No description available.

No description available.

The unique late Cretaceous to early Tertiary Great Tonalite Sill (GTS) of SE Alaska and British Columbia is a very long (c.1000km) and thin (<25km), orogen- parallel, composite batholith, which may separate two major superterranes in the western Cordillera: the Insular superterrane (including the Alexander and Wrangellia terranes) from the Intermontane superterrane (including the Stikine and Cache Creek terranes).The steeply NE dipping, sheet-like plutons of the Great Tonalite Sill are dominated by NW-SE striking concordant fabrics with steep lineations, which formed within a country rock shear zone of similar dimensions - the Great Tonalite Sill shear zone - prior to the complete crystallization of the calc-alkaline tonalitic magmas. The steep, multiple dyke like nature of this composite body and its emplacement during orogenic contraction, imply that ascent and emplacement have been achieved by dyke wedging mechanisms along the deep reaching, probably crustal scale, shear zone. The remarkable narrowness and yet persistence of the Great Tonalite Sill, is probably the result of petrogenesis associated with a very localised zone of crustal thickening, produced by the associated narrow shear zone extending along the orogen length. Deformation in the Great Tonalite Sill shear zone is dominated by NE-SW directed contraction orthogonal to the orogenic strike associated with a component of NE over SW high angle shear. Such a shear zone of late Cretaceous to early Tertiary age, lying along 800 km of the boundary between the Insular and Intermontane superterranes, strongly implies that it represents the actual boundary between them. That being the case, then terrane accretion during this interval was orthogonal and not obliquely dextral as in some current interpretations of paleomagnetic data. NE side up tilting of mid Cretaceous plutons may therefore be responsible for much of the anomalous palaeomagnetic data determined for these intrusions.

Balanced and restored cross sections demonstrate a minimum 16 km shortening below the Moine Thrust and minimum restored wi1ths of 17 km between the La Meije and Combeynot Thrusts (external Alps), and 54 km between the frontal Subbrianconnais Thrust and the western Brianconnais Zone (internal Alps). Imbricate thrusts within the Moine Thrust Zone branch off a floor thrust which cuts up stratigraphic section from basement (Lewisian gneiss) to Cambrian shelf sediments parallel to the ESE-WNW movement direction. Cut-off relationships and fold geometries within the Pelvoux-Brianconnais suggest a change in move-ment direction of thrust sheets during the evolution of the French Alps from ENE-WSW for the Brianconnais and Sub-brianconnais Zones to ESE-WNW for the External Zones. Higher thrust sheets are frequently flexured as a result of slip on lower thrust surfaces (in-sequence thrusting). Important examples occur of Imver thrust assemblages which are truncated by higher thrust surfaces (out-of-sequence thrusting). Strains within thrust sheets from the Kinlochewe region suggest 0%-20% layer parallel shortening may develop in the footwall to an abandoned thrust as a tip strain to a newly developing thrust. Variations in strain may reflect variable propagation rates; differential displacement has resulted in differential movement within thrust sheets. Fault-bend folds (structurally necessary folds) have developed following slip of thrust sheets across irregular thrust surfaces and buckle folds have grown during shortening within the sheet. Extensional fault sheets (surge zones) can be mapped out in the Briancon region which both truncate and are flexured by thrust structures; important extensional structures have developed during evolution of the thrust belt.

A total of 178 sites have been collected for palaeomagnetic analysis from within two strike-slip fault systems in northern Chile (25.4°S - 26.4°S); the Atacama Fault System in the Coastal Cordillera, and the Domeyko Fault System in the Andean pre-Cordillera. In the Coastal Cordillera, analysis of Middle Jurassic lavas (La Negra Formation) and Middle Jurassic to Early Cretaceous dyke swarms reveal a consistent clockwise rotation of approximately 42°. The remanence from the La Negra Formation passes both fold and reversal tests and is interpreted as a pre-folding remanence. Four of the five dyke swarms have mixed polarity, suggesting that they too carry a primary or very early remanence. The clockwise rotation of the area is interpreted as occurring due to a domino-type mechanism where the blocks are bounded by sinistral faults operating in a crustal scale strike-slip duplex structure. The rotation is the result of sinistral transpression during the middle Cretaceous as a result of the Peruvian Orogeny leading to abandonment of the Jurassic-Early Cretaceous magmatic arc in the Coastal Cordillera and its subsequent eastward migration. The Domeyko Fault System (DFS) comprises the Domeyko Fault Zone (DFZ), and a series of subsidiary faults to the east which define two distinct but slightly overlapping domains: a fold-and-thrust belt in the north and a domain o f sinistral strike-slip faults in the south. Samples were collected from the volcanic rocks of the Sierra Fraga Formation (Middle Jurassic) west of the DFZ, and from the lavas of the Quebrada Vicunita Formation (Late Jurassic) and Cerro Valiente Formation (Palaeocene) from both domains of the DFS. Also sampled in the southern domain, and in a small area between the two domains were sandstones o f the Quebrada Monardes Formation (Early Cretaceous). All of the volcanic units have been remagnetised and no tectonic interpretation is made from them. The sandstones in the southern domain record a clockwise rotation of approximately 24°. This is interpreted as being due to compression across the DFS causing the sinistral strike-slip faults, and the blocks between them to rotate clockwise towards the major tectonic structure, the DFS. The sandstones in the central area between the two domains record an anticlockwise rotation of approximately 28°. This is explained by small-scale rotation of thrust sheets or slip on minor dextral strike-slip faults. The deformation and rotation in the pre-Cordillera occurred in response to the Incaic Orogeny during the Miocene-Oligocene.

No description available.

No description available.

No description available.

Mud volcanoes are a poorly understood phenomenon whereby overpressured, fine-grained sediments extrude to the surface from depths of up to several kilometres. They are observed worldwide, usually in areas of tectonic compression and thick, rapidly-deposited sedimentary sequences. This research is aimed at achieving a better understanding of the type of sediment behaviour leading to mud volcanoes. The study combines a geological investigation of the origin of mud volcanoes with a geotechnical investigation of their behaviour. As part of the geological study, the mud volcano examples around the world are described, and possible mechanisms of overpressuring and extrusion are investigated. Particular attention is paid to the field description and the subsurface geology of the mud volcano areas visited in South Trinidad and Southwest Taiwan. The results of laboratory analyses on the muds sampled in the field, mineralogy, particle size and shapes, Atterberg limits and, where possible, micropalaeontology, are presented. These are aimed at understanding the origin of the clay and establishing any characteristics unique to mud volcanoes. The geotechnical investigation concentrates on the undrained shear behaviour of mud volcano clays, which originate in an environment where thick, low permeability sedimentary sequences are subjected to tectonic compression. The experiments were conducted in a high pressure triaxial cell, using confining pressures of 5 to 50 MPa. Particular attention is paid to the effects of consolidation path, stress magnitude and material properties on the behaviour of mud volcano clays. The results of the experiments are discussed in the light of the geological study, and equivalent testing results from low pressure soil mechanics studies. The discussion concentrates on the role of tectonic activity in the formation of mud volcanoes.

The present thesis is concerned with the volcanogenic sulphide deposits associated with the Troodos ophiolite, Cyprus. Extensive petrological, chemical and field data are presented and these suggest that the ophiolite has formed on a spreading axis which was located above a subduction zone. This tectonic regime was responsible for the enrichment of the Troodos primary magma in silica and the large ion lithophile elements. Formation of the Sheeted Complex and Lower Pillow Lavas is envisaged to have taken place close to the axis of spreading. The lavas are extrusive products of subaxial chambers which exhibited both open and closed system fractionation. The Upper Pillow Lavas are considered to be derived from the same primary magma as the other members of the ophiolite but exhibit only limited fractionation. The difference between the two suites is attributed to extrusion of the Upper Pillow Lavas on the flanks of the spreading axis, from diapirically rising magma chambers. A detailed geochemical study of the hydrothermal metamorphism of the ophiolite suggests extensive mobility of components in the Sheeted Complex. All geochemical changes are in agreement with experimental and field evidence of seawater-rock interaction; they suggest enrichment of the rocks in magnesium and sodium and extensive leaching of calcium, silica, barium and zinc. Strontium is locally leached and concentrated in secondary minerals. Calculations of element fluxes during hydrothermal metamorphism suggest that the amounts leached from the Sheeted Complex can account for the amounts deposited in the sulphide deposits and metalliferous sediments (umbers). A hydrothermal model is proposed according to which seawater trapped within the Sheeted Complex is heated under the influence of the geothermal gradient, enriched in metallic and other elements by breakdown of primary silicates and finally driven off by the intrusion of magma chambers at high levels in the crust. These chambers are deduced to be those which formed the Upper Pillow Lavas, as inferred from the association of sulphide deposits with the initial stages of eruption of these lavas. Examination of the field occurrence of several sulphide deposits suggests that these are most often localised by faults which were inherited from the tensional regime of the spreading axis. Localisation at sites of structural intersections is also described. The size of the deposits is determined to a great extent by the structural disturbance in the area in which they occur, smaller deposits being favoured at strongly tectonised areas. In the absence of tectonism ore localisation may take place within the lava pile. It is concluded that the presence of a heat source is more important than structural disturbance in the formation of deposits. The results suggest, also, that there was only one period of sulphide-producing hydrothermal activity and this is temporally placed at the early stages of extrusion of the Upper Pillow Lavas. The mineralogical characteristics of deposits are described, with emphasis on the behaviour of the hydrothermal fluids at discharge sites. The main conclusions suggest that extensive interaction of the fluids with the wall rocks and seawater takes place and this determines the final mineralogical character of the deposits and the formation of copper-rich and copper-poor orebodies. The extent of interaction is determined to a large extent by the structure and results to the formation of extensive alteration zones in deposits which formed deep within the lavas. Exchange between the fluids and wall rocks is mainly characterised by enrichment of the surrounding lavas in magnesium and sodium, depletion in calcium and strong enrichment in barium and potassium. The mineralized lavas are depleted in alkalies and enriched in magnesium. The implications of the above conclusions on exploration are briefly discussed, with emphasis on the future application of geochemical methods in the search for sulphide deposits.