Small-scale gravitational instabilities under the oceans: Implications for the evolution of oceanic lithosphere and its expression in geophysical observables
Small-scale gravitational instabilities under the oceans: Implications for the evolution of oceanic lithosphere and its expression in geophysical observables
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DOI：
10.1080/14786430802464248
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发表时间：
2008-10
影响因子：
1.6
通讯作者：
S. Zlotnik;J. Afonso;P. Díez;Manel Fernàndez
中科院分区：
文献类型：
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作者：
S. Zlotnik;J. Afonso;P. Díez;Manel Fernàndez
岩石圈下的小尺度对流（SSC）被认为是负责在成熟板块中观察到的海底深度和表面热流的平坦化。虽然SSC的存在被普遍接受，但它是否能够有效地产生一个恒定的岩石圈厚度（即观测值的平坦化）是一个有争议的问题。在这里，我们研究的发展和演变的SSC与二维热机械有限元代码。重点放在（一）各种流变学和热物理参数对SSC的影响，以及（二）其再现地球物理观测值（即海底深度，表面热流和地震速度）的能力。我们发现，剪切加热起不到显着的作用，无论是在开始的SSC或在减少岩石圈厚度。相反，辐射热源和绝热加热对SSC的活力和岩石圈的热结构都起主要控制作用。我们发现，无论是位错蠕变，扩散蠕变，或这些机制的组合，可以产生SSC的流变参数由实验室实验。然而，只有在相对较低的活化能下，才有可能出现剧烈的SSC和显著的岩石圈侵蚀。只有当地幔的前300公里具有1020 Pa·s的平均粘度时，才会发生发育良好的SSC;较高的值抑制SSC，而较低的值产生不切实际的高速度。我们的模型预测的地震结构类似于海洋地幔的层析成像研究。然而，对观测到的海底地形和表面热流的拟合仍然不能令人满意。这在两个数据集之间提出了一个基本的二分法。如果所观察到的海底地形变平的大部分是受SSC以外的过程的影响，这一点是可以调和的。
Sublithospheric small-scale convection (SSC) is thought to be responsible for the flattening of the seafloor depth and surface heat flow observed in mature plates. Although the existence of SSC is generally accepted, its ability to effectively produce a constant lithospheric thickness (i.e. flattening of observables) is a matter of debate. Here we study the development and evolution of SSC with a 2D thermomechanical finite-element code. Emphasis is put on (i) the influence of various rheological and thermophysical parameters on SSC, and (ii) its ability to reproduce geophysical observables (i.e. seafloor depth, surface heat flow, and seismic velocities). We find that shear heating plays no significant role either in the onset of SSC or in reducing the lithospheric thickness. In contrast, radiogenic heat sources and adiabatic heating exert a major control on both the vigour of SSC and the thermal structure of the lithosphere. We find that either dislocation creep, diffusion creep, or a combination of these mechanisms, can generate SSC with rheological parameters given by laboratory experiments. However, vigorous SSC and significant lithospheric erosion are only possible for relatively low activation energies. Well-developed SSC occurs only if the first ∼300 km of the mantle has an average viscosity of ≲1020 Pa s; higher values suppress SSC, while lower values generate unrealistic high velocities. Seismic structures predicted by our models resemble closely tomography studies in oceanic mantle. However, the fitting to observed seafloor topography and surface heat flow is still unsatisfactory. This puts forward a fundamental dichotomy between the two datasets. This can be reconciled if most of the observed flattening in seafloor topography is influenced by processes other than SSC.