The Middle Crust of the Wyoming Province – Ground-truthing above 2000 Meters Elevation in the Beartooth Mountains, Montana and Wyoming Darrell Henry, Dept. of Geology and Geophysics, Louisiana State University; David Mogk, Dept. of Earth Sciences, Montana State University and Paul Mueller, Dept. of Geological Sciences, University of Florida A significant challenge for EarthScope will be to develop geologic and geophysical studies that can distinguish crustal discontinuities at a range of scales and at resolutions high enough to discern structural and lithologic boundaries, and to relate these discontinuities to structures in the mantle lithosphere. To gain an insight into the lithologic and structural complexity of the middle crust that is likely to be imaged over large areas in the Northern Rocky Mountains, we can examine Archean crust exhumed from middle crustal levels in the cores of mountain ranges uplifted during the Laramide Orogeny. The Archean rocks of the Beartooth Mountains of Montana and Wyoming provide an opportunity to examine, first-hand, the lateral and vertical distributions of formerly middle-crustal lithologies and the structures superimposed on them. Introduction Regional Distribution and General Character of Archean Rocks of the Wyoming Province Archean rocks exposed in the eastern and central Beartooth Mountains, Bighorn Mountains, and samples from deep-drill cores in eastern Wyoming and Montana are dominantly Late Archean granitoids, members of the tonalite-trondhjemite- granodiorite suite, with inclusions of older supracrustal rocks preserved as tectonic slices or pendants in the younger magmatic rocks. These rocks are considered to be part of the Beartooth-Bighorn Magmatic Zone (BBMT). The Archean rocks in the western Beartooth Mountains and to the west are dominantly older high-grade gneisses with varying abundances of metasedimentary rocks. This area is generally termed the Montana Metasedimentary Province (MMP). The dominantly magmatic terrane in the eastern and central Beartooth Mountains and the high-grade gneiss terrane to the west are separated by a major discontinuity in the Archean basement marked by a mobile belt in the North Snowy Block, western Beartooth Mountains. Fig. 2: Geographic and geologically-distinct blocks of the Archean rocks of the Beartooth Mountains. Local Characteristics of Archean Rocks in the Beartooth Mountains The main Beartooth massif consists predominantly of voluminous Late Archean igneous granitic rocks ( Ga) with inclusions of metasupracrustal rocks, which exhibit wide ranges in sizes (cm to km), composition, metamorphic grade, and isotopic age (to 3.3 Ga). The peak metamorphic conditions likely associated with the early stages of granitic magmatism are considered to be 5-7 kbar and ºC. Swarms of mafic dikes (2-200 m wide) intruded the area during several distinct time periods beginning 2.55 Ga and extending until 0.75 Ga. Issues potentially addressed by EarthScope EarthScope could address several additional questions that are related to the Beartooth and other Wyoming Province uplifts: 1.What is the character of the faulting at depth that brought these middle crustal blocks to the surface, and what is the relation of these faults to Archean structures? 2.What is the nature of the crust below the 3000 m of mid-crustal level lithologies currently exposed in the Beartooth Mountains? 3.Are major subprovince boundaries (e.g., BBMZ-MMP) largely vertical or horizontal structures? 4.Do any of the geologically recognizable province or sub-province boundaries extend to the mantle? Fig. 1: General outline map of the Wyoming Province and its subprovinces showing major areas of exposure of Archean rocks (WGT, Wyoming greenstone province; BBMT, Beartooth-Bighorn magmatic zone; MMT, Montana metasedimentary province). The subprovinces are defined by the restricted occurrences of certain lithological associations and common geologic histories. Although not geologically homogeneous, major contrasts among subprovinces include metamorphic grade, ages of granitoids and granitic gneisses, structural style and the compositions and ages of supracrustal associations. Modified from Mueller et al. (1996). The Stillwater Block is dominated by the mafic layered Stillwater Complex (2.7 Ga) and its contact aureole that developed in older metasedimentary rocks (Fig. 5). On a finer scale in the Beartooth Mountains we recognize four geographically and geologically distinct domains: the main Beartooth massif, the Stillwater Block, the North Snowy Block and the South Snowy Block represent distinct lithologic, structural, and chronologic subdivisions (Fig. 2). The North Snowy Block is interpreted as a collage of several allochthonous units structurally juxtaposed against the main Beartooth Massif with latest Archean magmatism suggesting this juxtaposition occurred about 2.55 Ga. Fig. 3: Supracrustal xenoliths of varying size included in the Ga, late Archean granitic rocks of the eastern Beartooth Mountains. A representative P-T path of the migmatites associated with the granitic rocks is given on the right (Maas, 2004). Fig. 4: Large-scale isoclinal fold in the George Lake Marble that is cored by the Barney Creek Amphibolite with strong attenuation of the units in the lower limb. The Pine Creek Nappe Complex is overlain by the Heterogeneous Gneiss unit (Mogk, 1983). The South Snowy Block is dominated by lower grade metasedimentary rocks and a series of locally important 2.7 Ga old granitic plutons. Fig. 7: Highly interpretive cross-section of the northern Wyoming Province after the juxtaposition of the terranes at ~2.55 Ga. Modified after Mogk and Henry (1988). Maas, A. T. (2004) Migmatization of Archean Aluminous Metasediments from the Eastern Beartooth Mountains, Montana, U.S.A. MS Thesis, Louisiana State University, 141 p. Mogk, D. W. (1983) The Petrology, Structure and Geochemistry of an Archean Terrane in the North Snowy Block, Beartooth Mountains, Montana. PhD Thesis, University of Washington, 234 p. Mogk, D. W. and Henry, D. J. (1988) Metamorphic petrology of the northern Archean Wyoming Province, Southwestern Montana: Evidence for Archean Collisional Tectonics: In W. G. Ernst, ed., Metamorphism and Crustal Evolution of the Western U.S., Rubey Volume VII, Prentice-Hall, p Mueller, P. A., Wooden, J. L., Mogk, D. W., Nutman, A. P. and Williams, I. S. (1996) Extended history of a 3.5 Ga trondhjemitic gneiss, Wyoming Province, USA: evidence from U-Pb systematics in zircon. Precambrian Research, 78, Turner, A.R., Wolfgram, D. and Barnes, S.J., Geology of the Stillwater County sector of the J-M reef, including the Minneapolis adit. In: Czamanske, G.K. and Zientek, M.L. (Eds.), Stillwater Complex. Montana Bureau of Mines and Geology, Spec. Pub. 92, Fig. 5. Structural section through the complex at Mountain View (after Turner et al., 1985). Additional Major Geologic Features in the Beartooth Mountains and Archean terranes to the west Paleozoic – Archean unconformity is readily accessible (Fig. 6). The bounding thrust faults at the margins of the uplift are exposed (Fig. 5). Terrane distinctions to the west of the Beartooth Mountains are relatively distinctive (Fig. 7) Fig. 6: Paleozoic sedimentary rocks unconformably overlying the Archean rocks of the main Beartooth massif. References cited