Latent heat effects of the major mantle phase transitions on low-angle subduction

Very low to zero shallow dip angles are observed at several moderately young subduction zones with an active trenchward moving overriding plate. We have investigated the effects of latent heat for this situation, where mantle material is pushed through the major mantle phase transitions during shallow low-angle subduction below the overriding plate. The significance of the buoyancy forces, arising from the latent heat effects, on the dynamics of the shallowly subducting slab is examined by numerical modeling. When a 32-Ma-old slab is overridden with 2.5 cm/yr by a continent, flat subduction occurs with a 4–5 cm/yr convergence rate. When latent heat is included in the model, forced downwellings cause a thermal anomaly and consequently thermal and phase buoyancy forces. Under these circumstances, the flat slab segment subducts horizontally about 350 km further and for about 11 Ma longer than in the case without latent heat, before it breaks through the 400-km phase transition. The style of subduction strongly depends on the mantle rheology: increasing the mantle viscosity by one order of magnitude can change the style of subduction from steep to shallow. Similarly, an overriding velocity of less than 1 cm/yr leads to steep subduction, which gradually changes to flat subduction when increasing the overriding velocity. However, these model parameters do not change the aforementioned effect of the latent heat, provided that low-angle subduction occurs. In all models latent heat resulted in a substantial increase of the flat slab length by 300–400 km. Varying the olivine–spinel transition Clapeyron slope γ from 1 to 6 MPa/K reveals a roughly linear relation between γ and the horizontal length of the slab. Based on these results, we conclude that buoyancy forces due to latent heat of phase transitions play an important role in low-angle subduction below an overriding plate.     

The 3D shear experiment over the Natih field in Oman: the effect of fracture-filling fluids on shear propagation

This is the final paper in a series on the 3D multicomponent seismic experiment in Oman. In this experiment a 3D data set was acquired using three-component geophones and with three source orientations. The data set will subsequently be referred to as the Natih 9C3D data set. We present, for the first time, evidence demonstrating that shear waves are sensitive to fluid type in fractured media. Two observations are examined from the Natih 9C3D data where regions of gas are characterized by slow shear-wave velocities. One is that the shear-wave splitting map of the Natih reservoir exhibits much larger splitting values over the gas cap on the reservoir. This increase in splitting results from a decrease in the slow shear-wave velocity which senses both the fractures and the fracture-filling fluid. Using a new effective-medium model, it was possible to generate a splitting map for the reservoir that is corrected for this fluid effect. Secondly, an anomaly was encountered on the shear-wave data directly above the reservoir. The thick Fiqa shale overburden exhibits a low shear-wave velocity anomaly that is accompanied by higher shear reflectivity and lower frequency content. No such effects are evident in the conventional P-wave data. This feature is interpreted as a gas chimney above the reservoir, a conclusion supported by both effective-medium modelling and the geology.
With this new effective-medium model, we show that introduction of gas into vertically fractured rock appears to decrease the velocity of shear waves (S2), polarized perpendicular to the fracture orientation, whilst leaving the vertical compressional-wave velocity largely unaffected. This conclusion has direct implications for seismic methods in exploration, appraisal and development of fractured reservoirs and suggests that here we should be utilizing S-wave data, as well as the conventional P-wave data, as a direct hydrocarbon indicator.     

Stratigraphic and structural setting of the Griqualand West and the Olifantshoek Sequences at Black Rock,Beeshoek and Rooinekke Mines,Griqualand West,South Africa

The Black Rock to Rooinekke area has been influenced by the Kheis and Namaqua orogenies. Thrusting associated with the Kheis orogeny is well documented throughout the mining area. Thrusting at Rooinekke resulted in the Griqualand West Sequence rocks overlying parts of the Olifantshoek Sequence. At Black Rock lithologies of the Voëlwater Subgroup have been thrusted onto the Olifantshoek Sequence. A prominent N-S trending graben structure has been observed in the Rooinekke, Beeshoek and Black Rock mining areas. The thrusts are disrupted by these N-S faults. At Rooinekke supergene replacement and enrichment of the iron ore is associated with pre-Olifantshoek N-S faulting. The Namaqua orogeny may also have played a part in upgrading the existing ore bodies under discussion.     

Comagmatic lamprophyres and diabases on the south coast of Norway

Two lamprophyre-diabase associations on the south coast of Norway are described. The lamprophyres and the diabases contain feldspathic leucocratic segregations in the shape of ocelli, ocellar pipes, sheets, or irregular veins and schheren. The origin of the lamprophyres is considered to be related to the formation of the leucocratic segregations. It is believed that the behaviour of alkaline volatiles under the influence of tensional stress and gravity accounts for the differentiation.     

Holocene and late-Pleistocene sedimentation in the Adriatic Sea

The following paper is a summary of sedimentological data on the Adriatic Sea (with the exception of the areas along the Jugoslavian and Albanian coasts). Because it is difficult to summarize a summary, only a few of the main conclusions will be mentioned here.Geophysical investigations indicate that the top of the limestone series, underlying the clayey and sandy deposits of the Pliocene and the Quaternary in the Adriatic area has a very uneven topography. Its greatest depths (4–6 km) are found a) between Ravenna and Rimini, b) between San Benedetto and Pescara, and c) below the Albanian shelf.Recent sands are mainly limited to the littoral zone; pleistocene sand, originally supplied by rivers, covers the greater part of the deeper shelf. Between these zones a terrace-shaped pro-littoral mud belt is present, where the bulk of the recent terrigenous mud is deposited. The maximum rate of accumulation in this belt is probably about 4 1/2 mm per year.The remaining part of the recent mud is transported in the sea water as floccules of such small size that they remain suspended over the deeper zones of the shelf. Most of it is deposited in the basins of the Central Adriatic (maximum accumulation rate for the Holocene on the average circa 1/2 mm per year) and in the bathyal basin in the southeast. The deepest area of the latter basin is formed by an almost horizontal plain (circa 1218 m deep). The longest core from this plain (240 cm of Holocene and 400 cm of late Pleistocene) is composed for roughly 61% of turbidite material, 5% of volcanic ash (coarser than fine silt), 0,2% of organic carbonate remains (coarser than silt) and 34% of normal terrigenous mud. The ash falls were limited to the central and southeastern parts of the Adriatic.

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Zusammenfassung Eine kurze Übersicht wird gegeben über die sedimentologische Kenntnis der Adria (mit Ausnahme der jugoslawischen und albanischen Küstengewässer).Geophysikalische Untersuchungen zeigen, daß die Kalkstein-Oberfläche unter den tonig-sandigen Ablagerungen des Pliozäns und des Quartärs, ein starkes Relief besitzt. Sie hat ihre größten Tiefen (4–6 km) a) zwischen Ravenna und Rimini; b) zwischen San Benedetto und Pescara und c) im Untergrund des Albanischen Schelfes.Rezente Sande sind in der Hauptsache auf eine schmale Küstenzone beschränkt. Dagegen haben pleistozäne Residual-Sande, ursprünglich von Flüssen herbeigebracht, eine große Ausdehnung auf dem Schelf. Zwischen diesen beiden sandigen Zonen findet man einen pro-littoralen Schlicksaum, wo die Hauptmasse des rezent ins Meer gebrachten terrigenen Schlickes abgelagert wird. Die maximale Akkumulationsgeschwindigkeit in dieser Zone beträgt wahrscheinlich ungefähr 4 1/2 mm pro Jahr.Der Anteil des terrigenen Schlickes, der nicht in diesem prolittoralen Schlicksaum zur Ablagerung kommt, besteht aus Flocken von so kleinen Abmessungen, daß sie während ihres Transportes über den äußeren Schelf-Regionen suspendiert bleiben. Sie sedimentieren größtenteils in den Becken der Zentral-Adria (mittlere Ablagerungsrate während des Holozäns maximal etwa 1/2 mm pro Jahr) und im bathyalen Becken der Südost-Adria.Der tiefste Teil dieses südöstlichen Beckens wird von einer fast horizontalen Ebene (auf etwa 1218 m Tiefe) eingenommen. Der längste Kern, der in dieser Ebene entnommen wurde (640 cm, wovon 240 cm Holozän), hat ungefähr die folgende Zusammensetzung: 61% Turbidit-Material, 5% vulkanische Asche (Sand- und grobe Schluff-Fraktionen), 0,2% organische Kalkreste (gröber als Schluff) und 34% normaler terrigener Schlick. Die Aschenfälle waren auf die mittleren und südöstlichen Teile der Adria beschränkt.

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Résumé L'auteur donne un bref résumé de la connaissance sédimentologique de la Mer Adriatique (à l'exception des parties le long des côtes Jugoslaves et Albanaises).Des recherches géophysiques indiquent que la surface du calcaire couvert par les dépôts argileux-sableux du Pliocène et du Quaternaire a un relief prononcé. Cette surface atteint des profondeurs maximales (4–6 km) a) entre Ravenna et Rimini, b) entre San Benedetto et Pescara et c) au-dessous du plateau continental Albanais.Les dépôts sableux d'âge Holocène sont limités pratiquement à l'étroite zone du littoral. Par contre, des sables pléistocènes résiduels, d'origine fluviale, couvrent de vastes étendues du plateau continental sous-marin. Entre ces deux zones sableuses, on trouve la bande vaseuse «pro-littorale», où se dépose la plus grande partie de la matière vaseuse terrigène, apportée à la mer sous les conditions actuelles. L'accumulation maximale dans cette zone est probablement de l'ordre de 4 1/2 mm par an.La partie de la vase terrigène qui dépasse cette bande pro-littorale est transportée dans la mer à l'état de flocons d'une taille si petite qu'ils restent en suspension au-dessus des parties extérieures du plateau continental. Ils sont déposés surtout dans les bassins de l'Adriatique Centrale (vitesse moyenne d'accumulation pendant l'Holocène au maximum environ 1/2 mm par an), et dans le bassin bathyal du Sud-Est.La partie la plus profonde dans ce dernier bassin est formée par une plaine presqu' horizontale (à environ 1218 m). La carotte la plus longue, tirée de cette plaine (640 cm, dont 240 cm d'Holocène) est constituée approximativement de 61% de matériel turbiditique, de 5% de matière volcanique (fractions de sable et de silt grossier), 0,2% de restes calcaires organiques (plus grossier que du silt) et 34% de vase terrigène normale. Les chutes de matière volcanique étaient limitées aux parties centrales et sud-orientales de l'Adriatique.

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— . , . (4–6 ) : a) Ravenna Rimini; ) San Benedetto Pescara ) . , , - , , pro-litto-ralen , . 4,5 . , ( 1/2 ) - . 1218 . , 640 , 240 . : 61% , 5% , 0,2% 34% . - .

     

Plastic stress relaxation around solid inclusions in pyrope

Misfit between primary (syngenetic) inclusions and pyrope host is usually due to differences in the coefficients of thermal expansion and compressibilities. It is shown that the misfit stresses are relieved by the generation and rearrangement of dislocations. Zones of plastic flow have been recognized around inclusions in pyropes from Bohemian garnet peridotites and in kimberlite pyropes from South Africa and Yakutia. The extent of plastic yield is determined by the history of the pyrope in the ductile regime. Implications for using the piezothermometric method of Rosenfeld and Chase (1961) which is based upon the elastic strain, are discussed.Publications No. 38 in the Norwegian Geotraverse Project.     

The S-External pyrenees of Huesca

An Upper Cretaceous-Paleocene series is more or less detached from the Hercynian basement and piled up southward as a Younger Calcareous Chain. A broad belt of Eocene flysch with limestone intercalations terminates eastward against a contemporaneous cross-structure and is affected by a zone of persistent overturning.

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Zusammenfassung Eine Oberkreide-PalÄozÄn-Serie ist teilweise vom herzynischen Untergrund abgeschert und südwÄrts als Jüngere Kalkkette aufgestaucht. Eine breite Zone von EozÄn-Flysch mit Kalkeinlagerungen endet nach Osten an einer gleichaltrigen Querstruktur. Sie enthÄlt einen durchgehend überkippten Streifen.

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Résumé La série néocrétacée-paléocène est plus ou moins décollée du socle hercynien et déversée au sud comme «ChaÎne calcaire postérieure». Une large zone de flysch éocène à intercalations calcaires se termine vers l'est contre un accident transversal contemporain et est affectée d'une bande persistante de renversement.

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