Out-of-sequence Thrust Research Articles

Deformation history and processes during accretion of seamounts in subduction zones: The example of the Durkan Complex (Makran, SE Iran)

The Durkan Complex is a tectonic element of the Makran Accretionary Prism (SE Iran) that includes fragments of Late Cretaceous seamounts. In this paper, the results of map- to micro-scale structural studies of the western Durkan Complex are presented with the aim to describe its structural and tectono-metamorphic evolution. The Durkan Complex consists of several tectonic units bordered by mainly NNW-striking thrusts. Three main deformation phases (D1, D2, and D3) are distinguished and likely occurred from the Late Cretaceous to the Miocene–Pliocene. D1 is characterized by sub-isoclinal to close and W-verging folds associated with an axial plane foliation and shear zone along the fold limbs. This phase records the accretion of fragments of the seamount within the Makran at blueschist facies metamorphic conditions (160–300 °C and 0.6 – 1.2 GPa). D2 is characterized by open to close folds with sub-horizontal axial plane that likely developed during the exhumation of previously accreted seamount fragments. An upper Paleocene – Eocene siliciclastic succession unconformably sealed the D1 and D2 structures and is, in turn, deformed by W-verging thrust faults typical of D3. The latter likely testifies for a Miocene – Pliocene tectonic reworking of the accreted seamount fragments with the activation of out of sequence thrusts. Our results shed light on the mechanism of accretion of seamount materials in the accretionary prisms, suggesting that seamount slope successions favour the localization and propagation of the basal décollement. This study further confirms that the physiography of the subducting plates plays a significant role in the tectonic evolution of the subduction complexes.

Tectonic Influences on Trench Slope Basin Development via Structural Restoration Along the Outer Nankai Accretionary Prism, Southwest Japan

AbstractThree‐dimensional (3‐D) seismic reflection data and sediment cores record ~2.87 million years of structural and depositional history of a trench slope basin along the outer Nankai accretionary prism, southwest Japan. Numerous mass transport deposits (MTDs) and fault structures are present in the basin. Here, we investigate the links between slope failures, slope basin development, and movement along a prominent out‐of‐sequence thrust (OOST) fault and development of a large anticline. Three two‐dimensional (2‐D) cross sections are restored to ~2.2 Ma using stratigraphic and structural relationships interpreted in the 3‐D data. The restorations are then compared and combined to provide a 3‐D perspective of basin development. We find that total shortening across all faults and folds was accommodated by different displacement styles along multiple branches, with 5.3, 5.5, and 7.3 km of shortening from northeast to southwest over the past ~2.2 Ma. We believe that the majority of this displacement occurred prior to ~1.7 Ma, followed by a dramatic decrease in slip rate within the study area as slip was transferred to the more seaward portions of the prism. In the northeast, deformation is primarily accommodated by the main branch of the OOST and anticlinal faulting, while deformation in the southwest is primarily along deeper branches of the OOST. This differential motion explains the occurrences of various mass wasting events, and lateral differences in trench slope basin geometry within the study area.

Origin of the early Cenozoic belt boundary thrust and Izanagi–Pacific ridge subduction in the western Pacific margin

AbstractThe belt boundary thrust within the Cretaceous–Neogene accretionary complex of the Shimanto Belt, southwestern Japan, extends for more than ~ 1 000 km along the Japanese islands. A common understanding of the origin of the thrust is that it is an out of sequence thrust as a result of continuous accretion since the late Cretaceous and there is a kinematic reason for its maintaining a critically tapered wedge. The timing of the accretion gap and thrusting, however, coincides with the collision of the Paleocene–early Eocene Izanagi–Pacific spreading ridges with the trench along the western Pacific margin, which has been recently re‐hypothesized as younger than the previous assumption with respect to the Kula‐Pacific ridge subduction during the late Cretaceous. The ridge subduction hypothesis provides a consistent explanation for the cessation of magmatic activity along the continental margin and the presence of an unconformity in the forearc basin. This is not only the case in southwestern Japan, but also along the more northern Asian margin in Hokkaido, Sakhalin, and Sikhote‐Alin. This Paleocene–early Eocene ridge subduction hypothesis is also consistent with recently acquired tomographic images beneath the Asian continent. The timing of the Izanagi–Pacific ridge subduction along the western Pacific margin allows for a revision of the classic hypothesis of a great reorganization of the Pacific Plate motion between ~ 47 Ma and 42 Ma, illustrated by the bend in the Hawaii–Emperor chain, because of the change in subduction torque balance and the Oligocene–Miocene back arc spreading after the ridge subduction in the western Pacific margin.

Thermal maturity structures in an accretionary wedge by a numerical simulation

This study investigates the thermal maturity structure of the accretionary wedge along with the thermal history of sediments during wedge formation using a numerical simulation. The thermal maturity, which is described in terms of vitrinite reflectance, is determined using the temperature and duration of exposure based on the particle trajectories within the accretionary wedge. This study revealed the variability in the thermal maturity even though sediments are observed to originate at an identical initial depth and thermal conditions. We propose two end-member pathways of sediment movement in the accretionary wedge during wedge growth: a shallow, low thermal maturity pathway and a deep, high thermal maturity pathway. These shallow path sediments, which move into the shallow portion of the wedge during wedge growth through accretion, rarely experience high temperatures; therefore, their thermal maturity is low. However, the sediments subducted in the deep portion of the wedge experience high temperatures and obtain high thermal maturity as a function of the deep high thermal maturity pathway. Simultaneously, a geological deformation event, such as faulting, defines the steps of thermal maturity. The small step of thermal maturity is formed by the frontal thrusting and can be preserved as a function of the shallow low thermal maturity pathway. However, the step is overprinted and is observed to disappear through the deep high thermal maturity pathway. The large step of thermal maturity is formed by long-term displacement along an out-of-sequence thrust (OOST) in the deep portion of the wedge.

Radiolarian and U–Pb zircon dating of Late Cretaceous and Paleogene Shimanto accretionary complexes, Southwest Japan: Temporal variations in provenance and offset across an out-of-sequence thrust

In this study, we describe radiolarian fossils, analyze sandstone petrography and geochemistry, and perform detrital zircon U–Pb dating on the Late Cretaceous and Paleogene Shimanto accretionary complexes. These data are used to constrain temporal variations in provenance, which are related to volcanic arc activity. We also determine the behavior of the Aki Tectonic Line, which represents an out-of-sequence thrust. Based on lithology and radiolarian assemblages, we subdivide the Late Cretaceous to early Paleocene Mugi Unit into the Mg1, Mg2, and Mg 3 subunits, and the Eocene Naharigawa Unit into the Nh1 and Nh2 subunits. Detrital zircon U–Pb ages indicate that sandstones of the Mugi Unit were sourced mainly from Late Cretaceous to Paleocene igneous rocks. Nh1 sandstone records a single peak age in the Late Cretaceous, whereas Nh2 sandstone preserves multiple peaks from the pre-Jurassic to early Paleocene. The Naharigawa Unit lacks syn-depositional detrital zircons; however, felsic tuff within Nh1 records an age of 48.7 Ma, which is consistent with radiolarian ages. The Nh1 and Nh2 sandstones contain slightly higher Ba and Rb concentrations than those of the Mugi Unit, suggesting that basement had been uplifted and eroded in the source region, prior to deposition. We conclude that temporal variations in sandstone composition within the Late Cretaceous to early Paleocene Shimanto accretionary complex resulted from tectonic events that occurred in response to syn-depositional igneous activity. We infer that the evolution of the Eocene Shimanto accretionary complex was influenced by uplift and erosion of pre-Jurassic basement. Late Cretaceous radiolarians were identified in subunit Mg3 that forms the footwall of the Aki Tectonic Line, which was previously interpreted as a boundary fault between the Late Cretaceous and Paleogene Shimanto accretionary complexes. The Aki Tectonic Line is re-interpreted as an out-of-sequence thrust (OST) that was active under low-grade metamorphic conditions. We classify several OSTs close to the Late Cretaceous and Paleogene Shimanto accretionary complexes, based on contrasts in metamorphic grades across the faults. Within the study area, the Aki Tectonic Line is classified as a Type-2 OST, indicating that it developed within the Cretaceous Shimanto accretionary complex at temperatures of 250–270 °C.

Three-dimensional mapping and kinematic characterization of mass transport deposits along the outer Kumano Basin and Nankai accretionary wedge, southwest Japan

Three-dimensional (3D) seismic data from the southern Kumano Basin of southwest Japan image a nested series of moderately sized mass transport deposits (MTDs) that slid from a slope along the seaward side of the forearc basin. The deposits are dated to be approximately 0.3 to 0.9 Ma. These MTDs are likely linked to the movement along a prominent out-of-sequence thrust (OOST) fault, regionally steeper slopes that would have existed during deposition, and shifts in sedimentation over the past 0.9 Ma. The spatial resolution provided by the 3D seismic data permits the identification of kinematic characteristics and the internal geometries of the MTDs which total over 2.8 km3 in volume and cover more than 59 km2 of the seafloor at various stratigraphic levels. Each MTD is well imaged and exhibits various kinematic indicators while most of the basal glide planes and original headwall scarps above the deposits have been partially or fully eroded by subsequent MTDs. There are at least seven individual deposits that range in volume from 0.005 to 1.16 km3, in area from 0.2 to 21.8 km2; have runouts between 0.55 and 7.9 km, and generally translate downslope from the SE to NW. Basal, internal, and top surface kinematic indicators, such as grooves, thrust and fold systems, and pressure ridges, show that these MTDs originate from a prominent slide scar recognized in the high-resolution regional bathymetry. This, combined with a regionally shifting depocenter and faulting related to the earthquake cycle, points to regional tectonic activity as being the most likely failure trigger for these nested landslides.

Impact of sedimentation on evolution of accretionary wedges: Insights from high-resolution thermomechanical modeling

Syntectonic sedimentation history is a potential cause of differentiated accretionary wedge structures along the subduction margin. Recent efforts to model the role of sedimentation on wedge evolution have highlighted the importance of spatio-temporal history of sedimentation on the evolution of the wedge. Moreover, reconstruction of deformation history of the accretionary wedges using reflection seismic and borehole data have further substantiated the impact of sedimentation on wedge evolution. We conduct several numerical experiments using a high-resolution dynamic 2D thermo-mechanical plate subduction model to systematically investigate and quantify different effects of sedimentation on accretionary wedge evolution. Models with sedimentation suggest migration of deformation to parts of the wedge lying outside the sedimentation zone leading to emergence/reactivation of out-of-sequence thrusts (OOSTs). Frequency and length of new thrust sheets are correlated with sedimentation in the trench. Models undergo a transition period of ~1.5 Myr following the onset of sedimentation, after which they continue to grow under a new steady state. Stabilization of the wedge and increased load on the oceanic plate due to sedimentation create conditions in which smaller wedge-top basins combine to form a large and flat forearc basin. Last but not the least, emergence of OOST in models of accretionary wedges undergoing sedimentation provide important insights in to evolution of potentially tsunamigenic OOSTs like the Megasplay Fault seaward of the Kumano forearc basin.

Late stage exhumation of HP metamorphic rocks, progressive localization of strain, and changes in transport direction, Sambagawa belt, Japan

New geologic mapping delimits major structures associated with the later stages of exhumation of high-pressure (HP) metamorphic rocks of the Sambagawa belt of central Shikoku. The main sets of structures define an inclined N-dipping wedge, bounded by S-vergent thrust faults below and N-vergent normal faults above. Such faults bound every metamorphic mineral zone of the Sambagawa belt. Faults also occur within metamorphic mineral zones, such as the Asemigawa detachment, which is entirely within the oligoclase–biotite zone, the highest-grade metamorphic zone. This fault is the structurally lowest of the N-vergent normal faults. These D2 structures accommodate the exhumation of the higher-grade rocks relative to the lower-grade rocks within the Sambagawa belt, rather than relative to units bounding the belt, so they are termed an “internal” extrusional wedge. The extrusional wedge is truncated to the east against footwall chlorite zone rocks by the Hamegano out-of-sequence thrust (OOST). The OOST divides the traditional chlorite zone into an upper unit consisting of varicolored schist (L tectonite), and a lower unit consisting of regionally extensive pelitic schist. To the north, the metamorphic mineral zones are stacked over the OOST, forming the Tomisato duplex, and these structures are further modified by the Oboke anticline and Tsuneyama syncline. The evolution of exhumation structures of the Sambagawa suggests progressive localization of strain as exhumation proceeded, as well as a change in movement direction from D1 oblique (orogen-subparallel) exhumation to the D2 structures with dip-parallel (orogen-normal) transport that we describe in detail. The evolution of exhumation structures in the Sambagawa belt differs from that of the HP rocks of the Franciscan Complex of California in that the exhumation of the latter is accommodated entirely by discrete faults and the Franciscan lacks internal normal faults. The internal extrusional wedge structure of the Sambagawa HP rocks resembles that of HP rocks of Leti Island of eastern Indonesia.

Drainage migration and out of sequence thrusting in Bhalukpong, Western Arunachal Himalaya, India

In the foreland regions of the Western Arunachal Himalaya (WAH), geological studies along the Kameng river (between Tipi village and the Himalayan Frontal Thrust (HFT)) reveal four levels of unpaired terraces and a paired terrace. In WAH, wrench deformation of HFT zone resulted in a SE propagation of the Balipara anticline and it is suggested that the Mikir high basement controls its orientation. Ages of terrace surfaces from Siwaliks suggest that since the Late Pleistocene, Kameng River migrated at a rate varying between ∼7.5cm/yr in upper reaches and ∼13.5cm/yr towards northeast due to HFT related uplift. In the Brahmaputra plains, luminescence ages of abandoned paleochannel deposits suggest eastward shifting of the Kameng river at an average rate of ∼1m/yr. Field evidences between Bhalukpong and Tipi villages show Pliocene strath and Quaternary terrace surfaces, displaced by faults that do not correspond to the mapped faults in the foreland region. We interpret them as out-of-sequence thrusts (OOSTs). This is the first such report of OOST in the NE Himalaya. Presence of active OOST is inferred by similar age (∼1ka) and differing incision rates of the surface of same terrace (T2b) in adjacent locations. This suggests that OOSTs in the western Arunachal Siwalik are <1ka. Average slip rate and horizontal shortening rate on OOST during the Holocene, are calculated as ∼12mm/yr and 7mm/yr respectively. Thus any estimation of Holocene shortening in the Siwalik therefore, needs to incorporate slip along the OOSTs given that it accommodates a significant amount of N-S compression of the Himalayan fold-and-thrust belt. The reason for OOST in the WAH Siwalik foreland is discussed in terms of the critical wedge dynamics arising from erosion via tectonics-climate interaction. We estimate a minimum slip rate of Siwalik as ∼27mm/yr during the Holocene and suggest acceleration in shortening rates east of Bhutan.

Distribution of resistive and conductive structures in Nankai accretionary wedge reveals contrasting stress paths

In this article, we study the characteristics and spatial distribution of the deformation structures along the Kumano transect of the Nankai accretionary wedge, and use this information to interpret the stress path followed by the sediments. Deformation structures are identified from logging while drilling (LWD) resistivity images of the materials surrounding the drill hole and from 3-dimensional X-ray CT-images of cores acquired during the IODP NanTroSEIZE project. The relative resistivity of the structures identified on logs and the strike, dip, and density of structures identified on CT scan images are measured. The analysis of dip and strike of structures indicates that most of the resistive structures identified on logging data correspond to compactive shear bands. Results also indicate that conductive structures predominate at the toe of the prism and above the main out of sequence thrust, in locations where past and recent erosion occurred. We propose several mechanisms that could explain the relation between erosion and the absence of compactive shear bands. We conclude that sediments followed different stress paths depending on their location within the wedge, and that those differences explain the distribution of deformation structures within the wedge. We also show the coexistence of dilatant and compactant structures in fault zones including the frontal thrust and mega splay fault, and we interpret the coexistence of these structures as a possible consequence of a transient fluid pressure.

Fluid overpressure along an Oligocene out-of-sequence thrust in the Shimanto Belt, SW Japan

Out-of-sequence thrusts (OSTs) exposed in ancient accretionary prisms are considered as fossil analogs of present-day megasplay faults in subduction margins and can provide direct information about the conditions of deformation during thrust activity. In modern as well as in ancient accretionary prisms, first-order megasplay faults or OSTs truncate or merge with faults of lesser importance called second-order OSTs. Structural analysis of the Makinokuchi fault, a branch of an Oligocene to lower Miocene second-order OST in the Tertiary Shimanto Belt of central Kyushu, SW Japan, brings information about the conditions of deformation at the time of thrusting. The studied exposure shows that the fault footwall and, to a much lesser extent, the fault hanging-wall, consist of quartz-cemented syntectonic dilatant hydraulic breccias testifying to pore fluid pressures larger than the least principal stress component. The footwall sandstones are crossed by several centimeters thick quartz veins that merge with the footwall breccias. The continuity between the veins and the breccias suggest that the veins acted as conduits which likely collected fluids from the footwall side sandstones upward and toward the fault. Fluid inclusions indicate that the quartz cementing the breccias and that filling the feeder veins crystallized from similar fluids and under similar pressure and temperature conditions (245–285°C and 5–8km depth). These similarities suggest that the fluids responsible for syn-tectonic hydraulic brecciation were collected from the footwall through the conduits. The fluid inclusion trapping temperatures are close to the temperatures expected to be reached along the seismogenic zone. Our analysis shows that fluid overpressures can play a key role in the growth and activity of second-order OSTs in accretionary prisms and suggests that fluids collected along second-order OSTs or splay faults may flow upward along first-order OSTs or megasplay faults.

Some factors that can explain the particular complexity of the middle-southern Appennine chain

The interpretation of the central-southern sector of the Apennine mountain belt has always been considered as complex and difficult; this is why the literature shows the coexistence of several different structural and paleogeographic models. In the present work some factors are described, which should explain the structural complexity and the coexistence in the literature of such contrasting interpretative models. These factors are the following: a) the presence of several out of sequence thrusts, backthrusts and strike-slip faults; b) the occurrence of repeated tectonic events related to gravity; c) the absence of complete paleogeographic domains in the chain; d) “misleading evidences”; e) difficulty in the choice of the data presented in the literature; f) presence of a rather complex paleography predating deformation. Each of these factors is discussed. Specifically, since the model implying an articulated paleography is not easily accepted in the current literature, here a whole set of data and methodologies are proposed, which allow to hypothesize a Mesozoic and Tertiary paleography, previous to the tectogenesis. The most important methodology used is the study of Miocene and Pliocene deposits, which are in stratigraphic contact with the pre-tectonic substrate. This study allows to recognize a series of stratigraphic-structural units, each of them highlighting a sequence of the most relevant moments in the tectonic evolution (beginning with the flexure, with the concordant transgression in the platform domains and individuation of the foredeeps, deepening with disappearance of the platform facies, mature foredeeps with beginning of arkosic siliciclastic sedimentation, accretion of a portion of the foreland to the chain, first thrust top deposition, etc.). A similar kinematic history develops in different times for each paleogeographic domain, being more ancient in the most internal domains, and more recent in the external ones. By considering the reciprocal position of the platform facies, of basin and basin transition, and by positioning every kinematic unit in ascending order of the beginning age of the arkosic siliciclastic sedimentation, a quite articulated paleogeographic model is obtained, composed by a whole of six carbonate platforms separated by basins. As a corollary, the possible existence of two distinct tectonic phases is emphaisized. The first must be related to the opening of the Alboran sea, the other to the opening of the Tyrrhenian Sea; probably each of them are characterized by a different subduction angle.

Le front de chevauchement au Jebel Bou El Hanèche-Kalâat Khasba (Tunisie centro-septentrionale). Mise en évidence à partir des donnés géologiques et géophysiques

In Central-northern Tunisia, the septentrional border of Kalâat Khasba trough was considered as a normal fault which has functioned since the Cretaceous to Quaternary periods. A stratigraphic and geophysical recent study of the Kalâat Khasba trough and the Jebel Bou El Haneche fold area, allows us to propose a new tectonic model. The Northern border was reactivated as reverse fault during the Eocene and was considered as a thrust front. A compressive Eocene event resulted in formation of Bou El Haneche fold (fault propagation fold) and in determination of a decollement level situated in the Triassic series. This deformation was controlled by the reactivation of normal faults inherited from a distensive aptien phase. The compressive deformation was amplified by the propagation of this fold and, when a ramp reached the competent carbonates series of late Campanian-lower Maâstrichtien times, the fold was stacked, involving a breakthrough steep limb and creating a second thrust on the North. The order of evolution of these events shows an out of sequence thrust.

Implications of channel flow analogue models for extrusion of the Higher Himalayan Shear Zone with special reference to the out-of-sequence thrusting

The Higher Himalayan Shear Zone (HHSZ) contains a ductile top-to-N/NE shear zone—the South Tibetan detachment system-lower (STDSL) and an out-of-sequence thrust (OOST) as well as a top-to-N/NE normal shear at its northern boundary and ubiquitously distributed compressional top-to-S/SW shear throughout the shear zone. The OOST that was active between 22 Ma and the Holocene, varies in thickness from 50 m to 6 km and in throw from 1.4 to 20 km. Channel flow analogue models of this structural geology were performed in this work. A Newtonian viscous polymer (PDMS) was pushed through a horizontal channel leading to an inclined channel with parallel and upward-diverging boundaries analogous to the HHSZ and allowed to extrude to the free surface. A top-to-N/NE shear zone equivalent to the STDSU developed spontaneously. This also indirectly connotes an independent flow confined to the southern part of the HHSZ gave rise to the STDSL. The PDMS originally inside the horizontal channel extruded at a faster rate through the upper part of the inclined channel. The lower boundary of this faster PDMS defined the OOST. The model OOST originated at the corner and reached the vent at positions similar to the natural prototype some time after the channel flow began. The genesis of the OOST seems to be unrelated to any rheologic contrast or climatic effects. Profound variations in the flow parameters along the HHSZ and the extrusive force probably resulted in variations in the timing, location, thickness and slip parameters of the OOST. Variation in pressure gradient within the model horizontal channel, however, could not be matched with the natural prototype. Channel flow alone presumably did not result in southward propagation of deformation in the Himalaya.

Effect of increased shear stress along a plate boundary fault on the formation of an out-of-sequence thrust and a break in surface slope within an accretionary wedge, based on numerical simulations

We investigated the effect on accretionary wedge structure of increased shear stress, which describes the frictional sliding resistance along a decollement arising from an increase in material friction or reduction in pore pressure. To clarify the nature of the effect, we performed numerical simulations using two models: a Stable Friction model and an Increased Friction model. The Stable Friction model produced a low-angle, smooth, surface slope and an in-sequence thrust, whereas the Increased Friction model produced a break in surface slope (scarp) and an out-of-sequence thrust (OST) that cuts through the thrust sheet. The OST formed via the connection of segments of two adjacent thrusts, and its formation resulted in a change in the thickening mode of the wedge from thrust-sheet rotation and back-thrust activity to underplating. This contrast in thickening mode between the landward high-friction zone and seaward low-friction zone resulted in the formation of a clear break in slope, as the landward zone is steeper than the seaward zone, consistent with critical taper theory. The subduction of a basement slice or seamount can produce similar structures arising from an increase in resistance to basal shear sliding. However the distinctive structures arising in an accretionary wedge as a result of increased shear sliding resistance include a flat basal plane and absence of slope-failure sediments beneath the OST. These structural features are observed in accretionary wedges of the Nankai Trough off Muroto (Japan), the Sunda Strait, and the Barbados Ridge.

Origin and evolution of a splay fault in the Nankai accretionary wedge

Subduction zones are often characterized by wedge-shaped sedimentary complexes—called accretionary prisms—that form when sediments are scraped off the subducting plate and added to the overriding plate. Large, landward-dipping thrust faults can cut through such a prism: these faults, known as 'megasplay faults'1, 2, originate near the top of the subducting plate and terminate at the shallow, landward edge of the prism1, 3, 4, 5, 6. Megasplay faults have been the subject of numerous geological and geophysical studies4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, but their initiation and evolution through time remains poorly constrained. Here we combine seismic reflection data from the Nankai accretionary wedge with geological data collected by the Integrated Ocean Drilling Program (IODP) and find that the splay fault cutting this wedge initiated approx1.95 Million years (Myr) ago in the lower part of the prism as an out-of-sequence thrust (OOST). After an initial phase of high activity, the movement along the fault slowed down, but uplift and reactivation of the fault resumed about 1.55 Myr ago. The alternating periods of high and low activity along the splay fault that we document hint at episodic changes in the mechanical stability of accretionary prisms.

Style of fluid flow and deformation in and around an ancient out‐of‐sequence thrust: An example from the Nobeoka Tectonic Line in the Shimanto accretionary complex, Southwest Japan

AbstractTo understand the characteristics of deformation of an out‐of‐sequence thrust (OST) and the style of fluid flow along it, we investigated the Nobeoka Tectonic Line, which has been interpreted as a deep OST (7–9 km), in the Shimanto accretionary complex, Southwest Japan. The shear zone in the footwall differs significantly in the along‐strike direction not only in thickness, which varied from 100 to 300 m, but also in lithology and mineral vein development. These variations might reflect primarily differences in lithology; that is, the sandstone‐dominant shear zone with a large amount of mineral veins precipitated in microcracks is relatively thick, whereas the shale‐dominant shear zone with a small amount of veins and with textures indicating highly pressurized pore fluid, is thinner. By comparison with characteristics of a shallow OST (3–5 km), we conclude that the shallow OST has experienced repeated brittle failure with rapid slip and focused fluid flow whereas the deep OST has experienced both brittle and ductile deformation, followed by fluid flow of various styles, depending on the lithology.

Prediction of thrusting sequences in accretionary wedges

The main objective is to determine the three stages of the life of a thrust in an accretionary wedge which are the onset of thrusting along its ramp, the development with the construction of the relief, and the arrest because of the onset of another thrusting event. A simple kinematics is proposed for the geometry of the developing thrust fold based on rigid regions separated by velocity discontinuities along which work is dissipated according to the Coulomb criterion. The evolution of the thrust fold satisfies mechanical equilibrium and is optimized at every time of the three stages to provide the least upper bound in tectonic force according to the maximum strength theorem. The development of the thrust or its arrest because of the initiation of another thrust is decided by selecting the event which leads to the least upper bound in tectonic force. The approach is first validated by proving that the critical slope angle αc for the classical triangular wedge is properly captured. It is shown that a perturbation, in the form of an extra relief in this perfectly triangular geometry, leads to the onset of thrusting with the ramp or the back thrust outcropping either at the back or to the front of the perturbation, respectively, for a range of slope dip close to the critical angle αc. The study of normal thrust sequences (from the rear to the front in the wedge toe) reveals that weakening of the ramp, accounted for by changing its friction angle from an initial to a smaller final value, is necessary for each thrust to have a finite life span. This life span is longer with a larger relief buildup for more pronounced weakening. Decreasing the décollement friction angle results in an increase in the number of thrusts in the sequence, each thrust creating milder relief. The normal sequence is ended with the first out of sequence thrust which occurs earlier for smaller weakening over the ramp. The proposed methodology is partly used to construct an inverse method proposed to assess the likeliness for the transfer of activity from the active to the incipient thrust in a section of Nankai's accretionary wedge. The inverse method provides the initial friction angle over the incipient ramp and the final friction angle over the fully active ramp, from the geometry of the corresponding thrusts, and the topography. It is shown that the friction angle over the incipient ramp is most likely to be larger then the one over the active ramp, justifying a key hypothesis needed to predict discrete sequences of thrusting.

Out-of-sequence thrusting in Himalaya: Modification of wedge extrusion and channel flow models

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 130

Plio-Pleistocene rapid uplift process of the Nepal Himalaya revealed from fission-track ages

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 98-99