Resumen. La interpretacion de las megafracturas y fallas menores, la edad y distribucion de las volcanitas y sedimentitas terciarias, asociadas a la region occidental del Macizo y Cordillera Norpatagonica, han permitido establecer un modelo tectonico en el que fracturas de transcurrencia anastomosadas o paralelas entre si, originan cuencas sedimentarias de tipo "pull apart". Estas cuencas, de las que se distinguieron dos franjas principales en las que la compresion es simultanea con la extension, formarian parte del arco volcanico calcoalcalino desarrollado durante el Terciario, inmediatamente al este de un arco interno plutonico. Abstract. The interpretation of the main megafractures and other faults, the age and distribution of Tertiary volcanites and associated sediments of the western zone of the North-Patagonian Massil and North-Patagonian Cordillera allowed establishing a tectonic model in which transcurrent and anastomosed fractures originated volcanic-sedimentary basins of pull apart type. These basins, described as a two belt system, in which the compression was simultaneous with the extension, are related to the Tertiary calc-alkaline volcanic arc with marine and continental sedimentation developed as narrow belts east of the inner plutonic magmatic arc.