INTERPLAY OF FAULTING, VOLCANISM, AND SEDIMENTATION IN THE SOUTHEASTERN JEMEZ MOUNTAINS VOLCANIC FIELD, NEW MEXICO

The Neogene-Quaternary Jemez Mountains volcanic field (JMVF) has long been broadly associated with the intersection of N-striking Rio Grande rift (RGr) faults and the NE-striking Jemez lineament, a late Cenozoic volcanic alignment coincident with a Proterozoic suture. Quadrangle mapping and stratigraphic studies show that intersecting N- and NE-striking faults form a patchwork of polygonal fault blocks in the southeastern Jemez Mountains; most vents are located along these faults. The southernmost NE-striking faults coincide with the southern volcanic front of the JMVF, indicating a strong correlation between volcanism and basement structure. NE-striking faults form transfer and termination structures to the more continuous N-striking faults. NE-striking faults were mostly active prior to 1.6 Ma whereas N-striking faults have a longer Neogene-Quaternary history (except for a NE-striking segment of the Pajarito fault that also demonstrates large Quaternary displacement). Volcaniclastic sediments coeval with JMVF volcanism are thickest in the northward projection of RGr sub-basins but accumulation space for sediment accumulation was abruptly diminished northward across NE-striking cross faults.

The Bearhead basin is the best example of this relationship where a 25 km² polygonal basin subsided to accumulate 550 m of 6-7 Ma pyroclastic and sedimentary deposits along with local lava of the Bearhead Rhyolite. Although this brief magmatic episode is recorded by isolated intrusions and outcrops of coeval Peralta Tuff over a large area of the southern JMVF, the Bearhead basin is the only place where a complete stratigraphy of multiple eruptive events (>50) with interbedded sedimentary deposits accumulated and was preserved. The depositional pattern of the basin fill indicates that pyroclastic/sedimentary accumulation outpaced subsidence to maintain an overfilled state. The pyroclastic deposits provide a unique record of the fall, flow, and surge processes associated with Bearhead Rhyolite volcanism that was mostly eroded away elsewhere in the JMVF, including hydrovolcanism where rhyolite magma encountered aquifers in the RGr basin fill. Therefore, faults not only partly explain vent distributions but also controlled the nature of the preserved volcanic record and eruption processes.