Encyclopedia of geology, five volume set, volume 1 5 (encyclopedia of geology series) ( PDFDrive ) 1344
Bạn đang xem bản rút gọn của tài liệu. Xem và tải ngay bản đầy đủ của tài liệu tại đây (55.39 KB, 1 trang )
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
