Tectono-metamorphic events in the Paleozoic coastal metamorphic complexes of south-Central Chile: the Hualpén–Laraquete segment (36.5–37.5° S)

Metamorphic complexes along the coast of south-central Chile are interpreted as a Late Palaeozoic paired metamorphic belt constructed during ocean–continent subduction in the SW Gondwana margin. Low P/T units are referred to as the Eastern Series, while high P/T units are referred to as the Western Series. A detailed field and structural analysis (36.7–37.15° S) depicted three deformation events: (1) S₁ foliation interpreted as the onset of frontal accretion, followed by the intrusion of the Coastal Batholith (ca. 320–300 Ma) that generated a thermal overprint on the Eastern Series, with north–south-trending isograds; (2) S₂ foliation wrapping relict S₁ in albite porphyroblasts interpreted as basal accretion in Western Series; (3) D₃ event controlled by WNW–ESE post-accretionary sinistral faults that offset the Eastern/Western series contact and generated NE–SW to north–south-trending folds. This event was influenced by the rigid backstop of the Coastal Batholith, disrupting the isograds from the previous thermal overprint, and affected also Upper Triassic units. A ⁴⁰Ar/³⁹Ar age of 275 ± 6 Ma on syn-S₂ white mica records the retrograde metamorphic event of the Western Series. Although passive uplift by basal accretion and oceanic relief arrival are a recurrent model to explain exhumation, our data suggest that strain partitioning along strike-slip systems took place during the Late Palaeozoic in SW Gondwana margin.