DEFORMATION HISTORY AND STRUCTURAL KINEMATICS OF A QUARTZITE-SCHIST SEQUENCE: IMPLICATIONS FOR PROTEROZOIC TECTONICS IN THE SOUTHERN ROCKY MOUNTAINS

The Proterozoic evolution of the Southern Rocky Mountains is a subject of great interest among workers attempting to better understand the late Precambrian tectonic history of the western United States. The Blue Ridge area in the southern Front Range of Colorado consists of an elongate, northeast-southwest trending quartzite-schist sequence (QSS) juxtaposed against granitic rocks of the Paleoproterozoic (1705 Ma) Twin Mountain batholith. Although of limited geographic extent (~10 km²), the Blue Ridge quartzite-schist sequence has a similar orientation to other better-studied shear zones (e.g., Skin Gulch, Moose Mountain, Idaho Springs-Ralston) and may hold important clues to unraveling the Proterozoic history of central Colorado.

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.