Comparison of Field and Laboratory Measurements of Schist Anisotropy, Chester Dome, Vermont

Abstract:

Constraining crustal anisotropy gives insights into lithospheric deformation and allows for more accurate mantle observations. Seismic observations can be related to rock fabric with elastic properties determined from laboratory methods, e.g. ultrasound and electron backscatter diffraction (EBSD). However, common simplifications used to invert for anisotropic structure (elliptical assumption, transverse isotropy) are confounded by the presence of microcracks, shape-preferred orientation, and higher-order symmetries. Therefore, we seek to make direct comparisons between seismically and experimentally measured anisotropy to determine what symmetry can be observed and the effect of extrinsic factors. We compare anisotropic velocities from EBSD to those observed in situ by a refraction survey at Chester Dome, Vermont, an elongated gneiss dome in the northern Appalachians.

The field study was performed by an undergraduate research class on a vertically-dipping schist unit on the dome’s western flank. We acquired refraction data on a Geometrics ES3000 with 24 geophones using a sledgehammer source. Three profiles with baselines of 140 m were collected at 0°, 45°, and 90° to the strike of foliation. We analyze each profile with a trans-dimensional Bayesian 2D refraction code (Huang et al., in review) to produce an ensemble of velocity profiles from which we can estimate the velocity of the underlying schist, and its uncertainty, in each direction.

We compare the field results to EBSD measurements of rocks collected in and around the survey site. Directional velocity is estimated from mineral composition and alignment and microcrack properties. We investigate the following questions: 1) Can the refraction survey detect the non-elliptical component of anisotropy? 2) What microcrack density and alignment reconciles high EBSD-derived velocities with seismic observations? 3) Can the survey statistically distinguish the target schist from other rock types?