South-central Tibetan Plateau Research Articles

The Holocene Indian monsoon variability over the southern Tibetan Plateau and its teleconnections

A high-resolution stalagmite oxygen isotope record from Tianmen Cave, located in the south central Tibetan Plateau, characterizes detailed climatic variations between 8.7 and 4.3ka BP on centennial and decadal time scales, with a temporal resolution of 3 to 7 years. The Tianmen record is in good agreement with speleothem records from Asian monsoon regions (i.e., Dongge Cave, Dykoski et al., 2005; Wang et al., 2005; Qunf Cave, Fleitmann et al., 2003), upwelling records from the Arabian Sea (Gupta et al., 2003, 2005) and peat bog records from southeast Tibetan Plateau (Hong et al., 2003), indicating that the Indian summer monsoon (ISM) gradually weakened as Northern Hemisphere summer insolation declined during the early-mid Holocene and that ISM intensity, rather than temperature, dominates the precipitation δ18O on centennial to decadal time scales in the southern TP. On centennial to decadal time scales, the detrended Tianmen record correlates well with the Greenland ice core record, further confirming the mechanistic connection between the ISM and high northern latitude temperature changes. Meanwhile, the Tianmen record bears significant solar activity cycles, suggesting that the solar output changes may affect the variability of the ISM and likely the position of the Intertropical Convergence Zone (ITCZ) as well. Moreover, due to joint effects of changes in monsoon precipitation, moisture source and temperature, the Tianmen δ18O record shows much larger amplitude changes than speleothem δ18O records from low-elevation Asian Monsoon regions, which is similar to the previously reported Marine Isotope Stage (MIS) 5 record (Cai et al., 2010), suggesting heightened sensitivity of precipitation isotope composition to climate changes over the high-elevation regions and further demonstrating that the stable isotope lapse rate may change under different climatic conditions.

Middle to late Miocene extremely rapid exhumation and thermal reequilibration in the Kung Co rift, southern Tibet

The Kung Co rift is an approximately NNW striking, WSW dipping normal fault exposed in southern Tibet and is part of an extensive network of active approximately NS striking normal faults exposed across the Tibetan Plateau. Detailed new and published (U‐Th)/He zircon and apatite thermochronometric data from the footwall of the early Miocene Kung Co granite provide constraints on the middle Miocene to present‐day exhumation history of the footwall to the Kung Co fault. Inverse modeling of thermochronometric data yield age patterns that are interpreted as indicating (1) initiation of normal fault slip at ∼12–13 Ma and rapid exhumation of the footwall between ∼13 and 10 Ma, (2) acceleration of normal fault slip at rates of 21.9–6.9 mm/yr at ∼10 Ma, (3) rapid thermal reequilibration between 10 and 9 Ma, and (4) slow exhumation and/or quiescence from ∼9 Ma to the present day. Hanging glacial valleys in the footwall and fault scarps that cut late Quaternary till and moraine deposits indicate that fault slip continues today. Middle to late Miocene initiation of extension across the Kung Co rift is broadly the same as the documented initiation of EW extension across the south central Tibetan Plateau. Eastward flow of middle or lower crust from beneath Tibet accommodated by northward underthrusting of Indian crust beneath Tibet provides a plausible explanation for the onset of EW extension across the Tibetan Plateau.

Large variations of oxygen isotopes in precipitation over south-central Tibet during Marine Isotope Stage 5

Oxygen isotope (δ 18 O) records of two stalagmites from Tianmen Cave, on the south-central Tibetan Plateau, grew during much of Marine Isotope Stage 5. The Tianmen record, the fi rst cave record from the Tibetan Plateau, characterizes a precipitation δ 18 O history larger in amplitude but similar in structure to Asian Monsoon records from the adjacent regions, providing essential evidence that the Asian Monsoon system, including the East Asian and Indian Monsoon subsystems, responds largely to changes in Northern Hemisphere summer insolation. Extremely low δ 18 O values during Marine Isotope Stages 5a, 5c, and 5e suggest that precipitation, rather than temperature, was a major factor controlling δ 18 O in precipitation on orbital time scales in the south-central Tibetan Plateau. The Tianmen record may help in the interpretation of regional ice core δ 18 O records. The large range of orbital-scale shifts in meteoric δ 18 O (>9‰) raises important considerations related to reconstructing the uplift history of the plateau.

Hydrological and geomorphological basin and catchment characteristics of Lake Nam Co, South-Central Tibet

For Lake Nam Co, south-central Tibetan Plateau, geomorphological features and hydrological characteristics have been compiled to provide an overview of the landscape character. Information is based on primary data, collected during field campaigns in the summers of 2005 and 2006, and secondary data as available from satellite images. Nam Co is located at the intersection of the Westerlies, the winter monsoon, the South West Asian monsoon, and the East Asian monsoon. It is situated in a graben structure. Landforms of the drainage basin are highly influenced by tectonics, superimposed by fluvial and periglacial processes, and locally by glacial and aeolian processes. Its drainage basin is endorheic. Thus, lake level is primarily controlled by the balance between input through precipitation or inflow streams and evaporation as the output. The significance of lake water evaporation is evidenced by the salinity and measurable increase of 2H and 18O of the lake water when compared to the inflow streams and precipitation. Maximum conductivities total 431 μS mm −1 for the inflowing river waters and 1920 μS mm −1 for the lake water. Overall, the solute contents are low and characterise Nam Co as a pristine environment. Dominant ions are calcium and bicarbonate in river waters. Sulphate contents are relatively high in Nam Co, and the lake is also characterised by high boron values. A significant decrease in lake water salinity during the past two decades is likely related to increased freshwater input.

Research of the Conductive Structure of Crust and the Upper Mantle beneath the South-Central Tibetan Plateau

With the super-wide band magnetotelluric sounding data of the Jilong-Cuoqin profile (named line 800) which was completed in 2001 and the Dingri-Cuomai profile (named line 900) which was completed in 2004, we obtained the strike direction of each MT station by strike analysis, then traced profiles that were perpendicular to the main strike direction, and finally obtained the resistivity model of each profile by nonlinear conjugate gradients (NLCG) inversion. With these two models, we described the resistivity structure features of the crust and the upper mantle of the center-southern Tibetan plateau and its relationship with Yalung Tsangpo suture: the upper crust of the research area is a resistive layer with resistivity value range of 200–3 000 ω·m. The depth of its bottom surface is about 15–20 km generally, but the bottom surface of resistive layer is deeper in the middle of these two profiles. At line 900, it is about 30 km deep, and even at line 800, it is about 38 km deep. There is a gradient belt of resistivity at the depth of 15–45 km, and a conductive layer is beneath it with resistivity even less than 5 ω·m. This conductive layer is composed of individual conductive bodies, and at the south of the Yalung Tsangpo suture, the conductive bodies are smaller with thickness about 10 km and lean to the north slightly. However, at the north of the Yalung Tsangpo suture, the conductive bodies are larger with thickness about 30 km and also lean to the north slightly. Relatively, the conductive bodies of line 900 are thinner than those of line 800, and the depth of the bottom surface of line 900 is also shallower. At last, after analyzing the effect factors to the resistivity of rocks, it was concluded that the very conductive layer was caused by partial melt or connective water in rocks. It suggests that the middle and lower crust of the center-southern Tibetan plateau is very thick, hot, flabby, and waxy.

Cooling history and tectonic exhumation stages of the south-central Tibetan Plateau (China): Constrained by 40Ar/ 39Ar and apatite fission track thermochronology

Between the Qiangtang Block and Yalung-Zangpo Suture Zone in the south-central Tibetan Plateau, the following geological units and suture zones have been identified from south to north: the Gangdese Granitic Belt, the Lhasa Block, the Nyainqentanghla Shear Zone, the Dangxiong–Sangxiong Tectono-granitic Belt and the Bangong–Nujiang Suture Zone. To better constrain the tectonic evolution and cooling histories of these units, 40Ar/ 39Ar muscovite, biotite and K-feldspar, as well as apatite fission track dating and thermochronological analysis have been carried out. The analytical results indicate that the south-central Tibetan Plateau, with the exception of the Nyainqentanghla Shear Zone, provides a record of three cooling stages at 165–150, 130–110 and ∼45–35 Ma. Fission-track data modelling also indicates that the stages of cooling were different in the different tectonic belts or blocks. Very different cooling phases occurred in the south-central Tibetan Plateau, compared with southern Tibet, as well as along the Yalung–Zangpo Suture Zone. There is no thermochronological evidence to indicate that the south-central part of Tibetan Plateau was influenced by the underthrusting of Indian Plate. The three-stage cooling history and the stages of tectonic exhumation were controlled completely by the closure of the Bangong–Nujiang Suture Zone along its eastern segment during Middle–Late Jurassic (165–150 Ma) and its western segment in the Early–Late Cretaceous (130–110 Ma), as well as by the collision between the Indian and Asian plates in the Paleogene (45–35 Ma).

Seismic and GPS evidence for the kinematics and the state of stress of active structures in south and south-central Tibetan Plateau

Since the middle Miocene, widely distributed N-S striking rifts or rift-depressions and NE-NW-striking strike-slip faults have developed as major structures in southern and south-central Tibet. Active structures, seismic mechanisms and directions and rates of movement determined from GPS data are different in south and south-central Tibetan Plateau. In southern Tibet, N-S striking normal faults and a few transtensional/transpressional faults are the main structures, but none of the GPS measurements show eastward movement. In south-central Tibet, NE- and NW-striking transpressional/transtensional faults and several large-scale E-W and WNW-striking dextral and sinistral strike-slip faults are the main structures, and are consistent with the movement direction indicated by the GPS data. On the other hand, the pattern of earthquake data from the south-central Tibetan Plateau, and especially from the eastern and western syntaxes, are very complicated. These structures may be produced by N-S contraction which has resulted in the N-S striking linear structures, parallel to the direction of maximum compressive stress, and conjugate NE- and NW-striking transpressional/transtensional faults on the southern side of the Qiangtang Block, and in the eastern and western syntaxes, but the deformation is not continuous from the south to the south-central Tibetan Plateau. Underthrusting of the Indian Plate under the southern Tibetan Plateau after the Middle Miocene may be the main cause of these structural features.