MAGMATISM AND POLYPHASE DEFORMATION IN THE JURASSIC ARC OF CENTRAL CHILE: IMPLICATIONS FOR THE TECTONIC DEVELOPMENT OF THE EARLY ANDEAN MARGIN

We present a detailed structural analysis and new geo/thermochronological data from the Jurassic Papudo-Quintero plutonic complex near 32.5°S in central Chile. Structural data collected along ~12 km of nearly-continuous coastal exposures document three deformation events and at least six generations of structures in ~166–164 Ma plutonic rocks. The first event (D₁) includes: a) high-temperature, coaxial-dominated strain along NE- to N-striking subvertical shear zones; b) widespread emplacement of granitic dikes primarily dipping gently to steeply NE; and c) development of narrow (0.5–10 cm thick) strike-slip and oblique-reverse shear zones under amphibolite-facies conditions. These D₁ structures record NW-SE to WNW-ESE shortening with a component of sinistral shear parallel to the magmatic arc. Zircon U-Pb ages and cross-cutting relations tightly constrain all plutonism and development of high-temperature shear zones to 165.7–164.7 Ma. Apatite U-Pb data and field relations suggest that most of the narrow shear zones formed ~165–164 Ma. The second event (D₂) was characterized by coeval development of pervasive E-W-striking veins (±epi±chl±act±qtz) with alteration halos, minor strike-slip and normal faults, and rare brittle-plastic shear zones recording N-S extension in a transtensional regime. D₂ structures are cut by Late Jurassic to Early Cretaceous mafic dikes and likely overlapped with cooling through lower greenschist-facies conditions recorded by published ~162–154 Ma biotite K-Ar dates. Structures associated with the last event (D₃) include mafic dikes, veins, and epidote-bearing conjugate strike-slip faults that broadly record NW-SE to NNW-SSE shortening in a strike-slip regime. D₃ deformation began in the Late Jurassic and ended in Early Cretaceous, prior to cooling below ~180°C as recorded by a 105 ± 3 Ma zircon (U-Th)/He date. D₁ deformation is consistent with studies from other areas that document sinistral transpression along the arc during the Middle Jurassic, though our results demonstrate that NW-SE shortening associated with transpression was short-lived (~2 Myr). D₂ and D₃ likely record low-degrees of plate coupling as the arc migrated eastward. This study highlights how magmatic arcs are sensitive recorders of plate-related deformation and changes in plate dynamics.