How late Archean tectonics could be seen to have operated in the Slave Province is illustrated. Lithospheric thinning and stretching, with the formation of rifted margins (to continental or island arc fragments), and lithospheric flexural loading of the kind familiar in arcs and mountain belts could be discerned

Bradley, D. C.

Kidd, W. S. F.

Kusky, T. M.

English

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LATE ARCHEAN GREENSTONE TECTONICS -- EVIDENCE FOR THERMAL AND THRUST- 
LOADING LITHOSPHERIC SUBSIDENCE FROM STRATIGRAPHIC SECTIONS IN THE SLAVE 
PROVINCE, CANADA; W.S.F. Kidd, Dept. Geol. Sci., SUNYA, Albany, NY 12222, USA, 
T.M. Kusky, Dept. Earth Sci., Johns Hopkins Univ., Baltimore, MD 21218, USA, 
D.C. Bradley, LDGO, Palisades, NY 10964, USA 
Subsidence in rifts and passive continental margins is driven by stretching 
and subsequent cooling and thickening of the lithosphere(1); subsidence in 
foreland trough basins is a result of thrust-loading and flexure of the 
lithosphere(2). Sediment sequences localized by these different mechanisms 
have distinctive sequences of facies and are one of the more convincing forms 
of evidence for the operation of plate tectonics in the Palaeozoic and 
Proterozoic. Archean examples of such sequences have not been so obvious, 
perhaps because none are preserved in little deformed state, and the most 
likely candidates on the basis of lithological assemblages for passive margin 
sequences (3 .4 )  are not only highly deformed and structurally dismembered but 
are also strongly metamorphosed. We have identified intact stratigraphic 
sections of passive margin type, and others of foreland trough-type in 
deformed but low-grade Archean rocks in several extremely well-exposed areas 
of the Slave Province, N.W.T., Canada. These sequences are similar in most 
respects to younger examples and support the hypothesis ( 5 , 6 , 7 )  that tectonic 
processes similar to those operating now were active in the Archean. 
On both the north and south arms of Point Lake an intact stratigraphic 
sequence above a thick conglomerate containing shallow-water arenites and 
intercalated mafic and felsic volcaniclastics and volcanics starts with a 
unit of black pyritiferous slate and argillite of 60-200 m present thickness. 
This contains a minor proportion of siderite iron formation, calcarenite- 
siltite and limestone breccia beds and, adjacent to the volcanics at the 
base, local quartzose and tuffaceous silts and arenites. These black slates 
are conformably succeeded by a thick sequence of quartzofeldspathic turbidite 
greywackes and pelites, with local magnetite iron formation in some of the 
pelitic intervals. The turbidites coarsen and thicken upward through the 
basal 20-100 m. The overall sequence of turbidites is several km thick and 
is imbricated by thrusts directed westwards. We interpret this sequence to 
be the product of submarine thrust-loading subsidence, with the black slates 
being the outer trench slope deposits  and the turbidi tes  the trench f loor  
deposits, subsequently incorporated into an accretionary thrust stack. 
Sections in the Cameron River belt, in the area of Upper Ross Lake-Victory 
Lake-Detour Lake, lie with observed unconformity on tonalitic gneisses. At 
Detour Lake, deformed but low grade sediments consisting largely of quartz 
and carbonate arenites, with a basal biotite phyllite matrix conglomerate and 
a local upper unit of calcsilicates and marbles, form a section about 500 m 
thick. Similar sediments are much thinner nearby, along strike, but 500 m of 
quartzites in the same tectonic position are reported ( 8 )  in the Beaulieu River 
region farther north. These shallow water sediments, which we interpret as a 
passive margin sequence, are truncated by a major thrust carrying greenstone 
belt lithologies generally southwestward, except near Detour Lake, where the 
section passes up through a 100 m thick interval of pelite and iron formation 
to a quartzofelspathic turbidite sequence, of overall great thickness, and 
probably cut by many thrusts. This change to turbidite deposition we also 
interpret as the development of a submarine foreland-basin caused by thrust- 
loading. 
https://ntrs.nasa.gov/search.jsp?R=19890012845 2020-03-20T02:32:27+00:00Z
LATE ARCHEAN GREENSTONE TECTONICS 
W. S. F. Kidd et a l .  
Sequences like these are not unique to the Slave Province but the 
recognition of their significance depends on good outcrop and being able to 
identify which contacts are stratigraphic and which are important faults. 
Similar passive margin-type sequences are well documented from Zimbabwe (9) 
and the Superior Province (10); in both cases the sediments lie unconformably 
on older basement and are only a maximum of a hundred to a few hundred meters 
thick. We interpret adjacent volcanics, including komatiites in Zimbabwe, to 
be in tectonic contact with the sediments, as they are observed to be in the 
Slave Province. Within the Archean, the Witwatersrand basin has been 
suggested to be a foreland basin due to thrust-loading subsidence (11); within 
Archean greenstone belts, the only documented thrust-loading subsidence 
sequence is that of the Barberton Xtn Land (12); they are probably common, 
and several potential examples, which we have not yet had the opportunity to 
examine, occur elsewhere in the Slave Province (data in 13). The change 
between the Bababudan and Chitradurga Groups (14) of southern India, and much 
of the sedimentation in the Chitradurga Group, is possibly of the same origin. 
The thrust-subsidence sequences in the Slave Province are very similar in 
facies and thickness to Phanerozoic examples, particularly those from places 
where island arc terranes dominate, for example central Newfoundland (15). 
The combined effect of lithospheric thickness and the size of the load provided 
by the thrust stack must therefore have resembled the same combination of 
these two factors in more recent times. In contrast, the passive margin 
sequences, while of very similar lithologies in similar order to those in 
younger examples, are consistently thin in comparison with them, even allowing 
for thickness reduction by ductile strain. This difference can be interpreted 
I in at least two different ways; one is that a higher mantle heat production 
I caused slower lithospheric thickening and a smaller total equilibrium thickness 
after a rifting event, resulting in less overall thermal subsidence of rifted 
margins. Another is that rifted margins were more swiftly incorporated into 
convergent tectonic systems than in later times. Unless independent evidence 
on lithospheric thickness can be obtained from other aspects of the Archean 
record or these sequences can be much more precisely dated than at present, it 
I may prove difficult to distinguish between these possibilities. This evidence 
I does show, however, that tectonic processes active in the Archean had primary 
effects on the lithosphere indistinguishable from those of present plate 
tectonics. 
References ‘(1) XcKenzie, D., (1978) Earth Planet. Sci’. Lett., 40, 25-32. 
(2) Jordan, T., (1981) Am. Assoc. Pet. Geol. Bull. 65,  2506-2520. (3) Eriksson, 
K., and Kidd, W.S.F. (1985) Geol. SOC. Amer. Abstracts 17, 575. (4) Hogk, D. 
(1985) Geol. SOC. Amer. Abstracts 17, 666. ( 5 )  Burke, IC., and Dewey, J., 
(1972) African Geology, pp 583-608, Univ. Ibadan Press. (6) Burke, K., et al. 
(1975) Early History of the Earth, pp 113-129, Wiley. (7) Tarney, J., et al. 
(1975) Early History of the Earth, pp 131-146, Wiley. (8) Donaldson, A., et 
al. (1987) G.A.C. Summer Field Conf. Abstracts. (9) Bickle, X.J., et al. 
(1975) Earth Planet. Sei. Lett., 27, 155-162. (10) Wilks, X., and Uisbet, E., 
(1985) Can. J. Earth Sci. 22, 792-799. (11) Burke, K., et al. (1986) Tectonics 
2, 439-456. (12) Jackson, X., et al., (1987) Tectonophysics 136, 197-221. 
(13) Henderson, J., (1975) Geol. Surv. Can. Pap. 75-1A, 325-330. (14) Swami 
Uath, J., and Viswanatha, H., (1976) Rec. Geol. Surv. India 107, 149-175. 
(15) Dewey, J., et al., (1983) Profiles of Orogenic Belts, pp 205-241, A.G.U., 
Washington. 


Late Archean greenstone tectonics: Evidence for thermal and thrust-loading lithospheric subsidence from stratigraphic sections in the Slave Province, Canada

Late Archean greenstone tectonics: Evidence for thermal and thrust-loading lithospheric subsidence from stratigraphic sections in the Slave Province, Canada

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