Geological record of fluid flow and seismogenesis along an erosive subducting plate boundary

Tectonic erosion of the overriding plate by the downgoing slab is believed to occur at half the Earth's subduction zones. In situ investigation of the geological processes at active erosive margins is extremely difficult owing to the deep marine environment and the net loss of forearc crust to deeper levels in the subduction zone. Until now, a fossil erosive subduction channel-the shear zone marking the plate boundary-has not been recognized in the field, so that seismic observations have provided the only information on plate boundary processes at erosive margins. Here we show that a fossil erosive margin is preserved in the Northern Apennines of Italy. It formed during the Tertiary transition from oceanic subduction to continental collision, and was preserved by the late deactivation and fossilization of the plate boundary. The outcropping erosive subduction channel is approximately 500 m thick. It is representative of the first 5 km of depth, with its deeper portions reaching approximately 150 degrees C. The fossil zone records several surprises. Two décollements were simultaneously active at the top and base of the subduction channel. Both deeper basal erosion and near-surface frontal erosion occurred. At shallow depths extension was a key deformation component within this erosive convergent plate boundary, and slip occurred without an observable fluid pressure cycle. At depths greater than about 3 km a fluid cycle is clearly shown by the development of veins and the alternation of fast (co-seismic) and slow (inter-seismic) slip. In the deepest portions of the outcropping subduction channel, extension is finally overprinted by compressional structures. In modern subduction zones the onset of seismic activity is believed to occur at approximately 150 degrees C, but in the fossil channel the onset occurred at cooler palaeo-temperatures.     

A wide depth distribution of seismic tremors along the northern Cascadia margin

The Cascadia subduction zone is thought to be capable of generating major earthquakes with moment magnitude as large as M(w) = 9 at an interval of several hundred years. The seismogenic portion of the plate interface is mostly offshore and is currently locked, as inferred from geodetic data. However, episodic surface displacements-in the direction opposite to the long-term deformation motions caused by relative plate convergence across a locked interface-are observed about every 14 months with an unusual tremor-like seismic signature. Here we show that these tremors are distributed over a depth range exceeding 40 km within a limited horizontal band. Many occurred within or close to the strong seismic reflectors above the plate interface where local earthquakes are absent, suggesting that the seismogenic process for tremors is fluid-related. The observed depth range implies that tremors could be associated with the variation of stress field induced by a transient slip along the deeper portion of the Cascadia interface or, alternatively, that episodic slip is more diffuse than originally suggested.     

Cenozoic climate change as a possible cause for the rise of the Andes

Causal links between the rise of a large mountain range and climate have often been considered to work in one direction, with significant uplift provoking climate change. Here we propose a mechanism by which Cenozoic climate change could have caused the rise of the Andes. Based on considerations of the force balance in the South American lithosphere, we suggest that the height of, and tectonics in, the Andes are strongly controlled both by shear stresses along the plate interface in the subduction zone and by buoyancy stress contrasts between the trench and highlands, and shear stresses in the subduction zone depend on the amount of subducted sediments. We propose that the dynamics of subduction and mountain-building in this region are controlled by the processes of erosion and sediment deposition, and ultimately climate. In central South America, climate-controlled sediment starvation would then cause high shear stress, focusing the plate boundary stresses that support the high Andes.     

Reflection signature of seismic and aseismic slip on the northern Cascadia subduction interface

At the northern Cascadia margin, the Juan de Fuca plate is underthrusting North America at about 45 mm x yr(-1) (ref. 1), resulting in the potential for destructive great earthquakes. The downdip extent of coupling between the two plates is difficult to determine because the most recent such earthquake (thought to have been in 1700) occurred before instrumental recording. Thermal and deformation studies indicate that, off southern Vancouver Island, the interplate interface is presently fully locked for a distance of approximately 60 km downdip from the deformation front. Great thrust earthquakes on this section of the interface (with magnitudes of up to 9) have been estimated to occur at an average interval of about 590 yr (ref. 3). Further downdip there is a transition from fully locked behaviour to aseismic sliding (where high temperatures allow ductile deformation), with the deep aseismic zone exhibiting slow-slip thrust events. Here we show that there is a change in the reflection character on seismic images from a thin sharp reflection where the subduction thrust is inferred to be locked, to a broad reflection band at greater depth where aseismic slip is thought to be occurring. This change in reflection character may provide a new technique to map the landward extent of rupture in great earthquakes and improve the characterization of seismic hazards in subduction zones.     

Seismic reflection imaging of two megathrust shear zones in the northern Cascadia subduction zone

At convergent continental margins, the relative motion between the subducting oceanic plate and the overriding continent is usually accommodated by movement along a single, thin interface known as a megathrust. Great thrust earthquakes occur on the shallow part of this interface where the two plates are locked together. Earthquakes of lower magnitude occur within the underlying oceanic plate, and have been linked to geochemical dehydration reactions caused by the plate's descent. Here I present deep seismic reflection data from the northern Cascadia subduction zone that show that the inter-plate boundary is up to 16 km thick and comprises two megathrust shear zones that bound a >5-km-thick, approximately 110-km-wide region of imbricated crustal rocks. Earthquakes within the subducting plate occur predominantly in two geographic bands where the dip of the plate is inferred to increase as it is forced around the edges of the imbricated inter-plate boundary zone. This implies that seismicity in the subducting slab is controlled primarily by deformation in the upper part of the plate. Slip on the shallower megathrust shear zone, which may occur by aseismic slow slip, will transport crustal rocks into the upper mantle above the subducting oceanic plate and may, in part, provide an explanation for the unusually low seismic wave speeds that are observed there.     

A satellite geodetic survey of large-scale deformation of volcanic centres in the central Andes

Surface deformation in volcanic areas usually indicates movement of magma or hydrothermal fluids at depth. Stratovolcanoes tend to exhibit a complex relationship between deformation and eruptive behaviour. The characteristically long time spans between such eruptions requires a long time series of observations to determine whether deformation without an eruption is common at a given edifice. Such studies, however, are logistically difficult to carry out in most volcanic arcs, as these tend to be remote regions with large numbers of volcanoes (hundreds to even thousands). Here we present a satellite-based interferometric synthetic aperture radar (InSAR) survey of the remote central Andes volcanic arc, a region formed by subduction of the Nazca oceanic plate beneath continental South America. Spanning the years 1992 to 2000, our survey reveals the background level of activity of about 900 volcanoes, 50 of which have been classified as potentially active. We find four centres of broad (tens of kilometres wide), roughly axisymmetric surface deformation. None of these centres are at volcanoes currently classified as potentially active, although two lie within about 10 km of volcanoes with known activity. Source depths inferred from the patterns of deformation lie between 5 and 17 km. In contrast to the four new sources found, we do not observe any deformation associated with recent eruptions of Lascar, Chile.     

Laboratory models of the thermal evolution of the mantle during rollback subduction

The subduction of oceanic lithosphere plays a key role in plate tectonics, the thermal evolution of the mantle and recycling processes between Earth's interior and surface. Information on mantle flow, thermal conditions and chemical transport in subduction zones come from the geochemistry of arc volcanoes, seismic images and geodynamic models. The majority of this work considers subduction as a two-dimensional process, assuming limited variability in the direction parallel to the trench. In contrast, observationally based models increasingly appeal to three-dimensional flow associated with trench migration and the sinking of oceanic plates with a translational component of motion (rollback). Here we report results from laboratory experiments that reveal fundamental differences in three-dimensional mantle circulation and temperature structure in response to subduction with and without a rollback component. Without rollback motion, flow in the mantle wedge is sluggish, there is no mass flux around the plate and plate edges heat up faster than plate centres. In contrast, during rollback subduction flow is driven around and beneath the sinking plate, velocities increase within the mantle wedge and are focused towards the centre of the plate, and the surface of the plate heats more along the centreline.     

Pliocene eclogite exhumation at plate tectonic rates in eastern Papua New Guinea

As lithospheric plates are subducted, rocks are metamorphosed under high-pressure and ultrahigh-pressure conditions to produce eclogites and eclogite facies metamorphic rocks. Because chemical equilibrium is rarely fully achieved, eclogites may preserve in their distinctive mineral assemblages and textures a record of the pressures, temperatures and deformation the rock was subjected to during subduction and subsequent exhumation. Radioactive parent-daughter isotopic variations within minerals reveal the timing of these events. Here we present in situ zircon U/Pb ion microprobe data that dates the timing of eclogite facies metamorphism in eastern Papua New Guinea at 4.3 +/- 0.4 Myr ago, making this the youngest documented eclogite exposed at the Earth's surface. Eclogite exhumation from depths of approximately 75 km was extremely rapid and occurred at plate tectonic rates (cm yr(-1)). The eclogite was exhumed within a portion of the obliquely convergent Australian-Pacific plate boundary zone, in an extending region located west of the Woodlark basin sea floor spreading centre. Such rapid exhumation (> 1 cm yr(-1)) of high-pressure and, we infer, ultrahigh-pressure rocks is facilitated by extension within transient plate boundary zones associated with rapid oblique plate convergence.     

Subduction dynamics and the origin of Andean orogeny and the Bolivian orocline

The building of the Andes results from the subduction of the oceanic Nazca plate underneath the South American continent. However, how and why the Andes and their curvature, the Bolivian orocline, formed in the Cenozoic era (65.5?million years (Myr) ago to present), despite subduction continuing since the Mesozoic era (251.0-65.5?Myr ago), is still unknown. Three-dimensional numerical subduction models demonstrate that variations in slab thickness, arising from the Nazca plate's age at the trench, produce a cordilleran morphology consistent with that observed. The age-dependent sinking of the slab in the mantle drives traction towards the trench at the base of the upper plate, causing it to thicken. Thus, subducting older Nazca plate below the Central Andes can explain the locally thickened crust and higher elevations. Here we demonstrate that resultant thickening of the South American plate modifies both shear force gradients and migration rates along the trench to produce a concave margin that matches the Bolivian orocline. Additionally, the varying forcing along the margin allows stress belts to form in the upper-plate interior, explaining the widening of the Central Andes and the different tectonic styles found on their margins, the Eastern and Western Cordilleras. The rise of the Central Andes and orocline formation are directly related to the local increase of Nazca plate age and an age distribution along the margin similar to that found today; the onset of these conditions only occurred in the Eocene epoch. This may explain the enigmatic delay of the Andean orogeny, that is, the formation of the modern Andes.     

Evolution and diversity of subduction zones controlled by slab width

Subducting slabs provide the main driving force for plate motion and flow in the Earth's mantle, and geodynamic, seismic and geochemical studies offer insight into slab dynamics and subduction-induced flow. Most previous geodynamic studies treat subduction zones as either infinite in trench-parallel extent (that is, two-dimensional) or finite in width but fixed in space. Subduction zones and their associated slabs are, however, limited in lateral extent (250-7,400 km) and their three-dimensional geometry evolves over time. Here we show that slab width controls two first-order features of plate tectonics-the curvature of subduction zones and their tendency to retreat backwards with time. Using three-dimensional numerical simulations of free subduction, we show that trench migration rate is inversely related to slab width and depends on proximity to a lateral slab edge. These results are consistent with retreat velocities observed globally, with maximum velocities (6-16 cm yr(-1)) only observed close to slab edges (<1,200 km), whereas far from edges (>2,000 km) retreat velocities are always slow (<2.0 cm yr(-1)). Models with narrow slabs (< or =1,500 km) retreat fast and develop a curved geometry, concave towards the mantle wedge side. Models with slabs intermediate in width ( approximately 2,000-3,000 km) are sublinear and retreat more slowly. Models with wide slabs (> or =4,000 km) are nearly stationary in the centre and develop a convex geometry, whereas trench retreat increases towards concave-shaped edges. Additionally, we identify periods (5-10 Myr) of slow trench advance at the centre of wide slabs. Such wide-slab behaviour may explain mountain building in the central Andes, as being a consequence of its tectonic setting, far from slab edges.     

Coseismic and postseismic slip of the 2011 magnitude-9 Tohoku-Oki earthquake

Most large earthquakes occur along an oceanic trench, where an oceanic plate subducts beneath a continental plate. Massive earthquakes with a moment magnitude, M(w), of nine have been known to occur in only a few areas, including Chile, Alaska, Kamchatka and Sumatra. No historical records exist of a M(w) = 9 earthquake along the Japan trench, where the Pacific plate subducts beneath the Okhotsk plate, with the possible exception of the ad 869 Jogan earthquake, the magnitude of which has not been well constrained. However, the strain accumulation rate estimated there from recent geodetic observations is much higher than the average strain rate released in previous interplate earthquakes. This finding raises the question of how such areas release the accumulated strain. A megathrust earthquake with M(w) = 9.0 (hereafter referred to as the Tohoku-Oki earthquake) occurred on 11 March 2011, rupturing the plate boundary off the Pacific coast of northeastern Japan. Here we report the distributions of the coseismic slip and postseismic slip as determined from ground displacement detected using a network based on the Global Positioning System. The coseismic slip area extends approximately 400?km along the Japan trench, matching the area of the pre-seismic locked zone. The afterslip has begun to overlap the coseismic slip area and extends into the surrounding region. In particular, the afterslip area reached a depth of approximately 100?km, with M(w) = 8.3, on 25 March 2011. Because the Tohoku-Oki earthquake released the strain accumulated for several hundred years, the paradox of the strain budget imbalance may be partly resolved. This earthquake reminds us of the potential for M(w)?≈?9 earthquakes to occur along other trench systems, even if no past evidence of such events exists. Therefore, it is imperative that strain accumulation be monitored using a space geodetic technique to assess earthquake potential.     

Late Mesozoic–Cenozoic de′collement structure and its deep geological background in western Shandong, China

Data from seismic reflection profiles, drilling, stratigraphy, structural deformation studies and physical rock properties reveal the existence of de′col1ement structures in both shallow and deep levels in western Shandong, China. The most outstanding shallow de′col1ement structures occur along the regional unconformity surface between the Cambrian and Archean, and the disconformity surface between the Carboniferous and Ordovician. The de′col1ement structure surface manifests as a fault zone with cataclastic rocks and asymmetrical folds. Some of the cataclastic rocks underwent dynamic metamorphism and hydrothermal alteration, including silicification, marbleization and specularite mineralization. Above the de′col1ement structure, the bottom of the Cambrian might be missing or overprinted because of de′collement. The striations, asymmetrical folds and boudinage structures indicate the direction of the main de′collement to the NNW and NNE. A deep level de′collement structure occurs at a depth of 12–22 km and up to 30 km distance to the south. The early Cretaceous and Eocene are two main periods of activity, with the Cretaceous de′collement probably initiated by mantle upwelling derived from subduction and collision of the Yangtze Plate with the North China Plate along the Tancheng-Lujiang Fault in the late Triassic–early Jurassic. This circumstance implies a multidirection of subduction and collision of these two plates in the early late Mesozoic.     

Late Mesozoic-Cenozoic décollement structure and its deep geological background in western Shandong,China

Data from seismic reflection profiles,drilling,stratigraphy,structural deformation studies and physical rock properties reveal the existence of drcollement structures in both shallow and deep levels in western Shandong,China.The most outstanding shallow décollement structures occur along the regional unconformity surface between the Cambrian and Archean,and the disconformity surface between the Carboniferous and Ordovician.The drcollement structure surface manifests as a fault zone with cataclastic rocks and asymmetrical folds.Some of the cataclastic rocks underwent dynamic metamorphism and hydrothermal alteration,including silicification,marbleization and specularite mineralization.Above the decollement structure,the bottom of the Cambrian might be missing or overprinted because of decollement.The striations,asymmetrical folds and boudinage structures indicate the direction of the main décollement tothe NNW and NNE.A deep level decollement structure occurs at a depth of 12-22 km and up to 30 km distance to the south.The early Cretaceous and Eocene are two main periods of activity,with the Cretaceous decollement probably initiated by mantle upwelling derived from subduction and collision of the Yangtze Plate with the North China Plate along the Tancheng-Lujiang Fault in the late Triassicearly Jurassic.This circumstance implies a multidirection of subduction and collision of these two plates in the early late Mesozoic.(C) 2009 National Natural Science Foundation of China and Chinese Academy of Sciences.Published by Elsevier Limited and Science in China Press.All rights reserved.     

秦岭北缘巨型陆内俯冲带的深部物理状态

秦岭造山带是一个复合型大陆造山带。燕山末期——喜马拉雅初期由于华北板块相对于秦岭造山带的俯冲,在秦岭北缘形成巨型陆内俯冲带。反射地震剖面揭示该带为一向南倾斜的强反射波组带,并且在两侧显示出明显的差异;大地电磁测深剖面中该带表现为略向南倾的低阻带。南侧为高阻体,北侧为多层结构;流变学特征揭示,该俯冲带南部相似于造山带的核带,北部则相同于中、新生代的大陆汇聚带,充分证明了秦岭北缘巨型陆内俯冲带的存在。     

汤加-克马德克俯冲带的地质构造与地震火山特征

2022年1月15日西南太平洋的洪阿哈阿帕伊岛海底火山发生了爆炸式的剧烈喷发，吸引了全球的关注。洪阿哈阿帕伊岛海底火山位于汤加-克马德克俯冲带，综合前期研究结果，对汤加-克马德克俯冲带的地质构造特征、地震和火山分布进行初步分析，发现：（1）从汤加-克马德克俯冲带弧前向海方向直到俯冲的太平洋板块，构造上主要表现为大规模正断层。（2）路易斯维尔海山链的俯冲将汤加-克马德克俯冲带分为北部的汤加俯冲带和南部的克马德克俯冲带，沿汤加俯冲带板块汇聚率为67～84 mm/a，沿克马德克俯冲带板块汇聚率为41～58 mm/a，板块俯冲速度的差异造成汤加俯冲带和克马德克俯冲带目前俯冲深度的不同。（3）在路易斯维尔海山链以北，太平洋板块上覆沉积物厚度不足0.4 km，而在南侧达到1 km左右，由于俯冲板块上覆沉积物厚度的差异而造成北部的汤加俯冲带和南部的克马德克俯冲带孕育地震能力的差异。这些认识对研究该俯冲带的火山喷发机制、大地震成因机理及其灾害风险具有重要意义。     

Static slip model of the M w 9.0 Tohoku (Japan) earthquake: Results from joint inversion of terrestrial GPS data and seafloor GPS/acoustic data

Based on co-seismic displacements recorded by terrestrial GPS stations and seafloor GPS/acoustic stations, the static slip model of the 2011 Mw 9.0 Tohoku earthquake was determined by inverting the data using a layered earth model. According to a priori information, the rupture surface was modeled with a geometry that is close to the actual rupture, in which the fault dip angle increases with depth and the fault strike varies with the trend of the trench. As shown by the results inferred from the joint inversion, the "geodetic" moment is 3.68 × 10 22 Nm, corresponding to Mw 9.01, and the maximum slip is positioned at a depth of 13.5 km with a slip magnitude of 45.8 m. Rupture asperities with slip exceeding 10 m are mainly distributed from 39.6 to 36.97°N, over a length of almost 240 km along the trench. The slip was mostly concentrated at depths shallower than 40 km, up-dip of the hypocenter. "Checkerboard" tests reveal that a joint inversion of multiple datasets can resolve the slip distribution better than an inversion with terrestrial GPS data only-especially when aiming to resolve slip at shallow depths. Thus, the joint inversion results obtained by this work may provide a more reliable slip model than the results of other studies that are only derived from terrestrial GPS data or seismic waveform data.     

内蒙古达茂旗乌德缝合带特征及大地构造意义

乌德缝合带位于华北地块和北缘增生带之间，显示早古生代属于典型的沟－弧－盆体系。与碰撞造山作用同时，形成区域性构造混杂带，地层学和同位素地质年代资料表明，造山作用发生于加里东期，造山作用的动力学过程主要表现为由北向南的洋壳俯冲和向北的陆壳仰冲，并伴随右旋剪切滑移运动。     

Indonesian earthquake: earthquake risk from co-seismic stress

Following the massive loss of life caused by the Sumatra-Andaman earthquake in Indonesia and its tsunami, the possibility of a triggered earthquake on the contiguous Sunda trench subduction zone is a real concern. We have calculated the distributions of co-seismic stress on this zone, as well as on the neighbouring, vertical strike-slip Sumatra fault, and find an increase in stress on both structures that significantly boosts the already considerable earthquake hazard posed by them. In particular, the increased potential for a large subduction-zone event in this region, with the concomitant risk of another tsunami, makes the need for a tsunami warning system in the Indian Ocean all the more urgent.     

西太平洋俯冲带岩石圈变形研究

全球海洋岩石圈的最大弯曲与大地震发生在俯冲带。当弯曲应力超过岩石承受范围，就会产生正断层和地震，海水沿着正断层进入上地幔并发生蛇纹石化，引发浅源地震并可能造成灾难性海啸。选取西太平洋最具代表性的日本、伊豆-小笠原、马里亚纳和雅浦俯冲带以及汤加-克马德克俯冲带，归纳近些年的地球物理观测及地球动力学模拟的结果，对比分析了不同俯冲带挠曲正断层的分布特征，并探讨了俯冲板块变形与地震之间的相关性,以揭示俯冲板片弯曲变形及相应的正断层与潜在板块水化特征。     

Seismic tomography of the northwest Pacific and its geodynamic implications

High-resolution tomographic images across Japan Trenh-Changhai Mountains-lDong Ujimqinqi are displayed, showing the morphological feature of the subducted slab in the norhwestem Pacific margin and the eharaeter istics of lithosphere stmctures under the Changhai Mountains and the Da Hinggan Mnuntains. The Pacific plate began to penetrate into the deeper mantle after it subducted to the 660 km discontinuity with an underthmsting angle of 26°, but did not continue to mnve furrther westward. In contrast, there appeared a remarkable thermal upwelling zone to the west of the downward plate. In addition, the evidence frnm the subduction time and time lag between the subduetion and eon sequent magmatism indicates that there is no direct genetic correlatiom between the Mesoznic magmatism in eastern China ami subduction of the Pacific plate. In this work. we also emphasize that what the tomographic images reflect is the pre sent structure in the deep earth interior, which should preserve some Mesozoic lithospheric structure characteristics. In summary, we attribute the Mesozoic intense magmatic evolution in north China to the intraplate asthenosphere upwelling zone.