Earthquake And Provenance Analysis Of The Lacustrine Sediments In The Upper Reaches Of The Min River During The Late Pleistocene

Eastern Tibetan Plateau is characterised by the greatest topography gradient be1t,the most complex neotectonics and active tectonics and the densest active faults than anywhere else in the plateau.The upper reaches of the Min River is located in the central to north part of“the South-North Seismic Belt of China”.Under the influence of the Minjiang fault,Huya fault and Longmenshan thrust faults,earthquakes occurred frequently during the geological and historical period.The upper reaches of the Min River are characterized by elevations of 1500 m in valleys and 3000 m along ridges;consequently,the steep slopes along both sides of the river are high-risk regions for slumps,landslides and debris flows.Since several major earthquakes happened in the last few years,e.g.the 2008 Ms.8.0 Wenchuan,the 2010 Ms.7.1 Yushu,and the 2013 Ms.7.0Lushan earthquakes,it is urgent to study the historical seismicity of eastern Tibetan Plateau.However,the region is characterized by alpine valleys.It is difficult to study the paleoearthquake sequences by trenches due to long-term erosion,human activity and lack of the Quaternary sediments.What's more,a handful of palaeoearthquake studies almost concentrated in the Holocene,and only a few earthquakes can be identified.The 2008 Wenchuan and 1933 Diexi earthquakes result in wide distribution of dammed lakes in upper reaches of the Min River.Previous studies have shown that these lakes may well record the seismic signals.Some soft-sediment deformation structures can be readily observed in these lacustrine sediments,and also in the well-exposed field outcrops,indicating a strong earthquake liquefaction deformation.Fortunately,lacustrine sediments are continuous archives recording tectonic and climatic events,which can be used to reconstruct paleoearthquake sequences.Based on the collection of previous data,we determined the well-exposed lacustrine sediments in upper reaches of the Min River.The research methods are as follows.Firstly,we use sedimentology,including lithology description,stratigraphic classification,bedding recognition,to determine the sedimentary environment and facies.Secondly,we choose some soft-sediment deformation structures and stratification in lacustrine sediments,mainly research the soft-sediment deformation,from its classification,morphology description,genetic analysis,trigger mechanism,deformation process,deformable model,deformation intensity and the relationship with earthquake magnitudes and epicentral distance.Thirdly,we combine microcosmic indexes including grain size,SUS,geochemistry and end member model to distinguish earthquake event layers from non-earthquake event layers in these lacustrine sediments.In addition,using OSL and¹⁴C dating methods to establish a chronological framework,and obtain the paleoearthquake history,earthquake magnitudes.Finally,we integrate the major and trace elements and detrital zircon U-Pb chronology to better understand the detailed release,transport process and transportation routes of detrital material from the eastern Tibetan Plateau into the Sichuan Basin.Based on the above studies,we draw the following conclutions.1)OSL and ¹⁴C dating results indicate that Xinmocun?,Xinmocun?and Taiping sections of lacustrine sediments lasted from Last Glacial Maximum to Holocene.The Shawan section lasted from the last deglaciation to Holocene.2)Seventeen deformed beds were identified in four lacustrine sequences?Xinmocun?,Xinmocun?,Taiping,Shawan?,which can be divided into seven types of soft-sediment deformation structures,e.g.ductile structures?load casts,flame structures,pseudonodules,ball-and-pillow structures,and liquefied convolutes?and brittle structures?liquefied breccias,micro-faults?.Most of the soft-sediment deformation structures indicate deformation mechanisms related to liquefaction and/or fluidization processes,probably triggered by paleoseismic events.We also identified twelve medium-to-thick sand layers with normally graded vertical sequences.It is related to the landslides,debris and fine particulate dust caused by earthquakes,which were transported by wind or water into the lake and quickly deposited.3)The statistical results show that load,ball-and-pillow structures which induced by earthquake represent a minimum seismic magnitude of 5.0 and maximum magnitude of more than8.0,and mostly correspond to magnuitude of 6.0–7.0 and epicentral distance of 20–70 km.For a given intensity of load,ball-and-pillow structure,the maxium earthquake magnitudes required to generate these structures is the lacustrine sediments,followed by fluvio-lacustrine,marine sediments.The deformation layer width and thickness of load,ball-and-pillow structures and ball radius sizes is positively correlated to earthquake magnitudes.The lithology is not directly correlated to the earthquake magnitudes.It is possible to estimate the epicentral location or the maximum liquified distance by using the earthquake magnitude which is evaluted from the intensity of soft-sediment deformation structures;and vice versa.4)Integrated the type,form,maximum liquefaction distance and thickness distributions of these soft-sediment deformation structures in the Xinmocun?,Xinmocun?,Taiping,and Shawan sections,earthquake magnitudes can be obtained.The Shawan probably documents six strong earthquakes of magnitude 6.0-7.0 and one over 7.0.Xinmocun?probably documents three strong earthquakes of magnitude 5.0-6.0 and two 6.0-7.0.Xinmocun?probably documents two strong earthquakes of magnitude 5.0-6.0 and three 6.0-7.0.Taiping probably documents one strong earthquake of magnitude 6.0-7.0.5)According to grain-size variations,distribution,accumulation curves,clay-silt-sand triangular plots,C-M plots,Q1-Md-Q3 plots and Sahu's empirical judgement equation,the sediments of Xinmocun?,Xinmocun?and Taiping sections are fine,mainly clay,silt and fine sand,which is typical eolian deposits.The sediments of Shawan are coarse,mainly silt and sand,which is eolian and fluvial deposits.Two end members were inverted from grain size data from the Xinmocun?,Xinmocun?,Xinmocun?or Taiping sections by using the end member model.EM1 components are deposited during aseismic period;EM2 component is deposited in extreme events?e.g.earthquakes,landslides etc.?.Three end members were inverted from grain size data from the Shawan section by using the end member model.EM1 component is deposited during aseismic period;EM2 and EM3 component are deposited in extreme events?e.g.earthquakes,landslides etc.?.Integrated end member model,geochemistry,grain-size and SUS data in the five lacustrine sections?Xinmocun?,Xinmocun?,Xinmocun?,Taiping and Shawan?,we identified 20,18,26,23,10 possible earthquake event layers,respectively.Earthquake event layers are obviously differentfrom non-earthquake event layers.Although the evolution phases?AB,BC,and CD segments?of each event layer are different,the event layers have the same or similar pattern of evolution.The grain-size data in event layers are featured by abrupt coarsening and slowly upward fining,which is obviously different from non-earthquake event layers featured by silty clay.We suppose the land surface processes are like this:the sudden release of the coarse particles in the co-seismic event at first,then mixing period of coarse clastic and fine particles in post-seismic short time scales,and further well-sorted fine particles in long term seismic quiet period.6)Integrated soft-sediment deformation structures and event layers which from a plot of grain-size and SUS,with the age of lacustrine sediment,Xinmocun?probably documents fifteen earthquakes of magnitude 5.0-6.0,five 6.0-7.0,two 7.0-8.0,total twenty two earthquakes,.Xinmocun?probably documents eleven earthquakes of magnitude 5.0-6.0,five 6.0-7.0,two7.0-8.0,total eighteen earthquakes.Xinmocun?probably documents four earthquakes of magnitude 5.0-6.0,eighteen 6.0-7.0,four 7.0-8.0,total twenty six earthquakes.Shawan probably documents four earthquakes of magnitude 5.0-6.0,five 6.0-7.0,three 7.0-8.0,total twelve earthquakes.Taiping probably documents six earthquake of magnitude 5.0-6.0,fifteen 6.0-7.0,two 7.0-8.0,total twenty three earthquakes.7)We identified 227 hummocky cross-stratification?HCS?beds within three lacustrine sequences of Xinmocun?,Xinmocun?and Xinmocun?,which have 135,76 and 16 layers,respectively.Two dimensional shape results show HCS is featured by length of 1.7-24 cm,mean8.4 cm;thickness of 0.1-2.7 cm,mean 0.56 cm,length-to-width ratios of 10-20.Three dimensional shape results show wave plate-shapes with wave length of 2.6-10 cm,mean 5.9 cm;wave height of 0.4-6 cm,mean 2.3 cm,length-to-height ratios of 2-5.HCS is also featured by an obvious rust oxidation ring.Integrated geochemistry and grain-size data,the sediments in HCS beds are mostly silt and fine sand,which is a typical eolian deposits transported by wind and trapped in the lake.HCS is obviously affected by water and as an important sedimentary structure which most commonly repositioned between normal fair-weather wave base and storm-wave base,representing a storm wave in extreme weather conditions.It can also be preserved in the dammed lakes,which is first recorded in Xinmocun sections.8)From the upper reaches of the Min River in the eastern TP to the Sichuan Basin,eight samples were collected for zircon U-Pb chronology and grain-size analysis indicate that zircon grains are comparatively coarse in three bedrock samples,one fluvial sand sample and one dust sample,but are distinctly finer in three lacustrine samples.The zircon grain size parameters from the fluvial sand and dust samples are comparable.The analysis of U-Pb zircon ages indicates five major age populations,corresponding to known granitoid magmatic events within the Yangtze River?Lüliangian,Jinningian,Caledonian,Hercynian and Indo-Sinian/Yanshanian?.The Min River links lacustrine sediments at Lixian,fluvial sands at Wenchuan,Leshan,Yibin,Dadu River sediments and the Dayi conglomerate,implying that the Dayi conglomerate was transported by fluvial rather than glacial processes.The denuded sediment mainly derived from seismic events in the eastern TP was transported by rivers into the western Sichuan Basin,where they formed two sedimentary depocenters and the relatively fine grains were transported from the western to the northern Sichuan Basin by aeolian processes.Thus,the thick sediments in the western Sichuan Basin mainly transported by the Min River influenced dust deposition in the northern Sichuan Basin.In contrast,the Jialing and Dadu rivers made minor contributions.