Significance of Underplated Oceanic Melange in the Tectonic Evolution and Crustal Structure of Central Tibet

板下大洋混杂岩在藏中构造演化及地壳结构中的意义

• 批准号: 0438120
• 负责人: Paul Kapp
• 金额: $ 22.49万
• 依托单位: University of Arizona
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 2005
• 资助国家: 美国
• 起止时间: 2005-06-15 至 2008-05-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0438120&HistoricalAwards=false
• 关键词: Significance; Underplated; Oceanic; Melange; Tectonic

Within the middle of the Qiangtang terrane of central Tibet is an enormous (greater than 500-km-long and up to 100-km-wide) east-west trending belt of blueschist-bearing melange. This belt occurs structurally beneath continental margin strata, in the footwall of early Mesozoic domal low-angle normal faults, requiring the melange to have underplated continental margin strata prior to exhumation. The tectonic significance of the melange is highly uncertain, with two leading hypotheses proposed to date: (1) the melange marks the location of a suture zone separating formerly unrelated crustal fragments to the north and south. (2) the melange consists of Songpan-Ganzi sedimentary and subduction-accretion rocks which have been underthrust approximately 200 km from the Jinsha suture to the north. In an effort to test these hypotheses, this three-year investigation is conducting (1) regional and detailed geologic mapping and structural analysis along two N-S traverses across the Qiangtang terrane, (2) detailed structural analyses and thermochronologic and thermobarometric studies within the Qiangtang melange, (3) comparative studies of detrital zircon ages from the melange, Songpan-Ganzi strata, and northern and southern Qiangtang region strata, and (4) biostratigraphic studies, led by colleagues from the collaborating Institute of Tibetan Plateau Research in Beijing. Project findings will have a major impact on understanding the tectonic assembly and crustal structure of central Tibet and the general tectonic processes by which melange is incorporated into continental crust. If project results indicate that Qiangtang melange has been underthrust from the Jinsha suture to the north, then much of the deeper central Tibetan crust should include underplated melange. This relationship would also provide an unparalleled opportunity to directly examine processes of low-angle subduction which are rarely observed in the geologic record. Conversely, the conclusion of a major oceanic suture in the central Qiangtang region would require significant revisions to conventional thought regarding the tectonic framework and development of central Tibet. The project involves one Ph.D. student and is engaging a group of undergraduate students to assist in the collection and interpretation of detrital zircon data. In addition, a Tibetan Tectonics" seminar is being organized that will bring together University of Arizona faculty, graduate and undergraduate students, and researchers from Chinese and other U.S. institutions.

西藏中部羌塘地体中部有一条巨大的（长超过500公里，宽达100公里）东西走向的蓝片岩混杂岩带。 该带在结构上出现在大陆边缘地层下方，位于早中生代穹窿低角度正断层的下盘，要求混杂岩在折返之前具有底板大陆边缘地层。 混杂岩的构造意义高度不确定，迄今为止提出了两个主要假设：（1）混杂岩标志着缝合带的位置，该缝合带将以前不相关的地壳碎片分隔开来。 (2)混杂岩由松潘-甘孜沉积岩和俯冲增生岩组成，这些岩层俯冲于金沙缝合带以北约200公里处。 为了检验这些假设，这项为期三年的调查正在进行（1）沿羌塘地体的两条南北走向的区域详细地质测绘和结构分析，（2）羌塘混杂岩内的详细结构分析以及热年代学和热气压研究，（3）混杂岩、松潘-甘孜地层、羌塘南北地层碎屑锆石年龄对比研究，（4）生物地层学研究，由北京青藏高原研究所的同事领导。 项目研究结果将对了解西藏中部的构造组合和地壳结构以及混杂岩融入大陆地壳的一般构造过程产生重大影响。 如果项目结果表明羌塘混杂岩已从金沙缝合带向北逆冲，那么青藏高原中部地壳深处的大部分区域应包括底镀混杂岩。 这种关系还将提供一个无与伦比的机会来直接检查地质记录中很少观察到的低角度俯冲过程。 相反，羌塘中部地区大洋缝合带的形成需要对有关西藏中部构造框架和发展的传统思想进行重大修正。 该项目涉及一名博士。学生，并正在聘请一组本科生协助收集和解释碎屑锆石数据。 此外，正在组织一场“西藏构造”研讨会，亚利桑那大学的教师、研究生和本科生以及来自中国和其他美国机构的研究人员将齐聚一堂。