Larry S. Lane

Detrital zircon geochronology of the western Ellesmerian clastic wedge, northwestern Canada: Insights on Arctic tectonics and the evolution of the northern Cordilleran miogeocline

Detrital zircon provenance investigations of mid-Paleozoic sandstone from the western Ellesmerian clastic wedge and Cordilleran miogeocline in northern Yukon and Northwest Territories, northwestern Canada, provide critical new data on the source of foreland basin sedimentation attributed to terrane accretion and plate convergence along the ancestral Arctic margin of North America. Late Devonian and early Mississippian clastic wedge strata yield “exotic” ca. 360–390, 430–460, 530–680, and 1500–1600 Ma detrital zircon populations that are consistent with source rocks that originated near the Caledonian and Timanian orogenic belts. Specifically, the Pearya and Arctic Alaska–Chukotka terranes, the landmass of Crockerland, and Caledonian rocks in eastern Greenland are the inferred sources for exotic detrital zircons in clastic wedge strata. Progressive recycling of Ellesmerian foreland basin sediments into the continental margin environment along northwestern Laurentia is indicated by the presence of ca. 360–430 Ma and 1500–1600 Ma detrital zircons in post-tectonic, middle to late Mississippian miogeoclinal strata in Yukon. Provenance data from these Mississippian samples record a dramatic shift in the source of the Cordilleran miogeocline, since Caledonian and Baltican (Timanide) detrital zircon signatures are not recognized in pre–Late Devonian sedimentary rocks in western Canada. Devonian strata of the Alexander terrane and Yreka subterrane (eastern Klamath terrane) have Caledonian and Baltican detrital zircon age signatures similar to Ellesmerian clastic wedge sandstones, implying that several Cordilleran terranes originated in the paleo-Arctic realm. Speculative correlations suggest that the Arctic Alaska–Chukotka terrane was located to the west of Crockerland and the Canadian Arctic Islands in pre-Cretaceous time, prior to opening of the Amerasian basin. Rifting models for the western Arctic Ocean featuring counterclockwise rotation of the Arctic Alaska–Chukotka terrane away from the Canadian Arctic Islands may need reevaluation.

Crustal structure and tectonics of the southeastern Beaufort Sea continental margin

The structure of the southeast margin of the Canada Basin is synthesized from seismic reflection and refraction profiles in the southern Beaufort Sea and Mackenzie Delta, interpreted in conjunction with potential field data and the exploration seismic data base. The present margin was formed in the Jura-Cretaceous and comprises a complex pattern of rifted and transform faulted crustal segments. Thinning in the upper crust is bounded by the Eskimo Lakes Fault Zone (ELFZ), a series of extensional listric normal faults, and is controlled by preexisting structures. Lower crustal thinning and the transition to oceanic crust occurs outboard of the ELFZ. A thick (up to 16 km) Late Jurassic and younger synrift and postrift sedimentary succession overlies oceanic crust in the eastern part of the Canadian Beaufort Sea and thinned continental crust between the Mackenzie Delta and Alaska. Tertiary faulting in the sedimentary basin appears to be related to the crustal structure. Present-day seismicity in the southern Beaufort Sea is essentially limited to the area underlain by oceanic crust. Abrupt along-strike changes in crustal affinity and degree of thinning allow the recognition of a NW-trending transform fault. Regional gravity data, dominated by a series of coastline parallel highs, are used to extrapolate crustal features to the northeast along the Canadian polar continental margin. It is inferred that the Canadian polar margin consists of a number of 250-to-350-km-long stretched crustal segments separated by possible fracture zones. The orientation of the analogous tranform fault identified in the southeastern Beaufort Sea offers the possibility of kinematic constraints on models of ocean floor development within Canada Basin.

Structure of the southeast margin of the Beaufort-Mackenzie basin, Arctic Canada, from crustal seismic-reflection data

New seismic-reflection profiles image the crustal structure of the Beaufort-Mackenzie basin and adjacent features near the Mackenzie Delta in northwestern Canada. The Mesozoic to Quaternary sediments are nearly 12 km thick under Richards Island at the edge of the Beaufort Sea and are bounded by listric normal faults on the south edge of the Beaufort-Mackenzie basin. These faults parallel features that are Proterozoic in age; this suggests that the older features controlled the younger. Thrust faults are identified from offsets in reflections from Proterozoic strata underlying the Campbell uplift. The age of the compression is not yet clearly established; it may be either Late Proterozoic or late Paleozoic. A Late Proterozoic age would imply that Precambrian compressional structures underlie much of northwestern Canada. A late Paleozoic age would imply that Ellesmerian (Devonian–Carboniferous) compressional structures are buried beneath the Arctic coastal plain from the Mackenzie River delta to the Parry Islands fold belt, some 1000 km to the northeast in the Arctic Archipelago. The base of the crust is imaged on the south end of the profile at approximately 39 km, whereas in the north it is inferred, from gravity modeling, to be at approximately 28 km.

Latest Cretaceous-Tertiary Tectonic Evolution of Northern Yukon and Adjacent Arctic Alaska

Regional crustal shortening in northern Yukon and northeastern Alaska occurred episodically from the latest Cretaceous to the late Miocene, with a major culmination occurring in the Paleocene to middle Eocene, and a secondary culmination in the late Miocene. Structural trends are predominantly north to northeastward in northern Yukon and adjacent east-central Alaska, and generally east-west along the Brooks Range. The early Tertiary trend is strongly arcuate in the Beaufort fold belt and adjacent onshore areas. The continuity of structures southward from the Beaufort Sea region through northern Yukon and east-central Alaska supports the interpretation that the structures north of 65°N form a single orogenic entity. The Beaufort Sea region is 1000 km from the nearest plate margin. Northward shortening across Arctic Alaska and the northern Canadian cordillera reflects the convergent component of Kula (later, Pacific)-North America interactions throughout the Tertiary. North- to northeast-trending structures of latest Cretaceous to early Tertiary age in northern Yukon and east-central Alaska accommodate shortening of 180-240 km, and reflect Eurasia-North America convergence. The strongly arcuate offshore Beaufort fold belt and similar structures in adjacent northernmost Yukon and northeast Alaska were formed by the complex interplay of three factors: shortening of northern Yukon between Arctic Alaska and the craton to produce a north-trending orogenic welt; northward displacements propagated from the Kula plate margin; and local boundary conditions imposed by lithology and crustal structure, which aided lateral escape of the deforming supracrustal succession northward into the Beaufort Sea. In central Alaska, any kinematic linkage between the Kaltag fault in the west and the Tintina fault of the northern cordillera is more complex than was previously assumed. A new regional tectonic reconstruction of northern Yukon-Alaska quantifies the tectonic shortening in central Alaska south of the Kaltag and Tintina faults in an area where tectonic shortening is difficult to quantify due to complex geology. Complex deformation accommodated by folding, thrust and strike-slip faulting, and/or tectonic rotations accounts for an estimated 460 km of crustal shortening, approximately equivalent in magnitude to the total Tintina fault displacement of at least 450 km. The foreland of the Brooks Range, the Beaufort fold belt, and the northern cordillera contain proven petroleum basins. This regional synthesis validates a model of orogenic shortening for the Beaufort fold belt and provides a unifying tectonic setting for oil and gas plays throughout the region. The latest Cretaceous-Tertiary structural evolution of the Brooks Range, Beaufort, and northern Yukon fold belts is a case study of the temporally and kinematically complex far-field deformation arising from the convergence of four major tectonic plates.

Detrital zircon lineages of late Neoproterozoic and Cambrian strata, NW Laurentia

The Phanerozoic tectonic evolution of the Arctic is a field of escalating scientific interest. Detrital zircon provenance studies provide vital contributions to clarify the region’s tectonic evolution. Northwest Laurentia exposes a broad expanse of Proterozoic and Paleozoic sedimentary strata for which detrital zircon populations are poorly characterized. Moreover, the significance of sedimentary recycling is becoming better appreciated in light of detrital zircon studies. As more data become available, our understanding of the detrital zircon character of NW Laurentia improves, providing an increasingly reliable baseline at subcontinental resolution against which potentially allochthonous terranes, such as Arctic Alaska, can be assessed. Sandstones of late Neoproterozoic and Cambrian age from NW Canada yield detrital zircon signatures dominated by zircon grains recycled from Proterozoic sedimentary strata. Two Neoproterozoic sandstones from the northern Mackenzie Mountains yield zircon populations sourced from the Mackenzie Mountains Supergroup. Two Lower Cambrian sandstones sourced from the Yukon stable block and deposited in Richardson Trough have zircon populations nearly identical to those of the upper Wernecke Supergroup, locally exposed across the southern Yukon stable block, where they are unconformably overlain by Cambrian strata. Comparisons with similar studies from NW Canada permit generalizations of the patterns of zircon recycling. Four provenance lineages are described that characterize Laurentia-derived successions in NW Canada.

Provenance and paleogeography of the Neruokpuk Formation, northwest Laurentia: An integrated synthesis

The Neruokpuk Formation is a Neoproterozoic and Cambrian turbiditic succession in northwesternmost Yukon (Canada) and northeastern Alaska (USA), part of a latest Proterozoic to Early Devonian slope and basin succession that is correlated in detail with strata in Selwyn Basin of the northern Canadian Cordillera. It includes quartz-lithic sandstone, locally containing altered detrital feldspar and muscovite indicating that a metamorphic source contributed detritus to the unit. The muscovite yields disturbed Ar-Ar spectra suggesting ages of 1800–1900 Ma. Detrital zircon distributions are dominated by 1800–2000 Ma grains with subsidiary populations of 1000–1600 Ma, 2300–2500 Ma and 2600–2800 Ma grains, consistent with a hybrid provenance dominated by a Laurentian cratonic source. Additional populations are derived from recycled Mackenzie Mountains and possibly Wernecke Supergroups. Integrating the geochronology with the regional stratigraphic setting, structural history, and geochemistry leads to the conclusion that the Neruokpuk Formation was deposited near its present location as part of the autochthonous northwest Laurentian continental margin. Therefore, the eastern part of Arctic Alaska, underlain by the Neruokpuk Formation, has a history that is distinct from the allochthonous western part(s) of the Arctic Alaska terrane. However, the rest of Arctic Alaska is structurally and stratigraphically linked to the eastern part by Late Devonian time.

Regional Heat Flow Pattern and Lithospheric Geotherms in Northeastern British Columbia and Adjacent Northwest Territories, Canada

Abstract A new heat flow map has been constructed for northeastern British Columbia, using log-heading temperature records corrected for drilling disturbance, at depths ranging from 200 m to over 4000 m. Geothermal gradient varies from 20 to 90 mK/m (°C/km). A significant regional trend of southward and southwestward decrease of geothermal gradient is observed in the map area. Effective thermal conductivity, based on an analysis of an evenly distributed network of wells throughout the area, shows variation in the range 1.2 to 2.4 W/mK. Very large variations of terrestrial heat flow, from 40 to 130 mW/m2, suggest a large imprint of highly variable heat generation in the upper crust. The lowest heat flow is observed in the southern part of the study area and especially in the southwest. The highest heat flow is in the north, with values commonly exceeding 100 mW/m2. However, lower heat flow values (<60 mW/2) are also observed in the north. In the deep western part of the basin, low heat flow in the south contrasts with high heat flow north of 59°N. Reduced (deep) heat flow in the north is probably 10 to 20 mW/m2 higher than the average of 33 mW/m2 proposed previously. Higher regional reduced heat flow and higher than average heat generation (2-4 μW/m3) of the basement rocks likely accounts for part of the northern high heat flow. It is estimated that the average temperature at a depth of 5 km is 180°C; however, temperatures as high as 140°C are recorded at the much shallower depth of 2 km in the northern and northeastern part of the study area.

Kaltag fault, northern Yukon, Canada: Constraints on evolution of Arctic Alaska

The Kaltag fault has been linked to several strike-slip models of evolution of the western Arctic Ocean. Hundreds of kilometers of Cretaceous-Tertiary displacement have been hypothesized in models that emplace Arctic Alaska into its present position by either left- or right-lateral strike slip. However, regional-scale displacement is precluded by new potential-field data. Postulated transform emplacement of Arctic Alaska cannot be accommodated by motion on the Kaltag fault or adjacent structures. The Kaltag fault of the northern Yukon is an eastward extrapolation of its namesake in west-central Alaska: however, a connection cannot be demonstrated. Cretaceous-Tertiary displacement on the Alaskan Kaltag fault is probably accommodated elsewhere.

Scientific network to decipher crustal evolution of the Arctic

The tectonic evolution of the regions in and around the Arctic remains highly debated due to a lack of geologic knowledge, the regions geologic complexities, and the logistical difficulties of working at extreme latitudes. For example, the northward continuation of Paleozoic (∼545-to 250-million-year-old) mountain belts is predicted yet unrecognized, and the tectonic development of the Canada Basin—which defines the nature of the crust in and around the Arctic—is still debated, partly because few research projects are regional in scope or link the submarine and subaerial environments. This has led some to speculate, for example, that oceanic crust underlies the entire Amerasian Basin, whereas others indicate a much more limited extent within the Canada Basin. A contributing factor in the development of such conflicting hypotheses is the traditional segregation of land-and marine-based researchers: Over the past several decades, Arctic campaigns have conducted marine, aerogeophysical, and geological investigations [e.g., Lawver et al., 2010], but few of these have integrated onshore and offshore environments. To address this shortcoming, a new multinational, multidisciplinary science network called the Circum-Arctic Lithosphere Evolution (CALE) project seeks to integrate onshore geology with offshore geophysics in the Earths northernmost regions.

Foreword — Central Foreland NATMAP: Stratigraphic and Structural Evolution of the Cordilleran Foreland

This two-part special issue is a collection of synthesis papers derived from mapping, structural and stratigraphic analysis, and geophysical studies carried out as part of the Central Foreland NATMAP Project from 1998 to 2003. Accordingly, this issue has a geographic focus: the Cordilleran Foothills region between Fort St. John, B.C. and Nahanni Butte, N.W.T. Petroleum exploration and production have increased significantly in this region during the course of the project. Within this dynamic environment, new results from the project have prompted open discussion among participants, collaborators and industry. This has helped to propel important advances in our understanding of the regional geological evolution, putting an enormous amount of new geological data into the public domain, and helping to train dozens of young geoscientists. Earlier results have been published in this and other journals over the course of the project. Nine papers are presented in this special issue; and others will certainly follow as the data continue to be analyzed. The Bulletin continues to be the favoured venue for synthesis papers from the project, primarily because of their immediate relevance to the Canadian petroleum industry. Also, having a special issue devoted to a single geographic area (in this case) is convenient to the journal’s users because the various themes are available all …