Abstract
Cockades are clasts completely surrounded by spheroidal hydrothermal overgrowth rims. They are observed inside hydrothermal fault breccias and can provide insights into fault dynamics. The formation of cockades with spheroidal hydrothermal overgrowth rims is related to fast fracturing and dilation, and requires primary clasts to be suspended in a fluid. The rim growth is driven by drops in fluid pressure and related oversaturation. We use descriptions of cockades, their rims and cements in a fault breccia. Geometrical data are combined with mechano-chemical calculations to gain insights into seismic processes and estimate seismic magnitudes. Fast rates for formation of cockade cores and first rim growth are interpreted to be the result of an earthquake’s main shock. Younger growth rims represent subsequent aftershocks, while cemented cockades record interseismic periods. We propose that by considering growth rates of hydrothermal precipitates and cements, paleo-earthquake cycles can be unraveled and a link between geophysics and fault structures can be established.
Highlights
Cockades are clasts completely surrounded by spheroidal hydrothermal overgrowth rims
To gain information about these processes active during the seismic cycle, both rock deformation experiments and the study of ancient seismic activity recorded in exhumed natural rocks are needed[2,3]
This contribution focuses on the rock record of seismicity in exhumed natural fault rocks of the Grimsel Breccia Fault (Central Switzerland)
Summary
Cockade textures with their cores and different cementation stages provide a proxy to unravel paleo-seismic activity along large-scale faults[7]. The seismic formation of cockade breccias and their different cementation stages within fault zones require the creation of substantial pore space and the presence of considerable amounts of fluid that is injected instantaneously. Linking quantitative studies on cockades/cataclasite systems with rock deformation experiments can provide important information on (i) the effective chemo-mechanical feedback processes, (ii) evolution of pore fluid pressures and (iii) changes in fault rock strengths. It is these three fundamental parameters that mainly affect the loading of the fault rocks and define whether aseismic creep can persist or tectonic stresses will build up preparing the fault for the rupture. Understanding the dynamics of cockade breccias might be of great interest for the exploration of geothermal energy in active hydrothermal systems
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