Stuart Henrys

Characterizing the seismogenic zone of a major plate boundary subduction thrust: Hikurangi Margin, New Zealand

The Hikurangi subduction margin, New Zealand, has not experienced any significant (>Mw 7.2) subduction interface earthquakes since historical records began ∼170 years ago. Geological data in parts of the North Island provide evidence for possible prehistoric great subduction earthquakes. Determining the seismogenic potential of the subduction interface, and possible resulting tsunami, is critical for estimating seismic hazard in the North Island of New Zealand. Despite the lack of confirmed historical interface events, recent geodetic and seismological results reveal that a large area of the interface is interseismically coupled, along which stress could be released in great earthquakes. We review existing geophysical and geological data in order to characterize the seismogenic zone of the Hikurangi subduction interface. Deep interseismic coupling of the southern portion of the Hikurangi interface is well defined by interpretation of GPS velocities, the locations of slow slip events, and the hypocenters of moderate to large historical earthquakes. Interseismic coupling is shallower on the northern and central portion of the Hikurangi subduction thrust. The spatial extent of the likely seismogenic zone at the Hikurangi margin cannot be easily explained by one or two simple parameters. Instead, a complex interplay between upper and lower plate structure, subducting sediment, thermal effects, regional tectonic stress regime, and fluid pressures probably controls the extent of the subduction thrusts seismogenic zone.

Slow slip near the trench at the Hikurangi subduction zone, New Zealand

Applying pressure to plate tectonics The full range of deformation behavior of subduction zone faults that are responsible for great earthquakes and tsunamis is now clearer. Wallace et al. observed the heave of the ocean floor near the Hikurangi trench, off the east coast of New Zealand, with a network of absolute pressure gauges (see the Perspective by Tréhu). The gauges sit on the ocean floor and detect changes in pressure generated from slow-slip deformation events. Detailed geodetic observation of deformation events will finally clarify the role that such aseismic events play at major plate boundaries. Science, this issue p. 701; see also p. 654 Absolute pressure gauges detect a slow-slip event near the Hikurangi trench. The range of fault slip behaviors near the trench at subduction plate boundaries is critical to know, as this is where the world’s largest, most damaging tsunamis are generated. Our knowledge of these behaviors has remained largely incomplete, partially due to the challenging nature of crustal deformation measurements at offshore plate boundaries. Here we present detailed seafloor deformation observations made during an offshore slow-slip event (SSE) in September and October 2014, using a network of absolute pressure gauges deployed at the Hikurangi subduction margin offshore New Zealand. These data show the distribution of vertical seafloor deformation during the SSE and reveal direct evidence for SSEs occurring close to the trench (within 2 kilometers of the seafloor), where very low temperatures and pressures exist.

Numerical models of lithospheric deformation forming the Southern Alps of New Zealand

Compression of the entire continental lithosphere is considered using two-dimensional numerical models, in order to study the influence of the lithospheric mantle on the geometry of continental collision in its initial stages. The models are based on the central section of New Zealand Southern Alps, where continental collision has occurred along the Alpine Fault since about 7 Ma. They incorporate brittle-elastic-ductile rheology, heat transfer, surface processes, and fault localisation. The models are compared to the surface relief, the GPS convergence velocity, the measured electrical conductivity, and the geometry of the crustal root imaged from seismic velocity measurements. The crustal deformation is characterized by localized uplift at the plate boundary (Alpine Fault) and by two secondary zones of faulting. One is located about 60 to 80 km east of the Alpine Fault, at the start of upper crust bending (or tilting), and the other is located about 100 to 130 km east of the Alpine Fault as a result from shear strain propagating to the surface through the ductile lower crust. The observed asymmetric shape of the crustal root is best reproduced for a mantle lithosphere strength of the order of 200 MPa, and an intermediate rate of strain-softening. A lower strength of the mantle lithosphere can produce symmetric thickening, but provides an amplitude of the crustal root too small when compared to observations. The observed 20 km offset between the maximum in surface relief and the crustal root was not satisfactorily reproduced. This offset is most likely due to the three dimensionality of oblique collision in New Zealand Southern Alps.

Geophysical exploration and dynamics of the Alpine Fault Zone

The Alpine Fault of central South Island New Zealand, can be tracked with seismic reflection methods to depths of ∼35 km as a listric-shaped surface with strong reflectivity. Maximum dips of the surface are ∼60 degrees at 15 km depth and the dip then lessens with depth until the reflectivity is sub-horizontal at ∼35 km. Wide-angle seismic methods are used to show that the P-wave velocities of the rocks are up to 10% less than normal in the zone above the fault surface. In cross-section this low-velocity Alpine Fault Zone is elongate, sits above the fault surface, and has dimensions roughly 45 by 20 km. A magnetotelluric study shows a low-resistivity anomaly that is roughly coincident with the zone of low seismic velocity. A straightforward interpretation is that both the electrical and seismic anomalies are caused by interconnected fluids at lithostatic pressure. The inference of fluids in the lower crust is supported by an attribute analysis of seismic reflections on specific shot gathers where the Alpine Fault reflections can unequivocally be identified. We reference both the amplitude and phase of the fault-zone reflections to the distinctive side-swipe reflections generated at the far shore of Lake Pukaki. High reflection coefficients of ∼0.25 are estimated for the Alpine Fault reflections, which may require both anisotropy and fluid to explain. We interpret the source of water to be metamorphic dewatering of the schist-greywacke rocks that thicken into the orogen. A detachment surface along which the greywacke-schist rocks are obducted is recognised as a zone of strong reflectivity on an 80-km-long, unmigrated seismic reflection section. This zone of strong reflectivity, which apparently merges into the Alpine Fault reflections, does not correlate with depth to the Moho but rather with the boundary between the base of the schist-greywacke rocks (Vp ∼6-6.2 krn/s) and the lower crust (Vp ∼7-7.2 km/s). We interpret the strong reflectivity on this boundary as being due to a shear fabric. Both geological and geophysical observations imply deformation in the lower and mid-crust and mantle that appears to be caused by a combination of ductile and brittle behaviour, with no evidence of lithospheric flexure. We interpret the Alpine Fault Zone as a profoundly hot, wet, and weak region of continental crust.

Evolution of fluid expulsion and concentrated hydrate zones across the southern Hikurangi subduction margin, New Zealand: An analysis from depth migrated seismic data

Identification of methane sources controlling hydrate distribution and concentrations in continental margins remains a major challenge in gas hydrate research. Lack of deep fluid samples and high quality regional scale seismic reflection data may lead to underestimation of the significance of fluid escape from subducting and compacting sediments in the global inventory of methane reaching the hydrate zone, the water column and the atmosphere. The distribution of concentrated hydrate zones in relation to focused fluid flow across the southern Hikurangi subduction margin was investigated using high quality, long offset (10 km streamer), pre-stack depth migrated multichannel seismic data. Analysis of low P wave velocity zones, bright-reverse polarity reflections and dim-amplitude anomalies reveals pathways for gas escape and zones of gas accumulation. The study shows the structural and stratigraphic settings of three main areas of concentrated hydrates: (1) the Opouawe Bank, dominated by focused periodic fluid input along thrust faults sustaining dynamic hydrate concentrations and gas chimneys development; (2) the frontal anticline, with a basal set of protothrusts controlling permeability for fluids from deeply buried and subducted sediments sustaining hydrate concentrations at the crest of the anticline; and (3) the Hikurangi Channel, with buried sand dominated channels hosting significant amounts of gas beneath the base of the hydrate zone. In sand dominated channels gas injection into the hydrate zone favors highly concentrated hydrate accumulations. The evolution of fluid expulsion controlling hydrate formation offshore southern Hikurangi is described in stages during which different methane sources (in situ, buried and thermogenic) have been dominant.

Double-sided onshore–offshore seismic imaging of a plate boundary: “super-gathers” across South Island, New Zealand

Abstract Onshore–offshore seismic refraction profiling allows for the determination of crustal and mantle structures in the transition between continental and oceanic environments. Islands and narrow landmasses have the unique geometry of allowing for double-sided onshore–offshore experiments that favor the construction of composite “super-gathers” using the acquisition of onshore–offshore and ocean-bottom seismometer receiver gathers, land explosion shot gathers, and near-vertical incidence multichannel seismic (MCS) profiling. A number of sites at plate boundaries are amenable to the application of double-sided onshore–offshore imaging, including the Indo-Australian/Pacific transform boundary on South Island, New Zealand. By comparing the ratio of island width to mantle refraction (Pn) “maximum” crossover distance, using nondimensional distances, we provide an indicator of raypath “coverage” for crustal illumination. Islands or narrow land masses whose widths are less than twice their maximum crossover distance are candidates for double-sided onshore–offshore experiments. The SIGHT (South Island GeopHysical invesTigation) experiment in New Zealand is located where the width of South Island is sufficiently narrow with respect to its crustal thickness that a double-sided onshore–offshore experiment allows for complete crustal imaging of the associated plate boundary.

Three‐dimensional velocity structure of the northern Hikurangi margin, Raukumara, New Zealand: Implications for the growth of continental crust by subduction erosion and tectonic underplating

Traveltimes between shots from nine marine seismic reflection lines and nine onshore recorders were used to construct a 3-D P wave velocity model of the northern Hikurangi subduction margin, New Zealand. From north to south between Raukumara Basin and Raukumara Peninsula, the Moho of the overriding plate increases in depth from 17 to similar to 35 km. Low seismic P wave velocities of 3.5-5.0 km/s are localized within a similar to 10 km thick prism in the lower crust of the overriding plate immediately updip of the intersection between the subduction thrust and Moho and beneath the topographic crest of East Cape Ridge and the Raukumara Range. Southward, this region of low seismic velocities and surface uplift increases in distance from the trench as the thickness of the crust in the overriding plate increases. We interpret this low-velocity volume to be underplated sedimentary rocks and crustal materials that were tectonically eroded by subduction beneath the trench slope. The buoyancy and low strength of these subducted materials are proposed to assist the escape from a subduction channel near the base of the crust and drive local rock uplift. In the middle crust, our observations of very low velocity suggest high fluid-filled porosities of 12%-18%, and the implied buoyancy anomaly may enhance underplating. At greater depths the process is driven by the contrast between upper crustal quartz-feldspar mineralogy and the denser diabase or olivine-rich lithologies of the lower crust and mantle. We estimate a rate of lower crustal underplating at the northern Hikurangi margin of 20 +/- 7 km(3) Ma(-1) km(-1) since 22 Ma. We suggest that underplating provides an efficient means of accreting subducted sediment and tectonically eroded material to the lower crust and that the flux of forearc crustal rocks into the mantle at subduction zones may be systematically overestimated.

Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: implications for global growth and recycling of continents

We use seismic reflection and refraction data to determine crustal structure, to map a fore-arc basin containing 12 km of sediment, and to image the subduction thrust at 35 km depth. Seismic reflection megasequences within the basin are correlated with onshore geology: megasequence X, Late Cretaceous and Paleogene marine passive margin sediments; megasequence Y, a similar to 10,000 km(3) submarine landslide emplaced during subduction initiation at 22 Ma; and megasequence Z, a Neogene subduction margin megasequence. The Moho lies at 17 km beneath the basin center and at 35 km at the southern margin. Beneath the western basin margin, we interpret reflective units as deformed Gondwana fore-arc sediment that was thrust in Cretaceous time over oceanic crust 7 km thick. Raukumara Basin has normal faults at its western margin and is uplifted along its eastern and southern margins. Raukumara Basin represents a rigid fore-arc block > 150 km long, which contrasts with widespread faulting and large Neogene vertical axis rotations farther south. Taper of the western edge of allochthonous unit Y and westward thickening and downlap of immediately overlying strata suggest westward or northwestward paleoslope and emplacement direction rather than southwestward, as proposed for the correlative onshore allochthon. Spatial correlation between rock uplift of the eastern and southern basin margins with the intersection between Moho and subduction thrust leads us to suggest that crustal underplating is modulated by fore-arc crustal thickness. The trench slope has many small extensional faults and lacks coherent internal reflections, suggesting collapse of indurated rock, rather than accretion of > 1 km of sediment from the downgoing plate. The lack of volcanic intrusion east of the active arc, and stratigraphic evidence for the broadening of East Cape Ridge with time, suggests net fore-arc accretion since 22 Ma. We propose a cyclical fore-arc kinematic: rock moves down a subduction channel to near the base of the crust, where underplating drives rock uplift, oversteepens the trench slope, and causes collapse toward the trench and subduction channel. Cyclical rock particle paths led to persistent trench slope subsidence during net accretion. Existing global estimates of fore-arc loss are systematically too high because they assume vertical particle paths. Citation: Sutherland, R., et al. (2009), Reactivation of tectonics, crustal underplating, and uplift after 60 Myr of passive subsidence, Raukumara Basin, Hikurangi-Kermadec fore arc, New Zealand: Implications for global growth and recycling of continents, Tectonics, 28, TC5017, doi: 10.1029/2008TC002356.

Seismic images of gas conduits beneath vents and gas hydrates on Ritchie Ridge, Hikurangi margin, New Zealand

Abstract Recently acquired seismic reflection data across the southern edge of Ritchie Ridge, a prominent bathymetric high on the Hikurangi margin, display zones of high amplitudes and reflections that crosscut strata. We interpret the latter as bottom‐simulating reflections which are commonly associated with gas beneath gas hydrates. An analysis of reflection strength indicates the high‐amplitude zones are caused by free gas in the pore space of sediments, probably migrating upward along layers. One of the high‐amplitude regions is situated beneath the projected location of a known gas vent site. The seismic data appear to image the conduits that supply this vent site with gas. The seafloor in most of the study area is likely to be within the zone of gas hydrate stability, depending on bottom water temperatures and hydrate composition. Hence, gas appears to be venting through the gas hydrate stability zone, favouring locally high concentrations of gas hydrates.

Rift History of the Western Victoria Land Basin: A new Perspective Based on Integration of Cores with Seismic Reflection Data

The results of a stratigraphic study of the western Victoria Land Basin, Antarctica, are summarized. This analysis is based on all existing seismic reflection data integrated with lithological information from fully cored drillholes in the Cape Roberts area of western McMurdo Sound. A number of subsurface seismic reflectors were recognized in the Cape Roberts area and correlated to stratal interfaces previously recognized in the cores. These events were then traced regionally throughout the southern McMurdo Sound, and form the basis for a new seismic stratigraphic subdivision of the Cenozoic section. Key reflectors define boundaries of seismic stratigraphic units, each of which shows distinctive overall cross-sectional geometry and internal reflection character/facies. On this basis, we propose a new model for the evolution of the Victoria Land Basin, invoking five phases of tectonic activity and associated sediment accumulation patterns. Phase 1 (pre-latest Eocene) involved regional uplift and erosion of the Transantarctic Mountains to the immediate west of the basin. Phase 2 (latest Eocene to Early Oligocene) was an Early Rift stage characterized by sediment accumulation in laterally restricted grabens. Phase 3 (Early Oligocene to Early Miocene) was the Main Rift stage, in which sediment accumulation was no longer confined to grabens in the west of the basin, but rather formed an eastward-thickening wedge into the centre of the basin. Phase 4 (Early Miocene) was a consequence of passive thermal subsidence, producing a relatively even blanket of sediment across the entire basin. Phase 5 (post-Early Miocene) was associated with the “Terror Rift” and gave rise to a succession contain both young magmatic rocks and young faults and which thickens markedly into a central depocentre. The new framework allows recognition of thick, post-Early Miocene stratigraphic intervals as yet unsampled by stratigraphic drilling in McMurdo Sound.