CrustalVSstructure in northwestern Canada: Imaging the Cordillera-craton transition with ambient noise tomography

Abstract

[1] Crustal seismic velocity structure in northwest Canada is imaged using group velocity measurements derived from ambient noise cross correlations. The focus area surrounds the Canadian Northwest Experiment (CANOE) array, a 16 month deployment of 59 broadband seismic stations. The CANOE array extended from the Northern Cordillera on the west into the Archean Slave province to the east, crossing crustal terranes that span ∼4 Gyr of Earth history. Forty broadband stations from the Canadian National Seismograph Network and the POLARIS network are also included. The Green's function for each pair of stations is estimated by cross-correlating 24 h long time series of ambient noise for each day in the time period July 2004 to June 2005. Fundamental mode Rayleigh waves are observed on cross-correlated vertical component records and Love waves on the transverse components. The group velocity measurements are inverted for 3-D shear wave velocity within the crust. Local sensitivity kernels are used to account for the effects of a laterally variable sedimentary layer at shallow depths, and receiver function P-s differential times at the CANOE stations constrain total crustal thickness. Known sedimentary basins dominate lateral variations in wave speed in the shallow crust. In the middle crust, the model contains a strong low-velocity region that correlates spatially with the location of Proterozoic metasedimentary rocks that have been inferred from active source reflection profiles to occupy much of the crust beneath the Cordilleran terranes. Velocity variations in the lower crust, which are weakly resolved due to the limited period range of our data set (5–20 s), suggest a west-to-east transition from lower to higher wave speeds that aligns with the Cordilleran deformation front in parts of the study area. The results presented here are consistent with but do not confirm the notion of Proterozoic strata throughout much of the Cordilleran crust. These metasedimentary rocks, if present, must have experienced some deformation and metamorphism in order to also be consistent with upper mantle seismic models that contain a sharp transition from low to high velocity across the deformation front.