Geologic mapping, age determinations, and geochemistry of rocks exposed in the Abiquiu area of the Abiquiu embayment of the Rio Grande rift, north-central New Mexico, provide data to determine fault-slip and incision rates. Vertical-slip rates for faults in the area range from 16 m/m.y. to 42 m/m.y., and generally appear to decrease from the eastern edge of the Colorado Plateau to the Abiquiu embayment. Incision rates calculated for the period ca. 10 to ca. 3 Ma indicate rapid incision with rates that range from 139 m/m.y. on the eastern edge of the Colorado Plateau to 41 m/m.y. on the western part of the Abiquiu embayment. The Abiquiu area is located along the margin of the Colorado Plateau–Rio Grande rift and lies within the Abiquiu embayment, a shallow, early extensional basin of the Rio Grande rift. Cenozoic rocks include the Eocene El Rito Formation, Oligocene Ritito Conglomerate, Oligocene–Miocene Abiquiu Formation, and Miocene Chama–El Rito and Ojo Caliente Sandstone Members of the Tesuque Formation (Santa Fe Group). Volcanic rocks include the Lobato Basalt (Miocene; ca. 15–8 Ma), El Alto Basalt (Pliocene; ca. 3 Ma), and dacite of the Tschicoma Formation (Pliocene; ca. 2 Ma). Quaternary deposits consist of inset axial and side-stream deposits of the ancestral Rio Chama (Pleistocene in age), landslide and pediment alluvium and colluvium, and Holocene main and side-stream channel and floodplain deposits of the modern Rio Chama. The predominant faults are Tertiary normal high-angle faults that displace rocks basinward. A low-angle fault, referred to as the Abiquiu fault, locally separates an upper plate composed of the transitional zone of the Ojo Caliente Sandstone and Chama–El Rito Members from a lower plate consisting of the Abiquiu Formation or the Ritito Conglomerate. The upper plate is distended into blocks that range from about 0.1 km to 3.5 km long that may represent a larger sheet that has been broken up and partly eroded. Geochronology ( 40 Ar/ 39 Ar) from fifteen volcanic and intrusive rocks resolves discrete volcanic episodes in the Abiquiu area: (1) emplacement of Early and Late Miocene basaltic dikes at 20 Ma and ca. 10 Ma; (2) extensive Late Miocene–age lava flows at 9.5 Ma, 7.9 Ma, and 5.6 Ma; and (3) extensive basaltic eruptions during the early Pliocene at 2.9 Ma and 2.4 Ma. Clasts of biotite- and hornblende-rich trachyandesites and trachydacites from the base of the Abiquiu Formation are dated at ca. 27 Ma, possibly derived from the Latir volcanic field. The most-mafic magmas are interpreted to be generated from a similar lithospheric mantle during rifting, but variations in composition are correlated with partial melting at different depths, which is correlated with thinning of the crust due to extensional processes.

Abstract Volcanic, sedimentary, and granitic plutonic rocks that are part of the early Mesozoic Cordilleran continental magmatic arc are exposed in a belt from the southwestern United States to Guatemala. In the Mexican states of Chihuahua, Coahuila, Durango, Zacatecas, and San Luis Potosí, these rocks form a discontinuous southeast-trending belt across north-central Mexico. Whole-rock geochemical analyses of volcanic and intrusive rocks in north-central Mexico indicate a calc-alkaline suite formed in this continental volcanic arc along the convergent margin of western North America. Paleomagnetism, field relations, and isotopic ages ( 40 Ar/ 39 Ar, K-Ar, Rb-Sr, and U-Pb) of 73 volcanic and intrusive rocks document the Late Triassic–Middle Jurassic age of the arc. In the region, isotopic ages commonly are reset, apparently because of thermotectonic events during the “Laramide” orogeny that led to the development of the Sierra Madre Oriental fold and thrust belt and to deep burial of the arc rocks. Available evidence suggests that the arc underwent two main phases of subsidence. One phase of extensional subsidence created intra-arc basins and a peak of volcanism throughout the arc in the Early–Middle Jurassic. A second phase began in the Oxfordian, with subsidence and initial deposition of the Zuloaga and La Gloria Formations. Continued sedimentation during this phase led to accumulation of 5–7 km of strata above the arc, as Cretaceous seas transgressed westward over inland Mexico. The similarities in age, depositional environment, clastic composition, magma types, and geochemical affinity and, more importantly, the tectonic settings that gave rise to the Nazas Formation in Mexico and La Quinta and Girón Formations in Venezuela and Colombia suggest that these two volcanic-sedimentary sequences, now hundred of kilometers apart, were once part of the Late Triassic–Jurassic continental magmatic arc. This arc extended from Alaska to South America and evolved during simultaneous subduction along the western margin of Pangea, rifting in the Caribbean–Gulf of Mexico region, and associated large-scale transpressive activity.