Trench-normal seismic anisotropy generated by lawsonite eclogites from the southern Motagua fault zone, Guatemala

Abstract

We present the results of EBSD analyses on extremely fresh lawsonite eclogites from southern Motagua fault zone, Guatemala for understanding seismic properties of subducting oceanic crust. The CPO of omphacite is characterized by (010)[001] with AVS of 1.4-2.7%, whereas that of lawsonite defined by (001)[010] with 3.4-14.7% AVS. The seismic anisotropy of lawsonite eclogite is calculated as 1.2-4.1% AVP and 1.8-6.8% AVS. Based on an assumption of the high modal abundance and strong fabric of lawsonite, the AVS of eclogite is estimated as ~11.7% in the case of 75% lawsonite and 25% omphacite. For events that propagate more or less parallel to the maximum extension direction, X, the fast S-wave velocity (VS) causing trench-normal seismic anisotropy for orthogonal subduction. Our study suggests that lawsonite in both blueschist and eclogite may play the key role in the formation of complex pattern of seismic anisotropy observed in NE Japan: weak trench-parallel in the forearc basin and trench-normal in the backarc.

* Main contents of this study has been published in Tectonophysics (2016, vol 677-678, 88-98, doi: 10.1016/j.tecto.2016.04.012).