bluuebunny/crossref_metadata_2025 huggingface dataset:
Timing of the Avalonia-Baltica plate convergence as inferred from palaeogeographic and stratigraphic data of chitinozoan assemblages in west Pomerania, northern Poland
Authors: Samuelsson Joakim , Vecoli Marco , Bednarczyk Wiesław S. , Verniers Jacques
Published: April 2008

Tectonically disturbed Ordovician rocks penetrated by deep drillholes in Pomerania, NW Poland (Koszalin-Chojnice Zone) belong to the Heligoland-Pomerania Deformation Belt. Earlier data demonstrate that the Avalonia-Baltica collision occurred in Late Ordovician times, but in Pomerania, the timing of convergence has not been ascertained, and it is uncertain if the rocks underneath the Koszalin-Chojnice Zone belong to Avalonia or Baltica. Data from chitinozoans, organic-walled Palaeozoic microfossils with applications in biostratigraphy and palaeobiogeography, were assessed from ten boreholes (Brda 2; Brda 3; Chojnice 5; Karsina 1; Kościernica 1; Nowa Wieś 1; Okunino 1; Sarbinowo 1; Skibno 1; Wyszebórz 1) to address these problems. The results improve the biostratigraphy of the cores and demonstrate that the youngest Ordovician rocks are of a Burrellian (early mid Caradoc) to Cheneyan (late mid Caradoc) age. Because these rocks are interpreted as forming part of the deformation belt, the obtained ages put a lower age limit on the initiation of foreland basin sedimentation on the foreland of the orogeny, i.e. the Baltic platform. Quantitative comparison of chitinozoan assemblages demonstrates a high level of similarity between Pomerania and Avalonia. Together, Pomerania and Avalonia show greater similarity to Baltoscandia than to North Gondwana, supporting the idea that the Tornquist Ocean had narrowed significantly in early Caradoc times.
Structure and evolution of the Bohemian Arc
Authors: Franke W. , Żelaźniewicz A.
Published: April 2008

Tectonic zones and palaeogeographic units (terranes) in the German segment of the Variscides correlate with equivalents in the Sudetes at the NE margin of the Bohemian Massif. This correlation defines an arcuate structure with an opening angle of about 90°. The structure is truncated to the SE by a crustal scale. NE-trending fault zone with dextral transpression, the Moldanubian Thrust (MT). The arc cannot have been formed by northeastward indentation of the Bohemian Massif, since there is no evidence of a fault zone on the NW flank of the notional indenter, and little evidence for northeastward tectonic transport. Kinematic and age constraints on the main fault zones instead suggest that the structural array was formed by a complex sequence of events. Northwestward displacement along the margin of the East European Platform (EEP) with clockwise rotation was followed by large southwestward movements along the Moldanubian Thrust, and renewed northwestward displacement along the SW margin of the East European Platform.
Late Quaternary architecture of trough-mouth fans: debris flows and suspended sediments on the Norwegian margin
Authors: Taylor J. , Dowdeswell J. A. , Kenyon N. H. , Ó Cofaigh C.
Published: April 2008

Trough-mouth fans are the main marine depocentres for glacier-derived sediments in the Polar North Atlantic, but their growth through the Late Quaternary is complex. Glacigenic debris flows (GDFs) are sourced from a common and homogeneous part of the upper fan and only develop as coherent individual flows after downslope transport. Their genesis and mode of deposition mean that GDFs are confined to particular areas of trough-mouth fans; accumulation of these subglacial sediments is controlled by a combination of margin glaciology and fan morphology. Although most of the fan sediment is deposited as GDFs, during glacials considerable areas of trough-mouth fans are dominated by sedimentation of suspension deposits, associated with extensive meltwater release from a warm-based ice sheet and probable contour current activity. The depositional sequence of these two sediment types may be important in generating the long run-out distances of GDFs, which are initiated and sustained over low gradients. Furthermore, emplacement of GDFs is interpreted to be a relatively low-frequency event, and temporally, at least, fans are not dominated by this mode of sediment emplacement whilst ice sheets are at the shelf break. Large-scale trough-mouth fan development is therefore asynchronous and non-uniform, a result of the interaction between glaciology, morphology, and oceanography.
Experimental constraints on shear mixing rates and processes: implications for the dilution of submarine debris flows
Authors: Talling P. J. , Peakall J. , Sparks R. S. J. , Ó Cofaigh C. , Dowdeswell J. A. , Felix M. , Wynn R. B. , Baas J. H. , Hogg A. J. , Masson D. G. , Taylor J. , Weaver P. P. E.
Published: April 2008

Submarine debris flows show highly variable mixing behaviour. Glacigenic debris flows travel hundreds of kilometres along the sea floor without undergoing significant dilution. However, in other locations, submarine slope failures may transform into turbidity currents before exiting the continental slope. Rates and processes of mixing have not been measured directly in submarine flow events. Our present understanding of these rates and processes is based on experimental and theoretical constraints. Significant experimental and theoretical work has been completed in recent years to constrain rates of shear mixing between static layers of sediment and overlying turbulent flows of water. This work was driven by a need to predict transport of fluid mud and the erosion of cohesive mud beds in shallow water settings such as estuaries, docks and shipping channels. These experimental measurements show that the critical shear stress necessary to initiate shear mixing (around 0.1 to 2 Pa) is typically several orders of magnitude lower than the yield strength of the debris. Shear mixing should initiate at relatively low velocities (about 10–200 cm s
−1
) on the upper surface of a submarine debris flow, at even lower velocities at its head (about 1–10 cm s
−1
), and play an important role in mixing over-ridden water into the debris flow. Addition of small amounts of mud (approximately 3% kaolin) to a sand bed dramatically reduces the rate of mixing at its boundary, and changes the processes by which sediment is removed. Estimates are presented for rates of shear mixing at a given flow velocity, and for the critical velocity necessary for hydroplaning or a transition from laminar to turbulent flow. Although these estimates are crude, and highlight the need for further experimental work, they illustrate the potential for highly variable mixing behaviour in submarine flow events.
Late Pleistocene glacially-influenced deep-marine sedimentation off NW Britain: implications for the rock record
Authors: Davison Stephen , Stoker Martyn S.
Published: April 2008

A new deep-water borehole at the foot of the West Shetland Slope revealed a sequence of Late Pleistocene glacimarine sediments. This section comprises an interbedded sequence of muddy diamictons, with subordinate sandy muds with dropstones and matrix-poor sands and gravels. Integration of core data, seabed imagery and high-resolution seismic-reflection records indicate the succession is dominated by stacked glacigenic debris flows.

Individual debris flows are composed of massive, clast-poor muddy diamicton, and are seperated by sandy muds and the sands and gravels. The mud units are interpreted as a combination of hemipelagic-glacimarine and distal ice-rafted deposits. The sands and gravels are interpreted as the result of bottom-current reworking of glacial and pre-glacial sediments. The new data support previous studies which suggest that Pleistocene ice sheets reached the edge of the West Shetland Shelf on several occasions, delivering large volumes of glacigenic sediment directly to the shelf edge and upper slope. These deposits became unstable, forming debris flows which transported the glacigenic sediments to the deep-water environment. Seabed imagery shows that the area around the borehole formed a focus for this style of deposition.

A depositional model constructed from the new data is enhanced by the application of data from an ancient deep-water, glacially-influenced, succession from the Neoproterozoic Macduff Formation NE Scotland.
A glacial sequence stratigraphic model for temperate, glaciated continental shelves
Authors: Powell Ross D. , Cooper James M.
Published: April 2008

Temperate, glaciated continental shelves are by nature complex basins in that they not only have typical low-latitude siliciclastic processes acting to produce a sedimentary record and depositional architecture, but they also have the consequences of glacial action superimposed. For these settings glacial systems tracts are defined and related to glacial advance and retreat signatures, which can then be evaluated relative to changes in other external forces. We define glacial maximum (GMaST), retreat (GRST), minimum (GMiST) and advance (GAST) systems tracts separated by bounding discon-formities, which are respectively, the grounding-line retreat surface (GRS), the maximum (glacial) retreat surface (MRS) and the glacial advance surface (GAS). Each glacial advance and retreat sequence is bounded by a regionally significant unconformity, a glacial erosion surface (GES), or its equivalent conformity. Parasequence motifs vary across the shelf but include facies dominated by the movement of grounding lines. Facies motifs may have subglacial till above a GES and this is the main facies of the GMaST; however, tills are commonly absent. Although diamictic debrites are often associated with groundingline deposystems, the GRST succession is dominated by sorted deposits of gravel, rubble and poorly sorted sands due to the dominance of glacial meltwater. These deposits commonly have the geometry of banks or wedges/fans. They form the offlap-break at the outer continental shelf and also form a retrogradational stacking of bank systems on the shelf. Banks are capped by glacimarine rhythmites including thin debrites, turbidites, cyclopsams and cyclopels and perhaps iceberg-rafted varvites in fan to sheet-like geometries. Above these are draped sheets of bergstone muds that grade into paraglacial muds. The GMiST occurs with glacial retreat onto land or into fjords. The GMiST is represented in nearshore areas by progradational deposits of paralic systems dominated by deltaic and siliciclastic shelf systems, and in offshore areas by condensed sections. The GAST is represented by the inverse of the GRST facies succession, but is also the most likely interval to be eroded during readvance.
Geomorphology of buried glacigenic horizons in the Barents Sea from three-dimensional seismic data
Authors: Rafaelsen B. , Andreassen K. , Kuilman L. W. , Lebesbye E. , Hogstad K. , Midtbø M.
Published: April 2008

The glacigenic sequence of the southwestern Barents Sea shelf has for the first time been studied using 3-D seismic data. The close spacing of 3-D lines and powerful computer workstation interpretation techniques have allowed detailed mapping of the observed features. Several generations of subglacial lineations observed on four different palaeo-surfaces are interpreted to reflect the flow patterns of palaeo-ice sheets. To our knowledge, this is the first time that multiple levels of subglacial lineations have been observed. The lineations are 2.5–8 m in relief, 50 to 180 m wide and 0.5 to 20 km long. All four surfaces show a main lineation pattern comprising lineations with a N-S trend, suggesting that the dominant ice flow was directed northwards across the Barents Sea shelf at least four times during the last 0.8 Ma. Two of the surfaces display semi-circular to oblong depressions trending mainly in the same direction as the sub-parallel lineations. These depressions are 9–53 m in relief, 1.25–3.2 km wide and 1.9–9 km long. In contrast to the buried surfaces, the sea floor is dominated by 2.5–25 m deep cross-cutting iceberg plough-marks from the deglaciation phase of the last Barents Sea ice sheet. The 3-D seismic data are conventional industry data. Despite relatively low seismic frequencies and, hence, limited vertical resolution of seismic profiles, time slices and sub-horizontal time maps are of high spatial resolution, providing detailed images of different stages of buried Quaternary glacial geomorphology.
Millennial and sub-millennial-scale variability in sediment colour from the Barra Fan, NW Scotland: implications for British ice sheet dynamics
Authors: Wilson Lindsay J. , Austin William E. N.
Published: April 2008

Sediment colour, together with other proxy data, provides a novel, rapid and non-destructive tool in the investigation of glacier-influenced sedimentation on the Barra Fan, NW Scotland. Lightness (
L
*) and reflectance (400–700 nm) measurements at this site provide a quantitative estimate of changes in calcium carbonate and clay content. Interstadials are carbonate-rich/clay-poor (higher
L
* and reflectivity), whereas stadials are carbonate-poor/clay-rich (lower
L
* and reflectivity). Detailed sedimentological investigations suggest that the last British Ice Sheet (BIS) extended to the outer continental shelfbreak shortly after 30 ka
bp
. This climatic response of the BIS to global cooling at the Marine Isotope Stage (MIS) 3 – 2 transition marks a significant increase in sediment delivery to the Barra Fan. Prior to 30 ka
bp
, strong Dansgaard/Oeschger (D/O) cyclicity dominates the record. After 30 ka
bp
, shorter periodicities prevailed as the BIS reached its maximum extent. Glacier dynamics plays a significant role in the delivery of ice-rafted debris (IRD) across this margin, highlighting the inherent difficulties of correlating millennial-scale IRD events when the IRD is derived from different ice sheets. An event stratigraphy based upon carbonate-rich interstadials provides a more robust means of amphi-Atlantic correlation during this interval.
Observations of surge periodicity in East Greenland using molybdenum records from marine sediment cores
Authors: Woodward John , Carver Steve , Kunzendorf Helmar , Bennike Ole
Published: April 2008

This paper describes a unique record of glacier flow instability for East Greenland during the Little Ice Age. Trace metal analysis of sediment cores collected during 1998 from the Noret Inlet in the Mesters Vig area of East Greenland shows two peaks in the molybdenum (Mo) record at 495 ± 40 years
bp
and 95 ± 2 years
bp
. This is notable as there is no molybdenum mineralization in the geology of the Noret Inlet catchment area. Molybdenum is found, however, in the drainage basin of Mesters Vig Inlet, just to the south of the Noret Inlet. The molybdenum record in the Noret core provides a long-term surge record for the Östre Gletscher, a large surge-type glacier in the Werner Bjerge that drains into Mesters Vig Inlet. The two molybdenum peaks indicate surge termination for the glacier, indicating a surge recurrence interval of around 400 years.
Timing and tectonic controls in the evolving orogen of SE Asia and the western Pacific and some implications for ore generation
Authors: Macpherson Colin G. , Hall Robert
Published: April 2008

SE Asia lies at the convergence of the Eurasian, Pacific and Australian plates. The region is made up of many active arcs, extensional basins, and the remnants of similar tectonic environments developed throughout the Cenozoic. There are many important hydrothermal mineral deposits and prospects in SE Asia but their formation is often poorly understood due to the complicated tectonic history of this region and the knowledge of relationships between mineralization and tectonics. Plate reconstruction offers a framework to integrate geological and geochemical data that can be used to unravel the large-scale tectonic processes that affected mineralized provinces. We present examples of the information that can be derived from this approach and discuss the implications for understanding the origin of some hydrothermal mineral deposits in SE Asia.
Example of a structurally controlled copper deposit from the Hercynian western High Atlas (Morocco): the High Seksaoua mining district
Authors: Chauvet A. , Barbanson L. , Gaouzi A. , Badra L. , Touray J. C. , Oukarou S.
Published: April 2008

The mineralized district of the High Seksaoua (Western High Atlas, Morocco) is characterized by a lithological succession marked by an alternation of schists and limestones attributed to the Cambrian and affected by at least five deformational events. The copper mineralization herein analysed is systematically localized close to a dolomite/black schist level in which a top-to-the-NNW décollement-type tectonics (D
d
) has been identified. We demonstrate that the economic mineralization is a syn-tectonic stockwork formed in response of this top-to-the-NNW shearing event (D
d
). This tectonic event can be reasonably correlated with the late Hercynian tectonics responsible, in the same area, for the Tichka granite emplacement at
c.
291 ± 5 Ma, also under the control of a NW-SE shortening direction. Indeed, the Tichka granite represents a good candidate to explain the origin of the mineralized fluid. Such an hypothesis is confirmed by the Permo-Triassic age (
c.
270 Ma) given by
40
Ar/
39
Ar dating realized on white micas related to the stockwork formation (this study). This important result questions the syn-genetic interpretation accepted until now for this mineralization and allows us to propose a new model of formation for this kind of deposit that could contribute to exploration programmes.
Indo-Antarctic Correlation: a perspective from the Eastern Ghats Granulite Belt, India
Authors: Dasgupta Somnath , Sengupta Pulak
Published: April 2008

Available lithological, petrological and geochronological data on the rocks of the Eastern Ghats Belt (EGB), which has a key role to play in any model of Indo-Antarctic correlation in Precambrian and Cambrian times, have been synthesized and interpreted. Longitudinal lithological subdivisions of the belt are mostly not supported by recent geochronological data, which rather suggest a fourfold division based on protolith ages of the ortho- and paragneisses. Mineral ages show prominent tectonothermal events at Mesoproterozoic, Grenvillian and Pan-African times. However, not all of the imprints of all the events are preserved in all the geochronological domains. In the most well-studied Domain II, three phases of metamorphism are recorded — an early ultra high-temperature metamorphism with an anticlockwiseP-Tpath, a second granulite facies event atc.1000 Ma, with a retrograde trajectory of near-isothermal decompression, and a third of amphibolite facies. The available information, however, is not conclusive as to whether or not the entire EGB experienced polyphase UHT metamorphism. The propagation of the Grenvillian orogenic front in the EGB is identified.
Crustal architecture and evolution of the Eastern Ghats Belt and adjacent regions of India
Authors: Dobmeier Christoph J. , Raith Michael M.
Published: April 2008

Extending along the east coast of peninsular India, the Eastern Ghats expose a deep section through a composite orogenic belt that once formed part of the Proterozoic mobile belt system within East Antarctica and East India. The critical evaluation of the existing geological and isotopic data strongly suggests that this orogenic belt includes not only the granulite facies Eastern Ghats Belt but also the Nellore-Khammam Schist Belt and lower grade units at the southern margin of the Singhbhum Craton. The present authors propose its subdivision into four crustal provinces with widely different geological evolutions. The Rengali and Jeypore Provinces formed at the margin of the Bhandara Craton in the Late-Archaean. In the Krishna Province, volcanosedimentary rocks equivalent to the Cuddapah Supergroup accumulated, probably on the Dharwar Craton in the Palaeoproterozoic, and the major tectonometamorphic event took place between 1.67 and 1.55 Ga, subsequent to a short-lived igneous activity. The Eastern Ghats Province, which shows considerable similarities with the Rayner Province of East Antarctica, was strongly affected by pervasive deformation, high-grade metamorphism and crustal-derived magmatism between 1.1 and 0.9 Ga, which extensively modified the crustal structure of present eastern peninsular India. Neoproterozoic and Early Phanerozoic tectonothermal activities were largely restricted to pre-existing shear zones, but the present configuration of the composite orogenic belt may have been achieved only during the Pan-African Orogeny.
Proterozoic crustal evolution of southernmost India and Sri Lanka
Authors: Braun Ingo , Kriegsman Leo M.
Published: April 2008

The Southern Granulite Terrain of India comprises four Proterozoic domains (from north to south): the Madurai Block, and the Achankovil, Ponmudi and Nagercoil Units. All but the Achankovil Unit share similarities with the Highland Complex of Sri Lanka: (1) evidence for Palaeoproterozoic crust formation (from U-Pb ages and 2.0–3.0 Ga Nd model ages); and (2) Pan-African ultrahigh-temperature metamorphism between 610 and 550 Ma. More specifically, the Ponmudi Unit and the Madurai Block share lithological and petrological characteristics with the low-Pand high-Pdomains of the Highland Complex, respectively. The Achankovil Unit is correlated with the Wanni Complex of Sri Lanka on the basis of lithologies and Nd model ages (1.0–2.0Ga). These domains and the Vijayan and Kadugannawa Complexes of Sri Lanka represent Late Mesoproterozoic-Early Neoproterozoic crustal domains.

Similarities in tectonic style, degree of metamorphism and Neoproterozoic U-Pb ages, suggest a common Pan-African tectonothermal evolution for the lower crustal domains of southern India, Sri Lanka and the Lützow-Holm Bay of Eastern Antarctica. Sri Lanka may have been located at the junction between the Mozambique Belt and a second belt partly involving areas that underwent a Grenvillian high-grade event during the formation of Rodinia. Madagascar shows a similar Pan-African metamorphic overprint, but is located in a different position and lacks the influence of the second belt. Alternative views are discussed.
Archaean-Cambrian crustal development of East Antarctica: metamorphic characteristics and tectonic implications
Authors: Harley S. L.
Published: April 2008

The East Antarctic Shield consists of a variety of Archaean and Proterozoic-Cambrian high-grade terranes that have distinct crustal histories and were amalgamated at various times in the Precambrian-Cambrian. High-grade Pan-African tectonism at 600–500Ma is recognized from four distinct belts: the Dronning Maud Land, Lützow-Holm Bay, Prydz Bay and Denman Glacier Belts. These high-grade belts juxtapose distinct Mesoproterozoic and Neoproterozoic crustal provinces (Maud, Rayner and Wilkes), the Rauer Terrane, and have also marginally affected Archaean cratonic remnants in the Napier Complex and southern Prince Charles Mountains. The Wilkes Province experienced its principal tectonothermal events prior to 1130Ma and was not affected by the younger events that characterize the Maud Province (1150 and 1030–990Ma), the Rayner Province (990–920Ma) and the Rauer Terrane (1030–990Ma). These differences between the isotopic/event records of the basement provinces now separated by the Pan-African belts require that the older provinces were not formerly parts of a continuous ‘Grenville’ belt as proposed in the SW US-East Antartic model. East Antarctica was not a single unified crustal block within either East Gondwana or Rodinia until the Cambrian, which is now demonstrated to be the key phase of high-grade and ultrahigh-temperature (UHT) metamorphism associated with supercontinent assembly.

The high-grade Pan-African tectonism is characterized by extensive infracrustal melting, clock-wise
P-T
paths, rapid post-peak exhumation along isothermal decompression paths to shallow- or mid-crustal levels by 500 Ma and the generation, at least locally, of UHT conditions. A significant flux of heat from the mantle into the deep and initially overthickened crust is required to produce these observed metamorphic effects. Whilst the thermal evolution can be explained by models that invoke the removal of most of the lithospheric mantle following crustal thickening and prior to rapid extension of the remaining crust, these one-dimensional models are inconsistent with present crustal thicknesses of 25–35km in the Pan-African domains of the East Antarctic Shield.
Extensional collapse of the late Neoproterozoic-early Palaeozoic East African-Antarctic Orogen in central Dronning Maud Land, East Antarctica
Authors: Jacobs J. , Klemd R. , Fanning C. M. , Bauer W. , Colombo F.
Published: April 2008

The East African-Antarctica Orogen resulted from the continent-continent collision of East and West Gondwana, or parts thereof, during the Pan-African event at
c
. 650–510 Ma. The collision overprinted large areas of older, mainly Mesoproterozoic, crust up to granulite facies grade in East Antarctica. The collision history is well documented by folding and thrusting, isothermal decompression and metamorphic zircon growth at
c.
580–560 Ma (Pan-African I). The convergence was succeeded by an extensional phase, probably representing orogenic collapse. This Pan-African II event at
c
. 530–510 Ma is characterized by large-scale extensional structures, finally resulting in the post-tectonic intrusion of voluminous A2-type granitoids. In central Dronning Maud Land the Pan-African II event started with the intrusion of syntectonic igneous rocks within an overall extensional setting. Two new SHRIMP data from gabbro zircons of the Zwiesel Gabbro give ages of 521±5.6 and 527±5.1 Ma. These ages are interpreted as crystallization ages and confirm the interpretation that the gabbro was emplaced early during the Pan-African II event. The gabbro was intruded by a network of leucogranite dykes and veins. Whereas the gabbro appears entirely undeformed, the leucogranite dykes are strongly mylonitized along extensional shear zones, indicating pronounced strain partitioning of the gabbro complex. Within the leucogranite mylonites, large tension gashes developed during mylonitization, indicating very high strain rates. Quartz c-axis orientations from quartz of the tension gashes show a distinct cross-girdle that formed during pure shear deformation. Fluid inclusion data from the leucogranite mylonites and the associated tension gashes mainly reveal recrystallization-related intracrystalline CO
2
-dominant inclusions with relatively low densities of < 1 g cm
−3
. The fluid inclusion data are interpreted to represent the last stages of a retrograde
P-T
path that is characterized by simultaneous cooling and decompression during extensional exhumation, probably succeeding the collapse of overthickened crust. A comparable orogenic collapse of the East African-Antarctic Orogen is reported from other parts of the orogen, such as from western Madagascar and the northern Arabian-Nubian Shield.
Development of the Arabian-Nubian Shield: perspectives on accretion and deformation in the northern East African Orogen and the assembly of Gondwana
Authors: Johnson Peter R. , Woldehaimanot Beraki
Published: April 2008

The Arabian-Nubian Shield froms the suture between East and West Gondwana at the northern end of the East African Orogen (EAO). The older components of the shield include Archaean and Palaeoproterozoic continental crust, and Neoproterozoic (
c
.870–670Ma) continental-marginal and juvenile intraoceanic magmatic-arc terranes that accumulated in an oceanic environment referred to as the Mozambique Ocean. Subduction, starting
c.
870 Ma, and initial arc-arc convergence and terrane suturing at
c.
780 Ma, marked the beginning of ocean-basin closure and Gondwana assembly. Terrane amalgamation continued until
c.
600 Ma, resulting in the juxtaposition of East and West Gondwana across the deformed rocks of the shield, and final assembly of Gondwana was achieved by
c.
550 Ma following overlapping periods of basin formation, rifting, compression, strike-slip faulting, and the creation of gneiss domes in association with extension and/or thrusting. Most post-amalgamation basin contain molasse deposits, but those in the eastern Arabian Shield and Oman have marine to glaciomarine deposits, which indicate that seaways penetrated the orogen soon after orogeny. The varied character of the post-amalgamation events militate against any simple tectonic model of final Gondwana convergence at the northern end of the EAO, and requires that models accommodate alternating periods of Late Neoproterozoic extension and shortening, uplift and depression, deposition and erosion.
Neoproterozoic dextral faulting on the Najd Fault System, Saudi Arabia, preceded sinistral faulting and escape tectonics related to closure of the Mozambique Ocean
Authors: Kusky Timothy M. , Matsah Mohamed I.
Published: April 2008

The Neoproterozoic Najd Fault System extends for 2000km across the East African Orogen, yet its history of motion and tectonic significance are widely debated. The Halaban-Zarghat Fault is the northeastern-most of the major NW-striking Najd faults in the Arabian Shield. Several sedimentary basins of the Neoproterozoic Jibalah Group are bounded by strands of the Halaban-Zarghat Fault and other Najd faults, particularly along right steps in the fault trace. Among the largest of the basins is the Jifn. The geometry of the Jifn Basin and the sedimentary facies of Jibalah Group indicate that it is a dextral pull-apart basin between strands of the Halaban-Zarghat Fault. A zone of high-grade mylonitic gneiss is located along a left step in the fault zone and may be a deeply eroded pop-up structure related to dextral transpression. Analysis of structural data from around and within the Jifn Basin, the position of other pull-apart basins and high-grade mylonite zones along the Halaban-Zarghat Fault are all consistent with early dextral movement along the Halaban-Zarghat Fault. Offsets of distinctive older rock units and transection of the Jifn Basin by sinistral faults, however, show that the latest and most significant sense of offset on the Halaban-Zarghat Fault and other Najd faults was sinistral.

A U-Pb zircon date of 624.9 ± 4.2 Ma from rhyolitic basement of the Jifn Basin gives a lower limit for the formation of the basin and initiation of dextral movement along the Halaban-Zarghat Fault. This age is interpreted as the earliest age for the collision of East and West Gondwana. A 621 ± 7 Ma pluton is offset 10 km dextrally along the Halaban-Zarghat Fault, showing that dextral motions continued for some time past 621 Ma, before switching to sinistral motions, and accreted terranes caught between the two continents were forced toward an oceanic-free face to the north. A 576.6 ± 5.3 Ma U-Pb zircon date from an undeformed felsite dyke that intrudes the Jibalah Group gives an upper time limit for movement along the Halaban-Zarghat Fault. This may mark the time that collision and escape tectonics ended, or it may reflect the time that displacements were transferred to other Najd faults in more interior parts of the East African Orogen.
Neoproterozoic deformation in central Madagascar: a structural section through part of the East African Orogen
Authors: Collins Alan S. , Johnson Simon , Fitzsimons Ian C. W. , Powell Chris McA. , Hulscher Bregje , Abello Jenny , Razakamanana Théodore
Published: April 2008

The Itremo region of central Madagascar has an importance in the evolution of the East African Orogen (EAO) that belies its size. Unusually for the southern EAO (Mozambique Belt), it is made up of low-grade metasedimentary rocks and therefore preserves an almost unique window into upper crustal deformation during this key period of the Gondwana supercontinent cycle.

In this paper new field mapping of three linked regions in the Itremo Sheet and in the upper part of the underlying mid-crustal Antananarivo Block are presented. From these a complete structural section through the eastern Itremo Sheet is produced and the complex deformation record preserved there is then discussed.

An early deformation (D1) consists of 10 km scale recumbent isoclinal folds that predate intrusion of a
c.
780–800 ma igneous suite. Metamorphic aureoles around these plutonic bodies overprint D1-related fabrics. Local deformation accompanies intrusion of the
c.
780–800 Ma, suite (D2). Extensive E-W contractional deformation occurs between 780 and
c.
570 Ma, that is here amalgamated as a composite D3 event, which includes thrusts and at least two phases of upright folds. Post-551 Ma, normal shearing (D4) marks the boundary between the Itremo Sheet and the underlying Antananarivo Block (the Betsileo Shear Zone), and may have also been responsible for formation of the Saronara Shear Zone. Finally, E-W open folding and dextral shear zone development marks a late N-S contractional event that is interpreted as a far-field response to collision between the northern Bemarivo Belt and central Madagascar.
Tectonic evolution of the Proterozoic Itremo Group metasediments in central Madagascar
Authors: Fernandez Alain , Schreurs Guido
Published: April 2008

The major geologic units of the Itremo region in central Madagascar include: (1) upper amphibolite to granulite facies (higher grade) Precambrian rocks, mainly para- and orthogneisses, and migmatites; (2) the newly defined Itremo Nappes, a fold-and-thrust belt containing the Proterozoic Itremo Group sediments, metamorphosed at greenschist to lower amphibolite facies (lower grade) conditions: (3) Middle Neoproterozoic and Late Neoproterozoic-Cambrian intrusives. The stratigraphic succession of the Itremo Group in the eastern part of the Itremo region is, from bottom to top: quartzites, metapelites, metacarbonates and metapelites overlain by metacarbonates. During D1 the Itremo Group sediments were detached from their continental substratum, deformed into a fold-and-thrust nappe (Itremo Nappes), and transported on top of higher grade rocks that are intruded by Middle Neoproterozoic (
c.
797–780 Ma) granites and gabbros. A second phase of deformation shortening (D2) affected both the Itremo Sedimentary Nappes and structurally underlying higher-grade rocksunits, and formed large-scale N-S-trending F2 folds. S1 axial plane foliations in Itremo Group sediments are truncated by Late Neoproterozoic-Cambrian granites (
c.
570–540 Ma). The age of the formation of the Itremo Nappes is not well constrained: they formed in Neoproterozoic times between 780 and 570 Ma.
Sequence stratigraphy of the Baltic Silurian succession: tectonic control on the foreland infill
Authors: Lazauskiene J. , Sliaupa S. , Brazauskas A. , Musteikis P.
Published: April 2008

The Baltic basin overlaps the SW margin of the East European Craton (EEC). During the Silurian its subsidence was governed by the flexural bending of the EEC margin due to the collision of Eastern Avalonia and Baltica. Two mechanisms — orogenesis and dynamic loading — were responsible for the flexural subsidence of the basin. Lithofacies, sequence- and cyclo-stratigraphic analysis were applied to reveal the tectonosedimentary evolution of the Silurian Baltic Basin, focusing on the imprint of geodynamic processes in adjacent orogens on the sedimentary architecture. Adopting a sequence stratigraphic approach, 10 depositional sequences superimposed by the lower rank cycles were identified in the Silurian Baltic Basin. The Llandovery sequences correspond with the initial stage of flexuring. The low terrigenous influx to the basin is explained by the lack of relief in the fold belt and its location at a distance from the orogenic front. The Wenlock-Lower Ludlow sequences reflect the advancement of the Caledonian orogen. The Late Ludlow-Pridoli sequences comprise the final stages of flexuring and basin infilling. Two major provenances — cratonic and orogenic — competed to supply terrigenous sediment to the basin during Silurian times.
Geodynamic evolution and palaeogeography of the Polish Carpathians and adjacent areas during Neo-Cimmerian and preceding events (latest Triassic-earliest cretaceous)
Authors: Golonka J. , Krobicki M. , Oszczypko N. , Ślączka A. , Słomka T.
Published: April 2008

The aim of this paper is to place the geodynamic and palaeogeographical evolution and position of the major crustal elements of the Polish Carpathians within a global framework. Neo-Cimmerian movements and their synsedimentary consequences are the main objects of our elaboration in relation to sedimentary record. Five time-interval maps are presented, which depict the plate-tectonic configuration, palaeogeography and lithofacies for the circum-Carpathian region and adjacent areas from the Late Triassic through to the Early Cretaceous.

Almost simultaneous tectonic events proceeding within different types of Carpathian sedimentary basins (Pieniny Klippen Belt and Outer Carpathian Silesian Basins) indicate the very important role of the Neo-Cimmerian movements (mainly of the Osterwald Phase) in the geodynamic history of the northernmost margin of the Tethyan Ocean. The global plate reorganization is related to this Tethyan Neo-Cimmerian tectonic activity.
The Tamworth Belt in Southern Queensland, Australia: thrust-characterized geometry concealed by Surat Basin sediments
Authors: Wartenberg W. , Korsch R. J. , Schäfer A.
Published: April 2008

The subsurface geometry and tectonic development of the Devonian-Carboniferous Tamworth Belt, a fore-arc basin in the New England Orogen, Eastern Australia, has been examined using seismic reflection, aeromagnetic and gravity data. The Tamworth Belt is bounded to the west by the Moonie Fault, a thrust fault, which exhibits a fault-bend-fold geometry. Two major westward-dipping fauls form the main eastern boundary, with a series of eastward-dipping backthrusts located farther to the west. The eastern margin also coincides with a gravity and magnetic ridge, similar to the gravity and magnetic pattern of the serpentinites and iron-enriched rocks that are exposed along the Peel Fault to the south. In the investigated area, the Tamworth Belt is over 50 km wide and has been shortened by at least 10 km across strike. The sedimentary succession is at least 12 km thick and is moderately folded. Within the succession, six seismic sequences were identified, each of which is separated by a major sequence boundary.
Critical stress-related permeability in fractured rocks
Authors: Rogers Stephen F.
Published: April 2008

Conventional wisddom has always assumed that fluids prefer to flow along fractures that are orientated parallel to the maximum
in-situ
horizontal stress direction. These fractures have the lowest normal stresses across them and therefore provide the least resistance to flow movement. However more recent work (Barton
et al.
1995) has suggested that those fractures that are orientated such that they experience a high ratio of shear to normal stress are most likely to flow. These critically stressed fractures are in a state of stress that is close to failure, allowing them to undergo a degree of shear. Even small shear displacements can cause significant dilation along a fracture surface as the two surfaces un-mate during sliding. This dilation results in a significant increase in fracture permeability, resulting in an increase in flow from these features. A review of past work reveals that this shear mechanism may be more important than previously thought. A case study from Sellafield in the UK is presented, where the critical stress technique has been successfully applied for predicting zones of enhanced permeability at the well scale. Difficulties for implementing the technique on a wider field wide scale are discussed.
Data analysis, processing and 3D fracture network simulation at wellbore scale for fractured reservoir description
Authors: Aliverti E. , Biron M. , Francesconi A. , Mattiello D. , Nardon S. , Peduzzi C.
Published: April 2008

Three-dimensional fracture networks, combined from
seismic scale
to
wellbore scale
, can greatly enhance the knowledge of fracture contribution to hydrocarbon storage and flow inside the reservoir. This paper presents some techniques used at ENI-Agip for fracture network simulation at wellbore scale. First, new imaging techniques adopted for enhancing and facilitating the data acquisition from orientated cores will be shown. We will discuss how the fracture data, acquired from the combined analysis of orientated core and wellbore image logs, have to be processed in order to calculate the geometrical parameters of each feature (dip direction, dip, size, terminations) and to classify them according to their filling (oil, water, shale, calcite, etc.). Second, we will illustrate how these kind of data are processed in order to extract the fracture representative parameters needed for stochastic simulation of fracture network at the wellbore scale: spatial distribution along the cored/logged interval, number of fracture sets, representative orientation and statistical distribution of each set, distribution laws of the fracture length and relevant minimum radii and fracture aperture estimate. Fracture porosity evaluation, matrix block size, fracture network connectivity at wellbore scale constitute the outputs of such simulations: they are used to better characterize a fractured reservoir and to describe its behaviour. The synthetic results of the application of such a methodology on a real case (tight packstone/wackestone of carbonate platform from a southern Apennine italian oil reservoir) complete the paper.
Seismic volume attributes for fracture analysis
Authors: Trappe H. , Hellmich C.
Published: April 2008

Seismic volume attributes are introduced for the characterization of subtle fault patterns. These patterns are important in understanding the flow behaviour in reservoirs. The seismic attributes are based on neighbourhood analysis of the seismic signal and take 2D or 3D information into account. In the case of fracture systems, attributes such as coherency, that quantify change in the seismic signal are used. Case studies are presented to show the impact of the attribute analysis on the description of reservoirs.
Intraplate strike-slip deformation belts
Authors: Storti F. , Holdsworth R. E. , Salvini F.
Published: April 2008

Intraplate strike-slip deformation belts are typically steeply-dipping structures that develop in both oceanic and continental lithosphere where they form some of the largest and most spectacular discontinuities found on Earth. In both modern and ancient continental settings, intraplate strike slip deformation belts are of major importance in accommodating horizontal displacements where they additionally form very persistent zones of weakness that substantially influence the rheological behaviour of the lithosphere over very long time periods (up to 1 Ga or more). These deformation zones provide a fundamental geometric, kinematic and dynamic link between the more rigid plate-dominated tectonics of the oceans and the non-rigid, complex behaviour of the continents. During convergence, they help to transfer major displacements deep into the plate interiors. During divergence, they act as transfer zones that segment rifts, passive continental margins and, ultimately, oceanic spreading ridges. Such belts are also of great economic importance, controlling the location of many destructive earthquakes, offshore and onshore hydrocarbon deposits and metalliferous ore deposits. In the oceans, intraplate strike-slip movements are relatively minor along transform-related fracture zones, but there are an increasing number of documented examples that may reflect spatial and temporal variations in spreading rate along individual active ridge segments.
Wrench faults down to the asthenosphere: geological and geophysical evidence and thermomechanical effects
Authors: Vauchez A. , Tommasi A.
Published: April 2008

We review a set of geological and geophysical observations that strongly support a coherent deformation of the entire lithosphere in major intracontinental wrench faults. Tectonic studies of wrench faults eroded down to the middle to lower crust show that, even in cases in which the lower to middle crust is partially melted, strain remains localized (although less efficiently) in transcurrent shear zones. Seismic profiling as well as seismic tomography and magnetotelluric soundings provide strong argument in favour of major wrench faults crosscutting the Moho and deforming the upper mantle. Pn velocity anisotropy, shear-wave splitting and electric conductivity anisotropy measurements over major wrench faults and in transpressional domains support that a wrench fault fabric exists over most or even the entire lithosphere thickness. These seismic and electrical anisotropies are generated by a crystallographic preferred orientation of olivine and pyroxenes developed in the mantle during the fault activity, which is frozen in the lithospheric mantle when the deformation stops. The preservation of such a ‘wrench fault type’ fabric within the upper mantle may have major effects on the subsequent tectonothermal behaviour of continents, because olivine is mechanically and thermally anisotropic. Indeed, the association of numerical models and laboratory data on textured mantle rocks strongly suggests that the orogenic continental lithosphere is an anisotropic medium with regards to its stiffness and to heat diffusion. This anisotropy may explain the frequent reactivation, at the continents scale, of ancient lithospheric-scale wrench faults and transpressional belts during subsequent tectonic events.
Intraplate strike-slip tectonics as an alternative to mantle plume activity for the Cenozoic rift magmatism in the Ross Sea region, Antarctica
Authors: Rocchi S. , Storti F. , Di Vincenzo G. , Rossetti F.
Published: April 2008

The West Antarctic Rift System is one of the largest areas of crustal extension in the world. Current interpretations on its driving mechanisms mostly rely on the occurrence of one or more mantle plumes, active during the Cenozoic or the Mesozoic. Recent studies of structural-chronological relationships between emplacement of plutons, dyke swarms, and volcanic edifices since middle Eocene in northern Victoria Land imply that magma emplacement is guided by strike-slip fault systems that dissect the western rift shoulder in Victoria Land. These studies led to a critical re-examination of the arguments used to support plume models. In Victoria Land, the linear geometry of the uplift and the relative chronology of uplift and extension are inconsistent with the traditional concepts of lithospheric evolution above a mantle plume. The geochemical signature of the mafic rocks is equivocal, because both OIB and HIMU features cannot be exclusively interpreted in terms of plume activity. From a thermal point of view, magma production rates are low compared with the core part of plume-related provinces. Additionally, the hot mantle below the West Antarctic Rift System is not documented as deep as expected for mantle plumes and the shape of thermal anomaly is related to lithospheric geometry, being linear rather than having circular symmetry. The lack of any decisive evidence for plume activity is contrasted by evidence that large-scale tectonic features guide magma emplacement: the Cenozoic fault systems reactivated inherited Palaeozoic tectonic discontinuities and their activity is dynamically linked to the Southern Ocean Fracture Zones. As an alternative to both active, plume-driven rifting and passive rifting, we propose that lithospheric strike-slip deformation could have promoted transtension-related decompression melting of a subplate mantle already decompressed and veined during the late Cretaceous amagmatic extensional rift phase. Magma ascent and emplacement occurred along the main strike-slip fault systems and along the transtensional fault arrays departing from the master faults.