Investigating small scale transient deformation features in convergent settings- Insights from analogue modeling

The evolution of a convergent orogenic belt can be dissected into a combination of small scale events. Deformation in the orogenic belts can range in a timescale from earthquake cycle to millions of years. Moreover, long term deformation trends are a composition of the smaller events that together create the final geometry of an orogen. Therefore, it is important to understand the complexity of these events in order to further understand large scale mechanics of deformation. In this study, we employ analogue models of sand wedges representing the brittle upper crust to visualize temporal and spatial deformation in a convergent setting. The time-series evolution of these convergent sand wedges is monitored by Particle Image Velocimetry (PIV). In addition, the stress change within the wedge, especially at the localization of strain i.e. faulting events and between fault events is monitored by a force sensor. The sensor is attached to the back wall, in the experimental setup and against which the sand wedge grows. In these experiments the effect of basal friction on the final geometry of the wedge is tested. This parameter is varied twice. Results show that displacement data from the PIV system, analyzed in the form of strain correlates well with data from the force sensor. On a larger scale, force increases (indicating a linear trend) until strain is localized and a fault is formed causing a sudden drop in force and a release of stress. The magnitude of the force drop after a fault has occurred is related mainly to the horizontal length of the fault. However between fault events, recordings of the force measurements show a cyclic pattern with a decreasing frequency towards a fault event. Over time and as the wedge grows and matures, this intra fault frequency decreases as well. Varying basal friction demonstrates a cutoff in the maximum stress a wedge can handle due to the strength of its base. Time-series image analysis of strain combined with stress analysis provides an avenue to visualizing the minute deformation features that may shed more insight into the mechanics of orogenic belts.