Seismic anisotropy studies using both surface and body waves constrain the lithosphere thickness of Precambrian cratons to a maximum of about 200-220 km and provide information on large-scale fabric within the subcrustal lithosphere and asthenosphere. High lateral resolution of body-wave studies enable us to map orientations of anisotropic structures within smaller-scale blocks and to define their first order sutures separating blocks with different orientation of their large-scale fabric.
On the contrary, the surface-waves with their radial resolution and lateral integral effects, detect anisotropy beneath the Precambrian shields and platforms which exhibits its maxima at about 100 km depth with Vsv>Vsh signature, while beneath Phanerozoic regions it is characterized by Vsh>Vsv, with maximum at depths about 70 km. An interpretation of the observed seismic anisotropy by the preferred orientation of olivine crystals results in a model of the mantle lithosphere characterized by anisotropic structures with the a-c foliation planes plunging steeply beneath Precambrian shields and platforms, compared with less inclined anisotropies beneath Phanerozoicorogenic belts. The different olivine orientations can result from different processes that lead to the formation of the continental lithosphere in the Precambrian, compared with the Phanerozoic, and that probably changed considerably during the Proterozoic.
Babuska, V et al, Pageoph, 151, 257-280, (1998).
The LITHOPROBE Western Superior Transect was established to investigate the Archean crust generation and evolution in relation to tectonic processes. The transect includes a high resolution refraction and wide-angle reflection seismic survey along two lines running parallel and perpendicular to the strike of metaplutonic, metasedimentary and volcanoplutonic subprovinces, which are the basis of proposed accretionary models for the area. The high quality of the data allows placing constraints on the seismic structure of the upper 100 km of the lithosphere beneath the continental block. We have done ray-based travel time inversion to construct a 2D P-wave velocity model, with additional constraints provided by modeling the amplitude trends of the data via 1D synthetic seismograms.
In the upper crust Vp ranges from 6.0 to 6.4 km/s down to about 18 km depth. Along tectonic strike, the east-west transect reveals a sharp contact between the seismically distinct English river metasedimentary belt (Vp= 6.0 km/s) and the Wabigoon granite/greenstone belt (Vp=6.2 km/s). The top of the middle crust is a first order discontinuity that marks a jump in velocity from 6.4 km, above, to 6.7 km/s below. Moreover the middle crust appears to pinch out, from 18 to 25 km depth, in the southern end of the north-south line. The lower crust is modeled as a high vertical velocity gradient layer (from 6.9 to 7.2 km/s) and overlies a Moho (41 km depth) that exhibits only little topography. Within the upper mantle, Pn, Pn* and Pum phases are correlated laterally continuous along the two lines. Pn phases indicate that the seismic upper mantle starts with a sub-Moho velocity of 8.1 km/s within an 8 to 10 km thick layer. Pn* are phases showing an azimuthal dependence velocity variation that could be interpreted to be anisotropy. The velocity in the north-south direction is higher (8.7 km/s) than in the east-west direction (8.4 km/s). The hypothesis of anisotropy within this layer is under investigation. Pum are modeled as refracted waves travelling within a north-east dipping layer (from 60 to 85 km depth) with a velocity of about 9.2 km/s. We investigate the interpretation of the above lithospheric seismic structure within the frame of plate tectonic, comparing the Western Superior Province with the modern orogenic belts.
The high grade metamorphic mid-Proterozoic Grenville Province in southeastern Canada exposes the mid- to deep levels of a collisional orogen. In Ontario, the crustal scale Central Metasedimentary Belt boundary thrust zone (CMBbtz) represents a major Grenvillian suture, along which the ca. 1350-1100 Ma magmatic arc(s) of the Central Metasedimentary Belt (CMB) were emplaced on top of the pre-1400 Ma Laurentian craton, represented by Central Gneiss Belt (CGB).Previous interpretations concerning the tectonic style and the timing of Grenvillian metamorphism and ductile deformation are varied and partly inconsistent: I) Synkinematic pegmatites within the CMBbtz were dated at 1190-1180 Ma and at 1080-1050 Ma, and interpreted to reflect collision of the CMB with Laurentia and later reactivation of the CMBbtz, respectively (e.g. McEachern and van Breemen 1993, Nadeau and Hanmer 1992). II) In contrast, thrusting and metamorphism further west along Georgian Bay from 1080-1030 Ma was interpreted to reflect an in-sequence progression of the Grenville orogen into its foreland (e.g., Culshaw et al. 1994, 1997, Jamieson et al. 1995). In the study area, emplacement and stacking of lithologically and structurally different thrust sheets during the Grenvillian orogeny was accompanied by intense progressive deformation, high-grade metamorphism and anatexis. Peak-P-T conditions derived from mafic enclaves in the immediate footwall of the CMBbtz range from 10.2 to 11.2 kbar and 750 to 850°C, similar to peak temperatures derived from the amphibolite facies hosts rocks. U-Pb zircon data constrain the timing of this high-grade metamorphism and anatexis from ca. 1079 to 1063 Ma. As there is no earlier, ca. 1190 Ma tectonometamorphic event recorded in the study area, these data are interpreted to reflect the accretion of the Central Metasedimentary Belt onto the Central Gneiss Belt shortly before 1080 Ma.The retrograde P-T paths are characterised by near isothermal decompression, consistent with field evidence reflecting continental thickening and later extension, and with the occurrence of sillimanite as the stable alumosilicate. The timing of extension in the study area is interpreted to have happened at approximately the same time as extension in other areas of the Central Gneiss Belt in the range of ca. 1040 to 1020 Ma, and predated late-stage thrusting in the Grenville Front Tectonic Zone (GFTZ). In conclusion, the tectonic evolution in the study area is consistent with the previously proposed tectonic model of the normal northward propagation of the orogen into its foreland. This and the contemporaneous extension within the orogen during the later stages of the Grenvillian orogeny suggests that tectonic processes during the mid-Proterozoic operated similarly to those in modern collisional orogens such as the Himalayas.
Culshaw NG, Ketchum JWF, Wodicka N & Wallace P, Can J Earth Sci, 31, 160-175, (1994).
Culshaw NG, Jamieson RA, Ketchum JWF, Wodicka N, Corrigan D & Reynolds PH, Tectonics, 16 (6), 966-982, (1997).
Jamieson RA, Culshaw NG & Corrigan D, J Met Geol, 13, 1-23, (1995).
McEachern S & van Bremen O, Can J Earth Sci, 30, 1155-1165, (1993).
Nadeau L & Hanmer S, Tectonophysics, 210, 215-233, (1992).
The Trans-Hudson and Grenville orogens bear similar pre-collisional accretionary histories. However, contrasting post-collisional evolutions in each orogen has led to major differences in crustal architecture, metamorphic overprint and type of magmatism. The Trans-Hudson Orogen in its type area in central Canada consists of a collage of juvenile terranes flanked, and cored by moderately reworked Archean crust. Present-day crustal architecture outlines the collisional phase of the orogen and is consistent with the subduction polarity inferred from the location of arc magmas along the northwestern margin only. Metamorphic grade across the orogen is characterized by lower- to upper-amphibolite facies assemblages with metamorphic pressures generally not exceeding 600 MPa. Post-collisional magmatism is restricted to widely distributed but relatively small volumes of leucogranitic melts.
In contrast, the Grenville Orogen in Canada preserves only a small volume of accreted arc material and contains a much larger proportion of reworked basement. The crustal architecture is characterized by subhorizontal to southeast-dipping reflectors, which contradict assumptions of northwestward subduction polarity inferred by the distribution of continental arc magmas in the continental "upper plate". Metamorphic grade across the Grenville Orogen is dominated by uppermost-amphibolite to granulite facies assemblages and local presence of eclogites in the hanging wall of deeply rooted thrusts, with pan-orogen metamorphic pressures commonly exceeding 800 MPa. Post-collisional magmatism is characterized by the emplacement of anorthosite massifs and related hypersolvus granitoids (AMCG suites).
We attribute the fundamental differences in structural, metamorphic and magmatic styles between the two orogens to the duration and extent of post-collisional convergence. The longer period of convergence during the Grenvillian orogeny (~ 200 m.y. duration) may have led to more pronounced crustal and lithospheric thickening, facilitated exhumation of lower continental crust, and favoured the propagation of the orogen into the foreland. The anomalously thickened lithospheric root increased its potential of becoming detached or convectively removed, resulting in asthenospheric ascent and production of AMCG complexes. In contrast, less pronounced post-collisional convergence during the Trans-Hudson orogeny (~ 35 m.y. duration), followed mostly by transpression, favoured the preservation of supracrustal assemblages, limited the amount of crustal and lithospheric thickening, and minimized the production and emplacement of post-collisional magmas.
A number of deep seismic reflection profiles have now imaged Paleoproterozoic arc-continent collision zones within several of the world's cratonic regions. Prominent examples include the accretion of the 2.0-1.8 Ga Svecofennian terrane to the Baltic craton, the accretion of the 1.92-1.83 Reindeer zone and the Ungava belt of the Trans-Hudson orogen to the Superior craton and the accretion of the 1.92-1.90 Hottah terrane of the Wopmay Orogen to the Slave craton. Each example reveals new details of and variations in this accretion process, but the fundamental role of a wedge-shaped older craton flaking off parts of the crust of the juvenile terrane emerges in all cases studied to date. The crustal level at which the cratonic wedge causes detachment within the colliding terrane varies from the mid-crust to the Moho, with the implication that a significant portion of the colliding lithosphere is underthrust beneath the leading edge of the lower plate. Underplating of these deep crust and mantle portions of colliding terranes (typically intra-oceanic and continental arcs) against the lower plate lithosphere represents one possible mechanism for growing the lithospheric roots of continents, and may be manifest by some of the mantle reflectors imaged by the Hottah-Slave (Wopmay Orogen) collision zone. Late transpression often plays a major role in modifying the original collisional structures. More modern analogue studies from the Banda arc of Indonesia and the Caledonides of the UK, also anchored by deep seismic reflection profiles, provide insight into how these final wedge geometries develop. Late stage polarity reversal appears to play a key role and usually results in the prominent reflector geometries observed in the ancient orogens. The ramp and wedge geometries also are important in understanding many of the geochemical and isotopict patterns observed in late or post-orogenic plutons in the ancient orogens. Highly evolved isotopic signatures are found in plutons that intrude juvenile rocks at the surface but that overlie lower plate cratonic lithosphere at depth.
Like many other collisional orogens, southern Scandinavia is transected by several suture-like tectonic belts. The largest of these are the Mandal-Ustaoset and Great Breccia Zones in Norway, the Mylonite-, Protogine- and Göta Älv-Sillerud Zones in Sweden, and the Örje Zone which skirts the Swedish-Norwegian border.
Initially, a number of these N-S trending tectonic zones were interpreted as sites of extinct oceans. Somewhat later, most have been described as uniformly west-dipping, Moho-deep listric thrust-faults created by compression during the late Mesoproterozoic Sveconorwegian orogeny. New studies reconsidering the previous deep-seismic data by imposing geological and geochronological constraints have now substantially diversified these models.
The scene for the formation of essentially N-S trending sutures in southern Scandinavia was set in the Palaeoproterozoic. Around c. 1.8 Ga, the Fennoscandian and Sarmatian/Volgo-Uralian protocratons docked with each other, thus creating the beginnings of ancient Europe. In consequence, the Palaeoproterozoic accretionary belts growing from the Archaean core of Fennoscandia toward the present southwest and from Sarmatia toward the northwest were united (cf. S. Bogdanova's abstract in the present volume). Thereafter, general westward growth of essentially juvenile crust ensued, now within the framework of a giant accretionary belt extending from Europe to Laurentia. This transition from southwestwards "Svecofennian" to westward "Gothian" accretion was marked by the formation of the N-S trending Transscandinavian Igneous Belt (TIB).
Within the c. 1.7-1.55 Ga Gothian orogen west of the TIB, several structurally different sub-provinces can be distinguished. In the east there are largely east-west striking structures possibly related to earlier Svecofennian patterns. Farther west, the tectonic grain trends north-south. The separating "Mylonite Zone" is Sveconorwegian but may reproduce an earlier demarcation between structurally different crustal segments. At least, several of the tectonic belts in the west have "Gothian" precursors. Thus the Göta Älv-Sillerud Zone controlled intrusions of 1.56 - 1.57 Ga granites, while the Örje Zone, although marked by Sveconorwegian eastward thrusting, essentially follows the boundary of Gothian igneous belts against similarly Gothian accretionary-prism terranes.
In southern Norway, substantial remodelling is now required by finds of >1.7-Ga rocks (Ragnhildstveit et al., 1994, etc) and the discovery of formations indicating middle Mesoproterozoic rifting (Åhäll et al., 1998). In the light of these findings, the great suture zones in the west appear other than solely faults marking eastwards Sveconorwegian thrusting. Also in the east, Mesoproterozoic rifting along and west of the Protogine Zone occurred at c. 1.5 Ga and again at 1.25-1.20 Ga, while lithological continuity across that zone suggests uplift on its western side after the erosion of covering Sveconorwegian nappe piles rather than superposition of a crustal-thickness thrust slice onto an eastern foreland.
The present re-interpretation demonstrates the necessity of meticulous integrated geophysical-geological study, simultaneously showing the dire effects of single-method simplistic approaches.
Åhäll K-I, Cornell DH & Armstrong R, Precambrian Research, 87, 117-134, (1998).
Ragnhildstveit J, Sigmond EMO & Tucker RD, Terra Nova Abs Suppl, 2, 615-16, (1994).
The Zenaga inlier represents the northern margin of the Palaeoproterozoïc Eburnean domain of the West African Craton in Morocco. It is characterized by crystalline, metamorphic and intrusive rocks. The main granitoïds of the Zenaga inlier represent the Eburnean event (Azguemerzi granitoïds) in the south. The Pan-African event in the north affected mainly the (Eburnean) Tazenakht monzogranite to syenogranite. Petrographically, it is composed of microcline and plagioclase, quartz, biotite and muscovite with a porphyritic texture and characterized by a strong mylonitic to ultramylonitic foliation with layers of mica separated by quartz and feldspar ribbons. The measurements of different structural elements show a regular foliation which strikes NW-SE and dips toward the SW. The dips are shallower in the southern part of the Tazenakht granite and steeper in the north with some shear planes. Microscopic studies show broken mega alkali-feldspar which is re-oriented parallel with the mylonitic foliation, the elongate quartz crystals show dynamic recristallization with C/S surfaces, and plagioclase has bent lamellae. These observations indicate an intense and heterogeneous deformational patterns, transforming the granite, locally, to mylonite and blastomylonite, documenting the existence of a large zone with ductile shearing. The thermal conditions during deformation correspond to c 450°C according of FitzGerald and Stünitz (1993).
The internal structures of the Tazenakht granite reflect the Pan-African deformational patterns, which were dominated by a 4 km wide sinistral shear zone at its northern margin, which is related to the major fault (accident majeur) of the Anti-Atlas. It overprinted older structures in the Eburnean domain. The kinematic criteria in the Tazenakht granite (mega alkali feldspar, muscovite fish, pegmatitic pockets, sigmoïdal doleritic and gabbroïc dykes) indicate a transform sinistral movement along the E-W with striking Eburnean margin. Studies of the anisotropy of magnetic susceptibility also confirm this movement.
Our observations show the major fault of the Anti-Atlas as transform boundary between the juvenile Pan-African domains in the north and the pre-existing craton in the south. At the same time, the deformational patterns both in the Tazenakht granite with sinistral shearing are remarkably similar, pointing to a regionally homogeneous stress field.
FitzGerald JD & Stünitz H, Tectonophysics, 221, 299-314, (1993).
A palaeomagnetic study has been performed on Meso- and Palaeoproterozoic rocks from the Ukrainain Shield. Samples were collected from three crustal blocks and primary magnetizations were identified in 1.77 to 1.72 Ga old anorthosites and related basic dykes. Stable magnetizations were also isolated in a 2.0 to 1.8 Ga sedimentary rock and in basic dykes and a monzonite of ca 2.0 Ga age. On basis of these results a 2.0 to 1.72 Ga apparent polar wander path has been defined for the Ukrainian Shield. A tectonic comparison between the Ukrainian Shiled and the Fennoscandian Shield is possible if there are coeval rocks and palaeomagnetic pole calculated of original magnetizations from the two shield. Fennoscandia lacks poles of 2.0 Ga, however, there are data from ca 1.7 Ga and 1.8 to 1.90 Ga old rocks in the shield that may be used for a tentavive tectonic reconstruction versus the Ukraine. There is a significant difference between the palaeopoles of the Ukraine and those of similar ages from the Fennoscandian Shield. This suggests that the relative position and orientation of the Ukrainian Shield was different from its present position relative to Fennoscandia at 2.0 to 1.7 Ga ago. One pole of a yet not dated basic dyke falls on the 1.58/ 1.30 Ga part of the Fennoscandian APWP and it may represent a time when Fennoscandia was already accreted to the Ukraine. Contemporaneous rifting of the two shields at 1.35 Ga indicates that they were already joined to each other at that time, which means that the final accretion may have taken place during 1.72 - 1.58 Ga. After 1.58 Ga the Ukraine and Fennoscandia probably formed a part of a pre-Rodinian supercontinent that was assembled in the Mesoproterozoic.
During EUROBRIDGE'95 and '96 seismic data were acquired along a 700 km NW-SE profile, from the West Lithuanian Granulite Belt (WLG) to the Ukrainian Shield. Shotpoints at 30 km interval were recorded by 3-component seismographs deployed at 3-4 km intervals. Tomographic inversion and raytrace modelling established a 2-dimensional P-wave velocity lithospheric model and the spatial variations of Vp/Vs. Upper, middle and lower crystalline crust exhibit velocities of 6.1-6.3, 6.4-6.8 and 6.9-7.2 km/s. P-wave velocities immediately beneath Moho are 8.2-8.35 km/s. Crust below Lithuania is about 44 km thick, and below Belarus about 50 km thick with Moho elevations of a few km. A lower lithosphere reflector occurs at 65-70 km depth. S-wave velocities are relatively high in the upper crust and low in the lower crust. Correlation of our seismic structure with near surface geology tentatively suggests that the contact zones between the East Lithuanian Belt, Belarus-Baltic Granulite Domain, Central Belarussian Belt (CB), and the Osnitsk-Mikashevichi Igneous Belt all dip slightly to the south-east, consistent with successive docking of these terranes during craton growth. A spectacular feature of our model is high velocity body in the CB crust, which marks the Fennoscandia - Sarmatia suture. Here we observe change from typical shield/platform crust in the north-west to highly heterogeneous crust with pronounced lower crustal lower reflectivity in the south-east. Our results are consistent with CB uplift during continental collision. We discuss the implications of our model for the evolution of the Pripyat Trough.
The paper presents a detailed interpretation of deep seismic data along profiles in the Precambrian Craton in Poland based on the large scale seismic experiment POLONAISE '97. This paper covers the interpretation of seismic data from three profiles of total length of about 900 km. The recordings were of high quality, with seismic energy visible up to a 400 km offset. The crustal model was developed by tomographic inversion and 2-D raytracing forward modelling. The crystalline crust consists of 3 parts: the upper, middle and lower crust with P-wave velocities of 6.0-6.4, 6.4-6.8 and 7.0-7.15 km/s, respectively. We have found the high-velocity body which can be correlated with Mazury rapakivi-like complex. The PmP wave is usually correlated at distances from 100 km with variable reflection character. Moho depth varies from about 40 km towards south-west and to about 50 km at the north-eastern part of the study area. The Vp/Vs ratio was determined separately for the upper/middle and lower crust to 1.67 and 1.77, respectively. Behind the Pn wave with apparent velocity of 8.0-8.15 km/s, another phase with an apparent velocity of about 8.6 km/s is interpreted as a reflection from a discontinuity in the uppermost mantle.
Deep Seismic Sounding (DSS) profile EUROBRIDGE'97 traverses the north-western part of the Ukrainian Shield within the Sarmatian major crustal segment of the East European Craton. The field experiment was conducted in September 1997 along a 530 km N-S transect as an international co-operation between the Ukraine, Belarus, Poland, Germany, Denmark, Finland, Sweden and UK. Recording a total of eighteen shotpoints with shot sizes varying from 250 kg to 1000 kg was undertaken using 120 mobile 3-component seismographs in two deployments at a station spacing of 3-4 km. Most shots provided high quality traces with good signal-to-noise ratios, for both P- and S-waves, particularly where basement is at or near surface. Shots fired within the thick sediments show low signal-to-noise ratios.
The north end of the profile is located within the Osnitsk-Mikashevichi Igneous Belt, which is an igneous complex of mainly granodiorite-granitic batholiths with essentially Andean-type chemical signatures marginal to the Sarmatian crustal segment. Profile croses the Phanerozoic sediments of the Pripyat Trough which is a part of the extensive Pripyat-Dnieper-Donetsk Palaeorift. Further south EUROBRIDGE'97 traverses the Volyn Block, the anorthosite-rapakivi Korosten Pluton, and Podolian Block. The Korosten Pluton is of particular interest. It is a layered intrusion of some 6 km thickness, below which seismic reflection and gravity data indicate that the crust is extensively intruded by mafic intrusions.
We present 2-D crustal models based on both P- and S-waves with use of tomographic inversion and ray tracing technique.
Two dimensional P-T-t modeling was used to test quantitavely an existing terrane convergence hypothesis for the amalgamation of the western part of the East European Craton (EEC). Metamorphic P-T and radiometric data from two major Precambrian basement terranes in Lithuania, the West Lithuanian Granulite Domain (WLG) and East Lithuanian Belt (ELB), separated by the Middle Lithuanian Suture Zone (MLSZ), have been considered.In the P-T-t model, the tectono-metamorphic history began with the subduction of island arc terranes beneath an already accreted continetal nucleus ca. 1.85 Ga. In the WLG, metamorphic peak conditions of 800-850°C and 8-10 MPa are predicted to have been reached after a relaxation time of 50 Ma, i.e. ca. 1.8 Ga, given an assumed convergence rate of 0.5 cm/a. This was followed by a rapid exhumation phase (1.8-1.79 Ga) that is interpreted to be related to orogenic collapse processes. It is inferred that underthrusting of the ELB beneath the WLG ca. 1.7 Ga occured such that a phase of nearly isobaric cooling of the latter was interrupted and its cooling reversed. Simultaneously, the amphibolite facies rocks of the MLSZ reach their thermal peak of 650-700°. Heating of these was interrupted in the model by a second orogenic collapse and exhumation event. A substantial rise of the lithosphere-astenosphere boundary is predicted for the latter, which might have triggered the intrusion of rapakivi granites, which started ca. 1.6 Ga in the northwest of the WLG and were abundant at 1.5-1.45 Ga in the ELB and MLSZ. The model fails to explain the documented temperatures (mainly in the ELB and MLSZ), however, which are inferred to have been higher within the upper crust. This might be explained by an additional heat influx from the ascending rapakivi intrusions. Reasonable heat flow values (85mW/m2), comparable with present day values, were used in the calculations.
Karelia craton is a typical granite-greenstone area (GGA). The greenstone belts (GB) includes mostly epidote-amphibolite facies volcanics formed in various geodynamic environments. Their mutual relationships are believed as a result of putting together fragments of volcano-sedimentary successions that were initially originated in various geodynamic settings (mafic-ultramafic pillow-lavas of oceanic plateaus and back-arc basins, calc-alkaline series of island arc and active margin types, turbiditic-terrigenous with interbedding BIF rocks, acid pyroclastics, carbonates and carboniferous rocks that are similar to passive margin or back-arc basin assemblages). The geochronological data (Levchenkov et al., 1989; Lobach-Zhuchenko et al., 1986; Martin, 1987) manifest that age of the Late Archean assemblages becomes younger from east to west. The 3.2-3.1 Ga TTG gneisses examined in the Vodlozero block (South-Eastern Karelia) include some fragments of 3.6-3.5 Ga crust. Younger tonalites of 3.0-2.9 Ga age are concentrated in southern and south-eastern parts and the 2.9-2.8 Ga TTG gneisses are characteristic for central and western parts of GGA although some relic dates like 3.37 Ga are known (Kozhevnikov et al., 1987). The youngest granitoids (Naavala Gneisses) of 2.7-2.6 Ga localized in the latter area. Age of greenstone volcanics is characterized by similar regularity (Piirainen, 1988; Pukhtel et al., 1991, 1996; Samsonov et al., 1996, Sochevanov et al., 1991). Most ancient date (3.4 Ga) was obtained in the komatiitic section of Vodlozero block, Kamennoozero volcanics of the East-Karelian greenstone zone are 3.1-2.9 Ga old, Koykara andesitic-dacitic volcanics were originated 2.86 Ga ago. Kostamukcha GB have been constructed 2.8 Ga ago as a result of accretion of the 2.84 Kontock mafic-ultramafic terrain to passive margin of the Gimola terrain covered by terrigenous rocks (Puchtel et al., 1996). Similar dates were obtained for the Kuhmo-Suomussalmi GB in the western part of the Karelia craton.
Based on above considered geochronological data the evolutionary model of westward migration of riftogenesis across ancient basement have been elaborated (Levchenkov et al., 1989; Lobach-Zhuchenko, 1986; Glebovitsky, 1995). Keeping in mind not only geochronological data but the participation of oceanic and island-arc type assemblages in greenstone successions and suprasubduction conditions for TTG origin we interpret the Karelian GGA as the Late Archean accretionary orogen formed as a result of consequent eastward directed accretion of island-arc and oceanic terrains to multiply reproduced active margin. Final structure of the Karelian GGA resulted from rheomorphism and doming of stacked crust. Generally, the main stages of an origin and evolution of the Karelian GGA have duration of 300-400 Ma and have been completed by incorporation of the Karelia craton to marginal part of the Supercontinent that has been formed at the very end of the Late Archean time.
The geological structure of the Central-Kola granulite belt have been considered differently: as the most ancient area of consolidated continental crust of the Baltic Shield (Goryainov, 1980), as the Archean-Paleoproterozoic mobile fold belt (Zagorodny and Radchenko, 1987), as strongly folded and partially imbricated block of the Sírvaranger-Kola terrane (Dobrzhinetskaya et al., 1995) or of the Kola domain (Mitrofanov et al., 1995). Thrust-nappe pattern of the Central-Kola granulite belt has been developed on a basis of integrated study of space images, geological and geophysical data (Barzhitsky, 1988; Mints et al., 1992, 1996).
The real features of general structure of the Central-Kola belt up to 15 km depth may be observed at cross-sections elaborated in a result of reflection seismic sounding (CDP-CMP) and precision gravity measurements along 1-EV (Pechenga-Liinahamari-Murmansk-Monchegorsk) profile. Special processing using the differential seismics method (MDS) permitted to discover a system of northward dipping reflectors that can be traced up to 10 km depth. This reflector system may be observed especially well at the profile section from Kola Ultradeep well to Liinahamari. Some worse the same system is seen in northern and central parts of the Murmansk-Monchegorsk section. Corresponding seismic boundaries are to be related to the geologically mapped thrust type faults. In both sections this system is overshadowed by subhorizontal reflections which are especially multiple in the upper part of the crust up to 4-6 km depth. The suggested interpretation of the northward dipping seismic boundaries as thrust-nappe faults is supported strictly by the results of gravity field treating. The model ellipsoid-shaped bodies are northward-dipping too in certain concordance with seismic boundaries.
Suggested model of geological structure of the Late Archean Central-Kola granulite belt is based on the considered above seismic, gravity and geological data and the results of space image interpretation: the Central-Kola belt is to be considered as the northward descending thrust-nappe ensemble. According to plate-tectonic model of the Late Archean evolution of the northeastern Baltic Shield (Mints, 1992; Mints et al., 1996) thrusting and corresponding crustal thickening are attributed to the Late Archean collision between Central-Kola and Murmansk microcontinents 2.8-2.7 Ga ago. In frame of that model the Central-Kola belt was appeared as front thrust-nappe ensemble originated after closure of the Titovka-Keivy ocean and as a results of compression of the front part of the Central-Kola microcontinent caused by its subduction beneath an active margin of the Murmansk microcontinent.
Amazonite rand-pegmatite of Mt.Ploskaya occurs in exocontact zones of the alkaline granite of the East Keivy Structure, which is situated in the eastern Baltic Shield. Dipyramidal-prismatic zircon megacrysts are 1 to 10 cm in size, consist of monomineral aggregates or chains of single split crystals, and are found in interstices among rock-forming minerals. According to Ivanyuk et al. (1997), the zircons originated during a postmagmatic albitite stage of the crystallization of the Ploskogorskoye deposit.
For the U-Pb isotope dilution analysis we took one big crystal of zircon (8 cm) and cut it along the prism. One half of the crystal was analyzed with the use of conventional separation procedures (heavy liquids, magnetic separation and hand picking). It was crushed to 0.01 cm and subdivided by colour into three parts. To analyze the other half, we picked up visibly unaltered parts of the central and inner zones of the crystal with a needle and abraded these two parts following the technique of (Krogh, 1992).
The resulting five points gave a U-Pb age of 1682±35 Ma, which corresponds to the pegmatite-forming stage that is widespread in the Keivy Structure. The three points obtained by conventional techniques are in the upper part of the isochron diagram. The two points of visibly unaltered zircon are in the lower part and yield 446±35 Ma, which corresponds to the age of Palaeozoic tectonic-magmatic activation of the Baltic Shield (Kramm et al., 1993). Curiously, the coordinates of one point in the U-Pb diagram have a 207Pb/206Pb age of more than 2.4 Ga.
Thus, the conventional crushing technique is more recommended for U-Pb dating of zircon megacrysts, because the U-Pb system in visibly unaltered crystal zones appears to be more reset.
The study was supported financially by the Russian Foundation for Fundamental Investigations, grant N 98-05-64321.
BABEL line B in the Baltic Sea ran at a high angle to the dominant structures in the SW margin of the Palaeoproterozoic Svecofennian orogen. The original line was subsampled to 8 ms and processed with generally the same parameters as the rest of BABEL. Because line B was recorded with much noise, the final stack showed very little from the upper crust. To improve signal-to-noise ratios, particularly in the upper few seconds of the data, we applied strong filters to the data using the original 4 ms sample rate. Routine, but carefully selected top mutes were chosen. Most important was the application of a RADON filter, designed and applied in the tau-P transform domain, which removed 'noise' propagating at very low velocities through individual shot gathers and also any signal that did not have hyperbolic moveout characteristic of real reflections. After filtering, new velocity analysis was made, consistent with the improved velocity models that became available since the time of the original processing (Thybo, Barton, Abramovitz models). Only the upper half of the crust was reprocessed, but it revealed many more bifurcating reflections than did the original stacked sections. Such a pattern cannot be attributed easily to residual noise or processing artefacts.
In the northen half of line B two successive wedges indentate upper crust towards the north, at 3.5 and 4 s TWT respectively. As c. 10 kms of crust has been eroded since Palaeoproterozoic time, the structures were originally formed at a depth of c. 20 kms. The overlying upper crustal lids are internally deformed by numerous S-vergent, N-dipping thrusts. This fabric may be equated with penetrative 'older' Svecofennian structures in the exposed Svecofennian domain in S. Sweden. The N-dipping soles of the upper lids coincide with the seaward extension of major lithological boundaries and late Svecofennian deformation zones. Such consistencies of reflection patterns with mapped structures onshore give us added confidence that important new structural geometries were revealed by the reprocessing. A strongly reflective package in the northern wedge, near Gotland island, previously interpreted as post-orogenic, Jotnian sills (Beunk et al., 1992), has been involved in the thrust structures and presumably represents older, Palaeoproterozoic mafic intrusions or a remnant of oceanic crust. Earlier work on the profile has shown that, in the lower crust, the wedges are underlain by N-dipping reflectors of crustal dimension. Similar Z-like reflection geometries are known from other collisional belts.
Beunk FF, Meissner R, Sadowiak P & Thomas SA, The BABEL Project, Comm. Europ. Commun, 117-121, (1992).
The Southwest Swedish Gneiss Province has a complex geological history with a high-grade metamorphism during the Sveconorwegian-Grenvillian orogeny, approximately 0.9 Ga ago. However, this tectonothermal event is not confirmed by conventional multi-grain U-Pb dating of zircons on high-grade gneisses from the same area. In order to test the potential of chemical age dating of zircons by electron microprobe in high-grade rocks, we selected a quartz syenitic granulite which already have been dated by conventional U-Pb dating (Johansson et al. 1993). The isotopic multi-grain zircon data yield a poorly defined discordia with an upper and lower intercept age of 1452 +350/-50 Ma and 317 +813/-966 Ma, respectively. Selected zircons within polished thin-sections were analyzed using an automated Cameca CAMEBAX electron microprobe at 25 kV and 100 nA probe current using the method described in detail in Geisler and Schleicher (in review). Backscattered electron images of zircons show distinct dark cores, surrounded by complex irregular growth zones and finally by small bright mantle domain, the latter which obviously reflect recrystallized outer parts of the zircons. Preliminary U-Th-Pb analyses of the mantle domains and the cores yield a well defined sveconorwegian and svecofennian U-Th-total Pb age, respectively. In contrast highly scattered apparent ages of the irregular growth zones range between the age of mantle domains and cores. This scatter could be explained by (i) a disturbance of the U-Th-Pb system by the sveconorwegian metamorphic imprint or (ii) may be the result of more than one further growth event which recently have been established by ion microprobe dating of zircons from this area (Johansson, pers. communication). The mean age of these zones, however, is close to the conventional upper intercept age. The most striking feature of our result is the svecofennian age of the zircon cores which is in accordance with Nd model ages of the gneisses in this area (Johansson et al. 1993), and the first report of a svecofennian crustal component within the Southwest Swedish Gneiss Province.
Johansson A, Meier M, Oberli F, Wikman H., Precambrian Res, 64, 361-388, (1993).
Geisler T, Schleicher H, Submitted to Chem Geol
Panafrican tectonics (600-700 Ma) deformed the Arabian Shield into orogenic zones and intracontinental molasse basins, which have been studied at three structural levels: Domes of para- and orthoderived gneisses associated with both NW-SE left-lateral and N-S right-lateral shear zones are found in the domain of ductile deformation, which is characterized by a subhorizontal stretching lineation. The rise of the domes was accompanied by the exhumation of deep metamorphic (amphibolite) facies rocks. Shear zones at all scales in the domain of ductile-brittle deformation are characterized by kinks and a poorly developed fracture cleavage. The formation of foreland and intramontain molasse basins, the kinetics of which was complementary to that of the gneiss domes, are shown by the topographic effects at the surface of the orogenic belts. The corresponding sedimentary formations are of Murdama age (cf. deposits of the Murdama, Thalbah, Hadiyah, Ablah, Ghamr, etc.). These preliminary results are part of an ongoing research project on the Arabian Shield. On a larger scale they contribute to a reconstruction of the Panafrican orogeny.
In the frame of a global revaluation of the geology and metallogeny of the Arabian shield, a compilation of all the aeromagnetic surveys covering western Saudi Arabia has been completed. Ten aeromagnetic surveys (655000 km2), carried out from 1962 to 1983, stored on magnetic tapes, realised at different terrain clearance (150 m, 300 m and 500 m) and different line spacings (800 m, 500 m and 2000 m) have been compiled in a 1 km elementary squared grid at an altitude of 300 m. In order to interpret the magnetic anomalies in terms of geological structures a reduction to the pole to replace the magnetic anomalies at the top of the sources has been achieved. A vertical gradient has been computed in order to underline the short wavelength anomalies which are associated to shallow sources such as the basaltic dykes due to the opening of the red sea. The resulting maps show magnetic anomalies which are well correlated with structurally- and geochronologically-constrained geological features. They shed new light on the accretion, the panafrican tectonics and the intimate relationship between the N-S Nabitah fault zone and the NW-SE Nadj faults.
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