No description available.

No description available.

Mount Etna, Europe's largest active volcano, shows a long history of changing structure and environment, culminating in the modern volcanic edifice with its observed slope instability and region of past collapse (the Valle del Bove). The structure and evolution of the volcano and, more importantly, the Valle del Bove are poorly understood but have major effects on its present day behaviour. Several geophysical techniques have been used in order to determine new models for the sub-surface structure of the volcano, concentrating on the Eastern Flank and the Valle del Bove. The Valle del Bove is an important feature on the Eastern Flank. It measures 5 by 7 km and is 1200m deep at its maximum. Its formation and evolution are poorly understood and represent the main thrust of this work. The primary work has been gravity and aeromagnetic surveys combined with 2.5-D and 3-D modelling to develop a better understanding of the sub-surface structures of the area. This work has identified several large gravity and magnetic anomalies indicating areas of contrasting geophysical properties. A 16mGal positive gravity anomaly over the Southern Wall of the Valle del Bove is interpreted as a large (volume = 38km3) high density (2950 kg m-3) body, related to the old Trifoglietto centre, possibly representing the ancient feeder system. A second high density body is seen under the present day summit region and is interpreted as an area of shallow level magma storage within the upper flanks of the volcano. A negative gravity anomaly of 10mGal towards the coast, over the Chiancone sedimentary fan deposit, is interpreted as reworked debris flow material, derived from the Valle del Bove. The shape of the anomaly is strongly asymmetrical relative to the geographical extent of the deposit, with the centre of the anomaly sitting over its Northern extent. The resultant model of this material gives a thickness of 700m, with the material being deposited within a fault controlled basin, open to the sea. Such a shape suggests that a sizeable volume of material may be deposited off-shore. Below the Etnean volcanics lie sedimentary layers which slope in an easterly direction, providing a surface over which the Eastern Flank. is free to slide. Finite element modelling of the stress fields within the volcanic edifice and basement shows that the effects of sea-level variations and glaciations may be of a sufficient magnitude to affect the volcanic system, however, the time over which these changes occur may be more significant than the changes themselves. Similarly the Valle del Bove is shown to have had a significant effect on the stress patterns, and has resulted in a self-reinforcing process whereby the tension caused by the removal of mass encourages further collapse. Palaeomagnetic measurements show that rotational failure has not been important in the formation of the Valle del Bove, and combined with the results of the other methods, show the Valle's formation to have been via a series of small piecemeal collapses, possibly relating to oversteepening of the walls following dyke emplacement. These results are combined together to show that the Valle del Bove has been developing over much of the history of Etna, and is not a recent addition to the volcano.

No description available.

No description available.

The 6845±50 yrs BP caldera-forming eruption of Mt Mazama (Crater Lake, Oregon) was preceded within 200 years by two plinian eruptions producing voluminous airfall deposits followed by lava flows. This study concentrates on these two airfall deposits as well as the complex airfall deposits from the climactic eruption, which are distributed over = 1.7 million km2 of northwest America. Tephro-stratigraphic mapping of airfall units throughout south-central Oregon has revealed the presence of five lobes of coarse pumice deposits and two widespread ash units which are important marker horizons. Detailed grain size data have been generated by sieving and measurement of maximum clast sizes, and these are used to characterise each deposit and as input data for clast dispersal models of plinian airfall eruptions. Geochemical variations between each deposit generally support the models already developed for Mt Mazama, and geochemical techniques have been used to deduce the source of distal 'Mazama ash'. The role of volatiles in each eruption is reviewed and, along with the rate of vent and conduit erosion, is found to be vital in controlling eruptive evolution. Deduction of column height and mass eruption rate for various stages during each eruption has been possible using clast dispersal models and, when combined with eruptive velocity and vent and conduit dimensions, has produced a detailed physical model of eruptive development. This has then been linked to field characteristics to provide significant new information about the physical volcanology of plinian airfall eruptions. Revised volume estimates for the climactic airfall eruption, including distal fine ash, give a volume of = 20 km3 (dense rock equivalent), with a maximum column height of = 55 km occurring immediately prior to column collapse and ignimbrite generation. This eruption is thus one of the most intense and voluminous ultra -plinian eruptions yet documented.

Most passive margins display a prominent breakup unconformity coinciding with the rift to drift transition. The unconformity, as defined by Falvey, (1974) is of broad regional extent affecting both basins and highs and is easily recognised on seismic sections. Criteria for the recognition of the breakup unconformity include an inflection in the subsidence curve, fault terminations and volcanic strata (and/or evaporites) at the level of the unconformity. Falvey considered that it was caused by "erosion during the final uplift pulse associated with pre-breakup upwelling in the mantle". It is more likely that the uplift is caused by magmatic underplating in response to the passive upwelling of the mantle and the flexural isostatic effects of erosion throughout the syn-rift phase. The primary objective has been to quantify the amount of uplift and erosion associated with the breakup unconformity / breakup megasequence boundary. This is of particular importance in hydrocarbon exploration as it quantifies the potential loss of old reservoirs and predicts the provenance of new reservoir clastics. Two data sets, from the Grand Banks and the Northwest Shelf of Australia, have been studied. In both cases there are multiple breakup events and breakup megasequence boundaries form part of a complex tectono-stratigraphy. Regional seismic lines have been interpreted, depth converted and modelled using a new technique of combined reverse post-rift and forward syn-rift modelling. The results of this process, together with seismic megasequence analysis, show that the morphology of the breakup megasequence boundary varies systematically across a passive margin. It is strongly erosional at about 70 km landward of the continentocean boundary, where regional "breakup" uplift outweighs extensionally controlled subsidence, but may be depositional on either side of this zone. A coupled, quantitative magmatic-tectonic model has been constructed by combining the Bickle-McKenzie melt generation model with the flexural cantilever model for continental extension. The magnitude of underplating can be estimated using the Bickle-McKenzie model, in which the amount of melt produced is controlled by the extension factor, ß, and the proximity of a mantle plume convection cell.

A study of Devonian syn-extensional silicic volcanism in the Rhenohercynian Zone (RHZ) of Northern Europe was undertaken to determine their evolution and petrogenesis within the changing tectonic and sedimentological environment of a developing intra-plate rift zone. Silicic volcanics in the Lower Devonian of south Devon and Northern Germany are classified as subalkaline rhyolites-rhyodacites with minor peralkaline variants. Middle Devonian silicics from the Lahn-Dill syncline are peralkaline trachytes with minor subalkaline equivalents. All Lower and Middle Devonian Rhenohercynian silicic volcanics are tectonomagmatically classified as within-plate granitoids and are interpreted to represent syn-rift A-type silicic volcanics. However, a distinction is made with respect to age, in that all Lower Devonian silicics are N-type, whereas the Middle Devonian trachytes are AI-type. The McKenzie & Bickle (1988) theoretical evaluation of magmatism associated with crustal extension provides an excellent explanation for the changing geochemical and isotopic signatures of RHZ Lower and Middle Devonian silicic volcanics. The distinct geochemical and isotopic signatures between Lower and Middle Devonian volcanics reflects progressive lithospheric attenuation within the RHZ with the extension factor P<2 for Lower Devonian to P>2 (for Middle Devonian). The Lower Devonian southwest England silicic magmas, generated during the initial stages of extension, show evidence of crustal contamination and were probably derived from subduction-modified lithospheric mantle. Lower Devonian rhyolites from Northern Germany were generated via partial melting of crustal materials, the thermal energy for which was generated by the injection of basaltic melts which ponded at their equilibrated density level. Middle Devonian trachytes display mantle-dominated chemical patterns and represent extreme differentiates of alkali basalts derived from an omlike mantle source. The southwest-northeast trend of the major volcanic masses indicates the direction of faulting which has controlled the timing and extrusion of these syn-extensional melts. The volume of extrusive products appears strongly linked to the rate of extension and sediment accumulation. The greatest period of subsidence (Givetian) and sediment accumulation (200 metres per million years) coincides with the most volumetric period ofRHZ silicic volcanism

The Start-Perranporth Zone is one of a number of E-W trending zones in S.W. England, which are characterised by an anomalous and structurally complex deformation history, and which are thought to reflect the influence of pre-existing basin architecture. The SPZ straddles the Start Complex in S. Devon, and is approximately coincident with the northern margin of the Gramscatho Basin in S. Cornwall. It appears to coincide with significant sedimentological, geochemical and metamorphic transitions, and may mark the site of a pre- to Early Devonian terrane boundary. This terrane boundary may have formed the northern margin to a series of small possibly transtensional basins, including the Start and Gramscatho Basins, in which thick successions accumulated prior to inversion during the Variscan orogeny. The pelitic sequences in these basins (Gramscatho Group sandstones, Start greyschists) are geochemically similar to one another, and to other Rhenohercynian basinal sequences in mainland Europe. Both the Gramscatho and Start basins are characterised by the presence of incipient ocean crust (Lizard ophiolite, Start greenschists), with a strongly depleted N-type MORB signature and evidence of ridge-related sub-oceanic early deformation. The interlayered green and grey schists of the Start Complex are separated from the shallow marine Meadfoot shales to the north by a steep north dipping normal fault, the Start Boundary Fault, which bears evidence of a long-lived movement history. This fault is intimately associated with large volumes of highly altered and replaced basic intrusives, and appears to be the surface manifestation of the basin bounding fault at depth. Approaching the SBF, the strain intensifies, primary folds tighten, the primary cleavage steepens to sub-vertical and mineral stretching lineations switch from SSE plunging (sub-parallel to the Variscan transport direction) to sub-horizontal approximately E-W trending. Immediately adjacent to the SBF, sheath folds occur, suggesting very high along strike shear strains. Small scale structures, e.g. shear bands, refold relationships, etc. consistently indicate that dextral simple shear is important during Variscan shortening. Similar, though somewhat more cryptic, evidence for dextral shear is also seen in the L. Devonian shales north of the SBF. In S. Cornwall there is a similar focusing of high strain along the northern Gramscatho margin, with a tightening of folds, a backsteepening of the primary cleavage, and the development of overprinting late crenulations. Primary stretching lineations lie E-W. There is no evidence for sheath folding on either coast, although broad phyllonite zones bearing dextrally asymmetric quartz augen provide evidence of long-lived dextral shear. Many of the high strain fabrics on the east coast are absent, probably faulted out along a major NW-SE dextral strike-slip fault (the Pentewan Fault).The small scale structural evidence along this zone consistently indicates that dextral transpression was the dominant deformation mechanism during Variscan orogenesis. The structural transitions are also suggestive of fault butttressing, e.g. secondary backfolding, backthrusting, etc. and it appears that the ~E-W trending basin bounding fault acted as an oblique buttress to the NNW directed Variscan nappes, the high angle obliquity of this collision inducing dextral transpression in the shortening cover sequence. This fault buttressing mechanism readily accounts for all of the observed anomalous small scale structures, and the marked along strike persistence of the anomalous zone.

The thesis examines the geology and geomorphology of the town of Maratea, southern Italy, and their efffect on the development of the town. Maratea is situated at the mouth of a fault-controlled valley surrounded by high limestone mountains of Jurassic and Triassic age. Recent ground movements are associated with a 2km long fault plane scarp which runs along the eastern flank of the valley and the limestone strata above the Town Centre are affected by an excellent example of a deep-seated slope deformation known as a sackung. Six limestone blocks are located in the valley floor below the fault plane scarp and in addition the area is affected by earthquakes associated with the evolution of the Apennine chain. The area has experienced two phases of neotectonic activity. Evidence from the literature places the first of these at somewhere between 2.0 MY BP and 0.7 MY BP. Subsequently the valley was deepened by a further phase of neotectonic activity, fixed by a U-series date obtained by the author, at 46.4 +/-3.5 x 103 yrs BP. Since then landslide movements have predominated. The fault plane scarp is being exhumed from beneath covering deposits and lichenometric dating, based on the species Aspicilia calcarea, shows that up to 15m of scarp face has been exposed in the last 300 - 600 years. The movements are periodically accelerated by earthquake shaking. The limestone blocks are found to be stable features, although a piece of one block, which has broken away from the main block has tilted at various times. The dates of tilting have been determined by sectioning stalactites growing on the block and dating their observed growth phases by a first-order 14C method. Tilting apparently occurred at 2400 +/-300 yrs BP and within the last 400 years. Finally a questionnaire survey covering all 1008 buildings in Maratea indicated that earthquake shaking, exacerbated by the highly variable sub-surface geology of the valley, is the primary cause of damage. These preliminary findings reinforce the case for detailed neotectonic research as a prelude to development in unstable tectonic environments.