A new thermomechanical deformation apparatus for 3D coaxial deformation and its application to rheologically stratified models
Chowdhury, Baishali Roy; Dietl, Carlo; Zulauf, Gernold; Peinl, Mark; Zanella, Friedhelm
We present a new 3D coaxial thermomechanical deformation apparatus applicable to the scaled analogue modelling of lithospheric deformation with an attribution of a vertical thermal gradient into the experiments. We have conducted a few preliminary experiments in order to check the persistence of the thermal gradient produced by the machine and its possible effect on the resultant structures. We have used plasticine, a strain rate softening material, also having temperature dependent rheology, to model the middle and lower crust. Plasticine displays a non Newtonian pseudoplastic behaviour, as seen from our rheology measurements at 40 °C to 50 °C to estimate the effect of thermal softening. Our experiments, as far our knowledge goes, are the first to report plasticines as thermomechanical analogue model material. We have chosen the tectonic setting of inversion as it is commonplace in the worldwide orogenic mountain belts, e.g. Alps, Himalayas. Three sets of experiments are performed. All of them follow similar model configuration. In the first two sets, plasticine consists of lower two layers and sand the uppermost one, whereas the third set employs lowermost silicone putty layer, middle plasticine layer and upper sand layer. The attributed temperature not only applies a vertical thermal gradient but also a prominent mechanical gradient. The models are subjected to prior extension followed by shortening. The experiments come out with different structural features. Model 1 shows the development of tension gashes in the lower and middle plasticine layers. They later get folded and upheaved during shortening. Presence of normal faults and thrusting are observed in the upper sand layer. Model 2 shows least deformation. The lower and middle plasticine layer form only a marginal graben. Sand layer shows presence of thrusting and normal faulting. Model 3 displays considerable deformation. HK40 and middle plasticine sink down to create a graben during extension and during shortening the silicone putty rise up covering and engulfing both the upper two layers. Thus, our models show typical features for inversion tectonics in response to the rheologies of the individual materials. We assume that the temperature dependent rheology plays a considerable part in the analogue material behaviour and thus shaping up the structural features. Noteworthy, the vertical thermal gradient is maintained throughout the entire length of experiments.