地幔剥露型被动陆缘在碰撞造山过程中对岩石圈变形影响的数值模拟

Previous studies mainly focus on discussing the collision of uniform extension rifted margin with a continent-ocean boundary (COB), and the margin is modeled as a thin lithosphere commonly. However, as the continent-ocean transition (COT) is found in the Iberia-Newfoundland margin, researchers begin to know that the rifted margin consists of a transition zone, which has a width of hundreds kilometers and component of exhumed mantle. Comparing with the thin lithosphere, the mantle-exhumation rifted margin shows characteristics of non-layered lithosphere, strong rheological strength, abnormal thermal structure and serpentinization, affecting the thermodynamic structures of the margin and thus causing the differential lithosphere deformation during continent-continent collision. This proposal will set up a few numerical models of mantle-exhumation rifted margin, and then analyses the deformation of lithosphere quantitatively by varying several factors, such as lithosphere layers, rheological strength, thermal structure and serpentinization degree. Finally, we will apply the modeling results to interpret the characteristics of Alpine collisional orogeny, e.g. uni-modal stress distribution, high heat flux and high topography et al. Combing the passive margin with passion collision process, our research would improve the deformation system associated with collision of various rifted margin and the affecting factors.

前人对碰撞过程中岩石圈变形的模拟研究主要基于对洋陆转换边界的认识，将参与碰撞的被动陆缘设置为减薄的正常大陆岩石圈。然而随着洋陆转换带在伊伯利亚-纽芬兰陆缘的发现，人们认识到被动陆缘存在一个宽约数百公里、由剥露的地幔橄榄岩组成的过渡带。相对于减薄设置的正常大陆岩石圈模型，存在地幔剥露的被动陆缘具有无岩石圈圈层分异、高流变强度、热结构变化及浅部蛇纹石化等特点，这些结构特点会影响陆缘的热-力学性质，进而影响受力过程中岩石圈的构造演化与变形特征。本申请拟针对典型的地幔剥露型被动陆缘建立数学模型，定量分析圈层分异、流变强度、热结构、蛇纹石化等因素对陆陆碰撞过程中岩石圈变形的影响。最后，将模拟结果用于分析探讨阿尔卑斯造山带出现的单一应力环境、异常热流值、高地表抬升及其他变形特征。本申请将被动陆缘与主动碰撞过程结合起来研究，所得结果有助于完善不同类型被动陆缘类型在碰撞过程中岩石圈变形的研究。

被动陆缘的俯冲碰撞过程是研究地壳形成、演化、及其动力学机制的关键。本课题以动力学模拟为主要手段，将被动陆缘与主动碰撞过程结合起来研究，先认识碰撞前被动陆缘的形成过程，随后探讨被动陆缘参与碰撞时岩石圈出现的变形、岩相及热等特征，最终将结果用于解释典型被动陆缘及造山带出现的构造现象。研究发现中地壳流变结构、反向拖曳、Moho面温度等一级因素控制着超伸展陆缘的发育，岩石圈地幔先于地壳破裂同样可以导致地幔剥露型大陆边缘产生；通过与三维Parker‐Oldenburg反演得到的南海陆缘超减薄地壳分布范围、地温、典型剖面解释及穿时空破裂对比，得到东西陆缘差异性发育的成因机制。在被动陆缘认识基础之上展开的碰撞研究发现：地幔剥露及水化所致的低流变强度被动陆缘在碰撞过程中会发生明显的地壳增生现象，造成地表的抬升和挤压的应力环境，这种现象会随着强度变弱或圈层厚度增加而表现的更为明显；因大部分陆壳被增生在地表，故而很少的陆壳进入深部俯冲通道，这就造成后期折返过程中较少的出现超高压岩石，主要为高压岩石的出露。反之，流变强度强的被动陆缘在碰撞过程中会较为顺利的进入深部俯冲，甚至进入100km的超高压变质环境；后期折返过程中会有大量超高压岩石的出露。除此之外，碰撞过程的研究定量的给定了碰撞过程中增生及俯冲出现的地壳强度、厚度范围、热结构，并指出超高压岩石及异常热流值产生的必备条件。实验结果很好的吻合了阿尔卑斯、喜马拉雅等造山带的实际应力状态、异常热流、岩相分布及地表抬升等特征。将被动陆缘与主动碰撞过程结合起来研究，所得的研究结果有助于完善陆陆碰撞类型，补充碰撞变形过程的影响因素及岩石圈变形特征，为其他被动大陆边缘的俯冲碰撞研究提供典型案例。