Siddall, Ruth; (1993) Thermotectonic Evolution of the Continental Margins of the Bay of Biscay: Application of Apatite Fission Track Analysis. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The sensitivity of apatite fission tracks to cooling and unroofing episodes in Earth history is particularly suited to the tectonic processes of rifting and the long-term evolution of continental margins. Apatite fission track analyses have been made on ~70 samples of igneous, metamorphic and sedimentary rocks from coastal outcrops of Western France (the Armorican Massif) and north-western Spain (the Hesperian Massif) the aim being to unravel the potential thermal overprint associated with the opening of the North Atlantic Ocean, specifically in the Bay of Biscay. This thesis reviews the geological evolution of the continental margins of the Bay of Biscay region and presents the fission track sampling strategies and analytical results. The fission track ages range from ~70 to 270 Ma, with the youngest ages towards the Atlantic coasts, and become progressively older eastwards. The data are interpreted in terms of the thermal histories resulting from the Mesozoic postbreakup exhumation of the rift flanks in the westernmost regions of Spain and France, whilst eastwards the earliest detectable cooling episode is Early Permian, reflecting the exhumation of the Variscan mountain chain. The data from the Armorican Massif are consistent with 2 previously unrecognised periods of burial and subsequent exhumation during the Mesozoic. The most recent (Campanian) burial episode is inferred to represent the deposition of chalk. The data from north-western Spain do not appear to reflect this second burial, probably because at this time this region was outside the chalk depocentre. A second major aspect of the research is an assessment of the role of chemical composition as a control on variation in single grain ages within a sample. In particular, annealing of fission tracks in apatites rich in chlorine is expected to be retarded relative to a more fluorine-rich crystal, resulting in an older fission track age. A new application of infrared microspectroscopy has been developed to determine the chemical compositions of apatites in situ on the grain mount, enabling thesignificance of the relationship of fission track age to apatite chemistry to be assessed directly. This method relies on the progressive shift of infrared spectral peaks towards lower wavenumbers with the increasing chlorine. This phenomenon indicates that it is the structural rather than chemical adjustment of the crystal to the substitution of a large ion such as Cl that is important to the annealing of fission tracks in apatite. The methodology is an important advancement to the technique of fission track analysis being rapid and non-destructive.

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