Granitoides asociados a zonas de desgarre: modelos analógicos y aplicación al macizo de Lizio (zona de cizalla surarmoricana)

Abstract

Granite intrusion in the upper crust along a crustal-scale shear zone has been modeled by injecting a Newtonian fluid (low-viscosity silicone putty) into a sandpack containing a ductile layer of silicone putty which acted as a potential décollement level along which the injected material could spread. The strike-slip regime was obtained using a mobile rigid basal plate sliding horizontally. Boundary conditions were chosen in order to analyze the influence of different rheologic profiles of the upper crust on the pattern of the intrusion. Experiments showed that: 1) intrusions are elongate and their long axis tends to track the principal stretching direction associated with the strike-slip regime, 2) intrusions are asymmetric shaped in horizontal view, with the development of a sheared tail trailing behind the intrusion, 3) the strike slip environment allowed local rising of the injected fluid along faults formed in the overburden, 4) when the crustal rheologic profile is similar in both sides of the shear zone, intrusions are asymmetric drop shaped, 5) when the viscosity of materials above the feeding pipe is higher that viscosity of materials of the other side of the shear zone, the asymmetric drop shape not appears.
Lizio leucogranite 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.