Analysis of Long-term Borehole and Seafloor Pressure Data Recorded by the Subseafloor Observatories in Middle Valley, Northern Juan de Fuca Ridge

Pressure data recorded by long-term subseafloor observatories (CORKs) are a useful tool for understanding the state of the crustal hydrologic system. In Middle Valley, a sedimented rift at the northern end of the Juan de Fuca Ridge, thickly sedimented basaltic crust hosts an array of hydrologic regimes that have been monitored continuously by two CORKs since 1996. This dissertation analyzes both recent trends in borehole and seafloor pressures, as well as several older datasets, in concert with local seismicity, physical properties of the crust, and hydrothermal circulation in an effort to understand the region’s ongoing dynamic eruptive cycle and hydrogeologic connectivity.</p

A seismic swarm and regional hydrothermal and hydrologic perturbations: The northern Endeavour segment, February 2005

The February 2005 swarm at the overlapping spreading center (OSC) on the northern end of the Endeavour segment is the first swarm on the Juan de Fuca Ridge recorded on a local seafloor seismic network. The swarm included several larger earthquakes and caused triggered seismicity and a hydrothermal response in the Endeavour vent fields as well as regional‐scale hydrologic pressure perturbations. The spatial and temporal pattern of over 6000 earthquakes recorded during this seismic sequence is complex. Small‐magnitude events dominate, and seismicity rates wax and wane, indicating a magmatic process. The main swarm initiates at the northern end of the Endeavour ridge. However, most of the moment release, including six strike‐slip events, occurs in the southwest Endeavour Valley, where the swarm epicenters generally migrate south. The main swarm is contemporaneous with a hydrologic pressure response at four sealed seafloor boreholes, ∼25–105 km away. We infer that the seismic sequence is driven by a largely aseismic magma intrusion at the northern Endeavour axis. Resulting stress changes trigger slip on tectonic faults and possibly dike propagation at the opposing limb of the Endeavour OSC in the southwest Endeavour Valley, consistent with the eventual decapitation of the Endeavour by the West Valley segment. Furthermore, 2.5 days after the start of the main swarm, seismicity is triggered beneath the Endeavour vent fields, and temperature increases at a diffuse vent in the Mothra field. We infer that this delayed response is due to a hydrologic pressure pulse that diffuses away from the main magma intrusion

Integrated Ocean Drilling Program Expedition 327 Preliminary Report: Juan de Fuca Ridge-Flank Hydrogeology The hydrogeologic architecture of basaltic oceanic crust: compartmentalization, anisotropy, microbiology, and crustal-scale properties on the eastern flank of Juan de Fuca Ridge, eastern Pacific Ocean, 5 July–5 September 2010

Integrated Ocean Drilling Program (IODP) Expedition 327 and related experiments focus on understanding fluid–rock interactions in young, upper ocean crust on the eastern flank of the Juan de Fuca Ridge, delineating the magnitude and distribution of hydrologic properties; the extent to which crustal compartments are connected or isolated (laterally and with depth); the rates and spatial extent of ridge-flank fluid circulation; and links between ridge-flank circulation, crustal alteration, and geomicrobial processes. Expedition 327 built on the achievements of IODP Expedition 301 and subsequent submersible and remotely operated vehicle (ROV) expeditions. Both drilling expeditions installed subseafloor borehole observatories ("CORKs") in basement holes to allow borehole conditions to recover to a more natural state after the dissipation of disturbances caused by drilling, casing, and other operations; provide a long-term monitoring and sampling presence for determining fluid pressure, temperature, composition, and microbiology; and facilitate the completion of active experiments to resolve crustal hydrogeologic conditions and processes.During Expedition 327, two basement holes were cored and drilled at Site U1362. Hole U1362A was cored and drilled to 528 meters below seafloor (mbsf) (292 meters subbasement [msb]), subjected to geophysical logging and hydrologic testing, and instrumented with a multilevel CORK observatory. Hole U1362B was drilled to 359 mbsf (117 msb), subjected to a 24 h pumping and tracer injection experiment, and instrumented with a single-level CORK observatory. Both CORK observatories include monitoring of pressure and temperature and downhole fluid and microbiology sampling. Wellhead samplers will be added and a long-term cross-hole test will be initiated during a postdrilling ROV expedition scheduled for Summer 2011. In addition, part of an instrument string deployed in Hole U1301B during Expedition 301 was recovered, and a replacement string of thermal sensors was installed. Finally, a program of shallow sediment coring was completed adjacent to Grizzly Bare outcrop, a suspected site of regional hydrothermal recharge. Thermal measurements and analyses of pore fluid and microbiological samples from a series of holes aligned radially from the outcrop edge will elucidate rates of fluid transport and evolution during the initial stages of ridge-flank hydrothermal circulation