Transtensional rifting in the proto-Gulf of California near Bahia Kino, Sonora, Mexico

Continental rifts require focused strain to rupture and form an ocean basin. In oblique rifts, such as the Gulf of California, focused transtensional strain associated with strikeslip faulting may serve as a catalyst for rupture. To test this hypothesis, we analyzed structural mapping, geochronology, paleomagnetism, and fault kinematics of pre-and synrift rocks exposed in an similar to 200 km(2) coastal mountain belt fl anking the eastern rift margin of the northern Gulf of California. This coastal Sonora region hosts the onshore portion of the transform boundary between the Upper Tiburon and Adair-Tepoca marine basins-two early-formed oblique rift segments. Extension commenced here between 11.5 Ma and 7 Ma, resulting in 25 degrees-40 degrees of E-NE tilting, initiation of clockwise verticalaxis rotation of fault-bounded blocks, and minor basin sedimentation. Rates of deformation prior to 7 Ma are unconstrained due to a lack of exposed syntectonic deposits. Deformation after 7 Ma was associated with rapid tilting and the majority of observed clockwise vertical-axis rotation and strikeslip faulting. Nonmarine sedimentary basins accumulated coarse sediments above an unconformity eroded across older, tilted strata. By 5-6 Ma, deformation in coastal Sonora must have largely ceased and migrated westward into the Upper Tiburon marine basin. We document up to 120% total extension and total clockwise block rotations up to 53 degrees. In portions of the study area, extension and rotation were supplanted by strike-slip faulting as deformation proceeded. We develop a tectonic model for this Coastal Sonora fault zone, which is bounded by major NW-striking transform faults with > 10 km of displacement. Internal to the Coastal Sonora fault zone, the majority of an estimated 6.2 +/- 1.1 km of dextral deformation, associated with up to 5.7 km of WNW-directed extension, occurred over the fi nal 1-2 m. y. of its life span, at a strain rate approaching 10(-14) s(-1). This activity occurred as the plate boundary localized along nascent pull-apart basins in the northern Gulf of California, consistent with the hypothesis that late protoGulf of California dextral shear zones, such as the Coastal Sonora fault zone, acted to focus lithospheric-scale strain and promoted continental rupture in the wide-rift setting of the Mexican Basin and Range.