Permeability, Past and Present, in Continental Crustal Basement

Geology

Hay, Stephen John

University:University of Glasgow

关键词: Geology;   

Others  :  http://theses.gla.ac.uk/77749/1/10998220.pdf

来源: University of Glasgow

【 摘 要 】

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