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1.
The deep-sea trenches that occur between unstable chains of volanic arcs and the ocean floor attract great interest because they are the most inaccessible areas of our planet. Now, direct observations of trench faces have become possible by using deep-sea submersibles. This article describes features of the Nankai Trough and the Japan and Kuril Trenches at depths of up to 6000 m.  相似文献   

2.
Two subducting seamounts under inner trench slopes have been identified around Japan on the basis of magnetic anomalies, morphology and geological structure. The first one is located under the foot of the inner trench slope at the junction between the Japan Trench and the Kuril Trench. Another one occurs beneath the slope slightly seaward of the Tosabae (the basement high at the trench slope break along the Nankai Trough off Shikoku). The magnetic anomalies of seamount origin are accompanied by the characteristic morphology of a forearc wedge i.e., a swell landward and a depression seaward. The seamounts beneath the inner trench slopes have preserved magnetization showing reasonably consistent directions, which suggests that the subducting seamounts have kept roughly their original shapes. The morphology of the forearc wedge can be explained by a subducting seamount on the oceanic crust pushing the forearc material forward and upward. Deformation of the forearc wedge by the subducting seamount extends to the forearc basin. The seamounts are stronger and less deformable than the inner slope material and are not offscraped onto inner trench slopes.

Two other examples of deformed inner trench slopes around Japan which can be explained by subduction of topographic highs are presented. One example is a depression on the foot of the inner trench slope northeast of the junction between the Kyushu-Palau Ridge and the Nankai Trough. Another one is an area of complex morphology of the inner trench slope along the Japan Trench around the Daiichi-Kashima Seamount.  相似文献   


3.
The main structures of a subduction zone are as follows.

1. (1) On the outer wall: faults, formed either by reactivation of the structural grain of the oceanic plate, when the latter is slightly oblique to the trench, or by a new fault network parallel to the trench, or both. The width of the faulted zone is about 50 miles.

2. (2) On the inner wall: either an accretionary prism or an extensional fault network, or both; collapsed structures and slumps are often associated, sometimes creating confusion with the accretionary structures.

3. (3) The overall structure of the trench itself is determined by the shape of the edge of the continental crust or of the island arc. Its detailed structure, however, is related to the oceanic plate, namely when the structural grain of the latter is slightly oblique to the trench, which then takes an “en echelon” form. Collapsed units can fill up the trench which is, in that case, restricted to an irregular narrow depression; the tectonic framework of the trench can be buried under a sedimentary blanket when the sedimentation rate is high and the trench bottom is a large, flat area.

Two extreme types of active margins can be distinguished: convergent compressive margins, when the accretionary mechanism is strongly active; and convergent extensional margins where the accretionary mechanism is absent or only weakly active.

The status of a given margin between these two extreme types is related to the convergence rate of the plates, the dip of the subduction zone, the sedimentation activity and the presence of a continental obstacle, because oceanic seamounts and aseismic ridges are easily subducted.

Examples are taken from the Barbados, Middle America, Peru, Kuril, Japan, Nankai, Marianna, Manila, New Hebredes and Tonga trenches.  相似文献   


4.
A theory of two-dimensional geothermic problems is elaborated by the active temperature function at the vertical contact of two horizontally layered media. The approach offered before for oceanic ridges is extended to the case of continental margins and the upper part of a descending slab, i.e. “sink”, in island-arc areas. It is assumed that the plate motion in the oceanic area exists; in a descending area it is directed downward but remains zero on a continental side. Mathematically it symbolizes a “source—span—sink” thermal model. Numerical parameters are given for a theoretical thermal model of the heat-flow profile across the Kuril island arc, from the trench through Iturup Island, Sakhalin Island and the Tatarian Trough.  相似文献   

5.
ABSTRACT

We quantify flexural deformation of subducting oceanic plates at a global array of 15 ocean trenches, using a new approach of modelling spatial variations in flexural bending shape and curvature. The investigated trenches are chosen to represent a diverse range of subducting plate age of 24–150 Ma, including the Middle America, Peru, Chile, West and East Aleutian, Sumatra, North and South Philippine, Tonga, Kermadec, Kuril, Japan, Izu-Bonin, and South and North Mariana. The studied trenches show systematic intra- and inter-trench variability in the calculated flexural bending curvature, stress distribution, extensional brittle yield zone, and effective elastic plate thickness Te of the subducting plates. We find that subducting plate age is a critical factor controlling the bending curvature and the corresponding extensional yield zone. The width, depth, and area of the extensional yield zone are all calculated to increase systematically with the subducting plate age. The newly-developed curvature analysis can yield continuously varying apparent Te(x) from the trench axis to outer rise. The calculated extensional yield zones from the curvature analysis are in general consistent with the observed normal faulting earthquakes of magnitude≥6.0 at the 15 global trenches. Our analyses also reveal that the five deepest regions of the global trenches, i.e. the Challenger, Horizon, Serena, and Scholl Deeps and Galathea Depth, are associated with relatively large flexural bending and calculated yield zones comparing to their respective adjacent trench segments. The Serena and Challenger Deeps of the S. Mariana trench are calculated to have the largest flexural bending among the five deeps.  相似文献   

6.
Large benthic chemosynthetic communities have been observed at four main locations during the Kaiko submersible dives in the Japanese trenches. They appear to be associated with venting along fractures. The first site for our observation was along the Japan and Kuril trenches where the continental margin is eroded by the subducting plate and collapses into the trench. The benthic communities there seem to be related to tension gashes parallel to the subduction vector. The other communities were found on the toe of the Nankai accretionary prism, along the frontal thrust and tension gashes. The temperature anomaly associated with one of the communities is modeled to constrain the upward flow of interstitial water. As the anomaly has a small spatial extent and as the peak thermal gradient is high, the best fitting model is to be found in a vertical upward flow at a velocity of 100 m/yr in a cylindrical conduit leading out of an underlying shallow thrust.  相似文献   

7.
基于高精度机载Li DAR数据在GIS平台的地貌因子渲染分析,对海原断裂老虎山段松山地区古地震研究点进行高精度大比例尺(1∶1000)地貌填图,勾勒出研究点微地貌空间展布和断裂高精度几何形态。通过对松山古地震研究点2个新探槽的开挖,结合细致的探槽解译、地震事件识别与分期、年代学样品测试,得出5次37380±880BP以内的不连续古地震序列。通过对比此处已经开挖的各自相距不足150m、分布于断裂同一段落的4个古地震探槽的微地貌位置、沉积特征和地震事件信号强弱,发现即使相距不远,不同微地貌位置古地震探槽揭示的古地震现象也会有显著差别。这种差别凸显了古地震研究结果,如揭示的事件证据和个数等与探槽点位置的选取有较强的依赖性。综合对比分析表明,较低的地势、低能静水环境、高沉积速率、细粒的沉积物源区及连续的沉积环境是走滑断裂上开展古地震研究的优选地貌位置。实例表明,基于高精度地形数据对研究点开展精细地貌填图揭示微地貌时空演化,从而在探槽开挖前对古地震研究点的构造地貌优劣进行充分评价是提高古地震研究质量的必要程序,同时也显示出高精度机载Li DAR数据在活动构造研究中的重要新应用。  相似文献   

8.
Altimeter geoid profiles crossing trenches and island arcs typically exhibit a long-wavelength increase in geoidal height approaching the trench which, in many cases, reaches a maximum over the back-arc area. Bathymetry profiles across trenches show a similar regional increase in the mean depth of the ocean floor behind the trench. Filters corresponding to Airy and Pratt models of isostatic compensation were applied to bathymetry profiles crossing seven different trench systems in order to estimate how much of the observed geoid variation in these regions could be attributed to isostatically-compensated ocean-floor topography. The results indicate that short-wavelength, high-amplitude variations in the geoid, which in some cases account for over 50% of the total observed amplitude variation, can be reasonably reproduced assuming a Pratt model of isostatic compensation. An additional component of the geoid arises from the uncompensated outer rise seaward of the trench. It is therefore concluded that a large part of the geoid signal over trenches and island arcs may be related to variations in sea-floor topography. This topographic component should be removed from altimeter geoid profiles before using the data to infer details of the deeper structure of subduction zones.  相似文献   

9.
Seabeam, seismic and submersible surveys took place during the Kaiko Project and revealed significant compressive deformation at the northeastern end of the Philippine Sea plate, related to the recent collision of the Izu-Ogasawara Arc against Central Japan. Intraoceanic thrusting at the base of the Zenisu Ridge, a linear topographic high running a few tens of kilometers south of the Nankai Trough, is supported by tectonic, magnetic and gravimetric data. We investigate the formation of the Zenisu Ridge in terms of compressive mechanical failure of a thin elastic-perfectly plastic plate, subducting at a trench and subject to a regional compressive axial force. The rheological envelope concept is used throughout the numerical calculations. Based on a detailed study of flexure of the present-day bending far from the deformation zone, we evaluate the bending forces involved: the bulge is 120 to 150 m high and the compressive stress all along the Nankai Trough is about −100 MPa. In the Zenisu Ridge area, an additional compressive stress is superimposed due to the nearby collision at Izu-Peninsula. We compute the vertical distribution of the deviatoric stress before failure and find that the deviatoric stress is maximum at a depth of 20–25 km in the trench area, and again at the surface 60 to 100 km seaward, in the vicinity of the bulge. The development of a thrust joining these two maxima through the entire thickness of the lithosphere is discussed. The model predicts that the formation of the Zenisu Ridge did not occur before 4 Ma and is caused by progressive tectonic uplift due to the redistribution of bending stresses as the ridge approaches the Nankai Trough.  相似文献   

10.
The peculiarities of catastrophic earthquakes that occurred in the Northwest Pacific region on January 13, 2007, and January 15, 2007, east of the Kuril Islands and the Tohoku megaearthquake of March 11, 2011, east of Japan are considered and analyzed. It is revealed that these earthquakes, apart from the fact that they occurred in the transition zone from the Pacific to island arcs and the Eurasian continent, have common features and similar characteristics. The seismotectonic slip in the source of the chronologically first event, the 2006 Kuril earthquake, was a gentle thrust, while that of the second event of January 13, 2007, was a downthrow beneath the Kuril deep trench.  相似文献   

11.
Blue-schists are the product of an eo-alpine metamorphic event in former trenches (foredeeps), under conditions of high confining pressures and relatively low temperatures. The metamorphic sedimentary rock sequences are generally associated with basic to ultra-basic rocks of the ophiolitic suite (and their metamorphic equivalents), both appearing in belts of partly plastically deformed, partly highly imbricated Pennine-type nappes.The metamorphism and tectonization of the foredeeps (trenches) with their sedimentary and igneous contents occurred during the overriding of major Austroalpine nappes from the concave (inner) side of the orogenic arcs. The foreland is passively subducted during this major orogenic phase (the ‘Flysch phase’ of Alpine-type orogeny).In those cases where the foreland is a continental lithosphere (such as the foreland of the Alpine belt in the Mediterranean) the subduction is less deep than in those cases where the “foreland” is formed by an oceanic lithosphere (such as in the case of the Japan arc).The geological setting of blue-schists in the Alpine system of the western Mediterranean shows that the load pressures produced by the Austroalpine nappes were generally lower than the confining pressures required by experimental data. Tectonic overpressures, produced during phases of high strain-rates which temporarily sealed off the pore fluids, may have produced the supplementary amount of confining pressure. An additional advantage of this hypothesis is, that it accounts for the occurrence of glaucophane-type metamorphism without such unusually low thermal gradients as those prognosticated by petrological experiments.Circum-Pacific blue-schist belts might be the result of deep subduction. However, the problem of rapid exhumation is then the stumbling block for the geodynamic model of plate-tectonics (Model I). The field data of observation do not confirm the expectation of this model, that subsequently dozens of kilometers of isostatic uplift occurred, exposing erosion levels from such great depths.On the other hand, the model of mantle-diapirism (Model II) suggests, that even if the trench deposits were subducted to great depths (passive subduction) upthrusting movements radiating from ‘Stockwerke’ of gravitational instability would drag up the high-P, low-T metamorphism immediately after its formation, in one and the same major phase of orogeny (flysch phase).The geological case-history of the Japan arc is analysed and its diagnostic facts are compared with the predictions (prognoses) of Model II. This verification leads to a confirmation of the functional correctness of this model of mantle-diapirism.A geodynamic analysis of the tectonic evolution and the present structural and geophysical situation of the Japanese realm leads to the conclusion that the driving forces of orogeny and seismicity are produced by stress-fields of gravitational instabilities that may occur in various structural levels (“Stockwerke”). These Stockwerke range in depth from near-surface to the top of the lower mantle, that is over a range about one thousand kilometers. This is a more complicated, but yet more realistic interpretation of the driving forces of orogeny, than the oversimplified views of plate-tectonics.  相似文献   

12.
A.J. Vroman 《Tectonophysics》1981,80(1-4):271-283
The formation of the major rift-valleys is proposed to have been triggered off by the E—W oriented tensional “wave” caused by the slow rotation of the equatorial bulge passing as a stretching hoop through the Earth (Paleozoic—Mesozoic). This ‘wave’ follows the wandering of the polar axis through a great circle (e.g. Creer et al., 1969). The polar wandering is regarded as the readjustment of the Earth's rotational instability caused by the growth of a “weight” fixed on the surface of the Earth and endeavouring to increase its moment of inertia until the weight rotates on the new equator (Gold, 1950). This weight, which must topple the Earth through its fixed spacial axis of rotation, may be slowly developing Pangea. The “wave” of E—W tension is imposed on zones already under E—W tension, e.g., crests of N—S running welts, alias “craton ridges”. The intruding asthenosphere expands the crests and fractures them along tensional rift-valleys. These rifts may develop as spreading centers by gliding of the plates over a lubricating basalt magma.

The idea proposed by R. Schweickert (pers. commun., 1979) that the lithosphere is decoupled from the asthenosphere to an extent that the shell may rotate as a separate unit (as a means to explain how fixed plumes move in unison under the “roll” of the lithosphere) is dismissed. The subducted slabs act as braking flaps and cannot overcome the friction against the asthenosphere. The “roll” would be too young (50 m.y.), because the polar wandering according to Creer is much older.  相似文献   


13.
This paper addresses the geotechnical engineering problem of evaluating the ultimate bearing capacity of a strip foundation resting upon a reinforced soil, by means of the yield design homogenization approach. The analysis is notably focused on the determination of the macroscopic strength criterion of such reinforced soils, where both constituents are purely cohesive, which can be conveniently expressed through the notion of anisotropic cohesion. A comprehensive comparison is made between the classical configuration of reinforcing columns and the more original one of orthogonal reinforcing trenches. Among the most outstanding results of the analysis is the conclusion that the cross trench configuration is notably more efficient in terms of load bearing capacity than the reinforcement by columns, notably when significantly inclined loading is concerned. Copyright © 2014 John Wiley & Sons, Ltd.  相似文献   

14.
Some analytical methods have been proposed for the computation of the stability of plane open trenches supported by bentonite slurry. Nevertheless, there is no validated analytical method for the case of non-plane trenches which are required as stiff watertight corners in excavation pits. Simplified approaches replace the geometry of the corner (non-plane) trench by an equivalent plane trench. In this work it is shown, that some of these approaches are not adequate, as they oversimplify the geometry of the sliding body in the limit state and may overestimate the factor of safety against failure. Using a 3D finite element model it was possible to identify the geometry of the sliding body by simulating the stepwise excavation process of a corner trench in normal consolidated clay and its failure due to the reduction of bentonite support. The influence of the geometry of the trench, drainage conditions, material properties and anisotropy of the soil has been considered. In contrast to the simplified sliding body with the shape of a wedge on which analytical methods are based, the sliding body observed in the simulations has the shape of a quarter of a vertical cylinder, intersected by an inclined plane at its bottom.  相似文献   

15.
Migration of crustal deformation   总被引：2，自引：0，他引：2  
Observations on the migration rates of crustal deformation, as recently discovered in several tectonic areas, such as the south Kanto and central Tohoku districts, Japan and the West Cordillera Mts., Peru, has opened up a new opportunity for the study of crustal dynamics. Briefly, these examples from coastal areas are characterized by migration landwards with a velocity of about 10–100 km/yr. This agrees well with the velocity of mIgration of seismicity as previously known. Dispersion and dissipation of the deformation waveform are also noted as characteristics.Simple extrapolation of the migration path back toward the ocean may locate a possible origin of the event. In the case of the south Kanto district, for example, the deformation front seems to have originated in the early 1950s from the vicinity of the junction of the Japan and Izu—Mariana trenches. The deformation front in the central Tohoku district, on the other hand, is thought to have originated in the northern part of the Japan Trench in the late 1960s. One may suppose that either a repeated irregular aseismic plate motion generates the deformation events, or that it results from a periodic seismic slip at a plate boundary. In the latter case, the 1953 Boso-oki and the 1968 Tokachi-oki earthquakes might be suspected of generating the deformation fronts in the south Kanto and central Tohoku districts respectively.As Scholz speculated, the migration of a deformation front might trigger earthquakes, if it hits areas of high seismic potential. Studies of migration events can contribute significantly to earthquake prediction studies.  相似文献   

16.
The Ontong Java Plateau (OJP) in the western central Pacific is the largest and thickest oceanic plateau and one of a few oceanic plateaus converging on an island arc (Solomon island arc—SIA). To better understand the evolution of the North Solomon trench (NST), active oblique convergence between the OJP and SIA, and late Neogene development of Malaita accretionary prism (MAP), we present 850 km of multichannel seismic reflection data integrated with 7832 km2 of IZANAGI side-scan sonar coverage. We have focussed the study at the transition area between the well-defined northwestern end of the North Solomon trench and a diffusely deformed area where the trench is actively propagating in a northwestward direction. The deeper structure beneath the survey area is discussed by Phinney et al. [Oceanic plateau accretion in the Malaita accretionary prism inferred from multi-channel seismic reflection data, this issue] using deeper penetration, multichannel seismic reflection lines. The serial cross sections provided by multichannel seismic profiling combined with the IZANAGI backscattering imagery provides a time series evolution for the development of the North Solomon trench. The main evolutionary stages include (1) the incipient trench in the northern area marked by a diffuse zone of deformation above a broad arch in the crust. Deeper penetration profiles by Phinney et al. show the bulge is related to a deeper decollement fault that is propagating upward and seaward through the crust. (2) The formation of a continuous thrust front in the central area. Deeper penetration profiles by Phinney et al. show this thrust front is surface expression of the same decollement present at depth to the north. The boundary between the surface trace of the thrust and the diffuse area of deformation in the northern area is inferred as a vertical, high-angle tear fault with left-lateral offset. (3) The formation of a deep, elongate trench which controls gravitationally related slumping and sedimentation around the steep edges of the trench fill basin. The areas to the southeast are those that have undergone convergence for the longest period of time and therefore show better developed trench structures and a reduced width of the MAP. Areas to the northwest have undergone convergence for a shorter period of time and show less developed trench structures and a wide area of the MAP.  相似文献   

17.
Consideration of the paleogeography and large and small structures in the outer part (the Sakawa Fold Belt) of the Paleozoic—Mesozoic geosyncline of Japan suggests that the main part of the Japanese Islands has grown up not from an arc—trench system but from a marginal sea basin—microcontinent system: the Chichibu Geosyncline and the Kurosegawa — Ofunato Island Arc, Minor structures are superposed in a complicated way and they are analyzed in terms of the concepts of tectonic level and multiple deformation.Stratigraphic evidence shows that an early deformation was pre-middle Triassic in the Chichibu Terrain but northwards in the Sambagawa terrain it may have continued until early Cretaceous in relation to minor scale subduction within the marginal basin. A late Cretaceous phase of deformation produced the greatest crustal shortening in the microcontinent area of the Kurosegawa Tectonic Zone and was nearly contemporaneous with the intrusion of granitic rocks in the Ryoke Zone Younger secondary eugeosynclines were developed by progressive encroachment on the arc—trench gap south of the Kurosegawa zone from late Permian times onwards.  相似文献   

18.
Reliable dating is an essential element of palaeoseismological studies, yet whilst a suite of geochronological methods can now provide late Quaternary age control it remains very difficult to date modern events (i.e., those occurring within the last 150 years). This is significant because the starting point for many palaeoseismological investigations is a modern surface-rupturing event, whose geological effects need to be disentangled in trench stratigraphies from palaeoseismic ruptures. Two dating methods which, in combination, can provide robust dating control in recently deposited sediments are the 210Pb and 137Cs dating methods. Here, we test the applicability of using 210Pb and 137Cs to date colluvial sediments exposed in three trenches excavated across an earthquake fault—the Eliki fault, Gulf of Corinth, Greece—which ruptured in an earthquake in 1861. The 210Pb and 137Cs profiles observed in these colluvial sequences are relatively erratic due to the mixed nature of the sediments, i.e., their deposition in an environment where the supply of slope sediments is driven by seasonal rainfall, causing non-uniform sediment accretion and sediment reworking. In one trench, however, 210Pb dating, corroborated by 137Cs dating, indicates that a proposed post-1861 surface colluvial unit has been deposited over the period 1950 AD–present (at a rate of ca. 9 mm/year), and overlies a significantly older unit (>120 years old). The dating control provided here by 210Pb and 137Cs dating corroborates the published interpretation of the trench stratigraphy, and refines the 14C-based estimated dates for the upper unit. At two other trenches 210Pb and 137Cs dating only provided minimum ages (based on the presence or absence of 210Pbexcess and 137Cs). Such approximate ages, however, may still useful in corroborating interpretations made using the trench stratigraphy, or, at sites which have long earthquake recurrence intervals, determining which earthquake event was responsible for a particular bed offset.  相似文献   

19.
李建平  张晓磊  冯世进 《岩土力学》2020,41(9):3131-3138
受季节性降雨的影响,雨季和旱季存在明显的水位波动。为分析地下水位升降对多空沟隔振效果的影响,建立了上覆单相弹性层饱和地基中空沟有限元分析模型。模型中考虑了沟中水的作用,算例分析了等沟深和不等沟深多沟屏障、沟壁倾斜多沟屏障和连续起伏地形屏障在不同水位下的远场隔振效果。计算结果与相应单相弹性地基进行对比,结果表明：土体渗透性低时,多数水位下不利于多沟屏障隔振;沟深为0.3LR（LR为R波波长）的三沟屏障通常可获得较理想的隔振效果（75%隔振率）,但屏障在隔振效果最差的共振水位(0.6LR)下,空沟须贯穿上覆干土层才可获得75%隔振率;递减沟深屏障可有效降低上覆土层共振的不利影响;沟深较浅时,沟壁倾角越大,多沟的隔振效果越好,但对较深多沟的影响小;连续起伏的地形景观可作为一种有效的绿色隔振屏障。  相似文献   

20.
In the Hazara arc region of northern Pakistan, some of the active basements structures buried below a thick, detached sedimentary layer are inferred from the distribution of lineaments and the drainage patterns, as viewed in Landsat satellite imagery and from river profiles.A prominent set of NW-trending lineaments seen on satellite imagery, coincides approximately with the southwest or updip side of the Indus—Kohistan seismic zone (IKSZ) —the most active basement structure of the region, even though this structure is buried beneath and decoupled from a 12 km thick sedimentary layer. The IKSZ has been interpreted as an extension of the Himalayan Basement Thrust, and is also associated with a prominent topographic “step”.Knickpoints on major rivers in the region lie on or north of the IKSZ. All Indus River tributaries, examined north of the IKSZ, show prominent knickpoints, while two tributaries draining south of the IKSZ have no knickpoints. These results suggest ongoing uplift above and north of the IKSZ, and are consistent with the tectonic model obtained from the seismic data.Another prominent lineament set is detected along the north—south section of the Indus River. This set is probably related to the Indus River horst—anticline and associated reentrant.One of the two highest lineament concentrations occurs at the intersection between the NW-trending IKSZ lineament and the N-trending Indus River lineament. The other is along the west bank of the Indus Valley, 25 km north of Tarbela Dam.A topographic ridge (Swabi—Nowshera ridge) appears to be forming along the west side of the Indus River, in the Peshawar Basin. The rising ridge is ponding the Kabul River upstream of Nowshera, where the drainage is braided.  相似文献   

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