The presence of pre-existing rheological heterogeneities in the lithosphere plays a significant role during subsequent stages of deformation in essentially every geological process. Extensional basins located in foreland fold-and-thrust belts will alter the spatio-temporal evolution of its associated orogen. It remains unclear how far horizontal stresses can act and reactivate extensional structures due to their intrinsic irregular patterns of deformation deflection and localisation. Overprinting events and relative dating uncertainties in the geological record make it difficult to interpret how stresses were transferred across a heterogeneous crust. Here we examine the inversion of extensional basins in foreland fold-and-thrust belts by using three-dimensional analogue experiments that simulate first an extensional stage, followed by a shortening stage. Our results show how extensional basins proximal to the orogenic front effectively localise deformation in the shape of thrusts and prevent stress transfer beyond their location. Basins that are located at large distances from the orogenic front also show evidence of mild inversion at early stages but are characterised only by basin infill contraction and uplift. When multiple extensional basins are present, the degree and type of inversion will depend primarily on their relative location and distance to the orogenic front. Here we also prove that the presence of additional extensional features in the vicinity of a basin can be a first-order controlling factor in their overall reactivation history. We share additional insights of how a fold-and-thrust belt evolves once the extensional basins have been incorporated by the advancing wedge, and we provide comparisons with natural examples that shed light on some still unanswered questions related to the process of basin inversion in orogenic belts.

A series of influential papers in the 1980’s showed how the long-term evolution of fold-and-thrust belts and accretionary wedges (here collectively termed orogenic wedges) can quantitatively be described as striving towards a mechanical equilibrium defined by their internal and basal material strengths (Dahlen 1984, Dahlen et al. 1984, Davis et al. 1983). Unstable orogenic wedges will deform to adjust their basal and surface slopes to a critical taper angle, defined by a wedge that is at the verge of failure everywhere. Critical taper theory has been confirmed by analogue and numerical experiments and found numerous successful applications in field studies.The success of critical taper theory forms a framework that allows investigating non-critical behaviour of orogenic wedges. Previous numerical and analogue studies pointed out that: (1) Only portions of orogenic wedges may be at failure at any given time, separating critically stressed from non-critical segments (Lohrmann et al. 2003, Simpson 2011). As these wedges still observe a critical taper, this may indicate that it is the critically stressed segments that define the overall wedge shape. (2) Numerical experiments often attain a critical taper at lower shortening percentages than analogue experiments. We speculate that this may be related to larger amounts of strain softening generally used in numerical setups and/or the number of shear zones that forms at equivalent shortening (which is controlled by numerical resolution and analogue material properties). This non-criticality is thus likely only a transient state.We here ask the question whether structural inheritance from earlier compressional or extensional deformation phases may lead to longer-term non-critical wedge behaviour by favouring out-of-sequence thrusting or shear zone propagation into the foreland. To address this question, we combine a review of previous dynamic wedge experiments with new analogue experiments that investigate the influence of inherited shear zones and variations in material properties on wedge evolution. We shorten quartz sand layers overlying a weak basal microbeads layer with a non-deformable backstop. The backstop has two independently moving parts, allowing to alternate thin- and thick-skinned deformation. We find that the reactivation of basement shear zones formed in earlier deformation phases is short-lived and does not affect thrusting to a degree that would distinguish these wedges from those without inheritance. We extend these experiments by including variations in internal material properties and weaker shear zones, remaining however in the domain of brittle orogenic wedges.Non-critical wedge behaviour may only be a transient state, but could occur frequently owing to variations in material properties or structural inheritance, which are to be expected in regions of inter-plate shortening of former rift regions. Our contribution hopes to highlight the potential for future modelling studies of orogenic wedges to examine how non-critical wedge behaviour could play into the evolution of fold-and-thrust belts and accretionary wedges.

When orogeny reactivates extensional structures or uplifts pre-existing depocenters in the foreland (inversion), the overall nature, dimension, and geometry of these rheological heterogeneities represent one of the main controlling factors in the spatio-temporal evolution of foreland fold-and-thrust belts. Relationships between inversion structures in the foreland and far-field stresses caused by orogenic fronts have long been identified (e.g., Ziegler, 1989, Geol. Soc. Spec. Publ. 44). However, conditions that facilitate or hinder basin inversion in these settings remain unclear, mainly due to the intrinsic complexity of analysing multiple overprinted geological events.We use novel laboratory experiments of basin inversion to investigate how compressional stresses are transferred across a heterogeneous crust. More specifically, we determine how the presence of multiple extensional basins in the foreland controls the location, occurrence, and sequencing of foreland thrusts. Quantitative analysis of our experiments allows us to define conceptual models for comparison and application to natural examples where geological interpretation remains partially conjectural due to their intrinsic complexity, such as the permo-carboniferous troughs beneath the Swiss Molasse basin or the inverted Broad Fourteens Basin in the North Sea.Our experiments are built in a modelling apparatus with a mobile backstop, using quartz sand to model brittle crustal materials and glass microbeads to simulate a weaker basal detachment layer. Velocity discontinuities at the base are created by attaching multiple thin basal sheets to the mobile wall during extensional phases (pulling). The location of each extensional basin is defined by the lengths of the basal sheets. During extension, the resulting graben-like structures are progressively filled with microbeads to create a sedimentary infill that is less competent than the surrounding rock. The basal sheets are completely detached from the mobile wall before the initiation of the shortening phase (pushing). Topography, surface and lateral deformation is quantified employing a high-resolution particle imaging velocimetry (PIV) system.We present results of shortening multiple extensional basins at fixed distances from the orogenic front. Detailed analysis shows that extensional basin faults are not reactivated during shortening, but instead inversion is characterised by an initial squeezing of the basin fill and subsequent formation of either frontal or back thrusts that localise along the microbead-sand interface, leading to the overall uplift of the basins. This mechanism occurs independent of the distance of the basin to the orogenic front. However, when several grabens are present, the extent of shortening that each extensional structure localises differs greatly between experiments, showing variability according to the number of basins and their distance to the orogenic front.When compared to reference models with a homogeneous crust, our results show that the presence of multiple extensional basins in the foreland exerts a first-order control on the evolution of propagating fold-and-thrust belts. Thrust location and sequencing evolve differently, with frontal thrusts developing along pre-existing basins boundaries at early stages, and subsequent stages of back thrust formation characterising wedge thickening at the hinterland of the extensional basins.

During rifting, pre‐existing penetrative basement fabrics can affect new faults in cover rocks by a mechanism that does not appear to involve reactivation. This subtle form of inheritance can significantly impact fault network architecture in rift basins above laterally variable basement domains with geomechanical anisotropies. Here we use multi‐layer, brittle‐ductile, crustal‐scale analogue experiments to study the influence of penetrative basement anisotropies on fault patterns in the overlying cover during a single phase of orthogonal rifting. The experiments were designed to test whether basement anisotropies, oriented 45° to the extension direction, can lead to the formation of rift faults that are oblique to both the imposed extension direction and basement anisotropies. Our experiments show that a penetrative, vertically layered, mm‐wide basement anisotropy creates extension‐oblique faults in the overlying cover. We interpret this to arise when local strike‐slip kinematics along the interfaces of mechanically contrasting materials in the basement combine with the regional imposed orthogonal extension, creating a transtensional regime. The width and spacing of alternating “strong” and “weak” basement zones interact with rift kinematics, impacting the orientation, kinematics and spacing of new faults in the cover. New insights on the influence of penetrative, pre‐existing basement fabrics on localized re‐orientation of 3D strain in the cover have implications for understanding complex fault systems in rift basins and transfer zones.

Shortening in fold-and-thrust belts can be accommodated with little or substantial basement involvement, with the former, thin-skinned, style arguably being the more common (Pfiffner, GSA Special Paper, 2006). Experimental studies on thin-skinned fold-and-thrust belts have confirmed critical taper theory and have highlighted the roles of bulk rheology, embedded weak layers, décollement strength, and surface processes in structural evolution. However, analogue models of thick-skinned fold-and-thrust belts are less common, which may be related to practical challenges involved in shortening thick layers of brittle materials. Here we focus on basement fault reactivation, which has been suggested for several fold-and-thrust belts, such as the Swiss Alps, the Laramide belt in North America and the Sierras Pampeanas in South America, which show evidence of deep-rooted thrust systems, pointing to a thick-skinned style of shortening. Within an orogenic system, the shortening style may change between thin- and thick-skinned in space (foreland to hinterland) and time. This raises the question how inherited structures from one shortening phase may influence the next. We aim to use analogue experiments of multi-phase shortening to discuss the effects of deep-seated shortening-related inherited structures, such as thrusts and basement topography, on the structural evolution of fold-and-thrust belts. We employ a push-type experimental apparatus that can impose shortening in both thick- and thin-skinned style. The device has two independently moving backstops, permitting to change between these shortening styles over time, allowing the simulation of multiple contractional scenarios. We start with an initial stage of thick-skinned shortening, followed by either thin- or thick-skinned reactivation. We use quartz sand to simulate crustal materials and microbeads for embedded weak (sedimentary) layers. Surface and lateral strain, as well as topography, is quantified using a high-resolution particle imaging velocimetry and digital photogrammetry monitoring system. We will present preliminary results of this innovative experimental approach with the objective of discussing to what extent pre-existing conditions in the basement control the geometric, kinematic, and mechanical evolution of thick-skinned and basement-involved thin-skinned tectonics. In this presentation, we hope for a discussion of mechanisms of localisation of shortening in brittle analogue models, of sequences of thin- and thick-skinned deformation expected during multi-phase shortening, and comparisons to ongoing research and natural observations. Questions we aim to discuss are: Can weaknesses and anisotropies within the basement influence and control later structural evolution? Are pre-existing structures, such as thrusts or shear zones within the basement, responsible for subsequent fault nucleation, thin-skinned folding or basement uplift? What role does the rheology of the basement-cover interface play in the reactivation of basement thrusts? Can we model these reactivations with an analogue setup?

The Red Sea represents an incipient ocean basin that is in transition from continental rifting to ocean initiation. In this paper we assess how pre-existing mantle and crustal heterogeneities within the African and Arabian lithospheres influenced rift development and propagation using three-dimensional (3D) brittle-ductile analogue laboratory experiments with a rotational boundary condition. Here we investigate the effect of three different configurations: (i) pre-existing rheological heterogeneities in the crust, (ii) in the lithospheric mantle, and (iii) in both. The experimental outcomes show that a linear weak zone in the lithospheric mantle layer exerts a first-order control on the overall mechanical behaviour of extending lithosphere, rift architecture and style of rifting. Crustal heterogeneities control second-order features, such as the trends of the main boundary escarpments along the rift axis. Our experiments reproduce the geological history of the Red Sea and provide key insights on its deformation history in space and time. We provide, for the first time, 3D dynamic constraints on how pre-existing weak zones in the lithosphere have controlled the temporal evolution of the Red Sea and determined its distinctive present-day morphology. The results find that certain morphological elements of the Red Sea can only be explained by the combined effects of crustal and mantle lithosphere heterogeneities. When applied to other natural examples, our findings provide significant new insights on the role of crustal and lithospheric heterogeneities during continental rifting.

Improvements in seismic imaging, computing capabilities, and analytical methods, as well as a number of industry deep-water wells sampling distal offshore settings, have underpinned new concepts for rifted margin evolution developed in the last two decades; these mark significant progress in our understanding of extensional systems. For example, the tectonic, sedimentary, and magmatic processes linked to the formation of rifted margins have been overhauled, giving rise to more quantitative approaches and new concepts. However, these processes cannot be understood in isolation, requiring consideration of the continuum in which inheritance and physical processes are integrated within a plate tectonic framework. The major progress and fundamental developments of past research in rifted margins have been made hand-inhand with other domains of Earth Sciences and have fundamental implications for the understanding of key geological systems such as active rifts, the ocean lithosphere, subduction zones, and collisional orogens. The "IMAGinING RIFTING" workshop, organized in Pontresina-Switzerland in September 2017, gathered researchers from all disciplines working on rifts and rifted margins, and included participants from academia and industry. This contribution summarizes the workshop discussions, in addition to outlining our state-of-the-art knowledge of rifted margins. We highlight future challenges in unraveling the processes and conditions under which these extensional systems form and, ultimately, how tectonic plates rupture and new oceans are born. Our aims here are to provide a framework for future research endeavors and to promote collaboration not only within the rift and rifted margins communities, but across other Earth Science disciplines.

During rifting, pre-existing penetrative basement fabrics can affect new faults in cover rocks by a mechanism that does not appear to involve reactivation. This subtle form of inheritance can significantly impact fault network architecture in rift basins above laterally variable basement domains with geomechanical anisotropies. Here we use multi-layer, brittle-ductile, crustal-scale analogue experiments to study the influence of penetrative basement anisotropies on fault patterns in the overlying cover during a single phase of orthogonal rifting. The experiments were designed to test whether basement anisotropies, oriented 45° to the extension direction, can lead to the formation of rift faults that are oblique to both the imposed extension direction and basement anisotropies. Our experiments show that a penetrative, vertically layered, mm-wide basement anisotropy creates extension-oblique faults in the overlying cover. We interpret this to arise when local strike-slip kinematics along the interfaces of mechanically contrasting materials in the basement combine with the regional imposed orthogonal extension, creating a transtensional regime. The width and spacing of alternating “strong” and “weak” basement zones interact with rift kinematics, impacting the orientation, kinematics and spacing of new faults in the cover. New insights on the influence of penetrative, pre-existing basement fabrics on localized re-orientation of 3D strain in the cover have implications for understanding complex fault systems in rift basins and transfer zones.

Pre-existing structures in the crust such as shear zones, faults, and mobile belts are known to exert a significant control on the structural evolution of continental rifts. However, the influence of such features when the extension direction progressively changes over time remains uncertain. Here we present new results from three-dimensional lithospheric-scale laboratory experiments of rotational extension that provide key insights into the temporal evolution of propagating rifts. We specifically test and characterize how rifts propagate and interact with linear crustal rheological heterogeneities oriented at variable angles with respect to the extension direction. Results show that approximately rift-parallel pre-existing heterogeneities favor the formation of long, linear faults that reach near-final lengths at early stages. Low angles between the heterogeneities and the propagating rift axis may result in strong strike-slip reactivation of the pre-existing structures if they are suitably oriented with respect to the stretching direction. When the linear heterogeneities are oriented at intermediate to high angles rift branches become laterally offset as they propagate, resulting in complex rhombic fault patterns. Rift-perpendicular crustal heterogeneities do not affect fault trends during rift propagation, but cause stalling and deepening of laterally growing rift basins. Similarities between the analogue experimental results and selected natural examples provide insights on how nature finds the preferential pathway to breakup in heterogeneous continental lithosphere.

Microcontinents occur outboard of passive margins and stranded in ocean basins. Three-dimensional analogue laboratory experiments of continental rifting demonstrate that microcontinent formation at passive margins requires a combination of preexisting linear weaknesses in the lithosphere and rotational extension. Our results suggest that separation of microcontinents from passive margins occurs during the latest stages of continental breakup, before the onset of seafloor spreading, and that preexisting lithospheric weaknesses are a first-order control on where they form. These findings suggest that microcontinent formation may be restricted to localized regions along passive margins associated with zones of lithospheric weakness, providing a new structural and tectonic framework for the interpretation of microcontinents in the geological record.

When continents are pulled by moving lithospheric plates they break into pieces that drift apart to form new oceans. This is an integral part of the theory of plate tectonics, but many aspects of this process are still poorly understood. Using different materials, I physically simulate geological processes that last millions of years in nature in the laboratory. Comparison of my models with nature allows me to better understand how and why continents break up, with particular interest in cases when such continents are internally heterogeneous from a mechanical point of view.

The lateral propagation of rifts is a consequence of the relative divergence of lithospheric plates about a pole of rotation. Modern and ancient examples of rifts are known to overprint preexisting linear anisotropies in the crust and lithosphere, such as lithospheric boundaries, crustal sutures, and thermal anomalies. Here we investigate how propagating rifts interact with preexisting structures by using three-dimensional analogue experiments with rotational extensional boundary conditions and variably oriented linear weak zones in the lithospheric mantle. When linear weaknesses are oriented at low angles to the rift axis, early strain localization occurs in narrow domains, which merge at later stages, resulting in continental breakup by unzipping. Strong strain partitioning is observed when the linear heterogeneity is oriented at high angles with respect to the rift axis. In these experiments, early subparallel V-shaped basins propagate toward the pole of rotation until they are abandoned and strain is transferred entirely to structures developed in the vicinity of the strongly oblique weak lithosphere zone boundary. The experimental results are characterized in terms of their evolution, patterns of strain localization, and surface topography as a function of the lithospheric heterogeneity obliquity angle. Comparison of the experiments to ancient and modern examples in nature may help to elucidate the common but still poorly understood process of propagating rift-lithospheric heterogeneity interaction.

Combined field and gravimetric-magnetic data reveal a complex pattern of extensional structures superimposed to the late Carboniferous - Early Permian Gondwanan orogen at the inner sectors of the Southern Central Andes, in the westernmost part of the Chos Malal fold and thrust belt at the cordillera del Viento area. W-NW, NW basement structures of regional significance, segmented by minor NE structures are bounding Late Triassic depocenters and structural highs corresponding to the cordillera del Viento rifting, equivalent to the Precuyo cycle. A pattern of roughly N-S trending structures recognized in the field associated with Andean thrusts do not show evidences of previous structural controls, as they cut the magnetic anomalies. Field observations show that W-NW, NW and NE normal structures control changes in Late Triassic sedimentary thicknesses and are associated with synextensional geometries. Our model indicates that Late Triassic rifting in the area would have had a regional W-NW to NW trend being segmented by minor NE structures. Both sets were reactivated during Andean times, acting W-NW and NW structures as transfer zones between decoupled contractional panels and NE structures as frontal contractional structures. N-S contractional structures did not respect rifting architecture cutting through the depocenters and occasionaly exhuming synextensional geometries.