低温热年代学数据对龙门山推覆构造带南段新生代构造活动的约束
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摘要
龙门山位于青藏高原东边缘,地形陡变,剥蚀作用强烈。近年来先后经历了2008年汶川M W7.9地震和2013年芦山M S7.0地震,多位学者对龙门山地区做了大量的低温热年代学研究。文中在研究程度相对薄弱的龙门山南段补充了4个锆石裂变径迹年龄和4个磷灰石裂变径迹年龄,结合前人的低温热年代学数据结果得出,宝兴杂岩从新生代早期开始快速冷却,降温幅度超过225℃,而龙门山中段的彭灌杂岩降温幅度为185~225℃。宝兴杂岩的4个裂变径迹年龄分布在2.7~5Ma,相对彭灌杂岩较年轻,表明晚新生代以来宝兴杂岩的冷却速率要高于彭灌杂岩。在地表温度为15℃和古地温梯度为30℃/km的假设下,宝兴杂岩距今3~5Ma以来的平均剥露速率为0.63~1.17mm/a。低温热年代学数据揭示出龙门山中段的差异剥蚀集中在北川-映秀断裂和江油-灌县断裂上,而南段的差异剥蚀分散在更宽范围内的双石-大川断裂(南、北2个分支)及其东侧的断层和褶皱。
The Longmen Shan,located at the eastern margin of the Tibetan plateau,is a steep and high exhumation area.In recent years,the 2008 Wenchuan M W7.9 earthquake and the 2013 Lushan MS7.0earthquake occurred,and researchers presented a lot of low-temperature thermochronology data of the Longmen Shan and adjacent area.In this paper,we provide 4 ZFT ages and 4 AFT ages for the southern segment of the Longmenshan Thrust Belt(LTB),where the low-temperature thermochronology data are still few.Combining with previous researches,we get the Cenozoic exhumation history of the Baoxing Massif,located at the southern segment of the LTB,and new knowledge about the Cenozoic activity of the southern segment of the LTB.The Baoxing Massif began quickly cooling in the early Cenozoic,with the cooling range exceeding 225℃,while the cooling range of the Pengguan Massif in the central segment of the Longmen Shan is between 185~225℃.The four AFT ages in the Baoxing Massif are between 2.7~5.0Ma,which are younger than that in the Pengguan Massif,and it indicates that the late-Cenozoic cooling rate of the Baoxing Massif is bigger than that of the Pengguan Massif.Under this assumption that the surface temperature is 15℃ and the paleo-geothermal gradient is 30℃/km,the average exhumation rate from 3~5Ma to present is about 0.63~1.17mm/yr.The low-temperature thermochronology data indicate that the differential exhumation is concentrated in the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault in the central segment of the LTB,while it is dispersed in a wider region along the two branches of the Shuangshi-Dachuan Fault and the faults and folds to the east,in the southern segment.
The Longmen Shan,located at the eastern margin of the Tibetan plateau,is a steep and high exhumation area.In recent years,the 2008 Wenchuan M W7.9 earthquake and the 2013 Lushan MS7.0earthquake occurred,and researchers presented a lot of low-temperature thermochronology data of the Longmen Shan and adjacent area.In this paper,we provide 4 ZFT ages and 4 AFT ages for the southern segment of the Longmenshan Thrust Belt(LTB),where the low-temperature thermochronology data are still few.Combining with previous researches,we get the Cenozoic exhumation history of the Baoxing Massif,located at the southern segment of the LTB,and new knowledge about the Cenozoic activity of the southern segment of the LTB.The Baoxing Massif began quickly cooling in the early Cenozoic,with the cooling range exceeding 225℃,while the cooling range of the Pengguan Massif in the central segment of the Longmen Shan is between 185~225℃.The four AFT ages in the Baoxing Massif are between 2.7~5.0Ma,which are younger than that in the Pengguan Massif,and it indicates that the late-Cenozoic cooling rate of the Baoxing Massif is bigger than that of the Pengguan Massif.Under this assumption that the surface temperature is 15℃ and the paleo-geothermal gradient is 30℃/km,the average exhumation rate from 3~5Ma to present is about 0.63~1.17mm/yr.The low-temperature thermochronology data indicate that the differential exhumation is concentrated in the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault in the central segment of the LTB,while it is dispersed in a wider region along the two branches of the Shuangshi-Dachuan Fault and the faults and folds to the east,in the southern segment.
引文
谭锡斌.2012.龙门山推覆构造带新生代热演化历史研究及其对青藏高原东缘隆升机制的约束[D]:[学位论文].北京:中国地震局地质研究所.TAN Xi-bin.2012.Cenozoic thermal history of the Longmenshan thrust belt:Implication for the uplift mechanism in the eastern margin of Tibetan plateau[D].Ph D thesis.Institute of Geology,China Earthquake Administration,Beijing(in Chinese).
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Hubbard J,Shaw J H.2009.Uplift of the Longmen Shan and Tibetan plateau,and the 2008 Wenchuan(M S7 .9)earthquake[J].Nature,458(12):194—197.
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Kirby E,Reiners P W,Krol M A,et al.2002.Late Cenozoic evolution of the eastern margin of the Tibetan plateau:Inferences from40Ar/39Ar and(U-Th)/He thermochronology[J].Tectonics,21(1):1—19.
Li Z,Liu S,Chen H,et al.2012.Spatial variation in Meso-Cenozoic exhumation history of the Longmen Shan thrust belt(eastern Tibetan plateau)and the adjacent western Sichuan basin:Constraints from fission track thermochronology[J].Journal of Asian Earth Sciences,47:185—203.
Liu T K,Hsieh S,Chen Y G,et al.2001.Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating[J].Earth and Planetary Science Letters,186:45—56.doi:10.1016/S0012-821X(01)00232-1.
Meng Q R,Hu J M,Wang E,et al.2006.Late Cenozoic denudation by large-magnitude landslides in the eastern edge of Tibetan plateau[J].Earth and Planetary Science Letters,243(1-2):252—267.
Richardson N J,Densmore A L,Seward D,et al.2008.Extraordinary denudation in the Sichuan basin:Insights from low-temperature thermochronology adjacent to the eastern margin of the Tibetan plateau[J].Journal of Geophysical Research,113:B04409.doi:10.1029/2006JB004739.
Xu G,Kamp P J.2000.Tectonics and denudation adjacent to the Xianshuihe Fault,eastern Tibetan plateau:Constraints from fission track thermochronology[J].J Geophys Res,105:19231—19251.
Xu X W,Wen X Z,Yu G H,et al.2009.Coseismic reverse-and oblique-slip surface faulting generated by the 2008Wenchuan M W7 .9 earthquake,China[J].Geology,37(6):515—518.
Wang E,Kirby E,Furlong K P,et al.2012.Two-phase growth of high topography in eastern Tibet during the Cenozoic[J].Nature Geoscience,5:640—645.doi:10.1038/ngeo1538.
Wilson C J L,Fowler A P.2011.Denudational response to surface uplift in east Tibet:Evidence from apatite fissiontrack thermochronology[J].GSA Bulletin,123(9/10):1966—1987.
王卫民,郝金来,姚振兴.2013.2013年4月20日四川芦山地震震源破裂过程反演初步结果[J].地球物理学报,56(4):1412—1417.WANG Wei-min,HAO Jin-lai,YAO Zhen-xing.2013.Preliminary result for rupture process of Apr.20,2013,Lushan earthquake,Sichuan,China[J].Chines J Geophys,56(4):1412—1417(in Chinese).
徐锡伟,韩竹军,李传友,等.2013.四川芦山7.0级强震:一次典型的盲逆断层型地震[J].科学通报,58:1—1 .XU Xi-wei,HAN Zhu-jun,LI Chuan-you,et al.2013.Lushan M S7 .0 earthquake:A blind reserve-fault earthquake[J].Chin Sci Bull,58:1—1(in Chinese).
Arne D,Worley B,Wilson C,et al.1997.Differential exhumation in response to episodic thrusting along the eastern margin of the Tibetan plateau[J].Tectonophysics,280:239—256.
Burchfiel B C.2004.2003 presidential address:New technology,new geological challenges[J].GSA Today,14:4—9.
Brandon M T.1996.Probability density plot for fission-track grain-age samples[J].Nuclear Tracks and Radiation Measurements,26:663—676.
Chen Z,Burchfiel B C,Liu Y,et al.2000.Global Positioning System measurements from eastern Tibet and their implications for India/Eurasia intercontinental deformation[J].J Geophys Res,105:16215—16227.doi:10.1029/2000JB900092.
Cook K L,Royden L H,Burchfiel B C,et al.2013.Constraints on Cenozoic tectonics in the southwestern Longmen Shan from low-temperature thermochronology[J].Lithosphere.doi:10.1130/L263.1.
Gan W,Zhang P,Shen Z K,et al.2007.Present-day crustal motion within the Tibetan plateau[J].J Geophys Res,112:B08416.doi:10.1029/2005JB004120.
Godard V,Pik R,LavéJ,et al.2009.Late Cenozoic evolution of the central Longmen Shan,eastern Tibet:Insight from(U-Th)/He thermochronometry[J].Tectonics,28.doi:10.1029/2008TC002407.
Hubbard J,Shaw J H.2009.Uplift of the Longmen Shan and Tibetan plateau,and the 2008 Wenchuan(M S7 .9)earthquake[J].Nature,458(12):194—197.
King R W,Shen F,Burchfiel B C,et al.1997.Geodetic measurements of crustal motion in southwest China[J].Geology,25:179—182.doi:10.1130/0091—7613.
Kirby E,Reiners P W,Krol M A,et al.2002.Late Cenozoic evolution of the eastern margin of the Tibetan plateau:Inferences from40Ar/39Ar and(U-Th)/He thermochronology[J].Tectonics,21(1):1—19.
Li Z,Liu S,Chen H,et al.2012.Spatial variation in Meso-Cenozoic exhumation history of the Longmen Shan thrust belt(eastern Tibetan plateau)and the adjacent western Sichuan basin:Constraints from fission track thermochronology[J].Journal of Asian Earth Sciences,47:185—203.
Liu T K,Hsieh S,Chen Y G,et al.2001.Thermo-kinematic evolution of the Taiwan oblique-collision mountain belt as revealed by zircon fission track dating[J].Earth and Planetary Science Letters,186:45—56.doi:10.1016/S0012-821X(01)00232-1.
Meng Q R,Hu J M,Wang E,et al.2006.Late Cenozoic denudation by large-magnitude landslides in the eastern edge of Tibetan plateau[J].Earth and Planetary Science Letters,243(1-2):252—267.
Richardson N J,Densmore A L,Seward D,et al.2008.Extraordinary denudation in the Sichuan basin:Insights from low-temperature thermochronology adjacent to the eastern margin of the Tibetan plateau[J].Journal of Geophysical Research,113:B04409.doi:10.1029/2006JB004739.
Xu G,Kamp P J.2000.Tectonics and denudation adjacent to the Xianshuihe Fault,eastern Tibetan plateau:Constraints from fission track thermochronology[J].J Geophys Res,105:19231—19251.
Xu X W,Wen X Z,Yu G H,et al.2009.Coseismic reverse-and oblique-slip surface faulting generated by the 2008Wenchuan M W7 .9 earthquake,China[J].Geology,37(6):515—518.
Wang E,Kirby E,Furlong K P,et al.2012.Two-phase growth of high topography in eastern Tibet during the Cenozoic[J].Nature Geoscience,5:640—645.doi:10.1038/ngeo1538.
Wilson C J L,Fowler A P.2011.Denudational response to surface uplift in east Tibet:Evidence from apatite fissiontrack thermochronology[J].GSA Bulletin,123(9/10):1966—1987.
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