1. Phil Thurston Laurentian University
The balance between vertical & lateral tectonics in the Superior Province
Phil Thurston
Laurentian University

2. Conclusions
Early Greenstone belts: Largely vertical processes ~ 3 Ga, 2.8 Ga
Later greenstones final assembly controlled by lateral processes.
Some evidence for thrusting, but not large scale fold & thrust belts
Where are melanges, UHP, blueschists?

3. Types of subprovince
Plutonic: dominated by plutonic rocks, some greenstone fragments
Granite-greenstone: intact greenstone belts, low grade, granitoids
Sedimentary: accretionary prisms? Forearc basins?
Length of subprovinces = plate tectonics
Younging of ages of volcanism, sedimentation, plutonism major faults = plate tectonics
No melanges, blueschists

4. Evidence for Autochthonous Construction of Superior Province Greenstones
Diking Relationships
Crustal sections
Domes and Keels
Upward facing, upward younging stratigraphy
Contamination of mafic and ultramafic magmas
Isotopic Inheritance
Differing architecture north and south

5. Transition to North Caribou terrane
Transition to North Caribou terrane. Defined by presence of 3 Ga greenstones. Greenstones as young as 2.7 Ga.
Birch-Uchi
Abitibi

6. Muskasenda Gabbro Dikes
Deloro
2730
2705 Ma
Tisdale
Pacaud
2750
2705 Ma gabbro dike cuts 2730 Ma Deloro volcanics indicating authochthonous construction of Abitibi

7. Thurston 2002
Ca Ma dikes cut 2960 and 2832 Ma woman assemblage

8. Upper Crustal Patterns: Central Sync. Younging off batholiths
Stratigraphic younging
Revised stratigraphic map.
Central syncline produced by diapirism of granites
Younging of stratigraphy off batholiths = crustal sections

9. Transition to North Caribou terrane
Transition to North Caribou terrane. Defined by presence of 3 Ga greenstones. Greenstones as young as 2.7 Ga.
N. Caribou

10. Dome & Keel Patterns
Belt has old sequence, then orogeny?, younger greenstones start with qtzite-komatiite platform
Granites younger than belt.
Impose metamorphic aureole on belt at 2.87 Ga
Vertical stretching lineations at triangular syncline
Diapiric uprise of granites in northern 2.8 ga terrane
Reactivation of 3 Ga NCT by 2.8 event

12. Autochthonous Construction: Isotopic Inheritance

13. Evidence for Vertical tectonics
Stratigraphy developed as vertically stacked sequences
Domes and keels with outward directed stretching lineations.
Timing in northern Superior not consistent with lateral tectonics ( My/cycle)
Stratigraphy up-ended by diapiric emplacement of granites

14. Lower Crust? Berens River
N. Spirit Lake
Greenstone fragments, a little better preserved than Minto area. Older sequences fragmentary, younger sequences a little more complete.
Stone 2000

15. Berens River Greenstone Belts
3.02 Ga
N. Spirit Lake
Ga Arc
2.98 Ga Qtz. Kom.
Unconformity Ga
2.73 Ga Arc
Evidence for reactivation and/or multiple rifting events. Lies just southwest of Eyap belt.
Fragments of older sequences. Younger sequences unconformably overlie older sequences.
Reactivation again at 2.7 Ga based on ages of youngest sequence and ages of granitoids. 250 km inland from margin if lateral tectonics
However inheritance seen in Sm-Nd

16. Western Superior Onaman Abitibi-Grenville Types of subprovince
Plutonic: dominated by plutonic rocks, some greenstone fragments
Granite-greenstone: intact greenstone belts, low grade, granitoids
Sedimentary: accretionary prisms? Forearc basins?
Onaman
Abitibi-Grenville

17. Abitibi:
steep dips on margins of belt
shallow dips in center of belt
seismically transparent batholiths
Opatica:
north dipping reflections upper mantle
Abitibi & Pontiac: subhorizontal reflectors
no clear Moho reflection under Abitibi
Mafic restite, underplating beneath Abitibi
Pontiac:
crust ~35 km not 40+
Upper crust unreflective
strike-slip structures hard to see
Lack of lower crustal reflectors Abitibi & Pontiac but present in Opatica

18. This western Superior seismic reflection profile [70] has also been interpreted to indicate progressive accretion of slabs of material from the south onto the North Caribou continental margin. Note “subduction scars”: projections of reflective (crustal) material beneath the Moho into the underlying mantle (see also [71,72]).

19. Devaney & Williams 1989

20. Conclusions
Early Greenstone belts: Largely vertical processes ~ 3 Ga, 2.8 Ga
Later greenstones final assembly controlled by lateral processes.
Some evidence for thrusting, but not large scale fold & thrust belts
Where are melanges, UHP, blueschists?