The Hikurangi Margin east of New Zealand's North Island hosts an extensive gas hydrate province with numerous gas hydrate accumulations related to the faulted structure of the accretionary wedge. One ...such hydrate feature occurs in a small perched upper‐slope basin known as Urutī Basin. We investigated this hydrate accumulation by combining a long‐offset seismic line (10‐km‐long receiver array) with a grid of high‐resolution seismic lines acquired with a 600‐m‐long hydrophone streamer. The long‐offset data enable quantitative velocity analysis, while the high‐resolution data constrain the three‐dimensional geometry of the hydrate accumulation. The sediments in Urutī Basin dip landward due to ongoing deformation of the accretionary wedge. These strata are clearly imaged in seismic data where they cross a distinct bottom simulating reflection (BSR) that dips counterintuitively in the opposite direction to the regional dip of the seafloor. BSR‐derived heat flow estimates reveal a distinct heat flow anomaly that coincides spatially with the upper extent of a landward‐verging thrust fault. We present a conceptual model of this gas hydrate system that highlights the roles of fault‐controlled fluid flow at depth merging into strata‐controlled fluid flow into the hydrate stability zone. The result is a layer‐constrained accumulation of concentrated gas hydrate in the dipping strata. Our study provides new insight into the interplay between deep faulting, fluid flow and gas hydrate formation within an active accretionary margin.
Plain Language Summary
Gas hydrates are ice‐like substances in which natural gas molecules are trapped in a cage of water molecules. They exist where the pressure is high, temperature is cold, and enough methane is present. These conditions exist in the marine environment at water depths greater than 300–500 m near sediment‐rich continental margins and in polar regions. It is important to study gas hydrates because they represent a significant part of the Earth's carbon budget and influence the flow of methane into the oceans and atmosphere. In this study, we use the seismic reflection method to generate images of gas‐hydrate‐bearing marine sediments east of New Zealand. Our data reveal an intriguing relationship between deep‐sourced fluid flow upward along a tectonic fault, and shallower flow through dipping sediments. This complex fluid flow pattern has led to disruption of the gas hydrate system and the formation of concentrated gas hydrate deposits within the dipping sediments. Our study highlights the relationships between relatively deep tectonic processes (faulting and fluid flow) and the shallow process of gas hydrate formation in an active subduction zone.
Key Points
A distinct gas‐hydrate to free‐gas transition is mapped using high‐ and low‐frequency seismic data
Gas and hydrate accumulations in the Urutī Basin are controlled by the structural setting, ongoing deep‐sourced fluid flow, and near surface stratigraphy
Regions of high modeled heat flow can be directly related to accumulations of gas and gas hydrates
Bottom simulating reflections (BSRs) interpreted in seismic data are one of the most used indicators for the presence of gas hydrates. In numerous hydrate provinces, including the Hikurangi margin, ...east of New Zealand, distinct and anomalous gaps in reflectivity punctuate otherwise continuous BSRs. We undertake a seismic stratigraphic and structural interpretation of a dense grid of two‐dimensional reflection seismic data to investigate possible causes of widespread BSR gaps that occur near the hinge area of synclines. We explain these BSR gaps with a tectono‐sedimentary model where sedimentation into accretionary wedge synclines leads to an upward migration of the base of gas hydrate stability with respect to stratigraphy, causing dissociation of gas hydrates to water and free gas. A trough‐shaped syncline's radially dipping beds promotes along‐strata, upward migration of gas away from the hinge area, incrementally depleting synclinal hinges of gas and resulting in a BSR gap or muted BSR. This model may be applicable to observed gaps in BSRs in tectonically active slope basins worldwide.
Reflectivity gaps in across hydrate related bottom simulating reflections induce uncertainty around large‐scale estimations of gas hydrate occurrence. Using two‐dimensional seismic data from the southern Hikurangi subduction margin, we present a new model that explains the cause of BSR gaps within synclinal structures. The new model may have wide appliciability in hydrate provinces with similar tectonic histories
We present recently-acquired high-resolution seismic data and older lower-resolution seismic data from Rock Garden, a shallow marine gas hydrate province on New Zealand's Hikurangi Margin. The ...seismic data reveal plumbing systems that supply gas to three general sites where seeps have been observed on the Rock Garden seafloor: the ‘LM3’ sites (including LM3 and LM3-A), the ‘Weka’ sites (including Weka-A, Weka-B, and Weka-C), and the ‘Faure’ sites (including Faure-A, Faure-B, and Rock Garden Knoll). At the LM3 sites, seismic data reveal gas migration from beneath the bottom simulating reflection (BSR), through the gas hydrate stability zone (GHSZ), to two separate seafloor seeps (LM3 and LM3-A). Gas migration through the deeper parts of GHSZ below the LM3 seeps appears to be influenced by faulting in the hanging wall of a major thrust fault. Closer to the seafloor, the dominant migration pathways appear to occupy vertical chimneys. At the Weka sites, on the central part of the ridge, seismic data reveal a very shallow BSR. A distinct convergence of the BSR with the seafloor is observed at the exit point of one of the Weka seep locations (Weka-A). Gas supply to this seep is predicted to be focused along the underside of a permeability contrast at the BGHS caused by overlying gas hydrates. The Faure sites are associated with a prominent arcuate slump feature. At Faure-A, high-amplitude reflections, extending from a shallow BSR towards the seafloor, are interpreted as preferred gas migration pathways that exploit relatively-high-permeability sedimentary layers. At Faure-B, we interpret gas migration to be channelled to the seep along the underside of the BGHS — the same scenario interpreted for the Weka-A site. At Rock Garden Knoll, gas occupies shallow sediments within the GHSZ, and is interpreted to migrate up-dip along relatively high-permeability layers to the area of seafloor seepage. We predict that faulting, in response to uplift and flexural extension of the ridge, may be an important mechanism in creating fluid flow conduits that link the reservoir of free gas beneath the BGHS with the shallow accumulations of gas imaged beneath Rock Garden Knoll. From a more regional perspective, much of the gas beneath Rock Garden is focused along a northwest-dipping fabric, probably associated with subduction-related deformation of the margin.
The Pāpaku Fault Zone, drilled at International Ocean Discovery Program (IODP) Site U1518, is an active splay fault in the frontal accretionary wedge of the Hikurangi Margin. In ...logging‐while‐drilling data, the 33‐m‐thick fault zone exhibits mixed modes of deformation associated with a trend of downward decreasing density, P‐wave velocity, and resistivity. Methane hydrate is observed from ~30 to 585 m below seafloor (mbsf), including within and surrounding the fault zone. Hydrate accumulations are vertically discontinuous and occur throughout the entire logged section at low to moderate saturation in silty and sandy centimeter‐thick layers. We argue that the hydrate distribution implies that the methane is not sourced from fluid flow along the fault but instead by local diffusion. This, combined with geophysical observations and geochemical measurements from Site U1518, suggests that the fault is not a focused migration pathway for deeply sourced fluids and that the near‐seafloor Pāpaku Fault Zone has little to no active fluid flow.
Plain Language Summary
Faults are boundaries in the Earth where two different blocks of sediment or rock slide past each other. Offshore New Zealand, the Pāpaku Fault is very shallow and intersects the seafloor but connects to deeper faults kilometers below the seafloor where large earthquakes can occur. An ice‐like form of methane called hydrate also occurs within and surrounding the fault. We use scientific drilling data to understand the physical properties of the fault. Hydrate can affect fault properties and how fluid flows; however, based on the pattern of hydrate distribution and other geochemical and geophysical measurements, we suggest that the Pāpaku Fault does not have active fluid flow.
Key Points
The Pāpaku Fault Zone is a 33‐m‐thick near‐seafloor splay fault drilled at Site U1518 on the Hikurangi Margin
Multiple lines of observational, geophysical, and geochemical evidence suggest that there is little to no fluid flow along the Pāpaku Fault
Slow slip events (SSEs) accommodate a significant proportion of tectonic plate motion at subduction zones, yet little is known about the faults that actually host them. The shallow depth (<2 km) of ...well-documented SSEs at the Hikurangi subduction zone offshore New Zealand offers a unique opportunity to link geophysical imaging of the subduction zone with direct access to incoming material that represents the megathrust fault rocks hosting slow slip. Two recent International Ocean Discovery Program Expeditions sampled this incoming material before it is entrained immediately down-dip along the shallow plate interface. Drilling results, tied to regional seismic reflection images, reveal heterogeneous lithologies with highly variable physical properties entering the SSE source region. These observations suggest that SSEs and associated slow earthquake phenomena are promoted by lithological, mechanical, and frictional heterogeneity within the fault zone, enhanced by geometric complexity associated with subduction of rough crust.
Seafloor pockmarks are abundant around Aotearoa New Zealand, occurring across a diverse range of tectonic, sedimentological and geomorphological settings. Globally, the formation and source of ...pockmarks is widely researched because they: 1) have potential links to subsurface hydrocarbon systems, 2) can provide important habitats for benthic organisms and 3) may be indications of fluid escape pathways or areas of sediment disturbance, which influence seafloor stability and could pose a risk to infrastructure. Pockmarks are widely associated with fluid release (such as gas or water) from subsurface reservoirs. However, the formation of pockmarks, the processes that shape and modify their morphology over time, and the relative timing of these events, remains enigmatic. Here, we compile the first national database of over 30,000 pockmarks around Aotearoa New Zealand, allowing us to begin to comprehend the dynamic processes that shape and affect pockmarks by exploring regional and inter-regional patterns in pockmark geometry and seabed characteristics. This compilation reveals several significant trends, including a distinct lack of correlation between active seafloor seeps and pockmarks, and a strong association of pockmarks with mud-rich seafloor substrate. Furthermore, we highlight key knowledge gaps that require further investigation moving forward, including a lack of constraint on the timing of pockmark formation, and limited modelling of the processes involved in their formation.
Seafloor depressions are widespread on the present-day continental slope along the southeast coast of New Zealand's South Island. The depressions appear to be bathymetrically constrained to depths ...below 500m, correlating to the top of the gas hydrate stability zone, and above 1100m. Similar depressions observed on the Chatham Rise are interpreted to have formed as a result of gas hydrate dissociation, leading to the hypothesis that a similar origin can be applied for the depressions investigated in this study. Our investigation, however, has found limited geophysical or geochemical evidence to support this hypothesis.
The objective of this paper is to examine whether a causal relationship can be established between potential mechanisms of depression formation and the present-day seafloor geomorphology. Geostatistical analysis methods applied to multibeam bathymetry and interpretation of 3D seismic data have been used to empirically describe the geomorphology of the seafloor depressions and investigate potential correlations between geomorphology and other processes such as current flow along the shelf and slope in this region and underlying polygonal fault systems.
Although the results of our analysis do not preclude that the seafloor depressions formed as a result of gas hydrate dissociation, neither does our geophysical or geochemical evidence support the theory. Therefore, we propose an alternative mechanism that may have been responsible for the formation of these structures. Based on the evidence presented in this study, the most likely mechanism responsible for the formation of these seafloor depressions is groundwater flux related to the interaction of current systems and the complex geomorphology of submarine canyons on the southeast coast of the South Island.
•Geostatistical analysis methods applied to multibeam bathymetry and seismic data•Geomorphology of seafloor depressions has been quantitatively characterised.•No direct correlation between gas venting and formation of seafloor depressions•Likely mechanism of depression formation: groundwater flux linked to current flow
The quality of reservoir rocks, in particular their permeability, is likely to be a key factor for the economic viability of future gas production from gas hydrates. As for conventional gas ...resources, high-permeability sands are considered the economically most promising gas hydrate reservoirs. Studies of subsurface lithology however, are difficult without calibration from boreholes. We investigated seismic data from the Hikurangi Margin, a subduction zone east of New Zealand and New Zealand's largest gas hydrate province. We suggest that the strength of bottom simulating reflections (BSRs) from the base of the gas hydrate stability zone may support lithologic interpretations on this margin.
BSRs along large parts of this margin are exceptionally weak. Absolute reflection coefficients of a weak BSR on Puke Ridge, a thrust ridge in the accretionary wedge, are roughly between 0.01 and 0.02, an order of magnitude lower than those observed for many BSRs globally. A combination of rock physics modelling and seismic amplitude-versus-offset analysis leads to the conclusion that these weak BSRs are primarily caused by low saturation of gas with patchy distribution, i.e., gas that is only present in pores or fractures of some mesoscopic (i.e., larger than pore sizes but smaller than seismic wavelengths) sediment patches while other patches are fully water saturated. This type of distribution, combined with observed high seismic velocities, is compatible with lithified fine-grained reservoir rocks, similar to indurated mudstones dredged from a submarine outcrop close to the study area. We therefore suggest that weak BSRs may mark fine-grained reservoir rocks with usually low primary permeability. Even though these reservoir rocks may exhibit enhanced secondary permeability from fracturing, they would currently not be considered prime candidates for potential gas production from hydrates.
We identified several high amplitude bright spots along weak BSRs. Two possible lithologic explanation for this reflection pattern are that (1) the bright spots mark higher saturation of gas in high-permeability, probably sand-dominated layers, as found in the Gulf of Mexico, and (2) evenly distributed networks of pores may result in gas to be distributed more homogenously in the sediments even though permeability may still be relatively low, as suggested for highly reflective layers beneath the Blake Ridge. Elevated gas hydrate saturations above layers with high saturations of gas would be expected to lead to highly reflective layers in the gas hydrate stability zone and thus, reflection patterns above the BSR may allow distinguishing between both causes.
► Strength of bottom simulating reflections (BSRs) may be lithology indicator. ► Weak BSRs in study area likely to mark fine-grained, low-permeability sediments. ► Two models of gas distribution proposed for bright spots along weak BSRs.
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