The evolution of transform boundaries in response to plate motion changes

Abstract

Transform margins and transform faults are first order tectonic features that accommodate or have accommodated motion between tectonic plates in our planet. Changes in plate motion that occurred in the past as tectonic plates moved are imprinted and documented on the planet’s divergent and convergent plate margins. In a similar manner, these motion changes are expected to be found along transform systems. Thus, investigating transform margins and identifying structures that detail such plate motion changes is of great scientific interest to delineate their evolution. Moreover, areas around transform margins and faults have a significant resource potential (such as the hydrocarbon fields offshore Ghana or the geothermal potential of the Gulf of California) and earthquake hazard risk (such as the San Andreas or North Anatolian faults).
In this thesis, a multi-disciplinary and multi-scale approach using numerical and physical analogue modelling is applied to investigate the evolution of transform plate boundaries and faults when these are affected by changes in plate motions. The combination of analogue and numerical modelling was selected as these two methods complement each other by having different strong points. Numerical modelling offers fast and multiple iterations of experiments while analogue modelling offers straightforward observable physics.
First, a numerical modelling approach covering the lithosphere-scale is presented that highlights the differences between the effect obliquely inherited structures versus changes in plate motion have on transform margins. The main finding is that changes in plate motion affect transform margin evolution significantly both in duration and also in structure. Then, two analogue modelling studies follow. The first one focuses on the crustal scale and investigates what changes a transform margin and a rift-transform intersection undergo when a change in plate motion occurs. The most significant findings of this set of experiments are that the transform systems re-orient to accommodate the changes in the plate motion through the creation of new strike-slip faults and that faults in such systems display a dual character (i.e. oblique-normal or oblique-reverse). The second set of analogue modelling experiments represents an investigation into basin-scale transtensional rotations along releasing bends on transform faults (pull-apart basins). The key finding of this set of experiments is that the resulting pull-apart morphology from these models (such as an asymmetrical triangular shape and faults oblique to the extension trend) can be used as an identifying tool for pull-apart basins that have experienced a change in plate motion during their evolution.
The modelling results are compared against natural examples, such as the Gulf of California, the Tanzania Coastal Basin, and the Gulf of Aden. The very high degree of similarity between the models and nature, apart from validating the models, also indicates that changes in plate motion add a further degree of complexity to the evolution of transform plate boundaries. This complexity can be seen in the dual character of faulting along Principal Displacement Zones, or in oblique fault orientation in pull-apart basins, and even in rifting asymmetry in rift-transform intersections. Thus, plate motion changes should always be considered when investigating transform boundaries, as potentially they are the rule, and not the exception.