Description: Large uncertainties about the energy resource potential and role in global climate change of gas hydrates result from uncertainty about how much hydrate is contained in marine sediments. During Leg 204 of the Ocean Drilling Program (ODP) to the accretionary complex of the Cascadia subduction zone, we sampled the gas hydrate stability zone (GHSZ) from the seafloor to its base in contrasting geological settings defined by a 3D seismic survey. By integrating results from different methods, including several new techniques developed for Leg 204, we overcome the problem of spatial under-sampling inherent in robust methods traditionally used for estimating the hydrate content of cores and obtain a high-resolution, quantitative estimate of the total amount and spatial variability of gas hydrate in this structural system. We conclude that high gas hydrate content (30–40% of pore space or 20–26% of total volume) is restricted to the upper tens of meters below the seafloor near the summit of the structure, where vigorous fluid venting occurs. Elsewhere, the average gas hydrate content of the sediments in the gas hydrate stability zone is generally 〈2% of the pore space, although this estimate may increase by a factor of 2 when patchy zones of locally higher gas hydrate content are included in the calculation. These patchy zones are structurally and stratigraphically controlled, contain up to 20% hydrate in the pore space when averaged over zones ∼10 m thick, and may occur in up to ∼20% of the region imaged by 3D seismic data. This heterogeneous gas hydrate distribution is an important constraint on models of gas hydrate formation in marine sediments and the response of the sediments to tectonic and environmental change.

Description: Several cold vents are observed at the northern Cascadia margin offshore Vancouver Island in a 10 km2 region around Integrated Ocean Drilling Program Expedition 311 Site U1328. All vents are linked to fault systems that provide pathways for upward migrating fluids and at three vents methane plumes were detected acoustically in the water column. Downhole temperature measurements at Site U1328 revealed a geothermal gradient of 0.056 ± 0.004°C/m. With the measured in situ pore-water salinities the base of methane hydrate stability is predicted at 218–245 meters below seafloor. Heat-probe measurements conducted across Site U1328 and other nearby vents showed an average thermal gradient of 0.054 ± 0.004°C/m. Assuming that the bottom-simulating reflector (BSR) marks the base of the gas hydrate stability zone variations in BSR depths were used to investigate the linkages between the base of the gas hydrate stability zone and fluid migration. Variations in BSR depth can be attributed to lithology-related velocity changes or variations of in situ pore-fluid compositions. Prominent BSR depressions and reduced heat flow are seen below topographic highs, but only a portion of the heat flow reduction can be due to topography-linked cooling. More than half of the reduction may be due to thrust faulting or to pore-water freshening. Distinct changes in BSR depth below seafloor are observed at all cold vents studied and some portion of the observed decrease in the BSR depth was attributed to fault-related upwelling of warmer fluids. The observed decrease in BSR depth below seafloor underneath the vents ranges between 7 and 24 m (equivalent to temperature shifts of 0.07–0.15°C).

Description: In April and May 1996, a geophysical study of the Cascadia continental margin off Oregon and Washington was carried out aboard the German RV Sonne as a cooperative experiment between GEOMAR, the USGS and COAS. Offshore central Oregon, which is the subject of this study, the experiment involved the collection of wide-angle refraction and reflection data along three profiles across the continental margin using ocean-bottom seismometers (OBS) and hydrophones (OBH) as well as land recorders. Two-dimensional modelling of the travel times provides a detailed velocity structure beneath these profiles. The subducting oceanic crust of the Juan de Fuca plate can be traced from the trench to its position some 10 km landward of the coastline. At the coastline, the Moho has a depth of 30 km. The dip of the plate changes from 1.5° westward of the trench to about 6.5° below the accretionary complex and to about 16° further eastward below the coast. The backstop forming western edge of the Siletz terrane, an oceanic plateau that was accreted to North America about 50 Ma ago, is well defined by the observations. It is located about 60 km to the east of the deformation front and has a seaward dip of 40°. At its seaward edge, the base of the Siletz terrane seems to be in contact with the subducting oceanic crust implying that sediments are unlikely to be subducted to greater depths. The upper oceanic crust is thinner to the east of this contact than to the west. At depths greater than 18 km, the top of the oceanic crust is the origin of pre-critical reflections observable in several land recordings and in the data of one ocean bottom instrument. These reflections are most likely caused by fluids that are released from the oceanic crust by metamorphic facies transition.

Description: The 2014 Mw 8.1 Iquique earthquake ruptured the boundary between the subducting Nazca Plate and the overriding South American Plate in the North Chilean subduction zone. The broken segment of the South American subduction zone had likely accumulated elastic strain since an M~9 earthquake in 1877 and what therefore considered a mature seismic gap. The moderate magnitude of the 2014 earthquake and its compact rupture area, which only broke the central part of the seismic gap, did not result in a significant tsunami in the Pacific Ocean. To investigate the seismo-tectonic segmentation of the North Chilean subduction zone in the region of the 2014 Iquique earthquake at the shallow seismic/aseismic transition, we combine two years of local aftershock seismicity observations from ocean bottom seismometers and long- offset seismic reflection data from the rupture area. Our study links short term deformation associated with a single seismic cycle to the permanent deformation history of an erosive convergent margin over millions of years. A high density of aftershocks following the 2014 Iquique earthquake occurred in the up-dip region of the coseismic rupture, where they form a trench parallel band. The events spread from the subducting oceanic plate across the plate boundary and into the overriding continental crust. The band of aftershock seismicity separates a pervasively fractured and likely fluid-filled marine forearc farther seaward from a less deformed section of the forearc farther landward. At the transition, active subduction erosion during the postseismic and possibly coseismic phases of the 2014 Iquique earthquake leads to basal abrasion of the upper plate and associated extensional faulting of the overlying marine forearc. Landward migration of the seismogenic up-dip limit, possibly at similar rates compared to the trench and the volcanic arc, leaves behind a heavily fractured and fluid-filled outermost forearc. This most seaward part of the subduction zone might be too weak to store sufficient elastic strain to nucleate a large megathrust earthquake.

Description: On 1 April 2014, the Mw 8.1 Iquique earthquake broke the plate-boundary along the North Chilean margin in the region between 19.5°S and 21°S. During this event, seismic rupture concentrated under the marine forearc with an updip limit at a plate-boundary depth of 17 km under the middle continental slope. In late 2016, wide-aperture seismic reflection and refraction data were acquired aboard the R/V Marcus G. Langsethoffshore Northern Chile as part of the “Pisagua/Iquique Crustal Tomography to Understand the Region of the Earthquake Source” (PICTURES) project. Utilizing multiple suppression techniques and ray-based tomographic inversion, we have achieved enhanced pre-stack depth migrated images to a depth of 40 km. Seismic lines MC23 and MC25, located in the southern part of the 2014 rupture area, display a pronounced plate boundary reflection that can be tracked to a depth of ~16 km. In contrast, on line MC04, located north of the 2014 rupture area, a plate boundary reflection is clearly visible to ~40 km depth. We consider that changes in fluid pressure cause the observed spatial variations in the downdip extent of the reflective plate boundary and thus may exert an influence on seismic rupture. However, the processes that control the spatial variations in fluid pressure over short distances remain enigmatic. Temperature controlled dehydration processes within the shallow subduction zone are expected to change only gradually along the margin and may therefore not explain short wavelength changes in the downdip extent of high reflectivity between line MC04 in the north and the other lines farther south. We notice, however, that the vertical displacement induced by bending related normal faults in the oceanic plate is significantly smaller along line MC04 compared to lines MC23 and MC25. This may lead to a delayed vertical flow of pore-fluids from the oceanic basement towards the plate boundary along line MC04. In contrast to lines MC23 and MC25, where fluids are expelled from the oceanic basement at relatively shallow depth along the plate boundary (i.e. under the outermost wedge), they are subducted to greater depths at the location of line MC04.

Description: A seismic-reflection survey on the Oregon continental margin conducted in 1989 indicates the widespread presence of gas hydrate beneath the middle and lower slope of this accretionary margin. The seismic signature of gas hydrate, a bottom simulating reflector (BSR) with negative polarity that locally cuts across stratigraphic horizons, is especially well developed beneath Hydrate Ridge. This anomalously shallow accretionary ridge was drilled during Ocean Drilling Program Leg 146 to study fluid venting. In this paper we focus on the seismic data from the southern part of Hydrate Ridge, where little evidence of active venting has previously been reported but where the seismic data indicate a complicated subsurface plumbing system. Apparent disruptions of the BSR beneath the western ridge flank suggest dissociation of gas hydrate in response to slumping. A double BSR beneath the southern crest suggests hydrate destabilization in response to tectonic uplift and folding. On the basis of these and other observations, we propose a qualitative model for the evolution of a hydrate-bearing ridge in an active accretionary complex in which gas hydrate initially stabilizes the sea floor, permitting construction of large ridges that are then eaten away by slumps along their margins. The north-to-south variation in sea-floor venting and subsurface seismic structure along Hydrate Ridge may reflect different stages in the temporal evolution of one of these ridges.