ECLOGITE-FACIES SHEAR ZONES - DEEP-CRUSTAL REFLECTORS

Strongly foliated eclogite-facies rocks in 30-150 m thick shear zones of Caledonian age occur within a Grenvillian garnet granulite-facies gabbro-anorthosite terrain in the Bergen Arcs of Norway. The predominant eclogite-facies mineral assemblages in the shear zones are omphacite + garnet + zoisite + kyanite in gabbroic anorthosite and omphacite + garnet in gabbro. Eclogite-facies rocks in shear zones are generally fine-grained; alternating omphacite/garnet- and kyanite/clinozoisite-rich layers define gneissic layering. A strong shape preferred orientation of omphacite, kyanite, and white mica (phengitic muscovite and/ or paragonite) define the foliation. The anorthositic eclogites show omphacite b-axis maxima approximately normal to the foliation and c-axis girdles within the foliation plane. P-wave velocities (V(P)) determined at confining pressures to 600 MPa for samples from eclogite-facies shear zones range from 8.3 to 8.5 km s-1 and anisotropy ranges from 1 to 7%. The few samples with more pronounced anisotropy tend to be approximately transversely isotropic with minimum velocities for propagation directions normal to foliation and maximum velocities for propagation directions parallel to foliation. The fast propagation direction lies within the c-axis girdles (parallel to foliation) and the slow propagation direction is parallel to the b-axis concentration (normal to foliation) in samples for which omphacite crystallographic preferred orientation was determined. Vp for the granulite-facies protoliths average about 7.5 km s-1. High calculated reflection coefficients for these shear zones, 0.04-0.14, indicate that they are excellent candidates for deep crustal reflectors in portions of crust that experienced high-pressure conditions but escaped thermal reactivation.