Subsurface fracture patterns in the PACMANUS hydrothermal system identified from downhole measurements and their potential implications for fluid circulation

Ocean Drilling Leg 193 drilled 13 holes in an active hydrothermal vent system associated with felsic volcanism in the Manus Basin along the western margin of the Pacific plate. This hydrothermal field is located near the crest of the northeast trending Pual Ridge, which is a 500- to 700-m-high felsic neovolcanic edifice that appears to be constructed of stacked subhorizontal lava flows with negligible sediment cover. Wireline and logging-while drilling (LWD) measurements were obtained from 4 holes in order to characterize the subsurface volcanic architecture of the system, determine the structural relationships and associated hydrological processes, and assess the mineralization and alteration patterns of the active PACMANUS hydrothermal field. A series of downhole temperature profiles show a very high post-drilling thermal rebound in this area with changes in temperature of over 200 deg C in five days. Highly resistive subvertical features that may correspond to high concentrations of silica and/or anhydrate suggest potential fluid flow patterns. Downhole measurements show high gamma ray anomalies usually associated with high uranium content that corresponds to intensely fractured and brecciated intervals also suggesting potential hydrothermal fluid flow. Fracture patterns from Formation Micro Scanner (FMS) and LWD Resistivity-at-the-Bit (RAB) measurements show NNE-SSW strike orientations. Earthquake focal mechanism solutions show strike slip events in the proximity of the drilled area with approximately NE-SW orientations for the main compressional stresses. These relationships suggest that fluid flow in this area could be highly influenced by shallow volcanism underneath the Pual Ridge and to some lesser extent, to localized earthquake activity.