DEFORMATION HISTORY AND STRUCTURAL KINEMATICS OF A QUARTZITE-SCHIST SEQUENCE: IMPLICATIONS FOR PROTEROZOIC TECTONICS IN THE SOUTHERN ROCKY MOUNTAINS
The primary structural fabric in the QSS is northeast-southwest and steeply-dipping, as recorded by schist foliation (066°/85°S) and quartzite bedding (065°/89°N). Mineral lineations in the schist are mostly steep to the east-northeast with a mean orientation of 084°/74°. Field analyses suggest that a history dominated by brittle-ductile deformation is preserved along the southern contact of the QSS with the granitic rocks (unlike a previously reported intrusive relationship). Multiple exposures reveal a sharp contact where the QSS transitions to intensely-foliated (i.e., mylonitic) granite. The intensity of fabric development in the granite decreases rapidly away from the contact, but is still measurable at a distance of 50 m. The granite fabric (064°/83°S) is sub-parallel to the contact zone (063°/87°S) as well as the schist foliation. Field observations of mesoscale folds and S-C fabrics in the QSS and asymmetric porphyroclasts in the granite indicate a kinematic history dominated by high-angle reverse movement (south side up) with a lesser component of sinistral movement.
Work in progress is directed at more fully documenting the microscale kinematics within the quartzite-schist sequence and the granites near the southern contact. If appropriate mineral assemblages are present, constraints on the pressure/temperature conditions will also be sought. Together, this research should offer further insights into the Proterozoic structural/tectonic evolution of the Southern Rocky Mountain region in central Colorado.