R. M. Chamberlin, S.M. Cather, M.W. Nyman, and V.T. McLemore

SUMMARY-- The Lemitar quadrangle covers the western side of the Socorro Basin and adjacent eastern flank of the Lemitar Mountains in central New Mexico, about 15 kilometers north of the town of Socorro. The channel and floodplain of the Rio Grande lie along the eastern margin of the quadrangle. Interstate 25 parallels the western edge of the floodplain and provides access to the small farming communities of Lemitar and Polvadera. The broad piedmont slope east of Polvadera Mountain, highest point in the Lemitar Mountains, descends gradually to the floodplain. This is an anomalous geomorphic relationship compared to the stepped piedmont slopes observed southwest of Lemitar.

            The Lemitar quadrangle lies within the central Rio Grande rift, where WSW-directed lithospheric extension over the last 29-32 million years has broken an Oligocene volcanic plateau into a north-trending array of tilted fault-block ranges and alluvial basins. Contemporaneous dip slip and westward rotation of early rift domino blocks is recorded in the Lemitar Mountains by wedge-shaped prisms of basaltic-andesite lavas (Tl_1-3) found between progressively less tilted ignimbrite sheets of late Oligocene age (28.7-27.4 Ma; Tj, Tv, Tll-Tlu, Tsc). Volcanic rocks of the Mogollon-Datil field in the Lemitar Mountains record the transition from subduction-related andesitic volcanism at about 39-37 Ma to increasingly bimodal basalt-rhyolite volcanism associated with early rifting between 32-27 Ma. Two or three episodes of domino-style extension have rotated originally high-angle, east-dipping normal faults to near horizontal attitudes (e.g. circular fault block 2 km NE of Polvadera Mountain).

            A complex structural/tectonic evolution is apparent in the Lemitar quadrangle. Strongly tilted domino blocks of the early Rio Grande rift are superimposed on a Laramide highland of early Tertiary age. Absence of Mesozoic strata and limestone-cobble conglomerates of the Eocene Baca Formation (Tb) record uplift and erosion of the Lemitar Mountains area at this time (Sierra uplift of Cather, 1983). The Polvadera thrust fault, exposed SE of Polvadera Mountain, represents N-S crustal shortening associated with the Ancestral Rocky Mountains orogeny in middle Pennsylvanian (Atokan) time. Middle Proterozoic granites, diorites, gabbros, gneisses and metasediments form the crystalline core of the Polvadera Mountain block. Northeast trending high-angle contacts between these units may reflect ductile and brittle shear zones of middle to late Proterozoic age, locally reactivated or offset by younger tectonic events.

            Moderately tilted volcanic-rich conglomerates and playa claystones of the Miocene Popotosa Formation are preserved in tilt-block depressions within and adjacent to the Lemitar range. The ENE dipping Popotosa beds at Canoncito de las Cabras contain a 14.5 Ma rhyolitic ash bed correlative with a rhyolite lava dome located 35 km to the southwest near Magdalena. Local unconformities in the Popotosa section near las Cabras indicate that the low-angle Puerto fault was a major intrabasinal fault in Miocene time. Structurally higher, down-to-the-west normal faults repeat the metasomatized lower Popotosa conglomerates (Tpd) but then disappear northward into the finer-grained middle Popotosa strata. The anomalous northeasterly dip of Popotosa beds at Canoncito de las Cabras probably represents a large ramp structure associated with an eastward splay of the Silver Creek fault that was linked to a large north-trending low-angle fault zone farther to the east. The latter is now largely removed by erosion on the east-sloping face of Polvadera Mountain, but a few klippe-like remnants support its existence. Playa claystones west of Kelly Ranch (south of Canocito del Lemitar) contain a west-tilted 9.8-Ma basaltic-andesite lava that flowed onto the basin floor from the west. Gypsum beds within the thick claystone sequence west of Kelly Ranch demonstrate the closed basin nature of the Popotosa Formation. The correlation of Popotosa beds and facies between Kelly Ranch and Canoncito de las Cabras is presently uncertain.           

            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. Volcanic-rich conglomerates and sandstones of the lower Sierra Ladrones piedmont facies (Tsp at San Lorenzo arroyo) are distinguished from Popotosa conglomerates by a few sparse pebbles and cobbles of red, jasper-cemented volcanic conglomerate. These distinctive clasts were apparently recycled from metasomatized basal Popotosa conglomerates (Tpd) shortly after uplift and erosion of the northern Lemitar Mountains. Similar clasts of jasperized volcanic conglomerate occur in the lower Sierra Ladrones piedmont facies at Socorro Canyon, where it underlies a 3.8 Ma basalt flow.

Moderately deep wells (300-400 feet) west of I-25 tap the ancestral Rio Grande aquifer and provided over 36 million gallons of potable water to the Polvadera-Lemitar community water system in 1990. Map patterns imply that these high-yield aquifer beds (upper Sierra Ladrones axial-river facies, QTsf ) are thin or absent under the floodplain between San Acacia and San Lorenzo arroyo. At San Acacia, the relatively thin river deposits of Holocene age (Qvy, ~ 90 feet thick) represent the entire thickness of the high-yield aquifer. This Holocene aquifer is locally inset into less permeable, westerly transported distal piedmont facies of the lower Sierra Ladrones Formation (Tse), which is more than 5 million years old.

            The Socorro Canyon fault, an active high-angle fault zone of the Rio Grande rift, bounds the western subsided margin of the Socorro Basin and the eastern flank of the moderately west tilted Socorro-Lemitar uplift of Pliocene to Quaternary age. Map relationships along this fault zone, which is about 30 km long, suggest as much as 3000 feet of down-to-the-east displacement since about 5 million years ago. The Lemitar uplift and contiguous La Jencia Basin block, footwall to the Socorro Canyon fault, have been rotated about 5-10° to the west during this period. Late Pliocene to Quaternary doming and extensional subsidence, centered near San Acacia, is suggested by SW tilt of Sierra Ladrones beds (Tsp, Tsf) near Alamillo and of ESE tilt of the 4.9 Ma trachyandesite flow (Tas) exposed north of Bowling Green. The latter represents a significant local reversal of the dominant westerly tilt pattern previously active from 29-5 million years ago.

            The Lemitar quadrangle lies above a large pancake-shaped mid-crustal magma body, which is geophysically defined at a depth of 19 km.  Leveling surveys along rail lines and satellite-based radar interferometry data (InSAR) indicate the ground surface near San Acacia and Lemitar is rising at 2-3 mm per year over the last 80 years. This surface deformation may reflect recent inflation of the magma body by replenishment from the upper mantle, or it may reflect longer term doming of the upper crust in response to more buoyant secondary magmas.  Quaternary fault zones above and adjacent to the magma body primarily represent the long-term plate-tectonic stress field. However, the interaction of magmatic stresses with the regional tectonic stress field may locally increase the seismic risk in the Socorro region. The geomorphic anomaly east of Polvadera Mountain could reflect relatively rapid uplift of the mountain block in late Quaternary time.