Gibson, Dan

Peak metamorphism of the Wolverine metamorphic complex (WMC) occurred at conditions of 770°-830°C and 7.2-10.4 kbars, and was accompanied by partial melting and the development of tight to isoclinal northeast-vergent folds that are axial planar to a transposition foliation. The WMC is modelled as a diffuse northeast-vergent shear zone that formed beneath a southwest-vergent panel of rocks as the orogenic wedge detached and translated northeastward during Mesozoic contraction. Juxtaposition of Middle Jurassic greenschist-facies upper crustal rocks against upper amphibolite-facies rocks of the WMC, which contain Eocene 40Ar/39Ar cooling ages, suggests that the WMC remained at deep crustal levels until it was rapidly exhumed in the Eocene along the normal Wolverine fault. Mineral reaction and disequilibrium textures indicate a near- isothermal decompression path from 7.2-10.4 kbars to below 4 kbars, corresponding to a minimum of 11 km of exhumation prior to cooling below ~650°C.

The lowest structural and stratigraphic levels of the Yukon-Tanana terrane and the structurally underlying parautochthonous North American margin rocks were metamorphosed at similar conditions (7.5 – 9 kbar; 600 – 680 °C) and share a common style of deformation, characterized by the transposition of lithologic contacts and primary compositional layering into a regional ductile foliation (ST) with at least one generation of intrafolial isoclinal folds. Equilibrium assemblage modeling and in situ U-Th-Pb SHRIMP dating of monazite are used to place quantitative constraints on the conditions and timing of deformation, metamorphism and subsequent decompression. These data reveal that these fabrics, and the associated metamorphism, did not develop during a single tectono-metamorphic event. Rather, ductile deformation and high-grade metamorphism developed diachronously. Rocks deformed and metamorphosed in the Permo-Triassic and Early Jurassic were exhumed in the Early Jurassic, while rocks to the northeast in the Finlayson domain were buried, heated and ductiley deformed at mid- crustal levels (~25 km depth) from the Middle Jurassic to Early Cretaceous (c. 169 – 142 Ma). Metamorphism continued at an even deeper crustal level (~ 30 km depth, as recorded in the Australia Mountain domain), propagating downward into the parautochthonous North American crust in the Early Cretaceous (c. 146 – 118 Ma). Together, these data reveal a spatial and temporal pattern of structurally downward younging deformation and metamorphism that corresponds with the foreland-directed growth of a critically tapered orogenic wedge. In this model, rocks in front of the wedge are episodically underthrust downward into a distributed, high-grade transposition shear zone at 25 to 30 kilometres depth near the base of the overriding wedge. Rocks previously underthrust, buried and metamorphosed are progressively exhumed to higher structural levels within the wedge, as the upper crust enters a state of extension in order to maintain a critically tapered wedge. Rocks that occupied the mid-crustal shear zone in the Middle Jurassic and Early Cretaceous (Finlayson and Australia Mountain domains) were exhumed in the mid-Cretaceous along southeast-directed (orogen-parallel) extensional faults, from beneath a supracrustal ‘lid’ that had previously been metamorphosed and ductiley deformed at amphibolite facies in the Permo-Triassic and Early Jurassic.

Quaternary glacial and non-glacial sediment exposed at White River and Silver Creek provide a record of environmental change in southwest Yukon for much of the late- Middle to Late Pleistocene. Eighteen sites at White River, located beyond the marine oxygen isotope stage (MIS) 2 glacial limit, contain thick accumulations of till, loess, peat, gravel and glaciolacustrine silt and clay, with tephras, paleosols, plant and insect macrofossils and large mammal fossils. Radiocarbon ages and eleven tephra beds constrain two tills to MIS 4 and 6. These tills correlate to the Gladstone and Reid glaciations and represent the penultimate and maximum all-time limits of the St. Elias lobe of the northern Cordilleran Ice Sheet. Two peat beds located between these tills indicate that interglacial conditions existed in the area during MIS 5e and 5a. Pond sediment deposited during mid-MIS 5 suggests that the sites were covered by an open birch tundra at this time. The MIS 3/2 transition was marked by a treeless, dry steppe- tundra populated by mammoth, horse and bison. The eleven Silver Creek sites, located ~200 km up-ice, contain a similar record of glacial and non-glacial sediment. Infrared-stimulated luminescence (IRSL) and radiocarbon dating constrain the glacial deposits at these sites to MIS 2, 4, either MIS 7 or 6, and to two Early to Middle Pleistocene, Pre-Reid glaciations. Tilting of glaciolacustrine beds of up to 1.9 mm/yr may be from uplift along the Denali fault since MIS 7. Pollen and macrofossils analyses from overlying MIS 3-aged sediment suggest that the environment was dominated by herbs and forbs, with few shrubs and almost no tree pollen at this time. Combined, the White River and Silver Creek sites contain a record of glacial and non-glacial conditions in southwest Yukon since the Middle Pleistocene. The glacial limits in southwest Yukon are markedly different from those in central Yukon. In southwest Yukon, the glacial limits are closely-spaced and were more extensive in the Middle to Late Pleistocene than in the Late Pliocene and Early Pleistocene. In central Yukon, glacial limits are separated by up to 300 km and were most extensive in the latest Pliocene and Early Pleistocene. This suggests that different forcing mechanisms controlled the extents of the St. Elias and Selwyn lobes during successive glaciations. Boundary conditions such as varying substrates, topography, moisture pathways and atmospheric circulation likely had a greater affect than tectonics and sea level on these glacial limits throughout the Plio-Pleistocene.