27 Structural Geology/Tectonics (Posters)

DISTINCT ZONES OF DUCTILE DEFORMATION BETWEEN WHISTLER AND LILLOOET, BC

Paper number: 27-14
Program: Technical Sessions
Time: 9:00 AM – 5:00 PM

Authors

Abstract

Rocks along Duffy Lake Road (Rt. 99) between Whistler and Lillooet, British Columbia commonly record penetrative deformation. Regional stratigraphy within this transect includes rocks of the Cadwallader and Bridge River Terranes. Major units include the Taylor Ck. Group, Cayoosh Assem., Brew Fm., Chism Ck. Schist, Bridge River Cx., and Cadwallader G. Within this 110 km transect NW striking foliation, dipping 30° NE to vertical is common. A down dip crenulation lineation, is common in these rocks. Although they are commonly re-crystallized, locally developed microstructures reveal shear sense indicators in sections cut parallel to lineation and dip suggesting more complicated movement than that recorded by the rocks near Whistler and Lillooet. Units mapped as Cadwallader or included as Cretaceous volcanics may be unfoliated. Steep thrusts including the Bralorne set and Downton Creek fault are attributed to Cretaceous contraction before the intrusion of Cretaceous plutons the oldest of which is 113 Ma. Previous mapping shows that the Bridge River and Cayoosh rocks were thrust over the Brew Formation. The Brew Fm. shows distinctly different deformation characteristics than surrounding rock units including sub-horizontal foliation and shear bands.

Near Whistler and Lillooet foliation is closely spaced and mylonitic. At Lillooet steeply dipping mylonite along the dextral, Eocene Marshall Creek fault (MCF) consists of L-tectonites with distinct sub-horizontal mineral lineation. Microstructures record right-lateral shear sense. A releasing step along the MCF structurally exhumes the Brew in the footwall beneath the Bridge River rocks. Westward along the transect toward Duffy Lake, foliation cut parallel to dip shows stronger deformation and flattening of grains. These foliated rocks are cut by generally undeformed Cretaceous plutons of granodioritic composition including, from east to west, the Bendor (63-57Ma), Scuzzy (85Ma), Spetch Creek (102Ma) and Pemberton Diorite (113Ma) Complexes. Near Whistler outcrops of the strongly foliated volcanic rocks mark the western edge of the Bridge River Terrane at the Green Lake Fault. The rocks found in this Whistler shear zone are VFG and tend to show a much stronger lineation parallel to dip. The strongly foliated rocks extend for ~20 km southwest.