CENOZOIC EXTENSION ACROSS THE WESTERN RIO GRANDE RIFT MARGIN IN THE NORTHERN JEMEZ MOUNTAINS AND ADJOINING COLORADO PLATEAU, NORTH-CENTRAL NEW MEXICO

The northeast-striking Cañones fault zone (CFZ) has traditionally been viewed as the main boundary between the Colorado Plateau and the Rio Grande rift in the northern Jemez Mountains. However, normal faults with vertical stratigraphic offsets of ~3 to 110 m occur in the Chama Basin on the Colorado Plateau as far west as the village of Coyote, ~20 km west of the CFZ. A similar broad zone of distributed deformation ~17 km-wide has previously been recognized to the east of the CFZ in the Abiquiu embayment. The Cañones fault zone was an east-side-up reverse fault during Laramide deformation. Previous workers determined that the reverse fault was reactivated as an east-down normal fault in late Oligocene time (28-25 Ma). Deformation across the rift margin continued during Miocene deposition of the Santa Fe Group (SFG) and into Pliocene time. SFG sediments thicken dramatically from ~90 m on Cerro Pedernal to >300 m on Polvadera Mesa across the CFZ towards a prominent gravity low to the southeast. Thickening of the Tesuque Fm of the SFG, which includes, from oldest to youngest, the Chama El Rito Mbr, the Ojo Caliente Sandstone Mbr, and an unnamed fluvial unit, is particularly pronounced. For example, the Ojo Caliente Sandstone appears to be absent on Cerro Pedernal in the footwall of the Gonzales fault, a splay of the CFZ, but is ~120 m-thick in the hanging wall on Mesa Escoba. Basalt and andesite lava flows that are 7-8 Ma rest on the Chama-El Rito Mbr on Cerro Pedernal, on the Ojo Caliente Sandstone and younger Hernandez Mbr (Chamita Fm of the SFG) on Mesa Escoba, and solely on the Hernandez Mbr further east on Polvadera Mesa. Approximately 670 m of down-to-the-east faulting disrupted the 7-8 Ma lava flows. 2.8-3.0 Ma basaltic andesite and dacite lavas that flowed across the CFZ are generally not offset. Faults 3-10 km east of the CFZ have displaced a 3.8 Ma dacite by 100 m and a 5.6 Ma basalt by 11 m. In summary, a nearly 40-km-wide zone of faulting rather than a single structure defines the western margin of the Rio Grande rift, and this zone has experienced a long history of activity. We infer relatively high displacement rates for this zone in early to middle Miocene time, and a progressive decrease afterwards because: 1) the late Miocene Chamita Fm in the Abiquiu embayment is relatively thin; and 2) < 7-8 Ma lava flows are mildly deformed compared to >7-8 Ma flows.