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Large Coherent Block Versus Microplate Rotation of the Western Transverse Range: A New Examination of Paleomagnetic Constraints


Within the last 30 million years, the Southern California plate boundary has transitioned from a region of plate collision and subduction to a right-lateral translational boundary. This transition created many of the geologic features that are observed today along the Inner California Borderlands (ICB), including the Western Transverse Range Province (WTR). The WTR has long been considered a large coherent block (~200 by 100 km) that underwent 90-100 degrees of clockwise rotation in the early Neogene as a result of right-lateral transform motion along the margin in a process characterized as “book-shelf” faulting. Directional paleomagnetic data have been employed to support the hypothesis of coherent rotation of ‘books’, here referred to as a `large coherent block’. As the data collected in this region vary in quality, age, and locality, we set out to test how well the data support the large coherent block rotation hypothesis of the WTR when viewed as a composite. If the large coherent block rotation model is correct, it predicts the paleomagnetic directions of the same age within the WTR should record the same amount of rotation. To test this, we have compiled a large set of directional paleomagnetic data from relevant published papers, updating the age constraints to a consistent time scale where possible. By re-examining directional paleomagnetic data from same-age rocks within the WTR, our compilation shows different amounts of rotation across the WTR from formations that are similar in age. The directional paleomagnetic data are therefore inconsistent with the prediction of a large coherent block rotation model. We propose an alternative model for the deformation of the WTR, one that is consistent with paleomagnetic data from the region. In our model, rather than a single large block rotating coherently, many micro-plates that range from kilometers to tens of kilometers rotated individually, accommodating numerous right-lateral fault displacement. Our analog for the deformation of the WTR is a garnet schist, where the “garnets” are the microplates rotated by bounding right-lateral shear zones.

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