Graben Research Articles

Construction of a sheeted dyke complex: evidence from the northern margin of the troodos ophiolite and its southern margin adjacent to the arakapas fault zone

The structure of a sheeted dyke complex in relation to intrusion is important for an understanding of sea-floor spreading. Here, we provide data for a previously undescribed ~ E-W cross-strike section along the northern margin of the Troodos massif. For comparison, we also provide new information for a ~NS section along the southern margin of the Troodos massif, adjacent to the Arakapas Fault Zone, an inferred oceanic transform fault. The N-Troodos margin section is divided into two main segments with opposing dip directions, corresponding to the Solea graben in the east and the Stravros graben in the west. High-temperature, hydrothermally altered dykes occur relatively early in a relative intrusive chronology of both grabens, followed by gabbroic bodies and ubiquitous grey sheeted dykes; plagiogranite dykes intruded at relatively later stages. Chemical analysis reveals a trend towards a boninite-type composition, both in the sheeted complex as a whole and in several specific sections for which crosscutting relations were documented in detail. The relatively steeply dipping, late-stage, boninitic-type dykes are interpreted as feeders to the Upper Pillow Lavas. To the N of the Arakapas Fault Zone, large plagiogranite bodies intruded relatively early and were in turn transected by grey dykes, small gabbroic intrusions and late-stage isolated brown dykes. Chemical analyses show that both early-stage grey dykes and the later-stage brown dykes were fed by chemically similar magma. Highly depleted boninite-type dykes occur close to the Arakapas Fault Zone. The relatively late-stage brown dykes show a swing in orientation towards ~ E-W near the transform zone, reflecting an inferred up to ~ 90° relatively clockwise, vertical-axis rotation. The earlier-stage grey dykes are indicative of lesser amounts of vertical-axis rotation but show complex rotations about low-angle axes. Small-scale faults are mainly of normal and left-lateral strike-slip type. Taking alternatives into account, we infer that the faulting in the dyke section took place near a left-offset spreading axis within an overall right-slipping, transtensional transform fault zone. In addition, depleted mantle harzburgites were exposed and eroded on the seafloor within an inferred southward extension of the transform zone (now within the Late Cretaceous Moni Melange), proving additional evidence of a transtensional setting near or within the Arakapas transform. The combined evidence suggests that the Troodos sheeted dyke complex as a whole developed in response to unstable, asymmetrical spreading above a subduction zone in which relatively late-stage amagmatic extension is likely to have played an important role. The spreading crust was bounded to the south (in present coordinates) by a relatively broad transtensional transform fault zone, along which mantle rocks were locally exposed and eroded on the seafloor.

Determination of Site Factor in and Around Rhine Graben, France in the 1–11 Hz Frequency Band by Non-Reference Network Average Technique

The greatest challenge in estimating site response from instrumental recordings of seismic events is removing the source and path effects. Several methods have been suggested in the past in that direction which may be divided into two main categories, depending on whether or not they need a reference site, with respect to which the particular effects at other sites can be estimated. An alternative approach is to determine the site response relative to the network average. In the present paper we propose one such technique for estimating the site factor in the Rhine graben and its surrounding provinces. Judging the higher signal thresh-hold level (≥3 times the background) only sixteen earthquake events with local magnitude varying between 3 and 4.5 are used from the catalogue of 8 years during 1988–1995 recorded by the Fosse Rhenan network using short period single component seismometers belonging to the Ecole et Observatoire des Sciences de la Terre, Strasbourg, Prance and Survey Baden-Wurttenberg, Freiburg, Germany. Sixteen stations in and around Rhine graben are considered for the site factor computation in the 1–11 Hz frequency band. The contour maps prepared from the spatial distribution of non-reference network average site factor estimates at 1, 3, 5, 7, 9 and 11 Hz exhibit high values along the major faults and the minor tectonic features to the western (Vosges) and eastern (Foret-Noire) provinces. Irrespective of the magnitude of the earthquake, the Rhine graben is comparatively stable with low site factors at all the frequencies. As is expected from Coda site amplification studies, the network average contour patterns remained unchanged showing minor or no variation at all with frequencies.

STRUCTURAL PATTERN AND SEDIMENT FILLING IN HUAGUANG SAG OF SOUTHERN QIONGDONGNAN BASIN

A great number of Cenozoic petroleum basins have been discovered in the northern continental margin of the South China Sea.Two of them,the Qiongdongnan Basin and the Pearl River Mouth Basin are located in the deepwater area of the region.Recently medium and large petroleum discoveries have been obtained in the southern deepwater area of the Pearl River Mouth Basin.It verifies that favorable petroleum geological conditions occur in the deepwater area.Having similar tectonic and depositional evolutionary history with the Pearl River Mouth Basin,the southern deepwater area of the Qiongdongnan Basin should have the same potential to form medium to large gas reservoirs.It,therefore,has become a necessity to study this area in details.The accumulation of high resolution seismic data in recent years has made it possible to understand more about the structural pattern of the deepwater area of the Qiongdongnan Basin.In this regard,the Huaguang Sag was selected as a case for study.Our study reveals that there are four types of extensional structures in the area,namely graben,half graban,terraced garaben and terraced half graben,formed in two stages,corresponding to the two regional structural evaluation stages in the region.There is an obvious double-layer vertical architecture in the sag,including the rifting lower layer and the subsiding upper layer,and four units have been recognized in the sag according to the structural pattern,namely the western sag,eastern sag,middle horst and the southern slope.The distribution of Paleogene deposits or the lower layer is obviously controlled by rifting.The tectonic activities in Neogene or the upper layer were rather weak.There is little tectonic deformation.The double layer architecture has provided an excellent combination for hydrocarbon accumulation.

STRUCTURAL PATTERN DURING THE RIFTING STAGE OF THE XIHU SAG AND ITS CONTROL OF SEDIMENT INFILLING

The structures of the Xihu Sag in the rifting stage include those formed from the onset of rifting(boundary Tg) up to the Late Eocene(Boundary T 30)in this paper.The Structural pattern is the factor controlling over the development of stratigraphy and the spatial distribution of depositional systems.The prototype of the basin during the rifting stage is a complicated half graben controlled by three groups of fault zone,i.e.the fault zone on east steep slope,the fault zone in central depression,and the fault zone on west gentle slope,all extending in NE direction.The plaeo-structural pattern of the Xihu Sag during the rifting stage plays significant roles in the formation of sedimentary sequences,and the distribution and shifting of depositional and subsidence centers.The sedimentation and tectonic events are episodic as a local response to the subduction of the Pacific plate under the Eurasian plate starting from the late Mesozoic.

Geological and biological aspects of a find of natural alloy (Au-Cu-Ag) nanoparticles in Cenozoic zeolitized tuff of the Vanchinskaya basin (Primor’e Area)

Zeolites first described by Swedish researcher Kro� nshted more than 250 years ago still attract special attention today. These minerals are effectively applied in industry, agriculture, medicine, and for conserva� tion of surroundings with gradually spreading areas of their application. This is explained by unique adsorp� tive, ionexchange, and catalytic properties typical for this group of minerals, as well as by their thermal, chemical, and radiation stability. Artificial zeolites with constant composition and a strictly definite set of physicochemical properties are claimed in industry. Natural zeolites represented by polymineral zeolitized rocks (zeolitites) of various geneses are applied in agri� culture and medicine. In spite of their old origin, the mechanism of biological activity of these natural min� eral systems still is not sufficiently studied. The discov� ery of natural Au-Cu-Ag alloy microparticles in nat� ural zeolitites discussed in this paper is a rare mineral� ogical fact deserving special attention. The Vanchinskaya basin, one of the Cenozoic rift� ogene basins within the Eastern SikhoteAlin' volca� nic belt, has a relatively small size of 4 × 15 km. How� ever, the presence of coal and zeolites volcanogenic- sedimentary deposits filling it, high registered concen� trations of REE in coals (1), and its location on the southeastern flange of the Soyuznoe gold-silver deposit (2) are responsible for the unremitting scien� tific and practical interest in it over many years. Structurally this basin is a oneside riftogenic gra� ben of northwestern strike. Its southwestern side is separated by the Milogradovskii fault (thrust) clearly reflected in relief. The northeastern flange occurring along the fault as well is gentle. The western part of the basin is filled by pyroclastic (tephrogenic) and tuffo� gene-sedimentary coalbearing deposits of Pale� ocene-Early Eocene age. In the eastern part they are replaced and intruded by Middle Eocene volcanic rocks (tuffs, lavas, explosive breccias, and extrusive rhyolite and trachyandesite bodies). Zeolite mineralization within the basin has a strat� iform character. All zeolitite occurrences on the right side of the Vanchina-Ugol'nyi Brook including the Berezovoe manifestation studied in detail are of the same type by geological characteristics. They are rep� resented by outcrops (fragments) of the undercoal band of tuffaceous sandstone and tuffite in the compo� sition of tuffogene-sedimentary lake deposits and characterized by a significant admixture of mineral� ized plant detritus. According to the data of Xray structural analysis, the mineral composition of zeoli� tites of the Berezovoe manifestation comprises zeolites of the clinoptilolite-heulandite group (50-60%) and clay minerals of the smectite group (20-30%) with an admixture of quartz and feldspars. The visible thick� ness of the zeolitebearing band is ~20 m. Clearly reflected signs of active eating of rocks by wild ungulates as lick niches, as well as eaten away caves and individual nibbles on outcrops with typical tooth prints are observed in the coldfree period of the year on all natural outcrops of the zeolitized band within the Vanchinskaya basin. Traces of Manchurian deer are the most frequently observed among the traces left by animals in the places where they eat rocks and soils. As we mentioned before, most of the soils eaten by animals within the SikhoteAlin' are formed from zeolitized and argillized tuffogene-sedimentary rocks with clear signs of the presence of buried organic rem� nants (3). The hypothesis on the significance of organic materials in zeolitites as accumulators of bio� logically active chemical elements appeared long ago,

Geological nvestigations in the Chukchi Sea and the adjacent areas of the Arctic Ocean during the RUSALCA-2009 expedition

The Joint Russian-American LongTerm Census of the Arctic Project (RUSALCA) started in Novem� ber of 2003 for the longterm monitoring of the Arctic environment. The studied region comprises poorly studied areas of the Chukchi Sea and the adjacent seg� ment of the Arctic Ocean near the Russia-United States boundary. This area is characterized by maximal rates of the environmental changes in the Arctic dur� ing the last decades, which is expressed, for example, in the enhanced ice melting (Fig. 1). Therefore, this project was aimed at the longterm monitoring of the Arctic environment and searching for the factors responsible for these changes. In this connection, of importance is the testing of different hypotheses explaining such significant environmental changes in the region under consideration. The main of these hypotheses are the increased water and heat exchange via the Bering Strait, the strengthened air heat transfer in the northern direction, and the upwelling of rela� tively warm intermediate Atlantic waters, which pene� trate into the eastern Arctic Ocean along the Eurasian continental slope. There is also an assumption con� cerning the activated geological processes in this region based on the development of a graben-rift sys� tem of nearmeridional and nearlatitudinal riftogenic structures in the Chukchi Sea (Fig. 1) formed in the Mesozoic and activated in the Late Cenozoic. This system is best studied in the socalled Chukchi Gra� ben, a neotectonic structure extending from eastern Chukotka in the south to at least the Herald Trench in the Chukchi Sea in the north (Fig. 1). Its onshore part studied in eastern Chukotka (the Kolyuchino- Mechigment zone) hosts Late Cenozoic volcanics and many hydrothermal vents with a discharging water temperature up to 97

Devonian swarms of dolerite dykes in the northeastern Siberian craton and their relation to the Vilyui plume

Recently, many researchers have started to pay attention to the study of large dike belts. This is explained by the fact that such belts mark extended faults in the lithosphere and, thus, both play an indic� ative role in defining epochs of important lithospheric reorganizations and serve as reference points in paleo� geodynamic reconstructions, which serve as a basis for recent metallogenic and prognostic concepts and reconstructing past deepseated geodynamic pro� cesses. Like many others structures, the Siberian Craton was subjected to many reorganizations during its life. Its lithosphere was stabilized in the Paleoproterozoic approximately 1.9 Ga ago (1). During its subsequent history, the craton was repeatedly involved in different continental agglomerations (for example, during the formation of Rodinia) and then separated from them during destruction of supercontinents. The corre� sponding events are reflected in differentage dike swarms. The Late Riphean dike belts that appeared during the breakup of Rodinia and indicate at least a Late Riphean age of the southern boundary of the cra� ton are best known among them. Significant swarms and clusters of dykes were also formed in the Middle Paleozoic. Powerful eruptions of Siberian Trap with the formation of a spacious system of sills and dykes occurred at the Permian-Triassic transition, and kim� berlites of a new generation intruded the northeastern part of the craton in the Late Mesozoic One of the most impressive systems of dike swarms was formed in the eastern part of the Siberian Craton during the Devonian. Its development accompanied the formation of the Vilyui rift system. In the opinion of (2), this process crowned the breakup of the conti� nent and formed its eastern boundary. An important feature of this rift system is the radial distribution of its main structural elements, dike swarms included. Such a position and structural features of the rift system allowed its formation to be related to destructive pro� cesses above the mantle plume (3), the center of which was located in the area overlain in the presentday structure by the foreland of the Mesozoic Verkhoyansk fold-thrust belt. In compliance with data on the distribution of gra� bens and dike swarms, the plume head at the base of the rift system should be 1500 km across. The follow� ing questions naturally arise: what mantle substrate was involved in the plume formation and to what extent was the latter uniform in different areas of the giant asthenospheric lens above the plume? Our previ� ous studies of igneous rocks, dykes included, in the southwestern part of the Vilyui rift system (Vilyui rift proper) revealed the main geochemical and isotopic- geochemical parameters of their mantle sources (4).

Carving the History of East Asia′s East-Tilting Topography and East Asian Monsoon——An Introduction to IODP Proposal 683

Ocean drilling in the Bengal Fan has revealed the uplift history of the Himalayas and Indian monsoon,but there is no analogous deep-water fan in the Western Pacific marginal seas that can be drilled and used to constrain the Tibet uplift history from its eastern side.The Yangtze river,originating from northeastern Tibet and draining a large area prevailed by monsoon precipitation,is highly sensitive to plateau uplift and monsoon evolution.A systematical study of the Yangtze river deposits will test various hypotheses concerning plateau uplift and its link with monsoon evolution.Along the modern Yangtze river,the upper reach is decoupled from the lower reach by the Three Gorges.About 30% of the total sediment load from the Yangtze river is laid in the Jianghan Basin immediately out from the Gorges,~40% is deposited in the lower reach and delta,and only ~30% is transported to the East China Sea.Therefore,a combination of ocean and continental drilling along the Yangtze River will recover records useful for carving the history of uplift and monsoon.A close timing of tectonic and climatic events mainly at four time intervals has been proposed as evidence for a link between Tibet uplift and the Asian monsoon initiation: the late Pliocene(2~3 Ma),the late Miocene(~8 Ma),the early-middle Miocene(~15 Ma) and the latest Oligocene(~25 Ma).On the basis of the Three Gorges incision or delta development data,however,the history of the Yangtze River can be traced back only to the late Pliocene or early Pleistocene,and the East China Sea came into existence only in the latest Miocene.A younger Yangtze river is in a sharp contrast to the early uplift of Tibet.In the Paleogene,a broad belt of aridity stretched across China from west to east,and numerous lake basins developed in rift grabens,prohibiting any east-flowing large rivers.Large rivers like the Yangtze river could have developed only since Neogene with the advent of an east-tilting topography and monsoon in East Asia.The proposed joint IODP/ICDP project will provide sediment records from marine and terrestrial basins to help clarify the most puzzling issues in East Asian environmental evolution and to test the hypothetical link between Tibetan uplift and monsoon climate from the eastern side of the plateau.

Regressive and transgressive cycles in a rift-basin: Depositional model and sedimentary partitioning of the Middle Jurassic Hugin Formation, Southern Viking Graben, North Sea

The Jurassic Hugin Formation consists of shallow-marine sandstones that belong to a significant hydrocarbon reservoir in the Sleipner area in the Norwegian North Sea. The formation encompasses coarsening-upward units of mouth bar and shoreface facies, interpreted to record delta outbuilding during regression; and fining-upward units with tidal channel, dune, and tidal flat facies interpreted as part of an estuary environment during transgression. The correlations reveal that the studied part of the Hugin Formation consists of 8 sequences, each with a transgressive and a regressive unit, representing the transgressive systems tract and the highstand systems tract respectively. The sequences are stacked retrogradationally landward as a result of rapid tectonic subsidence and rifting of the Viking Graben. Rifting led to the development of an elongate graben where tidal currents were amplified, wave-action damped and longshore drift (as sediments supply) reduced or absent. Lowstand and forced regressive systems tracts are not identified, and their absence is interpreted to reflect suppression of relative sea level falls in a rapidly subsiding basin where the basin subsidence rate outpaced any potential fall in eustatic sea level. Through facies interpretation and sequence-stratigraphic correlations between wells, these regressive and transgressive units are shown to exhibit characteristic thickness trends in the form of sigmoidal-shaped wedges, stacked in an offset manner in a landward to basinward orientation. These thickness trends illustrate sediment partitioning within the sequences and are explained by the relationship between accommodation versus sediment supply in terms of mass-balance. During regression, the focus of sedimentation was pushed basinward, and during transgression it was pushed landward as sediments were trapped there. The mapping of these sequence-stratigraphic units serves as input to reservoir models and to help increase recovery and identify new exploration targets.

3 - SOUTHERN TUSCANY - C - EASTERN SECTOR OF THE VOLTERRA BASIN

The geological setting of Southern Tuscany, where the Volterra Basin is located, is due to two different tectonic phases. The first one, compressional, is linked to the convergence and collision between the Adriatic Microplate and the European Plate and occurred from the Late Cretaceous to the Early Miocene. The second one is consequence of the extensional tectonic regime active since the Early-Middle Miocene (Carmignani and Kligfield, 1990; Jolivet et al., 1990; Bertini et al., 1992; Carmignani et al., 1994; 1995; Elter and Sandrelli, 1995a; Baldi et al., 1995; Dal Mayer et al., 1996). According to Elter and Sandrelli (1995a; 1995b), the extensional phase included two main events. During the first one, late Burdigalian to early Tortonian in age, there was a rapid uplift of the lithosphere, thickened during both the Alpine and Apenninic orogeneses. The lithosphere never reached the temperature necessary to generate calc-alkaline magmas(600°-650°, Sonder et. al., 1987). This event, was accompanied by “II phase folds” with both east and west vergence, by the emplacement of the Cretaceous Ligurian Units directly on the Triassic evaporites (“Serie ridotta”: Signorini, 1949; Trevisan, 1955; Giannini et al., 1971; Bertini et al., 1992; Decandia et al., 1993) and by the deposition of the Epiligurian Units. During the second extensional event, which started in the late Tortonian, a “thermal re-equilibration” associated to generation of anatectic magmas initiated in a thinned continental crust characterized by a high thermal flow (Table 1 in Elter and Sandrelli, 1995a). The extensional regime led also to the formation of graben and half-graben basins where thick sedimentary successions were deposited. These basins are laterally separated by WSW-ESE transverse lineaments (transfer zones Fig. 1). A different hypothesis on the geodynamic evolution of Southern Tuscany, considers that the first compressional phase acted until the end of the Neogene. In such a case, the sedimentary basins of western Tuscany would have formed as piggy-back or thrust-top basins (Bonini et al., 1994; Boccaletti et al., 1994; Bonini and Moratti, 1995; Boccaletti et al., 1997). During the field trip, we will visit the Volterra Basin between Spicchiaiola and Pignano. This area is particularly important because it lies on the western margin of the Middle Tuscany Ridge and recorded most of the depositional events which occurred in Southern Tuscany from Late Miocene to Pliocene. The four stops will illustrate some of the relationships between tectonics and sedimentation: Stop 1 - Transition from Turolian (upper Tortonian) lacustrine deposits to lower Messinian marine deposits in distal areas of the basin. A transgression affected different formations, highlighting discontinuities and disconformities. Stop 2 - Gypsum deposits formed during the salinity crisis which affected the Mediterranean Sea at the end of early Messinian. A debris flow interrupts the continuity of these deposits, suggesting synsedimentary tectonic activity. Stop 3 - Limestone deposits tilted down near a fault which formed before the deposition of Lower Pliocene marine sediments. This fact documents activity of the border fault between Early Pliocene and late Messinian. This portion of the basin was uplifted during late Messinian and subsided again during Early Pliocene. Stop 4 - The marine deposits of the Early-Middle Pliocene, very well developed in the Mazzolla-Volterra area, where they reach a thickness of about 1200 m. In the western area of Spicchiaiola-Pignano the Pliocene succession is, instead, thin and discontinuous. This difference in thickness is due to a synsedimentary normal fault.

SHALLOW GEOLOGICAL STRUCTURES OF THE SOUTH SHETLAND TRENCH, ANTARCTIC PENINSULA

In order to study the differential passive subduction among the three oceanic segments of the Phoenix Plate remnant, the R/V OGS-Explora acquired new geophysical data in the South Shetland Trench (offshore to the Antarctic Peninsula). An integrated geophysical dataset was acquired in the trench across the incoming oceanic plate segmented by the “D” and “E” oceanic Fracture Zones, and along the frontal part of the Antarctic Peninsula accretionary prism. We present here a high resolution multibeam bathymetry map, Chirp sub-bottom profiles and marine magnetic data to study changes in the deformational style along strike and to better understand features and processes of passive subduction. The new multibeam image and Chirp data display in detail shallow structures affecting the seafloor as a response to crustal deformation. Normal faults, bounding horst and graben structures cutting the incoming plate, are detected and characterized by variable orientation with respect to the inferred “D” and “E” Fracture Zones. Normal faults, width and depth of the trench and morphology of the frontal prism are related to bending and roll-back of the Phoenix Plate, and to inherited structural discontinuities. Some of these oblique faults offset the seafloor, with variable throws and with local dip changes. The orientation of the oblique faults, the different depth of the three oceanic segments and narrowing and deepening of the trench indicate that differential coupling between the Hero, Shackleton, “E” and “D” Fracture Zones may locally modify the regional stress field orientation and the sinking of each oceanic segment, during bending and roll-back.

EXTENSION OF A CONTINENTAL LITHOSPHERE WEAKENED BY IMPREGNATION OF ASTHENOSPHERIC MELTS: ANALOGUE MODELLING APPROACH

Extension of the European-Adria plate resulted in continental break-up and the development of the oceanic lithosphere of the Ligurian Tethys. During continental extension, progressive thinning of the continental lithosphere was accompanied by asthenospheric upwelling which underwent decompressional partial melting; resulting fractional melts impregnated and reacted with substantial portions of the overlying lithospheric mantle (e.g., Muntener and Piccardo, 2003). This generated a significant heating of the extending subcontinental lithosphere, which induced a thermochemical erosion of the lithospheric mantle and a strong softening of the extending continental lithosphere (e.g., Piccardo et al., 2004). This decrease in lithosphere strength could have played an important role in the dynamics of extension and rifting, and favoured the transition from passive lithosphere extension to active oceanic spreading. In this work, the mechanical consequences of the impregnation of an extending continental lithosphere by asthenospheric melts is investigated by means of laboratory experimental models. Analogue experiments were performed in an artificial gravity field by using the large capacity centrifuge at the Tectonic Modelling Laboratory of the CNR-IGG and Earth-Sciences Dept. of Florence University. Similarly to previous works (e.g., Corti et al., 2003), models simulated extension of a three layer lithosphere (brittle and ductile crust, lithospheric mantle) floating above a low-viscosity fluid (representing the asthenosphere). The analogue brittle crust was simulated with K-Feldspar sand, whereas the lower crust and lithospheric mantle were modelled with mixtures of silicone and corundum sand (100:30 and 100:50 % in weight for the crust and the mantle, respectively). The asthenosphere was modelled with a low-viscosity mixture of glycerol and gypsum. In the central part of the models, the lithosphere was weakened in order to reproduce the softening of the subcontinental lithospheric mantle by impregnation of the asthenospheric melts. In particular, the mantle strength was decreased by replacing the relatively strong sand-silicone mixture with the low-viscosity fluid representing the asthenosphere. The size of the impregnated volume was parameterized as the ratio (g) between the thickness of the impregnated zone and the thickness of the lithospheric mantle (g=1/3 means that 1/3 of the mantle lithosphere upward from the lith/asth boundary is impregnated). Weakening of the continental lithosphere in the central part of the model was dependent on the factor g, with maximum reduction of ~70% for g=1 (i.e., the impregnation affects the whole lithospheric mantle up to the Moho). Experimental results display that the style and evolution of deformation are highly influenced by the amount of lithospheric weakening in central part of the models, i.e. by the parameter g. In case of g≤1/2, models are characterized by diffuse deformation, resulting in a regular array of closely-spaced faults and a Basin-and-Range style of extension. At depth, this deformation pattern is associated with no important lithospheric thinning and asthenospheric upwelling. Conversely, when g>1/2 (i.e., almost the whole lithospheric mantle is weakened by asthenospheric melts), strong strain localization is observed within the weakest areas of the model (i.e. above the impregnated mantle). Extension gives rise to the development of normal faults and a central graben structure; the deformation style can be described as narrow rifting with necking of the continental lithosphere. In this case, important thinning of the lithosphere and upwelling of the asthenosphere is observed within the necking area, resulting in an important strength reduction of the lithosphere that -with further extension- may eventually lead to continental rupture and development of a new oceanic basin. These results support that the thermomechanical erosion of the lithosphere caused by the upward percolation of asthenospheric melts and the resulting mantle softening may have represented a controlling factor leading to the continental extension/oceanic spreading transition in the Mesozoic Ligurian Tethys.

High-resolution geological AUV survey results across a portion of the eastern Sigsbee Escarpment

High-resolution geophysical data were acquired for an investigation across a portion of Sigsbee Escarpment using an autonomous underwater vehicle (AUV Hugin 3000), which allowed mapping of the seabed and near-seafloor features in detail and large two-dimensional data sets to be collected in a short time in deep water over rugged terrain. Complex seafloor structures are revealed in the survey area. These seafloor structures include a graben fault zone, rugged escarpment faces, slump deposits, and erosional furrows. Geological morphologies occurring in the survey area are associated with salt tectonics, gravitational driven failure, and ocean bottom-current activities. The Sigsbee Escarpment in the survey area is marked by an abrupt scarp on the order of 700 m (2300 ft) and a prominent increase in seafloor gradients as much as 30°. The Sigsbee Escarpment in the center and west of the survey area is generally scalloped, representing retrogressive slumps. The escarpment face is characterized by narrow and sharp ridges and numerous gullies. In the east of the survey area, the escarpment appears to be upturned, tilted, and eroded. A graben fault structure, representing a suture zone possibly associated with the joining of the two underlying salt sheets, is observed in the north-central survey area. In front of the escarpment, on the continental rise, a series of longitudinal furrows and slump deposits have been interpreted. The slump deposits at the base of the escarpment form aprons of sediment consisting of displaced and mixed sediments primarily of clay.

Episodic growth of normal faults as recorded by syntectonic sediments, July oil field, Suez rift, Egypt

The July oil field is a major normal-fault–bounded structural block in the Suez rift basin, Egypt. It is adjacent to a major structural transfer zone, which has controlled sediment influx to the rift basin center for the past 20 m.y. The lower to middle Miocene Upper Rudeis Formation, part of the synrift stratigraphic sequence, records deformation of the July structural block. The formation contains abrupt lateral changes in thickness and facies, which record earlier phases of fault movement and deformation in the July field area that do not conform to the present-day structural configuration.The Upper Rudeis Formation was deposited as turbidites in a submarine-fan system sourced from the western rift shoulder. It was deposited over and around bathymetric highs created by coeval fault displacement in the July field area. By studying thickness and facies patterns, we have determined that the present-day main bounding fault to the July block consisted of a series of unlinked fault segments, which linked after Upper Rudeis deposition. A subsidiary fault west of the block exerted the most control on thickness patterns, not the present-day main bounding fault. Thus, commonly used models of deposition in active half grabens are difficult to apply at July field.

STRUCTURAL CONSTRAINTS FOR A GEODYNAMIC MODEL IN THE NORTH TYRRHENIAN-NORTHERN APENNINES POST-COLLISIONAL SYSTEM

The North Tyrrhenian-Northern Apennines post-collisional system (NTAS) developed its extensional setting since the Middle-Late Miocene, dismembering the Cretaceous- Miocene Northern Apennines fold and thrust belt with a remarkable eastward migration of the extensional events (Fig. 1). In this work the NTAS has been analysed with the following principal aims: – to provide an up-to-date structural pattern of the extensional frame; – to furnish the geometric relationships between the shallow and the deep extensional features of the crust; and – to suggest a structural interpretation that better fits the geometric elements obtained both from marine seismic data and recent field studies. E and NE-dipping extensional faults are the new postcollisional geometric elements of the North Tyrrhenian Sea and of the inner part of the Northern Apennines chain, recently outlined in works dealing with offshore and onshore areas (e.g. Bartole, 1995; Barchi et al., 1997; Bertini et al., 1991; Carmignani and Kligfield, 1990; Keller and Pialli, 1990; Lavecchia et al., 1997; Pascucci et al., 2001). These faults, along with those which dip toward W and SW, belong to a rift system composed of rotated blocks, half grabens, listric master faults striking N and NW, and of anti- Apennine E and NE oriented transversal faults (Fig. 2). As a consequence, two opposite and centrifugal extension polarities are recognized in the NTAS: while some extensional basins are bound by W or SW facing master faults (Tyrrhenian extension polarity), others are bound by E or NE facing master faults (anti-Tyrrhenian polarity) (Fig. 2). The occurrence of E and NE-dipping shallow detachment planes in the studied area constitutes a structural constraint that bears the following geodynamic implications: 1 - a greater emphasis is given to the role of the anti- Apennines lineations with respect to other ones because they separate rifted crustal blocks with Tyrrhenian extensional polarity (SW) from those having anti-Tyrrhenian polarity (NE); 2 - the concept of extensional re-utilization of crustal thrust planes (in this case with Adriatic vergence, i.e. with W or SW-dipping thrust surfaces) previously formed during contraction phases is partly contrasted because the E and NE facing master faults are new significant extensional features which originated during post-collisional times; 3 - the above described structural elements seem in better agreement with the extension mechanisms predicted by the anastomosing shear model (Kligfield et al., 1984; Carmignani and Kligfield, 1990; Reston, 1990) rather than by the simple shear (Wernicke, 1985) or by the delamination (Lister et al., 1986) models previously applied (Boccaletti et al., 1985; Lavecchia and Stoppa, 1989) to the NTAS. According to this model (Fig. 3), brittle, ductile and pervasive pure-shear are the extension mechanisms respectively characterizing the upper, middle and lower crust. Both Tyrrhenian and anti-Tyrrhenian facing normal faults only affect the upper brittle crust and sole out into a regionally extended, waving detachment surface approximately located at the brittle-ductile transition. Extension in the metamorphosed middle crust originated low-strain, lozenge-shaped blocks, while in the remaining lower crust extension is supposed to be only of pure-shear type. The major benefits of applying the anastomosing shear model to the post-collisional history of the NTAS are: a- it has a better “flexibility” in explaining the great complexity of the structural frame with respect to other geometrically more “rigid” models; b- it removes any hierarchy in the normal faults of the extensional system, since the terms synthetic and antithetic, utilized in the simple-shear and in the delamination models for the definition of the lithospheric detachment plane, now pertain to a local context rather than to a regional one; and c- it allows a better balancing, with respect to the other models, between extension of the Tuscany-Tyrrhenian sector and the synchronous shortening of the Apennines-Adriatic one (Fig. 4).

ABSTRACT: Fault-Growth and Drainage Development in the Canyonlands Grabens, Utah

ABSTRACT: Fault-Growth and Drainage Development in the Canyonlands Grabens, Utah

Geological Interpretation of the Parnaíba Basin Magnetovariational Anomaly Using a 2D Sheet Current Inversion

The magnetovariational response of a 2D electrical structure in the southeastern part of the Parnaiba basin, north-northeast Brazil, has been inverted in terms of laterally variable sheet current. The resulting inverted current distribution helps to visualize the overall basement configuration as a graben-horst-graben structure or alternatively indicates the presence of a resistive intrusive within a broad graben infilled with conductive sediments. Carbon bearing sediments are suggested as an alternative to hydrated sediments to account for the high conductivity of the mapped anomalies. Heating effects associated with the Cretaceous magmatic activity may be responsible in producing carbon through the pyrolysis of the hydrocarbon-saturated Paleozoic sediments or through the coalification of terrestrial plants.

NEOGENE AND QUATERNARY SEDIMENTATION PATTERNS IN THE NORTHWESTERN HYBLEAN PLATEAU (SE SICILY): THE EFFECTS OF A COLLISIONAL PROCESS ON A FORELAND MARGIN

Miocene carbonates and marls in the northwestern margin of the Hyblean Plateau (SE Sicily) exhibit considerable thickness variations consistent with synsedimentary tectonism. In the Early-Middle Mio­cene carbonates (Ragusa Formation) a series of parallel swells dose to the platform edge encouraged the development of a coarse-grained bioclastic facies. The overlying Middle-Late Miocene marls (Tellaro Formation) thin over these seafloor highs and associated mafie Tortonian volcanics are particularly developed along the hinge zones between swells and local sea-floor depressions. The basic extrusives appear to have followed in­cipient NE-SW fault lines propagated from deeper basement fractures. During the succeeding Messinian these faults were activated to produce horst structures (Vallone Lamia horst, Monte Caratabia horst and Cameme horst). These separate locai NE-SW oriented narrow grabens in which thick Messinian evaporites were deposited, probably connected with the main Centrai Sicilian Basin. Further subacrial deposits and volcanics levelled the Hyblean horst and graben topography prior to the major Early Pliocene marine high-stand. The ensuing transgression covered the study area with a thick pelagic Early Pliocene chalk. Local regressions, the product of Middle Pliocene and Early Pleistocene major tectonic activity along the margins of the plateau, are associated with local unconformities. Final emergence along the margins of the plateau occurred at the close of Early Pleistocene times when the emplacement of the Gela Nappe, from the NE, partly filled the foredeep basin. The withdrawal of the sea from the foredeep occurred shortly afterwards.

Ergebnisse mikropaläontologischer Untersuchungen im Rheintalgraben

Ergebnisse mikropaläontologischer Untersuchungen im Rheintalgraben

Geology of the Lake Inawasiro District.,

The Tertiary strata of this district can be subdivided into Nakayama bed, Sirakata bed, Iwane bed, Katahira bed, and Hukura bed., The Nakayama bed consists of basal conglomerate, sandstone, green tuff, sandstone agglomerate, sandstone, and is older than at least the Middle Miocene., Over this lies conformably the Sirakata bed, which consists of green tuff and an alternation of conglomerate, sandstone, shale and tuff ; many remains of molluscs occur in this bed., The following marine fossils were found in the lower part of this bed ; Arca ninohensis OTUKA Pecten kaneharai YOKOYAMA Pecten kagamianus YOKOYAMA Lucina acutilineata CONRAD Cardium Shiobarense YOKOYAMA Venus yokoyamai MAKIYAMA Macoma optiva YOKOYAMA Cultellus izumoensis YOKOYAMA Panomya simotomensis OTUKA The following fossils occur in the upper part ; Lucina acutilineata CONRAD Dentalium yokoyamai MAKIYAMA Linthia nipponica YOSHIWARA This bed may be correlated with the Siobara, Tanagura, Yanagawa, Siogama shell beds, and the age is Middle Miocene., Over this lies with slight unconformity the Iwane bed, which consists of liparite and its agglomerate ; and over this, with marked unconformity lies the Katahira bed, consisting of conglomerate and tuff ; and the unconformably overlying Hukura bed consists of dacite and its tiff., This district suffered two crustal movements., The formation of the first movement may have dated from the late Iwate bed and early Hukura bed, that is, from late upper Miocene and probably early Pliocene., Nakayama and Sirakata beds are folded, faulted and dip with high angle., The formation of the second movement may have date later than the Hukura bed, that is, probably later than Pliocene ; this was mainly a fault in NS trend, and resulted in the construction of graben and horst.,