The imbricated frontal wedge of the central Hikurangi subduction margin is characteristic of wide (ca. 150 km), poorly drained and over pressured, low taper (not, vert, similar 4°) thrust systems associated with a relatively smooth subducting plate, a thick trench sedimentary sequence (not, vert, similar 3–4 km), weak basal decollement, and moderate convergence rate (not, vert, similar 40 mm/yr). New seismic reflection and multibeam bathymetric data are used to interpret the regional tectonic structures, and to establish the geological framework for gas hydrates and fluid seeps. We discuss the stratigraphy of the subducting and accreting sequences, characterize stratigraphically the location of the interplate decollement, and describe the deformation of the upper plate thrust wedge together with its cover sequence of Miocene to Recent shelf and slope basin sediments. We identify approximately the contact between an inner foundation of deforming Late Cretaceous and Paleogene rocks, in which widespread out-of-sequence thrusting occurs, and a 65–70 km-wide outer wedge of late Cenozoic accreted turbidites. Although part of a seamount ridge is presently subducting beneath the deformation front at the widest part of the margin, the morphology of the accretionary wedge indicates that frontal accretion there has been largely uninhibited for at least 1–2 Myr. This differs from the offshore Hawkes Bay sector of the margin to the north where a substantial seamount with up to 3 km of relief has been subducted beneath the lower margin, resulting in uplift and complex deformation of the lower slope, and a narrow (10–20 km) active frontal wedge.

Five areas with multiple fluid seep sites, referred to informally as Wairarapa, Uruti Ridge, Omakere Ridge, Rock Garden, and Builders Pencil, typically lie in 700–1200 m water depth on the crests of thrust-faulted, anticlinal ridges along the mid-slope. Uruti Ridge sites also lie in close proximity to the eastern end of a major strike-slip fault. Rock Garden sites lie directly above a subducting seamount. Structural permeability is inferred to be important at all levels of the thrust system. There is a clear relationship between the seeps and major seaward-vergent thrust faults, near the outer edge of the deforming Cretaceous and Paleogene inner foundation rocks. This indicates that thrust faults are primary fluid conduits and that poor permeability of the Cretaceous and Paleogene inner foundation focuses fluid flow to its outer edge. The sources of fluids expelling at active seep sites along the middle slope may include the inner parts of the thrust wedge and subducting sediments below the decollement. Within anticlinal ridges beneath the active seep sites there is a conspicuous break in the bottom simulating reflector (BSR), and commonly a seismically-resolvable shallow fault network through which fluids and gas percolate to the seafloor. No active fluid venting has yet been recognized over the frontal accretionary wedge, but the presence of a widespread BSR, an extensive protothrust zone (> 200 km by 20 km) in the Hikurangi Trough, and two unconfirmed sites of possible previous fluid expulsion, suggest that the frontal wedge could be actively dewatering. There are presently no constraints on the relative fluid flux between the frontal wedge and the active mid-slope fluid seeps.
Article Outline

Tectonic and geological framework for gas hydrates and cold seeps on the Hikurangi subduction margin, New Zealand

Abstract High salinities and high temperatures at the seafloor record the upward flow of water and hydrocarbons from depth at natural vents in the deepwater Gulf of Mexico. We present a multiphase heat- and solute-transport model, in which water supplied from depth transports heat and salt, and hydrocarbon transports heat. We show that there is a unique water and hydrocarbon flux that simulates the observed salinity and temperature. We estimate the hydrocarbon flux to be 3.2 – 15 × 10 4 t yr − 1 and 1.8 – 8.0 × 10 4 t yr − 1 from two vents at lease blocks MC852/853 and GB425. These fluxes are 1–4 orders of magnitude greater than previous estimates from individual deepwater vents. If these results are extrapolated to the entire Gulf of Mexico, then we estimate the regional hydrocarbon flux to be at least 100× greater than previous estimates and 14–120% of the hydrocarbon flux from the Macondo oil spill. Large natural seepage may inoculate marine basins such as the Gulf of Mexico from oil spills like the 2010 Deepwater Horizon blowout by sustaining populations of hydrocarbon-degrading bacteria.

Hydrocarbon flux from natural deepwater Gulf of Mexico vents

[1] Geochemical profiles were coupled with seismic information to examine subsurface hydrocarbon source, migration, and fate at a Gulf of Mexico carbonate-gas hydrate mound (Woolsey Mound). Three seafloor features were investigated in detail: (1) major faults resulting from a rising salt body, (2) an acoustic backscatter anomaly, and (3) a pockmark associated with a major fault. We analyzed sulfate, chloride, dissolved inorganic carbon, and hydrocarbon concentrations, and carbon isotopes in pore water extracted from 20 m piston cores to characterize gas source and calculate methane flux. Dissolved biogenic methane dominated the off-fault sites, while the contribution of thermogenic methane increased near a major fault where thermogenic gas hydrates were recovered. Within the pockmark, methane concentrations were low and isotopes indicated a biogenic source. Since pockmarks are typically formed from expulsive fluid flow, this suggests that either the pockmark is the legacy of a conduit that has become plugged or that the expulsed fluid is confined within the fault walls. At the acoustic anomaly, nonsteady state sulfate profiles suggested temporal variability in methane flux. Estimates from >75 gravity cores collected across Woolsey Mound since 2002 were mapped to display the spatial variability in methane flux relative to the faults. Methane flux to the seafloor was generally low, but increased several fold near the faults suggesting that the faults may provide conduits for hydrocarbons to bypass the “microbial biofilter” and cross the sediment water interface.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2013GC004888

Subsurface methane sources and migration pathways within a gas hydrate mound system, Gulf of Mexico

Hydrocarbon migration pathway and methane budget for a Gulf of Mexico natural seep site: Green Canyon 600

Marine seepage variability and its impact on evaluating the surface migrated hydrocarbon seep signal

Green Canyon Lease Block 600 is a well-documented cold seep site characterized by near-perennial oil and gas slicks large enough to reach the sea surface and to be detected on satellite imagery. This site has been selected by the Ecosystem Impact of Oil and Gas Inputs to the Gulf consortium to study the impacts of natural hydrocarbon seepage versus anthropogenic inputs of hydrocarbons into the Gulf of Mexico ecosystem following the Deepwater Horizon oil spill disaster that occurred in 2010. Here we present the first high-resolution characterization of the site. We investigated the seafloor and shallow subsurface combining autonomous underwater vehicle-borne seismo-acoustics with photo data. Our records show vigorous hydrocarbon flux occurring over a wide area of the complex seafloor morphology. Several mounds, which are the most prominent features of the site, are aligned along a ridge structure and are actively venting hydrocarbons. Detailed bathymetry of the mound’s surface highlights the presence of numerous pockmarks, cones, and micro-mounds of variable sizes and shapes. High reflectivity seafloor backscatter characterizes the entire ridge surface; ground-truthing photo surveys confirm the presence of authigenic carbonate hardgrounds, gas hydrates, and chemosynthetic communities. The shallow stratigraphy indicates that the ridge represents the seafloor expression of normal faults, probably rooted to a deep salt body. Salt tectonics generated a complex network of normal faults, many of which displace the upper part of the Holocene sediments up to the seafloor, forming long linear scars. Acoustic anomalies, interpreted as active migration pathways, underlie mounds and venting structures, suggesting a link between the deep hydrocarbon reservoir and seafloor expulsion features.

High-resolution seismo-acoustic characterization of Green Canyon 600, a perennial hydrocarbon seep in Gulf of Mexico deep water

Improving 3D water column seismic imaging using the Common Reflection Surface method

Discovery and Characterization of Cold Seep Vents Using a Mass Spectrometer Operating Aboard an Autonomous Underwater Vehicle

. Seismic oceanography is a new cross-discipline between geophysics and oceanography that uses seismic reflection data to image and study the oceanic water column. Previous work on seismic oceanography was largely limited to two-dimensional (2D) seismic data and methods. Here we explore and quantify temporal and spatial variations in three-dimensional (3D) seismic oceanography to address whether 3D seismic imaging is meaningful in all directions and how one can take advantage of the variations. From a 3D multichannel seismic survey acquired for oil and gas exploration in the Gulf of Mexico over a 6-month period, a 3D oceanic seismic volume was derived. The 3D seismic images exhibit both temporal and spatial variations of the ocean, and theoretical and data analyses were used to quantify their contribution. Our results suggest that temporal variation is more prominent in the crossline direction than in the inline direction, causing discontinuities in crossline images. However, a series of 3D inline images can be seen as snapshots of the water column at different times, capturing temporal variation of thermohaline structures induced by ocean dynamics. Our findings suggest the potential uses of marine 3D seismic data in studying time-evolving mesoscale ocean dynamics.

/pdf/temporal-and-spatial-variations-in-three-dimensional-seismic-4zzngi9hoc.pdf

Leonardo Macelloni

Papers

Hydrocarbon migration pathway and methane budget for a Gulf of Mexico natural seep site: Green Canyon 600

High-resolution seismo-acoustic characterization of Green Canyon 600, a perennial hydrocarbon seep in Gulf of Mexico deep water

Improving 3D water column seismic imaging using the Common Reflection Surface method

Discovery and Characterization of Cold Seep Vents Using a Mass Spectrometer Operating Aboard an Autonomous Underwater Vehicle

Temporal and spatial variations in three-dimensional seismic oceanography