STRUCTURAL DEVELOPMENT OF THE PALEOPROTEROZOIC HIGHLAND MOUNTAINS DOME, MONTANA

Metamorphic rocks of the Highland, Ruby, and Tobacco Root Mtns. (Montana) record Paleoproterozoic collisional processes along the NW margin of the Archean Wyoming province, however, the tectonic significance of many of the Proterozoic structures and fabrics in the region is unclear. Mapping by O'Neill et al. (1996) showed that the Highlands contain a NE-trending dome with a gneissic core flanked by augen mylonites. O'Neill et al. (1986) suggested that the dome formed from downdip extensional shearing along the flanks of the dome. A detailed structural study was undertaken to test this model and to better understand the tectonic history along the NW margin of the Wyoming province.

Structural fabrics include lineations defined by quartz rodding, sillimanite, and elongate felspar. Lineation orientations show a systematic change between the west flank and the southern end of the dome. On the west flank, lineations trend NE (~040°) and become more NNE (~020°) toward the south; plunges are gentle and ~parallel to strike in these areas. Farther south, where layering wraps around the south end of the dome, lineations trend 140°-150° and plunges change to oblique and downdip. Shear-sense indicators along the W and SW flanks indicate ~strike-parallel dextral shear along the mylonitic foliation. NW-directed updip (present orientation) shearing is suggested by a small number of shear-sense indicators on the south end of the dome. Subvertical, ENE-trending shear zones are also common near the southern end of the dome. A U-Pb age of ~1.75 Ma, from a late-synkinematic granitic dike indicates that deformation was synchronous with high-grade metamorphism in the Tobacco Root Mtns. to the east.

Although the significance of the observations is not yet clear, the lineation patterns and preliminary kinematic observations suggest that a simple model of extensional downdip shearing is not warranted. Additional structural analysis is being undertaken to further delineate the lineation pattern, kinematics, and relationships between the steep shear zones and folds to better understand the mechanisms of doming. The structural history of the Highland Mtns. will be compared with that of the adjacent ranges and the tectonic significance in terms of collisional processes along the NW margin of the Wyoming province will be discussed.