The Crust and Mantle Structure of the Mid-Continent Rift System from Ambient Noise and Earthquake Surface Wave Analysis
Tuesday, 16 December 2014
An investigation of the crust and mantle structure beneath the northern part of the Mid-Continent Rift Zone (MCRZ) is carried out using through seismic tomography from both teleseismic earthquake data and ambient seismic noise, using data from the EarthScope USArray Transportable Array (TA) and from the Flexible Array (FA) project SPREE (Superior Rifting EarthScope Experiment). The SPREE project deployed 83 temporary broadband seismic stations in Minnesota, Wisconsin, and Ontario during 2011-2013. The goal is to study the ancient Superior Province rifting via the related crustal and upper mantle modification to the petrologic and thermal structures. Continental rifting 1.1 Ga ago produced an enigmatic horseshoe-shaped rift system that is only exposed in the Lake Superior Region, but shows strong gravity and magnetic anomalies. Cross-correlation between all station pairs of the SPREE and concordant TA stations are used to obtain ambient noise Rayleigh-wave Green’s functions, which are in turn used to obtain group and phase velocity dispersion curves in the 8 – 50 s period range. In addition, the two-plane-wave method with finite frequency kernels is used to determine teleseismic earthquake-generated Rayleigh wave phase velocities in the 20 – 182 s period range. Combining the phase velocity measurements from both techniques provides an opportunity to invert for 1-D shear wave velocity structure in the 8 – 182 s period range. Phase velocities generally agree well where the period bands overlap. A three-dimensional S-wave velocity model of the crust and upper mantle is obtained from the 1-D shear wave velocity profiles at each location, also incorporating receiver function estimates of crustal thickness. For the shear inversion the overlapping phase velocities are averaged, with noise correlation results preferentially weighted at short periods and teleseismic results at longer periods. The model shows slow shear velocities at shallow depths along the rift zone due to sediment infill. Slightly slow velocities are also seen in the uppermost mantle along much of the rift zone, perhaps due to Compositional variation. The other clear feature is fast velocities at deeper lithospheric levels in the Superior craton.