the Creative Commons Attribution 4.0 License.
the Creative Commons Attribution 4.0 License.
| 24 Mar 2026
Structural drivers of exhumation in compressional orogens: Examples from western Nepal
Abstract. The magnitude and location of vertical uplift in fold-thrust belts is a function of the geometry, duration, and timing of faults and how these structures have evolved over time. Yet in the Himalaya, uncertainties persist in whether vertical uplift and exhumation are driven by sustained displacement over mid-crustal ramps in the basal décollement, pulses of more rapid exhumation during periods of out-of-sequence fault displacement, or a combination of these drivers. In western Nepal, the well-defined zone of steep slopes and high relief that marks the high Himalaya in central Nepal splits into two zones: a northern zone ~10 km south of the Main Central thrust (MCT) and a southern zone ~80 km south of the MCT. While geomorphic metrics indicate active uplift in the southern zone, ~5–10 Ma apatite fission track and (U-Th)/He ages limit the amount of young exhumation. In the northern zone, <6 Ma muscovite 40Ar/39Ar ages indicate significant exhumation. We evaluate variations in ramp geometry and kinematic sequence, particularly the importance of out-of-sequence faults, necessary to reproduce the observed cooling ages, topography, and geomorphic metrics along the Simikot transect by integrating new and published cooling ages, basin accumulation data, and geomorphic uplift indicators with thermokinematic and landscape evolution models of three balanced cross-sections. Model results demonstrate that the northern zone of high relief and young exhumation is a combination of sustained uplift over an active ramp and recent motion on an out-of-sequence fault at ~5 km south of the MCT. The southern zone of high relief is produced by active (<0.6 Ma), but low displacement, surface breaking and subsurface faults. Thermokinematic model results emphasize the importance of a northernly ramp location, co-located with the youngest measured cooling ages at ~13 km north of the MCT, and of the out-of-sequence thrusting at ~6–5 Ma and <1 Ma.
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Status: final response (author comments only)
- RC1: 'Comment on egusphere-2026-1560', Anonymous Referee #1, 23 Apr 2026
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RC2: 'Comment on egusphere-2026-1560', Anonymous Referee #2, 25 Apr 2026
General comments
Overall, this is a well-executed study that compiles an impressive thermochronological dataset, (new and published) with basin accumulation rates, thermokinematic and landscape evolution models to investigate structural controls on exhumation in western Nepal. The integration of balanced cross-sections with thermal constraints is a methodologically sound approach, and the effort required to synthesize and iterate across this volume of data is commendable. In general, the work makes a valuable contribution to understanding Himalayan thrust belt tectonics and is relevant to EGU Solid Earth. The paper is well structured and written, the title and abstract accurately represent the content, and the figures are clear and easy to follow. The central conclusion of the work is that the structure, especially MHT ramp geometry, is a primary driver of observed cooling dates, basin accumulation rates and topography from Simikot transect in western Nepal Himalaya. The authors evaluate this with thermokinematic models, which are in turn compared with landscape evolution models to support the main conclusion. However, I have several questions and comments below.1. Averaging and treatment of thermochronology data
A study of this scope inevitably requires averaging and generalization of data, especially when compilations span a large spatial extent and datasets from multiple published work. However, my biggest concern is the treatment (mean/averaging) of ZHe and AHe ages (as mentioned in Line 96). If all samples rapidly cooled, then the ages can be justifiably averaged but if there is inter-grain variability, averaging cooling ages can yield geologically meaningless dates. These are important for ensuring that the ages input into the thermokinematic models genuinely reflect cooling rather than grain-scale artifacts. Additional information on the treatment of thermochronology data (screening of outliers? age-eU correlations?) would help strengthen the authors use of averaged datasets.2. Pecube modeling details
As I (and many readers) are not deeply familiar with Pecube, can the authors clarify the use of constant and /or variable shortening rates? Is this routine or deliberate methodological choice? For variable shortening, the authors mention that it was based on cooling ages (depth, depo ages) from the foreland which is reasonable, but what were the range/uncertainty of the variable shortening? This context would strengthen the reasoning. Broadly, please add more information about the iterative process that produced Model C. Were Models A and B evaluated and adjusted until Model C yields a satisfactory fit? Or was Model C independently assessed and yielded a better fit? Additional details about what changes were made between iterations, how many iterations were attempted, and why >80% fit threshold was adopted to select models for CASCADE will be good for clarification. Also, in Figure 6, both displacement and velocity varies between each model slip. How was the required displacement evaluated? How sensitive are the models are sensitive to these parameters, especially the velocity (varies 24, 48, 45, 16, 15, 18, 20 mm/yr in Figure 6)?On a different note, the authors should move the discussion of the South Tibetan Detachment System and Tethyan Himalayan rocks from supplementary in the main text. Given that Soucy La Roche et al. (2016, 2018) document STDS and THS rocks in western Nepal in the Dadeldhura klippe, the authors should briefly clarify it in the main text.
3. Discussion of alternate drivers of cooling and exhumation
Firstly, the authors do an intensive interrogation of existing structural models (Model A-B) to test with observed independent thermochronology and basin accumulation rates, but the authors do not account for an erosional signal that could be independent of ‘tectonics’. For instance, other work adjacent to the study area in western Nepal (e.g., Sherpa et al., 2022) suggests, based on preservation of a an anomalous high-elevation low-relief surface and cooling ages, that low-T thermochronology results highlights an erosional signal due to a northward propagating fluvial system. Do the authors think that the ramp is the strongest control or there is no erosional control at all? I recognize that the authors do briefly talk about fluvial incision required to replicate topography in Lines 425-427 but I encourage the authors to discuss alternate explanations.Specific comments
Line 15. Are there any ZHe, AFT, ZFT, AHe data from the northern zone? Apatite, zircon fission track and (U-Th)/He thermochronometers have vastly different closure temperatures from muscovite Ar-Ar thermochronometer so the cooling dates may be documenting something different, unless they are all within same age range (within uncertainty).
Line 17. Add ‘in western Nepal’ after Simikot transect.
Line 42. Please add citation for the AHe, AFT study.
Line 46. Please add citation to the thermochronology studies or figure in the manuscript.
Line 110-113. Please rethink this line. It is unclear how prograde path documented for rocks in NW Indian Himalaya (Corrie) and Bhutan Himalaya (Long et al., 2016; Long and Kohn, 2020) can be meaningfully related to this sector of the western Nepal Himalaya. Metamorphic histories are often sample specific, depending on when the rock equilibrated, and need to be carefully evaluated given the lateral heterogeneity in deformation and metamorphism across the orogen.
Line 155. Were the input ZHe and AHe ages raw or corrected ages? Please add.
Line 156. Please reconsider these lines. The model excludes distributed ductile shearing and the justification is that the model reproduces peak temperature gradients well but thermal fit alone cannot confirm the absence of a deformation mechanism.
Line 159. Some samples deviate spatially laterally from the transect significantly, why is a lateral
error of ±0.5 km applied?
Line 163. For the MAr ages, are the ages used as input in the models plateau ages, integrated or isochron ages?
Line 238. What is considered a best-fit variability velocity and how is it determined? Please add more details for readers to follow.
Line 433. Please replace dates with ages for consistency.
Line 449. Please specify U-Th or Th-Pb ages reported for monazite by Braden et al., 2020.
Line 472. A sentence about why the conclusions of this work is scientifically relevant can be valuable for the general audience to understand the novelty and importance of this work.Figure comments
Figure 1. Please remove coordinates so that it’s not repeated. There is a lot of information in the figure and it adds to visual load. Maybe increase the size or bold the outline of the new sample locations so it’s immediately visible. It was hard to spot which of the samples are new datapoints.
Figure 3. Please add briefly in the label or add citations (TopoToolbox etc) to show how the swath profile, mean, max/min and median Ksn were plotted in panel 3b.
Figure 4. The Fit % within the panels are different from Total Fit%. Please describe why they are different or have brief additional information in the figure label.
Figure 5. Same comment as for Figure 4.
Figure 6. Regarding the thermal field, as deformation propagates toward the frontal thrust belt, what is the assumed effect on the geothermal gradient, or is there any? The modeling appears to treat heat flow from the basal fault as the primary thermal control, with deformation propagation on individual having minimal influence. Is there model sensitive to geothermal gradient variations in the individual sheets?
Figure 8. How is the exhumation rate calculated? Please add brief details for the reader either in text or label.
Table 1. What is the reasoning for averaging a mean cooling age for samples with inter-grain variability (eg., 98)? These can be geologically meaningless age.Citation: https://doi.org/10.5194/egusphere-2026-1560-RC2
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- 1
This is interesting contribution - where the authors reconstruct the deformation in space and time in an improved way. Also they illustrate by using the extended thermochronologic dataset - advanced thermal modelling across the range of Himalayan transects in western Nepal to helps to prove or disprove two previous published cross sectional modells - favor a reconstructed new cross sectional model..
Compared to other studies - they provide detailed solution, how the MHT looks like and have geologic/stuctural solution, what kind of processes form ramps in the MHT and how they look like ( a much debated topic and first order structural features of the Himalaya - still not well constrained.
Also there modells have very interesting implication - showing dramatic changes within deformation of the orogenic wedge dispite a plate tectonic mostly constant underthrusting rate of the India versus Eurasia.
Fig. 6 is one of the key figures of the study - where the authors show reconstructed and favoured balanced cross section at various time steps and the proposed tectonic evolution and change of the crustal thermal field related to the deformation history.
However, its a bit unfortunate that the authors miss to define all parameteres presented in figure capture of Fig. 6 - used for optaining the modelling results. Crusial parameters are velocity and displacement - what do they exactly mean. This needs to be introduced better and referred to the other studies of the authors - where these parameters has been studied in more detail.
As the reviewer most likely understand - displacement is total fault offset of the active fault. Velocity is more challing to understand - but is likely the shortening rate of section studien of the orogen.
If the reviewer understand to results correctly - the authors suggest that this compartment of the orogen has accomodated changing ammount of the total shortening of the Himalayan over time since the Miocene until today. This is fundamental constraine to obtain good agreement between model predicted data and measured thermochron results. This imply that at middle Miocene time-window - the Himalaya accomodates nearly the entire plate-tectonic shorting betweem India and Eurasia. The authors have studied several studies of other segments of the orogen - which come to similar results - however - there assumptions/result are so fundamental - that they need to be introduced properly and make the reader aware of these temporal change in shortening. Even if they refer to two independent parameters - flexural banding and sediment filling of the forland basin, as well as cooling pattern of the orogen.
With this revised version of the manuscript they nicely show - that is potential agreement with other recent studies of the region - and they do not nicely explain - why they agree. Fig. 8 panel and based on their results, they suggest significant reorganisation of deformation across the range since the middle Miocene. Most prominent over the
Fig. 8 is very important - but the authors lack to discuss the implications - but rather leave the interested author making his own view - therefore the manuscript will only be appreacited by small community - that focus on this kind of research.
Last 5 Ma of cross section history forms super well constrained and important time window because it provides the best resolution of both geomorphic and thermal constraints. What do we learn from this - with respect to the earlier evolution? For instance the authors propose that out-of-sequence deformation from there studied section is only recognized over the last ~five million years of Himalanyan evolution in western Himalaya. Also major tectonic reorganzation are proposed within this time fame - due to avaiabillity of the geomorphic parameters - which are lacking for earlier time windows. In figure 6 they have to add a new ramp at ~-70 km south of the MCT into the lower thrust nappe - with out any geologic evidence - just to match topographic parameters from the surface. This illustrates how much interpretation the presented section are.
Even if the first order topography is modelled - this will be avaraged to mean topography. The dramatic changes they propose of the exhumation rates related to changes in evolving ramps (Fig. 8 upper panel) - implies that the models topography is oversimplied - and most likely orogenic wedge formed actovity out-of-sequence fault or reactivated form ramps several time - however this information is lost and most likely due the lowering of resolution of the thermal history during middle and early Miocene evolution. Here only the realy first order deformation processes are recognized. Also the mega-ramp for the onset of modelling - is simplied solution to generate usful thermal field for the stating of the modelling - however have no geologic evidences for this set-up. All these implication - need to be presented in some why or other - to help the reader to understand and appreaciate the great work of the authers.
In Fig. 8 the authors present exhumation pattern of MBT hanging wall - however have poor or no data to constrain this history in more detail. After there reference frame the MBT is exposed at the surface at -120 km south of the MCT. First ZHe and AFT are at ~110 km south of MCT. In other segements of the Himalaya obtained thermochron data (ZHe and AFT) in vicinity of the MBT yield reset ZHe (< 3 Ma) and the pattern of young AFT ages imply deep ramp likely linking directly to the MHT - horizontally running about ~40 km into MBT hannging wall.
Furthermore in Fig. 8 south of the MCT (-40 km to the MCT) they are not able to reconstruct the tectonic evolution of early Miocene - as most likely vigerous erosion has removed cover units of the LH duplex. Also the studied section in Western Nepal provided very little thermochronologic constraints for the hanging wall of the MCT (data are <10 Ma). However - other sections of the Himalaya data more data are awailbable. Therefore the presented modells of exhumation pattern from the MCT hanging wall for the early Miocene is not well constraint by the presented data and a lot interpretation is used here. Therefore much more out-of-sequence deformation of direct MCT foot- and hanging wall (RMT-duplex and 20 km of MCT hanging wall) during Early and Middle Miocene might not be recognized by the modelling and available data. Ditrital Ar/Ar data from Siwaliks from Szulc et al., 2006 a study the author refer to - shows that the maximum exhumation of Greater Himalaya during early Middle Miocene is 3.6 mm/yr - based to short lag times (<4 Myrs) between depositional and cooling age between 16-13 Ma. This do not match with exhumation proposed from the modelled sections - see Fig. 6. However - this is not discussed. All these are limitation the authors do not or partly miss to discuss or present in any way. The reviewer gets to some degree the impressiong the authors do a kind of cherry picking - just choosing studies that support obtained results - neglecting studies contradicting obtained results.
Tectinal suggestion and comments:
Line 68-90 - 22. Basin Record:
Introduce also other parts of the study by Szulc et al.: Ditrital Ar/Ar data from Siwaliks from Szulc et al., 2006 and discuss later the mismatch between your results and their results.,
Line 135 - how can you publish thermokinomatic modelling results without having published new thermochron-data ???
Line 175: OK, very good. What are amd where in manuscript are these these predictions tested and confirmed?. Is this also confirmed and varified for the "RMT duplex and MCT-hanging wall ? What are the measured peak-temperatures in "RMT duplex" and MCT-hanging wall of Western Nepal???
Line 322-324: This is most tricky part of the reconstruction - because authors have very little independent information to constaine this. Entire new faults and not reactivation are suggested by the authors - to improve the fit between Modell and topographic parameters.
Thus are we at a transistion to major reorganisation of the deformation with Western Nepal over the last million year - as others have suggested? Even if the location of ramps of MHT are different in this solution - other (e.g. Harvey et al., 2015) have concluded that Western Nepal might in process of tectonic transistion???
Line: 325-329: Results of Model C shown in 'Figure 5. Is the revised geometry and kinematic sequence new - or is it based on earlier work?
Line: 335: Figure 6: Please define - Displacement and Velocity in figure capture - so interessted reader better unterstand - how to read the figure. The result are very interesting - and potentieally have a lot of implication - for a overall understand of the Himalayan wedge deformation - not only for western Nepal Himalaya - but rather Himalayan evolution in general.
These changes in velocity of the model - which relates to shorting rate within this segment of the orogen - is some degree different to the results presented of models of the best results yield in parallel but earlier published study in Tectonics 2023 by the authors - why is that?
In (a) the authors present the Initial model configuration. Try introduce here a kind of "Mega Ramp" in the MHT at 25 Ma - however do not define, if the have field observation or other constraints justify this. What is motivation - or is it way to try set up crustal thermal field fo the sections modelled later??? This has strong impacts of the modeled thermal field for the entire Miocene - therefore the reader needs more information about this and motivation - explanation why this is justified.
Line 432: Exhumation drivers in the western Himalaya - Please explain better what the effect of the shortening rate changes shown in Fig. 6 between ~15 to 13.5 Ma - Model step 32 - 39. This is indeed very interesting - if you have good reasons. Is this change fundamental to get a good fit between modelel and measured data???? But also discuss the results other thermochronologic approaches such as Szulc et al.,
2006 or DeCelles et al., 2020. Aren't the models fits very strongly supporting work by Wobus et al., 2006 - - Model step 56 - Out-of-sequence deformation in the vicinity of the MCT with Late Miocene reset Ar/Ar-Mica data???
Line 455-460: This the most interesting part of the study - however much to surfically and short - this needs to be significantly extended - also to really appreciate the lovely findings of the study and all the implications - if the setting favour by the authors are getting close to how the tectonic evolution of the frontal Himalaya have evolved since the Middle Miocene - Pleiocene - until today.
This part the of the discussion is the most important part - to better understand the results - and implication the authors have worked out.
It is good choice to focus on the last five Ma - show here the entire strength of the thermochronolgic approach in active orogen such as the Himalaya - vigerouse erosion and exhumation rates. Combining both geomorphic and other constaints and setting to the results in context to the model results but also to other studies illustrates its strength. It is most likely here - where the models the authors present are best justified - because the input parameters have their highest resolution.
At the same time these results have a lot of implication - which are lost and not mentions by the authors.
They illustrate the spatial deformation pattern has changes on million to two million year time scales in different compartments of the orogen. Some of the parameters here available are much less well constraint for the earlier time steps of the modells. Also detailed resolution of the thermochronolic dataset lowers significantly at earlier time windows studied here. This implies that the presented results might show nicely the first order cooling pattern - however lack to exactly reconstract earlier evolution in the same detail - as this apporach is able to do of the last five Ma. The reviewer is convinced that the authors are aware of the this - however not readers will be aware of this. 'This has also strong implication, how reliable the results presented in Figure 8 are justified - especially of the earlier history or the orogen. Compare the again differences ob the obtained model results with Szulc et al., 2006 or DeCelles et al., 2020. Not only with Harvey and Burbank, 2024)
Unterstanding the last five million in this detail means - for instance also earlier time windows were affected most likely by significant deformation switches forth and back in million to two million year timescales - but this likely beyound the resolution to the presented datasets for earlier timescales. Here plays the critiacal collomb wedge models an important role - how the Himalayan orogenic wedge deformed during shortening and erosion through time.
Therefore beside the first order discussion on the architecture of the MHT and this part should be discussed in much greater detail - to better appreciate the strength of this approach and what we can learn from it. This is sometimes a bit lost in just listing the first order aspects of the study.
Also please consider a small paragraph on temporal resolution of the study and which details (parameters) can be considered.