Borehole Televiewer Data Analysis from the New Hebrides Island Arc: The State of Stress at Holes 829A and 831B

Abstract

Analysis of borehole televiewer and Formation MicroScanner data from Ocean Drilling Program (ODP) Holes 829A and 83 IB, located in the Vanuatu subduction zone, reveals that both boreholes are elongated. However, borehole televiewer data from neither borehole show the classical form of borehole breakouts (enlargements on opposite sides in an otherwise circular wellbore); rather, the data indicate smooth, varying borehole surfaces. The orientation of the borehole elongation in Hole 829A (123° ± 16°) parallels topography and structural lineaments. In Hole 83 IB the orientation of elongation (148° ± 19°) is roughly perpendicular to the direction of plate convergence (76°). Focal mechanism solutions show that in the area of investigation the overriding plate is characterized by an east-northeastwest-southwest directed compressional stress regime. This indicates, that the orientation of the minimum horizontal stress direction, where compressive shear failure occurs, is parallel to the strike of the Australia/Pacific Plate boundary. Calculation of finite element modelling also show that high compressive stresses are assumed in accretionary wedges (Bott et al., 1989). The correlation of the orientation of the elongations in both wellbores with the local and regional stress field implies that the orientation of borehole elongation results from the present day stress field. Orientation of borehole elongation of Hole 83 IB strikes parallel to the Australia/Pacific Plate boundary indicating that it is consistent with the stress field deduced from earthquake focal mechanisms. However, orientation of borehole elongation of Hole 829A is affected by the local rather than the regional stress field. Observed local tectonics and topography superimpose the regional stress field at this drill site and lead to a 43° northward rotation of the maximum horizontal stress field. Additionally, the existence of a smooth borehole surface rather than borehole breakouts implies that despite the assumed high compressive stresses, additional processes, for example decoupling along thrust faults, must exist to reduce these stresses.