We use a prestack depth migration reflection image and magnetic anomaly data across the northern Hikurangi subduction zone, New Zealand, to constrain plate boundary structure and geometry of a ...subducting seamount in a region of shallow slow slip and recent International Ocean Discovery Program drilling. Our 3‐D model reveals the subducting seamount as a SW‐NE striking, lozenge‐shaped ridge approximately 40 km long and 15 km wide, with relief up to 2.5 km. This seamount broadly correlates with a 20‐km‐wide gap separating two patches of large (>10 cm) slow slip and the locus of tectonic tremor associated with the September–October 2014 Gisborne slow slip event. Largest slow slip magnitudes occurred where the décollement is underlain by a 3.0‐km‐thick zone of highly reflective subducting sediments. Wave speeds within this zone are 7% lower than adjacent and overlying strata, supporting the view that high fluid pressures within subducting sediments may facilitate shallow slow slip along the north Hikurangi margin.
Plain Language Summary
Using a suite of geophysical data from the northern Hikurangi margin, New Zealand, we determine the location and geometry of a subducting seamount on the subducting Pacific Plate and establish its spatial relationship with slow slip and tremor that occurred on the plate boundary in September–October 2014. We infer that slow slip appears to occur preferentially where there are sediments with high fluid pressure in pore fluids subducting adjacent to the seamount but is reduced above the seamount itself. This observation has implications for understanding what physical conditions contribute to spatial variation in frictional properties of the plate interface that may control fault slip behavior on large, plate boundary subduction thrusts.
Key Points
Seismic images reveal Hikurangi margin accretionary wedge architecture and seismic velocity distribution
Magnetic anomaly modeling shows seismic tremor focused on the landward flanks and downdip of subducting seamounts
Structural heterogeneity of the plate interface may influence the distribution of slow slip and tremor
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
The imbricated frontal wedge of the central Hikurangi subduction margin is characteristic of wide (ca. 150 km), poorly drained and over pressured, low taper (∼
4°) thrust systems associated with a ...relatively smooth subducting plate, a thick trench sedimentary sequence (∼
3–4 km), weak basal décollement, and moderate convergence rate (∼
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 décollement, 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 décollement. 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.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
A new interpretation of active faulting in central Cook Strait, New Zealand, reveals tectonic structures associated with the spatial transition from subduction to continental transform faulting. ...Marine seismic reflection profiles and multibeam bathymetric data indicate that there are no throughgoing crustal faults connecting the North Island Dextral Fault Belt and the Marlborough Fault System in South Island. The major faults terminate offshore, associated with 5–20 km wide step‐overs and a change in regional fault strike. This structure implies that propagation of strike‐slip earthquake ruptures across the strait is not probable. Faulted sedimentary sequences in the Wairau Basin (Marlborough shelf), correlated to glacioeustatic sea level cycles, provide a stratigraphic framework for fault analysis. A high‐resolution study of the postglacial (<20 ka) vertical displacement history of the Cloudy and Vernon faults reveals up to six and five paleoearthquakes since 18 ka, respectively. These long‐timescale records indicate variable recurrence intervals and possibly variable stress drop, thus conforming to the variable slip model of earthquake behavior. Integration of these data with other submarine and terrestrial paleoearthquake records indicates the presence of clustered earthquake sequences involving multiple faults. Different sequences do not always involve the same faults. It appears that earthquake clustering is driven by fault interactions that lead to specific loading conditions favoring the triggering of earthquakes on major faults in relatively short time intervals. Present‐day regional Coulomb stress distribution has been calculated in two scenarios considered to represent minimum and maximum loading conditions. The models, incorporating secular tectonic loading and stress changes associated with major crustal earthquakes, indicate high stress loading in a large part of central Cook Strait. These conditions may favor the triggering of future damaging earthquakes in this region.
Morphological and seismic data from a submarine landslide complex east of New Zealand indicate flow‐like deformation within gas hydrate‐bearing sediment. This “creeping” deformation occurs ...immediately downslope of where the base of gas hydrate stability reaches the seafloor, suggesting involvement of gas hydrates. We present evidence that, contrary to conventional views, gas hydrates can directly destabilize the seafloor. Three mechanisms could explain how the shallow gas hydrate system could control these landslides. (1) Gas hydrate dissociation could result in excess pore pressure within the upper reaches of the landslide. (2) Overpressure below low‐permeability gas hydrate‐bearing sediments could cause hydrofracturing in the gas hydrate zone valving excess pore pressure into the landslide body. (3) Gas hydrate‐bearing sediment could exhibit time‐dependent plastic deformation enabling glacial‐style deformation. We favor the final hypothesis that the landslides are actually creeping seafloor glaciers. The viability of rheologically controlled deformation of a hydrate sediment mix is supported by recent laboratory observations of time‐dependent deformation behavior of gas hydrate‐bearing sands. The controlling hydrate is likely to be strongly dependent on formation controls and intersediment hydrate morphology. Our results constitute a paradigm shift for evaluating the effect of gas hydrates on seafloor strength which, given the widespread occurrence of gas hydrates in the submarine environment, may require a reevaluation of slope stability following future climate‐forced variation in bottom‐water temperature.
Key Points
Low‐velocity active landslides are proposed to occur on the seafloor
Gas hydrates provide a perturbation mechanism for ongoing landslide mobility
We propose an active, mixed hydrate‐sediment seafloor glacier
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Three depth‐converted and geologically interpreted seismic profiles provide a clear image of the offshore outer accretionary wedge associated with oblique subduction of the Pacific Plate beneath the ...central Hikurangi margin. Plio‐Quaternary turbidites deposited over the pelagic cover sequence of the Hikurangi Plateau have been accreted to the margin by imbrication along E‐verging thrust faults that propagated up‐section from the plate boundary décollement. Growth stratigraphy of piggy‐back basins and thrusting of progressively younger horizons trace the eastward advance of the leading thrust front over ∼60 km in the last 2 Myr. Moderate internal shortening of fault‐bounded blocks typically 4–8 km wide reflects rapid creation of thrust faults, with some early formed faults undergoing out‐of‐sequence reactivation to maintain critical wedge taper. Multistage structural restorations show that forward progression of shortening involves: (1) initial development of a ∼10–25 km wide “proto‐thrust” zone, comprising conjugate sets of moderately to steeply dipping low‐displacement (∼10–100 m) reverse faults; and (2) growth of thrust faults that exploit some of the early proto‐thrust faults and propagate up‐section with progressive break‐through of folds localized above the fault tips. The youngest, still unbreached folds deform the present‐day seafloor. Progressive retro‐deformations show that macroscopic thrust faults and folds account for less than 50% of the margin‐perpendicular shortening imposed by plate convergence. Arguably, significant fractions of the missing components can be attributed to mesoscopic and microscopic scale layer‐parallel shortening within the wedge, in the proto‐thrust zones, and in the outer décollement zone.
Key Points
Structural restorations reveal widening of the Hikurangi accretionary wedge by 60 km since 2 (± 0.8) Ma
The low‐taper wedge geometry is consistent with a weak and forward propagating megathrust décollement
Macroscopic thrusting and folding accommodate less than 50% of the total shortening
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
SUMMARY
The southcentral Hikurangi subduction margin (North Island, New Zealand) has a wide, low-taper accretionary wedge that is frontally accreting a >3-km-thick layer of sediments, with ...deformation currently focused near the toe of the wedge. We use a geological model based on a depth-converted seismic section, together with physically realistic parameters for fluid pressure, and sediment and décollement friction based on laboratory experiments, to investigate the present-day force balance in the wedge. Numerical models are used to establish the range of physical parameters compatible with the present-day wedge geometry and mechanics. Our analysis shows that the accretionary wedge stability and taper angle require either high to moderate fluid pressure on the plate interface, and/or weak frictional strength along the décollement. The décollement beneath the outer wedge requires a relatively weaker effective strength than beneath the inner (consolidated) wedge. Increasing density and cohesion with depth make it easier to attain a stable taper within the inner wedge, while anything that weakens the wedge—such as high fluid pressures and weak faults—make it harder. Our results allow a near-hydrostatic wedge fluid pressure, sublithostatic fluid overpressure at the subduction interface, and friction coefficients compatible with measurements from laboratory experiments on weak clay minerals.
Although the global flux of sediment and carbon from land to the coastal ocean is well known, the volume of material that reaches the deep ocean-the ultimate sink-and the mechanisms by which it is ...transferred are poorly documented. Using a globally unique data set of repeat seafloor measurements and samples, we show that the moment magnitude (
) 7.8 November 2016 Kaikōura earthquake (New Zealand) triggered widespread landslides in a submarine canyon, causing a powerful "canyon flushing" event and turbidity current that traveled >680 km along one of the world's longest deep-sea channels. These observations provide the first quantification of seafloor landscape change and large-scale sediment transport associated with an earthquake-triggered full canyon flushing event. The calculated interevent time of ~140 years indicates a canyon incision rate of 40 mm year
, substantially higher than that of most terrestrial rivers, while synchronously transferring large volumes of sediment 850 metric megatons (Mt) and organic carbon (7 Mt) to the deep ocean. These observations demonstrate that earthquake-triggered canyon flushing is a primary driver of submarine canyon development and material transfer from active continental margins to the deep ocean.
Direct, on‐fault submarine paleoearthquake records can be derived from high‐resolution seismic reflection profiles of active fault growth sequences. Coseismic vertical increments of displacement are ...best preserved in the architecture of the growth sequence when the long‐term rate of sedimentation exceeds the rate of fault vertical displacement. Postseismic stratigraphic intervals can be recognized on vertical displacement history curves, from which estimates of the earthquake timing and vertical displacements can be made. The strike‐slip Wairau Fault is a major seismic hazard in central New Zealand and is partially submarine. Analysis of 10 high‐resolution seismic profiles spanning a 20 km section of the offshore fault trace reveals two types of postseismic growth sequences and a composite paleoearthquake record with up to eight surface‐rupturing earthquakes since 18 ka. The recurrence intervals range from ∼0.9 to 3.8 ka (mean is ∼2.2 ka), while the coseismic vertical displacements range from 0.5 to 5.3 m (mean is ∼2.5 m). These data conform to the variable slip model of earthquake behavior. The vertical coseismic displacement is not always predictable from the recurrence interval preceding the event. The data indicate that the seismic moment release has not been constant between earthquakes or that the ratio of horizontal to vertical coseismic displacement at a given site has varied over multiple earthquake cycles. The offshore data are consistent with onshore paleoearthquake records, long‐term dextral slip rate of >2 mm/yr (compared to 3–5 mm/yr, 40 km in land), and paleoearthquakes of >Mw 7.5. The last earthquake occurred on the fault 2.0 ± 0.3 kyr ago, indicating that significant elastic strain has now accrued.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
Contractional fore‐arc faulting and deformation is a characteristic feature of many subduction systems. Definition of the three‐dimensional geometry and displacement rates of active, upper plate, ...out‐of‐sequence thrust faults along ∼250 km of the upper Hikurangi Margin enables us to examine the relationship between fore‐arc deformation and the subduction interface in light of interseismic coupling estimates and distribution of slow slip events, both modeled from GPS measurements. These mid‐fore‐arc structures include the seaward vergent, outer shelf Lachlan and Ariel faults, with vertical separation rates up to 5 mm/yr, and several other major inner shelf faults with rates that are up to 3.8 mm/yr and comparable with Holocene coastal uplift rates. Seismic reflection imaging and geometric projection of these faults at depth indicate that they splay from the region of the plate interface where geodetic inversions for interseismic coupling and slow slip events suggest that the plate boundary undergoes aseismic slip. This observation may indicate either (1) that frictional properties and interseismic coupling on the plate interface are independent and unrelated to the active splay fault deformation in the inner‐middle fore arc or (2) that the active splay faulting reflects long‐term mechanical coupling related to higher shear stress, or the relative yield strength of the plate interface to the overriding plate, and that the current pattern of interseismic coupling may not be persistent over geological time scales of 20 ka. We compare structure and processes on the northern Hikurangi and Costa Rican margins and find similarities and significant differences astride these subduction systems.
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FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SAZU, SBCE, SBMB, UL, UM, UPUK
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