Mathematics – Logic
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp44a..07h&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #GP44A-07
Mathematics
Logic
1520 Magnetostratigraphy, 1525 Paleomagnetism Applied To Tectonics: Regional, Global, 1527 Paleomagnetism Applied To Geologic Processes, 4918 Cosmogenic Isotopes (1150)
Scientific paper
Sedimentary rocks in the western Salton Trough record the evolution of Late Cenozoic tectonic processes within this active plate margin zone. Studies of magnetostratigraphy and paleomagnetism from these rocks provide important constraints on the timing and kinematic nature of these processes. The results for several years of study, which integrates new stratigraphic, structural, and geochronologic work, will be summarized here, with emphasis on new findings and opportunities for future work in synorogenic basins such as this. Our work is based on studies of ~250 paleomagnetic sites collected in several stratigraphic sections. In the ~5.5 km thick FCVB, we have determined that the age of initial sedimentation in this basin, which likely marks the onset of regional extension, is ~8.1 Ma. Analysis of basin subsidence, from our combined stratigraphic work, points to three distinct phases of subsidence: a moderate (~0.4 mm/yr) rate from 8.1 to 4.6 Ma, a rapid (~2 mm/yr) rate from 4.6 to 3.1 Ma, and a return to moderate rates of ~0.4 mm/yr from 3.1 to ~1.0 Ma. Magnetostratigraphy of rocks exposed in the Borrego and Ocotillo Badlands north of the FCVB show that these sections range from ~0.6 to ~1.3 Ma in age. A widespread contact that changes laterally from a conformity to an angular unconformity (coinciding with the base of the Jaramillo sub-chron) marks the ~1.1-1.3 Ma initiation of significant dextral slip along the San Jacinto and related faults in this area. In terms of rotation, our work indicates that the most significant rotations (~20° CW) are recorded in the older (> 3 Ma) rocks of the FCVB. The results from rocks < 2.0 Ma in the FCVB, and rocks < 1.3 Ma in the Borrego and Ocotillo Badlands, all indicate minimal rotations (0 to 10°). These results are in contrast to the larger rotations inferred to have occurred after 1.0 Ma by previous work (e.g., Johnson et al, 1983), but our results do agree well with GPS-based models that suggest CW rotation rates of 0.5 to 2°/m.y. for this area. Combining these results, we tentatively conclude that the more rapid subsidence and larger rotations recorded in the older rocks took place during slip on the Western Salton Detachment Fault. Paradoxically, the transition from oblique extension to strike-slip/transpression at ~1 Ma seems to have not produced significant vertical axis rotations. Our ongoing work is focused on using the sedimentologic record in the FCVB to test competing hypotheses for tectonic and climatic influences on basin filling. To this end, we are developing higher-resolution magnetostratigraphic studies (including use of relative paleointensity) in conjunction with cosmogenic-isotopic concentrations of detrital quartz (Oskin et al, this meeting). To better understand the nature and timing of kinematic variations, traditional directional analyses combined with analyses of the shape-distribution of post- Brunhes overprints will be employed to decipher the spatial and temporal pattern of rotations in these rocks.
Dorsey R. J.
Housen Bernard A.
Janecke S. U.
Oskin Michael E.
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