TALES OF THE DEADMAN THRUST: NORTHEAST VERGING FOLDS, INJECTION BRECCIAS, AND HIGH-ANGLE NORMAL FAULTS IN THE RIFT-FLANKING BASEMENT OF THE SANGRE DE CRISTO MOUNTAINS, COLORADO

The role of pre-existing structures in the inception and evolution of the Rio Grande rift has long been postulated. However, the rift-flanking basement is structurally heterogeneous due to many episodes of pre-rift tectonism. The Deadman thrust, near Crestone, comprises a set of well-exposed structures providing relative timing and kinematic data useful for exploring reactivation, rift evolution, and associated processes.

Previously proposed genetic links between the Deadman thrust and an inferred low angle, normal slip detachment fault 75 km south at the San Luis mine were based on exposures of green gouge and enigmatic breccias found at both localities. However, NE directed contraction is indicated at Deadman. The thrust has a shallow SW dip and NW strike, demarcated by a folded quartz-chlorite mylonite with NW striking axial planes that verge to the NE. Co-planar brittle slip surfaces are superimposed on the mylonites, and are also indicative of NE slip. The upper plate is a brittley deformed sandstone of middle Ordovician age. At the mouth of Deadman Creek the thrust surface is planar, but isoclinally folded to the SE, also with a NE verging axial plane. Within the isocline, the basement core shows a pervasive network of chlorite-rich cataclastic veins.

At Deadman Creek, the thrust is clearly cross-cut and intruded by a chlorite-rich breccia. The breccia body thickens from a few to tens of meters, with several emanating dikes. Small-displacement, normal faults strike NW and cut the breccia as well as the thrust. Borehole, seismic and potential-field data show a set of NW striking, steeply SW dipping normal faults that progressively drop the Deadman thrust surface into the rift basin. These data also indicate that basin-fill strata dip shallowly toward the basin axis to the west.

No evidence of low angle extensional faulting has been found to date at the Deadman locality–kinematic data are consistently indicative of NE directed Laramide shortening. Sub-rounded gneissic clasts in the breccias may have formed in response to high pore fluid pressures associated with cataclastic flow during progressive fold tightening of the thrust. High-angle normal faults that cut the thrust and breccias, and lack of strata back-tilting suggest that extensional reactivation was not involved in the evolution of this portion of the rift.