SYNTECTONIC INTRUSION OF THE TORRES-DEL-P AINE GRANITE, PATAGONIAN ANDES OF CHILE

The Miocene Torres del Paine Granite is a sill-like subhorizontal lacc olithic body. It is elliptical in plan view and its major axis extends for more than 12 km. In profile the section is strongly asymmetric, w ith a maximum thickness decreasing from 2.5 km in the west towards zer o in the east. The laccolith is emplaced along a shallow to subhorizon tal thrust fault (the Rio Nutrias Fault), that is associated with inte rnal folding and faulting developed during an early, pre-intrusion, th in-skinned tectonic phase that affected the Cerro Toro Formation. Wher e the intrusive shows its thicker section it connects downwards with a feeder dyke that dips more than 70 degrees west. This feeder dyke is emplaced along and reutilizes an old high-angle reverse fault (probabl y an inherited normal fault) that juxtaposes the Cerro Toro (to the ea st) and the Punta Barrosa (to the west) formations. This fault, though t to be related with a period of thick skinned tectonic shortening, ca uses a several kilometer uplift of the western block and consequent er osion of the stratigraphic cover. The compressional reactivation of fa ults at an angle equal to or greater than the mechanical frictional lo ck-up angle (approximately 60 degrees) requires, at least locally, the development of prefailure supralithostatic fluid pressures (lambda gr eater than or equal to 1 or P-m greater than or equal to S-v = S-3) Th e principal stress orientation during high-angle reverse fault reactiv ation implies the synchronous generation of dilation-extension structu res. In this case, these would have been lenticular and horizontal, gi ving rise to several lenses that were connected with and filled by mag ma along the fault. In the case of Torres del Paine, faulting and synt ectonic intrusion were possible only in those faults that were large e nough to connect themselves with a reservoir or magma chamber that pro vided the required fluid or lubricant to enable displacement. The form ation of the laccolith corresponds to the contemporaneous dilation of the extensional structure associated with the reverse faulting, develo ped at a depth of about 5 km below the paleosurface, an amount that is is significantly larger than the undeformed stratigraphic thickness o f the overlying rocks. From these calculations, and from the geologica l evidence, it is concluded that the overburden consisted of preintrus ion structural repetitions of the stratigraphy.