Analogue models of laccolith formation

Abstract

This paper describes dynamically scaled analogue models of laccolithic intrusions. Experiments consisted of the injection of a Newtonian fluid (low-viscosity silicone putty) into a sandpack, with or without an interbedded ductile layer of silicone putty acting as a potential décollement level. Boundary conditions were chosen to analyze the influence of the thickness of the brittle cover and of the décollement layer on the pattern of intrusion. Further experiments were made to examine the effects of an extensional regime during intrusion. Experiments showed that: (1) laccolith formation requires the occurrence of a décollement layer between two competent units, (2) the critical thickness of the décollement layer necessary for laccolith formation decreases with increasing depth, (3) laccoliths change from lenses to bell-shaped with decreasing overburden, (4) for a constant thickness of the décollement layer, the largest diameter of laccoliths is proportional to the thickness of the overburden, and (5) a syn-injection gravitational sliding regime results in an asymmetric laccolith, with amplification of the bell shape.