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