4D laboratory experiments of oblique extension and scissor tectonics, structural inheritance and sedimentation: implications for rift evolution, rift propagation and rift segment interaction

Zwaan, Frank (2017). 4D laboratory experiments of oblique extension and scissor tectonics, structural inheritance and sedimentation: implications for rift evolution, rift propagation and rift segment interaction. (Dissertation, Institut für Geologie, PHILNAT)

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Extension of the continental lithosphere leads to the formation of continental rift basins. When extension persists, continental break-up and oceanic spreading ensue, creating conjugate passive margins on both sides of a new oceanic basin. Rifts and passive margins have been intensely studied by geologists because of their vast hydrocarbon potential, representing the bulk of the global proven oil and gas resources and are of great importance for human society, housing a considerable share of the world’ s population. Passive margins, as their name suggest represent the remainders of a successful rifting event leading to continental break-up and have since ceased to experience active extension. In contrast, many continental rifts are still actively deforming. These rift basins are prone to various types of natural hazards but also provide economic opportunities for geothermal energy production.

However, despite the numerous geological studies, the complex tectonic evolution and structural frameworks of rifts and passive margins remain poorly understood; most rifts and passive margins have experienced oblique extension and/or scissor extension as well as the reactivation of pre-existing structures and significant syn-tectonic sedimentation during their history.

The aim of this Thesis, funded by the Swiss National Science Foundation and carried out at the University of Bern, is to improve our understanding of rifting processes using state- of-the-art analogue model experiments. These analogue models simulate the development and propagation of young continental rifts and the interaction between individual rift segments in the context of oblique extension and scissor extension settings, and aslo investigates the role of structural inheritance and sedimentation on the system. In addition, this thesis includes a comparison of various experimental methods involving foam, rubber and base plate set-ups, commonly used for simulations of extensional tectonics. The models are among others analysed with cutting-edge X-Ray computed tomography (XRCT or CT) and digital volume correlation (DVC) techniques for unparalleled 4D visualisation and quantification of internal model evolution.

The experimental method comparison illu- strates that structural inheritance is key to focus deformation leading to rift development. In the rubber and foam base set-ups, a viscous basal layer facilitates localization as it buffers distributed deformation, and reduces coupling, allowing the brittle cover to behave as rigid blocks, whereas it prevents the strong and probably exaggerated concentration of faulting along the edge of the base plates that develops in the rigid plate models. The experiments illustrate furthermore that the standard brittle-ductile foam set-up with a seed focussing deformation produces the least amount of boundary effects and is thus respresents best of the set-ups tested for studies of rift basin development. A rubber base setting could in theory be similarly efficient, yet a rubber or foam sidewall will be necessary to reduce boundary effects. The brittle-ductile base plate models for instance develop too much friction along the sidewalls, a problem that may be solved with the use of a lubricant.

Various factors are shown to be of influence on the evolution of a rift basin. Firstly, the application of a linear viscous seed, simulating a linear weak zone in the upper crust, produces a narrow rift basin, where previous authors have often applied wider patches of viscous material or rubber sheets to create a wider rift zone. Furthermore, the rate of extension is shown to control the degree of brittle-ductile coupling, either producing a localized rift along the seed or multiple rift basins throughout the model. Also the amount of extension determines the type of structures present in a rift: a small amount of extension only results in minor deformation along the boundary faults whereas a large amount of extension produces additional faulting within the rift wedge, accompanied by a rising viscous layer. The amount of extension itself is depends on the degree of extension obliquity, as increasing oblique extension results in progressively narrower basins, with steeper boundary faults and oblique internal structures.

Moreover, along-strike variations in extension rate in both time and space may lead to a complex structural configuration, as illustrated by scissor extension models. It appears that such gradients in the rate (and thus amount) of extension are a crucial factor controlling rift propagation.

A final influence on rift basin development is syn-rift sedimentation. The presence of sediments in the rift basin does not affect the initial rift configuration set by large-scale tectonic forces, but its additional weight does influence the structural style within the rift basin, concentrating deformation along a set of major faults instead of a myriad of minor faults and causing increased total subsidence, preventing the viscous layer from rising. High syn-rift sediment influx might even delay continental break-up, but larger-scale plate tectonic processes determine whether a continent is actually broken up in the end.

Most of the factors that influence the evolution of individual rift segments, affect the interaction and linkage between them as well. For instance, a weak seed may localize deformation, but a high extension rate and associated brittle-ductile coupling results in the development of multiple rift basins with little amounts of extension in each, limiting their capacity to interact. Instead, low extension rates focus deformation in a single rift, allowing it to propagate and interact with other segments.

Furthermore, the models illustrate that large rift offsets hinder rift linkage, especially when the extension direction is such that the segments propagate parallel to each other or even apart, although in some cases a strike- slip transfer zone may develop. The interplay between initial seed geometry and extension direction proves to be a crucial factor for rift linkage establishment.

Within this context, secondary structural weaknesses, connecting the main rifts, are of minor importance and are only activated when oriented favourably to the regional extension direction. Syn-rift sedimentation, although of importance for internal rift structures, does not significantly affect the large-scale rift linkage zone formation, especially since these zones experience relatively little subsidence, thus less accommodation space is available for sediment deposition. In contrast to the situation with large-scale rift structures, along- strike extension gradients associated with scissor extension have no strong impact on rift interaction zones, since the extension gradient between the two rift segments is generally minor.

Although top view photographs and CT data allow a thorough insight into the 3D external and internal model evolution, 4D DVC analysis has provided an unprecedented understanding of 3D internal displacement and deformation within the brittle and viscous parts of the models. These results illustrate the strong difference between brittle and viscous behaviour, the sand acting as rigid blocks interrupted by discrete zones of faulting, floating on the viscous layer that shows a distributed flow pattern. Not only does the DVC analysis capture the rising viscous material beneath the rift basins, it also reveals out-of-plane motion of both brittle and viscous material due to interacting rift segments. This last observation is crucial for 2D structural reconstructions of orthogonal extension settings, as both viscous and brittle material can move out of section.

The models are compared with natural examples of continental rifts and rift interaction structures, resembling various structures observed in e.g. the East African Rift System, the North Sea Viking Graben and the Cenozoic European Rift System. However, a comparison with previous models and natural examples also shows a reasonable fit with the geometries of oceanic spreading centres, suggesting that these brittle-viscous set-ups can also be used for the modelling of mid-oceanic ridge settings.

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