Pulsed deformation and variable slip rates within the central Himalayan thrust belt
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Forward modeling reconstructions and data derived from the Himalayan thrust belt and the foreland basin of far western Nepal tie the erosional unroofing and associated deposition to the kinematics and age of fault motion. We reproduce the deformation identified at the surface through a forward-propagating, linked fold-and-thrust belt–foreland basin system. This approach permits estimates of the magnitude of erosion at each time step and the extent, depth, and age of the associated foreland basin. The model reconstructions reveal that the units that supplied the sediment to the foreland basin changed through time: 25–13 Ma, erosion of the Tethyan Himalaya; ca. 12 Ma, first exposure of the Greater Himalaya; ca. 11 Ma, first exposure of the Lesser Himalaya. In our model, exposure of Greater Himalaya and Lesser Himalaya rock is associated with the formation of a thrust ramp that cuts through 7 km of footwall Lesser Himalaya stratigraphy and translates >7 km of Lesser Himalaya rock over the ramp, forming a Lesser Himalaya duplex. An increase in structural relief focuses erosion over the region of the ramp and facilitates exposure of Greater Himalaya and Proterozoic Lesser Himalaya rocks. As the Lesser Himalaya ramp propagates southward, more Lesser Himalaya thrust sheets are incorporated into the Lesser Himalaya duplex. Although uniquely dating thrust events is challenging, these model reconstructions allow us to associate time steps with an age of deposition or exhumation. What emerges is a tempo of deformation that varies with time, marked by periods of rapid shortening during propagation of the Main Central thrust, Ramgarh thrust, and middle stages of the development of the Lesser Himalaya duplex (∼25–30 mm/yr). After emplacement of the Ramgarh thrust, early and late stages of Lesser Himalaya duplex development are marked by periods of slow shortening (∼13–14 mm/yr). Although long-term and modern (geodetic) rates of deformation agree at ∼20 mm/yr, rates of shortening through time have varied from 4 to 33 mm/yr.Keywords:
Main Central Thrust
Thrust fault
The Himalayan thrust belt is often cited as an example of a thrust system that propagated from hinterland to foreland; however, this kinematic sequence is not well documented, and the process of formation of the thrust belt has not been well supported. This study uses forward modeling and timing data to reveal a detailed view of the evolution of the central Himalayan thrust belt from the footwall of the South Tibetan detachment system southward to the Main Frontal thrust. By using a reasonable configuration of undeformed stratigraphy, the surface deformation in western Nepal can be dynamically reproduced, confirming that the cross sections from which the undeformed sections were derived are viable and propagated from hinterland to foreland. In addition, this study yields detailed step-by-step reconstructions of three cross sections and is the first of its kind in any thrust belt system. These detailed views are useful for understanding and bracketing erosion data, the basin sediments, and geodynamic models. Modeling shortening estimates are between 495 and 733 km from the Main Frontal thrust to the South Tibetan detachment system, and are within the range predicted for shortening in western Nepal obtained from balanced cross sections (485–743 km). Thus, the Himalayan thrust belt in western Nepal is essentially a forward-propagating thrust belt from hinterland to foreland, with minor out-of-sequence (<5 km) thrust and normal faults. The data and the forward modeling support a conventional wedge model for the development of the central Himalayan thrust belt.
Main Central Thrust
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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.
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Thrust fault
Mylonite
Allochthon
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In the Central Pyrenees, the influence of syn- to post- orogenic deposits on the southern foreland may have influenced the late evolution. During middle to late Eocene, thrust deformation in the Pyrenean fold-and-thrust belt migrated to the south with in-sequence piggy-back thrust development. Then from Oligocene to Miocene, conglomerates sourced from the axial zone buried the fold and thrust belt until the Ebro foreland basin. At the same time thrust activity migrated from the front to the internal parts of the orogen reactivating major thrust in the foreland fold and thrust belt and in the south of the Axial zone. The reason for the out of sequence thrust activity is still a matter a debate. Moreover, thermal modeling of thermo-chronometric data and recent (U-Th)/He analysis on apatites in this area indicates conglomerate infill in excess of 2 km thick. Although the effect of changing orogen critical taper through wedge top sedimentation is known from theoretical studies, it has been difficult to demonstrate this behavior for natural system. The main objective of this study is to understand coupling between tectonics and surface processes during formation of the Pyrenean fold and thrust belt, the causes of out of sequence thrust activity, and the possible relationship with conglomeratic wedge top sedimentation. We use an Arbitrary Lagrangian Eulerian (ALE) numerical model called Sopale to model the thin-skinned fold and thrust belt at upper crustal scales (7 km depth and 200 km long). Sopale takes into account the main parameters that influence the development of a fold and thrust belt such as flexure, strain softening of materials, erosion and sedimentation. Main controlling factors in these models include a detachment horizon, strain-softening of strong layers, flexural isostasy and the addition of erosion and sedimentation processes. The modeling is first focusing on the syn-orogenic part with wedge development coupled with syn-orogenic sedimentation. The sedimentation, affecting the taper angle, clearly modifies the behavior of the wedge with the development of long thrust sheets, over the decollement level. Then, a sediment cover that progrades towards the south with time is added to reproduce the syn- to post- orogenic infilling of the conglomerates. The numerical models of FTB formation show a strong sensitivity to syn-tectonic deposition. This presentation was supported by the EUROCORES programme TOPO-EUROPE of the European Science Foundation.
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Seismotectonics
Thrust fault
Main Central Thrust
Stress field
Fault plane
Focal mechanism
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The Rioni foreland fold-and-thrust belt is part of the Greater Caucasus pro-wedge and is one of the most important examples of the collision-driven far-field deformation of the Arabia-Eurasia convergence zone. Here we show the deformation structural style of the Rioni foreland fold-and-thrust belt based on seismic reflection profiles and regional balanced cross-section. The main style of deformation within the Rioni foreland fold-and-thrust belt is represented by a set of fault-propagation folds, duplexes, and triangle zone. The regional balanced cross-section shows that fault-propagation folds above the upper detachment level can develop by piggyback and break-back thrust sequences. Formation of fault-bend fold duplex structures above the lower detachment is related to piggyback thrust sequences. A balanced section restoration of compressional structures across the Rioni foreland fold-and-thrust belt provides a minimum estimate of shortening of −40%, equivalent −42.78 km. The synclines within the Rioni foreland fold-and-thrust belt are filled by the Middle Miocene-Pleistocene shallow marine and continental syn-tectonic sediments, forming a series of typical thrust-top basins. Fault-propagation folds and duplex structures formed the main structure of the thrust-top basin. The evolution of the thrust-top basins was mainly controlled by the kinematics of thrust sequences. Using end-member modes of thrust sequences, the thrust-top basins are divided into: 1) Type I-piggyback basin, 2) Type II-break-back basin, and 3) Type III—formation of thrust-top basin characterized by bi-vergent geometry and related to combined, piggyback and piggyback back thrust sequences.
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Syncline
Anticline
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Forward modeling reconstructions and data derived from the Himalayan thrust belt and the foreland basin of far western Nepal tie the erosional unroofing and associated deposition to the kinematics and age of fault motion. We reproduce the deformation identified at the surface through a forward-propagating, linked fold-and-thrust belt–foreland basin system. This approach permits estimates of the magnitude of erosion at each time step and the extent, depth, and age of the associated foreland basin. The model reconstructions reveal that the units that supplied the sediment to the foreland basin changed through time: 25–13 Ma, erosion of the Tethyan Himalaya; ca. 12 Ma, first exposure of the Greater Himalaya; ca. 11 Ma, first exposure of the Lesser Himalaya. In our model, exposure of Greater Himalaya and Lesser Himalaya rock is associated with the formation of a thrust ramp that cuts through 7 km of footwall Lesser Himalaya stratigraphy and translates >7 km of Lesser Himalaya rock over the ramp, forming a Lesser Himalaya duplex. An increase in structural relief focuses erosion over the region of the ramp and facilitates exposure of Greater Himalaya and Proterozoic Lesser Himalaya rocks. As the Lesser Himalaya ramp propagates southward, more Lesser Himalaya thrust sheets are incorporated into the Lesser Himalaya duplex. Although uniquely dating thrust events is challenging, these model reconstructions allow us to associate time steps with an age of deposition or exhumation. What emerges is a tempo of deformation that varies with time, marked by periods of rapid shortening during propagation of the Main Central thrust, Ramgarh thrust, and middle stages of the development of the Lesser Himalaya duplex (∼25–30 mm/yr). After emplacement of the Ramgarh thrust, early and late stages of Lesser Himalaya duplex development are marked by periods of slow shortening (∼13–14 mm/yr). Although long-term and modern (geodetic) rates of deformation agree at ∼20 mm/yr, rates of shortening through time have varied from 4 to 33 mm/yr.
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Thrust fault
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Main Central Thrust
Thrust fault
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The Rioni foreland fold-and-thrust belt which is part of the western Greater Caucasus pro-wedge is located between the Lesser Caucasus and the Greater Caucasus orogens and is one of the most important examples of the collision-driven far-field deformation of the Arabia-Eurasia convergence zone (Alania et al., 2022). The Rioni foreland fold-and-thrust belt sedimentary infill consists of pre-and syn-orogenic sequences. Moreover, recent GPS and earthquake data indicate that the Rioni foreland fold-and-thrust belt is still tectonically active and the earthquakes’ focal mechanisms are mainly thrust faults (e.g., Tibaldi et al., 2017; Tsereteli et al., 2016).Fault-related folding and wedge thrust folding theories were used to interpret 2D depth-migrated seismic reflection profiles and to construct the regional balanced and restored cross-sections across Rioni foreland fold-and-thrust belt. The balanced cross-section is approximately parallel to the trust transport direction and has a total length of 64 km. On the other hand, the amount of shortening obtained for this part of the regional balanced cross-section is 40% (-42.78km).The main style of the deformation within the thin-skinned Rioni foreland fold-and-thrust belt is represented by a set of growth fault-propagation folds, duplexes, triangle zone, and a series of thrust-top basins. The evolution of the trust-top basins was mainly controlled by the kinematics of thrust sequences and competing growth fault-propagation folds and building compressional structures of the Rioni foreland fold-and-thrust belt was governed by the Greater Caucasus basement crustal-scale duplexes propagation along detachment horizons within the cover-generating thin-skinned structures.Acknowledgment: This work was supported by Shota Rustaveli National Science Foundation (SRNSF) [Structural model of the Rioni foreland fold-and-thrust belt and the Southern Slope of the Greater Caucasus (The Tekhuri river gorge area) Grant #: PHDF-21-087]References:Alania, V., et al. (2022). Deformation structural style of the Rioni foreland fold-and-thrust belt, western Greater Caucasus: Insight from the balanced cross-section. Frontiers in Earth Science, 10:968386.Tibaldi, A., et al. (2017). Active inversion tectonics, simple shear folding and back-thrusting at Rioni Basin, Georgia. Journal of Structural Geology 96, 35-53.Tsereteli, N., et al. (2016). Active tectonics of central-western Caucasus, Georgia. Tectonophysics 691, 328-344.
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Décollement
Wedge (geometry)
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Syn-orogenic sediments provide fundamental information on the timing and modes of deformation in fold and thrust belts. In this study, biostratigraphic and structural analyses on syn-orogenic deposits have been carried out in a key area of the southern Apennines in order to constrain the evolution of the Miocene thrust front and adjacent foreland basin. Our data indicate that, by middle Miocene times, a thin-skinned fold and thrust belt had developed in the study area. This was characterised by a complex tectonic setting, including: (i) a wide foreland basin area to the NE, ahead of the thrust front, where volcaniclastic and then quartz-rich ("Numidian") sandstones were deposited; and (ii) a series of thrust-top basin depocentres—hosting different siliciclastic and mixed detrital carbonate-siliciclastic successions— SW of the thrust front. Foreland propagation of the deformation produced intense folding of the foredeep succession. Later shortening and refolding around steeply dipping axial surfaces affected all of the tectonic units exposed in the study area. The latter deformation could be associated with Pliocene "en-masse" emplacement of the whole thin-skinned fold and thrust belt — as a major allochthonous detachment sheet—on top of the Apulian foreland sequence which presently underlies the exposed thrust sheets.
Siliciclastic
Imbrication
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