华北克拉通晚中生代壳-幔拆离作用: 岩石流变学约束

大陆岩石圈的流变学结构对于岩石圈深部过程(壳/幔过程)有着深刻的影响,直接表现在岩石圈壳-幔结构与浅部构造上.本文注意到华北克拉通晚中生代岩石圈减薄期间地壳的伸展、拆离与减薄在不同地区的宏观、微观构造及地壳岩石流变学等方面的差异表现与区域变化,以及现今和晚中生代时期岩石圈厚度的不均匀性.讨论了以水为主体的地质流体的存在对于岩石圈流变性的影响.综合克拉通东部与西部地壳/地幔厚度变化特点以及下地壳和上地幔含水性特点,阐述了晚中生代时期华北克拉通岩石圈内部壳幔耦合与解耦的规律,提出了华北岩石圈壳-幔拆离作用模型以解释华北克拉通晚中生代岩石圈减薄的基本现象与深部过程.提出区域性伸展作用是岩石圈减薄的主要动力学因素,东部地区在晚中生代伸展作用过程中壳-幔具有典型的解耦性,上部地壳、下部地壳和岩石圈地幔的变形具有显著差异性.而西部区壳幔总体具有耦合性,下地壳与岩石圈地幔共同构成流变学强度很高且难以变形的岩石圈根.

Crustal-mantle detachment of the North China craton in Late Mesozoic: Rheological constraints

Rheological structure of the continental lithosphere fundamentally influences on the deep processes of the lithosphere (crust/mantle process), which is directly shown by the crust-mantle structure of the lithosphere and also by shallow structures. Crustal extension, detachment faulting and thinning of the crust during thinning of the lithosphere of North China craton in Late Mesozoic are exactly shown by occurrence and variation of macro- and micro-structures, and due to changing rheology of the crust. The inhomogeneity of Late Mesozoic lithosphere is deduced from the present geophysical lithosphere structure and regional structural analysis. Lithosphere rheology is influenced by several factors including the absence and presence of H₂O dominated geological fluids. Coupling and decoupling relationships of the crust and mantle during regional extension of the lithosphere of North China craton are discussed from the variation of thicknesses of the crust, the lithosphere mantle and the lithosphere, and the varying contents of water in rocks from the lower crust and upper mantle. We present the crust-mantle detachment model to interpret the basic phenomena and deep processes for lithosphere thinning of North China craton during Late Mesozoic. We would argue that regional extension is the dominant dynamic factor of lithosphere thinning during Late Mesozoic. During this event, the crust and mantle are decoupled in the eastern part of the craton, and therefore, the upper crust, the lower crust and the upper mantle are detached at the same time, but in different ways. There is, however, a coupling relationship in the western part of the craton. Thus the lower crust and the upper mantle constitute a layer of high rheological strength and they are not strongly detached during extension of the lithosphere in Late Mesozoic.