No description available.

No description available.

No description available.

The GLORIA and other geophysical data obtained during this study have enabled the entire present-day tectonic pattern of the Easter Microplate to be determined. The complex nature of all of the plate boundaries has been revealed, and there is evidence of rapid plate boundary evolution. Earthquake focal mechanisms together with GLORIA data have provided the first unequivocal evidence that thrust faulting is taking place along the northern boundary of the microplate. On the Nazca plate to the north of the Easter Microplate, the GLORIA images have disclosed a series of prominent east-west trending ridges which earthquake focal mechanisms and plate tectonics of the region imply are the first known case of intraplate thrusting in young oceanic lithosphere. Slightly younger Nazca plate lithosphere to the north is overthrust over marginally older lithosphere to the south, in order to accommodate the tectonic shortening predicted in the region. Interpretation of magnetic anomaly data and the complex structural information offered by the GLORIA images have permitted a detailed history of the microplate to be determined. Three-plate Nazca-Pacific, Nazca-Easter and Pacific-Easter Euler poles have been calculated and used to rotate the Easter Microplate and surrounding major plates back in time since the East Rift of the microplate first started to propagate northwards at around 5.25 Ma. The microplate has evolved in two stages. The first is from 5.25 Ma to around 2.5 Ma, and involved continuous northward propagation of the East Rift, while the West Rift became segmented along new transform faults as it adjusted to keep the overall Nazca-Pacific velocities constant. The transpressive northern boundary migrated with the East Rift tip, while the transtensional southern boundary remained more or less still. At sometime after 2.5 Ma, the East Rift ceased northward propagation and the Southwest Rift began to open up along the transtensional Pacific-Easter transform. A convergent northern boundary formed involving southward overthrusting of the Nazca plate to the north over the microplate. No more Nazca plate lithosphere from the north of the microplate was transferred to the microplate interior, and the Pito Deep region was caused to open up by pervasively rifting apart pre-existing Nazca plate lithosphere. This study predicts that the Easter Microplate will be transferred onto the Pacific plate within the next 2-5 myr, when the Southwest Rift has propagated through to the southern EPR, and the shear couple on the microplate has ceased.

The Hveragerði Region is situated at the Hengill Triple junction, SW Iceland, where there are three volcanic systems. The crust in the area is constructed from both fissure (elongate) and lava shield (conical) eruptive units. Hengill is the presently active spreading zone with the Hengill Central Volcano, whereas the Hveragerði region is inactive with the extinct Grensdalur Central Volcano. Recent geophysical research indicates the presence of high and low density volumes within the upper 5 km of the crust in this area. The location of the density anomalies coincides with surface geological features, such as Recent lava shields, and the extinct and active central volcanoes. A geological map of the Hveragerði Volcanic System has been constructed, and approximately 450 basaltic samples have been examined petrographically and analysed for whole- rock, volcanic glass and mineral chemical data from this region. Observations from these data, coupled with the geological and geophysical observations, suggest that the lava shields are fed straight from the base of the crust, whereas fissure eruptions originate from shallow crustal reservoirs The character of the crustal reservoir has been highly variable in the past 1 Ma, and has varied from a melt-dominated reservoir, to a crystal mush-dominated one. Each lava shield is compositionally distinct and is thought to preserve the mantle-melting signature. The compositional variation amongst the lava shields suggests that instantaneous melts are able to segregate from the mantle without complete mixing with accumulated melts from the entire length of the melting column. The depleted instantaneous melts from the crest and top of the melting column will either form picritic lava shields or they may interact with more fractionated crustal reservoirs and undergo quench crystallisation of megacrystic plagioclase (An 80-90). These crystals with associated pyroxene and olivine become flototion cumulates. There are episodes within the stratigraphy where off-axis lava shield and highly megacrystic fissure eruptions dominate, and such periods may represent periods of low magma supply. On the submerged mid-ocean ridges, linear and conical features are also observed, and these may be analogous to the fissure and lava shields, respectively. The basalt types reported here from the Hveragerdi region have also been reported off-shore, and they may therefore represent basalts derived from similar magmatic processes in a similar magmatic plumbing system. However, an initial observation of the relationship of 12 dredged basalts from 63˚ 10'N on the Reykjanes Ridge suggests that this is not the case at this locality.

The Carajás-Cinzento fault system is centred within the Itacaiunas Shear Zone, the northern tectonic margin of the Archaean Sul do Pará Granite Greenstone Terrain of the Amazonian Craton, Brazil. The regional tectonostratigraphy can be subdivided based on the geological relationship of units to the main phase of ductile movements along the Itacaiúnas Shear Zone. A Basement Assemblage includes an older group of orthogneisses, migmatites, and granitic to dioritic plutons (Xingu Complex) and a later volcano-sedimentary sequence of ironstones, quartzites. amphibolites and schists (Igarapé Salobo Group). Intense ductile shearing and high grade metamorphism along the Itacaiúnas Shear Zone has led to widespread tectonic interleaving of the gneisses and volcano-sedimentary rocks and has obliterated all traces of the original unconformity between these units. A Cover Assemblage is represented by very low-grade volcanic and sedimentary rocks that are inferred to rest unconformably on the Basement Assemblage rocks deformed within the Itacaiúnas Shear Zone. Older clastic, volcanic and ironstone sequences (Igarapé Pojuca & Grao Pará Groups; ca.2.7 Ga) are overlain by a sequence of shallow-water marine to fluvial clastic deposits (Águas Claras Formation). Both Cover and Basement assemblages are intruded by ca.1.8 Ga A-type granitic plutons and basic dykes. All units are unconformably overlain by a thin, localised sequence of polymictic conglomerates (? Gorotire Formation).During the formation of the upper amphibolite facies within the Itacaiúnas Shear Zone, sub-vertical mylonitic fabrics, generally E-W-trending, were variably developed in the Basement Assemblage rocks. They preserve widespread sinistrally transpressional kinematic indicators. Radiometric dating suggests that the metamorphism and deformation in the shear zones occurred toward the end of the Archaean (ca.2.8 Ga). At least three cycles of brittle-ductile strike-slip reactivation at low metamorphic grades appear to post-date the development of the Itacaiúnas Shear Zone, leading to the formation of the Carajás and Cinzento faults. There is no stratigraphic, structural or sedimentological evidence to suggest that these faults were active during the deposition of the Cover Assemblage sequences. However, as most outcrops of Cover Assemblage rocks are presently localised within bends, branches and offsets of these fault systems it appears that, following their deposition, they were faulted down into dilational jogs formed during an initial phase of brittle dextral movements. The effects of a later episode of brittle-ductile sinistral transpression are widely preserved in both Basement and Cover assemblages, with intense deformation localised in the region of the major fault strands. The 1.8 Ga granites and dykes appear to relate to a regional extensional or dextral transtensional episode recognised in the Middle Proterozoic throughout the Amazon region. There is some circumstantial evidence for further minor fault reactivation during the Phanerozoic, and the region appears to be tectonically active in the present day, as illustrated by the occurrence of recent small-scale earthquakes and hot springs centred along the major fault traces. The influence of the basement architecture and the intensity of later reactivations appears to wane after a time of ca.1.0 Ga following the existence of a weakening effect on a lithospheric-scale with a finite life span, possibly originating in the underlying lower crust and mantle. There is widespread evidence that the mylonitic fabrics of the Itacaiúnas Shear Zone have controlled the orientation of later structures, including the Carajás and Cinzento fault systems. Long-term fault zone weakening mechanisms are recognised in the region. Brittle fracturing processes have caused increases in fault zone permeabilties allowing extensive ingress of fluids, some of which have caused e.g. gold and copper mineralisation.

No description available.

This research investigates a previously unidentified geological deposit at Cotopaxi Volcano, Ecuador: the Rio Pita Formation. The thesis is in two parts. The first defines and identifies the Formation, and shows that it is a volcanic debris avalanche deposit. It also investigates the nature of other deposits in the Rio Pita area, and thereby builds up a revised evolutionary history of Cotopaxi volcano. The second part questions more closely two controversial aspects of volcanic debris avalanches: their mode of travel, including the reason for their great mobility; and the development of their characteristic topography, the mound field. Despite earlier studies which suggested otherwise, Cotopaxi is shown to have produced highly silicic magmas (72&'37 SiO2) as well as more basic material. The Rio Pita volcanic debris avalanche occurred about 4000 years ago, probably as a result of a high level silicic magma intrusion. It was associated with a blast and a rhyo-dacite eruption, and travelled at more than 28ms-1 (100kmh-1). Its H/L ratio was 0.1, and it expended a total energy of more than 9.5 x 1016J. After the avalanche, the volcano was dormant or non-explosive for about 1000 years, before producing a series of increasingly more basaltic tephras (59&'37 - 56&'37 SiO2). Cotopaxi's last major eruption was in 1904. This study tests various hypotheses proposed for volcanic debris avalanche movement by comparison with the field evidence at the Rio Pita Formation. The movement hypotheses best able to explain the sedimentary features of the Formation are sliding, for the block facies, and a combination of fluidised flow with grain flow at the base for the matrix facies. It is proposed that the Rio Pita volcanic debris avalanche was fluidised by a mixture of gas and dust. This fluidising medium was created partly by the avalanche itself, due to friction and clast comminution during movement, and partly by the volcanic blast which accompanied the avalanche. The avalanche was only rapidly mobile for as long as the depressurising fluidising agent stayed within the avalanche: a period of between 3 and 12 minutes. During this time, the avalanche was inflated by a factor of five, and had developed a head. The escaping gas and dust tended to blow through the avalanche debris along preferential routes, a process termed 'perflation'. Perflation is also able to account for formation of the mound-field. In the past, there had been various differing ideas regarding mound-field development. However, the fact that the mounds differ in character to those of non-volcanic debris avalanches and landslides suggests that they are formed by a process peculiar to volcanic events. Perflation would enhance mound development because the escaping gas and dust follows preferential routes through the avalanche, throwing up debris as it does so. Areas from which the debris has been thrown up are left as hollows, and the other areas from mounds. Simple experiments show that this effect does occur, but more experimental and theoretical work is required before the hypothesis can be generally accepted.

No description available.

No description available.