Impact of erosion and décollements on large-scale faulting and folding in orogenic wedges: analogue models and case studies

Deformation mechanisms, long-term kinematics and evolution of fold and thrust belts subjected to erosion are studied through 2D analogue experiments involving large convergence. First-order parameters tested include (1) décollements and/or plastic layers interbedded at different locations within analogue materials and (2) synconvergence surface erosion. Weak layers, depending on their location in the model, favour deformation partitioning characterized by the simultaneous development of underplating domains in the inner part of the wedge (basal accretion) and frontal accretion where the wedge grows forward. Interaction between tectonics and surface processes influences this behaviour. Development of antiformal thrust stacks controlled by underplating shows small- and large-scale cyclicity. Thin plastic layers induce folding processes, which are studied at wedge scale. Recumbent and overturned folds, with large inverted limbs, develop in a shear-induced asymmetric deformation regime via progressive unrolling of synclinal hinges. Surface erosion and underplating at depth induce further rotation (passive tilting) and horizontalization of fold limbs. Model results give insights to discuss the mechanisms responsible for the large-scale structures (i.e. antiformal nappe stacks, klippen and kilometre-scale recumbent fold–nappes) encountered in several mountain belts such as the Montagne Noire (French Massif Central), the Galicia Variscan belt (Spain) and the northern Apennines (Italy).

本研究通过开展大收敛量二维相似模拟实验，探究受侵蚀作用影响的褶皱冲断带（fold and thrust belts）的变形机制、长期运动学特征与演化过程。本次实验测试的一级控制参数包括：（1）在相似材料不同层位互层的滑脱层（décollement）与/或塑性层；（2）同汇聚阶段地表侵蚀作用。弱层依据其在模型中的赋存位置，会促进变形分区作用的发育，其典型特征为楔体内部区域同步形成底垫（underplating）域（即基底增生（basal accretion）），以及楔体向前生长的前缘增生（frontal accretion）域。构造作用与地表过程之间的相互作用会对该变形行为产生调控影响。受底垫作用控制的背形冲断堆叠体（antiformal thrust stacks）的发育过程，呈现出小尺度与大尺度的旋回性特征。薄塑性层会引发褶皱作用，相关过程可在楔体尺度下开展研究。具大型倒转翼的平卧褶皱（recumbent fold）与倒转褶皱（overturned fold），会在剪切诱导的非对称变形体制中，通过向斜枢纽（synclinal hinge）的逐步展开而形成。地表侵蚀与深部底垫作用会进一步引发褶皱翼部的旋转（即被动倾斜）与水平化作用。本模型实验结果可为探讨多个造山带中发育的大型构造（即背形推覆堆叠体、飞来峰（klippen）以及千米级平卧褶皱-推覆体）的形成机制提供启示，这些造山带包括法国中央地块（Massif Central）的黑山（Montagne Noire）、西班牙的加利西亚华力西造山带（Galicia Variscan belt）以及意大利的北亚平宁造山带（northern Apennines）。