Spatial Variations in Crustal and Mantle Anisotropy Across the North American‐Caribbean Boundary on Haiti

Daniel Possee,Graham Stuart,Catherine Rychert,Claude Prépetit,Jordane Corbeau,Dominique Boisson,Frédérique Rolandone,Caroline Eakin,Nicholas Harmon,Roberte Momplaisir,Derek Keir,Sylvie Leroy

doi:10.1029/2019jb018438

Abstract

AbstractHaiti, on the island of Hispaniola, is situated across the North American‐Caribbean plate boundary at the transition point between oblique subduction in the east and a transform plate boundary in the west. Here we use shear wave splitting measurements from S waves of local (0–50 km) and intermediate depth (50–150 km) earthquakes as well as SK(K)S phases from teleseismic earthquakes to ascertain good spatial and vertical resolution of the azimuthal anisotropic structure. This allows us to place new constraints on the pattern of deformation in the crust and mantle beneath this transitional region. SK(K)S results are dominated by plate boundary parallel (E‐W) fast directions with ~1.9 s delay times, indicating subslab trench parallel mantle flow is continuing westward along the plate boundary. Intermediate depth earthquakes originating within the subducting North American plate show a mean fast polarization direction of 065° and delay time of 0.46 s, subparallel to the relative plate motion between the Caribbean and North American plates (070°). We suggest a basal shear zone within the lower ductile crust and upper lithospheric mantle as being a potential major source of anisotropy above the subducting slab. Upper crustal anisotropy is isolated using shear wave splitting measurements on local seismicity, which show consistent delay times on the order of 0.2 s. The fast polarization directions indicate that the crustal anisotropy is controlled by the fault networks in close proximity to the major strike‐slip faults, which bisect the north and south of Haiti, and by the regional stress field where faulting is less pervasive.

Highlights

The island of Hispaniola has seen a complex tectonic evolution due to its position along the North American‐Caribbean Plate boundary (Figure 1), with its present‐day position being at the transition from oblique subduction in the east to oblique collision to the west
Using data from the Trans‐Haiti network and permanent seismic stations, we extend the lateral extent of shear wave splitting observations along the Caribbean‐North American plate boundary (Figure 2)
Seventy‐eight of these measurements were obtained from two permanent stations (LGNH, SDDR), the remainder were obtained from the Trans‐Haiti temporary network

Summary

Introduction

The island of Hispaniola has seen a complex tectonic evolution due to its position along the North American‐Caribbean Plate boundary (Figure 1), with its present‐day position being at the transition from oblique subduction in the east to oblique collision to the west. Observations of the present‐day crustal stress state and its link to the underlying mantle dynamics of the plate boundary play an important role in our understanding of the plate tectonics. One of the most common methods to observe seismic anisotropy is shear wave splitting, whereby the shear wave becomes polarized into two orthogonal components as it passes through the anisotropic medium. By measuring the orientation of the fast polarization direction (φ) and the delay time (dt) between the components inferences can be made about the anisotropic system. Seismic anisotropy is typically interpreted as the result from strain‐induced lattice‐preferred‐orientation (LPO) of olivine caused by past or POSSEE ET AL

Results

Discussion

Conclusion