Geology topics

Samantha Malavarca

Publications and source records attributed to Samantha Malavarca.

2 recordsLinked to original sources

USGS2026

Ductile and brittle Rio Grande Rift deformation in Oligocene granite records a two-stage rift history in southern Colorado

The timing and nature of early deformation in the Rio Grande Rift remains poorly constrained. We present evidence for the earliest structural signature of rift extension in the Sangre de Cristo Range, southern Colorado, based on new geologic mapping, structural analysis, rock magnetic data, and thermochronology. These analyses focus on the ~30.0 Ma granite of Chokecherry Canyon, which hosts discrete low-angle mylonitic shear zones and a distributed, gently SW-dipping protomylonitic fabric. Incremental stretching axes, stretching lineations, and Kmax magnetic lineations plunge gently WSW. Quartz microstructures and crystallographic orientations indicate dominantly coaxial strain in the protomylonite and general shear in the discrete shear zones. Quartz c-axis opening-angle thermometry suggests deformation at ~420–540°C. Thermal modeling of ⁴⁰Ar/³⁹Ar K-feldspar data indicates rapid post magmatic cooling below the brittle–plastic transition, supporting shear-zone formation immediately after emplacement. Slow cooling from ~20–13 Ma was followed by renewed rapid cooling at ~13 Ma, interpreted as the onset of extensional exhumation along the Sangre de Cristo Fault System. These results show that extension in the northern Rio Grande Rift was active by ~30 Ma, earlier than previously recognized. We propose a two-stage model for northern Rio Grande Rift evolution: Stage I (30–23 Ma) records ENE–WSW extension localized in low-angle mylonitic shear zones associated with mid-crustal intrusions; Stage II (≤18 Ma) reflects brittle high-angle normal faulting, focused exhumation, and rift narrowing. Stage I magmatism and deformation along the western range front likely established crustal weaknesses that guided later fault development.

Colorado↗

USGS2026

Exhumation history of the Sangre de Cristo Range, Colorado, from mid- to low-temperature thermochronology: New insights into Laramide contraction and Oligocene–Miocene extension in the northern Rio Grande rift, USA

The Sangre de Cristo Range in southern Colorado records a complex tectonic history that includes Late Cretaceous–Eocene Laramide contraction and Oligocene–Quaternary Rio Grande rift extension. We present new thermochronologic data ( 40 Ar/ 39 Ar, fission-track, (U-Th)/He) and thermal history models that provide insight into this polyphase history and spatiotemporal patterns of extensional exhumation. Our data indicate that the Alvarado fault, which bounds the northeastern flank of the range, initiated as a NE-dipping reverse fault during the early stages of the Laramide orogeny and was subsequently reactivated as a normal fault in the early Miocene. The onset timing of rapid cooling associated with extensional exhumation systematically youngs southwestward across the range from 20–16 Ma along the northeastern flank, 18–14 Ma near the range crest, and 14–10 Ma along the southwestern flank. We attribute this exhumation pattern to earlier onset of normal slip on the Alvarado fault, with extension shifting to the SW-dipping Sangre de Cristo fault system along the southwestern flank of the range and adjacent San Luis Basin at 14–10 Ma. The southwestern flank of the range was exhumed from temperatures of ∼240 ± 25°C at the onset of Miocene rapid cooling, consistent with ∼7–9 km of displacement across the Sangre de Cristo fault system. Thermal history models indicate limited cooling between the Oligocene inception of rift extension and the Miocene onset of rapid cooling, suggesting that development of rift flank topography and adjacent extensional basins was primarily associated with Miocene normal faulting.

Colorado↗