Constraints on the Upper Mantle Structure of the Slave Craton From Surface Wave Tomography

The Slave craton in the NW Canadian Shield, because of its high degree of preservation and ideal location relative to global seismicity, has been the focus of many seismological studies that have enhanced our understanding of the structure and evolution of Archean cratons. Recent body-wave tomography has imaged a low P-wave velocity structure (2.8% lower than the surrounding mantle) beneath the high-velocity cratonic lithosphere of the Slave, implying persistent processes in the upper mantle relevant to kimberlite magmatism. While body-wave tomography provides good constraints on lateral velocity variations, the vertical extent of anomalous structures in the lithosphere and upper mantle may be further constrained by surface wave tomography due to its frequency-dependent sensitivity to Earth's structure as a function of depth. In this study, we analyze teleseismic Rayleigh wave phase velocities to investigate upper mantle structure of the Slave craton. We measured phase velocities for periods between 28s-143s of 3833 crossing seismic rays from 44 events recorded by the POLARIS broadband seismic network and the Yellowknife array. To better determine the velocity structure under this region, we employ an array-analysis technique that takes into account the multipathing effect of surface wave propagation by modeling the incoming wavefield with a two plane-wave approximation. Our preliminary results show slightly higher phase velocities at shorter periods compared to those of other cratons. Conversely, at periods greater than 100s, the phase velocities are reduced by up to 5% relative to average cratonic values. 1-D inversion of phase velocities yields high S-wave velocities ( ∼4.65±0.2 km/s) associated with cratonic lithosphere between 30-130 km depths, underlain by a negative velocity gradient that reaches average velocities of ∼4.0 km/s in the 220-350 km depth range. While such negative gradients have been observed between 120 km and >200 km depth beneath most other cratons, the S-wave velocities inferred here are lower than elsewhere by at least 7%. These anomalous values, in conjunction with the low P-wave velocity observed in the body-wave tomography, indicate that a heterogeneous structure which may be related to kimberlite genesis persists in the upper mantle beneath the central Slave craton. We investigate the source of this anomaly by considering the influence of water content, thermal and compositional variations, and partial melting.