162 GRENVILLIAN OROGENY large and complex as the Grenville Province is The preponderance of available data indicate that such suites are a likely consequence of orogenesis and result from deep-seated lithospheric mechanisms that originated in response to previous orogenic processes Viewed in this light, the Grenvillian AMCG suites represent postorogenic magmatism with origins that relate directly, albeit locally, to Elzevirian orogenesis Ottawan Orogeny Final closure of the ocean bordering proto-North America occurred as a result of continent-tocontinent collision beginning at 1080 Ma, involving a series of events referred to as the Ottawan Orogeny (also as the ‘Grenvillian Orogeny’ by some geologists; see Figure 2) Compressional tectonic forces resulted in telescoping of the continental margin and the formation of large thrust slices and ductile folds with dominant direction of transport towards the north-west (Figure 5D) Multiple lines of evidence indicate that the orogen propagated north-westward, with the youngest faults located in the vicinity of the Grenville Front Tectonic Zone (Figure 4) Metamorphic assemblages in the Grenville Province generally indicate recrystallization at granulite-facies conditions in the hinterland (region located towards the south-east), where large-scale, recumbent nappes were developed and most plutonism occurred Thrusting was more common in the foreland (region located towards the north-west), where metamorphic conditions were more variable, ranging from greenschist to high-pressure amphibolite facies, with eclogite-facies conditions developed locally Many of the north-west-directed thrusts were of large scale, including the Allochthon Boundary Thrust (Figure 3) that is associated with high-pressure mineral assemblages Results from many studies throughout the province indicate that metamorphism and deformation were ongoing across the entire province during the interval from 1080 to 980 Ma However, disagreement exists regarding the nature of these tectonic processes Some researchers suggest that metamorphism and deformation occurred in at least three discreet pulses (see column C in Figure 2) separated by intervals of extension The first two pulses of crustal shortening were concentrated in the hinterland, whereas the latest pulse caused north-westward propagation of the orogen into its foreland Abundant magmatism, including emplacement of anorthosite complexes, occurred during the intervening extensional periods A different interpretation of the tectonic evolution of the eastern Grenville Province suggests that orogenesis peaked at different times in different places, and that, as the crust yielded through thrusting in one location, stresses were transferred elsewhere Thus, instead of separate orogenic pulses, this model proposes that the ‘pressure point’ responsible for compressional tectonism shifted periodically as the crust yielded locally to the forces of deformation The effects of the Ottawan Orogeny were generally more intense in the Adirondack massif, where evidence of ductile structural fabrics and high-grade metamorphism is widespread Studies of metamorphic rocks indicate that the crust beneath the Adirondack Highlands nearly doubled in thickness as a result of Ottawan orogenesis Such extreme crustal thickening, considered together with the general lack of Ottawan-age calc-alkaline rocks of possible arc affinity in the Adirondacks and contiguous Grenville Province, constitutes important evidence in support of continent–continent collision as the primary cause of the orogeny The Adirondack Highlands represent one of the world’s classic granulite-facies terranes, where peak metamorphic conditions during the Ottawan Orogeny reached temperatures of 750–800 C at pressures of 6–8 kbar Metamorphism was associated with recumbent isoclinal folding and development of intense penetrative fabrics that typically overprinted features developed during the Elzevirian orogenesis throughout the Adirondacks (Figure 7A and B) However, the anorthosite complexes in this area, all of which were emplaced at about 1150 Ma, are typically not structurally overprinted (Figure 7C) because these generally anhydrous, dominantly monomineralic plutons acted as rigid bodies that deflected stresses during Ottawan orogenesis Such kinematics that result in plutonic rocks that appear to be relatively undeformed when, in fact, they predate deformation, must be considered carefully by field geologists interested in deciphering the sequence of local structural events Field relations and isotopic ages of plutonic rocks indicate that Ottawan orogenesis occurred from 1090 to 1030 Ma in the Adirondacks The occurrence of thick allochthons and associated highpressure belts in the Canadian Grenville Province stands in contrast to the dominantly ductile deformation and large recumbent fold nappes of the Adirondacks Such differences are attributed to variations in style, metamorphic grade, and ductility between the mobile interior of the orogen to the south-east and the foreland to the north-west, as well as to the thermal softening effects imparted on the Adirondacks by intrusion of the Hawkeye granitic suite at 1100 Ma Post-Ottawan Activity Tectonic activity associated with the Ottawan Orogeny did not terminate simultaneously throughout