SUMMARY

The anomalous density structure at subduction zones, both in the wedge and in the upper mantle, is analysed to shed light on the processes that are responsible for the characteristic gravity fingerprints of two types of subduction: ocean–continent and ocean–ocean. Our modelling is then performed within the frame of the EIGEN-6C4 gravitational disturbance pattern of two subductions representative of the above two types, the Sumatra and Mariana complexes, finally enabling the different characteristics of the two patterns to be observed and understood on a physical basis, including some small-scale details. A 2-D viscous modelling perpendicular to the trench accounts for the effects on the gravity pattern caused by a wide range of parameters in terms of convergence velocity, subduction dip angle and lateral variability of the crustal thickness of the overriding plate, as well as compositional differentiation, phase changes and hydration of the mantle. Plate coupling, modelled within a new scheme where the relative velocity at the plate contact results self-consistently from the thermomechanical evolution of the system, is shown to have an important impact on the gravity signature. Beyond the already understood general bipolar fingerprint of subduction, perpendicular to the trench, we obtain the density and gravity signatures of the processes occurring within the wedge and mantle that are responsible for the two different gravity patterns. To be compliant with the geodetic EIGEN-6C4 gravitational disturbance and to compare our predictions with the gravity at Sumatra and Mariana, we define a model normal Earth. Although the peak-to-peak gravitational disturbance is comparable for the two types of subductions, approximately 250 mGal, from both observations and modelling, encompassing the highest positive maximum on the overriding plates and the negative minimum on the trench, the trough is wider for the ocean–ocean subduction: approximately 300 km compared to approximately 180 km for the ocean–continent subduction. Furthermore, the gravitational disturbance pattern is more symmetric for the ocean–ocean subduction compared to the ocean–continent subduction in terms of the amplitudes of the two positive maxima over the overriding and subducting plates. Their difference is, for the ocean–ocean type, approximately one half of the ocean–continent one. These different characteristics of the two types of subductions are exploited herein in terms of the different crustal thicknesses of the overriding plate and of the different dynamics in the wedge and in the mantle for the two types of subduction, in close agreement with the gravity data.

中文翻译：

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海洋-大陆和海洋-海洋俯冲的静态和时变特征：苏门答腊和马里亚纳复合体的案例研究

概要

分析了楔形和上地幔中俯冲带的异常密度结构，以揭示造成两种俯冲特征性重力指纹的过程：海洋-大陆和海洋-俯冲。然后在代表上述两种苏门答腊和马里亚纳复合体的两个俯冲的EIGEN-6C4重力扰动模式的框架内进行我们的建模，最终使这两种模式的不同特征在物理基础上得以观察和理解。 ，包括一些小范围的细节。垂直于沟槽的二维粘性模型说明了由收敛速度方面的各种参数所引起的对重力模式的影响，俯冲俯冲角和上覆板地壳厚度的横向变化，以及地幔的成分差异，相变和水化作用。板耦合在新方案中建模，其中板接触处的相对速度是由系统的热机械演化自洽地产生的，显示出对重力签名有重要影响。除了已经被理解的，垂直于沟槽的俯冲的一般双极指纹之外，我们还获得了楔形和地幔内部发生的，引起两种不同重力模式的过程的密度和重力特征。为了符合大地测量EIGEN-6C4重力扰动，并将我们的预测与苏门答腊和马里亚纳的重力进行比较，我们定义了一个模型法线地球。尽管从观测和建模来看，两种俯冲的峰-峰重力扰动均相当，大约为250 mGal，包括上覆板的最高正最大值和沟槽上的负最小值，但波谷的宽度较宽。海洋-海洋俯冲：大约300公里，而海洋-大陆俯冲大约为180公里。此外，就俯冲和俯冲板块上两个正最大值的振幅而言，与海洋-大陆俯冲相比，重力干扰模式对于海洋-海洋俯冲而言更加对称。对于海洋-海洋类型，它们的差异大约是海洋-大陆类型的一半。