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Deformation at low and high stress-loading rates

Geoscience Frontiers 2019年第1期10卷 43-54页

作者： Claudia A.Trepmann Lina Seybold Department of Earth and Environmental Sciences Ludwig-Maximilians University

In an extensional shear zone in the Talea Ori, Crete, quartz veins occur in high-pressure low-temperature metamorphic sediments at sites of dilation along shear band boundaries, kink band boundaries and boudin necks. Bent elongate grains grown epitactically from the host rock with abundant fluid inclusion trails parallel to the vein wall indicate vein formation by crack-seal increments during dissolutionprecipitation creep of the host rock. The presence of sutured high-angle grain boundaries and subgrains shows that temperatures were sufficiently high for recovery and strain-induced grain boundary migration, i.e. higher than 300 -350℃, close to peak metamorphic conditions. The generally low amount of strain accumulated by dislocation creep in quartz of the host rock and most veins indicates low bulk stress conditions of a few tens of MPa on a long term. The time scale of stress-loading to cause cyclic cracking and sealing is assumed to be lower than the Maxwell relaxation time of the metasediments undergoing dissolution-precipitation creep at high strain rates(10^(-10) s^(-1) to 10^(-9) s^(-1)), which is on the order of hundred years. In contrast, some veins discordant or concordant to the foliation show heterogeneous quartz microstructures with micro-shear zones, sub-basal deformation lamellae, shortwavelength undulatory extinction and recrystallized grains restricted to high strain zones. These microstructures indicate dislocation glide-controlled crystal-plastic deformation(low-temperature plasticity) at transient high stresses of a few hundred MPa with subsequent recovery and strain-induced grain boundary migration at relaxing stresses and temperatures of at least 300 -350℃. High differential stresses in rocks at greenschist-facies conditions that relieve stress by creep on the long term, requires fast stress-loading rates, presumably by seismic activity in the overlying upper crust. The time scale for stress loading is controlled by the duration of the slip event along a fault, i.e. a few seconds to minutes.

关键词： High-stress crystal plasticity crack-seal Seismic cycle Stress-loading rates Talea Ori Crete

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EVOLUTION OF FLUID PRESSURE AND HYDROFRACTURING IN MESOTHERMAL GOLD-QUARTZ DEPOSITS

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Geotectonica et Metallogenia 1999年第1期23卷 51-57页

作者： Tang Shijia Peng Ensheng Sun Zhenjia and Gao Guangming(Dept. of Geology Central South University of Technology, Changsha 410083, China)

The formations of many mesothermal gold-quartz deposits are closely re1ated with fluid pressure. In the course of ore-forming of gold deposits, fractures act as values, promoting cyclic fluctuations in fIuid pressure from lithostatic to hydrostatic values. Once the fluid pressure satisfies the condition of hydrofracture: P- q>T, the cracks undergo fracturing and extension. By hydrofracturing, the pre-existing fau1ts reactivate, forming steep or flat dipping shear zones. At the same time, deposition within fau1t veins is attributed to the immediate postfailure discharge phase.Fault self-sealing leads to reaccumulation of fluid pressure and a repetition of the cycle, During fracturing, many structures are formed, such as, banded compound veins, breccia that can be pieced together, and massive quartz veins in the Haopinggou and Woxi gold-quartz deposits.

关键词： Hydrofracture high-pressure fluid crack-seal gold-quartz deposit

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