Reservoir-Scale Faulting Patterns in Transtensional Rift Zones: the Influence of Lithology and Pre-Existing Structures

De Paola, N.¹, R.E. Holdsworth², K.J.W. McCaffrey², R.R. Jones³ (1) University of Perugia, Perugia, Italy (2) Durham University, Durham, United Kingdom (3) Geospatial Research Limited, Durham, United Kingdom

In transtensional deformation zones, the bulk 3-D strains are often kinematically partitioned into regions of wrench- and extension-dominated faulting. Most strain models assume ideal incompressible materials with a Poisson's ratio (ν) of 0.5. It is well known from experimental and geophysical data, however, that natural rocks have values of ν < 0.5 and that significant variations in the values of ν occur for different lithologies. We demonstrate that for non-coaxial, 3-D transtension and transpression, this should lead to an expansion of the wrench-dominated strain field. The effect is especially marked in lithologies with very low Poisson's ratios (ν ≤ 0.15), where wrench-dominated deformation can occur even where the regional direction of divergence or convergence is only modestly oblique (e.g. 52°). The Carboniferous basin-bounding 90 Fathom Fault, NE England was reactivated as a dextral transtensional structure during NE-SW regional stretching in post-Carboniferous times. The preferential dip-slip reactivation of pre-existing E-W structures in the underlying Carboniferous basement led to kinematic partitioning of the transtensional bulk strain. In addition, however, the geometric, spatial and kinematic patterns of minor faulting in Permian rocks located in the fault hangingwall are markedly influenced by the host lithology. Quartz-rich sandstones (ν = 0.12) preserve complex faulting patterns consistent with a wrench-dominated transtension while immediately overlying dolostones (ν = 0.29) preserve simpler patterns of Andersonian conjugate faults consistent with a more extension-dominated regime. Our findings illustrate that micro- to meso-scale faulting patterns are likely to be substantially influenced by lithology in all regions of oblique divergence or convergence.