Stacked uppermost mantle layers within the Slave craton of NW Canada as defined by anisotropic seismic discontinuities

Abstract

A 20‐station seismic array in NW Canada recorded 336 teleseismic events with distribution in back azimuth and epicentral distance sufficient to characterize uppermost mantle discontinuities between depths typical of the Mohorovicic and Lehman discontinuities. Following wavefield decomposition, groups of seismograms were source‐normalized through simultaneous deconvolution to estimate the near‐receiver impulse response and thus detect major discontinuities beneath each seismic station. Stations within the Lac de Gras kimberlite field display an unusually strong negative impulse on the radial component within the NW quadrant and two moderate impulses on the transverse component. Forward modeling of these impulses suggests a mantle layer dipping at 22° to the southeast with a mildly anisotropic (2%) upper discontinuity at 120–135 km depth and another mildly anisotropic (2%) discontinuity at about 170 km depth. Superimposed on these layers is another, stronger anisotropic (4%) layer between 110 and 180 km depths that dips to the west. Stations outside of the Lac de Gras field, but within the southeastern Slave craton, display more numerous, but weaker, impulses. The most prominent of these occurs at about 150 km depth on the transverse component and has opposite polarity to that observed farther north. The prominent negative impulse observed on the radial component is interpreted to arise from structural‐preferred orientation in the form of a stockwork of wehrlite dykes beneath the Lac de Gras field. Interpretation of the other layers in the context of known surface geology as well as xenolith petrology and garnet geochemistry of diamondiferous kimberlites favors previous suggestions that they represent 4000–2900 Ma depleted harzburgite and eclogite layers underthrust from the northwest at 2600 or 1880 Ma. The layer beneath the SE Slave craton has a similar, but distinct, tectonic history of NW‐verging underthrusting associated with the 2635–2615 Ma Defeat Suite of plutonism. Taken together, these interpretations indicate that the Slave craton was assembled from at least four lithospheric blocks prior to its cratonization about 2580 Ma: each block is 90–120 km thick, and all four blocks abut across a steep suture boundary beneath MacKay Lake in the central Slave craton. Significant amounts of carbon and eclogites could have become incorporated into the central Slave mantle within the diamond stability field during the proposed underthrusting of lithosphere to explain their relatively common occurrence in kimberlite eruptions within the Slave craton.

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

“…Dipping or tapered fabrics associated with some MLDs in cratonic lithosphere may relate to craton root thickening involving plate tectonics (Bostock, 1997;Cook et al, 1999;Chen et al, 2007;Snyder 2008;Miller and Eaton, 2010;Ashchepkov et al, 2014;Cooper and Miller, A C C E P T E D M A N U S C R I P T ACCEPTED MANUSCRIPT 25 2014; Wirth and Long, 2014) (Fig. 2C).…”

“…Subduction-accretion of oceanic slabs has been a popular model for the construction of cratonic roots (Helmstaedt and Schulze, 1989;Canil, 2004;Wittig, 2008, 2014) and has been invoked to explain dipping seismic discontinuities, and in some cases spatially coincident seismic and conductive anomalies, in some cratonic mantle samples (Cook et al, 1999;Snyder, 2008;Chen et al, 2009;Cooper and Miller, 2014). …”

Spatial distribution of eclogite in the Slave cratonic mantle: The role of subduction

“…This would explain why the layered anisotropic structure the southeastern Slave craton 24 (east of the MCKN station on Fig. 1) is distinctly different, demonstrating more numerous, 25 weaker impulses, with the opposite polarity to that observed farther north (Snyder, 2008). interval.…”

“…10A) with the mid-lithosphere 13 discontinuity (Fig. 11) and the presence of two overlapping layers defined by teleseismic 14 discontinuities with the different dipping directions (Snyder, 2008). These two layers were 15 …”

“…This craton in the western Canadian Shield (Fig. 1) has a well constrained geological 16 history (Snyder, 2008;Helmsteadt, 2009) and a thoroughly studied mantle, mapped 17 geophysically (e. g. Snyder et al, 2004; and petrologically (e. g. Kopylova and Caro,18 2004; Heaman and Pearson, 2010). The long sequence of tectonomagmatic events pre-and post-19 dating craton stabilization is mirrored in the complex architecture of the Slave mantle, which is 20 compositionally stratified, separated in multiple domains and hosts zones distinct in geophysical 21 and geochemical properties.…”