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 expose some of the deepest Laramide and Rio Grande rift structural levels in the Rocky Mountain region, offering a rare opportunity to investigate deformation near the brittle-plastic transition. We hypothesize that fluid-rock interactions weakened faults in this region, allowing contractional deformation to take place at relatively low stresses during the Laramide orogeny and localizing extensional reactivation during development of the Rio Grande rift. This project uses a multidisciplinary research approach that includes geologic mapping, microstructural analysis, EBSD, geochemistry, and geo/thermochronology. Data generated in this project will provide key constraints on the stresses, temperatures, deformation mechanisms, and fluid involvement associated with intraplate shortening near the strongest part of the crust. This research will also shed light on the relationships between Laramide contraction and Rio Grande rift extension and the role of shear zone geometry and rheology on extensional reactivation.

This impressive diversity of geology in the Sangres lends itself to a wide array of research topics, and as part of this project we are also investigating several additional topics, including: a) polyphase exhumation history across the range, b) deformation and exhumation associated with the late Paleozoic Ancestral Rocky Mountains, c) and Cenozoic metamorphism in Pennsylvanian strata along the western flank of the range.

Preliminary results: Sitar et al. 2022 GSA abstract,  Malavarca et al. 2023 GSA abstract,  Ghamedi et al., 2023 GSA abstract

Structural evolution of the Atacama fault system, northern Chile: Insights into rheology and strain in a magmatic arc during oblique convergence 

Funding: NSF Tectonics program (EAR 1822064, $308,743, including $83,298 in total supplements for the NSF INTERN program, 2018–2022)

John Singleton (PI), CSU Students: Nikki Seymour, Skyler Mavor, Rachel Ruthven, Emily Perman, Chilean geologists: Rodrigo Gomila, Gert Heuser, and Gloria Arancibia

This project addresses the structural evolution of the Cretaceous Atacama fault system, which is considered a classic example of an intra-arc strike-slip system. Intra-arc strike-slip fault systems play a critical role in accommodating oblique subduction convergence, yet significant uncertainty remains surrounding their rheology and deformation histories.  The main goals of this project are to: a) determine the timing, magnitude, and rate of slip across the AFS, b) determine the spatial and temporal relationship between magmatism and deformation along the AFS, and c) determine the rheology and deformation conditions near the brittle-plastic transition along the AFS. Through a major field mapping effort, geo/thermochronology, and microstructural analysis we have established the slip history of this structure, identifying the first clear geologic offset markers along the fault (indicating ~54±6 km of sinistral displacement from 133–110 Ma), and we have documented in detail the how plutonism is intimately related to the timing and rheological development of the AFS. In addition, our work has provided new insight into the pre- and post-AFS deformation history in the Coastal Cordillera. Throughout this project we have worked closely with Chilean geologists at Pontificia Universidad Católica. Fieldwork has been completed, and we are now working to complete analyses and finishing publishing results.

Publications

Mavor, S.P, Singleton, J.S., Heuser, G., Gomila, R., Seymour, N.M., Williams, S., and Arancibia, G.,2022, Sinistral shear during Middle Jurassic emplacement of the Matancilla Plutonic Complex in northern Chile (25.4°S) as evidence of oblique plate convergence during the early Andean orogeny: Journal of

South American Earth Science, v.120, 104047 link

Seymour, N.M., Singleton, J.S., Gomila. R., Mavor, S.P., Heuser, G., Arancibia, G., Williams, S., and Stockli, D.F., 2021, Magnitude, timing, and rate of slip along the Atacama fault system, northern Chile: Implications for Early Cretaceous slip partitioning and plate convergence: Journal of the Geological Society, v.178 link

Mavor, S.P., Singleton, J.S., Gomila, R., Heuser, G., Seymour, N.M., Williams, S.A., Arancibia, G., Johnston, S.M., Kylander-Clark, A.R.C., and Stockli, D.F., 2020, Timing, kinematics, and displacement of the Taltal fault system, northern Chile: Implications for the Cretaceous tectonic evolution of the Andean margin: Tectonics, v.39 link

Seymour, N.M., Singleton, J.S., Mavor, S.P., Gomila, R., Stockli, D.F., Heuser, G., and Arancibia, G., 2020, The relationship between magmatism and deformation along the intra-arc strike-slip Atacama fault system, northern Chile: Tectonics, v.39 link

Ruthven, R., Singleton, J., Seymour, N., Gomila, R., Arancibia, G., Stockli, D.F., Ridley, J., Magloughlin, J., 2020, The geometry, kinematics, and timing of deformation along the southern segment of the Paposo fault zone, Atacama fault system, northern Chile: Journal of South American Earth Sciences, v.97 link

Seymour, N.M., Singleton, J.S., Gomila, R., Arancibia, G., Ridley. J.,Gevedon, M.L., Stockli, D.F., and Seman, S.M., 2024, Sodic-calcic alteration and transpressional shear along the Atacama fault system during IOCG mineralization, Copiapó, Chile: Mineralium Deposita, doi.org/10.1007/s00126-024-01259-2. link

In preparation

Mavor, Seymour, Gomila, Singleton, Heuser, Arancibia, and Williams; Geologic map of the Atacama and Taltal fault systems near Taltal, Chile: new constraints for the evolution of the Andean margin: to be submitted to Journal of Maps

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 synorogenic Pennsylvanian to Permian strata, and relatively little is known about the structural geology of this cryptic intraplate orogeny. In particular, information on 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 will 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. Our proposed research will test kinematic models for deformation that are based largely on subsurface data, providing a new structural framework for the ARM system.

Preliminary results: Johnson & Singleton 2022 GSA abstract