Synkinematic evolution of stylolite porosity

Abstract

Stylolites form by the transportation and precipitation of dissolved materials in pore fluids due to concentration gradients that develop in rocks under stress. This is a coupled chemical-hydraulic-mechanical (CHM) process, resulting from the interplay between chemical dissolution, pore fluid flow and stress during stylolite growth. Although the CHM feedbacks that develop around stylolite seams lead to porosity compartmentalization, the dynamic processes controlling porosity evolution during stylolite growth remain unknown. Here, thrust-induced stylolites in micritic limestones of the Eilean Dubh Formation that crop out in the footwall of the Caledonian Ullapool/Moine Thrust in NW Scotland are used to investigate the dynamic processes controlling stylolite growth. Stylolite microstructures observed in detail, using a backscatter electron image mosaic (102,600 × 18,239 pixels; 0.17μm/pixels), both reveal stylolite seams (core zones) surrounded by zones of increased-porosity (damage zones) and suggest a four stage self-organization process for stylolite growth and evolution. In the first stage, stress concentration in the micritic matrix led to the partial dissolution of the matrix, forming new pores adjacent to pyrites grains. In the second stage, the new pores facilitated local dissolution, which in turn promoted their growth, establishing a positive feedback loop between texture and solubility that enlarged the local porous zone by self-organization. In the third stage, the porous zones facilitated the influx of externally derived fluids and differentiated into core zones, with syn-kinematic precipitation of secondary pyrites and clay minerals, and damage zones, with ongoing dissolution. In the fourth stage, the porous zones propagated perpendicular to the shortening direction and became interlinked, forming mature stylolites with a core zone in the centre surrounded on both sides by damage zones with high porosities. The fluid flux along the stylolites during their formation was likely controlled by episodic movements along the Ullapool Thrust.