Site U1518

Abstract

Site U1518 is located on the lower continental slope near the Hikurangi Trench approximately 62 km from shore in ~2630 m water depth. The site lies on the frontal accretionary wedge ~6.5 km west of the deformation front. Drilling at Site U1518 targeted a westward-dipping thrust fault, the Pāpaku fault, that ramps from the décollement below and reaches the seafloor along an escarpment 500–1000 m east of the drilling site. The fault is thought to accommodate a significant component of plate motion and possibly to host slow slip events (SSEs). Coring and logging at Site U1518 during International Ocean Discovery Program (IODP) Expeditions 372 and 375 were intended to penetrate the fault and terminate 150–200 m in the footwall. Based on predrilling interpretation of seismic data, drilling was expected to encounter accreted Pleistocene trench-fill sediments comprising sand and mud turbidites, ash, and mass transport deposits (MTDs) in both the hanging wall and footwall of the fault. The thrust fault was expected to lie between 295 and 325 meters below seafloor (mbsf). The hanging wall sequence was expected to include 65–82 m of moderately reflective sediment, assumed to be Plio–Pleistocene in age, that overlies an interval characterized by irregular weak seismic reflections. The hanging wall sequence apparently dips to the east in the forelimb of an anticlinal fold associated with the fault. The footwall sequence (i.e., below the fault) to the 600 mbsf target drilling depth was expected to include a strongly reflective interval about 145–180 m thick immediately below the fault, overlying a less reflective sequence below. The footwall sequence has an apparent westerly dip on the seismic profile, and the drilled interval was expected to be Plio–Pleistocene in age. The primary objectives at Site U1518 were to (1) core and log to total depth with the highest priority of sampling the lower ~100 m of the hanging wall, the fault zone, and the footwall of the thrust (including possible additional subsidiary faults in the footwall) and (2) install a subseafloor observatory to monitor deformation, pore fluid pressure, and temperature and to sample fluids over time through the slow slip cycle. With shallow SSEs on this northern segment of the Hikurangi margin recurring every 1–2 y, it is anticipated that the borehole observatory will record pressure, temperature, and fluid flow and fluid chemistry transients associated with SSEs. The objectives of logging and coring at Site U1518 were to define the structures and deformation, physical properties, age, thermal state, lithology and composition, and interstitial fluid geochemistry of the fault and surrounding sediments. Coring results were used in combination with logging-while-drilling (LWD) data to define the depth interval for observatory pore pressure monitoring and geochemical sampling in the fault zone and to select optimal locations for pore pressure monitoring in the hanging wall and footwall. Key foci for post-expedition studies on core samples and LWD data sets include (but are not limited to) the following: structural analyses to characterize deformation mechanisms and style, fracture and fault orientations, and wellbore failures; Experimental investigation of rheology and friction to test hypotheses linking fault constitutive properties to slip behavior; geomechanical and thermal properties measurements to define poroelastic, strength, and heat transport properties of the formation to guide the interpretation of observatory data; and strength, permeability, and elastic moduli measurements to provide context for the interpretation of borehole failures as indicators of in situ stress magnitude, parameterization of hydrological models, and core-log-seismic integration. The observatory will monitor formation pore pressure (as a proxy for volumetric strain) to document possible hydraulic transients linked to SSEs and to define ambient pore pressure and effective stress state, as well as hydrological, thermal, and chemical properties and conditions throughout the slow slip cycle. These observations will be accomplished using an integrated observatory design that includes multilevel pore pressure sensing, a string of temperature sensors spanning the borehole, and an OsmoSampler and OsmoFlowmeter installed in the fault zone.