Inversion structures in rift basins: real shortening events or products of strain partitioning during transtension ?

R.E. Holdsworth, N. de Paola, R.R. Jones*, K.McCaffrey & P.Clegg

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

* = CognIT a.s., P.B.610, N-1754 Halden, Norway

So-called ‘inversion structures’ (e.g. folds, reverse faults) spatially associated with basin-bounding faults are very widely recognised in rift basins in both onshore and offshore settings worldwide. Leaving aside those examples related to halokinesis, the great majority of inversion structures are attributed to local or regional crustal shortening events. There is, however, an alternative possibility which is investigated in this presentation: that inversion reflects horizontal contractional deformation during progressive transtension.

It has long been recognised that many regions of rift-related deformation should experience directions of divergence that are significantly oblique to the main basin-bounding faults, i.e. transtensional strain. This can arise either because the causative plate motions are non-orthogonal to the plate margins and/or because the basin-bounding faults reactivate pre-existing structures that lie significantly oblique to the regional direction of extension. In transtension zones with moderate to high angles of divergence, the axis of maximum finite contraction (z) should always be vertical, as is the case in orthogonal extensional basins. However, at low angles of divergence (a < 20º), the contractional z axis is initially horizontal, swapping orientation with the vertical intermediate (y) axis with increasing finite strain. In such cases, it has been demonstrated both theoretically and in analogue modelling studies that the horizontal contraction should be sufficient to generate structures such as folds.

Using examples from Palaeozoic and Mesozoic basins in NE England, we suggest that contractional structures can also form in moderate- to high-angle obliquely divergent zones of rifting where the bulk strain undergoes kinematic partitioning into distinct regions of wrench-dominated (a < 20º) and pure-shear dominated (a >> 20º) transtension. Such partitioning will be favoured in basins where strike-slip displacements become increasingly focused along certain basin-bounding faults during oblique divergence. This may occur where such faults formed due to reactivation of pre-existing structures that may be zones of long-lived weakness and/or are in an orientation particularly favourable to the preferential accommodation of strike-slip displacements. Our findings could have profound implications for the routine interpretation of inversion structures in any rift basin where the direction of extension may be significantly oblique to the basin margins.