Development of Detachment Faults at Intermediate Magma Supply

现在人们认识到，长寿命（拆离）断层和岩心杂岩的形成是常见的，占中大西洋洋脊北部2500公里延伸的约50%。拆离断层不仅使我们能够研究下海洋地壳和上地幔的结构，而且还改变了我们对海洋地壳的组成和结构、流体流动、热液喷口和相关生态学的看法。但仍然存在许多关键问题：为什么一个断层继续延伸，翻滚形成一个核心复合体，而另一个断层只延伸了很短的时间就停止了？拆离断层的形成是否与整体和/或局部岩浆预算有关？相邻的分遣队是否相连？这个项目的目标就是试图回答这些问题。特别是，1)约束控制中段滑脱断层形成、演化和连接的过程；2)考察沿脊轴岩浆供给与滑脱断裂形成的关系。在南纬16.5度附近的研究区，在中轴线附近出现了一条新的中段拆离断裂，打断了一条更长的、更古老的、仍然活跃的、离轴更远的拆离断裂的整体发育。PIs的计划是获得区域多波束测深和磁数据，以了解5毫安的扩散历史。高分辨率多波束测深、磁、CTD和光学传感器数据将从自主水下航行器（AUV） Sentry收集，并从WHOI Towcam收集照片。这些数据将用于描述分离断层及其末端，评估它们的活动位置，并检查它们如何沿着轴线连接。这些数据还将有助于评估地轴上火山活动的性质。疏浚计划将确认从水深分析得出的推论，如地幔、深部或火山基底的起源，地壳结构在整个地区的变化，并将确定区域岩浆供应在滑脱断层形成过程中的恒定程度，以及与局部岩浆供应变化的可能关系。这项工作将对滑脱断层形成及其与岩浆供应关系的模型提出强有力的限制。更广泛的影响：拟议的研究解决了AGU查普曼会议上社区确定的几个问题。海洋岩石圈中的分离：变形、岩浆作用、流体流动和生态系统？对于推进我们对缓慢扩张的洋中脊断层的理解至关重要。通过J. Escartin及其研究生（巴黎IPGP）和H. Zhou（中国同济大学）的参与，该项目具有国际合作的组成部分。三名麻省理工学院/WHOI联合项目的学生将获得航海经验，其中两名将收集数据，将有助于他们的博士论文。朱先生和来自马里兰大学的两名本科生将参加这次航行。

It is now recognized that long-lived (detachment) faults and the formation of core complexes are common, accounting for ~50% of the extension along 2500 km of the northern Mid-Atlantic Ridge (MAR). Not only do detachment faults allow the architecture of the lower ocean crust and upper mantle to be examined, but they change the way we think about the very composition and structure of ocean crust, fluid flow, and hydrothermal venting and associated ecology. But many critical questions remain: Why does a fault continue to extend, rolling over to form a core complex, while another stops after extending only for a short time? Is detachment fault formation associated with overall and/or local magma budget? Are adjacent detachments linked? The goal of this project is to try to answer these questions. In particular, to 1) constrain the processes controlling the formation, evolution, and linking of mid-segment detachment faults and; 2) examine the relationship between magma supply along the ridge axis and detachment fault formation. the chosed study area near 16.5 degrees N at the MAR presents a dramatic demonstration of a new mid-segment detachment fault near to the axis interrupting the overall development of a longer, older and still active detachment fault whose breakaway is farther off-axis. The PIs plan is to obtain regional multibeam bathymetry and magnetic data to understand the spreading history out to 5 Ma. High-resolution multibeam bathymetry, magnetic, CTD and optical sensor data will be collected from the autonomous underwater vehicle (AUV) Sentry, and photographs from the WHOI Towcam. These data will be used to characterize the detachment faults and their terminations, assessing where they are active and examining how they might link along the axis. The data also will allow assessment of the nature of volcanism at the axis. A dredging program will confirm the inferences made from bathymetric analysis as to the mantle, plutonic or volcanic origin of basement, how crustal architecture varies across the region, and will establish the degree to which the regional magma supply has been constant over detachment fault formation and possible relationships to local magma supply variations. This work will place strong constrains on models for detachment fault formation and its relationship to magma supply.Broader Impacts: The proposed study addresses several issues identified by the community at the AGU Chapman Conference ?Detachments in Oceanic Lithosphere: Deformation,Magmatism, Fluid Flow, and Ecosystems? as critical to move forward our understanding of faulting at slow-spreading mid-ocean ridges. The project has a component of international cooperation through the participation of J. Escartin and his graduate student (IPGP, Paris) and H. Zhou (Tonji University, China). Three MIT/WHOI Joint Program students will receive sea-goingexperience, and two of them will collect data that will contribute to their PhD theses. W. Zhu and two undergraduate students from U. Maryland will participate on the cruise.