Seismic Pre-Site Survey for ICDP Drilling Locations at Lake Nam Co, Tibet
Geophysics
Final Report Abstract
The Tibetan Plateau is a key location to understand Asian and global climate evolution in the geologic past as it received signals from different air masses and is reacting sensitively to the monsoon intensity. For accurate climate models, data of past climatic changes are required, but prolonged and continuous archives on the Tibetan Plateau are rare. To identify suitable locations for lake drilling Nam Co was visited during a 2014 reconnaissance survey, and the potential of a significant sediment fill of the enclosed basin lead to this DFG project, which was embedded in the DFG priority program ICDP. The project confirmed that Nam Co represents a unique lacustrine archive on the Tibetan Plateau comprising potentially more than one million years of sedimentary history. Based on acquired sediment echosounder and multichannel seismic data in a field campaign in 2016, several research topics have been investigated, all targeting to understand lake level variations, sediment delivery and depositional processes and history and its interaction with the complex tectonic setting. Sediment echosounder profiles of 1400 km length were combined with the chronology and logs of the sediment core NC01/08 from the centre of Nam Co, to develop an acoustic stratigraphy for the last 20.5 kyr. Modern deposition at high lake level is restricted to water depth of 66 m and deeper, marked by an onlap, indicating interflows controlled by water column stratification delivering fine-grained sediment to the deeper lake. Lake level seems to control onlap depth and thus acoustic stratigraphy and mapping provided a lake level curve throughout the Holocene. With lake level drops, hyperpycnal flows, paleo beach ridges and reduced shoreline distance lead to grain size and acoustic amplitude increase together with acoustic facies variations. The data indicate lake level rise from 21 to 17 ka by 22 meters and to 13 ka by another 40 meters, high lake levels prevail afterwards, with a maximum being reached at 12 ka. On longer time scales, multichannel seismic data (MCS) (970 km) were utilized. The seismic stratigraphy indicates a sediment infill at Nam Co exceeding 680 m in thickness. The seismic facies analysis revealed a cyclic alternation in seismic reflection amplitude, similar to echosounder data, which we directly correlated to lake level, e.g. low seismic amplitudes indicate the sedimentation of fine-grained material in a low energy environment during a high lake level. This relation was used to establish a relative lake-level curve representative for Nam Co, which in turn could be linked to 8 global glacial-interglacial cycles (Marine Isotope Stages) back to 712 ka. The environmental changes at Nam Co are similar throughout the late Pleistocene, as the mean sedimentation rates for full glacial-interglacial cycles show uniform values of 0.52 mm/yr. Extrapolation of these sedimentation rates for the entire sediment infill may exceed more than one million years in age. Comparing the relative lake-level curve with climate-proxy records of global and supra-regional scale exposed that the lake-level behaviour of Nam Co correlates to the shift of the Asian summer monsoon on short (20.5 kyrs) and long (700 kyr) time scales. Deviations possibly indicate local climate feedback caused by enhanced meltwater supply and thawing permafrost and triggered quick, high amplitude lake-level variations as observed for the interglacial substages of MIS 5. Overall, precipitation steered lake-level variations at Nam Co. The MCS data moreover exposed a previously unknown amount of faults in the lacustrine infill of Nam Co and documented, for the first time, tectonic extension since the Middle Pleistocene in southern Tibet. The analysis of the fault structures revealed 1) primary strike-slip faults along a NW-SE directed wrench zone, parallel to the dextral Beng Co Fault, NE of Nam Co, and 2) secondary faults with normal or strike-slip faults characteristics. A transtensional regime with a dominating extensional W-E direction is proposed to represent the overall tectonic setting at Nam Co. Cumulative throw rates stay in the range of 0.3 mm/yr in WSW-ENE direction to 0.9 mm/yr in WNW-ESE direction, similar to the southern Yadong-Gulu rift system. The WNW-ESE cumulative throw sums up to a total of 315 m. Lateral extension rates range between 0.5 and 0.7 mm/yr and comprise nearly half of the total extension compared to the southern Yadong-Gulu rift system, or ~7% compared to the whole south Tibet extension. Faulting was initiated in Nam Co at ~350 ka. This result is a novel perception of the tectonic extension in southern Tibet. Faulting was continuously active with short exceptions for periods of very high lake levels during wet, interglacial conditions in MIS 5e and since the Holocene. Regarding the depositional setting, tectonically induced extension has formed an enclosed basin and subsidence created accommodation space in confined zones of the deeper lake, which have ensured continuity in the sedimentary archive even at low lake levels. Overall, this project comprises new findings regarding the seismo-stratigraphic framework, precipitation-driven lake-level variations, and extensional regime of Nam Co and set the essential basis for selecting suitable drill sites for the ICDPdrilling campaign 'NamCore'.
Publications
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ICDP workshop on scientific drilling of Nam Co on the Tibetan Plateau: 1 million years of paleoenvironmental history, geomicrobiology, tectonics and paleomagnetism derived from sediments of a high-altitude lake. Scientific Drilling, 25(c(2019, 6, 12)), 63-70.
Haberzettl, Torsten; Daut, Gerhard; Schulze, Nora; Spiess, Volkhard; Wang, Junbo & Zhu, Liping
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The Sedimentary and Tectonic Evolution of the Nam Co Basin, Tibetan Plateau, since the Middle Pleistocene – A Seismoacoustic Study on Lake Sediments. Dissertation
Schulze, Nora
