上部地壳岩石流动与显微构造演化--天然与实验岩石变形证据

岩石流动性和变形显微构造的发育直接受温度、压力、应变速率和流体相等制约 ,致使在不同地壳层次岩石的流动性表现出很大的差异。对上部地壳环境条件下天然和实验变形岩石的显微构造分析揭示出一系列具有不同特点以及由不同的成核、扩展和联合方式形成的破裂与微破裂型式的存在。讨论了在上部地壳环境中 ,温度与围压的变化对岩石破裂的影响 ,并阐述了高压破裂与低压破裂及其力学、流变学和显微构造特点 ,提出高压破裂对应于天然变形环境下出现的剪切 (挤压 )破裂 ,而碎裂岩带是典型的天然低压破裂 ,其低压环境的出现可以是浅部低围压或深部高流体压力所致。流体相的存在不仅可以引起石英 ,也可以引起方解石类碳酸盐岩矿物的水解弱化 ,并进而导致岩石流动机制的转变。岩石变形及流体等因素所致的岩石粒度变化 ,则从另一个方面影响着上部地壳岩石流动性的变化。从变形环境考虑 ,随着深度的加大 ,温度和压力升高 ,导致岩石由脆性向韧性转变 ;转变域内岩石的变形是一个复杂过程 ,是多种不同脆性和晶质塑性机制的综合。

Flow and microstructural evolution of rocks in the upper crust:Evidence from naturally and experimentally deformed rocks

The flow and occurrence of deformation microstructures in rocks are strongly dependent on conditions of deformation, e.g. temperature, confining pressure, strain rate and fluid involvement. Hence, there is a great variation in flow behavior of rocks at different crustal levels. A sequence of fractures and microfractures with distinct characteristics and mechanisms of nucleation, propagation and coalescence are present in the deformed rocks at upper crustal level. This paper discusses the effects of temperature, confining pressure and fluid phases on fracture micromechanics. Further discussion is concentrated on the mechanical features, flow properties of rocks and microstructural characteristics during high-pressure and low-pressure fracturing. It is suggested that the equivalents of high-pressure type fractures are straight shear fracture and that of low-pressure type fractures are cataclastic fault zones. Occurrence of low-pressure conditions is attributed either to the low confining pressures at shallow crustal level or to the effect of high fluid pressures at depths. The presence of minor amount fluid phases may result in hydrolytic weakening of quartz as well as carbonate minerals, further leading to a transition of deformation mechanisms. Moreover, grain size variation due to deformation and involvement of fluid phases may also affect the variation of deformation mechanisms and flow properties of rocks at high crustal levels. The transition from brittle to ductile of rocks involves a complicated sequence of deformation processes or a combination of deformation mechanisms of brittle and ductile regimes.