Imbricate Structure Research Articles

Paleozoic structural deformation of Bachu uplift, Tarim basin of Northwest China: Implications for plate drifting

Based on high-resolution 2D seismic profiles, the Paleozoic structural deformation characteristics of Bachu (巴楚) uplift of northwestern Tarim basin, NW China, are exhibited in this article. The deformation happened during three main geological periods: the end of Middle-Late Ordovician (O2–3), the end of Early-Middle Devonian (D1–2), and the end of Late Permian (P2). In the Bachu uplift, there developed a series of NW-trending thrust faults and imbricate structures due to the effect of the NW-SE compression stress towards the end of Middle-Late Ordovician (O2–3) (middle Caledonian movement), and there developed some NNE-trending thrust faults and fault blocks under the control of the NEE-SWW compression stress at the end of Early-Middle Devonian (D1–2) (early Hercynian movement). However, at the end of Late Permian (P2) (late Hercynian movement), some NE-trending thrust faults and associated folds developed as a result of the NE-SW compression stress. The first-stage (O2–3) deformation is obviously more violent than those of the latter two stages (D1–2 and P2), which implies that the Tarim plate drifted quickly to the north at around the same time basin.

Discovery of Compressional Structure in Wuerxun–Beier Sag in Hailar Basin of Northeastern China and Its Geological Significance

The Hailar Basin is a Mesozoic-Cenozoic basin, which is superimposed on the Paleozoic collision orogenic belt of Inner Mongolia–Greater Hinggan Mountain. The basin, trending north-east, consists of five first-order structural units (three depressions and two uplifts), they are, from west to east, the Zhalenuoer Depression, the Cuogang Uplift, the Beier Lake Depression, the Bayanshan Uplift and the Huhe Lake Depression, respectively. The Wuerxun Sag and the Beier Sag are two second-order structural units in the south Beier Lake Depression. Research indicates that in the Early Cretaceous Epoch, the Wuerxun-Beier sag underwent two long-term compressions: NW-SE trending during the Damoguaihe Period to the early Yimin Period, and near EW trending during the end of the Yimin Period, respectively. The compressions led to the inversion of some depression-controlling normal faults in the extensional faulted depression, and formed new fault-propagation fold, fault-bend fold, imbricate structure, duplex structure, pop-up structure, and triangular zone etc. Because of the differences in geometric and kinematic features and the variance in the ratio of the uplift rate of compressional structures to the deposition rate of isostructural growing strata in space, syn-compression strata show different depositional features in different structural positions. These structural features are very similar to those of the orogenic belt. The discovery of two phases of compressional structures in the Wuerxun–Beier Sag proved that the crust of Northeast China was not always in the state of extension and thinning in the Early Cretaceous, but showed the features of superimposition and thickening during the two compression periods, and thus formed the structural framework of the alternative distribution of the compression-depression basins and the intra continental (intra plate) orogenic belt in space.

Coseismic reverse- and oblique-slip surface faulting generated by the 2008 Mw 7.9 Wenchuan earthquake, China

The Mw 7.9 Wenchuan, China, earthquake ruptured two large thrust faults along the Long-menshan thrust belt at the eastern margin of the Tibetan Plateau. This earthquake generated a 240-km-long surface rupture zone along the Beichuan fault and an additional 72-km-long surface rupture zone along the Pengguan fault. Maximum vertical and horizontal offsets of 6.5 m and 4.9 m, respectively, were measured along the Beichuan fault. A maximum vertical offset of 3.5 m was measured along the Pengguan fault. Coseismic surface ruptures, integrated with aftershocks and industry seismic profiles, show that two imbricate structures have ruptured simultaneously, resulting in the largest continental thrust event ever documented. Large oblique thrusting observed during this earthquake indicates that crustal shortening is an important process responsible for the high topography in the region, as everywhere along the edge of Tibetan Plateau.

Morphotectonics of the central Muertos thrust belt and Muertos Trough (northeastern Caribbean)

Multibeam bathymetry data acquired during the 2005 Spanish R/V Hespérides cruise and reprocessed multichannel seismic profiles provide the basis for the analysis of the morphology and deformation in the central Muertos Trough and Muertos thrust belt. The Muertos Trough is an elongated basin developed where the Venezuelan Basin crust is thrusted under the Muertos fold-and-thrust belt. Structural variations along the Muertos Trough are suggested to be a consequence of the overburden of the asymmetrical thrust belt and by the variable nature of the Venezuelan Basin crust along the margin. The insular slope can be divided into three east–west trending slope provinces with high lateral variability which correspond to different accretion stages: 1) The lower slope is composed of an active sequence of imbricate thrust slices and closed fold axes, which form short and narrow accretionary ridges and elongated slope basins; 2) The middle slope shows a less active imbricate structure resulting in lower superficial deformation and bigger slope basins; 3) The upper slope comprises the talus region and extended terraces burying an island arc basement and an inactive imbricate structure. The talus region is characterized by a dense drainage network that transports turbidite flows from the islands and their surrounding carbonate platform areas to the slope basins and sometimes to the trough. In the survey area the accommodation of the ongoing east–west differential motion between the Hispaniola and the Puerto Rico–Virgin Islands blocks takes place by means of diffuse deformation. The asymmetrical development of the thrust belt is not related to the geological conditions in the foreland, but rather may be caused by variations in the geometry and movement of the backstop. The map-view curves of the thrust belt and the symmetry of the recesses suggest a main north–south convergence along the Muertos margin. The western end of the Investigator Fault Zone comprises a broad band of active normal faults which result in high instability of the upper insular slope.

Intraoceanic thrusts in the Nankai Trough off the Kii Peninsula: Implications for intraplate earthquakes

We identified intraoceanic thrusts developed as imbricate structures within the subducting Philippine Sea plate off the Kii Peninsula in central Japan manifesting as strong‐amplitude reflections observed in an industry‐standard three‐dimensional (3D) seismic reflection data set. These imbricate intraoceanic thrusts cut through the oceanic crust as a discontinuous thrust plane striking approximately parallel to the trench. In our survey area, large intraplate earthquakes with moment magnitudes (Mw) over 7 occurred on 5 September 2004, causing strong ground motions on the islands of Japan and tsunami waves. The locations of the intraoceanic thrusts recognized in the seismic data are distributed around the estimated hypocenters of the mainshocks and aftershocks of the 2004 earthquakes. Furthermore, their geometry extracted from the 3D seismic data could explain the kind of complex rupture pattern observed during the 2004 events. Therefore we propose that the intraoceanic thrusts are seismogenically active.

Crystal structure and optical properties of 4‐nitrophenyl 4‐butoxybenzoate

AbstractThe title compound, C17H17N1O5, has been synthesized and characterized by single crystal X‐ray analysis and UV‐Vis spectra. The crystals are monoclinic, P 21/c, a = 17.994 (5) Å, b = 4.0592 (9) Å, c = 21.625 (5) Å, β = 99.634 (5)°, V = 1557.2 (6) Å3 and Z = 4. The molecule has an almost stretched form with a molecular length of 17.505 Å and an imbricated structure known for liquid crystals. The structure contains no direction‐specific intermolecular interactions like aromatic π‐π stacking and C‐H… π(arene) other than two weak C‐H…O hydrogen bonds. Good optical transmittance in the entire visible region of the UV‐Vis spectrum suggests that it is a potential candidate for optoelectronic applications. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Influence of palaeokarst morphology on the formation of ice-pushed ridges: a case study from Rejowiec, eastern Poland

This article describes the well-exposed landform/sediment assemblage of a Pliocene palaeouvala and Pleistocene (Saalian) ice-pushed ridge in a large quarry in Upper Cretaceous marls near Rejowiec, eastern Poland. The site provides a rare opportunity to study the development of glacial depositional and deformational processes on morphologically and structurally variable bedrock. The internal structures of both chronostratigraphic units which comprise this assemblage provide evidence of two principal stages of deformation associated with an ice advance on a karstified foreland, each stage preceded by stabilization of the ice mass. The first deformational stage was characterized by the development of imbricate structures in outwash sediments (the initial ice-pushed ridge) and listric faults at the contact between the bedrock and palaeouvala fills (caused by an increasing ice load). Shallow subglacial folding of the proximal fan deposits, and a listric thrust which limited the extent of the ice-pushed ridge under development occurred in the second deformational stage. The data assembled indicate that the formation of push moraines (in a general sense) during ice-sheet advance is controlled by the substratum undulations. Closed depressions with loose fills in the foreland of advancing ice sheets can control the deformational style, geometry and extent (both lateral and vertical) of the push moraines.

Successive, large mass‐transport deposits in the south Kermadec fore‐arc basin, New Zealand: The Matakaoa Submarine Instability Complex

Four >100 km3 mass‐transport deposits (MTDs) identified from their morphology and seismic facies across the Matakaoa Margin and Raukumara fore‐arc basin, NE New Zealand, constitute the Matakaoa Submarine Instability Complex (MSIC). MSIC originates from a 45‐km‐wide, 1100‐m‐high reentrant in the continental slope. The deposits resulted from three mass‐failure events: (1) The Raukumara Slump is identified from the collapsed NW flank of an anticline at the northern end of the reentrant and imbricate structures at its distal end, overlying a flat décollement over a >50‐km distance. The slump age is roughly estimated between upper Miocene and lower Pleistocene. (2) The Matakaoa Debris Avalanche (MDA) is subdivided into a ∼260‐km3 blocky unit and a ∼170‐km3 weakly reflective unit, overlying a high‐amplitude seismic reflector truncating the underlying sedimentary units. The MDA is dated 600 ± 150 ka. It originated as a slump, as indicated by back‐tilted blocks overlying a rotational failure surface, and evolved during transport as a debris avalanche. The failure of sedimentary basement blocks released Plio‐Pleistocene shelf‐basin infill, thus producing the blocky and weakly reflective units. Fore‐arc basin sediments deformed in front of the MDA for approximately 20 km. (3) The Matakaoa Debris Flow (MDF) occurred 38–100 ka ago and extends 200 km northward from the reentrant headwall and consists of a 150‐m‐thick layer with a chaotic seismic facies. Scouring beneath the MDF and a <250‐m‐high east bounding scarp indicate basal and lateral erosion associated with the flow displacement. Incorporation of eroded material into the debris flow accounts for >30% of the flow's 1250 km3 volume. Factors facilitating failures include the following: slope oversteepening associated with margin uplift and fore‐arc subsidence, large‐scale folding related to shortening between the Pacific and Australia plates, high‐discharge rivers draining the region, and rapid sediment accumulation on the margin. Large to great earthquakes along the plate interface are a likely trigger mechanism controlling the recurrence of large margin failures. Unlike other MTDs along plate boundaries, which are destined to be consumed into the subduction factory, the MSIC provides an opportunity to investigate mega‐instabilities at an active margin over million year timescales.

Oil and Gas Occurrences in Southwest Turkey

In SW Turkey, on both sides of the Antalya basin, two continental micro-plates can be distinguished: the Anatolian micro-plate to the NE and Bey Dağlari micro-plate to the W. These micro-plates are composed of a Paleozoic basement of Gondwana type, surrounded by Mesozoic carbonate platforms. From south to north the Bey Dağlari platforms are made of shallow marine carbonates from Triassic time up to Cenomanian. The southern part of this carbonate platform remained under shallow marine conditions up to the Eocene. On the contrary, the northern part subsided during Late Cretaceous time. In the Bey Dağlari platform, occurrences of organic materials are known in the Triassic and in the Early Cretaceous-Cenomanian carbonates. Rock-eval analysis indicate 0.25 to 1.46% total organic carbon (TOC). These facies could have represented a source rock for oil and gas. In the same area the uncomformable Early Miocene carbonates are made of bioclastic calcarenites. In some areas these calcarenites are dark colored and contain oil. These carbonates represent a reservoir rock The Anatolian micro- plate is also composed of shallow marine carbonates from Triassic time up to Eocene. Organic materials are known in the Akkuyu formation, Late Jurassic in age. According to organic geochemical characteristics, this unit is a good source rock. The total organic carbon (TOC) content of this unit is up to 12%. In the same area, other occurrences of oil and gas are known in two locations, Çirali to the SW of Antalya Bay and the Kizildag to the NE of Antalya. The Çirali location is known since the ancient civilizations as Chimera. Gas of this area is pouring out from a complex tectonic imbricate structure including ultrabasic rocks. The source rock and the reservoir are poorly known. The Kizildag is a large klippen of peridotites in the Antalya nappes complex. Liquid petroleum seepage is observed in this ophiolitic unit.

Rapid evolution of the Eocene accretionary complex (Hyuga Group) of the Shimanto terrane in southeastern Kyushu, southwestern Japan

Abstract Detailed geologic examination of the Eocene accretionary complex (Hyuga Group) of the Shimanto terrane in southeastern Kyushu revealed that the oceanic plate was composed of Paleocene to Lower Eocene mudstone and siliceous mudstone, lower Middle Eocene red mudstone, and mid‐Middle Eocene trench‐fill turbidite with siltstone breccia, successively overlying the pre‐Eocene oceanic plate. This oceanic plate sequence was overlain by Upper Eocene siltstone. Deposition of the lower Middle Eocene red mudstone was accompanied by basalt flows and it is interbedded with continental felsic tuff, which indicates that the basalt and red mudstone were deposited near the trench just before accretion. The Hyuga Group has very similar geological structure to that of the chert–clastic complexes found in the Jurassic accretionary complexes in Japan: that is, a decollement fault formed in the middle of an oceanic plate sequence, and an imbricate structure formed only in the upper part of the sequence. Thus, it appears that the Hyuga Group was formed by the same accretionary process as the Jurassic accretionary complexes. No accretion occurred before the Middle Eocene, and the rapid accretion of the Hyuga Group was commenced by the supply of coarse terrigenous sediments in the mid‐Middle Eocene, when the direction of movement of the Pacific Plate changed. The pre‐Eocene oceanic basement and lower Middle Eocene volcanic activity suggest that the oceanic plate partly preserved in the Hyuga Group was very similar to the northern part of the present West Philippine Sea Plate.

Imbricate structure of Luobusa ophiolite, southern Tibet

sequence has a fault contact with the Triassic flysch-type sedimentary rocks. The northern and the southern boundaries dip gently to the south. Mesoscopic structures were observed on outcrop surfaces which are nearly perpendicular to the foliation and nearly parallel to the lineation. Microscopic observations were made on polished slabs and thin sections of oriented samples taken from the shear zones near to the top and bottom boundaries of the Luobusa ophiolite. The slabs and thin sections were made on the section perpendicular to the foliation and parallel to the mineral lineation. A serpentine melange zone, which lies along the northern margin of ophiolite sequence occupies the structural bottom of Luobusa ophiolite. The serpentine melange zone contains lenses of gabbroic rocks, pillow lavas and Cretaceous marine sedimentary rocks. These rocks occur as blocks in a partly serpentinized ultramafic matrix. Individual lithologies are irregularly distributed and the serpentinite matrix is intensively deformed. In the outcrops and oriented samples sets of subparallel shear and minor shear zones occur in the serpentinite. The shear are oblique to foliation in serpentinite and are regarded as C'-type shear bands (Figure 1). Consistent top-to-the-north displacements are observed at three localities in the serpentine melange zone.

New data on the composition of island-arc volcanic rocks in the Eastern mountain range, Kamchatka

Neotectonic uplifts of the Vostochnyi (Eastern) mountain range of Kamchatka are composed of Cretaceous‐Paleogene complexes with fold-nappe and imbricate structures that represent fragments of the southern Ozernov‐Valagin segment of the Achaivayam‐Valagin paleoarc [1]. The composition of island-arc volcanic rocks developed within the uplifts has been studied to a variable extent. Among the best studied are the volcanic rocks of the Ozernoi Peninsula and Tumrok Ridge [2‐4]. We obtained new data on the composition of the least studied volcanic rocks of the Kumroch Ridge, as well as on the Tumrok and Valagin ridges, which allowed us to refine the geodynamic conditions of the volcanic sequences of this segment of the paleoarc. The sampling localities of the studied volcanic rocks are shown in Fig. 1 and Table 1. In the Kumroch Ridge, the Late Campanian‐Danian island-arc rocks compose the tectonic nappes of an autochthon (rocks of the Khapitsk sequence) overlain by a serpentinite melange nappe. The nappe includes blocks of Albian‐Cenomanian and Santonian‐Early Campanian siliceous rocks, basalts, diabases, gabbros, and serpentinized peridotites. Volcanic rocks of the Khapitsk sequence are mainly composed of clinopyroxene‐plagioclase porphyritic basalts and basaltic andesites. In the P 2 O 5 ‐TiO 2 diagram [5], these volcanic rocks are plotted in the island-arc rock field. In terms of REE distribution patterns (Fig. 2), the studied samples are ascribed to the calc-alkaline and tholeiitic series. The location of volcanic rocks in the section suggests the simultaneous formation of these two rock groups. In the REE patterns (Fig. 2), the samples of calc-alkaline series demonstrate LREE enrichment relative to HREE, while LREE and MREE contents in tholeiitic rocks show a similar (almost chondrite-type) pattern, with an insignificantly elevated LREE content. The spidergrams of the studied volcanics have island-arc signatures: deep negative anomalies of HFSE (Nb, Ta, Zr, Ti) and REE enrichment relative to LILE (Ba, U, Rb) (Fig. 3). The calc-alkaline rocks have higher contents of Nb, Ta, Ba, U, and Rb. The two rock groups also differ in bulk composition. Relative to the tholeiitic basalts, the calc-alkaline rocks have a more potassic composition. In general, they are depleted in Ti and enriched in P (Table 1).

Structural and fabric analyses of basal slip zone of the Jin’nosuke-dani landslide, northern central Japan: its application to the slip mechanism of decollement

We describe in detail the deformation structures and textures of a large-scale landslide body that developed in the Betto-dani Valley in northern central Japan. We studied the shape-preferred orientation of clasts and clay flakes and the development of internal shear planes within the slip zone of the landslide. The slip has an average rate of 5–10 cm/year under the overburden pressure of approximately 1.6 MPa; these values are similar to those of the proto-decollement zone of the Nankai accretionary prism in SW Japan. The anisotropy of magnetic susceptibility of samples obtained from the slip zone reveals that the long axes of clay flakes define an imbricate structure. The slip was due to a long-term periodical creep, which occurs during the thaw seasons with an average slip rate of 0.16–0.32 μm/min. During the creep, the long axes of grains including clay flakes in the slip zone are developed from parallel to perpendicular to the slip direction. The observed textures provide a clue to elucidate the deformation textures and process in the decollement zone of the Nankai prism.

Thrusting and transpressional shearing in the Pan-African nappe southwest El-Sibai core complex, Central Eastern Desert, Egypt

The Wadi El-Shush area in the Central Eastern Desert (CED) of Egypt is occupied by the Sibai core complex and its surrounding Pan-African nappe complex. The sequence of metamorphic and structural events in the Sibai core complex and the enveloping Pan-African nappe can be summarized as follows: (1) high temperature metamorphism associated with partial melting of amphibolites and development of gneissic and migmatitic rocks, (2) between 740 and 660 Ma, oblique island arc accretion resulted in Pan-African nappe emplacement and the intrusion of syn-tectonic gneissic tonalite at about 680 ± 10 Ma. The NNW–SSE shortening associated with oblique island arc accretion produced low angle NNW-directed thrusts and open folds in volcaniclastic metasediments, schists and isolated serpentinite masses (Pan-African nappe) and created NNE-trending recumbent folds in syn-tectonic granites. The NNW–SSE shortening has produced imbricate structures and thrust duplexes in the Pan-African nappe, (3) NE-ward thrusting which deformed the Pan-African nappe into SW-dipping imbricate slices. The ENE–WSW compression event has created NE-directed thrusts, folded the NNW-directed thrusts and produced NW-trending major and minor folds in the Pan-African nappe. Prograde metamorphism (480–525 °C at 2–4.5 kbar) was synchronous with thrusting events, (4) retrograde metamorphism during sinistral shearing along NNW- to NW-striking strike-slip shear zones (660–580 Ma), marking the external boundaries of the Sibai core complex and related to the Najd Fault System. Sinistral shearing has produced steeply dipping mylonitic foliation and open plunging folds in the NNW- and NE-ward thrust planes. Presence of retrograde metamorphism supports the slow exhumation of Sibai core complex under brittle–ductile low temperature conditions. Arc-accretion caused thrusting, imbrication and crustal thickening, whereas gravitational collapse of a compressed and thickened lithosphere initiated the sinistral movement along transcurrent shear zones and low angle normal ductile shear zones and consequently, development and exhumation of Sibai core complex.

Recent activity of the regional geologic structures in western Slovenia

Several important geological structures in the western Slovenia were identified as active and their activity was quantified. Geologic interpretation is based on the analysis of repeated leveling line campaigns data along the Sečovlje-Bled polygon. Taking into account the limitations of the method - only the vertical component of displacement is measured - the following structures were identified as active: a juvenile syncline between Strunjan and Koper, the Kras Imbricate Structure, the Divača fault, the Raša fault, the Southalpine Front and the Julian Alps thrust. Vertical movement rate is relative, calculated with respect to the benchmark in Sečovlje. The largest uplift rate difference between Sečovlje and Bled is 7 mm/a.Vertical Geodynamic Activity (VGA) is introduced as a link between geologic interpretation of geodetic measurements on one side and possible applications on the other as well as a mean of comparison between tectonically active regions.

Application of Geophysical Survey to Study the Rising of Ground Water Level (A Case Study of Beiji Refinery

At Beiji Refinery the level of ground water raised from 13m below ground surface to less than 1m within fifteen years. Number of boreholes was drilled to monitor the level and movement of ground water. Geophysical techniques (Seismic refraction and resistivity) have been used to detect subsurface cavities and channels in the refinery. It has been found number of subsurface cavities and holes most of them related to the presence of manholes and subsurface pipes and cables. The cavities are shallow (less than 5m deep) and had been formed due to the leakage water from these manholes and pipes which lead subsequently to the leaching of soil that is contain secondary gypsum. The most important result, however, which is not known before and lead to the clarification of ground water movement in the refinery is that the refinery had been built over a subsurface channel (old meander of Tigris river). This channel had been filled with river sediments (Pebbles, Sand, silt and clay). These sediments are presents as lentils. The direction of the subsurface channel is from east towards the west bending southwards, where the refinery had been built at the bending area where the thickness of the river sediments in this part is over 20m. It has been found that the water leakage from manholes and pipes are accumulating in the bending area where it forms as subsurface lake since it is the lowest part in the channel. From this area the water discharges to the southern refinery. The subsurface channel has been followed outside the refinery where it has been observed number of springs, cross-bedding and imbricate structures. An advice has been forward to the refinery to dig channels in the spring’s area to help discharging subsurface water.

Structural evolution of the Achnashellach Culmination, southern Moine Thrust Belt: testing the duplex model

Abstract The Achnashellach Culmination is one of the major structures of the Moine Thrust Belt. As with other culminations in the belt, it is formed by a stack of imbricate thrusts. Up to 1 km of Torridon Group sediments, together with a further 200–250 m of Cambrian strata, are repeated up to 10 times, but with ramp-on-ramp thrust geometries. Thus structural thickening was chiefly achieved by thick thrust sheets with individually and aggregated displacements that are substantially lower than elsewhere in the thrust belt. The culmination is limited on its flanks by lateral ramps that climb section out of Torridon Group and up into Cambrian strata. To the north the imbricate thrusts may be deduced to branch onto the major Kinlochewe Thrust. To the south the imbricates are represented only by stacked Durness Limestone. The northward-climbing lateral ramp coincides with a major Precambrian structure, the Loch Maree Fault, which controls the thickness of Torridonian strata preserved beneath the sub-Cambrian unconformity, a rare example of basement influence on thrust system geometry within the Moine Thrust Belt. The imbricates of the Achnashellach Culmination show back-steepening and have bulged up the overriding Kishorn and Kinlochewe thrust sheets. However, these structurally higher level tectonic units slice across imbricate structures in their footwalls. Elsewhere high-level thrusts are folded by some parts of underlying imbricates. Collectively these relationships are not compatible with classical duplex models. They are explained better by models of quasi-synchronous slip on imbricate thrusts. Discordant relationships beneath major thrust sheets, including those that cut down stratigraphic section in the transport direction, can be explained by such models without necessitating low-angle extensional faulting within the thrust belt.

Cenozoic post-collisional brittle tectonic history and stress reorientation in the High Zagros Belt (Iran, Fars Province)

The Zagros fold-and-thrust belt of SW-Iran is among the youngest continental collision zones on Earth. Collision is thought to have occurred in the late Oligocene–early Miocene, followed by continental shortening. The High Zagros Belt (HZB) presents a Neogene imbricate structure that has affected the thick sedimentary cover of the former Arabian continental passive margin. The HZB of interior Fars marks the innermost part of SE-Zagros, trending NW–SE, that is characterised by higher elevation, lack of seismicity, and no evident active crustal shortening with respect to the outer (SW) parts. This study examines the brittle structures that developed during the mountain building process to decipher the history of polyphase deformation and variations in compressive tectonic fields since the onset of collision. Analytic inversion techniques enabled us to determine and separate different brittle tectonic regimes in terms of stress tensors. Various strike–slip, compressional, and tensional stress regimes are thus identified with different stress fields. Brittle tectonic analyses were carried out to reconstruct possible geometrical relationships between different structures and to establish relative chronologies of corresponding stress fields, considering the folding process. Results indicate that in the studied area, the main fold and thrust structure developed in a general compressional stress regime with an average N032° direction of σ 1 stress axis during the Miocene. Strike–slip structures were generated under three successive strike–slip stress regimes with different σ 1 directions in the early Miocene (N053°), late Miocene–early Pliocene (N026°), and post-Pliocene (N002°), evolving from pre-fold to post-fold faulting. Tensional structures also developed as a function of the evolving stress regimes. Our reconstruction of stress fields suggests an anticlockwise reorientation of the horizontal σ 1 axis since the onset of collision and a significant change in vertical stress from σ 3 to σ 2 since the late stage of folding and thrusting. A late right-lateral reactivation was also observed on some pre-existing belt-parallel brittle structures, especially along the reverse fault systems, consistent with the recent N–S plate convergence. However, this feature was not reflected by large structures in the HZB of interior Fars. The results should not be extrapolated to the entire Zagros belt, where the deformation front has propagated from inner to outer zones during the younger events.

Imbricate structure of the Luobusa Ophiolite and surrounding rock units, southern Tibet

The Cretaceous Luobusa Ophiolite is a tectonic slice less than 1.2 km thick. The structurally underlying unit is the Tertiary Luobusa Formation and the overlying unit is composed of feebly metamorphosed Triassic sedimentary rocks. The top and bottom contacts of the ophiolite dip gently to the south. The major part of the Luobusa Ophiolite is mantle peridotite, including podiform chromitite. This chromitite has received much attention because it contains an ‘unusual mineral assemblage’ which includes diamond and moissanite. A serpentinite mélange zone, including clasts of basaltic and sedimentary rocks, occurs underneath the mantle peridotite. Mesoscopic–microscopic structures were observed in the sheared rocks. Shear-indicating structures (C′-type shear bands and σ-type porphyroclasts) in the rocks near the top and bottom boundaries of the Luobusa Ophiolite show consistent top-to-the-north (or northeast) reverse displacement. The results reveal that the Luobusa Ophiolite was overturned and intercalated into an imbricate structure. The thrust faults on the top and bottom of the ophiolite can be correlated with north-vergent back-thrusting which was associated with crustal shortening along the Main Central Thrust due to the continued northward movement of India, after the welding of India to Asia.

Pleistocene rapid uplift of the Himalayan frontal ranges recorded in the Kathmandu and Siwalik basins

Core-drilling project carried out in the southern margin of the Kathmandu Basin revealed that muddy debris flow deposits dammed up the Proto-Bagmati river to form the Paleo-Kathmandu Lake during the Jaramillo subchron from 1.07 to 0.97 Ma. Subsequent deposition of the alluvial fanglomerate, derived from the uplifting Mahabharat Range to the south, raised the dam deepening the lake-water. After 1 Ma, in the southern part of the basin, palaeo-current directions changed from southward to northward and deposition of gneissose and granitic detritus are replaced by meta-sediments derived from the Mahabharat Range. During the same time, at about 1 Ma, the boulder conglomerates were deposited on top of the Siwalik Group as piggy-back basins in front of an intra-basinal high, along the Main Dung Thrust in Nepal and NW India. Onset of movement of the Main Dung Thrust is dated back to 3 to 2.4 Ma [Mugnier, J.L., Huyge, P., Leturmy, P., Jouanne, F., 2003, Episodicity and rates of thrust-sheet motion in the Himalayas, Western Nepal. Am. Assoc. Petrol. Geol., Mem. 82, 1–24]. The Main Frontal Thrust is most active at present suggesting that imbricated structure of the Siwalik Group was formed by convergence of the Indian plate during the last 3 myr. The accretionary wedge of the Siwalik Group, stacked beneath the Main Boundary Thrust, might have started to jack up the frontal range of the Lesser Himalaya since 1 Ma. Coeval uplift and erosion of the frontal range of the Lesser Himalaya and the intra-basinal high in the Siwalik since 1 Ma are possible causes of an abrupt increase in both sedimentation rate and grain size of detrital quartz, and changes in composition of clay minerals, recorded in the sediments of the Bengal Deep Sea Fan at 0.9 Ma.