Abstract: Tectonic Setting of the Axial Basins of the Northern and Central Rio Grande Rift

Charles E. Chapin, Steven M. Cather

The axial basins of the northern and central Rio Grande rift evolved since late Oligocene as a chain of half grabens between the Colorado Plateau on the west and the interior of the craton on the east. The basins range from 80 to 240 km in length and from 5 to 95 km in width, with an average width of approximately 50 km. Basin-fill sedimentary deposits range up to 5-6 km in thickness. The oldest dated volcanic rocks interbedded with synrift sediments are about 26 Ma. Extension in the rift was left-oblique and increases southward from 8-12% in the San Luis Basin, to about 17% in the northern Albuquerque Basin, to at least 28% in the southern Albuquerque Basin, to an average of 50% in the Socorro area. The most rapid phase of regional extension was middle to late Miocene. Sedimentary de osits of this age are volumetrically dominant within the rift basins and on the rift flanks (Ogallala and Bidahochi formations).

Basin asymmetry shifts back and forth from east-tilted to west-tilted across complex northeast-trending accommodation zones which developed along pre-existing transverse structural lineaments. These accommodation zones lie on small circles about the Miocene Euler pole of rotation for the Colorado Plateau relative to the stable craton. Rotation about this Euler pole in northeastern Utah caused synchronous episodes of Miocene extension and sedimentation in the Rio Grande rift and in the central Wyoming-northern Colorado area, largely by the collapse of Laramide uplifts. Late Cenozoic rotation of the Colorado Plateau was 1.0-1.5° clockwise.

Structural subdivision of basin floors within the rift by longitudinal master faults increased with increasing extension until at some point in excess of 28% extension one or more longitudinal fault blocks was uplifted isostatically through the basin surface to form an intrabasin tilted-block mountain range which subdivided the basin. Faulting and subsidence tended to migrate towards the basin axis as newer master faults cut off, or merged downdip with, earlier faults. This deactivated the outer longitudinal blocks and left them stranded as shallow suballuvial benches covered by relatively thin sections of the older synrift sediments.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994