Geology of the Do�a Ana Mountains, south-central New Mexico: A summary

Abstract

Located a few kilometers north of Las Cruces in south-central New Mexico, the Dona Ana Mountains are part of a late Tertiary fault block that extends from the northern Franklin Mountains to San Diego Mountain. Uplift along the Jornada fault on its eastern side and subsidence along the Robledo fault into the northern Mesilla half graben has resulted in a ~15° westward tilt of the Dona Ana block. Early Pliocene to Quaternary axial-fluvial and piedmont-slope deposits, paleocanyon fill, and pediment veneers onlap bedrock around the perimeter of the range. Two easterly trending, dike-filled faults or fracture zones, both downthrown to the south, divide the range into three structural blocks. The northern, structurally highest block primarily consists of easterly trending, folded and thrust-faulted upper Pennsylvanian and Lower Permian sedimentary rocks, deformed during the latest Cretaceous-early Tertiary Laramide orogeny. The Permo-Pennsylvanian strata are intruded by the Summerford Mountain syenite sill. The central block exposes a thick sequence of southerly dipping, upper Eocene andesite and dacite lava flows and volcaniclastic rocks assigned to the Palm Park Formation. Deeply eroded remnants of the Dona Ana caldera constitute the structurally lowest, southern block. Eruption of the ~36 Ma Dona Ana Rhyolite, an ash-flow tuff sequence at least 440 m thick, triggered caldera collapse. Post-caldera fallout tuffs and ash-flow tuffs hundreds of meters thick accumulated within the caldera, most notably in the Red Hills graben, where chaotic megabreccia, rhyolite flows, and rhyolite dome-flow complexes and other intrusives were emplaced. Rhyolite and syenite sheets or dikes were emplaced along the northern margin of the caldera, as well as along faults that earlier had broken both caldera tuffs and post-caldera rocks. The Summerford Mountain sill may have extended below both the northern and central blocks, as well as beneath the Dona Ana caldera, where it may have been the source of felsite to syenite dikes at the northern margin of and within the caldera. Because it is buried by younger rocks or alluvium, the western, southern, and eastern limits of the caldera are unknown. However, the western boundary may have been hinged rather than broken, as suggested by the gradual westward decrease in the number and thickness of dikes along the northern margin of the caldera. 71