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

辐合边缘的范例涉及一个均匀特征的大洋板块，它以一个相对陡峭的（30至45度）角度俯冲。然而，大洋板块不是均匀的，而是由厚地壳的宽（高原）和窄（地震脊）区域组成，以及熔融的地幔物质到达表面并形成新地壳的扩展脊。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浮力更强，并以较浅或接近平坦的角度俯冲。在世界上的一些地区，浮力海洋高原和地震和扩张脊的平板俯冲发生。S主收敛边。在这些平板俯冲带之上的沉积盆地的反应尚不清楚。本项目研究了阿拉斯加中南部Cook Inlet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。拟议研究的一个重要影响将是更好地了解上部板块和沉积记录对平板俯冲的反应，这些结果将为未来潜在平板位置的研究提供信息。该项目将通过以下方式促进预期的社会成果：1)通过支持两名早期职业女性pi和向少数民族高中生伸出援手，使女性充分参与STEM；2)通过公众外展提高公众科学素养和公众对科学技术的参与；3)通过在辛辛那提高中开展的活动、在安克雷奇为学生和专业人士开设的短期课程以及对研究生和本科生的培训，培养一支多元化的、具有全球竞争力的STEM劳动力队伍。与平板俯冲有关的上板块作用的顺序、时间和幅度仍然知之甚少。古地质记录中对平板俯冲作用的识别主要基于火山弧岩浆活动的变化，较少基于上板块变形、挖掘和沉积盆地演化的向内迁移。然而，很少有研究试图量化地表隆起和侵蚀的时空格局，因为这与浮力板块的俯冲有关，并导致邻近盆地主要沉积物来源的类型和位置发生变化。在这里，来自爱荷华大学和辛辛那提大学的研究小组将对阿拉斯加中南部库克湾盆地的晚白垩世-新生代沉积沉积物进行研究，这些沉积沉积物构成了三种不同俯冲模式下弧前盆地地层的完整沉积记录。晚白垩世洋壳正常俯冲后是扩张脊的平板俯冲（约62 ~ 50 Ma），之后是海洋高原的俯冲（约40 ~ 0 Ma）。对两个假设进行了验证：(1)古新世晚期的伸展脊俯冲？始新世导致了地形的变化和沉积物来源从邻近的弧向弧后地区的转移；(2)渐新世至今海洋高原的平板俯冲作用导致平板区域上方地形形成，弧前盆地流域整体收缩。研究人员通过传统的地层分析，结合碎屑锆石双测年技术（铀铅测年和裂变径迹测年）、泥岩稀土元素分析和砂岩岩石学技术，对库克湾弧前盆地岩心样品中的晚中生代和新生代地层进行了研究，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我们能够研究整个地区的区域挖掘、岩浆活动和沉积物运输模式，并根据区域构造和时间变化来解释这些模式。该研究将为经平板俯冲作用改造的弧前盆地新的构造模式和物源模式奠定基础。

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.