A seismic study on the structural evolution of the North Celtic Sea Basin, offshore Ireland

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Byrne, Keith B.
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University College Cork
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The North Celtic Sea Basin (NCSB) is one of a number of basins related to regional Mesozoic extension across north-west Europe. Previous authors have described the NCSB as having a conventional “steer’s head” geometry or alternatively a half graben geometry. Modern 2D and 3D seismic data has now allowed interpretation of faulting at depth within the NCSB. In particular it has demonstrated the importance of intra-basinal faulting and results in a robust updated structural evolution of the NCSB. Rifting is believed to have commenced in the Triassic with the development of an asymmetric simple shear rift. Extension was accommodated by several reactivated Variscan thrust faults with a detachment between the upper and lower crust at 18-20 km (11-12.5 miles) depth. Rifting continued through the Lower Jurassic and extension was accommodated primarily on the most northern of the reactivated Variscan thrusts, the Morrigan Fault. A deep extension of the Morrigan Fault has been mapped by previous authors on deep refraction seismic data as a south -easterly dipping low angle detachment. The other Variscan thrusts became locked, possibly against the granites within the Labadie Bank High – Pembrokeshire Ridge to the south. Halokinesis initiated within the Lower Jurassic, caused by movement on underlying faults and differential loading of the overburden. Renewed rifting in the Upper Jurassic and Lower Cretaceous was accommodated by a symmetric pure shear rift as extension was accommodated on the Morrigan Fault and new mid-basinal normal faults, antithetic to the Morrigan Fault, resulting in a conventional full graben geometry. These antithetic faults (Dagda, Brigit and Aonghus Faults) detach in the underlying Triassic halites. The post rift sag phase in the Upper Cretaceous yielded thick deep marine deposits which overstepped the basin bounding faults to yield a classic “steer’s head” geometry. Subsequent Alpine compression in the Oligo-Miocene and uplift in the Paleocene reactivated and reversed the mid-basin antithetic faults, creating broad mid-basinal anticlines and flower structures. These faults were preferentially reactivated as they detached in Triassic halites. Recognition of this revised structural evolution of the NCSB is critical to predicting the spatial distribution of sedimentary facies and de-risking hydrocarbon prospectivity of the basin.
Structural geology , North Celtic Sea Basin , Basin evolution , Structural evolution
Byrne, K. B. 2020. A seismic study on the structural evolution of the North Celtic Sea Basin, offshore Ireland. MRes Thesis, University College Cork.
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