Granite intrusion along strike-slip zones in experiment and nature

Abstract

Granite intrusion in the upper crust along a crustal-scale shear zone has been modelled by injecting a Newtonian fluid (low-viscosity silicone putty) into a sandpack containing a ductile layer of silicone putty which acted as a weak level along which the injected material could spread. The strike-slip regime was obtained using two mobile rigid basal plates sliding horizontally. Boundary conditions were chosen in order to analyse the influence of thicknesses of the brittle cover and of the volume of injected material on the pattern of the intrusion. Experiments showed that (1) the presence of a soft layer between two competent units allowed the formation of laccolithic intrusions by lateral expansion in the emplacement site, (2) intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike-slip regime, (3) this direction controls the emplacement beginning with the first stages of injection, (4) the lateral expansion of the intrusion is locally controlled by arrays of Riedel faults formed in the overburden, (5) the control of faults on the intrusion pattern increases with increasing brittle/ductile ratio, i.e. with increasing depth and (or) decreasing thickness of the weak layer, (6) intrusions are sigmoidal or lozenge shaped in horizontal view, (7) the greater the brittle/ductile ratio, the less sigmoidal are the intrusions, (8) feeding pipes shifted with respect to the strike-slip zone result in asymmetric intrusions, with the development of a sheared tail trailing behind the intrusion. Examples of leucogranites of the South Armorican Shear Zone (South Brittany, France) emphasize that our experiments can explain the geometry of many syntectonic granites emplaced along strike-slip zones. They further shed some light on mechanisms of pluton intrusion in the upper crust.