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The Cascadia convergent margin is a first‐order research target to study the impact of rapid sedimentation processes on the mechanics of frontal subduction zone accretion. The near‐trench part of the accretionary prism offshore Washington is affected by strongly increased glacial age sedimentation and fan formation that led to an outstanding Quaternary growth rate with landward vergent thrust faulting that is rarely observed elsewhere in accretionary wedges. Multichannel seismic reflection data acquired on the ORWELL project allows us to study the structure and dynamics of the atypical frontal accretion processes. We performed a kinematical and mechanical analysis of the frontal accretion structures, and developed a dynamic Coulomb‐wedge model for the landward‐verging backthrust formation. Backthrusting results from heterogeneous diffuse strain accumulation in the mechanically heterogeneous Cascadia basin sediment succession entering the subduction zone, and strain partitioning along a midlevel detachment that is activated by gravitational loading caused by rapid glacial age sedimentation. These complex deformation processes cause the passive “upward” delamination of the upper turbidite beds from the basal pelagic carbonate section similar to triangle‐zone formation and passive backthrust wedging in foreland thrust belts caused by rapid burial beneath syntectonic sediment deposits. The deformation mechanism at the tectonic front of the Cascadia margin is an immediate response to the strongly increased late Pleistocene sediment flux rather than to atypical physical boundary conditions as generally thought.

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Please cite this article as: Frank Zwaan, Guido Schreurs, Jürgen Adam , Effects of sedimentation on rift segment and transfer zone evolution in orthogonal and oblique extension settings: Insights from analogue models analysed with 4D X-ray computed tomography and digital volume correlation techniques. AbstractDuring the early evolution of rift systems, individual rift segments often develop along preexisting 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 distance between rift segments on rift interaction structures. 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 transfer zone evolution in orthogonal and oblique extension settings, by means of 3D brittleductile analogue models, analysed with 4D X-ray computed tomography (XRCT or CT) methods and 3D digital volume correlation (DVC) techniques.Our models show that syn-rift sedimentation does not significantly influence the large-scale evolution of rift and transfer zone structures. 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 ACCEPTED MANUSCRIPT extension promotes the formation of oblique rift-internal structures (Fig. 4c).The above-mentioned general features also occur in models with syn-rift sedimentation (Fig. 4g-j). However, the regular filling of the rift basins results in significantly less pronounced rift topography. In models without sedimentation the seed below the rift segments rises towards the model surface as is visible after sand removal ( Fig. 4d-e). In models with sedimentation the same "diapirism" occurs but an additional trough structure develops within the seed, which continues partially into the propagating rift branch (Fig. 4i, j). CT-analysisThe use of X-Ray CT techniques allows a more detailed analysis of our models (Figs. 5-7).Almost immediately after experiment initiation faults localize above the seed (Figs. 6d, j, 7d, ACCEPTED MANUSCRIPT