Resolving the space problem by downward host rock transport during magma reservoir emplacement in a contractional arc setting: The case study of three Miocene plutons in central Chile

Abstract:

This contribution aims to document a case of contractional arc, exploring how the host rock was displaced by ascending magma during the construction of shallow reservoirs. Based on geological mapping, field observations and fabric measurements, the cases of Miocene plutons in Central Chilean Andes were documented. This contractional arc is characterized by shortening and crustal thickening from Miocene, giving place to a sequence of major reverse fault systems (e.g. Chacayes fault) and the fold and thrust belt.

La Gloria (LGP), San Gabriel (SGP) and Alfalfalito (ALP) plutons (9-13 Myr) are three similar felsic-intermediate intrusive bodies, shallowly emplaced (< 5km) in a NS plutonic belt, and 10-20 km long and 4-6 km wide. The plutons intruded on Cenozoic rocks at 5, 3 and 0 km westward Chacayes fault, respectively. In the three cases, thermal and mechanical aureole of the plutons has small dimensions (<0.1 plutons’ width), characterized by the preservation of the host rock stratification oblique on plutons’ contact. A few meters wide of ductile deformation zones, which are superimposed by fragile deformation (stoped blocks), are recognized on the roof host rock parallel to the contacts. Preservation of ductile deformation is also preserved in lateral contacts of ALP, increasing towards the Chacayes fault. No significant evidences of lateral shortening or ductile downward flow is recognized in LGP and SGP, as suggested by: (1) gently-dipping granitic dykes intruding the host rock from both intrusives, engulfing host rock blocks, and (2) low-anisotropic magnetic fabrics of the SGP’s host rock, consistent with the stratification.

Mechanical aureole analysis shows that downward rock flow controlled the making-space during the emplacement of the Miocene plutons, which occurred mainly in a rigid state (probably by magmatic stoping). Downward ductile flow could take place during early stages as recorded in the LGP roof, but evidence was erased by later rock displacement. The ALP was probably controlled by ductile downward flow to a higher extent, due to the proximity to a regional fault zone that favored the vertical magma transport. The cases are interpreted as a record of an ancient magmatic arc constructed by the downward host rock transport, as expected in contractional and crustal thickening tectonic settings.