Paläomagnetische Quantifizierung neogener Blockrotationen an einer aktiven, transtensionalen Plattengrenze, Baja California, Mexiko
Final Report Abstract
Compared to oceanic plate boundaries which are generally narrow zones of deformation, intra-continental plate boundaries appear as widespread areas in which fault bounded rigid blocks accommodating deformation by vertical-axis rotations are common. Whereas motion of crustal blocks within these plate boundaries causes rather small-scale lithospheric deformation along the boundary zone, the main plates behave more rigidly. Complex deformation patterns of interacting blocks separated by a variety of active faults are the consequence. In order to study the dynamic implications of deformation along an oblique-divergent plate boundary, the southern part of the Baja California peninsula, Mexico, was chosen as subject. Despite located in a major active transtensional zone, paleomagnetic studies in Baja California Sur are still rather scarce and have concentrated on volcanic rocks from the central part of the peninsula. During the duration of the project more than 500 oriented paleomagnetic drill cores were taken from Neogene volcanic and sedimentary rocks from three distinct study areas in the eastern borderlands of the Baja California peninsula. Whereas paleomagnetic results from Miocene sedimentary rocks from the San José del Cabo basin perfectly agree with the expected directions based on the Apparent Polar Wander Path for North America results from coeval sediments and volcanic rocks from the Sierra La Trinidad, further east, show significant clockwise rotations with respect to the expected direction in the order of 75°. Results from the Bay of La Paz region show a similar pattern with no indication for rotations within the basin, but significant clockwise rotations on its southern margin. Paleomagnetic results from the San José and San Francisco islands separated from mainland Baja by the San José Channel also indicate clockwise rotations. All this suggests that right-lateral motion along the main plate boundary in the Gulf of California and along active faults located in the Baja California Peninsula Borderland resulted in clockwise rotations of individual blocks causing the scissor-like opening of triangular shaped basis along the Baja California Borderlands. This interpretation is consistent with a GPS interseismic velocity field calculated for the area, indicating extension between a stable Baja California microplate and an area including the Baja California Peninsula Borderland and the offshore islands. We also note that the paleomagnetic findings in the San José del Cabo basin (no rotation) and the neighboring Sierra La Trinidad (70° clockwise rotation) are in excellent agreement with the GPS velocity field for this region and the GPS-derived clockwise rotations in the order of 4°/Ma.
Publications
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Deformation and rotation in southern Baja California, MX. American Geophysical Union, Fall Meeting 2008, abstract #T11A-1855
Malservisi, R.; Plattner, C.; Hackl, M.; Weber, J.; Bachtadse, V.; Umhoefer, P.
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Paleomagnetic Quantification of Neogene Block Rotations within an Active Transtensional Plate Boundary, Baja California, Mexico. American Geophysical Union, Fall Meeting 2009, abstract #GP11A-0754
Weber, J.; Umhoefer, P. J.; Pérez Venzor, J. A.; Bachtadse, V.
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(2012). Paleomagnetic quantification of neogene block rotations within an active transtensional plate boundary, Baja California Sur, Mexico. Dissertation, LMU München: Fakultät für Geowissenschaften
Weber, Josef
