Evidencias geocronologicas 40Ar/39Ar y K-Ar de un hiatus cretacico superior-eoceno en Chile central (33-33°30'S)

Abstract

RESUMEN. Se entregan 17 nuevas edades 40Ar/39Ar y K-Ar para las rocas volcanicas de las formaciones Lo Valle y Abanico y de intrusivos hipabisales ('stocks', diques y 'silis') que cortan estas secuencias. Los depositos de la Formacion Lo Valle, fueron datados en ca. 71-72 Ma (Maastrichtiano). Le sobreyace la Formacion Abanico (correlacionable con la Formacion Coya-Machali Occidental), cuyas lavas, diques y 'silis' tienen una edad de ca. 34-19 Ma (Oligoceno-Mioceno Inferior). La Formacion Abanico en la zona estudiada, esta caracterizada por facies volcanicas proximales y mas jovenes respecto de su equivalente oriental (Formacion Coya-Machali Oriental). Se detecta un hiatus entre ambas unidades, de aproximadamente 37 Ma, que comprende desde el Cretacico Superior alto al Eoceno. Este lapso representaria una etapa tardia del alzamiento progresivo y erosion de la Cordillera de la Costa, exponiendo, en los niveles mas superficiales, rocas de edad cretacica superior. Posteriormente a este evento, se desarrollo el volcanismo y sedimentacion del Oligoceno-Mioceno Inferior. Algunos diques que se emplazan paralelos a estructuras interpretadas como ramas de la Zona de Falla Pocuro, indican una edad minima oligocena superior-miocena inferior para estas. Las mismas estructuras controlaron, en parte, el emplazamiento de intrusivos hipabisales del Mioceno Inferior-Mioceno Medio (20-14 Ma). ABSTRACT. 40Ar/39Ar and K-Ar geochronological evidences of an Upper Cretaceous-Eocene hiatus in Central Chile (33-33°30'S). Seventeen new 40Ar/39Ar and K-Ar ages have been obtained from volcanic rocks of Lo Valle (Upper Cretaceous) and Abanico (Oligocene-Lower Miocene) formations, and associated shallow intrusions (stocks, dykes and sills). The Lo Valle Formation deposits were dated in ca. 71-72 Ma (Maastrichtian). The lava flows of the overlying volcanic-sedimentary sequence, the Abanico Formation (equivalent to the Western Coya-Machali Formation), as well as the associated dykes and sills were dated in 34-19 Ma. In the studied area, the Abanico Formation is characterized by proximal and younger volcanic facies with respect to its eastern equivalent (Eastern Coya-Machali Formation). A hiatus of nearly 37 my (late Upper Cretaceous to Eocene) is detected between both rock units. This lapse represents a late stage of the gradual uplift and erosion of the Coast Cordillera, which exposed Upper Cretaceous rocks in the shallower levels. Afterwards, the volcanic arc and continental sedimentary basin of Oligocene-Lower Miocene were developed. Dykes emplaced parallel to structures interpreted as branches of the Pocuro Fault Zone, indicate an Upper Oligocene-Lower Miocene age for the structure. These same structures partly controlled the emplacement of Lower lo Middle Miocene hypabissal intrusives (20-14 Ma).