Time of re-emergence of Christmas Island and its biogeographical significance
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Seamount
Atoll
Faunal assemblage
Seamount
Atoll
Faunal assemblage
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Abstract The initial stages of seamount subduction and associated deformation in an overriding accretionary wedge is rarely documented. Initial subduction of Bennett Knoll seamount and faulting of the overlying strata along the Hikurangi subduction margin, New Zealand, are here studied using multibeam swath bathymetry, subbottom profiles, and regional seismic reflection lines. Our results provide new insights into the earliest stages of seamount collision at sediment-rich margins. Differential shortening along the subduction front induced by seamount subduction is initially accommodated in the accretionary wedge by conjugate strike-slip faults that straddle the buried flanks of the seamount and offset the frontal thrusts by as much as 5 km. The geometries of the strike-slip faults are controlled by the seamount’s dimensions and aspect, the obliquity of plate convergence, pore-fluid pressure, and the thickness and rheology of the incoming sedimentary section. Strike-slip faults in such settings are ephemeral and overprinted by the formation of new structures as seamount subduction advances.
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Accretionary wedge
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Abstract We conducted numerical experiments to simulate elastoplastic deformation of the overriding plate caused by a subducted seamount. Calculations revealed development of a distinct pair of fault‐like shear zones, including a landward dipping forethrust fault initiated from the seamount top and a seaward dipping backthrust fault from the landward base of the seamount. Significant dome‐shaped surface uplift was predicted above the thrust faults. Lesser‐developed seaward dipping backthrust faults were calculated to develop under certain conditions. The overriding plate was calculated to deform in two stages: In Stage I , elastic deformation leads to the formation of fault‐like shear zones. After major faults have cut through the entire plate, plastic deformation on faults dominates Stage II . On the subduction interface, compressional normal stress was calculated to increase on the landward leading flank of the seamount and decrease on the seaward trailing flank. These changes, together with associated stress singularities at seamount edges, could affect earthquake processes.
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Thrust fault
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Seamount
Basement
Margin (machine learning)
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Abstract Seamounts are ubiquitous on the oceanic plate; those situated near convergent margins will eventually undergo subduction. Using six prestack depth migrated MCS profiles transecting the Aleutian Trench, we investigate deeply buried seamounts offshore Kodiak Island, within 145–155°W and 55–58°N. A distinct sedimentary horizon exists in all six seismic profiles, at or above the average height of seamounts, which appears to be the preferred structural detachment zone. Where drilled, this horizon contains gravel‐sized debris interpreted to be ice rafted and marks the onset of intensification of Northern Hemisphere glaciation at ~2.7 Ma. Beneath this horizon, sediments prior to the Surveyor Fan development were deposited, all or the majority of these sediments will eventually be subducted. Despite the subducted seamounts being deeply buried, these features cause enhanced surface slope of the accretionary prism. Our observations lead us to propose a model for the stages of subduction for deeply buried seamounts. These stages include the following: (1) Prior to subduction, the protothrust zone undergoes enhanced shortening, (2) frontal thrust steepening and enhanced backthrusting occurs during subduction with a potential décollement step down seaward and a steeping outward of the deformation front to the limit of the protothrust zone, and (3) further subduction results in a pattern of uplift farther into the wedge resulting in enhanced out‐of‐sequence thrusting and persistence of the more seaward deformation front position. This pattern is distinct from the dominance of embayments and effective removal of prism material during seamount subduction described along margins with less deeply buried edifices.
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Accretionary wedge
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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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Pacific Plate
Underplating
Volcanic arc
Convergent boundary
Back-arc basin
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Sediment volume at the trench and topographic highs on the incoming plate are two of the main factors controlling whether a forearc will undergo subduction erosion or accretion. On oceanic plates, topographic highs such as large seamount complexes are commonly associated with significant volumes of flanking volcaniclastic sediments in the form of >100-km-wide debris aprons, with the largest deposits found in flexural moat basins. We propose that subduction of these sediment accumulations promotes localized frontal accretion, even in otherwise non-accretionary margins. The Osa mélange in southwestern Costa Rica is a field example that provides new insights into the nature and occurrence of this interaction. The southwestern margin of Central America is punctuated by accreted Late Cretaceous–middle Eocene seamounts that formed at the Galápagos hotspot and accreted throughout the late Miocene. In contrast to most accreted seamounts along this margin, which retained their overall structure, the Osa mélange is a chaotic mixture of seamount lithologies. It consists of basalt, chert, and carbonate blocks in a fine-grained pelitic matrix composed predominantly of feldspar and pyroxene grains with rare quartz. This lithology is consistent with sediment from a seamount chain's debris apron, such as the Hawaiian moat sampled during Ocean Drilling Program (ODP) Leg 136 and the Canary Islands moat sampled by ODP Leg 157. Subduction of seamounts and their debris aprons promotes concurrent accretion and erosion over short distances along the trench. This introduces heterogeneity into the subduction channel, with implications for deformation within the subduction zone plate interface.
Seamount
Accretionary wedge
Forearc
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Abstract 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.
Seamount
Fracture zone
North American Plate
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Deformation patterns caused by the subduction of a single seamount or aseismic ridge have been well studied in analogue and numerical models. However, the effects of sequential multi-seamount subduction on accretionary wedge deformation have rarely been investigated in details. We performed a series of analogue modelling experiments of sequential subduction involving two seamounts of variable shape, spacing and rheological properties of strata to better understand the deformation mechanisms of an accretionary wedge with multi-seamount subduction. The results demonstrate that a seamount significantly hinders the seaward propagation of the accretionary wedge and facilitates lateral propagation. Two structural quiet zones form at the leading and trailing edges of the subducted seamount in the early stage of collision. As the seamount deeply penetrates into the wedge, the structural quiet zone in the leading edge is remoulded by a duplex structure, which may help the upward transport of deep subducted sediments back into the shallow area. Comparatively, the structural quiet zone in the trailing edge remains undeformed because it is situated in the stress shadow of the seamount. Deformation of the strata between two subducting seamounts may occur by thrusts laterally propagating into the seamount gap, which is facilitated by the décollement layer. The modelling results provide insights into the complex deformation mechanisms related to seamounts collision observed in the accretionary wedge offshore from the Costa Rica margin.
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Accretionary wedge
Wedge (geometry)
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Abstract Millions of seamounts on modern and past seafloor end up being subducted, and only small pieces are recovered in suture zones. How they are metamorphosed and deformed is, however, critical to understand how seamount subduction can impact subduction zone geometry, fluid circulation or seismogenic conditions, and more generally to trace physical conditions along the subduction boundary. Since geophysical studies mostly reach the shallowest subducted seamounts and miss internal structures due to low resolution, there is a high need for fossil seamount exposures. We herein report on a fully exposed, 3D example of seamount that we discovered in the Siah Kuh massif, Southern Iran. Through a series of sections across the whole massif and the combination of magmatic-metamorphic-sedimentary petrological data, we document several distinct stages associated with seamount build-up on the seafloor and with subduction. In particular, we constrain different stages of metamorphism and associated mineralogy, with precise conditions for subduction-related metamorphism around 250 °C and 0.7 GPa, in the middle of the seismogenic zone. Extensive examination of the seismogenic potential of the Siah Kuh seamount reveals that it was not a large earthquake asperity (despite the report of a rare example of cm-scale, high-pressure pseudotachylyte in this study), and that it possibly behaved as a barrier to earthquake propagation. Finally, we discuss the nature of high-pressure fluid circulation preserved in this seamount.
Seamount
Seafloor Spreading
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