PLATEAU FORMATION IN A SANDBOX – MODELLING THE ANDEAN STRAIN EVOLUTION

Our present knowledge still lacks details on the distribution of strain accumulation and its variations on different temporal and spatial scales. Insights into scaling properties of the lithosphere are fundamental for understanding issues ranging from crustal long-term deformation to seismogenic hazard probability. For this reason, we examine multiscale strain patterns for the Central Andes (17°-23°S) both in nature and in analogue models by means of geostatistical methods such as variogram and fractal analysis.

As a first result, we show that the long-term deformation sampled at intervals of 1 Ma suggests active deformation periods of 3-5 Ma with significant fluctuations in its spatial extent, whereas a bigger sample interval of >5 Ma shows a laterally continuous synchronicity of deformation.

Due to the lack of a continuous strain record from field data, we utilise analogue models and a special monitoring system allowing for a high spatial and temporal resolution to examine when and how the patterns of strain accumulation change. Thus, any break of scale-dependent behaviour can be detected.

For the translation of nature into a model first, characteristics such as the “indenting” forearc, the low-velocity zone beneath the plateau, pre-existing weakness zones in both Cordilleras and the oroclinal bend are transferred via frictional parameters of granular materials typical for simulating the upper crust.

Different combinations of material and external factors such as erosion and indenter geometry yield distinct strain patterns, of which only one is comparable to the plateau. A major factor seems to be the strength of the basal horizon, possibly causing the decoupling of deformation in the upper from that of the lower crust when the base is weak. However, in the absence of the lower crust the decoupling remains a kinematic boundary condition. Future experiments need to account for this by implementing viscous material simulating the lower crust.