Showing papers in "Geological Society, London, Special Publications in 1989"

Kinematics of the western Mediterranean

Abstract: Summary The kinematic understanding of the relationship between relative plate motion and the structure of orogenic belts depends upon a knowledge of relative plate motion across the plate boundary system, the relative motion of small blocks and flakes within the system, an evaluation of orogenic body forces, and an understanding of the thermomechanical evolution of the upper part of the orogenic lithosphere in determining strength and detachment levels. We have built a preliminary model for the Cenozoic kinematic evolution of the western Mediterranean oceanic basins and their peripheral orogens that integrates (1) the motion of Africa relative to Europe based upon a new study of Atlantic fracture zones using SEASAT data and the Lamont-Doherty magnetic anomaly database, (2) a new interpretation of the rotation of Corsica/Sardinia and the opening of the Balearic and Tyrrhenian oceanic basins, (3) sedimentary facies sequences in the Apennines, Calabria, and Sicily, and (4) Apennine/Calabrian structure and structural sequence.

Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism

Abstract: SUMMARY: The SW Indian and American-Antarctic Ridges are two of the world's slowest spreading ocean ridges (less than 1 cm a-l), making them the low end-members for rate of ocean ridge magma supply. Two-thirds of the rocks dredged at the numerous large offset transforms along the ridges are residual mantle peridotites. Gabbroic rocks, however, representing layer 3 and possible palaeo-magma chambers are rare. This suggests a highly segmented crustal structure, with anomalously thin crust near fracture zones that may consist of only a thin veneer of pillow basalt erupted over mantle peridotite. The dredged peridotites underwent high degrees of melting, spanning the range believed to produce abyssal basalt. Their depleted compositions show that the melt was almost entirely removed. At the same time, the spatially associated basalts have a large range of compositions, similar to those from the rift valleys, requiring extensive shallowlevel fractional crystallization. Since there is little evidence for magma chambers at these fracture zones, it is concluded that melts formed in the underlying mantle flowed laterally through the mantle beneath the crust towards a magmatic centre at the midpoint of an adjacent ridge segment. Magma was then subsequently intruded down the rift valley fissure system from the magmatic centre to erupt onto the fracture zone floor. Alternatively, the melt was drained from a mantle diapir beneath the midpoint of a ridge segment, prior to lateral flow of the residual peridotite beneath the ridge axis to the fracture zone. These processes suggest behaviour of the partially molten layer beneath ocean ridges analogous to Rayleigh-Taylor fluid instability, where a light less viscous fluid layer floating upwards in a denser medium goes unstable and drains at regularly spaced points into protrusions which rise rapidly to the surface. Evidence for such dynamically driven non-uniform melt flow in the mantle is seen in locally-abundant plagioclase peridotites, where the plagioclase crystallized from impregnated trapped melt. These rocks can contain up to 30% trapped melt, contrasting sharply with the typical abyssal peridotite which contains virtually none. Basalts erupted along these ridges provide a classic case of trace- and major-element decoupling during magma genesis. Despite trace-element and isotopic diversity, basalts from individual ridge segments were derived from primary magmas with similar majorelement compositions. These observations can be explained if melt flows locally through the depleted mantle at the end of melting towards the midpoint of a ridge segment. This would cause melts originating at different points in an initially heterogeneous mantle to migrate through and equilibrate with the same section of mantle immediately prior to segregation--which, for the most part, would homogenize the melt's major-element compositions. However, by virtue of the lever rule, this would have little effect on critical incompatible-trace-element or isotopic ratios of the migrating melts because of the very low incompatible-trace-element content of residual peridotite. Ocean ridges, then, appear to be marked by strings of regularly spaced volcanic centres overlying instability points in the partially molten upwelling asthenosphere much as has been postulated for arc volcanism and early continental rifting. Unlike arcs, the asthenosphere upwells to the base of the crust and the magmatic centres undergo continuous extension. Thus, large volcanoes are not constructed, and instead, ribbons of basaltic crust form parallel to the spreading direction. This is most evident at the SW Indian and American-Antarctic Ridges because of their highly attenuated magma supply. Where the magma supply is more robust and the magma chambers are correspondingly larger, the chambers may merge and eliminate the surficial morphological and chemical expression of punctuated magmatism at ocean ridges.

Geometry and kinematics of inversion tectonics

Abstract: Summary Positive inversion tectonics involves the reversal of extensional fault movement during contractional tectonics. Basin stratigraphy developed before, during and after extensional fault movements may be described as pre-, syn- and postrift sequences. Growth fault activity may be graphically displayed using thickness changes in stratigraphic intervals from fault footwall to hanging wall. Alternatively, it may be recorded using a hanging wall displacement/distance plot. Contractional reactivation of extensional faults puts progressively older synrift markers into net contraction. The point of change from net extension to net contraction is the null point. Its position in the synrift stratigraphy may be used to quantify the inversion ratio, which is defined as the ratio of contractional to extensional movement. Negative inversion is the reactivation in extension of a significant portion of an existing contractional system. Stratigraphic separation diagrams constructed from geological maps may be used to define the null point of individual faults and to quantify their inversion ratio.

The role of the Periadriatic Line in the tectonic evolution of the Alps

Abstract: Summary The Periadriatic Line and related lineaments formed as a result of post-collisional deformations which severely modified the Alpine chain. This post-late Oligocene deformation is the result of dextral transpression between the Adriatic sub-plate and the European foreland. Indentation of the western edge of the southern Alps caused uplift, related to backthrusting and associated deformations of the Lepontine region combined with E-directed escape of the central Alps. In the eastern Alps the response to dextral transpression is mainly by lateral escape along conjugate strike slip zones with minor or no vertical movements. Older deformations along this essentially late Alpine lineament can still be inferred locally and include: extension and transfer faulting in the late Palaeozoic to early Mesozoic, Cretaceous deformations, and Tertiary phases of compression (Eocene) and possibly extension (Oligocene). The geometry of crustal thinning associated with the formation of the passive continental margin of the southern Alps (associated with initial uplift of the Ivrea zone) has a profound influence on strain localization and the kinematics of movements along and north of the present day Periadriatic Line.

Alpine tectonics — an overview

Abstract: Summary This overview summarizes aspects of 150 years of research in Alpine tectonics and in particular introduces the tectonic setting for the more detailed papers in this volume. The Alpine Mesozoic ocean, Tethys, formed as a large elongate pull-apart basin in the Jurassic, as a consequence of the opening of the Atlantic and of the movement of Africa towards the east relative to a fixed Europe. The NNE trending Tethys was bounded by WNW trending transforms, by the European/Iberian margin in the W and by the Adriatic promontory of Africa in the E, and its shape determined the present day configuration of the arcs of the Alpine chain. The closing of this ocean and the collision tectonics began during the Cretaceous, as Africa moved to the NE relative to Europe and as the N Atlantic gradually opened, to drive Iberia and the southern part of the European plate to the E. Subduction of oceanic crust and adjacent continental crust led to high pressure metamorphism of Cretaceous age. Ophiolites were obducted over the southern continental margin, but after collision the shear sense reversed so that the Austro-Alpine nappes of the African Adriatic promontory overthrust Europe in a WNW direction. During the main Tertiary deformation the overall anticlockwise rotation of Africa led to a change-over from N to NW and WNW-directed collisional structures. The E-W striking sector of the Alps in Switzerland and Austria is therefore a diffuse transpressive dextral shear belt, approximately reworking the northern transform boundary of Tethys, modifying it by compression related to the rotation of the African Adriatic promontory. Approximately 250 km of European lithosphere were involved in the building of the western Alps. As Alpine nappes consist largely of rock material confined to the upper crust, a large amount of lower crust and lithospheric mantle of the two continental blocks must be duplicated and/or subducted during the Alpine collision history.

Rift basin evolution in Africa: the influence of reactivated steep basement shear zones

Abstract: Summary Phanerozoic rift basin evolution in Africa has been strongly influenced by the pre-existing basement structure inherited from Precambrian tectonics. Two modes of basement reactivation may be identified; the largely dip-slip reactivation of gently dipping shear zone and thrust structures and the largely strike-slip reactivation of steep shear zones and faults. In particular, crustal scale steep basement shear zones have exercised a profound control on rift basin formation on a regional scale. Such shear zone structures are seen to govern the location, trend and structural style of rifts and appear to constrain the orientation of displacements generating the rifts. The shear zones are relatively narrow, high strain zones formed during ductile deformation. They occur as steep, crustal scale ramps that define a pronounced mechanical anisotropy within the African continental crust. These structures are difficult to reactivate by dip-slip movement and thus tend to act as major strike-slip transfer zones during later extensional events. This paper discusses examples of the steep shear zone-rift relationship from three of the rift systems of Africa: the Upper Palaeozoic (Karoo) rifts of Central Africa, the Mesozoic rifts of the Sudan and the Cenozoic rifts of East Africa. These discussions indicate that steep shear zones are sites of rift nucleation and play a fundamental role in the mechanisms of rift propagation and subsequent continental breakup.

The origin of the Southern Ocean marine fauna

Abstract: Abstract Current knowledge of the break-up of Gondwana during the Tertiary indicates that shallow water marine habitats may have been present continuously, and on occasions were considerably more extensive than at present. Although direct fossil evidence is sparse after the Eocene, geophysical evidence suggests that shallow waters have been present since the late Mesozoic, and possibly much longer. The break-up of Gondwana was accompanied by a more or less steady lowering of both surface and bottom temperatures in the Southern Ocean from about 15°C in the Late Cretaceous to the present range of roughly +2 to −1.8°C. Microfossils in deep-sea drilling cores indicate that temperature drops were particularly sharp in the early Oligocene (c. 38 Ma), mid-Miocene (10–14 Ma) and Pliocene (c. 4 Ma BP). Geological evidence suggests that the Drake Passage opened, and the present oceanographic regime established, about 25–30 Ma BP. This is now known to be about the time of full-scale development of the East Antarctic ice cap. Subsequently ice sheets extended across, and deeply eroded, the continental shelves but the effects of these glacial maxima on the marine biota are not fully understood. Late Cretaceous/early Tertiary marine fossils from the James Ross Island group indicate a diverse shallow water marine fauna, including two groups notably lacking in diversity in the living fauna: decapods and teleost fish. In several genera occurrences in this fauna predate first occurrences in lower latitudes by as much as 40 Ma, suggesting the possibility that a number of groups originated at high southern latitudes. The living fauna exhibits a high biomass in many areas, and within-site diversity can be as high as anywhere in the world. Some individual taxonomic groups, however, (notably bivalves and gastropods) have a lower diversity than in the tropics, supporting the concept of a latitudinal cline in diversity. Studies of physiological adaptation to temperature suggest that the decline in seawater temperature during the Cenozoic has not presented a particularly severe evolutionary problem. The reasons for the absence of large decapods and the low diversity of fish in the present fauna are unclear. Most of the biological features of the modern fauna are more likely a response to the seasonality of the ecosystem rather than low temperature per se. Overall the evidence suggests that the present Southern Ocean shallow water marine fauna largely evolved in situ, having been present since at least the Late Cretaceous, and possibly much longer. Some groups have invaded, for example along the Scotia arc, but the isolation of the Southern Ocean by the present oceanographic regime and the limited dispersal ability of many forms means that exchange with lower latitudes is very slow.

The current status of thermobarometry in metamorphic rocks

Abstract: Summary Information on pressure (P) and temperature (T) is a fundamental aspect of research on metamorphic terrains. Unfortunately, many workers employ thermobarometers that are not experimentally calibrated, are insensitive or too sensitive to P-T changes, depend on a priori assumptions of water pressure (such as most petrogenetic grids), or are rapidly reset on cooling. Many systems are based on inaccurate thermodynamic data, involve solids with inadequately characterized structural states, neglect effects of thermal expansion and compressibility, or require long extrapolations in P-T-X space. For instance, application of the widely used garnet-clinopyroxene KD thermometer may require extrapolation to temperatures where current thermodynamic models of pyroxenes and garnets remain uncertain. Current versions of the Mg/Fe exchange thermometer for biotite-garnet involve substantial compositional extrapolations for many applications and the biotite is easily reset while cooling from higher T. The most widely employed barometer is based on dilution of the reaction grossular + kyanite + quartz = anorthite, but failure to correct molar volumes for P-T-X may yield systematic errors of 1–2 kbar for barometry of crustal metamorphites. Application of this barometer to rocks equilibrated at T < 600–650°C is presently unwarranted in view of unknown a-X relations of garnets and plagioclases at these T. However, by careful selections, thermobarometry may be accurate to ±50°C and ±1 kbar in many metamorphic terrains if a variety of different equilibria can be applied. Well-calibrated barometers that are useful for T τ 600–650°C rely on continuous reactions based on equilibria such as almandine + rutile = ilmenite + sillimanite + quartz, garnet + quartz = ferrosilite + plagioclase, garnet + rutile = ilmenite + anorthite + quartz, and almandine + sillimanite = hercynite + quartz. An extensive survey of the recent literature on thermobarometry of individual metamorphic facies reveals the range of P-T encountered in each facies. Temperature estimates are in good agreement with the inferences of Turner (1968). Barometry reveals that the blueschist, amphibolite and granulite facies give way to the eclogite facies over the pressure range of 12–16 kbar.

Characterization of the St Helena magma source

Abstract: Summary Combined Sr-Nd-Pb isotope and trace-element data for St Helena are interpreted in terms of changing thermal and chemical fluxes impinging on and interacting with the base of the lithosphere over a period of 6 Ma. The data reveal the existence of two geochemically distinct components in the St Helena source region: (i) a HIMU (high U/Pb) component which has extremely radiogenic lead isotopes (206Pb/204Pb >20.8) with 87Sr/86Sr and 143Nd/144Nd ratios displaced below the mantle array; (ii) a component with less radiogenic lead and strontium isotopic compositions and more radiogenic neodymium compositions. Coupled trace-element and isotope variations are evident during the activity of each volcano. During shield development an increase in incompatible-trace-element enrichment occurs. This is coupled to a decrease in 143Nd/144Nd, whilst strontium and lead isotope ratios become progressively more radiogenic with time. The time-dependent variations are thought to be consistent with high-level processes occurring at the base of, or within, the lithosphere. A decrease in the signature of the depleted component with time is shown to be the result of a decreasing thermal flux acting on the base of the lithosphere. As a consequence the degree of partial melting of the depleted component (which may reside in the lithosphere or asthenosphere) is reduced, increasing the signature of the HIMU component in the erupted magmas.

Some geometrical characteristics of inversion

Abstract: Summary The inversion of extensional fault systems results in the reversal of slip on the faults and expulsion of the synrift fill. During inversion the beds in the cover sequence shorten before the net extension at the basement level has been cancelled. Shortening of the sedimentary cover generates folding and backthrusting in the still downthrown hanging wall block. Intracratonic inverted basins in different parts of the Alpine Foreland show similar structural geometries with the major extensional faults which controlled basin development reactivated during subsequent compression. We use examples from the Western Approaches and offshore Holland (Broad Fourteens Basin) to illustrate the structural styles developed during inversion. The fundamental control on compressional structural geometry exerted by pre-existing extensional structures is also visible in more complexly deformed orogenic belts, like the Western Alps and the Pyrenees. In these areas inversion also occurs, but more commonly extensional faults which may not have inverted act as an indirect control on the location of ramps, and/or thrust orientation. Seismic data are normally required to establish these effects with certainty. However, as the body of knowledge builds up, it is possible to recognize certain geometrical characteristics which suggest the control of extensional faults in thrust belts. These include footwall shortcuts, out of sequence structures and arcuate thrust-fold traces.

Nile Delta: a review of depositional environments and geological history

Abstract: Summary The distribution of the Holocene-Recent sedimentary environments of the Nile Delta is related to secular variations of the semi-arid climate and of the Nile River sediment input and flux, and to coastal subsidence and high wave energy, together with an eastward littoral drift. The delta started to form in the Late Pliocene, with its main development taking place in the Pleistocene, associated with progradation of large volumes of coarse coastal delta sands and the offshore accumulation of turbidities (Nile Cone). In the Oligocene-Early Miocene, an ancestral Nile discharged to the NW. Fluvio-deltaic deposits appeared in the present delta area in the Late Miocene, where they built a narrow prism close to an active faulted flexure, which in the subsurface now separates the South Delta Block from the deep North Delta Basin. The stratigraphy, structure and depocentre development of the region are functions of the interaction between the E-W- to WSW-ENE-trending Mediterranean margin structures, active since the end of the Cretaceous, and the NW-, NNW- and NNE-trending Red Sea-Gulf of Suez structural fabric associated with arching and rifting from the Oligocene to Pleistocene. The main phases of tectonic activity and of clastic deposition, which also correlate with lowered sea levels, were in Late Oligocene-Early Miocene, late Middle Miocene, Tortonian-Messinian, Middle Pliocene and Early-Middle Pleistocene. Petroleum exploration in the Nile Delta has produced modest results to date with a few medium-sized gas fields and minor oil discoveries.

Geology and metamorphic evolution of the Sanbagawa metamorphic belt, Japan

Abstract: Summary The Cretaceous (pre-Japan Sea) Sanbagawa metamorphism affected the Japanese Jurassic complex south of the Median Tectonic Line in the regions now recognized as the Sanbagawa, Mikabu and Chichibu belts. The metamorphic peak (116 Ma) was reached and passed during the tectonic ‘D1’ deformation, corresponding to sinistral shear N30°E along the eastern margin of the Asian continent. This was followed by ‘D2’ (c. 85 Ma) fold and thrust deformation, the vergence of which is normal to the ‘D1’ trend. These deformational events established the present thermal structure. The final regional deformation formed upright ‘D3’ folds. The four metamorphic zones based on pelitic assemblages can be enhanced by using basic schists to subdivide the pelitic chlorite zone. Apparent Fe-Mg partition coefficients between chlorite and garnet show an essential regional continuity of metamorphism and that thrust-offsets do not juxtapose elements from different mineral zones. Peak conditions of metamorphism ranging from 250°C and 6 kbars to 600°C and 10 kbars are consistent with simple P-T-t loops which progress at higher pressures and return at lower pressures to the surface.

The influence of pre-existing basin structure on thrust system evolution in the Western Alps

Abstract: Summary Collision mountain belts have recently been interpreted as being large scale stacked sedimentary basins, where the wholesale reactivation of pre-existing normal faults exerts the primary control on orogenic style. This model is explored here using examples from the Western Alps. The use of stratigraphic separation across faults, in the absence of corroborative sedimentological data, provides an ambiguous criterion to determinate fault reactivation. Variations between ‘compressional’ and ‘extensional’ stratigraphic separations across the Frontal Pennine thrust are more likely to be due to regional unconformity patterns developed in lower Tertiary times rather than normal fault reactivation along the eastern margin of the external zones. Mesozoic-Tertiary basin structure has been important in controlling the sites of major thrust ramps where stratigraphic horizons suitable for activation as thrust detachment zones have been offset. Examples include the thrust front in the Jura (Triassic detachment dropped to W by faults bounding the Bresse basin) and Vercors (basement uplifted to W at Faille de l’Isere, bounding the Mesozoic sub-Alpine basin to W). Buttressing structures, where thrust displacement is converted into vertical pure shear, may indicate buried normal faults but can occur at either hinterland or foreland-directed basin faults. The overall structural style in the sub-Alpine thrust belt indicates detachment between basement and cover with basement involvement along the Sole thrust which carries the external basement massifs. There is no indication from surface geology that this thrust originates from reactivation of a Mesozoic basin fault although this process may have occurred at depth. Similar styles of detachment-dominated thrusting with large horizontal relative to vertical displacements also characterize internal Alpine structure. Preliminary results from the ECORS-CROP deep seismic reflection profile across the Western Alps support the detachment thrusting model although they do not rule out minor normal fault reactivation.

Analogue models of inversion tectonics

Abstract: Summary Inversion of dip-slip fault systems by horizontal recompression has been modelled using sand, sand-mica and sand-clay analogues. Three extension-inversion systems have been tested: uniform basal detachment, simple listric detachment and ramp/flat detachments. Extension above a uniform basal detachment produces a domino or conjugate array of near planar faults. Upon inversion only faults at angles < 60° to the layering and suitably oriented with respect to the compression direction undergo reactivation. With increased contractional strains all faults are rotated to high angles and reactivation ceases. Shortcut faults may develop in the footwall of steeply dipping faults. Extension above a simple listric detachment fault produces a characteristic rollover anticline and associated crestal collapse graben structure. Upon inversion asymmetric uplift is associated with the major detachment. For models with no competency contrast the rollover anticline is accentuated. The crestal collapse graben faults are reactivated and the graben tightens producing a ‘flower-like’ fault system. Reactivation of synthetic faults ceases when they become rotated into angles too steep for contractional deformation. Extension of ramp/flat detachment systems produces characteristic rollover anticlines and crestal collapse graben systems above each concave-up segment of the listric fault system. It also produces a hanging wall syncline associated with the convex-up segment of the detachment. Inversion results in asymmetric uplift at the emergence of the reactivated detachment and above the convex-up portion of the detachment surface. The crestal collapse graben faults become reactivated and rotated into steep angles resulting in flower-like geometries. The rollover anticline is accentuated resulting in nappe-like structures. The results of these analogue studies of inversion structures indicate that considerable further research is needed to understand the geometries and kinematics of inversion fault systems.

Inversion tectonics — a discussion

Abstract: The term ‘inversion’ to describe an inverted basin was first used by Glennie & Boegner (1981) although inverted basins had been recognized many years before e.g. Lamplugh (1920) and Stille (1924). During this meeting it became apparent that the application of the term had broadened to such an extent that the understanding of ‘inversion’ in the petroleum industry was incompatible with much of the current usage. The discussion that follows illustrates many of the points of disagreement, perhaps the most contentious of which is the use of the term ‘negative inversion’ although this was also introduced by Glennie & Boegner (1981). Most of the discussion was presented verbally at the meeting and was recorded, transcribed and returned to speakers for their corrections. In addition, a number of written contributions were received. All contributions to the discussion have been edited as gently as possible so as to retain the exact meaning intended by the contributor. All contributors are included as co-authors in this discussion article but clearly this does not mean that individuals necessarily accept all the points made by other contributors. The editors have identified individual contributions. The discussion commenced with some proposals by the editors which are briefly reproduced here. This discussion article concludes with a considered revision of the proposals on nomenclature which aims to satisfy some of the shortcomings identified in the discussions. Our initial definition of inversion relied on the concept of regional elevation. The regional elevation of a marker horizon is the structural elevation of the horizon

Inversion, reactivated faults and related structures: seismic examples from the southern North Sea

Abstract: Summary This study examines the geometry and timing of inversion features in offshore Quadrants 53/54 of the UKCS. The area studied lies south of the Zechstein salt basin and is dominated by two major and temporally-persistent structural elements, the Wales-Brabant Massif (in the SW of the area) and the South Hewett Fault (in the centre of the area). The Wales-Brabant Massif is a stable basement block, flanked to the north by an inverted Triassic basin. This Triassic basin in turn forms the footwall block to the South Hewett Fault. NE, and locally SW, of the South Hewett Fault lies a series of Jurassic tilted fault blocks symmetrically disposed about a central ‘keystone’. These central blocks contain no synextension sediment fill and are interpreted as outer-arc extensional structures parasitic on a large wavelength uplift produced by an underlying thermal anomaly. All major structural features lying to the NE of the Wales-Brabant Massif were inverted during the late Cretaceous. This inversion produced forced folds in the cover rocks above deeper basement faults, and harpoon-shaped structures at the sites of earlier rotational half-grabens. This account considers the effects of both this late Cretaceous inversion and a succeeding period of inversion during the Miocene on earlier extensional structures.

The P-T-t path associated with crustal extension, Naxos, Cyclades, Greece

Abstract: Metamorphism associated with crustal thickening is well documented in the geological literature, whereas metamorphism during crustal extension has received much less attention. In this paper, we describe the structural and metamorphic evolution of Barrovian style metamorphism on Naxos, Greece. Metapelitic textural evolution and P-T estimates are consistent with decompression during heating. Non-coaxial fabric development occurs as a continuum from the prograde to the retrograde segments of the P-T-t path. We suggest that Barrovian metamorphism on Naxos developed during non-coaxial mid-lower crustal extension. The island of Naxos is one of the Cycladic group of eastern Greece. These islands form a curved belt of metamorphic rocks, known as the Attic-Cycladic Massif (ACM) which may be traced northwards on to the Greek mainland and eastwards into the Menderes Massif of Turkey. The ACM is a structurally complex nappe pile (Jansen & Schuiling 1976) and metamorphic complexes within the ACM have been subdivided into at least two major tectonic units (Andriessen et al. 1987). A lower unit of Palaeozoic age, containing relict amphibolitegrade Hercynian assemblages is exposed on the island of Ios, and probably on the islands of Naxos and Sikinos (Henjes-Kunst & Kreuzer 1982, van der Maar & Jansen 1983, Andriessen et al. 1987). The upper unit consists of an alternating series of pelitic and psammitic schists and gneisses, metavolcanics and marbles of probable Mesozoic sedimentary age. The geology of Naxos may be divided into three components (Fig. 1). 1 The metamorphic complex. The ACM has undergone intense deformation and metamorphism of Alpine

A combined chemical and Pb-Sr-Nd isotope study of the Azores and Cape Verde hot-spots : The geodynamic implications

Abstract: The mafic lavas of the Azores and Cape Verde Islands are of a highly varied composition, reflecting a complex history of magma genesis and a variety of source compositions. The lavas of the Cape Verde Islands are characteristically highly silica undersaturated, with alkali-rich ankaramites, larnite-normative melilitites and carbonatites. In contrast, the lavas of the Azores vary from strongly nepheline- to hypersthene-normative types. Isotopic ratios and trace elements also show considerable variation, consistent with derivation from multicomponent mantle sources. Three distinct groupings of lava compositions can be seen in Pb-Sr-Nd isotope space, and each is characterized by its own trace-element enrichment. (i) The majority of the lavas from the Azores and northern Cape Verdes Islands have isotope and trace-element systematics similar to basalts recovered from elevated segments of the mid-Atlantic Ridge (eg during Deep Sea Drilling Project Leg 82) (Ba/La <12, 206Pb/ 204Pb=19.3-20.0). (ii) The islands of Sao Miguel and Faial in the Azores are characterized by radiogenic strontium and lead isotope ratios and marked fractionation of the most incompatible trace elements (eg 87Sr/ 86Sr=0.70522, 206Pb/ 204Pb=19.88, 207Pb/ 204Pb=15.75, Ba/Nb=12, La/Nb=0.84). (iii) In contrast, the southern Cape Verde Islands have relatively unradiogenic Pb-Sr-Nd isotope ratios (eg 87Sr/ 86Sr=0.70393, 206Pb/ 204Pb=18.74, 207Pb/ 204Pb=15.53, Ba/La=22, Ba/Nb=15, La/Nb=0.67).Mixing relationships between isotope and trace-element ratios indicate the involvement of at least four, and possibly five, chemically distinct source regions in the petrogenesis of mafic lavas. It is shown that these different source regions represent combinations of the depleted upper mantle (mid-ocean ridge basalt source), recycled oceanic lithosphere and two components of the subcontinental lithospheric mantle. No direct unequivocal evidence is found for a fifth component, namely undepleted primitive mantle reservoir, although helium isotope data suggest influxes of material and heat from the lower mantle. Similarly, it is suggested that continental crust has no direct contribution to the petrogenesis of these ocean island basalts and its role in the mantle is limited to the production/modification of the subcontinental lithosphere above subduction zones.

Unravelling the thermo-tectonic evolution of the Alps: a contribution from fission track analysis and mica dating

Abstract: Summary Fission track analysis of apatite and zircon, and K-Ar and Rb-Sr dating of white mica and biotite provide time and temperature constraints for direct dating of deformation phases and in establishing cooling and uplift rates and patterns. In the western Alps, timing of the Sesia-Lanzo zone Eoalpine metamorphic temperature peak is confirmed between 110 and 85 Ma, possibly preceded by the pressure maximum. The slowly cooling Sesia zone reached 200–250°C by ∼33 Ma and ∼100°C by ∼25 Ma, with fast uplift during the late Oligocene to Miocene Insubric phase; cooling in the adjacent Gran Paradiso Massif occurred slightly later, with fission track zircon and apatite ages of 30±2 and ∼22 Ma respectively. No evidence to support an inverted metamorphism of the Gran Paradiso was found. In the Silvretta nappe (E Alps) three different uplift styles and periods are distinguished: uparching between 110 and 35 Ma, homogeneous uplift during 35 and 2 Ma and recent eastward tilting. The data imply that updoming of the Engadine Window occurred before ∼31 Ma. In the central Lepontine Alps, a uniform late Neogene to recent mean uplift rate of ∼0.5 mm/a succeeded early Miocene uplift at ∼2.2 mm/a in the southern steep belt (root zone), the latter resulting from a major phase of backthrusting of the central Alps along the Insubric Line. Clear-cut time-temperature paths are not found for the Suretta Nappe (central Alps): fission track data delimit the Miocene cooling, but speculative interpretation of phengite-mica and K-Ar amphibole ages suggests that the main deformation (possibly Ferrera phase) occurred between 55 and 36 Ma. Schematic models are advanced to describe the asynchronous thermal evolution of different parts of the central Lepontine Alps.

New plesiosaurs from the Upper Cretaceous of Antarctica

Abstract: Abstract New plesiosaur remains from the Upper Cretaceous Lopez de Bertodano Formation (late Campanian-Maastrichtian) of Seymour Island, Antarctic Peninsula include Cryptoclididae and Elasmosauridae. The occurrence of Cryptoclididae is reported from the Antarctic region for the first time. The taxon represents a new genus and species, based on a skull and associated cervical vertebrae. The long, slender and delicate teeth may have formed a ‘trapping’ device that enabled cryptoclidids to feed on small fish and crustaceans that abound in the same deposits. The cryptoclidids had a restricted distribution, being known so far from the Middle and Late Jurassic of England, and the Late Cretaceous of Chile, Argentina, and Antarctica. Other specimens, represented by several postcranial skeletons, are taxonomically indeterminate, but they share some features with other contemporary elasmosaurid genera such as Hydrotherosaurus, Morenosaurus, Thalassomedon, and Mauisaurus. Unlike the cryptoclidids, the elasmosaurids had a cosmopolitan distribution during the Jurassic and Cretaceous periods. Trophic diversity within guilds of marine predators is examined in the Lopez de Bertodano palaeocommunities. Three predator guilds are recognized on the basis of tooth morphology and prey preference. The mosasaurs composed the ‘Cut guild’, and were the principal predators. The elasmosaurids constituted the ‘Pierce guild’, and the cryptoclidids formed the ‘Trap guild’. These marine reptiles exploited the various pelagic resources such as sharks, bony fish, soft cephalopods and crustaceans, and survived until the end of the Cretaceous. The plesiosaurs were excellent swimmers, and used their hyperphalangic paddles for subaqueous flight in the manner of modern sea lions.

Partial melting of a lithologically heterogeneous mantle: inferences from crystallization histories of magnesian abyssal tholeiites from the Siqueiros Fracture Zone

Abstract: Summary Complex processes of magma mixing are revealed by the crystallization histories of several magnesian, porphyritic abyssal tholeiites dredged from the Siqueiros Fracture Zone at about 8°30′ N near its intersection with the East Pacific Rise. The compositions of chromian spinels, olivines, plagioclases and glass inclusions in these rocks indicate that polybaric crystallization and mixing occurred at high temperatures involving a range of refractory (low-TiO2, low-Na2O) melts. These later coalesced with cooler much less refractory components to produce the bulk rocks. The crystallization histories and comparisons with experimental data indicate that mantle sources of these basalts were lithologically heterogeneous, ranging from fertile (lherzolitic) to refractory (in the extreme, harzburgitic) with respect to a basaltic melt fraction. The separate magma strains going into a typical magnesian abyssal tholeiite were produced by simultaneous partial melting of the different lithofacies, after which they coalesced and mixed. Such mixing is more likely than batch melting to explain the ranges in CaO/Al2O3, and abundances of Na2O and TiO2, within the fairly small range and low (non-picritic) magnesium numbers (0.75–0.64) exhibited by all liquids including those inferred from mineral compositions. The early crystallization of minerals from refractory magma strains suggests that melting domains beneath spreading ridges consist of cores of refractory peridotite surrounded by cooler, more fertile peridotite. Sequential mixing of refractory and fertile magma strains is an unavoidable consequence of the ascent of magma batches from the interiors of complex melting regimes (diapirs) in the mantle. The diapirs may have been triggered into ascent by the buoyancy of low-density (low-iron) refractory peridotite. Several of the Siqueiros magnesian abyssal tholeiites are among the most depleted basalts ever obtained from the eastern Pacific, and yet alkalic basalt was also dredged from the fracture zone a few kilometres away. The tholeiites are even more depleted than typical, more fractionated, summit eruptives of the East Pacific Rise. The contrast may arise because of the homogenizing effects of axial magma chambers, within which truly depleted tholeiites are mixed with enriched alkalic magmas instead of erupting separately, as they do in the fracture zone. Mantle in this region thus appears to be segregated into strongly contrasting depleted and enriched peridotites, with depleted peridotite also ranging from refractory to fertile lithofacies. Primary lithological variability in the mantle thus was probably imposed by one or more ancient melting events that left distinctively enriched and depleted reservoirs in their wake.

Phanerozoic ironstones: an introduction and review

Abstract: S U M M A R Y: Ironstones, particularly ooidal ironstones, have long fascinated sedimentary geologists and have generated an enormous variety of interpretations, but have remained poorly understood. A recent upsurge in interest in this group of rocks has generated much new information derived from many different disciplines within the earth sciences. The first part of this introduction examines recent advances, and attempts to produce a synthesis of them, with particular reference to the genesis of marine ooidal ironstones. Current models for the formation of marine oolitic ironstone-formations are discussed, and a working model based on various lines of recent research is proposed. This model invokes the intrasedimentary formation of berthierine ooids in marine environments during postoxic diagenesis. The ooids may be mechanically modified and mineralogically transformed by reworking, as well as by later diagenesis. Goethite ooids may be oxidized berthierine ooids, reworked lateritic ooids or primary grains. The formation of oolitic ironstoneformations is favoured by a break in clastic sediment supply, continued supply of iron and physical reworking. Many sedimentary environments may supply these parameters, but changes in sea level leading to the flooding of land masses and the reworking of suitable terrestrial soils may be both an important source for the iron and provide a break in clastic sediment supply. In the second part of this introduction the terminology and classification are discussed, and conventions proposed. The term berthieroid is introduced as a non-specific term for material of undetermined berthierine or chamosite composition. The term ironstone is proposed as a petrological term and as an informal lithostratigraphic term. A deposit composed of ironstones may be termed an ironstone-formation. It is recommended that a 'Dunham' style of classification is employed petrographically, with -ironstone replacing the -stone of the original carbonate classification. The nomenclature of allochems in ironstones is also discussed. A glossary of ironstone terminology is provided.

P-T histories of peridotite and amphibolite tectonic blocks in the Sanbagawa metamorphic belt, Japan

Abstract: The Sanbagawa (Sambagawa) metamorphic belt in south-west Japan suffered a high-pressure intermediate metamorphism of Cretaceous age. The regionally highest grade rocks (epidote-amphibolite facies) of the belt occur in the Besshi district in central Shikoku, and locally contain higher grade masses of coarse-grained metabasic and ultrabasic lithologies, such as the Higashiakaishi peridotite, Iratsu epidote-amphibolite, Nikubuchi peridotite and Sebadani metagabbro masses (Kunugiza et al. 1986) (Fig. 1). These masses have been regarded as large-scale tectonic blocks (maximum of 7 × 3 km2) in a mélange zone which formed at the initial stage of the uplifting of the Sanbagawa belt (Takasu 1984, Kunugiza et al. 1986). In this paper, quite a diverse origin and metamorphic history of these blocks will be revealed, and the tectonics associated with the development of the tectonic blocks will be discussed. Metamorphic history of the tectonic blocks Sebadani (SB) metagabbro mass The Sebadani (SB) metagabbro mass, 0.3 × 0.2 km2, is located in the albite-biotite zone, and consists of garnet-epidote amphibolites and relict eclogite which survived epidote amphibolization. The banding of melanocratic hornblende-rich and leucocratic epidote-rich layers records the layering of previous pyroxene and plagioclase layers of cumulate gabbro. Takasu (1984) recognized two stages of eclogite equilibrium in the SB mass; a core-core pair of 720–750°C and 12–20 kbar, and a rim-rim pair of 610–630°C and 10–17 kbar. The basic Sanbagawa schists, up to 20 m from the SB mass, underwent contact metamorphism at high pressure that produced garnet and omphacite (maximum Jd 36%) porphyroblasts from

Geodynamic model for Alpine intra-plate compressional deformation in Western and Central Europe

Abstract: Summary The Mesozoic grabens and wrench induced troughs of Western and Central Europe developed in response to Triassic to Early Cretaceous intra-plate tensional stresses which affected the Arctic — North Atlantic and the Tethys borderlands during the rifting phases preceding the opening of the respective oceans. During the Alpine orogeny, collision-related compressive stresses exerted on the foreland induced the reactiviation of pre-existing fracture systems, and caused the inversion of Mesozoic grabens and the uplifting of major basement blocks at distances up to 1300 km to the north of the present Alpine deformation front. The structural style of these inversion structures is indicative of compression and transpression. Total crustal shortening is unlikely to exceed a few tens of kilometres. These displacements require a coupling between the foreland and the orogen at the Alpine A-subduction zones and, within the foreland, a decoupling at intra-crustal levels between the crust and mantle and/or at deeper lithospheric levels.

Basin inversion in and around the British Isles

Abstract: Summary The development of basins of Mesozoic and Tertiary age both onshore and offshore the British Isles is largely a consequence of episodic rifting that led to the progressive northward opening of the North Atlantic. In these basins, extension was controlled by the structure of underlying Palaeozoic terrains and succeeded by passive thermal subsidence. In this context, however, the British Isles represent an anomaly both in terms of their present elevation and in the widespread exposure of Pre-Cambrian to Tertiary rocks. The outcrop geology of the onshore and offshore basins is an obvious demonstration of uplift although the timing and origin and magnitude of the uplift have remained uncertain. Evidence from onshore and offshore the British Isles is used to document the influence of compressional inversion of Tertiary age in shaping the present day outcrop geology.

Antarctica: Cretaceous cradle of austral temperate rainforests?

Abstract: Although it is well known that certain Gondwanic elements of present-day austral temperate rainforests occurred on Antarctica during latest Cretaceous to early Tertiary times, there has been insufficient factual evidence for pinpointing the cradle of these forests. Fossil evidence from Antarctica and closely associated regions in the Creataceous southern Gondwanan assembly confirms that Antarctica was a Cretaceous origination and dispersal region of certain elements of today's southern hemispheric humid and perhumid forests. Antarctic origins are indicated for the fern Lophosoria, the podocarp gymnosperms Lagarostrobus and Dacrydium, Nothofagus, Ilex, and several lineages of the Proteaceae; migration to their present regions of distribution was probably step-wise. Antarctica also served as a Cretaceous dispersal corridor for other angiosperms represented today in mid to low latitude austral regions. These include Ascarina (or its stock), Myrtaceae, Gunneraceae, and Winteraceae, all of which had earlier histories in northern Gondwana or southern Laurasia. Origination and dispersal appears to be related to changing environmental circumstances associated with fragmentation of Gondwana and opening and enlargement of the southern oceans.

The inverted margin of the French Alps and foreland basin inversion

Abstract: Summary The whole of the Western Alpine realm can be considered to result from the inversion of the European margin of Ligurian Tethys as a consequence of the collision between Apulia and Europe. The margin was developed due to Tethyan rifting during the Liassic and Middle Jurassic and regional thermal subsidence as the Ligurian ocean was spreading in the Late Jurassic and Early Cretaceous. Coevally, the margin underwent additional stretching in response to North Atlantic rifting. Inversion was initiated by Late Cretaceous times when the Ligurian oceanic crust began to be subducted, with localized obduction, beneath the previously passive margin. The most recent inversion of the Western Alps occurred in the external Molasse basin of Manosque-Digne-Valensole during the Late Miocene and Pliocene. Regions of mild inversion allow useful comparisons to be made with more strongly inverted basins. The geometric characteristics of the inverted basins depend on the lithology, geometry of the extensional structures, orientation of extensional faults with respect to the compressive Alpine stresses and the amount of compression. Comparisons of the external and interal zones suggest that the ductilely deformed thrust sheets may have originated from inverted half-grabens.

Geothermobarometry of high-grade metapelites: simultaneously operating reactions

Abstract: S U M M A R Y : Experimental data on equilibria involving rock-forming minerals in highgrade metapelites (garnet, orthopyroxene, cordierite and plagioclase) have been used to derive an internally consistent system of mineralogical thermometers and barometers based on component reactions and excess mixing properties of solid solutions. Application of these data to a granulite-facies metapelite sample reveals that compositional inhomogeneity of minerals on a thin-section scale is likely to be a result of several reactions opening simultaneously at the retrograde stage of metamorphic evolution. The widely accepted practice of estimating pressure and temperature from separate reactions may thus lead to errors and misinterpretation of P T paths.