STRUCTURAL EVOLUTION OF THE TALLY HO SHEAR ZONE, SOUTHERN YUKON TERRITORY Amy Tizzard* and Stephen Johnston University of Victoria, *email@example.com STRUCTURES At least 3 episodes of deformation are recorded in the rocks of the Tally Ho INTRODUCTION Mountain area. Style of The Tally Ho Shear Zone is located along the PRELIMINARY LEGEND * Episode Deformation Field Evidence N western boundary of the Whitehorse Trough in GEOLOGICAL MAP CRETACEOUS Mylonitization, schistosity and elongation southern Yukon, separating the Stikine Terrane to 0 300 D1 ductile shear lineations developed in Triassic volcanic, W E OF THE TALLY HO Wheaton Valley Granodiorite plutonic and volcaniclastic rocks. the east and the Nisling Terrane to the west. kilometres MOUNTAIN AREA Wheaton River Volcanics Thrusting of pyroxenite and gabbro onto S D2 compression NWT Parts of NTS Map Sheets 30 mylonitinized sediments Whitehorse Pluton Complex geologic structures, plutonism and YT 105 D/2, 3, 6 and 7 40 (see cross-section). Millhaven Conglomerate abundant Tertiary volcanism obscure the nature Mount Wheaton D3 compression Elongation lineations are folded into east- r k vergent crenulations and kink bands e ee iv of this boundary and its relation to adjacent D/6 D/7 JURASSIC R Cr found in mylonitic rocks. ad n Ro to Stikine Terrane k Ho a terranes. ee 84 Bennett Granite e he k brittle to Cr ly La W al 88 D/3 D/2 Discrete shears and brittle faulting in mid- e T D4 brittle-ductile ni Nisling Terrane Cretaceous and younger rocks. An 26 TRIASSIC shear Detailed geological mapping, with associated 65 56 Lewes River Group geochronology and geochemistry sampling was River s 76 en 09 58 Dawson City v 11 04 massive agglomerate Ste 86 65 ALASKA undertaken to better understand the Tally Ho 47 49 B variably strained volcaniclastic YT 80 12 Shear Zone and thus the relationship between 74 84 and volcanic rocks Stikinia and the Nisling Assemblage. 38 31 Mount massive augite-pheric basalt Stevens Tin 64 tin a Fa 58 44 19 moderately strained augite ult 72 83 38 basalt and volcaniclastic rocks De Tally Ho na li Fa 105 Mountain 48 20 37 81 marble ul 48 t 88 N = 168 Tally Ho Mountain 29 12 43 highly strained and mylonitic Contour Interval = 3 N = 63 1 0.60% 8 4.76% in relation to the 67 30 volcanic and volcaniclastic rocks 2 3 1.19% 1.79% 9 10 5.36% 5.95% pole to girdle mean elongation lineation Contour Interval = 3 1 1.59% 4 2.38% 11 6.55% 2 3.17% Annie Lake Road 79 5 2.98% 12 7.14% 3 4.76% A 68 Pa 6 3.57% 13 7.74% 4 6.35% and Wheaton Whitehorse 18 foliated leucogabbro 7 4.17% 5 7.94% r tr 76 37 River. Photo faces STUDY AREA id YT 38 Contoured stereoplot of poles to Contoured stereoplot of Wheaton TALLY HO D Watson Lake ge ANNIE LAKE southwest. SHEAR ZONE 56 66 massive pyroxenite BC foliation planes in the Tally Ho elongation lineations in the Tally Cr ROAD 67 17 WHEATON RIVER ee Limit of mapping/outcrop Mountain area. A weak girdle is Ho Mountain area. A weak cluster k 66 Location of study area and simplified tectonic boundaries of Stikinia Geological boundary definable by the methods of is definable by the methods of (defined, approximate, assumed) and the Nisling Assemblage. Tectonic boundaries are modified after 07 Dickson Woodcock and Naylor (1983). Woodcock and Naylor (1983). 74 Fault Hill 64 Wheeler and McFeely (1991). 34 31 (defined, approximate, assumed) 66 48 Fault (transcurrent) 34 45 Fault (thrust) 45 07 29 25 Foliation, schistosity Schematic representation of 85 68 02 25 deformed rocks on Tally Ho Lineation (elongation, rodding, mineral) 38 25 Mountain. Mylonitized MAJOR LITHOLOGIES 81 Minor fold axis Topographic 2 rocks typically have: 70 contour Synform, antiform 1) southeast trending and shallowly Road plunging elongation lineations Dyke pyroxenite ol? Water N 1 3 2) south plunging and east vergent The majority of rocks in the study area Contour Interval 100 feet B Line of cross section Swamp W S E crenulations A belong to the Triassic Lewes River Arc 10 cm 3) moderate to steeply dipping * Refer to Hart and Radloff (1990) for rock unit descriptions foliation (Hart and Radloff 1990). Major lithogies include leucogabbro, pyroxenite, mylonite, greenschist, augite basalt, marble and Augite porphyroclast on Dickson Hill volcaniclastic rocks. Dark grey, coarse-grained pyroxenite on Southwest GEOLOGICAL CROSS SECTION OF TALLY HO MOUNTAIN Northeast SUMMARY Tally Ho Mountain. Serpentinization is showing sinistral sigma-type augen. pervasive and cumulate textures with A B Rocks in the study area have experienced at Plutonic rocks of the region are part of the relict olivine (ol) may be present. 1900 least 3 phases of deformation since the Whitehorse Plutonic Suite, and include the 6000 late Triassic. metres feet Bennett Granite and Wheaton Valley 1600 5000 Granodiorite. Stereoplot analyses suggest that sub- 1300 4000 horizontal upright folding may be present in Cretaceous and Eocene volcanism cross- 1000 En-echelon tension gashes indicating the sheared rocks with movement cut structures throughout the study area. 500 metres sinistral semi-brittle shear in to the southeast. granodiorite on Mt. Wheaton. Crenulations may reflect folding of a thrust fault which juxtaposes ultramafic rocks Preferrential alignment of clasts in a against mylonite. nite le gr rb lo Characteristic coarse-grained texture of ma my strained volcaniclastic rock on the foliated leucocratic gabbro on Tally Future work will examine these and nite Mt. Stevens. lo Ho Mountain and Dickson Hill. my other field relations to help further the marble mylonite understanding of the tectonic evolution of the ma nite e northern Canadian Cordillera. rb l lo my ble ite m ylon m ar e bl ar m REFERENCES East facing photo of a scarp on the north face of Tally Ho East-vergent crenulation in mylonite on Crenulation lineations in augite mylonite South-vergent duplex structures on the Hart, C.J.R. and Radloff, J.K., 1990. Geology of Whitehorse, Alligator Lake, Fenwick Creek, Carcross and part of Robinson Map Areas (105D/11, 6, 3, 2 & 7). Indian and Northern Affairs Canada: Yukon Region, Open File Mountain. Intercalated marble and mylonite are in the Tally Ho Mountain. on Dickson Hill. north face of Tally Ho Mountain. 1990-4. Wheeler, J.O. and McFeely, P., 1991. Tectonic Assemblage map of the Canadian Cordillera and adjacent parts of the foreground with Wheaton Valley Granodiorite in the background. Photo faces east. United States of America. Geological Survey of Canada, Map 1712A, 1:2,000,000 scale. Woodcock, N.H. and Naylor, M.A., 1983. Randomness testing in three-dimensional orientation data. Journal of Structural Geology, vol 5, no 5.
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