Seismic properties of lawsonite eclogites from the southern Motagua fault zone, Guatemala

Abstract

We present new data on the crystal preferred orientation (CPO) and seismic properties of omphacite and lawsonite in extremely fresh eclogite from the southern Motagua fault zone, Guatemala, to discuss the seismic anisotropy of subducting oceanic crust. The CPO of omphacite is characterized by (010)[001], and it shows P-wave seismic anisotropies (AV_P) of 1.4%-3.2% and S-wave seismic anisotropies (AV_S) of 1.4%-2.7%. Lawsonite exhibits (001) planes parallel to the foliation and [010] axes parallel to the lineation, and seismic anisotropies of 1.7%-6.6% AV_P and 3.4%-14.7% AV_S. The seismic anisotropy of a rock mass consisting solely of omphacite and lawsonite is 1.2%-4.1% AV_P and 1.8%-6.8% AV_S. For events that propagate more or less parallel to the maximum extension direction, X, the fast S-wave velocity (V_S) polarization is parallel to the Z in the Y-Z section (rotated from the X-Z section), causing trench-normal seismic anisotropy for orthogonal subduction. Based on the high modal abundance and strong fabric of lawsonite, the AVS of eclogites is estimated as ~ 11.7% in the case that lawsonite makes up ~ 75% of the rock mass. On this basis, we suggest that lawsonite in both blueschist and eclogite may play important roles in the formation of complex pattern of seismic anisotropy observed in NE Japan: weak trench-parallel anisotropy in the forearc basin domains and trench-normal anisotropy in the backarc region.