KECK PROPOSAL: Eocene Tectonic Evolution of the Teton-Absaroka Ranges, Wyoming (Year 2) Project Leaders: John Craddock (Macalester College; [email protected]) and Dave Malone (Illinois State University; [email protected]) Host Institution: Macalester College, St. Paul, MN Project Dates: ~July 15-August 14, 2011 Student Prerequisites: Structural Geology, Sedimentology. Preamble: This project is an expansion of a 2010 Keck project that was funded at a reduced level (Craddock, 3 students); Malone and 4 students participated with separate funding. We completed or are currently working on three 2010 projects: 1. Structure, geochemistry and geochronology (U-Pb zircon) of carbonate pseudotachylite injection, White Mtn. (J. Geary, Macalester; note that this was not part of last year’s proposal but a new discovery in 2010 caused us to redirect our efforts), 2. Calcite twinning strains within the S. Fork detachment allochthon, northwest, WY (K. Kravitz, Smith; note because of a heavy snow pack in the Tetons this past summer, we chose a different structure to study), and 3. Provenance of heavy minerals and detrital zircon geochronology, Eocene Absaroka volcanics, northwest, WY (R. McGaughey, Carleton). We did not sample the footwall folds proposed in the previous proposal (under snow) and will focus on this project and mapping efforts of White Mountain and the 40 x 10 km S. Fork detachment area near Cody, WY, in part depending on the results (calcite strains, detrital zircons) of the 2010-11 effort. All seven students are working on the detrital zircon geochronology project, and two abstracts are accepted at the 2011 Denver GSA meeting. Overview: This proposal requests funding for 2 faculty to engage 6 students researching a variety of outstanding problems in the tectonic evolution of the Sevier-Laramide orogens as exposed in the Teton and Absaroka ranges in northwest Wyoming. Although the projects that we propose are quite varied in theme, it is important to note that all faculty and students will work as a group for the entire field season, and that all students will participate in at least some level on all of the projects. The Eocene was a time when the thin-skinned Sevier orogen was replaced with the thick-skinned Laramide orogen in the Cordillera, producing a variety of complex, overlapping structures. We will 1. study the strain history of footwall Paleozoic sediments overthrust by Archean gneisses in the Teton range and, 2. produce a detailed geologic map of the S. Fork detachment (SFD) allochthon exposed in the S. Fork of the Shoshone River valley, and 3. complete a geologic map of White Mtn., where we discovered 1 enormous vertical injection of carbonate pseudotachylite related to the Heart Mountain detachment in 2010. We have also included budget for detrital zircon analyses which will allow us to 1. Finish dating samples collected in 2010 (we’re on the laser in Tucson in early Nov.) or 2. Collect additional samples in 2011 depending on our results from this year, especially the White Mtn. CUC rocks (we’ve found 2 zircon populations). Introduction Orogenic shortening is accommodated by hanging wall folding above thrust faults, and the Sevier belt (late Jurassic-Eocene) records this shortening as an orderly “younging toward the craton” sequence of thrust motions with dated synorogenic sediments (Armstrong and Oriel, 1965; Dorr et al., 1977, Wiltschko and Dorr, 1983, Craddock, 1992). The transition from Sevier, east-vergent thin- skinned shortening, with 45° slab dip to the west, to west-vergent, thick-skinned shortening, occurred with decreasing slap (5°) dip in the Eocene (Bird, 1988). These crustal-scale offsets of Archean crust are dated by synorogenic deposits (Gries, 1983; DeCelles et al., 1991; Fuentes et al., 2009) and fission track studies (Roberts and Burbank, 1993; Crowley et al., 2002), and many Laramide uplifts preserve a curious peneplain surface at high elevation (Smith and Seigel, 2000). The zone of overlap between thin and thick-skinned structures is in the vicinity of Jackson Hole and the Teton Range. The Heart Mountain detachment system, arguably the largest volcanic landslide deposit (Malone, 1995 and 1996; Craddock et al., 2009; Malone and Craddock, 2008) in the world, is also an Eocene event in the vicinity of Jackson Hole and the greater Absaroka volcanic province. The South Fork detachment system is contemporaneous with the HMD event. Figure 1 is a location map with specific field sites identified; this is correlated to the student project table below (and vice versa). 2 Figure 1: Digital elevation (DEM) base of northwest Wyoming. Projects include footwall folds (1a and 1b), mapping the S. Fork detachment (2), and White Mountain (3). Proposed Projects 1. Laramide Footwall Folds (2 students) Orogenic shortening is accommodated by hangingwall folding above thrust faults. Uplift of the hangingwall ramp-anticline buries the footwall leading geologist’s to presume the footwall is undeformed. Where the footwall rocks are exposed, which is rare, the underlying sediments are usually layered parallel to the thrust with one exception, where complex folds are exposed (Craddock et al., 1985). The Laramide Fourellen fault (and Buck Mtn. fault to the south; Smith, 1991) strikes N-S and dips east along the length of the Teton Range where spectacular exposures of Archean gneisses are in thrust contact with an overturned footwall syncline in Cambrian-Mississippian sediments (Figures 2a- b). The exposures near Fourellen Peak and Alaska Basin are ideal for studying the strain history of this folded structure (N-S trend, shallow plunges). Both sandstones (quartzites; finite strains) and carbonates (calcite twin analysis on limestone and calcite veins) will preserve the deformation history around the fold curvature and along strike, and results will be compared with strain analyses nearby in the southern Cache Creek thrust sheet (Craddock et al., 1988) as well as the fold strain history of the nearby Derby Dome Laramide uplift (Craddock and Relle, 2003). We may also find syn-faulting calcite that can be 3 used to record the stress-strain field when the fault was active (Craddock et al., in review). Oriented samples will be collected throughout for thin section work on the universal stage. Each student project will include field mapping of part of the synclinal structure, sampling and measuring of fault-fold kinematic indicators, and collection of oriented samples for 3-D strain analysis. Strain analysis will involve either using 3 orthogonal oriented sections (ACF method for finite strain ellipsoids) or 1-3 oriented sections for measuring mechanical twins in calcite (limestones and veins). Fig. 2: Fourellen fault (left), with Archean gneisses over the Cambrian-Pennsylvanian section (photo from Love et al., 2007) and, right, the trace of the same fault placing Archean gneisses over the Cambrian Flathead Sandstone (circled) on the west side of the Teton’s (photo from Smith, 1991). 2. South Fork Detachment Strains and Mapping (2 students) The Heart Mountain Detachment in northwest Wyoming has been the focus of scientific inquiry for more than 100 years. The lesser known South Fork detachment, which is temporally and spatially related to the Heart Mountain detachment, has had much less attention over the years. The South Fork detachment (SFD) is exposed in the drainage of the South Fork of the Shoshone River and along Rattlesnake Creek. Dake (1918) initially described and named the SFF and Pierce further defined and mapped its extent (Pierce, 1957, 1966, 1970; Pierce and Nelson, 1968, 1969). Bucher (1936) first suggested gravity as a driving mechanism for movement and formation of this 10 X 40 km rootless, folded structure (Fig. 3). 4 Figure 3: Geologic map of the South Fork Detachment area (from Love and Christianson, 1985, above), and and air photo composite (1”=10 km) of the S. Fork detachment southwest of Cody, WY. The decollement surface is at the base of the Jurassic Sundance Formation. It ramps up section to the Cretaceous Cody Shale, and ramps once again up section to the Eocene Willwood Formation. The South Fork detachment is obscured by younger volcanic rocks to the west and south. As much as 10 km of displacement is indicated and the hanging wall structures (folds) are oriented SW‐NE with shallow plunges. Some bedding is overturned, indicating nappe‐ 5 like folds, in association with “circular” faults (Fig. 4). Although the motion direction of the South Fork fault seems to align with the presumed displacement direction the Heart Mountain slide block (to the SE), the orientation of pre‐ Cenozoic rocks in the lower plate and those to the southeast and southwest of the upper plate of the fault are consistent with the variable pattern of E‐W Sevier‐ Laramide shortening. The South Fork detachment has been interpreted to be the front of a gravity detachment that is older than and unrelated to the Heart Mountain Detachment (Blackstone, 1985; and Pierce, 1957, 1986), the easternmost expression of the Cordilleran overthrust belt (Clarey, 1990), or the toe of the Heart Mountain detachment (Beutner and Hauge 2009). The data supporting either interpretation are so poorly constrained that new detailed mapping of critical exposures is warranted. Most of the reinterpretations used by Clarey, Beutner, and Hauge used existing mapping of Pierce and his colleagues, which has been called into question by all of the workers in the area for the past 30+ years. The principal scientific question advanced with this subproject is: Does the new detailed geometric (through geologic mapping) and kinematic (through calcite strain analysis) advance the Sevier‐Laramide shortening or the Heart Mountain‐related gravity slide hypothesis? Mapping done as part of this project will be at the 1:24,000 scale on modern base maps in the Twin Creek and Belknap Creek 7.5 Minute Quadrangles. Reconnaissance mapping will be conducted in critical surrounding areas.