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The tectonic exhumation of mantle material is a well-known phenomenon and may occur during both rifting and subsequent (large-scale) basin inversion. However, the processes leading to the exhumation of dense and therefore negatively buoyant (sub-)lithospheric mantle material remain poorly understood. We therefore conducted a series of thermomechanical simulations using the geodynamics code ASPECT (coupled with FastScape for the inclusion of surface processes) testing the impact of various parameters on mantle exhumation in inverted rift systems.We find that rift duration strongly impacts mantle exhumation, both during the rift phase, as well as during subsequent inversion. When only limited rifting is applied, the dense mantle material cannot reach the surface as the overlying crustal layers remain connected. Basin inversion then tends to create a symmetric pop-up structure by reactivating rift boundary faults, and the dense mantle material is forced down by the thickening of low-density crustal layers on top of it. Only after certain amount of extension, the crust is sufficiently thinned so that mantle material can be exhumed. This mantle material may then remain near the surface or be further exhumed during basin inversion. Such further mantle exhumation is favoured if asymmetric reactivation of the rift basin occurs, so that mantle material is thrust on top of the downgoing plate.The establishment of such asymmetric orogenic systems allowing for efficient mantle exhumation is further promoted by having only short-lived tectonic quiescence between rifting and inversion, so that no thermal equilibration of the exhumed mantle domain can occur. As a result, the rift basin remains a weakness that is readily exploited during inversion. Longer periods of tectonic quiescence restore the strength of the lithosphere, so that delayed inversion generates more symmetric structures, with limited opportunities for mantle exhumation.Within this tectonic context, erosion efficiency is another key factor. First, more efficient erosion during inversion removes crustal material so that the mantle can be exhumed (even in symmetric orogenic systems). Second, efficient erosion also leads to the development of asymmetric orogenic systems, thus doubly contributing to mantle exhumation. Somewhat similarly, high plate velocities during inversion introduce larger amounts of crustal material into the system, which erosion cannot remove in a timely manner, whereas slow plate velocities allow erosion more time to remove material. Hence, mantle exhumation is positively correlated to erosion efficiency, and is negatively correlated to plate velocities during inversionFinally, serpentinization of mantle material can occur close to the Earth’s surface (i.e. in the uppermost kilometres) and strongly reduces the material’s density and brittle strength. Although our models so far only show a limited effect of serpentinization, the overall weakness of serpentinized mantle material at the rift basin floor seems to reduce localization of inversion-related deformation, thus generating more symmetric inversion systems with limited mantle exhumation.

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.

The East African Rift System (EARS) represents a major tectonic feature splitting the African continent apart into the Nubian Plate situated to the west, and the Somalian Plate to the east. The EARS comprises various rift segments and two microplates (the Victoria and Rovuma plates) and represents a key location for studying rift evolution. Researchers have proposed various scenarios for the evolution of the EARS, but the impact of continental-scale rotational rifting, caused by 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 the broader rotational rifting framework. Our models show that rotational rifting leads to the lateral propagation of deformation towards the rotation axis, but we must distinguish between the propagation of distributed deformation, which can move very rapidly, and localized deformation, which can significantly lag behind. The various structural weakness arrangements in our models (representing pre-existing structural heterogeneities) lead to a variety of different structures. Laterally overlapping weaknesses are required for localizing parallel rift basins to create rift pass structures, possibly leading to the rotation and segregation of micro-plates, as is the case for the Victoria Plate in the EARS. 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 advanced 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 are a good fit with the large-scale features of the EARS, providing constraints on the timing of rift development and on the segregation and rotation of the Victoria plate within the broader rotational rifting framework of the EARS.

Rifting and continental break-up are fundamental tectonic processes, the understanding of which is of prime importance. However, the vast temporal and spatial scales involved pose major limitations to researchers. Analogue tectonic modelling represents a great means to mitigate these limitations, but studying the complex internal deformation of lithospheric-scale models remains a challenge. We therefore present a novel method for lithospheric-scale rifting models that are uniquely monitored in an X-ray CT-scanner, which combined with digital image correlation (DIC) techniques, provides unparalleled insights into model deformation. Our first models illustrate how the degree of coupling between competent lithospheric layers, which are separated by a weak lower crustal layer, strongly impacts rift system development. Low coupling isolates the upper crust from the upper lithospheric mantle layer below, preventing an efficient transfer of deformation between both layers. By contrast, fast rifting increases coupling, so that deformation in the mantle is efficiently transferred to the upper crust, inducing either a symmetric or asymmetric (double) rift system. Furthermore, oblique divergence may lead to en echelon graben arrangements and delayed exhumation of the lower crustal layer. The successful application of our novel modelling approach, yielding these first-order insights, provides a clear incentive to continue running lithospheric-scale rifting models, and to apply advanced monitoring techniques to extract as much information from models as possible. There is indeed a broad range of opportunities for follow-up studies within (and beyond) the field of rift tectonics.

Basin inversion is a process that takes place when a sedimentary basin is subjected to compressional stresses and may result in the reactivation of pre-existing faults and/or the localization of deformation along new reverse faults. The Araripe Basin (NE Brazil) is an example of a Cretaceous intracontinental aborted rift with its sedimentary infill found at ca. 1000 m altitude in the present day. Post-rift basin inversion is proposed as the cause of this topographic high, however how inversion mechanisms affected this basin is a matter of debate with two end member scenarios: reactivation of pre-existing normal faults leading to local uplift, or regional tectonic uplift. In this study, we conducted analogue models of basin inversion to test these scenarios. We present two series of crustal-scale brittle-viscous experiments: i) extension followed by compression without sedimentation, with a variation of rifting and inversion directions (orthogonal or 45° oblique) and ii) extension and compression with syn-rift sedimentation, with the same variation in rifting and inversion directions. We used a seed representing a structural weakness that was applied at the base of the brittle layer to localize deformation along the model axis. We found that orthogonal rifting without sedimentation forms through-going border faults, whereas oblique rifting creates initial en-echelon faults that eventually link up creating large border faults. Rift basins with syn-rift sedimentation evolved in a similar fashion, however sedimentary loading increased subsidence. During inversion, most deformation is accommodated along new low angle reverse faults. Within that framework, significant intra-graben fault reactivation occurred in models without sedimentation. By contrast, syn-rift sedimentation caused only minor reactivation of rift faults in oblique inversion since the sediments acted as a buffer during compression; no rift fault reactivation occurred in orthogonal compression situations. Comparing the existing scenarios for inversion in the Araripe Basin with our model results and field data show that these scenarios do not fully explain the natural example. Therefore, we propose an alternative scenario based on our models, involving oblique compression and the development of low angle reverse faults, which better explains inversion in the Araripe Basin.

The southern Dead Sea Transform (SDST) is an active left-lateral transform plate boundary that extends from the Sinai triple junction to the Lebanon restraining bend, separating the Arabian and Sinai plates. In this study, we analyze structural variations along the SDTS, and reproduce these variations in a 4D analogue model. From south to north, the structural styles along the SDTS indicate (1) rotational transtension within the Gulf of Aqaba, (2) pure strike-slip in Wadi Araba and Jordan River valley, and (3) pull-apart basins in the Dead Sea, Sea of Galilee and Hula basin. These different structural styles were replicated experimentally in an analogue model incorporating transtension with minor rotation along a kinked plate boundary. Our 4D model produced a deep southern depression with en echelon faults corresponding to the Gulf of Aqaba, a simple strike-slip fault system without vertical displacement reflecting the Wadi Araba and Jordan Valley, and a set of pull-apart basins reminiscent of the Dead Sea, Sea of Galilee and Hula basins. The accurate reproduction of the structural styles along this 600km-long plate boundary segment constrains the relative movement between the Arabian and Sinai plates to a simple combination of transtension with minor rotation, thereby negating the earlier hypothesis of Euler pole shift during the tectonic evolution of the SDST.

When simulating lithosphere-scale rifting processes, analogue modellers have their model lithosphere float on top of a dense fluid representing the sub-lithospheric mantle (i.e. the asthenosphere). Such models provide crucial insights into rift evolution, but monitoring model-internal deformation has always been a major challenge. Here we present the results of new rifting experiments performed with a novel lithospheric-scale modelling machine that allows for X-ray CT-scanner, uniquely revealing the models’ internal evolution.Our models involve a 4-layer lithosphere, with brittle layers for the competent upper crust and upper lithospheric mantel, and viscous layers for the ductile lower crust and lower lithospheric mantle. This model lithosphere is placed in a basin of glucose syrup simulating the asthenosphere and contained by mobile sidewalls. When stretching the model by moving these sidewalls apart (inducing either orthogonal or oblique extension), deformation is accompanied by syrup flow and isostatic compensation. A weakness within the upper mantle serves to localize deformation along the central axis of the model. We use photogrammetry and PIV techniques for detailed analysis of surface deformation, whereas CT imagery and PIV analysis of CT-sections provide unprecedented insights into internal model evolution.We find that early on in orthogonal extension models, deformation initiates along the weakness in the upper mantle layer. This deformation is then transferred into the upper crust via shear zones in the lower crust, generating a dual graben structure there. In parts of the model, one of the grabens can become dominant and as extension progresses, so that a large shear zone cutting through the whole lithosphere forms (asymmetric, simple-shear rifting). In other parts of the model deformation may be more distributed so that both grabens are well-developed (symmetric, pure shear rifting). Meanwhile, the on-going stretching and thinning of the lithosphere splits the upper mantle layer, and the simulated lower mantle (and especially the asthenosphere) rises towards the model surface, bringing the lower mantle layer in contact with the lower crustal layer (i.e. necking of the lithosphere).In oblique extension models initial deformation also localizes in the upper mantle layer, but no clear surface structures develops (except for a broad topographic depression along the central model axis). By increasing the extension velocity and thus the coupling between the upper mantle and upper crust, faulting initiated in the upper crust, creating two bands of en echelon grabens. Also in these models, we observe lithospheric necking.Our (final stage) model results are similar to previous works. Yet the new CT-imagery provides the first-ever direct insights (both qualitative and quantitative) into the internal evolution of lithospheric-scale rift models. Furthermore, this new and versatile modelling machine in combination with our CT-scanning abilities provides a broad range of opportunities for advanced future lithospheric-scale modelling studies. Figure: 3D CT image of an oblique extension model. UC: upper crust, LC: Lower crust, ULM: upper lithospheric mantle, LLM: lower lithospheric mantle, As: asthenosphere

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.

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.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.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.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.

During extension of the continental lithosphere, rift basins develop. These are often initially offset, and must interact and connect in order to create a continuous rift system that may ultimately achieve break-up. When simulating extensional tectonics and rift interaction structures, analogue and numerical modellers often apply a continuous extension rate along the strike of a rift or rift system. Yet in nature significant extension velocity variations occur along rifts and plate boundaries as a natural consequence of tectonic plates moving apart about a pole of rotation, resulting in rotational extension, and associated rift propagation and structural gradients. Here we present various analogue tectonic experiments to assess rift interaction structures forming in orthogonal extension settings versus rotational extension settings. Our modelling efforts show that rotational extension and orthogonal extension produce significantly different large-scale structures. Rotational extension can cause important variations in rift maturity between rift segments, delay rift interaction zone development, and make rift segments propagate in opposite directions. Still, local features in a rotational extension system can often be regarded as evolving in an orthogonal extension setting. Furthermore, we find that various degrees of rift underlap produce three basic modes of rift linkage structures. Low underlap distance (high angle φ) experiments develop rift pass structures. With increasing underlap distance (φ = ca. 40°), transfer zone basins develop. High degrees of underlap (φ ≤ 30°) tend to result in en echelon sub-basins. Our results match with data from previous modelling efforts and natural examples. We furthermore propose a large-scale tectonic scenario for the East African Rift System based on rotational extension and associated rift propagation. These insights may also be applicable when studying other large-scale rift systems.

The Afar region represents a unique opportunity for the study of ongoing rift development and the various phases of continental break-up. In this work we discuss the geological and geomorphological characteristics of the Western Afar Margin (WAM) and the various scenarios proposed for its evolution. A drastic decline in topography and crustal thickness from the Ethiopian Plateau into the Afar Depression, as well as a series of marginal grabens and a general presence of antithetic faulting characterize the WAM. Present-day extension is mostly accommodated at the rift axis in Afar, yet the margin is still undergoing significant deformation.Models for the evolution of the WAM involve either isostatic loading effects due to erosion, rifting-induced block rollover, large-scale detachment fault development or crustal flexure due to lithospheric stretching or magmatic loading. This wide variation of potential mechanisms for WAM development may reflect a general structural variation along the margin and in Afar, involving different stages of rift formation and possibly indicating two distinct pathways leading to continental break-up.In order to better understand the rifting mechanisms and to fully exploit the research potential of the region, further assessment of the WAM and its relation to Afar will be necessary. The findings of such future work, combined with data from rifts and passive margins from around the globe will be of great importance to assess the processes involved in continental breakup and to better constrain the sequence of events leading from initial rifting to break-up and oceanic spreading.

This multidisciplinary study focuses on the tectonics of the Western Afar Margin (WAM), which is situated between the Ethiopian Plateau and Afar Depression in East Africa. The WAM represents a developing passive margin in a highly volcanic setting, thus offering unique opportunities for the study of rifting and (magma-rich) continental break-up, and our results have both regional and global implications.Earthquake analysis shows that the margin is still deforming under a ca. E-W extension regime (a result also obtained by analysis on fault measurements from recent field campaigns), whereas Afar itself undergoes a more SW-NE extension. Together with GPS data, we see Afar currently opening in a rotational fashion. This opening is however a relatively recent and local phenomenon, due to the rotation of the Danakil microcontinent modifying the regional stress field (since 11 Ma). Regional tectonics is otherwise dominated by the rotation of Arabia since 25 Ma and should cause SW-NE (oblique) extension along the WAM. This oblique motion is indeed recorded in the large-scale en echelon fault patterns along the margin, which were reactivated in the current E-W extension regime. We thus have good evidence of a multiphase rotational history of the WAM and Afar.Furthermore, analysis of the margin’s structural architecture reveals large-scale flexure towards Afar, likely representing the developing seaward-dipping reflectors that are typical for magma-rich margins. Detailed fault mapping and earthquake analysis show that recent faulting is dominantly antithetic (dipping away from the rift), bounding remarkable marginal grabens, although a large but older synthetic escarpment fault system is present as well. By means of analogue modelling efforts we find that marginal flexure indeed initially develops a large escarpment, whereas the currently active structures only form after significant flexure. Moreover, these models show that marginal grabens do not develop under oblique extension conditions. Instead, the latter model boundary conditions create the large-scale en echelon fault arrangement typical of the WAM. We derive that the recent structures of the margin could have developed only after a shift to local orthogonal extension. These modeling results support the multiphase extension scenario as described above.Altogether, our findings are highly relevant for our understanding of the structural evolution of (magma-rich) passive margins. Indeed, seismic sections of such margins show very similar structures to those of the WAM. However, the general lack of marginal grabens, which are so obvious along the WAM, can be explained by the fact that most rift systems undergo or have undergone oblique extension, often in multiple phases during which structures from older phases control subsequent deformation.

Much of the future hydrocarbon exploration potential in the North Sea lies in locating stratigraphic traps and discrete reservoir intervals. This study assesses the potential for Lower Cre- taceous reservoirs, with particular focus on the Norwegian Central Graben and methods to identify future prospects over a wider area. Seismic interpretation and well data reveal the structure and sed- imentology of the study area. Although the region was isolated from a large hinterland in the Early Cretaceous, potential local sediment sources, sediment transport routes and areas with possible res- ervoir development are identified. The greater Mandal High area, where Lower Cretaceous shore- face deposits and submarine fan systems are postulated, is suggested for primary focus. Similar deposits may have developed around the other exposed highs in the region, although several were drowned towards the end of the Early Cretaceous. Detailed seismic and stratigraphic analysis will be necessary to identify individual reservoir units. Since comparable settings may have occurred in the adjacent South Viking Graben and Southern Permian Basin regions during the Early Creta- ceous, further reservoir assessment is recommended for the North Sea in general.

In rift settings, extension rates often vary along strike, due to rotation about a vertical axis or Euler pole, yet tectonic modelers traditionally apply constant along-strike deformation rates. Here we compare rift development and propagation under traditional orthogonal extension versus rotational extension conditions. The set-ups involve brittle-viscous layering and localize deformation through structural weaknesses (seeds). Our models provide first-order insights into the differences in rift development between both boundary conditions: orthogonal extension produces a rift basin with constant synchronous along-strike features, whereas rotational extension induces along-strike structural gradients, diachronous rift development causing rift propagation and the development of V-shaped basins. We observe important viscous flow associated with differential pressure gradients in rotational extension. We also describe the important effects of strain partitioning between rift axis and model boundaries, the quantifying of which is crucial to avoid incorrect model interpretations. Although our model results are first-order only, they are in good agreement with various natural examples and previous modeling studies and highlight the importance of considering the third dimension when studying tectonic systems.

The Afar region in East Africa is a key locality for studying continental break-up. Within Afar, passive margins are developing, of which the Southern Afar Margin (SAM) contains synthetic (basinward) faulting, whereas crustal flexure, antithetic faulting and marginal grabens occur along the Western Afar Margin (WAM). Numerous conflicting scenarios for the evolution of the WAM exist. In this analogue modelling study we test various factors that may affect the development of a WAM-style passive margin: brittle crustal thickness, (en echelon) rheological contrasts, sedimentation and oblique extension. Our experimental results illustrate how marginal flexure due to a weak lower crust below Afar can elegantly account for the structural features of the WAM. Brittle crustal thickness controls what structures occur: a thinner brittle crust accommodates flexure internally, whereas increasing brittle thicknesses lead to faulting. Large escarpment faults develop early on, followed by late-stage antithetic faulting and marginal grabens. A thicker brittle crust also causes enhanced subsidence, and increased strength contrasts between lower crustal domains leads to more localized deformation. Basin-wide sedimentation causes enhanced subsidence, as well as longer activity along large synthetic (escarpment) faults. Finally, oblique extension clearly prevents the development of marginal grabens, which only form in near-orthogonal extension. These results support a tectonic scenario involving initial oblique extension due to Arabian plate motion, creating echelon synthetic escarpment faults along the WAM. After the Danakil Block started its independent rotation, near-orthogonal extension conditions were established, allowing (enhanced) marginal flexure, antithetic faulting and marginal graben formation along the older en echelon escarpment. Differences in extension obliquity may also explain the differences in structural architectures between the WAM and SAM. The characteristics of the WAM are typical of magma-rich passive margins, and the margin has great potential for studying continental break-up and (magma-rich) passive margin formation.

The Afar region in East Africa represents a key location to study continental breakup. We present an integrated structural analysis of the Western Afar Margin (WAM) aiming to better understand rifted margin development and the role of plate rotation during rifting. New structural information from remote sensing, fieldwork, and earthquake data sets reveals that the N-S striking WAM is still actively deforming and is characterized by NNW-SSE normal faulting as well as a series of marginal grabens. Seismicity distribution analysis and the first-ever borehole-calibrated sections of this developing passive margin show recent slip concentrated along antithetic faults. Tectonic stress parameters derived from earthquake focal mechanisms reveal different extension directions along the WAM (82°N), in Afar (66°N) and in the Main Ethiopian Rift (108°N). Fault slip analysis along the WAM yields the same extension direction. Combined with GPS data, this shows that current tectonics in Afar is dominated by the local rotation of the Danakil Block, considered to have occurred since 11 Ma. Earlier stages of Afar development (since 31–25 Ma) were most likely related to the large-scale rotation of the Arabian plate. Various authors have proposed scenarios for the evolution of the WAM. Any complete model should consider, among other factors, the multiphase tectonic history and antithetic fault activity of the margin. The findings of this study are not only relevant for a better understanding of the WAM but also provide insights into the role of multiphase rotational extension during rifting and passive margin formation in general.

Continental rifts often develop from linkage of distinct rift segments under varying degrees of extension obliquity. These rift segments arise from rift initiation at non-aligned crustal heterogeneities and need to interact to develop a full-scale rift system. Here, we test the effects of 1) oblique extension and 2) initial heterogeneity (seed) offset on continental rift interaction with the use of an improved analogue model set-up. X-ray computer tomography (CT) techniques are used to analyse the 3D models through time and the results are compared with additional numerical models and natural examples. The experimental results reveal that increasing extension obliquity strongly changes rift segment structures from wide rifts in orthogonal settings to narrower rifts with oblique internal structures under oblique extension conditions to narrow strike-slip dominated systems towards the strike-slip domain. We also find that both decreasing seed offset and increasing extension obliquity promote hard linkage of rift segments through the formation of continuous rift boundary faults at the surface. (Initial) soft linkage through the formation of relay ramps is more likely when seed offset increases or extension is more orthogonal. Rather than linking at depth, the rift boundary faults curve around each other at depth and merge towards the surface to form a continuous trough. Orthogonal extension promotes the formation of intra-rift horsts, which may provide hydrocarbon traps in nature.

Strike-slip motion is a fundamental tectonic process around the globe often resulting in prominent surface expressions. The full reconstruction and comprehension of strike-slip dominated structures can be a major challenge due to the complexity of the associated fault patterns. Further complexities occur when strike-slip dominated structures in the foreland interact with thrust belt fronts. Starting from a well-studied natural example (the Sciacca Fault Zone in the Sicilian Channel, Italy) we ran a set of analogue models simulating the interaction between strike-slip dominated fault zones and thrust structures using quartz sand as the analogue material and X-Ray Computed Tomography as a technique to carry out a 4D analysis of model internal structures. During a first phase, a thrust belt was created in our models, which was subsequently affected by a perpendicular fault system that underwent either pure strike-slip, 10°-, 20°-, 30°-transtensional, or 10°-, 20°-, 30°-transpressional motion. In general, transpressional models form pop-up structures, while the pure strike-slip model develops one sub-vertical fault bounded by two converging reverse faults. The 10˚-transtensional model generates a set of Riedel shear faults, which merge during the later stages of deformation. This model also shows a positive elevation change along the strike-slip dominated structures near the intersection with the thrust belt. The 20˚-transtensional model contains some Riedel shear faulting as well, but is dominated by two normal faults with steep dip angles and some minor sub-vertical strike-slip faults in between. This fault architecture is also observed in our 30˚-transtensional model. Three different tectonic regimes interacting with pre-existing and newly formed thrust front led to very different structural settings. These results were of help in better defining the tectonic regime that controlled the strike-slip dominated part of the Sciacca Fault Zone, but they show a fair match with other natural examples as well.

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 subsidence 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.

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.