造山带下岩石圈地幔小尺度减薄的动力学成因

Dynamics of small-scale removing of mantle lithosphere under orogens

地质与地球物理观测数据表明青藏高原、安第斯山、以及帕米尔等典型造山高原之下均有明显的岩石圈地幔小尺度/分段式减薄现象.这些小尺度岩石圈减薄难以用经典的拆沉或对流减薄理论来解释,一方面,拆沉预示大尺度岩石圈地幔的剥离过程,而对流减薄则在黏度相对低的地幔岩石圈中发生,其主要以小尺度的局部增厚触发并仅减薄地幔岩石圈的底部区域.另一方面,拆沉或对流减薄模型都预测造山带尺度的地幔岩石圈拆离,都假设造山带岩石圈横向均一,然而实际的造山带岩石圈往往由多个不同的地块构成,块体之间岩性、物性、流变结构可能大有差别,即横向不均一性.这些造山带岩石圈地幔的横向不均一性,能否有效解释观测到的局部小尺度减薄现象?为此,我们构建了一系列高精度动力学数值模型,系统模拟了碰撞造山过程中岩石圈地幔的形变和不稳定性.结果表明,在塑性屈服强度很低的情况下,横向不均一的造山带岩石圈有发生分段式/小尺度减薄的可能性;其主要机理是由位错蠕变与强塑性作用所导致的应变集中使得地块间及壳幔间耦合弱化,从而使得较弱地块的岩石圈地幔在增厚时由于重力不稳定性而产生局部剥离,进而诱发小尺度软流圈上涌.模拟结果可以良好地解释发生在青藏高原东北缘、安第斯中部高原、以及帕米尔高原之下岩石圈的局部小尺度/分段式减薄现象.

Geological and geophysical data suggests that small-scale fragmental lithospheric thinning,some all the way to the base of the crust,has occurred under some large orogens,including Tibetan Plateau,central Andes,and Pamir.Such localized small-scale removal of mantle lithosphere under orogens cannot be readily explained by the existing models of delamination and convective thinning(dripping)—the formal would predict large-scale peeling of the mantle lithosphere,and latter could occur in small scale(drips)when the viscosity is low,but in that case it tends to occur near the base of the mantle lithosphere rather than thinning the lithosphere all the way to the Moho.On the other hand,the previous models usually predict orogenic scale removal of mantle lithosphere,and it assume laterally homogeneous rheological structure of the lithosphere,but natural orogens often consists of numerous terranes with different lithology,history,and thermal structure,and hence laterally heterogenous rheological structure.Could such rheological heterogeneity explain the observed small-scale lithosphere thinning?To answer this question,we used a high-resolution thermomechanical model to simulate strain evolution and gravitational instability of collisional orogenic lithosphere.Our results indicate that lateral rheological heterogeneity could cause small-scale/fragmental lithospheric thinning under orogens when plastic yielding strength is rather low.The key factors are the combined weakening effects,by power-law viscous rheology and strong plastic yielding,which promote strain localization and lead to small-scale removal of mantle lithosphere.The numerical results provide useful insight into the cause of the observed small-scale lithosphere thinning under northeastern Tibetan Plateau,central Andes,and the Pamir.