Ocate volcanic field

The Ocate volcanic field (also known as the Mora volcanic field) is a monogenetic volcanic field located in the eastern Sangre de Cristo Mountains of northern New Mexico.[1] About 8.12 million years ago, basaltic volcanoes began to erupt in the Ocate area. Fourteen eruptive pulses have been identified using Ar-Ar dating, with the most recent taking place around 0.67 to 0.95 Mya.[2][3]

The lavas erupted in the field are mostly alkali olivine basalt or transitional olivine basalt, with smaller quantities of basaltic andesites, olivine andesites, and dacites. The basaltic lavas were generated from heterogeneous source regions in the mantle, while the more silica-rich andesites and dacites formed from fractional crystallization of basaltic magma and mixing with silica-rich magma from partial melting of crust.[4]

The volcanic field lies along the Jemez Lineament, a zone of young volcanic activity stretching from central Arizona to northeastern New Mexico. The lineament is thought to be a zone of weakness in the lower crust and upper mantle. The vents in the eastern part of the field are aligned roughly east to west, perpendicular to the direction of least compression in the southern Great Plains. Vents in the western part of the field are aligned more randomly, probably because of recent tectonic disturbance of this area.[3]

An unusual feature of the Ocate volcanic field is that the oldest basaltic flows form the highest mesas, and the youngest flows form surfaces lower in elevation. This reverse stratigraphy occurred because after the older flows were erupted about 8.3–5.7 m.y. ago on a relatively flat alluvial plain, the entire area was uplifted, and canyons were cut into the older basalt flows. Younger basalt flows then filled the major stream canyons. When uplift ceased, the youngest basalt flows of the Ocate volcanic field flowed on top of intermediate-aged flows.[2][3]