Mantle roots of major Precambrian shear zones inferred from structure of the Great Slave Lake shear zone, northwest Canada

Abstract

Preserved, ancient orogens provide important perspective on modern continental convergent zones. Relatively deep levels of these systems are exposed at the surface, and these crystalline rocks promote the recording of seismic signals from great depths. In this study, multiazimuthal teleseismic observations reveal gently dipping discontinuities and wedge-shaped structures beneath the Great Slave Lake shear zone, Canada, which formed the Paleoproterozoic convergent boundary between the Archean Slave and Rae cratons during assembly of the Nuna supercontinent. This ancient shear zone experienced two distinct tectonic phases. An early convergence phase involved intracontinental underthrusting of the Slave craton beneath the Rae margin with little evidence of subduction. A later phase was characterized by brittle strike-slip faulting. Receiver functions exhibit discontinuities that imply the latter faults to be listric in the crust. A high-velocity layer below the Moho may mark eclogitic residues of melts that formed “calc-alkaline” granitoids within the crust of the shear zone, without invoking subduction-induced melting within the mantle. Uppermost mantle discontinuities and xenolith suites from nearby kimberlite pipes together defi ne lithological and structural geometries interpreted as two phases of wedging by the central Slave craton lithosphere, associated with either Slave craton construction or early Slave-Rae convergence. The new teleseismic data contain no direct or indirect evidence of faulting along near-vertical suturing structures within the crust or the mantle. The observed wedge-shaped cratonic margins imply that lithospheric blocks grew primarily by lateral accretion of similar blocks, with the strongest layer at 100–150 km depths. In the study area, the diamondbearing Slave lithospheric mantle wedge underlies the entire shear zone at depths where diamonds are stable, and data indicate that the Slave craton’s lithospheric mantle was intact and strong at these depths as early as 2.0–1.9 Ga, when this tectonic boundary was active.