State of Stress and Structure of Volcanic Shield Penetrated by the HSDP Deep Core Hole

As part of the Hawaii Scientific Drilling Project (HSDP), a pilot hole was drilled in 1993 to a depth of 1056 meters below sea level (mbsl) and a deeper hole was drilled to 3098 mbsl in 1999. Complementary borehole deviation and acoustic televiewer logs were obtained in the deep hole that provide fundamental information regarding the structure and the state of stress that exist within a volcanic shield. Fracture planes observed in the Mauna Kea basalts between 1829 and 2896 mbsl display a fairly uniform strike whose direction transitions smoothly into the fracture orientations observed in the shallow pilot hole. The combined directional data from both holes provides a continuous record of fracture strike with depth and also with age to 540 ka. It depicts a clockwise rotation through the surficial Mauna Loa basalts that stabilizes to a constant heading in the underlying Mauna Kea rocks. This information corresponds to the direction of lava flows and the location of volcanic sources relative to the drill site. The deviation log mimics this behavior because the drill bit methodically drifts in a direction perpendicular to the dominant fracture strike; the wellbore trajectory, in effect, becomes a surrogate for the evolution of fracture orientation and, thus, for the direction of basalt deposition with time. Breakouts observed in the televiewer log identify the orientations of the maximum (S_HMAX) and minimum (S_hmin) horizontal principal stresses to be north-south and east-west, respectively. The additional appearance of ovals cut into the borehole walls implies that S_HMAX >> S_hmin at this site, and examination of local topographic conditions supports this conclusion. A sharp break in onshore-offshore slope reduces stress east-west and a prevailing north-south slope associated with the emergence of Kilauea increases stress north-south. These two effects combine to amplify the differences in the magnitudes of the horizontal principal stresses. Consequently, breakouts are extensive and appear over about 30 percent of the open hole.