Stress distribution in the Winters Pass thrust plate

Abstract

The Winters Pass thrust plate carries a section of Precambrian basement and its sedimentary cover rocks over Precambrian and lower Paleozoic sedimentary rocks. The thrust plate has undergone two periods of movement: the first to the southeast, and the second to the northeast. The basal 3 to 4 meters of this thrust plate consists of basement rocks that are cataclastically deformed, locally blastomylonitic and contain a well-developed cataclastic foliation and mineral lineation due to feldspar and quartz streaking. In the higher parts of the thrust plate, the cataclastic rocks present in the basement are mylonitic gneisses and mylonites. Relatively undeformed gneisses are present above the mylonites and throughout the thrust plate interspersed with cataclastic rocks. Closely spaced shear fractures are present throughout the gneiss. However, they are not well-developed in the blastomylonites in the lower parts of the thrust or in the upper part where they become progressively less well developed. The shear fractures are also absent in areas of massive quartzo-feldspathic rocks. Microshear fractures, however, are found in all thin sections studied throughout the plate. Stress distribution was determined by using conjugate shear fractures present both in the field and in thin-section. The earliest stress field had with a NWSE subhorizontal orientation and varying in orientation between northeast plunging 55 degrees and southwest plunging 4 degrees. The second stress field had a subhorizontal NE-SW orientation for o oriented in a southeastern direction with plunges varying between 4 and 9 degrees. These two different stress distributions indicate two separate thrusting events. Overturned folds in the area agree with this interpretation and also give direction of movement. These folds indicate the first movement of the Winters Pass thrust plate was to the southeast and the second movement was to the northeast.