Chapter 17 - 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 structure remains 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 point depth 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 over the Nazca plate and higher heat flow areas. Besides, the Moho depth was inverted from the high-resolution gravity GECO geopotential model using different densities contrasts. We then compared these inversions with the Moho determinations from previous seismological studies observing a significant difference between them over the flat-slab region. We computed three 2D models using as constraints local seismological studies of receiver functions and tomographies to understand these discrepancies. Areas with discrepancies in Moho depth values and with shallower Curie point depths support the presence of an eclogitized lower crust beneath the thickest orogenic 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 mantle lithosphere linked to the dragged plume head of the Juan Fernández hot-spot as imaged by a recent seismic tomography survey.