Australia's lithospheric density field, and its isostatic equilibration
Australia's lithospheric density field, and its isostatic equilibration
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澳大利亚的岩石圈密度场及其均衡平衡
DOI：
10.1093/gji/ggv396
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发表时间：
2015
影响因子：
2.8
通讯作者：
L. Gross
中科院分区：
文献类型：
--
作者：
A. Aitken;C. Altinay;L. Gross
密度是构造过程的一个关键驱动因素，但在岩石圈中很难很好地定义密度，因为重力方法是不唯一的，而且因为从地震速度模型转换为密度本身也是不唯一的，也是高度不确定的。本文采用一种新的方法确定了澳大利亚100°E ~ 165°E、5°N ~ 55°S、地壳表层~ 300 km深度范围内的岩石圈密度场。参考模型主要来自最近公布的澳大利亚地震参考模型，并使用沉积盆地厚度和地壳厚度的其他模型进一步完善。一种新形式的有限元法为基础的确定性重力反演应用于大地坐标，在开放源代码脚本建模环境中实施。模拟了三种空间分辨率：半度、四分之一度和八分之一度纬度和经度，垂直分辨率分别为5、2.5和1.25公里。关键反演正则化参数的参数扫描表明，参数选择不依赖于尺度。扫描结果还表明，更好的分辨率是更敏感的最上地壳，但不敏感的中下地壳和最上地幔比低分辨率。所有分辨率在100公里深度以下都显示出类似的灵敏度。最终的密度模型表明，澳大利亚的岩石圈密度场是强烈的分层，但也有很大的横向密度差异在所有深度。在大陆地壳内部，中地壳和下地壳的结构与结晶上地壳有很大的不同，这表明保存在深部地壳中的构造过程或事件与保存在浅部地壳中的构造过程或事件不同。岩石圈地幔结构并没有从参考模型中进行广泛的修改，但结果加强了海洋和大陆区域密度之间的系统差异，并有助于确定每个区域内的细分。利用重力场和密度场，在三维空间中求解岩石圈静压场。压力场模型还突出了海洋和大陆区域之间的根本区别，在模型的大部分时间里，前者的压力较低。总的压力变化在上地壳很大（60 MPa），但在海拔-30 km处显著降低（20-30 MPa）。到海拔-50 km时，厚的下地壳产生进一步的不平衡（25-35 MPa），直到海拔-125 km（10-20 MPa）才得到补偿。在海拔-125公里以下，在大陆区域观察到更高的压力，延伸到模型的底部。这表明岩石圈在很大程度上均衡补偿的基础附近的长英质-中间大陆地壳，并再次接近成熟的海洋岩石圈的理论基础。
Density is a key driver of tectonic processes, but it is a difficult property to define well in the lithosphere because the gravity method is non-unique, and because converting to density from seismic velocity models, themselves non-unique, is also highly uncertain. Here we use a new approach to define the lithospheric density field of Australia, covering from 100°E to 165°E, from 5°N to 55°S and from the crust surface to 300 km depth. A reference model was derived primarily from the recently released Australian Seismological Reference Model, and refined further using additional models of sedimentary basin thickness and crustal thickness. A novel form of finite-element method based deterministic gravity inversion was applied in geodetic coordinates, implemented within the open-source escript modelling environment. Three spatial resolutions were modelled: half-, quarter- and eighth-degree in latitude and longitude, with vertical resolutions of 5, 2.5 and 1.25 km, respectively. Parameter sweeps for the key inversion regularization parameters show that parameter selection is not scale dependent. The sweep results also show that finer resolutions are more sensitive to the uppermost crust, but less sensitive to the mid- to lower-crust and uppermost mantle than lower resolutions. All resolutions show similar sensitivity below about 100 km depth. The final density model shows that Australia's lithospheric density field is strongly layered but also has large lateral density contrasts at all depths. Within the continental crust, the structure of the middle and lower crust differs significantly from the crystalline upper crust, suggesting that the tectonic processes or events preserved in the deep crust differ from those preserved in the shallower crust. The lithospheric mantle structure is not extensively modified from the reference model, but the results reinforce the systematic difference between the density of the oceanic and continental domains, and help identify subdivisions within each. The lithospheric static pressure field was resolved in 3D from the gravity and density fields. The pressure field model also highlights the fundamental difference between the oceanic and continental domains, with the former possessing lower pressure through most of the model. Overall pressure variability is large in the upper crust (60 MPa) but reduces significantly by −30 km elevation (20–30 MPa). By −50 km elevation, thick lower-crust generates further disequilibria (25–35 MPa) that are not compensated until −125 km elevation (10–20 MPa). Beneath −125 km elevation higher pressure is observed in the continental domain, extending to the base of the model. This indicates a lithosphere that is to a large degree isostatically compensated near the base of the felsic-intermediate continental crust, and again near the theoretical base of mature oceanic lithosphere.