Deformation modes of subducted lithosphere at the core‐mantle boundary: An experimental investigation
Deformation modes of subducted lithosphere at the core‐mantle boundary: An experimental investigation
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核幔边界俯冲岩石圈的变形模式：实验研究
DOI：
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发表时间：
2009
期刊：
影响因子：
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通讯作者：
Y. Gamblin
中科院分区：
文献类型：
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作者：
N. Loubet;N. Ribe;Y. Gamblin
古代俯冲岩石圈可能是D″层地震观测非均匀性和各向异性的重要来源。因此，对地震观测的正确解释需要了解俯冲岩石圈在到达核幔边界（CMB）时如何变形。在这里，我们研究这个过程的实验，使用一个模拟模型系统，其中一张粘性糖浆（代表俯冲岩石圈）福尔斯垂直下降到一个不可渗透的表面（“CMB”）在底部的水库粘度较低的糖浆（“环境地幔”）。当薄层/环境粘度比γ小于临界值γc（γc取决于下落高度与薄层厚度之比）时，薄层以两侧对称的滞止流形式撞击CMB。当γ > γc时，则发生周期性的折叠不稳定性。我们通过实验确定了折叠幅度δ随两种流体的粘度和密度以及进入折叠区的薄片的厚度和垂直速度变化的普适标度律。为了将这一定律应用于地球，我们假设地幔岩片相对于周围地幔的过量粘度和密度都由相同的温度异常ΔT控制。利用估算的地幔底部扩散蠕变流变参数，我们发现褶皱判据γ > γc要求ΔT > 300 K。最近Hutko等人[2006]根据地震数据提出，在Cocos板块下方的CMB处存在褶皱岩石圈，这不太可能是原位褶皱的结果，因为解释所观察到的后钙钛矿相变深度变化所需的温度异常（400-700 K）可能太大，无法保存在深穿透板中。然而，地震观测结果可能以另一种方式解释，即在660公里深处形成了一个折叠的岩石圈“堆”，随后沉入CMB。
Ancient subducted lithosphere may be an important source of the seismically observed heterogeneity and anisotropy in the D″ layer. Proper interpretation of the seismic observations therefore requires an understanding of how subducted lithosphere deforms upon reaching the core‐mantle boundary (CMB). Here we study this process experimentally, using an analog model system in which a sheet of viscous sugar syrup (representing the subducted lithosphere) falls vertically onto an impermeable surface (the “CMB”) at the bottom of a reservoir of less viscous syrup (the “ambient mantle”). For sheet/ambient viscosity contrasts γ less than a critical value γc that depends on the ratio of the fall height to the sheet thickness, the sheet impinges on the CMB as a stagnation flow with bilateral symmetry. For γ > γc, however, the sheet undergoes a periodic folding instability. We determine experimentally a universal scaling law for the folding amplitude δ as a function of the viscosities and densities of the two fluids and the thickness and vertical velocity of the sheet where it enters the folding region. To apply this law to the Earth, we assume that the sheet's excess viscosity and density relative to the ambient mantle are both controlled by the same temperature anomaly ΔT. Using estimated rheological parameters for diffusion creep in the lowermost mantle, we find that the folding criterion γ > γc requires ΔT > 300 K. The presence of folded lithosphere at the CMB beneath the Cocos plate, suggested recently on the basis of seismic data by Hutko et al. [2006], is unlikely to be the result of in situ folding because the temperature anomaly (400–700 K) required to explain the observed variations in the depth of the postperovskite phase transition is probably too large to be preserved in a deeply penetrating slab. However, the seismic observations might be explained in an alternative way by a “pile” of folded lithosphere that formed at 660 km depth and subsequently sank to the CMB.