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Aspects of precambrian cyclic growth in the Atlantic region
Journal of African Earth Sciences, Vol. 7, No. 2, pp. 499-505, 1988 Printed in Great Britain
0731-7247/88 $3.00+ 0.00 Pergamon Press pie
Aspects of Precambrian cyclic growth in the Atlantic region A . M. GOODWIN* Institut de G6ologie, Universit6 de Rennes, 35042-Rennes C6dex, France (Received for publication 23 February 1987)
Abstract--Recently compiled sub-Phanerozoic geologic maps provide the areal proportions by Precambrian era]eon (Archean eon, Early-, Mid-, and Late Proterozoic eras) of exposed and buried Precambrian crust in the four Precambrian platforms of the Atlantic region. Because a tectonic-based time-rock classification is used, areal proportions of Precambrian crust by era/eon do not reflect original growth rates of continental crust, but rather accumulated orogenic histories by era, with younger eras gaining at the expense of the older. Notable differences by continent include the high proportions of Late Proterozoic crust in Africa and of Early Proterozoic crust in North America, manifestations of the relative abundances of Pan-African and Hudsonian fold belts respectively in the southern and northern continents. The four Atlantic continents are considered to have developed during at least six horizontal crustal oscillations or cycles, each cycle of about 400 Ma duration. The three younger cycles, back to 1000 Ma, can be readily interpreted in terms of the Wilson Cycle; and the two preceding cycles, back to 1800 Ma, less readily so. In the still older cycles, the nature of the operating plate tectonic process is highly controversial and uncertain. The present state of paleomagnetic studies, so important to continental paleoreconstructions, does not provide unequivocal answers to many critical problems. Models for continental cyclicity advocate a thermal instability mechanism involving excessive heating of the upper mantle due to the presence of an insulating supercontinent (Pangea), leading to supercontinental dispersal and eventual reassemblage. The complete cycle from one supercontinent to the next takes about 400 Ma. Resum6--Un document cartographique pr6sente la r6partition du socle pr6cambrien des continents bordant l'Atlantique, dont l'Afrique, en boucliers affleurants et plate-formes masqu6es par des couvertures plus r6centes. Il montre 6galement la r6partition en Arch6en, Prot6rozoique inf6rieur, moyen et sup6deur, ii partir des donn6es g6ochronologiques, chaque cat6gorie comportant la crofite nouvellement form6e et la croflte ant6rieure remani6e. Une analyse comparative des surfaces occup6es par ces 4 cat6gories pour chaque continent, montre l'importance qu'y joue chaque cycle. Pour l'Afrique, l'analyse montre le r61e important jou6 par le Pr~cambrien, dont notamment le d6veloppement de la crofite au Prot6rozoique sup6rieur et inversement la faible repr6sentation du Prot6rozoi'que moyen, h l'inverse de l'Am6rique du Nord. Les continents atlantiques semblent avoir subi au moins 6 grandes oscillations horizontales ou cycles, chacun de l'ordre de 400 Ma. Les 3 derniers cycles jusqu'~t environ I milliard d'ann6es peuvent s'interpr6ter facilement en termes de cycle de Wilson, mais cela est beaucoup moins clair pour les cycles plus anciens. Pour expliquer ce caract~re cyclique de la formation continentale, les modules 6voquent une instabilit6 thermique due h la pr6sence d'une calotte super-continentale (type Pang6e), qui entralne une dispersion des continents, leur mobilisation et de nouveaux assemblages. INTRODUCTION THE FOUR c o n t i n e n t s o f the A t l a n t i c r e g i o n - - E u r o p e , N o r t h A m e r i c a with G r e e n l a n d , S o u t h A m e r i c a with P a t a g o n i a , a n d A f r i c a with A r a b i a a n d M a d a g a s c a r - c o n t a i n l a r g e P r e c a m b r i a n p l a t f o r m s which, t a k e n t o g e t h e r with a d j o i n i n g P h a n e r o z o i c m o b i l e b e l t s , give e v i d e n c e o f e p i s o d i c c o n t i n e n t a l g r o w t h involving regular ca 400 M a - l o n g cycles. P r e c a m b r i a n c o n t i n e n t a l p l a t f o r m s , which c o n s t i t u t e the core of each continent, comprise both exposed shields (also c a l l e d massif, uplift, rise, c r a t o n , etc . . . . ) and buried basement. R e c e n t l y c o m p i l e d subPhanerozoic geologic maps provide the areal prop o r t i o n s b y P r e c a m b r i a n e r a / e o n ( D . A r c h e a n e o n , C. E a r l y - , B. M i d - , a n d A . L a t e - P r o t e r o z o i c eras) o f e x p o s e d a n d b u r i e d P r e c a m b r i a n crust in t h e f o u r contin e n t a l p l a t f o r m s o f t h e A t l a n t i c r e g i o n . B e c a u s e a tect o n i c - b a s e d t i m e - r o c k classification is u s e d , b a s e d o n i s o t o p i c d a t i n g in s t r u c t u r a l p r o v i n c e s ( S t o c k w e l l , 1982), r o c k s a s s i g n e d to a P r e c a m b r i a n e r a / e o n t y p i c a l l y i n c l u d e b o t h n e w l y f o r m e d crust a n d r e w o r k e d ( m e t a m o r p h o s e d o r r e c r y s t a l l i z e d ) o l d e r crust. T h u s
a r e a l p r o p o r t i o n s of P r e c a m b r i a n crust by e r a / e o n d o n o t reflect o r i g i n a l g r o w t h r a t e s o f the c o n t i n e n t a l crust. R a t h e r t h e y reflect a c c u m u l a t e d o r o g e n i c h i s t o r i e s b y e r a , t h e y o u n g e r e r a s g e n e r a l l y g a i n i n g at t h e e x p e n s e o f the o l d e r . In this r e g a r d , the r e m a r k a b l e f e a t u r e o f t h e c o n t i n e n t a l crust is the survival o f significant A r c h e a n
* Permanent address: Department of Geology, University of Toronto, Toronto, Canada M5S IA1. 499
Pfecambrian Era and Eon (%) Proterozoic Era
Precambriat7 Platform 003 km 2 }
A. Late J B. Mid- C. Early (0.6I (1.0(1.8t.0 Ga) 1.8 Ga 2.6 Ga)
D. Archean Eon (> 2,6 Ga)
A. Exposed Crust Only in P/atform EUROPE
(1,595)
35
17
28
20
NORTH AMERICA
(5,969)
10
;)3
37
30
SOUTH AMERICA AFRICA Total Atlantic
Region B.
(5.366)
33
36
15
16
(10,684)
54
8
18
20
(23,614)
36
19
23
22
Combined Crust (exposed + buried) in Platform 45
11
20
24
NORTH AMERICA
EUROPE
(19,470)
(7,572)
4
30
49
17
SOUTH AMERICA
(12,969)
48
28
17
7
AFRICA
(28,381)
75
6
7
12
Total Atrantic Region
(68,392)
46
18
22
14
Table 1. Areal proportions by era/eon of exposed and buried Precambrian crust in Precambrian continental platforms, Atlantic region. (A) Exposed crust only; (B) Combined (exposed + buried) crust.
500
A . M . GOODWIN
and Early Proterozoic crust (14 and 22% respectively, Table 1), survivals requiring enduring cratonization of early-formed Precambrian crust. These high survival rates may be reasonably attributed to early development of deep sub-shield tectospheric roots in the growing continental masses, roots of demonstrated resistance to ensuing tectonic assault, (Goodwin, 1985).
PRECAMBRIAN PROPORTIONS
Areal proportions by Precambrian continentalplatform Areas of exposed only Precambrian crust in the four Precambrian platforms of the Atlantic region are listed in Table 1A. Thus the total exposed Precambrian crust amounts to 23,614,000 km 2. Of this amount Africa contains 45.2%, by far the largest proportion; North America, 25.3;% South America, 22.7%; and Europe, 6,8%. Areas of total (exposed and buried) Precambrian crust in the Atlantic region by platform are listed in Table lB. Thus the total Precambrian crust amounts to 68,392,000 km 2. Again, the four platforms listed in order of decreasing amount, with areal proportions in brackets, are: Africa (41.4%), North America (28.5%), South America (19.0%), and Europe (11.1%). Thus the proportions of exposed and of total Precambrian crust by continental platform correspond closely. In summary, Africa contains slightly less than half the total; the two Americas each about a quarter; and Europe about one-tenth.
Proportions by Precambrian era/eon The proportions of exposed only Precambrian crust by era/eon (A, B, C, D in Table 1) in the four combined Precambrian continental platforms of the Atlantic region are listed in Table 1A. The combined proportions are A : B : C : D = 36:19:23:22. That is to say, the proportion of Late Proterozoic crust (era A) is highest at 36%; and of Mid- and Early-Proterozoic crust) (eras B and C respectively) and of Archean crust (eon D) are each about equal at 19, 23 and 22% respectively. These proportions are very close to those of the earth's total Precambrian crust (unpublished data) i.e. including all Precambrian platforms of the world. Considered by individual continental platform, significant differences in the proportions of exposed Precambrian crust by eraJeon emerge. Firstly, however, Europe (A : B : C: D = 35 : 17 : 28 : 20) corresponds closely to the combined Atlantic region average discussed above. On the other hand, North America (A : B : C : D = 10: 23 : 37 : 30) is especially high in both Early Proterozoic (C) and Archean (D) crust, and very low in Late Proterozoic (A) crust. South America (A: B : C: D = 33 : 36: 15 : 16) is exceptionally high in Mid-Proterozoic (B) crust and low in both Early Proterozoic (C) and Archean (D) crust. Africa (A : B : C : D = 54: 8 : 18 : 20) is exceptionally high in Late Proterozoic
(A) crust, very low in Mid-Proterozoic (B) crust, and about average in both Early Proterozoic (C) and Archean (D) crust. The proportions of combined (i.e. exposed + buried) Precambrian crust by era are presented in Table lB. For the total Atlantic region, i.e. the four combined Precambrian platforms, the proportions are A : B : C: D = 46 : 18 : 22 : 14. Thus inclusion of buried Precambrian components (i.e. Table 1B versus Table 1A) significantly (1) increases Late Proterozoic (A) proportion and (2) decreases Archean (D) proportion, without however substantially altering the other two (i.e. B + C) proportions. Otherwise stated, buried and exposed Precambrian crusts are respectively rich in Late Proterozoic and Archean components, but uniform in Early- and Mid-Proterozoic components. Considered by Precambrian platform: (1) Europe, at A : B : C : D = 45 : 11 : 20: 24, again is close to the proportions present in exposed crust (Table 1A) with only modest gain in Late Proterozoic (A) and loss in MidProterozoic (B) proportions; (2) North America, at A : B : C : D = 4 : 30: 49 : 17, shows a dramatic drop in Late Proterozoic crust, an equally dramatic gain in Early Proterozoic crust, and a substantial gain in MidProterozoic crust; (3) South America at A: B : C : D = 48:28:17:7, is comparatively rich in crust of the two later Precambrian eras (A + B), and low in that of the two earlier eras (C + D) notably the latter; and (4) Africa, at A : B : C : D = 75 : 6: 7 : 12, has a remarkably high Late Proterozoic content, and correspondingly low Early- and Mid-Proterozoic contents. Geologically, these results express the following geologic relations: (1) the European Platform contains a more or less average global mix of Precambrian crust, both exposed and buried; (2) the North American Platform with abnormally high (C + D) proportions, contains both (a) the unusually large Early Proterozoicdominated Churchill + Bear + Southern + Rinkian + Naqssuqsoqidian + Ketilidian belts and (b) unusually large Archean domains, notably Superior Province; (3) the South America Platform with high A + B proportions, includes (a) the large Mid-Proterozoicdominated Rio Negro-Juruena and Rondonian belts plus significant adjoining coeval platform cover and (b) the Late Proterozoic-dominated Atlantic Shield with widespread Brasiliano (Late Proterozoic) imprint; and (4) the African Platform, with high A proportion, is dominated by Late Proterozoic rocks in the form of both (a) widespread Pan-African mobile belts, and (b) extensive tabular to mildly folded platform cover e.g. Taoudeni and Congo Basins.
Proportions of Precambrianplatforms and Phanerozoic belts The sizes of the four Precambrian platforms by continent and of their adjoining combined Phanerozoic mobile belts and platform cover, which collectively constitute the complete continents, are listed in Table 2. Thus the total combined continental crust in the Atlantic
A s p e c t s of Precambrian cyclic growth in the Atlantic region
~
~
501
x
;
LEGEND 600 Ma
i~i~ ~..
a ~ b
PRECAMBRIAN A 1000Ma
a~;~b
PRECAMBRIAN B 18OOMa
a~b
PRECAMBRIAN C 2600Ma
a ~ b 0 I
1000 I
2000km I
a _outcrops / b. covered crust
PRECAMBRIAN D (Archean)
Fig. 1. Preliminary sub-Phanerozoic geologic map of Precambrian continental platforms in the Atlantic region (BuUard et al., 1965 reconstruction). Peripheral Phanerozoic fold belts are shown by coarse rectangular pattern. Precambrian inliers in the Andrean Belt are of uncertain age. &gS
7:2-N
502
A . M . GOODWIN
Continent
Precambrian Platform
Phanerozoic Mobile Belts
Size Ratio ' 03km 2 }
Size I Ratio [103 km2} ;
Combined Continental Crusl Size [10 a km 2 ) Ratio
EURCPE NORTH AMERICA SOUTH AMERICA AFRICA
7,572 19.470 12,969 28.381
1 2.6 1.7 3.7
15.846 13.122 4,152 1.396
1 0.8 0,3 0.1
23,418 32.592 17.121 29,777
1 1.4 0.7 1.3
Total
68.392
9.0
34,516
2.2
102,908
4.4
Table 2. Relative size by continentsof (1) Precambriancontinental platforms, (2) adjoining Phanerozoic mobile belts, (3) combined continentalcrust. region is 102,908,100 km 2 which comprises, in decreasing continental size: North America followed closely by Africa, then Europe, and South America. Phanerozoic belts of Europe (Caledonides + Hercynides + Alpides) occupy the largest area at 15,846,000 km 2, and those of Africa (Cape + Mauritanides + Atlas) the smallest at 1,396,000 km2; North and South American belts are intermediate in size as shown. In all four continents but Europe the Precambrian platform is substantially larger than the enclosing or adjoining Phanerozoic mobile belts; this is especially so in Africa. In brief, each continent is an aggregate of "interior" Precambrian platform (partly covered) of designated construction by era/eon, and of peripheral Phanerozoic mobile belts, the Precambrian and Phanerozoic proportions varying by continent.
CYCLES IN CONTINENTAL GROWTH Gastil (1960) established that periods of abundant global igneous and metamorphic dates, corresponding to periods of orogeny, are about 210 Ma in length, and alternate with like periods of mineral date scarcity (tectonic quiescence) for a mean 417 Ma-long cyclic distribution pattern extending back to 2600 Ma. This pattern, based on a sample of about 400 K-Ar, Rb-Sr, and U-Pb dates, is considered to correspond to long cycles in earth's orogenic history, the peaks in the number of radiometric ages corresponding to terminal events of the major crustal processes. Dearnley (1965) soon expanded the investigation on a sample of 3400 mineral ages determinations. A cumulative curve from his histogram showed three particularly well-defined changes of slope at ca 1950, 1075, and 180 Ma, and two other less abrupt changes at 2750 and 750 Ma. All the changes were regarded as signifying the onset of worldwide tectonic regimes each with a duration of several hundred million years. From a survey of age determinations relating to rockforming events, particularly Rb-Sr whole rock isochron ages and initial 87Sr/86Sr ratios, Moorbath (1976) suggested that short periods of accelerated crustal growth may have occurred episodically and that they may correspond with the groupings of relevant radiometric dates which are approximately 3800-3500; 2800-2500; 1900-1600; 1200-900; and 500-0 Ma ago. Condie (1976) surveyed global dates and concluded that world-wide Precambrian orogenic periods are
episodic averaging 200 to 400 million years in length and occurring about every 500 to 600 million years. Major orogenic periods occur at 3.8-3.0, 2.7-2.5, 2.0-1.5, 1.2--0.9, and 0.7--0.5 Ga. Earlier, York and Farquhar (1972) averaged the estimates of significant dates in Earth's history by numerous authors, and arrived at the following numbersm2.60, 1.84, 1.09 and 0.57 Ga. These they delightfully called "Magic Numbers", to express the impossibility of assigning their recognition to any one individual, and the constancy of the interpretations during an exponential growth in the number of radiometric ages, but also as a caution to the practice of lumping large numbers of ages into one histogram, and attaching major geological significance to ensuing structures. Moorbath (1984) further cautions that the analysis of histograms of isotopic dates can be subject to serious misinterpretation unless the geological nature and significance of the events under investigation are fully understood. Furthermore, although episodic crustal events may be reasonably wellestablished in certain Precambrian platforms, their global synchroneity is in serious question (Moorbath, 1984). In spite of these reservations, these "Magic Numbers" retain their apparent significance in terms of crustal processes. In a related sense, culminations of the same or related crustal processes are also reflected in the post-Archean paleomagnetic record by "hairpin" turns which appear to mark sudden reversals in the sense of polar motion of continental plates (York and Farquhar, 1972). Furthermore, Cahen et al. (1984), following an exhaustive survey of events dated in Africa, the largest Precambrian continent, select the following as marking "chronological milestones" in African evolution: 3.5, 2.9, 2.5, 2.1, 1.75, 1.1 and 0.57 Ga. For the present purpose, then, the following four dates (each +100 Ma): 2.6, 1.8, 1.0 and 0.6 Ga are used to demarcate the Archean eon, and Early-, Mid-, and Late Proterozoic eras respectively in Atlantic region Precambrian crust (Table 1).
ATLANTIC REGION CYCLES The Atlantic region of the collective EuropeAmericas-African platforms has experienced repeated horizontal crustal oscillations. Including the modern Atlantic opening, at least three (to 1.0 Ga) and possibly four (to 1.4 Ga) coherent Wilson cycles have been tracked, each involving early divergence with supracrustal accumulation followed, in all but the modern cycle, by convergence and orogeny, the complete cycle lasting about 400 Ma (Goodwin, 1985, Table 4). The four cycles are named, in order of increasing age, I-Current (0-200 Ma), II--Appalachian (200-600 Ma), Ill--Pan-African (600-1000 Ma), and IV--Grenvillian (1000-1400 Ma). Still older cycles are named V--Elsonian (1400-1800 Ma) and VI--Hudsonian (1800-2200), the specific names marking the culminating orogenies respectively of the cycles.
Aspects of Precambrian cyclic growth in the Atlantic region
503
For the most part, Late Proterozoic-Paleozoic crust therefore cause changes in the patterns of mantle concycles (II-III) are readily interpreted in terms of modern vections. Runcorn considered that at the time of tranplate tectonic processes involving ocean floor consump- sition from one mode of convection to the next higher, tion and subduction zones with active and passive conti- the continents would be under great stress and those nental margins. Even within these cycles, however, transitions at critical values would be manifest in the whereas certain 1000-600 Ma-old belts e.g. the Pan- geologic record. Sutton (1963) postulated the existence of chelogenic African Pharusides and Hijaz, carry unmistakable Wilson cycle signatures involving ophiolites, aulacogens cycles consisting of a sequence of events leading to the and calc-alkaline extrusive and intrusive rocks, other early disruption of the continents and later to the coeval belts in Africa and South America, e.g., Dama- regrouping of these disrupted fragments of continental ran-Katangan, Ribeira, and Paraguay-Araguaia, are crust. A cycle is considered to last for 750 to 1250 Ma. More recent models link continental cyclicity directly more readily interpreted as predominantly if not completely ensialic in origin, thereby suggesting a somewhat with the plate-tectonic process. Worsley et al. (1984) summarize a "non-random" crustal model to account for different perhaps modified plate tectonic origin. Mid-Proterozoic (B) crust (cycles IV-V) is charac- long term tectonic cyclicity. Plate-motion is attributed to terized by (a) widespread anorogenic magmatism (anor- a thermal instability mechanism (Busse, 1978) resulting thosites, rapakivi granites, rhyolites) (b) continental from the repeated assembly of supercontinents (Pangea) rifts and aulacogens together with (c) major mobile that never completely disperse. A pattern of plate tecbelts, both marginal "accretionary" (e.g. Grenville, tonic cycles, each cycle of about 400 Ma duration, is Rondonian) and "interior", some of the latter appa- recognized back to 2000 Ma. The popular "random" rently ensialic (e.g. Kibarides, Espinhaco). The nature plate motion model (Anderson, 1982), however, advoof the operating plate tectonic process(es), probably cates that Mesozoic Pangea represents assemblage of continental fragments dispersed from a still-earlier complex, is highly controversial and uncertain. Early Proterozoic (C) crust (cycle VI plus still older supercontinent centred on the Pacific Ocean, and now post-Archean units) features highly deformed, inter- marked by the Central Pacific residual geoid high. Le Pichon and Huchon (1984), in turn, interpret mediate to high grade metamorphic gneiss terrains together with commonly asymmetric fold belts which evidence pertaining to the geoid and supercontinent in appear, more often than not, to be at least partly super- terms of a weak coupling of (a) a separate steady-state posed on an ensialic basement (e.g. Labrador belt). lower mantle, which is responsible for the present geoid, Some belts, however, (e.g. Coronation and Svecofen- to (b) upper mantle convection leading to hemispheric nian) closely resemble Phanerozoic equivalents includ- continental configuration (Pangea). Pangean configuring local ophiolites. Still others (e.g. Birrimian) contain ation ends when excessive heating of the upper mantle, Archean-style greenstone belts. Most belts are severely due to the insulating continental (i.e. Pangean) cap, deformed as a result of low-angle foreland transport. leads to continental dispersal, the complete cycle from This tectonic mobility was frequently followed by inten- one supercontinent to the next being in the order of sive and repeated granitoid intrusions, commonly with 400 Ma. These models point to regular cyclicityin the evolution ring-structures, generally high-level, often alkaline, and of continental crust. linked to lava extrusion, all conducive to cratonization. Finally Archean (D) crust contains the well-known low-to-medium grade granitoid-greenstone belts and PALEOMAGNETISM adjoining higher grade gneiss-migmatite terrains, commonly granulitic. Up to three generations of greenstone Paleomagnetic studies of Precambrian rocks offer the belts are known in some regions. In some platforms, repeated "accretion-differentiation" episodes, potential of providing constraints on the paleorepresenting major builders of continental crust, occur- reconstruction of continental masses during Precambred in near-continuous succession. Widespread tonalite- rian time and, thereby, on the formative tectonic procestrondhjemite-granodiorite (TTG) plutonism led to ses. Unfortunately, as illustrated below, the present rapid growth of at least locally stable cratons. In keeping state of the studies does not provide unequivocal answers with its antiquity and uniqueness, the nature of the plate to many critical problems. Piper (1976) proposed that during much of the Protectonic processes is largely unresolved. terozoic paleomagnetic results are consistent with a fixed position for Africa immediately "southwest" of Laurentia. Piper (1983) re-examines the case for a ProMODELS FOR CONTINENTAL CYCLICITY terozoic supercontinent and contends that both the Runcorn (1962) was an early exponent of correlating polarities and positions of the Proterozoic paleopoles mineral date peaks with changes in mantle convection from the major Precambrian shields conform to a single patterns. Thus the pattern of convection to be expected "apparent polar wander path" (a.p.w.p.). These data in the earth's mantle is determined by the ratio (n) of the imply that the continental crust was amassed together as radius of the earth's core to the radius of the earth. With a single large lens-shaped supercontinent from late a slowly growing core, a gradually changing n would Archean time to ca 600 Ma ago.
504
A . M . GOODWIN
However, Morris et al. (1979) contend that the same data are consistent with Africa situated immediately "southeast" of Laurentia during the Proterozoic (as in the Bullard et al. (1965) reconstruction). Embleton and Schmidt (1979) have argued that data for much of the Proterozoic are consistent with Africa and Laurentia in their p r e s e n t relative positions. Irving and McGlynn (1979) believe, however, that the data provide few constraints and, in particular, that there is no basis for the proposition that the continental crust was assembled into a single unchanged Pangea during the ca 2200 to ca 1300 Ma period. Furthermore, Dunlop (1981) contends that Laurentia, Africa and Australia, each internally assembled as we know them today by early Proterozoic time, drifted largely independently, and not as a coherent supercontinent, during the Proterozoic. He further contends that some major "interior" provinces, e.g. Churchill, are ensialic in origin; others, e.g. Bear, may have accreted marginally. McElhinny and McWilliams (1977) likewise agree that the evidence for a Proterozoic supercontinent is no longer tenable. They further contend that, although the paleomagnetic data are considered to preclude platetectonic models involving the convergence of previously widely separated cratons to explain Proterozoic provinces such as the Churchill, the data do not exclude models involving the opening and closing of small (5001000 km) intercratonic oceans, provided that the cratons return to their same relative positions following ocean closure. Finally Irving and McGlynn (1981) contend that paleomagnetic results from Laurentia are in disarray in the interval 2300 to c a 1650 Ma and that it is not possible to determine whether the component cratons (Slave and Superior) had separate or common a.p.w.p.s, and hence whether or not they moved relative to one another, prior to and during the Hudsonian Orogeny. They conclude that the idea of the operation of plate tectonic processes in the Proterozoic is not contradicted nor proven by the paleomagnetic evidence from Laurentia.
CONCLUSIONS Africa alone contains almost 50% of the total exposed and buried Precambrian crust in the combined Precambrian platforms of the Atlantic region; North America and South America each contain about 25%; and Europe contains about 10%. Considered by era/eon (i.e. Archean, Early-, Midand Late-Proterozoic), Africa contains exceptionally high Late Proterozoic and very low Mid-Proterozoic proportions. North America (including Greenland), on the other hand, contains exceptionally high Early Proterozoic and very low Late Proterozoic proportions, the reverse of the African situation. The reasons for these crustal imbalances by continent though basically unknown, are related to the relative abundances of Late Proterozoic (Pan-African) and Early Proterozoic (Hud-
sonian) fold belts in the southern and northern continents respectively. Regarding radiometric ages and continental cycles, despite an exponential increase in the data, the interpretations have remained strikingly constant. Thus certain dates, notably 2.6, 1.8, 1.0 and 0.6 Ga, may mark recurring long-term orogenic cycles. Although cyclicity in a particular continental block may be readily established, synchroneity between continents is uncertain. In this vein, the Atlantic region, represented by the collective Europe-Americas-African platforms, appears to have experienced repeated (at least 6) horizontal crustal oscillations or cycles, each cycle of about 400 Ma duration. Cycles 1-3, back to about 1.0 Ga ago, are readily interpreted in terms of the Wilson Cycle; and cycles 4-5 less readily so. In the older cycles, however, the nature of the operating plate tectonic processes is highly controversial and uncertain. Major unresolved problems remain. Current models for continental cyclicity advocate a thermal instability mechanism involving excessive heating of the upper mantle due to the presence of an insulating supercontinental (Pangea) cap. This leads to supercontinental dispersal, followed, by either "nonrandom" or "random" motions of the continental fragments, before eventual reassemblage. The complete cycle from one supercontinent to the next is in the order of 400 Ma. The current disarray in paleomagnetic interpretations together with other fundamental problems, demonstrate that a new level of interdisciplinary studies is required, involving coordinated field, isotopic and paleomagnetic studies. Rigorous integration of these disciplines is required for meaningful interpretations.
REFERENCES Anderson, D. L. 1982. Hotspots, polar wander, Mesozoic convection and the geoid. Nature, Lond. 297,391-393. Bullard, E., Everett, J. E. and Smith, A. G. 1965. The fit of the continents around the Atlantic. Roy. Soc. London Phil. Trans. A258, 41-51. Busse, F. H. 1978. A model of time-periodic mantle flow. Royal Astron. Soc. J. Geophys. 53, 1-12. Cahen, L., Snelling, N. J., Delhal, J. and Vail, J. R. 1984. The Geochronology and Evolution o f Africa. Clarendon Press, Oxford, 512p. Condie, K. C. 1976. Plate Tectonics and Crustal Evolution. Pergamon Press, New York. Dearnley, R. 1965. Orogenic fold-belts and continental drift. Nature, Lond. 2116, 1083-1087. Dunlop, D, J. 1981. Paleomagnetic evidence for Proterozoic continental development. Phil. Trans. R. Soc. Lond. A301, 265-277. Embleton, B. J. J. and Schmidt, P. W. 1979. Recognition of common Precambrian polar wandering. Nature, Lond. 282,705-707. Gastil, G. 1960. The distribution of mineral dates in time and space. Am. J. Sci. 258, 1-35. Goodwin, A. M. 1985. Rooted Precambrian ring-shields: growth, alignment and oscillation. Am. J. Sci. 285,481-531. Irving, E. and McGlynn, J. C. 1979. Paleomagnetism in the Coronation Geosyncline and arrangement of continents in the middle Proterozoic. Geophys. J. R. Astron. Soc. 58, 309-336. Irving, E. and McGlynn, J. C. 1981. On the coherence, rotation and paleolatitude of Laurentia in the Proterozoic. In: Precambrian Plate Tectonics (edited by KrOner, A.), pp. 561-598. Elsevier, Amsterdam.
Aspects of Precambrian cyclic growth in the Atlantic region Le Pichon, X. and Huchon, P. 1984. Geoid, Pangea and convection. Earth Planetary Sci. Letters 67, 123-135. McElhinny, M. W. and McWilliams, M. O. 1977. Precambrian geodynamics--a paleomagnetic viewpoint. Tectonophysics40, 137158. Moorbath, S. 1976. Age and isotopic constraints for the evolution of Archaean crust. In: The Early History of the Earth (edited by Windley, B. F.), pp. 351-360. Wiley, London. Moorbath, S. 1984. Patterns and geological significance of age determinations in continental blocks. In: Patterns of Change in Earth Evolution (edited by Holland, H. D. and Trendall, A. F.), Dahlem Konferenzen, pp. 207-219. Springer-Verlag, Berlin. Morris, W. A., Schmidt, P. W. and Roy, J. L. 1979. A graphic approach to polar path: paleomagnetic cycles and global tectonics. Geol. Surv. Canada, Paper 75-54.
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Piper, J. D. A. 1976. Paleomagnetic evidence for a Proterozoic super-continent. Royal Soc, London Philos. Trans. A280, 469-490. Piper, J. D. A. 1983. Dynamics of the continental crust in Proterozoic times. Geol. Soc. America, Memoir 161, 11-34. Runcorn, S. K. 1962. Towards a theory of continental drift. Nature, Lond. 193,311-314. Stockweil, C. H. 1982. Proposals for time classification and correlation of Precambrian rocks and events in Canada and adjacent areas of the Canadian Shield. Part I: a time classification of Precambrian rocks and events. Geol. Surv. Canada Paper. 80-19, 1-135. Sutton, J. 1963. Long-term cycles in the evolution of continents. Nature, Lond. 198,731. Worsley, T. R., Nance, D. and Moody, J. B. 1984. Global tectonics and eustasy for the past 2 billion years. Marine Geol. 58,373-400. York, D. and Farquhar, R. M. 1972. The Earth's Age and Geochronology. Pergamon Press, New York, 178 pp.
0731-7247/88 $3.00+ 0.00 Pergamon Press pie
Aspects of Precambrian cyclic growth in the Atlantic region A . M. GOODWIN* Institut de G6ologie, Universit6 de Rennes, 35042-Rennes C6dex, France (Received for publication 23 February 1987)
Abstract--Recently compiled sub-Phanerozoic geologic maps provide the areal proportions by Precambrian era]eon (Archean eon, Early-, Mid-, and Late Proterozoic eras) of exposed and buried Precambrian crust in the four Precambrian platforms of the Atlantic region. Because a tectonic-based time-rock classification is used, areal proportions of Precambrian crust by era/eon do not reflect original growth rates of continental crust, but rather accumulated orogenic histories by era, with younger eras gaining at the expense of the older. Notable differences by continent include the high proportions of Late Proterozoic crust in Africa and of Early Proterozoic crust in North America, manifestations of the relative abundances of Pan-African and Hudsonian fold belts respectively in the southern and northern continents. The four Atlantic continents are considered to have developed during at least six horizontal crustal oscillations or cycles, each cycle of about 400 Ma duration. The three younger cycles, back to 1000 Ma, can be readily interpreted in terms of the Wilson Cycle; and the two preceding cycles, back to 1800 Ma, less readily so. In the still older cycles, the nature of the operating plate tectonic process is highly controversial and uncertain. The present state of paleomagnetic studies, so important to continental paleoreconstructions, does not provide unequivocal answers to many critical problems. Models for continental cyclicity advocate a thermal instability mechanism involving excessive heating of the upper mantle due to the presence of an insulating supercontinent (Pangea), leading to supercontinental dispersal and eventual reassemblage. The complete cycle from one supercontinent to the next takes about 400 Ma. Resum6--Un document cartographique pr6sente la r6partition du socle pr6cambrien des continents bordant l'Atlantique, dont l'Afrique, en boucliers affleurants et plate-formes masqu6es par des couvertures plus r6centes. Il montre 6galement la r6partition en Arch6en, Prot6rozoique inf6rieur, moyen et sup6deur, ii partir des donn6es g6ochronologiques, chaque cat6gorie comportant la crofite nouvellement form6e et la croflte ant6rieure remani6e. Une analyse comparative des surfaces occup6es par ces 4 cat6gories pour chaque continent, montre l'importance qu'y joue chaque cycle. Pour l'Afrique, l'analyse montre le r61e important jou6 par le Pr~cambrien, dont notamment le d6veloppement de la crofite au Prot6rozoique sup6rieur et inversement la faible repr6sentation du Prot6rozoi'que moyen, h l'inverse de l'Am6rique du Nord. Les continents atlantiques semblent avoir subi au moins 6 grandes oscillations horizontales ou cycles, chacun de l'ordre de 400 Ma. Les 3 derniers cycles jusqu'~t environ I milliard d'ann6es peuvent s'interpr6ter facilement en termes de cycle de Wilson, mais cela est beaucoup moins clair pour les cycles plus anciens. Pour expliquer ce caract~re cyclique de la formation continentale, les modules 6voquent une instabilit6 thermique due h la pr6sence d'une calotte super-continentale (type Pang6e), qui entralne une dispersion des continents, leur mobilisation et de nouveaux assemblages. INTRODUCTION THE FOUR c o n t i n e n t s o f the A t l a n t i c r e g i o n - - E u r o p e , N o r t h A m e r i c a with G r e e n l a n d , S o u t h A m e r i c a with P a t a g o n i a , a n d A f r i c a with A r a b i a a n d M a d a g a s c a r - c o n t a i n l a r g e P r e c a m b r i a n p l a t f o r m s which, t a k e n t o g e t h e r with a d j o i n i n g P h a n e r o z o i c m o b i l e b e l t s , give e v i d e n c e o f e p i s o d i c c o n t i n e n t a l g r o w t h involving regular ca 400 M a - l o n g cycles. P r e c a m b r i a n c o n t i n e n t a l p l a t f o r m s , which c o n s t i t u t e the core of each continent, comprise both exposed shields (also c a l l e d massif, uplift, rise, c r a t o n , etc . . . . ) and buried basement. R e c e n t l y c o m p i l e d subPhanerozoic geologic maps provide the areal prop o r t i o n s b y P r e c a m b r i a n e r a / e o n ( D . A r c h e a n e o n , C. E a r l y - , B. M i d - , a n d A . L a t e - P r o t e r o z o i c eras) o f e x p o s e d a n d b u r i e d P r e c a m b r i a n crust in t h e f o u r contin e n t a l p l a t f o r m s o f t h e A t l a n t i c r e g i o n . B e c a u s e a tect o n i c - b a s e d t i m e - r o c k classification is u s e d , b a s e d o n i s o t o p i c d a t i n g in s t r u c t u r a l p r o v i n c e s ( S t o c k w e l l , 1982), r o c k s a s s i g n e d to a P r e c a m b r i a n e r a / e o n t y p i c a l l y i n c l u d e b o t h n e w l y f o r m e d crust a n d r e w o r k e d ( m e t a m o r p h o s e d o r r e c r y s t a l l i z e d ) o l d e r crust. T h u s
a r e a l p r o p o r t i o n s of P r e c a m b r i a n crust by e r a / e o n d o n o t reflect o r i g i n a l g r o w t h r a t e s o f the c o n t i n e n t a l crust. R a t h e r t h e y reflect a c c u m u l a t e d o r o g e n i c h i s t o r i e s b y e r a , t h e y o u n g e r e r a s g e n e r a l l y g a i n i n g at t h e e x p e n s e o f the o l d e r . In this r e g a r d , the r e m a r k a b l e f e a t u r e o f t h e c o n t i n e n t a l crust is the survival o f significant A r c h e a n
* Permanent address: Department of Geology, University of Toronto, Toronto, Canada M5S IA1. 499
Pfecambrian Era and Eon (%) Proterozoic Era
Precambriat7 Platform 003 km 2 }
A. Late J B. Mid- C. Early (0.6I (1.0(1.8t.0 Ga) 1.8 Ga 2.6 Ga)
D. Archean Eon (> 2,6 Ga)
A. Exposed Crust Only in P/atform EUROPE
(1,595)
35
17
28
20
NORTH AMERICA
(5,969)
10
;)3
37
30
SOUTH AMERICA AFRICA Total Atlantic
Region B.
(5.366)
33
36
15
16
(10,684)
54
8
18
20
(23,614)
36
19
23
22
Combined Crust (exposed + buried) in Platform 45
11
20
24
NORTH AMERICA
EUROPE
(19,470)
(7,572)
4
30
49
17
SOUTH AMERICA
(12,969)
48
28
17
7
AFRICA
(28,381)
75
6
7
12
Total Atrantic Region
(68,392)
46
18
22
14
Table 1. Areal proportions by era/eon of exposed and buried Precambrian crust in Precambrian continental platforms, Atlantic region. (A) Exposed crust only; (B) Combined (exposed + buried) crust.
500
A . M . GOODWIN
and Early Proterozoic crust (14 and 22% respectively, Table 1), survivals requiring enduring cratonization of early-formed Precambrian crust. These high survival rates may be reasonably attributed to early development of deep sub-shield tectospheric roots in the growing continental masses, roots of demonstrated resistance to ensuing tectonic assault, (Goodwin, 1985).
PRECAMBRIAN PROPORTIONS
Areal proportions by Precambrian continentalplatform Areas of exposed only Precambrian crust in the four Precambrian platforms of the Atlantic region are listed in Table 1A. Thus the total exposed Precambrian crust amounts to 23,614,000 km 2. Of this amount Africa contains 45.2%, by far the largest proportion; North America, 25.3;% South America, 22.7%; and Europe, 6,8%. Areas of total (exposed and buried) Precambrian crust in the Atlantic region by platform are listed in Table lB. Thus the total Precambrian crust amounts to 68,392,000 km 2. Again, the four platforms listed in order of decreasing amount, with areal proportions in brackets, are: Africa (41.4%), North America (28.5%), South America (19.0%), and Europe (11.1%). Thus the proportions of exposed and of total Precambrian crust by continental platform correspond closely. In summary, Africa contains slightly less than half the total; the two Americas each about a quarter; and Europe about one-tenth.
Proportions by Precambrian era/eon The proportions of exposed only Precambrian crust by era/eon (A, B, C, D in Table 1) in the four combined Precambrian continental platforms of the Atlantic region are listed in Table 1A. The combined proportions are A : B : C : D = 36:19:23:22. That is to say, the proportion of Late Proterozoic crust (era A) is highest at 36%; and of Mid- and Early-Proterozoic crust) (eras B and C respectively) and of Archean crust (eon D) are each about equal at 19, 23 and 22% respectively. These proportions are very close to those of the earth's total Precambrian crust (unpublished data) i.e. including all Precambrian platforms of the world. Considered by individual continental platform, significant differences in the proportions of exposed Precambrian crust by eraJeon emerge. Firstly, however, Europe (A : B : C: D = 35 : 17 : 28 : 20) corresponds closely to the combined Atlantic region average discussed above. On the other hand, North America (A : B : C : D = 10: 23 : 37 : 30) is especially high in both Early Proterozoic (C) and Archean (D) crust, and very low in Late Proterozoic (A) crust. South America (A: B : C: D = 33 : 36: 15 : 16) is exceptionally high in Mid-Proterozoic (B) crust and low in both Early Proterozoic (C) and Archean (D) crust. Africa (A : B : C : D = 54: 8 : 18 : 20) is exceptionally high in Late Proterozoic
(A) crust, very low in Mid-Proterozoic (B) crust, and about average in both Early Proterozoic (C) and Archean (D) crust. The proportions of combined (i.e. exposed + buried) Precambrian crust by era are presented in Table lB. For the total Atlantic region, i.e. the four combined Precambrian platforms, the proportions are A : B : C: D = 46 : 18 : 22 : 14. Thus inclusion of buried Precambrian components (i.e. Table 1B versus Table 1A) significantly (1) increases Late Proterozoic (A) proportion and (2) decreases Archean (D) proportion, without however substantially altering the other two (i.e. B + C) proportions. Otherwise stated, buried and exposed Precambrian crusts are respectively rich in Late Proterozoic and Archean components, but uniform in Early- and Mid-Proterozoic components. Considered by Precambrian platform: (1) Europe, at A : B : C : D = 45 : 11 : 20: 24, again is close to the proportions present in exposed crust (Table 1A) with only modest gain in Late Proterozoic (A) and loss in MidProterozoic (B) proportions; (2) North America, at A : B : C : D = 4 : 30: 49 : 17, shows a dramatic drop in Late Proterozoic crust, an equally dramatic gain in Early Proterozoic crust, and a substantial gain in MidProterozoic crust; (3) South America at A: B : C : D = 48:28:17:7, is comparatively rich in crust of the two later Precambrian eras (A + B), and low in that of the two earlier eras (C + D) notably the latter; and (4) Africa, at A : B : C : D = 75 : 6: 7 : 12, has a remarkably high Late Proterozoic content, and correspondingly low Early- and Mid-Proterozoic contents. Geologically, these results express the following geologic relations: (1) the European Platform contains a more or less average global mix of Precambrian crust, both exposed and buried; (2) the North American Platform with abnormally high (C + D) proportions, contains both (a) the unusually large Early Proterozoicdominated Churchill + Bear + Southern + Rinkian + Naqssuqsoqidian + Ketilidian belts and (b) unusually large Archean domains, notably Superior Province; (3) the South America Platform with high A + B proportions, includes (a) the large Mid-Proterozoicdominated Rio Negro-Juruena and Rondonian belts plus significant adjoining coeval platform cover and (b) the Late Proterozoic-dominated Atlantic Shield with widespread Brasiliano (Late Proterozoic) imprint; and (4) the African Platform, with high A proportion, is dominated by Late Proterozoic rocks in the form of both (a) widespread Pan-African mobile belts, and (b) extensive tabular to mildly folded platform cover e.g. Taoudeni and Congo Basins.
Proportions of Precambrianplatforms and Phanerozoic belts The sizes of the four Precambrian platforms by continent and of their adjoining combined Phanerozoic mobile belts and platform cover, which collectively constitute the complete continents, are listed in Table 2. Thus the total combined continental crust in the Atlantic
A s p e c t s of Precambrian cyclic growth in the Atlantic region
~
~
501
x
;
LEGEND 600 Ma
i~i~ ~..
a ~ b
PRECAMBRIAN A 1000Ma
a~;~b
PRECAMBRIAN B 18OOMa
a~b
PRECAMBRIAN C 2600Ma
a ~ b 0 I
1000 I
2000km I
a _outcrops / b. covered crust
PRECAMBRIAN D (Archean)
Fig. 1. Preliminary sub-Phanerozoic geologic map of Precambrian continental platforms in the Atlantic region (BuUard et al., 1965 reconstruction). Peripheral Phanerozoic fold belts are shown by coarse rectangular pattern. Precambrian inliers in the Andrean Belt are of uncertain age. &gS
7:2-N
502
A . M . GOODWIN
Continent
Precambrian Platform
Phanerozoic Mobile Belts
Size Ratio ' 03km 2 }
Size I Ratio [103 km2} ;
Combined Continental Crusl Size [10 a km 2 ) Ratio
EURCPE NORTH AMERICA SOUTH AMERICA AFRICA
7,572 19.470 12,969 28.381
1 2.6 1.7 3.7
15.846 13.122 4,152 1.396
1 0.8 0,3 0.1
23,418 32.592 17.121 29,777
1 1.4 0.7 1.3
Total
68.392
9.0
34,516
2.2
102,908
4.4
Table 2. Relative size by continentsof (1) Precambriancontinental platforms, (2) adjoining Phanerozoic mobile belts, (3) combined continentalcrust. region is 102,908,100 km 2 which comprises, in decreasing continental size: North America followed closely by Africa, then Europe, and South America. Phanerozoic belts of Europe (Caledonides + Hercynides + Alpides) occupy the largest area at 15,846,000 km 2, and those of Africa (Cape + Mauritanides + Atlas) the smallest at 1,396,000 km2; North and South American belts are intermediate in size as shown. In all four continents but Europe the Precambrian platform is substantially larger than the enclosing or adjoining Phanerozoic mobile belts; this is especially so in Africa. In brief, each continent is an aggregate of "interior" Precambrian platform (partly covered) of designated construction by era/eon, and of peripheral Phanerozoic mobile belts, the Precambrian and Phanerozoic proportions varying by continent.
CYCLES IN CONTINENTAL GROWTH Gastil (1960) established that periods of abundant global igneous and metamorphic dates, corresponding to periods of orogeny, are about 210 Ma in length, and alternate with like periods of mineral date scarcity (tectonic quiescence) for a mean 417 Ma-long cyclic distribution pattern extending back to 2600 Ma. This pattern, based on a sample of about 400 K-Ar, Rb-Sr, and U-Pb dates, is considered to correspond to long cycles in earth's orogenic history, the peaks in the number of radiometric ages corresponding to terminal events of the major crustal processes. Dearnley (1965) soon expanded the investigation on a sample of 3400 mineral ages determinations. A cumulative curve from his histogram showed three particularly well-defined changes of slope at ca 1950, 1075, and 180 Ma, and two other less abrupt changes at 2750 and 750 Ma. All the changes were regarded as signifying the onset of worldwide tectonic regimes each with a duration of several hundred million years. From a survey of age determinations relating to rockforming events, particularly Rb-Sr whole rock isochron ages and initial 87Sr/86Sr ratios, Moorbath (1976) suggested that short periods of accelerated crustal growth may have occurred episodically and that they may correspond with the groupings of relevant radiometric dates which are approximately 3800-3500; 2800-2500; 1900-1600; 1200-900; and 500-0 Ma ago. Condie (1976) surveyed global dates and concluded that world-wide Precambrian orogenic periods are
episodic averaging 200 to 400 million years in length and occurring about every 500 to 600 million years. Major orogenic periods occur at 3.8-3.0, 2.7-2.5, 2.0-1.5, 1.2--0.9, and 0.7--0.5 Ga. Earlier, York and Farquhar (1972) averaged the estimates of significant dates in Earth's history by numerous authors, and arrived at the following numbersm2.60, 1.84, 1.09 and 0.57 Ga. These they delightfully called "Magic Numbers", to express the impossibility of assigning their recognition to any one individual, and the constancy of the interpretations during an exponential growth in the number of radiometric ages, but also as a caution to the practice of lumping large numbers of ages into one histogram, and attaching major geological significance to ensuing structures. Moorbath (1984) further cautions that the analysis of histograms of isotopic dates can be subject to serious misinterpretation unless the geological nature and significance of the events under investigation are fully understood. Furthermore, although episodic crustal events may be reasonably wellestablished in certain Precambrian platforms, their global synchroneity is in serious question (Moorbath, 1984). In spite of these reservations, these "Magic Numbers" retain their apparent significance in terms of crustal processes. In a related sense, culminations of the same or related crustal processes are also reflected in the post-Archean paleomagnetic record by "hairpin" turns which appear to mark sudden reversals in the sense of polar motion of continental plates (York and Farquhar, 1972). Furthermore, Cahen et al. (1984), following an exhaustive survey of events dated in Africa, the largest Precambrian continent, select the following as marking "chronological milestones" in African evolution: 3.5, 2.9, 2.5, 2.1, 1.75, 1.1 and 0.57 Ga. For the present purpose, then, the following four dates (each +100 Ma): 2.6, 1.8, 1.0 and 0.6 Ga are used to demarcate the Archean eon, and Early-, Mid-, and Late Proterozoic eras respectively in Atlantic region Precambrian crust (Table 1).
ATLANTIC REGION CYCLES The Atlantic region of the collective EuropeAmericas-African platforms has experienced repeated horizontal crustal oscillations. Including the modern Atlantic opening, at least three (to 1.0 Ga) and possibly four (to 1.4 Ga) coherent Wilson cycles have been tracked, each involving early divergence with supracrustal accumulation followed, in all but the modern cycle, by convergence and orogeny, the complete cycle lasting about 400 Ma (Goodwin, 1985, Table 4). The four cycles are named, in order of increasing age, I-Current (0-200 Ma), II--Appalachian (200-600 Ma), Ill--Pan-African (600-1000 Ma), and IV--Grenvillian (1000-1400 Ma). Still older cycles are named V--Elsonian (1400-1800 Ma) and VI--Hudsonian (1800-2200), the specific names marking the culminating orogenies respectively of the cycles.
Aspects of Precambrian cyclic growth in the Atlantic region
503
For the most part, Late Proterozoic-Paleozoic crust therefore cause changes in the patterns of mantle concycles (II-III) are readily interpreted in terms of modern vections. Runcorn considered that at the time of tranplate tectonic processes involving ocean floor consump- sition from one mode of convection to the next higher, tion and subduction zones with active and passive conti- the continents would be under great stress and those nental margins. Even within these cycles, however, transitions at critical values would be manifest in the whereas certain 1000-600 Ma-old belts e.g. the Pan- geologic record. Sutton (1963) postulated the existence of chelogenic African Pharusides and Hijaz, carry unmistakable Wilson cycle signatures involving ophiolites, aulacogens cycles consisting of a sequence of events leading to the and calc-alkaline extrusive and intrusive rocks, other early disruption of the continents and later to the coeval belts in Africa and South America, e.g., Dama- regrouping of these disrupted fragments of continental ran-Katangan, Ribeira, and Paraguay-Araguaia, are crust. A cycle is considered to last for 750 to 1250 Ma. More recent models link continental cyclicity directly more readily interpreted as predominantly if not completely ensialic in origin, thereby suggesting a somewhat with the plate-tectonic process. Worsley et al. (1984) summarize a "non-random" crustal model to account for different perhaps modified plate tectonic origin. Mid-Proterozoic (B) crust (cycles IV-V) is charac- long term tectonic cyclicity. Plate-motion is attributed to terized by (a) widespread anorogenic magmatism (anor- a thermal instability mechanism (Busse, 1978) resulting thosites, rapakivi granites, rhyolites) (b) continental from the repeated assembly of supercontinents (Pangea) rifts and aulacogens together with (c) major mobile that never completely disperse. A pattern of plate tecbelts, both marginal "accretionary" (e.g. Grenville, tonic cycles, each cycle of about 400 Ma duration, is Rondonian) and "interior", some of the latter appa- recognized back to 2000 Ma. The popular "random" rently ensialic (e.g. Kibarides, Espinhaco). The nature plate motion model (Anderson, 1982), however, advoof the operating plate tectonic process(es), probably cates that Mesozoic Pangea represents assemblage of continental fragments dispersed from a still-earlier complex, is highly controversial and uncertain. Early Proterozoic (C) crust (cycle VI plus still older supercontinent centred on the Pacific Ocean, and now post-Archean units) features highly deformed, inter- marked by the Central Pacific residual geoid high. Le Pichon and Huchon (1984), in turn, interpret mediate to high grade metamorphic gneiss terrains together with commonly asymmetric fold belts which evidence pertaining to the geoid and supercontinent in appear, more often than not, to be at least partly super- terms of a weak coupling of (a) a separate steady-state posed on an ensialic basement (e.g. Labrador belt). lower mantle, which is responsible for the present geoid, Some belts, however, (e.g. Coronation and Svecofen- to (b) upper mantle convection leading to hemispheric nian) closely resemble Phanerozoic equivalents includ- continental configuration (Pangea). Pangean configuring local ophiolites. Still others (e.g. Birrimian) contain ation ends when excessive heating of the upper mantle, Archean-style greenstone belts. Most belts are severely due to the insulating continental (i.e. Pangean) cap, deformed as a result of low-angle foreland transport. leads to continental dispersal, the complete cycle from This tectonic mobility was frequently followed by inten- one supercontinent to the next being in the order of sive and repeated granitoid intrusions, commonly with 400 Ma. These models point to regular cyclicityin the evolution ring-structures, generally high-level, often alkaline, and of continental crust. linked to lava extrusion, all conducive to cratonization. Finally Archean (D) crust contains the well-known low-to-medium grade granitoid-greenstone belts and PALEOMAGNETISM adjoining higher grade gneiss-migmatite terrains, commonly granulitic. Up to three generations of greenstone Paleomagnetic studies of Precambrian rocks offer the belts are known in some regions. In some platforms, repeated "accretion-differentiation" episodes, potential of providing constraints on the paleorepresenting major builders of continental crust, occur- reconstruction of continental masses during Precambred in near-continuous succession. Widespread tonalite- rian time and, thereby, on the formative tectonic procestrondhjemite-granodiorite (TTG) plutonism led to ses. Unfortunately, as illustrated below, the present rapid growth of at least locally stable cratons. In keeping state of the studies does not provide unequivocal answers with its antiquity and uniqueness, the nature of the plate to many critical problems. Piper (1976) proposed that during much of the Protectonic processes is largely unresolved. terozoic paleomagnetic results are consistent with a fixed position for Africa immediately "southwest" of Laurentia. Piper (1983) re-examines the case for a ProMODELS FOR CONTINENTAL CYCLICITY terozoic supercontinent and contends that both the Runcorn (1962) was an early exponent of correlating polarities and positions of the Proterozoic paleopoles mineral date peaks with changes in mantle convection from the major Precambrian shields conform to a single patterns. Thus the pattern of convection to be expected "apparent polar wander path" (a.p.w.p.). These data in the earth's mantle is determined by the ratio (n) of the imply that the continental crust was amassed together as radius of the earth's core to the radius of the earth. With a single large lens-shaped supercontinent from late a slowly growing core, a gradually changing n would Archean time to ca 600 Ma ago.
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However, Morris et al. (1979) contend that the same data are consistent with Africa situated immediately "southeast" of Laurentia during the Proterozoic (as in the Bullard et al. (1965) reconstruction). Embleton and Schmidt (1979) have argued that data for much of the Proterozoic are consistent with Africa and Laurentia in their p r e s e n t relative positions. Irving and McGlynn (1979) believe, however, that the data provide few constraints and, in particular, that there is no basis for the proposition that the continental crust was assembled into a single unchanged Pangea during the ca 2200 to ca 1300 Ma period. Furthermore, Dunlop (1981) contends that Laurentia, Africa and Australia, each internally assembled as we know them today by early Proterozoic time, drifted largely independently, and not as a coherent supercontinent, during the Proterozoic. He further contends that some major "interior" provinces, e.g. Churchill, are ensialic in origin; others, e.g. Bear, may have accreted marginally. McElhinny and McWilliams (1977) likewise agree that the evidence for a Proterozoic supercontinent is no longer tenable. They further contend that, although the paleomagnetic data are considered to preclude platetectonic models involving the convergence of previously widely separated cratons to explain Proterozoic provinces such as the Churchill, the data do not exclude models involving the opening and closing of small (5001000 km) intercratonic oceans, provided that the cratons return to their same relative positions following ocean closure. Finally Irving and McGlynn (1981) contend that paleomagnetic results from Laurentia are in disarray in the interval 2300 to c a 1650 Ma and that it is not possible to determine whether the component cratons (Slave and Superior) had separate or common a.p.w.p.s, and hence whether or not they moved relative to one another, prior to and during the Hudsonian Orogeny. They conclude that the idea of the operation of plate tectonic processes in the Proterozoic is not contradicted nor proven by the paleomagnetic evidence from Laurentia.
CONCLUSIONS Africa alone contains almost 50% of the total exposed and buried Precambrian crust in the combined Precambrian platforms of the Atlantic region; North America and South America each contain about 25%; and Europe contains about 10%. Considered by era/eon (i.e. Archean, Early-, Midand Late-Proterozoic), Africa contains exceptionally high Late Proterozoic and very low Mid-Proterozoic proportions. North America (including Greenland), on the other hand, contains exceptionally high Early Proterozoic and very low Late Proterozoic proportions, the reverse of the African situation. The reasons for these crustal imbalances by continent though basically unknown, are related to the relative abundances of Late Proterozoic (Pan-African) and Early Proterozoic (Hud-
sonian) fold belts in the southern and northern continents respectively. Regarding radiometric ages and continental cycles, despite an exponential increase in the data, the interpretations have remained strikingly constant. Thus certain dates, notably 2.6, 1.8, 1.0 and 0.6 Ga, may mark recurring long-term orogenic cycles. Although cyclicity in a particular continental block may be readily established, synchroneity between continents is uncertain. In this vein, the Atlantic region, represented by the collective Europe-Americas-African platforms, appears to have experienced repeated (at least 6) horizontal crustal oscillations or cycles, each cycle of about 400 Ma duration. Cycles 1-3, back to about 1.0 Ga ago, are readily interpreted in terms of the Wilson Cycle; and cycles 4-5 less readily so. In the older cycles, however, the nature of the operating plate tectonic processes is highly controversial and uncertain. Major unresolved problems remain. Current models for continental cyclicity advocate a thermal instability mechanism involving excessive heating of the upper mantle due to the presence of an insulating supercontinental (Pangea) cap. This leads to supercontinental dispersal, followed, by either "nonrandom" or "random" motions of the continental fragments, before eventual reassemblage. The complete cycle from one supercontinent to the next is in the order of 400 Ma. The current disarray in paleomagnetic interpretations together with other fundamental problems, demonstrate that a new level of interdisciplinary studies is required, involving coordinated field, isotopic and paleomagnetic studies. Rigorous integration of these disciplines is required for meaningful interpretations.
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