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B L N Kennett - One of the best experts on this subject based on the ideXlab platform.
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tears or thinning subduction structures in the Pacific Plate beneath the japanese islands
Physics of the Earth and Planetary Interiors, 2010Co-Authors: B L N Kennett, Takashi FurumuraAbstract:Abstract The nature of a subduction zone at depth is affected by the evolution of its tectonic system, and the geometry of the trench line can change over time due to slab roll-back or the arrival of a distinctive feature with the incoming oceanic lithosphere. The configuration of the Plate has to accommodate such changes with buckling, thinning or the formation of tears depending on the rate of influx to the trench. Tomographic imaging is commonly used to recognise the presence of such tears through marked reductions in wavespeed anomalies in localised zones. A good example is provided by Pacific Plate subduction beneath the Japanese Islands. A horizontal tear in the Plate below 300 km depth can be recognised at the southern end of the Izu-Bonin arc associated with the change in slab morphology to the much steeper Mariana arc. Beneath southern Honshu a break in the fast wavespeeds associated with the Pacific Plate has been described as a tear based on the evidence of converted phases from the edge of the zone and tensional focal mechanisms for seismic events in the tear zone. In the north, close to the Hokkaido bend in the subduction zone, the reduction in the shear wavespeed anomaly is just as dramatic, but here the characteristics of high frequency guided waves from deep earthquakes indicate continuity of slab material with thinning of the slab. The thinned slab has less wavespeed contrast within the affected cells and so appears in the tomographic images as a weakened anomaly. The various modes of slab deformation represent different ways in which the subducted material accommodates the strains imposed by the evolution of the geometry of the subduction scenario. Not all significant reductions in wavespeed anomalies represent tears and thus it is important that such interpretations be checked against the characteristics of wave propagation through the zone.
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spatial and temporal evolution of the subducting Pacific Plate structure along the western Pacific margin
Journal of Geophysical Research, 2006Co-Authors: Meghan S Miller, B L N KennettAbstract:[1] Tomographic images of the subducting Pacific Plate beneath the Izu-Bonin-Mariana arc illustrate a progression of geometries from shallow dipping to vertical from north to south along the arc. Recent advances in technology and inversion techniques have improved resolution of slab structure beneath the western Pacific island arcs, but reasons for the variation in geometry and morphology are still poorly understood. By comparing high-resolution tomographic images of the western Pacific and updated paleogeographic reconstructions, we are able to link the spatiotemporal evolution of the subducting Pacific Plate back to the mid-Miocene. We have reconstructed tectonic motions along the Kurile-Japan-Izu-Bonin-Mariana arc system and the Ryukyu arc to provide an independent, additional interpretation of the subducting Pacific Plate using the most current Plate motion data. We then investigate the plausibility of our model and three other proposed models based on the interpreted slab structure from tomographic images. The new reconstruction agrees with the basic characteristics of former trench retreat models but illustrates the importance of the collision of the Ogasawara Plateau with the trench in the mid-Miocene and its subsequent effect on the slab structure at depth and the impact of other aseismic ridge collisions along the Plate boundary. The combination of evidence in changed physical properties imaged with tomography and the current interpreted slab morphology can be analyzed with the past Plate motions to understand subduction zone processes.
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imaging changes in morphology geometry and physical properties of the subducting Pacific Plate along the izu bonin mariana arc
Earth and Planetary Science Letters, 2004Co-Authors: Meghan S Miller, B L N Kennett, Gordon S ListerAbstract:Abstract Recent studies on slab structure in the northwest Pacific have imaged variable geometries of the subducted oceanic lithosphere along the Plate margin. Tomographic images show portions of the subducted Pacific Plate penetrate vertically into the lower mantle below the Mariana arc, whereas below the Izu–Bonin arc, the slab appears to be lain down horizontally on top of the 670-km discontinuity. Using new technology to grid and visualize regional seismic tomography data even more detail about the structure of the subducting Plates can be extracted from the three-dimensional images. We investigate the morphology and geometry of the subducting Pacific Plate as it changes between horizontal to nearly vertical using P-wave tomographic images. The most profound result from these new models is the presence of a distinct change of seismic property in the downgoing Pacific slab beneath the Izu–Bonin arc at a depth of 350–400 km. The position of this anomaly corresponds to a region north and west of the Ogasawara Plateau. We propose the change in morphology and physical property of the slab is related to the distortion of the Pacific Plate geometry as its shape converts from near horizontal to vertical, together with the subduction of the Ogasawara Plateau.
Bernhard Steinberger - One of the best experts on this subject based on the ideXlab platform.
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Pacific Plate motion change caused the hawaiian emperor bend
Nature Communications, 2017Co-Authors: Trond H Torsvik, Pavel V Doubrovine, Bernhard Steinberger, Carmen Gaina, Wim Spakman, Mathew DomeierAbstract:A conspicuous 60° bend of the Hawaiian-Emperor Chain in the north-western Pacific Ocean has variously been interpreted as the result of an abrupt Pacific Plate motion change in the Eocene (∼47 Ma), a rapid southward drift of the Hawaiian hotspot before the formation of the bend, or a combination of these two causes. Palaeomagnetic data from the Emperor Seamounts prove ambiguous for constraining the Hawaiian hotspot drift, but mantle flow modelling suggests that the hotspot drifted 4–9° south between 80 and 47 Ma. Here we demonstrate that southward hotspot drift cannot be a sole or dominant mechanism for formation of the Hawaiian-Emperor Bend (HEB). While southward hotspot drift has resulted in more northerly positions of the Emperor Seamounts as they are observed today, formation of the HEB cannot be explained without invoking a prominent change in the direction of Pacific Plate motion around 47 Ma. The Hawaiian-Emperor Chain has a 60° bend that has been interpreted as the result of Pacific Plate motion at 47 Ma or drift of the Hawaiian hotspot. Here, the authors show that hotspot drift cannot be the dominant mechanism for bend formation, but involves a change in the direction of Pacific Plate motion at ∼47 Ma.
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on the role of slab pull in the cenozoic motion of the Pacific Plate
Geophysical Research Letters, 2012Co-Authors: Claudio Faccenna, Thorsten W Becker, Serge Lallemand, Bernhard SteinbergerAbstract:We analyze the role of slab pull acting on the Pacific Plate during its early Tertiary change in motion. Slab pull forces are estimated by integrating the negative buoyancy of a 700 km long slab along a revised subduction boundary model adopting the Muller et al. (2008) seafloor age reconstructions. Our results indicate that torques predicted from a simple slab pull model match the Pacific Plate Euler vectors during the Tertiary fairly well. The change of the Pacific motion at similar to 50-40 Ma appears to be driven by the onset of the Izu-Bonin-Mariana system and, soon afterwards, by the Tonga-Kermadec subduction zones.
Masanao Shinohara - One of the best experts on this subject based on the ideXlab platform.
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normal faulting earthquakes beneath the outer slope of the japan trench after the 2011 tohoku earthquake implications for the stress regime in the incoming Pacific Plate
Geophysical Research Letters, 2012Co-Authors: Koichiro Obana, Gou Fujie, Tsutomu Takahashi, Yojiro Yamamoto, Yasuyuki Nakamura, Shuichi Kodaira, Narumi Takahashi, Yoshiyuki Kaneda, Masanao ShinoharaAbstract:[1] After the 2011 Mw 9.1 Tohoku earthquake, numerous intraPlate earthquakes occurred beneath the outer slope of the Japan Trench. Based on ocean bottom seismograph observations, these earthquakes occurred in the oceanic crust and uppermost mantle of the Pacific Plate at depths shallower than about 40 km and had normal-faulting focal mechanisms at all depths. Before the 2011 earthquake, normal-faulting earthquakes beneath the outer trench slope occurred only at depths shallower than 20 km, whereas those at depths of around 40 km had reverse-faulting mechanisms. These observations suggest that the stress regime in the Pacific Plate was changed by the 2011 earthquake. The tensional stresses that now extend to depths of about 40 km may play an important role not only in the occurrence of large normal-faulting earthquakes but also in hydration of the uppermost mantle of the incoming Pacific Plate prior to the subduction.
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Normal‐faulting earthquakes beneath the outer slope of the Japan Trench after the 2011 Tohoku earthquake: Implications for the stress regime in the incoming Pacific Plate
Geophysical Research Letters, 2012Co-Authors: Koichiro Obana, Gou Fujie, Tsutomu Takahashi, Yojiro Yamamoto, Yasuyuki Nakamura, Shuichi Kodaira, Narumi Takahashi, Yoshiyuki Kaneda, Masanao ShinoharaAbstract:[1] After the 2011 Mw 9.1 Tohoku earthquake, numerous intraPlate earthquakes occurred beneath the outer slope of the Japan Trench. Based on ocean bottom seismograph observations, these earthquakes occurred in the oceanic crust and uppermost mantle of the Pacific Plate at depths shallower than about 40 km and had normal-faulting focal mechanisms at all depths. Before the 2011 earthquake, normal-faulting earthquakes beneath the outer trench slope occurred only at depths shallower than 20 km, whereas those at depths of around 40 km had reverse-faulting mechanisms. These observations suggest that the stress regime in the Pacific Plate was changed by the 2011 earthquake. The tensional stresses that now extend to depths of about 40 km may play an important role not only in the occurrence of large normal-faulting earthquakes but also in hydration of the uppermost mantle of the incoming Pacific Plate prior to the subduction.
Richard G. Gordon - One of the best experts on this subject based on the ideXlab platform.
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Global Earth Physics - Mean Paleomagnetic Poles for the Major Continents and the Pacific Plate
AGU Reference Shelf, 2013Co-Authors: Richard G. GordonAbstract:Apparent polar wander is the motion of Earth’s spin axis relative to a reference frame that is usually fixed relative to a tectonic Plate or stable interior of a continent. The main method for estimating the past position of Earth’s spin axis is through paleomagnetism, the investigation of the magnetic memory of rocks. Mean paleomagnetic poles, which average poles over about half a geologic period and typically differ in mean age by -20 to 30 millions of years, are presented for the major cratons over Phanerozoic time and are presented for more closely spaced ages for the Pacific Plate since Late Jurassic time.
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True polar wander since 32 Ma B.P.: A paleomagnetic investigation of the skewness of magnetic anomaly 12r on the Pacific Plate
Journal of Geophysical Research, 2010Co-Authors: B. C. Horner-johnson, Richard G. GordonAbstract:[1] We test the fixed hot spot and fixed spin axis hypotheses through a paleomagnetic investigation of the skewness of crossings of magnetic anomaly 12r (32 Ma B.P.) between the Galapagos and Clarion fracture zones on the Pacific Plate. We focus on this region for three reasons. First, numerical experiments show that these crossings, of all those available from the Pacific Plate, should contain the most information about the location of the 32 Ma B.P. paleomagnetic pole for the Pacific Plate. Second, many of the available crossings are from vector aeromagnetic profiles, which have superior signal-to-noise ratios. Third, the rate of seafloor spreading recorded in these crossings exceeds the threshold (half rate of 50 mm a−1) above which anomalous skewness is negligible. The new pole (83.5°N, 44.6°E) has compact 95% confidence limits (ellipse with major semiaxis length of 3.1° toward 84° clockwise from north and minor semiaxis length of 1.2°) and is not subject to the biases inherent in other methods for estimating Pacific Plate paleomagnetic poles. The pole differs significantly by ≈5° from the pole predicted if the Pacific hot spots have been fixed with respect to the spin axis, thus demonstrating, for the first time from paleomagnetic data, that Pacific hot spots have moved relative to the spin axis since the formation of the elbow in the Hawaiian-Emperor chain. The pole is consistent, however, with previously published paleomagnetic poles in a reference frame fixed relative to Indo-Atlantic hot spots. Thus, the new results require no motion between Pacific and Indo-Atlantic hot spots since 32 Ma B.P. Instead, superimposed on whatever motion occurs between hot spots, as expected for true polar wander.
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A Maastrichtian palaeomagnetic pole for the Pacific Plate from a skewness analysis of marine magnetic anomaly 32
Geophysical Journal International, 1999Co-Authors: Katerina E. Petronotis, Richard G. GordonAbstract:SUMMARY The asymmetry (skewness) of marine magnetic anomaly 32 (72.1‐73.3 Ma) on the Pacific Plate has been analysed in order to estimate a new palaeomagnetic pole. Apparent eVective remanent inclinations of the seafloor magnetization were calculated from skewness estimates of 108 crossings of anomaly 32 distributed over the entire Pacific Plate and spanning a great-circle distance of ~12 000 km. The data were inverted to obtain a palaeomagnetic pole at 72.1°N, 26.8°E with a 95 per cent confidence ellipse having a 4.0° major semi-axis oriented 98° clockwise of north and a 1.8° minor semi-axis; the anomalous skewness is 14.2°±3.7°. The possible dependence of anomalous skewness on spreading rate was investigated with two empirical models and found to have a negligible eVect on our palaeopole analysis over the range of relevant spreading half-rates, ~25 to ~90 mm yr’1. The new pole is consistent with the northward motion for the Pacific Plate indicated by coeval palaeocolatitude and palaeoequatorial data, but diVers significantly from, and lies to the northeast of, coeval seamount poles. We attribute the diVerence to unmodelled errors in the seamount poles, mainly in the declinations. Comparison with the northward motion inferred from dated volcanoes along the Hawaiian‐Emperor seamount chain indicates 13° of southward motion of the Hawaiian hotspot since 73 Ma. When the pole is reconstructed with the Pacific Plate relative to the Pacific hotspots, it diVers by 14°‐18° from the position of the pole relative to the Indo‐Atlantic hotspots. This has several possible explanations including bias in one or more of the palaeomagnetic poles, motion between the Pacific and Indo‐Atlantic hotspots, and errors in Plate reconstructions relative to the hotspots.
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A 57 Ma Pacific Plate palaeomagnetic pole determined from a skewness analysis of crossings of marine magnetic anomaly 25r
Geophysical Journal International, 1994Co-Authors: Katerina E. Petronotis, Richard G. Gordon, G ActonAbstract:SUMMARY An increase in the accuracy and age resolution of the apparent polar wander path of the Pacific Plate could be important for testing reconstructions that relate the motion of Pacific basin Plates to other Plates, for testing if hotspots in different ocean basins are stationary relative to one another, and for estimating the motion of hotspots relative to the spin axis. With these goals in mind, herein we investigate how accurately a palaeomagnetic pole can be estimated from skewness analysis of many crossings of a single magnetic anomaly on the Pacific Plate. Apparent effective remanent inclinations of the sea-floor magnetization were estimated from the skewnesses of 132 useful (out of 149 total) crossings of anomaly 25r (56.5–57.8 Ma) distributed over a distance of more than 11000 km across the Pacific Plate. These estimates were inverted to obtain a best-fitting palaeomagnetic pole latitude, pole longitude, and anomalous skewness for this single reversed-polarity chron. The best-fitting model gives a pole of 78.2°N, 4.8°E with a 95 per cent confidence ellipse having a 6.4° major semi-axis oriented 93° clockwise of north and a 4.1° minor semi-axis; anomalous skewness is 16.2°± 4.6° (95 per cent confidence limits). We also investigated the effect of the dependence of anomalous skewness on spreading rate by correcting our data using an empirical model. The pole obtained from the inversion of this alternative data set lies a statistically insignificant 0.6° from the pole obtained using no correction. That a pole with usefully compact confidence limits and a narrowly resolved, precisely estimated age can be so determined suggests that an accurate apparent polar wander path with a fine-age resolution can be determined for the Pacific Plate by applying the same approach to the shapes of other marine magnetic anomalies. Comparison of our chron 25r pole with other Pacific palaeomagnetic and palaeoequatorial sediment facies data indicates that the Pacific Plate remained nearly stationary relative to the spin axis during the Eocene (-0.05°Myr−1± 0.28° Myr−1), but probably moved rapidly northward during the Paleocene (0.83° Myr−1± 0.46° Myr−1). Comparison of these data to latitudes of dated volcanic edifices along the Hawaiian-Emperor chain indicates that the Hawaiian hotspot drifted southward by 10.2°± 3.4° (95 per cent confidence limits) since 57 Ma, but only by 1.7°± 1.9° since 39 Ma, which gives a southward displacement of 8.5°± 3.9° (95 per cent confidence limits) between 57 and 39 Ma, corresponding to a rate of southward motion of 52°24mm yr−1. Incorporation of realistic uncertainties of volcano ages would increase these uncertainties considerably, however. We also examined the distance between the crossings of anomalies 25 and 27 on all the profiles we analysed; along the palaeo-Pacific-Farallon boundary these distances are inconsistent with the joint hypotheses of symmetric spreading and single Pacific and Farallon Plates between 62 and 56 Ma, indicating that the evidence for a single Pacific Plate in early Tertiary time is not as compelling as it had previously seemed.
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Test for bias in paleomagnetically determined paleolatitudes from Pacific Plate Deep Sea Drilling Project sediments
Journal of Geophysical Research, 1990Co-Authors: Richard G. GordonAbstract:Paleolatitudes determined from paleomagnetic study of azimuthally unoriented sediment cores recovered by deep sea drilling on the Pacific Plate have been previously used in determining reference poles for the Pacific Plate apparent polar wander (APW) path. From discordant paleoinclinations and corresponding paleolatitudes determined from these data, some workers have inferred tectonic tilting or large horizontal motion of some deep-sea drilling sites relative to other parts of the Pacific Plate. Here the reliability of these paleolatitudes is evaluated by comparing them with paleolatitudes determined from a Pacific Plate reference APW path determined from seamount poles, skewness and amplitudes of magnetic profiles across magnetically lineated seafloor, and paleocolatitudes from azimuthally unoriented basalt cores and from equatorial sediment facies. The reference APW path includes a new determination of the Early Cretaceous pole from several types of paleomagnetic data, which agree well with one another. Sediment paleolatitudes are found to be systematically less southerly than those predicted from the reference APW path, a discrepancy that corresponds to inclinations shallower than expected. Thus most deep-sea drilling sediment paleomagnetic paleolatitudes are unreliable and should be omitted from the data used in determining reference APW paths. Inclination and paleolatitude discrepancies noted in prior studies may be due not to tectonic causes but to shallowly biased inclinations.
Takashi Furumura - One of the best experts on this subject based on the ideXlab platform.
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Tears or thinning? Subduction structures in the Pacific Plate beneath the
2020Co-Authors: Japanese Islands, Brian Kennett, Takashi FurumuraAbstract:The nature of a subduction zone at depth is affected by the evolution of its tectonic system, and the geometry of the trench line can change over time due to slab roll-back or the arrival of a distinctive feature with the incoming oceanic lithosphere. The configuration of the Plate has to accommodate such changes with buckling, thinning or the formation of tears depending on the rate of influx to the trench. Tomographic imaging is commonly used to recognise the presence of such tears through marked reductions in wavespeed anomalies in localised zones. A good example is provided by Pacific Plate subduction beneath the Japanese Islands. A horizontal tear in the Plate below 300 km depth can be recognised at the southern end of the Izu-Bonin arc associated with the change in slab morphology to the much steeper Mariana arc. Beneath southern Honshu a break in the fast wavespeeds associated with the Pacific Plate has been described as a tear based on the evidence of converted phases from the edge of the zone and tensional focal mechanisms for seismic events in the tear zone. In the north, close to the Hokkaido bend in the subduction zone, the reduction in the shear wavespeed anomaly is just as dramatic, but here the characteristics of high frequency guided waves from deep earthquakes indicate continuity of slab material with thinning of the slab. The thinned slab has less wavespeed contrast within the affected cells and so appears in the tomographic images as a weakened anomaly. The various modes of slab deformation represent different ways in which the subducted material accommodates the strains imposed by the evolution of the geometry of the subduction scenario. Not all significant reductions in wavespeed anomalies represent tears and thus it is important that such interpretations be checked against the characteristics of wave propagation through the zone. © 2010 Elsevier B.V. All rights reserved.
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tears or thinning subduction structures in the Pacific Plate beneath the japanese islands
Physics of the Earth and Planetary Interiors, 2010Co-Authors: B L N Kennett, Takashi FurumuraAbstract:Abstract The nature of a subduction zone at depth is affected by the evolution of its tectonic system, and the geometry of the trench line can change over time due to slab roll-back or the arrival of a distinctive feature with the incoming oceanic lithosphere. The configuration of the Plate has to accommodate such changes with buckling, thinning or the formation of tears depending on the rate of influx to the trench. Tomographic imaging is commonly used to recognise the presence of such tears through marked reductions in wavespeed anomalies in localised zones. A good example is provided by Pacific Plate subduction beneath the Japanese Islands. A horizontal tear in the Plate below 300 km depth can be recognised at the southern end of the Izu-Bonin arc associated with the change in slab morphology to the much steeper Mariana arc. Beneath southern Honshu a break in the fast wavespeeds associated with the Pacific Plate has been described as a tear based on the evidence of converted phases from the edge of the zone and tensional focal mechanisms for seismic events in the tear zone. In the north, close to the Hokkaido bend in the subduction zone, the reduction in the shear wavespeed anomaly is just as dramatic, but here the characteristics of high frequency guided waves from deep earthquakes indicate continuity of slab material with thinning of the slab. The thinned slab has less wavespeed contrast within the affected cells and so appears in the tomographic images as a weakened anomaly. The various modes of slab deformation represent different ways in which the subducted material accommodates the strains imposed by the evolution of the geometry of the subduction scenario. Not all significant reductions in wavespeed anomalies represent tears and thus it is important that such interpretations be checked against the characteristics of wave propagation through the zone.