Apatite Fission Track (AFT) Analysis Of The Cenozoic Extensional Exhumation And Uplift Of The Helan Shan And The Qinling Mountains, And Frictional Heating Along Active Faults

This thesis focuses on application of apatite fission track (AFT) analysis to determining the onset time, the distribution of extension-related uplift and exhumation, and revealing the pattern of the Cenozoic uplift and exhumation of the Helan Shan and the Qinling mountains. It provides constraints on the mechanisms of their deformations, and discusses the magnitude, tectonic significance of the Cenozoic rapid uplift of Qinling mountains, and its effect on the climate and environment of China. In addition, AFT analysis is used to reveal frictional heating along the active faults on the northeastern margin of the Tibetan Plateau, and a discussion is made on the primary problems and prospect of application of this method to detect or/and measure frictional heating along active faults. The major research results of this thesis are as follows:1. Timing and pattern of Cenozoic extension-relate rapid uplift and exhumation of the Helan ShanThe Helan Shan is located at the conjuction zone between the tectonically stable Alashan block and Ordos block, and the northern end of the China south-north tectonic zone( earthquake zone), as a intraplate deformation zone in the interior of North China Craton. It experienced long-term evolution of tectonic deformation. The Helan Shan region is characterized by extensional tectonic deformation under the control of normal faults since the Cenozoic, which resulted in the formation of the uplifted Helan Shan and fault-bounded depressed Yinchuan Basin. Based on the characteristics of the Cenozoic tectonic deformation of the Helan Shan, samples were collected from the Suyukou transect in the south-central Helan Shan and Dawukou transect in the north Helan Shan, respectively. Besides, some samples were from western Helan Shan and Zhengyiguangou in the north Helan Shan. Combined with the results of apatite fission track from Xiaosongshan-Nvqigou reported by Zhao et al (2007), the purpose is to investigate the time, distribution and pattern of uplift and exhumation of the Helan Shan in Cenozoic, and to discuss the mechanism of Cenozoic extensional deformation of the Helan Shan region. The main conclusions are as follows:(1) The samples HL07-01, HL07-02 and HL07-3a collected from Zhengyiguangou, HL07-05 and HL07-06 collected from Dawukou transect in the north Helan Shan are characterized by concordant cooling ages(about10~12Ma), long mean track lengths (>13.87um) and small standard deviations of track lengths (<1.1um), indicating the rapid cooling resulted from uplift and exhumation of the Helan Shan onsetting at 10~12Ma(2) The spatial distribution of the AFT data of the Helan Shan revealed that the rapid exhumation of Helan Shan onsetting at 10-12Ma was more rapid closer to the Eastern Helan Shan fault (EHSF) and slower away from the EHSF, and decreased from the north (east) Helan Shan to the south (west) Helan Shan.(3) The spatial distribution of the exhumation of the Helan Shan onsetting at 10-12Ma indicates that it experienced westward and southward tilt uplift and exhumation associated with the extension of the EHSF(4) The westward and southward tilt uplift and exhumation of the Helan Shan can be attributed primarily to the late Cenozoic strong NW-SE extension on the northwestern margin of the Ordos block. The foot wall and hang wall of the dextral-slip normal EHSF moved upward and downward, respectively during extensional faulting, which resulted in forming the uplifted Helan Shan and sunk Yinchuan Basin, respectively.2. Timing and pattern of Cenozoic rapid uplift and exhumation of the Qinling Mountains, and their tectonic significance and effects on climate and environment Qinling Mountains, trending W-E, is located in central China, is a typical multiple-stage orogenic belt which experienced plate-collision and intracontinent orogen. In the Cenozoic the parallel–orogen extension deformation on the northern margin and in the interior of Qinling Mountains are prominent, which produced Weihe Basin and post-orogen intracontinental extensional uplift of the Qinling Mountains as the natural boundary of the geology, biology and climate between North and South China. On the basis of the previous studies, AFT samples were collected from the Taibai Shan transect, Hua Shan transect, Zhongtiao Shan transect, Li Shan, Fenyukou and the south-side of the Qinling Mountains, respectively. By use of the new AFT data combined with published AFT data, this thesis has determined the onset time, spatial distribution and pattern of Cenozoic uplift and exhumation, and discusses its tectonic significance, uplift magnitude and effects on climate and environment. The main conclusions are as follows:(1) The results of AFT analysis revealed that the rapid uplift and exhumation cooling of the Qinling Mountains began at 10.5~7Ma.(2) The spatial distribution of AFT data indicates that the Cenozoic exhumation was slow in the South Qinling and rapid in the North Qinling, more rapid closer to the boundary faults between Weihe Basin and Qinling Mountains and slower denudation away from the boundary faults.(3) The spatial distribution of the Cenozoic denudation indicates the Cenozoic southward tilted uplift and exhumation of the Qinling Mountains in response to the intracontinental parallel-orogen extension.(4) The timing of rapid uplift and exhumation onsetting at 10.5~7Ma is consistent with the time of the outward growth of the Tibetan Plateau ca.8Ma and the tectonic location of the Qinling Mountains adjacent to the northeastern margin of the Tibetan Plateau suggest that the rapid extensional uplift and exhumation of the Qinling Mountains onsetting at 10.5~7Ma are associated with late Cenozoic rising and outward growth of the Tibetan Plateau. Obviously, the rapid extensional uplift and exhumation of Qinling Mountains onsetting at 10.5~7 provides the important evidence and a constraint on the timing of late Cenozoic uplift and outward growth of the Tibetan Plateau, with the indicative significance of tectonics.(5) The rapid uplift and exhumation of the Qinling Mountains onsetting at 10.5~7Ma revealed by AFT analysis, combined with the formation time and distribution area of Aeolian Red Clay in Northwest and North China and the distinct regional differentiations of mammals between South and North China, demonstrate the rapid uplift of the Qinling Mountains initiated in the late Miocene, and its elevation might reach or be close to the present elevation of the Qinling Mountains becoming the natural boundary of biology, climate and environment between South and North China. Furthermore, the uplift of the Qinling Mountains accelerated the formation of the eastern Asian monsoons, and reinforced formation and eastward extent of arid climate of North China, and is one of the important factors and bases of formaton of the eastern Asian monsoon.3. Preliminary application of apatite fission track thermochronology analysis to detect or/and measure frictional heating along active faultsAlthough fault-slip frictional heating is common along active faults, it is a still unresolved difficulty to detect or/and measure frictional heating of natural faulting, especially single earthquake faulting. The last section of this thesis to attempt to use AFT analysis to detect or and measure fault-slip frictional heating on samples from the active faults zones on the northeastern margin of the Tibetan Plateau. Due to the unique thermal sensitivity of the AFT, AFT ages and confined horizontal track length distribution of samples within and adjacent to fault zones can provide information of fault-slip frictional heating during individual earthquake fault-slip events. Totally 13 samples were collected from the following 3 transects: The Dagoumen transect perpendicular to the Haiyuan active fault, and the Xiaohonggou transect and Hongguliang transect perpendicular to the Xiangshan-Tianjingshan active fault on the northeastern margin of the Tibetan Plateau. Samples include fault gouges and ultracataclasites on principle slip planes of the faults, and ambient rocks adjacent to the faults. No localized thermal anomaly has been detected by AFT analysis. Samples from the Dagoumen transect show damaged AFT system, probably caused by fault activities which resulted in very low U content of some samples. The results indicate that the magnitudes of frictional heat or increased temperature are too limited to cause annealing of AFT. Based on this study and previous work of other people, this thesis concludes that AFT analysis is only suitable for detecting or/and measuring frictional heat caused by large fault displacement and high strength friction which are generally caused by large earthquakes. Furthermore, this thesis concludes that AFT analysis technique would be a potential"calorimeter or thermometer"to detect or/and measure quantitatively the magnitude of fault-slip frictional heat generated by individual earthquake events.