Abstract
AbstractThe competition between the impact of inherited weaknesses and plate kinematics determines the location and style of deformation during rifting, yet the relative impacts of these ‘internal’ and ‘external’ factors remain poorly understood, especially in 3D. In this study, we used brittle‐viscous analogue models to assess how multiphase rifting, that is changes in plate divergence rate or direction, and the presence and orientation of weaknesses in the competent mantle and crust, influences rift evolution. We find that the combined reactivation of mantle and crustal weaknesses without any kinematic changes already creates complex rift structures. Divergence rates affect the strength of the weak lower crustal layer and hence the degree of mantle‐crustal coupling; slow rifting decreases coupling, so that crustal weaknesses can dominate deformation localisation and surface structures, whereas fast rifting increases coupling and deformation related to mantle weaknesses can have a dominant surface expression. Through a change from slow to fast rifting mantle‐related deformation can overprint structures that previously formed along (differently oriented) crustal weaknesses. Conversely, a change from fast to slow rifting may shift deformation from mantle‐controlled towards crust‐controlled. When changing divergence directions, structures from the first rifting phase may control where subsequent deformation occurs, but only when they are sufficiently well developed. We furthermore place our results in a larger framework of brittle‐viscous rift modelling results from previous experimental studies, showing the importance of general lithospheric layering, divergence rate, the type of deformation in the mantle, and finally upper crustal structural inheritance. The interaction between these parameters can produce a variety of deformation styles that may, however, lead to comparable end products. Therefore, careful investigation of the distribution of strain localisation, and to an equal extent of basin depocenter locations over time is required to properly determine the evolution of complex rift systems, providing an incentive to revisit various natural examples.
Highlights
During the early stages of continental rifting, deformation is often localised along structural weaknesses inherited from previous tectonic phases (e.g. Bonini et al, 1997; Corti, 2012; Morley et al, 1990; Nelson et al, 1992; Wilson, 1966)
The results from the second, fast rifting phase in the models from Series B suggest that fast rifting localises deformation along the velocity discontinuity (VD) (Figure 3), and we found a similar effect in our Model C1 with a model-a xis parallel VD and no seeds (Figure 4a)
We present an analogue modelling study involving brittle-viscous set-ups to study how multiphase rifting in a continental lithosphere containing pre-existing weaknesses in the competent mantle and crust may affect the evolution of a rift system
Summary
During the early stages of continental rifting, deformation is often localised along structural weaknesses inherited from previous tectonic phases (e.g. Bonini et al, 1997; Corti, 2012; Morley et al, 1990; Nelson et al, 1992; Wilson, 1966). Bonini et al, 1997; Corti, 2012; Morley et al, 1990; Nelson et al, 1992; Wilson, 1966) These inherited weaknesses may be situated anywhere in the lithosphere, but their impact is more significant when they are located in competent layers. In a subsequent publication, Zwaan et al (2021a) improved upon this study by systematically testing how mantle and crustal weaknesses interact under constant kinematic settings. Their model results revealed the development of complex rift structures with different structural orientations under a constant kinematic setting, showing that structural weaknesses can be a highly dominant factor in a rift system. The authors pointed out, as was previously suggested by Reeve et al (2015), that the reactivation of pre-existing crustal and mantle weaknesses during a single phase of rifting could establish a rift system with structural trends that would otherwise suggest a multiphase rifting history involving changes in large-scale plate divergence directions over time
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