Deformation of the northern Sumatra accretionary prism from high-resolution seismic reflection profiles and ROV observations
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Accretionary wedge
Forearc
Seafloor Spreading
Two 3D seismic reflection surveys across the Nankai Trough have imaged the deep structure of the subduction zone megathrust from the trench into the up dip end of the seismogenic zone where slip occurred during the most recent megathrust earthquakes. In 1999 we acquired a 3D seismic volume across the Muroto transect in the vicinity of the Muroto peninsula, offshore Shikoku Island, Japan. These data imaged 70 km of the subduction thrust from the toe of the accretionary wedge, across the up dip rupture area of the 1946 Nankaido M 8.1 earthquake, to 8 km depth below seafloor. At the down dip edge of the 3D volume we see a 1 km high seamount that has subducted to 7 km below seafloor, producing several very distinctive structures that imply uplift of the overriding accretionary wedge, removal of ∼ 1 km of material from the overriding plate, and a 1 km thick underthrust sediment sequence that appears to have been subducted to 5 km below seafloor in the wake of the subducted seamount.
Seafloor Spreading
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The Manila Trench subduction zone is characterized by varying tectonic structures from north to south. Analyses of new seismic reflection data and bathymetric data indicate distinct morphological and deformational patterns in the forearc region. Differences in the nature of the subducting oceanic lithosphere (i.e. seafloor relief related to seamounts and ridges, sediment supply, reactivated features and faults associated with the South China Sea opening) cause along-strike heterogeneity in the Manila Trench and the Luzon forearc region. The northern segment is classified as an accretionary margin while the southern segment is mainly an erosive margin (with a narrow, steep, and often eroded frontal wedge). This sharp contrast is attributed to abundant sediment supply to the trench in the north and to the highly eroded frontal wedge in the south due to scarce sediment supply. The southern trench segment is prone to submarine slope failures and mass wasting processes. The 17°N latitude boundary also separates the forearc basin into the North Luzon Trough and the West Luzon Trough. Associated with this is the initiation of the Scarborough Seamount Chain (South China Sea extinct spreading ridge) subduction at 16°N latitude. A combination of forearc uplift and submarine mass movements attributed to subduction of bathymetric highs near and south of 17°N latitude produced the Stewart Bank. Seamount and other seafloor spreading features induced complex responses arising to diverse forearc architectures in the southern segment. Seamounts and other seafloor spreading-related features in the subducting slab induce slope steepening and significant vertical movement in the frontal wedge and the forearc region, respectively. Deformation associated with the subduction is overprinted by shearing related to the Philippine Fault Zone splay faults with the frontal wedge shortening associated with the ongoing subduction, further complicating the forearc development in the trench and marine forearc region.
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Abstract The southern Ryukyu subduction zone is one of the potential sources for tsunamigenic earthquakes. Despite a great seismic risk, the deformation pattern remains poorly known, primarily due to the absence of seafloor constraints. With GNSS‐acoustic measurements over years, we characterize the convergence rate across this margin growing from 92 mm/yr offshore eastern Taiwan to 123 mm/yr near the Gagua Ridge. The new data suggest the subduction interface is capable of hosting M w 7.5–8.4 earthquakes. The orientations of seafloor movement and P‐ axes in the Nanao Basin are both subnormal to the trench, notably deviate from the direction of plate convergence. By considering the combined effect of plate convergence and backarc rifting, different trends between the forearc convergence, P ‐axes, and seafloor movement may indicate some degree of slip‐partitioning. The trench‐parallel component is likely accommodated in part by earthquakes near Taiwan, lower plate deformation, and strike‐slip faults within the accretionary wedge.
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Convergent boundary
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Abstract The M w 8.8 megathrust earthquake that occurred on 27 February 2010 offshore the Maule region of central Chile triggered a destructive tsunami. Whether the earthquake rupture extended to the shallow part of the plate boundary near the trench remains controversial. The up-dip limit of rupture during large subduction zone earthquakes has important implications for tsunami generation and for the rheological behavior of the sedimentary prism in accretionary margins. However, in general, the slip models derived from tsunami wave modeling and seismological data are poorly constrained by direct seafloor geodetic observations. We difference swath bathymetric data acquired across the trench in 2008, 2011 and 2012 and find ~3–5 m of uplift of the seafloor landward of the deformation front, at the eastern edge of the trench. Modeling suggests this is compatible with slip extending seaward, at least, to within ~6 km of the deformation front. After the M w 9.0 Tohoku-oki earthquake, this result for the Maule earthquake represents only the second time that repeated bathymetric data has been used to detect the deformation following megathrust earthquakes, providing methodological guidelines for this relatively inexpensive way of obtaining seafloor geodetic data across subduction zone.
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Forearc
Seamount
Seafloor Spreading
Accretionary wedge
Submarine canyon
Continental Margin
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We present the results from a new grid of deep penetration multichannel seismic (MCS) profiles over the 280-km-long north-central segment of the Lesser Antilles subduction zone. The 14 dip-lines and 7 strike-lines image the topographical variations of (i) the subduction interplate decollement, (ii) the top of the arcward subducting Atlantic oceanic crust (TOC) under the huge accretionary wedge up to 7 km thick, and (iii) the trenchward dipping basement of the deeply buried forearc backstop of the Caribbean upper plate. The four northernmost long dip-lines of this new MCS grid reveal several-kilometres-high topographic variations of the TOC beneath the accretionary wedge offshore Guadeloupe and Antigua islands. They are located in the prolongation of those mapped on the Atlantic seafloor entering subduction, such as the Barracuda Ridge. This MCS grid also provides unexpected evidences on huge along- strike topographical variation of the backstop basement and of the deformation style affecting the outer forearc crust and sediments. Their mapping clearly indicates two principal areas of active deformation in the prolongation of the major Barracuda and Tiburon ridges and also other forearc basement highs that correspond to the prolongation of smaller oceanic basement highs recently mapped on the Atlantic seafloor. Although different in detail, the two main deforming forearc domains share similarities in style.
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Accretionary wedge
Décollement
Basement
Convergent boundary
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Abstract Sandy trench-fill sediments at accretionary margins are commonly scraped off at the frontal wedge and rarely subducted to the depth of high-pressure (HP) metamorphism. However, some ancient exhumed accretionary complexes are associated with high-pressure–low-temperature (HP-LT) metamorphic rocks, such as psammitic schists, which are derived from sandy trench-fill sediments. This study used sandbox analogue experiments to investigate the role of seafloor topography in the transport of trench-fill sediments to depth during subduction. We conducted two different types of experiments, with or without a rigid topographic high (representing a seamount). We used an undeformable backstop that was unfixed to the side wall of the apparatus to allow a seamount to be subducted beneath the overriding plate. In experiments without a seamount, progressive thickening of the accretionary wedge pushed the backstop down, leading to a stepping down of the décollement, narrowing of the subduction channel, and underplating of the wedge with subducting sediment. In contrast, in experiments with a topographic high, the subduction of the topographic high raised the backstop, leading to a stepping up of the décollement and widening of the subduction channel. These results suggest that the subduction of stiff topographic relief beneath an inflexible overriding plate might enable trench-fill sediments to be deeply subducted and to become the protoliths of HP-LT metamorphic rocks.
Accretionary wedge
Seamount
Seafloor Spreading
Underplating
Décollement
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Forearc
Seafloor Spreading
Accretionary wedge
Décollement
Convergent boundary
Basement
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Abstract The Nankai Trough, Japan, is a subduction zone characterized by the recurrence of disastrous earthquakes and tsunamis. Slow earthquakes and associated tremor also occur intermittently and locally in the Nankai Trough and the causal relationship between slow earthquakes and large earthquakes is important to understanding subduction zone dynamics. The Nankai Trough off Muroto, Shikoku Island, near the southeast margin of the rupture segment of the 1946 Nankai earthquake, is one of three regions where slow earthquakes and tremor cluster in the Nankai Trough. On the Philippine Sea plate, the rifting of the central domain of the Shikoku Basin was aborted at ~15 Ma and underthrust the Nankai forearc off Muroto. Here, the Tosa‐Bae seamount and other high‐relief features, which are northern extension of the Kinan Seamount chain, have collided with and indented the forearc wedge. In this study, we analyzed seismic reflection profiles around the deformation front of accretionary wedge and stratigraphically correlated them to drilling sites off Muroto. Our results show that the previously aborted horst‐and‐graben structures, which were formed around the spreading center of the Shikoku Basin at ~15 Ma, were rejuvenated locally at ~6 Ma and more regionally at ~3.3 Ma and have remained active since. The reactivated normal faulting has enhanced seafloor roughness and appears to affect the locations of slow earthquakes and tremors. Rejuvenated normal faulting is not limited to areas near the Nankai Trough, and extends more than 200 km into the Shikoku Basin to the south. This extension might be due to extensional forces applied to the Philippine Sea plate, which appear to be driven by slab‐pull in the Ryukyu and Philippine trenches along the western margin of the Philippine Sea plate.
Forearc
Accretionary wedge
Seamount
Seafloor Spreading
Trough (economics)
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Forearc
Accretionary wedge
Martinique
Slab
Volcanic arc
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