Three-dimensional P- and S-wave velocity structures beneath the Japan Islands obtained by high-density seismic stations by seismic tomography
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Keywords:
Convergent boundary
Seismic Tomography
Pacific Plate
Seismic array
Seamount
Low-velocity zone
Underplating
Receiver function
The Tonga-Kermadec forearc is deforming in response to on-going subduction of the Pacific Plate beneath the Indo-Australian Plate. Previous research has focussed on the structural development of the forearc where large bathymetric features such as the Hikurangi Plateau and Louisville Ridge seamount chain are being subducted. Consequently, knowledge of the 'background' forearc in regions of normal plate convergence is limited. We report on an ∼250-km-long multichannel seismic reflection profile that was shot perpendicular to the Tonga-Kermadec trench at ∼28°S to determine the lateral and temporal variations in the structure, stratigraphy and deformation of the Kermadec forearc resulting solely from Pacific Plate subduction.
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Seafloor Spreading
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The structure and seismicity of the subduction zone of central Costa Rica have been investigated with local earthquake tomography down to ca. 50 km depth. Seismic traveltime data sets of three on- and offshore seismic networks were combined for a simultaneous inversion of hypocentre locations, 3-D structure of P-wave velocity and Vp/Vs ratio using about 2000 high-quality events. The seismicity and slab geometry as well as Vp and Vp/Vs show significant lateral variation along the subduction zone corresponding to the changes of the incoming plate which consists of serpentinized oceanic lithosphere in the northwest, a seamount province in the centre and the subducting Cocos Ridge in the southeast of the investigation area. Three prominent features can be identified in the Vp and Vp/Vs tomograms: a high-velocity zone with a perturbation of 4–10 per cent representing the subducting slab, a low-velocity zone (10–20 per cent) in the forearc crust probably caused by deformation, fluid release and hydration and a low-velocity zone below the volcanic arc related to upwelling fluids and magma. Unlike previously suggested, the dip of the subducting slab does not decrease to the south. Instead, an average steepening of the plate interface from 30° to 45° is observed from north to south and a transition from a plane to a step-shaped plate interface. This is connected with a change in the deformation style of the overriding plate where roughly planar, partly conjugated, clusters of seismicity of regionally varying dip are observed. It can be shown that the central Costa Rica Deformation Belt represents a deep crustal transition zone extending from the surface down to 40 km depth. This transition zone indicates the lateral termination of the active part of the volcanic chain and seems to be related to the changing structure of the incoming plate as well.
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The Seismic Array Hikurangi Experiment (SAHKE) investigated the structure of the forearc and subduction plate boundary beneath the southern North Island along a 350 km transect. Tomographic inversion of first‐arrival travel times was used to derive a well‐resolved 15–20 km deep P wave image of the crust. The refracted phases and migrated reflection events image subducting slab geometry and crustal structure. In the west, Australian Plate Moho depth decreases westward across the Taranaki Fault system from 35 to ∼28–30 km. In the east, subducted Pacific Plate oceanic crust is recognized to have a positive velocity gradient, but becomes less distinct beneath the Tararua Ranges, where the interface increases in dip at about 15 km depth from <5° to >15°. This bend in the subducted plate is associated with vertical clusters in seismicity, splay fault branching, and low‐velocity high‐attenuation material that we interpret to be an underplated subduction sedimentary channel. We infer that a step down in the decollément transfers slip on the plate interface at the top of a subduction channel to the oceanic crust and drives local uplift of the Tararua Ranges. Reflections from the Wairarapa Fault show that it is listric and soles into the top of underplated sediments, which in turn abut the Moho of the overriding plate at ∼32 km depth, near the downdip end of the strongly locked zone. The change in dip of the Hikurangi subduction interface is spatially correlated with the transition from geodetically determined locked to unlocked areas of the plate interface.
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Multichannel seismic (MCS) profiles and bathymetric data from the central Mariana and Izu‐Bonin subduction systems image the subducting Pacific Plate from the outer trench slope to beneath serpentinite seamounts on the outer fore arc. Subducting oceanic crust varies along the Mariana margin from 5.3 to 7 km thick and is covered by 0.5–2 km thick sediments and numerous seamounts. Oceanic crustal thickness east of the Izu‐Bonin Trench is ∼6 km. Faulting resulting from flexure of the incoming Pacific Plate begins up to 100 km east of the trench axis, near the 6 km depth contour. The plate is cut by normal faults that reactivate inherited tectonic fabric where that fabric strikes <25° to the trench. Where the strike is >25°, incoming crust breaks along new faults with a trench‐parallel strike. The Mariana Trench axis is commonly a graben that accommodates an abrupt change (within <25 km) of plate dip from <4° (commonly ≤2°) on the incoming plate to >8° beneath the outer fore arc. We infer that the plate fails there rather than simply bends under the applied loads. Along portions of the Mariana margin, subducting seamounts displace the trench axis westward and uplift the toe of the slope. Surprisingly, west of the toe, there is no geophysical evidence of disturbance of the upper plate in response to seamount subduction, nor of significant subduction erosion or sediment underplating. MCS profiles across the base of the Mariana inner trench slope provide evidence for both complete subduction and small‐scale accretion of Pacific Plate sediments; however, we found no evidence for long‐term sediment accretion. The subducting plate dips 9–12° beneath serpentinite seamounts on the Izu‐Bonin and Mariana fore arcs. Along the Mariana margin, the majority of these seamounts are located ∼50–70 km west of the trench where the mantle wedge is 3–7 km thick between 8–10 km thick fore‐arc crust and the top of the subducting plate. The apparent lack of significant deformation of the Mariana fore arc crust by subducting seamounts may be the result of a weak serpentinized mantle wedge and/or progressive fracturing as the subducting plate increases in dip as it passes through the trench graben.
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Abstract Seismic tomography has revealed two large low shear velocity provinces (LLSVPs) in the lowermost mantle beneath the central Pacific and Africa. The LLSVPs are further shown to be compositionally different from their surroundings. Among several hypotheses put forth in recent years to explain the cause of the LLSVPs, one postulates that they are thermochemical piles caused by accumulation of subducted oceanic crust at the core‐mantle boundary (CMB). Mineral physics experiments indicate that oceanic crust becomes denser than the surrounding mantle at lower mantle pressures. In addition, seismic observations provide evidence of subducted slabs arriving at the CMB. However, a major question pertains to whether subducted oceanic crust can survive viscous stirring associated with mantle plumes and accumulate into piles with the same spatial scale as LLSVPs. We perform a set of high‐resolution convection calculations to examine this hypothesis by investigating the interaction of thin oceanic crust (6 km) with mantle plumes. Our results show that as subducted oceanic crust is swept toward upwelling plume regions, the majority of it is viscously stirred into the surrounding mantle. Only a small amount of oceanic crust may accumulate at the base of plumes, but it is consistently entrained away into the plume at a rate equal to or greater than it is accumulated. We find that it is difficult for subducted oceanic crust to accumulate into large thermochemical piles at the CMB.
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Core–mantle boundary
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Abstract A reorganization centered on the Pacific plate occurred ~53–47 million years ago. A “top‐down” plate tectonic mechanism, complete subduction of the Izanagi plate, as opposed to a “bottom‐up” mantle flow mechanism, has been proposed as the main driver. Verification based on marine geophysical observations is impossible as most ocean crust recording this event has been subducted. Using a forward modeling approach, which assimilates surface plate velocities and shallow thermal structure of slabs into mantle flow models, we show that complete Izanagi plate subduction and margin‐wide slab detachment induced a major change in sub‐Pacific mantle flow, from dominantly southward before 60 Ma to north‐northeastward after 50 Ma. Our results agree with onshore geology, mantle tomography, and the inferred motion of the Hawaiian hot spot and are consistent with a plate tectonic process driving the rapid plate‐mantle reorganization in the Pacific hemisphere between 60 and 50 Ma. This reorganization is reflected in tectonic changes in the Pacific and surrounding ocean basins.
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