Scale-related strain evolution at convergent margins and effects due to parameter changes - insights from nature and experiment
Scaling relations of deformation
By means of statistics and geostatistics, we can quantify characteristic scale lengths of active structures and their typical duration for the regional scale, which are on the order of 150-200 km width and 200-300 km length and 2-4 Ma, respectively. On the orogen scale, the scale lengths are multiples of these values, as the orogen scale represents a summary of the active structures on the regional scale, being adjacent and coevally active. These scale lengths are artefacts resulting from the current resolution of the data set.
In spite of such an unusually well resolved data set on strain accumulation both in space and time from nature, we cannot conclusively distinguish the varying deformation frameworks, possibly due to a still insufficient data resolution. Depending on the applied resolution, we might unintentionally integrate data from different scales, so that their original strain pattern cannot be identified. It is nevertheless likely that dominant frameworks alternate over time. The strain distribution pattern does not provide conclusive information on the underlying deformation mechanisms. For example, both strain weakening and strain hardening can affect a deformation system that is basically fractal. Such deformation modes can coincide with the different deformation frameworks. Both likely alternate in space and time and so does their effect on different scales. Generally, the lack of highly resolved data precludes the identification of the respective patterns and deformation modes.
Analogue modelling (plateau formation)
Furthermore, we analyzed the effect of both system intrinsic and external parameters on the resulting strain pattern for the above mentioned scales. In a first experimental series that simulates only the upper crust using granular media, we show that deformation patterns are controlled by mechanical heterogeneities. Threshold values exist for coupled parameters of both basal (20%) and internal (35%) strength contrasts, which determine if either wedge-like or plateau- style settings will result. These threshold values indicate the absence of gradual transitions between the two end members. We were also able to reproduce the spatiotemporal strain evolution of the Central Andean plateau. Yet, the controlling parameter combination in the analogue models is very different from those proposed for the natural example. This indicates that a resulting strain pattern is possibly explained by more than only one parameter combination. Thus, we cannot conclusively infer information from the resulting picture on the controlling factors to relate causes and effects.
Analogue modelling (plateau initiation)
A second analogue series simulates the upper crust to the asthenosphere to study the crucial parameters that have to be met in a young system to initiate the plateau formation. A plateau is initiated when two anticlinal hinges enclose an undeformed basin, which is subsequently drained. This is bound to a critical lateral strength contrast within the system combined with a curved geometry. Furthermore, a normal density profile is required, as well as decoupling of the upper crust deformation from layers below.
Comparing three experiments of plateau initiation clearly shows that controlling factors can be of first or second order. The first order parameters actively change the resulting deformation patterns on the orogen scale (e.g., plate geometry) whereas the second order factors become effective only on the next smaller “regional” scale (e.g., additional decoupling horizons), while the orogen scale pattern remains the same. Thus, the effect of controlling parameters is scale- dependent.