ROLE OF TECTONIC INHERITANCE AND MODE OF CONTINENTAL EXTENSION IN THE UPPER RIO GRANDE RIFT

The Arkansas River valley and the San Luis basin form two kinematically distinct segments of the Upper Rio Grande rift (RGR) and show an intimate relationship with preexisting Laramide thrusts. The two half-graben rift segments are separated by the Poncha Springs accommodation zone, which coincides well with a prominent right-step (~50 km) of the eastern limit of the Front Range that in turn marks a major reversal in vergence of Laramide thrusting. To the north of Poncha Springs both the Laramide thrusts and Neogene normal faults predominantly dip east, whereas to the south both generations of faults dip west. Reconnaissance mapping in the Sangre de Cristo Mts (Bruce and Johnson, 1991) show that some of the Laramide thrusts cross cut Early Miocene (?) dikes and others place Permian strata on Proterozoic basement. Therefore, it is likely that Laramide thrusts have been reactivated as west-directed low-angle normal faults in the Sangre de Cristo Mts and we propose that a similar reactivation of Laramide structures is responsible for the change in vergence of normal faulting across the Poncha Springs accommodation zone.

Continental extension in the RGR is widely considered to be a narrow-rift mode (Buck 1991) because the zone of extension rarely exceeds 75 km in width. However, numerous intrinsic parameters indicate that the northern RGR instead could have experienced core-complex mode of continental extension. High surface heat flow, high mantle temperatures(Gao et al., 2004), slow extension rates and relatively thick continental crust (~45 km; Gilbert and Sheehan, 2004) would permit lower crustal flow, which is considered necessary for core-complex mode extension. Additionally the Sangre de Cristo range front shows well developed corrugations that become progressively larger in amplitude as the total extension and basin width increase to the south from Poncha Springs. The elevation of the Sangre de Cristo footwall is systematically higher in antiformal corrugations compared to synformal corrugations, which may suggest that they have been amplifying during the formation of the rift. The corrugated range front fault and abundant syn-extensional magmatism strongly suggest that the northern RGR may be better classified as an incipient metamorphic core complex rather than a narrow rift.