Results of Borehole Televiewer Observations in the Celebes and Sulu Seas

Abstract

Borehole televiewer data were collected during Ocean Drilling Program Leg 124 to investigate stress orientations in the Sulu and Celebes sea basins. At Site 768C, in the Sulu Sea, televiewer logs were collected from 809-870 and 950-1250 meters below sea floor (mbsf). At Site 770C, in the Celebes Sea, televiewer logs were collected between 405-524 mbsf. No borehole breakouts were found during examination of these data. Calculation of the allowable stresses using a range of basaltic rock strengths shows that ambient stresses at the depths logged are lower than those necessary to cause breakouts in basalt. Analysis of fractures visible in the borehole televiewer logs showed strong groupings of fracture orientations at both sites. At Site 768, the main fracture trend is N 120° E, dipping 60° SW, with smaller groupings at N 117° E, 62° NE; N 64° E, 68° SE; and N 35° E, 64° E. At Site 770, the main fracture orientations are N 119° E, 68° NE and N 143° E, 70° SW. The preferred orientations of these high-angle fractures may represent a response to the present stress field, either by formation of new fractures or reactivation of older zones of weakness. The observed fracture pattern is compatible with extensional stresses related to subduction at the Cotabato and Negros trenches. Alternatively, the fractures may be relict features recording earlier stress conditions.