Earth’s geological structure and processes - Fundamentals of the Physical Environment - page 303

355-310 Ma ago, responding to the trench suction force of a subduction zone 1,000 km
to the south. The resulting crustal extension caused one-sided or half-graben rifting of the
eroded Caledonian terrane, with crustal slabs tens of kilometres long behaving like a huge
rotational landslide. One of these, the Askrigg block, underlies the western Yorkshire
Dales and its fault-controlled south-west and north-west margins (the Craven and Dent
Fault systems respectively) are clearly visible on the satellite image. Driven by extension
to the south, the surface of each block dipped slowly and intermittently northwards,
whilst its southern margin reared up simultaneously to form a more abrupt scarp front,
creating a series of shallow marine basins and deeper troughs between blocks (Plate 1).
The Yorkshire Dales today reflect the control of tectonics and equatorial climates on
the sedimentary environments of the Carboniferous period. Carbonate platforms and
coral reefs 300-500 m deep developed in the shallow basins, much as they do in the
Caribbean today, whilst marine muds 2-4 km thick infilled the deeper troughs.
Intermittent rotation of the
Plate 1 Near-vertical Lower Palaeozoic basement rocks of the
Askrigg block (lower ground) are overlain unconformably by
horizontally bedded Carboniferous limestone (Upper Palaeozoic),
forming higher ground in Twisleton Dale, North Yorkshire.
Photo: Ken Addison.
rigid blocks and changing sea levels meant that sedimentation was not continuous but
cyclical. Moreover, marine conditions gradually gave way to deltaic and then estuarine
environments as the sea closed and terrestrial sources of sediment began to dominate The