Revealing the Early Mesozoic tectonic inheritance of the western Central Andes (Domeyko Range, northern Chile): A structural, geochronological and 40Ar-39Ar thermochronological approach

MAURICIO ESPINOZA; VERÓNICA OLIVEROS; PAULINA VÁSQUEZ; LAURA BEATRIZ GIAMBIAGI; FLORENCIA BECHIS; LEAH MORGAN

Concepción

Congreso; XV Congreso Geológico Chileno; 2018

Universidad de Concepción

The Domeyko Range defines the western edge of the Central Andes (northern Chile 22º - 26º S) and hosts some of the largest porphyry Cu-Mo deposits in the world. It is a deformed belt formed by a core of Late Paleozoic rock thrusted over Mesozoic strata. The building of this range seems to have been strongly controlled by the inversion of the former Mesozoic basin (Domeyko Basin) but to what extent the primary structure of the basin influenced the deformation is unknown. This study aims to unravel the tectonic evolution of the Triassic rifting and its relationship to the building of the Domeyko Range. U-Pb data show that the Domeyko Basin was formed during two synrift stages. The Synrift I (~240-225 Ma) initiated the Sierra Exploradora subbasin (SESB), while the Synrift II (~217-200 Ma) originated the Sierra de Varas sub-basin (SVSB). During the Synrift II, rift-climax sequences were developed at the SVSB evidencing a fault-linkage process which created a major N-S fault array in the present position of the Domeyko Fault System (Eocene-Oligocene). Structural mapping and kinematic analyses indicate a transtensional rifting kinematic with a main ~NE-SW stretching direction, where major N-S faults interacted with NW-SE basement structures producing deep releasing bends. U-Pb detrital zircon ages (DZ) from SVSB deposits, show the main peak at ca. 290 Ma and much younger 40Ar-39Ar detrital muscovites ages (DM), suggesting the beginning of source exhumation at ~245 Ma. On the contrary, along the SESB source exhumation would have taken place at ~265 Ma (DM ages), shortly after their emplacement within the crust (DZ peak at ~270 Ma). This suggests a distinct exhumation history for each sub-basin, likely due to the segmentation of the basement structure prior to ~265 Ma. Moreover, main spatial variations in the structural style along the range axis correlate with the sub-basins edges (vergence direction, thick- versus thin- skin and shortening ratios), with thick-skinned belts occurring at the site of former synrift border faults. We propose that the rifting architecture strongly determined the main structural style of the Domeyko Range and controlled the inception of the prolific Domeyko Fault System. The main structure which segmented the rift sub-basins would have been present at least since the Late Paleozoic (~265 Ma) and correlates with a major NW lineament of the present forearc.