AR-40 AR-39 AND K-AR GEOCHRONOLOGICAL EVI DENCES OF AN UPPER CRETACEOUS EOCENE HIATUS IN CENTRAL CHILE (33-33-DEGREES-30'S)/

Seventeen new Ar-40/Ar-39 and K-Arages have been obtained from volcani c rocks of Lo Valle (Upper Cretaceous) and Abanico (Oligocene-Lower Mi ocene) formations, and associated shallow intrusions (stocks, dykes an d sills). The Lo Valle Formation deposits were dated in ca. 71-72 Ma ( Maastrichtian). The lava flows of the overlying volcanic-sedimentary s equence, the Abanico Formation (equivalent to the Western Coya-Machali Formation), as well as the associated dykes and sills were dated in 3 4-19 Ma. In the studied area, the Abanico Formation is characterized b y proximal and younger volcanic facies with respect to its eastern equ ivalent (Eastern Coya-Machali Formation). A hiatus of nearly 37 my (la te Upper Cretaceous to Eocene) is detected between both rock units. Th is lapse represents a late stage of the gradual uplift and erosion of the Coast Cordillera, which exposed Upper Cretaceous rocks in the shal lower levels. Afterwards, the volcanic are and continental sedimentary basin of Oligocene-Lower Miocene were developed. Dykes emplaced paral lel to structures interpreted as branches of the Pocuro Fault Zone, in dicate an Upper Oligocene-Lower Miocene age for the structure. These s ame structures, partly controlled the emplacement of Lower to Middle M iocene hypabissal intrusives (20-14 Ma).