Do fault systems impede fluvial incision in active orogens?

doi:10.21203/rs.3.rs-2096959/v1

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Do fault systems impede fluvial incision in active orogens?

https://doi.org/10.21203/rs.3.rs-2096959/v1

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It is widely accepted that tectonism generally enhances the incision of river drainage systems. However, the question as to why on a global-scale, rivers have not incised further or more deeply into orogenic plateaus to destroy terranes over long-timescales remains ambiguous. Here we hypothesize the diverse nature of regional tectonics could have impeded river erosion to maintain plateau topography, taking Yarlung River in southern Tibetan Plateau as a case. We constrain the incision history and effect of a N-S trending tectonic rift on the evolution of fluvial incision by the low-temperature thermochronology. Results show a fast cooling since ~ 7 Ma focused near the rift structures, but markedly reduced cooling in upstream and downstream regions. This indicates that the steep flank fault of the rift controlled local rapid exhumation as a knickpoint since that time. This coincides with an episode of rapid exhumation of Eastern Himalaya Syntaxis downstream. We propose that these two co-phased tectonic systems with the accelerated late Miocene extension of southern Tibet prevented upstream migration of river knickpoints, and impeded fluvial incision. Our study highlights that the activity of fault systems may hinder regional erosion, thereby facilitating the preservation of topography and high plateau in active orogenic belts.

Earth and environmental sciences/Solid Earth sciences/Geomorphology

Earth and environmental sciences/Planetary science/Geomorphology

Earth and environmental sciences/Hydrology

The landscape of an orogenic belt is constantly shaped by competition between tectonic uplift and bedrock river incision¹. It is generally accepted that tectonic uplift increases stream gradient, which conveys more water and sediment via orographic precipitation that facilitates faster incision^2–4, driving the headward propagation of river knickpoints^4–7. However, why rivers have not incised further or more deeply into active orogenic plateaus (e.g., Tibetan Plateau, Fig. 1a) to destroy uplifted low-relief terrane over geologic time remains uncertain. The traditional explanation is that pronounced aridity in the interior plateau defeats river incision, and concomitant formation of internally drained basins⁸. In addition, the bidirectional coupling between bedrock river incision and landslide and the resulted drop in landslide frequency along time could leads to a corresponding decrease in the rate of fluvial incision⁹. Further, highly local rock uplift^10–11, glacier and/or landslide damming^12–14, or dynamic feedbacks between tectonics uplift and erosion (i.e. ‘tectonic aneurysm’ model)^15–16 on the plateau margins have been also viewed as constraining features for the dissection of the plateau morphology by rivers. Thus, the controls on the erosional decay of orogenic plateaus remain controversial.

This debate exists especially in the southern Tibetan Plateau, where is characterized by a relatively low-relief and high-elevation landscape (Fig. 1b). The Yarlung River, the largest river in the southern Tibetan Plateau, flows eastward along the Indus-Yarlung Suture zone and then turns southward around the Eastern Himalayan Syntaxis (EHS), creating one of the deepest gorges (Fig. 1b) on Earth^11,15−17. Evidence from paleo-altimetry data¹⁸ and the sediments in the Himalaya foreland¹⁹ and Bengal basins²⁰ show that similar to present-day external drainage systems and high elevation in the southern Tibetan plateau persisted since at least 15 Ma. Meanwhile, the Tibetan plateau since the Miocene also underwent active east-west extension¹⁷, resulting in a series of N-S trending rifts developed across the plateau, especially in southern Tibet. However, it is uncertain how high elevations in the southern plateau have been sustained in the face of river erosion and fault activity.

Thermochronological ages show a younging trend from the central part of the Tibetan Plateau to the margins (e.g., northeastern and southeastern margins), suggesting that the plateau may have progressively grown and propagated northeastwards and southeastwards^21–22. This expanding uplift has been thought to accelerate river incision from the southeastern margin to the interior plateau^23–26, driving the propagation of river knickpoints into the plateau²⁷. However, some large knickpoints along river valleys coincide in space with local tectonic deformation. For example, knickpoints along the Yarlung River locally coincide with the late Cenozoic N-S trending tectonic rifts (Fig. 1b) that accommodate E-W extension in southern Tibet²⁸. This suggests that formation of these knickpoints may have been controlled by active fault-flank uplift. Whether the diverse activity of regional tectonics could have also inhibited knickpoint migration and thus reduced river erosion to maintain topography in southern Tibet remains unclear.

To further address this issue, the incision history of the eastern Yarlung River valley in the southern Tibetan Plateau was constrained by low-temperature thermochronology. We collected 23 granite samples along the Gyaca gorge (knickpoint) and the broad valley of the Yarlung River (Supplementary Note 1). A total of 22 apatite fission track (AFT), 6 apatite and 8 zircon (U-Th)/He (AHe and ZHe) ages were obtained (Supplementary Tables 1–3), and previously published thermochronological ages in the eastern Yarlung River valley were also compiled (Fig. 1c, Supplementary Table 4 and Fig. S1). Three-dimensional (3D) thermokinematic modeling was then undertaken to explore the effect of the N-S trending rift on the formation and evolution of the knickpoint and the incision of the Yarlung River.

Thermochronological data

New and published thermochronological ages (Supplementary Table 4) along the eastern Yarlung River (from west to east spanning ~ 330 km) are presented in Fig. 1d (see Fig. S1 for further details). Zircon (U-Th)/He (ZHe), apatite fission track (AFT), and apatite (U-Th)/He (AHe) ages range from 23.1 ± 1.4 to 7 ± 1.1 Ma, 17.5 ± 2.6 to 4.2 ± 0.4 Ma, and 9.25 ± 1.49 to 1.75 ± 0.22 Ma, respectively (Supplementary Table 4). The ZHe and AFT ages, except for the relatively young ages in the Gyaca gorge, generally show a gradually increasing trend from downstream to upstream (Fig. 1d). The AHe ages show remarkable youngest ages in the Gyaca gorge and older ages downstream and upstream of this gorge (Fig. 1d).

Thermal History

Cooling histories of the new and previous samples in the study area (divided into 6 Zones, see Fig. 2) were reconstructed using inverse thermal history modelling by the QTQt software³² (Fig. 2a-2f). The spatial and temporal variations of the rock cooling rate since ~ 12 Ma were calculated from the modeling results (Fig. 2g). The results indicate rapid cooling of the basement rocks in the eastern Yarlung valley between ~ 12 Ma and ~ 7 Ma, but the cooling rate has increased significantly in the Gyaca gorge and decreased dramatically in its upstream and downstream since ~ 7 Ma (Fig. 2g).

3d Thermo-kinematic Modeling

To further explore the mechanisms governing the regional erosion history, 3D thermo-kinematic modeling strategy using Pecube was applied (detailed setting of the models described in Supplementary Note 5). We constructed two scenarios in Pecube model, i.e. once without (Scenario A), and then with the influence of the Woca fault that bounds the rift structure (Scenario B). Based on our thermal history modeling results above, the first model (Scenario A) was set-up with a two-stage (12 − 7 Ma and 7 − 0 Ma) evolution history without Woca fault activity. More than 20000 iterations were run to find the best fitting model that constrains the incision rate of the Yarlung River. The best-fitting results exhibit a first stage of rapid incision rates of 1.98 km/Ma, followed by a second stage with a decreased rate of 0.45 km/Ma (Supplementary Table 6 and Fig. S3). The model predicted thermochronological ages are generally consistent with those actually observed for samples located > 25 km away from the Gyaca gorge. However, for the samples located nearby the Gyaca gorge (in the footwall of the Woca fault), predicted AFT and AHe ages are much older than those measured (Figs. 3a and 3e). These large differences suggest that Scenario A, which did not integrate normal fault movements, were unable to fit the spatial distribution of observed AHe and AFT ages (Figs. 3a and 3e).

In Scenario B, we added a normal fault to simulate a more complex uplift velocity field. The time of initial normal fault activity was set at 7 Ma based on our thermal history modeling results (Fig. 2g), and the fault trace at the surface follows that of the Woca fault (see Fig. 1c). Due to the lack of geophysical data constraining the deeper sections of the fault, we ran three distinct inversions in which we fixed the deepest segment to be horizontal at depths of 20, 30, and 50 km, respectively. The results show that the three models in Scenario B successfully reproduced the AHe and AFT ages in the Gyaca gorge (Figs. 3b-3d and 3f-3h), and that the spatial distribution of predicted ages is generally consistent with measured ages (Figs. 3b-3d and 3f-3h). This confirms that incision of the eastern Yarlung River was indeed strongly influenced by a normal fault since ~ 7 Ma.

Comparing the misfit values of the inversion for the three models in scenario B (Fig. 3f-3h, Supplementary Table 6), we found that values for the model coupled with a 30 (3.92) or 50 km-depth fault (4.07) are lower than that for a 20 km-depth fault (4.17). This would indicate that the depth of the Woca fault could reach up to ~ 30 km or more. Furthermore, these models predict a steep (68°∼75°) fault in the shallow crust, near the surface (Fig. 3c-3d), and a fault-slip velocity rate of 1.19 ± 0.03 km/Ma since 7 Ma (Supplementary Table 6 and Figs. S4-6). This finding is in very good agreement with the observed high-angle fault geometry of 60°~75°³³and the Quarternary slip rate of 1.2 ± 0.6 km/Ma³⁴. The model also shows that the incision rate along the eastern Yarlung River was up to 1.73 ± 0.07 km/Ma between 12 Ma and 7 Ma, followed by a rather slow incision rate of 0.18 ± 0.03 km/Ma upstream and downstream of the Gyaca gorge since 7 Ma (Supplementary Table 6 and Figs. S4-6). This is in general consistent with the thermochronology results as well.

Fault activity controls the formation of knickpoint

In a previous study²⁷, Schmidt et al proposed that the Gyaca knickpoint resulted from upstream migration of erosional waves starting from the Yarlung Tsangpo Grand Canyon (Fig. 1b) in response to uplift of the southeastern Tibet prior to ∼10 Ma²⁷. We confirmed that only headward incision of the Yarlung River since ~ 12 Ma due to uplift of the southeastern Tibetan Plateau²⁷ or intensified monsoon precipitation^35–36 cannot fit the remarkable young AHe and AFT ages in the Gyaca gorge (Fig. 3e). In addition, the rock outcrops in the Gyaca gorge and its downstream valley are mainly granitoids (Fig. 1c), thus lithological variation is not a major controlling factor for the formation of the Gyaca gorge. In contrast, a high-angle normal fault is required as indicated by the well-constrained Pecube model (Scenario B), to fit with an increased rock cooling rate at the Gyaca gorge since ~ 7 Ma (Fig. 3g). Therefore, we suggest that activity of the Woca fault has controlled the formation of the Gyaca knickpoint since ~ 7 Ma, and that a similar mechanism could also explain the formation of other knickpoints in the middle and upper reaches of the Yarlung River (Fig. 1b).

Mechanisms For Impeding River Incision In The Late Miocene

Based on QTQt and Pecube modeling results, a relatively high incision rate (~ 1.73 ± 0.07 km/Ma) of the eastern Yarlung River initiated at ∼12 Ma, with a slightly decreasing trend in thermochronological ages from east to west along the Yarlung River. Published thermochronometric data from the externally drained portion of the eastern^10,37−38 and central^39–40 Lhasa terrane, as well as from large rivers in the southeastern Tibetan Plateau^36,41, demonstrate that rapid exhumation rates (> 1 km/Ma) were pervasive across the southern and southeastern plateau between ∼17 and 10 Ma. These spatially large-scale synchronous rapid incision events most likely reflect enhanced Asian summer monsoon precipitation in the mid-Miocene^35–36 that promoted the headward erosion of the Yarlung River channels (Fig. 4a). However, the incision rate upstream and downstream of the Gyaca gorge has decreased dramatically with the rapid activity of the Woca fault since 7 Ma (Fig. 2g and Supplementary Table 6).

Thermochronometric data from rifts in the Tibetan Plateau^42–43 suggest that southern Tibet has experienced rapid late Miocene to Pliocene rift acceleration²⁸ (see Fig. S7). This rapid rift activity controls knickpoints, such as the Gyaca knickpoint, in the plateau interior. Further, the high rate of rift extension in south Tibet also facilitates thinning of the upper crust while its lower crust is thickened by ongoing compression^28,44−45. This could be a main driving force accelerating eastward crustal flow⁴⁶. The accelerated crustal flow at the Eastern Himalayan Syntaxis (EHS) drives the localized deformation and uplift^47–49, leading to the active coupling between crustal rock advection and river erosion in the EHS⁵⁰ since ~ 7 Ma (Fig. 4b). A stationary knickpoint in the EHS develops initially. In fact, Zeitler et al. suggested a sustained high-elevation base level for the Yarlung River in the eastern syntaxis since ∼10 Ma¹⁵. Although Wang et al. argued for a rapid uplift of the EHS at ∼2.5 Ma based on sediment fill immediately upstream of the Yarlung Zangbo gorge¹¹, we consider this as only one of several stages of EHS uplift, rather than the initial uplift episode. Furthermore, detrital thermochronological data from foreland basin sediments downstream of the EHS suggested an efficient coupling between tectonics uplift and erosion starting at 8 Ma⁵⁰, 7 − 5 Ma⁵¹ and/or 6 − 4 Ma⁵².

A current paradigm for the initiation of rapid exhumation in the EHS is the Tectonic Aneurysm model, in which spatially focused surface erosion driven by the Yarlung River locally accelerates rock uplift and exhumation of hot and weak crust at the syntaxes^{15–16, 53}. Evidence from sediments in the Himalaya foreland¹⁹, Bengal basins²⁰ and our thermochronometric data (Fig. 2) show that the Yarlung river was definitely set in course before ~ 12 Ma. This does not specifically address the role of the Yarlung River in driving initial rapid exhumation of the EHS, which is thought to have begun 8 − 6 Ma^50–52. In contrast, we find that a synchronous exhumation pulse in the rifts (Fig. S7) and EHS⁵⁰ since the late Miocene. This synchronous rapid exhumation implies that the tectonic system with the accelerated late Miocene extension of southern Tibet drives regional fault activity to control rapid exhumation and formation of knickpoints in the rifts and EHS.

With the stabilization of the knickpoints, river gradients above the knickpoints are commonly so low that fluvial incision may generally have difficulty in keeping up with increase in tectonic uplift rates. As a result, channel slope and stream power would decrease transiently above these Yarlung River knickpoints (Fig. 4b). Simultaneously, the reduced river gradient and sediment flux could promote upstream aggradation, burial of bedrock valley floors^13,54−55, and valley widening (Fig. 4b), which could have caused widespread backwater aggradation, forming broad valley trains occupied today by braided river systems (Figs. 1d and 4b). This braided river only incised when the stripping of deposited alluvial material exposes bedrock to processes of abrasion, weathering and plucking^55–56.

Hence, coeval tectonic systems (rifting upstream and uplift and exhumation of the EHS downstream) with the accelerated extension of southern Tibet drove the onset of knickpoints formation in the Gyaca and EHS since the late Miocene. These were subsequently adjusted and reduced the regional river gradient, which retarded the headward incision of the knickpoints, thereby facilitating stabilization of southern Tibetan Plateau topography since the late Miocene.

Actually, many other active orogenic belts grow first to a certain height, and then experienced laterally outward propagation⁵⁷. This expanding uplift generally increases the steepness of rivers, and promotes upstream erosion^24–26. But this expansion is not a simple one-dimensional process, it is also accompanied by strong regional tectonic deformation of the upper crust^58–59 (e.g., rifting, subduction, and strike-slip faulting) and lower crustal flow⁴⁶. Lower crustal flow in its turn will promote and further drive regional tectonic deformation of the upper crust. Thus, we propose that the activity of the local fault systems limited the migration of these river knickpoints as that on the eastern Yarlung River, which are still located in the location of regional tectonic deformation (Fig. 5). This process subsequently reduced fluvial incision in the upstream of these knickpoints, stabilized the topography of the Plateau and protected high-elevation, low-relief surfaces over geologic time in active orogenic belts.

Our finding suggests that tectonic activity does not always necessarily enhance fluvial incision, and that the diverse activity can also decrease these rates by the adjustment of fault systems in active orogenic belts. This discovery also provides a new mechanism for explaining the universal stability of long-term topography and high plateau of orogenic belts worldwide.

Thermochronological data and thermal history. Apatite and zircon separates were concentrated using standard heavy liquid and magnetic separation techniques. Analyses were performed at the School of Geography, Earth and Atmospheric Sciences, University of Melbourne (see Supplementary Note 2–3 for further details). The thermal histories based on these thermochronological data were determined using the QTQt software³² (details in Supplementary Note 4), and spatial and temporal cooling pattern and rates along the Yarlung River were calculated based on the model results.

3D thermo-kinematic modeling. To identify detailed effects of rift activity on Yarlung River incision, we used the thermokinematic finite-element Pecube⁶⁰ code, which has been developed to interpret thermochronological data by solving a 3D crustal heat-transport equation. We constructed two scenarios in Pecube, i.e. without (Scenario A) and with the influences of the Woca fault (Scenario B) (detailed model parameters are described in Supplementary Note 5). In Scenario A, the Yarlung River was incised from a flat plateau surface to form the current topography, ignoring the influence of the Woca normal fault in the plateau interior. This scenario mainly reflects the upstream migration of erosional waves of the Yarlung River to form the Gyaca gorge (Fig. S2). In Scenario B, we considered not only the incision of the Yarlung River, but also incorporated the effect of the Woca normal fault (see Fig. S2).

Data availability

All thermochronological data used in this study are available in the Supplementary materials. Results of thermo-kinematic inversion in this study have been deposited in the Figshare Data Repository (https://figshare.com/s/dd1203e7834449827c74).

Code availability

The code Pecube, used in this work, is open source and can be downloaded at https://github.com/jeanbraun/Pecube

Acknowledgments

This research is supported by National Natural Science Foundation of China (41971005, 42021001) and Second Tibetan Plateau Scientific Expedition Program (2019QZKK0205, 2019QZKK0204). The University of Melbourne thermochronology laboratory receives support under the AuScope program (auscope.org.au) of the Australian Government's National Collaborative Research Infrastructure Strategy (NCRIS).

Author contributions

X.W. and G.L. developed the conceptual idea, designed and organized the study, and wrote the manuscript with D.C.. G.L. conducted fieldwork and processed data. X.W., G.L., and D.C. analyzed the data, tested the numerical simulations, and created all figures. X.W., G.L., D.C., R.J., B.K., W.Z., J.D.G and H.L. discussed and revised early versions of the ideas. All authors contributed to the discussion and writing of the manuscript.

Competing interests

The authors declare no competing interests.

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Do fault systems impede fluvial incision in active orogens?

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Abstract

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Introduction

Results

Thermochronological data

Thermal History

3d Thermo-kinematic Modeling

Discussion

Fault activity controls the formation of knickpoint

Mechanisms For Impeding River Incision In The Late Miocene

Methods

Declarations

Data availability

Code availability

Acknowledgments

Author contributions

Competing interests

References

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