himalayan tibetan analogies for the evolution of the zimbabwe craton and limpopo belt abstract the limpopo orogeny which lasted from 2700 to 2600 ma was characterised by nnw sse directed shortening this resulted in both crustal thickening accommodated by folding and nnw directed thrusting and associated crustal extrusion whereby a number of crustal blocks including the central zone of the limpopo belt was displaced to the west southwest along wsw ene trending shear zones this was part of a regionally distributed phase of orogenic deformation which affected the adjoining kaapvaal and zimbabwe cratons from the witwatersrand basin in the south to the harare greenstone belt in the north this deformation which migrated northwards with time postdated an earlier phase of crustal accretion during which the archaean crustal unit now marked by the kaapvaal and zimbabwe cratons and the limpopo belt grew towards the north northwest shortening of this unit during the limpopo orogeny may be regarded as the last stage of the crustal accretion process this tectonic process of continental growth by accretion followed by crustal shortening with associated crustal extrusion has many features in common with tibetan tectonics from the mesozoic to the present

finite strain calculations of continental deformation 2 comparison with the india asia collision zone numerical experiments on a thin viscous sheet model for deformation of continental lithosphere subjected to an indenting boundary condition yield distributions of crustal thickness of stress and strain rate and of latitudinal displacements that may be compared with observations in the india asia collision zone a simple indenting boundary condition applied to initially laterally homogeneous sheets obeying a power law rheology produces results that are in broad agreement with the observations provided that the power law exponent is three or greater and the sheet can support vertically integrated stress differences of 2 1013 5 1012 n m 1 in regions in front of the indenter under these conditions the calculated deformation shows accommodation of convergence primarily by crustal thickening to produce a plateau in front of the indenter palaeomagnetic data from india and tibet and the observed distribution of topography suggest that much of the post eocene convergence of india with asia has been taken up by deformation within asia that involved crustal thickening the principal difference between calculation and observation is the absence from the calculated strain rate fields of east west extension of the plateau in front of the indenting boundary the calculations show that once such a plateau is formed the buoyancy force associated with the crustal thickness contrast inhibits further thickening and the plateau strains at less than half the rate of its immediate surroundings seismically determined regional strain rates exhibit a similar distribution with the tibetan plateau straining at about one quarter the rate of the tien shan and ningxia gansu regions calculated principal compressive stress orientations and regional strain rates agree with the seismically determined quantities in the mongolia baikal tien shan tibet and ningxia gansu regions of asia to within the uncertainty of the latter the vertically integrated stresses that are calculated for the viscous sheet are comparable with those that can be supported by a theologically stratified continental lithosphere obeying laboratory determined flow laws we suggest that the thin viscous sheet model described in this paper and its companion gives a simple and physically plausible description of the observed deformation in central asia in this description the predominant mechanism of accommodation of continental convergence is diffuse crustal thickening with shear on vertical planes playing a subsidiary role once large crustal thickness contrasts have been established

evolution of continental crust in southern africa abstract nd isotopic data from the zimbabwe and kaapvaal cratons and the limpopo kalahari namaqualand and damara mobile belts imply that over 50 of present day continental crust in this region had separated from the mantle by the end of the archaean and that

the vredefort structure a perspective with regard to new tectonic data from adjoining terranes abstract the deformational history of the area surrounding the vredefort structure in south africa is long and complex involving several periods of contraction and extension a general chronology of events covering a time span of 1000 ma is established the vredefort deformation is identified as a separate structural event which is distinct from the earlier deformations it is essential therefore that deformational structures related to the pre vredefort event should be identified and subtracted from the total spectrum of tectonism so that only those structures which can be related with certainty to the formation of the vredefort structure are defined this approach could influence interpretations of the origin of the vredefort structure which attribute most structural and geological associations to the deposition of sediments onto a granitoid basement followed by a vredefort deformational event

metamorphic p t profiles from the zimbabwe craton to the limpopo belt zimbabwe abstract metamorphic p t profiles in the eastern and western parts of the limpopo belt zimbabwe were examined on the basis of mineralogic and petrologic data from areas which include the southernmost zimbabwe craton the northern marginal zone and the central zone the zimbabwe craton consists of a granite greenstone terrane of 2900 ma to 3600 ma old the northern marginal zone represents a high grade equivalent of the granite greenstone lithologies the central zone has suffered granulite facies metamorphism during the late archean metamorphic temperature of the zimbabwe craton adjacent to the northern boundary of the limpopo belt is calculated to be 650 680 c for amphibolite and 710 720 c for quartzofeldspathic gneiss in the eastern part these temperatures are slightly lower than that calculated from the same assemblages in the northern marginal zone and central zone peak metamorphic conditions in the eastern part of the northern marginal zone are estimated at about 740 790 c and 7 5 7 9 kbar from the assemblage garnet core orthopyroxene plagioclase quartz in pelitic gneiss these conditions were followed by a retrograde event of about 630 660 c and 3 8 4 5 kbar which were calculated from the assemblage garnet rim cordierite sillimanite quartz in the same rock the p t path is very similar to the retrograde path from the southern marginal zone it implies that both the northern marginal zone and southern marginal zone may have shared a similar metamorphic evolution garnet bearing mafic gneiss in the central zone indicates somewhat higher p t conditions than those in the northern marginal zone central zone metamorphic conditions are estimated at 790 890 c and 11 0 13 3 kbar for garnet clinopyroxene plagioclase quartz assemblages and 750 870 c and 7 6 8 8 kbar for garnet hornblende plagioclase quartz assemblages the difference of p t conditions between the northern marginal zone and central zone may be related to movement along the pre triangle shear zone the northern marginal zone appears to have been displaced along the shear zone relative to the central zone during uplift carrying the deeper crust to the surface

neoarchean arc magmatism and subsequent collisional orogenesis along the eastern rae domain western churchill province implications for the early growth of laurentia abstract the athabasca granulite terrane on the eastern margin of the rae domain of the western churchill province contains a well exposed record of neoarchean magmatism deformation and metamorphism circa 2 63 2 60 ga plutonic rocks range from 58 01 to 73 71 sio2 and vary from ferroan to magnesian they form shallow linear trends on modified alkali lime index diagram plotting largely in the calc alkaline field on tectonic discrimination diagrams all samples plot in the volcanic arc field samples are lree enriched and contain relatively steep negative slopes and flat depleted hree profiles when normalized to chondrites the entire neoarchean plutonic complex independent of lithology contains strong enrichments in the liles and pb and depletions in hfses monazite u th total pb results from a migmatitic paragneiss suggest that partial melting occurred at ca 2 57 ga within the chipman subdomain monazite results from the adjacent northwestern subdomain suggest that deformation occurred at ca 2 57 ga in sims and icp ms zircon u pb geochronology corroborates these conclusions although the region is dominated by paleoproterozoic localized shear zones internal to each subdomain is evidence of a widespread tectonothermal event during the neoarchean that involved arc plutonism crustal thickening lower crustal flow at pressures exceeding 1 0 gpa and extensive partial melting of the lower continental crust we interpret this sequence of magmatic deformational and anatectic processes to reflect the transition from a continental arc to a collisional tectonic setting the recognized series of events presented herein is likely correlative to the 2 60 2 50 ga macquoid orogen which has been recognized further to the northeast and likely parallels the eastern margin of the rae domain of the western churchill province these data support an important component of modern style plate tectonics during continental magmatism and accretion in the neoarchean

shear zones bounding the central zone of the limpopo mobile belt southern africa abstract contrary to previously suggested north directed thrust emplacement of the central zone of the limpopo mobile belt we present evidence indicating west directed emplacement the central zone differs from the marginal zones in rock types structural style and isotopic signature and is an allochthonous thrust sheet it is bounded in the north by the dextral tuli sabi shear zone and in the south by the sinistral palala shear zone which are crustal scale lateral ramps published gravity data suggest that the lateral ramps are linked at depth and they probably link at the surface in a convex westward frontal ramp in the vicinity of longitude 26 30 e in eastern botswana two phases of movement the first between 2 7 and 2 6 ga and the second between 2 0 and 1 8 ga occurred on both the tuli sabi and the palala shear zones

right lateral shear and rotation as the explanation for strike slip faulting in eastern tibet the convergence of two continents can be accommodated by crustal thickening and by lateral transport of crust out of the path of the converging continents1 3 the principal features suggesting that lateral transport is important are major strike slip faults striking roughly orthogonally to the orientation of convergence between the continents displacement on such faults is imagined to allow lateral transport of material with respect to the converging continents possibly accounting for a large fraction of the convergence between the two continents the most spectacular example of such strike slip faulting is in the eastern part of the tibetan plateau where three major left lateral faults with slip rates of 10 mm yr 1 and several smaller roughly parallel faults dominate the regional strain field2 3 cobbold and davy4 suggested that these faults may be rotating in a clockwise fashion here we put bounds on that rate of rotation and conclude that the image of lateral transport on such faults2 known also as continental escape extrusion or expulsion is an illusion and that instead the left lateral slip on east striking planes in eastern tibet is a manifestation of north striking right lateral simple shear if this conclusion is correct the east striking left lateral faults and the crustal blocks between them are rotating clockwise at 1 2 myr 1 the east west dimension of eastern tibet is shortening at 10 20 mm yr 1 and little material is moving eastward out of india s path into eurasia by left lateral simple shear

reaction history of garnet sapphirine granulites and conditions of archaean high pressure granulite facies metamorphism in the central limpopo mobile belt zimbabwe sequential reaction textures in archaean garnet corundum sapphirine granulites from the central zone of the limpopo belt document a progression from early coarse grained high pressure p 9 5 kbar granulite facies assemblages m1 to late low pressure p 6 kbar granulite facies sub assemblages m2 the stable m1 assemblage was garnet 57 pyrope mg mg fe 62 sapphirine corundum gedrite phlogopite rutile late m1 boron free kornerupine grew at the expense of garnet and corundum and coexisted with garnet sapphirine and gedrite partial or complete breakdown of coarse garnet and kornerupine during m2 resulted in the development of pseudomorphs and coronas consisting of fine grained symplectic intergrowths of cordierite gedrite and sapphirine later spinel the majority of reaction textures can be explained in terms of a stable reaction sequence and a model time sequence of mineral facies can be constructed when compared with a qualitative petrogenetic grid of fe mg discontinuous reactions in the fmash multisystem sapphirine garnet corundum spinel cordierite gedrite kornerupine the facies sequence indicates decompression at essentially constant t assuming constant a h2o exhumation of m1 corundum inclusions during m2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite a second disequilibrium reaction of the spinel with cordierite produced sapphirine the operation of such reaction while pressure was decreasing the opposite dp from that implied by the texture if assumed to be the product of an equilibrium reaction has serious implications for the use of reaction textures in the construction of p t vectors garnet biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca 850 c for the m1 stage a minimum late m1 pressure of ca 7 kbar is indicated by the former association of kornerupine and corundum relict m1 kyanites reported by other workers indicate a minumum early m1 pressure of 9 5 kbar implying metamorphism at depths of at least 33 km probably 38km the high pressure granulite facies metamorphism was followed by an almost isothermal pressure decrease of 5 kbar indicative of rapid uplift the p t path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite facies terranes

the limpopo mobile belt a result of continental collision the 600 km long limpopo mobile belt is discussed within the frame of a proterozoic supercontinent model piper 1976 evidence is presented that the rhodesian and kaapvaal cratons may have been separated by distances of more than 1000 km of oceanic crust from about 3350 ma ago the kaapvaal craton appears to have been driven intermittently n nw and then wnw against the rhodesian craton forming the ne sw trending collision zone the limpopo mobile belt and all the major fold and fracture patterns found this movement would be similar to the oblique movement of the pacific plate into the aleutian trench when collision ceased around 2500 ma ago it is likely that the great dyke and other complexes intruded along release fractures formed at right angles to the compression

sapphirine kornenipine bearing rocks and crustal uplift history of the limpopo belt southern africa sapphirine kornerupine bearing rocks occur within the anorthosites of the messina layered intrusion in the limpopo mobile belt of zimbabwe the xmg range of the major minerals is as follows cordierite 0 98 0 93 enstatite 0 97 0 86 chlorite 0 98 0 92 phlogopite 0 98 0 90 sapphirine 0 98 0 86 kornerupine 0 94 0 88 gedrite 0 96 0 85 spinel 0 92 0 78 there are four rock types the constituent minerals of which have different x m g values which decrease in the above mineral order other minerals are corundum sillimanite and relict kyanite we recognise twenty reactions without phlogopite and nine reactions involving phlogopite the textural relations and the plots of the microprobe data of coexisting minerals in the mgo al2o3 sio2 h2o system are consistent with the following sequence of main reactions 1 enstatite corundum cordierite sapphirine 4 sapphirine sillimanite cordierite corundum 8 kornerupine corundum cordierite sapphirine 13 kornerupine cordierite sapphirine enstatite 15 enstatite spinel chlorite sapphirine 18 cordierite sapphirine chlorite corundum 20 sapphirine chlorite corundum spinel the early reactions are shown by coarse grained reaction intergrowths kornerupine and gedrite breakdown is shown by finer grained symplectites and the latest reactions by very fine grained products in micro fractures these selected reactions illustrate a remarkably steep trajectory from thept peak close to 10 kbar and 800 c to the minimum observable at 3 5 4 5 kbar and 700 c as indicated by the pure mash system very rapid uplift took place under nearly isothermal conditions the protolith of this material was possibly sedimentary derived from altered volcanic rocks the bulk composition is close to the composition of kornerupine or to a mixture of alunite chlorite and pyrophyllite these texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history

the significance of 3000 ma granulite facies mafic dikes in the central zone of the limpopo belt southern africa abstract structural petrographic and chemical relationships indicate that a suite of mafic dikes possibly 3000 ma old were emplaced at high crustal levels deformed and metamorphosed epicontinental supracrustal rocks of the central zone of the limpopo belt these dikes were subsequently metamorphosed at granulite grade conditions probably during the limpopo orogeny about 2700 ma ago the earlier metamorphism was under conditions of at least amphibolite grade and isotopic data suggest that it may have occurred between about 3200 ma and 3100 ma ago these relationships indicate that the rocks comprising the central zone did not follow a single p t time loop but instead were involved in two distinct orogenies the orogenies were separated by a period of crustal stability during which the supracrustal rocks presently and originally exposed at the surface were again moved near to the surface this tectonic history also implies that the supracrustal rocks of the central zone and the sand river gneisses are of great antiquity if the sand river gneisses with their unique suite of older mafic dikes form a previously deformed basement to the supracrustal rocks then the sand river gneisses have been involved in three orogenies pb isotopic data indicate that the mafic dikes were derived from a mantle source with a high average ratio at some stage in its evolution prior to 3200 ma all archean lithologic units within the central zone of the limpopo belt are characterized by having been derived from rocks that had a large mu ratio at some time in their history and this relationship suggests that a link existed between the crustal rocks of the central zone and this mantle source from at least 3200 ma ago until it was destroyed by or after the limpopo orogeny the tectonic history of the central zone is different from those of the lithologies forming the adjacent terranes and suggests that it evolved separately from these terranes until they were juxtaposed during the limpopo orogeny

erosional and depositional history of the atlantic passive margin as recorded in detrital zircon fission track ages and lithic detritus in atlantic coastal plain sediments comparison of fission track ft ages of detrital zircons recovered from atlantic coastal plain sediments to ft ages of zircons from bedrock in source terranes in the appalachians provides a key to understanding the provenance of the sediments and in turn the erosional and depositional history of the atlantic passive margin in appalachian source terranes the oldest zircon fission track zft ages from bedrock in the western appalachians defined for this paper as the appalachian plateau valley and ridge and far western blue ridge are notably older than the oldest ages from bedrock in the eastern appalachians piedmont and main part of the blue ridge the age difference is seen both in zft sample ages and in individual zircon grain ages and reflects differences in the thermotectonic history of the rocks in the east zft data indicate that the rocks cooled from temperatures high enough to partially or totally reset zft ages during the paleozoic and or mesozoic the majority of the rocks are interpreted to have cooled through the zft closure temperature 235 c at various times during the late paleozoic alleghanian orogeny in contrast most of the rocks sampled in the western appalachians have never been heated to temperatures high enough to totally reset their zft ages reflecting their contrasting thermotectonic histories nearly 80 percent of the sampled western rocks yield one or more zircon grains with very old ft ages in excess of 800 ma zircon grains yielding ft ages this old have not been found in rocks in the piedmont and main part of the blue ridge the zft data suggest that the asymmetry of zircon ages of exposed bedrock in the eastern and western appalachians was in evidence by no later than the early cretaceous and probably by the late triassic detrital zircon suites from sands collected in the atlantic coastal plain provide a record of detritus eroded from source terranes in the appalachians during the mesozoic and cenozoic in virginia and maryland sands of early cretaceous through late early oligocene age do not yield any old zircons comparable in age to the old zircons found in bedrock in the western appalachians very old zircons yielding ft ages 800 ma are only encountered in coastal plain sands of middle early miocene and younger age miocene and younger fluvial deltaic deposits associated with the major mid atlantic coastal plain rivers that now head in the western appalachians the hudson delaware susquehanna potomac james and roanoke contain abundant clasts of fossiliferous chert and quartzite and other distinctive rock types derived from paleozoic rocks of the western appalachians these distinctive clasts have not been reported in older coastal plain sediments the zft and lithic detritus data indicate that the drainage divide for one or more east flowing mid atlantic rivers migrated west into the western appalachians and the river s began transporting western appalachian detritus to the atlantic coastal plain sometime between the late early oligocene and middle early miocene by no later than late middle miocene most if not all of the major rivers that now head west of the blue ridge were transporting western appalachian detritus to the coastal plain prior to the drainage divide migrating into the western appalachians the zft data are consistent with the dominant source of atlantic coastal plain sediments being detritus from the piedmont and main part of the blue ridge with possible input from distant volcanic sources the zft data suggest that the rapid increase in the rate of siliciclastic sediment accumulation in middle atlantic margin offshore basins that peaked in the middle miocene and produced almost 30 percent of the total volume of post rift siliciclastic sediments in the offshore basins began in the early miocene when atlantic river s gained access to the relatively easily eroded paleozoic sedimentary rocks of the western appalachians

constraints on the form of the p t loop in the southern marginal zone of the limpopo belt south africa abstract a generalised clockwise p t path consisting of a prograde portion a high grade isothermal decompression portion and an essentially isobaric cooling portion has been suggested for the metamorphic evolution of the southern marginal zone previous studies have concluded that the high grade decompression path started in the kyanite stability field peak metamorphic mineral compositions have been reset during the equilibrium decompression event and hydration during the cooling dominated isobaric portion occurred in the kyanite stability field the present petrographic study from the bandelierkop quarry a locality in the granulite zone of the southern marginal zone identified a series of sequential anatectic reactions which record a period of prograde heating from approximately 700 c to 850 c during this period pressure is constrained to within the sillimanite stability field by the production and preservation of prograde generations of sillimanite the maximum temperature melting event is closely followed by two divariant reactions ga qz sil cord and ga qz cord hy which occur in response to a period of general isothermal decompression a subsequent cooling dominated rehydration period is indicated by a crystallisation hydration reaction cord ksp melt bi ky qz which occurred at a minimum temperature of approximately 640 c and pressure greater than 6 5 kbar the present data confirms the clockwise form of the loop and also supports the previously suggested flat slope of the retrograde cooling portion additional mineral chemistry data relating to the second decompression reaction indicate that the zonation profiles of cordierite and garnet as well as the growth of small quantities of garnet after the reaction are in strong support of the suggested p t path the present mineral compositions in the southern marginal zone reflect a near total re equilibration during the high grade cooling dominated portion of the p t path and as a result reflect p t conditions considerably lower than those of the peak metamorphism

deep crystal response to continental collision the limpopo belt of southern africa the limpopo belt is an example of an archean 2700 ma continental collisional zone in which the thickened crust 70 km has virtually attained equilibrium with the adjacent non thickened plates by a process of upward movement and lateral spreading of the underthrust high grade metamorphosed plates

partial melting and the origin of metapelitic granulites in the southern marginal zone of the limpopo belt south africa abstract two potentially realistic models exist to explain the origin of the granulite facies rocks of the southern marginal zone i co2 fluxing inducing the necessary high temperature dehydration reactions and ii melting of hydrous minerals to yield an h2o undersaturated granitic melt in equilibrium with a dehydrated granulitic assemblage petrographic and field evidence from the bandelierkop quarry in the southern marginal zone indicate a sequence of melting reactions involving muscovite and biotite relict textural evidence indicates that the reaction responsible for the generation of most of the melt bi sil qz plag melt ga ksp was fluid absent the melting reactions can be shown to have occurred during the prograde metamorphic path with the highest temperature reaction bi qz plag hy cord melt occurring at approximately peak metamorphic conditions 850 c and less than 9 5 kbar this suggests a fluid absent mode of granulite formation a subsequent retrograde reaction cord ksp h2o ky qz related to a rise in h2o during in situ crystallisation of the melt did not destroy the granulite assemblages and hypersthene remains stable in areas of the southern marginal zone that record pressures of greater than 6 5 kbar during retrogression

first direct radiometric dating of archaean stromatolitic limestone here we report the first direct dating of the depositional age of a sedimentary carbonate rock using long lived radioactive decay schemes the mushandike stromatolitic limestone from the masvingo formerly fort victoria greenstone belt of southern zimbabwe1 2 yields a pb pb isochron age of 2 839 33 myr published claims3 for an age of 3 5 gyr for the mushandike limestone were based on its supposed intrusion by the mushandike granite for which a rb sr whole rock age of 3 445 260 myr had been reported4 new rb sr and pb pb isotope data for the mushandike granite indicate a probable emplacement age of 2 900 myr and new field evidence j l o p t j f w in preparation suggests that the granite forms part of the basement on which the limestone was deposited thus the new dates do not support an age of 3 5 gyr for the algal stromatolites at mushandike the pb pb isochron method may have wide applicability to dating the time of deposition of precambrian fossiliferous and unfossiliferous limestones and thus to the problem of the antiquity of life

evolution of the northern high grade margin of the kaapvaal craton south africa the northern kaapvaal craton is divided into three distinctive crustal zones 1 the high grade southern marginal zone smz of the limpopo belt in the north 2 a transitional zone of technically stacked metamorphic rocks which includes the area from the sutherland belt in the north to the pietersburg and murchison belts in the south and 3 a low grade granite greenstone terrane farther to the south the change across the smz boundary is expressed by an overall increase in the grade of metamorphism a strip of retrogression related to a zone of thrusting abrupt changes in intensity of deformation rock densities magnetic lineament patterns and isotopic ages in compositionally similar rocks regional metamorphism in the northern kaapvaal craton is demonstrably not a continuum in the sense that the highest grade rocks underwent all earlier stages of metamorphism rather that rapid local tectonic processes must have occurred a proposed evolutionary path for the smz was initiated with crustal thicke