Intrusion sintectonica del Granito de las Torres del Paine, Andes patagonicos de Chile

Abstract

RESUMEN. El Granito de las Torres del Paine es un lacolito subhorizontal de edad miocena, que intruye a rocas sedimentarias marinas del Cretacico Superior de la Formacion Cerro Toro. Es un cuerpo asimetrico, con una planta eliptica de eje mayor cercano a los 12 km y una potencia que decrece desde un maximo de 2.5 km al oeste, hasta desaparecer al este. Esta emplazado en el plano de una falla subhorizontal preexistente (la Falla del Rio Nutrias) desarrollado durante una tectonica del tipo escamas delgadas, anterior a la intrusion. En el sector donde el intrusivo presenta la mayor potencia, este se conecta hacia abajo con un dique o conducto alimentador de mas de 70° de inclinacion al oeste, que ocupa una falla inversa de alto angulo (posiblemente una antigua falla normal heredada) que alza y yuxtapone a la Formacion Punta Barrosa (al oeste) con la Formacion Cerro Toro (al este). Este fallamiento esta relacionado con una tectonica del tipo escamas gruesas que produjo un alzamiento de varios kilometros y la consecuente erosion del bloque occidental. La reactivacion compresional de fallas de inclinacion cercana o superior a su angulo de bloqueo friccional (aproximadamente 60°) requiere que, previo a su movimiento, se acumulen presiones de fluidos o magmaticas supralitostaticas, al menos localizadamente (lamda mayor o igual que 1 o Pm mayor o igual que Sv=S3). La orientacion de los esfuerzos principales durante el fallamiento inverso de alto angulo lleva implicita la generacion simultanea de estructuras de dilatacion y extension, lenticulares y horizontales, conectadas con las fallas, las que son rellenadas durante el movimiento de ella. En el Paine, el fallamiento y la intrusion sintectonica implican la actividad selectiva solo de aquellas fallas que tenian acceso a un reservorio o camara magmatica que proporciono el fluido o lubricante necesario para su desplazamiento. El emplazamiento del lacolito se desarrolla asociado con una extension en la direccion vertical simultanea con la reactivacion contraccional de la falla de alto angulo, a ca. 5 km bajo la superficie. Esta profundidad de emplazamiento, es compatible con el espesor de la sobrecarga total de rocas sobre el pluton al momento de su emplazamiento, que es significativamente mayor que los espesores estratigraficos de las rocas sobreyacentes Estos calculos, sumados a la evidencia geologica, permiten concluir que esta sobrecarga esta compuesta, ademas de la estratigrafia, por repeticiones estructurales anteriores a la intrusion. ABSTRACT. Syntectonic intrusion of the Torres del Paine Granite, Patagonian Andes of Chile The Miocene Torres del Paine Granite is a sill-like subhorizontal laccolithic body. It is elliptical in plain view and its major axis extends for more than 12 km. In profile the section is strongly asymmetric, with a maximum thickness decreasing from 2.5 km in the west towards zero in the east. The laccolith is emplaced along a shallow to subhorizontal thrust fault (the Rio Nutrias Fault), that is associated with internal folding and faulting developed during an early, pre-intrusion, thin-skinned tectonic phase that affected the Cerro Toro Formation. Where the intrusive shows its thicker section it connects downwards with a feeder dyke that dips more than 70° west. This feeder dyke is emplaced along and reutilizes an old high-angle reverse fault (probably an inherited normal fault) that juxtaposes the Cerro Toro (to the east) and the Punta Barrosa (to the west) formations. This fault, thought to be rel3ted with a period of thick skinned tectonic shortening, causes a several kilometre uplift of the western block and consequent erosion of the stratigraphic cover. The compressional reactivation of faults at an angle equal to or greater than the mechanical frictional lock-up angle (approximately 60°) requires, at least locally, the development of prefailure supralithostatic Fluid pressures (lamda more or equal than 1 or Pm more or equal than Sv=S3). The principal stress orientation during high-angle reverse fault reactivation implies the synchronous generation of dilation-extension structures. In this case, these would have been lenticular and horizontal, giving rise to several lenses that were connected with and filled by magma along the fault. In the case of Torres del Paine, faulting and syntectonic intrusion were possible only in those faults that were large enough to connect themselves with a reservoir or magma chamber that provided the required fluid or lubricant to enable displacement. The formation of the laccolith corresponds to the contemporaneous dilation of the extensional structure associated with the reverse faulting, developed at a depth of about 5 km below the paleosurface, an amount that is significantly larger than the undeformed stratigraphic thickness of the overlying rocks. From these calculations, and from the geological evidence, it is concluded that the overburden consisted of preintrusion structural repetitions of the stratigraphy.