Abstract

The Rio Grande rift is one of the major late Cenozoic continental rifts of the world, sharing most geophysical, geochemical, and geological characteristics with other rifts. Cenozoic evolution of the rift was synchronous with lithospheric plate interactions along and under the western North American margin: Paleocene-Eocene: Laramide primarily amagmatic compression related to flat-slab subduction; Oligocene: intermediate to silicic volcanism related to collapse of the slab; Miocene to present: rifting related to complex plate interactions overprinted on the previous history. Cenozoic paleogeographic and paleotectonic characteristics are consistent with a passive-mantle mode of rifting.

North-central New Mexico provides a unique opportunity to constrain models for rift initiation and evolution. It is one of the few locations within any rift where excellently exposed pre-rift and syn-rift basin fill has been studied thoroughly enough to allow detailed paleogeographic reconstruction for almost the entire Cenozoic.

Cenozoic paleogeography for the study area is summarized as follows: (1) Eocene (58-37 Ma): a single amagmatic sedimentary basin (El Rito-Galisteo) trended northwest-southeast with Laramide basement uplifts on three sides; (2) early to late Oligocene (37-28 Ma): intermediate magmatism with volcaniclastic aprons derived from the San Juan and Ortiz-Cerrillos volcanic fields, and residual Laramide uplifts; (3) late Oligocene-early Miocene (28-21 Ma): initiation of bimodal volcanism with widely dispersed volcaniclastic aprons derived from the predominantly silicic San Juan and Latir volcanic fields; (4) early to middle Miocene (21-15 Ma): continued volcaniclastic dispersal from silicic volcanic centers, concurrent with initiation of block faulting to form half grabens, internal drainage, and erosion of Phanerozoic strata and Precambrian basement; (5) middle to late Miocene (15-8 Ma): continued deepening of half grabens, widespread exposure of Precambrian terranes, formation of complex depositional environments in basin centers, and continued bimodal volcanism; (6) late Miocene to present (8-0 Ma): concentration of extension in central grabens linked by accommodation zones, major bimodal volcanism (for example, Taos Plateau and Jemez Mountains), regional uplift, and integration of Rio Grande drainage.

Dispersal patterns, petrofacies analysis, K-Ar ages, and chemical analyses of volcanic clasts provide details concerning three primary volcanic centers: (1) San Juan Mountains (27-29 m.y., high-K andesite and rhyodacite); (2) Latir volcanic field (25-28 m.y., high-K andesite and rhyolite); and (3) the previously unrecognized Servilleta Plaza center (22-23 m.y., latite, high-K andesite and rhyodacite), which may have been a southern extension of the Latir field. These three volcanic centers provided detritus to the following units, respectively: (1) Esquibel Mbr. of Los Pinos Fm., upper Abiquiu Fm., middle Picuris Fm., Bishops Lodge Mbr. of Tesuque Fm. and volcaniclastic units in the northern Albuquerque basin (Zia and Abiquiu); (2) Cordito Mbr. of Los Pinos Fm. and uppermost Abiquiu Fm.; and (3) Chama-El Rito Mbr. of Tesuque Fm and upper Picuris Fm. Use of petrofacies (1. Esquibel, 2. Cordito, and 3. Plaza) simplifies the chaos of stratigraphic nomenclature and promotes regional correlations of poorly exposed units.

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