Oblique Extension and Basinward Tilting along the Ca?ones Fault Zone, West Margin of the Rio Grande Rift

摘要

The Ca?ones fault zone in north-central New Mexico is a boundary between the Colorado Plateau to the west and the Rio Grande rift to the east. It consists of a major fault, the Ca?ones fault, and a series of synthetic and antithetic normal faults within the Abiquiu embayment in the northwestern Espa?ola basin. The Ca?ones fault is a southeast-dipping high-angle normal fault, striking ~N20°E in the south, N40°E in the middle, and east-west at its northern end. The synthetic and antithetic faults are sub-parallel to the major fault. Detailed fault kinematic studies from the master fault reveal that the trends of slickenlines range S85°E - S70°E, and average approximately S76°E. Slickenlines on antithetic faults trend S20°W - N30°W， clustering at ~ N70°W. The attitude of fault surfaces and slickenlines indicate east-southeast/west-northwest extension within the Ca?ones Fault Zone. The sense of motion on the major fault is normal dominantly with left-slip. Fault throw is at least 225 m, based on Mesozoic units as hanging wall and footwall cutoffs. Thus, the heave is as ~143 m and the left-lateral displacement is ~60 m, given the averaged fault attitudes. In contrast to sub-horizontal Permian-Triassic units in its footwall, hanging wall strata of the Ca?ones fault zone dips in two directions: west-dipping Jurassic Entrada, Todilto, and Morrison formations; and southeast-dipping Eocene El Rito, Oligocene Ritito, and Oligocene-Miocene Abiquiu formations. Tilted Jurassic strata suggest that the overall structure is monoclinal, probably resulting from Laramide orogeny shortening. The Eocene-Miocene basin fill sediments, surprisingly, dip 10° - 30° away from the Ca?ones Fault， instead of dipping northwest towards the fault. This phenomenon, in contrast to the prediction of the rollover structure, suggests a different mechanism on this fault zone. Field observation provides direct evidence that basinward tilting is accommodated by multiple antithetic normal faults that cut through Permian to Miocene units. We propose that extensional fault-propagation folding model is a possible mechanism to result in the regional tilting of the basin fill. During upward propagation of the fault tip, horizontal-axis rotation and antithetic and synthetic faulting occur within a triangle zone above the fault tip. Alternatively, a buried large-scale lowangle normal fault can also generate such basinward tilting. In this scenario, the Ca?ones fault and other southeast-dipping normal faults are antithetic faults that grow on the detachment. These hypothetical mechanisms take into account the antithetic faulting within a rift-bounding fault zone and can be indicative of the evolution of other rift basins in which basin fills dip to the axis, such as the eastern Espa?ola basin and San Luis basin in northern New Mexico and southern Colorado.