Main Folding Phase Research Articles

GEOCHEMICAL ANALYSES OF SEEP OILS IN THE SOUTHERN GAFSA BASIN, SW TUNISIA: REASSESSMENT OF REGIONAL HYDROCARBON POTENTIAL

The recent discovery of surface oil seeps in the Tamerza area in the west‐central Gafsa Basin (southern Tunisia) has prompted a re‐evaluation of the hydrocarbon potential of the region. In this paper, we report the results of analyses of seep oils by Rock‐Eval pyrolysis (n = 6) and gas chromatography – mass spectrometry (n = 4). The goals of the study were to assess the composition of the seep oils, to investigate the relationship between the seep oils and potential source rocks, and to highlight the significance of the seep oils for oil exploration in the region.In the Tamerza area, surface oil seeps have been recorded in numerous formations ranging between the Upper Cretaceous Abiod Limestone Formation and the lower Miocene Sehib Siltstone Formation. The results of this study showed that all the seep oil samples analysed in general had a similar geochemical fingerprint: for example, Pr/Ph values are lower than 1; a plot of Pr/n‐C17 (0.27‐ 0.36) versus Ph/n‐C18 (0.3‐0.8) indicates a marine source rock deposited under reducing anoxic conditions; and Ts/(Ts+Tm) ratios indicate that the source rock was thermally mature. Correlation studies suggest that the oils originated from Cenomanian‐Turonian shales corresponding to the informally‐named Bahloul equivalent formation. Oil expulsion from this source rock at the seep locations is inferred to have ended by middle Miocene time. However the main phase of folding occurred here in the Pliocene – early Quaternary, and the resulting anticlinal folds are not therefore prospective structural traps for hydrocarbons because they developed after migration had already ceased. Stratigraphic traps and salt structures in the region may be of greater exploration interest.A surface oil seep sample was also recovered from the Quaternary upper Segui Formation at Jebel Orbata in the east of the Gafsa Basin. Analysis of this oil sample showed that it has similar geochemical characteristics to the seep oils from the Tamerza area, but that it appears to have had a much more recent migration history. In the eastern Gafsa Basin, Pliocene – early Quaternary anticlinal structures could therefore constitute effective structural traps charged by the same Bahloul equivalent formation source rock.

СТРУКТУРА КЕРЧЕНСКО-ТАМАНСКОЙ СКЛАДЧАТОЙ ЗОНЫ АЗОВО-КУБАНСКОГО ПРОГИБА

The nrotectonic structure of the Kerch-Taman folded zone, the southern deformed part of the Azov-Kuban piedmont trough, has a number of structural features and a history of deformation development. The authors divide the zone into five transverse segments, differing in the age of the main phase of folding. In the central Taman segment, folding is still going on, while in the framing segments, the main phases of folding took place in the Pliocene. In the peripheral segments, the main phase of folding is older - Early-Middle Miocene. The boundaries of the Kerch-Taman zone, as well as its youngest, the Taman segment, coincide with the transverse fault (fault-flexure) zones. It is shown that the southern frame of the Kerch-Taman folded zone is the continuation of the structures of the Crimean Mountains and the Northwestern Caucasus. The correlation of mud volcanism manifestations with tectonic elements of the Taman segment is noted to be ambiguous.

Eocene Paleoseismodislocations of Mount Ak-Kaya (Belogorsk District, Crimea)

The peculiarities of the structure of Cretaceous–Eocene boundary deposits in the Belogorsk district, Central Crimea, are considered. Tectonic structures interpreted as paleoseismodislocations are revealed and described; their age, structures, and evolutionary history are characterized; the magnitude and intensity of the seismic paleoevent are estimated. The position of steeply dipping fractures in Cretaceous rocks is regular and related to the bedding along the dip and strike of the structure. Their formation was caused by transverse extension in the background of general uplift at the beginning of Eocene, coincided in time with the main folding phase in North Turkey.

Eocene paleoseismic dislocations of the Ak-Kaya Mountain (Belogorskiy district, Crimea)

The article considers features of boundary Cretaceous–Eocene deposits in Belogorskiy district of Central Crimea. Structures interpreted as paleoseismic dislocations are described, their age, features of composition, history of formation are specified, magnitude and intensity of paleoearthquake are estimated. Steeply dipping fractures in Cretaceous rocks are regular and associated with dip and strike of the main regional structures. Their formation was caused by a transverse stretching during the main uplift of the structures at the beginning of the Eocene coinciding with the main phase of folding in Northern Turkey.

The ages and tectonic setting of the Faja Eruptiva de la Puna Oriental, Ordovician, NW Argentina

The Ordovician Faja Eruptiva de la Puna Oriental is a magmatic, predominantly intrusive belt in the Puna of northwestern Argentina with a N-S extension of ca. 400km. Scarce isotope geochemical ages and biostratigraphic data on some of the folded Faja Eruptiva country rocks assign the magmatism either to the Lower and lower Middle Ordovician, or to the latest Ordovician. Interpretations of origin and tectonic framework of the Faja Eruptiva are controversial and vary between arc, back-arc and collisional-orogenic settings.We present high-resolution La-ICP-MS U–Pb age and Hf isotope data on zircons from 10 plutonic samples covering the magmatic belt along a length of 200km in the northern Argentinian Puna. The xenocrystic and magmatic zircon age data have a wide spread between 2700Ma and 440Ma. Concordia and weighted mean age data document protracted magmatism in two phases between 480 and 460Ma, and between 453 and 444Ma, and constrain the time of the last intrusions at 444±3Ma and at 445±2Ma thus defining this last and main phase of intrusion at 444Ma.εHf(t) values define a main vertical trend centered at 500Ma with εHf(t) values between +3 and −16 indicating significant mixing of juvenile early Paleozoic melts with Paleoproterozoic crustal components. A second trend is formed by zircons with ages between 1.1Ga and c. 500Ma and predominantly positive εHf values between +8 and −3 and originates in juvenile mantle compositions between 1.6 and 1.1Ga. The spread of the zircon and Hf data document that the Faja Eruptiva intrusives have experienced large-scale contamination by the hosting crustal basement. It follows that the basement of the Puna is formed either by the upper Proterozoic-lower Cambrian Puncoviscana Formation as an erosional product of the Proterozoic orogenic belts of SW Amazonia or that the Puna including its Puncoviscana basement is underlain by a crust shaped by these orogenies.The main intrusive event at 444Ma has been linked to the Oclóyic orogeny in the Late Ordovician. The plutons intruded very likely in a sinistral strike-slip regime after the main folding phase of the Oclóyic orogeny had deformed the Ordovician sedimentary country rocks.

Fabrica Magnetica y Microestructuras a traves de los Andes de Tierra del Fuego, Argentina.

An anisotropy of magnetic susceptibility (AMS) transect was carried out across the Fuegian Andes, in Argentina, with the aim of studying its tectonic evolution. Two-hundred and forty oriented samples were collected from 27 sites distributed between the Paso Garibaldi to the north and the Canal Beagle to the south. The study was restricted to the Upper Jurassic Lemaire Formation, with a single site located in the Lower Cretaceous Yahgan Formation. Studied rocks comprised basaltic, andesitic, volcaniclastic and sedimentary rocks affected by low-grade metamorphism. AMS measurements were complemented with thin section analyses of representative samples in order to characterize the microstructures and metamorphic assemblages. In general, the magnetic fabric shows dominant oblate shapes and a large variation in the anisotropy degree from 1.04 up to 2. The anomalously high values were observed to be associated to growth of secondary pyrrhotite, which was identified by rock magnetic tests. Magnetic foliation was generally consistent with slaty cleaveage as observed in the field, confirming the tectonic origin of the magnetic fabric. Three geographic domains were distinguished in the study region on the basis of the pattern of the AMS axes distribution. In the northern domain, from Paso Garibaldi to Valle Carbajal, the orientation of the maximum susceptibility axis (k1), or magnetic lineation, is N-S to NE-SW with moderate plunge towards the S-SW and coincides with previous determination of mineral lineations associated with the Andean deformation and very low grade metamorphism. The magnetic fabric pattern can be correlated with the main deformational phase responsible for the development of slaty cleavage (main Andean deformational phase) and the tectonic transport due to progression of the Fuegian fold and thrust belt in the Late Cretaceous. A different character is shown along the Valle Carbajal domain, where subvertical E-W magnetic foliation planes and roughly E-W to ESE-WNW subhorizontal magnetic lineations are more difficult to correlate with the main folding phase and suggest its relation to an E-W, possibly localized, strike-slip regime during the main deformational and metamorphic phase. The magnetic fabrics in the third domain, close to the Canal Beagle, displays a more heterogeneous character with both E-W and N-S striking foliations; in this case a population of subhorizontal E-W magnetic lineation (k1) suggests the existence of a significant component of strike-slip deformation.

The lithostratigraphic subdivision of the Proterozoic basement rocks of the Mudiyah–Mukalla area, Yemen

The Precambrian basement rocks between Mudiyah and Mukalla, Yemen are lithostratigraphically divided into the Ahwar and Gharish supergroups and the Tha’lab and Ghabar groups. The age of the basement rocks range from Paleoproterozoic to Neoproterozoic. The late Archean–Paleoproterozoic Ahwar supergroup comprises the Khashab and Barak groups, which were metamorphosed to the upper amphibolite facies and folded within the ‘Irqah phase. It was representing old crystalline zone, marked by monotonous lithological succession, and shows irregular folding and boudinage. The Gharish supergroup is of Meso- to Neoproterozoic age and displays a broad range of metamorphic assemblages from upper amphibolite to greenschist facies. It comprises the Fahman, the Al-Aswad, and the Hatib groups and was subjected to monoclinal and sigmoidal folding. The lithologic succession of the Gharish supergroup reflects a young crystalline zone, punctuated by episodes of orogenic tectonism among the Mahfid, Ludiyah, and ‘Awarid phases. The Tha’lab and Ghabar groups represent the uppermost lithostratigraphical units of Neoproterozoic to Early Cambrian age and formed by low to non-metamaorphic volcanics of the Tha’lab group and by anchi-metamorphic sediments of the Ghabar group. The Tha’lab group contains the youngest volcanic rocks of the basement complex, which were formed after the ‘Awarid main folding phase. The Ghabar group consists of molasse-like deposits formed within small late orogenic basin. It represents a transitional stage between Proterozoic basement and sedimentary platform development, which faulted, folded and partly cleaved within the Ghabar folding phase. The geological and structural evolution of the intrusive rocks recognized that the granitoids can generally divided into the following stages; the anatectic stage of the Ahwar supergroup, the syntectonic stage of the Gharish supergroup, the posttectonic stage of the Tha’lab group and the late orogenic stage of the Ghabar group.

Where did rotational shortening occur in the Himalayas? – Inferences from palaeomagnetic remagnetisations

In metacarbonates of the Lesser (LH) and Tethyan (TH) Himalayas of Kumaon/Garhwal (N-India) characteristic remanent magnetisations carried by pyrrhotite (unblocking temperatures: 250–330°C) and magnetite (demagnetising spectra: 15–50 mT) have been identified. Negative fold tests indicate remanence acquisition after the main folding phase, which is of short-wavelength character and occurs during the early orogenese of the Himalayas. A thermal or thermochemical origin of magnetisation is likely and the age of remanence acquisition is indicated to be about 40 Ma by 40K/39Ar cooling and 40Ar/39Ar crystallisation ages. In the Kumaon LH a long-wavelength tilting is indicated by a distribution of the remanence directions along a small-circle in N–S direction. Steepening of the remanence directions in the TH related to ramping on the Main Central Thrust (MCT) was not observed, in contrast to other related studies. In the Alaknanda valley of LH a 38±8 Ma age of remanence acquisition is supported by comparison of observed inclinations to the apparent polar wander path of India. Clockwise rotation of 20.3±11.7° (LH/Alaknanda valley) and 11.3±8.5° (TH) with respect to the Indian plate is observed, indicating that there is no significant evidence for rotational shortening along the MCT since about 40 Ma. Our results suggest that most of rotational underthrusting and oroclinal bending has not been accommodated by the MCT, but by the main thrusts south of it. The latest Miocene/Pliocene age of the Main Boundary Thrust indicates that oroclinal bending is a late-orogenic process.

Thermo-tectonic history of the Tethyan Himalayas deduced from the palaeomagnetic record of metacarbonates from Shiar Khola (Central Nepal)

Palaeomagnetic measurements were carried out on low-grade metamorphic carbonates, of Mesozoic age from the Shiar area (85.1°E, 28.6°N) of the Tethyan Himalaya (TH) in north central Nepal. Two characteristic remanence components carried by pyrrhotite (ChRM1) and magnetite (ChRM2) could be identified by their unblocking temperature spectra of 270–340 and 430–580°C, respectively. Fold tests are not significant, due to the uniform bedding of all sites. However, according to results from other areas of the TH, the pyrrhotite component has been probably acquired as a secondary (p)TRM during exhumation and cooling; thus the age of remanence acquisition can be related to the last cooling event (25–17Ma in the surrounding areas). The inclination of the magnetite component matches the value expected from the Indian APWP. This may the primary origin of the ChRM2.Pyrrhotite site-mean directions show a small-circle distribution, with a best fit parallel to the N–S direction. Backtilting to the expected inclination (Iexp) by intersection of the remanence small-circle with the small-circle of constant Iexp yields a clockwise block rotation of 30–35° with respect to the Indian Plate. Characteristics of the pyrrhotite component (small-circle distribution of site-means, secondary origin, (p)TRM with unblocking temperatures below about 300°C), allow the interpretation of the chronologic order of the thermo-tectonic history: (i) an earlier main folding phase at elevated temperatures; (ii) a later event of cooling through about 300°C coinciding with the acquisition of ChRM1; (iii) clockwise block rotations with respect to the Indian Plate and (iv) long-wave folding as the youngest tectonic event.

Temperature and Driving Force Dependence of the Folding Rate of Reduced Horse Heart Cytochrome c

Utilizing the stability difference between the ferro and ferri forms of horse heart cytochrome c (cyt c), folding of reduced cyt c was triggered by laser-induced reduction of unfolded oxidized cyt c. Measurements were made of the kinetics of the main folding phase (1 ms-10 s) in which collapsed reduced cyt c transforms to the native conformation. The folding rates were studied extensively as a function of temperature (5-75 degrees C) and guanidine hydrochloride (GdnHCl) concentration (1.6-4.9 M). At constant [GdnHCl], the Arrhenius plot of the folding rate constant (k) is nonlinear. At temperatures above 40 degrees C, the decrease in protein stability counteracts the expected increase in folding rate. Introducing free energy (DeltaG), derived from protein stability data, into the Eyring and Arrhenius equations leads to: ln k = ln(k(b)T/h) + DeltaS()/R - DeltaH()/RT - theta(m)DeltaG/RT = ln A - E(a)/RT - theta(m)DeltaG/RT, where theta(m) is the ratio between the denaturant dependence of the folding rate and the stability. By using this equation at constant DeltaG [or constant equilibrium constant (K)], linear Arrhenius plots are obtained. For the main folding phase of reduced cyt c, a positive DeltaS() is obtained indicating that the transition state is less ordered than the reactant. A model is proposed in which reduced cyt c first collapses into a compact intermediate, which needs to expand to reach the transition state of the rate-limiting folding reaction.

La déformation post-Miocène en Provence occidentale

The structure of western Provence (SE France) is the result of successive deformations connected to the building of the Pyrenees and the Alps. It is a seismically active region still undergoing deformation. The aim of this study is to characterize the recent deformation in western Provence and to integrate the cumulated displacements in a coherent deformation model. In order to do this, we identified the recent structures that concentrate the deformation. We used the Miocene as a sedimentary marker to estimate the discontinuous deformation over the last 20 Ma and geomorphic surfaces to evaluate the amount of the post-Miocene deformation. Miocene terrains are deformed along south-vergent thrusts such as Le Luberon, Les Costes, La Trévaresse or Les Alpilles, and along sinistral strike-slip faults such as the Durance and Nîmes faults. North-vergent Pyrenean thrusts such as L’Étoile-Sainte Baume, Sainte Victoire or the Eguilles thrusts were not reactivated during the Alpine phase. Field evidence shows that in the Luberon, the main folding phase occurred during, or immediately after the Burdigalian (20.5–16 Ma). The shortening measured on a regional N–S cross-section is of a few kilometres, implying a deformation of 0.1–0.2 mm·year –1 since the beginning of the Miocene. Geomorphic surfaces have been reported on cross-sections of the E–W thrusts. The intensity of the deformation decreases southward and through time during the Miocene. Pliocene surfaces are not deformed near the active structures, except at the front of the Digne thrust. Furthermore, Quaternary geomorphic markers such as alluvial fans are not affected by the Durance strike-slip fault. Our results show that from Miocene to the Present, Provence was not intensively deformed (0.1–0.2 mm·year –1), and occurred in a short period of time during the Miocene. It is coherent with the southward emplacement of the Alpine Digne thrust being the cause of this deformation. Since the end of the Miocene, there have been no major displacements on any of the active structures.

Equilibrium and transient intermediates in folding of human macrophage migration inhibitory factor.

Acid, guanidinium-Cl and urea denaturations of recombinant human macrophage migration inhibitory factor (MIF) were measured using CD and fluorimetry. The acid-induced denaturation was followed by CD at 200, 222, and 278 nm and by tryptophan fluorescence. All four probes revealed an acid-denatured state below pH 3 which resembled a typical molten globule. The pH transition is not two-state as the CD data at 222 nm deviated from all other probes. Urea and guanidinium-Cl denaturations (pH 7, 25 degrees C) both gave an apparent DeltaGU app H2O of 31 +/- 3 kJ.mol-1 when extrapolated to zero denaturant concentration. However, denaturation transitions recorded by fluorescence (at the same protein concentration) occurred at lower urea or guanidinium-Cl concentrations, consistent with an intermediate in the course of MIF denaturation. CD at 222 nm was not very sensitive to protein concentration (in 10-fold range) even though size-exclusion chromatogryphy (SEC) revealed a dimer-monomer dissociation prior to MIF unfolding. Refolding experiments were performed starting from acid, guanidinium-Cl and urea-denatured states. The kinetics were multiphasic with at least two folding intermediates. The intrinsic rate constant of the main folding phase was 5.0 +/- 0.5 s-1 (36.6 degrees C, pH 7) and its energy of activation 155 +/- 12 kJ.mol-1.

Present‐day vertical displacements in the north‐western Alps and southern Jura Mountains: Data from leveling comparisons

Two high‐precision leveling networks were successively surveyed in France, the NGF, measured during the 1886–1907 period, and the IGN69, measured from 1965 to 1979. The accuracy of these levelings (standard deviation of 1.8mm/√km to 3.8mm/√km) allows us to compute the vertical displacements of the benchmarks between two different eras. The results indicate the occurrence of discrete zones of uplift and subsidence: (1) a regional uplift (up to 1.4 mm/yr) of the Subalpine Massifs; (2) an important uplift of the internal Jura (up to 2 mm/yr); (3) a relative subsidence of the southern part of the Jura (0.8 mm/yr); and (4) a relative subsidence of the Bresse Basin with respect to the external Jura. Comparing the spatial distribution of zones of uplift and their respective vertical displacement rates with a regional structural cross section leads to the conclusion that present‐day uplift of the Belledonne and Bornes Massifs and of the internal parts of the Jura Mountains, can be explained by crustal shortening along a major basement‐involving thrust fault. This fault ramps up under the Bornes Massif from a depth of 12 km to 7 km, turns into a flat under the Molasse Basin, ramps up to the top of the basement at the north‐eastern margin of the internal Jura Mountains, and reaches the surface in the external Jura. The Salève ramp‐anticline is carried by a bifurcation of this thrust. Horizontal displacement rates of 6 mm/yr at the Bornes ramp, 2 mm/yr at the Salève ramp, and 4 mm/yr at the internal Jura ramp have been determined by inversion of profiles of uplift rates. Whether this basement‐involving thrust fault was already active during the Miocene main folding phase of the Jura Mountains or whether it was activated only during Pliocene‐Pleistocene times is subject to debate.

Petrochemical characterisation of the Late Alpine orogenic acid volcanism of the Carpathian-Balkan area

Late Apine orogenic volcanism appears immediately after the main phase of folding and thrusting in the Carpathian-Balkan area. The age of the volcainic manifestations is Priabonian-Oligocene (after Ilirian phase) in the Balkan segment and Miocene-Pliocene (after Savian phase) in the Carpathian segment, respectively. Most of the acid volcanics are formed by crustal melting, whereas a smaller part are differentiates of basaltic and intermediate magmas. The first one has higher SiO2 content than the second. K2O content increases from the North to the South: in the Carpatian segment the acid volcanics are Ca-alkaline and high-K Ca-alkaline, whereas in the Balkan segment they are high-K Ca-alkaline and shoshonitic. The Na2O content of the acld volcanics of both segments show less significant variations. The amount of K2O correlates with the thickness of the crust in the different volcanic regions. The petrochemical data of the Carpathian-Balkan acld volcanlcs permit to draw a boundary between high-K Ca-alkaline and shoshonitic series at a level of K2O=4.75 wt % (at 70 wt % SiO2) and K2O=4.9 wt % (at 78 wt % SiO2) in the Peccerillo & Taylor diagam.

Gold mineralization in shear zones within a turbidite terrane, examples from Central Victoria, S.E. Australia

Central Victoria, in S.E. Australia, is regarded as the classical area for gold mineralization associated with saddle reefs, and with reverse faults, within a turbidite terrane. However, there is also an important contribution by steep, brittle shear zones to the mineralization and two such areas, Fosterville and Heathcote are the subject of this article. The shear zones hosting mineralization are sinistral transpressional structures that, while accommodating regional shortening, do it by both crustal thickening and lateral expulsion. The mineralization occurs at dilational jogs or bends within the zones. The shearing and mineralization postdate the main phase of folding and thrusting but predate widespread post-tectonic granitoid intrusion. It is postulated that the shears are a part of the initial phases of orogenic collapse which culminated in the emplacement of the granites, as well as sedimentation and acid to bimodal vulcanicity. Furthermore, it is argued that mineralization within the saddle reefs and reverse faults that characterize Central Victoria are also related to the initial phase of orogenic collapse. A final postulate is that orogenic collapse provides a common tectonic thread to some turbidite hosted, epithermal and Archean lode types of gold mineralization.

Chronology and orientation of N2–CH4, CO2-H2O, and H2O-rich fluid-inclusion trails in intrametamorphic quartz veins from the Cuiabá gold district, Brazil

AbstractThe upper Proterozoic Cuiabá group of Mato Grosso, Brazil, is composed of low-grade clastic meta-sediments which have been folded by several successive tectonic events. Three generations of quartz veins are associated with the structural evolution of this area. The first veins are deformed by the main tectonic phases and show a complex deformational patterns. The second set is parallel to the cleavage and was formed syntectonically during the main folding phase, whilst the last quartz veins are related to a later stage of deformation. A systematic study of fluid inclusions in relation with a statistical study of microstructural markers (fluid inclusion trails, opened microcracks) was carried out on quartz veins from three localities. On the basis of microthermometric studies and Raman spectrometry analysis, four differents types of fluids have been distinguished, each trapped in specific fluid inclusion trails: (i) CO2-rich liquids and vapours (Lc, Vc) at Casa de Pedra, (ii) Lc and Vc inclusions with variable amounts of CO2, CH4, N2 in the vapour phase at BR-70, (iii) CH2-N2-rich vapours (Vn-m), and (iv) aqueous inclusions (L) with variable salinities representing the last fluid generations at all localities.At Casa de Pedra and BR-70, most fluids are observed within the three generations of quartz veins, indicating an important fluid circulation associated with the last phase of brittle deformation. Fluid inclusions of type (iii) and (iv) are oriented along several well defined directions. The study shows the importance of integrated microstructural and fluid-inclusion studies for understanding the geometry and chronology of fluid circulation.

Effects of Deformation Mechanisms on Reservoir Potential in Central Appalachian Overthrust Belt

Finite strain associated with various deformation mechanisms can significantly alter the porosity and permeability of reservoir rocks in overthrust belts. Mechanisms such as pressure solution and cataclasis reduce porosity and permeability, whereas extension fracturing and brecciation increase them. A theoretical derivation of the relationship between finite strain and original and final porosity indicates that little porosity is preserved in rocks whose strain (R) exceeds 1.5. Studies of variations in the relative importance of these mechanisms are used to define regional and local variations in reservoir properties in the central Appalachian overthrust belt. Regional limits of hydrocarbon potential are defined by combining finite-strain data with thermal-maturation (con dont color alteration index or CAI) data. On the basis of these studies, I conclude that the potential for oil production is primarily restricted to the Appalachian Plateau province. The potential for gas production decreases from west to east in the Valley and Ridge and Plateau provinces, with its approximate eastern limit defined by the North Mountain thrust. Local variations in reservoir properties depend on original depositional and diagenetic fabric of the reservoir, the relative timing of different mechanisms, lithologic and textural characteristics, and structural and lithotectonic position. The possibility of retaining open fractures depends on the relative timing of fracturing and pressure solution. Fractures formed during the main phase of folding are more likely to be sealed by pressure-solved material, whereas fractures formed or reactivated late in the deformation are more likely to remain open. Lithologic units that enhance fracturing and inhibit pressure solution most likely have higher fracture and matrix porosity. Fine-grained argillaceous limestone is more susceptible to pressure solution than orthoquartzite, whereas fractu ing is more intense in brittle, thin-bedded and fine-grained units. Structural positions with high fracture potential include angular fold hinges, fault zones, and the steep forelimbs of folds. The steep forelimbs of folds also are characterized by intense pressure solution and cataclasis, which reduce porosity. Consideration of the above criteria can be useful in evaluating the reservoir potential of prospective structures in the central Appalachian and other overthrust belts.

Contribution à l'étude du métamorphisme des gisements cupro-cobaltifères stratiformes du Sud-Shaba, Zaire. Le district minier de Lwishia

The late precambrian copper-bearing strata of southern Shaba (‘Série des mines’) have undergone metamorphism during the latest proterozoic to early paleozoic times. Structural, textural and crystallochemical analysis of this metamorphic event shows a very peculair evolution. During a first phase, prior to folding, a static recrystallization is accompanied by several chemical chanes such as Na, then KMg metasomatism. This hydrothermal character is due to ‘hydrostatic’ stress conditions and fluid circulation linked with surrounding evaporites. During this prograde stage of metamorphism, kyanite + carbonate + sulphide paragenesis suggest low water partial pressure and high pH-low Eh conditions. Geothermometers and fluid inclusions indicate moderate to medium temperatures (300–400°C). Sparse synergetic CuCo sulphide mineralization is overconcentrated to its present disposition. A second hydrothermal event following the main folding phase is probably linked with halokinetic or halotectonic structures. It is essentially retrograde with oxidizing and low-temperature (200°C) conditions, resulting in the total leaching of evaporites and the forming of clay minerals, such as kandites or smectites.

Structural Styles and Tectonic Implications of Richmond-Taylorsville Rift System, Eastern Virginia: ABSTRACT

Recent drilling and seismic surveys in the Richmond basin have revealed unexpected results. Alternating episodes of extension and compression have created overprints of structural styles, and generated syndepositional unconformities as sediments were uplifted and rotated. Eroded Triassic sediments from structural highs were redeposited in adjacent synclinal or fault-controlled lows. In addition, lithologic correlation has revealed the existence of large deltaic complexes prograding across the basins, followed by Mississippi-size meander-belt channel deposits in the subsurface. The early lacustrine-deltaic deposits (Vinita beds, etc) appear to have been deposited in a basin much larger than the apparent outcrop limits of the Richmond or Taylorsville basins. As much as 3,000 ft of these deposits accumulated prior to a compressional event that folded them into enormous anticlines and synclines, as Paleozoic thrust sheets were reactivated and sub-Triassic buried hills grew in height, raising the overlying Triassic sediments by as much as 2,000 ft over a distance of less than 2 mi. End_Page 1434------------------------------ The synclines contain a nearly continuous sedimentary sequence, whereas the anticlines have the uppermost Otterdale Sandstone facies as a blanket deposit over their truncated tips and steeply dipping Vinita beds below the unconformity on their flanks. Extensional events followed the main phase of folding and pulled the limbs of the anticlines apart, causing normal listric faults to form and blocks of Vinita beds to separate into North Sea-type fault-block structure. Based on maturation and porosity trends with depth, at least 7,000 ft of overburden have apparently been removed from the basins. Either the structural style changed dramatically from listric faults to horst and graben mechanics, thereby dropping the Richmond-Taylorsville sequences below the present erosional level, or an enormous gravity slide from the east covered the basins, and erosion has created windows through this detached Paleozoic thrust sheet into a much larger Triassic rift system below it. End_of_Article - Last_Page 1435------------

Early palaeozoic palaeomagnetism in Australia I. Cambrian results from the Flinders Ranges, South Australia II. Late Early Cambrian results from Kangaroo Island, South Australia III. Middle to early-Late Cambrian results from the Amadeus Basin, Northern Territory

I. Cambrian results from the Flinders Ranges, South Australia A total of 460 samples from six sequences spanning the Cambrian succession of the Flinders Ranges (Adelaide “Geosyncline”, South Australia) has been analyzed through thermal demagnetization studies. All samples showed a recent field component, generally constituting more than 50% of the initial intensity, which in most cases was removed by 200–400°C. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Cambro-Ordovician age (S-pole at 75.3°E 26.0°N, α 95 = 7.4°, N = 5 localities) interpreted as having been induced by thermochemical activity during a period of enhanced heat flux prior to the Late Cambrian-Early Ordovician diastrophistic phases of the Delamarian Orogeny. 2. (B) A primary magnetic component, which indicates rapid polar motion during the Early Cambrian and a much reduced polar motion during the Middle Cambrian. Representative palaeomagnetic pole positions for the primary component are: 1. (1) Basal Hawker Group (earliest Cambrian): S-pole at 2.3°E 26.7°S, d p = 8.1°, d m = 14.3°, N = 10 (sites). 2. (2) Billy Creek Formation— Wirrealpa Limestone— Aroona Creek Limestone (late Early Cambrian to early Middle Cambrian): S-pole at 20.1°E 37.4°S, d p = 7.2°, dm = 14.4°, N = 11(sites). 3. (3) Basal Lake Frame Group (Middle Cambrian): S-pole at 26.1°E 29.3°S, d p = 6.6°, d m = 13.1°, N =10 (sites). 4. (4) Pantapinna Formation (late Middle Cambrian?): S-pole at 29.2°E 36.4°S, d p = 8.4°, dm = 16.7°, N = 4 (sites). Available data suggest that deposition of the Lake Frome Group beds probably did not continue into the Late Cambrian. II. Late Early Cambrian results from Kangaroo Island, South Australia A total of 108 block samples from a late Early Cambrian red-bed sequence on Kangaroo Island (Adelaide “Geosyncline”, South Australia) has been analysed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity. Two characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Cambrian—Early Ordovician age (S-pole at: 75.8°E 17.4°N, d p = 4.2°, d m = 1.9°, N = 54 specimens), attributed to thermochemical activity predating the main folding phases of the Delamarian Orogeny. 2. (B) A primary magnetic component corresponding to a S-pole position at 15.1°E 33.8°S (d p = 6.2°, d m = 12.3°, N = 16 sites). Both the primary and the secondary magnetic component are in very good directional agreement with the magnetization pattern from the correlated Billy Creek Formation of the Flinders Ranges (I). Consequently, noticeable rotational movement since late-Early Cambrian times between Kangaroo Island and the northwestern part of the Adelaide “Geosyncline” can be ruled out. III. Middle to early-Late Cambrian results from the Amadeus Basin (Northern Territory) A total of 328 samples from a Middle Cambrian red-bed succession and a Middle to early-Late Cambrian carbonate succession in the Amadeus Basin (Central Australia) have been analyzed through thermal demagnetization studies. All samples contained a recent field component of considerable intensity which became broken down, respectively below 200°C in the carbonate samples and between 300°C and 500°C in the red-bed samples. Another recent field component, broken down between 600°C and 675°C, was noted in some of the red-bed samples. Three characteristic magnetic components have been identified: 1. (A) A secondary magnetic component of Late Devonian—Early Carboniferous age (S-pole at 110.5°E 46.9°S, N = 2 localities) which predates the main folding phase of the Early Carboniferous Alice Springs Orogeny. 2. (B) Another secondary magnetic component (S-pole at 60.8°E 33.8°N, N = 2 localities) which is very similar to a thermo-chemically induced Cambro-Ordovician magnetic component, noted in rocks from the Adelaide “Geosyncline”. 3. (C) A primary magnetic component which suggests reduced polar motion during the Middle Cambrian (S-pole at 16.6°E 35.0°S, α 95 = 7.4°, N = 5 sites) and rapid polar motion during the early-Late Cambrian (S-pole at 34.9°E 14.9°N, d p = 6.3°, d m = 11.6°, N = 9 specimens).