Main Thrust Fault Research Articles

A faixa de dobramento Paraguai na Serra da Bodoquena e depressão do Rio Miranda, Mato Grosso do Sul

The Southern Paraguay Belt is a typical fold-and-thrust belt. Its geological evolution began with a continental rifting process, probably at the end of the Cryogenian, evolving into a restricted sea and an extensive marine transgression at the end of the Ediacaran. The final collisional event occurred during the Lower Cambrian, with post-collisional magmatism during the Upper Cambrian. The Corumba Group is divided into five formations (Cadiueus, Cerradinho, Bocaina, Tamengo and Guaicurus). The Puga Formation is correlated to the Cerradinho and Cadiueus formations. This stratigraphy can be seen even in the most deformed parts located in central-eastern area. In the far west of the area, the Corumba Group is deposited above an unconformity in the Rio Apa cratonic block. The name Agachi Schists is suggested for the schists located at the easternmost portion of the area. During the Ediacaran period, whereas, in the east part of the area, the Cuiaba Group was affected by deformation, metamorphism and arc-related granites, in the west part of the area, the Tamengo and Guaicurus formations were deposited, probably in a context of foreland basin. Up to three superimposed coaxial folding events are observed associated with greenschist metamorphism and thrust faults, showing a westward tectonic vergence. WSW - ESE convergence vectors indicate that the collision with the Rio Apa block was slightly oblique. Changes in metamorphism and structural style can be explained by the westward migration of the deformational front. The main thrust faults coincide with important basin boundaries, suggesting that the listric faults of the rift stage were reactived by them.

Stability Assessment and Quantitative Evaluation of H/V Spectral Ratios for Site Response Studies in Kumaon Himalaya, India Using Ambient Noise Recorded by a Broadband Seismograph Network

The Kumaon Himalaya region in India has accumulated considerable seismic risk potential from moderate to high seismicity due to ongoing tectonic evolutionary processes. To assess conditions of high seismic risk arising from local site effects at populated locations, we applied the empirical horizontal to vertical (H/V) spectral amplitude ratio method (Nakamura in Quarterly Reports of the Railway Technical Research Institute Tokyo, 30:25–33, 1989) using ambient seismic noise recorded by a network of 32 digital broadband seismographs from June 2005 to June 2008. The data and the estimated parameters were subjected to stability tests to assess the effect of seasonal variations. Seasonal variations in the ambient noise data seemed to show a lesser effect on fundamental frequency estimates and a slightly greater effect on the peak H/V amplitudes. Validation of the ambient noise results was done by complementary tests using H/V ratios of local and regional earthquakes. The ‘peak’ corresponding to the fundamental resonance frequency is prominently present in both the ambient noise and the earthquake datasets. The study locations showed distinct H/V curve topologies, corresponding to the effects of both velocity contrast at well-defined frequencies and characteristic topographic effect around a certain frequency range. The clearly indicated ‘peaks’ in the H/V curves possibly correspond to velocity contrasts created by weathered sediments overlying hard rock basements in rocky hills. Our study indicates high site responses at many populated locations near the surface trace of the seismically active Main Central Thrust (MCT) and other fault boundaries. The fundamental resonance frequencies estimated from the site response studies at the 32 locations could be useful in preliminary site characterization, ground motion prediction and seismic hazard analysis.

Probabilistic seismic hazard analysis of Islamabad, Pakistan

Pakistan is prone to seismic activity, and its capital, Islamabad, is located close to the Main Boundary Thrust (MBT) fault. On October 8th, 2005 the disastrous Muzaffarabad earthquake shook Islamabad and damaged many high-rise buildings. A probabilistic seismic hazard analysis technique was used to estimate strong ground motion parameters for a closely spaced 1 km grid. Traditionally, PGA is calculated, which is then used in structural earthquake resistant design or seismic safety assessment. However, Peak Ground Acceleration (PGA) is not sufficient to design for seismic load or to account for the modern building code’s emphasis on the use of spectral acceleration values. Therefore, a seismic hazard analysis was performed for Islamabad, and the design parameters that are required by codes to account for seismic loading were derived.

Evaluation of high local groundwater inflow to a rock tunnel by characterization of geological features

High local groundwater flow into rock tunnels is a technical and environmental problem for underground constructions. Geological features such as fault zones, open fractures and dykes can be the source of very high local inflows. However, prediction of possible groundwater inflow from these features using analytical and numerical tools often failed due to given hydrogeological assumptions and simplification of these heterogeneous media. In fact, the characteristics of geological features are not often detected by normal exploratory methods and missed from geological models. In order to identify factors that affect inflows from the geological features, a detailed investigation has been carried out at a tunnel excavated to supply potable water to the city of Semnan in Iran. The tunnel passes through formations of Cambrian age with thin and thickly laminated limestone, sandstone, shale and siltstone, which thrust on tuff Formation of Tertiary age by a main thrust fault. It was found out that: (1) High local inflows (from 20 up to 750 l/s) come from five geological features and much of tunnel is dry or has less inflow. (2) Maximum inflow occurs in an open fracture with amount of 750 l/s. (3) Faults have different hydraulic effect; they may act as conduits, barriers or combined conduit-barrier systems. (4) Dykes are generally barriers along which a large volume of water can be stored. (5) Evaluation of high local groundwater inflow to rock tunnels based on characterization of geological features is more reliable compared to available analytical and empirical estimation.

The Kumaun and Garwhal Lesser Himalaya, India: Part 2. Thermal and deformation histories

An integrated petrologic, thermochronologic, and numerical modeling study constrains the thermal evolution of the Lesser Himalayan Sequence in the Kumaun and Garwhal regions of India. South of the Tons thrust, peak metamorphic temperatures do not exceed 330 °C. On the northern side, a transition toward higher temperatures adjacent to the Main Central thrust hanging wall is documented. In the immediate footwall of the Main Central thrust, peak temperatures average ~550 °C. An inverted thermal field gradient of ~30 °C/km beneath the Vaikrita thrust is calculated. An inverted thermal field gradient of ~20 °C/km is also documented beneath a klippe of the Main Central thrust hanging wall ~25 km south of the Vaikrita thrust, and a peak footwall temperature of 530 °C is recorded close to the shear zone at the base of the klippe. The mechanism for thermal metamorphism in the Lesser Himalayan Sequence is interpreted to be conduction of heat from the Main Central thrust fault and/or hanging wall to the footwall. A localized thermal field gradient is also associated with the Tons thrust. The ^(40)Ar/^(39)Ar thermochronology reveals that the exposed rock in the Kumaun and Garwhal Lesser Himalaya underwent cooling below the closure temperature (Tc) for white mica at different times in the late Tertiary. The proximal footwall of the Munsiari thrust experienced cooling below the white mica Tc at ca. 8.5 Ma. South of the Munsiari thrust, in the proximal footwalls of the Chaukori and Askot klippen, muscovites ages are ca. 11.5 Ma, ~3 Ma older than samples to the north. The age differences in samples from northern and southern exposures are interpreted to document cooling related to thermal relaxation following passage of the Main Central thrust hanging wall atop the Lesser Himalayan Sequence. Results of thermal modeling achieve good fits to the data when scenarios involve an early Miocene phase of overthrusting of a hot hanging wall over a downgoing footwall, followed by the initiation of a duplex within Lesser Himalayan Sequence rocks.

Kinematics of the Greater Himalayan sequence, Dhaulagiri Himal: implications for the structural framework of central Nepal

Abstract: In the Dhaulagiri region of central Nepal quartz-rich specimens sampled from the Greater Himalayan sequence yield well-defined quartz c -axis fabrics with a dominant top-to-the-SW sense of shear. These fabrics reflect pervasive crystal-plastic deformation extending more than 8 km structurally below previously mapped locations of the Main Central thrust fault. Quartz c -axis fabric opening angles suggest deformation temperatures of c . 500 ± 50 °C within the lower portion of the Greater Himalayan sequence and up to c . 670 ± 50 °C within the migmatitic upper portion. These temperatures generally plot within error of garnet–biotite temperature estimates for metamorphic assemblages and are interpreted to reflect Tertiary deformation during extrusion of the mid-crust. The quartz c -axis data, and a new, detailed tectonostratigraphic map, constrain the position of the Ramgarh thrust in the Dhaulagiri region to be coincident with the Main Central thrust at the base of the pervasively deformed exhumed mid-crustal core. Mean kinematic vorticity numbers ( W m ) measured in specimens sampled from the lower portion of the Greater Himalayan sequence range between 0.49 and 0.80 ( c . 66–41% pure shear) with an average value of 0.67 ( c . 53% pure shear). These data indicate that extrusion of the mid-crust was facilitated by a significant component of pure shear strain.

Late Quaternary alluvial fans and paleosols of the Kangra basin, NW Himalaya: Tectonic and paleoclimatic implications

Abstract The paper describes Late Quaternary pedosedimentary sequences of the alluvial fans from the Kangra basin of NW Himalayas for tectonic and paleoclimatic implications. In the proximal part of the Kangra basin three coalescing alluvial fans, namely Rait-Rihlu fans (~ 65 km2), Kangra fans (~ 200 km2), and Palampur fans (~ 170 km2) from west to east evolved due to reactivation of longitudinal and transverse faults and climatic changes during the Late Quaternary. The fans are characterised by subsidence of Rait-Rihlu fans, uplift of Kangra fans and tilting of Palampur fans. The thick (~ 90 m) pedosedimentary sequences exposed along the rivers characterise the dominant formative processes over the fans. The stream flow sediments dominate the Rait-Rihlu fans, whereas the debris flow sediments dominate the Kangra and Palampur fans. The fan sequences are also marked by the formation of strongly developed paleosols on loess (L1–L3 loess paleosols) and weakly developed paleosols on fluvial deposits in response to the tectonics and climate change in NW Himalayas. Formation of the loess in proximal and distal settings of the alluvial fans is related to the cool–dry climates during the advance of glaciers in the adjoining areas at 78 ka, 44 ka, 30 ka and 20 ka when climate was approaching towards the last glacial maxima. Field characters, micromorphology, grain size and clay minerals of these loessic deposits suggest strong pedogenesis of the loess during a warm–humid climate after the retreat of glaciers in the NW Himalayan region and are likely related to marine isotope stages (MIS4/5 in proximal and MIS2/3 in distal). The loess-paleosols show some degree of syndepositional pedogenesis in upper horizons and are accretionary in nature. Weakly expressed pedogenic features in fluvial and debris flow deposits suggest rapid sedimentation over unstable surfaces related to the reactivation of Main Boundary Thrust (MBT) and other regional faults.

A comparison of Main Central Thrust and other Himalayan fault systems from central and west Nepal with some two-dimensional stress fields

Himalayan Journal of Sciences Vol.2(4) Special Issue 2004 pp. 123-4

Plate Motion of India and Interseismic Strain in the Nepal Himalaya from GPS and DORIS Measurements

We analyse geodetically estimated deformation across the Nepal Himalaya in order to determine the geodetic rate of shortening between Southern Tibet and India, previously proposed to range from 12 to 21 mm yr^(−1). The dataset includes spirit-levelling data along a road going from the Indian to the Tibetan border across Central Nepal, data from the DORIS station on Everest, which has been analysed since 1993, GPS campaign measurements from surveys carried on between 1995 and 2001, as well as data from continuous GPS stations along a transect at the logitude of Kathmandu operated continuously since 1997. The GPS data were processed in International Terrestrial Reference Frame 2000 (ITRF2000), together with the data from 20 International GNSS Service (IGS) stations and then combined using quasi observation combination analysis (QOCA). Finally, spatially complementary velocities at stations in Southern Tibet, initially determined in ITRF97, were expressed in ITRF2000. After analysing previous studies by different authors, we determined the pole of rotation of the Indian tectonic plate to be located in ITRF2000 at 51.409±1.560°N and−10.915± 5.556°E, with an angular velocity of 0.483±0.015°. Myr^(−1). Internal deformation of India is found to be small, corresponding to less than about 2 mm yr^(−1) of baseline change between Southern India and the Himalayan piedmont. Based on an elastic dislocation model of interseismic strain and taking into account the uncertainty on India plate motion, the mean convergence rate across Central and Eastern Nepal is estimated to 19 ± 2.5 mm yr^(−1), (at the 67% confidence level). The main himalayan thrust (MHT) fault was found to be locked from the surface to a depth of about 20km over a width of about 115 km. In these regions, the model parameters are well constrained, thanks to the long and continuous time-series from the permanent GPS as well as DORIS data. Further west, a convergence rate of 13.4 ± 5 mm yr^(−1), as well as a fault zone, locked over 150 km, are proposed. The slight discrepancy between the geologically estimated deformation rate of 21 ± 1.5 mm yr^(−1) and the 19 ± 2.5 mm yr^(−1) geodetic rate in Central and Eastern Nepal, as well as the lower geodetic rate in Western Nepal compared to Eastern Nepal, places bounds on possible temporal variations of the pattern and rate of strain in the period between large earthquakes in this region.

Structural geology and regional tectonic significance of the Ramgarh thrust, Himalayan fold‐thrust belt of Nepal

The Ramgarh thrust is one of the major fault systems of the Himalayan thrust belt in Nepal and northern India. The Ramgarh thrust sheet is ∼0.2–2.0 km thick and can be traced along strike the entire length of the Himalaya in Nepal. The fault generally places the oldest Paleoproterozoic rocks in the Lesser Himalayan series upon younger Lesser Himalayan rocks or lower Miocene foreland basin deposits. Regional balanced cross sections suggest that the Ramgarh thrust had at least ∼120 km of initial south vergent displacement. Subsequently, the frontal part of the thrust experienced further slip as the roof thrust for a large duplex in underlying Lesser Himalayan rocks. Ramgarh hanging wall strata are greenschist‐grade phyllite, quartzite, and augen gneiss, all of which locally exhibit phyllonitic and mylonitic fabrics that indicate a top‐to‐the‐south sense of shear. Structural fabrics in the Ramgarh thrust sheet are generally parallel to the fabrics in rocks above and below the thrust sheet. Regional and local mapping of the Ramgarh thrust in Nepal demonstrates that the fault always places a hanging wall flat upon a footwall flat, except where local lateral ramps complicate its geometry. Similarly, the structurally overlying Main Central thrust always places a hanging wall flat in Greater Himalayan series rocks upon the regionally flat Ramgarh thrust sheet. These geometric relationships preclude kinematic and thermal models that elevate Greater Himalayan and lower Lesser Himalayan rocks along high‐angle thrust ramps in the vicinity of the present traces of the Ramgarh and Main Central thrust faults. Instead, the corresponding footwall ramps for these thrusts must be located more than 100 km north of the current trace of the Main Central thrust. The present steep dips of the Ramgarh and Main Central thrust sheets can be attributed to tilting during emplacement of structurally lower thrust sheets within a large antiformal duplex that occupies most of the Lesser Himalayan zone. The Ramgarh thrust sheet overlaps a bed length of at least 100 km in lower Miocene foreland basin deposits, indicating that a significant amount of displacement on the thrust must have occurred after ∼15 Ma. Growth of the Lesser Himalayan duplex and additional slip on the frontal part of the Ramgarh thrust occurred from ∼12 to 5 Ma. The presence of a major greenschist‐grade metasedimentary thrust sheet composed of Lesser Himalayan rocks directly below the Main Central thrust suggests that the famous “inverted metamorphism” in this region is a result of structural inversion. Similarly, the concept of a broad zone of intense shear strain related exclusively to emplacement of the Main Central thrust sheet is probably invalid in Nepal.

Discussion of “The polygenetic Au–Ag bearing veins of Bou Madine (Jbel Ougnat, Eastern Anti-Atlas, Morocco): tectonic control and evolution of a Neoproterozoic epithermal deposit” [Abia et al., Journal of African Earth Sciences, 36 (2003) 251–271

In order to better understand the orogenic evolution of the Rif chain in the Eocene-Miocene interval, we provide new structural and kinematic data for the Jebha area, a key-sector of the Central Rif. Here the thrust sheet superposition occurs along the well-known Jebha-Chrafate lineament, widely considered as a major left-lateral transfer fault that enabled the Miocene westward migration of the internal thrust front. Our structural analysis was mainly focused on (i) the internal deformation of stacked nappes and (ii) the kinematics of the main thrust faults. Five main deformation stages were recognized for the Eocene-Miocene tectonic evolution of this area. The first orogenic pulse (D1), which occurred in the Eocene-Oligocene interval, was responsible for the tectonic stacking of the Ghomaride Nappes. Subsequently between the late Aquitanian and the late Burdigalian, imbrication (stage D2) occurred for some Internal Dorsale Calcaire thrust sheets within a dominant regional ENE-WSW shortening. At the Rif scale, different displacements of the WSW-migrating thrust front were accommodated by transfer structures including the Jebha-Chrafate fault. The following late Burdigalian-Langhian stage (D3) was defined, on the contrary, by a prevalence of the radial thrust front migration. In the Jebha area the early thrusting (stage D3a) was characterized by a main SE-vergence. In this phase the External Dorsale Calcaire and the Maghrebian Flysch Basin Units were included in the accretionary wedge. Two late D3 regional deformation phases were probably related to the buttressing effect that followed the collision of the thrust sheet pile against the crustal ramp of the External Rif domain. The first stage (D3b) consisted of an out-of-sequence thrusting recorded in the western sector of the Jebha area with the superposition of the Ghomaride Unit onto the External Dorsale Calcaire Unit, and in the eastern sector with the stacking of the Internal Dorsale Calcaire Unit directly onto the Predorsalian Unit. The second stage (D3c) included a late back-thrusting affecting the whole orogenic chain and deforming all the tectonic contacts. The fourth stage (D4) was characterized by strike-slip faulting and SW-verging folding. This latter mostly affected the successions located to the East of the Jebha village and was partially synchronous with the D3 stage. It was most probably related to the SW-migration of the internal thrust-front of the Bokkoya Dorsale Calcaire Complex and a renewed activity of the Jebha-Chrafate fault zone. The last tectonic stage (D5) included a radial extension expressed by high and low-angle normal faults.

Miocene faulting at plate tectonic velocity in the Himalaya of central Nepal

Combination of geochemical zoning in metamorphic garnet and monazite plus in situ Th–Pb isotopic dating of monazite yields P– T conditions, ages and convergence rates for the Main Central Thrust (MCT) and affiliated faults in central Nepal. Inferred rates were 1.5±0.9 cm/yr (Langtang Thrust, ∼19 Ma), 2.2±0.7 cm/yr (Main Central Thrust, ∼15 Ma) and 7±3 cm/yr (Ramgarh Thrust, ∼10 Ma). The lower values are similar to modern convergence rates across the Himalaya, but the Ramgarh Thrust may have briefly absorbed all Indo–Asian convergence at ∼10 Ma, when foreland and marine sedimentation rates markedly increased, and at least one major strike slip fault in Tibet experienced a hiatus in movement. Variable rates of convergence across the Himalaya on Myr timescales imply Myr variations in strain rates throughout all components of the Indo–Asian orogen.

Role of synkinematic ductile levels on the evolution of compressive zones – analogue modelling

In foldbelt faults, layers with ductile behaviour can form levels of decollement [Byerlee, 1978]. When these levels are prekinematic, they play a significant role in the genesis, evolution and final geometry of the foldbelt faults, as, for example in the Appalachian Mountains [Davis and Engelder, 1985], the Jura [Sommaruga, 1999], or the Pyrenees [Verges et al., 1992]. Previous studies based on analogue modelling have shown how a prekinematic decollement level can influence the geometry of foldbelt faults and structures [Ballard, 1989; Colletta et al., 1991; Letouzey et al., 1995; Merle et Abidi, 1995]. However, no study has yet described the influence of synkinematic sedimentation of incompetent levels on the genesis and evolution of compressive structures. The laboratory experiments presented here are designed to explore some of the mechanisms of formation of synsedimentary thrust faults, in relation with the occurrence of a decollement layer during syntectonic sedimentation.

Quantitative analysis of movement along an earthquake thrust scarp: a case study of a vertical exposure of the 1999 surface rupture of the Chelungpu fault at Wufeng, Western Taiwan

A vertical exposure across the principal thrust scarp of the 1999 Mw 7.6 earthquake allows quantification of fault slip. The exposure is located on the active Chelungpu fault near Wufeng, along the range front of the fold-and-thrust belt in western Taiwan. The 1999 surface ruptures at the Wufeng site are characterized by a west-facing 2 to 3 m high principal thrust scarp and an east-facing lesser backthrust scarp. We mapped a 15 m-long, 5 m-deep exposure across the principal thrust scarp and characterized complex deformation structures, which include a main basal thrust fault, a wedge thrust, and a pop-up anticlinal fold with two secondary opposing thrust faults. The vertical displacement across the principal thrust scarp is measured directly from the offsets of the same sedimentary horizons between the hangingwall and the footwall. The average vertical displacement is 2.2±0.1 m, and the maximum displacement is 2.5 m, at the crest of the small pop-up fold. Horizontal displacement estimates were determined using line- and area-balancing methods. With line-length methods we estimated a horizontal displacement of 3.3±0.3 m across the principal scarp for four sedimentary horizons. For area balancing, first we selected three horizontal soil/sand deposits with a total thickness of about 0.5 m. The estimate yields an average horizontal displacement of 4.8±1.0 m. Using these individual and relatively thin stratigraphic layers yielded significant standard deviations in displacement estimates as a result of thickness variations. Second, we used the 3 m-thick overbank soil/sand and the lower part of fluvial pebble/cobble to calculate a horizontal displacement of 2.6±0.2 m with the area-balancing technique. According to the geometry of the dip angle (35–40°) of the basal thrust, the line-length measurement and the 3 m-thick package area balancing both provided reasonable results of horizontal displacement. By comparing the different deposits applied to the line- and area-balancing methods, we interpret that decoupling of deformation occurred between the lower fluvial gravels and the upper overbank sand and mud deposits. Due to lesser confining pressure at the surface, additional deformation occurred in the upper 1–2 m thick overbank deposits. This additional deformation yielded further vertical uplift of 0.3–0.5 m and horizontal displacement of 0.2–0.8 m around the core of the pop-up fold. Our work suggests that determination of slip across surface thrust ruptures varies as a function of the mechanical behavior of young late Quaternary deposits.

Along‐depth stress rotations and active faults: An example in a 5‐km deep well of southern Italy

We have analyzed a 1500 m section at 3.9 to 5.4 km depth in a well of the southern Apennines, in order to better characterize the local active stress field and its correlation with tectonic structures. In this paper we present and discuss the results obtained from the comparison between breakouts and structural analysis from dipmeter data. We have found that the mean breakout direction is in agreement with the regional stress field that in this area is characterized by normal faulting (σ1 = σv) with NE‐SW trending extension (horizontal σ3). Since the regional stress field is relatively well known in this region, we could detect and study some anomalous horizontal stress directions along the well, which we interpret as due to faults crosscutting the borehole. A detailed comparison between the breakout‐inferred stress variations along depth and the faults identified by the dipmeter analysis reveals that some of these faults are associated with stress rotations, whereas others do not show any variation. The former can be interpreted either as “open” fractures or as faults that slipped recently with a near‐complete stress drop, and the latter can be interpreted as “sealed” faults. In particular, we found that the main thrust faults of the area, mainly active in Pliocene times, appear to be sealed, whereas ∼E‐W trending high‐angle (normal?) faults determine strong stress rotations, suggesting that they are the main active structures of the region. This suggests that the study area is located in a transfer zone between the two main “Apenninic” (NW‐SE trending) fault systems which ruptured in the last 150 years. This study has shown that a detailed analysis of the structural and geometrical characteristics of deep wells can be used for the reconnaissance of active structures. This approach can contribute to seismic hazard studies and, if carried out in an oil‐bearing section, can help to maximize the hydrocarbon production.

Horizontal tectonic deformation geometries in a late Archaean sedimentary sequence, Belingwe greenstone belt, Zimbabwe

In the Belingwe greenstone belt of Zimbabwe, structural evidence from the circa 2.65 Ga old sedimentary Cheshire Formation which overlies and is imbricated with a mafic volcanic unit is consistent with thrusting of the greenstone sequence. The Cheshire Formation consists of a karstified carbonate ramp sequence overlain by siliciclastic turbidite deposits that formed in a southeast deepening basin. The earliest deformational structures formed during a syndepositional to postdepositional, thin‐skinned thrusting event (D1) that affected poorly consolidated sediments and is recorded in bedding‐parallel ductile shear zones, boudins, folds, and block‐in‐matrix structures. D1 shear zones separate the volcanic sequence and the overlying sediments and occur between the carbonate and siliciclastic units. Syntectonic sulphide mineralization and silicification of mainly fine‐grained sediments along the main thrust faults gave rise to the formation of rocks similar in appearance to banded iron formations. Stratigraphic units were locally duplicated along the D1 thrust faults, including a tectonic slice of mafic volcanics that was emplaced onto carbonates along a chaotic unit similar to a tectonic melange. The elongation of inclusions in the melange zone and lineations in ironstones together with kinematic indicators suggest that stratigraphic duplication resulted from northwestward directed tectonic transport. Deposition of the Cheshire Formation took place in an asymmetric, foreland‐type basin contemporaneously with thrusting. Soon after deposition the formation was incorporated into the thrust stack. Subsequent deformation events include tight upright folding, gentle cross‐folding, and dextral strike‐slip faulting of the greenstone succession. Evidence for early thrusting suggests that horizontal tectonic processes played an important role in the evolution of the Belingwe greenstone belt.

Dynamic modelling of the subduction zone of central Mexico

In central Mexico some significant normal faulting events have occurred within the subducted oceanic Cocos plate, just below or near the down-dip edge of the strongly coupled interface. These normal faulting shocks followed large shallow thrust earthquakes. In other subduction zones such events generally precede the up-dip thrust events. A vertical 2-D finite element modelling has been used to simulate the subduction of the Cocos plate beneath the North American plate when the slab is driven by an active convergence velocity or slab pull. We find that the latter mechanism plays only a minor role due to shallow subduction. The modelling results show that the stress pattern is very sensitive to the geometry of the plates. In particular, normal faulting earthquakes that follow large thrust events can be explained on the basis of the flexural response of the overriding and subducting plates to the peculiar geometry of this subduction zone, where the subducting slab becomes horizontal at about 100 km from the trench. This horizontal part of the subducting plate, down-dip with respect to the main thrust zone, is under an extensional stress field. This provides an alternative explanation to the slab pull for the occurrence of normal faulting intraplate earthquakes. In order for normal faulting earthquakes to occur in the early part of the seismic cycle, it is necessary that the large up-dip thrust events have a partial stress drop. We find that for small fractional stress drop, a wide region of extension remains below the down-dip edge of the main fault plane following a large thrust earthquake. Thus, the main thrust earthquakes do not invert the polarity of the active stress field, which is compressional and extensional up-dip and down-dip, respectively, with respect to the main thrust fault. Larger fractional stress drops result in larger delays in the occurrence of normal faulting events after the main thrust events.

Modeling mountain building and the seismic cycle in the Himalaya of Nepal

A host of information is now available regarding the geological and thermal structure as well as deformation rate across the Himalaya of central Nepal. These data are reconciled in a two‐dimensional mechanical model that incorporates the rheological layering of the crust which depends on the local temperature and surface processes. Over geological timescale (5 Ma) the ∼20 mm/yr estimated shortening rate across the range is accommodated by localized thrust faulting along the Main Himalayan Thrust fault (MHT). The MHT reaches the surface along the foothills, where it is called the Main Frontal Thrust fault (MFT). The MHT flattens beneath the Lesser Himalaya and forms a midcrustal ramp at the front of the Higher Himalaya, consistent with the river incision and the anticlinal structure of the Lesser Himalaya. Farther northward the MHT roots into a subhorizontal shear zone that coincides with a midcrustal seismic reflector. Aseismic slip along this shear zone is accommodated in the interseismic period by elastic straining of the upper crust, increasing the Coulomb stress beneath the front of the Higher Himalaya, where most of the microseismic activity clusters. Negligible deformation of the hanging wall requires a low apparent friction coefficient (μ) less than ∼0.3 on the flat portion of the MHT. On the ramp, μ might be as high as 0.6. Sensitivity tests show that a rather compliant, quartz‐rich rheology and a high radioactive heat production in the upper crust of ∼2.5 μW/m3 is required. Erosion affects the thermal structure and interplays with crustal deformation. A dynamic equilibrium is obtained in which erosion balances tectonic uplift maintaining steady state thermal structure, topography, and deformation field. Using a linear diffusion model of erosion, we constrain the value of the mass diffusivity coefficient to 0.5–1.6×l04 m2/yr. This study demonstrates that the data are internally consistent and compatible with current understanding of the mechanics of crustal deformation and highlight the role of viscous flow in the lower crust and of surface erosion in orogeny processes on the long term as well as during interseismic period.

Active folding of fluvial terraces across the Siwaliks Hills, Himalayas of central Nepal

We analyze geomorphic evidence of recent crustal deformation in the sub‐Himalaya of central Nepal, south of the Kathmandu Basin. The Main Frontal Thrust fault (MFT), which marks the southern edge of the sub‐Himalayan fold belt, is the only active structure in that area. Active fault bend folding at the MFT is quantified from structural geology and fluvial terraces along the Bagmati and Bakeya Rivers. Two major and two minor strath terraces are recognized and dated to be 9.2, 2.2, and 6.2, 3.7 calibrated (cal) kyr old, respectively. Rock uplift of up to 1.5 cm/yr is derived from river incision, accounting for sedimentation in the Gangetic plain and channel geometry changes. Rock uplift profiles are found to correlate with bedding dip angles, as expected in fault bend folding. It implies that thrusting along the MFT has absorbed 21±1.5 mm/yr of N‐S shortening on average over the Holocene period. The ±1.5 mm/yr defines the 68% confidence interval and accounts for uncertainties in age, elevation measurements, initial geometry of the deformed terraces, and seismic cycle. At the longitude of Kathmandu, localized thrusting along the Main Frontal Thrust fault must absorb most of the shortening across the Himalaya. By contrast, microseismicity and geodetic monitoring over the last decade suggest that interseismic strain is accumulating beneath the High Himalaya, 50–100 km north of the active fold zone, where the Main Himalayan Thrust (MHT) fault roots into a ductile décollement beneath southern Tibet. In the interseismic period the MHT is locked, and elastic deformation accumulates until being released by large (Mw > 8) earthquakes. These earthquakes break the MHT up to the near surface at the front of the Himalayan foothills and result in incremental activation of the MFT.

Climatic and tectonic controls on fluvial incision and aggradation in the Spanish Pyrenees

The influences of tectonic and climatic changes on upland river systems are investigated using data from Plio-Pleistocene terraces of the Rio Cinca river in the southern Pyrenees. This river runs transverse to the main thrust fault structures and is a major conduit for sediment delivery to the Ebro Basin. Detailed field mapping, combined with long-profile and palaeohydraulic reconstructions, yields a comprehensive picture of changes in palaeoriver character during the Plio-Pleistocene. As the area is over 150 km from the basin outlet in the Mediterranean Sea, changes in base level are unlikely to have influenced terrace development. Although tectonic activity has exerted a strong control on the position of the river, the main period of thrust propagation pre-dates the terraces and activity has waned from the Pliocene through to the present. It is concluded that the main control on incision in this area is climate, through its influence on sediment supply. Rivers which are starved of sediment by climate change will have the power to incise, whereas aggradational phases are linked to periods of increased sediment flux.