Abstract Mapping on the microstructural scale can contribute significantly to conventional field and larger scale mapping and understanding of spatial, temporal and process-oriented relationships. Here, electron backscattered diffraction (EBSD)-based microstructural maps are presented of subgrain and Dauphiné twin boundaries in undeformed sedimentary quartzite. ‘Plane matching’ analysis permits determination of the complete orientation of boundaries. A workflow is presented to facilitate the necessary crystallographic calculations. The resulting maps indicate: (1) boundary plane rotation angle/axis pairs, including tilt–twist components; (2) boundary migration vectors; and (3) conventional EBSD misorientation angle/axis pairs. Subgrain boundaries are general with small misorientations and boundary plane normal directions sub-parallel to grain boundary stress concentrations; rotation axes are oriented sub-parallel to the bedding dip (017°/10°E). Most exhibit bi-direction boundary migration vectors parallel to bedding normal. The EBSD misorientation analysis results are different as they only recognize the parallelism of adjacent crystal lattices. The differences are especially apparent for Dauphiné twin boundaries. Maps are presented of twin boundaries using matched plane analysis, including explanation for lateral twin migration. Driving forces to move twin boundaries are also estimated by mapping variations in Young's modulus between parents and twins; differences are significant, indicating that the Young's modulus and driving forces do not need to be large, explaining the propensity for twinning in many quartzites.

Abstract Rock successions in Britain and Ireland, and more especially those in Wales, were instrumental in the founding and naming of the Ordovician System, and the Anglo-Welsh series established both initially and subsequently were used widely as a standard for Ordovician chronostratigraphy. Although now largely superseded in the global scheme of series and stages, they retain their local and regional importance. The Ordovician System in Britain and Ireland documents the history of a segment of the Earth's crust that incorporated opposing peri-Gondwanan and peri-Laurentian/Laurentian margins of the Iapetus Ocean during its closure, and is accordingly complex. The complexity arises from the volcanic and tectonic processes that accompanied oceanic closure coupled with the effects of eustatic sea-level changes, including the far-field effects of the Late Ordovician glaciation. For the past three decades, Ordovician successions in Britain and Ireland have been discussed in terms of terranes. Here we review Ordovician successions in each terrane, incorporating the results of recent research and correlating those successions via biostratigraphical schemes and radiometric dates to the global Ordovician series and stages.