23. Kent, BAP; Dashtgard, SE; Huang, CQ; MacEachern, JA; Gibson, HD; Cathyl-Huhn, G.Initiation and early evolution of a forearc basin: Georgia Basin, Canada.Basin Res., 2020, 32: 163-185 Initiation and early evolution of a forearc basin: Georgia Basin, Canada
coal; forearc basins; Late Cretaceous; sequence stratigraphy; stratigraphy; T-R sequences; transgression
The lower Nanaimo Group was deposited in the (forearc) Georgia Basin, Canada and records the basin's initiation and early depositional evolution. Nanaimo Group strata are subdivided into 11 lithostratigraphic units, which are identified based on lithology, paleontology, texture and position relative to both the basal nonconformity and to each other. Significant topography on the basal nonconformity, however, has resulted in assignment of lithostratigraphic units that are not time correlative, and hence, cannot reliably be used to accurately reconstruct basin evolution. Herein, we present a sequence stratigraphic framework for lower Nanaimo Group strata in the Comox Sub-Basin (northern Georgia Basin) that integrates both facies analysis and maximum depositional ages (MDAs) derived from detrital zircon. This stratigraphic framework is used to define significant sub-basin-wide surfaces that bound depositional units and record the evolution of the basin during its early stages of development. Seven distinct depositional phases are identified in the lower 700 m of the lower Nanaimo Group. Depositional phases are separated by marine flooding surfaces, regressive surfaces, or disconformities. The overall stratigraphy reflects net transgression manifested as an upwards transition from braided fluvial conglomerates to marine mudstones. Transgression was interrupted by periods of shoreline progradation, and both facies analysis and MDAs reveal a disconformity in the lowermost part of the Nanaimo Group in the Comox Sub-Basin. Stratigraphic reconstruction of the Comox Sub-Basin reveals two dominant depocenters (along depositional strike) for coarse clastics (sandstones and conglomerates) during early development of the Georgia Basin. The development and position of these depocenters is attributed to subduction/tectonism driving both subsidence in the north-northwest and uplift in the central Comox Sub-Basin. Our work confirms that in its earliest stages of development, the Georgia Basin evolved from an underfilled, ridged forearc basin that experienced slow and stepwise drowning to a shoal-water ridged forearc basin that experienced rapid subsidence. We also propose that the Georgia Basin is a reasonable analogue for ridged forearc basins globally, as many ridged forearcs record similar depositional histories during their early evolution. DOI
22. Huang, C; Dashtgard, SE; Kent, BAP; Gibson, HD; Matthews, WA.Resolving the Architecture and Early Evolution of a Forearc Basin (Georgia Basin, Canada) Using Detrital Zircon.Sci Rep, 2019, 9: 15360 Resolving the Architecture and Early Evolution of a Forearc Basin (Georgia Basin, Canada) Using Detrital Zircon
Convergent-margin basins (CMBs) are commonly associated with active arcs, and hence are rich in detrital zircon (DZ) whose ages closely reflect the timing of deposition. Consequently, maximum depositional ages (MDA) from DZ geochronology can be employed to resolve the stratigraphy and evolution of CMBs. Herein, we use DZ to revise the internal architecture of the lower Nanaimo Group, which partially comprises the fill of the (forearc) Georgia (or Nanaimo) Basin. Maximum depositional ages and multi-dimensional scaling of DZ age distributions are employed to determine chronologic equivalency of strata and assess sediment provenance variability within the pre-existing lithostratigraphic framework. The results are compared to a recently developed sequence stratigraphic framework for the lower Nanaimo Group. The basal lithostratigraphic unit of the Nanaimo Group, the Comox Formation (Fm), comprises strata that are neither time correlative nor genetically related. The three lithostratigraphic units directly overlying the Comox Fm (Haslam, Extension, and Protection formations) comprise strata with similar genetic affinities and MDAs that indicate deposition of these units was not always sequential and locally was contemporaneous. Through this work, we provide an example of how MDAs from DZ geochronology in CMBs can resolve basin-scale stratigraphic relations, and identify chronological changes in sediment provenance. DOI PubMed
21. Monger, JWH; Gibson, HD.Mesozoic-Cenozoic deformation in the Canadian Cordillera: The record of a "Continental Bulldozer"?Tectonophysics, 2019, 757: 153-169 Mesozoic-Cenozoic deformation in the Canadian Cordillera: The record of a "Continental Bulldozer"?
Cordillera; Craton trajectory; Structures; Terrane accretion; Tectonic evolution
A possible Mesozoic-Cenozoic trajectory for the North American craton is outlined from latitude changes of the craton derived from a revised apparent polar wander path, and from westward movement of the craton based on the assumption that Africa has been the least mobile continent geographically since the latest Paleozoic. During each of five time intervals that span 220 million years, the craton trajectory had a different vector. Each vector appears to be reflected in the Canadian Cordillera by the dominant style and orientation of structures formed during that interval. For much of the past similar to 220 million years, the similar to meridionally-oriented western margin of the Pangea-Laurasia-North America Plate has been the site of arc magmatism where weak arc/back arc lithosphere, sandwiched between strong ocean-floor and craton lithospheres at times coupled across convergent or transform plate boundaries, focused and recorded strain. When the craton apparently moved due westward, between similar to 180-160 Ma an similar to 120-60 Ma, the dominant structures formed then record orogen-normal compression and were accompanied by orogeny. Structures formed during the earlier episode are mainly in the eastern and interior Cordillera and their formation shortly followed or coincided with accretion of most terranes to the craton margin. In the later episode, compressional structures span the entire Cordillera, which emerged as a tectonic and physiographic entity. Before and between these intervals, when the craton apparently moved mostly northwestward, mainly geological and paleomagnetic considerations indicate some terranes moved southward (sinistrally) relative to the craton. After similar to 60 Ma, southwestward movement of the craton was concurrent with large northward (dextral) strike-slip faults that disrupted the newly-established ancestral Cordillera. The coincidence between the age of structures that record dominant orogen-normal compression at times when the craton apparently moved due westward, and orogen-parallel displacements when the craton had either northward or southward components of motion, suggests the craton, acting as a "Continental Bulldozer", was the primary driver of Cordilleran deformation and orogenesis. DOI
20. Schultz, SK; MacEachern, JA; Gibson, HD.Late Mesozoic reactivation of Precambrian basement structures and their resulting effects on the sequence stratigraphic architecture of the Viking Formation of east-central Alberta, Canada.Lithosphere, 2019, 11: 308-321 Late Mesozoic reactivation of Precambrian basement structures and their resulting effects on the sequence stratigraphic architecture of the Viking Formation of east-central Alberta, Canada
The Lower Cretaceous Viking Formation is a siliciclastic unit that occurs in the subsurface of Alberta in the Western Canadian sedimentary basin. This study focuses on a lowstand paleoshoreline trend extending along strike between two hydrocarbon-producing fields, Joarcam and Judy Creek (250 km NW). The Viking Formation in these fields records depositional thicknesses ranging from 20 to 30 m. Between these two fields, however, the formation is anomalously thick (45-60 m), complicating the recognition and correlation of key stratigraphic surfaces. Marine flooding surfaces above and below the Viking Formation are routinely employed as stratigraphic datums in order to remove postdepositional deformation and facilitate the development of a sequence stratigraphic framework. However, as each successive surface is employed as the datum, the other flooding surfaces within the formation become distorted, resulting in unrealistic depositional geometries. These geometries are best explained to be the result of structural readjustments during Viking deposition. The Precambrian lithosphere of the Canadian Shield forms the Western Canadian sedimentary basin basement, with major structures previously mapped using gravity and magnetic anomaly studies. Locally, the increased accommodation observed within the Viking Formation of central Alberta is attributed to differential reactivation of the Paleoproterozoic Snowbird tectonic zone basement structures, which flank the areas of anomalously thick deposits and trend approximately normal to the regional strike of the Western Canadian sedimentary basin. The Snowbird tectonic zone faults are interpreted to have been reactivated during renewed tectonic loading in the southern Canadian Cordillera during Aptian-Albian time, causing subtle readjustments along basement faults that caused variable syndepositional subsidence. By selecting successive datums, the gross Viking interval can be recognized to have accumulated prior to, during, and following structural reactivation. DOI
19. Thiessen, EJ; Gibson, HD; Regis, D; Pehrsson, SJ; Cutts, JA; Smit, MA.High-grade metamorphism flying under the radar of accessory minerals.Geology, 2019, 47: 568-572 High-grade metamorphism flying under the radar of accessory minerals
Uranium-lead (U-Pb) accessory mineral petrochronology has been increasingly used to constrain the timing of tectonometamorphic events. However, because mafic rocks commonly lack minerals with a high U/Pb ratio, they may be underrepresented in the chronologic record. This study on polymetamorphic mafic granulites from the Archean Rae craton (northern Canada) provides a striking example of a metamorphic cycle that has been entirely overlooked. We utilized Lu-Hf garnet geochronology and equilibrium phase diagram modeling to characterize two high-pressure granulite-facies mineral assemblages that affected the 2.6 Ga protolith. Zircon and garnet recrystallization occurred at 1.87 Ga within a gneissic foliation, while a coarse-grained garnet precursor nucleated 230 m.y. earlier during a stage of high heat flow within thickened lower crust, the latter of which is nearly absent in the zircon and monazite age record except for rare igneous occurrences. Combined garnet geochronology and petrological modeling reinforce a ca. 1.9 Ga age for high-grade overprinting in the southern Rae craton and clearly show within the same sample that U-Pb accessory minerals did not grow during a newly identified 2.11 Ga granulite-facies event. DOI
18. Parsons, AJ; Coleman, MJ; Ryan, JJ; Zagorevski, A; Joyce, NL; Gibson, HD; Larson, KP.Structural evolution of a crustal-scale shear zone through a decreasing temperature regime: The Yukon River shear zone,Yukon-Tanana terrane, Northern Cordillera.Lithosphere, 2018, 10: 760-782 Structural evolution of a crustal-scale shear zone through a decreasing temperature regime: The Yukon River shear zone,Yukon-Tanana terrane, Northern Cordillera
We present the first detailed structural analysis of the Yukon River shear zone (YRSZ), which forms an important structural break within the Yukon-Tanana terrane of the Northern Cordillera in Yukon (Canada). The YRSZ is a NW-SE-striking shear zone that juxtaposes Mississippian orthogneiss hanging-wall rocks (Simpson Range suite) against pre-Late Devonian metasedimentary footwall rocks (Snowcap assemblage). Field and microstructural analyses, including quartz c-axis fabric investigation, indicate that the YRSZ initiated as a top-ESE mid-crustal shear zone active through a temperature range of >= 650-500 degrees C to similar to 540-440 degrees C. Constraints from the footwall associated with top-ESE shearing on the YRSZ at mid-crustal conditions record a decrease in deformation temperature toward the shear zone, coincident with a transition from coaxial to non-coaxial deformation and an increase in fabric intensity, strain rate, and differential stress estimates. Collectively, these spatial trends represent a classic example of a narrowing shear zone that progressively localizes and intensifies deformation as ambient temperature decreases. U-Pb zircon geochronometry of a deformed Permian orthogneiss from within the YRSZ combined with previously published thermochronometry bracket the timing of top-ESE mid-crustal shearing between 259 +/- 2 Ma and 176-168 Ma, either during Late Permian-Middle Triassic metamorphism and lithospheric extension or latest Triassic-Early Jurassic metamorphism and crustal thickening. The YRSZ was subsequently reactivated as a top-WNW upper-crustal thrust fault zone during or after Early to Middle Jurassic cooling and exhumation at 176-168 Ma. This top-WNW thrusting within the YRSZ may be responsible for structural separation of Late Triassic and Early Jurassic plutonic rocks in the hanging wall of the YRSZ from Permian plutonic rocks in its footwall. DOI
17. Thiessen, EJ; Gibson, HD; Regis, D; Pehrsson, SJ.Deformation and extensional exhumation of 1.9 Ga high-pressure granulites along the Wholdaia Lake shear zone, south Rae craton, Northwest Territories, Canada.Lithosphere, 2018, 10: 641-661 Deformation and extensional exhumation of 1.9 Ga high-pressure granulites along the Wholdaia Lake shear zone, south Rae craton, Northwest Territories, Canada
The origin of high-pressure granulites in the south Rae craton and Snowbird tectonic zone (STZ) is highly enigmatic. Current models for their formation and exhumation envisage continental collision at 2.55 Ga and intracratonic orogenesis at 1.9 Ga, or collision and exhumation at ca. 1.9 Ga. As an attempt to reconcile these disparate models, we conducted a regional and detailed mapping program along a geophysical discontinuity 100 km west of the STZ within the south Rae craton of the Northwest Territories, Canada. This work presents the discovery of a new crustal-scale shear zone, the Wholdaia Lake shear zone (WLsz), which deformed and transposed host rocks into a 20-km-wide and 300-km-long ductile high-strain zone. U-Pb zircon geochronology was utilized to establish host-rock crystallization ages, timing of deposition of metasedimentary rocks, and age constraints of metamorphism and ductile shearing. Hanging-wall metasedimentary rocks have a new depositional range of 1.98-1.93 Ga and contain abundant metamorphic zircon at 1.91 Ga. The protoliths of the footwall mafic granulite orthogneisses crystallized at 2.6 Ga and were metamorphosed at 1.9 Ga, which extends the known footprint of 1.9 Ga metamorphism 100 km west of the STZ. During and after 1.9 Ga metamorphism, the WLsz began progressively exhuming footwall rocks in three distinct stages, associated with (1) normal-oblique shearing at high-pressure granulite-facies conditions, (2) normal-oblique shearing accompanied by mylonitization at amphibolite-facies conditions, and (3) normal-oblique shearing with ultramylonite development at amphibolite-to greenschist-facies conditions. Ductile shearing was waning by 1.86 Ga, based on ages obtained from late syn- to postkinematic crosscutting dikes. Collectively, the WLsz in concert with other regional structures aided both extensional and thrust-sense exhumation of a large high-grade terrane at 1.9 Ga in the south Rae craton. DOI
16. Verbaas, J; Thorkelson, DJ; Crowley, J; Davis, WJ; Foster, DA; Gibson, HD; Marshall, DD; Milidragovic, D.A sedimentary overlap assemblage links Australia to northwestern Laurentia at 1.6 Ga.Precambrian Res., 2018, 305: 19-39 A sedimentary overlap assemblage links Australia to northwestern Laurentia at 1.6 Ga
Columbia; Nuna; Proterozoic; Paleogeography; Laurentia; Australia supercontinent; Detrital zircon; Wernecke Breccia; Olympic Dam
The Columbia (Nuna) supercontinent existed from approximately 1.9 Ga to 1.3 Ga. Laurentia was part of Columbia, and the western edge of Laurentia (current coordinates) was likely proximal to a large landmass during parts of this interval. Here, we present detrital zircon ages of a Paleoproterozoic sedimentary succession in northern Yukon, Canada, that bear on the evolution of Columbia. The sedimentary succession is preserved as lasts within 1.60 Ga hydrothermal megabreccias. Analyses of detrital zircon reveal abundant 1.78-1.68 Ga zircon with evolved Hf isotope values (-16.1 < epsilon Hf(t) < +1.4). Sm-Nd isotope analysis on clasts yields epsilon Ndi from -5.3 to -5.5 and model ages from 2.4 to 2.2 Ga. The detrital zircon age distribution is strikingly similar to those from sedimentary megaclasts in the ca. 1.59 Ga Olympic Dam Breccia Complex on the Gawler Craton of Australia. The whole rock Sm-Nd ratios are consistent with derivation from the Gawler Craton. We propose that the sedimentary material contained in both breccia complexes was derived from an overlap assemblage deposited on Australia and Laurentia at ca. 1.6 Ga. This model supports a previous hypothesis that the Gawler Craton was connected to northwestern Laurentia at ca. 1.6 Ga, and that these regions shared a single hydro thermal province that is recognized in northwestern Laurentia as the Wemecke Breccia and in the Gawler Craton as the Olympic Dam Breccia Complex and associated IOCG deposits. The sedimentary overlap succession was deposited after collision between Australia and Laurentia. Australia was subsequently translated southward along the Laurentian margin, placing the Gawler Craton next to southwestern Laurentia and the Mt. Isa Inlier adjacent to northwestern Laurentia by 1.5 Ga. DOI
15. Verbaas, J; Thorkelson, DJ; Milidragovic, D; Crowley, JL; Foster, D; Gibson, HD; Marshall, DD.Rifting of western Laurentia at 1.38 Ga: The Hart River sills of Yukon, Canada.Lithos, 2018, 316: 243-260 Rifting of western Laurentia at 1.38 Ga: The Hart River sills of Yukon, Canada
The Hart River sills are a set of mafic to intermediate intrusions that occur in northern Yukon, Canada. The largest sills are over 500 m thick and over 200 km long. New U-Pb dates of 1382.15 +/- 0.39 Ma and 1382.14 +/- 0.36 Ma were obtained via chemical abrasion thermal ionization mass spectrometry on zircon. Whole rock initial neodymium isotopic compositions of the Hart River sills are juvenile and have epsilon Ndi from +1.5 to +4.0. The primary mineralogy of the Hart River sills is predominated by clinopyroxene and plagioclase. Geochemical modeling indicates that the Hart River sills lie on a common liquid line of descent defined by a fractionating assemblage of plagioclase, clinopyroxene and minor olivine. The Hart River sills have rare earth element and high field strength abundances similar to normal mid-ocean ridge basalts (N-MORB) but are enriched in large ion lithophile elements. The Sm/Yb and Dy/Zr ratios indicate >8% partial melting of spinel-bearing mantle. During the emplacement of the Hart River sills, western Laurentia was juxtaposed with Australia and eastern Antarctica within the supercontinent Columbia. The degree of partial melting, similarity to N-MORB, and juvenile isotopic signature are consistent with an episode of rifting at 1.38 Ga. Coeval magmatism and intracontinental rift basins farther south on Laurentia provide additional evidence for rifting of supercontinent Columbia at 1.38 Ga. (C) 2018 Elsevier B.V. All rights reserved. DOI
14. Brown, SR; Andrews, GDM; Gibson, HD.Corrugated architecture of the Okanagan Valley shear zone and the Shuswap metamorphic complex, Canadian Cordillera.Lithosphere, 2016, 8: 412-421 Corrugated architecture of the Okanagan Valley shear zone and the Shuswap metamorphic complex, Canadian Cordillera
The distribution of tectonic superstructure across the Shuswap metamorphic complex of southern British Columbia is explained by east-west-trending corrugations of the Okanagan Valley shear zone detachment. Geological mapping along the southern Okanagan Valley shear zone has identified 100-m-scale to kilometer-scale corrugations parallel to the extension direction, where synformal troughs hosting upper-plate units are juxtaposed between antiformal ridges of crystalline lower-plate rocks. Analysis of available structural data and published geological maps of the Okanagan Valley shear zone confirms the presence of <= 40-km-wavelength corrugations, which strongly influence the surface trace of the detachment system, forming spatially extensive salients and reentrants. The largest reentrant is a semicontinuous belt of late Paleozoic to Mesozoic upper-plate rocks that link stratigraphy on either side of the Shuswap metamorphic complex. Previously, these belts were considered by some to be autochthonous, implying minimal motion on the Okanagan Valley shear zone (<= 12 km); conversely, our results suggest that they are allochthonous (with as much as 30-90 km displacement). Corrugations extend the Okanagan Valley shear zone much farther east than previously recognized and allow for hitherto separate gneiss domes and detachments to be reconstructed together to form a single, areally extensive Okanagan Valley shear zone across the Shuswap metamorphic complex. If this correlation is correct, the Okanagan Valley shear zone may have enveloped the entire Shuswap metamorphic complex as far east as the east-vergent Columbia River-Slocan Lake fault zones. DOI
13. Furlanetto, F; Thorkelson, DJ; Rainbird, RH; Davis, WJ; Gibson, HD; Marshall, DD.The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China.Gondwana Res., 2016, 39: 14-40 The Paleoproterozoic Wernecke Supergroup of Yukon, Canada: Relationships to orogeny in northwestern Laurentia and basins in North America, East Australia, and China
Wernecke Supergroup; Laurentia; Columbia; Paleoproterozoic; Detrital zircon geochronology; Geochemistry
The Paleoproterozoic Wernecke Supergroup of Yukon was deposited when the northwestern margin of Laurentia was undergoing major adjustments related to the assembly of the supercontinent Columbia (Nuna) from 1.75 to 1.60 Ga. U-Pb detrital zircon geochronology coupled with Nd isotope geochemistry and major and trace element geochemistry are used to characterize the evolution of the Wernecke basin. The maximum depositional age of the Wernecke Supergroup is reevaluated and is estimated at 1649 +/- 14 Ma. Detrital zircon age spectra show a bimodal age distribution that reflects derivation from cratonic Laurentia, with a prominent peak at 1900 Ma. Going upsection, the late Paleoproterozoic peak shifts from 1900 Ma to 1850-1800 Ma, and the proportion of Archean and early Paleoproterozoic zircon decreases. These modifications are a consequence of a change in the drainage system in western Laurentia caused by early phase of the Forward orogeny, several hundred km to the east. The exposed lower and middle parts of the Wernecke Supergroup are correlated with the Hornby Bay Group. Zircon younger than 1.75 Ga appear throughout the sedimentary succession and may have originated from small igneous suites in northern Laurentia, larger source regions such as magmatic arc terranes of the Yavapai and early Mazatzal orogenies in southern Laurentia, and possible arc complexes such as Bonnetia that may have flanked the eastern margin of East Australia. Basins with similar age and character include the Tarcoola Formation (Gawler Craton) and the Willyama Supergroup (Curnamona Province) of South Australia, the Isan Supergroup of North Australia, and the Dongchuan-Dahongshan-Hondo successions of southeast Yangtze Craton (South China). Nd isotope ratios of the Wernecke Supergroup are comparable with values from Proterozoic Laurentia, the Isan and Curnamona assemblages of east Australia, the Gawler Craton, and the Dahongshan-Dongchuan-Hondo successions of the Yangtze Craton of South China. These similarities are compelling evidence for a shared depositional system among these successions. Western Columbia in the Late Paleoproterozoic may have had a dynamic SWEAT-like configuration involving Australia, East Antarctica and South China moving along western Laurentia. (C) 2016 International Association for Gondwana Research. Published by Elsevier B. V. All rights reserved. DOI
12. Staples, R; Gibson, HD; Colpron, M; Ryan, JJ.An orogenic wedge model for diachronous deformation, metamorphism, and exhumation in the hinterland of the northern Canadian Cordillera.Lithosphere, 2016, 8: 165-184 An orogenic wedge model for diachronous deformation, metamorphism, and exhumation in the hinterland of the northern Canadian Cordillera
Development of amphibolite-facies transposition fabrics in the northern Canadian Cordilleran hinterland occurred diachronously in the Permian-Triassic, Early Jurassic, Middle Jurassic to Early Cretaceous, and Early to mid-Cretaceous. Rocks tectonized in the Permian-Triassic and Early Jurassic were exhumed in the Early Jurassic, while rocks immediately to the northeast (toward the foreland) were not buried and heated until the Middle Jurassic to mid-Cretaceous. Early Jurassic to mid-Cretaceous emplacement of the Yukon-Tanana terrane on the North American continental margin, together with the imbrication of parautochthonous rocks, formed a foreland-propagating orogenic wedge. Cooler rocks in front of the wedge were progressively buried and metamorphosed to amphibolite facies from the Jurassic to mid-Cretaceous as they were underthrust into a spatially and temporally transient distributed ductile shear zone near the base of the overriding wedge. Rocks previously incorporated into this zone were displaced upward and exhumed through the combined effects of renewed underplating at depth and compensating extensional and erosional denudation above to maintain a critically tapered wedge. Extensional exhumation of the metamorphic hinterland in the mid-Cretaceous marked the collapse and end of orogen-perpendicular wedge dynamics in operation since the Early Jurassic. Rocks incorporated into the midcrustal shear zone in the Middle Jurassic to mid-Cretaceous were exhumed in the mid-Cretaceous along southeast-directed (orogen-parallel) extensional faults from beneath a supracrustal "lid" tectonized in the Permian-Triassic and Early Jurassic. Like the Himalayan orogen and eastern Alps, orogen-parallel extension developed in an orthogonal plate-convergent setting, simultaneous with, and bounded by, orogen-parallel strike-slip faulting that facilitated northwestward lateral extrusion of rocks normal to the direction of convergence. DOI
11. Mayer, JM; Allen, DM; Gibson, HD; Mackie, DC.Application of statistical approaches to analyze geological, geotechnical and hydrogeological data at a fractured-rock mine site in Northern Canada.Hydrogeol. J., 2014, 22: 1707-1723 Application of statistical approaches to analyze geological, geotechnical and hydrogeological data at a fractured-rock mine site in Northern Canada
Fractured rocks; Hydraulic testing; Geotechnical data; Statistical modeling; Canada
Mine site characterization often results in the acquisition of geological, geotechnical and hydrogeological data sets that are used in the mine design process but are rarely co-evaluated. For a study site in northern Canada, bivariate and multivariate (hierarchical) statistical techniques are used to evaluate empirical hydraulic conductivity estimation methods based on traditional rock mass characterisation schemes, as well as to assess the regional hydrogeological conceptual model. Bivariate techniques demonstrate that standard geotechnical measures of fracturing are poor indicators of the hydraulic potential of a rock mass at the study site. Additionally, rock-mass-permeability schemes which rely on these measures are shown to be poor predictors of hydraulic conductivity in untested areas. Multivariate techniques employing hierarchical cluster analysis of both geotechnical and geological data sets are able to identify general trends in the data. Specifically, the geological cluster analysis demonstrated spatial relationship between intrusive contacts and increased hydraulic conductivity. This suggests promise in the use of clustering methods in identifying new trends during the early stages of hydrogeological characterization. DOI
10. Medig, KPR; Thorkelson, DJ; Davis, WJ; Rainbird, RH; Gibson, HD; Turner, EC; Marshall, DD.Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic.Precambrian Res., 2014, 249: 88-99 Pinning northeastern Australia to northwestern Laurentia in the Mesoproterozoic
Yukon; Mesoproterozoic; Columbia; Nuna; Supercontinents; Geochronology
Two supercontinents have been proposed for the latter half of the Precambrian: Columbia (or Nuna) from ca. 1.9 to 1.3 Ga, and Rodinia from ca. 1.1 to 0.75 Ga. In both supercontinents, Laurentia and Australia are regarded as probable neighbours, although their relative positions are contentious. Here we use detrital zircons ages from unit PR1 of the lower Fifteenmile group in Yukon, Canada, to demonstrate that northeastern Australia and northwestern Laurentia were firmly connected in the Mesoproterozoic. The zircon ages define a near-unimodal population with a peak at 1499 +/- 3 Ma, which lies in an interval of magmatic quiescence on Laurentia, known as the North American magmatic gap (NAMG), and abundant magmatism in Australia. Sediment compositions and textures suggest the sediment was derived from a proximal metaplutonic source. We suggest that the Williams and Naraku batholiths in the Mt. Isa inlier in northeastern Australia, with crystallization ages ranging from 1493 +/- 8 Ma to 1508 +/- 4 Ma, are the most probable sources of sediment for the PR1 basin. The plutons were exhumed between 1460 and 1420 Ma, and likely formed an active, eroding highland in the Australian part of Columbia. Sediment derived from these plutons was carried eastward by a short, direct river system and deposited into the PR1 marine basin. Formation of the PR1 basin coincides with the formation of the southern Cordilleran Belt-Purcell, Hess Canyon, and Trampas basins. These basins, formed on the western margin of Laurentia, also have detrital zircon populations that fall into the NAMG, suggesting that sediment was derived from a non-Laurentian westerly source. The PR1 basin is herein correlated with the Belt-Purcell, Hess Canyon, and Trampas basins to the south, and together these basins record the onset of Columbia breakup along the length of the western margin of Laurentia from as far north as Yukon to as far south as Arizona. Crown Copyright (C) 2014 Published by Elsevier B.V. All rights reserved. DOI
9. Staples, RD; Murphy, DC; Gibson, HD; Colpron, M; Berman, RG; Ryan, JJ.Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphism in the northern Canadian Cordillera: Recording foreland-directed migration of an orogenic front.Geol. Soc. Am. Bull., 2014, 126: 1511-1530 Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphism in the northern Canadian Cordillera: Recording foreland-directed migration of an orogenic front
In situ sensitive high-resolution ion microprobe monazite geochronology and garnet isopleth thermobarometry reveal a previously unrecognized Middle Jurassic to earliest Cretaceous mid-crustal tectono-metamorphic event in the eastern part of the Yukon-Tanana terrane (Finlayson Lake district, southeast Yukon) in the northern Canadian Cordillera. Intersection of garnet end-member compositional isopleths applied to single-stage, growth-zoned garnet records progressive garnet growth from 550 degrees C and 6.1-6.6 kbar to 600 degrees C and 7.5 kbar. Monazite textures, chemical zoning, and in situ U-Pb ages record a single protracted episode of monazite growth from ca. 169 to 142 Ma coeval with the development of transposition fabrics and the late stages of garnet growth. This event post-dates widespread Early Jurassic exhumation of Yukon-Tanana terrane rocks west of the Tintina fault in west-central Yukon, which were previously ductily deformed and metamorphosed in the Permo-Triassic. The lack of evidence for Permo-Triassic ductile deformation and high-grade metamorphism within the Finlayson Lake district, and its position east of the Permian arc center and west of Permian blueschists and eclogites, suggests this eastern part of the terrane occupied the cool forearc at this time. These data indicate younger, more protracted mid-crustal orogenesis in the northern Cordillera than was previously recognized, with deformation and metamorphism migrating toward the foreland and downwards in the Middle Jurassic to Early Cretaceous, in part contemporaneous with and analogous to that in the southeastern Canadian Cordillera. DOI
8. Furlanetto, F; Thorkelson, DJ; Gibson, HD; Marshall, DD; Rainbird, RH; Davis, WJ; Crowley, JL; Vervoort, JD.Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis.Precambrian Res., 2013, 224: 512-528 Late Paleoproterozoic terrane accretion in northwestern Canada and the case for circum-Columbian orogenesis
Wernecke Supergroup; RackIan; Paleoproterozoic; Bonnetia; Columbia
The reconstruction of the paleocontinental configuration involving ancestral North America (Laurentia) at the Paleoproterozoic-Mesoproterozoic boundary has been developed in the last 30 years with different scenarios being proposed and different combinations of landmasses assembled together. However, the lack of information for the northwestern side of the North American craton has so far been an obstacle for the complete paleocontinental reconstruction and its tectonic history. Here we provide new age determinations on rocks of the Wemecke Supergroup and of the Wernecke Breccia of the Wernecke Mountains in Yukon to provide a more complete picture of the entire North American craton and its possible conterminous at 1600 Ma. The six youngest U-Pb ages of the detrital zircon from quartz sandstones of the Wernecke Supergroup suggest that the sedimentary succession is as old as 1640 Ma. Lu-Hf garnet ages on garnet bearing schists of the Fairchild Lake Group (lower Wemecke Supergroup) give a bimodal population of ages of approximately 1600 Ma and 1370 Ma: the first age is related to the Racklan Orogeny, and the younger event is likely attributable to a reheating episode (Hart River Sills emplacement). The younger age of the Wernecke Supergroup puts into question the previous model concerning the emplacement of the Bonnet Plume River Intrusions, and requires the development of a new tectonic model for the northwestern margin of Laurentia. This new model involves obduction of an exotic terrane on top of the Wemecke Supergroup during the latest phases of the Racklan Orogeny (ca. 1600 Ma). This exotic terrane, herein called Bonnetia, contains rocks of the Bonnet Plume River intrusions and of the Slab volcanics. During the hydrothermal event that led to the emplacement of the Wernecke Breccia, clasts and megaclasts of the overlying Bonnetia foundered down to the breccia pipes to the level of the Wernecke Supergroup, and this dynamic explains the existence of older rocks engulfed within a younger sedimentary succession. The Racklan Orogeny is now interpreted as a northwestern expression of the Mazatzal Orogeny of southwestern United States, and of the Labradorian Orogeny of eastern Canada which was in turn connected with the Gothian Orogeny of Scandinavia. The connection among these orogenic events makes plausible the hypothesis of a circum-Laurentian orogenic belt with possible extensions in other landmasses (Australia, Antarctica, Siberia, or China) where coeval deformation belts are present. (c) 2012 Elsevier B.V. All rights reserved. DOI
7. Nielsen, AB; Thorkelson, DJ; Gibson, HD; Marshall, DD.The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia.Precambrian Res., 2013, 238: 78-92 The Wernecke igneous clasts in Yukon, Canada: Fragments of the Paleoproterozoic volcanic arc terrane Bonnetia
Geochemistry; Tectonics; Wernecke; Proterozoic; Yukon; Metasomatism
The Wernecke igneous clasts consist of blocks of plutonic and volcanic rock that range up to hundreds of metres in size. These clasts occur exclusively within zones of hydrothermal breccia (Wernecke Breccia) which are widespread in central and northern Yukon. The breccia zones are hosted by the Wernecke Supergroup and have been dated by U-Pb titanite at 1599 Ma. Four U-Pb zircon ages on the Wernecke igneous clasts (1714-1706 Ma) demonstrate that the clasts are older than the Wernecke Supergroup (< 1.64 Ga) and indicate that the clasts were not derived from dykes within the Wernecke Supergroup. Instead, the clasts were derived from an obducted terrane named Bonnetia. Geochemical characteristics of the Wernecke igneous clasts infer that Bonnetia formed as a volcanic arc with a component of within-plate magmatism. Neodymium mantle depletion ages of 2080-2760 Ma suggest that the arc was built on older continental crust. Consequently, Bonnetia may have been a volcanic arc, possibly Wilt on a rifted fragment of Laurentia, on another continental fragment, or possibly on the leading edge of another continent. The subsequent event of breccia-formation may represent a hydrothermal response to abduction-caused tectonic loading of the crust. The characterization of Bonnetia as a volcanic arc complex that underwent obduction requires that northwestern Laurentia was flanked by an ocean basin in the late Paleoproterozoic. (C) 2013 Elsevier B.V. All rights reserved. DOI
6. Staples, RD; Gibson, HD; Berman, RG; Ryan, JJ; Colpron, M.A window into the Early to mid-Cretaceous infrastructure of the Yukon-Tanana terrane recorded in multi-stage garnet of west-central Yukon, Canada.J. Metamorph. Geol., 2013, 31: 729-753 A window into the Early to mid-Cretaceous infrastructure of the Yukon-Tanana terrane recorded in multi-stage garnet of west-central Yukon, Canada
in situ monazite geochronology; multi-stage garnet; P-T-t path; SHRIMP; Yukon-Tanana terrane
Amphibolite facies metasedimentary schists within the Yukon-Tanana terrane in the northern Canadian Cordillera reveal a two-stage, polymetamorphic garnet growth history. In situ U-Th-Pb Sensitive High Resolution Ion Microprobe dating of monazite provide timing constraints for the late stages of garnet growth, deformation and subsequent decompression. Distinct textural and chemical growth zoning domains, separated by a large chemical discontinuity, reveal two stages of garnet growth characterized in part by: (i) a syn-kinematic, inclusion-rich stage-1 garnet core; and (ii) an inclusion-poor, stage-2 garnet rim that crystallized with syn- to post-kinematic staurolite and kyanite. Phase equilibria modelling of garnet molar and compositional isopleths suggest stage-1 garnet growth initiated at similar to 600 degrees C, 8 kbar along a clockwise P-T path. Growth of the compositionally distinct, grossular-rich, pyrope-poor inner portion of the stage-2 overgrowth is interpreted to have initiated at higher pressure and/or lower temperature than the stage-1 core along a separate P-T loop, culminating at peak P-T conditions of similar to 650-680 degrees C and 9 kbar. Stage-2 metamorphism and the waning development of a composite transposition foliation (S-T) are dated at c. 118 Ma from monazite aligned parallel to S-T, and inclusions in syn- to post-S-T staurolite and kyanite. Slightly younger ages (c. 112 Ma) are obtained from Y-rich monazite that occurs within resorbed areas of both stage-1 and stage-2 garnet, together with retrograde staurolite and plagioclase. The younger ages obtained from these texturally and chemically distinct grains are interpreted, with the aid of phase equilibria calculations, to date the growth of monazite from the breakdown of garnet during decompression at c. 112 Ma. Evidence for continued near-isothermal decompression is provided by the presence of retrograde sillimanite, and cordierite after staurolite, which indicates decompression below similar to 4-5 kbar prior to cooling below similar to 550 degrees C. As most other parts of the Yukon-Tanana terrane were exhumed to upper crustal levels in the Early Jurassic, these data suggest this domain represents a tectonic window revealing a much younger, high-grade tectono-metamorphic core (infrastructure) within the northern Cordilleran orogen. This window may be akin to extensional core complexes identified in east-central Alaska and in the southeastern Canadian Cordillera. DOI
5. Brown, SR; Gibson, HD; Andrews, GDM; Thorkelson, DJ; Marshall, DD; Vervoort, JD; Rayner, N.New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone.Lithosphere, 2012, 4: 354-377 New constraints on Eocene extension within the Canadian Cordillera and identification of Phanerozoic protoliths for footwall gneisses of the Okanagan Valley shear zone
The Okanagan Valley shear zone delineates the SW margin of the Shuswap metamorphic complex, the largest core complex within the North American Cordillera. The Okanagan Valley shear zone is a major Eocene extensional fault zone that facilitated exhumation of the southern Shuswap metamorphic complex during the orogenic collapse of the SE Canadian Cordillera when convergence at the western margin of North America switched from transpression to transtension. This study documents the petrology, structure, and age of the Okanagan gneiss, the main lithology within the footwall of the Okanagan Valley shear zone, and constrains its history from protolith to exhumed shear zone. The Okanagan gneiss is an similar to 1.5-km-thick, west-dipping panel composed of intercalated orthogneiss and paragneiss in which intense ductile deformation of the Okanagan Valley shear zone is recorded. New U-Pb zircon ages from the gneiss and crosscutting intrusions constrain the development of the Okanagan gneiss to the Eocene, contemporaneous with widespread extension, intense deformation, high-grade metamorphism, and anatexis in the southern Canadian Cordillera. Thermobarometric data from the paragneiss domain indicate Eocene exhumation from between 17 and 23 km depth, which implies 64-89 km of WNW-directed horizontal extension based on an original shear zone angle of similar to 15 degrees. Neither the Okanagan gneiss nor its protolith represents exhumed Proterozoic North American cratonic basement as previously postulated. New U-Pb data demonstrate that the protolith for the gneiss is Phanerozoic, consisting of Mesozoic intrusions emplaced within a late Paleozoic-Mesozoic layered sequence of sedimentary rocks. DOI
4. Crowley, JL; Brown, RL; Gervais, F; Gibson, HD.Assessing Inheritance of Zircon and Monazite in Granitic Rocks from the Monashee Complex, Canadian Cordillera.J. Petrol., 2008, 49: 1915-1929 Assessing Inheritance of Zircon and Monazite in Granitic Rocks from the Monashee Complex, Canadian Cordillera
Zircon and monazite from granitic sheets and dikes in the Monashee complex, Canadian Cordillera, were investigated to determine whether igneous crystallization occurred at 19 Ga or 50 Ma with 19 Ga inherited zircon and monazite. Four of the five samples are weakly deformed to undeformed, despite occurring in a gneiss dome at the structurally deepest exposed level of the orogen that elsewhere was strongly deformed and partly melted at 50 Ma. Based on U(Th)Pb ages from zircon and monazite, field relationships, and mineral composition and zoning, we conclude that the granitic rocks crystallized at 19 Ga and were metamorphosed at 50 Ma. All dated zircon is 19 Ga (except for 2320 Ga inherited cores) and 19 Ga monazite makes up 90 of the population in four samples. The remainder of the monazite is 50 Ma and all monazite in one sample is 50 Ma. Composition and zoning of 19 Ga zircon and monazite are uniform within samples, yet differ between samples, indicating growth from 19 Ga magmas that are unique to each sample. This relationship is unlikely if the grains are inherited because the host rocks are heterogeneous 2321 Ga gneisses. The 19 Ga zircon and monazite have zoning that is consistent with growth from magmas, whereas the 50 Ma monazite has variable composition and zoning that suggest growth from diverse metamorphic fluids. The results demonstrate that part of the Monashee complex was last strongly deformed and partly melted at 19 Ga, and thus largely escaped Cordilleran tectonism. DOI
3.Gibson, HD; Brown, RL; Carr, SD.Tectonic evolution of the Selkirk fan, southeastern Canadian Cordillera: A composite Middle Jurassic-Cretaceous orogenic structure.Tectonics, 2008, 27 Tectonic evolution of the Selkirk fan, southeastern Canadian Cordillera: A composite Middle Jurassic-Cretaceous orogenic structure
The eastward transition from penetrative ductile deformation, metamorphism, and plutonism in the hinterland of the southern Canadian Cordillera to the "thin-skinned'' deformation of the foreland represents a significant change in tectonic style and process. The transition is also marked by a zone of structural divergence across which SW vergent hinterland structures pass into NE vergent foreland structures. The Selkirk fan within the Selkirk Mountains of the southern Canadian Cordillera is considered a type locality for this zone. This structure trends SE-NW for more than 120 km in greenschist to upper amphibolite facies rocks that have at least three generations of superposed deformation. The kinematic development of the Selkirk fan has been controversial, but its age generally has been inferred to be Middle to Late Jurassic. However, new U-Th-Pb dates obtained by ID-TIMS and SHRIMP indicate a more protracted history. The data demonstrate that a protofan developed in the Middle Jurassic (172-167 Ma) and that the west flank of the fan was exhumed to upper crustal levels at this time. Conversely, the eastern flank was progressively buried to > 25 km depth and pervasively overprinted by Cretaceous deformation (104-84 Ma) and Cretaceous-Paleocene metamorphism (144-56 Ma). A new tectonic model is proposed to reconcile the composite nature and protracted development (> 100 Ma) of the Selkirk fan within an orogenic system that evolved in response to periods of terrane accretion on the western margin of the North American craton. DOI
2. Brown, RL; Gibson, HD.An argument for channel flow in the southern Canadian Cordillera and comparison with Himalayan tectonics.Geol. Soc. Spec. Publ., 2006, 268: 543-+ An argument for channel flow in the southern Canadian Cordillera and comparison with Himalayan tectonics
Crustal thickening in excess of 55 km, and high heat flow, suggest that a high-standing plateau region in the Cordilleran hinterland was present in the Late Cretaceous. A low strength middle crust developed beneath the plateau, and parts of this layer were exhumed to upper crustal levels in Late Cretaceous to Eocene time. During Late Cretaceous time, structures in the hinterland were reactivated. Strata, buried to mid-crustal depths since the Jurassic, began to flow upward to higher levels; earlier structures were refolded and tightened, and a new transposition fabric developed. Some 10-20 km of the middle crust was involved in high temperature ductile flow. The lower boundary of the ductile zone lies with thrust sense on top of Precambrian rocks of Canadian Shield affinity, and splays upwards to the NE where it closely coincides with highly strained rocks in the hanging wall of the Purcell Thrust Fault. The upper boundary is marked by a normal-sense high strain zone, above which only minor Cretaceous deformation occurred. The boundaries were reactivated at upper crustal levels after cessation of flow in the mid-crustal channel. This reactivation resulted in formation of ductile to brittle extension faults such as the Okanagan Fault System. During final stages of flow, the Precambrian basement gneisses at the base of the channel became domed and exhumed to upper crustal levels. Comparisons with Himalayan tectonics are clearly drawn, but there are significant contrasts such as the long residence time of the proposed Cordilleran channel, and the nature of the channel boundaries. DOI
1.Gibson, HD; Brown, RL; Carr, SD.U-Th-Pb geochronologic constraints on the structural evolution of the Selkirk fan, northern Selkirk Mountains, southern Canadian Cordillera.J. Struct. Geol., 2005, 27: 1899-1924 U-Th-Pb geochronologic constraints on the structural evolution of the Selkirk fan, northern Selkirk Mountains, southern Canadian Cordillera
structural fan; Canadian Cordillera; U-Th-Pb geochronology; IDTIMS; SHRIMP; diachronous deformation
In the southern, Canadian Cordillera a zone of structural divergence marks the eastward transition from penetrative ductile deformation and metamorphism in the Omineca belt to the more brittle 'thin-skinned' style of deformation typical of the Foreland belt. In the Selkirk Mountains of southern British Columbia, this zone includes a regional-scale structure termed the Selkirk fan. The fan trends northwest, consists primarily of medium- to high-grade metamorphic rocks, and comprises at least three generations of superposed structures. IDTIMS and SHRIMP analyses provide new U-Th-Pb age constraints for the structural evolution of the Selkirk fan. The data demonstrate that the thermo-structural development of the fan's west flank occurred principally in the Middle Jurassic (ca. 172-167 Ma), whereas in the east flank significant Cretaceous (ca. 104-84 Ma) deformation was superimposed on an early transposition fabric. These data require revision of previous models that concluded fan formation occurred primarily during Middle Jurassic time. Rather, the Selkirk fan is a composite structure comprising Middle Jurassic and Cretaceous deformation. Development of the fan during the Early-Middle Jurassic accretion of the Intermontane Superterrane was followed by extensive reworking and tightening of structures in the fan's east flank during the Cretaceous accretion of the Insular Superterrane. (C) 2005 Elsevier Ltd. All rights reserved. DOI
