A multi-disciplinary geophysical approach reveals a Paleoproterozoic cryptic suture in the Amazon Craton – Implications for Proterozoic Supercontinent reconstructions

Abstract

The Amazon Craton evolved in the Paleo-Mesoproterozoic through successive continental magmatic arcs, which ages decrease westwards. However, when and if these magmatic arcs collided with another continent, closing an ocean basin is a subject of ongoing debate. It is speculated that the Baltica and Amazon cratons formed a purely long-lived accretionary margin for at least 900 Ma and only collided during the formation of the Rodinia Supercontinent at 1.2 – 1.0 Ga. Identifying relicts of suture zones is challenging since Proterozoic orogens tend to preserve the continental arc and the reworked components so that less than 10 % of the orogens are composed of oceanic evidence. Moreover, the Phanerozoic cover overlays more than 50 % of the proterozoic rocks in the Amazon Craton. Therefore, we propose a robust geophysical assessment to investigate the upper and lower crusts. We acquired new magnetotelluric data and integrated it to the available potential field (satellite, airborne, and ground), geochronological data (magmatic and metamorphic ages), crustal thickness and depth to Curie temperature estimates. A major crustal-scale structure (more than 1000 km in extension) separates two tectonic fragments with remarkable differences in the magnetic gradient and structure trend. We also find along this major boundary high and low Bouguer anomalies of long-wavelength paired, a step in the Mohorovic discontinuity, and a lower crust conductor. This evidence supports the hypothesis of a collision in Paleoproterozoic times. Our findings suggest that the orogen did not remain purely accretionary and evolved to a collisional type between 1.63 and 1.69 Ga. This contrasts with the purely accretionary orogens described in the Baltica Craton, thereby undermining the hypothesis that the Amazon and Baltica cratons shared the same peri-Nuna subduction system from 2.0 to 1.3 Ga.