Journal of Conference Abstracts

Session D01:1B

D01 : 1B/25 : G2

Evolution of the Meso- to Neoproterozoic Sedimentary Basin of Southeastern Siberia

Andrei K. Khudoley (khudoley@AH3549.spb.edu)¹,

Robert H. Rainbird (rrainbir@nrcan.gc.ca)²,

Richard A. Stern (rstern@nrcan.gc.ca)²,

Anatoli P. Kropachev (vsegei@mail.wplus.net)¹,

Larry M. Heaman (larry.heaman@ualberta.ca)³ &

Victor N. Podkovyrov⁴

¹ All Russian Geological Research Institute (VSEGEI), Sredny Prospect 74, St.Petersburg, 199106, Russia

² Geological Survey of Canada, 615 Booth Street,Ottawa, Ontario, K1A 0E6, Canada

³ Dept. of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, T6G 2E3, Canada

⁴ Institute of Geology and Geochronology of Precambrian, Makarova 2, St.Petersburg, 199034, Russia

The Meso- to Neoproterozoic sedimentary succession of southeastern margin of the Siberian platform consists of terrigenous-carbonate units termed from oldest to youngest as Uchur, Aimchan, Kerpyl, Lakhanda, Uy and Yudoma groups. Total thickness of succession is about 12-14 km. Group boundaries typically are regional unconformities; local angular unconformities occur at the base of Aimchan and Yudoma groups. Sedimentation mostly occurred in an intracratonic sea dominated by tidal- to supratidal sedimentary environments. Only the uppermost Uy Group contains evidence of deeper water sedimentation.

Paleocurrent measurements and facies trends show that provenance of the Uchur, Aimchan and most of Kerpyl groups was from the Siberian platform. This corresponds with the mineralogical and chemical composition of sandstones, which suggests significant recycling. A U-Pb SHRIMP study of detrital zircons at the Geological Survey of Canada shows that only 3 among 64 grains are of Archean age. Grains ca. 2050 Ma predominate at the base of Uchur Group. At the base of Kerpyl Group ca. 1880-2060 Ma zircons predominate with youngest grains ca. 1300 Ma. The latter may represent some unknown source, as rocks younger ca.1700 Ma are not reported from the basement of the Siberian platform. Sandstones from Uy and Yudoma groups were derived from both western (Siberian) and eastern exotic (?) sources. Zircons in the uppermost part of Uy Group range in age from 1050 to 1500 Ma suggesting a non-Siberian provenance, perhaps from Grenvillian orogeny. Conventional U-Pb analysis of a few zircon grains from the Yudoma Group sandstones gave ages ca. 2000-2200 Ma that is significantly older than in underlying formations. Although Yudoma Group sandstones are mature in composition, REE distribution implies their source includes Archean rocks, in contrast to older sandstones that have sources of dominantly Proterozoic age.

The lower part of Uy Group hosts numerous mafic sills with U-Pb (baddeleyite) age ca. 975-1000 Ma. Chemistry of these mafic rocks is similar to that of MORB. Sill intrusion precedes deep-water sedimentation in the upper Uy Group suggesting that this mafic magmatism may have been the initial stage of rifting and formation of passive margin that existed during Paleozoic.

Research of A.K. Khudoley and V.N. Podkovyrov was partly supported by RFBR grants 98-05-65561 and 96-05-64781.

D01 : 1B/26 : G2

Proterozoic Post-Orogenic Cratonic Evolution Based on Multi-Method (Pb-Pb, Rb-Sr, K-Ar) Dating of Argillites from Gunflint Iron Formation (Lake Superior Region)

Theofilos Toulkeridis (theo@illite.u-strasbg.fr) &

Norbert Clauer (nclauer@illite.u-strasbg.fr)

Centre de Géochimie, (OEST-CNRS), 1, Rue, Blessig, 67084 Strasbourg, France

Many dating studies of Precambrian sedimentary rocks produced uncertain results owing to a lack of independent constraints on the actual timing of deposition or of disturbance. Among the controversial aspects are the ages and the used methods.

We have investigated argillaceous sediments of the Proterozoic Gunflint Iron Formation being part of the Animikie Group in the Lake Superior region, which overlies Archaean granite-gneiss-greenstone terranes of the Superior Province. The Gunflint Iron Formation is supposed to be slightly older than or contemporaneous to the Penokean Orogeny which occurred between 1870 and 1820 Ma. The rocks are virtually unmetamorphosed allowing preservation of the primary textures. Clay minerals separated from the rocks confirm this observation, indicating crystallinity indices representative of a diagenetic recrystallization without temperature increase above 150°C.

Pb-Pb isotopic results of the argillaceous whole rocks (WR) provide two isochrones identical in age at 1734 ± 17 Ma (n = 10) and 1743 ± 41 Ma (n = 5). This mean age is much lower than the inferred depositional age of 2076 ± 248 Ma based on a Sm-Nd isochron determination made on 4 WR and one chlorite fraction (Stille & Clauer, 1986). The Pb-Pb age most probably displays a thermal and/or chemical event which reseted the isotopic system at an apparent temperature of about 150°C. It is higher than a previously determined Rb-Sr age of about 1.46 Ga and K-Ar ages scattered between 1283 and 1536 Ma. One of the samples yields a K-Ar age of 1839 ± 68 Ma that is similar to the Pb-Pb ages within analytical uncertainty. It also yields the highest total organic carbon content at 9.28% and the lowest Rb/Sr ratio of all studied samples. In addition, four separated clay fractions yield the same K-Ar age scatter between 1376 and 1567 Ma.

The Pb-Pb ages provide evidence for cryptic low-thermal and/or chemical overprints in the Proterozoic sediments of this area; the Pb-age corresponding to a recently documented widespread metamorphism (Holm et al., 1998), and confirming a rapid crustal stabilization of the Penokean Orogen on the southwestern part of the sampling area. The younger K-Ar and Rb-Sr ages seem to reflect later metamorphic activities which were recognized locally, and which extend to be as low as 1100 Ma. This integrated approach allows to recognize and characterize postdepositional metamorphic activities, especially of low thermal intensity, which is essential for understanding the post-orogenic cratonic evolution in Proterozoic time.

Stille P & Clauer N, Geochim. Cosmochim. Acta, 50, 1141-1146, (1986).

Holm DK & Darrah KS & Lux DR, Amer. J. Sci., 298, 60-81, (1998).

D01 : 1B/27 : G2

Provenance Ages and Alteration Histories of Shales from the Middle Archean Buhwa Greenstone Belt, Zimbabwe: Pb and Nd Isotopic Evidence

Eirik J. Krogstad (ejk@dlc.ku.dk)¹,

Christopher M. Fedo² &

Kenneth A. Eriksson³

¹ Danish Lithosphere Centre, Oester Voldgade 10, 1350 Copenhagen K, Denmark

² Dept. of Geology, George Washington Univ., Washington, D.C. 20052, USA

³ Dept. of Geological Sci., Virginia Polytechnic Inst. and State Univ., Blackburg, VA 24061, USA

Pb isotopic compositions of whole rock samples of shales from the ca. 3.1 to 3.0 Ga Buhwa Greenstone Belt, Zimbabwe, are quite radiogenic, with a range of ²⁰⁶Pb/²⁰⁴Pb from 25.5 to 154. An array of ten samples lies scattered about a line with a ²⁰⁷Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb slope age of about 2.73 Ga. Five individual samples were sequentially leached to further test the timing and characteristics of this U-Th-Pb alteration event. These arrays of a whole rock, three leach steps, and a residue also form linear Pb-Pb arrays with ages ranging from 2251 ± 140 Ma to 2824 ± 170 Ma, suggesting that all samples experienced a latest Archean to earliest Proterozoic enrichment in U/Pb. All samples, both whole rocks and leached samples, lie grouped on a ²⁰⁸Pb/²⁰⁴Pb - ²⁰⁶Pb/²⁰⁴Pb diagram around a line with ²³²Th/²³⁸U = 3.1 (2.9 to 3.9). Because of the lack of difference in the Th/U of the samples through large ranges of U/Pb, we interpret this consistency in Th/U to mean that the shales of the Buhwa belt experienced Pb loss, rather than U and Th gain. Circumstances that may be responsible for Pb loss in a sedimentary basin include loss of saline fluids during basin dewatering. The timing of this event would likely have been in the late Archean. Values of <epsilon> _Nd at the age of sedimentation range from +0.6 to -10.8, consistent with a range of provenance ages. T_DM values range from 4.46 Ga to 2.99 Ga. The oldest crustal formation ages, up to 0.7 Ga older than known detrital components, indicate that the Sm-Nd system of the several samples of the sediments may have experienced open system behavior. The inferred alteration of samples with anomalous T_DM would have included an increase in Sm/Nd by about 20-25 percent. This may have been accomplished by preferential loss of Nd with respect to Sm.

D01 : 1B/28 : G2

Accelerated Denudation, and Evolution of Lithospheric Mantle Beneath the Kaapvaal Craton During the Mid-Cretaceous

Kerry Gallagher (kerry@ic.ac.uk)¹ &

Roderick Brown (rod.brown@latrobe.edu.au)²

¹ T.H. Huxley School of Environment, Earth Science and Engineering, Imperial College, London, SW7 2AS, England

² Dept. of Earth Science, University of Melbourne, Victoria, Australia.

New apatite fission track data from the Kaapvaal Craton in Southern Africa indicates that rocks forming parts of the present land surface resided at palaeotemperatures in excess of 110°C as recently as 80-100 Ma. There is a general trend of increasing palaeotemperature to the eastern and northern margins of the craton. Model results imply cooling of 40-50°C in the interior of the craton in the Mid-Cretaceous. Data from a deep borehole allow us to determine the palaeogeotherm at the time of this denudation. The estimate of 13±5°C/km is essentially the same as the present data estimate of 15°C/km. substantial denudation. This implies ~1.5 to 3.5 km of denudation since the mid-Cretaceous, the actual value depending on the thermal properties of the eroded material. The inferred denudation episode correlates strikingly with a dramatic, but short-lived increase in clastic sedimentation in basins off both the eastern and western margins of southern Africa (see figure), and in cratonic margin conglomerates. Furthermore, the mid-Cretaceous timing is coincident with a period of widespread kimberlite intrusion in the craton, and many of these kimberlites were exposed by the Latest Cretaceous.

We suggest that enhanced denudation reflects regional uplift and changes in drainage base-level as a consequence of a decrease in the mean density of the lithosphere. This is achieved by some kind of delamination process, whereby the lowermost lithosphere is replaced hot asthenosphere. The associated buoyancy leads to regional uplift. Provided this happens quickly, the process is effectively isothermal, and can lead to local melting of the lower lithosphere, producing the kimberlites.

D01 : 1B/29 : G2

Intracontinental Crustal Doming and Regional Extension in Central Mongolia: Structural Evidence for an Active Plume Beneath Central Asia

Dickson Cunningham (wdc2@le.ac.uk)

Dept. of Geology, U. of Leicester, Leicester, UK

The Hangay Dome in central Mongolia is a mountainous region covering over 200,000 km² with numerous peaks over 3000 m and some glacier-covered summits exceeding 3900 m. It is one of the world's least understood mountain ranges in terms of its structural evolution, but represents an important kinematic link between the Baikal rift province to the north and Altai transpressional ranges to the south and west. Field work was undertaken during summer, 1998 to investigate the mechanism of uplift along the southern flank and crest of the Dome. The results indicate that Central Mongolia is regionally domed without any evidence for crustal thickening due to thrusting. The highest elevations are flat plateau summits which represent remnants of an early Tertiary erosion surface. Mountainous relief is due to river incision into the Dome with greater precipitation and downcutting on the north side of the Dome. Normal fault scarps of Neogene-Recent age also cut the southern flank and crest of the Dome and bound small extensional basins. The most recent fault activity appears to be at the highest elevations where Quaternary alluvial sediments are cut by active normal faults at 2500-m+ elevations. Normal faults along the southern flank of the Dome are dominantly NE-striking whereas to the north at higher elevations, NE and NW-striking normal faults occur together. There is a close spatial association between normal faulting and linearly aligned Quaternary volcanic centres. The contrast between active left-lateral transpressional mountain building in the northern Gobi Altai, and doming and extension immediately to the north in the Hangay region suggests that the crustal stress field changes from dominantly horizontal NE-directed maximum compressive stress in the Altai region (stresses derived from the Indo-Eurasian collision 2000 km to the south) to vertical maximum stress combined with subordinate NE-directed horizontal stress in the Hangay region. The origin of the dominant vertically oriented stress in the Hangay region is believed to be a mantle plume which dynamically supports the domed topography and generates the regional volcanism. It is speculated that a plume beneath the Hangay Region may act to partially deflect NE-flowing lower lithospheric material which is believed to be driving the shallow crustal deformation in the adjacent Altai.

D01 : 1B/30 : G2

Rifting and Denudation of the Mahanadi Rift, India: An Apatite Fission-track Study

Frank Lisker (flisker@uni-bremen.de)

FB 5, Universität Bremen, PF 330440, 28334 Bremen, Germany

Following a NW-SE trending Archaic suture zone perpendicular to the eastern coastline of Orissa (India), the Mahanadi Rift transects ~500 km of the metamorphic basement of the Precambrian Indian Shield. It consists of two main sedimentary basins, which are filled with Upper Carboniferous to Upper Triassic coal-bearing sediments. Morphologically, the rift displays no remarkable expression. It is characterised only by the flat basin depressions and the course of the Mahanadi river.

Forty one samples of the Meso- to Neoproterozoic metamorphic basement (gneisses, granite, charnockite) and eight sandstone samples of the Gondwana sediments have been dated using apatite fission-track technique to analyse the low temperature history (<150°C). The apparent ages range from 443 ± 28 to 133 ± 8 Ma. There is a distinct correlation between fission-track ages and distance to the basin margin, and the samples can be divided in different tectonic domains. The fission-track data reveal an at least three-stage cooling sequence in the Mahanadi region. The cratonic basement of the eastern Indian Shield cooled in the late Palaeozoic to temperatures of ~200°C due to post - pan Indian (Pan African) crustal cooling and long-term denudational unroofing. Around the Mahanadi, the development of an alternating asymmetric rift from late Carboniferous to early Triassic (280-220 Ma) led to differentiated denudation with the formation of sedimentary basins. The maximum burial depth of the sediments can be estimated as ~2 km. The fission-track length data and the common occurrence of Mid Cretaceous single grain ages in the Mahanadi Rift-samples indicate regional denudation of up to 3 km since the last 100 Ma. This Mesozoic denudation stage might be mainly related to the spreading between India and Australia/ Antarctica as well as the intracontinental volcanic Rajmahal activity.

D01 : 1B/33 : G2

Proterozoic Sedimentary Tectono-Magmatic Evolution of the Intracratonic Cuddapah Basin, India

Mahesh Anand (anand@esc.cam.ac.uk) &

Sally Gibson (sally@esc.cam.ac.uk)

Department of Earth Sciences, Downing Street, University of Cambridge, Cambridge, UK

The Cuddapah Basin is a Proterozoic intracratonic sedimentary basin, situated in the eastern part of the Dharwar Craton of southern India, covering an area of around 44,500 km². It is infilled by >10 km of sediments of the Cuddapah Super Group. Each formation in the sedimentary succession invariably starts with a conglomerate horizon which is overlain by alternating sequences of shales, quartzites and stromatolitic dolomites. Igneous activity in the form of tholeiitic lava flows and basic sills is predominantly concentrated in the lower part of the sedimentary sequence and ultrapotassic igneous rocks occur in the younger formations. The time of emplacement of these igneous rocks ranges from 1850 to 1420 Ma (Bhaskara Rao et al., 1995; Chalapathi Rao et al., in press). The source of the deviatoric stresses within the crust that cause the initiation of sedimentary basins is a problem currently being debated. There are several lithospheric mechanisms by which an intracratonic basin can be created such as an active thermal uplift (Sleep, 1971), passive lithospheric stretching (M^cKenzie, 1978), and phase change mechanisms (Middleton, 1980). Based on the integrated results from field observations, geochemical studies and published geophysical measurements, the lithospheric stretching model of McKenzie best explains the evolution of the Cuddapah Basin. The lithospheric extension may be associated with melt generation events during which basaltic melt may either extrude/intrude or underplate the crust. This model seems to conform with the presence of lava flows and sills and a large lopolithic body which has been identified below the southwestern part of the basin.

The presence of tholeiitic lavas suggests that relatively large degrees of partial melting were occurring in the underlying mantle during the early formation of the intracratonic Cuddapah Basin. This hypothesis is supported by the fact that a large stretching factor (ß >3) is required for sediment loaded subsidence of >10 km. The presence of relatively small-degree melts (lamproites) in the uppermost parts of the sedimentary sequence suggests that the lithosphere had re-thickened after the initial phase of extension.

Bhaskara Rao, YJ, Pantulu, JBC, Damodar Reddy, V and Gopalan, K, Memoir of Geol. Soc. of India, 33, 307-328, (1995).

Chalapathi Rao, NV, Miller, JA, Gibson, SA, Pyle, DM, and Madhavan, V, in press, Geol. Soc. of India

M^cKenzie, D, Earth and Planetary Sc. Letters, 40, 25-32, (1978).

Middleton, MF, Geophy. J of the Royal Astr. Soc, 62, 1-14, (1980).

Sleep, NH, Geophy. J of the Royal Astr. Soc, 24, 325-350, (1971).

D01 : 1B/34 : G2

The Intra-Plate Palmyride Trough ­ Syria: Formation by Lithospheric Folding

Barry Wood (barryw@earth.ox.ac.uk)

40 Oak End Way, Gerrards Cross, Buckinghamshire, U.K. Sl9 8BR

The Palmyride Trough presently occupies a position in west-central Syria between the Rutbah High, straddling the Jordanian-Syrian border, and the Aleppo High, straddling the Syrian-Turkish border. The Aleppo High separates the trough from the Taurus Thrust belt of southeast Turkey placing it within the northwestern Arabian Plate. A rift origin for the intra-plate Palmyride Trough of central Syrian is not supported by newly released seismic and well data nor is a thin-skinned tectonic model. The trough has been the site of deposition of six successive, stacked basins since the Late Palaeozoic. Each basin records a low sedimentation rate. Sedimentary fill consists of laterally consistent, low energy, dominantly shallow water carbonates and fine clastics recording an intra-cratonic shelf environment and a record of periodic drowning and drying of the basin interior. Each drying period was followed by exposure and erosion. Lateral facies continuity is remarkable with units as thin as 25 meters recognizable on e-logs over hundreds of kilometres. Late cycle and post-cycle volcanism suggests a tectonic influence on cycle termination. Isopach maps suggest a clockwise rotation of the axes of successor basins and a northwest-southeast narrowing of the trough through time.

Basin margin faults are not observed. Marginal feathering of basin sedimentary packages and the centralized position of the depocenter of terminal sedimentary units further precludes marginal block faulting. No deep-seated basement deformation is indicated from published refraction data yet shallow deformation (less than 4000 metres) portrays a strong tripartite structural division to the trough. The Central Uplift marks a structurally inverted area located centrally in the trough bound by high angle reverse faults. To the southeast lies the Addaw Depression and to the northwest the Homs Depression. Disharmonic buckle-folding characterizes deformation within these lateral depressions.

The absence of typical rift structures such as basin margin faults and internal block faulting and rotation precludes extension as a model for basin formation. The absence of low angle reverse faults precludes typical thrust models. Further, the low sedimentation rates recorded in the trough and non-clastic terminal facies preclude sediment loading as a mechanism of basin subsidence. A model based on lithospheric folding due to distant horizontal stress is proposed to account for trough formation. Fold wavelength is in the order of 300 kilometres. As folding of the lithosphere progressed, cyclical sedimentation and deformation of the sedimentary column occurred as a natural reaction to the periodic decrease in flexural wavelength and increase in flexural amplitude. This would translate into trough formation and flooding with subsequent deformation (inversion) due to a progressive loss of volume (space) within the trough. Cycles of deformation and sedimentation appear to be in concert with regional orogenic events strengthening acceptance of the model.

D01 : 1B/35 : G2

Styles of Tectonic Deformations Within the East-European Craton (EEC) During Phanerozoic Times

Anatoly Nikishin (nikishin@geol.msu.ru)¹,

Randell Stephenson (ster@geo.vu.nl)² &

Sierd Cloetingh (cloeting@geo.vu.nl)²

¹ Geological Faculty, Moscow State University, 119899, Moscow, Russia

² Vrije Universiteit, De Boelelaan 1085, 1081 HV Amsterdam, The Netherlands

The work is based on the following data: new series of palaeogeographic maps for the East European Craton with the time slices from 10 to 1 million years; 1D burial subsidence modelling for more then 100 wells; 2D burial subsidence modelling for seismic profiles; palaeotectonic reconstructions for the EEC and adjacent areas for different time intervals; analyses of magmatism; detailed correlations of tectonic events round the EEC and adjacent areas. We can recognise the following main intracratonic tectonic processes: rifting with different geodynamics; inversion of rifted basins; syncompressional long-wavelenght arching of topography (lithospheric buckling); wide regional subsidence and uplift. Regional correlations show that main intracratonic events took place due to regional unified stresses which changed from one to another tectonic epoch. Rift events took places during E. Ordovician, M.-L. Devonian, and in minor style at the Carboniferous/Permian and Permian/Triassic boundaries, and at the end of the Cretaceous. The reasons of rifting were different: oceanic opening (Ordovician), back-arc extension together with mantle plumes (Devonian), superregional extension events (Carboniferous/Permian and Permian/Triassic boundaries), compression related impactogen-like tension (L. Cretaceous - ?). Different scale inversion tectonics took place nearly in all rifted basins during polyphase compressional epochs (L. Silurian to E. Devonian, L. Devonian to Permian, Triassic/Jurassin boundary, Senonian to Palaeocene, Neogene). Syncompressional long-wave topographic arching with wavelengths 400-600 km (proposed lithospheric buckling) took place many times: in Mid-Devonian, Carboniferous to Permian, Late Triassic-Liasic, Senonian-Palaeocene, Neogene. The pattern of lithospheric buckling changed from time to time. The reasons of a wide regional vertical movements with amplitudes 100-300 meters with wave-lengths more than 600-1000 km are unknown; they are not in every case correlate with stress regime and global sea-level. Main tectonic events inside the EEC are in a good correlation with tectonic processes round the EEC and with a global plate kinematic reorganisation phases. EEC is a good example to study the styles of the intracratonic tectonics.

D01 : 1B/36 : G2

Far-Field Stress Transmission Effect in the Structural Evolution of Baltic Basin During the Early Palaeozoic

Saulius Sliaupa (sliaupa@geologin.lt)

Institute of Geology, Sevcenkos 13, 2600 Vilnius, Lithuania

The Baltic Basin (BB) is situated on the western margin of EEP. During Early Palaeozoic the subsidence pattern and structuring style was strongly affected by far-field stress transmission from adjacent Caledonides into stabile platform interiors at a distance exceeding 1000 km. This influence was just a minor during Cambrian. In Ordovician the increasing stresses transmitted from Scandinavian Caledonides evoked crustal-scale long-wave folding that dominated the whole architecture of the BB. The wavelength ranges in order of 80 km, the amplitudes are of 60-100 m (against background Ordovician thickness of ca. 100-200 m). Moreover, the overall shape of the Ordovician basin elongated WSW-ENE has been controlled by reactivation of coring mantle-rooted Saldus-Liepaja fault zone trending the same direction. The regular structural pattern was obliterated in middle Silurian due to flexural banding of the Baltica plate margin along the German-Polish Caledonides (GPC). Still, in Late Silurian Scandinavian stresses increased enough to initiate the family of forced folds (flexures) along the ancient basement faults striking SW-NE and WSW-ENE. In earliest Devonian it climaxed in the rapture of the sedimentary pile and establishment of regular network of transpressional faults. Fault amplitudes exceed locally 200-500 m. The fault pattern was controlled by structural grain of the crystalline basement. The main faulting was confined to the areas showing prevailing W-E and WSW-ENE structural trends in the basement. The structural evolution of the Baltic Basin points out to dominant influence of the Scandinavian Caledonides, while basin subsidence was more strongly affected by GPC development. It implies strong tectonic coupling between Laurentia and Baltica plates, whereas it was much weaker between E.Avalonia and Baltica during Early Palaeozoic. Stresses generated in Scandinavia and transmitted into plate interiors evidently increased in a course of Early Palaeozoic.

D01 : 1B/37 : G2

Late Caledonian Compressional Tectonics in the Intracratonic Baltic Basin

Väino Puura (puura@math.ut.ee)¹,

Tom Flodén (tom@geo.su.se)²,

Albertas Monkevicius (albertas.monkevicius@geo.su.se)² &

Rein Vaher (vaher@gi.ee)³

¹ Dept. of Geology, University of Tartu, Vanemuise Street 46, EE2400, Tartu, Estonia

² Dept of Geology and GeoChemistry, Stockholm University, 106 91, Stockholm, Sweden

³ Institute of Geology, Tallinn Technical University, Estonia pst 7, 101 43 Tallinn, Estonia

The early Palaeozoic Baltic sedimentary basin contains Late Vendian to earliest Devonian sedimentary deposits. The main sedimentary body infilling the basin is of Silurian age. Shallow shelf carbonates form marginal belts along the erosional boundaries north-west, north-east and south-east of the basin. The deposits grade into deep shelf shale deposits at the continental margin in the southwest. Including the 400-500 m thick Vendian to Ordovician deposits, the thickness of the platform cover increases from 100-200 m along the slope of the Baltic Shield to some 4000 m, including around 3500 m of Silurian, near the south-west border of the East European Craton. The Silurian Baltic Basin was closed during the Late Caledonian tectonic compression.

During its early Paleozoic evolution, the Baltic Basin extended parallel to the Scandinavian Caledonian active margin some 1000 km towards the north-west. Rare compressional faults occurred in northern Estonia already in the early Palaeozoic. The main dislocation processes occurred during the late Silurian and Early Devonian, i.e. during the Late Caledonian phase. The pre-Mid-Devonian compressional structures are reverse faults and fault-related linear uplifts of the crystalline basement and the sedimentary cover - anticlines. Together they form asymmetric assemblages. On the downside of the faults there occurs a gentle asymmetric syncline. The anticline on the uplifted side of the faults is usually much higher than the syncline, it has a steep slope facing the fault, and a very gentle opposite slope.

In the sedimentary cover, the flexure connecting the syncline and the anticline is often dissected by fracture zones and real faults. In the crystalline basement, the flexure consists of fractured, cataclastic rocks. Near the post-Silurian erosional palaeosurface, karst processes have substantially complicated the morphology of the deformation zones. Despite of drilling of tens of thousands of deep wells, and detailed geophysical mapping, the real morphologies of the dislocation zones have only been documented in very few cases. In the Ordovician oil shale mines of north-eastern Estonia and in the Silurian of the Baltic seabed there has been observed coupling of competent layers within compressionally shortened soft rock packages. The strike of the linear fault-and-anticline deformation zones in the central deep part of the basin follows the direction of the pre-Silurian gentle depressions and internal swells. In the shallow marginal parts of the Silurian Basin, the deformation zones have fan-shaped general distributions. Generally speaking, the deformation pattern has an inversional style.

The inversional tectonics in the Baltic Basin was most probably connected with the north-west to south-east directed Late Caledonian compression, which formed the thrust belts of the presently only partly preserved Scandinavian fold belt. The lateral transmission of the compression was mainly conducted in the crystalline basement, and only in lesser extent along competent layer packages in the sedimentary cover. The easternmost evidences of compressional tectonics, at St. Petersburg in Russia, occur as far as 1300 km from the north- western edge of the Precambrian basement under the Caledonides. Selective regeneration of older basement faults during the Caledonian intracratonic deformation phase has been observed.

D01 : 1B/37 : G1

Sedimentary Record of Post-Collisional Events of the Scandinavian Caledonides in the Baltic Devonian Basin in Estonia

Piret Plink-Björklund (piret@geo.gu.se) &

Lennart Björklund (lennartb@geo.gu.se)

Dept of Earth Sciences; Geology, Göteborg University, Box 460, SE-40530 Göteborg, Sweden

The Baltic Basin was an epicontinental sea, dominated by marine carbonate deposition during the Ordovician and Silurian times. In Early Devonian, episodic continental siliciclastic sedimentation started. The main phase of the siliciclastic deposition was from the Emsian time until the end of the Middle Devonian. In the Upper Devonian, carbonate and evaporitic sedimentation prevailed.

We have studied deposits of the main siliciclastic phase in outcrops in Estonia. The measured palaeocurrent directions indicate sediment influx from the north-western sector throughout the Middle Devonian. The outcrop studies indicate that the siliciclastic deposition in the Middle Devonian dominantly occurred in shallow water deltas. However, accommodation and sediment supply rate vary. The lower Middle Devonian deposits are texturally and compositionally immature compared to the middle and upper portions of the Middle Devonian. The quartz content of the lower units is up to 75-85%. The sedimentological studies suggest accommodation increase and sediment supply decrease during the early Middle Devonian. The sedimentation style changed from the deposition of sandstones and clay conglomerates on the subaqueous delta plains, to the deposition of mudstones and domerites in shallow marine conditions. The textural and compositional maturity increases upwards within the middle and late Middle Devonian deposits. The sandstones in upper Middle Devonian are quartz arenites with up to 94% quartz. The sedimentary environment changed from subaqueous delta front to subaerial delta plains. Thus, the late Middle Devonian deposits indicate infilling of the basin.

The ubiquitous occurrence of intrabasinal clay conglomerates is evidence that there was enough energy for transport of gravel-size clasts. The high maturity of the deposits together with the lack of extrabasinal conglomerates suggest a texturally and compositionally mature source rock. A sedimentary environment change from subaqueous to subaerial normally results in decrease of maturity. The observed upwards increasing maturity is interpreted to correspond with a downwards increasing maturity in the of the eroded source rocks.

The interpreted maturity development together with the northwesterly palaeocurrent directions suggest that the Middle Devonian deposits in Estonia are the product of a cannibalisation of the Scandinavian Caledonian foreland basin. In the Early Devonian, Estonia was an erosion and bypass area, while contemporaneous siliciclastic deposition occurred in southern portions of the Baltic Basin. A with time migrating forebulge axis may have constrained sediment transport southward along the axis of the foredeep. In the Emsian time extensional collapse and uplift occurred in the Scandinavian Caledonides. This event uplifted the foredeep sediments, exposed them to erosion and caused southeast transport across the foreland to Estonia. This redeposition continued until the basin in Estonia was filled and ended by evaporite deposition in the Upper Devonian.

Session D01:1P

D01 : 1P/01 : PO

Panafrican Molasse Basins of Saudi Arabia

Antonin Genna (a.genna@brgm.fr)¹,

Pierre Nehlig &

Mohamed Shanti²

¹ BRGM/DR, BP6009, 45060 Orleans, France

³ BRGM, Jeddah, Saudi-Arabia

Panafrican tectonism in the Arabian Shield is marked by the development of ortho- and para-gneiss domes, and intracontinental molasse basins showing complementary geometries within a fold system of crustal scale. Within these basins, sedimentary wedges reflect tilting of the country rocks, which is in agreement with the formation kinematics of the crustal folds. The basin fill can be as much as 10,000 m thick, which is similar to the amount of uplift in the axial zone of the domes. Sedimentary megasequences, comprising basal conglomerates overlain by a siliciclastic succession, reflect the pulsated nature of the tectonism, which was accompanied by alkaline, essentially rhyolitic, volcanic activity. The sedimentary structures reflect a shallow-water depositional environment. A single deformation phase, common to the domes and the edge of the Proterozoic basins, marks the transcurrent nature of the deformation with a subvertical foliation and a horizontal stretching lineation. Kilometre-scale folds, the tightness of which decreases from the inner to the outer zones, affect the basins and are overfolded towards the outer zones. These dome-basin systems are affected by a metamorphic gradient that is weak in the outer zones of the basins and increases to amphibolite facies (amphibole, garnet) in the axial zone of the domes. A major structure, the Murdama basin, constitutes a large foreland basin (today extending 600 km N-S) to the east of the Panafrican orogenic belt (Nabitah Belt). Various secondary basins (Talbah, Hadiyah, Ablah, Garm and Furayh) record activity in the peripheral mountains.

D01 : 1P/02 : PO

Sedimentary Record of Late Cretaceous Strike-Slip Movement of Tan-Lu Fault Zone, East China

Jingpeng Hong (hong@shida.planet.kobe-u.ac.jp) &

Takao Miyata

Graduate School of Science and Technology, Kobe University, Rokkodai1-1, Nada-ku, Kobe, Japan

The Tan-Lu Fault Zone (TLF) is the largest NNE-striking transcurrent fault along the eastern margin of Asia Continent. It is still remained as a disputed topic about the active time and offset distance. The sinistral strike-slip movement of TLF has been suggested to take place at Proterzoic (e.g. Fletcher et al., 1996), Middle-Late Triassic (e.g. Wan et al., 1991) Late Jurassic to Early Cretaceous (e.g. Xu et al., 1995), but few have discussed the Late Cretaceous displacement of TLF because there are several elongated Late Cretaceous "extensional grabens" along it. The Mazhan Basin (MB) is one of these grabens. But our study illustrate that it is a strike-slip basin. The MB, Shandong Province, China, locate between the main faults, F3 and F4, of the TLF. It is an elongated basin over 60 km in length and 8 km in width and contains typical continental sediments (the Upper Cretaceous Wangshi G.). The main lithofacies of MB are composed of fluvial to lacustrine deposits which have a rapid lateral facies change. We divided them into three sedimentary facies associations: conglomerate and sandstone facies association of alluvial fan to lake margin environment, and siltstone facies association of lacustrine origins. Their zonal distribution represent a contemporaneous heterotopic facies due to a lateral facies change from margins to axis of the basin. Their stratigraphic sequence becomes younger northward, suggesting that the depocenter of the fan-lake system migrate northward along the boundary faults. The thickness measured along the axis of the MB is more than 18,000 m, and the basin has distinctive asymmetric features on shape as well as the distribution of lithofacies. The change of pebble composition along margin area of the MB shows a sinistral movement of main fault F3. Ages of 82.4±3.5 Ma and 81.8±3.6 Ma had obtained by zircon fission-track dating from tuff interbeds of the basin. From the upper features the MB is thought to have been formed by the sinistral strike-slip of the east main boundary fault F3. We conclude that the TLF, at least part of it, such as F3 and F4 faults, occurred evident sinistral strike-slip displacement during the Late Cretaceous.

D01 : 1P/03 : PO

Controls on Paleocene Sedimentation in the Crimea

Victoria Gayduchok (geomin@geof.franko.lviv.ua) &

Alexander Emetz (geomin@geof.franko.lviv.ua)

37/3 Levitsky St., Lviv, Ukraine

There are three big tectonic parts in the Crimea Peninsula: Skythian platform, Crimea Mountains, and Kerch alpide folding zone (Kazantzev, 1982). Two latest of them have the Cainozoic sediments at sub-horizontal attitude. Deposits represent the sediment complex from Cretaceous to Quaternary.

Often the Paleocene sediments are observed in crop out along the north border of the Crimea Mountains. Also they were discovered by holes here long ago.

On the Novoselovsk paleo-uplift there were a land for the Lower Paleocene time. Around it and in top the sediments are rerepresented by pure carbonate facies for this time. Predominantly it is white crinoide-bryozoan limestone. In the south crop out there are fossils of corals, molluscs, algae. In bottom around this uplift the limestones are enriched by clastic material, aleurolite (quartz, glauconite, chalcedony) interbeds appear sometimes; to the east (the Kerch Peninsula) and the south-east the argillite, aleurolite and marlstone bedding are observed. To the north the sediment are represented by carbonate micrites and marlstones bedding. Therefore for Lower Paleocene time a decline of the erosion level controlled the sedimentation on the territory. At the beginning there were formation of terrigenous sediments with periods of cleaning of the marine water when sedimentation of carbonate sediments was. Gradually the ablation was decreased and pacific environment have led to the situation when paleo-relief controlled carbonate sedimentation. On the higher sites occurred organic sedimentation, and in the marine valley were deposited carbonate oozes. Clastic material was carried by submarine streams sometimes. The basin shallowed gradually.

For the Upper Paleocene the land has extended on the Tarchankutsk paleo-uplift also. Around it there were the accumulation of quartz-glauconite sands which are replaced by marlstone and clay higher. To the west the sediments are more calcareous (clayey limestones, marlstones bedding) than to the east (clay and marlstones bedding). In the Kerch peninsula the holes have struck non-calcareous clays. Therefore for the Late Paleocene time the territory was broken down to two separated parts: to the east from the Tarchankutsk-Novoselovsk paleo-land the Earth crust was more stable than to the west. Possibly for this time such situation have reflected the future Caucasian collision.

Kazantzev YV, Tectonic of Crimea. Moscow, Nauka, 112

D01 : 1P/04 : PO

Sedimentary Bodies as Gauges of the Rheology of the Crust Under the Paris Basin During Dogger

Pascal Allemand

(Pascal.Allemand@univ-lyon1.fr)¹,

Gilles Allemand (gdromart@ens-lyon.fr)¹,

Jean-Pierre Garcia

(jpgarcia@satie.u-bourgogne.fr)³,

Fabrice Gaumet (Fabrice.Gaumet@ifp.fr) &

Cécile Robin (Cecile.Robin@ccr.jussieu.fr)³

¹ Université Lyon 1, UFR Sciences Terre, UMR CNRS 8517, France

² Université de Bourgogne, CST, UMR 5561, France

³ Université P. et M. Curie Paris 6, ESA 7073 France

Intracratonic basins as the Paris basin are characterised by duration of subsidence exceeding 200 Ma, a thickness of sediments larger than 3000 m and a deficit of horizontal tectonics relative to the amount of subsidence. One model which can explain this paradox consists in decoupling the extensive global tectonics resulting from external forces from the local vertical tectonics produced by the load of the sedimentary bodies on the crust. These bodies would induced the flow of the lower crust.

From core and well-log data obtained on 173 wells of the Paris basin, 25 time surfaces have been correlated in the Dogger of the Paris Basin. These surfaces, which are maximum of base level, are defined in terms of sedimentary thickness and depth of deposit. From these data, map of relative tectonics have been drawn at a time scale around 500 ky. These maps show 2 different tectonic behaviours: (1) narrow regions with a high horizontal gradient of tectonics (faults) and (2) domains with a diffuse subsidence correlated with topographic domes and high velocity of sedimentation.

The geometrical and temporal characteristics of the regions of diffuse subsidence are compatible with a model of flow of the lower crust if the thickness of the flowing channel is at least of 20 km with a viscosity of 10¹⁰ Pas. The thickness of the elastic crust will be less than 5 km.