The Colorado River Extensional Corridor: An Analog for the Early Stages of Passive Margin Formation

BERATAN, KATHI K., Jet Propulsion Laboratory, Pasadena, CA, and JANE E. NIELSON, Jet Propulsion Laboratory and U.S. Geological Survey, Menlo Park, CA

Structural asymmetry of many passive continental margins has recently led to the concept that the initial stages of rifting (extension and thinning of continental crust) occur on large-scale, low-angle normal (detachment) faults. Similarities in geometry and evolution of passive margins and detachment-related basins in the Colorado River extensional corridor (CRec), California-Arizona, suggest that development of the CRec resembles early stages of passive margin formation.

Two major sets of high-angle synrift faults have been described from the upper plates of passive margins--rotational normal faults and an orthogonal set of transfer faults. Transfer faults, oriented approximately parallel to extension direction, divide an extending terrane into structural blocks which are tilted by the rotational normal faults. Similarly, the generally rectilinear shapes of syn-detachment sedimentary basins or subbasins within the CRec suggest control by two orthogonal fault sets. One set, oriented perpendicular to tectonic transport direction, broke the region into half-graben. These faults, and thus the basins, terminated against faults oriented approximately parallel to tectonic transport direction.

The CRec basins experienced a two-stage history, with an early stage of active extension, which brought mid-crustal rocks to near-surface positions, followed by an adjustment stage, during which topographic relief between the range cores and the basins increased. If this adjustment resulted from differential subsidence within the extended region rather than from uplift of range cores, then the two-part evolution resembles that of classic rifted margins: an initial active stage followed by subsidence.

AAPG Search and Discovery Article #91009©1991 AAPG-SEPM-SEG-SPWLA Pacific Section Annual Meeting, Bakersfield, California, March 6-8, 1991 (2009)