Investigation of a Lower-plate Continental Margin: A Proposal for Drilling in the Great Australian Bight Region by the Ocean Drilling Program

The southern margin of Australia, formed by the separation of Australia and Antarctica in theCretaceous, has long been considered an example of a classic, non-volcanic, passive, rifted margin. In recent times it has been cited as an excellent example of a 'lower plate' margin inthe terminology of detachment models of passive margin formation. As the margin has been relatively sediment-starved since breakup, and particularly in the Tertiary, it represents anexcellent opportunity to test detachment models via reflection seismic data and drilling.

In this document, we propose that a total of seven holes be drilled by the Ocean DrillingProgram in the central Great Australian Bight (GAB). Five of these holes are located close tothe previously interpreted continent-ocean boundary (COB), which high-quality seismic datasuggest may actually be a metamorphic core complex. The holes have the following basic aims:
1.Test the applicability of detachment tectonics to passive margin formation by drillingthrough a master detachment to extended lower continental crust;
2.Sample acoustic basement at several sites to characterise the tectonic and magmaticprocesses that have been influential at high rates of continental extension and/or duringearly and slow oceanic spreading.

In the event that the interpretation of a metamorphic core complex proves to be incorrect and oceanic crust is encountered, then two additional holes are proposed to:
1.Date the oldest sediments above oceanic crust in the South Australian Abyssal Plain, andhence constrain the currently ill-defined age of onset of spreading between Australia andAntarctica and refme the breakup history of eastern Gondwanaland.

2.Date and characterise the extensional listric faulting that is prominent in possible oceaniccrust on the South Australian Abyssal Plain.

As the data on which these sites are proposed is sparse and, in some cases, old, the AustralianGeological Survey Organisation (AGSO) proposes to carry out a deep-seismic program in thecentral GAB that will allow a refmal interpretation and consequent up-grading or modificationof the proposed sites in 1995/96.

澳大利亚南缘由白垩纪时期澳大利亚与南极洲的分离作用形成，长期以来被视作经典非火山型被动裂谷边缘的典型范例。近年来，依据被动边缘形成拆离模型的术语体系，该边缘被列为“下盘”边缘的绝佳实例。自裂谷分离以来，该区域（尤其在第三纪）沉积物补给相对匮乏，为通过反射地震数据与钻探实验验证拆离模型提供了绝佳契机。

本文件提议由大洋钻探计划（Ocean Drilling Program）在澳大利亚大湾（Great Australian Bight，简称GAB）中部部署共计7个钻探孔位。其中5个孔位紧邻此前解释的大陆洋壳边界（continent-ocean boundary，COB），而高质量地震数据显示该边界实际可能为一处变质核杂岩（metamorphic core complex）。本次钻探孔位的核心目标如下：
1. 通过钻穿主控拆离面至拉伸后的下陆壳，验证拆离构造学对被动边缘形成机制的适用性；
2. 在多个点位采集声学基底（acoustic basement）样本，以表征在大陆高速拉伸阶段及/或早期慢速洋壳扩张过程中起到主导作用的构造与岩浆作用过程。

若变质核杂岩的解释被证实有误，且实际钻遇洋壳，则需额外增设2个孔位，以实现以下目标：
1. 测定南澳大利亚深海平原洋壳上方最古老沉积物的年代，进而约束当前尚不明晰的澳-南极洋壳扩张起始年龄，并完善东冈瓦纳大陆（Gondwanaland）的分离演化历史；
2. 测定并表征南澳大利亚深海平原内疑似洋壳区域中广泛发育的伸展铲状断层（extensional listric faulting）。

由于支撑本次孔位提议的现有数据较为匮乏，且部分数据年代久远，澳大利亚地质调查局（Australian Geological Survey Organisation，AGSO）提议在GAB中部开展深海地震勘探项目，以实现精准解释，并于1995/1996年度对提议孔位进行后续优化或调整。