INVERSION TECTONICS AT CORDILLERA DE DOMEYKO (NORTH CHILE) AND ITS CONTROL ON GIANT PORPHYRY COPPER EMPLACEMENT: NEW INSIGHTS ON FLAT-SLAB SUBDUCTION KINEMATICS DURING THE TERTIARY

The Cordillera de Domeyko, a complex Mesozoic-Tertiary structural belt inboard of the present-day subduction margin, is part of the north Chilean Pre-Cordillera morpho-tectonic unit. Giant porphyry copper deposits, with ages similar to the host intrusions (40-30 Ma), are aligned N-S within this structural belt. N-S alignment of deposits has lead to the widespread hypothesis that emplacement of this Eocene-Oligocene plutonic complex was controlled by regional strike-slip faults developed in a transtensional regime related to oblique subduction.

Landsat TM analysis and field observations indicate that the structure of the Precordillera is dominated by several elongate, N-S trending, basement ridges that are exhumed along steep reverse faults deforming the Triassic-Cenozoic cover. The vergence of the system changes from west to east along the trend giving a doubly-vergent “pop-up” geometry to the axial zone. Many of these geometries and structures result from reactivation of basement extensional faults related to Triassic-Jurassic extensional events. Basement short-cut faults such as the Sierra Castillo basement block are also typical, uplifting Palaeozoic rocks and shortening the footwall Mesozoic cover; showing a strong genetic relationship between thin and thick-skinned structures. Fieldwork shows no evidence of strike-slip motion on these basement faults.

Tertiary porphyry intrusions are aligned N-S, and are strongly controlled by basement-involved reverse faults that facilitated ascension of the magma and intrusion into the Mesozoic sedimentary cover as sills, mainly located at the anticlines hanging-walls.

Late Eocene-Early Oligocene giant porphyry copper bodies (Chuqicicamata & La Escondida) emplaced within Cordillera de Domeyko show adakitic affinities. This affinity combined with the structural evidence of emplacement at the end of the basement-involved deformational stage, suggest the existence of a subducting flat-slab under the Central Andes (22º-26ºS latitude), during the Tertiary. Shallowing slab would result in an easterly-migrating compressional regime in the over-riding plate characterized by thick-skinned tectonics. The resulting flat-slab would be the likely source of adakitic rocks by oceanic crust partial melting.