Anisotropy Measurements and the Deep Structure of a Passive Margin: Southwestern Greenland

Abstract

The non-uniqueness of the laboratory measured physical properties (P-wave velocities and densities) can be partly overcome by the inclusion of anisotropy. Seismic anisotropy estimation can constitute an additional constraint in the determination of the mineralogical composition of the deep crust from indirect seismic measurements. Resolution analysis studies reveal that p-and S-wave velocity measurements derived from wide-angle seismic reflection/refraction data are usually characterized by large error estimates (i.e., very low resolution ). These error estimates render physically meaningless Poisson’s ratio profiles. Also the duality in S-wave velocities due to anisotropy complicates the estimation of Poisson’s ratio. The almost general use of single component (vertical) seismic instruments and low resolution of refraction/wide-angle reflection experiments (mostly due to spatial under sampling) have prevented the use of a nisotropy as a constraint for the determination of lower crustal composition in favor of estimates on Poisson’s ratio. Therefore, we suggest that anisotropy estimates can place new and more relevant constraints on the different rock types present in the deep crust. In order to assess indirect seismic anisotropy measurements we employed three component, densely space large aperture (0-250 km offset, at 100-150 m spacing) seismic recordings acquired along the south west coast of Greenland utilizing REFTEK PASSCAL instruments deployed by the University of Wyoming. With the aid of an inversion scheme that uses reflected and converted energy we determined P- and S-wave velocity-depth functio ns for the passive margin of southwestern Greenland. The inversion suggests a P-wave velocity structure characterized by two gradient zones: a relatively high gradient from the surface to approximately 5 km depth where velocities exceed 6.0 km/s, followed by a low gradient to the base of the crust where velocities reach 7.0 km/s. A high P-wave velocity layer (7.2-7.4 km/s) can be identified between 6-8 km above the Moho. GraVity modeling suggests relatively high densities 3 .0-3.1 kg/m3 for this layer. Independent analysis of the radically and transversely polarized horizonta l components revealed average velocities of 4.9 ± 0.1 km/s and 4.5 ± 0.1 km/s respectively suggesting a seismically anisotropic crust. A time delay of 0.25 s between the radial and the transverse horizontal components of the SIS phase is observed at offsets of 70 km. The radically polarized S-wave is parallel to the southwest coast of Greenland. From the S-wave analysis. the ocean-continent transitional crust is clearly seismically anisotropic above a high velocity layer in the lower continental crust. The density and velocity values suggested for this high velocity structure above the Moho, and the anisotropy measured just above it seem to favor an accretion of hot. mafic mantle material (underplating) at the base of the crust during a rifting episode. Possibly. magmatic underplating during Late Cretaceous rifting of the Labrador Sea heated the preexisting lower crust promoting plastic flow and enabling alignment of anisotropic minerals to produce the seismic anisotropy.