THE PHANEROZOIC GEOLOGIC RECORD ALONG THE CRESTAL CHILEAN ANDES ABOVE THE FLAT SUBDUCTING SLAB BASED ON 1:50,000 SCALE MAPPING BY SERNAGEOMIN

Geologic mapping accompanied by K-Ar geochronology and geochemistry in the Chilean crestal Andes (29°-30° S lat.) reveals a long-lived Phanerozoic record of active margin geology culminating with the flattening of the subduction zone in the Neogene. The basement complex consists of pelitic schists, felsic volcanic and volcaniclastics, and granitoids associated with active Late Carboniferous-Early Permian subduction along the western margin of Gondwana. Latest Paleozoic and Triassic bimodal volcanics and cogenetic intrusions relate to extension and crustal melting. E. Jurassic shallow marine limestones and M. Jurassic to E. Tertiary continental deposits and intermediate volcanics are consistent with active sedimentation within a back-arc setting east of the eastward migrating Andean arc. Paleocene to E. Oligocene units comprise intermediate volcanic and continental sediments and intermediate to felsic porphyries emplaced as the arc continued its eastward migration such that near the end of this interval it was near the present-day continental divide. Early deformation and uplift in the region associated with N-striking high-angle reverse faults is coeval with emplacement of E. Oligocene porphyries.

The U. Oligocene to U. Miocene records nearly continuous volcanic activity along the crest of the Andes with both volcanic activity and deformation sweeping eastward during flattening of the subducting slab. Five distinct volcanic units include: Tilito Fm. (felsic ashflows and sediments; 27-23 Ma); Escabroso Fm. (andesite flows and breccias, 22-17 Ma); Cerro de las Tortolas Fm. (intermediate, isolated and coalesced volcanic centers and cogenetic Infiernillo intrusions, 17-14 Ma), Tambo Fm. (intermediate to felsic tuffs and domes, 12.7-9.6 Ma); and the Vallecito Fm. (volcaniclastic sediments and rhyolitic ashflows, 6.6-5.4 Ma). A second phase of contraction and vertical uplift also associated with N-striking high-angle reverse faults is younger than the Escabroso Fm. and older than the Cerro de las Tortolas Fm. Initiation of the shallowing of the subducting slab may be associated with the onset of this phase of deformation, whereas waning of volcanism by Middle to Early Miocene time and geochemical trends suggest that the subducting slab had shallowed to near its present day geometry by 10-12 Ma.