CSU Structural Geology - Current Projects

Glacial valley along the axial trace of the Gibson Peak syncline, Sangre de Cristo Mountains

MS student Cole Sitar inspecting the Independence Mine shear zone

Brittle-plastic deformation, fluid-rock interaction, and extensional reactivation along Laramide thrust faults in the Sangre de Cristo Mountains, Colorado

Funding: NSF Tectonics program (EAR 2115719, $313,270, 2021–2024), USGS EDMAP program (G21AC10493, $21,250, 2021–2022)

John Singleton (PI, CSU), Jeffrey Rahl (co-PI, Washington & Lee University), Jonathan Caine (USGS), Cole Sitar (CSU), Sammy Malavarca (CSU), Miriam Primus (CSU), plus several other collaborators

The Sangre de Cristo Mountains in southern Colorado exposes some of the deepest Laramide and Rio Grande rift structural levels in the Rocky Mountain region, offering a unique opportunity to investigate deformation near the brittle-plastic transition and relations between ARM+Laramide contraction and Rio Grande rift extension. This project has used a multidisciplinary research approach to provide key constraints on the kinematics, deformation conditions, and fluid involvement associated with intraplate shortening and extension near the strongest part of the crust. Research led by MS students Cole Sitar and Miriam Primus have demonstrated the brittle-plastic Independence Mine shear zone, Deadman shear zone, and related (previously unmapped) shear zones record complex structural histories that include both top-NE reverse-sense shear and subsequent top-SW normal-sense shear. Extensional overprint is locally pervasive and focused primarily in mica-rich domains formed from reaction softening, highlighting an important rheological control on reactivation. In other areas, mylonitization associated with extension is spatially associated with late Oligocene intrusions or is localized along geometrically favorable portions of contractional structures. Thermochronology and U-Pb zircon and monazite geochronology have provided key constraints on the timing of late Oligocene extensional reactivation and the onset of Middle Miocene exhumation associated with the range-bounding normal fault system. Additional work on this project led by PhD student Samantha Malavarca has provided detailed timing and temperature constraints on the previously unrecognized major Oligocene thermal event that affected much of the range. Her work has also provided key insight into the transition between Eocene Laramide contraction and Oligocene Rio Grande rift extension.

Publications: Sitar, M.C., Singleton, J.S., Rahl, J.M., Caine, J.S., King, J., Kylander-Clark, A., O’Sullivan, P., 2025, Structural analysis of brittle-plastic shear zones in the Sangre de Cristo Range, southern Colorado, USA: Superposition of Rio Grande rift extension on Laramide contraction: Geosphere, doi.org/10.1130/GES02772.1.

(several addition papers are in prep.)

The Ouray fault with Mississippian Leadville Limestone in the footwall

Kinematics of the Ancestral Rocky Mountain system in Colorado

The Ancestral Rocky Mountain (ARM) system between Utah and Oklahoma consists of several late Paleozoic uplifts and adjacent basins. The majority of studies on the ARM system have focused on the synorogenic (Pennsylvanian-Permian) sedimentary record, and relatively little is known about the structural geology of this cryptic intraplate orogeny. In particular, knowledge of the geometry and kinematics of faults responsible for ARM uplift is extremely limited, as most ARM structures are either not exposed or have been overprinted by Late Cretaceous-early Paleogene (Laramide) or Miocene (Rio Grande rift) deformation. We have identified several field areas in southern and central Colorado that clearly preserve late Paleozoic structures, offering an outstanding opportunity to investigate ARM deformation. This research will document in detail the geometry, kinematics, and field relationships associated with faults and growth folds in these regions, and map key portions of the fault systems to evaluate how ARM deformation was influenced by basement structures. In addition, we plan to use U-Pb geochronology of synkinematic calcite to directly date the timing of slip on numerous faults, allowing us to establish a clear ARM origin for several structures. This research will test kinematic models for deformation that are based largely on subsurface data, providing a new structural framework for the ARM system.

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