Early Proterozoic Research Articles

Precambrian evolution of the climate system

Climate is an important environmental parameter of the early Earth, likely to have affected the origin and evolution of life, the composition and mineralogy of sedimentary rocks, and stable isotope ratios in sedimentary minerals. There is little observational evidence constraining Precambrian climates. Most of our knowledge is at present theoretical. Factors that must have affected the climate include reduced solar luminosity, enhanced rotation rate of the Earth, an area of land that probably increased with time, and biological evolution, particularly as it affected the composition of the atmosphere and the greenhouse effect. Cloud cover is a major uncertainty about the early Earth. Carbon dioxide and its greenhouse effect are the factors that have been most extensively studied. This paper presents a new examination of the biogeochemical cycles of carbon as they may have changed between an Archean Earth deficient in land, sedimentary rocks, and biological activity, and a Proterozoic Earth much like the modern Earth, but lacking terrestrial life and carbonate-secreting plankton. Results of a numerical simulation of this transition show how increasing biological activity could have drawn down atmospheric carbon dioxide by extracting sedimentary organic carbon from the system. Increasing area of continents could further have drawn down carbon dioxide by encouraging the accumulation of carbonate sediments. An attempt to develop a numerical simulation of the carbon cycles of the Precambrian raises questions about sources and sinks of marine carbon and alkalinity on a world without continents. More information is needed about sea-floor weathering processes.

Growth and accretion of continental crust: Inferences based on Laurentia

Nd isotopic data and U-Pb zircon ages from Laurentia (North America and the Baltic Shield) indicate that continental growth (extraction from the mantle) and accretion (collision of arcs and microcontinents) have been episodic with maxima at 2.7, 1.9, 1.8 and 1.7 Ga ago and a major period of accretion at ∼ 1.9 Ga ago. In general, accretion follows mantle extraction by <100 Ma. Archean and Early Proterozoic continental growth rates are 0.5–0.7 km 3 a −1, whereas Late Proterozoic and Phanerozoic rates are 0.2–0.3 km 3 a −1. Laurentia reached 40% of its present volume by the end of the Archean and 75% by 1.6 Ga ago. Reworking of continental crust does not become significant until after 1.2 Ga. Calculated worldwide continental growth rates for the Late Archean, Early Proterozoic and Late Proterozoic-Phanerozoic are ∼3, ∼1.5 and ∼ 1.1km 3 a −1, respectively. These rates are in good agreement with growth rates predicted by assuming that continental growth rate is proportional to the decrease in radiogenic heat production in the Earth with time.

Sources of Hercynian granitoids from the French Massif Central: Inferences from Nd isotopes and consequences for crustal evolution

The French Massif Central is characterized by a great abundance and variety of granitoids emplaced during Hercynian times (360-280 Ma). Distinct ϵ Nd(i) and ( 87 Sr 86 Sr) i are not associated with particular petrographic types; however, lower ϵ Nd(i)-values (−8.5) occur among the younger leucogranites from the western part of the massif. Intermediate ϵ Nd( i) (−6) correlate with aluminous monzogranites and higher values (−6 to −3) with calc-alkaline granodiorites. In the eastern Massif Central ϵ Nd(i) are homogeneous at the regional scale. In the western Massif Central, the leucogranites and the monzogranite-granodiorites display different trends of ϵ Nd( i) with emplacement ages. From these results, extreme models involving a depleted mantle, or an old (Archean or Early Proterozoic) source may be ruled out. The trend of increasing ϵ Nd( i) with decreasing age observed in the monzogranite-granodiorite group suggests either an increasing contribution of a mantle component, or the progressive melting of more mafic, more refractory crustal sources, or both. The reverse trend displayed by the leucogranites might be explained in terms of the upward migration of partial melting across an isotopically zoned metasedimentary pile. The large amount of granites may be the result of favourable processes during the collision tectonics, combined with the unusual abundance of fertile materials, enriched in water and heat-producing elements, in the pre-Carboniferous crust.

Origin of Late Archean and Early Proterozoic rocks and associated mineral deposits from the Zhongtiao Mountains, east-central China

Abstract Late Archean and Early Proterozoic supracrustal rocks in the Zhongtiao Mountains of east-central China include five successions separated by unconformities or intrusive contacts. The oldest successions, the Shushui Complex (⩾ 2500 Ma), and the Jiangxian (2400-2200 Ma) and Zhongtiao Groups (∼ 2100 Ma) are composed chiefly of metapelites, quartzites and marbles with small amounts of bimodal felsic-mafic volcanics in which felsic ash flow tuffs dominate. The overlying Danshanshi Group includes chiefly quartzites and conglomerates and the Xiyanghe Group (∼ 1830 Ma) comprises bimodal volcanics and red beds. Granitoids range from Late Archean to Early Proterozoic in age and vary from tonalite to granite in composition. Precambrian igneous rocks show a subduction zone geochemical component (depletion in Nb-Ta relative to LILE), and Early Proterozoic granitoids range in composition from I to S types. Geochemical and isotopic constraints favor an origin for Early Proterozoic Zhongtiao felsic magmas either by melting of sediments or tonalites in the crust or by melting of amphibolite/eclogite in a descending slab. Zhongtiao pelites are similar in composition to Phanerozoic shales and exhibit negative Eu anomalies on chondrite-normalized graphs. Stratiform copper deposits in the Zhongtiao Proterozoic rocks are of three types: volcanic-hosted deposits of hydrothermal or exhalative origin; primary sedimentary deposits; and remobilized sedimentary deposits. The Jiangxian and Zhongtiao lithologic assemblages of quartzite-pelite-carbonate together with primary textures and structures in these rocks suggest deposition in shallow-marine, tectonically stable basins, whereas coarse terrigenous sediments in the Danshanshi and Xiyanghe Groups appear to reflect near-shore marine to fluvial deposition in tectonically active basins. Most data are consistent with deposition of the Early Proterozoic supracrustal rocks in a continental-margin back-arc basin.

Early Precambrian within-Plate Basalts from the Kaapvaal Craton in Southern Africa: A Case for Crustally Contaminated Komatiites

Archean and Early Proterozoic basalts from the Kaapvaal Craton in southern Africa have geochemical characteristics of subduction-related basalts. These basalts, however, cannot be produced in the subcontinental lithosphere, if ultramafic xenoliths depleted in Fe, Ti, and Ca are representative of this lithosphere. Geochemical and isotopic data are consistent with an origin by assimilation-fractional crystallization of Al-undepleted komatiite in the mid-to upper crust with Archean granitoids as the major contaminants. The proportion of granite to tonalite needed in the contaminant is higher for most of the Proterozoic basalts than it is for the Archean basalts. Widespread 2.3 Ga old within-plate, anorogenic granites in the Kaapvaal Craton may be crustal melts produced by komatiitic magmas heating the crust. Komatiitic magmatism in southern Africa could have lasted for (), and many Archean and Early Proterozoic within-plate basalts from other regions also may be crustally contaminated komatiites.

Petrogenetic applications of the [formula omitted] radiogenic decay scheme — A reconnaissance study

Ca isotopic data are presented for oceanic basalts, potassic igneous rocks, carbonatites, marine sediments and chemical and clastic sediments from Archaean and Proterozoic terrains. An overall external precision of ±0.02% (95% confidence) was obtained for 40Ca 42Ca measurements. By comparison with the mid-ocean ridge basalt, no resolvable enrichments in 40Ca from radiogenic decay of 40K were detected in the Hawaiian ocean-island or Banda and Aleutian island-arc samples, the marine sediments or the carbonatite samples. Potassic igneous rocks from the West Kimberley region of Western Australia, Gaussberg (Antarctica) and southeastern Spain also possess 40Ca 42Ca ratios which are analytically indistinguishable from the value found in oceanic basalts and carbonatites, suggesting that: (a) the K/Ca ratio of the magmas has been increased substantially during partial melting or melt extraction, and/or (b) the Ca in the magmas has been diluted by mixing with non-radiogenic Ca derived from other sources. In contrast, enrichments in 40Ca were detected in genetically unrelated kimberlitic and lamproitic intrusions from the Sisimiut (formerly Holsteinsborg) region of central west Greenland. The excess 40Ca in the Greenland kimberlites is unsupported, indicating that the K/Ca ratio has been reduced, probably during extraction of the kimberlite magmas from carbonate-rich sources. The radiogenic 40Ca 42Ca of the Greenland kimberlites is not accompanied by elevated 87Sr 86Sr , suggesting that the kimberlite major- and trace-element components have been decoupled, possibly with much of the major elements derived from the subcontinental lithosphere and the trace elements largely derived from sources located within the asthenosphere. The radiogenic 40Ca 42Ca of the Greenland lamproites is consistent with their derivation from subcontinental lithospheric sources which have been enriched in K for > 1 Ga prior to their emplacement. Gypsum samples and pelitic clastic sediments from Archaean and Early Proterozoic terrains possess substantial enrichments in 40Ca relative to oceanic basalts. Application of the 40K- 40Ca decay scheme using presently available technology can provide useful petrogenetic and chronologic information which complements that provided by other techniques.

Geophysical investigation of the Oslo Graben, the proterozoic continental earth crust of South-East Norway and the Western Gneiss region

Geophysical investigation of the Oslo Graben, the proterozoic continental earth crust of South-East Norway and the Western Gneiss region

Structure and composition of the Precambrian crust along the POLAR Profile in the northern Baltic Shield

Along the 500-km long POLAR Profile, the continental crust of the northern Baltic Shield consists of several Archaean and Early Proterozoic crustal segments.In the south, in the Karelian Province, the upper crust consists of a 3.1–2.6 Ga old Archaean substratum with a 0–6-km thick, low- to medium-grade volcanosedimentary cover. Here, shallow-basin facies rocks of the Central Lapland Complex have been thrust northwards onto the Karasjok-Kittilä Greenstone Belt. The repeatedly folded cover rocks form linear troughs between domes and culminations of granitoids. The 47–48-km thick crust of the Karelian Province is of a shield type. It is characterised by a low-velocity layer interpreted as a horizon of décollement below the supracrustal and granitoid rocks.In the Lapland Granulite Belt, Early Proterozoic high-grade metapelites and metapsammites with minor metaigneous intercalations have been thrust to the southwest. They are separated from the underlying Karelian Province by the tectonic mixture zone of the Tanaelv Belt. In the interpretations, two alternative models are given for the shape of the Lapland Granulite Belt: it is either a synformal nappe or a NE-dipping body pinching out at depth and overthrust from the north by the Archaean Inari Terrain. Both the Lapland Granulite Belt and the Inari Terrain have a 42–44-km thick crust with a relatively thin lower crust marked by lower seismic velocities.The Early Proterozoic Polmak-Pasvik-Pechenga Belt separates the Archaean Inari Terrain from the Archaean Sørvaranger Terrain in the north. This belt is interpreted as a suture zone which continues to depth. It may either dip steeply towards irregularities in the Moho or form a SW-dipping low-angle plane below the Lapland Granulite Belt. The northernmost part of the Baltic Shield is concealed beneath a 0–7-km thick, very low-grade metamorphic Late Proterozoic cover. This cover is divided into two parts by the Trollfjord-Komagelv strike-slip fault. Here, the crust is again of a normal shield type with a thickness of 42–46 km.

Is the equatorial atlantic discordant?

The equatorial Atlantic is often considered discordant because the margins of South America and Africa, which elsewhere follow the trends of Late Proterozoic mobile belts, here intersect those mobile belts at high angles and apparently truncate Archean and Early Proterozoic cratons. The Amazonian and São Luís Cratons of northern French Guiana and Brazil, however, exhibit Early and Late Proterozoic structures which are parallel to and concordant with the coastline. In Gondwana reconstructions, the clash between these structures and discordant structures of the West African Craton implies that the equatorial Atlantic coincides with a major Proterozoic structural boundary. One thousand km of dextral offset observed between lithologic/geochronologic provinces and major fault zones of the two continents agrees with the offset in Proterozoic paleopoles from Venezuela and Liberia. We propose three models to account for this offset. Our preferred model includes the São Luís Craton with the Amazonian, rather than the West African Craton, and accommodates motion along a zone parallel to the northern margin of South America. Movement must have been completed between the Amazonian/Eburnean (2.2-1.9 Ga) and the Brazilian/Pan-African orogenies (600-500 Ma). This zone of weakness was reactivated during the Cretaceous as the site of the equatorial Atlantic fracture zones.

The development of the Central European continental crust since the Early Archaean based on conventional and ion-microprobe dating of up to 3.84 b.y. old detrital zircons

Archaean components up to 3.84 b.y. old have been detected by ion-microprobe dating of zircon from various sediments and metasediments of the European Hercynides (Moldanubian of northeastern Bavaria and the Massif Central). The Early Archaean age of 3.84 b.y. was obtained within the euhedral core of a detrital grain from a Moldanubian paragneiss. The same detrital zircon grain reveals at least three metamorphic overprints at 2.59 b.y. B.P., 1.94 b.y. B.P. and 460 m.y. B.P., and at least two cycles of mechanical abrasion during surface recycling in the Early Proterozoic and after the Pan-African cycle. It is, thus, the crystal with the most complex geological history ever detected. Further Archaean ages for the crystallization of primary magmatic zircons, probably in the course of continental crust formation in the first cycle provenances of the two studied areas, were found at 3.15, 2.9, 2.76 and 2.65 b.y. B.P., and in a few cases between 2.6 and 2.5 b.y. B.P. Early Proterozoic crust-forming events between about 1.75 and 2.15 b.y. B.P. play an important role, especially for the sediments and metasediments of the Montagne Noire at the southern rim of the Massif Central. Middle and Late Proterozoic ages cluster around 1.0 b.y. B.P. and ≈ 600 m.y. B.P. for all analysed samples. Therefore, the deposition of the metasedimentary precursors took place after the Pan-African cycle, which is in line with the Rb-Sr whole-rock systematics of Central European metasediments. The ages of metamorphism in the different source areas could be inferred in some cases by lower intercept ages obtained on single detrital grains. The following sequence coinciding with times of magmatic events has been so far established: 2.59,1.94 and 1.0 b.y. B.P., and 600 m.y. B.P. Thus, ion-probe dating is also capable of defining secondary effects in the respective provenances which are typical for orogenic cycles. The ubiquitous presence of Pan-African detrital zircons in European metasediments is a strong indication that the detrital material was derived from Gondwana. However, from the presence of metamorphic units including eclogitefacies rocks which probably formed at different continental margins and at various times between about 500 m.y. and 330 m.y. ago it is probable that various Gondwana-derived microcontinental plates contributed to the different basin fillings. The Gondwana-derived detritus is also supported by the observation of two periods of relative quiescence or non-activity around 2.3 b.y. ago, and especially around 1.5 b.y. ago, which is a typical feature observed in Gondwana (Cahen et al., 1984) but not in Laurasia, the other possible source of the detritus. The lack of zircons crystallized between 3.84 and 3.15 b.y. ago might have different causes. Thus, the continental crust of the European Hercynides probably developed since the Early Archaean and grew via many crustal accretion events to its present state.

The Goodman Swell: A lithospheric flexure caused by crustal loading along the midcontinent rift system

Rb‐Sr biotite ages of Archean and Early to Middle Proterozoic crystalline rocks in northern Wisconsin and adjacent Upper Peninsula of Michigan describe a regionally systematic pattern related to differential uplift. An “age low” occurs in northern Wisconsin where values range from 1070–1172 Ma for rocks with crystallization ages of 1760 to 1865 Ma. These values overlap with the main episode of mafic igneous activity (1090 to 1120 Ma) along the Midcontinent rift system (MRS). We interpret these low biotite ages as registering closure due to cooling below the 300°C isotherm as a consequence of uplift and rapid erosion of an area that we are informally naming the Goodman swell. The position of the Goodman swell is independent of the Archean and Early Proterozoic regional geologic framework but closely linked with the curvilinear (concave to the south) MRS. We interpret the swell to be a forebulge imposed on an elastic crust by loading of mafic igneous rocks along and within the axis of the MRS. The southward concavity of load axes resulted in flexural interaction that localized the swell. Because of rapid uplift the swell may have been a source area for clastic sediments in the rift sequence.

Proterozoic plate tectonics, glaciation and iron-formations

Glacigenic diamictites and associated rocks are common in Early and Late Proterozoic sequences. Most of the preserved Proterozoic glacigenic rocks formed in marine basins in an extensional tectonic regime at or close to the rift-drift transition. The reasons for glaciation in this tectonic setting are not known but two possible contributing factors, given an initially cool climatic regime, are the formation of new seaways by continental fragmentation and uplift of the rift shoulders during early stages of breakup. The absence of such glacigenic deposits in the classical miogeoclines of the peri-Atlantic region may be due to an anomalously warm global climatic regime in the late Mesozoic. Iron-formations in the Archean and Early Proterozoic formed in many different environments; the critical factor was a dearth of siliciclastic deposition. Starved basins commonly developed during the transition from miogeocline to foreland basin in the Wilson cycle of ocean opening and closure, when drainage from the craton was replaced by that from a rising orogen. Early Proterozoic iron-rich sediments in the Great Lakes region, in southeast Wyoming and in the Northwest Territories of Canada are interpreted as the earliest deposits of such foreland basins. They are considered to be chemical precipitates from iron-rich sea water. Late Proterozoic iron-formations formed under an extensional regime, possibly in small Red Sea-type ocean basins where hydrothermal circulation produced metal-rich brines. These brines were displaced by movement of cold waters derived from sea-going glaciers, leading to dilution and precipitation of Fe and Si. Thus, in the Late Proterozoic, there was a critical association of continental rifting, glaciation and hydrothermal circulation. This combination of environmental parameters led to the production of the unique Late Proterozoic class of hydrothermal iron-formations.

The Proterozoic biosphere: a multidisciplinary study

List of contributors Preface Part I: 1. Geology and paleobiology of the Archean Earth 2. Geological evolution of the Proterozoic Earth 3. Proterozoic biochemistry 4. Proterozoic atmosphere and ocean 5. Proterozoic and selected early Cambrian microfossils: prokaryotes and protists 6. Modern mat-building microbial communities: a key to the interpretation of Proterozoic stromatolitic communities 7. Proterozoic and earliest Cambrian carbonaceous remains, trace and body fossils 8. The Proterozoic-early Cambrian evolution of metaphytes and metazoans 9. Molecular phylogenetics, molecular paleontologym and the Proterozoic fossil record 10. Biostratigraphy and paleobiology of the Proterozoic 11. Biotic diversity and rates of evolution during Proterozoic and earliest Phanerozoic time 12. A paleogeographic model for Vendian and Cambrian time 13. Evolution of the Proterozoic biosphere: benchmarks, tempo and mode Part II: 14. Geographic and geologic data for PPRG rock samples 15. Flow chart and processing procedures for rock samples 16. Procedures of whole rock and kerogen analysis 17. Abundances and isotopic compositions of carbon and sulphur species in whole rock and kerogen samples 18. Procedures for analysis of extractable organic matter 19. Composition of extractable organic matter 20. Modern mat-building microbial communities: methods of investigation and supporting data 21. Construction and use of geological, geochemical and paleobiological databases 22. Proterozoic and selected early Cambrian microfossils and microfossil-like objects 23. Described taxa of Proterozoic and selected earliest Cambrian Carbonaceous remains, trace and body fossils 24. Atlas of representative Proterozoic microfossils 25. Informal revised classifications of Proterozoic microfossils 26. Models for Vendian-Cambrian biotic diversity and for Proterozoic atmospheric and ocean chemistry 27. Glossary of technical terms References cited Subject index Index to geologic units Taxonomic index.

Compositional characteristics of the Kenora–Kabetogama dyke swarm (Early Proterozoic), Minnesota and Ontario

Over a 100 new major-element analyses and 48 analyses for a restricted suite of minor and trace elements have been made on samples from 31 localities in the Kenora–Kabetogama (KK) dyke swarm of Minnesota and northwestern Ontario. A subset of 14 analyses, selected to minimize the aberrations of secondary hydrous alteration, flow layering, and oversampling bias, characterizes the range of dyke compositions. The dykes are interpreted as having been derived from a single magma type; their compositions span a smooth continuum of MgO and TiO2 variations. The magmatic system was tapped most vigorously at two stages: at a relatively early stage when the magma contained about 7.6% MgO and 0.9% TiO2, and again at a later stage when the magma contained about 5.5% MgO and 2.2% TiO2. Large master dykes (~100–190 km long) and many smaller dykes are of these two compositions. Dykes throughout the compositional spectrum are indistinguishable paleomagnetically. Available radiometric data suggest that KK magmatism may have been prolonged (&gt; 100 Ma?), but the paleomagnetic results suggest a much shorter probable life-span.The KK swarm is related to tectonic activity in the Penokean orogen (Early Proterozoic). It may have developed as a manifestation of peripheral foreland bulging initiated by thrust loading of the crust in the hinterland of the Animikie basin.

Back-arc spreading in the proterozoic — a theoretical approach

Abstract The early Proterozoic Earth probably had a hotter mantle and a lower residence time for oceanic plates at the surface than the modern one. The hotter mantle may have led to thinner oceanic plates. Young ocean floor has low negative buoyancy and therefore exerts low slab-pull forces in subduction zones. This effect would have been greater if the plates were thinner, since they would have warmed up more quickly as they descended into the mantle. Thus the negative buoyancy of slabs and the slab-pull forces were probably substantially lower in the early Proterozoic. Calculations show that this applies even if subduction rates were higher and the crust was thicker. Lower slab-pull leads to lower rates of ‘rollback’ in subduction zones and lower rates of retreat of upper plates from trenches. The limited age-range of oceanic crust and the more rapid heating of descending slabs in subduction zones probably leads to a smaller range of slab-profiles, since this range is controlled by thermal properties of the slab, and therefore to more stability in subduction zones. Thus, many of the proposed mechanisms for back-arc spreading may have been less effective, and backarc spreading may have been less common, in early Proterozoic subduction zones. This may explain the apparent absence of ophiolite complexes of that age.

Late Archaean/Early Proterozoic CO2 streaming through the lower crust and geochemical segregation

Considerable evidence exists for large‐scale migration of CO2‐rich fluids through the lower continental crust in the Late Archaean and Early Proterozoic, with transfer of heat producing elements and H2O to higher levels. The evidence manifest in the large charnockitic terrains of this age includes deformation‐related charnockitic alteration of gneisses, nearly ubiquitous CO2‐rich fluid inclusions in minerals, depletion of large‐ion‐lithophile elements (LILE), particularly Rb, relative to typical upper crustal rocks, similar depletion of 18O in paragneisses and petrographic and geochemical evidence of open‐system replacement of amphibole gneiss by charnockite. A conveyor‐belt upper mantle source appears to have been necessary for prolonged CO2 supply in high‐grade crustal metamorphism, most plausibly by deep subduction of marine carbonate under continental interiors. This plate tectonic mode seems consonant with crustal thickening and orogenic patterns inferred for most charnockitic terrains and with CO2 transport mechanisms constrained by experimental petrology.

The metamorphic internal zone of Wopmay Orogen (Early Proterozoic), Canada: 30 km of structural relief in a composite section based on plunge projection

In Wopmay Orogen, an early Proterozoic volcano‐sedimentary marginal prism and its underlying Archean structural basement, deformed together during the 1885 Ma Calderian orogeny, are exposed in oblique view as a result of regional cross folding. A composite cross section displaying over 30 km of structural relief provides a framework in which to examine the geologic relationships in the metamorphic‐internal zone of the orogen. A sole thrust separates Archean basement and a thin mantle of autochthonous, early Proterozoic strata from the eastward transported Calderian allochthon. The allochthon is characterized by a transition from external thrust‐fold belt geometries in the east to polyphase, isoclinal recumbent folds and thrusts in the west. The center of the zone contains the Hepburn Batholith which has been been transported, along with the deforming, hot allochthon, over the exposed basement. Both basement and allochthon are deformed by late, large‐scale, upright folds. The metamorphic‐internal zone is interpreted to record the closing of a rift basin during which the basin fill was shortened, thickened (to over 30 km), metamorphosed and thrust eastward while still hot over the relatively cool Archean craton. The basement presently exposed beneath the sole thrust remained relatively nondeformed during thrusting but underwent large‐scale folding due to continued compression during uplift of the internal zone.

Paleoclimatology of Nonacho Basin (early proterozoic), Northwest territories, Canada

Numerous features of paleoclimatic significance are preserved in terrestrial strata of Nonacho Basin (Early Proterozoic). Minimal chemical weathering is indicated by relic corestones, grus and talus, and the absence of chemical alteration zones at the basal unconformity. Humid conditions are indicated by: (1) dominance of siliciclastic lacustrine deposits (even in lakes that had restricted drainage); (2) paucity of caliche in extensive alluvial fan deposits; (3) near-absence of debris flows in alluvial fan deposits; (4) absence of large-scale high-angle cross-stratification in eolian deposits; and (5) penecontemporaneous variegated redbeds (drab sandstones; red mudstones). Minimal chemical weathering under humid conditions implies a cold climate. This is supported by published paleomagnetic data humid conditions implies a cold climate. This is supported by published paleomagnetic data which indicate that Nonacho Basin probably formed at paleolatitudes of 40° – 55°, by talus deposits (formed as a result of frost action), and by penecontemporaneous stratiform sandstone breccias for which the early cementing agent probably was ice. High elevations probably contributed to the low temperatures. The Nonacho Group probably was deposited in a moist continental climate with cold winters characterized by repeated freeze—thaw cycles. Local chemically precipitated lacustrine sediments and caliche indicate brief dry intervals.

Tectonics of the Kola collision suture and adjacent Archaean and Early Proterozoic terrains in the northeastern region of the Baltic Shield

As preparation for the deep-seismic and other geophysical experiments along the Polar Profile, which transects the Granulite belt and the Kola collision suture, structural field work has been performed in northernmost Finland and Norway, and published geological information including data from the neighbouring Soviet territory of the Kola Peninsula, have been compiled and reinterpreted. Based on these studies and a classification according to crustal and structural ages, the northeastern region of the Baltic Shield is divided into six major tectonic units. These units are separated and outlined by important low-angle, ductile shear or thrust zones of Late Archaean to Early Proterozoic age. The lateral extension of these units into Soviet territory and their involvement in large-scale crustal deformation structures, are described. Using the “view down the plunge” method, a generalised tectonic cross-section that predicts the crustal structures along the Polar Profile is compiled, and the structures around the Kola deep drill-hole are reinterpreted. The Kola suture belt, through parts of which the Kola deep bore-hole has been drilled, is considered to represent a ca. 1900 Ma old arc-continent and continent-continent collision suture. It divides the northeastern Shield region into two major crustal compartments: a Northern compartment (comprising the Murmansk and Sörvaranger units) and a Southern compartment (including the Inari unit, the Granulite belt and the Tanaelv belt, as well as the more southernly situated South Lapland-Karelia “craton” of the Karelian province of the Svecokarelian fold belt). The Kola suture belt is outlined by a 2–40 km wide and ca. 500 km long crustal belt composed of 1. (1) Early Proterozoic (ca. 2400-2000 Ma old) metavolcanic and metasedimentary sequences which originally formed part of the attenuated margin of the Northern Archaean compartment, and 2. (2) the remains of a ca. 2000-1900 Ma old, predominantly andesitic island-arc terrain. This island-arc terrain was built up above a SW-plunging subduction zone, initiated ca. 2000 Ma ago in the southern part of a newly formed oceanic domain, the Kola ocean. Due to continued subduction and complete consumption of this ocean, the northern passive margin deposits and the island-arc terrain were brought into tectonic juxtaposition, and during the final arc-continent and continent-continent collision, they were overthrusted onto the northern Archaean continent. Along its southern boundary, the Kola suture belt is tectonically overlain by the Archaean rocks of the Inari unit. This unit was derived from a microcontinent split from the Southern compartment, the depositional basin of the protoliths of the Granulite belt being formed to the south of the microcontinent. The Inari microcontinent appears to have wedged out towards the southeast, as the continuation of the Granulite belt north of the White Sea is in direct tectonic contact with the Kola suture belt. The Granulite belt is composed of high-grade paragneisses and minor amounts of meta-igneous rocks. The paragneisses formed from thick turbidite and mass flow deposits lain down in a back-arc basin south of the Inari microcontinent. A thermal anomaly beneath the partly oceanic basement of the back-arc basin is believed to have contributed to the ca. 2000-1900 Ma old granulite facies metamorphism of the granulite assemblages. Granulite facies conditions still prevailed when the Inari microcontinent overrode the granulites and when the Granulite belt as such was formed and was overthrusted (for at least 100 km) towards the southwest. In conjunction with the latter event, the rocks of the basement of the basin also became involved in thrust movements. These now form the Tanaelv belt, which shows gradational tectonic contacts towards underlying cover and basement rocks of the South Lapland-Karelia craton. Although not all parts of this craton were affected by the Svecokarelian deformation, it is considered to belong to the Karelian province of the Svecokarelian fold belt. A ca. 1900-1800 Ma old episode of wrench faulting and the intrusion of 1790-1770 Ma old post-kinematic granites concluded the Svecokarelian evolution of the northeastern Shield region.

An introduction to the European Geotraverse Project: First results and present plans

The European Geotraverse (EGT) is an international, multidisciplinary project focused on a north-south orientated lithospheric profile, 4000 km long and of varying width, extending from northernmost Scandinavia to North Africa. This profile consists of three interlinking Segments (Northern, Central, and Southern) comprising a continuous succession of tectonic provinces ranging from the oldest Precambrian areas of the Baltic Shield to the currently active area of the Western Mediterranean. The broad aim of the EGT Project is to obtain a better three-dimensional picture of the structure, state, and composition of the continental lithosphere to use as a basis for an understanding of its evolution and dynamics. All of the 12 major projects that constitute the EGT “Joint Programme” have now been initiated, and several of these projects are nearing completion. Along the Northern Segment, data from “The Fennoscandian Long-Range Project” (FENNOLORA), a 2000 km long seismic profile across the Precambrian Baltic Shield, show that except beneath southern Sweden, the Shield is characterized by a high-velocity, 40–50 km thick crust—including a 5–10 km thick crust-mantle transition zone. An alternating series of 4–6 high- and low-velocity zones is present in the subcrustal lithosphere, the base of which increases in depth from ca. 110 km to ca. 230 km from south to north beneath the Shield. The top of the mantle transition zone lies at a depth of about 450 km. The second major project along this Segment, the EUGENO-S (European Geotraverse Northern Segment—Southern Part) project, is a multidisciplinary study of the Fennoscandian Border Zone, and was largely completed in 1984 with the realization of a large-scale seismic experiment. Preliminary interpretation of the excellent data obtained indicate the presence of strong lateral variations in internal crustal structure beneath the Danish Basin. Field work for a third major project, a multidisciplinary transect of the Archaean and Early Proterozoic terrains in the northernmost part of the Shield (the “Polar Profile”), was carried out in 1985. A series of deep seismic reflection lines has so far been realized in the area of the Central Segment in the context of German national programmes. First interpretations of the seismic data from a 260 km long profile across the two main intra-Variscan (Hercynian) lineaments have shown the presence of numerous horizons making up a highly reflective zone in the lowermost 10 km of the crustal section studied, and distinct changes in reflectivity between the main Variscan tectonic zones. In 1986, the entire Segment will be investigated in detail in an ambitious international programme of integrated geological and geophysical studies. A series of seismic experiments (termed EGT-S) have been carried out across the Southern Segment (in 1982, 1983, and 1985). Interpretation of data from the 1982 and 1983 experiments have led to several interesting results, including: 1. (1) the suggestion that two “crust-mantle”-like interfaces exist beneath the Po Basin (at depths of about 35 and 50 km) and adjacent tectonic units, these interfaces marking a deep contact zone between the Adriatic and European plates, 2. (2) in the area between Genoa and Corsica, the Ligurian Sea is underlain by a greatly thinned, distinctly layered section of continental crust, and 3. (3) Corsica and Sardinia are underlain by bowl-shaped, “typically” Variscan continental crusts. The 1985 phase of seismic surveys focused on crustal structure beneath Tunisia and the adjacent seas. In addition, two off-traverse projects are being realized. First, a wide-aperture network of autonomously recording seismic stations (“NARS”), installed along the line Gothenburg-Málaga between 1982 and 1984, is already yielding high-quality data on the upper 600–700 km of the mantle. Second, an investigation of lithospheric seismic anisotropy in the area of the Iberian Peninsula is being organized for 1987–1988. Finally, of great importance are the systematic compilation of existing data and, where needed in critical regions, collection of new geophysical and geological data presently being carried out for the entire area encompassed by the EGT. It is expected that these compilations will be completed by 1987, at about the same time that full results from the main large-scale seismic experiments become available, enabling the construction of an integrated lithospheric cross-section along the EGT, requiring a final phase of intensive multidisciplinary collaboration.