Subduction Margin Research Articles

Jurassic failed rift system below the Filchner-Ronne-Shelf, Antarctica: New evidence from geophysical data

During the austral summer of 1994/95, reasonable ice conditions in the Weddell Sea allowed the acquisition of new high quality seismic refraction data parallel to the Filchner-Ronne Ice Shelf (FRS), Antarctica. Although pack ice conditions resulted in some data gaps, the final velocity-depth/2D-density models cover the entire FRS in E-W direction using all available deep seismic data/picks from this remote area. The velocity-depth model shows a sedimentary basin with a thickness up to 12km and a large velocity inversion in the lowermost sedimentary unit. The crustal thickness reaches a maximum of 40km along the basin's margins in the Antarctic Peninsula and East Antarctica. In the central shelf area, numerous interfering seismic phases occur from the crust-mantle boundary at decreasing distances indicating a thinning of the crust. Here, the modelled velocities and densities reveal a thickness of 20km for the igneous crust. This corridor of overthickened oceanic or close to oceanic crust is 160km wide. The corridor is characterized by weak, but in general continuous magnetic anomalies, which we interpret as isochrons developed during the rifting or the initial formation of oceanic crust. If the crustal composition represents an old stripe of oceanic crust, a minimum estimate for the early formation of the oceanic crust is 145/148Ma (Late Jurassic). However, based on the velocity of rift propagation during the initial opening of the adjacent Weddell Sea the oceanic crust is likely to have formed around 160Ma. The onset of rifting and development of a thick igneous crust can be related to stresses developed between the interior and the southwestern paleo-Pacific subduction margin of the fragmenting Gondwana supercontinent in combination with additional melt supply from a deeper mantle source that arrived and spread in the period 183–155Ma.

Tectonic role of margin-parallel and margin-transverse faults during oblique subduction in the Southern Volcanic Zone of the Andes: Insights from Boundary Element Modeling

Obliquely convergent subduction margins develop trench-parallel faults shaping the regional architecture of orogenic belts and partitioning intra-plate deformation. However, transverse faults also are common along most orogenic belts and have been largely neglected in slip partitioning analysis. Here, we constrain the sense of slip and slip rates of differently oriented faults to assess whether and how transverse faults accommodate plate-margin slip arising from oblique subduction. We implement a forward 3D boundary element method (BEM) model of subduction at the Chilean margin evaluating the elastic response of intra-arc faults during different stages of the Andean subduction seismic cycle (SSC). Our model results show that the margin-parallel, NNE-striking Liquine-Ofqui Fault System (LOFS) accommodates dextral-reverse slip during the interseismic period of the SSC, with oblique slip rates ranging between 1-7 mm/yr. NW-striking faults exhibit sinistral-reverse slip during the interseismic phase of the SSC, displaying a maximum oblique slip of 1.4 mm/yr. ENE-striking faults display dextral strike-slip, with a slip rate of 0.85 mm/yr. During the SSC coseismic phase, all modeled faults switch their kinematics: NE-striking fault become sinistral whereas NW-striking faults are normal-dextral. Because coseismic tensile stress changes on NW faults reach 0.6 MPa at 10-15 km depth, it is likely that they can serve as transient magma pathways during this phase of the SSC. Our model challenges the existing paradigm wherein only margin-parallel faults account for slip partitioning: transverse faults are also capable of accommodating a significant amount of plate-boundary slip arising from oblique convergence.

Origin of the late Early Cretaceous granodiorite and associated dioritic dikes in the Hongqilafu pluton, northwestern Tibetan Plateau: A case for crust–mantle interaction

We present a detailed study of geochronology, mineral chemistries, bulk-rock major and trace element abundances, and Sr–Nd–Hf isotope compositions of the granodiorite and associated dioritic dikes in the Hongqilafu pluton at the northwestern margin of the Tibetan Plateau. The granodiorite and dioritic dikes yielded zircon U–Pb ages of ~104Ma and ~100Ma, respectively. The dioritic dikes comprise varying lithologies of gabbroic diorite, diorite porphyry and granodiorite porphyry, exhibiting a compositional spectrum from intermediate to felsic rocks. Their mineral compositions display disequilibrium features such as large major element compositional variations of plagioclase, clinopyroxene and amphibole crystals. These dioritic dikes are enriched in incompatible elements (Ba, Rb, Th, U, K) and Sr–Nd–Hf isotopes (87Sr/86Sri: 0.7066 to 0.7071, εNd(t): −5.3 to −7.4, εHf(t): −3.6 to −6.2). We suggest that the dioritic dikes were most likely derived from partial melting of mantle wedge metasomatized by the subducted/subducting seafloor with a sediment component, followed by AFC processes with fractional crystallization of clinopyroxene, amphibole and plagioclase and assimilation of lower continental crust. The mantle-wedge derived magma parental to the dioritic dikes underplated and induced the lower continental crust to melt, forming the felsic crustal magma parental to the granodiorite with mantle melt signatures and having more enriched isotope compositions (87Sr/86Sri: 0.7087 to 0.7125, εNd(t): −9.5 to −11.6, εHf(t): −10.3 to −14.1) than those of the dioritic dikes. The Hongqilafu pluton is thus the product of mantle–crust interaction at an active continental margin subduction setting over the period of several million years. This understanding further indicates that the closure timing of the Shyok back-arc basin and the collision between the Kohistan–Ladakh Arc and the Karakoram Terrane may have taken place later than ~100Ma.

Origin and dynamics of depositionary subduction margins

AbstractHere we propose a new framework for forearc evolution that focuses on the potential feedbacks between subduction tectonics, sedimentation, and geomorphology that take place during an extreme event of subduction erosion. These feedbacks can lead to the creation of a “depositionary forearc,” a forearc structure that extends the traditional division of forearcs into accretionary or erosive subduction margins by demonstrating a mode of rapid basin accretion during an erosive event at a subduction margin. A depositionary mode of forearc evolution occurs when terrigenous sediments are deposited directly on the forearc while it is being removed from below by subduction erosion. In the most extreme case, an entire forearc can be removed by a single subduction erosion event followed by depositionary replacement without involving transfer of sediments from the incoming plate. We need to further recognize that subduction forearcs are often shaped by interactions between slow, long‐term processes, and sudden extreme events reflecting the sudden influences of large‐scale morphological variations in the incoming plate. Both types of processes contribute to the large‐scale architecture of the forearc, with extreme events associated with a replacive depositionary mode that rapidly creates sections of a typical forearc margin. The persistent upward diversion of the megathrust is likely to affect its geometry, frictional nature, and hydrogeology. Therefore, the stresses along the fault and individual earthquake rupture characteristics are also expected to be more variable in these erosive systems than in systems with long‐lived megathrust surfaces.

A recent phase of accretion along the southern Costa Rican subduction zone

Abstract In 2011 we acquired a 3D seismic reflection volume across the Costa Rica margin NW of the Osa Peninsula to investigate the complex structure and the development of the seismogenic zone within the Costa Rican subduction zone in the vicinity of recent International Ocean Drilling Program (IODP) drilling. In contrast to previous interpretations, these newly acquired seismic images show that the margin wedge is composed of a layered fabric that is consistent with clastic sediments, similar to materials recovered from IODP drilling, that have been thrust and thickened into thrust-bounded folded sequences. These structures are consistent with a balanced sequence that has been frontally accreted in the context of an accretionary model. We interpret these sequences as sediment originally deposited on the subducting crust in a trench basin created by the southward migration of the Cocos–Nazca–Caribbean triple junction, and accreted during recent margin subduction that also accelerated with passage of the triple junction. The margin is composed of relatively rapidly accreted sediment that was added to the margin during a phase of accretion within the last ∼5 Ma that was probably preceded throughout the Neogene by periods of non-accretion or erosion.

Devonian magmatism in the accretionary complex of southern Chile

Supposed or potential Devonian igneous rocks in the accretionary complex of southern Chile were investigated using sensitive high-resolution ion microprobe U–Pb dating of zircon, with Hf- and O-isotope analyses of selected grains. Ages of 384 ± 3 and 382 ± 2 Ma are confirmed for two igneous bodies (another having been previously dated at 397 ± 1 Ma). Detrital zircon ages in the host rocks, some associated with Devonian marine fossils, indicate maximum possible sedimentation ages of c . 330 – 385 Ma. Devonian ages of 391 ± 10 and 374 ± 3 Ma for plutonic rocks at the western edge of the North Patagonian Massif are somewhat older than those of orthogneisses in the western flank of the Andes near Chaitén (361 ± 7 and 364 ± 2 Ma). O and Hf isotopes indicate that the Devonian intrusions in the accretionary complex crystallized from mantle-derived magmas, whereas those in the North Patagonian Massif show a strong crustal influence, corresponding to oceanic and continental margin subduction environments of magma genesis, respectively. Devonian zircon provenance in the accretionary complex was from the North Patagonian Massif and not from the mantle-derived intrusions, suggesting that the accretionary complex formed an integral part of the Gondwana margin during Devonian–Carboniferous times. Supplementary material: Description of analytical methods and tables of isotope analytical data are available at http://doi.org/10.6084/m9.figshare.c.2336728 .

Unusual stress rotations within the Philippines possibly caused by slip heterogeneity along the Philippine fault

AbstractTo understand the complex tectonic interactions between subduction margins and transcurrent faults, we investigated the spatial distribution of stress orientations in the Philippines by using focal mechanisms derived from the waveform data of regional broadband seismic stations and Global Centroid Moment Tensor solutions. We investigated the spatial distribution of stress orientations by dividing the region into a central region containing the Philippine fault (PF) and regions to the east and west of the PF. Our stress tensor inversion results show that the σ1 axes in the central and eastern regions are WNW‐ESE oriented and parallel to the relative plate motion orientations. However, in the western region, the orientations of σ1 axes were different. In particular, the orientations of the σ1 axes in the southern part (Bohol region) differ substantially (differences exceed approximately 60°) from those of the relative plate motion. Furthermore, the orientations of the σ1 axes in the northern part (Mindoro and southwestern Luzon regions) differ by approximately 30° from those found throughout the Philippines. The σ1 axes in the Bohol region and the σ3 axes in the Mindoro and southwestern Luzon regions are parallel to the strike of the PF. It was demonstrated that our numerical model incorporating slip on the PF and interplate coupling is able to successfully reproduce the features of the observed stress pattern.

Gas migration through Opouawe Bank at the Hikurangi margin offshore New Zealand

This study presents 2D seismic reflection data, seismic velocity analysis, as well as geochemical and isotopic porewater compositions from Opouawe Bank on New Zealand’s Hikurangi subduction margin, providing evidence for essentially pure methane gas seepage. The combination of geochemical information and seismic reflection images is an effective way to investigate the nature of gas migration beneath the seafloor, and to distinguish between water advection and gas ascent. The maximum source depth of the methane that migrates to the seep sites on Opouawe Bank is 1,500–2,100 m below seafloor, generated by low-temperature degradation of organic matter via microbial CO2 reduction. Seismic velocity analysis enabled identifying a zone of gas accumulation underneath the base of gas hydrate stability (BGHS) below the bank. Besides structurally controlled gas migration along conduits, gas migration also takes place along dipping strata across the BGHS. Gas migration on Opouawe Bank is influenced by anticlinal focusing and by several focusing levels within the gas hydrate stability zone.

High-resolution seismic velocity analysis as a tool for exploring gas hydrate systems: An example from New Zealand’s southern Hikurangi margin

The existence of free gas and gas hydrate in the pore spaces of marine sediments causes changes in acoustic velocities that overprint the background lithological velocities of the sediments themselves. Much previous work has determined that such velocity overprinting, if sufficiently pronounced, can be resolved with conventional velocity analysis from long-offset, multichannel seismic data. We used 2D seismic data from a gas hydrate province at the southern end of New Zealand’s Hikurangi subduction margin to describe a workflow for high-resolution velocity analysis that delivered detailed velocity models of shallow marine sediments and their coincident gas hydrate systems. The results showed examples of pronounced low-velocity zones caused by free gas ponding beneath the hydrate layer, as well as high-velocity zones related to gas hydrate deposits. For the seismic interpreter of a gas hydrate system, the velocity results represent an extra “layer” for interpretation that provides important information about the distribution of free gas and gas hydrate. By combining the velocity information from the seismic transect with geologic samples of the seafloor and an understanding of sedimentary processes, we have determined that high gas hydrate concentrations preferentially form within coarse-grained sediments at the proximal end of the Hikurangi Channel. Finer grained sediments expected elsewhere along the seismic transect might preclude the deposition of similarly high gas hydrate concentrations away from the channel.

Initiation of a thrust fault revealed by analog experiments

To reveal in detail the process of initiation of a thrust fault, we conducted analog experiments with dry quartz sand using a high-resolution digital image correlation technique to identify minor shear-strain patterns for every 27μm of shortening (with an absolute displacement accuracy of 0.5μm). The experimental results identified a number of “weak shear bands” and minor uplift prior to the initiation of a thrust in cross-section view. The observations suggest that the process is closely linked to the activity of an adjacent existing thrust, and can be divided into three stages. Stage 1 is characterized by a series of abrupt and short-lived weak shear bands at the location where the thrust will subsequently be generated. The area that will eventually be the hanging wall starts to uplift before the fault forms. The shear strain along the existing thrust decreases linearly during this stage. Stage 2 is defined by the generation of the new thrust and active displacements along it, identified by the shear strain along the thrust. The location of the new thrust may be constrained by its back-thrust, generally produced at the foot of the surface slope. The activity of the existing thrust falls to zero once the new thrust is generated, although these two events are not synchronous. Stage 3 of the thrust is characterized by a constant displacement that corresponds to the shortening applied to the model. Similar minor shear bands have been reported in the toe area of the Nankai accretionary prism, SW Japan. By comparing several transects across this subduction margin, we can classify the lateral variations in the structural geometry into the same stages of deformation identified in our experiments. Our findings may also be applied to the evaluation of fracture distributions in thrust belts during unconventional hydrocarbon exploration and production.

Formation mechanism of the global Dupal isotope anomaly

AbstractThe Dupal anomaly has been a frequently discussed feature since it was first proposed three decades ago. We here limit the distribution of the Dupal anomaly based on more than 10 000 Sr–Nd–Pb isotopic composition analyses and classify the anomaly into three types, located in three different oceans and with different mechanisms of formation. The Dupal anomaly in the East Asian Continental Margin subduction zone is related to enriched mantle II, which may originate from the recycling of the subducting plate or continental mantle. The high μ and enriched mantle I are the reason for the Dupal anomaly in the Pacific Ocean, which is related to the superplume. However, the source of the Dupal anomaly in the Indian Ocean and the Southern Atlantic Ocean is a mixture of enriched mantle I and enriched mantle II, and they come from the African superplume and the recycling of subcontinental mantle or continental crust of Gondwana. In consideration of all the above factors, we suggest that the superplume from the core–mantle boundary, the recycling subducting plate and continental mantle are the main characteristics of the global Dupal anomaly. Copyright © 2016 John Wiley & Sons, Ltd.

High-resolution view of active tectonic deformation along the Hikurangi subduction margin and the Taupo Volcanic Zone, New Zealand

ABSTRACTUnderstanding the mechanisms and dynamics of deformation at plate boundaries requires high-resolution images of strain and deformation sources. Vertical derivatives of horizontal stress (VDoHS) rates are the horizontal-component surface manifestation of all subsurface deformation sources, and have substantially higher spatial resolution than GPS velocities or strain rates. We calculate VDoHS rates from GPS data at the Hikurangi subduction margin. We evaluate our results in the context of interseismic coupling on the subduction interface and upper plate deformation processes. Instead of the expected rifting signal we find strong contraction within the Taupo Volcanic Zone, indicating non-tectonic effects from magmatic and/or hydrothermal processes. Differences between SHmax directions from historical seismicity (Townend et al. 2012) and our maximum contraction directions demonstrate that at the Hikurangi margin historical seismicity and active faulting are strongly controlled by long-term processes, such as rotation of the forearc, rather than short-term, elastic strain from interseismic subduction interface locking.

Salt-marsh foraminiferal record of 10 large Holocene (last 7500 yr) earthquakes on a subducting plate margin, Hawkes Bay, New Zealand

Sudden changes in microfossils and lithologies in Holocene sediments of a former tidal inlet on the Hikurangi subduction margin provide evidence of 10 large earthquakes. Studies were focused in three former embayments where intertidal shelly sediment interfingers with freshwater and salt-marsh peat. Paleoelevation histories were reconstructed using the modern analogue technique with foraminiferal assemblages. Land elevation record analysis indicates 8−9 m of mid- to late Holocene tectonic subsidence occurred prior to 1.5 m of uplift during the A.D. 1931 Hawkes Bay earthquake. Chronologies of displacement events were constrained using 50 radiocarbon dates and three widespread air-fall tephras. We infer the following earthquakes: earthquake 1: 7.3−7.0 ka (−1.1 ± 0.3 m), earthquake 2: 5.6−5.1 ka (+0.4 ± 0.4 m), earthquake 3: 5.2−4.9 ka (−0.5 ± 0.5 m), earthquake 4: 4.4−3.8 ka (−0.6 ± 0.5 m), earthquake 5: 2.8−2.4 ka (−0.9 ± 0.5 m), earthquake 6: 1.73−1.70 ka (−1.0 ± 0.3 m), earthquake 7: 1.5−1.3 ka (−0.7 ± 0.5 m), earthquake 8: 1.04−0.89 ka (−1.2 ± 0.4 m), earthquake 9: 0.60−0.44 ka (−0.8 ± 0.6 m), and earthquake 10: A.D. 1931 (+1.5 ± 0.3 m). A further 1.6−2.6 m of subsidence could have occurred by gradual aseismic slip or in smaller earthquakes. The age ranges of four of the recognized earthquakes (earthquakes 1, 6, 8, and 9) overlap with other documented displacement events onshore along 250−600 km of the Hikurangi subduction margin, and with turbidites offshore 100−300 km to the north. These four are considered strong candidates for large subduction-interface earthquakes. The other five inferred earthquakes are less strongly correlated with along-margin displacement events and offshore turbidites. These could have been caused by upper-plate fault ruptures (like historic earthquake 10), but subduction-interface sources cannot be ruled out. This evidence for repeated coseismic vertical deformation suggests large coseismic slip on a part of the subduction interface beneath Hawkes Bay that is currently dominated by aseismic creep processes, such as transient slow-slip events. This clearly indicates multiple slip processes are possible in a single location on a subduction interface.

Clusters of megaearthquakes on upper plate faults control the Eastern Mediterranean hazard

AbstractThe Hellenic subduction margin in the Eastern Mediterranean has generated devastating historical earthquakes and tsunamis with poorly known recurrence intervals. Here stranded paleoshorelines indicate strong uplift transients (0–7 mm/yr) along the island of Crete during the last ~50 kyr due to earthquake clustering. We identify the highest uplift rates in western Crete since the demise of the Minoan civilization and along the entire island between ~10 and 20 kyr B.P., with the absence of uplifted Late Holocene paleoshorelines in the east being due to seismic quiescence. Numerical models show that uplift along the Hellenic margin is primarily achieved by great earthquakes on major reverse faults in the upper plate with little contribution from plate‐interface slip. These earthquakes were strongly clustered with recurrence intervals ranging from hundreds to thousands of years and primarily being achieved by fault interactions. Future great earthquakes will rupture seismically quiet areas in eastern Crete, elevating both seismic and tsunami hazards.

Earth’s rotation variability triggers explosive eruptions in subduction zones

The uneven Earth’s spinning has been reported to affect geological processes, i.e. tectonism, seismicity and volcanism, on a planetary scale. Here, we show that changes of the length of day (LOD) influence eruptive activity at subduction margins. Statistical analysis indicates that eruptions with volcanic explosivity index (VEI) ≥3 alternate along oppositely directed subduction zones as a function of whether the LOD increases or decreases. In particular, eruptions in volcanic arcs along contractional subduction zones, which are mostly E- or NE-directed, occur when LOD increases, whereas they are more frequent when LOD decreases along the opposite W- or SW-directed subduction zones that are rather characterized by upper plate extension and back-arc spreading. We find that the LOD variability determines a modulation of the horizontal shear stresses acting on the crust up to 0.4 MPa. An increase of the horizontal maximum stress in compressive regimes during LOD increment may favour the rupture of the magma feeder system wall rocks. Similarly, a decrease of the minimum horizontal stress in extensional settings during LOD lowering generates a larger differential stress, which may enhance failure of the magma-confining rocks. This asymmetric behaviour of magmatism sheds new light on the role of astronomical forces in the dynamics of the solid Earth.

Magnetotelluric imaging zone in western Victoria of a Palaeozoic Andean margin subduction

SUMMARY A 450 km long transect of broadband (200 Hz – 2000 s) magnetotelluric (MT) sites spaced between 1 and 5 km apart, has been collected across the Palaeozoic Delamerian-Lachlan Orogens in western Victoria. The bandwidth of responses yields resistivity constraints between a few tens of metres in near-surface cover to sub-Moho depths. The passive nature of the source-field means that the MT responses have been collectively assembled in several tranches over ten years, with the last section across the transition between the Orogens collected in June 2014. The MT coverage now completely coincides with a deep crustal reflection seismic transect to generate complementary insight of the crustal structure. We report on preliminary modelling and interpretation.

Farallon plate dynamics prior to the Laramide orogeny: Numerical models of flat subduction

The Laramide orogeny (~80–50Ma) was an anomalous period of mountain-building in the western United States that occurred more than 1000km inboard of the Farallon Plate subduction margin. It is widely believed that this orogeny is coincident with a period of flat (subhorizontal) subduction. However, the factors that caused the Farallon Plate to evolve from a normal (steep) geometry to flat subduction are not well understood. Three proposed factors are: (1) a westward (trenchward) increase in North America motion, (2) an increased slab suction force owing to the presence of thick Colorado Plateau lithosphere, and (3) subduction of a low-density oceanic plateau. This study uses 2D upper mantle scale numerical models to investigate these factors. The models show that trenchward continental motion is the primary control on subduction geometry, with decreasing slab dip as velocity increases. However, this can only create low-angle subduction, as the Farallon Plate was old (>100Myr) and denser than the mantle. A transition to flat subduction requires: (1) subduction of a buoyant oceanic plateau that includes an 18-km-thick crust that does not undergo metamorphic densification and an underlying depleted harzburgite layer, and (2) a slab break-off at the landward side of the plateau. The break-off removes the dense frontal slab, and flat subduction develops as the buoyant plateau deflects the slab upward. The slab suction force has only a minor effect on slab flattening, but the thickness of the Colorado Plateau lithosphere controls the depth of the flat slab. With a continental velocity of 4cm/yr and a 400-km-wide oceanic plateau, flat subduction develops within 15Ma after plateau subduction. The flat slab underthrusts the continent at ~200km depth, eventually extending >1500km inboard of the trench.

Subsidence-driven environmental change in three Holocene embayments of Ahuriri Inlet, Hikurangi Subduction Margin, New Zealand

Three paleo-embayments on the southwest side of the uplifted Ahuriri Inlet, Hawkes Bay, contain complex interfingering sequences of Holocene terrestrial and saltmarsh peat and intertidal shelly sand and mud. We use 295 foraminiferal samples from 45 short cores (up to 8 m deep) to reconstruct the paleoenvironmental history of the bays. We infer that the strongest influence on their paleoenvironmental history was 8–10 m of net tectonic subsidence since 7.3 ka, which provided the accommodation space for sediment deposition prior to the c. 1.5 m of uplift in the AD 1931 Hawkes Bay Earthquake. Much of the subsidence appears to have occurred in eight large co-seismic events (0.4–2 kyr recurrence time) which caused marine transgressions of varying amounts into each bay. Sea-level changes (eustatic and isostatic) have been a secondary driver over the middle–late Holocene with an interval (c. 2.6–1.7 ka) of widespread erosion or slow sedimentation, caused by a sea-level fall of c. 1 m following the middle Holocene highstand. The interval of fastest sedimentation and maximum marine transgression occurred within the last 1 kyr and was driven by increased tectonic subsidence, rising sea level and enhanced compaction of thicker sequences of Holocene sediment, particularly peat. The supply of fluvial mud and cliff-eroded sand was the main, relatively constant source of clastic sediment. Airfall and reworking of three pumiceous tephras had a minimal impact on the paleoenvironments of the bays.

Seismic stratigraphy and paleogeographic evolution of Fairway Basin, Northern Zealandia, Southwest Pacific: from Cretaceous Gondwana breakup to Cenozoic Tonga–Kermadec subduction

AbstractWe present the first comprehensive seismic‐stratigraphic analysis of Fairway Basin, which is situated on the rifted continent of Zealandia in the Tasman Sea, southwest Pacific, between Australia and New Caledonia. The basin is 700 km long, 150 km wide, and has water depths of 500–3000 m. We describe depositional architecture and paleogeographic evolution of this basin. Basin formation was concurrent with two tectonic events: (i) Cretaceous rifting during eastern Gondwana breakup and (ii) initiation and Cenozoic evolution of Tonga–Kermadec subduction system to the east of the basin. To interpret the basin history we compiled and interpreted 2D seismic‐reflection profiles and make correlations with DSDP boreholes and the geology of New Caledonia. Five seismic‐stratigraphic units were defined. The deepest and oldest unit, FW3, folded and faulted can be correlated with volcaniclastic sediments and magmatic rocks in New Caledonia that are associated with Mesozoic Gondwana margin subduction. Alternatively, given the basin location 200–300 km west of New Caledonia and inboard of the ancient plate boundary, the unit could have formed as Gondwana intra‐continental basin with no known correlative. The overlying unit FW2b records syn‐rift deposition, probably associated with Cretaceous Gondwana breakup. Subaerial erosion supplied terrigenous sediment into the deltas in the northern part of the basin, as suggested by the truncation surfaces on the basement highs and sigmoid reflector geometries within unit FW2b respectively. Above, unit FW2a records post‐rift sedimentation and passive subsidence as the Tasman Sea opened and the Fairway Basin drifted away from Australia. Subsidence led to the flooding of the basement highs and burial of wave‐cut surfaces. Eocene compressive deformation resulted in minor folding and tilting within the Fairway Basin and was associated with the formation of many diapiric structures. The top of unit FW2 is an extensive unconformity that is associated with erosion and truncation on surrounding ridges. Above this unconformity, unit FW1b is interpreted as a turbidite system sourced from topography created during the Eocene tectonic event, which we interpret as being related to Tonga–Kermadec subduction initiation. Pelagic carbonate sedimentation is now prevalent. Unit FW1a has progressively draped the basin during Oligocene to Pleistocene subsidence. Many small volcanic cones were erupted during this final phase of subsidence, either as a delayed consequence of subduction initiation, or related to Tasmantid and Lord Howe hotspot trails. The northern Fairway Ridge remains close to sea level and its reef system continues to supply carbonate detrital sediments into the basin, most likely during sea‐level lowstands. Fairway Basin contains a nearly continuous record of tectonic and paleoclimatic events in the southwest Pacific since Cretaceous time. Its paleogeographic history is a key piece in the puzzle for understanding patterns of regional biodiversity in the southwest Pacific.

What 2‐D multichannel seismic and multibeam bathymetric data tell us about the North Sumatra wedge structure and coseismic response

AbstractRecent large earthquakes have prompted studies to reevaluate seismicity and rupture propagation behavior along the world's major subduction margins. Our study area covers the entire fore arc from northwest of northern Sumatra to west of Simeulue Island, the southern portion of the 2004 tsunamigenic earthquake rupture zone. The accretionary prism width is up to ~180 km, water depths between ~4.5 km near the Sunda Trench and <1 km on fore‐arc high. The wedge consists of a steep outer slope (5–12°), a plateau ~100–120 km wide with anticlinal folds spaced 2–15 km apart, and a steep inner slope adjacent to the Aceh Basin. Analysis of seismic profiles and bathymetry reveal three main structural zones consistent along‐strike, from the trench landward: (1) predominantly landward vergent folds, (2) mixed vergent folds, and (3) predominantly seaward vergent folds. This paper uses those zones to propose a geometry of an underlying rigid backstop. This backstop is seaward dipping and extends from under the Aceh Basin to beneath the mixed vergence zone. A dynamic backstop possibly exists seaward of the rigid backstop and is responsible for the steep slope of the outer prism. Indurated accreted sediments form the landward vergence zone. Along with the possible dynamic backstop beneath the outer wedge, and the rigid backstop in the inner wedge, all behave as a solid block coseismically. This block allows great earthquake rupture to propagate farther seaward toward the Sunda Trench, with resultant hazardous tsunamigenic potential.