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Earthquake source characteristics provide a valuable constraint on fault behaviour, crustal stress, and regional plate tectonics. In southwestern Yukon, a region of complex active tectonics, studies of earthquake sources have historically been limited by sparse seismic network coverage. In this work, we leverage recent improvements in station coverage to estimate focal mechanisms for small and moderate-magnitude (M ≥ 2.0) earthquakes from P-wave first-motion polarity data. We invert these data using a probabilistic method that rigorously quantifies mechanism uncertainties. We present preliminary solutions for 363 events, which improve the spatial coverage of the focal mechanism catalogue for this region. We observe contrasting P-axis orientations for events on either side of the Fairweather fault. For events within southwestern Yukon, the distribution of faulting mechanism types and P-axis orientations are relatively consistent. Our focal mechanism solutions support the existence of an unmapped fault south of the Duke River fault. Finally, our results provide a valuable input for subsequent detailed analysis of crustal stress throughout the region.

The southwestern Yukon Coast Belt mapping project is a joint Yukon Geological Survey/Geological Survey of Canada initiative operated under Natural Resources Canada’s GEM (Geomapping for Energy and Minerals) program. This project is aimed at investigating the geological relationships and mineral potential of the Kluane Schist, the Ruby Range batholith and the Yukon-Tanana terrane in southwestern Yukon. Bedrock mapping at 1:50 000-scale followed a 400 m line-spaced aeromagnetic survey flown in the winter of 2010. Preliminary results indicate the presence of a northeast-dipping structural stack through an ~40 km-thick crustal section, whereby the Kluane Schist occupies the lowest structural level and the Yukon-Tanana terrane the highest. The Ruby Range batholith intruded along the contact between the Kluane Schist and the Yukon-Tanana terrane, and was emplaced late in the deformation history. An orthogneiss/paragneiss unit of unknown tectonic affinity was mapped structurally between the Ruby Range and the Kluane Schist. Detrital zircon analyses from two samples of Kluane Schist indicate that the onset of deposition for this metasedimentary sequence occurred after ca. 94 Ma. Two significant metamorphic events, dated at 82 and 70 Ma, affected the Kluane Schist. This indicates that original structural juxtaposition between the Kluane Schist and the Yukon-Tanana terrane pre-dated intrusion of the Ruby Range batholith. Mineral potential in the Coast Belt area is significant and includes porphyry Cu-Mo-Au, epithermal Au-Ag and orogenic Au occurrences. The upper level of the Ruby Range batholith is most prospective for porphyry and epithermal mineralization, while the Kluane Schist is most prospective for orogenic Au mineralization.

In collaboration with the Yukon Geological Survey, Innovate Geothermal Ltd. performed a multi-component geoscientific investigation in southwestern Yukon to initiate the search for subsurface geothermal energy resources that could be used for direct use applications and, possibly, the generation of electricity. The study area for this project is located near the community of Burwash Landing and straddles the eastern Denali fault zone. The aim of this project is to analyze and interpret a variety of pre-existing and newly-acquired geological and geophysical data sets to identify favourable subsurface targets for a shallow, exploratory geothermal drilling program that could take place in the future. The geoscience work accomplished here includes both 2D map interpretation as well as construction of a 3D geologic model that was tested by geophysical inversion modelling of gravity and magnetic survey data. In addition, a literature review was conducted to identify analogous geothermal structural environments located in similar crustal-scale transform fault zones for comparison with geothermal systems that may be present in the vicinity of the Denali fault. Importantly, geophysical data from this study discovered a right-step in the Denali fault that has the appropriate orientation to form a small pull-apart zone in the Earth’s crust within the project area south of Duke River. Such crustal extension may generate fractures and permeability in rocks in the subsurface. Subsurface permeability in geothermal systems provides the pathway for hot geothermal fluids to ascend to drillable depths. This study has identified seven drilling targets, all located in the vicinity of the previously mentioned right-step in the Denali fault. The 3D geologic model generated for this study was utilized to help understand the lithologic domains and structures likely to be encountered by the proposed exploratory boreholes. The distribution of temperature in the subsurface, however, remains a significant unknown. Regional-scale, Curie point depth estimates suggest an average geothermal gradient of ~40°C/km near the eastern Denali fault, but drilling is required to measure actual subsurface temperatures. Based upon the encouraging results of this study, it is recommended that at least two of the seven targets are drilled to depths of 500–1000 m to obtain data on subsurface temperatures, fluids and geology.

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