Investigating mechanisms of stylolite formation

The Israel Science Foundation Individual Research Grant Research Grant Application no. 571/08 Pressure solution is considered the most important ductile deformation mechanism operating in the Earth’s upper crust. Although operating at the grain scale, pressure solution controls basic large-scale processes such as strength and healing of faults, compaction of sedimentary basins, and storage and flow of oil. Pressure solution is a process by which rock mass is dissolved at highly stressed regions, transported through the fluid phase, and re-precipitated at lower-stress regions. Pressure solution is macroscopically manifested either as pervasive dissolution in the rock, or as localized solution seams and stylolites. Stylolites play a crucial role in determining both the deformation and permeability of rocks, yet their evolution remains enigmatic. This is probably due to the fact that macroscopic localized dissolution was not yet reproduced in the lab nor fully modeled. Existing numerical and conceptual models of stylolites focus on different parts of the physical picture of stylolite evolution (e.g. porosity evolution feedbacks, stress or strain controlled dissolution), and a coherent understanding is still much needed. We propose a new visco- elasto model of pressure solution that attempts to consider a more complete picture of stylolite formation. In our model pressure solution is driven by both stress and strain-energy, and is enhanced by the presence of clays. The model is capable of studying growth of a single stylolite, as well as stylolite interactions. Recently we used this model, accompanied by analytical calculations, to show that the stress distribution around an initial dissolution defect, an ‘embryo stylolite’, does not promote a dissolution-stress feedback that spontaneously localizes pressure solution. Following many observations that clays enhance pressure solution, we thus propose to test localization by a new feedback – a feedback between clay content and pressure solution. In this feedback regions with high clay content undergo enhanced pressure solution, and thus accumulate even more insoluble clay residue and enhance their dissolution even further. Initial modeling results suggest that this feedback indeed produces stylolite morphology similar to that found in the field. Further analysis of this feedback is proposed. In addition to studying single stylolite growth, we propose to study stylolite - stylolite interactions, and stylolite-fracture interactions, which will require adding to our model the ability to model shear fracturing. Modeling results will be continuously compared with field and experimental observations.