Collaborative Research: Investigating the Sedimentary Record of Differing Modes of Flat-Slab Subduction

收敛边缘的范例包括一个性质统一的大洋板块，以相对陡峭的角度(30度至45度)俯冲。然而，大洋板块并不是统一的，而是由加厚的地壳的宽(高原)和窄(无地震脊)区域以及扩张的山脊组成，在那里熔融的地幔物质到达地表并创造新的地壳。与这些特征相关的地壳厚度或热流的增加导致板块比典型的海洋板块更浮力，并以较浅或接近平坦的角度俯冲。沿着世界上一些主要的会聚边缘，出现了浮力大洋高原和无地震扩张海脊的平板式俯冲。这些平板俯冲带上方的沉积盆地的响应还不是很清楚。本项目研究阿拉斯加中南部库克湾盆地对俯冲角度变化的响应，特别关注从陡峭俯冲到平坦俯冲的转变。拟议研究的一个重大影响将是更好地了解上板块和沉积记录对平板俯冲的响应，这一结果将为未来对潜在平板位置的研究提供信息。该项目将通过以下方式推动预期的社会成果：1)通过支持两名职业早期女性个人指导员和接触少数族裔高中生，使妇女充分参与STEM；2)通过公众宣传提高公众的科学素养和公众对科学技术的参与；3)通过辛辛那提高中的活动、为学生和专业人员在安克雷奇举办的短期课程以及对研究生和本科生的培训，发展一支多样化的、具有全球竞争力的STEM劳动力队伍。在古代地质记录中，对平板俯冲的认识主要是基于火山弧岩浆作用的变化，而较少基于上板块变形、折返和沉积盆地演化的内向迁移。然而，很少有研究试图量化地表抬升和侵蚀的空间和时间模式，因为它与浮力板块的俯冲有关，并导致主要泥沙来源的类型和位置改变到邻近盆地。在这里，来自爱荷华大学和辛辛那提大学的一个研究小组将调查阿拉斯加中南部库克湾盆地的晚白垩世-新生代沉积沉积，这些沉积构成了三种不同俯冲模式期间沉积的弧前盆地地层的完整沉积记录。晚白垩世洋壳的正常俯冲之后是扩张海脊的平板式俯冲(约62-50 Ma)，后来是大洋高原的俯冲(约40-0 Ma)。验证了两种假设：(1)晚古新世-始新世伸展海脊的俯冲导致地形变化，沉积物来源从邻近弧区向弧后地区转移；(2)渐新世至今大洋高原的平板式俯冲导致平板区以上地形的形成和弧前盆地流域的整体收缩。研究人员通过对库克湾弧前盆地岩心样品中晚中生代和新生代地层的传统地层学分析，结合铀铅和裂变径迹双测年碎屑锆石的物源和热年代学技术，泥岩的稀土元素分析和砂岩岩石学，研究了阿拉斯加南部平板俯冲的沉积记录。这些数据集的整合使得能够调查区域折返、岩浆作用和整个区域的沉积物运移模式，并从区域构造和随时间变化的角度解释这些模式。这项研究将为经平板式俯冲作用改造的弧前盆地的新的构造和物源模式提供基础。

The paradigm of convergent margins involves an oceanic plate of uniform character that subducts at a relatively steep (30 to 45 degrees) angle. Oceanic plates are not uniform, however, and instead consist of broad (plateaus) and narrow (aseismic ridges) regions of thickened crust, as well as spreading ridges where molten mantle material comes to the surface and creates new crust. An increase in crustal thickness or heat flow associated with these features causes the plates to be more buoyant than typical oceanic plates and to subduct at shallower or nearly flat angles. Flat-slab subduction of buoyant oceanic plateaus and aseismic and spreading ridges occurs along some of the world?s major convergent margins. The response of sedimentary basins above a these zones of flat slab subduction is not well understood. This project investigates the response of the Cook Inlet basin in south central Alaska to changes in subduction angle with special focus on the transition from steep subduction to flat subduction. A significant impact of the proposed research will be to provide a better understanding of the response of the upper plate and the sedimentary record to flat slab subduction, results which inform future studies of potential flat slab localities. The project will advance desired societal outcomes through: 1) full participation of women in STEM through support of two early career female PIs and outreach to minority high school students; 2) increased public scientific literacy and public engagement with science and technology through public outreach; and 3) development of a diverse, globally competitive STEM workforce though activities at Cincinnati high school, a short course for students and professionals in Anchorage, and training of graduate and undergraduate students.The sequence, timing, and magnitude of upper plate processes related to flat-slab subduction are still poorly understood. Recognition of flat-slab subduction in the ancient geologic record is based primarily on changes in volcanic arc magmatism and less frequently on inboard migration of upper plate deformation, exhumation, and sedimentary basin evolution. However, very few studies have attempted to quantify the spatial and temporal patterns of surface uplift and erosion as it relates to subduction of a buoyant slab, and which result in changes to the types and locations of major sediment sources to adjacent basins. Here, a research team from the University of Iowa and University of Cincinnati will investigate the Late Cretaceous-Cenozoic sedimentary deposits of the Cook Inlet basin in south central Alaska, which constitute a complete sedimentary record of forearc basin strata deposited during three different modes of subduction. Normal subduction of oceanic crust in the Late Cretaceous was followed by flat-slab subduction of a spreading ridge (ca. 62-50 Ma), and later by subduction of an oceanic plateau (ca. 40-0 Ma). Two hypotheses are tested: (1) subduction of a spreading-ridge in late Paleocene?Eocene time resulted in a change in topography and a shift in sediment sources from the adjacent arc to the retroarc region; (2) flat-slab subduction of an oceanic plateau from Oligocene time to the present resulted in the creation of topography above the flat-slab region and an overall contraction of the forearc basin drainage area. The researchers investigate the sedimentary record of flat slab subduction in southern Alaska by examining Late Mesozoic and Cenozoic strata in core samples from the Cook Inlet forearc basin through traditional stratigraphic analysis combined with provenance and thermochronologic techniques of double-dating detrital zircons using uranium-lead and fission track dating, rare earth element analyses of mudstones, and sandstone petrography. The integration of these datasets enables the investigation of the patterns of regional exhumation, magmatism, and sediment transport across the area and interpretation these patterns in terms of regional tectonics and changes through time. This study will provide a foundation for new tectonic and provenance models of forearc basins that have been modified by flat-slab subduction processes.