Proterozoic plate tectonics

Early tectonic models of Proterozoic lithosphere envisaged that the Archean cratons were subdivided by mobile belts in which deformation was wholly ensialic, with no rock associations that could be equated with ancient ocean basins. These interpretations where Pro-terozoic orogenies occurred far from continental margins have since fallen out of favor. Most studies now indicate that Proterozoic orogens (Fig. 11.12) evolved along the margins of lithospheric plates by processes that were similar to those of modern plate tectonics.

One of the best-studied examples of an Early Proterozoic orogen that formed by plate tectonic processes lies between the Slave craton and the Phanerozoic Canadian Cordillera in northwestern Canada. This region provides a record of nearly 4 billion years of lithospheric development (Clowes et al., 2005). Deep seismic reflection data collected as part of the Lithoprobe SNORCLE (Slave-Northern Cordillera Lithospheric Evolution) transect of the Canadian Shield (see also Section 10.6.2) provide evidence of a modern, plate tectonic-style of arc-continent collision, terrane accretion, and subduction along the margin of the Archean Slave craton between 2.1 and 1.84 Ga (Cook et al., 1999). These processes formed the Early Proterozoic Wopmay Orogen (Fig. 11.15a) and resulted in continental growth through the addition of a series of magmatic arcs, including the Hottah and Fort Simpson terranes and the Great Bear magmatic arc.

Final assembly of the Slave craton occurred by ~2.5 Ma. Cook et al. (1999) suggested that low-angle seismic reflections beneath the Yellowknife Basin (Fig. 11.15b) represent surfaces that accommodated shortening during this assembly. Some of these reflections project into the upper mantle and represent the remnants of an east-dipping Late Archean subduction zone. Following amalgamation of the craton, the Hottah terrane formed as a magmatic arc some distance outboard of the ancient continental margin between 1.92 and 1.90 Ga. During the Calderian phase (1.90-1.88 Ga) of the Wopmay Orogen this arc terrane collided with the Slave craton, causing compression, shortening, and the eastward translation of exotic material (Fig. 11.15c). In the seismic profile (Fig. 11.15b), the accreted Protero-zoic crust displays gently folded upper crustal layers overlying reflectors that appear to be thrust slices above detachment faults that flatten downward into the Moho. Remnants of the old, east-dipping subduction zone associated with the collision are still visible today as reflections that project to 200 km or more beneath the Slave craton.

Once accretion of the Hottah terrane terminated, the subduction of oceanic lithosphere to the east beneath the continental margin created the 1.881.84 Ga Great Bear Magmatic arc and eventually led to the collision of the Fort Simpson terrane some time before 1.71 Ga (Fig. 11.15c,d). Mantle reflections that record subduction and shortening during this arc-continent collision dip eastward beneath the Great Bear magmatic arc from the lower crust to depths of 100 km (Fig. 11.15b,c,d). Where the mantle reflections flatten into the lower crust, they merge with west-dipping crustal reflections, producing a litho-spheric-scale accretionary wedge that displays imbricated thrust slices. This faulted material and the underthrust lower crust represent part of a Early Proterozoic subduction zone that bears a remarkable resemblance to structures that record subduction and accretion within the Canadian Cordillera (Fig. 10.33) and along the Paleozoic margin of Laurentia (Fig. 10.34). Seismic refraction and wide-angle reflection data (Fernández-Viejo & Clowes, 2003) indicate the presence of unusually high velocity (7.1 km s-1) lower crust and unusually low velocity (7.5 km s-1) upper mantle in this zone (Fig. 11.16c) compared to other parts of this section (Fig. 11.16a,b,d). This observation indicates that the effects of collision, subduction, and the accompanying physical changes in rocks of the mantle wedge remain identifiable 1.84 billion years after they formed.

In western Australia, distinctive patterns of magnetic anomalies provide direct evidence for the collision and suturing of the Archean Yilgarn and Pilbara cratons beginning by ~2.2 Ga (Cawood & Tyler, 2004). The Capricorn Orogen (Fig. 11.17a,b) is composed of Early Proterozoic plutonic suites, medium- to high-grade metamorphic rocks, a series of volcano-sedimentary basins, and the deformed margins of the two Archean cratons. Late Archean rifting and the deposition of passive margin sequences at the southern margin of the Pilbara craton is recorded by the basal sequences of the Hamersley Basin. Following rifting between the cratons, several major pulses of contractional deformation and metamorphism took place during the intervals 2.001.96 Ga, 1.83-1.78 Ga, and 1.67-1.62 Ga. These events resulted in basin deformation and the juxtaposition of cratons of different age and structural trends (Fig. 11.17b,c). The episodic history of rifting followed by multiple episodes of contraction and collision corresponds to at least one and probably two Wilson cycles (Section 7.9) involving the opening and closing of Late Archean-Early Proterozoic ocean basins (Cawood & Tyler, 2004). The presence of similar collisional orogens in Laurentia, Baltica, Siberia, China, and India suggests that the early to mid-Early Proterozoic marks a period

Cordilleran Front

Extent of Phanerozoic cover

Shot points

1000 Simpson 2000

-1140

3000

Junction Ft. Providence Hwy. 1 & 3

Yellowknife

1000 Simpson 2000

3000

Junction Ft. Providence Hwy. 1 & 3

Yellowknife

Cordillera

Fort Simpson Terrane

Wopmay Orogen

Hottah Terrane

Great Bear Magmatic Arc

Slave Province

Yellowknife Basin

Moho rise due to Middle Proterozoic extension

Proterozoic mantle of Fort Simpson Terrane and Basin

Fort Simpson Terrane

Hottah Terrane

Great Bear Magmatic Arc

Yellowknife Basin

Moho rise due to Middle Proterozoic extension

Proterozoic mantle of Fort Simpson Terrane and Basin

Underthrusted lower crust of Fort Simpson

Underthrusted lower crust of Fort Simpson

~400 km convergence minimum

West

"70 th

"70 th

Fort Simpson Fort Simpson Accretion

Hottah-Great Bear Terrane (1.92-1.85 Ga)

Fort Simpson Fort Simpson Accretion

Hottah-Great Bear Terrane (1.92-1.85 Ga)

Calderian

Proterozoic Mantle

Fig. 11.15 (a) Map showing tectonic elements of the Wopmay Orogen and location of the SNORCLE transect (after Fernández-Viejo & Clowes, 2003, with permission from Blackwell Publishing). Straight black lines (circled letters) show division used for interpretation of crustal structure. Orogen includes Coronation, Great Bear, Hottah, Fort Simpson and Nahanni domains. BC, British Colombia; AB, Alberta; YK, Yukon; NWT, Northwest Territories; GSLsz, Great Slave Lake shear zone. (b) Seismic profile, (c) interpretation and (d) reconstruction of the Wopmay Orogen and Slave Province (modified from Cook et al., 1999, by permission of the American Geophysical Union. Copyright © 1999 American Geophysical Union). Reconstruction is made along major faults (bold black lines) and shows a minimum estimate.

Proterozoic Mantle

West

100 km

Calderian

Fig. 11.15 (a) Map showing tectonic elements of the Wopmay Orogen and location of the SNORCLE transect (after Fernández-Viejo & Clowes, 2003, with permission from Blackwell Publishing). Straight black lines (circled letters) show division used for interpretation of crustal structure. Orogen includes Coronation, Great Bear, Hottah, Fort Simpson and Nahanni domains. BC, British Colombia; AB, Alberta; YK, Yukon; NWT, Northwest Territories; GSLsz, Great Slave Lake shear zone. (b) Seismic profile, (c) interpretation and (d) reconstruction of the Wopmay Orogen and Slave Province (modified from Cook et al., 1999, by permission of the American Geophysical Union. Copyright © 1999 American Geophysical Union). Reconstruction is made along major faults (bold black lines) and shows a minimum estimate.

Anton terrane Yellowknife Basin

(a) 1104 .1103 1102 1101

pe D

pe D

0 20 40 60 80 100 120 140 Distance (km)

0 20 40 60 80 100 120 140 Distance (km)

1108 0

Hottah

,1107

Great Bear

10 20 30 40 50

Contwoyto 1105 1104

1108 0

Hottah

,1107

Great Bear

Contwoyto 1105 1104

10 20 30 40 50

60 80 100 120 140 160 180 200 220 Distance (km)

Fort Simpson 1111 1110 0

1109

Hottah

1108

10 20 30 40 50

1109

Hottah

1108

10 20 30 40 50

5.4 5.6 6

-> V

5.8

6.2

6.4-6.6-

6.8 7

6.8

W

7.8

Segment C

20 40

60 80 100 120 140 160 180 200 Distance (km)

Nahanni

0J

-10

m)

(k

- 20

th

pt e

- 30

D

- 40

- 50

0

Fort Simpson .1111 1110

Nahanni

5.8

■—■

V___'

6

6.2

6.4 6.6

6.8

SW

Segment D

60 80 Distance (km)

100 120

Fig. 11.16 P-wave velocity model for (a) Slave province, (b) eastern part of the Wopmay Orogen and transition to Slave Province, (c) Hottah and Fort Simpson terranes, and (d) Fort Simpson terrane and Nahanni domain (after Fernández-Viejo & Clowes, 2003, with permission from Blackwell Publishing). Line segments shown in Fig. 11.15a. Contours are drawn at 0.2 km s- increments. Heavy black lines are locations from which wide-angle reflections were observed.

of supercontinent assembly by plate tectonic processes (Fig. 11.12) (Section 11.5.4).

The appearance of the metamorphic products of subduction and continental collision during the Early Proterozoic, including eclogites and other >1 GPa high-pressure metamorphic assemblages (e.g. Sections 9.9,

10.4.2), represents an important marker of the onset of tectonic processes similar to those seen in the Phanero-zoic Earth (O'Brien & Rotzler, 2003; Collins et al., 2004; Brown, 2006). Phanerozoic eclogites commonly preserve evidence of having been partially subducted to depths greater than ~50 km and then rapidly exhumed

Fig. 11.17 (a) Tectonic map (after Hackney, 2004, with permission from Elsevier) and (b) magnetic anomaly image emphasizing gradients in total magnetic intensity (after Kilgour & Hatch, 2002, with permission from Geoscience Australia, image provided by M. Van Kranendonk, Geological Survey of Western Australia). Magnetic anomaly image shows total magnetic intensity measured in nanoteslas (nT) compiled from airborne, marine and land-based geophysical surveys. The distinctive magnetic anomaly patterns from the Pilbara and Yilgarn cratons reflect different structural trends that resulted from Precambrian plate tectonic processes. (c) Interpretive cross-section of the Capricorn Orogen showing the sutured cratons (after Cawood & Tyler, 2004, with permission from Elsevier).

Fig. 11.17 (a) Tectonic map (after Hackney, 2004, with permission from Elsevier) and (b) magnetic anomaly image emphasizing gradients in total magnetic intensity (after Kilgour & Hatch, 2002, with permission from Geoscience Australia, image provided by M. Van Kranendonk, Geological Survey of Western Australia). Magnetic anomaly image shows total magnetic intensity measured in nanoteslas (nT) compiled from airborne, marine and land-based geophysical surveys. The distinctive magnetic anomaly patterns from the Pilbara and Yilgarn cratons reflect different structural trends that resulted from Precambrian plate tectonic processes. (c) Interpretive cross-section of the Capricorn Orogen showing the sutured cratons (after Cawood & Tyler, 2004, with permission from Elsevier).

(Fig. 11.18) (Collins et al., 2004). In the modern Earth, these unusual conditions are met in subduction-accretion complexes and at the sites of continental collision where relatively cold crust is buried to subcrustal depths. For these subducted rocks to return to the Earth's surface with preserved eclogite facies mineral assemblages, they must be exhumed rapidly before tec-tonically depressed isotherms can re-equilibrate and overprint the assemblages with higher temperature granulite facies minerals.

The oldest examples of in-situ eclogites (i.e. rocks other than xenoliths) include 1.80 Ga and 2.00 Ga varieties (<1.8 GPa, 750°C) from the North China craton and the Proterozoic orogens surrounding the Tanzanian craton (Zhao et al., 2001; Möller et al., 1995). Eclogites recording conditions of ~1.2 GPa and 650-700°C at 1.9-1.88 Ga also occur in the Lapland Granulite Belt of Finland (Tuisku & Huhma, 1998). The Aldan Shield in Siberia (Smelov & Beryozkin, 1993) and the Snowbird tectonic zone between the Rae and Hearne cratons of Canada (Baldwin et al., 2003) preserve retrogressed eclogites of 1.90 Ga. The absence of Archean eclogite facies rocks suggests that before Early Proterozoic times either the conditions to produce such rocks did not exist, the processes to exhume them at a sufficient rate to preserve eclogite-facies mineral assemblages did not exist, or all pre-existing examples have been obliterated by subsequent tectonic events.

The presence of ophiolitic assemblages in Precambrian orogens provides another possible marker of tec-

Phanerozoic subduction zones

High pressure granulites

Granulite p e

400 800

Temperature (°C)

1200

Fig. 11.18 Pressure-temperature plot showing the published paths of the moderate temperature eclogite and high-pressure granulite facies metamorphic rocks older than 1.5 Ga (after Collins et al., 2004, with permission from Elsevier). I, Usagaran Belt, Tanzania; 2, Hengshan Belt, China; 3, Sanggan Belt, China; 4, Ubendian Belt, Tanzania; 5, Jianping Belt, China; 6, Sare Sang, Badakhshan Block, Afghanistan; 7, Snowbird tectonic zone, Canada; 8, Lapland Granulite Belt. Thick arrows refer to Usagaran/Ubende eclogites. Field for Phanerozoic subduction zone metamorphism and metamorphic facies indicated by thick gray curves. Aluminosilicate polymorph fields plotted for reference. And, andalusite; Ky, kyanite; Sill, sillimanite; Amph, amphibolite.

tonic processes similar to those operating during the Phanerozoic (de Wit, 2004; Parman et al., 2004). Some Archean greenstone belts have been interpreted as ophiolites, although these interpretations typically are controversial. The oldest unequivocal examples are Early Proterozoic in age and support interpretations that seafloor spreading and associated ocean crust formation was an established mechanism of plate tectonics by this time. One of the best preserved and least equivocal of the Early Proterozoic examples is the Pur-tuniq Complex (Scott et al., 1992; Stern et al., 1995) of the Trans-Hudson Orogen between the Hearne and Superior cratons of northern Canada (Plate 11.1a between pp. 244 and 245). Other examples occur in the Arabian-Nubian Shield (Kröner, 1985) and the Yavapai-Mazatzal orogens of the southwestern USA (Condie, 1986). The presence of these features suggests that complete Wilson cycles (Section 7.9) of ocean opening and closure and ophiolite obduction were occurring by at least 2.0 Ga.

Together these observations strongly suggest that plate tectonic mechanisms became increasingly important after the Late Archean. Most authors link this development to the increased stability of continental crust during this Eon (Section 11.4.2). Nevertheless, it is important to realize that many uncertainties about Proterozoic tectonics remain. The age of formation of the rock units and the timing of regional meta-morphism, deformation, and cooling are poorly known in many regions. The sources of magmatism and the amounts and mechanisms of crustal recycling also commonly are unclear. In addition, there continues to be a need for high resolution paleomagnetic and geochronologic data to enable accurate reconstructions of the continents and oceans during Pro-terozoic as well as Archean times (Section 11.5.3). These data are crucial for determining the tectonic evolution of Proterozoic lithosphere and for detailed comparisons between Proterozoic and Phanerozoic orogens.

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