Geology of the Luis Lopez 7.5' Quadrangle, Socorro County, New Mexico:

Richard M. Chamberlin, Ted L. Eggleston and William C. McIntosh         

            SUMMARY-- The Luis Lopez quadrangle lies within the central Rio Grande rift, where WSW-directed lithospheric extension over the last 29 million years has broken an Oligocene ignimbrite caldera cluster into a north-trending array of tilted fault-block ranges and alluvial basins by a process called domino-style extension. The Socorro accomodation zone (SAZ of Chapin, 1989), a regional ENE-trending tilt-block domain boundary, is here reinterpreted to cross the northern segment of the Chupadera Mountains, about 5km south of its previous interpretation. Major topographic elements of the quadrangle include the narrow north-trending Chupadera Mountains and the 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 central and southern Chupadera block provides a cross-section like view of the southeastern sector of the 24-km diameter Socorro caldera, source of the 1200 km3 31.9-Ma Hells Mesa Tuff. As much as 2 km of xenolith-rich, phenocryst-rich, rhyolite ignimbrite in the lower caldera-facies Hells Mesa Tuff defines the collapsed core of the caldera at Red Canyon. Moderate volume (~50km3) upper caldera-facies Hells Mesa Tuff was formed by many small pulsed ash-flow deposits that contain comagmatic lag breccias with abundant clasts of phenocryst-rich spherulitic rhyolite (representing Hells Mesa magma, rapidly chilled along shallow vent walls and then explosively erupted). Many thin ash-fall beds in the upper Hells Mesa Tuff mark periods of repose as the Hells Mesa magma chamber became depleted in exsolved gases that drove the waning-stage eruptions (Chamberlin 2001a). Post-collapse volcanism was anomalously brief. Only one small (~0.3 km3) 31.9-Ma ring-fracture lava dome, associated with incipient resurgent uplift, is present in the southeast sector of the caldera, near the Esperanza mine.        

            Sedimentary and volcanic units that back filled the Socorro caldera, prior to the next major ignimbrite eruption (28.7-Ma La Jencia Tuff), are collectively assigned to the Luis Lopez Formation. Basal Luis Lopez volcaniclastic sediments filled the shallow moat during an extended lull in volcanism (31.9 to ~ 30.5 Ma). Relatively primitive trachybasalt lavas (9.3% MgO, 170 ppm Ni) ponded in the southeastern moat shortly after renewed magmatic uplift of the caldera core at about 30.5 Ma. Basaltic eruptions signaled replenishment or initiation of a new crustal magmatic system under the now crystallized Hells Mesa magma body. Minor eruption of a more evolved basaltic andesite lava occurred in the northeast moat, just prior to eruption of a series of moderate volume (>10km3) pumiceous, phenocryst-poor, rhyolite ignimbrites from a small collapse structure (Black Canyon vent area) nested in the northern moat, at 30.0 Ma. Mafic-rhyolite lava (69 % SiO2, 70 ppm Cr), which formed by mixing the earlier basaltic andesite magma with the pumiceous rhyolite magma, was then erupted on the flanks of a central horst block during a brief lull in the pumiceous tuff eruptions. Following a second period of minor erosion and sedimentation, medium porphyritic and coarsely porphyritic intermediate lavas were erupted from NE-striking fissure vents in the northeast moat. Several high-silica rhyolite lava domes were then erupted from reopened ring-fractures of the Socorro caldera between 28.8-28.7 Ma, just prior to massive eruptions of the 28.7-Ma La Jencia Tuff (1250 km3) from the adjacent Sawmill Canyon caldera, which obliterated most of the western half of the Socorro caldera.

            The eastern topographic wall of the Sawmill Canyon caldera is partially exhumed near the Tower mine.  Ancient colluvial breccias, conglomerates and sandstones of the Sawmill Canyon Formation, derived from Luis Lopez volcanics, are overlapped by 27.9-Ma Lemitar Tuff at the caldera wall. Distal lavas of the La Jara Peak Basaltic Andesite and the South Canyon Tuff outflow sheet were the last regional Oligocene volcanic units to flow across the mostly buried moat of the Socorro caldera at 27.3 Ma.

            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. An intercalated dacite flow and two trachybasalt flows of late Miocene age (~ 11, 9.7 & 8.4 Ma, respectively) help define several local unconformities associated with fault-block highlands in the early rift basins. The 8.4-Ma basalt of Broken Tank flowed westward across the mostly covered Chupadera block on inset piedmont gravels of the Popotosa Formation near Walnut Creek; it then flowed northward down the axis of the Broken Tank basin onto playa muds near Bear Canyon. The topographically highest point in the Chupadera Mountains (1984 m, 6510 ft above msl.) is formed by caldera-facies Hells Mesa Tuff immediately adjacent to an 8.4 Ma paleovalley wall locally preserved by the basalt of Broken Tank.

            Episodes of hydrothermal alteration and mineralization were locally associated with shallow silicic magmatism and volcanism of Oligocene age (31.9, 30.0 and 28.7 Ma) and late Miocene age. Late Miocene potassium metasomatism at ~ 7.4 Ma (Dunbar and Miggins, 1996), and slightly younger manganese mineralization (6.6 Ma; Lueth et. al., in press) in the northern Chupadera Mountains appear to be temporally and spatially linked to five pulses of silcic to intermediate magmatism and volcanism (at 9.5, 8.7, 7.9, 7.4, 7.0, and 6.9 Ma) in the Socorro Peak area, about 6 km north of the quadrangle. Popotosa playa mudstones presumably formed a thick impermeable cap on the late Miocene hydrothermal system, allowing the upwelling hot waters to migrate laterally from their source.

            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 (Tsft) of early Pliocene age apparently emanating from the eastern Magdalena Mountains, and well-known sands and gravels of the ancestral Rio Grande (axial facies of Sierra Ladrones Formation, QTsf). 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. Moderately deep wells that tap the ancestral Rio Grande aquifer west of I-25 provide over 27 million gallons of potable water to the San Antonio water system each year (Ramsey, 1994). The solid-waste landfill for the city of Socorro, a potential source of groundwater pollution,  is located in the unsaturated zone of this primary aquifer (QTsf) about 1 km northwest of Luis Lopez and about 30m above the water table.

            The north-trending Socorro Canyon fault, an active fault zone of the Rio Grande rift, displaces middle to late Pleistocene piedmont-slope deposits in the area west of  I-25, about 2 km west of Luis Lopez (Chamberlin and Harrison, 1996). Recent isotopic-dating studies of this active fault at Socorro Canyon (Ayarbe, 2000) indicate the most recent major displacement events (~2 m each) occurred at 92 16 ka and 28 20 ka, equivalent to a maximum slip-rate of about 0.3 mm/yr. Earthquake risk may be increased by ongoing uplift (~ 2-3 mm/yr at San Acacia; Fialko and Simmons, 2001) and stretching of the Socorro region above the extensive modern-day Socorro magma body, which has been  geophysically mapped at a depth of 19 km (Balch.et.al., 1997).