Geology of Organ Mountains and southern San Andres Mountains, New Mexico

The Organ and southernmost San Andres Mountains in south-central New Mexico are part of a 150-mi long, west-tilted fault block extending from El Paso, Texas, northward to central New Mexico. The Organ Mountains tower nearly a mile above the floor of the Tularosa Basin on the east and the Rio Grande valley on the west. Oldest rocks exposed are Precambrian granite, overlain by as much as 8,500 ft of mostly marine Paleozoic and Cretaceous strata. In Laramide time, these rocks were deformed along the faulted margins of a basement-cored block uplift which was ancestral to the modern Organ-San Andres range. Lower Tertiary fanglomerate, at least 2,000 ft thick, records the erosional "unroofing" of the Laramide uplift and was sharply deformed as uplift progressed. Magmatism profoundly affected the area in middle Tertiary time. Intermediate composition lava and mudflow breccia of late Eocene or early Oligocene age buried the deeply eroded Laramide uplift. The Organ batholith (about 32.8 m.y. old) is interpreted to be an exposure of the upper, outer part of a magma chamber whose silicic cap erupted as pyroclastic flows and lavas 33.0-33.7 m.y. ago. The pyroclastic rocks, approximately 2 mi thick, subsided to form the Organ cauldron, only a piece of which is exposed in the Organ Mountains today. The pyroclastic rocks also formed the roof beneath which the batholith crystallized. Compositional zoning within the volcanics indicates that the erupted magma volume was also zoned, ranging from 77 percent SiO2 at the top to about 65 percent at lowest levels. The batholith represents deeper, less silicic (56-68 percent) levels of the magma chamber, and late-stage mineralized plutons within the batholith are interpreted to be products of the progressive crystallization of the magma chamber. Undated tristanite-trachybasalt flows interbedded with fanglomerate may indicate onset of late Tertiary block faulting in the area. Subsequent uplift of the modern ranges involved an early stage of closely spaced faulting associated with moderate west tilting or, locally, east downwarping. Some faults, initially steep, were rotated into low-angle positions as tilting progressed. The more recent stage of uplift is distinguished by development of the modern, widely spaced range-boundary faults and their associated horsts, west-tilted blocks, and grabens. Movement on the eastern range-boundary fault of the Organ and southern San Andres fault block has persisted to within the last 4,000-5,000 yrs. Several generations of alluvial fans, as old as middle Pliocene, are products of repeated movement on these faults. Mineral deposits related to the Organ batholith include base-metal deposits as replacements in limestone and dolomite, and as disseminations, vein fillings, and pegmatite minerals in the batholith.