SEISMIC ANISOTROPY VARIATIONS ACROSS GREENLAND: EVIDENCE FROM SHEAR WAVE SPLITTING

Much of Greenland is a stable craton containing Precambrian rocks associated with the Laurentian continent. Despite the current stability of the region, Greenland’s geology preserves orogenic belts and traces of ancient subduction processes dating from Precambrian to Mesozoic times. In order to find out how these deformation zones are preserved in the lithospheric mantle, we investigate upper-mantle seismic anisotropy using the method of shear wave splitting. When a shear wave meets an anisotropic medium, it separates into two orthogonal components, “fast” and “slow”, aligned along, and transverse to, the orientation of anisotropy. We measure the time delay between the two waves, which depends on the thickness of the layers and the strength of the anisotropy, as well as the azimuth of the fast wave, which aligns with the preferential direction of deformation of the medium.

We use PKS, SKS and SKKS phases from large (magnitude >6) earthquakes from distant (>80°) earthquakes recorded at 35 seismograph stations distributed across Greenland. The recording period covers the period 1999-present, with at least 6 years of operation for the majority of the stations.

The resulting anisotropy measurements show great variability in the value of dt, from 0.35 to 1.98 seconds, suggesting several different sources. Comparison of the dominant fast directions with regional tectonics and with geodynamic models show some correlations, but no clear distinction. Distinct periodicity in the relation between the fast direction of anisotropy and the backazimuth of the incoming wave indicates the presence of more than one layer of anisotropy. Two-layer modelling suggests that the shallower layer is mostly correlated with tectonic features (i.e. “fossil” lithospheric anisotropy), whereas the deeper layer is more correlated with mantle convective flow. The models also suggest that the layers are dipping, adding more complexity to the interpretation. Greenland’s upper mantle is highly complex, with several anisotropic sources recorded by the splitting measurements, and further work is needed to constrain the depth-dependence and origins of the anisotropy.