Competition between 3D structural inheritance and kinematics during rifting: insights from analogue models

The 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, i.e., changes in plate divergence rate or direction, and the distribution of weaknesses in the competent mantle and crust influence rift evolution. We find that the combined reactivation of mantle and crustal weaknesses without kinematic changes creates complex rift structures. Divergence rates affects the strength of the weak lower crustal layer and hence the degree of mantle-crustal coupling. In this context slow rifting decreases coupling, so that crustal weaknesses can easily localize deformation and dominate surface structures, whereas fast rifting increases coupling so that deformation related to mantle weaknesses can have a dominant surface expression. Through a change from slow to fast rifting mantle-related deformation can overprint previous structures that 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 well developed. Alternatively, they are ignored during subsequent rifting. We furthermore place our results in a larger framework of brittle-viscous rift modelling results from previous experimental studies, showing the importance of genral lithospheric layering, divergence rate, the type of deformation in the mantle, and finally upper crustal structural inheritance. The interaction between these parameters can lead to a large variety of deformation styles that may often lead to comparable end products. Therefore, detailed investigation of faulting and to an equal extent basin depocenter distribution over time is required to properly determine the evolution of complex rift systems. These insights provide a strong incentive to revisit various natural examples.

Are complex rift patterns the result of interacting crustal and mantle weaknesses, or of multiphase rifting? An analogue modelling study

<p>During extension of the continental lithosphere, deformation often localizes along pre-existing weaknesses originating from previous tectonic phases. When simulating such structures with analogue or numerical methods, modellers often focus on either crustal or mantle heterogeneities. By contrast, here we present results from 3D analogue models to test the combined effect and relative impact of (differently oriented) mantle and crustal weaknesses on rift systems.</p><p>Our model set-up involves a rigid base plate fixed to a mobile sidewall. When this sidewall moves outward, the edge of the base plate induces a “velocity discontinuity” (VD) that acts as an upper mantle fault/shear zone in a strong upper mantle. The VD is either parallel to the model axis, or 30˚ oblique. On top of this base plate, we apply a viscous layer representing the ductile lower crust, followed by a sand cover that simulates the brittle upper crust. Crustal weaknesses were either imposed by implementing “seeds” (i.e. ridges of viscous material at the base of the sand layer), or by pre-cutting the sand. Similar to the basal plate edge, we apply different crustal weakness orientations as well.</p><p>Without weaknesses in the model crust, an axis-parallel VD forms an axis-parallel rift basin above along the VD. When adding oblique seeds, they strongly localize deformation, creating a series of obliquely oriented graben. Yet the VD still induces faulting along the model axis, leading to the development of offset axial graben as well. Pre-cut faults also localize deformation but are less dominant than the seeds. As a result, the VD has more control and the axial rift structures are much more pronounced. In the oblique VD case, the reference model develops a series of en echelon graben along the VD. Axis-parallel seeds strongly localize faulting, to such a degree that the effect of the VD is very much overruled. Pre-cut faults allow more influence from the VD, but still dominate the system. Doubling the extension rate increases the strength of the viscous layer, enhancing coupling between the VD and sand cover, so that a series of en echelon graben crosscutting the seed-induced structures develop.</p><p>We find that the orientation and relative weakness of inherited weaknesses in the mantle and crust, as well as extension rates control subsequent rift structures. These structures and their relative evolution can be complex due to the interplay of the above factors, and importantly, all develop under the same pure shear extensional boundary condition. Our results show that very differently oriented rift structures can form during one phase of extension without the need to invoke multiple rift phases. Furthermore, coupling can change over time due to changes in extension velocity or gradual thinning of the lower crust, thus affecting rift evolution. These findings provide a strong incentive to reassess the tectonic history of various natural examples.</p>

Optical lace for synthetic afferent neural networks

Whereas vision dominates sensing in robots, animals with limited vision deftly navigate their environment using other forms of perception, such as touch. Efforts have been made to apply artificial skins with tactile sensing to robots for similarly sophisticated mobile and manipulative skills. The ability to functionally mimic the afferent sensory neural network, required for distributed sensing and communication networks throughout the body, is still missing. This limitation is partially due to the lack of cointegration of the mechanosensors in the body of the robot. Here, lacings of stretchable optical fibers distributed throughout 3D-printed elastomer frameworks created a cointegrated body, sensing, and communication network. This soft, functional structure could localize deformation with submillimeter positional accuracy (error of 0.71 millimeter) and sub-Newton force resolution (~0.3 newton).

A systematic comparison of experimental set-ups for modelling extensional tectonics

Analogue modellers investigating extensional tectonics often use different machines, set-ups and model materials, implying that direct comparisons of results from different studies can be challenging. Here we present a systematic comparison of crustal-scale analogue experiments using simple set-ups simulating extensional tectonics, involving either a foam base, a rubber base, rigid basal plates or a conveyor base system to deform overlying brittle-only or brittle-viscous models. We use X-ray computed tomography (CT) techniques for a detailed 3-D analysis of internal and external model evolution. We find that our brittle-only experiments are strongly affected by their specific set-up, as the materials are directly coupled to the model base. Experiments with a foam or rubber base undergo distributed faulting, whereas experiments with a rigid plate or conveyor base experience localized deformation and the development of discrete rift basins. Pervasive boundary effects may occur due to extension-perpendicular contraction of a rubber base. Brittle-viscous experiments are less affected by the experimental set-up than their brittle-only equivalents since the viscous layer acts as a buffer that decouples the brittle layer from the base. Under reference conditions, a structural weakness at the base of the brittle layer is required to localize deformation into a rift basin. Brittle-viscous plate and conveyor base experiments better localize deformation for high brittle-to-viscous thickness ratios since the thin viscous layers in these experiments allow deformation to transfer from the experimental base to the brittle cover. Brittle-viscous-base coupling is further influenced by changes in strain rate, which affects viscous strength. We find, however, that the brittle-to-viscous strength ratios alone do not suffice to predict the type of deformation in a rift system and that the localized or distributed character of the experimental set-up needs to be taken into account as well. Our set-ups are most appropriate for investigating crustal-scale extension in continental and selected oceanic settings. Specific combinations of set-up and model materials may be used for studying various tectonic settings or lithospheric conditions. Here, natural factors such as temperature variations, extension rate, water content and lithology should be carefully considered. We hope that our experimental overview may serve as a guide for future experimental studies of extensional tectonics.