![Sample our Geography journals, sign in here to start your FREE access for 14 days]({"type":"image" , "src" : "/sda/5937/TOC-Subject-Sample-2021-Geography.jpg"})
Abstract
Recent seismic reflection studies of large-volume, anorogenic basaltic provinces at passive continental margins have challenged the traditional viewpoint that erupted magmas predominantly ascend through the lithosphere via dykes that exploit high-angle faults. However, such seismic-based methods are yet to be applied to identify the magmatic plumbing systems of low-volume basaltic provinces, such as the Cretaceous–Cenozoic Newer Volcanic Province (NVP) in southeastern Australia (total volume <20 000 km3). The South Australian Quaternary Basalt (SAQB) represents the most recent phase of activity both within the NVP and on the Australian continent. This province is located within the Otway Basin, and a large amount of seismic data from gas exploration is available for this region. Consequently, the SAQB represents a superb natural laboratory in which geochemical and seismic data can be combined in order to study the magmatic plumbing system of a low-volume continental basalt province and discriminate between the competing hypotheses for magma transport through the Earth's crust. Geochemical analyses and thermodynamic modelling suggests that the magma that fed the SAQB was generated by adiabatic decompressional melting of a secondary mantle plume. These models imply that melt segregation took place at successively lower pressures from about ∼4000 to 3000 MPa during the ca 1 Ma SAQB eruptive history that culminated in the Northern Group, Mt Schank and Mt Gambier eruptions. During ascent, the magma underwent 34–41% fractional crystallisation and cooled ∼200 °C, while residing in the crust for a time period on the order of days to weeks. Interpretation of a 3D seismic survey that overlaps with the northeastern part of the SAQB reveals a saucer-shaped sill with an unusual morphology, exhibiting a series of vertical concentric steps towards its outer rim. This sill appears to be fed by magma that intruded along steep normal faults from a feeder dyke that is hosted by the regional, NW–SE-trending Hungerford-Kalangadoo Fault. Our results suggest that the melt that fed the SAQB rose through the crust via dykes and high-angle normal faults, with less evidence for significant horizontal transport of magma than observed in large-volume basaltic provinces in sedimentary basins at rifted continental margins, possibly highlighting a correlation between the extent, volume and magma supply rate within a basaltic province and the nature of magma ascent.
最近对被动大陆边缘的大体积非造山玄武岩区域进行的地震反射研究，挑战了这样一种传统观点：岩浆上涌的主要通道是岩石圈的岩脉并产生高角度断层。然而，这样的以地震为基础的方法尚待用于鉴定小体积玄武岩区的岩浆管道系统，如澳大利亚东南部的白垩纪 - 新生代较新火山岩区（ NVP ） （总体积< 20 000立方公里） 。南澳大利亚第四纪玄武岩（ SAQB ）代表NVP之内和澳大利亚大陆之上的最近的活动阶段。这个区域位于Otway盆地内，得自天然气勘探的大量地震数据可用于该地区。因此， SAQB代表一个极好的天然实验室。在这个实验室里，我们可以综合地球化学和地震资料来研究小体积大陆玄武岩区的岩浆管道系统，并将若干有竞争力的岩浆运输地壳说区别开来。地球化学分析和热力学模型表明，流入SAQB的岩浆是由二次地幔柱的绝热减压熔融产生的。这些模型意味着，在约1Ma的SAQB喷发期间，融化隔离发生在持续低压力（大约_4000至3000兆帕）的条件下，在North Group群、Mt Schank 山和Mt Gambier山喷发期达高峰。在上升过程中，岩浆经历了34-41 ％的粒级结晶，在 200℃左右冷却 ，期间在地壳里滞留的时间从数天至数周。通过对在SAQB东北部进行的三维地震勘探的解读，发现了一个形态不寻常的碟形岩床，具有一系列的朝向外缘的垂直同心阶梯。流入这个岩床的岩浆可能沿着陡峭的正断层而侵入，该岩浆来自区域性西北 - 东南走向的Hungerford-Kalangadoo断层中的岩脉。我们的研究结果表明，侵入SAQB的熔岩经由岩脉和高角度正断层穿过地壳而上流，比在张裂大陆边缘的沉积盆地内的大体积玄武岩区所观察到的岩浆大范围水平运移的证据要小的多，这点可能突显玄武岩区之内的岩浆的范围、体积和供给率与岩浆上升的性质之间的相关性。
ACKNOWLEDGEMENTS
SJH would like to thank both the School of Geology and Geophysics and the Australian School of Petroleum at the University of Adelaide for their support and use of facilities, and Ernest Swierczek for his invaluable advice during the seismic interpretation. The authors would like to thank Nick Schofield and Chris Jackson for helpful discussions, and the journal reviewers for their assistance. SPH acknowledges funding support from the ARC (DP0897612). We thank IHS for access to the Kingdom seismic interpretation software, and DMITRE for provision of seismic data. This paper represents TRaX record #267.