Erosion-tectonic Sandbox Models of the Structural and Fluvial Evolution of Transpressional Systems

Many analog transpression studies focus on the structural development of the system without including the effects of surface processes. Considering the high number of transpressional systems globally, the lack of these studies restricts our ability to representatively constrain, interpret, and model the crust and surface through time. Here, we present a new set of analog models to investigate how tectonic and surface processes at transpressive plate boundaries interact to shape topography. Experiments were conducted in a 2 × 1 × 0.5 m plexiglass box, with ends left open for drainage. Inside the box, we fix a plexiglass board cut to 20º obliquity to the sidewall. A mylar sheet is pulled under the board, forming a velocity discontinuity between the fixed board and the moving sheet. We load the board–sheet set up with a ~5 cm thick package of the experimental material (cf. CMII in Reitano et al., 2020, doi: 10.5194/esurf-8-973-2020 at 20 wt. % H20). Surface processes are initiated using commercial misting nozzles aligned with the trend of the wedge. We used a laser scanner to generate digital elevation models incrementally throughout the models and cameras (1 min photo intervals) for particle image velocimetry analysis. Here we focus on three experiments that we conducted using this system across various rainfall and convergence settings. Two tests represent end member CR# (the ratio between convergence and rainfall rate) settings. The third is a dry reference model. By analyzing these models, we attempt to identify the potential feedback between drainage and fault networks to explain morphological differences between experimental wedges with high, low, and no erosion. In all experiments, a bivergent wedge forms, and strain partitioning broadly evolves following previously established models. We find that erosion may influence the structural evolution of transpressional mountain belts leading to accelerated strike-slip partitioning. We also highlight how incision along main structures may localize exhumation in the system. We apply this model to assist in understanding uplift, deformation, and erosion patterns in natural transpressional systems, including the central Transverse Ranges of the San Andreas, the Merida Andes of Venezuela, and the Central-Western Cordillera of Colombia.