Fission Track Thermochronology Of Tectonic Evolution In Taiyuan Xishan Syncline Area

Located in the central portion of the eastern limb of Luliang anticlinorium,the understanding in Meso-Cenozoic uplift history of Taiyuan Xishan syncline is not only the component part to study the Luliang mountain uplift evolution,the foundation to understand the tectonic evolution of the study area and adjacent area,but also the important issue to explore the crustal reflection owing to the destruction and later evolution of the North China Craton.Previously,researches of Taiyuan Xishan syncline area are mainly focused on coal geology,constructional morphology,petrology and geochemistry of Huyan alkaline complex and some limited chronologic data from adjacent area,lacking of direct geochronologic evolution research of this area.As a common used low-temperature thermochronology method nowadays,fission track dating could perfectly reflect the crustal tectonic evolution.Employing fission track chronology as the theoretical basis,from the aspect of mountain(denudation area),we take advantage of Age-Height difference and thermal history modeling methods in this study.Depending on comprehensive analysis and summary of regional geological evolution in Taiyuan Xishan syncline and adjacent areas,six different age of rock samples are finally confirmed and collected from both western and eastern limbs for FT analysis.And eight AFT and ZFT ages are lastly acquired to preliminarily study the issues such as this area's uplifting episodes,amplitude,time,and to inverse the thickness of overlying strata.The major research results of this thesis are as follows:(1)The samples SL and BJT collected from top and bottom of Huoshan formation near Baijiatan in northern portion of the western limb of Taiyuan Xishan syncline,the central age are 398±48Ma(SL-Z)?291±21Ma(BJT-Z)?49±6Ma(BJT-A)respectively;the maximum depth of samples is still within the Zircon Partial Annealing Zone(ZPAZ);in combine with the existing maximum thickness of strata,we could infer that the lower limit of this maximum depth is approximately 3.63km;BJT-A is characterized by P(x2)>0,mean track length 10.8±2.3?m and a wide unimodal distribution of track length histogram,it could be inferred that the sample remained within the Apatite Partial Annealing Zone(APAZ);the thermal history modeling results reveal that the northern portion was uplifting during 200Ma?120Ma with a mean uplift rate of 0.01mm/y,remained at a constant temperature during 120Ma?80Ma,uplifting apparently during 80Ma?20Ma with a mean uplift rate of 0.012mm/y and 20Ma?0Ma with a mean uplift rate of?0.105 mm/y.(2)The samples HY and XY collected from top and bottom of Huyan alkaline complex in southern portion of the western limb of study area,the central age are 64±6Ma(HY-A)?62±7Ma(XY-A)?112±7Ma(HY-Z)?105±5Ma(XY-Z)respectively;HY-A and XY-A are characterized by P(x2)? 5%,mean track length 12.4±1.9?m and 11.6±2.9?m respectively,it could be inferred that the samples remained within the Apatite Partial Annealing Zone(APAZ)for a long time;HY-Z and XY-Z,P(x2)<5%,it could be inferred that the rock mass emplaced within the Zircon Partial Annealing Zone(ZPAZ)(3.16km?7.3 7km);thermal history modeling results reveal that the rock mass was uplifting as a whole during 90Ma?80Ma with a mean uplift rate of 0.149mm/y;using the Age-Height difference method,the rock mass was uplifting as a whole during 62±7Ma-64±6 with a mean uplift rate of 0.442mm/y;and 20Ma-0Ma with a mean uplift rate of?0.088mm/y.(3)The sample WJG-Z collected from Shanxi formation in southern portion of the western limb,the central age is 236±19Ma,P(x2)= 0,the single grain ZFT ages are too disperse,showing the central age is a mixed age and the sample had experienced partial annealing with its maximum depth is approximately 3.16km-7.37km.(4)Based on the fission track ages and thermal history modeling results acquired,it could be considered that the northern portion of western limb was uplifting during 200Ma?120Ma;104Ma?119Ma,the rock mass which samples HY and XY belong to began to emplace,while this event didn't show to affect the northern portion;during 90Ma?80Ma,after the emplacement of the rock mass,the southern portion shows apparently rapid uplift,while the northern portion remained at a constant temperature,this time span could represent the onset of the structural framework of Xishan syncline;about 80Ma?20Ma,the western limb shows uplifting as a whole with a mean uplift rate of 0.005?0.012mm/y;20Ma?0Ma,the western limb shows rapid uplift as a whole with a mean uplift rate of 0.088?0.105mm/y;eventually a steeper western limb was formed.The rapid uplift event from 20Ma in the study area is also the response to the uplift of the entire Luliang mountain area.(5)Combining WJG-Z age with the 2#coal metamorphism level data acquired from the northern portion,it could be inferred that the maximum depth of sample WJG could be remained at is at least about 3.16km during its geologic history.And considering about the actual situation of sedimentary strata in the study area,we could deduce that there could at least have Yanchang formation(T3y),even could have the Upper Jurassic strata deposited in the study area.Then the area entered a period of apparent tectonic activity,with rock mass intruding,land surface uplift and denuded.