Structure and evolution of deformation-band-dominated faults in porous volcaniclastic rocks. Insights from Eastern Taiwan

Abstract

This MSc project documents fault evolution in porous volcaniclastic rocks of Miocene age in Shihtiping, Eastern Taiwan. Sequential fault evolution in porous sandstone, and to some extent in carbonates, has been studied for several decades. It is widely recognised that formation of faults in porous rocks includes a sequential evolution as a result of progressive shear. Fault formation initiates from simple deformation band structures with small displacements to form more complex band structures. By progressive shear and strain hardening deformation bands may evolve into faults with continuous slip surfaces. Despite extensive studies on deformation bands and fault evolution in other porous rocks, there are significantly less studies conducted on the subject in volcaniclastic rocks. This study aims to fill the knowledge gap and expand our understanding of fault evolution in porous volcaniclastic rocks. In order to do so, an outcrop-based study on strike-slip deformation bands and faults in volcaniclastic rocks along the eastern coast of Taiwan has been conducted. By documenting and quantifying the geometry of deformation bands and faults in the study area, a sequential fault evolution model for porous volcaniclastic rocks was developed. In the presented model, displacement is used as a proxy for its division. The initial stage of fault formation is characterised by small deformation bands with mm-cm-scale displacements accommodated through granular flow and cataclastic flow mechanisms. Deformation bands at the initial stage are comprised of single or a few strands. By progressive shear, linking and interaction, deformation bands may evolve into the intermediate stage of fault development. This stage is characterised by several-stranded deformation band clusters and networks with patchy slip surfaces. Displacements at this stage range between 5 and 50 centimetres and is accommodated through a combination of cataclastic flow and discrete brittle failure. Formation of through-going slip surfaces and accumulation of significant shear displacement (>0.5 metres) characterise the final fault stage. Discrete brittle failure replaces cataclastic flow as the dominating deformation mechanism at the final fault stage. This study documents fault architecture in a porous volcaniclastic succession by detailed mapping and topological analysis. Fault architecture is a function of displacement lithology and structural position along strike. The fault core is either dominated by slip surfaces within a shear-zone of deformation bands or discrete brittle failure. In terms of lithological control, the porosity, volcanic glass content and number of lithic fragments in the host rock control the fault core architecture. Linking of deformation bands and fault segments are associated with structural complex zones and high structural connectivity at all stages of fault evolution. This study does not only contribute to the understanding of deformation band and fault evolution, it may also improve the understanding of structurally controlled fluid flow in volcaniclastic reservoirs/aquifers.