GRENVILLE DEFORMATION OF THE WESTERN AMAZON CRATON; STRUCTURAL AND METAMORPHIC ASYMMETRY IN A COLLISIONAL OROGEN

The collision of the Amazon craton with southernmost Laurentia in the late Mesoproterozoic (ca. 1.15-1.2 Ga) marks a major event in the assembly of Rodinia. This collision resulted in very different structural styles and metamorphic histories for these two cratons. The North American Grenville Province (NAGP) is dominated by gently dipping foliations, demonstrated by numerous field and seismic surveys to be crustal-scale nappe structures. The complex metamorphic history preserved in the NAGP includes many occurrences of high-pressure (>10 kbar) rocks, including eclogites, which record substantial crustal thickening resulting from the collision. Geochronological studies indirectly confirm these observations of crustal thickening, since most metamorphic minerals with blocking temperatures <750C record a post-collisional cooling history during exhumation and uplift. The Grenville history of the Amazon craton suggests a different orogenic architecture, one dominated by steeply-dipping foliations and large strike-slip fault zones. 40Ar/39Ar analyses of hornblendes from Amazon basement rocks preserve both older, ca. 1.35 Ga ages and younger ages of ca. 1.15-1.20 Ga. The younger ages are found in shear zones that record deformation during the Grenville collision. In contrast with these deformation ages, both 40Ar-39Ar and Rb-Sr analysis of biotite reveal more widespread isotopic resetting, suggesting that these ages represent cooling at ca. 1.1 Ga. The association of strike-slip zones with the preservation of older ages from a previous metamorphic event suggests that crustal thickening was less important on the Amazon side of the Grenville collision. We suggest that the NAGP-Amazon architecture represents a collision zone asymmetry that characterizes the deep roots of other collisional boundaries.