STAGES OF PROTEROZOIC DUCTILE DEFORMATION IN ARCHEAN ROCKS ALONG THE NORTHWESTERN MARGIN OF THE LARAMIE ANORTHOSITE COMPLEX, SOUTHEASTERN WYOMING

Along the SE margin of the Wyoming Archean province, granitic gneiss and supracrustal rocks of the Elmers Rock greenstone belt (ERGB) were reworked during two periods of Paleoproterozoic deformation, and were subsequently reoriented during emplacement of the Red Mountain and Sybille plutons of the Laramie Anorthosite Complex (LAC, 1.43 Ga). This study defines fabric geometries of the pre-LAC deformation events and their reorientation along the northwestern margin of the LAC.

The older Paleoproterozoic event (D2) formed in response to a pre-Cheyenne belt E-W shortening event that produced a series of westerly verging folds and imbricate basal shear zones that transported the ERGB units to the west. Outside of the LAC contact zone, the shear zones and a locally well-developed S2 axial plane foliation dip shallowly to moderately to the north. L2 lineations, which are well developed in the hinge zones of the F2 folds, plunge to the north in S2. The younger Paleoproterozoic event (D3) is best developed in metasediment and metabasalt along the eastern and southeastern margin of the ERGB. In these areas, F3 folds verge to the SSE in WSW striking axial planes and may have formed during the Cheyenne belt collision.

The Emplacement of the LAC reoriented both D2 and D3 features; however, no new (D4) fabrics were produced by the LAC intrusion. S2 foliation planes and F2 axial planes progressively steepen into the contact from shallow to moderate dips to vertical dips directly adjacent to the LAC. The strikes of both features progressively rotate clockwise and become parallel to the NNE contact with the Red Mountain pluton and ENE contact with the Sybille pluton. L2 lineations and F2 fold hinges, which plunge shallowly to the north outside the Red Mountain contact zone, are only reoriented within ~1 km of the pluton where they steepen to near vertical orientations. Starting about 3 km from the Sybille pluton, the L2 lineations and F2 fold hinges are progressively re-oriented into a more NW-SE trend and steepen toward the contact.

D3 features are most common in the northern part of the area. Toward the LAC contact, S3 rotates to a NNW strike and westward dip. Like L2, L3 fabrics are relatively undisturbed by the Red Mountain Intrusion and trend NNE except directly adjacent to the contact where L3 Steepens and trends at an acute angle towards the contact.