Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.278258
Title: Permeability, past and present, in continental crustal basement
Author: Hay, Stephen John
ISNI:       0000 0001 3547 8467
Awarding Body: University of Glasgow
Current Institution: University of Glasgow
Date of Award: 1988
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Abstract:
The examination of microcracks from the Lewisian of the northwest highlands of Scotland, has revealed the sequential development of microcrack networks, sealed by minerals deposited from fluids passing along them. Comparisons, between open and sealed microcracks, indicate that both thermal and mechanical stresses were active in the formation of the cracks. Several mechanisms which may allow a crack to remain open under compressive stress have been seen, and it is found that microcracks may act as recurring fluid pathways. Observations on the crack-sealing minerals have allowed inferences on the nature of fluid flow within the Lewisian crystalline basement to be made. The sealed cracks were formed as a result of both localized, and regional, periods of fluid flow through the basement. This fluid flow took place over a period spanning some two billion years (from the Archaean Badcallian metamorphic event, to the late-Proterozoic Torridonian rifting). Several magnetite crack sealing events, specific to lithologies and structures, occurred during pre-Torridonian times. These were associated with the Scourian and Laxfordian metamorphic cycles. However, most sealed cracks are post-tectonic. Four main post-Laxfordian regional sealing events have been identified, and are characterized by the sealing mineral sequence of; K-feldspar + calcite; prehnite + albite + calcite; pumpellyite + quartz + calcite; and stilpnomelane. Crack-crack intersections indicate that this sequence of crack sealing is consistent throughout the Lewisian's central region. Investigation of these sealed cracks, using mineral PTX considerations, stable isotopes, fluid inclusions and radiometric isotopes, suggest that this sequence relates to a period of hydrothermal activity within the Lewisian complex, associated with Torridonian basin subsidence and rifting. The study suggests that the fluid passing along the cracks was originally a surface derived meteoric water. This permeated into the basement gneisses along subvertical, NE-SW, microcracks and joints, which were dilating during the Torridonian extension. Pressure estimates suggest that the currently exposed Lewisian surface was in the region of 3 km deep when the fractures were sealing. The precipitated mineral sequence was the result of a cooling and evolving hydrothermal system, progressing from about 350°C to 250°C with time. Changes in fluid character, over the time of fracture sealing, can be accounted for by a combination of fluid/wall-rock interaction and deepening of the Torridonian rift valley. The present day porosities and permeabilities of Lewisian gneisses are low, averaging less than 1% for porosity, and in the region of 100 nd to 10 μd (10⁻¹⁹-10⁻¹⁷m²) for permeability. Both of these properties are inhomogeneous between lithologies and within samples. Estimates of porosity, using S.E.M. photographs, have shown that porosity can be considered at two separate levels within crystalline rocks. Firstly, porosity due to background cracks (grain boundary and intragranular cracks), and secondly, porosity due to intergranular cracks. Using S.E.M. pictures, the porosity can be quite closely estimated. A model has been developed whereby the present permeability of a crystalline rock can be accurately estimated by calculations involving the width and length of the fractures, and the linear crack density. Application of this model, to sealed cracks found in the Lewisian, suggest that most crack arrays would have accounted for permeabilities of between 1 and 10 μd. This is within the present day limits for most of the Lewisian rocks. The sealed cracks present within the Lewisian (and used in palaeopermeability calculations) largely formed when the presently exposed gneiss was close (< 5 km) to the surface. For this reason little information has been gained on the possible nature of fluid flow within the lower crust.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.278258  DOI: Not available
Keywords: Crustal structure of the Earth
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