KINEMATIC ANALYSIS OF NEOGENE AND QUATERNARY FAULTS IN THE SANTA ROSALIA BASIN ALONG THE MARGIN OF THE GULF OF CALIFORNIA, BAJA CALIFORNIA SUR, MEXICO

The Santa Rosalía Basin is located on the western shore of the Gulf of California in central Baja California Sur, México. This study identifies fault types and orientations and differs from previous investigations by taking advantage of excellent new exposures created by Minera Boleo mining operations. Volcanic rocks were measured in arroyo outcrops, and most other measurements were collected from mine cuts. Fault rocks including fault gouge and breccia were sampled and analyzed with petrographic, XRD, and SEM methods, revealing abundant mineralization of sepiolite within fault zones. We collected orientation data on over 250 faults and slickenlines in multiple unconformity bounded formations in the Santa Rosalía Basin. Fault orientation data were processed in QGIS, Microsoft Excel, Stereonet 9, and Fault Kin 7. Most faults are oblique faults with a normal component, and very few were thrust faults. The data suggest temporal changes in fault orientations and slip directions. Middle to Late Miocene volcanic rocks are dominated by NW- to N-striking oblique-normal faults that dip dominantly SW with moderately-plunging slickenlines. Late Miocene sedimentary rocks (Boleo Formation) are cut by steeply-dipping, NNW- to NNE-striking oblique-slip faults, with a wide range in plunge of slickenlines. Faults that cut the Pliocene sedimentary rocks (Tirabuzón and Infierno Formations) are mostly steeply-dipping NW-striking faults with dominant normal displacement determined from stratigraphic offset and steep plunge of striations. Quaternary marine terrace deposits are cut by predominantly NNE-striking normal faults with no preserved striations on the fault plane. The changes in fault orientations record evolving stress directions during development of the Pacific-North America plate boundary through time. The data are generally consistent with a proposed change from late Miocene ENE-WSW extension to the modern phase of ESE-WNW extension related to dextral offset on NW-striking transform faults in the Gulf of California (Angelier et al., 1981; Stock and Hodges, 1989). This tentative conclusion needs to be tested with future fault-kinematic analysis.