Thermal and lithospheric structure of the Chilean-Pampean flat-slab from gravity and magnetic data

Abstract

Flat subduction settings have a direct impact on the lithospheric structure and thermal state of upper plates in active margins. In particular, in the Chilean-Pampean flat subduction zone, the second largest flat subduction setting on earth, these effects have not yet been quantified and hence, its thermal state remain poorly understood. In order to shed light into the thermal structure and its sublithospheric controls, we analyzed high resolution aerial and land magnetic data to determine the Curie Depth Point and subsequently a regional heat flow map. The comparison between the obtained thermal structure above the Flat-slab and previous thermal models and seismic tomographic data shows a correlation between low velocity zones in Nazca plate and higher heat flow areas. In addition, the Moho depth was inverted from the high-resolution gravity GECO geopotential model using different densities contrasts. These inversions were then compared with the Moho determinations from previous seismological studies observing a significant difference between them over the flat-slab segment. To understand these discrepancies, several 2D models were computed using local seismological studies of receiver functions and tomography asconstraints. Areas with discrepancies in Moho values, shallow Curie point depth and high heat flow, support the presence of an eclogitized lower crust beneath the thickest orogenic region region and the existence of a low velocity zone beneath the flat-slab found in previous studies. Finally, these observations could indicate a potential local thinning of the subducted Nazca plate linked to the dragged plume head of the Juan Fernández hot-spot as imaged by a recent seismic tomography survey.