Seismic and gravity constraints on flexural models for the origin of seaward dipping reflectors
Seismic and gravity constraints on flexural models for the origin of seaward dipping reflectors
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向海倾斜反射器起源的弯曲模型的地震和重力约束
DOI：
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发表时间：
2018
影响因子：
2.8
通讯作者：
A. Watts
中科院分区：
文献类型：
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作者：
R. Morgan;A. Watts
向海倾斜反射体（SDR）是火山裂谷大陆边缘近海地区普遍存在的特征，其中它们由主要是喷出熔岩流的楔形包组成。然而，它们的起源一直存在争议，一些学者认为它们是由扩展的地壳逐渐沉降形成的，而另一些学者则认为它们是由一个或多个大陆倾斜的正断层容纳在地壳内的。我们在这里提出一个简单的模型，其中特别提款权是由一系列侵入地壳并加载的堤坝形成的。这些载荷导致表面弯曲，随后被火山材料填充并加载。该模型不需要边界正断层，它解释了在地震反射剖面数据中观察到的 SDR 的弓形、有限的断层几何形状和下倾增厚。通过比较观测到的和计算出的倾角，我们能够约束弹性板块模型类型和扩展岩石圈的有效弹性厚度 T e 。结果表明，需要破碎板或显着弱化的连续板模型才能产生特征性的弓形形状。随着裂谷的进展，降低连续载荷的 T e 会产生子包的脱落，而增加 T e 则会产生重叠。我们使用面向过程的重力建模验证了我们的结果，其中计算了地表火山填充荷载的重力效应，并将其与埋藏堤坝荷载的重力效应及其均衡补偿的重力效应相结合。纳米比亚近海奥兰治盆地的案例研究结果显示，观测到的艾里均衡异常与计算得出的 T e 在 1-3 km 范围内的重力异常之间具有良好的总体一致性。因此，在裂谷边缘的艾里均衡重力异常中经常观察到的陡峭梯度是海底扩张之前侵入地壳的堤坝向海边缘的有用代表，而不是像以前的工作人员所提出的那样，是大洋和大陆地壳之间边界处的基底高程的变化。
Seaward dipping reflectors (SDRs) are ubiquitous features of the offshore regions of volcanic rifted continental margins where they comprise wedge-shaped packages of mainly extrusive lava flows. However, their origin has been disputed with some workers suggesting they form by progressive subsidence of extended crust while others propose they are accommodated within the crust by one or more continent-dipping normal faults. We present here a simple model in which SDRs are formed by a succession of dykes which intrude and load the crust. These loads cause a surface flexure, which is subsequently infilled and loaded by volcanic material. The model, which does not require a bounding normal fault, explains the arcuate shape, limited offlap geometries and downdip thickening of SDRs as observed in seismic reflection profile data. By comparing observed and calculated dips we are able to constrain the elastic plate model type and the effective elastic thickness of extended lithosphere, T e . Results suggest a broken plate or significantly weakened continuous plate model is required to produce the characteristic arcuate shape. Decreasing the T e for successive loads as rifting progresses produces offlap of subpackages, while increasing the T e produces onlap. We have verified our results using process-oriented gravity modelling, in which the gravity effect of surface volcanic infill loads is calculated and combined with the gravity effect of buried dyke loads and the gravity effect of their isostatic compensation. Results from a case study in the Orange Basin, offshore Namibia show good general agreement between observed Airy isostatic anomalies and calculated gravity anomalies with T e in the range 1–3 km. The steep gradient that is often observed in the Airy isostatic gravity anomaly at rifted margins is therefore a useful proxy for the seaward edge of the dykes that intrude the crust prior to seafloor spreading, rather than a change in basement elevation at the boundary between oceanic and continental crust, as had been proposed by previous workers.