New Mexico Geological Society Annual Spring Meeting — Abstracts

Relationships between Neogene volcanism and faulting in the Jemez Mountains have been only poorly defmed. Recent mapping indicates, however, that upper Miocene Bearhead Rhyolite magmatism was coeval with, and spatially related to substantial subsidence along NNW intra-rift faults and NE-striking reactivated basement structures along the Jemez Lineament Extrusion of Bearhead Rhyolite flows/domes and cogenetic Peralta Tuff Member pyroclastic deposits occurred mostly between 7.0-6.7 Ma. Although eroded in many places and buried to the south beneath younger fill of the Santo Domingo basin, Bearhead-Peralta eruptive products comprise a minimum volume of 35 km³ and may have I exceeded 100 km³. Several workers (most notably L. Justet, UNL V M.S. thesis, 1999) have suggested that the Bearhead Rhyolite may represent eruptions from a single magma chamber. Most vents are concentrated in an ~165 km², NE-elongated elliptical region parallel to, and just north of, a discontinuous zone of NE-striking faults, at least 30 km long, that coincides with the volcanic front of the southern Jemez Mountains and a regional geophysical lineament Northeast-striking faults are common north of, but are rare south of, this zone. This zone of faults and vents may more appropriately represent the southern margin of a broadly defmed Jemez Lineament, rather than the narrow line through the Valles Caldera that is typically depicted on maps. Most Bearhead Rhyolite magmatism was focused along the margins of the Bearhead basin, a 6.5 km-wide asymmetric, west-tilted graben that accommodated at least 700 m of volcaniclastic strata between about 7.0 and 6.2 Ma. Erosional remnants of relatively thin Peralta Tuff pyroclastic aprons are found near rhyolite vents outside of the basin but a thick sequence of pyroclastic deposits and tuffaceous sedimentary strata accumulated within the basin, demonstrating subsidence contemporaneous with late Miocene volcanism. The restricted extent of hypabyssal rocks of the Cochiti Mining District can now be explained by a footwall uplift along the NW margin of the Bearhead basin. Pre-Miocene(?) roof pendants in mineralized intrusions suggest > 1 km of uplift. Alteration in the mining district, and broadly through the southern Jemez Mountains, has been dated at 5-7 Ma (Woldegabriel and Goff, 1989; Geology, 11:986).

All of these observations suggest that a large, late Miocene magma chamber developed along the crustal-scale boundary represented by the Jemez Lineament. This magma chamber may have induced hydrothermal mineralization/alteration of a large region of the southern Jemez Mountains. Movement along intra-rift faults intersected this chamber and tapped it incrementally prior to sufficient evolution of a gas-rich cap that might have led to a single, voluminous pyroclastic eruption. Subsidence along these faults enhanced accumulation and preservation of volcaniclastic deposits recording this magmatic episode.