Onno Oncken

The impact of analogue material properties on the geometry, kinematics, and dynamics of convergent sand wedges

Abstract Simulation of geodynamic processes in sandbox experiments requires analogue materials with a deformation behaviour that reproduces the deformation mechanisms of typical crustal rocks. We present data on the frictional strength of different sand types employing static and dynamic shear tests. The sand types analysed are characterised by an elastic/frictional plastic mechanical behaviour with a transient strain-hardening and strain-softening phase prior to transition to stable sliding. This is in conflict with the standard assumption of an ideal cohesionless Coulomb-material with constant frictional properties. The influence of the identified transient material properties on the kinematics, growth mechanisms, and internal deformation patterns of convergent sand wedges results in characteristic wedge segments which vary—depending on material compaction—between wedges with well defined segments (i.e. frontal-deformation zone, frontal-imbrication zone and internal-accumulation zone) with straight slopes and wedges with a continuous convex topographic profile. For most materials, only the frontal part of the wedge is critical during experimental runs. Taper and strength of the wedge segments can be shown to be controlled by the frictional properties of active faults. Wedge segmentation is controlled by a bulk-strength increase toward the rear of the wedge due to fault rotation in mechanically less-favourable orientations and plastic material hardening. The limit between the frontal critical parts of a wedge and internal stable parts is largely controlled by a critical state of stress upon which either renewed failure or fault inactivation occurs. On this basis, we suggest that critical-taper analysis of wedges must be restricted to specific kinematic segments. Comparison of the experimental results with the Nankai accretionary wedge suggests that our interpretation also applies to natural convergent wedges. Moreover, we provide constraints for the selection of adequate granular analogue materials to simulate typical crustal rocks in natural convergent wedges.

Subduction Erosion — the “Normal” Mode of Fore-Arc Material Transfer along the Chilean Margin?

Subduction erosion shapes at least half of the world’s convergent margins. However, its rates, and modes as well as spatial and temporal variation are poorly understood. Based on a compilation of published and newly derived estimates of subduction erosion along the Chilean part of the Andean margin, we discuss possible loci and modes of subduction erosion and also address the potential of subducting topographic highs for accelerating subduction erosion.

Central and Southern Andean Tectonic Evolution Inferred from Arc Magmatism

Patterns of spatial distribution, and geochemical and isotopic evolution from subduction-related igneous rocks provide tools for scaling, balancing and predicting orogenic processes and mechanisms. We discuss patterns from two Andean key arc segments, which developed into fundamentally different types of orogens: (1) A plateau-type orogen with thick crust in the central Andes, and (2) a non-plateau orogen with normal crust in the southern Andes.

Subduction Channel Evolution in Brittle Fore-Arc Wedges — a Combined Study with Scaled Sandbox Experiments, Seismological and Reflection Seismic Data and Geological Field Evidence

With a series of scaled sandbox experiments, we investigated the mass-flux patterns at the interface of convergent plates, with emphasis on the upper (brittle) part of subduction channels. Analysis of the particle displacement field integrated over short time periods shows that both types of simulated subduction channels (accretive and tectonically erosive) are characterized by episodically active thrusts (roof thrusts) at the top and a continuously active basal detachment. The short-term material flux reveals a complex temporal and spatial variability in the active mass-transfer processes within the subduction channel, and is particularly influenced by the activity of fore-arc structures (e.g. reactivation of backthrusts or duplexes). In the subduction channel, the localization of deformation also shows temporal and spatial fluctuations, which range from periodic kinematic cycles to unpredictable, apparently chaotic behaviour involving the activation and reactivation of shear zones. However, the location of the roof thrusts and their reactivation pattern during the periodic cycles is indicative of either tectonically erosive or accretive mass-transfer modes. In contrast to the short-term observations, the longterm material flux integrated over one kinematic cycle exhibited diagnostic patterns for the location of sediment accretion and subduction erosion. The series of accretive experiments shows that the combination of several parameters (initial wedge thickness, absence/presence of upper-crustal structures, and depth-dependent softening of the top of the subduction channel) can cause the same bulk effect in the upper plate (i.e., the migration of the center of uplift as an indicator of the position of rearward accretion). This experimental result precludes the determination of controlling parameters in nature.