NEOARCHEAN(?) TO PALEOPROTEROZOIC METAMORPHIC AND STRUCTURAL EVOLUTION OF THE HARTVILLE UPLIFT, SOUTHEASTERN WYOMING

The Hartville Uplift (HU) of southeastern Wyoming lies near the intersection of two major Paleoproterozoic crustal boundaries, the Cheyenne Belt (CB) and the Dakotan Orogen. Previous structural and geochronologic studies in the HU, Laramie Mountains, and the Black Hills show that temporal and kinematic relationships are complex and suggest an earlier episode of east-west compression and metamorphism prior to accretion along the CB and a younger (ca. 1715 Ma) suturing of the Wyoming and Superior Province (Chamberlain et al., 2003, Can. J. Earth Sci. ; Krugh, 1997, MS. thesis, U. of Wyoming). This study aims to unravel the polydeformational and polymetamorphic tectonic history of the HU by analyzing geologic structures and the metamorphic history of metapelites and other rocks during the Paleoproterozoic orogenies.

The HU is bisected by the Hartville Fault (HF) that separates high-pressure, low-temperature assemblages on its western side from the high-temperature assemblages on its eastern side. The first documented deformational event on the western side is a nappe-forming event, D₁, that is associated with high-pressure metamorphism that is inferred from 12-25% of grs component in garnet. Although there are no clear noses of nappes exposed, north-trending hinges of parasitic folds suggest an east-west vergence. D₂ structures refold D₁ nappes about vertical W-SW fold axes, developing axial-planar foliation only in fold hinges.

On the eastern side of the HF, the dominant structural grain is defined by 1715 Ma D₃ faults and folds, which have subvertical N-NE trending foliation and shear zones exhibiting east side up movement (Krugh, 1997). These faults juxtapose sillimanite-grade rocks on the eastern side against lower greenschist-facies rocks on the western side. Late D₄ open folds refold D₃ foliation with hinges defined by fibrolite lineations.

D₁, west-verging nappes may be correlative with 1780 Ma nappes in the central Laramie Range and suggest a pre-CB, east-west compression (Bauer et al., 1998, Basement Tectoncis, 12.). North-south D₂ compression is consistent with accretion of the Green Mountain Terrane along the CB. The large amount of vertical uplift and high-temperature metamorphism associated with D₃ and D₄ are attributed to the terminal collision of the Wyoming and Superior provinces.