Quaternary Faulting and Fault-Related Geomorphology along the Orogenic Retro Arc Wedge-Front Structure of the Central Andes: The Mandeyapecua Thrust System, Southeastern Bolivia

Actively deforming orogens are significant seismic hazard zones, especially in areas with steadily growing populations and infrastructure. An essential and yet oftentimes poorly understood attribute for a coherent hazard and risk assessment is whether the responsible tectonic fault systems are subject to permanent, creeping deformation or episodic, seismogenic rupture processes. In the southern Bolivian Subandes recent regional geodetic surface velocities measurements indicate that the décollement beneath the eastern orogen is the primary contributor to its lateral and vertical growth. Its surface manifestation is the Mandeyapecua Thrust Fault System (MTFS), which marks the active front of the Subandean fold-and-thrust belt in the Chaco foreland basin of Bolivia. Despite significant surface offsets within Quaternary landforms its geomorphic features and tectonic activity remain poorly understood. This study focuses on its longest fault segment – the ~300 km-long Mandeyapecua Fault (MF) located between 19° and 21°S. To evaluate its role in accommodating Quaternary deformation we used high-resolution DEMs, field-based mapping, and morphometric analyses, to document uplifted terraces, drainage anomalies, and fault-related landforms indicative of Quaternary tectonic activity. Electrical Resistivity Tomography surveys at two key sites reveal near-surface structures consistent with blind thrusting and folding. Where faults have reached the surface, the expressions of scarps suggest that the Mandeyapecua Fault (MF) may be segmented. Geochronological data along the front indicate fault activity during the past 12,000 years, with ruptures possibly spanning ~100 km, but the complex, distributed surface deformation indicates that the MF might not fit a standard thrust-fault model.