Neoproterozoic to lower Paleozoic basin and platform strata that formed during and after rifting along the western Laurentian margin are preserved in the northern Cordillera. Several pulses of magmatism occur within margin strata and are concentrated along the Dawson fault in central Yukon. Magmatism is dated as late Cambrian and Late Ordovician using: 1) U-Pb zircon geochronology of volcaniclastic rocks; and 2) fossil ages from strata interbedded with, and enclosing, volcanic rocks. Volcanic rocks from both pulses are predominantly alkaline and basic and erupted in subaqueous environments. The trace element geochemical compositions of the rocks suggest that they formed from partial melting of enriched lithosphere from the garnet stability field.

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@yukon.ca.

The breakup of the supercontinent Rodinia and development of the western Laurentian rifted margin are in part recorded by Neoproterozoic to mid-Paleozoic igneous and sedimentary rock successions in the Canadian Cordillera. New bedrock mapping and volcanic facies analysis of Early Ordovician mafic rocks assigned to the Menzie Creek Formation in central Yukon allow reconstruction of the depositional environment during the volcanic eruptions, whole-rock geochemical data constrain the melting depth and crust-mantle source regions of the igneous rocks within the study area, and zircon U-Pb age studies provide determination of the precise timing of submarine eruptions. Menzie Creek Formation volcanic rocks are interlayered with continental slope strata and show lithofacies consistent with those of modern seamount systems. Representative seamount facies contain several kilometers of hyaloclastite breccia and pillow basalt with rare sedimentary rocks. Menzie Creek Formation seamounts form a linear array parallel to the Twopete fault, an ancient extensional or strike-slip fault that localized magmatism along the nascent western Laurentian margin. Zircon grains from two volcanic successions yielded high-precision chemical abrasion–thermal ionization mass spectrometry (CA-TIMS) dates of ca. 484 Ma (Tremadocian), which are interpreted as the age of eruption. Menzie Creek Formation rocks are alkali basalt and have oceanic-island basalt–like geochemical compositions. The whole-rock trace element and Nd-Hf isotope compositions are consistent with the partial melting of subcontinental lithospheric mantle at ~75–100 km depth. Post-rift, Early Ordovician seamounts in central Yukon record punctuated eruptive activity along a rift-related fault, the separation of a continental fragment from western Laurentia, or the oblique post-breakup kinematics from the counterclockwise rotation of Laurentia that facilitated local extension in the passive margin.

Basalt and minor rhyolite flows and breccias in the northwestern extension of Selwyn Basin are stratigraphically above maroon argillite (Lower Cambrian Hyland Group) and beneath black chert (Middle Ordovician Road River Group). All are intensely folded and repeated by shallow thrust faults. The lower part of the volcanic succession is dominated by subaqueous flows, and the upper part includes shallow and subaerial breccias and limestone pods. The volcanic unit is interpreted as many small overlapping seamounts fed by gabbroic dykes. The basaltic rocks are alkalic and contain high concentrations of TiO2 (1.7 - 3.6%), P2O5 (0.5 - 1.2%) and Zr (140 ppm). They resemble the volcanic Marmot Formation in the northeastern part of Selwyn Basin, and are consistent with extension and thinning of continental crust. The volcanic rocks lack significant sulphide mineralization, except where hornfelsed near Cretaceous intrusions. Local high barium concentrations suggest that volcanism may have contributed to stratabound barite in overlying Devonian shales.

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.

Mesozoic to Tertiary rocks in the Rapid depression on the Yukon North Slope are dissected by a N-NNE striking fault array. Two phases of Tertiary deformation are recorded across the Rapid depression east of the Barn fault: D1 is characterized by faults with either sinistral or dextral strike-slip displacement and overall ~W-E to NW-SE contraction; D2 faults developed under a regime of ~W-E-oriented contraction and N-S extension. The southern segment of the Barn fault is interpreted as an oblique dextral fault. The structural style is inconsistent with the propagation of a large-scale strike-slip fault zone such as the Kaltag fault through the Rapid depression, as previously suggested, but rather may indicate reactivation of older structural heterogeneity in the subsurface.

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

In the southeastern Yukon Territory, Quaternary continental alkaline basalts have erupted across an important crustal suture, the Tintina Trench, which separates the accreted terranes of the Canadian Cordillera from the ancestral North American craton. The lavas from the Rancheria region from the west side of the Tintina Trench are basanites (BASAN), alkaline olivine basalts (AOB), and hypersthene-normative basalts (HYN). They display fractionated rare earth element (REE) profiles and are enriched in light rare earth elements (LREE) and high field strength elements (HFSE). The compositional spectra of the Rancheria alkaline magmas appears to represent the progressive melting of an amphibole-bearing garnet lherzolite. The involvement of amphibole in the petrogenesis of the Rancheria alkaline magmas indicates that these magmas were generated within the lithosphere. At the eastern end of the Rancheria suite, on the east side of the Tintina Trench, the AOB from Watson Lake have higher Zr contents than Rancheria AOB to the west of the Trench. The high Zr contents of the Watson Lake AOB are similar to those observed in the Hoole Eocene tholeiitic basalts, on the east side of the Tintina Trench, further to the north. The Eocene basalts from the Hoole River region are olivine tholeiites which have experienced closed-system crystal fractionation of olivine at low pressure. The estimated primary magma for these Eocene basalts appears to have been derived by partial melting of an incompatible-element enriched lithospheric mantle source, during which garnet was not a residual phase. The Nb-Zr systematics of the Watson Lake basalts indicate that they may be derived by mixing between melts produced by melting of an amphibole-bearing residue and a lithospheric mantle similar in composition to that of the Hoole basalts. Therefore, these compositional differences in the alkaline basalts across the Tintina Trench appear to reflect the juxtaposition of chemically distinct continental lithospheric mantles, indicating that the Tintina Fault is a steep lithospheric suture.

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.