The role of basement architecture in the spatial and temporal development of tectonic deformation in an orogenic foreland – An example from the Variscides of SW Ireland.

Determining the sequencing of different deformation mechanisms and styles is crucial to understanding how the Earth’s crust responds to tectonic stress.  The sequence of near surface compressional deformation in peripheral orogenic forelands typically starts with an early stage of bulk homogenous shortening, i.e. layer parallel shortening (LPS) followed by folding and contractional faulting. The temporal and spatial distribution of strain is also very much influenced by the inherited basement structure and pre-collisional basin architecture in such foreland settings, in particular inherited extensional fault systems. Tectonic contraction in such settings tends to promote the positive inversion of basins through LPS, folding and reverse faulting by the exploitation of pre-existing bounding structures and facilitated by the lower bulk density and strength of sedimentary fill sequences when compared to the surrounding basement.

This study will combine conventional structural field studies from the Variscides of SW Ireland with the spatial characterisation of finite strain using clast shape analysis and anisotropy of magnetic susceptibility (AMS) studies on deformed sandstones as a means of elucidating the temporal and spatial evolution of deformation in an orogenic foreland that is heavily influenced by pre-existing basement architecture and structures. AMS is increasingly seen to be a particularly sensitive proxy for tectonic strain signals in low to medium strain settings on the periphery of orogens.

The Variscan of SW Ireland offers an excellent opportunity to investigate the full foreland strain cycle from early bedding parallel oblate fabrics to more prolate intermediate fabrics and finally cleavage parallel oblate fabrics in an overall setting that incorporates marked basement control of the spatial distribution of finite strain. The adopted multi-technique approach will also allow for insights into the relative timing of deformation events.