Analogue modelling of fracturing in cooling plutonic bodies

Abstract

Fractures formed when igneous rocks cool below the surface of the Earth are of considerable current interest in studies of hydrology, mineral systems and radioactive waste disposal. However, little is known about the geometry and kinematics of such fractures. Conceptual models suggest that early fractures result from emplacement forces (the Cloos model) while two-dimensional numerical modelling suggests that fractures may result from the contraction of cooling bodies. We use analogue modelling to investigate the geometry of fractures formed in a cylindrical static contracting body. Our experiments show that dehydration of buried starch flour is a workable analogue. Fractures in the analogue material result from drying and contraction of a cylindrical volume which differs from previous experiments in which drying was from a single planar surface and resulted in the formation of columnar joints. In buried analogue models two key fracture sets form internally, producing sub-vertical radial fractures, and concentric fractures that curve downwards with depth. Fracture density decreases towards the centre and bottom of the samples. Samples that were buried more deeply have fewer and less curved concentric fractures than those with shallow burials. Radial fractures have similar orientations to those predicted from numerical models, and concentric fractures are comparable to marginal fissure orientations of the Cloos model. The analogue models suggest that both radial and concentric fractures may result from the contraction of plutonic bodies.