the San Andreas Fault could be either. The difference between the two models is far more substantial: it is the difference between strong and weak faults. Which model better reflects the geology alters our perception of both the neotectonic hazards of and the geometry produced in these folds, geometry that is responsible for several super giant oilfields in California alone. Furthermore, if these are slip-partitioned folds, then they are produced with a fold axis not normal to the most compressive horizontal stress, indicating that current theories of folding are incomplete. In a wrench-folding environment, the folds (and any underlying faults) rotate with time as the finite shortening strains rotate normal to the strike-slip fault. Paleomagnetic rotation should be equal everywhere at the same distance from the strike-slip fault. Micropolar rotation, observable from focal mechanisms, will be small, because small and large scale rotations are the same. On a partitioned system, the underlying faults are not rotating. However, a component of strike-slip on underlying faults has yet to be recognized in surface folding. Most likely, this shear strain produces vertical-axis rotations near the anticlinal axes of the folds paralleling the San Andreas, as is observed in physical models and in the Yakima Fold Belt in Washington. Both the inhomogeniety of rotation and the absence of rotation of the macroscale structures will produce laterally varying paleomagnetic rotations and larger micropolar rotations than for the wrench folding case. This project addresses this problem through analysis of the paleomagnetic and seismological characteristics of the folds adjacent to the San Andreas west of the San Joaquin Valley. These folds, initiated in the Neogene over active blind thrust faults, have been the focus of debate between those inferring strain partitioning and low shear stress on the San Andreas fault and those inferring wrench faulting and high stresses. Paleomagnetic and seismological investigation of the Coalinga, Kettleman Hills, and the Wheeler Ridge anticlines should determine which kinematic model of regional deformation best fits these structures. These two techniques complement each other, and the work will proceed in parallel so that refined analysis strategies can address questions arising from the other approach. Additionally, while the paleomagnetic analysis provides a time-integrated view of the deformation, use of some of the dense aftershock datasets in these folds will permit us to apply the micropolar analysis to different depth slices and different volumes near and far from the axial plane of the overlying anticline. This will constrain the three-dimensional variations in strain and micropolar rotation, which will provide important constraints for understanding the physics of deformation in these fold systems. Broader impacts of this work include studying active, super giant petroleum fields that continue to be exploited. Improved understanding of the genesis of these structures is likely to improve the ability to manage this resource most efficiently. Second, these structures are seismogenic and have already produced a damaging earthquake. This work directly addresses the mechanism of deformation, which in turn is likely to effect studies of return times and seismic hazard. This work also addresses the seismogenic character of the San Andreas Fault, one of the greatest seismic hazards in the United States. The PI will also be building the infrastructure of science and education. The project largely supports female graduate student Joya Tetreault, an underrepresented class in earth science.