Reaction-assisted hierarchical fracturing during serpentinization

Abstract

Hierarchical fracture patterns are the result of a fracturing process that successively divides rocks into smaller domains. In a 2D (outcrop) section such fracture patterns are characterized by four sided domains, and T-junctions where new fractures stop growing at right angles to pre-existing fractures. Here, we describe fracturing of mm to dm thick serpentinized orthopyroxenite dykes in a dunite matrix from the Leka Ophiolite Complex in Nord-Trøndelag, Norway. The fracturing process is driven by volume changes associated with the serpentinization of the dunites. During this process the orthopyroxenite dykes are extended by the expanding dunite and undergo considerable fracturing. The resulting fracture patterns are hierarchical. The cumulative distributions of fracture lengths show a scaling behavior that lies between a log–normal and power law (fractal) distribution. This is consistent with a simple fragmentation model in which domains are divided according to a ‘top hat’ distribution of new fracture positions within unfractured domains. Reaction-assisted hierarchical fracturing is also likely to be responsible for other structures commonly observed in serpentinized ultramafic rocks, including the mesh-textures observed in individual olivine grains, and the high abundance of rectangular domains at a wide range of scales. This mechanism may provide first-order controls on fluid migration rates in a wide variety of geological settings.