Phanerozoic burial and unroofing history of the western Slave craton and Wopmay orogen from apatite (U–Th)/He thermochronometry

Abstract

Low temperature thermochronometry of cratonic regions can illuminate relationships among burial and unroofing patterns, surface subsidence and uplift, and lithosphere–asthenosphere interactions. The Slave craton, initially stabilized by the development of a thick lithospheric mantle root in late Archean time, is an excellent location in which to examine these connections. Although the Slave craton currently lacks Phanerozoic cover, Phanerozoic sedimentary xenoliths entrained in ca. 610 to 45 Ma kimberlites indicate that the region underwent a more dynamic history of burial and unroofing than widely recognized. We report new apatite (U–Th)/He thermochronometry data along a southeast to northwest transect from the interior of the Slave craton into the adjacent Paleoproterozoic Wopmay orogen to resolve the region's depositional and denudational history. Six samples from the western Slave craton and three samples from Wopmay orogen yield mean dates from 296 ± 41 Ma to 212 ± 39 Ma. Individual apatite dates are broadly uniform over a wide span of apatite [eU], and this pattern can be used to more tightly restrict the spectrum of viable temperature–time paths that can explain the dataset. When coupled with geologic and stratigraphic information, temperature–time simulations of the thermochronometry results suggest complete He loss from the apatites at minimum peak temperatures of ~ 88 °C in Devonian–Pennsylvanian time, cooling to near-surface conditions by the Early Cretaceous, followed by reheating to ≤ 72 °C during Cretaceous–Early Tertiary time. Consideration of modern and ancient geotherm constraints implies ≥ 3.3 km of burial during the first Phanerozoic heating phase, with an ancillary phase of reburial in late Mesozoic–Cenozoic time. The uniformity of the apatite (U–Th)/He dates indicates that the rocks encompassed by our > 250 km-long sample transect experienced similar Phanerozoic thermal histories. Despite the distinctly different lithospheric architecture on either side of the Paleoproterozoic suture between the Slave craton and Wopmay orogen, the region behaved as a single, broadly coherent cratonic unit since at least ca. 250 Ma. The Phanerozoic burial and unroofing patterns across the craton may be a response to far-field convergent activity at the northern and western margins of North America and processes associated with episodic kimberlite emplacement.

Introduction

Cratons are stable and relatively resistant to disruption, largely attributable to their cold, thick, chemically depleted lithospheric mantle roots (Jordan, 1978). Some cratonic regions, such as the interior of North America, have undergone repeated episodes of burial and unroofing as recorded by thick sequences of Phanerozoic strata interspersed with widespread unconformities (e.g., Sloss, 1963, Bond, 1978). Phases of denudation and aggradation may reflect low amplitude, long wavelength vertical cratonic motions, superimposed on the record of eustatic sea level change. These patterns may suggest a more dynamic history of cratonic regions than commonly recognized, possibly linked with deep-seated mantle and far-field plate boundary processes. For example, dynamic topography, or vertical displacement of the Earth's surface by up to several kilometers in response to mantle flow, has been invoked to explain low amplitude subsidence and uplift in some continental interiors (e.g., Mitrovica et al., 1989, Gurnis, 1993, Burgess et al., 1997, Pysklywec and Mitrovica, 1998, Pysklywec and Mitrovica, 2000). Most cratons are pierced by kimberlites and mafic dike swarms that were emplaced following craton stabilization, and reflect younger thermal and mechanical disruptions to the cratonic lithosphere that may be linked with burial and unroofing patterns at the surface. The discrete lithotectonic blocks of variable age and architecture that make up cratons could respond differently to these younger perturbations, manifested as differing depositional and erosional histories across a cratonic region.

The Archean Slave craton and adjacent Paleoproterozoic Wopmay orogen in the northwestern Canadian shield together present an excellent location in which to address these issues (Fig. 1). The Canadian shield is a vast cratonic region consisting of a collage of Archean cratons that were amalgamated in the Paleoproterozoic. The “Slave craton” is an Archean craton that was assimilated into the Canadian shield; “Wopmay orogen” is a Paleoproterozoic orogenic belt that was active in the assembly of the Canadian shield and ultimately became part of the larger cratonic region. The Slave craton has been extensively studied and was repeatedly disrupted by kimberlites in Phanerozoic time (e.g., Heaman et al., 2003). Although the craton currently lacks Phanerozoic cover, some kimberlites contain sedimentary xenoliths that record snapshots of the extent and, in some cases, the thicknesses of Phanerozoic strata across the region. We acquired apatite (U–Th)/He thermochronometry data along a southeast to northwest transect extending over 250 km from the interior of the Slave craton into the adjacent Wopmay orogen to more fully constrain the magnitude, extent, and timing of past burial and unroofing episodes, and to assess whether differential unroofing occurred across the Slave–Wopmay lithospheric boundary. If lithospheric architecture exerts a fundamental control on the lithospheric response to younger thermal and mechanical perturbations, then this would predict contrasts between the low temperature histories of Archean and Proterozoic terranes that were affected by such perturbations. We incorporate our thermochronological data with numerous geologic and stratigraphic constraints to decipher the burial and unroofing history of the northwestern Canadian shield. These results allow a preliminary assessment of potential relationships among phases of cratonic sedimentation and denudation, plate margin tectonism, and kimberlite generation and emplacement in the region.