How stylolite tips crack rocks

Abstract

Stylolites are surfaces of localized dissolution, ubiquitous in sedimentary rocks. They belong to a family of sub-planar defects caused by localized volume reduction (LVR), alongside compaction bands, localized negative-volume phase transitions, and dehydration structures. In the past such LVR structures have been labeled as “anti-cracks”, but recent works showed that their stress field is actually that of an Eshelby Inclusion. This work first reviews the stress field near the tips of LVR's, focusing on the fact that while they induce significant compression at certain regions around their tip, they also induce significant extension and shear in other regions around the tip. We thus show that the stress field induced by the LVR tip may give rise to what seem to be counter-intuitive structures - specifically tension cracks, pull-aparts, and shear fractures. We demonstrate this by comparing the stress field of an LVR to a complex network of fractures, veins and sedimentary stylolites in a rock slab from the Calcare Massiccio Formation of the Umbria–Marche, Central Italy. We show that fractures near stylolite tips can be fully rationalized by the stress field induced by the tips, even though in some cases the veins and the stylolites are nearly parallel. Our work suggests that 1) stylolite tips may induce vein formation, creating interconnected network, 2) strain is accommodated in the coupled network, similarly to a mid-ocean ridge and transform fault system, and 3) tension and so-called “pressure-solution” may operate in unison and in the same direction, implying that sedimentary stylolites may sometimes form through textural/chemical heterogeneity, rather than by stress.