OBLIQUE EXTENSION AND BASINWARD TILTING ALONG THE CAñONES FAULT ZONE, WEST MARGIN OF THE RIO GRANDE RIFT

The Cañones 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 Cañones Fault, and a series of synthetic and antithetic normal faults within the Abiquiu Embayment in the northwestern Española Basin. The Cañones Fault is a SE-dipping high-angle normal fault, striking ~ N20E in the south, N40E in the middle, and E-W in the northern end. The synthetic and antithetic faults are sub-parallel to the major fault. Detailed fault kinematic studies reveal that the trends of slickenlines on the Cañones Fault range N75E - S70E, and average ~ S80E. Slickenlines on antithetic faults trend S70W - N50W, clustering at ~ N80W. The attitude of fault surfaces and slickenlines indicate nearly E-W extension within the Cañones Fault Zone. The sense of motion on the major fault is normal with left-slip. Given the fault throw of 355 m, the heave is calculated as ~ 206 m and the sinistral displacement is ~ 118 m.

In contrast to sub-horizontal Permian-Triassic units in its lower plate, the upper plate of the Cañones fault zone is tilted in two directions: W-dipping Jurassic Entrada, Todilto, and Morrison Formations, and SE-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 dip 10°~40° away from the Cañones Fault, instead of dipping NW to the Cañones Fault. Field observation provides direct evidence that basinward tilting is accommodated by multiple antithetic normal faults that cut through Ritito to Entrada Formations. Shear strain ranges 0.176 - 0.839, providing the Ritito and Abiquiu Formations were deposited horizontally. The basinward tilting may indicate an asymmetric half-graben in the Abiquiu Embayment, with its master fault to the SE of the Cañones Fault, possibly buried beneath the Jemez Volcanic Field. Alternatively, basal shearing from upwelling asthenosphere may trigger the bedding-parallel sliding and horizontal axis-rotation in the upper crust. This study highlights a rift-bounding fault zone where tilting of the upper plate is not controlled by the master fault, but instead by antithetic faulting.