Causes for axial high topography at mid-ocean ridges and the role of crustal thermal structure
Causes for axial high topography at mid-ocean ridges and the role of crustal thermal structure
复制标题
大洋中脊轴向高地貌的成因及地壳热结构的作用
DOI：
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发表时间：
2001
期刊：
影响因子：
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通讯作者：
W. Buck
中科院分区：
文献类型：
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作者：
A. Shah;W. Buck
洋中脊地形被建模为对载荷的弯曲响应，使用薄板近似，并设置岩石圈的热结构，以允许（但不要求）在靠近轴的区域快速冷却。岩石圈上的负荷来自于低密度熔体的存在，由于离脊轴距离较远而冷却的致密化，以及热收缩应力。我们发现在东太平洋隆起（EPR）可以产生轴向高地形和重力的两种端元温度和熔体结构。一类与以前的模型非常相似，要求在地幔和岩石圈内有一个狭窄的熔体柱，延伸到至少30公里的深度，并随着距离脊轴的距离而逐渐冷却和增厚。另一类是一个新的类别，预测岩石圈在离地轴几公里的范围内迅速冷却，然后在离地轴更远的地方缓慢冷却，融化主要包含在地壳内。后一种解决方案与EPR的层析成像和顺应性研究相一致，该研究预测，由于热液循环，在地轴几公里内的地壳快速冷却。该解决方案还允许熔体区域与地壳热结构耦合，并且要求地幔内没有熔体异常。根据岩石圈内的温度分布以及假定热收缩应力对地形的影响程度，模型拟合预测下地壳的熔点为0-30%。该模型通常预测岩石圈的轴向高度较宽，岩石圈在靠近轴的较宽区域内较薄。这与先前高的大截面积与高熔体存在或更温暖的地壳热状态之间的相关性是一致的。对于距离地轴超过~ 5公里的岩石圈冷却速度稍慢的情况，模型预测在地轴高的底部有一个槽。以前在南太平洋环流西侧的高压底部观察到过这样的槽，那里的沉降率异常低。最后，厚轴向岩石圈降低了高振幅，使其有时难以与长波长沉降区分开。这种形态与一些中间伸展脊的形态相当，那里的地形相对平坦，表明从快速风格到中间风格的转变。
Mid-ocean ridge topography is modeled as the flexural response to loads using a thin plate approximation and setting thermal structure of the lithosphere to allow, but not require, a region of rapid cooling near the axis. Loads on the lithosphere arise from the presence of low-density melt, densification due to cooling with distance from the ridge axis, and thermal contraction stresses. We find two end-member classes of temperature and melt structure that can produce axial high topography and gravity observed at the East Pacific Rise (EPR). One class is very similar to previous models, requiring a narrow column of melt extending to at least 30 km depth within the mantle and lithosphere which cools and thickens very gradually with distance from the ridge axis. The other is a new class, predicting lithosphere which cools rapidly within a few kilometers of the axis and then slowly farther from the axis, with melt which is contained primarily within the crust. The latter solution is consistent with tomography and compliance studies at the EPR which predict rapid crustal cooling within a few kilometers of the axis that is attributed to hydrothermal circulation. This solution also allows the melt region to be coupled to crustal thermal structure and requires no melt anomaly within the mantle. Model fits predict 0–30% melt in the lower crust, depending on how temperatures are distributed within the lithosphere and the degree to which thermal contraction stresses are assumed to contribute to topography. The model generally predicts a wider axial high for lithosphere which is thin over a wider region near the axis. This is consistent with previous correlations between large cross-sectional area of the high and indicators of higher melt presence or a warmer crustal thermal regime. For a slightly slower rate of lithospheric cooling at distances more than ∼5 km from the axis the model predicts a trough at the base of the axial high. Such troughs have been previously observed at the base of the high on the western flank of the southern EPR, where subsidence rates are anomalously low. Finally, thick axial lithosphere reduces the amplitude of the high, making it sometimes difficult to distinguish from long-wavelength subsidence. This morphology is comparable to that of some intermediate spreading ridges, where topography is relatively flat, suggesting a transition from fast to intermediate style morphology.