Study of the Structure and Tectonics of the South Shetland Trench and Bransfield Backarc Using Ocean Bottom Seismographs

该奖项由美国国家科学基金会极地项目办公室提供，支持调查南设得兰弧和布兰斯菲尔德海峡的地震活动和构造的研究。该地区呈现出迷人而独特的构造环境，俯冲速度减慢，岛弧火山活动停止，以及在过去400万年中出现的明显弧后裂谷。这个项目将进行为期5个月的14个海底地震仪的部署，以补充和扩展在1997年年初成功安装的6个宽频陆地地震台站的部署。OBSs包括2个带宽带传感器的仪器，所有仪器都有用于测量和采样热液流体的流量计。海底地震仪将用于检查设得兰-布兰斯菲尔德构造系统的许多特征，包括：设得兰板块的存在及其渗透深度：设得兰板块的存在将通过地震位置和地震层析成像来确定。地震活动的最大深度和板块速度异常的深度将约束板块的当前构造，并可能有助于阐明俯冲板块与弧火山活动停止之间的关系。——设得兰浅海沟地震活动？根据现有的地震资料，在南设得兰海沟未观察到远震浅层逆冲断层地震活动。利用OBS数据，将确定沿浅层逆冲带的小地震活动水平，并与其他经历年轻海洋岩石圈缓慢俯冲的地区（如卡斯卡迪亚）进行比较，后者通常也显示出非常低的逆冲带地震活动水平。—沿布兰斯菲尔德裂谷的变形模式：布兰斯菲尔德后弧的中心有一个活动裂谷，但有相当多的裂谷外断裂的证据。关于她的弧后延伸是沿着离散的裂谷带发生，还是在地理上更分散，一直存在争议。该项目将准确定位小地震，以更好地确定布兰斯菲尔德延伸是离散的还是扩散的。—火山活动和热液活动的识别：地震记录将用于识别沿Bransfield裂谷的海底火山活动的位置。附在OBSs上的流量计将记录并采样沉积物中的流体通量。——布兰斯菲尔德的上地幔结构——部分熔融的证据？其他弧后盆地表现出非常慢的上地幔地震速度和高地震衰减，这是由于存在部分熔融物质造成的。该项目将使用地震层析成像技术来解析布兰斯菲尔德后弧的上地幔结构，允许与其他后弧区域进行比较，并对部分熔融物质的存在以及部分熔融作为该地区地幔过程的重要性进行限制。合作奖项：OPP 9725679、OPP 9726180

This award, provided by the Office of Polar Programs of the National Science Foundation, supports research to investigate the seismicity and tectonics of the South Shetland Arc and the Bransfield Strait. This region presents an intriguing and unique tectonic setting, with slowing of subduction, cessation of island arc volcanism, as well as the apparent onset of backarc rifting occurring within the last four million years. This project will carry out a 5-month deployment of 14 ocean bottom seismographs (OBSs) to complement and extend a deployment of 6 broadband land seismic stations that were successfully installed during early 1997. The OBSs include 2 instruments with broadband sensors, and all have flowmeters for measuring and sampling hydrothermal fluids. The OBSs will be used to examine many of the characteristics of the Shetland- Bransfield tectonic system, including: --- The existence and depth of penetration of a Shetland Slab: The existence of a downgoing Shetland slab will be determined from earthquake locations and from seismic tomography. The maximum depth of earthquake activity and the depth of the slab velocity anomaly will constrain the current configuration of the slab, and may help clarify the relationship between the subducting slab and the cessation of arc volcanism. -- Shallow Shetland trench seismicity?: No teleseismic shallow thrust faulting seismicity has been observed along the South Shetland Trench from available seismic information. Using the OBS data, the level of small earthquake activity along the shallow thrust zone will be determined and compared to other regions undergoing slow subduction of young oceanic lithosphere, such as Cascadia, which also generally shows very low levels of thrust zone seismicity. -- Mode of deformation along the Bransfield Rift: The Bransfield backarc has an active rift in the center, but there is considerable evidence for off-rift faulting. There is a long-standing controversy about whet her back-arc extension occurs along discrete rift zones, or is more diffuse geographically. This project will accurately locate small earthquakes to better determine whether Bransfield extension is discrete or diffuse. -- Identification of volcanism and hydrothermal activity: Seismic records will be used to identify the locations of active seafloor volcanism along the Bransfield rift. Flowmeters attached to the OBSs will record and sample the fluid flux out of the sediments. -- Upper mantle structure of the Bransfield - evidence for partial melting?: Other backarc basins show very slow upper mantle seismic velocities and high seismic attenuation, characteristics due to the presence of partially molten material. This project will use seismic tomography to resolve the upper mantle structure of the Bransfield backarc, allowing comparison with other backarc regions and placing constraints on the existence of partially molten material and the importance of partial melting as a mantle process in this region. Collaborative awards: OPP 9725679 and OPP 9726180