Abstract: The Finlayson Lake fault zone forms the boundary between autochthonous North American rocks and rocks of the innermost accreted Slide Mountain and Yukon-Tanana terranes in southeastern Yukon. Geological mapping at
1::50 000 scale in a well exposed area of the Campbell Range, southeastern Yukon, was undertaken to examine the kinematics of the Finlayson Lake fault zone and rock types of the Slide Mountain terrane.

Five units were identified: (1) chloritic schist and phyllite, (2) laminated metachert and carbonaceous black slate, (3) tan weathering metachert and maroon siliceous and argillaceous metasiltstone, (4) greenstone and associated breccia, gabbro, metagreywacke, metachert and maroon metasiltstone and (5) serpentinite. Unit 2 is structurally interleaved with submap-scale bodies or layers of serpentinite, hornblende-plagioclase porphyry, plagioclase-potassium fledspar porphyry, quartz-eye muscovite-chlorite phyllite or schist, chloritic schist and minor grey, calcareous metacarbonate. Serpentinite is also exposed in unit 4 and as small slivers along the thrust contact between units 3 and 4.

Lithologically, units 4 and 5 are similar to the upper division of the Slide Mountain terrane in east-central and north-central British Columbia. Unit 2 has similarities with the lowest division of the Sylvester allochthon and is tentatively correlated with the Slide Mountain terrane. Maroon metasiltstone in unit 3 is indistinguishable lithologically from metasiltstone in the overlying greenstone unit suggesting that the eastern thrust fault juxxtaposes parts of the same depositional sequence, ie. The Slide Mountain terrane. Regional correlation of unit 1 is unclear.

Unit 2 is inferred to be bounded to the east and west by northwest-striking faults and to the south, by an east-striking, steeply dipping, normal (north-side down) fault. The northern boundary of unit 2 is unconstrained. Greenstone (unit 4) is thrust towards the southwest over unit 1 in the western part of the map area along a northwest-striking, gently northeast-dipping thrust fault. In the eastern part of the map area, greenstone is thrust towards the northeast over unit 3 along a northwest-striking, moderately southwest-dipping thrust fault. Outcrop data and topographic patterns suggest that the eastern thrust fault is truncated by a northwest-striking, steeply dipping fault and that the normal fault truncates the westernmost northwest-striking fault. The northwest-striking faults are poorly exposed and their kinematics have yet to be determined. However, if they are steep faults, they are likely dextral strike-slip faults.

Field data indicate that the Finlayson Lake fault zone consists of diverging thrust faults and subparallel strike-slip(?) faults. These structures are consistent with the interpretation of the Finlayson Lake fault zone as a transpressive fault zone. More constraints on the relative timing of faulting and the kinematics of the steep faults are required to test this hypothesis,

Authors: Plint, H.E.

Map Scale: 1 : 0

NTS Mapsheet(s): 115P16, 105G08, 105G09, 105H05, 105H12

Citation: Plint, H.E. , 1995. Geological Mapping in the Campbell Range, Southeastern Yukon (Parts of 105 G/8, G/9 and 105 H/5, H/12). In: Yukon Exploration and Geology 1994, Yukon Geological Exploration , Indian & Northern Affairs Canada/Department of Indian & Northern Development: Exploration & Geological Services Division, 47-57.

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