[S-GL33] [EE] Geodynamics of convergent margins: theoretical, laboratory and natural examples

[SGL33-02] Scaled laboratory experiments on sequential thrusting in a mechanically two-layered system and its implications in fold-and-thrust belts

*Puspendu Saha¹, Santanu Bose², Nibir Mandal³ (1.Visiting Post-doctoral researcher, Earthquake Research Institute, The University of Tokyo, 2.Experimental Tectonics laboratory, Department of Geology, University of Calcutta, 3.Department of Geological Sciences, Jadavpur University)

Keywords:Mechanical layers, Multi-storied thrust sequences, Topographic slopes, Thrust wedge

Thin-skinned tectonic models have been widely used to explain the process of sequential thrusting in convergent settings. These models generally treat the crustal horizon as a single mechanical layer on a weak basal detachment. However, many fold-and-thrust belts display multi-storied thrust sequences, characterizing a composite architecture of the thrust wedges. For example, the Himalayan wedge has produced a set of continental scale (first order) thrusts, namely Main Crystalline Thrust (MCT), Main Boundary Thrust (MBT) and Main Frontal Thrust (MFT), covering the entire strike length of the mountain belt. Taking a close-view to different sectors in this belt, one can find numerous closely spaced (higher order) thrusts in between any two first order thrusts.Despite dramatic progress in sandbox modelling over the last three decades, our understanding of such composite thrust-wedge mechanics is limited and demands a re-visit to the problem of sequential thrusting in mechanically layered systems. This study offers a new approach to sandbox experiments, designed with a two-layered sandpack simulating a mechanically weak Coulomb layer, resting coherently upon a stronger Coulomb layer. Experimental runs reproduce strikingly similar styles of the multi-storied frontal thrust sequences observed in natural fold-and- thrust belts. Our results show that the upper weak horizon undergoes sequential thrusting at a high spatial frequency, forming numerous, closely spaced thrusts, in contrast to widely spaced thrusts produced preferentially in the lower strong horizon. We investigated the development of such composite thrust styles by varying frictional strength (μ_b) at the basal detachment and thickness ratio (T_r) between the weak and strong layers. For any given values of T_r and μ_b, the two thrust sequences progress at different rates; the closely-spaced upper thrust sequence advances forelandward at a faster rate than the widely-spaced, lower thrust sequence. The stable elevation of a thrust wedge in a mechanically layered setup depends on the thickness ratio (T_r) between the weak and strong layers. The wedge can attain a stable hinterland elevation only when T_r ≥ 1 (Fig. 1). Basal friction (μ_b) has little effects on the thrust vergence in the upper weak layer and they always verge towards foreland, irrespective of Tr values. But, back-vergent thrusts develop in the lower strong layer when μ_b is low (~ 0.36) (Fig. 2). In our experiments, closely spaced thrusts in the upper sequence (US) experience intense reactivation due to their interaction with widely spaced thrusts in the lower sequence (LS). This interaction eventually affects the wedge topography, leading to two distinct parts: inner and outer wedges, characterized by steep and gentle surface slopes, respectively.