藏南帕里至达吉地带的上地壳结构特征──REFTEK顺带广角地震观测结果分析
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摘要
本文主要介绍１９９２年中美喜马拉雅和青藏高原深剖面与综合研究项目第一阶段（ＩＮＤＥＰＴＨＩ－１）广角地震观测资料利用Ｓｅｉｓ８１程序（Ｃｅｒｖｅｎｙ，１９８１）进行二维解释所获得的帕里一达吉地带的上地壳结构特征。主要成果为：（１）前寒武结晶基底之顶界表现为Ｒ１界面。据本项目地质调查，藏南拆离系（ＳＴＤＳ）在帕里以北约１０ｋｍ处出露，向北缓倾并向地下延伸。认为Ｒ１界面不仅是结晶基底之顶界的反映，而且ＳＴＤ可能沿着Ｒ１界面展布，也即Ｒ１界面同时是一条沉积盖层和基底之间的拆离层。Ｒ１界面埋深３±０．６—１１±０．６ｋｍ。（２）上述拆离层在萨马达一达吉之间，以Ｒ１界面之上的负速度梯度楔状体（ＬＶＬ）为特征。ＬＶＬ可能是ＳＴＤＳ活动时拖带下沉的中生代特提斯沉积，或可能是含水破碎带。（３）在结晶基底内部存在第二条拆离带（ＬＶＺ），表现为Ｔ２反射波组。ＬＶＺ在帕里埋深８．５±０．６ｋｍ，向北陡倾，至萨马达为２２．５±０．６ｋｍ深；再向北，倾角变缓，至达吉之下，达２７±０．６ｋｍ深。ＬＶＺ在帕里─萨马达之间，厚仅０．５ｋｍ；至达吉，增厚至５ｋｍ；也具楔形负速度梯度带性质；它可能是花岗岩局部融熔体之反映。（４）在萨马达─达吉?
The upper crustal structure constructed on the basis of 2-D interpretation with Seis81 program (Cerveny, 1981 ), based upon the piggyback wide-angle seismic data obtained during INDEPTH-I in 1992 is introduced in the paper. The main results discussed here are: (1 )The top of the Precambrian crystalline basement is marked by R1 interface. Based upon the geological investigation of the present programme INDEPTH-I the South Tibetan Detachment The project INDEPTH jointly funded by NSFC, MGMR of China & NSF of AInerican.'. Chinese side: Wu Charade, Gao Rut, Xu Zhongxin, Cul Junwen, Chen Honghai, Li Huiming, Yue Yoboun, Li Jixiang,Xiong Jiayu & Lu Jie (CAGS); Che Jingkai, Guo Jingru (Beijing Computing Center, MGMR); Liu Xianwen (Chang -- ChunCollege of Geology); Meng Fame, Li Ming (China University of thescince (Beijing) ). US side: Nelson, K. D., Brown, L.D., Hauck, M. L. (Chmell University); Kuo, J. (Columbia University).System (STDS) is cropped out north￣10 km of Pagri, gently dipping northward and continuing downward. It is suggested that the interface R1 not only marks the top of the crystalline basement but also represents the possible one, along which STD is spreaded, i. e. the R1 interface at the same time is a detachment between the sedimentary cover and the basement. The R1 interface is buried at a depth of 3±0. 6-11±0. 6 km; (2)The above mentioned detachment is characterized by wedge with negative velocity gradient (shown with LVL) above R1 interface between Samada and Dagyi. The LVL seems like to be some Mesozoic Tethyan sediment, towed down, when the STDS was being active, or, plausibly, a water-bearing fracture zone; (3)A second detachment(shown with LVZ), represented by reflection wave-group T2 may occur within the crystalline basement. The LVZ is buried at a depth of 8. 5±0. 6 km beneath Pagri,steeply dipping northward, to Samada at a depth of 22. 5±0. 6 km, further northward it flattens, up to Dagyi at a depth of 27 ±0. 6 km. Between Pagri-Samada the LVZ is only 0. 5km thick, but at Dagyi its thickness is increased to about 5 km; it is also characterized by a wedged negative velocity gradient zone, plausibly due to granite partial melt; (4) A high velocity(up to 6. 5-7. 0 km/s )wedge(Shown with HVZ), occurring between Samada-Dagyi,at a depth of 27-36km, and sandwiched between interfaces R2 and R3, may represent the top of the Tethys-Himalayan lower crust or truncated bed from the underthrusting Indian continental crust; (5) MHT (Main Himalayan Thrust), sandwiched between reflection wave-groups T3 and T4, is characterized by a low velocity zone with 3-9 km in thickness and 5. 8 km/s interval velocity, dipping of 6° N in average. For the MHT we observe its bottom from 24 ±0. 6 km depth at Pagri to 40±0. 6 km depth beneath Dagyi. The MHT is the third detachment zone,along which the indian continental crust is underthrusting Tethyan Himalaya. The bottom interface R4 of MHT appears to be the interface between the upper crust and the lower one. The average velocity of P-wave of the upper crust is about 6. 0-6. 3 km/s.The authors would like to thank engineers Meng Fame, Wu Ling, Lu Jie, Li Qiusheng, Dr.Li Ming, and Profs. Wu Xuanzhi, Zhang Zhaoyuan for their help in data-processing and 2-Dcomputation; arid also Dr. Yue Yongjun for his very instructive consultation in discussions during the study.
The upper crustal structure constructed on the basis of 2-D interpretation with Seis81 program (Cerveny, 1981 ), based upon the piggyback wide-angle seismic data obtained during INDEPTH-I in 1992 is introduced in the paper. The main results discussed here are: (1 )The top of the Precambrian crystalline basement is marked by R1 interface. Based upon the geological investigation of the present programme INDEPTH-I the South Tibetan Detachment The project INDEPTH jointly funded by NSFC, MGMR of China & NSF of AInerican.'. Chinese side: Wu Charade, Gao Rut, Xu Zhongxin, Cul Junwen, Chen Honghai, Li Huiming, Yue Yoboun, Li Jixiang,Xiong Jiayu & Lu Jie (CAGS); Che Jingkai, Guo Jingru (Beijing Computing Center, MGMR); Liu Xianwen (Chang -- ChunCollege of Geology); Meng Fame, Li Ming (China University of thescince (Beijing) ). US side: Nelson, K. D., Brown, L.D., Hauck, M. L. (Chmell University); Kuo, J. (Columbia University).System (STDS) is cropped out north￣10 km of Pagri, gently dipping northward and continuing downward. It is suggested that the interface R1 not only marks the top of the crystalline basement but also represents the possible one, along which STD is spreaded, i. e. the R1 interface at the same time is a detachment between the sedimentary cover and the basement. The R1 interface is buried at a depth of 3±0. 6-11±0. 6 km; (2)The above mentioned detachment is characterized by wedge with negative velocity gradient (shown with LVL) above R1 interface between Samada and Dagyi. The LVL seems like to be some Mesozoic Tethyan sediment, towed down, when the STDS was being active, or, plausibly, a water-bearing fracture zone; (3)A second detachment(shown with LVZ), represented by reflection wave-group T2 may occur within the crystalline basement. The LVZ is buried at a depth of 8. 5±0. 6 km beneath Pagri,steeply dipping northward, to Samada at a depth of 22. 5±0. 6 km, further northward it flattens, up to Dagyi at a depth of 27 ±0. 6 km. Between Pagri-Samada the LVZ is only 0. 5km thick, but at Dagyi its thickness is increased to about 5 km; it is also characterized by a wedged negative velocity gradient zone, plausibly due to granite partial melt; (4) A high velocity(up to 6. 5-7. 0 km/s )wedge(Shown with HVZ), occurring between Samada-Dagyi,at a depth of 27-36km, and sandwiched between interfaces R2 and R3, may represent the top of the Tethys-Himalayan lower crust or truncated bed from the underthrusting Indian continental crust; (5) MHT (Main Himalayan Thrust), sandwiched between reflection wave-groups T3 and T4, is characterized by a low velocity zone with 3-9 km in thickness and 5. 8 km/s interval velocity, dipping of 6° N in average. For the MHT we observe its bottom from 24 ±0. 6 km depth at Pagri to 40±0. 6 km depth beneath Dagyi. The MHT is the third detachment zone,along which the indian continental crust is underthrusting Tethyan Himalaya. The bottom interface R4 of MHT appears to be the interface between the upper crust and the lower one. The average velocity of P-wave of the upper crust is about 6. 0-6. 3 km/s.The authors would like to thank engineers Meng Fame, Wu Ling, Lu Jie, Li Qiusheng, Dr.Li Ming, and Profs. Wu Xuanzhi, Zhang Zhaoyuan for their help in data-processing and 2-Dcomputation; arid also Dr. Yue Yongjun for his very instructive consultation in discussions during the study.
引文
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