Data report: Seismic structure beneath the North Cascadia drilling transect of IODP Expedition 311

Between 1999 and 2004, new seismic data became available for the study of gas hydrates on the northern Cascadia margin. These data consist of multi- and single-channel data with two- and partly three-dimensional subsurface coverage and were acquired and used in support of the proposal for Integrated Ocean Drilling Program (IODP) Expedition 311 carried out in 2005. The working area lies across the continental slope off the coast of central Vancouver Island, British Columbia, Canada, with water depths ranging from 2600 m in the trench to 500 m on the upper slope, where it is well above the minimum depth for gas hydrate stability. This paper gives the details of the data acquisition and conventional processing and then focuses on describing the new data at six individual sites along a transect across the gas hydrate zone. Five of the sites were drilled during the Expedition 311. The transect of sites commences at the almost undeformed incoming sediments seaward of the region where gas hydrates are observed; these ocean basin sediments were drilled at a site 40 km southeast during Ocean Drilling Program (ODP) Leg 146. The transect continues up the continental slope into the area of hydrate stability, with a site on top of the frontal accretionary ridge where normal faulting indicates margin parallel extension; a site in the first slope basin overlying a buried ridge near a reflectivity wipe-out zone; a site adjacent to Site 889 of Leg 146 and therefore acting as a tie hole; the most landward site at the shallowest end of the hydrate stability field; and a cold vent site at one of several blank zones close to a bright spot region in the seismic records

Alteration of the subducting oceanic lithosphere at the southern central Chile trench-outer rise

Hydrothermal circulation and brittle faulting processes affecting the oceanic lithosphere are usually confined to the upper crust for oceanic lithosphere created at intermediate to fast spreading rates. Lower crust and mantle rocks are therefore relatively dry and undeformed. However, recent studies at subduction zones suggest that hydration of the oceanic plate is most vigorous at the trench–outer rise, where extensional bending-related faulting affects the hydrogeology of the oceanic crust and mantle. To understand the degree of hydration, we studied the seismic velocity structure of the incoming Nazca plate offshore of southern central Chile (∼43°S); here the deep-sea trench is heavily filled with up to 2 km of sediments. Seismic refraction and wide-angle data, complemented by seismic reflection imaging of sediments, are used to derive a two-dimensional velocity model using joint refraction and reflection traveltime tomography. The seismic profile runs perpendicular to the spreading ridge and trench axes. The velocity model derived from the tomography inversion consists of a ∼5.3-km-thick oceanic crust and shows P wave velocities typical for mature fast spreading crust in the seaward section of the profile, with uppermost mantle velocities as fast as ∼8.3 km/s. Approaching the Chile trench, seismic velocities are significantly reduced, however, suggesting that the structures of both the oceanic crust and uppermost mantle have been altered, possibly due to a certain degree of fracturing and hydration. The decrease of the velocities roughly starts at the outer rise, ∼120 km from the deformation front, and continues into the trench. Even though the trench is filled with sediment, basement outcrops in the outer rise frequently pierce the sedimentary blanket. Anomalously low heat flow values near outcropping basement highs indicate an efficient inflow of cold seawater into the oceanic crust. Hydration and crustal cracks activated by extensional bending-related faulting are suggested to govern the reduced velocities in the vicinity of the trench. Considering typical flow distances of 50 km, water might be redistributed over most of the trench–outer rise area. Where trapped in faults, seawater may migrate down to mantle depth, causing up to ∼9% of serpentinization in at least the uppermost ∼2 km of the mantle between the outer rise and the trench axis

Tidally controlled gas bubble emissions: A comprehensive study using long-term monitoring data from the NEPTUNE cabled observatory offshore Vancouver Island

Long-term monitoring over one year revealed high temporal variability of gas emissions at a cold seep in 1250 m water depth offshore Vancouver Island, British Columbia. Data from the North East Pacific Time series Underwater Networked Experiment observatory operated by Ocean Networks Canada were used. The site is equipped with a 260 kHz Imagenex sonar collecting hourly data, conductivity-temperature-depth sensors, bottom pressure recorders, current meter, and an ocean bottom seismograph. This enables correlation of the data and analyzing trigger mechanisms and regulating criteria of gas discharge activity. Three periods of gas emission activity were observed: (a) short activity phases of few hours lasting several months, (b) alternating activity and inactivity of up to several day-long phases each, and (c) a period of several weeks of permanent activity. These periods can neither be explained by oceanographic conditions nor initiated by earthquakes. However, we found a clear correlation of gas emission with bottom pressure changes controlled by tides. Gas bubbles start emanating during decreasing tidal pressure. Tidally induced pressure changes also influence the subbottom fluid system by shifting the methane solubility resulting in exsolution of gas during falling tides. These pressure changes affect the equilibrium of forces allowing free gas in sediments to emanate into the water column at decreased hydrostatic load. We propose a model for the fluid system at the seep, fueled by a constant sub-surface methane flux and a frequent tidally controlled discharge of gas bubbles into the ocean, transferable to other gas emission sites in the world's oceans

Three-dimensional lithospheric deformation and gravity anomalies associated with oblique continental collision in South Island, New Zealand

Isostatic considerations exhibit differences between the northern, central and southern parts of the Pacific–Australian plate collision in South Island, New Zealand. In the northern part mean elevations are moderate and the gravity low is small; the central part contains the highest elevations, and gravity and elevations correspond to each other relatively well; and in the southern part the gravity low is strongest whereas the mean elevations are moderate again. These differences indicate changes in the character of the isostatic compensation and are explained by increased thickening and widening of the crustal root from north to south, and also by the long wavelength gravity response to a mantle density anomaly that increases towards the south. A simple 3‐D gravity model is derived that includes the detailed crustal structures from the South Island GeopHysical Transect (SIGHT) experiment as well as a high‐density anomaly in the mantle inferred from teleseismic data. The model indicates that cold and, therefore, dense upper mantle material penetrates the asthenosphere to a greater extent in the south, similar to the behaviour of an apparently highly ductile lower crust. As plate reconstruction suggests more lithospheric shortening in the north, our model corresponds to lithospheric material escaping laterally to the south, almost perpendicular to the compression caused by lithospheric shortening of the mantle. Therefore, in addition to the prevailing mantle shear in New Zealand, there may also be a component of extrusional mantle creep beneath the Southern Alps orogen, which could have caused some of the observed large seismic anisotropy in this region. We may have also found evidence for submerged Eocene–Miocene oceanic lithosphere beneath the southeastern part of South Island that has been unaccounted for after plate reconstruction

Variability of Marine Methane Bubble Emissions on the Clayoquot Slope, Offshore Vancouver Island, Between 2017 and 2021

Seabed methane gas emissions occur worldwide at cold seeps located along most continental margins. Fluxes of methane gas released from the seabed in the form of bubbles can be extremely variable even over short time intervals. Some factors controlling the variability are still poorly understood. Here, we report on the results of continuous long-term sonar monitoring of bubble emissions at a depth of 1,260 m on the Clayoquot Slope, northern Cascadia margin. With a total monitoring duration of 4 years and a sampling period of 1 h, this is by far the longest high temporal resolution monitoring of seabed methane gas release ever conducted. Our results provide evidence that the diurnal and semi-diurnal tides influence the timing of the onset and cessation of bubble emissions. However, gas emissions within the monitoring area are active more than 84% of the time, indicating that tides alone are not sufficient to make venting pause. We hypothesize that the gas fluxes are transient but generally sufficiently high to maintain ebullition independently of the tidally-induced bottom pressure variations. Results also show that the tides do not seem to modulate the vigor of active gas emissions

A comparison between the transpressional plate boundaries of South Island, New Zealand, and Southern California, USA: the Alpine and San Andreas fault systems

There are clear similarities in structure and tectonics between the Alpine Fault system (AF) of New Zealand’s South Island and the San Andreas Fault system (SAF) of southern California, USA. Both systems are transpressional, with similar right slip and convergence rates, similar onset ages (for the current traces), and similar total offsets. There are also notable differences, including the dips of the faults and their plate-tectonic histories. The crustal structure surrounding the AF and SAF was investigated with active and passive seismic sources along transects known as South Island Geophysical Transect (SIGHT) and Los Angeles Region Seismic Experiment (LARSE), respectively. Along the SIGHT transects, the AF appears to dip moderately southeastward (~50 deg.), toward the Pacific plate (PAC), but along the LARSE transects, the SAF dips vertically to steeply northeastward toward the North American plate (NAM). Away from the LARSE transects, the dip of the SAF changes significantly. In both locations, a midcrustal decollement is observed that connects the plate-boundary fault to thrust faults farther south in the PAC. This decollement allows upper crust to escape collision laterally and vertically, but forces the lower crust to form crustal roots, reaching maximum depths of 44 km (South Island) and 36 km (southern California). In both locations, upper-mantle bodies of high P velocity are observed extending from near the Moho to more than 200-km depth. These bodies appear to be confined to the PAC and to represent oblique downwelling of PAC mantle lithosphere along the plate boundaries