Quaternary Landscape Evolution Of The Eastern Tibetan Plateau

Earth surface processes that transform rock into landscapes with distinctive features are critical to maintain the function and existence of nutrient delivery,climate change,life activity,natural hazards and biogeochemical cycles.The recognition that climatically modulated erosion governs the geodynamic evolution of active mountain ranges remains one of the most transformative conceptual shifts in geosciences in the past few decades.The interaction of landscape,climate,tectonics and river dynamics is attracting scientists to contribute to the rates of climate change,tectonic uplift,surface denudation,river incision,relief evolution and sedimentation.With these combined quantifications and disciplinary modeling,we can more confidently predict the landscape evolution among these interactions.Here,I present multiple in-situ cosmogenic nuclides(¹⁰Be,²¹Ne,²⁶Al and ³⁶Cl)in surface/subsurface rocks and fluvial/burial sediments（1）to estimate the surface denudation rate in a non-steady state surface using depth profile modeling;（2）to quantify the carbonate surface denudation rate and fluvial incision rate in the most margin of the southeastern Tibetan Plateau;（3）to illustrate the formation and destruction mechanisms of the low-relief and high altitude relicts in the central-eastern Tibetan Plateau combining with topographic analysis;（4）to decipher the reorganization of the paleo-Dudu-Anning River（a tributary of the Middle Yangtze River）and its indication for the deeply dissected canyons in the eastern Tibetan Plateau;and（5）to summarize the surface denudation rate,exposure age,fluvial incision rate and landscape evolution processes across the eastern Tibetan Plateau in different timescales and its interactions with climate change,tectonic uplift and river dynamics.In-situ cosmogenic nuclides are receiving growing attention in surface Earth system science in which steady-state denudation is an important assumption for estimating the surface denudation of long-lived（10³–10⁶ years）surfaces.However,it is difficult to distinguish steady-state denudation and decipher a reliable denudation rate from non-steady state denudation regions.Recent depth profile models for investigating the denudation rate,exposure age and inheritance are widely used for sediment and regolith of steady-state denudation regions.Here I present in-situ cosmogenic nuclides ¹⁰Be and ²⁶Al from two typical granite regoliths from Jixian in Tianjin and Tengchong in Yunnan to model non-steady landscape evolution process by using steady state（Model 1）and non-steady state（Model 2,continuous exposure;Model 3a,changed denudation rate;and Model 3b,abrupt denudation event）models.The results of the steady state model demonstrate that both regoliths are in a non-steady state.¹⁰Be non-steady state models of the Jixian regolith reveal that the surface has likely experienced a denudation rate change or an abrupt denudation event during the past several millennia,resulting in a non-steady state.Similar Models3a and 3b denudation rates of 14.6_-1⁺⁹.₂⁴.₉ and 14.7_-1⁺⁹.₄⁰.₇ mm kyr^-1,respectively,might demonstrate the long-term denudation rate rather than the exaggerated rates of 39.0_-2⁺²..₃⁶ and 39.0_-2⁺²..₄⁵ mm kyr^-1 calculated by assuming steady-state denudation and ignoring inheritance.The non-steady state model results of the Tengchong regolith suggest that the surface has likely experienced strong denudation about 157 ka(¹⁰Be)or 127 ka(²⁶Al)ago.This timing is consistent with the termination of the MIS 6（TII:130 ka）and climate change events in the Tibetan Plateau,suggesting a strong surface landscape denudation during the transition from glacial to interglacial.A compilation of basin-wide denudation rates in the Tibetan Plateau shows a good positive correlation with the regional tectonic activity,indicating that the denudation rate derived from non-steady state regions with strong tectonic activity is likely to be overestimated by assuming steady state.I suggest that a combination of steady state and non-steady state depth profile models should be used to quantify denudation rate and exposure age more accurately and effectively in regions of strong tectonic activity or climate change.Multiple cosmogenic ¹⁰Be,²¹Ne,²⁶Al and ³⁶Cl concentrations in quartz or carbonate rock from cave deposits,modern river sediments and bedrocks were measured to elucidate Quaternary landscape evolution history in the Guizhou Plateau,Southeast China.Using the ²⁶Al-¹⁰Be and²¹Ne-¹⁰Be pairs to distinguish the cosmogenic ²¹Ne concentration from excess ²¹Ne,I found that the nucleogenic ²¹Ne produced by U and Th decay in quartz is significant in the samples although there is the possibility of inherited cosmogenic ²¹Ne.Combined with previous studies,I suggest that a robust approach for applying cosmogenic ²¹Ne could be achieved by（1）estimating the contribution from nucleogenic ²¹Ne,（2）avoiding samples with complex burial histories to exclude inherited cosmogenic ²¹Ne and（3）combining the ¹⁰Be-²⁶Al-²¹Ne nuclides methods for the Quaternary samples.Paleo-denudation rates and burial ages derived from the ²⁶Al-¹⁰Be pair and carbonate surface denudation rates calculated from ³⁶Cl were used to determine the different timescale surface denudation rate and fluvial incision rate.The consistency of the different timescales pre-burial basin denudation（paleo-denudation）rate,³⁶Cl surface denudation rate and modern basin denudation rate indicates that the landscape-scale surface denudation has been likely stabilized since the Quaternary in the Guizhou Plateau area.The slightly higher river incision rates than the local surface denudation rate show that the river dynamics may not have reached a steady state due to the regional tectonic uplift in the Guizhou Plateau.Compiling the carbonate surface denudation rates in different climate zones in China and previous worldwide carbonate surface denudation studies,I find that:（1）the denudation rate increases systematically with mean annual precipitation（MAP）in arid climatic zones（MAP lower than～700 mm）and is dominated by long-term climatic gradients;（2）humid climatic zone（MAP greater than～700 mm）denudation rates are constrained to 34.1±11.7 mm kyr^-1 by carbonate dissolution kinetics;and（3）denudation rate is also influenced by short-term physical erosion or abrupt erosive events.Fluvial incision,regarded as one of the fundamental geomorphic processes,drives the evolution of mountainous landscapes.The transitional landscape from low-relief to high-relief in the central Tibetan Plateau is rapidly evolving as it is influenced by river dynamics,climate change and tectonic uplift.Combining cosmogenic ¹⁰Be depth profile dating and topographic analysis,this thesis provides new constraints on the formation and destruction of low-relief surfaces in the central Tibetan Plateau.I find that the high-relief landscape in the Suoqu area（a major tributary of the upper Nu River）shows a rapid fluvial incision rate of 710±70 mm kyr^-1 since the late Pleistocene,while the low-relief topography in the adjacent Xiaqiuqu area presents an order of magnitude lower incision rate of 70±10 mm kyr^-1.These results are consistent with the long-term（multi-million-year）exhumation rates derived from low-temperature thermochronology,suggesting that this region has experienced an evolving incision history.I interpret that the higher relief was caused by enhanced fluvial incision and the lower relief was slowly developed by sedimentation and a relatively steady low exhumation rate.The presence of a knickzone appears to mark the boundary between these differentially incising landscapes,which may be caused by rapid headward retreat and higher river discharge in the Suoqu River.The coincidence of fluvial terraces ages with climate-driven events,in addition to paleo-denudation rates,indicates that the formation of fluvial terraces in the Xiaqiuqu and Suoqu areas might be associated with the rapid sedimentation of weathered materials in early warming periods.Understanding the drainage history of the Middle Yangtze River is critical for determining its interaction of tectonics,climate and denudation.Previous studies focused on interpreting the provenance of Xigeda lacustrine sediments,however,the poorly constrained ages of these sediments mean the evolutionary history remains elusive.Here present new cosmogenic ¹⁰Be,²¹Ne and ²⁶Al data from the Dadu and Anning river sediments,indicating deposition ages of paleo-lake sediments between～1 and～0.5 Ma.Paleo-denudation rates together with neon isotopic composition and topographic analysis suggest that there was no connection between the Dadu and Anning rivers during the Middle Pleistocene.Combined with field investigations and cosmogenic nuclides ages,I suggest that paleo-lakes formed upstream of channel blocking landslides,which in turn could have been the result of tectonic uplift and/or increased precipitation.My results and previous thermochronology data constrain that the fluvial incision of the Dadu and Anning rivers enhanced between～13-9 Ma;subsequently paleo-lakes were formed（～1 Ma）after the paleo-Dadu-Anning river capture（～2 Ma）;finally,the draining of paleo-lakes occurred at～0.5 Ma.Based on cosmogenic ¹⁰Be and ²⁶Al constraints from depth profiles of one granitic regolith from Wumingshan and five fluvial terraces from Xianshuihe and Zagunao rivers,I discuss the timing of the last deglaciation,the landscape-scale denudation and fluvial incision rates across the eastern Tibetan Plateau,in relation to previous work.The exposure age（older than 19.4 ka）of the Wumingshan regolith corresponds to the retreat of the paleo-Daocheng ice cap,which covered the Yidun terrane during the Last Glacial Maximum（LGM）.Most basin-wide denudation rate data in the eastern Tibetan Plateau are lower than 130 mm kyr^-1（47%,n=90）,which is consistent with the Wumingshan regolith denudation rate(lower than 52.8 mm kyr^-1)and thus indicates that the landscape-scale denudation has been stabilized after the last deglaciation.Considering the reduction of integrated bulk density due to the accumulation of lower-density loess,I estimate mean exposure ages of Xianshuihe and Zagunao river terraces of 4.0±0.7 ka,5.9±0.3 ka,13.4±2.0 ka and 16.6±1.4 ka.The observed increase in incision rate from 390 mm kyr^-1 over a long timescale（～600 ka）to 5,880 mm kyr^-1 over the last 15 ka at the Xianshuihe river site is probably due to the transition from glacial to interglacial climatic conditions.The fewer abandoned terraces along the Zagunao river after the Heinrich event 1（H1）indicates that climate change during the latest glacial-interglacial transition hardly impacted landscape evolution in a relatively lower elevation area.Comparison with previously reported fluvial incision rates elsewhere across the eastern margin of the Tibetan Plateau indicates that incision rates are mainly influenced by abrupt climate change or intensified summer monsoon since the early Holocene but are probably controlled by regional tectonic uplift or fluvial headward retreat as the timescale increases.I propose a synthetic pattern of landscape evolution mainly dominated by long-term tectonic uplift together with fluvial headward erosion,episodically influenced by climatic change throughout the late Quaternary.Overall,I provide an alternative approach（TCNCal calculator）for modeling of surface,subsurface and buried cosmogenic nuclides samples.The existing basin-wide denudation rates derived from¹⁰Be in the eastern Tibetan Plateau suggest a peak denudation rate of～50 mm kyr^-1,which contrasts with the abnormally high denudation rate of>1000 mm kyr^-1 in regions of tectonic activity.I propose that the denudation rate derived from the non-steady-state surface is likely overestimated because episodic exhumation processes such as earthquakes,landslides and glaciations expose deep rocks with low cosmogenic nuclides abundances to the surface.The surface denudation rates of low relief regions in the central Tibetan Plateau and the Guizhou Plateau are 84.2±8.8 mm kyr^-1 and 18-69 mm kyr^-1,respectively,which are consistent with the surface denudation rate of high relief region in the eastern margin(～50 mm kyr^-1,excluding the abnormally high denudation rate from regions of tectonic activity).The similar surface denudation rates in the three regions contrast with the significantly different river incision rates,which vary from 70±10 mm kyr^-1 in the central Tibetan Plateau to>1000 mm kyr^-1 in the marginal region and then extend southeast to 43-74 mm kyr^-1 in the Guizhou Plateau.The incision patterns contrasted with the gentle gradient of the topography in the southeastern Tibetan Plateau are correlated with surface relief,demonstrating that the marginal region incised more deeply than did the central Tibetan Plateau and the most southeastern Tibetan Plateau（Guizhou）.Comparing the relatively stable surface denudation rate and the different incision patterns reveals that the landscape of the eastern Tibetan Plateau is evolving from the stable low relief in the central to the high-relief in the margin,to the low-relief steady surface of the Guizhou Plateau,which is likely controlled by the rapid headward retreat of external drainage.The denudation and transport of surface materials in the eastern Tibetan Plateau mainly occur in the marginal regions with high incision rates.The main reasons for the relatively stable surface of the Guizhou Plateau and the central Tibetan Plateau and the active surface in the margin can be summarized as（i）differential tectonic uplift controlled by fault zones in the eastern Tibetan Plateau（such as the Yalong-Yulong thrust belt,the Jinsha strike-slip fault and the Xianshuihe-Xiaojiang strike-slip fault）;（ii）climate-driven headward retreat of external drainage controlling the retreat of the knickzone;（iii）the response time of fluvial incision to landscape evolution being significantly faster than that of surface denudation.