Evidence for 3-D transtensional strain and reactivation in the Northumberland Basin (UK): an alternative to basin inversion models ?

N. De Paola, R.E. Holdsworth & K. McCaffrey

Reactivation Research Group, Dept of geological Sciences, University of Durham, DH1 3LE

nicola.depaola@durham.ac.uk

In common with all of the Carboniferous basins in the UK, tThe Northumberland Basin liesis situated in the foreland region of the Variscan Orogeny and has been so long been considered to have experienced considered as an inverted basin. In particular a compressional inversionevent, related to the far-field effects of Variscan collision.Orogeny The inversion event is manifested by folding and contractional faulting and is bracketed, is sandwiched between the early extensional Dinantian rifting and intrusion of the Whin Sill and associated dykesintrusion during the late Carboniferous.

Detailed mapping has been carried out of, conducted in two key-outcrops close toin the proximity of the Whin Sill and associated dDykes outcrops along the coast. This reveals a 3-D strain deformation with an quasi-orthorhombic, quadrimodal brittle-fault pattern with a close association between compartmentalised folds, faults and minor structures such as veins arrays, en-echelon tension gashes, stylolites, etc. Kinematic analyses indicate that early extensional structures have been reactivated during a later strike-slip or transtensional deformation. Quantitative strain analyses using the fault and slickenline patterns, following the odd-axis construction of Krantz (Krantz,1988) for orthorhombic symmetry, suggest prolate bulk strain ellipsoids that are consistent with transtensional deformation.

The recognition of a 3-D strain permits a somewhat simplified deformation history to be proposed for the Northumberland Basin involving early Dinantian extension, followed by late Carboniferous transtension with reactivation of some pre-existing faults. Thus, there is no need for an "inversion" event to explain the presence of contractional structures since a horizontal component of shortening can arise due to strain partitioning in a transtensional regime (see also Holdsworth et al. this meeting). The latter deformation may be directly associated with events responsible for the generation and emplacement of the Whin Sill and associated intrusions.

Further work is required to obtain a more complete strain distribution in the basin which can be used to test for transtensional strain and kinematic partitioning into extension- and wrench-dominated (‘inversion’) domains.