ANALOG MODELS OF LACCOLITH FORMATION

This paper describes dynamically scaled analogue models of laccolithic intrusions. Experiments consisted of the injection of a Newtonian flu id (low-viscosity silicone putty) into a sandpack, with or without an interbedded ductile layer of silicone putty acting as a potential deco llement level. Boundary conditions were chosen to analyze the influenc e of the thickness of the brittle cover and of the decollement layer o n the pattern of intrusion. Further experiments were made to examine t he effects of an extensional regime during intrusion. Experiments show ed that: (1) laccolith formation requires the occurrence of a decollem ent layer between two competent units, (2) the critical thickness of t he decollement layer necessary for laccolith formation decreases with increasing depth, (3) laccoliths change from lenses to bell-shaped wit h decreasing overburden, (4) for a constant thickness of the decolleme nt layer, the largest diameter of laccoliths is proportional to the th ickness of the overburden, and (5) a syn-injection gravitational slidi ng regime results in an asymmetric laccolith, with amplification of th e bell shape.