流变边界层及其对华北克拉通热/地震岩石圈底界差异的意义

The Rheological boundary layer and its implications for the difference between the thermal and seismic lithospheric bases of the North China Craton

二维热传导/对流数值模型显示,纯传导的固体岩石圈与纯对流的流体软流圈之间存在一过渡层,即流变边界层,其间传导与对流共同作用来传递热量.流变边界层厚度主要由软流圈黏性系数(η)控制,而受固体岩石圈厚度及热状态影响很小.随着η从1×1021Pa·s降低至1×1019Pa·s,流变边界层也随之减薄,流变边界层的厚度与lg(η)成正比.流变边界层的存在是造成热岩石圈与地震岩石圈厚度差异的重要因素.全球典型克拉通岩石圈的对比结果表明,地震岩石圈厚度普遍大于热岩石圈厚度,二者的差异多数在70~90km,很好地验证了流变边界层的存在.研究发现二者的差异在华北克拉通自西向东逐渐减小:由西部鄂尔多斯的约80km减少至渤海湾盆地的约20km.反映出华北克拉通岩石圈下部流变边界层厚度自西向东减薄,意味着软流圈黏性系数自西向东逐渐降低.这可能与中生代太平洋俯冲脱水形成的低黏大地幔楔有关,从一侧面印证了太平洋俯冲对华北克拉通破坏的影响.

The 2D thermal conductive/convective model indicates that a transition layer exists between the conductive solid lithosphere and the convective fluid asthenosphere, which is named as the rheological boundary layer （RBL）. Conduction and convection both transfer heat within this region. The thickness of the RBL is mainly controlled by the asthenospheric viscosity （r/）, but not or slightly influenced by the thickness and thermal regime of the solid lithosphere. RBL thins in direct proportion to lowering of lg（η） when η decreases from 1 X 1021 Pa · s to 1 × 1019 Pa · s. The existence of RBL results in the difference between the thermal and seismic lithospheric bases. A difference of 70 - 90 km is indicated by comparison between the thermal and seismic lithospheric thicknesses of global typical cratons, which verifies the existence of RBL. Such difference also occurs in the North China Craton, but decreases from ~80 km in the Ordos to^20 km in the Bohai Bay basin. This phenomenon implies an eas thickness, suggesting the asthenospheric viscosity decreases from related to the low-viscosity big mantle wedge formed by the stagnan zone, and confirms from one side the influence of Pacific subduction on Craton. tward thinning trend of RBI. west to east. It is probably t Pacific slab at the transition destruction of the North China