ANALYSIS OF FRACTURES INTERSECTING KAHI-PUKA WELL-1 AND ITS RELATION TO THE GROWTH OF THE ISLAND OF HAWAII

As part of the Hawaii Scientific Drilling Project, Kahi Puka Well 1 pe netrated about 275 m of Mauna Loa basalts overlying a sequence of Maun a Kea flow units as it was drilled and cored to a total depth of 10-53 m below land surface. A borehole televiewer (BHTV) was run in most of the well in successive stages prior to casing in order to obtain magn etically oriented acoustic images of the borehole wall, A total of 283 individual fractures were identified from this log and characterized in terms of strike and dip. These data are divided into three vertical sections based upon age and volcanic source, and lower hemisphere ste reographic plots identify two predominant, subparallel fracture subset s common to each section, Assuming that most of the steeply dipping fr actures observed in the BHTV log are tensile features generated within basalt flows during deposition and cooling, this fracture information can be combined with models of the evolution of the island of Hawaii to investigate the depositional history of these Mauna Loa and Mauna K ea basalts over the past 400 kyr. The directions of high-angle fractur es appear to be generally parallel to topography or to the coastline a t the time of deposition, as is supported by surface mapping of modern flows. Consequently, an overall counterclockwise rotation of about 75 degrees in the strike of these fractures from the bottom to the top o f the well represents a systematic change in depositional slope direct ion over time. We attribute the observed rotation in the orientations of the two predominant fracture subsets over the past 400 kyr to chang es in the configurations of volcanic sources during shield building an d to the structural interference of adjacent volcanoes that produces s hifts in topographic patterns.