Cockade structures as a paleo-earthquake proxy in upper crustal hydrothermal systems

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.