Origin and role of major strike-slip transfers during plate collision in the central Mediterranean

Variations in the crustal structure along the northern African plate margin have caused different modes of collision with Eurasia. Lateral density variations along the central Mediterranean collision zone are expressed in a change of the angle of the downbending African Plate and lead to the formation of strike-slip transfers in these transition zones that are roughly perpendicular to the trend of the collisional zone. In some cases these transfer zones are developed into hinge faults, while in others they can be developed into transform faults. This process governs the segmentation of the collision zone in the central Mediterranean region south of the Maghrebian thrust belt in Tunisia and Sicily through the Calabrian Arc to the northeastern Hellenic Arc, extending further to the Cyprian Arc and to the Taurus-Zagros chain.     

The sediment infill of subglacial meltwater channels on the West Antarctic continental shelf

Subglacial meltwater plays a significant yet poorly understood role in the dynamics of the Antarctic ice sheets. Here we present new swath bathymetry from the western Amundsen Sea Embayment, West Antarctica, showing meltwater channels eroded into acoustic basement. Their morphological characteristics and size are consistent with incision by subglacial meltwater. To understand how and when these channels formed we have investigated the infill of three channels. Diamictons deposited beneath or proximal to an expanded grounded West Antarctic Ice Sheet are present in two of the channels and these are overlain by glaciomarine sediments deposited after deglaciation. The sediment core from the third channel recovered a turbidite sequence also deposited after the last deglaciation. The presence of deformation till at one core site and the absence of typical meltwater deposits (e.g., sorted sands and gravels) in all three cores suggest that channel incision pre-dates overriding by fast flowing grounded ice during the last glacial period. Given the overall scale of the channels and their incision into bedrock, it is likely that the channels formed over multiple glaciations, possibly since the Miocene, and have been reoccupied on several occasions. This also implies that the channels have survived numerous advances and retreats of grounded ice.     

Intrusion of igneous rocks — Physical aspects

Igneous rocks occur in two predominant modes. The plutonic mode is characterized by intrusive emplacement under compressional tectonic stress regimes. The volcanic mode is dominated by the eruption of large volumes of magma through fault controlled dyke structures in extensional tectonic stress regimes.Those typical intrusive phenomena are discussed in the light of the physical parameters like the dynamic-thermal regime of the lithosphere, the nature and source of buoyant forces as well as mechanical aspects of subsolidus flow of rocks.Twodimensional numerical models on the basis of typical parameter specifications and by means of a coupled dynamic-thermal physical approximation are presented. The simulation of buoyantly-driven diapiric intrusions has been adopted to derive parameters which are critically controlling the transport and emplacement of intrusion under surroundings of variable viscosity. Both sill-type and batholith-type structures are presented and discussed with respect to their predominant dependence on mechanical and physical conditions.

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Zusammenfassung Magmatische Gesteine treten in zwei vorherrschenden Erscheinungsformen auf. Die Plutonite sind charakterisiert durch eine intrusive Platznahme unter kompressiven tektonischen Spannungsbedingungen. Die Vulkanite werden beherrscht von gro\en Volumina eruptierten Magmas, das vorzugsweise durch weiträumige Bruchsysteme unter Zugspannungsbedingungen aufsteigt.Derartige magmatische Phänomene werden aus der Sicht physikalischer Parameter wie dynamisch-thermische Bedingungen der Lithosphäre, der Natur und des Ursprungs von Auftriebskräften sowie der Mechanik von Gesteinsformationen diskutiert.Es werden zweidimensionale numerische Modellrechnungen auf der Grundlage typischer Parameterbereiche vorgestellt. Die Rechnungen basieren auf einer dynamisch-thermischen Approximation von Flie\vorgängen. Sie werden eingesetzt zur Untersuchung von diapirartigen Intrusionsvorgängen, die durch Dichteinversionen angetrieben sind. Hierbei sollen die Parameter bestimmt werden, die Aufstieg und Platznahme der Intrusionen bestimmen. Die Umgebung ist charakterisiert durch eine vorgebbare variable Viskosität. Sowohl linsenförmige Sill-Strukturen als auch typische Batholith-Strukturen werden mathematisch simuliert. Ihr Zusammenhang mit den strukturbestimmenden Parametern wird aufgezeigt.

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Résumé Les roches magmatiques se répartissent en deux catégories principales: la catégorie plutonique est caractérisée par une mise en place dans des régimes de contraintes tectoniques compressives; la catégorie volcanique est dominée par l'éruption de volumes importants de magma à travers des systèmes de cassures dans des régimes de contraintes tectoniques d'extension.Ces phénomènes intrusifs sont discutés à la lumière de paramètres physiques tels que: le régime thermique et dynamique de la lithosphère, la nature et l'origine des forces ascensionnelles, ainsi que les aspects mécaniques du fluage sub-solidus des roches.Les auteurs présentent des modèles numériques bidimensionnels basés sur des domaines paramétriques typiques, au moyen d'une approximation dynamo-thermique de phénomène d'écoulement. La simulation d'intrusions diapiriques permet de déduire les paramètres qui déterminent le transport et la mise en place d'une intrusion dans diverses conditions de viscosité de l'encaissant. Les structures de type sill et de type batholite sont discutées en relation avec les conditions mécaniques et physiques dont elles dépendent.

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Bedform signature of a West Antarctic palaeo-ice stream reveals a multi-temporal record of flow and substrate control

The presence of a complex bedform arrangement on the sea floor of the continental shelf in the western Amundsen Sea Embayment, West Antarctica, indicates a multi-temporal record of flow related to the activity of one or more ice streams in the past. Mapping and division of the bedforms into distinct landform assemblages reveals their time-transgressive history, which implies that bedforms can neither be considered part of a single downflow continuum nor a direct proxy for palaeo-ice velocity, as suggested previously. A main control on the bedform imprint is the geology of the shelf, which is divided broadly between rough bedrock on the inner shelf, and smooth, dipping sedimentary strata on the middle to outer shelf. Inner shelf bedform variability is well preserved, revealing information about local, complex basal ice conditions, meltwater flow, and ice dynamics over time. These details, which are not apparent at the scale of regional morphological studies, indicate that past ice streams flowed across the entire shelf at times, and often had onset zones that lay within the interior of the Antarctic Ice Sheet today. In contrast, highly elongated subglacial bedforms on sedimentary strata of the middle to outer shelf represent a timeslice snapshot of the last activity of ice stream flow, and may be a truer representation of fast palaeo-ice flow in these locations. A revised model for ice streams on the shelf captures complicated multi-temporal bedform patterns associated with an Antarctic palaeo-ice stream for the first time, and confirms a strong substrate control on a major ice stream system that drained the West Antarctic Ice Sheet during the Late Quaternary.     

Orientation and nature of active crustal stresses determined by electromagnetic measurements in the Patagonian segment of the South America Plate

Based on electromagnetic measurements we determined the current stress directions in the uppermost continental crust of Patagonia between the active plate margin of the Chilean Pacific coast and the Argentinean passive Atlantic margin. Regional variations of the observed stress pattern are giving details onto the acting tectonic processes. We distinguish five regional stress domains with different prevailing horizontal stress directions (S_H): 1. Southern Coastal Cordillera and Longitudinal Valley (S_H = SSW–NNE), 2. Chiloé Island (S_H = SW–NE), 3. Northern Patagonian Andes (S_H = WSW–ENE), 4. Argentinean Pampa and Atlantic margin (S_H = WNW–ESE) and 5. Southern Patagonian Andes (S_H = WNW–ESE). These stress regimes can be related to the geometry of the subducting Nazca- and Antarctic plates, to the transform fault between the South America and Scotia plates and to passive margin processes along the Atlantic coast. Absolute plate motion and rapid relative plate convergence control the subduction geometry and therefore the stress directions along the convergent margin of the South America Plate and the structural style within and landward of the Magmatic Arc. The knowledge of current local stress directions permits the characterisation of potential fault kinematics. By in situ measuring of electromagnetic emissions from rocks we determined the maximum horizontal stress orientation in the uppermost crust using a new geophysical tool. Our investigations on the orientation of the stress regimes also allow conclusions about the causative forces of either tectonic or gravitational origin in this part of the South-America Plate.     

Exploring biological constraints on the glacial history of Antarctica

The evolutionary and biogeographic history of the contemporary Antarctic terrestrial and marine biotas reveals many components of ancient origin. For large elements of the terrestrial biota, long-term isolation over timescales from hundreds of thousands to tens of millions of years, and thus persistence through multiple glacial cycles, now appears to be the norm rather than the exception. For the marine biota there are some parallels with benthic communities also including ancient components, together with an incidence of species-level endemism indicating long-term isolation on the Antarctic continental shelf. Although it has long been known that a few ice-free terrestrial locations have existed in Antarctica for up to 10–12 million years, particularly in the Dry Valleys of Victoria Land along with certain nunataks and higher regions of large mountain ranges, these do not provide potential refugia for the majority of terrestrial biota, which occur mainly in coastal and/or low-lying locations and exhibit considerable biogeographic regionalisation within the continent. Current glacial models and reconstructions do not have the spatial resolution to detect unequivocally either the number or geographical distribution of these glacial refugia, or areas of the continental shelf that have remained periodically free from ice scouring, but do provide limits for their maximum spatial extent. Recent work on the evolution of the terrestrial biota indicates that refugia were much more widespread than has been recognised and it is now clear that terrestrial biology provides novel constraints for reconstructing the past glacial history of Antarctica, and new marine biological investigations of the Antarctic shelf are starting to do likewise.     

Pantelleria Rift — crustal extension in a convergent intraplate setting

The Pantelleria Rift system is a wide zone of post-Miocene northwest-trending grabens and horsts beneath the Sicily Strait. The central grabens host volcanics of predominantly alkalic composition which are exposed on the islands of Pantelleria and Linosa. On the Maltese Islands, along the northeastern shoulder of the rift, an Oligocene-Miocene carbonate succession exposed above sea level allows structural analysis and determination of shallow crustal stresses within the otherwise largely submarine rift system. An older northeast-trending set of normal faults is probably the expression of an Oligocene-Miocene crustal extension event which produced continental rifts in western Europe and led to passive margin formation in the western Mediterranean. Younger northwest-trending grabens of the Pantelleria Rift system cut the older faults almost at right angles and define a zone of lithospheric stretching between Tunisia and Sicily. The northwest-trending grabens which subsided dramatically since the beginning of Pliocene time appear to be connected by east-trending dextral and, more rarely, north-trending sinistral transforms. Displacement along the transforms is probably in the order of a few kilometres. In-situ stress measurements carried out on the Maltese Islands show maximum horizontal compression (S_H) parallel to the rift. This suggests that in general ₁ (vertical) and ₂ (horizontal and parallel to the rift) are of about the same magnitude; both exceed ₃ (S_h) which trends northeasterly. Slight intraplate convergence in a NW-SE direction seems to be more than balanced by extension in a NE-SW direction. Neotectonics of the region possibly reflects an asthenospheric flow pattern which became established during the Messinian salinity crisis. The mechanism of recent intraplate deformation of the Pelagian shelf has relevance for the understanding of more anciently subsided platforms of the Apulian Plate.

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Zusammenfassung Das Pantelleria Rift System ist eine breite Zone von post-Miozänen NW-streichenden Gräben und Horsten in der Straße von Sizilien. Die überwiegend alkalischen Vulkanite der zentralen Gräben sind auf den Inseln Pantelleria und Linosa aufgeschlossen. Mit den Maltesischen Inseln ragt ein Teil der nordöstlichen Schulter des Rifts über den Meeresspiegel. In der Oligozänen-Miozänen Karbonatabfolge können oberflächennahe Krustenspannungen in dem sonst vom Meer weitgehend bedeckten Rift System gemessen werden. Die NO-orientierten älteren Abschiebungen sind vermutlich die Folge eines Krustendehnungsereignisses im Oligozän/Miozän, das in West-Europa kontinentale Rifts geschaffen hat und im Westlichen Mittelmeer zur Bildung von passiven Kontinentalrändern führte. Die jüngeren NW-streichenden Gräben des Pantelleria Rift Systems schneiden die älteren Abschiebungen fast rechtwinklig und charakterisieren einen Krustendehnungsbereich zwischen Tunesien und Sizilien. Die NW-orientierten Gräben, die seit dem Beginn des Pliozäns rapide abgesunken sind, scheinen durch O-W steichende, rechtsseitige und untergeordnet durch nordwärts orientierte linksseitige Horizontalverschiebungen verbunden zu sein. Der horizontale Versatz an den Blattverschiebungen dürfte nur wenige Kilometer betragen. In situ Spannungsmessungen, die auf Malta durchgeführt wurden, ergeben eine maximale Horizontalkompression (S_H) parallel zum Rift. Die deutet an, daß ₁ (vertikal) und ₂ (horizontal und parallel zum Rift) sich in der Höhe des Spannungsbetrages annähernd entsprechen. Beide Beträge liegen über dem der nordostwärts orientierten minimalen Horizontalkompressionsrichtung ₃ (S_h). Eine schwache nordwestwärts gerichtete Intraplattenkonvergenz scheint durch eine NO-SW gerichtete Krustendehnung mehr als ausgeglichen zu sein. Die Neotektonik dieses Raumes ist möglicherweise die Reaktion auf eine Umorientierung von Fließvorgängen in der Asthenosphäre während der messinischen Salinitätskrise. Der Mechanismus der rezenten Intraplattendeformation des Pelagischen Schelfs bietet einen Schlüssel zum Verständnis der in früheren geologischen Zeiträumen abgesunkenen Plattformen der Apulischen Platte.

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Résumén Le Rift-système pantellérien est une large zone de grabens et horsts post-miocènes d'orientation NW, située dans le détroit de Sicile. Les grabens centraux comportent des volcanites essentiellement alcalines qui affleurent dans les îles Pantellaria et Linosa. Dans le Rif-système, essentiellement sous-marin, une détermination de l'état de contraintes de la croûte supérieure est possible par l'analyse structurale d'une série carbonatée oligomiocène qui affleure dans les îles maltaises, le long de la lèvre NE du rift. Un train de failles normales anciennes, orientées NE est probablement l'expression de l'extension crustale oligo-miocène responsable des rifts continentaux d'Europe occidentale et de la formation d'une marge passive en Méditérranée occidentale. Les grabens, plus jeunes, du systmè pantellérien, orientés NW, coupent les failles plus anciennes à peu près à angle droit et définissent une zone d'extension entre la Tunisie et la Sicile. Les grabens d'orientation NW, rapidement affaissés depuis le début du Pliocène, semblent reliés par des failles transformantes E-W dextres et, plus rarement, N-S senestres. Le déplacement le long de ces failles ne semble pas excéder quelques Km. Les mesures de contraintes effectuées in situ dans les îles maltaises montrent une compression horizontale maximale (S_H) parallèle au rift. Ceci suggère que, en général, ₁ (vertical) et ₂ (horizontal et parallè au rift) ont a peu près la même valeur, supérieure à elle de ₃ (S_h) qui est orientée NE. Une légère convergence intraplaque de direction NW-SE semble être contre-balancée, et au-delà, par une extension NE-SW. On peut voir, dans la néotectonique de la région, le reflet d'un régime de flux asthénosphérique qui se serait établi lors de la crise évaporitique messinienne. Le mécanisme de la déformation intra-plaque récente permet de mieux comprendre les affaissements plus anciens des plates-formes de la plaque apulienne.

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Contribution No. 87, SFB 108 Universität Karlsruhe.     

No evidence for a Pleistocene collapse of the West Antarctic Ice Sheet from continental margin sediments recovered in the Amundsen Sea

Records of glaciomarine deposition recovered from the West Antarctic continental margin in the Amundsen Sea allow the reconstruction of the behaviour of the West Antarctic Ice Sheet (WAIS) in response to the natural climatic changes of the last 1.8 million years. Contents of gravel-sized and lithogenic components represent the input and redeposition of glaciogenic debris, whereas variations in the proportions of the calcareous sediment fraction reflect palaeoproductivity changes. All proxies, which are regarded as sensitive to a WAIS collapse, changed markedly during the global climatic cycles, but do not confirm a complete disintegration of the WAIS during the Pleistocene.