Geologic Map of the Sierra Bacha, Coastal Sonora, Mexico
Michael H. Darin and Rebecca J. Dorsey

Data originator: Michael H. Darin, Northern Arizona University,
doi: 10.1130/2014.DMCH021
Year: 2014
Area: Coastal Sonora, Mexico, between Bahia Kino and Puerto Libertad
Scale: field mapping = 1:10,000 and 1:20,000; published at 1:30,000

Projection: WGS 1984, UTM Zone 12N

False Easting: 500000.0
False Northing: 0.0
Central Meridian: -111.0
Scale Factor: 0.9996
Latitude of Origin: 0.0
Linear Unit: meter

Geographic Coordinate System: GCS_WGS_1984
Angular Unit: Degree (0.0174532925199433)
Prime Meridian: Greenwich (0.0)
Datum: D_WGS_1984
Spheroid: WGS_1984
Semimajor Axis: 6378137.0
Semiminor Axis: 6356752.314245179
Inverse Flattening: 298.257223563

This geologic map covers an area of ~500 km2 in coastal Sonora along the eastern rifted margin of the northern Gulf of California. Detailed geologic mapping of rare exposures of late Miocene rocks in the Sierra Bacha provides a unique opportunity to investigate the timing, magnitude, and style of deformation during the tectonic transition from subduction to oblique rifting and continental rupture that created the Gulf of California ca. 6 Ma.

A >2000-m-thick composite sequence of volcanic and volcaniclastic strata is exposed in the Sierra Bacha in a series of uniformly NE-tilted fault blocks, revealing a history of NE-SW-directed extension in the late Miocene Proto-Gulf of California, immediately adjacent to a documented latest Miocene transtensional shear zone, the Coastal Sonora Fault Zone (Bennett et al., 2013). Extension and tilting occurred between 10.4 and 6.4 Ma in the Sierra Bacha, based on a number of radiometrically dated flows and tuffs, including the 6.4 Ma sub-horizontal capping Basalt of Arivaipa (Tba), which lies above an angular unconformity with older, uniformly NE-tilted strata. Limited indirect evidence of late Miocene dextral shear in the Sierra Bacha includes minor faults with oblique-slip fault kinematics, several mapped N-S to NE-SW-oriented sinistral-normal faults, and negligible clockwise vertical-axis rotation from paleomagnetic data (Darin, 2011). This suggests that latest Miocene dextral shear was likely localized in the adjacent Coastal Sonora Fault Zone, which may have served as a catalyst for Pacific-North America plate boundary localization and continental rupture in the northern Gulf of California ca. 6 Ma.

References Cited:
Bennett, S.E.K., 2013, The role of rift obliquity in formation of the Gulf of California [Ph.D. thesis]: University of California - Davis, 205 p.
Darin, M.H., 2011, Late Miocene extensional deformation in the Sierra Bacha, coastal Sonora, Mexico: Implications for the kinematic evolution of the proto-Gulf of California [M.S. Thesis]: Eugene, Oregon, University of Oregon, 95 p.

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