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-Arages 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 are 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 to Middle Miocene hypabissal intrusives (20-14 Ma).