Revista Brasileira de Geocíências 12(1-3): 8-14, Mar.-SeL, 1982 - São Paulo

MOBILE BELT-CRATON TECTONIC RELATIONSHIPS lN PRECAMBRIAN GONDWANALAND: MEGA-ANALOG OF STRUCTURAL FEATURES FOUND lN SHEAR AND MYLONITE ZONES?

MICHAEL B. KATZ'

ABSTRACT Archean cratons-Proterozoie mobile belts mega-structural relationships in Pre~ cambrian Gondwanaland can be eompared to meso-microstructures found in ductile mylonite and shear zones. The large Arehean cratons are comparable to the much smaller scale, porphyro­ clasts in shear and mylonites. The late Archean-Proterozoie mobile belts resemble large seale equivalents of the fine-grained. ductile matrix of shear and mylonite zones. The relationships are 6 excellent examples of high strain, deformation of vastly different magnitudes (in the order of 10 ). The processes of deformation within a shear 01' mylonite zone have been described in some detail and may serve as a deformation scalc model for craton-mobile belt relationships. The craton-mobile belts recognized in Precambrian GondwanaJand and Austrália are analys­ ed using whole rock strain methods that are usually applied to meso-microstructures. This assumes simple, ideal eonditions of deformation, of originally circular era tons. The particle (01' craton} -centre to centre technique ~applied on the mega-scale suggests that the greatest relative extension in Precambrian Gondwanaland is between the South American and Australian era tons. Within Precambrian Austrália a similar exercisc results in greatest extension betwcen Pilbara and Gawlcr cratons. Thesc results lead to lhe development of a concept that the Archcan cratons were once originally part of a larger craton unit , 01' were close neighbours. and have been subsequently def­ ormed and disrupted in their transform mobile belt matrix, in late Archean-Proterozoic times.

INTROOUCTION The teclonic evolution of the conti­ and relationships of cratons to mobile belts, ean be under­ nental crust in the early Precambrian has been described on taken using techniques developed for small seale whole the basis of relatively passive Archean cratons penetrated rock strains. and disrupted by late Archean-Proterozoic marginal mobile Preeambrian Gondwanaland was selected for investígat• belts (Sutton and Watson, 1974)" This process of deform­ ion, as this supercontinent has been reeognized, by paleo­ ation can be described as intraplate teclonics where the magnetics, as a relatively coherent unit for rnost of the late Archean-Proterozoic mobile belts can be considered as, .Precambrian (Piper , 1976; Embleton and Schrnidt , 1979). ensialic transform zones of high strain, geometrieally des­ Some paleornagnetic models incorporate North America in eribing small eirele traces about apoie of rotation (Katz, Gondwanaland, but it isnot included inthisexercise because 1976,1980,1981). The older Archean cratons are separated of its uncertain position, Other models suggest intercontin­ and penetrated by these mobile belts and these cratons are ental, as well as intereratonic, movement and drift. APWP disrupted and internally deformed. The processes involved interpretations pro pose an East and a West Gondwanaland, in the deformation of global seale cratons and mobile belts that eonverged in the late Precambrian-Early Paleozoie (Pan ean be eompared to lhe development oflocal, ductile, mylon­ African event) (McWilliams, 1981), and other interpretat­ ite and shear zones. ions suggest large displaeements and rotations that result Although these structures are of vast1y different magni­ in a eomplieated tectonie evolution of Preeambrian Gondw­ 6 tudes (inthe order of 10 ), the similarities in structure bet­ analand (Onstott and Hargraves, 1981). These models ween these different scales are interpreted in terms ofsirnilar suggest that the present distribution of cratons and mobile deformation and strain parameters (e.g. Tchalenko, 1970). belts does not, neeessarily. reflect the original Precambrian Thus, the porphyroelasts of mesomicroscale shear and distribution. However, these other palaeomagnetic models mylonite zones are similar to the mega-scale cratons. The do not explaín the single continent, eoherence and continuity ductile matrix of the shears and mylonites are similar to the ofvarious Arcbean-Protcrozoic tectoniclinesintheGrondw­ straightening zones found in mobile belts. Assuming simpie, analand reassernbly (e.g. Hurley and Rand, 1969; Engel ideal conditions, deformation analysis of large areas of and Kelm, 1972), whieh are considered to be original feat­ continental crust, on the basis of the distribution, geometry ures. The single supercontinent model for Preeambrian Gondwanaland is adopted in this scheme, but this model does not deny the role of cratonic, "srnall scale" disp1aeem­ ** The terms craron and mobile belt are uscd in the sense defined ents and rotations, and openings and closings, of a Iimited by Anhaeusser et ai. (1969): "Craton is used to describe stable nuclei withir'Í shield arcas which consist of complex granitic terrains style, of a Wilson eyc1e. incorporating early Precambrian greenstonc bclts (... ). Mobile belt is uscd to describe lhe youngcr. linear, ... ctamorphic belts which OISTRIBUTION ANO GEOMETRV OF CRATONS­ tend to surround lhe ancicnt cratonic nuclei of shield arcas and MOBILE BELTS The general shape, size and distribut­ which are characterized by high grade metamorphisrn. granitiza­ ion ofArchean era tons and their intervening marginal mob­ ticn and often by transcurrent dislocauon (... I". ile belts in Precam brian Gondwanaland have been describ-

'" School of Applied Gcology, University of New South Wales, Kensington, N.S.W., Australia Revista Brasileira de Oeocténctas, volume 12 (1~3). 1982 9 ed by Hurley and Rand (1969) and Katz (1974). Their spe­ This postulatedextension results in a more linear pattern cific locations, geometry and geology are given by Wernick for Precambrian Gondwanaland, with a length-width of and Almeida (1979) for South America; Krôner (1977) about 20,000 km and 7,000 km, respectively. for Africa; Naqvi, Rao and Narain (1979) for India; Ravich and Kamenev (1975) for Antarctica; and Plumb (1979) for DEFORMATION MODELS This global deformation Austr""ia (Fig. I). ln general, the cratons (blocks) have process can be compared to the development of a shear or elliptical (sigmoidal) forms and cover areas, as much as mylonite zone, where the originally undeformed, coarser­ lOó km'. The intervening, marginal mobile belts sweep and -grained, isotropic textures are progressively modified into flow around the cratons (e.g. Katz and Premoli, 1979), and an anisotropic, finer-grained, linear zone of high strain. ln are as much as 200 km wide and 1,000 km long, The original the mylonite, the coarser members are fragmented, separat­ shapes of the Archean era tons are not known for certam, ed and deformed into eIliptical relicts known as porphyroc­ but recent work suggests an initial, basin-like tectonic setting lasts. This process, affecting the coarser elements, is (Goodwin, 1977). These basin-craton complexes are now. similar to the ductile deformation in the finer grained, strain mainly. subcircular to elliptical. but this elongation is not softened, mylonite matrix expressed as a penetrative foliat­ believed to be an original feature, but due lo substantial ion, which sweeps and tlows around the relict porphyr­ deformation of originally more circular, basin-craton com­ oelasts (White et ai., 1980). Thus, the porphyroclasts can plexes in the late Archean-Proterozoic (Park, 1980). be considered to be deformed, more or less homogeneously, Hurley and Rand (1969) recognized that these cratons with its matrix, developing a resultant whole rock strain. formed a coherent group, but they were not certain of their This scale model analog compares the development of the original shape or whether they were originally a single rnylonite porphyroelast to lhe craton (cratonoelast) and the nueleus. The present distribution of the cratons in Precamb­ mylonite, foliated, matrix to the mobile belt (mobile belt rian Gondwanaland can be approximated by a large ellipse matrix). whose major axis extends from the Guyana craton of South The structural relationships between the porphyroclast America to the Australian cratons. and whose minor axis and the matrixcan be com plicated, as there rnay be porphyr­ extends down the length of Africa (Fig. I). This distribut­ oelast rotation, fracture, as well as plastic distortion, dep­ ion rnay suggest ao original, morecircular pattern ar nucleus ending on the PT conditions and competency contrasts. in the Archean, which has been subsequently deformed and ln a similar manner, the sigmoidal shape of the Zambia fragmented and separated by mobile belts in the ·Iate Arch­ craton (Krôner, 1977) suggests rotation and the elliptical ean-Proterozoic, ln this scheme, the direction of greatest shape of the Rhodesian craton (Krõner, 1977) suggests extension or strain is parallel to the major axis ofthe ellipse. distortion.Cratonic lineamentsjwhich may be fracture contr-

I \

) J

Figure J - General distríbution of Archean cratons (shaded) and marginal, íntervening mobile belt matrix trcnds (Ilow Jines) ln Precambrian Gondwana/and. Thedístributíon carl be containedlnan ellípse whosemajoraxts extends Iram theGuyanacraton 01South America to the Austra­ /ian cratons and whose minor axís extends down the Iength of East A/rica 10 Revista Brasileira de Gecciênàias, Volume 12 (1·3), 1982 olled, such as greenstone belts, dykes and linear granites Inlier (A) have some cratonic properties and are tentatively may be evidences for internal, more brittle, cratonic failure considered as cratons in this exercise (Fig. 2). The problem (Katz, 1976). The geometry ofthe mylonite matrix foliation of separating cratons from mobile belts are in many cases may also be cornplicated by the PT conditions and the hom­ a difficult task, due to poor exposure and lack of detailed ogeneity of the deformation. Apparently, intersecting mo­ geological and geochronological data. Some ofthe divisions bile belts seen in the East African Shie1d (Krõner, 1977) are arbitrary and are based on the, often, incomplete inforrn­ may be a case where earlier directions ofmobility are overpr­ ation available. inted by oblique directions during changing deformation The cratons are surrounded by (younger) mobile belts. conditions akin to strain hardening structures observed in For example, the Yilgarn (Y) craton is bounded on the south mylonites (White et ai., 1980). Bearing ali these complic­ and southeast by the Albany Fraser mobile belt and on the ations in mind, the strain model utilized is substantially north by the Gascoyne mobile belt. The exact configuration simplified and assumes ideal conditions of deformation of of these marginal mobile belts are not known surely for initially circular cratons. The whole Gondwanaland Prec­ they are covered up, in many places, by more recent ambrian strain is considered to be homogeneous 00 this sediments. Their general trends, by extrapolation, seem to very large scale, although smaller scale local strains in ind­ indicate a continuous flow matrix, which can also be det­ ividuai cratons. and mobile belts may be inhomogenous. ected in gravity maps (Wellman, 1976). The present day Australian cratons have been somewhat fragmented by cont­ STRAIN ANALYSIS The distribution ofcratons and inental drift but, in general, they form elliptical (sigmoidal) mobile belts in Precambrian Gondwanaland can be cornp­ shapes (Fig. 2). ared, in the ideal case, to the small-scale distribution of The craton centre to centre technique, with the tangent strained elliptical partieles, such as deformed oolites, in a criterion, is utilized to evaluate the whole Austrahan Shield ductile deforming matrix. Analytical methods developed, strain. The tangent criterion applied to the craton distribut­ for small scale, whole rock strains rnay be used to estimare ion, indicates that the oldest unit, the Pilbara (P) craton, global, cratonic strains. ln particular, the partiele (craton} was probably the centre of the original Archean cratonic -centre to centre technique described by Ramsay (1967) and nucleus, as it neighbours most other cratons (Fig. 3). Two Pfiffner (1980) can be utilized, An important assumption groups may be recognized, a Pilbara (P), Yilgarn (Y), Gawler is made that the era tons were initially circular in shape and (O), Musgrave (M) group, and an Arunta (A), Kimberley distributed equally, or had a known unequal distribution. (K), Pine Creek Basement (PCB) group (Fig. 3). From the This method affords a valuable check on the other assurnpt­ centre to centre (d), (a) plot of the distribution ofAustralian ion that the cratons are deforming homogeneously with cratons (Fig. 4), the directions of maximum (a, = 110")and Q their mobile belt matrix. Evidence for this assumption can minimum ((X2 = 20 ) extensions can be deterrnined, as well be gleaned from structural studies on mobile belt-craton as an estimate of the ratio of principie strains (dX/dY = 3). relationships which indicate, in cases like the Limpopo­ The plot of (d), (a) also suggests that-the originally, more -Rhodesian, Kaapvaal situation, that the cratons are deforrn­ coherent (circular), cratons have been deformed hornoge­ ing, quasi-homogeneously, with their mobile belt matrix neously, with their mobile belt matrix. These (d), (a) values (Coward, 1980). are presented in Fig. 2; (dX/dY) as major and minor axes The centre to centre method consists of measuring dist­ of, and (a" a2) defining the orientation of, a strain ellipse, ances (d) between the centres of originally neighbouring here, schernatically representing the resultant Late Archean­ cratons and are plotted up as a function of the direction of -Proterozoic deformation ofthe Australian Shield. This spe­ the centre to centre line (a). ln undeformed material the culative strain ellipse, apparently, parallels the elongation points on the graph would scalter along some mean distance, of the Pilbara (P), Musgrave (M) and Arunta (A) cratons but if the material is deformed, the distances between adjac­ and, in general, is conformable with the shapes and distrib­ ent craton centres are altered and plots of (d) against (a) utions of the other cratons and their mobile belt matrices show a distribution whieh is proportional to the amount (Fig. 2). of longitudinal strain in the direction (a). From the graph, Some internal late Archean-Proterozoie cratonie tensional directions of maximum and minimum extension (a,. (X2) features, such as the basic dykes in the Yilgarn (Y) craton and the ratios ofprincipal strains (dX/dY) can be determined. (shown as dark dashed lines in Fig. 2) are not oriented norm­ One problem with this technique is that it may be difficult al to the direction of maximum extension, as would be to decide whieh era tons were nearest neighbours in the oríg• expected. This may suggest possible, internal, shear fracture inal state. This may be partly overcome by using a tangent control, or rotation, of about 90", of the Yilgarn craton, construction technique as acriterion (Pfiffner. 1980). Tang­ which does have a rough, sigmoidal formo As further disc­ ents are drawn from the centre of the cratons to the neighb­ ussed below, the results of this analysis may not represent ouring cratons. If the tangent lines to further removed the whole Australian Precambrian Shield strain, as it does era tons cut across one or more of the neighbouring cratons, not take into account the distribution of conjugare cratons then these further removed era tons are not considered as in the Indian and Antarctic Shields. neighbours. ln these cases (d) values are not measured. Strain analvsis of Precambrian Gondwanaland The era­ Strain analvsis of the Australian Precambrian An exam­ too centre to centre technique, with the tangent criterion, is pie of this strain analytical method can be demonstrated on now applied to the Oondwanaland Precambrian (Fig. 5). the distribution of cratons-mobile belts of the Australian The craton-rnobile belt distribution of this very large area Precambrian. According to Plumb (1979), at least three is ao approximation, in places, arbitrary, and their locations Archean cratons are recognized; the Yilgarn (Y), the Pilbara are based on the often incomplete, available data. Areas (P) and the Gawler (O), with the possibility of one other, of questionable nature are indicated in Fig. 5 and are, esp­ the Kirrrberley (K). Three other blocks, the Pine Creek ecially, evident in East Antarctica. The tangent criterion, Basement (PCB), the Musgrave Block (M), and the Arunta and many of the large vaiue centre to centre measurements RevistaBrasileira de Geoctências, Volume 12 (1-3),1982 11

PCB

Figure 2 - Dístríbutíon of cratons (outlined) and mobile belts (flow !ines) in 'he Australían Precambrian. Strain analysis results ín a struín eílipse with maxímum (o:, = llOf» and minimum (0:2 = 20f» extensions and ratío ofprincípal sttains (3.00) (Figs. 3 and 4). Y (Yilgarn), P (Pilbara), K (Kimberley)"G (Gawler), M (Musgral'es), A (Aruma), peB (Pine Creek Basements (heavy dashed linesin Y, basic dykes)

through these questionable areas are fraught with error. The results for Precambrian Gondwanaland, covering a However, even with these limitations, the results of the (d), very large area, are not comparable, or similar, to the results (a), plot (Fig. 6) seem reasonable. The questionable values obtained from the Australian Precambrian. The Australian seem to fit on the (d), (a) curve, although they are not used Precambrian may only be a sub-sarnple of a larger cratonic in determining the estimated values for (a" a,) and (dX/dY). nucleus, including the Indian and possibly the Antarctic crat­ The tangent criterion suggests various cratonic near ons. Results from only this Australian sub-sample would neighbours or groupings, South-American-West African, a showa highly biased result. Examination of Fig. 5 indicates well-defined African and an Indian-Australian groupings that the (d), (a) values for the Australian Precambrian would may be recognized (Fig. 5). These groupings may indicate be much different if the Indian cratons were included in three original, more coherent, supercratonic Archean nuclei the analyses. Also in this particular exercise the Musgrave (Fig. 7) which have been subsequently deformed and sep­ and Arunta cratons of the Australian Precambrian were arated in their mobile belt matrix, in late Archean-Prote­ not considered. The whole Precambrian Gondwanaland rozoic times. The·(d), (a) plot indicares that the cratons hàve strain analysis is, however, quite compatible with the oriento been deformed homogeneously in their mobile belt matrix. ation of late Archean-Proterozoic dykes in the Yilgarn (Y) The values of the direction of maximum (a, = 100') and craton of the Australian Precambrian and, in general, with minimum (a, = 10') extensions and the estimate of tlie the trends of similar dated major dykes (e.g. Great Dyke) ratio of principal strains (dX/dY = 4.5) for Precambrian in other era tons, which line ltp, sub-parallel.jo the tensional Gondwanaland are shown schematically in Fig. 7. The ratio direction of the strain ellipse (Fig. 7). of principal strains is indicated by the major and minor axes ofa strain ellipse, with (ad and (a,) defining the orient­ PROCESSES Of DEfORMATION Assuming that ation. This strain ellipse is, in general, compatible with the craton-mobile belt distribution, ofPrecambrian Gondw­ the craton shapes and distributions, and the configurations analand,.is the result ofthe deformation ofa more coherent of the mobile belt matrices. Thus, the analytical results Archean cratonic nucleus, or at least three neighbouring' compare favourably with the qualitative distribution pattern cratonic nuclei, some speculations on the processes of this defined in Fig. I. deformation can be entertained. The whole Precambrian 12 Revista Brasileira de Geociências, Volume 12 (1.3), 1982

Figure 3 - Constructio,:/ of craton, centre to centre and tangent fines for the Australian Precambrían. Tangem criteríon indicates that the Pil­ bara (P) craton was probably lhe nucleus of a larger cratonic group, as most of lhe other cratons can be considered near neíghbours (see text for details]

Gondwanaland strain analysis indicates that the, postulated, originaJly circular, cratons or cratonic groups deformed, more or less, homogeneously with their mobile belt matrix, The deterrnination of the strain eJlipse, using the centre to centre technique, results in a maximum extension from the Guyana craton to the Australian cratons, and a minirnum extension along the East Africa era tons, under modera te strains (Fig. 7). This deformation, on this global scale, could be a result of pure shear strain or shortening, by crustal forces oriented paraJleJ to the direction of minimum extens­ ion, or lengthening, by crustal forces oriented paraJlel to the direction of maximum extension. A more reasonable possibility would be, to consider, a simpie shearstrain deformation, by crustal shear couple forces oriented at oblique angles to the deformation axes. This simple shear strain deformation could be related to transforrn-type mobile belts that are beJieved to penetra te Precambrian Gondwanaland (Katz, 1980, 1981). At least two important transform mobile belts have been postulated o 20 40 60 80 100 120 140 160 180 for this area; the Limpopo-Albany Fraser (L-AF) and the r}. Kasila-Goiás-Namaqua (KGN) structures. Their distribut­ íon, geometry and poles of rotalion are shown in Fig. 7 and additional small-circle belts are also indicated, about Figure 4 - (d) and (0:) plot for the Australían Precambrian cralans. The (d), (0:) curve suggests that the cratons are dejormíng homogen­ the same poles of rotation. These conjugate, intersecting, eously with theír mobile beít matrix. Estímates can be made for ensialic, shear-type, transform mobile belts are oriented at maximum (0:1 = J/0°) and minimum (0:2 = 20°) extensions and ratío various angles to the direction ofmaximum extension, and o/ principal straíns (dX/dY = 1,500/500 = 3) could conceivably have contributed to the overaJl deformat- Revista Brasileira de Geociências, Volume 12 (1-), 1982 13

Figure 5 ~ Constructíon ofcraton, centre to centretinesfor the Precambrian Gondwanatand. Tangem fines have been omittedfor clarity, howev­ er, lhe tangent críteríon índícates that at teast three cratonic groupings can be recognized (see Fig. 7 and text for details}

large scale development of oceans, deformation of this crust 7 may result in the development of a circular, mosaic of basin­ -cratons, which jostled and adjusted to mantle and crustal 6 forces. The ductile Archean cratons deformed aIong with ""1 = 100 0 5 their mobile belt matrix, which developed along the cratonic boundaries. The relative motion of these cratons on a spher­ d ...... - '"'=' • dX = 4.5 4 ,/. \ ical earth means that a displacement between any two era­ / \. tons can be described by a rotation about apoIe ofrotation. ../ .. ..).. 3 0 "" =10 . /. •\ Relative surface motion between cratons proceeds along 2 • /. .\. "small circles" of rotation, or "transform zones of mobil­ 2 ... I \ ity", As the Archean crust was ductile, the cratons cannot ~/ ... / . ,"- be considered as being, exactly, similar to thick, rigid plates, .. / ...... dY=1 and they were, essentially, deforming homogeneously with their marginal, transforrn mobile belts. The cratons were o 20 40 60 80 100 120 140 160 180 distorted, from their original circular, to more elliptical J., forms and the mobile belts were not developed into, rather narrow, brittle transform faults, but into wide, ductile zon­ Figure 6 - (d} and(lX)pIOI for lheGondwanaland Precambrian cratons. The (d), (IX) curve suggests'that lhe cratons are deforming homogen­ es of mobility. The observation that many of these mobile eously with theír mobile belt matríx. Estímates can be made for belts fit the trace of small cireles about apoIe of rotation, maximum (1X1 = 100°) and mínimum (0:2 = 10°) extensíons and ratío indicates that these mobile belts may be Archean equivalents ofprincipal strains (dXjdY = 4.5). Questíonable values are ploued of transform faults. The cratons can also be considered to (outlined) but not used ín lhe estimates (see Fig. 7 and text for be Archean equivalents of plates. Thus a modified model details) ofplate tectonics may be applied to Archean-early Proteroz­ oic tectonics, where crusta! andmantle forces areaccornmo­ ion. These belts make up the mobile belt matrix along prim­ dated by the deformation of cratonic plates along with their ary small cirele traces, or along secondary shears, fold belts transform mobile belt matrices. Later Proterozoic events and rifts, as has been developed for the KGN structure involved more "rigid" plate tectonic processes which are (Katz, 1981). rellected in the Pan African belts (Krõner, 1980).

SPECULATIVE EARLY PRECAMBRIAN TECTO­ Acknowledgements K. Plumb (Bureau of Mineral NICS If the Archean is envisaged as having a, more or Resources, Canberra) critically read the paper ; however the less, continuous, thin, hot, continental-sialic crust, without author is responsible for the views presented. 14 RevistaBrasileira de Geocténctas, Volume 12 (l~3). 1982

•KGN

• L-A F

Figure 7 - Strain analyses of the Gondwanaland Precambrian resutts in a strain ellípse wíth maximum (ai = 100°) and minímum (

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