Reviewing the correlation of two exhumed basement shear zones: The geometry, structure and kinematics of the Lleyn Shear Zone, northwest Wales and the Berw Shear Zone, Anglesey

Abstract

The multiply reactivated Berw Fault/Shear Zone (BSZ) on Anglesey and Lleyn Shear Zone (LSZ) on the Lleyn Peninsular, northwest Wales, are strands of the steeply-dipping, southwest-northeast-trending Menai Strait Fault Zone (MSFZ). The MSFZ originated as a crustal-scale fault and shear zone system during sinistral strike-slip along the north-western margin of Gondwana in the early Ordovician (ca. 486-479 Ma).
It is widely believed, despite a lack of direct kinematic evidence, that the deformed and sheared rocks of the BSZ on Anglesey can be traced directly along strike onto mainland Wales to the southwest and are exposed in the LSZ, representing a single surface lineament. In this study, this hypothesis is tested by contrasting and comparing the kinematic and textural evolution of the LSZ and BSZ. Overprinting relationships and fault rock distributions, observed at localities along the two shear zones, are used to propose interpretations of the kinematic histories. Microstructural analysis of significant fault rocks is offered to support the argument for the kinematic correlations and to understand the textural evolution of the fault rocks associated with the two shear zones.
The BSZ and LSZ share similar kinematic characteristics and were likely both initiated during the same Ordovician D1 deformation and were also associated with similar regional tectonic processes. Outcrop and thin section analysis of key fault rocks from the BSZ and LSZ show that the initial D1 sinistral transpressional deformation led to the development of greenschists and mylonites within the BSZ and LSZ. This was followed by a D2 deformation that involved northwest-directed shearing and continued compression with some southeast-directed local backfolding and thrusting. Both shear zones preserve evidence for a later D3 dextral strike-slip deformation. The Berw Fault alone records at least one episode of D4 deformation involving normal, northwest-side-down displacement that is thought to be related to extensional tectonics in the Carboniferous and Permo-Triassic. Final phases of brittle sinistral strike-slip displacement along the Berw Fault and LSZ are thought to be correlated and are likely responsible for left-lateral offsets of Cenozoic dykes seen in geophysical data.
The early history of the LSZ and BSZ, including the D1 sinistral transpression, D2 northwest-directed shearing, and D3 dextral strike-slip, is likely related to deformation during the ‘Penobscot’ orogeny, which affected peri-Gondwanan microcontinents, and occurred at approximately 480 Ma. Differences in their post-D3 movement history suggest that the two fault zones are not directly equivalent structures along strike and perhaps represent separate strands of the same crustal scale fault zone.