Central Nepalese Himalaya Research Articles

The orographic influence on storm variability, extreme rainfall characteristics and rainfall-triggered landsliding in the central Nepalese Himalaya

The orographic influence on storm variability, extreme rainfall characteristics and rainfall-triggered landsliding in the central Nepalese Himalaya

Eocene Metamorphism and Anatexis in the Kathmandu Klippe, Central Nepal: Implications for Early Crustal Thickening and Initial Rise of the Himalaya

AbstractThe continental collision between India and Asia has been ongoing since early Eocene time, but the orogenic record is typically dominated by Miocene and younger deformation and metamorphism that largely overprinted earlier Eocene‐Oligocene events. This hinders our understanding of how crustal thickening responds to initial collision and when the Himalayan mountains initially rise. The advancement of spatially precise petrochronology techniques, however, has provided the means to see through the Miocene overprint and enabled the characterization of Eocene metamorphism in different parts of the Himalaya. The current study presents new monazite petrochronology and paired thermobarometry from the Kathmandu klippe in the central Nepalese Himalaya. These data reveal Eocene prograde metamorphism (44‐38 Ma) and partial melting (38‐35 Ma) under peak P‐T conditions of 730 °C–760 °C and up to 10.5 kbar. The migmatites within the Kathmandu klippe is equivalent to the Upper or Uppermost Greater Himalayan Crystallines and should have been exhumed during Eocene‐Oligocene. The new evidence of Eocene metamorphism and anatexis presented herein adds to a growing body of data detailing initial crustal thickening during the early continent collision. The mid‐Eocene crustal thickening event indicates that the Himalayan felsic crust was thickened to a depth of ∼35 km shortly within 10–20 Myr of the initial collision, which was probably responsible for the initial topographic rise of the Himalayan proto‐mountains. Characterizing the effects of this early orogenesis is critical in understanding the Himalayan architecture prior to the better‐preserved Miocene metamorphism and anatexis record and how the orogen may have been preconditioned for the younger stage.

Interseismic Coupling in the Central Nepalese Himalaya: Spatial Correlation with the 2015 Mw 7.9 Gorkha Earthquake

Geodetic measurements conducted in the Himalaya over the last two decades have shown that the shallow portion of the main himalayan thrust (MHT) was entirely locked during the interseismic period. The induced elastic strain accumulated on the MHT beneath the Lesser Himalaya was not released until the 2015 Gorkha Mw 7.9 earthquake, which ruptured the north edge of the locked portion of the MHT. We utilized our own Global Positioning System (GPS) data from southern Tibet, combined with published geodetic velocities, to quantify the spatial variations of the coupling that prevailed before the Gorkha earthquake. The refined coupling model shows that the MHT was strongly locked (coupling > 0.5) in the uppermost 15 km of crust, corresponding to a downdip width of ~ 100 km. This model suggests a sharp transition zone of strain accumulation, with a rapid decrease in the coupling coefficient from 1.0 to less than 0.2 along ~ 50 km of the MHT, coinciding with the locations of microseismicity. We also determined slip models for the 2015 Gorkha earthquake and its Mw 7.3 aftershock, considering the ramp–flat–ramp–flat structure of the MHT. We found that ~ 85% of the total moment released by the Gorkha earthquake was concentrated on the partially coupled transition portion of the MHT, indicating that the earthquake mainly ruptured the brittle/ductile transition zone. The coseismic Coulomb failure stress increased along the southern and western parts adjacent to the rupture zone, pushing these two regions closer to failure. The moment deficits that have accumulated in these regions could trigger Mw 8.0 and Mw 8.3 earthquakes, respectively.

Correlations of fluvial knickzones with landslide dams, lithologic contacts, and faults in the southwestern Annapurna Range, central Nepalese Himalaya

We investigate the role of landslide dams, spatial changes in lithology, and rock uplift on faults in the formation of knickzones on bedrock rivers. We focus our analysis in the southwestern Annapurna Range of the central Nepalese Himalaya where detailed geologic maps, topographic data, field observations, and aerial photographs are available. We identified knickzones in our study area from normalized river steepness indices (ksn values) extracted from river longitudinal profiles derived from a 25 m digital elevation model we interpolated from digitized topographic map contours. We compared the location of these knickzones with (1) lithologic contacts and faults from a detailed geologic map of the Modi Khola valley and (2) inferred ancient landslide dam features mapped from field observations and aerial photographs. The steepest location on the Modi Khola occurs near the same latitude as the steepest reach on the Mardi Khola located directly to the east, potentially highlighting a major topographic transition across the Annapurna. However, we find that landslide dams once blocked the flow of the Modi Khola, and damming followed by incision after landslide breaching can explain the location of these knickzones without the need for active faulting near the Main Central thrust. We also conclude that (1) knickzones do not correlate with any spatial changes in lithology and (2) knickzones generated by rock uplift on unmapped faults cannot be ruled out. We emphasize that disentangling the processes responsible for knickzone formation remains challenging even when high‐resolution geologic and topographic data are available.

A Late Miocene acceleration of exhumation in the Himalayan crystalline core

Unraveling the relative roles of erosion and tectonics in shaping the modern topography of active orogens requires datasets documenting spatial and temporal patterns of exhumation, surface uplift and climatic forcing throughout orogenic growth. Here we report the results of biotite 40Ar/ 39Ar incremental heating and single-grain laser-fusion experiments from a nearly vertical, ∼ 1000 m age-elevation transect in the central Nepalese Himalaya. Age-elevation relationships constructed from these data suggest very slow cooling in this part of the Himalayan crystalline core during the Early Miocene, accelerating to only moderate rates at ∼ 10 Ma. If we assume purely vertical exhumation and a steady-state thermal structure, the exhumation rates implied by these data are ≪ 0.1 mm/yr prior to 10 Ma and ∼ 0.5 mm/yr from ∼ 10–7 Ma. The acceleration in cooling rate at 10 Ma requires a change in kinematics that may be linked to large-scale changes in climate, or to more local tectonic perturbations. Although we do not presently have enough data to assess the relative roles of regional vs. local drivers, these data provide a new constraint on exhumation through the Miocene that must be honored by any model of Himalayan evolution.

Topography, exhumation pathway, age uncertainties, and the interpretation of thermochronometer data

The relationship between thermochronometer age and structural elevation is commonly used to infer long‐term exhumation histories. Previous studies suggest that inferred exhumation rates from the conventional (one‐dimensional, 1‐D) age‐elevation approach are sensitive to topography and variations in exhumation rate and pathway. Here we evaluate the magnitude of these effects by (1) using a 3‐D thermal‐kinematic model of the central Nepalese Himalaya to predict age‐elevation profiles for multiple thermochronometers as a function of exhumation rate and pathway (vertical, oblique, or thrust fault), and (2) calculating the probability that the true exhumation rate will be recovered from an age‐elevation profile for sample uncertainties of different magnitudes. Results suggest that profiles oriented orthogonal to long‐wavelength topography and the direction of lateral transport are relatively insensitive to their influence. For profiles oriented parallel to the transport direction, horizontal transport during exhumation partly counteracts topographic effects. The difference between model imposed and 1‐D exhumation rates from the slope of a best fit line through an age‐elevation plot is greatest when rocks are exhumed vertically and low‐temperature thermochronometers are used. The magnitude of error in 1‐D exhumation rate estimates varies dramatically as a function of sample uncertainty, particularly when exhumation is rapid. The nature of this variation can be used to design sampling strategies for which 1‐D interpretations of age‐elevation gradients are likely to be within error of the true exhumation rate. Alternatively, if sample uncertainties can be reduced, studies that combine thermal modeling with age‐elevation data can potentially provide important constraints on thermal and kinematic fields at depth.

Plio‐Quaternary exhumation history of the central Nepalese Himalaya: 2. Thermokinematic and thermochronometer age prediction model

In the Himalaya and other active convergent orogens, linear relationships between thermochronometer sample age and elevation are often used to estimate long‐term exhumation rates. In these regions, high‐relief topography and nonvertical exhumation pathways may invalidate such one‐dimensional (1‐D) interpretations and lead to significant errors. To quantify these errors, we integrate apatite fission track (AFT) ages from the central Himalaya with a 3‐D coupled thermokinematic model, from which sample cooling ages are predicted using a cooling‐rate‐dependent algorithm. By changing the slip partitioning between faults near the Main Central thrust and the Main Frontal thrust system at the Himalayan range front, we are able to explore the significance of different tectonic scenarios on predicted thermochronometer ages. We find that the predicted AFT cooling ages are not sensitive to the different slip partitioning scenarios but depend strongly on erosion rate: Predicted ages are most consistent with kinematic models that produce erosion rates of 1.8–5.0 mm/yr. This range is considerably smaller than that derived from regression lines through the data (−2.6–12.2 mm/yr). The pattern of observed AFT ages shows more complexity than our models predict. None of the kinematic scenarios are able to fit >80% of all of the AFT data, most likely because erosion is spatially variable. Such complexities notwithstanding, we conclude that the use of thermokinematic modeling and thermochronologic data sets to explore detailed fault kinematics in rapidly eroding active orogens has great promise but requires integration of higher‐temperature (>∼350°C) data sets to maximize effectiveness.

Plio‐Quaternary exhumation history of the central Nepalese Himalaya: 1. Apatite and zircon fission track and apatite [U‐Th]/He analyses

New apatite and zircon fission track and (U‐Th)/He analyses serve to document the bedrock cooling history of the central Nepalese Himalaya near the Annapurna Range. We have obtained 82 apatite fission track (AFT), 7 zircon fission track (ZFT), and 7 apatite (U‐Th)/He (AHe) ages from samples collected along the Marsyandi drainage, including eight vertical relief profiles from ridges on either side of the river averaging more than 2 km in elevation range. In addition, three profiles were sampled along ridge crests that also lie ∼2 km above the adjacent valleys, and a transect of >20 valley bottom samples spans from the Lesser Himalaya across the Greater Himalaya and into the Tethyan strata. As a consequence, these data provide one of the more comprehensive low‐temperature thermochronologic studies within the Himalaya. Conversely, the youthfulness of this orogen is pushing the limits of these dating techniques. AFT ages range from >3.8 to 0 Ma, ZFT ages from 1.9 to 0.8 Ma, and AHe ages from 0.9 to 0.3 Ma. Most ridges have maximum ages of 1.3–0.8 Ma at 2 km above the valley bottom. Only one ridge crest (in the south central zone of the field area) yielded significantly older ZFT and AFT ages of ∼2 Ma; we infer that a splay of the Main Central Thrust separates this ridge from the rest of the Greater Himalaya. ZFT and AFT ages from a vertical transect along this ridge indicate exhumation rates of ∼1.5 km Myr−1 (r2 > 0.7) from ∼2 to 0.6–0.8 Ma, whereas AHe ages indicate a faster exhumation rate of ∼2.6 km Myr−1 (r2 = 0.9) over the last 0.8 Myr. Exhumation rates calculated for six of the remaining seven vertical profiles ranged from 1.5 to 12 km Myr−1 (all with low r2 values of <0.6) for the time period from ∼1.2 to 0.3 Ma, with no discernible patterns in south to north exhumation rates evident. The absence of a trend in exhumation rates, despite a strong spatial gradient in rainfall, argues against a correlation of long‐term exhumation rates with modern patterns of rainfall. AFT ages in the Tethyan strata are, on average, older than in the Greater Himalaya and may be a response to a drier climate, slip on the South Tibetan Detachment, or a gentler dip of the underlying thrust ramp. These data are further evaluated with thermokinematic modeling in the companion paper by Whipp et al.

Influence of groundwater flow on thermochronometer-derived exhumation rates in the central Nepalese Himalaya

Research Article| September 01, 2007 Influence of groundwater flow on thermochronometer-derived exhumation rates in the central Nepalese Himalaya David M. Whipp, Jr.; David M. Whipp, Jr. 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar Todd A. Ehlers Todd A. Ehlers 1Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109, USA Search for other works by this author on: GSW Google Scholar Geology (2007) 35 (9): 851–854. https://doi.org/10.1130/G23788A.1 Article history received: 09 Feb 2007 rev-recd: 27 Apr 2007 accepted: 03 May 2007 first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation David M. Whipp, Todd A. Ehlers; Influence of groundwater flow on thermochronometer-derived exhumation rates in the central Nepalese Himalaya. Geology 2007;; 35 (9): 851–854. doi: https://doi.org/10.1130/G23788A.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Mountain topography creates variations in water-table elevation that drive groundwater flow. Consequently, advective heat transport by topography-driven fluid flow can modify the crustal thermal field and bias exhumation rates calculated from thermochronometer data. Although previous studies have considered the thermal effects of fluid flow, none has quantified the influence on thermochronometer ages. We use a steady-state three-dimensional coupled hydraulic thermokinematic finite-element model to simulate the influence of fluid flow on exhumation rates derived from thermochronometer data in the Nepalese Himalaya. Local hot springs suggest substantial heat transport by fluid flow and are adjacent to apatite fission-track samples. Model hydraulic conductivity controls the rate of fluid flow, and values characteristic of fractured rock (>>10−9 m/s) yield a fluid advection–dominated thermal field. Hydraulic conductivity is estimated by minimizing the misfit between predicted and observed hot spring thermal power. The best-fit hydraulic conductivity value of ∼5 × 10−7 m/s produces a fluid advection–dominated thermal field and older predicted apatite fission-track ages. To fit the observed age-elevation relationship, model-predicted ages require denudation rates that are ∼5 mm/yr, ∼200% higher than predictions from thermal models that do not simulate fluid flow. Thus, true exhumation rates can be substantially underestimated in orogenic systems where fluid advection is significant. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Neotectonics of the central Nepalese Himalaya: Constraints from geomorphology, detrital 40Ar/39Ar thermochronology, and thermal modeling

The southern flanks of the central Nepalese Himalaya correspond to a sharp transition in landscape morphology and bedrock mineral cooling ages that suggests a change in rock uplift rate. This transition can be explained by either (1) accretion of footwall material to the hanging wall across a ramp in the décollement separating India from Eurasia, thereby enhancing rock uplift rates above the zone of accretion or (2) out‐of‐sequence surface thrust faulting at the physiographic transition. Here we use geomorphic data, 649 new detrital 40Ar/39Ar cooling ages, and a simple thermokinematic model to evaluate which of these tectonic configurations is most appropriate for the central Nepalese Himalaya. We first define and delineate the physiographic transition in central Nepal using maps of knickpoints, river steepness indices, local relief, and the distribution of thick alluvial fill deposits. We then report new detrital 40Ar/39Ar data from two trans‐Himalayan transects, each of which suggests a rapid northward increase in the total amount of exhumation across the physiographic transition. Thermokinematic modeling suggests that either of the two developmental scenarios for the transition is plausible but that an accretion model is viable only under an extremely narrow range of conditions. We contend that the physiographic and thermochronologic data in our study area are most simply explained by recent out‐of‐sequence surface thrusting within the Lesser Himalayan metasedimentary sequence, approximately 15–30 km south of the mapped surface trace of the Main Central Thrust system. An important finding of this work is that there are substantial along‐strike variations in physiography and thermal history that reflect along‐strike changes in the degree and location of out‐of‐sequence surface thrusting.

The use of detrital mineral cooling ages to evaluate steady state assumptions in active orogens: An example from the central Nepalese Himalaya

The distribution of detrital mineral cooling ages in modern sediments has been proposed as a proxy for long‐term, catchment‐averaged erosion rates in developing orogens. However, the applicability of this potentially valuable tool hinges on restrictive assumptions regarding a catchment's steady state thermal and topographic evolution. In this paper, we outline a method by which these assumptions can be tested through statistical comparisons of cooling age distributions for detrital minerals and the hypsometric curves for their source regions using cumulative synoptic probability density functions. Our approach is illustrated with new detrital muscovite 40Ar/39Ar dates from the Marsyandi River valley, in the central Nepalese Himalaya. One of three studied catchments (Nyadi Khola) showed the strong correlation of hypsometry and cooling ages expected for steady state conditions over the 11 to 2.5 Ma time frame. The pattern of mismatch between hypsometry and cooling age distribution in the other catchments suggests that spatially nonuniform and transient erosional processes may be responsible for departure from steady state. Cooling age distribution comparisons for samples collected from nearby localities, samples collected in different years, and different grain size fractions from the same sample were used to evaluate sampling fidelity over a range of spatial scales (200 to 2590 km2). We found that approximately 50 analyses from a single sediment sample adequately characterize the cooling age signal for tributary catchments with simple erosional histories. However, because of temporally and spatially transient erosion, a specific detrital sample is unlikely to adequately characterize the complex signal in trunk stream sediments that integrate information from several large tributaries.

Active out-of-sequence thrust faulting in the central Nepalese Himalaya

Recent convergence between India and Eurasia is commonly assumed to be accommodated mainly along a single fault--the Main Himalayan Thrust (MHT)--which reaches the surface in the Siwalik Hills of southern Nepal. Although this model is consistent with geodetic, geomorphic and microseismic data, an alternative model incorporating slip on more northerly surface faults has been proposed to be consistent with these data as well. Here we present in situ cosmogenic 10Be data indicating a fourfold increase in millennial timescale erosion rates occurring over a distance of less than 2 km in central Nepal, delineating for the first time an active thrust fault nearly 100 km north of the surface expression of the MHT. These data challenge the view that rock uplift gradients in central Nepal reflect only passive transport over a ramp in the MHT. Instead, when combined with previously reported 40Ar-39Ar data, our results indicate persistent exhumation above deep-seated, surface-breaking structures at the foot of the high Himalaya. These results suggest that strong dynamic interactions between climate, erosion and tectonics have maintained a locus of active deformation well to the north of the Himalayan deformation front.

Kinematic model for the Main Central thrust in Nepal: Comment and Reply

In the Main Central thrust zone of the central Nepalese Himalaya, monazites decrease in age downsection between the Main Central thrust and Main Central thrust 1 (MCT-1 in [Fig. 1][1]) from ca. 22 to 3.3 Ma ([Catlos et al., 2001][2]). This astonishingly systematic result ([Kohn et al., 2001][3]) has

The landslide in the Surma Khola Valley, High Mountain Region of the Central Himalaya in Nepal

The Surma Khola catchment is situated between 2200–3410 m a.s.l. in an unsettled area with a quasi-natural Abies-Rhododendron arboreum - Rhododendron barbatum forest.The High Mountain Region of the Central Nepalese Himalaya is well known for its active geomorphodynamic processes caused by the Himalayan uplift, the steepness of the slopes and the enormous precipitation amounts during the monsoon.In the early hours of the morning of August 10th 1990, a landslide triggered by a light earthquake occurred over a width of 35 m on the western ridge of the Surma Khola Valley. For days great amounts of water had infiltrated into the slope from a leaking irrigation channel. Within minutes, 9000 m3 of material slipped and tumbled, dragging along the rhododendron-fir-fbrest and buried old slide material from earlier landslides 120 m further down. During the following 3 days, the suspended sediment concentration in the Surma Khola increased from 0.1 gl−1 measured before to a maximum of 8.1 gl−1. The calculated sediment transport increased from 66 gs−1 to approximately 23 kgs−1. During these 3 days, the specific suspension delivery totalled 4 tha−1, twice the annual suspended sediment delivery.

Rb-Sr ages of micas from the Kathmandu complex, Central Nepalese Himalaya: implications for the evolution of the Main Central Thrust

The Kathmandu complex, comprising the Bhimphedi and Phulchauki groups which range in age from Precambrian to Lower Palaeozoic, is situated in the frontal part of the Main Central Thrust sheet. Although it is likely that a tectonothermal event antedated the intrusion of early Ordovician granites, Rb-Sr dating of muscovites in deformed pegmatites suggests that the high-grade metamorphism in the Bhimphedi Group is of Tertiary age. These pegmatites cooled through c. 500°C at 18–14 Ma. Biotite Rb-Sr data indicate cooling through c. 300°C at 7.5 Ma. Thermobarometric and radiometric data from central Nepal suggest an evolutionary model for the emplacement history of the Main Central Thrust sheet involving cooling from a peak temperature c. 660°C to c. 500°C in 7 Ma, and loss by erosion or tectonic denudation of c. 4–11 km of section.

Monazite controlled [formula omitted] fractionation in leucogranites: An ion microprobe study of garnet phenocrysts

Rare earth element (REE) abundances in four garnet phenocrysts from two monazite-bearing leucogranites from southern British Columbia have been measured with an ion microprobe. This study examines the effect of monazite crystallization on the LREEs by using garnet as a monitor of elemental variations in the liquid. This approach virtually eliminates uncertainties associated with source-induced LREE fractionation. The garnets examined are spessartine-rich (Alm 70–75Sps 18–22Pyp 3–4Grs 3–4) phenocrysts that show primary igneous zoning and lack monazite inclusions. Garnet rims have zoning profiles (i.e., Eu to Ce) that are depleted by factors of five to ten relative to cores. Of the three models investigated to constrain processes controlling LREE zoning in garnet— 1. (1) variations in the garnet/melt distribution coefficients, 2. (2) liquid diffusion controlled partitioning and 3. (3) crystal diffusion controlled partitioning—the latter model best explains the observed zoning. This model is consistent with experimental studies suggesting that monazite has a low solubility in granitic melts. These results suggest that Nd model ages (calculated using measured 147Sm 144Nd ) in leucogranites should be viewed with caution, as is demonstrated by neodymium isotopic data from southern British Columbia and the central Nepalese Himalaya.

Possible thermal buffering by crustal anatexis in collisional orogens: Thermobarometric evidence from the Nepalese Himalaya

One-dimensional thermal models suggest that large thrust faults may exert first-order control on the thermal structure of collisional orogenic belts, and an increasing body of petrologic data may be interpreted as consistent with these models. We present new thermobarometric data from the central Nepalese Himalaya which imply that large sections of the Himalayan orogen may have been at roughly uniform temperatures during active thrusting, a phenomenon that is not predicted by existing models. We attribute this phenomenon to thermal buffering as a consequence of widespread in situ anatexis. If this process is common in the intermediate and lower crust of active orogens and if it is localized spatially, one-dimensional thermal models may produce an extremely oversimplified picture of heat transfer processes during collisional orogenesis.