Collaborative Research: Rift dynamics during the formation of the Carolina Trough and Blake Plateau

在这个项目中，科学家们将调查美国东南部近海的深层沉积物和基岩，以更好地了解大约1.9亿年前大西洋是如何开始开放的。早侏罗纪时期，随着美国东南部大陆边缘从非洲西北部断裂，大西洋开始形成。在大西洋中脊开始适应海底扩张之前，地壳和岩石圈的机械拉伸可能伴随着广泛的加热和火山活动，从而产生了裂谷。阿巴拉契亚基底的旧断层线和缝合线可能在这一大陆伸展阶段被重新激活，熔岩流可能填充了早期裂谷盆地。这次火山活动是北美和非洲分裂的原因还是结果尚不清楚。这个问题可以用新的海洋地球物理数据来解决，这些数据将覆盖从大陆架到邻近的深海平原的部分边缘，距离约为400公里。通过深层沉积物和基底的地震图像，科学家们可以确定基底断层与卡罗来纳海槽（沿着卡罗来纳海岸）和布莱克高原（沿着乔治亚州和佛罗里达州更南边）的熔岩流之间的关系。大西洋大陆边缘的这两个部分特别令人感兴趣，因为布莱克高原比卡罗莱纳海槽宽得多。要么布莱克高原形成了一块大陆地壳，在大陆裂谷期间没有拉伸太多，要么它是一个主要由火山岩组成的高原，在泛大陆超大陆破裂期间形成。海洋地球物理考察将包括两艘船，R/V马库斯·朗塞斯号和R/V尼尔·阿姆斯特朗号，以及海底地震仪。海底地震仪将用于记录地震波，绘制海底基底结构。调查小组将邀请其他大学的学生和早期职业科学家参加这次考察，并将组织两门短期课程，让早期职业科学家参与数据的分析和解释。虽然这项研究的重点是美国东南部边缘，但其结果将为全球其他大陆边缘正在进行和未来的研究提供信息。大陆裂谷下的软流圈在深层热异常存在的情况下，或由于变薄的岩石圈下的减压，可以产生大量的熔体。在这一背景下，岩石圈板块伸展与岩浆的产生和输送之间的相互作用尚不清楚。这组科学家将对美国东南部的裂谷边缘进行一次主动震源地震调查，以更好地了解大陆分裂期间构造伸展和岩浆活动之间的反馈，以及大西洋中部海底扩张的开始。布莱克高原的大陆边缘远宽于其北部的卡罗莱纳海槽的相邻边缘，表明在这两个相邻裂谷段，大陆岩石圈的伸展和破裂进程不同。地幔温度异常、地幔熔融和大陆岩石圈结构继承的差异可能都影响了这两种截然不同的裂陷样式。研究人员试图解决以下三个假设：1)由于岩石圈厚度的差异，布莱克高原盆地在分裂前比卡罗莱纳海槽盆地伸展得更宽。2)裂陷时期，岩浆供给与布莱克高原地壳伸展同步，而大陆地壳迅速变薄，南卡罗莱纳海槽下没有广泛的岩浆活动。3)裂陷时期，布莱克高原深部地幔明显比卡罗莱纳海槽下地幔热，这将影响岩石圈流变学和地幔熔融程度。为了评估与裂谷相关的过程如何影响地壳结构和沉积物地层学，科学团队将利用R/V Marcus Langseth收集大约4500公里的二维地震反射数据和900公里的海底地震仪（OBS）折射数据。在这次考察中，来自国家仪器中心（OBSIC）的39台obs将与R/V尼尔·阿姆斯特朗一起部署。地震速度和反射率的图像将为沉积物地层学、玄武岩流动和地壳尺度断层的几何形状以及火成岩地壳的厚度和组成提供新的见解。利用现有玄武岩样品的活动震源数据和地球化学分析，推断卡罗莱纳海槽和布莱克高原过去的变形和侵入、挤压地壳的侵位。来自其他美国机构的学生和早期职业科学家将被邀请参与R/V Neil Armstrong或R/V Marcus Langseth的海洋地震反射和折射数据的采集和分析。这些海上参与者将获得地震反射和折射数据的采集和船上处理经验。此外，本项目还提供了为期两周的海底地震仪折射数据分析和多道地震（MCS）反射数据处理短期课程，有助于培训学员如何使用这些新的地震数据。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

In this project scientists will investigate the deep layers of sediments and bedrock offshore the southeastern United States to better understand how the Atlantic Ocean started opening approximately 190 million years ago. In Early Jurassic, the Atlantic Ocean began to form as the continental margin of the southeastern United States rifted from northwest Africa. Rifting occurred by mechanical stretching of the crust and lithosphere, potentially with extensive heating and volcanism, before the Mid-Atlantic Ridge started to accommodate extension by seafloor spreading. Old fault lines and sutures of the Appalachian basement may have been reactivated during this continental stretching phase, and lava flows may have filled the early rift basin. Whether this episode of volcanism was a cause or consequence of the breakup between North America and Africa is not yet clear. This question can be addressed with new marine geophysical data that will cover parts of the margin from the continental shelf to the adjacent abyssal plain, a distance of approximately 400 km. With seismic images of the deep sediments and basement, scientists can determine the relationship between basement faults and lava flows on the Carolina Trough, along the coast of the Carolinas, and Blake Plateau, farther south along Georgia and Florida. These two sections of the Atlantic continental margin are of particular interest because the Blake Plateau is much wider than the Carolina Trough. Either the Blake Plateau forms a block of continental crust that did not stretch much during continental rifting, or it is a plateau that largely consists of volcanic rock that formed during rupture of the supercontinent Pangea. The marine geophysical expedition will involve two vessels, the R/V Marcus Langseth and the R/V Neil Armstrong, and ocean-bottom seismometers that will be used to record seismic waves to map subseafloor basement structure. The investigating team will invite students and early-career scientists from other universities to participate in the expedition, and two short courses will be organized to engage early-career scientists in the analysis and interpretation of the data. Although this study focuses on the southeastern United States margin, results will inform ongoing and future studies at other continental margins worldwide. The asthenosphere beneath continental rifts can produce large volumes of melt in the presence of deep-seated thermal anomalies, or due to decompression beneath thinning lithosphere. The interactions between lithospheric plate extension and the generation and delivery of magma in this setting are not yet well known. The team of scientists will conduct an active-source seismic investigation of the rifted margins of the southeastern United States to better understand the feedbacks between tectonic extension and magmatism during continental breakup and the onset of seafloor spreading in the central Atlantic Ocean. The continental margin of Blake Plateau is much wider than the adjacent margin of Carolina Trough to the north, suggesting that extension and rupture of continental lithosphere progressed differently at these two adjoining rift segments. Differences in mantle temperature anomalies, mantle melting, and structural inheritance of the continental lithosphere may all have influenced these two contrasting styles of rifting. The investigators seek to address the following three hypotheses: 1) The Blake Plateau Basin stretched much wider than the Carolina Trough Basin before breakup due to a difference in lithospheric thickness. 2) During rifting, the magma supply kept pace with extension of Blake Plateau crust, whereas the continental crust rapidly thinned without extensive magmatism beneath the southern Carolina Trough. 3) At the time of rifting, the deep mantle beneath the Blake Plateau was significantly hotter than the mantle beneath Carolina Trough, which would affect the lithospheric rheology, and the degree of mantle melting. To assess how rift-related processes affected crustal structure and sediment stratigraphy, the science team will gather approximately 4500 km of 2-D seismic reflection data and 900 km of ocean-bottom seismometer (OBS) refraction data with the R/V Marcus Langseth. During this expedition, 39 OBSs from the national Instrument Center (OBSIC) will be deployed with the R/V Neil Armstrong. Images of seismic velocity and reflectivity will provide new insights into the sediment stratigraphy, the geometry of basalt flows and crustal-scale faults, and the thickness and composition of igneous crust. The active-source seismic data and geochemical analyses of existing basalt samples will be used to infer past deformation and the emplacement of intrusive and extrusive crust at the Carolina Trough and Blake Plateau. Students and early-career scientists from other US institutions will be invited to engage in the acquisition and analysis of marine seismic reflection and refraction data on the R/V Neil Armstrong or R/V Marcus Langseth. These sea-going participants will obtain experience in the acquisition and on-board processing of seismic reflection and refraction data. In addition, two week-long short courses on OBS refraction data analysis and on processing of multichannel seismic (MCS) reflection data from this project will help train the participants in the use of these new seismic data.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.