Analogue modelling of extensional reactivation of thrust wedges
Summary
Analogue modelling techniques are used to explore the reactivational behaviour of thrust wedges in extensional tectonic settings. Multiple experiments with distinct rheology’s are tested under various conditions such as flat and tilted. Between these experiments, the reactivation behaviour of the pre-existing faults is compared to explore in what scenario reactivation is most dominant. A new modelling technique is developed where a rubber sheet is used to model extension, producing a homogeneously extended model. This extensional modelling is the second part of the experiment as negative inversion is modelled (shortening followed by extension). Previous researches have typically made use of a velocity discontinuity (VD), forcing extension to occur at a certain location in the model. As this thesis aims to explore the reactivational behaviour during a negative inversion tectonic setting, a VD would produce irrelevant results as the model is not free to react in the most energy favourable way.
Three experimental series are presented where i) makes use of solely brittle materials (quartz sand), ii) makes use of brittle materials which are quartz sand and (lower friction) glass beads and iii) makes use of both brittle and ductile materials (quartz sand and PDMS silicone putty). Each experimental series has a flat model and a 5o inclined model. Series ii and iii present an extra experiment where the effects of an extra low friction layer and a lower strain rate are explored respectively.
The experiments show that all experimental series adhere to the rule that tilting the system localizes deformation. Further, reactivation is only significant when a weak zone is present in the model. Series iii experiments demonstrate this by dragging silicone putty up in the model during shortening, along which during extension reactivation occurs. The model accommodate extension in two ways: i) By distributed extension that effects the entire model and ii) by localized deformation at pre-existing thrust contact or at the locations where new normal faults develop. Series i and ii demonstrate that (slight) reactivation is only visible by using particle tracing techniques and studying the relative strain rate variations during the experiments. The structures produced by series iii experiments have a high resemblance to naturally existing structures where the same interplay of (back) thrusts and (antithetic) normal faults is visibly in both the model and nature. Brittle/ductile models show that forward thrust become steeper during extension, as significant reactivation is occurring along a single plane. This causes large blocks to rotate whilst sliding down the reactivated contacts. The main conclusion of this thesis is that in order for extensional reactivation of thrust structures to occur, a weak area must be present within or below the thrust structure. If this is not the case, it will be energy favourable for the model to create new normal faults and accommodate extension that way. Future analogue modelling research on this topic should exclusively model brittle/ductile systems.