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

在西藏中部羌塘地块中部，有一条巨大的（长500多公里，宽100多公里）东西向的含蓝片岩杂岩带。该带在构造上位于早中生代穹状低角正断层下盘的大陆边缘地层之下，要求在挖掘前混杂岩必须有下覆大陆边缘地层。混杂岩的构造意义是高度不确定的，目前提出了两种主要假设：(1)混杂岩标志着一个缝合带的位置，该缝合带将以前不相关的地壳碎片南北分开。(2)该混杂岩由松潘—甘孜沉积—俯冲—增生岩组成，由金沙缝合带向北约200 km的逆冲作用形成。为了验证这些假设，这项为期三年的研究开展了以下工作：(1)沿着两条横贯羌塘地体的N-S线进行区域和详细的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和热年代学和热气压学研究；(3)对混合岩、松潘-甘子地层和羌塘北部和南部地层的碎屑锆石进行比较研究；该研究由北京青藏高原研究所的同事领导。项目研究成果将对认识西藏中部的构造组合和地壳结构以及混杂岩并入大陆地壳的一般构造过程产生重大影响。如果工程结果表明羌塘混杂岩是由金沙缝合带向北的逆冲构造，那么藏中地壳深部大部分应该包括下镀混杂岩。这种关系也为直接研究地质记录中很少观察到的低角度俯冲过程提供了无与伦比的机会。相反，如果断定羌塘中部地区存在大洋缝合带，则需要对有关西藏中部构造格局和发展的传统思想进行重大修正。该项目涉及一名博士生和一组本科生，他们将协助收集和解释碎屑锆石数据。此外，正在组织一次“西藏构造学”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.