Segmentation and rejuvenation of the Greater Himalayan sequence in western Nepal revealed by in situ U–Th/Pb monazite petrochronology
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Main Central Thrust
Main Central Thrust
Classification of discontinuities
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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.
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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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Lineation
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Mylonite
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Tectonite
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Abstract Understanding, and ideally quantifying, the relative roles of climatic and tectonic processes during orogenic exhumation is critical to resolving the dynamics of mountain building. However, vastly differing opinions regarding proposed drivers often complicate how thermochronometric ages are interpreted, particularly from the hinterland portions of thrust belts. Here we integrate three possible cross‐section geometries and kinematics along a transect through the eastern Bhutan Himalaya with a thermal model (Pecube‐D) to calculate the resulting thermal field and predict potential ages. We compare predicted ages to a suite of new and published cooling ages. Our results argue for ramp‐focused exhumation of the Main Central thrust from 16 to 14 Ma at shortening rates of 40–55 mm/year, followed by slower rates (25 mm/year) during the last 50 km of Main Central thrust displacement and growth of the Lesser Himalayan duplex from 14 to 11 Ma. Emplacement of frontal Lesser Himalayan thrust sheets occurred rapidly (55–70 mm/year) between ~11 and 9 Ma, followed by a decrease in shortening rates to ~10 mm/year during motion on the Main Boundary thrust. Modern shortening rates (17 mm/year) and out‐of‐sequence motion on the Main Boundary thrust from 0.5 Ma to present reproduce the young cooling ages near the Main Boundary thrust. We show that the dominant control on exhumation patterns in a fold‐thrust belt results from the evolution of ramps and emphasize that the geometry and kinematics of structures driving hinterland exhumation need to be evaluated with their linked foreland structures to ensure the viability of the proposed geometry, kinematics, and thus cooling history.
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Thermochronology
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It is well documented that in tectonically active regions, fluvial morphology responds to changes in base level. Vertical incision rates are adjusted through changes in channel morphology to balance imposed rates of rock uplift. It is common for responses in channel width, slope, grain size distribution and stream power to reflect spatial and temporal changes in rock uplift rates. Channels within tectonically active orogenic belts, such as the Himalayas, may be influenced by changes in discharge or sediment supply in addition to tectonic controls. Identifying the causes of morphological response in such areas is therefore of first order importance to enhance our understanding of landscape evolution. The Nepalese Himalayan foreland presents the ideal location to undertake this investigation, with its distinct tectonic frameworks heavily influenced by the style of foreland basin development. Thin-skinned thrust faulting over the past 1.6 Ma, has facilitated the recycling of foreland basin fill into hanging wall deposits of the frontal thrust (HFT), producing topographic entities now recognised as the Siwalik Hills. Above weak basal decollements, Dun valleys separate the frontal Siwalik Hills, and have rapidly filled with erosional detritus from the rising Himalaya. Poorly consolidated lithologies within the Siwalik Hills and Dun valleys are now being remobilised by modern incision of Himalayan River systems. It is unknown whether patterns of sediment storage and release within the foreland affect river morphology.
To understand the controls behind Himalayan river morphology, longitudinal profiles and channel slope were extracted from 90 m digital elevation models along the Gandak and Kosi Rivers about the Himalayan mountain front. Remotely sensed channel width measurements have also been made, and further supplemented with grain size data derived in the field and analysed using photo sieving techniques. Short-lived increases in channel slope are noted at the Main Boundary Thrust and Main Dun Thrust (MDT) of both rivers, in addition to a decrease in slope upstream of the HFT and Kosi Main Central Thrust (MCT). Increases in channel width upstream of the HFT and MCT (Kosi) are also consistent with morphological response to tectonic uplift. Where characteristic responses in morphology are absent at identified tectonic structures, it is likely that changes in lithology or anthropogenic modification of flow have overwhelmed tectonic influences. No increase in grain size upstream of recognized fault locations was noted on the Gandak, and it is proposed that an alternative mechanism dictates grain size patterns at the mountain front. On passing downstream of the MDT, an absence of direct hill slope inputs and a greater proportion of seasonal tributary inputs is reflected by a narrowing of grain size distributions, loss of Greater Himalayan lithologies, and decrease in D84 (by 50 mm).
These differences between geometry and grain size of the Gandak and Kosi Rivers are interpreted in terms of the style of foreland basin evolution. A lack of foreland accommodation above the strong basal decollement of the Kosi River facilitates continuous exportation of erosional detritus out of the mountain front. Widely spread D84 grain size distributions along the Kosi are dominated by regular inputs of coarse hill slope material from unstable relief produced by exceptional rates of uplift, above closely spaced frontal tectonic structures. It is interpreted that the weak basal decollement characterising the Gandak region has produced more stable hillslopes and accommodation for sediment within the Chitwan Dun. The fine grained and well sorted grain-size distributions of the Gandak noted between the MDT and HFT reflect an absence of direct hill slope inputs and a presence of seasonal tributary derived material. This study concludes that active tectonic structures strongly influence channel geometry at the mountain front. Grain size patterns are believed independent to differential uplift, and are considered a function of lateral sediment inputs, the nature of which reflects the structural evolution and tectonic history of the foreland basin.
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Stream power
Tectonic uplift
Lithology
River morphology
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Main Central Thrust
Anatexis
Migmatite
Geochronology
Leucogranite
Batholith
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