The Bala Fault

The Bala Fault is a south-west to north-east trending geological fault in Wales that extends offshore into Cardigan Bay. The resulting valley is occupied by the town of Bala, Bala Lake (Llyn Tegid) and further south and east by Tal-y-Llyn and the Mawddach Estuary.

The Bala Fault has an extremely long and complex history - it has been intermittently active between about 800 My (million years) and 10 My before the present!

Diagram showing the Bala FaultIt was probably initiated as a tensional fracture during the opening of the Iapetus Ocean in late Precambrian times. The fault is very deep, perhaps extending down to the mantle lithosphere, and could be better termed a 'geofracture'.

The Bala fault has had both vertical and sideways transverse motions at various times under different crustal stress patterns.

During the Cambrian, the fault probably formed a sea bed escarpment, separating shallower water to the south east from deeper water to the north-west. It was over this sea bed scarp that turbidites were discharged - evidence comes from the very coarse proximal turbidite grits, above Barmouth.

In Ordovician times, the Bala fracture zone was a major pathway for the rise of magma, producing a series of volcanic centres from the Arans, through Rhobell Fawr to Cader Idris. The fault then continued across the present Cardigan Bay to reappear on the tip of Pembrokeshire, where it was responsible for the Strumble Head basalt pillow lava eruptions. The diagram shows the positions of the Ordovician volcanic centres in relation to the Bala Fault.

West of Dolgellau, the main fault seems to spilt into a series of parallel fractures, along the Tal y Llyn valley, the valley of the Gwernan Lake, and the Mawddach estuary. This is typical of a major deep fault, which can split upwards into a 'flower structure' of parallel fractures at the surface.

In late Lower Palaeozoic times with closure of the Iapetus Ocean, metamorphism below Wales released hydrothermal fluids which rose along the major fracture zones. This can explain the location of gold deposits along the line of the Bala fault branch through the Mawddach estuary (e.g. Clogau mine), with further deposits following the north-south Rhobell fracture to form the gold deposits in Coed y Brenin (e.g. Gwynfynydd).

The final closure of Iapetus in the Silurian seems to have been oblique, which may have caused sideways motion on the Bala fault to allow sideways adjustment of crustal blocks.

The Bala fault may well have been active again at various times up to the Tertiary, as blocks of crust in the Wales area moved up and down in response to various plate movements. Evidence for these recent movements comes from the huge movement along the Mochras fault which runs north-south along the coast and has down-faulted and preserved Triassic, Jurassic and Tertiary sediments under Cardigan Bay.

 

Information provided by Dr Graham Hall with additional information by Ray Hind (Mountain Guide)