End-on arc collision and onset of the northern Cordilleran orogen is recorded in Late Triassic to Jurassic plutons in the Intermontane terranes of Yukon, and in development of the synorogenic Whitehorse trough (WT). A synthesis of the extensive data set for these plutons supports interpretation of the magmatic and tectonic evolution of the northern Intermontane terranes. Late Triassic juvenile plutons that locally intrude the Yukon-Tanana terrane represent the northern extension of arc magmatism within Stikinia. Early Jurassic plutons that intrude Stikinia and Yukon-Tanana terranes were emplaced during crustal thickening (200–195 Ma) and subsequent exhumation (190–178 Ma). The syn-collisional magmatism migrated to the south and shows increasing crustal contributions with time. This style of magmatism in Yukon contrasts with coeval, juvenile arc magmatism in British Columbia (Hazelton Group), that records southward arc migration in the Early Jurassic. Exhumation and subsidence of the WT in the north were probably linked to the retreating Hazelton arc by a sinistral transform. East of WT, Early Jurassic plutons intruded into Yukon-Tanana record continued arc magmatism in Quesnellia. Middle Jurassic plutons were intruded after final enclosure of the Cache Creek terrane and imbrication of the Intermontane terranes. The post-collisional plutons have juvenile isotopic compositions that, together with stratigraphic evidence of surface uplift, are interpreted to record asthenospheric upwelling and lithospheric delamination. A revised tectonic model proposes that entrapment of the Cache Creek terrane was the result of Hazelton slab rollback and development of a sinistral transform fault system linked to the collision zone to the north.

for a copy of this paper please contact the Yukon Geological Survey; geology@gov.yk.ca.

Lower Paleozoic continental margin rocks of the North American Cordillera, from Yukon to Nevada, include coeval platformal carbonate and basinal clastic strata that are offset along rift‐transfer faults, including the Liard, St. Mary‐Moyie, and Snake River structures. The Dawson fault is a prominent east‐weststriking structure in central Yukon that is interpreted herein to have been active as a rift‐transfer fault by late Cambrian time. This hypothesis is supported by new zircon U‐Pb dates that range from 501.98 ± 0.17 Ma to 497.57 ± 0.70 Ma from alkaline mafic volcanic rocks concentrated along the Dawson fault. The development of a sub‐Jiangshanian unconformity immediately post‐dates this alkaline magmatism and indicates that final continental breakup and establishment of the northern Cordilleran margin occurred by the late Miaolinginan. Alkaline magmatism caused by local decompression partial melting of the mantle may have been triggered by the release of in‐plane tensile stresses during lithospheric rupture and edge‐driven mantle convection. Upper Ordovician alkaline mafic volcanic and plutonic rocks that occur along a northwest‐southeast striking segment of the Dawson fault erupted ∼50 Myr after breakup and represent an example of post‐rift magmatism along a rift‐transfer fault. New bedrock mapping, and geochronological, paleontological, and petrological results from Upper Ordovician rocks indicate that there was localized basin development and punctuated volcanism along the Dawson fault from 453 to 447 Ma. Late Ordovician extension and post‐breakup magmatism in central Yukon is compatible with dextral strike‐slip reactivation of the Dawson rift‐transfer fault associated with counterclockwise rotation of Laurentia.

The early Tertiary magmatic episode in the northern Canadian Cordillera is linked to the restructuring of the Kula-North American plate system from orthogonal to oblique convergence. Resultant volcanism was widespread, and remnant successions outcrop along the eastern margin of the Coast Plutonic Complex (CPC). The Sifton Range volcanic complex of southwestern Yukon is a member of the Paleogene Sloko-Skukum Group, and comprises a 900-m thick, shallow-dipping, volcanic succession dominated by intermediate to evolved lava and pyroclastic rocks deposited in a northwesterly trending half-graben. Locally, the volcanic sequence is intruded by alkali-feldspar granites of the CPCs Nisling Plutonic Suite dated at 57.5 Ma. Felsite sills radiate from the main intrusive body, and together with numerous basaltic to dacitic dykes traverse the volcanic package. Both the felsic volcanic rocks and epizonal granitoids exhibit anomalous enrichments in large-ion lithophile elements indicating crustal contributions during the late-stage petrogenesis of the complex. In addition, the Sifton Range intrusive rocks exhibit modal mineralogy reflective of lower ambient pressures relative to the compositionally similar Annie Ned granites along the Alaska Highway between Stony Creek and Mendenhall, 20 km south of the complex. The amount of post-Eocene uplift (ca. 30 m/Ma) that exposed the contact between the intrusive and corresponding volcanic rocks is constrained by the presence of a calc-silicate bed at an elevation of 1830 m within the upper volcanic stratigraphy.

for a copy of this paper please contact the Yukon Geological Survey; geology@yukon.ca.

Seismic investigations were carried out at the confluence of the North and Central Arms of the Kaskawulsh Glacier, St. Elias Mts. Low velocities near the glacier surface are apparently due to melting, fracturing, and high porosity. It is concluded that velocity anisotropy is mainly caused by the foliation structure of alternating layers of clear and bubbly ice; it also occurs where surface ice has a strong fracture pattern. Greatest depth of ice in the Central Arm, 3,000 m wide, and in the combined glacier, 5,000 m wide, is about 1,000 m. The North Arm is less deep; both arms are roughly parabolic in cross section. The base of the ice is lower than the glacier terminus, but no bedrock depression was found at the confluence. It is concluded that only under especially favorable circumstances can seismic measurements be used practically to study crystallographic fabrics in glacier ice.

The Logan fault is one of many dextral faults that strike generally parallel to the northwest trend of the Northern Canadian Cordillera. Though widely documented, the timing and magnitude of displacement on many of these orogen-parallel faults are not well-constrained. Here, we present new low-temperature thermochronology data and thermal history models from the Upper Hyland River Valley area in the Logan Mountains in southeastern Yukon that document accelerated cooling between ca. 60–50 Ma in localized regions adjacent to faults that strike obliquely to and likely connect with the Logan fault. We propose that this phase of localized cooling was driven by the activation of a network of faults in the Upper Hyland River Valley, resulting in ∼4–5.5 km of local exhumation. We suggest that faulting in the Upper Hyland River Valley was synthetic with dextral slip on the Tintina fault, a lithospheric-scale structure thought to have accommodated ∼430 km of dextral strike-slip.

This field trip crosses southwestern Yukon and examines the geology of the Intermontane and Insular terranes, and synorogenic sedimentary basins that developed during Mesozoic terrane accretion.

Late Triassic to Early Jurassic age (~220-185 Ma) intrusions comprise one of the most widespread and volumetrically significant plutonic suites in central and western Yukon, and eastern Alaska, but have received very limited study thus far. A new research project has been initiated that will examine the temporal, geochemical and petrotectonic evolution of this magmatic event, and the nature and origin of associated Cu, Au and PGE mineralization. The intrusions are mainly hornblende- and biotite-bearing granodiorites and quartz monzonites, although granitic phases and rare ultramafic phases (as at Pyroxene Mountain) are also present. Several bodies of coarse-grained muscovite granite that are included within the suite have been recognized in southwestern Dawson, and central and western Stewart River map areas. Most intrusions give preliminary U-Pb zircon and titanite ages of ~195 Ma to ~185 Ma, although scattered bodies give ages up to 218 Ma. Geochemical studies completed thus far indicate that most intrusions are metaluminous and formed in a volcanic arc environment, although some of the muscovite-granite phases in western Yukon are peraluminous and trend into the anorogenic (within-plate) granite field on various tectonic discriminant plots. Dating studies at Minto and Williams Creek indicate that copper-gold mineralization in both areas is hosted in part by deformed intrusions dated at ~194 Ma and is crosscut by massive, post-mineralization Granite Mountain batholith dated at ~190 Ma. The mineralization is therefore intimately associated with the Triassic-Jurassic magmatism, and we tentatively interpret the deposits as deformed copper-gold porphyries.

A copy of this thesis is available at the EMR library – QE195.S748 1994.