Southwestward growth of plateau surfaces in eastern Tibet

16 Both the kinematics and dynamics for topographic growth of the Tibetan Plateau remain debated despite 17 their significance for understanding the evolution of continental lithospheric geodynamics, climate, and 18 biodiversity in Asia. Morphometric analysis reveals the continuity of high-elevated peneplains through the 19 Songpan-Garze-Yidun, Qiangtang and Lhasa terranes in eastern Tibet. Inverse thermal-history modeling of 20 thermochronological data indicates slow cooling of these terranes since 80-60 Ma, 40-35 Ma and 20-5 Ma, 21 respectively, which is interpreted as marking tectonic and topographic stabilization of the plateau surfaces. 22 The diachronous stabilization of flat plateau surfaces and early encroachment suggests decoupling of 23 plateau surface formation from Neogene river incision and tectonics. This southwestward piecemeal 24 expansion of small plateaus suggests that the high-elevation, low-relief landscape of eastern Tibet has been 25 constructed during distinct orogenic episodes prior and during the early stages of India-Asia collision. A 26 late stage of tectonic activity during Neogene only moderately remodeled the outer rims of the plateaus and 27 the valleys that delineate the transcurrent faults, while lower crustal channel flow only leveled the distinct 28 plateaus to a unique elevation, thereby triggering river incision in eastern Tibet. and published thermochronological data to reconstruct thermal histories of selected different in (A) prole of following cross-sections. (B) preexisting relief of the proto-Tibetan plateau relict during the Mesozoic orogenesis in the Songpan-Garze-Yidun and Qiangtang terranes. (C) Stabilization of the low-relief landscape in the Songpan-Garze-Yidun terranes with minor erosion and signicant surface uplift of the Qiangtang-Lhasa terranes due to crustal shortening and thickening during the India-Asia collision. (D) Stabilization of the low-relief landscape in the northeastern Qiangtang and Songpan-Garze-Yidun terranes with limited tectonic exhumation and erosion and moderate surface uplift of the southwestern Qiangtang-Lhasa terranes synchronous with block extrusion since the Oligo-Miocene. Heating of the thickened crust lead to lower crustal ow that passively uplifted the surfaces until distinct plateau surfaces are leveled, triggering a late stage of river incision and plateau dissection. Gray thrusts and folds are preexisting structures.


34
The Tibetan Plateau, with vast flat interiors and narrow steep margins (Fig. 1), is the highest and largest 35 orogenic plateau on Earth and mainly rose in the aftermath of the India-Asia collision ca. 60-50 million 36 years ago (Ma) 1,2 . The topographic evolution of the plateau remains debated and under-constrained, despite 37 its significance for understanding the geodynamics of continental lithospheric deformation 3,4 , the Asian 38 monsoon system 5,6 , and biodiversity evolution 7 . In particular, the origin of high-elevation, low-relief 39 surfaces that are key elements of the Tibetan landscape has attracted growing attention in recent years but 40 their geodynamic origin remains elusive [8][9][10][11] . Two end-member hypotheses have been proposed to explain

64
A common tool to estimate palaeoaltimetry are stable isotopes, which provide direct quantitative 65 paleoelevation estimates of Cenozoic basins. However, they do not necessarily indicate the timing of 66 plateau uplift at a regional scale. In addition, stable isotope paleoaltimetry relies largely on sampling of 67 unaltered paleosoil and pedogenic carbonates in basin sediments having precise age control 23 , which is 68 challenging in terrestrial sediments 24

76
Thermochronological data are complementary to paleoaltimetry data, as they allow quantifying the 77 thermal-tectonic histories of the plateau surfaces and are applied to constrain landscape evolution at a 78 broader scale. In principle, the onset of extremely slow cooling and exhumation for the low-relief plateau 79 surfaces rules out significant orogenic relief growth or erosion since then, and thus can be interpreted as 80 recording the formation of relict landscapes preserved at high elevations (e.g., ref. 17  The present topography of eastern Tibet is characterized by large areas of high-elevation, low-relief 92 landscape that were interpreted as relict plateau surfaces 8 (Fig. 1B). In order to examine the topographic 93 features of different terranes, we extracted a topographic swath profile from the Lhasa to Songpan-Garze   Table S1; Datasets S1-S4). The three low-temperature thermochronometric systems record cooling through 128 a temperature window of~220-40°C 42 . In the Zuogong-Markam plateaus, 11 samples were collected at 129 elevations between 4,300 m and 5,000 m on the plateau surfaces, as well as on the rim of the plateau cut by 130 the Lancang River draining to the southeast. Samples from the late Triassic Zuogong batholith (Table S1) 131 yield AFT ages ranging between 15 ± 2 Ma and 26 ± 3 Ma with mean track lengths between 14.2 ± 1.0 µm 132 and 14.9 ± 0.8 µm ( Fig. 2A; Table S1; Dataset S1). The youngest and oldest ages are from samples 133 collected in the center and near the top of the batholith, respectively. The lower-temperature AHe 134 weighted-mean ages overlap with their corresponding AFT ages within error and vary between 12.5 ± 0.9 135 Ma in the northern pluton and 28.7 ± 1.5 Ma on the western plateau rim, broadly consistent with 10 136 published single-grain AHe ages clustering at 18-20 Ma 43 ( Fig. 2A; Table S1; Dataset S2). For the higher-137 temperature ZHe thermochronometer, weighted-mean ages from replicates vary from 37.0 ± 0.3 Ma for a 7 sample collected on the northeast plateau rim to 60.2 ± 5.2 Ma on the eastern rim. In addition, one sample 139 from Mesozoic sandstone of the Markam fold-and-thrust belt ( Fig. 2A) Table S2 (Fig. 4C). The high topography that we termed "relict plateau" (Fig. 5A Neogene-Quaternary tectonic activity, respectively (Fig. 3). Samples from high-elevation (>4,000 m) and   Table S2.  Table S1 and Datasets S1-5).  Table S1). Abbreviations for basins: CB, Chuxiong basin; GB, Gonjo basin; NB, Nangqian basin; SB, Sichuan basin; XB, Xichang basin; YB, Yanyuan basin. Abbreviations for mountains, peaks and rivers: DR, Dadu River; G, Gongga; L, Longmen Shan; LCR, Lancang River; N, Namche Barwa; NR, Nu River; JR, Jinsha River; K, Kawagebo; Y, Yulong; YR, Yulong River. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Simpli ed geology of the studies areas and cooling histories of selected plateau surfaces. The Zuogong-Markam (A) and Weixi (B) plateaus with geochronological and thermochronological data superimposed on shaded elevation map. Data include zircon U-Pb (black), ZHe (blue), AFT (green) and AHe (red) ages (in Ma, see Table S1 and Datasets S1-5). Cooling histories of the Zuogong (C), Markam (D) and Weixi (E) plateaus in eastern and southeastern Tibet. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 3
A synthesis of Cenozoic rapid exhumation and cooling events related to faulting and river incision, as well as formation ages of low-relief landscapes in eastern Tibet. (A) Timing of Neogene tectonic activity and river incision, as well as formation of low-relief plateau surfaces in each terrane. For abbreviations and sources for onset timing of rapid incision of major rivers and faulting, refer to the caption of Fig. 1C. (B) Bimodal pattern of exhumation and erosion rates for stabilized plateau surfaces and areas affected by Neogene tectonics and river incision, correlated with relief across the swath pro le A-B in Fig. 1C. The estimated exhumation and erosion rates are derived from the original publications, corresponding to the constraints on the onset of neo-tectonic activity and river incision in Fig. 1C (see details in Table S3).
Average exhumation or erosion rates of Cenozoic tectonic activity, Neogene river incision and stabilized plateau surfaces are highlighted by red, orange and gray lines, respectively.

Figure 4
Comparison of inter-and intra-continental tectonic events, syn-tectonic sedimentation in the hinterland and peripheral basins, and sediment budgets in marginal seas of the southeastern and eastern Asia with development of low-relief plateau surfaces, major river incision and tectonic activity in eastern Tibet. Correlation of regional tectonism, syn-tectonic deposition and thermochronological peak ages for Lhasa (A), Qiangtang (B), Songpan-Garze-Yidun (C) terranes and South China (D) in eastern and southeastern Tibet is used to explain construction and destruction of high-elevation, low-relief plateau surfaces. The peak ages of low temperature thermochronological data are derived from the Kernel density estimate plots by DensityPlotter 74. (E) Cenozoic sedimentation rate in the marginal seas in southeastern and eastern Asia 75. For details, see the discussion. Abbreviations as in Fig. 1.

Figure 5
Proposed model for growth of plateau surfaces inherited from preexisting landscape elements related to multi-phased crustal thickening from inter-to intra-continental orogenesis in the cycles of the Paleo-, Meso-and Neo-Tethys Oceans in eastern Tibet (see a synthesis in Fig. 4). (A) Synthetic age contours of plateau surface formation based on this study and refs. 4,20,21,76. White curved line delineates the pro le of the following cross-sections. (B) Planation of preexisting topographic relief of the proto-Tibetan plateau and relict plateau that have been attained during the Mesozoic orogenesis (see Fig. 4 for details) in the Songpan-Garze-Yidun and Qiangtang terranes. (C) Stabilization of the low-relief landscape in the Songpan-Garze-Yidun terranes with minor erosion and signi cant surface uplift of the Qiangtang-Lhasa terranes due to crustal shortening and thickening during the India-Asia collision. (D) Stabilization of the low-relief landscape in the northeastern Qiangtang and Songpan-Garze-Yidun terranes with limited tectonic exhumation and erosion and moderate surface uplift of the southwestern Qiangtang-Lhasa terranes synchronous with block extrusion since the Oligo-Miocene. Heating of the thickened crust lead to lower crustal ow that passively uplifted the surfaces until distinct plateau surfaces are leveled, triggering a late stage of river incision and plateau dissection. Gray thrusts and folds are preexisting structures.
Gray dashed lines indicate topographic relief lowered by erosion. Abbreviations as in Fig. 1. Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

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