Origins of Curved Mountain Belts- Sevier Fold-Thrust Belt
Most mountain belts display curvature over a range of scales, recording complex, three-dimensional deformation histories.
However, few examples exist where deformation histories and processes responsible for forming curved mountains have been adequately quantified from geologic evidence. These histories are recorded in regional structural patterns, paleomagnetic directions, and grain-scale fabrics of deformed rocks.
The Wyoming salient of the Sevier thrust belt is an ideal location to study processes that produce curvature over a range of scales. Regional trends of major thrust faults and folds curve through 90 degrees from the north to south end of the salient. Rocks in the salient also display systematic structures (including cleavage, fracture networks, and deformed fossils) and carry multiple paleomagnetic components, providing a detailed record of internal strain and rotations. Work (funded by a National Science Foundation collaborative research grant with Arlo Weil at Bryn Mawr College) focused on integrated paleomagnetic and structural studies of two stratigraphic levels, the Jurassic Twin Creek Limestone and redbeds of the Triassic Ankareh Formation.Three-dimensional models, constrained by seismic data and incorporating new strain and rotation data, are being constructed to develop a robust kinematic model of the salient.
Yonkee, W.A., and Weil, A.B., 2011, Evolution of the Wyoming salient of the Sevier fold-thrust belt, northern Utah to western Wyoming: Utah Geological Association Publication 40, p. 1-55.
Yonkee, W.A., and Weil, A.B., 2010, Quantifying vertical-axis rotation in curved orogens: integrating multiple data sets with a refined weighted least-squares strike test: Tectonics, v. 29, TC3012, doi:10.1029/2008TC002312.
Yonkee, W.A., and Weil, A.B., 2010, Reconstructing the kinematics of curved mountain belts: internal strain patterns in the Wyoming salient, Sevier thrust belt, U.S.A.: Geological Society of America Bulletin, v. 122, p. 24-49, doi:10.1130/B26484.1.
Weil, A.B., Yonkee, W.A., and Sussman, A.J., 2010, Reconstructing the kinematic evolution of curved mountain belts: a paleomagnetic study of Triassic redbeds from the Wyoming salient, Sevier thrust belt, U.S.A.: Geological Society of America Bulletin, v. 122, p. 3-23, doi:10.1130/B26483.1.
Weil, A.B., and Yonkee, W.A., 2009, Anisotropy of magnetic susceptibility in weakly deformed red beds from the Wyoming salient, Sevier thrust belt: Relations to layer-parallel shortening and orogenic curvature: Lithosphere, v. 1, p. 235-256, doi:10.1130/L42.1
Processes of Foreland Deformation- Wyoming Laramide
Understanding the origins of foreland mountain belts far from plate margins, including evolution of diversely oriented arches, mechanisms of basement and cover deformation, and relationships to plate dynamics, are key issues in tectonics.
Using a similar approach to work in the Sevier fold-thrust belt, integrated structural, anisotropy of magnetic susceptibility (AMS), and paleomagnetic studies of Triassic redbeds in the Laramide foreland of Wyoming are underway (funded by a National Science Foundation collaborative research grant with Arlo Weil at Bryn Mawr College).
The redbeds carry a near primary remnant magnetization for quantifying vertical-axis rotations, have interpretable AMS fabrics, and contain a thin limestone unit cut by minor faults that formed during early layer parallel shortening (LPS), overprinted by more complex fault systems in steep fold limbs. Kinematic analysis of minor fault data indicates overall SW-NE-trending LPS directions in structurally simple settings, subperpendicular to regional NW structural trends. In detail, fault data record minor spatial and temporal changes of shortening directions, associated with basement heterogeneity and an evolving stress field.
AMS lineations are widely developed, providing additional estimates of LPS directions that are consistent with minor fault data. LPS directions estimated from minor fault and AMS data in steep forelimbs of folds display systematic changes in orientation that correlate with changes in structural trend. Paleomagnetic declinations also display changes with trend some steep fold limbs consistent with localized wrench shear along steep forelimbs, and stress/strain refraction along curved folds. Ongoing studies will further test and refine this model, and evaluate links between thick-skin Laramide foreland deformation, thin-skin deformation in the Sevier fold-thrust belt, and plate dynamic processes.
Weil, A.B., and Yonkee, W.A., 2012, Layer-parallel shortening across the Sevier fold-thrust belt and Laramide foreland of Wyoming: spatial and temporal evolution of a complex geodynamic system: Earth and Planetary Science Research Letters, v. 357-358, p. 405-420, http://dx.doi.org/10.1016/j.epsl.2012.09.021.
Weil, A.B., Yonkee, W.A., and Kendall, J.A., in review, Towards a better understanding of the influence of basement heterogeneities and lithospheric coupling on foreland deformation: a structural and paleomagnetic study of Laramide deformation in the southern Bighorn Arch, Wyoming: Geological Society of America Bulletin, 44 mp.
Several manuscripts are in preparation based on inititial results given in the following abstracts
Weil, A.B., Yonkee, W.A., Schultz, M.., and Zhi Yi, A., 2011, Early LPS patterns along the Sweetwater arch-Shirley Mountain system of the Laramide foreland: regional refraction of stress field and development of multiple structural trends: Geological Society of America Abstracts with Programs, v. 43, no. 5, p. 99.
Yonkee, W.A., and Weil, A.B., 2011, Unraveling the early LPS-stress history of the Laramide foreland: integrating field studies of minor faults with stress inversion and AMS studies: Geological Society of America Abstracts with Programs, v. 43, no. 5, p. 365.
Deformation During Flat-slab Subduction- Argentina
We have begun collaborative studies with colleagues at Mendoza CONICET on the Andean Cordillera to Precordillera fold-thrust belt and Sierra Pampeanas deformed foreland in Argentina, which have been proposed as analogues to the Sevier fold-thrust belt and Laramide foreland.
By studying the active Andean orogenic system, including changes in tectonic styles related to along-strike variation in subduction geometry, we can better understand relations between mountain building and plate margin dynamics. The study region includes multiple active faults, deformed synorogenic strata, as well as the highest peak in the western hemisphere- Aconcagua.
The photo shows highly deformed Miocene strata being thrust over Quaternary gravels along an active Las Penas fault north of Mendoza. This city was largely destroyed by a M~7 earthquake on a related fault in 1861. Better understanding of active faults in the region may lead to improved planning and hazard mitigation.
Fluid-rock interaction and deformation in thrust sheets and faults zones, implications for rheology and strain softening
Understanding the rheology of earth materials is a first-order issue in tectonics. Aspects of rock deformation, from behavior of individual fault zones to regional deformation along plate boundaries, depend on rheology of rocks over geological time scales. Characterization of rheology requires an understanding of how progressive deformation is accomplished, including nature of fluid-rock interaction and associated changes in deformation mechanisms, metamorphic reactions, and fabrics.
Ongoing work (funded by a National Science Foundation collaborative research grant with Dyanna Czeck at University of Wisconsin- Milwaukee), combines detailed strain, geochemical and microtextural analysis of diamictite deformed in the footwall and hanging wall of a major thrust (Willard thrust) in northern Utah. The diamictite provides a natural laboratory to study how a variety of rock types (granitic gneiss, schist, and quartzite preserved as clasts) respond during progressive deformation and fluid-rock interaction.
Detailed characterization of strain in different clasts of the diamictite is being used to understand relative strengths of the different components and whether the relative strengths change with progressive strain. The project will result in further understanding of the linkages between water content, deformation mechanisms, fluid pathways, hydrolytic weakening, and reaction softening. This knowledge should fundamentally help us to understand more about the strength of the crust and shear zone localization.
Publications to date:
Yonkee, W.A., Czeck, D., Nachbor, A.C., Barszewski, C., Pantone, S., Balgord, L., and Johnson, K.R., 2012, Strain accumulation and fluid-rock interaction in a naturally deformed diamictite, Willard thrust system, Utah (USA): Implications for crustal rheology and strain softening: Journal of Structural Geology, http://dx.doi.org/10.1016/j.jsg.2012.10.012.
Yonkee, W.A., 2005, Strain patterns within part of the Willard thrust sheet, Idaho-Utah- Wyoming thrust belt: Journal of Structural Geology, v. 27, p. 1315-1343.
Neoproterozoic-Cambrian Rifting History of Western North America and Snowball Earth
Widely exposed sections of Neoproterozoic to Cambrian sedimentary and associated volcanic rocks from Utah to Idaho comprise a key part of the rifted margin of western Laurentia and subsequent North American Cordilleran mountain belt. Although a general stratigraphic framework has been established for these rocks, depositional ages, spatial and temporal variations in provenance, and petro-tectonic setting of volcanic rocks are incompletely understood. Thus, fundamental concepts of Cordilleran geology, such as rift timing, paleogeographic evolution of the margin, and influence of sedimentary architecture on subsequent deformation, remain debated. Importantly, these sections also include diamictites and associated cap carbonates that record Cryogenian (635–850 Ma) glaciations (possibly of global extent), disruptions to geochemical cycles, and subsequent rapid deglaciations. Work funded by a National Science Foundation grant with collaborators Carol Dehler at Utah State University, Paul Link at Idaho State University, and Mark Fanning at Australia National Lab, has focused on geochronologic, geochemical, and sedimentalogic analyses of Neoproterozoic strata in central Utah to SE Idaho, which has revealed two episodes of rifting, igneous activity, and final development of a passive margin with subsequent thermal subsidence. Available geochronologic data indicate that glaciation likely spanned ~700 to 670 Ma, and that “Marinoan-style” cap carbonates in Utah with distinctive stable isotopic geochemical signatures, tube structures, and other unusual features, also formed at this time.
Prior to this work, all “Marinoan-style” cap carbonates were assumed to have been deposited globally at 635 Ma, but our results indicate these unusual rocks may have formed at different times. Ongoing work is focused on integrating additional detailed CA-ID-TIMS geochronologic analysis, detailed sampling for geochemical changes, and analysis of preserved microfossils to evaluate changes in the biosphere during major shifts in Earth global climate and ocean water chemistry. The upper photograph shows a large boulder in glacial diamictite, and the lower photo shows a dropstone in pebbly slate.
Publications to date:
Balgord, E., Yonkee, W.A., Link, P.K., and Fanning, C.M., in review, Stratigraphic, geochronologic, and geochemical record of the Cryogenian Perry Canyon Formation, northern Utah: Implications for Rodinian rifting and glaciations: Geological Society of America Bulletin, 51 ms p.
Keeley, J.K., Link, P.K.,, and Fanning, C.M., 2013, Pre- to synglacial rift-related volcanism in the Neoproterozoic (Cryogenian) Pocatello Formation, SE Idaho: New SHRIMP and CA-ID-TIMS constraints: Lithosphere, v. 5, p. 128-150.
Yonkee, W.A., Link, P.K., Dehler, C.M., Balgord, E., Keeley. J.K., Fanning, C.M., Wells, M.A., and Johnston, S., near ready for submission, Tectono-sedimentary framework of Neoproterozoic to Cambrian strata, Utah and southern Idaho: Protracted rifting, glaciation, and evolution of the Cordilleran margin: Lithosphere.
Geochronology and Thermochronology of Thrust Systems
Quantifying fault slip history, strain rates, and exhumation/cooling patterns within mountain belts is important for understanding rates of mountain building and erosion and potential feedbacks.
This projects is funded by a National Science Foundation grant with collaborator Michael Wells at the University of Nevada Las Vegas, along with support from Shari Kelley at New Mexico Tech and Danny Stockli at the University of Texas at Austin.
Initial work has focused on laserprobe 40Ar/39Ar geochronology of syndeformational mica grains from a range of microstructrual settings (deformed clasts along strain gradients, strain shadows, veins), zircon U/Pb-He and zircon fission-track thermochronology on vertical and horizontal transects across the Willard thrust sheet in northern Utah, and detrital zircon geochronology of associated synorogenic deposits to constrain sediment sources. Initial results indicate the hanging wall of the Willard thrust experienced low-grade metamorphism and internal strain from ~140 to 110 Ma, with large-scale thrust slip, exhumation, and cooling from ~120 to 90 Ma; synorogenic strata record an overall unroofing pattern with sources from progressively deeper levels of the thrust sheet as younger synorogenic strata were deposited.
The footwall experienced low-grade metamorphism and internal strain from ~120 to 90 Ma as the thrust sheet was emplaced and tectonically buried rocks below. The figure below shows Neoproterozoic to Jurassic source rock types in Willard-Paris-Meade thrust sheet with distinctive DZ age-probability distributions. DZ patterns for Cretaceous synorogenic strata reveal upward changes, with significant input from Jurassic and Triassic rocks eroded from the thrust sheet for the lower part of the Gannett Group, to significant input from Cambrian rocks eroded from the thrust sheet for the overlying Frontier Formation.