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.

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.

A copy of this thesis is available at the EMR library – QE195.C633 2011. This thesis is available online at http://hdl.handle.net/2429/38156.

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

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.

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.

This is a multidisciplinary study which combines sedimentology and sedimentary petrology, palynology, organic petrology and structural analysis of the middle Cenozoic Amphitheatre Formation in the St. Elias Mountains, and documents syntectonic deposition in strike-slip basins along the Denali fault system. The outcrops of the Amphitheatre Formation can be divided into two discrete basins:: the northern Burwash basin and the southern Bates Lake basin. Sedimentological analysis of the Burwash basin identified fault-controlled depocentres which allowed the development of several different types of non-marine depositional environments in close proximity. Palynology and organic petrology data indicate that the Amphitheatre Formation is diachronous and spans the Eocene-Oligocene boundary. Light-mineral provenance studies of sandstones, clast-counts in conglomerates and paleocurrent analyses suggest that the Wrangellia and Yukon Crystalline Terranes were sources for the Amphitheatre Formation in the Burwash basin. In contrast, preliminary work suggests that Wrangellia and possibly the Gravina-Nutzotin Terrane may have been important sources for the Amphitheatre Formation in the Bates Lake basin. Structural data combined with geologic mapping indicate a predominance of strike-slip deformation during and after deposition of the Amphitheatre Formation in the Burwash basin. The presence of syndepositional faults with subhoriziontal slickenside indicates that strike-slip deformation occurred during deposition of the Amphitheatre sediments. The Burwash basin contains structures indicative of both contractional and strike-slip deformation, whereas the Bates Lake basin contains structures indicative of both extensional and strike-slip deformation. The change in structural style between the two basins suggests that the Amphitheatre Formation may have been deposited in transpressional as well as transtensional tectonic settings along individual segments of the Denali fault system during the middle Cenozoic.

Reconnaissance geological mapping in the Coal River map area of southeastern Yukon investigated several small mid-Cretaceous plutons. The intrusions are composed of unfoliated or incipiently foliated, fine to coarse-grained, equigranular and porphyritic, biotite ± hornblende quartz monzodiorite to granodiorite. They are metaluminous to peraluminous and have reduced to oxidized geochemical characteristics. The composition of selected samples is consistent with magma formation from partial melting of infracrustal source rocks.U-Pb ages were obtained for nine plutons from five or six zircon single-grain analyses by the isotope dilution thermal ionization mass spectrometry method with chemical abrasion (CA-TIMS). All interpreted ages are concordant within statistical uncertainty. The plutons range in age from 99.80 ± 0.03 to 97.70 ± 0.03 Ma. Given the primarily unfoliated nature of the plutons, contractional, fabric-forming deformation within the Cordilleran orogeny must therefore have largely ceased at the present level of exposure in the Coal River area by the time of intrusion (ca. 98 Ma).The ages and compositions of the plutons in Coal River map area are consistent with their being part of the Tay River plutonic suite, a northwest-trending belt of coeval and compositionally similar plutons and local volcanic rocks (South Fork volcanic suite) that, when augmented by the addition of the Coal River plutons, extends approximately 465 km with a width of up to 150 km.

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

These data include structural measurements used to assess the evolution of Neogene faulting in the lower Colorado River region near Blythe, California. These datasets are associated with a manuscript accepted for publication: Mavor, S.P., Bennett, S.E.K., Crow, R.S., Singleton, J.S., Langenheim, V., Stockli, D., Stelten, M., Brickey, T.A., Sr., Umhoefer, P.J., and Beard, L.S., 2023, Evolution of Miocene normal and dextral faulting in the lower Colorado River region near Blythe, California, USA: Geosphere, v. 19, https://doi.org/10.1130/GES02608.1.