Structural Analysis and Tectonic Significance of the Carmel Head Thrust Belt, Anglesey, North Wales.
2009-06-22T10:59:16Z (GMT) by
The Carmel Head Thrust Belt (CHTB) is a major tectonic feature that records a complex history of polyphase deformation in N Anglesey, NW Wales, British Isles. This thesis tests the hypothesis that the CHTB developed by S-directed contraction that was subsequent to SE-directed ductile contraction (D1-D2) which is recorded elsewhere in western Anglesey, and on Holy Island. Also, this study tests whether the CHTB was formed as part of a Palaeozoic accretionary wedge complex. Detailed structural analysis and synthesis of data collected along three coastal transects in NE, N and NW Anglesey form the basis for the study. Major lithological units were examined in thin-section for petrographic textures, microstructures and deformation processes. The macro- and the micro-structural data indicate that lithological units in N Anglesey underwent S-directed brittle contractional deformation (D3), associated with anchizonal metamorphism. Structures associated with brittle shortening cross-cut SE-directed folds and shear zones (D1-D2) recorded in the greenschist-grade New Harbour and South Stack groups schists and phyllites. D1-D2 occurred after early Cambrian time, synchronous with the Caledonian orogeny. The D3 event occurred during the early Devonian Acadian orogeny and involved southward propagation of the CHTB with a dextral component. Since Ordovician time and prior to Acadian thrusting, transtensional basins were developed which were then locally inverted within a S-vergent thrust wedge. D3 CHTB structures are cut by E-W and N-S extensional faults. The E-W normal faults are related to a Permo-Triassic extensional event and considered D4, whereas the N-S normal faults are considered D5 and formed subsequent to emplacement of a regional Tertiary dyke swarm that affected Anglesey and NW Wales. The Gwna Group and Ordovician rocks were not affected by pre-D3 tectonism and regional metamorphism. The Gwna Group is affiliated with Ordovician rather than Monian rocks. The Gwna mélange is the youngest unit of the Gwna Group, underlain by the Fydlyn beds and the Skerries Formation. The Gwna mélange contains crude stratigraphy, was originally olistostromal and became tectonosomal during CHTB development. The CHTB in N Anglesey documents a major S-directed collisional event, whereas western, central and SE Anglesey record two separate and opposite dipping lower Palaeozoic subduction zones. The NW-dipping subduction zone is beneath NW Wales, and is in the same polarity as the SE-directed D1-D2 deformation. Central Anglesey represents a micro-continent, and the SE-dipping subduction zone was beneath southeastern-most Anglesey and NW Wales. The results of this study shed new light on the Palaeozoic amalgamation history of the British Isles.