Abstract:

Field studies and seismic data show that semi-brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so-called frictional-plastic transition. As the bottom of seismogenic fault, the dynamic characteristics of the frictional-plastic transition zone and plastic zone are very important for the seismogenic fault during seismic cycles. Granite is the major composition of the crust in the brittle-plastic transition zone. Compared to calcite, quartz, plagioclase, pyroxene and olivine, the rheologic data of K-feldspar is scarce. Previous deformation studies of granite performed on a quartz-plagioclase aggregate revealed that the deformation strength of granite was similar with quartz. In the brittle-plastic transition zone, the deformation characteristics of granite are very complex, temperature of brittle-plastic transition of quartz is much lower than that of feldspar under both natural deformation condition and lab deformation condition. In the mylonite deformed under the middle crust deformation condition, quartz grains are elongated or fine-grained via dislocation creep, dynamic recrystallization and superplastic flow, plagioclase grains are fine-grained by bugling recrystallization, K-feldspar are fine-grained by micro-fractures. Recently, both field and experimental studies presented that the strength of K-feldspar is much higher than that of quartz and plagioclase. The same deformation mechanism of K-feldspar and plagioclase occurred under different temperature and pressure conditions, these conditions of K-feldspar are higher than plagioclase. The strength of granite is similar to feldspar while it contains a high content of K-feldspar. High shear strain experiment studies reveal that granite is deformed by local ductile shear zones in the brittle-plastic transition zone. In the ductile shear zone, K-feldspar is brittle fractured, plagioclase are bugling and sub-grain rotation re-crystallized, and quartz grains are plastic elongated. These local shear zones are altered to local slip-zones with strain increasing. Abundances of K-feldspar, plagioclase and mica are higher in the slip-zones than that in other portions of the samples (K-feldspar is the highest), and abundance of quartz is decreased. Amorphous material is easily formed by shear strain acting on brittle fine-grained K-feldspar and re-crystallized mica and plagioclase. Ductile shear zone is the major deformation mechanism of fault zones in the brittle-plastic transition zone. There is a model of a fault failed by bearing constant shear strain in the transition zone: local shear zones are formed along the fractured K-feldspar grains; plagioclase and quartz are fine-grained by recrystallization, K-feldspar is crushed into fine grains, these small grains and mica grains partially change to amorphous material, local slip-zones are generated by these small grains and the amorphous materials; then, the fault should be failed via two ways, 1)the local slip-zones contact to a throughout slip-zone in the center of the fault zone, the fault is failed along this slip-zone, and 2)the local slip-zones lead to bigger mineral grains that are in contact with each other, stress is concentrated between these big grains, the fault is failed by these big grains that are fractured. Thus, the real deformation character of the granite can’t be revealed by studies performing on a quartz-plagioclase aggregate. This paper reports the different deformation characters between K-feldspar, plagioclase and quartz under the same pressure and temperature condition based on previous studies. Then, we discuss a mode of instability of a fault zone in the brittle-plastic transition zone. It is still unclear that how many contents of weak mineral phase(or strong mineral phase)will control the strength of a three-mineral-phase granite. Rheological character of K-feldspar is very important for study of the deformation characteristic of the granitic rocks.

Key words: granite, K-feldspar, brittle-plastic transition, deformation mechanism, instability of fault

摘要：

地震精定位结果显示, 大陆地震多数集中于大陆地壳的多震层内, 该多震层向下收敛于中部地壳的脆塑性转化带。 地壳脆塑性转化带的主要成分为花岗质岩石, 前人通常用石英-斜长石的组合代替花岗岩进行变形研究, 反演转化带的深度和变形特征, 并且认为花岗岩的变形强度由弱项矿物——石英的塑性变形控制。 近年来, 实验和野外研究均表明钾长石的变形强度高于石英和斜长石。 大应变量变形实验和野外韧性剪切带的研究结果显示, 在中地壳脆塑性转化带内, 钾长石变形以脆性破裂为主, 斜长石和石英通常表现为动态重结晶。 因此, 用石英和斜长石的组合体代替花岗岩来反演断层的变形特征, 无法全面、 真实地解释断层深部脆塑性转化带的变形特征。 文中总结了花岗岩在野外和实验变形条件下的研究结果, 并分析了花岗岩的主要组成矿物——石英、 斜长石和钾长石的变形特征以及其温压条件的不同步性, 讨论了断层深部脆塑性转化带的失稳条件。

关键词: 花岗岩, 钾长石, 脆塑性转化, 变形机制, 断层失稳