Fission-track Thermochronological Analysis And Its Geological Significance In The Southwestern Ordos Block And Its Surroundings

After more than half a century of development,the fission track thermochronology has been improved and widely used in many fields,such as landscape scale exhumation,basin thermal evolution and petroleum exploration.Especially in complex tectonic areas,it has become a useful method to reveal the activity time of fault,tectonic evolution and exhumation process.The tectonic activity,the late uplift and denudation in mainland China is intense,and the deep process is active.The southwestern Ordos Block and its surroundings is located at the junction of multiple tectonic units such as the North China Craton,Qinling-Qilian Orogenic Belts,etc.,and has undergone multiple tectonic implication.Since the Cenozoic,it has been affected by the outward growth of the Tibetan Plateau.Thus,it is an ideal place to reveal the continental tectonic activities by using of fission-track thermochronology.Although many apatite fission-track(AFT)data are published in southwestern Ordos Block,these data are poor interpretation due to the complexity of the structure of the region and its surroundings.In this paper,it sets southwestern margin of Ordos Basin,Liupanshan Arcuate Tectonic Belt and northeastern Qilianshan as the key area and uses the AFT thermochronology to discuss the tectonic uplift events,time of major fault activity and evolution process of the southwestern Ordos Block and its surroundings since the Mesozoic.In this paper,30 AFT samples were collected in the southwestern Ordos Block and its surroundings.The measured AFT ages and lengths were between 222.5-37.4Ma and 14.79-12.67?m.The age and altitude relationship shows that the uplift process is different and complex.A detailed thermochronology analysis of the southeastern section of the Haiyuan Fault Zone shows that initial uplift of the southern region occurred in Late Triassic,while the northern part of the zone experienced strong cooling during the Late Jurassic.Then,entire exhumation occurred at the Early Cretaceous,and the southern uplifted at Late Cenozoic again and formed the current pattern.Based on new data,it counts the existing 126 AFT data and other several low-temperature thermochronological data of the study region.The ‘boomerang' plot analysis and thermal history modeling implied that the area is mainly experienced four phases of rapid cooling events: Late Triassic-early Jurassic(ca.220-175 Ma),Late Jurassic--Early Cretaceous(ca.1656-118 Ma),Late Cretaceous-Early Cenozoic(ca.95-40 Ma)and the late Cenozoic(ca.14-7 Ma).The regional full annealed samples' AFT age and MTCL distribution maps show the following characteristics:(1)AFT age and length were evenly distributed within the basin,while the values are more dispersed at the margin of the Ordos Bain,indicating a regional difference cooling process;(2)AFT ages of three N-NE swaths across the Haiyuan-Liupanshan Fault Zone tends to decrease to ca.70-7 Ma as it is closer to the different part of fault,indicating the multistage activity of the Haiyuan-Liupanshan Fault Zone during ca.70-7 Ma;(3)the AFT thermochronological study of the long section across Liupanshan Arcuate Tectonic Belt suggests that the forming of the thrust fault belt can be traced to the Late Jurassic,while the arcuate belt was shaped since the Cenozoic and is still active today.Based on the analysis of the geological characteristics in the study area,it is considered that the cooling event and distribution characteristics of the AFT should be a response to the fault activity and regional tectonic events,and the tectonic evolution characteristics of the study area are then analyzed accordingly.The starting time of the cooling events in the Late Triassic-Early Jurassic was gradually delayed from the south(ca.220Ma)to the north(ca.190Ma),and their intensity was gradually weakened from the south to the north,which were responses to the subduction,collision and orogeny between the North China and Yangtze Block.The Late Jurassic-Early Cretaceous tectonothermal events occurred under the background of East Asian plates convergence,and there were many strong eastward thrust and deformation structures in the study area.In the Late Cretaceous-early Cenozoic,due to the far-field effect of Pacific subduction and flat rollback,the long-term denudation and levelling of the area triggered by NW-SE compression.Subsequently,extensional fault depression occurred in the Eocene(ca.40-30Ma)and formed the Cenozoic Ningnan Basin due to the Pacific flat rollback.The different fault activities developed in Haiyuan-Liupanshan fault zone during late Cenozoic(ca.14-7 Ma)that was thought to be influenced by the northeastward growth of the Tibetan Plateau.In addition,multiple tectonic events have inevitably influence on hydrocarbon conditions of source,reservoir and cap of the study area.The analysis of Gucan-1 well shows that its Cretaceous source rocks have potential for oil generation and reached the peak of oil generation in the late Early Cretaceous,but there is no significant exploration breakthrough.It is interpreted that the oil and gas reservoirs cannot be well preserved due to the influence of the above structural transformation and tectonic rework since Late Triassic,and wide distribution of Cretaceous oil seedlings in the study area should be the result of these tectonic activities.