The link between Somalian Plate rotation and the East African Rift System: an analogue modelling study

The East African Rift System (EARS) represents a major tectonic feature that splits the African continent between the Nubian Plate situated to the west and the Somalian Plate to the east. The EARS comprises various rift segments and microplates and represents a key location for studying rift evolution. Researchers have proposed various scenarios for the evolution of the EARS, but the impact of continent-scale rotational rifting, linked to the rotation of the Somalian Plate, has received only limited attention. In this study we apply analogue models to explore the dynamic evolution of the EARS within its broader rotational-rifting framework. Our models show that rotational rifting leads to the lateral propagation of deformation towards the rotation axis, which reflects the general southward propagation of the EARS. However, we must distinguish between the propagation of distributed deformation, which can move very rapidly, and localized deformation, which can significantly lag behind the former. The various structural-weakness arrangements in our models (simulating the pre-existing lithospheric heterogeneities that localize rifting along the EARS) lead to a variety of structures. Laterally overlapping weaknesses are required for localizing parallel rift basins to create rift pass structures, leading to the rotation and segregation of microplates such as the Victoria Plate in the EARS, as well as to the simultaneous north- and southward propagation of the adjacent Western Rift. Additional model observations concern the development of early pairs of rift-bounding faults flanking the rift basins, followed by the localization of deformation along the axes of the most developed rift basins. Furthermore, the orientation of rift segments with respect to the regional (rotational) plate divergence affects deformation along these segments: oblique rift segments are less wide due to a strike-slip deformation component. Overall, our model results generally fit the large-scale present-day features of the EARS, with implications for general rift development and for the segregation and rotation of the Victoria Plate

Effects of sedimentation on rift segment evolution and rift interaction in orthogonal and oblique extensional settings:Insights from analogue models analysed with 4D X-ray computed tomography and digital volume correlation techniques

During the early evolution of rift systems, individual rift segments often develop along pre-existing crustal weaknesses that are frequently non-continuous and laterally offset. As extension progresses, these initial rift segments establish linkage in order to develop a continuous rift system that might eventually lead to continental break-up. Previous analogue and numerical modelling efforts have demonstrated that rift interaction structures are influenced by structural inheritances, detachment layers, magma bodies, rate and direction of extension, as well as the distance between rift segments. Yet to date, the effects of syn-tectonic sediments have been largely ignored or only modelled in 2D. In this study we therefore assess the influence of sedimentation on rift segment and rift interaction zone evolution in orthogonal and oblique extensional settings, by means of 3D brittle-ductile analogue models, analysed with 4D X-ray computed tomography (XRCT or CT) methods and digital volume correlation (DVC) techniques. Our models show that syn-rift sedimentation does not significantly influence the initial large-scale evolution of rift segments and rift interaction zones. Nevertheless, syn-rift sedimentation can strongly affect rift-internal structures: sedimentary loading reinforces the rift wedge, decreasing rift wedge faulting and increases sub- sidence within the rift basin. These effects are strongest in areas where most accommodation space is available, that is, along the main rift segments. In contrast, rift segments that undergo high degrees of oblique extension develop less accommodation space and are therefore less influenced by sedimentary loading. Rift interaction structures are least affected by sediment influx, as they experience relatively low amounts of subsidence so that little accommodation space is available. Our conclusions are valid for the early stages of rift development, when a high sediment influx could delay continental break-up, as other processes are likely to become dominant during later stages of continental extension. Finally, state-of-the-art DVC analysis of CT data proves to be a powerful tool to extract and fully quantify 3D internal model deformation in great detail and could be useful for comparing and calibrating analogue and numerical models

How initial basin geometry influences gravity-driven salt tectonics: Insights from laboratory experiments

As a rifted margin starts to tilt due to thermal subsidence, evaporitic bodies can become unstable, initiating gravity-driven salt tectonics. Our understanding of such processes has greatly benefitted from tectonic modelling efforts, yet a topic that has however gotten limited attention so far is the influence of large-scale salt basin geometry on subsequent salt tectonics. The aim of this work is therefore to systematically test how salt basin geometry (initial salt basin depocenter location, i.e. where salt is thickest, as well as mean salt thickness) influence salt tectonic systems by means of analogue experiments. These experiments were analyzed qualitatively using top view photography, and quantitatively through Particle Image Velocimetry (PIV), and 3D photogrammetry (Structure-from-Motion, SfM) to obtain their surface displacement and topographic evolution. The model results show that the degree of (instantaneous) margin basin tilt, followed by the mean salt thickness are dominant factors controlling deformation, as enhancing basin tilt and/or mean salt thickness promotes deformation. Focusing on experiments with constant basin tilt and mean salt thickness to filter out these dominant factors, we find that the initial salt depocenter location has various effects on the distribution and expression of tectonic domains. Most importantly, a more upslope depocenter leads to increased downslope displacement of material, and more subsidence (localized accommodation space generation) in the upslope domain when compared to a setting involving a depocenter situated farther downslope. A significant factor in these differences is the basal drag associated with locally thinner salt layers. When comparing our results with natural examples, we find a fair correlation expressed in the links between salt depocenter location and post-salt depositional patterns: the subsidence distribution due to the specific salt depocenter location creates accommodation space for subsequent sedimentation. These correlations are applicable when interpreting the early stages of salt tectonics, when sedimentary loading has not become dominant yet

An HI selected sample of galaxies - The HI mass function and the surface brightness distribution

Results from the Arecibo HI Strip Survey, an unbiased extragalactic HI survey, combined with optical and 21cm follow-up observations, determine the HI Mass Function and the cosmological mass density of HI at the present epoch. Both are consistent with earlier estimates, computed for the population of optically selected galaxies. This consistency occurs because, although the distribution of optical central surface brightnesses among galaxies is flat, we fail to find a population of galaxies with central surface brightnesses fainter than 24 B-mag/arcsec^2, even though there is no observational selection against them.Comment: 5 pages, including 3 encapsulated postscript figures. Presented at the workshop `HI in the Local Universe', Sydney, May 13-15 1996. Accepted for publication by PASA. Also available from http://www.atnf.csiro.au/Publications/HI_workshop/proceedings.htm

Images and videos of analogue centrifuge models exploring marginal flexure during rifting in Afar, East Africa

This data set includes images and videos depicting the evolution of deformation and topography of 17 analogue experiments c passive margin development, to better understand the ongoing tectonics along the western margin of Afar, East Africa. The tectonic background that forms the basis for the experimental design is found in Zwaan et al. 2019 and 2020a-b, and references therein. The experiments, in an enhanced gravity field in a large-capacity centrifuge, examined the influence of brittle layer thickness, strength contrast, syn-rift sedimentation and oblique extension on a brittle-viscous system with a strong and weak viscous domain. All experiments were performed at the Tectonic Modelling Laboratory of of the Istituto di Geoscience e Georisorse - Consiglio Nazionale delle Ricerche (CNR-IGG) and of the Earth Sciences Department of the University of Florence (CNR/UF). The brittle layer (sand) thickness ranged between 6 and 20 mm, the underlying viscous layer, split in a competent and weak domain (both viscous mixtures), was always 10 mm thick. Asymmetric extension was applied by removing a 1.5 mm thick spacer at the side of the model at every time step, allowing the analogue materials to spread when enhanced gravity was applied during a centrifuge run. Differential stretching of the viscous material creates flexure and faulting in the overlying brittle layer. Total extension amounted to 10.5 mm over 7 intervals for Series 1 models that aimed at understanding generic passive margin development in a generic orthogonal extension setting, whereas up to 16.5 mm of extension was applied for the additional Series 2 models aiming at reproducing the tectonic phases in Afar. In models involving sedimentation, sand was filled in at time steps 2, 4 and 6 (i.e. after 3, 6 and 9 mm of extension). Detailed descriptions of the experiments, monitoring techniques and tectonic interpretation of the model results are presented in Zwaan et al. (2020c) to which these data are supplementary

The HI Mass Function of Galaxies from a Deep Survey in the 21cm Line

The HI mass function (HIMF) for galaxies in the local universe is constructed from the results of the Arecibo HI Strip Survey, a blind extragalactic survey in the 21cm line. The survey consists of two strips covering in total 65 square degrees of sky, with a depth of cz = 7400 km/s and was optimized to detect column densities of neutral gas N_HI > 10^18 cm^-2 (5 sigma). The survey yielded 66 significant extragalactic signals of which approximately 50% are cataloged galaxies. No free floating HI clouds without stars are found. VLA follow-up observations of all signals have been used to obtain better measurements of the positions and fluxes and allow an alternate determination of the achieved survey sensitivity. The resulting HIMF has a shallow faint end slope (alpha ~ 1.2), and is consistent with earlier estimates computed for the population of optically selected gas rich galaxies. This implies that there is not a large population of gas rich low luminosity or low surface brightness galaxies that has gone unnoticed by optical surveys. The cosmological mass density of HI at the present time determined from the survey, Omega_HI = (2.0 +/- 0.5) x 10^-4, is in good agreement with earlier estimates. We determine lower limits to the average column densities of the galaxies detected in the survey and find that none of the galaxies have below 10^19.7 cm^-2, although there are no observational selection criteria against finding lower density systems.Comment: 34 pages, including 8 figures. To appear in The Astrophysical Journa

Pre-existing basement faults controlling deformation in the Jura Mountains fold-and-thrust belt: Insights from analogue models

Pre-existing faults in the mechanical basement are believed to play an important role in controlling deformation of the thin-skinned Jura Mountains fold-and-thrust belt, which constitutes the northernmost extension of the European Alps. We use brittle-viscous analogue models to investigate the influence of frontal and oblique basement steps on the subsequent evolution of structures during thin-skinned shortening. Vertical offset between two rigid baseplates (simulating the mechanical basement) causes the formation of reverse faults and grabens in the overlying brittle layers that are not reactivated during subsequent thin-skinned shortening. However, baseplate steps localise deformation, causing a temporary frontward propagation of deformation in an early stage and inhibiting propagation afterwards. Downward baseplate steps induce very strong deformation localisation and foster the formation of fault-bend folds. Models featuring upward steps develop step-controlled pop-up structures with imbricated fronts and viscous ramps that shorten dynamically with progressive contraction. We find that deformation localisation increases both with higher step-throws and lower obliquity (α) of the strike of the step (e.g. frontal step α = 0°). With increasing step-throws, α = 30° and α = 45° oblique upward-steps lead to a characteristic imbrication of the brittle cover with laterally confined thrust-slices and step-parallel oblique-thrusts, which rotate up to 15° about a vertical axis over time. Step-controlled backthrusts preceding the formation of thrust-slices do not show notable rotation and hence constitute excellent indicators for the orientation of oblique upward-steps. The topographic patterns of oblique-step models resemble individual thin-skinned structures of the Internal Jura (i.e. Pontarlier and Vuache fault zones, the nappe system SE of Oyonnax and the Chasseral anticline), strongly suggesting that pre-existing NNE-SSW and NW-SE striking oblique upward-steps in the basement controlled deformation in the overlying cover. Our model results may be applied to other thin-skinned fold-and-thrust belts worldwide that formed above pre-existing basement structures

Influence of rheologically weak layers on fault architecture: insights from analogue models in the context of the Northern Alpine Foreland Basin

We present a series of analogue models inspired by the geology of the Zürcher Weinland region in the NorthernvAlpine Foreland Basin of Switzerland to explore the influence of rheological weak, i.e. (partially) ductile layers on the 3D evolution of tectonic deformation. Our model series test the impact of varying weak layer thickness and rheology, as well as different kinematics of an underlying “basal fault”. Model analysis focuses on deformation in the weak layer overburden and, uniquely, within the weak layer itself. We find that for low to moderate basal fault displacements, the above-mentioned parameters strongly influence the degree of coupling between the basal fault and the weak layer overburden. Coupling between the basal fault and overburden decreases by reducing the strength of the weak layer, or by increasing the weak layer’s thickness. As a result, basal fault displacement is less readily transferred through the weak layer, leading to a different structural style in the overburden. By contrast, increasing the amount, or rate, of basal fault slip enhances coupling and leads to a more similar structural style between basal fault and overburden. Moreover, dip-slip displacement on the basal fault is more readily transferred to the overburden than strike-slip displacement of the same magnitude. Our model results compare fairly well to natural examples in the Northern Alpine Foreland Basin, explaining various structural features. These comparisons suggest that rheological weak layers such as the Jurassic Opalinus Clay have exerted a stronger control on fault zone architecture than is commonly inferred, potentially resulting in vertical fault segmentation and variations in structural style. Furthermore, the novel addition of internal marker intervals to the weak layer in our models reveals how complex viscous flow within these layers can accommodate basal fault slip. Our model results demonstrate the complex links between fault kinematics, mechanics and 3D geometries, and can be used for interpreting structures in the Alpine Foreland, as well as in other settings with similar weak layers and basal faults driving deformation in the system

How oblique extension and structural inheritance influence rift segment interaction: Insights from 4D analog models

Rifting of the continental lithosphere involves the initial formation of distinct rift segments, often along preexisting crustal heterogeneities resulting from preceding tectonic phases. Progressive extension, either orthogonal or oblique, causes these rift segments to interact and connect, ultimately leading to a full-scale rift system. We study continental rift interaction processes with the use of analog models to test the influence of a range of structural inheritance (seed) geometries and various degrees of oblique extension. The inherited geometry involves main seeds, offset in a right-stepping fashion, along which rift segments form as well as the presence or absence of sec- ondary seeds connecting the main seeds. X-ray computer tomography techniques are used to analyze the 3D models through time, and results are compared with natural examples. Our experiments indicate that the extension direction exerts a key influence on rift segment interaction. Rift segments are more likely to connect through discrete fault structures under dextral oblique extension conditions because they generally propagate toward each other. In contrast, sinistral oblique extension commonly does not result in hard linkage because rift segment tend to grow apart. These findings also hold when the system is mirrored: left-stepping rift segments under sinistral and dextral oblique extension conditions, respectively. However, under specific conditions, when the right-stepping rift seg- ments are laterally far apart, sinistral oblique extension can produce hard linkage in the shape of a strike-slip-domi- nated transfer zone. A secondary structural inheritance between rift segments might influence rift linkage, but only when the extension direction is favorable for activation. Otherwise, propagating rifts will simply align perpendicu- larly to the extension direction. When secondary structural grains do reactivate, the resulting transfer zone and the strike of internal faults follow their general orientation. However, these structures can be slightly oblique due to the influence of the extension direction. Several of the characteristic structures observed in our models are also present in natural rift settings such as the Rhine-Bresse Transfer Zone, the Rio Grande Rift, and the East African Rift System