Reactivation of the San Marcos fault during mid-to-late Tertiary extension, Chihuahua, Mexico
José Jorge Aranda-Gómez, Todd B. Housh, James F. Luhr, Christopher D. Henry, Tim Becker, Gabriel Chávez-Cabello, 2005. "Reactivation of the San Marcos fault during mid-to-late Tertiary extension, Chihuahua, Mexico", The Mojave-Sonora Megashear Hypothesis: Development, Assessment, and Alternatives, Thomas H. Anderson, Jonathan A. Nourse, James W. McKee, Maureen B. Steiner
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The San Marcos fault is a >300-km-long, WNW- to NW-trending structure that has been repeatedly reactivated during Mesozoic and Tertiary pulses of deformation. The San Marcos fault has previously been demonstrated to extend across Coahuila in a trend roughly parallel to the proposed Mojave-Sonora megashear. Exposures in this broad region are dominated by Mesozoic marine sedimentary rocks. Northwest of the border between Coahuila and Chihuahua a thick cover of Tertiary volcanic rocks masks the fault trace. We believe that the San Marcos fault trace projects across the eastern branch of the Bolsón de Mapimí, a major Basin and Range structure, into the northern portion of the Sierra del Diablo and into the central part of the Pliocene-Quaternary Camargo volcanic field (40Ar/39Ar ages are 4.7–0.09 Ma). In the same area, upper Tertiary normal faults that bound both branches of the Bolsón de Mapimí show a marked change in orientation. South of the projected San Marcos fault the extensional structures strike ∼north-south. Northwest of the San Marcos fault, a conspicuous northwest-trending graben (ca. 4.7–2.1 Ma) crosses the Camargo volcanic field. A lineament identified in digital elevation models suggests that the San Marcos fault extends beneath the Camargo volcanic field, passes west of Sierra Aguachile, and can be traced at least an additional 300 km to the northwest, which effectively doubles the known or inferred length of the San Marcos fault.
Mid-Tertiary volcanic rocks (ca. 32–14 Ma) are folded into NNW-trending, SSE-plunging synclines in areas immediately northwest and southeast of the Camargo volcanic field. A short distance (∼5 km) west of Sierra El Morrión in central Chihuahua another NNW-trending syncline, observable in satellite images and in regional geologic and topographic maps, is developed in mid-Tertiary volcanic rocks. Miocene folding in the volcanic sequence at Peña Blanca has been reported in the literature near the NNW projection of the San Marcos fault.
The central graben of the Camargo volcanic field, the upper Tertiary synclines, and the abrupt change in trend of the Tertiary normal faults are all closely associated with a distinct bend in the San Marcos fault, marking a change in trend from WNW to NW. The Camargo volcanic field central graben and the adjacent synclines were developed in the same northwest-trending segment of the fault. We interpret the central graben of the Camargo volcanic field as a pull-apart basin, whereas the synclines were formed in areas where a small left-lateral component in the fault movement produced shortening. We believe this deformation was accomplished in at least two pulses of late Tertiary extension. An older event that occurred between ca. 14 Ma and 5 Ma produced the left-lateral movement and associated folding. This pulse corresponds to a short episode of regional northwestern extension documented in northern Mexico and the southern United States. The younger event (≤5 Ma) was in part synchronous with the activity in the Camargo volcanic field and was caused by NE to ENE extension.