Structure and Petrology along a Segment of the Shuksan Thrust Fault, Mount Shuksan Area, Washington
The structural geology of the Mt. Shuksan area is dominated by the Shuksan thrust fault. This fault juxtaposes the mid to late Paleozoic Chilliwack Group with the structurally overlying, early Cretaceous Shuksan Metamorphic Suite. The Shuksan thrust fault is a complex imbricate zone, approximately 1...
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| Format: | Text |
| Language: | English |
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Western CEDAR
1986
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| Online Access: | https://cedar.wwu.edu/wwuet/837 https://cedar.wwu.edu/cgi/viewcontent.cgi?article=1859&context=wwuet |
| Summary: | The structural geology of the Mt. Shuksan area is dominated by the Shuksan thrust fault. This fault juxtaposes the mid to late Paleozoic Chilliwack Group with the structurally overlying, early Cretaceous Shuksan Metamorphic Suite. The Shuksan thrust fault is a complex imbricate zone, approximately 1.5 km wide in map view, in which rocks of the Shuksan Suite and Chilliwack Group are imbricated with lesser amounts of exotic tectonic slices. Mesoscopic structures are chaotic and mostly disordered although a dominant shear fabric is pervasive. The macroscopic structure of the fault zone is relatively simple: it forms a shallowly-dipping plane with a general attitude of N30°-35°W, 25°-35°NE. The low-angle structures are slightly modified by Tertiary(?) high-angle faults and shear zones which exhibit small displacements. No evidence of a "root zone", as postulated by Misch (1966), was discovered in this study. The imbricate zone is composed of tectonic slices and fragments of exotic and autoclastic rocks, commonly in a sheared pelitic matrix or in other sheared rock of the Chilliwack Group. Exotic slices present include: serpentinite, ultramafic tectonite (forsterite-tremolite-talc rock), schist of the Vedder Complex, blueschist termed the Baker Lake blueschist in this study, and meta-hornblende gabbro of uncertain affinity. Forsterite-tremolite-talc rock contains a mineral assemblage indicative of the upper amphibolite facies of regional metamorphism. It, and the meta-hornblende gabbro, are the only rock units of the tectonic slices that occur north of Day Creek. The other tectonic slices occur in the south of the study area in association with a possible large tectonic fragment of titanaugite-bearing meta-basalt and meta-chert which may be correlative with the Haystack Mountain unit of Cruver (1983). Radiometric dating of albite-muscovite schist similar to that on Vedder Mountain yields a K/Ar date of 274 ± 9 Ma and a Rb/Sr date of 273 ± 6 Ma. These ages are within the range of dates for the Vedder Complex ... |
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