Tectonics and Collisional Architecture of the Grenville Margins of Laurentia and Amazonia

The Grenville Orogen of Ontario, New York and SW Brazil documents the heterogeneous effects of late Mesoproterozoic oblique collision between the Laurentia and Amazonia cratons and subsequent exhumation. Rocks of the North American segment mostly preserve ages that range from 1.2 Ga to ~0.9 Ga, whereas basement rocks of the Amazon and adjacent Paragua cratons mostly preserve pre-Grenvillian ages (older than 1.3 Ga). Localized isotopic age resetting at 1.18–1.12 Ga is caused by Grenvillian activation of widespread, sinistral strike-slip shear zones in the Amazon basement, whereas widespread 1.0-0.9 Ga ages in the North American basement reflect late to post-collisional processes. Paleomagnetism of 1.20 and 1.15 Ga rocks in SW Amazonia records initial collision with SE Laurentia and 2000km of left-lateral displacement along the eastern margin of Laurentia. Regional growth and cooling ages from U/Pb and 40Ar/39Ar thermochronologic data that were collected over a period of two decades show characteristic exhumation patterns that match this history. Paleodepths calculated for 1.0 Ga along a restored 1300-km-wide section vary from uniformly deep levels (15–30 km) exposed in North America to shallower levels (5– 15 km) in southwest Amazonia. This pattern reflects variation in collisional style along a transpressional plate margin, with thrust-dominated deformation in Laurentia progressively giving way to strike-slip dominated deformation that dominates Amazonia, commensurate with variation in crustal thicknesses. The contrast in tectonic styles explains the widespread preservation of both pre-Grenvillian ages and collisional ages in the Amazon craton and the more homogeneous array of cooling ages from the North American Grenville. The asymmetric orogenic architecture that emerges for the reconstructed root of Grenville orogen is similar to cross sections of modern orogenic belts, indicating that collisional asymmetry is a fundamental feature that persists into the deep crust.