Analogue and numerical modelling of continental rifting: New insights from quantitative deformation analyses

Schmid, Timothy Chris (2022). Analogue and numerical modelling of continental rifting: New insights from quantitative deformation analyses (Unpublished). (Dissertation, University of Bern, Faculty of Science)

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Extension of the continental lithosphere leads to the formation of rift basins. If extension succeeds to the point of lithospheric rupture eventually, new oceanic basins form with the continental remnants as conjugate passive margins on either side. The initial and early stages of continental break-up refer to the rifting phase, which shows a great diversity of rift geometries across the Earth’s continents. Continental rifts host great potential for hydrocarbon, geothermal, and metal exploration and are the source of natural hazards such as earthquakes and volcanism. Continental rifts have been extensively investigated by numerous geological studies. However, inferences on deformation mechanisms are often made under the assumption that rifts develop in plane strain cross sections, perpendicular to the extension direction and neglect the 3D complexity of natural rift settings. Modelling experiments pose a useful tool to capture deformation processes of continental rifting in 3D. Analogue modelling experiments inherently simulate mechanical (brittle and ductile) deformation processes in 3D but require further measures to monitor and quantitatively investigate deformation. Numerical experiments provide direct access to quantities such as stresses but, especially in 3D, are limited by computational power that affects the model resolution. Taking advantages of the strengths of both modelling approaches, the combination of both can overcome limitations of each technique and provides useful information for a better understanding of deformation processes of continental rifting.

The aim of this thesis is to investigate and analyse quantitatively both analogue and numerical model experiments simulating early stages of continental rifting leading to new insights rift formation processes. 3D numerical experiments validate results from analogue experiments and provide additional information on stress regimes and stress orientations. The numerical results demonstrate the effect of stress distribution on rift segment interaction under orthogonal rifting. Pre-existing heterogeneities in material strength play an important role in the location of rift initiation and rift segment linkage. As individual rift segments propagate, they can cause a notable reorientation of the local stress field in deformed areas with stress orientations deviating from the regional trend. In return, stress re-orientation affects progressive deformation and may deflect propagating rift segments. Strain localisation and stress reorientation are closely linked and influence each other mutually. This suggests that stress reorientation occurs under constant plate motions and depicts a transient stage that changes with progressive deformation.

Analogue experiments performed under rotational extension further investigate deformation features in the brittle upper and viscous lower crust and the high-resolution quantitative analysis provides novel in- sights on near-surface fault growth and associated rift propagation, deep-seated viscous flow, and coupling of deformation in both crustal levels. Normal fault growth under rotational extension is characterised by an early stage of bidirectional stepwise growth in length by fault linkage with pulses of high growth rates, followed by a longer and continuous stage of unidirectional linear fault growth. The hybrid fault growth model has direct implications on rift propagation depicting initial high propagation rates that subsequently decrease as the rift approaches the rotation axis. Model internal deformation analyses document a complex flow field in the ductile lower crust that is characterised by extension-parallel horizontal flow towards the rift axis and vertical upward flow that compensate thinning of the brittle upper crustal layer and in addition rift-parallel flow towards regions that have undergone higher amounts of extension.

Comparisons of the analogue and numerical model results to selected natural examples of continental rifts i.e., segments of the East African Rift System, the Taupo Rift, or the Woodlark Rift demonstrate the value of quantified modelling experiments in contributing to a better understanding of natural settings of continental rifts.

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