Rayleigh-Taylor instability, lithospheric dynamics, surface topography at convergent mountain belts, and gravity anomalies
Rayleigh-Taylor instability, lithospheric dynamics, surface topography at convergent mountain belts, and gravity anomalies
复制标题
瑞利-泰勒不稳定性、岩石圈动力学、汇聚山带的表面地形和重力异常
DOI：
10.1002/jgrb.50203
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发表时间：
2013
期刊：
影响因子：
3.4
通讯作者：
Molnar P
中科院分区：
文献类型：
--
作者：
Molnar P
一个类似大陆岩石圈的层上方的表面地形和相关的重力异常，在很大程度上取决于低密度地壳部分与地幔部分的粘度之比。对于线性稳定性分析，Rayleigh-Taylor不稳定性的增长率在很大程度上取决于扰动对岩石圈底部的波数或波长，而弱于这个粘性比，取决于地壳、地幔岩石圈和软流圈之间合理的密度差，以及地壳与总岩石圈厚度的比率。对于所有可能的密度、粘度和厚度，莫霍面在岩石圈底部下沉(上升)的地方被拉低(向上推)。与地幔岩石圈相比，地壳的粘性较大( >； ~30)，下沉和增厚的地幔岩石圈也将地表拉低。对于较小的粘度比，地壳增厚盖过了莫霍面的下降，地表在地幔岩石圈增厚和下降(变薄和上升)的区域上方上升(下降)。忽略地壳和地幔岩石圈内粘度的垂直变化，我们发现最大地表高度出现在地壳和地幔岩石圈粘度大致相等的情况下。对于大的地壳/地幔岩石圈粘性比，重力异常跟随地表地形，在下降(上升)区域具有负(正)自由空气异常。在这种情况下，地形异常比岩石圈处于均衡状态时可能出现的异常要小。因此，流动引起的应力--动压和偏应力--产生的地形比均衡状态下预期的要小。然而，对于小的壳幔粘性比(~lt；~10)，在长波长下计算的地表地形大于岩石圈柱处于均衡状态时的地表地形，而在短波长下局部均衡预测了错误符号的地表偏转。对于适合于会聚山带的波长范围(~15 0-6 0 0 km)，计算的重力异常在岩石圈增厚区为负，特别是当考虑表层的弯曲刚性时。相应地，导纳的计算值，即地表地形和自由空气重力异常的傅里叶变换的比率，也是与山脉带相关的波数的负值。然而，对于基本上所有的山脉地带，测量的自由空气异常和导纳都是正数。无论岩石圈的重力不稳定性是否影响收敛带的结构，与地壳厚度变化均衡预测的贡献相比，它对山带地形的贡献似乎很小。
Surface topography and associated gravity anomalies above a layer resembling continental lithosphere, whose mantle part is gravitationally unstable, depend strongly on the ratio of viscosities of the lower‐density crustal part to that of the mantle part. For linear stability analysis, growth rates of Rayleigh‐Taylor instabilities depend largely on the wave number, or wavelength, of the perturbation to the base of the lithosphere and weakly on this viscosity ratio, on plausible density differences among crust, mantle lithosphere, and asthenosphere, and on ratios of crustal to total lithospheric thicknesses. For all likely densities, viscosities, and thicknesses, the Moho is drawn down (pushed up) where the base of the lithosphere subsides (rises). For large viscosities of crust compared to mantle lithosphere (ratios > ~30), a sinking and thickening mantle lithosphere also pulls the surface down. For smaller viscosity ratios, crustal thickening overwhelms the descent of the Moho, and the surface rises (subsides) above regions where mantle lithosphere thickens and descends (thins and rises). Ignoring vertical variations of viscosity within the crust and mantle lithosphere, we find that the maximum surface height occurs for approximately equal viscosities of crust and mantle lithosphere. For large crust/mantle lithosphere viscosity ratios, gravity anomalies follow those of surface topography, with negative (positive) free‐air anomalies over regions of descent (ascent). In this case, topography anomalies are smaller than those that would occur if the lithosphere were in isostatic equilibrium. Hence, flow‐induced stresses—dynamic pressure and deviatoric stress—create smaller topography than that expected for an isostatic state. For small crust/mantle viscosity ratios (< ~10), however, calculated surface topography at long wavelengths is greater than it would be if the lithospheric column were in isostatic equilibrium, and at short wavelengths local isostasy predicts surface deflections of the wrong sign. For the range of wavelengths appropriate for convergent mountain belts (~150–600 km), calculated gravity anomalies are negative over regions of lithospheric thickening, especially when allowance for flexural rigidity of a surface layer is included. Correspondingly, calculated values of admittance, the ratio of Fourier transforms of surface topography and free‐air gravity anomalies, are also negative for wave numbers relevant to mountain belts. For essentially all mountain belts, however, measured free‐air anomalies and admittance are positive. Whether gravitational instability of the lithosphere affects the structure of convergent belts or not, its contribution to the topography of mountain belts seems to be small compared to that predicted for isostatic balance of crustal thickness variations.