Analogue Modelling of Inverted Oblique Rift Systems

The geometric evolution of brittle fault systems in inverted oblique and offset rift systems has been simulated using scaled sandbox analogue models. Dry fine-grained quartz sand was used to represent the brittle upper crust. Extensional faults geometries in the models were governed by the geometry and orientation of a stretching zone at the base of the models. Oblique rift models were characterized by segmented en-echelon border fault systems trending parallel to the rift axis and the underlying zone of basement stretching. Offset rift models promoted highly-segmented border faults as well as offset sub-basins within the rift. In both types of models, intra-rift fault arrays were oriented sub-perpendicular to the extension direction. Inversion of the oblique and offset extensional models was achieved by horizontal shortening. This resulted in partial inversion of the border and intra-rift faults as well as the formation of new reverse faults. The geometries, distribution, orientations and number of these new reverse faults were strongly controlled by the earlier-formed fault extensional architectures. At the margins of the rift zone, shortening was mainly accommodated by partial inversion of the border faults together with the formation of hanging-wall bypass faults and footwall shortcut thrusts. Inversion of the offset rift models produced reactivation of the extensional accommodation zones as soft-linked transfer zones between new thrust faults. The analogue model results have been compared with natural inversion structures in the Atlas Mountains of Morocco and the Ukrainian Donbas fold belt. The analogue modelling results suggest that the High Atlas formed as the result of oblique inversion of an oblique rift system, and the contractional structures in the Ukranian Donbas belt were generated by partial inversion of the earlier-formed Donbas extensional graben via two major newly developed short-cuts that uplifted and exhumed the basin

Weld kinematics of syn-rift salt during basement-involved extension and subsequent inversion: Results from analog models

Scaled analog models based on extensional basins with synrift salt show how basement topography exerts a control factor on weld kinematics during the extension and inversion phases. In the case of basement-involved extension, syn-rift salt thickness differences may lead to variable degrees of extensional decoupling between basement topography and overburden, which in turn have a strong impact on the development of salt structures. With ongoing extension and after welding, the basin kinematics evolves toward a coupled deformation style. The basin architecture of our experimental results record the halokinetic activity related to growing diapirs and the timing of weld formationduring extension. Moreover, the structures that result from anysubsequent inversion of these basins strongly depends on the inherited welds and salt structures. While those basins are uplifted,the main contractional deformation during inversion is absorbed by the pre-existing salt structures, whose are squeezed developing secondary welds that often evolve into thrust welds. The analysis of our analog models shows that shortening of diapirs is favored by: i) basement topography changes that induce reactivation of primary welds as thrust welds; ii) reactivation of the salt unit as a contractional detachment and iii) synkinematic sedimentation during basin inversion. Finally, in this article, we also compare two natural examples from the southern North Sea that highlight deformation patterns very similar to those observed in our analog models

Role of pre-existing topography and overburden on strain-partitioning of oblique doubly-vergent convergent wedges

Scaled sandbox modeling was used to analyze the three- dimensional aspects of strain partitioning along obliquely convergent margins that underwent a subduction polarity reversal event. In the experiments, the first phase of shortening produced a pre-existing topography that affected wedge kinematics during the second phase. Increasing degrees of obliquity during the first shortening phase produced different amounts of inherited topography and overburden, giving way to different effects on wedge response during the second phase of shortening. During the second phase, the models with an inherited heavy overburden show an anomalous orientation for R shears predating the development of a margin-parallel strike-slip fault. These shears are oriented at an angle that is higher than kinematically predicted with respect to the subduction slot, suggesting a transient rotation of the stress applied by the obliquity of subduction. In contrast, experiments with a lighter overburden do not show this inconsistency. In addition, all the models displayed an unexpectedly high accretion rate during the second phase of shortening, which we interpret to be dependent on topographical slope breaks inherited after the first phase of shortening and the critical asymmetrical architecture of doubly-vergent wedges

The effect of tectonic overloading on strain partitioning of doubly-vergent Coulomb wedges: a 2D-3D sand-box modelling approach

The preliminary results of a sand-box experimental program are presented in this paper. The aim of the study is to analyse the effect of a pre-existing load and wedge taper angle on the development and strain partitioning of a doubly-vergent Coulomb wedge. By simulating a subduction polarity reversal, two distinct and interacting thrust systems with opposite vergence were produced, and their relationship evaluated. The presence of previously built topography strongly alters the way in which deformation is distributed inside the wedge. In particular, 2D models indicate that the axial zone inherited from the first phase of contraction diminishes the block uplift concentrated above the step-up shear (or retro-thrust) during the second phase. Therefore, a high accretion rate characterises the initial stages of deformation. In 3D, this accretion produces an anomalously wide prowedge compared to theoretical model predictions for highly oblique convergent settings. Moreover, the whole wedge is not characterised by total strike-slip partitioning on a single fault

Effect of unbalanced topography and overloading on Coulomb wedge kinematics: Insights from sandbox modeling

This study addresses the effect of variable unbalanced topography and overload on the kinematics of a fold and thrust belt developed within a collisional belt that underwent a subduction polarity reversal event. This was done by physical modeling of doubly vergent Coulomb wedges, using sand as an analogue material. During the experimental procedure a preexisting topography was generated by a first phase of subduction in one direction. A second phase of subduction was then initiated in the opposite direction ( simulating a subduction flip). An anomalous, strong frontal growth for the wedge during the second phase was experimentally shown to be dependent upon surface slope breaks and the critical asymmetrical architecture achieved by the wedge at mature stages of deformation. The latter is suggested to be the rule for doubly vergent orogens at steady state, even after a subduction flip. During the experiments, surface processes, like syntectonic erosion and sedimentation, markedly altered mass transfer within the wedge. In particular, lowering the surface slope by syntectonic erosion favored cycling between accretion and underthrusting modes. By contrast, a sudden syntectonic sediment load in the prowedge region promoted prolonged phases of underthrusting, retarding accretion of new imbricates at the prowedge toe. However, at later stages of deformation the prowedge was forced to regain its characteristic minimum critical taper as predicted by theory and did so by the sudden nucleation of long, flat thrust units, rapidly rebalancing the asymmetry between prowedge and retrowedge regions. The experiments suggest that given a fixed critical taper related to time-invariant frictional properties along the wedge bounding surfaces, deformation does vary within the wedge according to time-varying location of normal stress surpluses and unbalanced topography acting on potential failure surfaces. These, in turn, alter the equilibrium between prowedge and retrowedge basal detachments that is here considered to be a major factor controlling the self-regulating dynamics of collisional orogens

Unbalanced topography. Localised denudation/sedimentation and dynamic strain partitioning in 3D experimental Coulomb wedges: a synopsis.

In this paper, sandbox modelling was used to investigate the dynamic effect of growing topography on the deformation mechanics of accretionary systems. High obliquity was chosen as the main boundary condition to analyse 3D strain partitioning of a doubly-vergent Coulomb wedge, for the particular case in which a subduction polarity reversal event affected wedge development. In the experiments, the first phase of shortening produces a pre-existing topography affecting wedge kinematics during the second phase. Different degrees of obliquity set for the first phase produced different amounts of inherited topography, thus giving way to variable effects on wedge response to shortening during the second phase. In addition, the elevation potential during second phase could be varied in the models via sequential events of syn-tectonic denudation and/or sedimentation, performed on distinct sectors of the deforming wedge. Experimental results suggest that both timing and mode of retrothrust activation are crucial factors in influencing critical wedge mechanics as a whole. These may control localisation of compressive stress surpluses in the axial region, thus temporarily suppressing margin-parallel total strike-slip partitioning. Differential disequilibrium characterizing the topographic profile with respect to a given basal/internal coefficients of friction ratio will have a strong impact on the wedge growth history. Moreover, the parallelism between the imbricates at the thrust front and the strike-slip fault at the rear of the prism predicted by theoretical models is valid only at steady-state, when failure conditions exist everywhere at the basal d\ue9collement. Before this stage, uneven mechanical conditions at the base of the prowedge region produced an irregular distribution of the velocity fields within the deforming wedge, which caused, in turn, non-collinear mass transfer vectors between the prowedge and retrowedge regions. This led to unexpected wedge behaviour during which superficial extension or compression, both located in the axial zone, predated the full-development of a strike-slip fault

Complex faulting sequences controlled by dynamic topography in 3D experimental deformation of doubly-vergent coulomb wedges

In this paper sandbox modelling was used to investigate the dynamic effect of localised syntectonic erosion and\or sedimentation on the deformation mechanics of accretionary systems. High obliquity was chosen as the main boundary condition to analyse 3D strain partitioning of a doubly-vergent Coulomb wedge for the particular case in which a subduction po1artv reversal event affected wedge development. In the experiments the first phase of shortening ( P1) produces a preexisting topography affecting wedge development during second phase ( P2). The elevation potential can be varied in the models via sequential events of syntectonic denudation and/or sedimentation performed on distinct sectors of the deforming wedge. Experimental results suggest that the parallelism between the imbricates at the thrust front and the strike-slip fault at the rear of the prism predicted by theoretical models is valid only at steady-state when failure conditions exist everywhere at the basal d\ue9collement. Before this stage different velocity fields characterise mass transfer in distinct sectors leading to unexpected wedge behavior during which superficial extension or compression, both located i n the axial zone, predate the full development of a strike-slip fault

Analogue Modelling of Inverted Oblique Rift Systems

The geometric evolution of brittle fault systems in inverted oblique and offset rift systems has been simulated using scaled sandbox analogue models. Dry fine-grained quartz sand was used to represent the brittle upper crust. Extensional faults geometries in the models were governed by the geometry and orientation of a stretching zone at the base of the models. Oblique rift models were characterized by segmented en-echelon border fault systems trending parallel to the rift axis and the underlying zone of basement stretching. Offset rift models promoted highly-segmented border faults as well as offset sub-basins within the rift. In both types of models, intra-rift fault arrays were oriented sub-perpendicular to the extension direction. Inversion of the oblique and offset extensional models was achieved by horizontal shortening. This resulted in partial inversion of the border and intra-rift faults as well as the formation of new reverse faults. The geometries, distribution, orientations and number of these new reverse faults were strongly controlled by the earlier-formed fault extensional architectures. At the margins of the rift zone, shortening was mainly accommodated by partial inversion of the border faults together with the formation of hanging-wall bypass faults and footwall shortcut thrusts. Inversion of the offset rift models produced reactivation of the extensional accommodation zones as soft-linked transfer zones between new thrust faults. The analogue model results have been compared with natural inversion structures in the Atlas Mountains of Morocco and the Ukrainian Donbas fold belt. The analogue modelling results suggest that the High Atlas formed as the result of oblique inversion of an oblique rift system, and the contractional structures in the Ukranian Donbas belt were generated by partial inversion of the earlier-formed Donbas extensional graben via two major newly developed short-cuts that uplifted and exhumed the basin