Retrograde Metamorphism in the Greek Cycladic Islands as a Window into Exhumation Mechanisms of High Pressure Terranes

俯冲带将海洋和大陆岩石圈带到弧岩浆生成的深处及更远的地方。然而，并非所有俯冲物质都到达这些深度，岩石经常返回地球表面并作为造山带中挖出的高压和超高压地体保存下来。这些岩石返回地表的机制仍然是个谜。了解折返机制是全面了解俯冲带动力学的基础。尽管研究重点是爱琴海的特定部分，但研究结果将对其他发掘的俯冲复合体以及现代和古代俯冲带中的一般构造和地球化学过程产生影响。例如，对俯冲带的现代观测揭示了沿俯冲界面的一系列变形和地震行为，这些行为是界面有效宽度、流变和变质异质性以及流体分布的函数。锡罗斯岛和蒂诺斯岛挖掘出的岩石记录对这个俯冲界面进行了采样，使我们能够更好地了解物质回流过程中质量传输、流体、变形和地震行为之间的联系。除了提高对俯冲带物质转移的理解所带来的智力影响外，更广泛的影响还包括两个关键组成部分：1）与雅典大学同事的国际合作，2）通过培训两名博士生来支持/推广少数族裔学生。学生（一名少数民族和一名女性），并支持波多黎各大学学生的少数民族本科暑期实习计划。人们普遍认为，锡罗斯岛和蒂诺斯岛的变质岩记录了由于始新世（约 50 Ma）期间俯冲而发生的前变质作用，随后在中新世（约 25 Ma）伸展拆离断层期间从中地壳水平折返。然而，在俯冲和核心复合体俘获之间的相当长的时间窗口（~25米）期间，负责将这些岩石从地幔深处挖出到中地壳的过程仍然是个谜，而且压力和温度条件也很少受到限制。此外，逆变质作用的几个神秘阶段可能与这个时间间隙有关，并记录了不同的矿物组合、变形结构以及流体渗透程度和流体来源。研究人员将确定锡罗斯岛和蒂诺斯岛几个露头的压力-温度-时间-变形-流体源路径，以破译造成早期逆行历史的折返机​​制，并提供基克拉迪蓝片岩单元始渐新世折返历史的高分辨率快照。研究人员将使用多学科方法，包括现场构造地质学、岩石学、地质年代学和地球化学，来破译锡罗斯岛和蒂诺斯岛高压变质岩的早期折返机制。现场构造地质学将重点关注确定变形阶段、应变分布和运动学；岩石学、热压测量和微观结构分析将用于将露头中发现的特定织物与其变形的压力-温度条件联系起来； 原位和多矿物等时地质年代学将为特定逆行织物的开发时间提供限制；稳定同位素地球化学将使我们能够破译促进回归过程的流体来源。

Subduction zones carry oceanic and continental lithosphere to depths of arc magma genesis and beyond. However, not all subducted material reaches these depths and rocks are often returned to Earth's surface and preserved as exhumed high-pressure and ultra-high-pressure terranes in orogenic belts. The mechanism(s) by which these rocks return to the surface remains enigmatic. Knowledge of exhumation mechanisms is fundamental for a complete understanding of subduction zone dynamics. Although the research focus is on a particular part of the Aegean, the results will have implications for the general tectonic and geochemical processes operative in other exhumed subduction complexes as well as modern and ancient subduction zones. For example, modern observations of subduction zones reveal a spectrum of deformational and seismic behaviors along the subduction interface that are a function of interface effective width, rheological and metamorphic heterogeneities, and fluid distribution. The exhumed rock record on Syros and Tinos samples this subduction interface allowing us to better understand the linkage among mass transport, fluids, deformation and seismic behavior during the return-flow of material. In addition to the intellectual impacts of improved understanding of material transfer in subductions zone, the broader impacts consist of two key components: 1) international collaboration with colleagues at the University of Athens and 2) minority student support/outreach via training of two Ph.D. students (one minority and one female) and support for a minority undergraduate summer internship program for students from the University of Puerto Rico.It is commonly accepted that metamorphic rocks on Syros and Tinos record prograde metamorphism due to subduction during the Eocene (~ 50 Ma), followed by exhumation from mid-crustal levels during Miocene (~25 Ma) extensional detachment faulting. However, the processes responsible for exhuming these rocks from mantle depths to the middle crust during substantial window of time (~25 m.y.) between subduction and core-complex-capture remain enigmatic and the pressure and temperature conditions are poorly constrained. In addition, there are several cryptic phases of retrograde metamorphism that are likely associated with this time gap, and that record different mineral assemblages, deformation fabrics, and degrees of fluid infiltration and fluid sources. THe researchers will determine the pressure-temperature-time-deformation-fluid-source paths for several outcrops on Syros and Tinos, in order to decipher the exhumation mechanisms responsible for the early retrograde history and provide a high resolution snapshot of the Eo-Oligocene exhumation history of the Cycladic Blueschist Unit. The researchers will use a multi-disciplinary suite of approaches, including field structural geology, petrology, geochronology, and geochemistry, to decipher the early exhumation mechanisms of high pressure metamorphic rocks on Syros and Tinos. Field structural geology will focus on establishing deformation stages, strain distributions and kinematics; petrology, thermobarometry and microstructural analysis will be used to link particular fabrics identified in outcrop to their pressure-temperature conditions of deformation; in-situ and multi-mineral-isochron geochronology will provide constraints on the timing of development of particular retrograde fabrics; and stable isotope geochemistry will allow us to decipher the fluid sources that facilitated the retrogression process.