Ups and downs of the Guiana Shield and Amazon Basin over the last 500 Myr

Abstract

Cratons such as the Guiana Shield are often considered as stable regions, undergoing long-term emergence and denudation due to buoyancy. However, by integrating geological and geomorphological observations with apatite fission-track analysis, we define a history involving repeated episodes of burial and exhumation over the last 500 Myr. Over much of the shield, the thermal history is dominated by the effects of earliest Jurassic magmatism, followed by Early Cretaceous exhumation coincident with the onset of seafloor spreading in the southern South Atlantic. Further episodes of regional exhumation occurred in Aptian-Albian time coincident with a global-scale plate reorganisation and in Eocene times coincident with a slowdown in the movement of the South American plate. Results from the Amazon Basin also define these four episodes. Thermal data from a deep well in the Amazon Basin show that the Early Cretaceous and Eocene exhumation episodes were preceded by burial by kilometre-scale thicknesses of cover, subsequently removed. Continuity of data from basin to shield suggests that burial extended across the shield. Early Cretaceous exhumation led to formation of a base-Cretaceous peneplain across the entire continent, from the Andes (during post-orogenic collapse) to the Amazon Basin and the Guiana Shield. This peneplain was then buried beneath Cretaceous–Paleogene sediments (remnants of which are preserved in the Amazon well) prior to the onset of Eocene exhumation, which also extended into in the offshore Casiporé Basin (based on our results from a deep well). The Eocene episode also correlates with post-orogenic collapse of the Andes. The history of repeated burial and exhumation defined for the Guiana Shield appears to be a common property of supposedly stable cratons. The correlation between Andean tectonics, episodes of exhumation defined here and changes in the motion of the South American plate, shows that plate-tectonic changes governed the vertical movements across the continent.