Geologic map of the Luis Lopez 7.5 minute quadrangle, Socorro County, New Mexico

Richard M. Chamberlin and Theodore L. Eggleston

New Mexico Bureau of Geology and Mineral Resources Open-File Report 421, 1996 (revised 1999)

ABSTRACT- The Luis Lopez quadrangle lies within the southern section of the Rio Grande rift, where two pulses of WSW directed lithospheric extension have broken a subduction-related Oligocene caldera complex into a north-trending array of tilted fault-block ranges and alluvial basins. Topographic elements of the quadrangle include the narrow north-trending Chupadera Mountains, and adjacent piedmont slopes of the Broken Tank and Socorro basins. The latter is represented by a dissected and faulted bajada-slope that descends to the floodplain of the Rio Grande at the east margin of the quadrangle.

The strongly east-tilted Chupadera block, mostly of Miocene age, locally provides a cross-section like view of the southeastern sector of the 31.9 Ma Socorro caldera. As much as 2 km of lithic-rich caldera facies Hells Mesa Tuff exposed at Red Canyon locally defines the collapsed core of the Socorro caldera. Bedded autoclastic ignimbrite breccias and intercalated fall deposits at the top of the intracaldera Hells Mesa probably reflect a brief resurgent phase. Autoclasts in the upper ignimbrite are texturally and compositionally identical to a coarsely porphyritic, spherulitic lava flow of Hells Mesa age, which is partially exposed in the southern moat-like area, near Nogal Canyon.

Sedimentary and volcanic units that back filled the Socorro caldera, prior to the next major ignimbrite eruption (28.65 Ma La Jencia Tuff), are collectively assigned to the Luis Lopez Formation. Distribution patterns of rhyolitic to andesitic conglomeratic sandstones in the lower Luis Lopez are consistent with derivation from a resurgent dome and the southern caldera wall. Clasts of pre-caldera rhyolite (33.7 Ma) occur in the wall-derived facies of this moat-fill unit, which was deposited during a lull in volcanism (31.9-30.2 Ma). Volcanic strata of the Luis Lopez formation consist of early trachybasalt lavas, medial lithic-rich rhyolitic ignimbrites (30.04 Ma), andesitic porphyry lavas, and late-stage flow-banded rhyolite lava domes (28.8-28.65 Ma). Ages of Luis Lopez volcanic units, imply that they are premonitory events related to a developing magma system that culminated in eruption of the 28.65 Ma La Jencia Tuff from the adjacent Sawmill Canyon caldera. The eastern topographic wall of the Sawmill Canyon caldera is partially exposed near Black Canyon; here breccias and conglomerates derived from the Luis Lopez formation are overlapped by 27.7 Ma Lemitar Tuff at the caldera wall.

Moderately tilted volcanic-rich conglomerates and playa claystones of the Miocene Popotosa Formation are preserved in tilt-block depressions within and adjacent to the Chupadera range. Intercalated dacite and trachybasalt flows of late Miocene age help define local unconformities associated with fault-block highlands in the early rift basins. The 8.42 Ma basalt of Broken Tank flowed westward across the Chupadera block on inset piedmont gravels of the Popotosa Formation near Walnut Creek and Broken Tank; it then flowed northward onto playa muds near Bear Canyon.

Episodes of hydrothermal alteration and mineralization were locally associated with shallow silicic magmatism and volcanism of Oligocene and late Miocene age. Potassium-metasomatism and slightly younger manganese-oxide vein deposits in the northern Chupadera Mountains appear to be temporally and spatially linked to late Miocene rhyolitic magmatism in the Socorro Peak area, about 6 km north of the quadrangle.

Pliocene and Pleistocene sedimentary deposits of the Sierra Ladrones Formation record the transition from early-rift closed basins to late-rift, valley-fill deposits of the ancestral Rio Grande and its tributary drainages. Important aquifers within the quadrangle include a newly recognized fluvial-fan deposit of early Pliocene age (ca. 5 Ma) apparently emanating from the eastern Magdalena Mountains, and well known sands and gravels of the ancestral Rio Grande (axial facies of Sierra Ladrones Formation). Aquifer units, possibly as much as 300 m thick, are locally preserved on subsiding, high-angle fault blocks near the eastern margin of the quadrangle.