The role of the heating mode of the mantle in intermittent reorganization of the plate velocity field
The role of the heating mode of the mantle in intermittent reorganization of the plate velocity field
复制标题
地幔加热模式在板块速度场间歇重组中的作用
DOI：
--
复制
发表时间：
2003
期刊：
影响因子：
--
通讯作者：
C. Gable
中科院分区：
文献类型：
--
作者：
J. Lowman;S. King;C. Gable
地质记录表明，相对稳定的板块运动阶段被相对较短的板块速度重组时期所打断。地幔中浮力源的分布通常被认为演化太慢，无法解释板块速度的快速转变。我们用二维和三维地幔对流模型研究了地幔对流和板块速度之间的反馈关系。我们着重研究了地幔内部加热的影响，并考虑了地幔粘性层结和不同板块几何形状对板块速度时间依赖性的影响。当瑞利数或内部加热率增加到接近地幔数值的量级时，我们计算的板块运动记录变得具有方向的间歇性变化的特征。这种现象是由于板材对固有的不稳定内部加热对流系统的热损失产生影响的结果。在二维和三维计算中，板块运动在底层对流上灌输一种反映板块几何形状的组织模式，并导致在收敛的板块边界形成片状下流结构(相反，三维模型中的上升流不是片状的)。在观察到的板块速度的阶段性重组中，片状下沉的作用是至关重要的。板块下方的温暖物质被板块运动带入包围着下沉冰盖的区域。在板块运动相当稳定的时期，与下沉板块周围的热量积累相关的浮力导致了不稳定对流模式的产生。这种热量的积累在计算类似地幔的内部加热率时是戏剧性的，并抵制了板块继续向成熟的下沉运动的长期运动。当板块运动方向的自由度受到限制时，例如在2-D模型中，这些影响变得更加明显。因此，在二维计算中，板块对全球平均热量的影响比在三维计算中更为显著。然而，结合了不同大小的板块的3-D计算确实显示了它们的对流模式的快速重组，因为年轻的板状特征的拉力取代了成熟的下流板的拉力。我们在计算中将重组事件的时间和频率与板块重建研究确定的板块运动的一般特征进行了比较。
SUMMARY The geological record indicates that stages of relatively steady plate motion have been punctuated by comparatively brief periods in which plate velocities have reorganized. The distribution of buoyancy sources in the mantle has generally been regarded as evolving too slowly to explain these rapid transitions in plate velocity. We investigate the feedback between mantle convection and plate velocity using 2-D and 3-D mantle convection models that incorporate mobile dynamic plates. We focus on the influence of internal heating in the mantle and consider the effect of mantle viscosity stratification and different plate geometries on the plate velocity time dependence. As either the Rayleigh number or the internal heating rate is increased to magnitudes approaching mantle values, the record of the plate motion from our calculations becomes characterized by intermittent changes in direction. This behaviour is a result of the influence of plates on heat loss from the inherently unsteady, internally heated convecting system. Plate motion instills a pattern of organization on the underlying convection that reflects the plate geometry and results in the formation of sheet-like downwelling structures at convergent plate boundaries in both 2-D and 3-D calculations (in contrast, upwellings in 3-D models are not sheet-like). The role of the sheet-like downwellings is critical in the observed episodic reorganization of the plate velocities. Warm material below the plates is entrained by plate motion into regions enveloping the downwelling sheets. During periods of fairly steady plate motion, buoyancy associated with the build-up of heat around the downwelling sheets leads to the creation of an unstable convection pattern. This build-up of heat is dramatic in calculations with mantle-like internal heating rates and resists continued long-term plate motion towards mature downwellings. When there are limitations on the degree of freedom of the direction of plate movement, such as in 2-D models, these effects become even more pronounced. Accordingly, the effect of plates on mean global thermal quantities is more dramatic in 2-D calculations than it is in 3-D calculations. Nevertheless, 3-D calculations incorporating plates of different sizes do exhibit rapid reorganizations in their convection patterns as the pull of young slablike features supersedes the pull of mature downwelling sheets. We compare the timing and frequency of the reorganization events in our calculations with the general characteristics of plate motions determined from plate reconstruction studies.