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

该奖项由国家科学基金会极地计划办公室提供，支持研究南设得兰弧区和布兰斯菲尔德海峡的地震活动和构造。该区域提供了一个耐人寻味的独特的构造环境，俯冲减缓，岛弧火山活动停止，以及过去400万年内弧后裂谷的明显开始。该项目将对14台海底地震仪进行为期5个月的部署，以补充和扩大1997年初成功安装的6个宽带陆地地震台的部署。OBSS包括两个带宽带传感器的仪器，所有仪器都有测量和采样热液流体的流量计。OBSS将被用来检查设得兰-布兰斯菲尔德构造系统的许多特征，包括：-设得兰板块的存在和穿透深度：设得兰板块下行的存在将根据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制约板块的现今形态，并有助于阐明俯冲板块与弧火山活动停止之间的关系。--设得兰海沟浅层地震活动：根据现有的地震资料，在南设得兰海沟沿线没有观测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震活动水平，并将其与其他经历年轻大洋岩石圈缓慢俯冲的地区进行比较，例如卡斯卡迪亚，后者通常也显示逆冲带地震活动水平很低。--布兰斯菲尔德裂谷的变形方式：布兰斯菲尔德背弧中心有一个活跃的裂谷，但有相当多的证据表明裂谷外的断层作用。关于她弧后伸展是沿着离散的断裂带发生，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伸展是离散的还是弥漫的。--确定火山活动和热液活动：将利用地震记录确定布兰斯菲尔德裂谷一带活跃的海底火山活动的位置。安装在OBSS上的流量计将记录和采样沉积物中的流体流量。-布兰斯菲尔德的上地幔结构--部分熔融的证据？：由于存在部分熔融的物质，其他弧后盆地表现出非常慢的上地幔地震速度和很高的地震衰减特征。该项目将使用地震层析成像来解决布兰斯菲尔德弧后的上地幔结构，从而能够与其他弧后地区进行比较，并限制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。协作奖：OPP 9725679和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