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

在西藏中部的羌塘盆地中部，有一条巨大的东西向蓝片岩混杂岩带（长500多公里，宽100多公里）。 该带在构造上位于大陆边缘地层之下，在中生代早期穹隆低角度正断层的下盘，要求混杂岩在折返之前具有底侵的大陆边缘地层。 混杂岩的构造意义是高度不确定的，有两个主要的假设提出日期：（1）混杂岩标志着一个缝合带的位置分离以前不相关的地壳碎片的北部和南部。(2)该混杂岩系由松潘-甘孜沉积岩和俯冲-增生岩组成，从金沙缝合带向北俯冲约200 km。 为了验证这些假设，本次为期三年的调查正在进行：（1）沿沿着两条横贯羌塘的南北走向进行详细的区域地质填图和构造分析;（2）对羌塘混杂岩进行详细的构造分析、热年代学和热压力学研究;（3）对混杂岩、松潘-甘孜地层、以及北方和南部羌塘地区地层;（4）由北京青藏高原研究所同事领导的生物地层学研究。 项目成果将对理解西藏中部的构造组合和地壳结构以及混杂岩进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是从金沙缝合带向北俯冲而来的，那么藏中地壳深部的大部分应该包括底侵混杂岩。 这种关系也将提供一个无与伦比的机会，直接检查低角度俯冲的过程中很少观察到的地质记录。 相反，在羌塘中部地区发现一条主要的大洋缝合线，就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改。 该项目涉及一名博士。目前，该研究所正在聘请一批本科生协助收集和解释碎屑锆石数据。 此外，正在组织一个"西藏构造”研讨会，将汇集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。

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.