: We report a direct comparison of scaled analogue experiments to test the reproducibility of model results among ten different experimental modelling laboratories. We present results for two experiments: a brittle thrust wedge experiment and a brittleviscous extension experiment. The experimental set-up, the model construction technique, the viscous material and the base and wall properties were prescribed. However, each laboratory used its own frictional analogue material and experimental apparatus. Comparison of results for the shortening experiment highlights large differences in model evolution that may have resulted from (1) differences in boundary conditions (indenter or basal-pull models), (2) differences in model widths, (3) location of observation (for example, sidewall versus centre of model), (4) material properties, (5) base and sidewall frictional properties, and (6) differences in set-up technique of individual experimenters. Six laboratories carried out the shortening experiment with a mobile wall. The overall evolution of their models is broadly similar, with the development of a thrust wedge characterized by forward thrust propagation and by back thrusting. However, significant variations are observed in spacing between thrusts, their dip angles, number of forward thrusts and back thrusts, and surface slopes. The structural evolution of the brittle-viscous extension experiments is similar to a high degree. Faulting initiates in the brittle layers above the viscous layer in

[1] High-resolutionseismictomography and magnetotelluric (MT) soundings of the shallow crust show strong changes in material properties across the Dead Sea Transform Fault (DST) in the Arava valley in Jordan. 2D inversion results of the MT data indicate that the DST is associated with a strong lateral conductivity contrast of a highly conductive layer at a depth of approximately 1.5 km cut-off at a position coinciding with the surface trace of the DST. At the same location, we observe a sharp increase of P wave velocities from <4 km/s west of the fault to >5 km/s to the east. The high velocities in the east probably reflect Precambrian rocks while the high electrical conductivity west of the DST is attributed to saline fluids within the sedimentary filling. In this sense, the DST appears to act as an impermeable barrier between two different rock formations. Such a localized fluid barrier is consistent with models of fault zone evolution but has so far not been imaged by geophysical methods. The situation at the DST is remarkably different from active segments of the San Andreas Fault which typically show a conductive fault core acting as a fluid conduit. INDEX TERMS: 1515

We propose a conceptual model of frontal accretion within bivergent wedges, which is based on two‐dimensional sandbox simulations and the analysis of particle displacement fields. Each frontal accretion cycle consists of a thrust initiation, an underthrusting phase, and a reactivation phase. The location and magnitude of deformation within a bivergent wedge and its associated surface uplift vary systematically with the phase of the frontal accretion cycle and are thus predictable. Therefore the frontal accretion cycle can be considered as an internal clock for wedge‐scaled deformation and surface uplift. We further demonstrate that the geometry of the deformation front and the spatial distribution of surface uplift can be used to infer the currently active phase within a frontal accretion cycle. Surface uplift of the axial zone and the retrowedge may reach up to half of the thickness of the incoming sedimentary layer during a frontal accretion cycle. We estimate cycle duration to range between 104 and 105 years, which is thus similar to the time frame of climatic cycles. The proposed conceptual model provides an explanation for the commonly observed transience of deformation and surface uplift and for the discrepancy between geodetic, paleoseismologic, and geologic estimates of fault slip.

We compare recent magnetotelluric investigations of four large fault systems: (i) the actively deforming, ocean-continent interplate San Andreas Fault (SAF); (ii) the actively deforming, continent-continent interplate Dead Sea Transform (DST); (iii) the currently inactive, trench-linked intraplate West Fault (WF) in northern Chile; and (iv) the Waterberg Fault/Omaruru Lineament (WF/OL) in Namibia, a fossilized intraplate shear zone formed during early Proterozoic continental collision. These fault zones show both similarities and marked differences in their electrical subsurface structure. The central segment of the SAF is characterized by a zone of high conductivity extending to a depth of several kilometres and attributed to fluids within a highly fractured damage zone. The WF exhibits a less pronounced but similar fault-zone conductor (FZC) that can be explained by meteoric waters entering the fault zone. The DST appears different as it shows a distinct lack of a FZC and seems to act primarily as an impermeable barrier to cross-fault fluid transport. Differences in the electrical structure of these faults within the upper crust may be linked to the degree of deformation localization within the fault zone. At the DST, with no observable fault-zone conductor, strain may have been localized for a considerable time span along a narrow, metre-scale damage zone with a sustained strength difference between the shear plane and the surrounding host rock. In the case of the SAF, a positive correlation of conductance and fault activity is observed, with more active fault segments associated with wider, deeper and more conductive fault-zone anomalies. Fault-zone conductors, however, do not uniquely identify specific architectural or hydrological units of a fault. A more comprehensive whole-fault picture for the brittle crust can be developed in combination with seismicity and structural information. Giving a window into lower-crustal shear zones, the fossil WF/OL in Namibia is imaged as a subvertical, 14 km-deep, 10 km-wide zone of high and anisotropic conductivity. The present level of exhumation suggests that the WF/OL penetrated the entire crust as a relatively narrow shear zone. Contrary to the fluid-driven conductivity anomalies of active faults, the anomaly here is attributed to graphitic enrichment along former shear planes. Once created, graphite is stable over very long time spans and thus fault/shear zones may remain conductive long after activity ceases.

No abstract