Abstract

The pattern and history of normal faulting in a portion of northeastern Baja California strongly suggest that the area developed as a result of east-west crustal extension during late Cenozoic time. Geometric and kinematic data strongly suggest that this area developed in concert with the opening of the proto–Gulf of California and the distension of the Basin and Range Province. In addition, these data, coupled with regional-tectonic timing relations, preclude the application of an oblique rifting model to this area.

Two intervals of extensional tectonics have affected the region since the early Miocene. At present it is not known if strain was continuous or discontinous with respect to time. The first and most intense period, initiated between 17 and 9 m.y. ago, was east-west directed, as determined from tilt patterns, orientation of intra-terrane transcurrent faults, and fault-plane kinematic indicators. This deformation was dominated by high-angle normal faulting and resulted in the disruption of a region-wide, low-relief erosion surface. New patterns of sedimentation were established and controlled by north-trending grabens that filled with alluvial-fan and marine deposits. Sporadic silicic eruptions occurred in conjunction with the extension, thus allowing resolution of the complex history of tilting that accompanied normal faulting. Tilting and antithetic faulting resulted from the volume compensation required by the downward flattening of normal faults and by fault-related monoclinal flexing and reverse-drag effects. Extension during this interval was complete or substantially reduced by 6 m.y. B.P. The second interval of east-west extension, younger than 6 m.y. B.P., has been much less intense than the earlier event, yet it seems to have utilized the same structures. Silicic volcanism also waned during this interval, giving way to mafic eruptions related to rifting of the present Gulf of California.

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