Age and evolution of diachronous erosion surfaces in the Amazon: Combining (U-Th)/He and cosmogenic 3He records

(U-Th)/He geochronology of two weathered plateaus in the Carajas Mountains, Para, Brazil, reveals a history of weathering spanning from ca. 80 Ma to the present for this high elevation (similar to 720 m) land surface. Cosmogenic He-3 measurements of hematite pebbles and blocks cemented onto the plateaus at two sites, N1 and S11D, yield erosion rates as low as 0.09 and 0.08 m Ma(-1), respectively. Thus, these results confirm that the plateau surfaces are nearly immune to physical erosion for tens of millions of years. (U-Th)/He geochronology of ferruginous duricrusts blanketing the low elevation (250-100 m) plains surrounding the Carajas Mountains yield results consistently younger than similar to 10 Ma. The geochronology results also reveal that the low elevation plain is diachronous, becoming progressively younger towards the receding plateaus. The spatial distribution of (U-Th)/He ages permits reconstruction of the history of scarp retreat for the Carajas landscape, showing that scarp retreat along major river valleys may have been as fast as 20 km Ma(-1) during tectonically active and humid periods in the Cenozoic. The cessation of scarp retreat at some sites suggests that metamorphosed banded iron-formations and quartzites provide effective barriers to retreating escarpments, helping to preserve some of the oldest continuously exposed land surfaces on Earth. (C) 2018 Published by Elsevier Ltd.