Berw Fault

The Berw Fault is a SW-NE trending fault in North Wales. It forms part of the Menai Strait Fault System, with the Dinorwic Fault and the Aber Dinlle Fault. It has a long history of movement with early ductile fabrics preserved from a sinistral (left lateral) strike-slip sense shear zone active at the end of the Precambrian and into the early Cambrian. Any Caledonian deformation is unclear but the fault zone was reactivated in the Carboniferous as a NW-throwing normal fault with seismic reflection data showing the formation of a half-graben in its hanging wall. There are no indications of inversion during the Variscan Orogeny, but the fault was reactivated in a normal sense during the Permian and Triassic and again during the Cenozoic with a sinistral strike-slip sense.^[1]

The Berw Fault zone represents a major mylonitic (intensely deformed and sheared) boundary that separates different rock units of varying ages and compositions. To the northwest of the fault lie sheared slivers of gneisses (banded metamorphic rocks), Gwna sediments, and volcanic lavas. To the southeast, the fault borders the Penmynydd schists of the Aethwy Unit, which contain blueschist facies metamorphic rocks.^[2]

Radiometric dating of the gneissic and granitic rocks near the Berw Fault has revealed they are of late Precambrian age (approximately 595 ± 12 million years old), while several granitic and tonalitic rocks in the fault zone have yielded younger ages (around 562 ± 31 million years old). These younger igneous rocks are interpreted as having intruded into the slightly older gneisses.^[2]

The Berw Fault zone is significant in understanding the tectonic evolution of North Wales as it represents a boundary between rock units with different metamorphic and deformational histories. Some researchers have compared its tectonic position to that of the Dover Fault in Newfoundland, which separates the Gander Zone from the Avalon Zone in Canada's geological framework.^[2]

The Berw Fault probably played an important role in the early Cambrian tectonic evolution of the region (approximately 520–542 million years ago). It may represent part of a system of mylonitic shear zones that juxtaposed slices of continental crustal basement against low-grade mélange (a chaotic mixture of rock types). This boundary marks a striking contrast between the geology of Anglesey and the Welsh mainland, suggesting that the Mona Complex may represent an "exotic terrain" – a fragment of crust that originated elsewhere and was brought to its present position through large-scale tectonic movements. Some researchers have proposed that significant strike-slip faulting (horizontal movement along the fault) occurred during the early evolution of the Mona Complex, potentially moving crustal fragments considerable distances.^[2]

• List of geological faults of Wales

53°13′N 4°20′W / 53.22°N 4.34°W / 53.22; -4.34