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PRECAMBRIAN TECTONICS AND THE SUPERCONTINENT CYCLE 365

ancient ocean basins. These interpretations where Pro- to the collision of the Fort Simpson terrane some
terozoic orogenies occurred far from continental time before 1.71 Ga (Fig. 11.15c,d). Mantle refl ections
margins have since fallen out of favor. Most studies now that record subduction and shortening during this
indicate that Proterozoic orogens (Fig. 11.12) evolved arc–continent collision dip eastward beneath the Great
along the margins of lithospheric plates by processes Bear magmatic arc from the lower crust to depths
that were similar to those of modern plate tectonics. of 100 km (Fig. 11.15b,c,d). Where the mantle refl ec-

One of the best-studied examples of an Early Pro- tions flatten into the lower crust, they merge with

terozoic orogen that formed by plate tectonic processes west-dipping crustal reflections, producing a litho-
lies between the Slave craton and the Phanerozoic spheric-scale accretionary wedge that displays imbri-
Canadian Cordillera in northwestern Canada. This cated thrust slices. This faulted material and the
region provides a record of nearly 4 billion years of underthrust lower crust represent part of a Early
lithospheric development (Clowes et al., 2005). Deep Proterozoic subduction zone that bears a remarkable
seismic reflection data collected as part of the Litho- resemblance to structures that record subduction and

probe SNORCLE (Slave-Northern Cordillera Litho- accretion within the Canadian Cordillera (Fig. 10.33)
spheric Evolution) transect of the Canadian Shield (see and along the Paleozoic margin of Laurentia (Fig.
also Section 10.6.2) provide evidence of a modern, plate 10.34). Seismic refraction and wide-angle refl ection
tectonic-style of arc–continent collision, terrane accre- data (Fernández-Viejo & Clowes, 2003) indicate the
−1
tion, and subduction along the margin of the Archean presence of unusually high velocity (7.1 km s ) lower
−1
Slave craton between 2.1 and 1.84 Ga (Cook et al., 1999). crust and unusually low velocity (7.5 km s ) upper
These processes formed the Early Proterozoic Wopmay mantle in this zone (Fig. 11.16c) compared to other
Orogen (Fig. 11.15a) and resulted in continental growth parts of this section (Fig. 11.16a,b,d). This observation
through the addition of a series of magmatic arcs, indicates that the effects of collision, subduction, and
including the Hottah and Fort Simpson terranes and the the accompanying physical changes in rocks of the

Great Bear magmatic arc. mantle wedge remain identifiable 1.84 billion years
Final assembly of the Slave craton occurred by after they formed.
∼2.5 Ma. Cook et al. (1999) suggested that low-angle In western Australia, distinctive patterns of mag-
seismic reflections beneath the Yellowknife Basin (Fig. netic anomalies provide direct evidence for the collision

11.15b) represent surfaces that accommodated shorten- and suturing of the Archean Yilgarn and Pilbara cratons
ing during this assembly. Some of these refl ections beginning by ∼2.2 Ga (Cawood & Tyler, 2004). The
project into the upper mantle and represent the rem- Capricorn Orogen (Fig. 11.17a,b) is composed of Early
nants of an east-dipping Late Archean subduction zone. Proterozoic plutonic suites, medium- to high-grade
Following amalgamation of the craton, the Hottah metamorphic rocks, a series of volcano-sedimentary
terrane formed as a magmatic arc some distance out- basins, and the deformed margins of the two Archean
board of the ancient continental margin between 1.92 cratons. Late Archean rifting and the deposition of
and 1.90 Ga. During the Calderian phase (1.90–1.88 Ga) passive margin sequences at the southern margin of the
of the Wopmay Orogen this arc terrane collided with Pilbara craton is recorded by the basal sequences of the
the Slave craton, causing compression, shortening, and Hamersley Basin. Following rifting between the cratons,
the eastward translation of exotic material (Fig. 11.15c). several major pulses of contractional deformation and

In the seismic profile (Fig. 11.15b), the accreted Protero- metamorphism took place during the intervals 2.00–
zoic crust displays gently folded upper crustal layers 1.96 Ga, 1.83–1.78 Ga, and 1.67–1.62 Ga. These events
overlying reflectors that appear to be thrust slices above resulted in basin deformation and the juxtaposition of

detachment faults that flatten downward into the Moho. cratons of different age and structural trends (Fig.
Remnants of the old, east-dipping subduction zone 11.17b,c). The episodic history of rifting followed by
associated with the collision are still visible today as multiple episodes of contraction and collision corre-
reflections that project to 200 km or more beneath the sponds to at least one and probably two Wilson cycles

Slave craton. (Section 7.9) involving the opening and closing of Late
Once accretion of the Hottah terrane terminated, Archean–Early Proterozoic ocean basins (Cawood &
the subduction of oceanic lithosphere to the east Tyler, 2004). The presence of similar collisional orogens
beneath the continental margin created the 1.88– in Laurentia, Baltica, Siberia, China, and India suggests
1.84 Ga Great Bear Magmatic arc and eventually led that the early to mid-Early Proterozoic marks a period