Tectonic geomorphology and active faults in the Bolivian Amazon

• Published in: Global and planetary change 2021-08, Vol.203, p.103544, Article 103544
• Main Authors: Lombardo, Umberto, Grützner, Christoph
• Format: Article
• Language: English
• Subjects: Active fault; Amazon basin; Bolivia; Forebulge; Landscape evolution
• ISSN: 0921-8181; 1872-6364
• DOI: 10.1016/j.gloplacha.2021.103544

The Bolivian Amazon foreland basin is controlled by i) the sediment input from the Andes and the topographic load; ii) the basinal uplift in the forebulge; and iii) the uplifting Fitzcarrald Arch in the north. These boundary conditions should lead to bending and active faulting of the crust. However, although the presence of active faults has been inferred from geomorphological studies, neither seismicity studies nor geodetic methods and geomorphological constraints have been successful in identifying active faults so far. Reasons for this are mainly the dense vegetation and the extremely dynamic fluvial landscape of the Bolivian Amazon. In this study we use the TanDEM-X and SRTM digital elevation models to show that there are at least five faults, several tens of kilometres long, that are active. We present offset measurements from fault scarps and combine these data with river system analyses. Our results suggest that tilting of tectonic blocks occurred in the Late Pleistocene or Holocene. The reversal of fluvial longitudinal profiles and incipient creek erosion inside paleochannels, markers of young tectonic activity, allow us to reconstruct the tilting directions. We find two orthogonal main faulting directions: WNW-ESE trending faults parallel to the Andean uplift and NNE-SSW trending faults tangent to the bulge of the uplifting Fitzcarrald Arch. Thus, our study area records the interplay between these two main drivers of crustal deformation. We show that some fault scarps were caused by large single earthquakes with magnitudes of around MW7, while others have recorded multiple strong seismic events. These magnitudes exceed by far the magnitudes known from instrumental seismicity in the last 40 years. Our results shed light on the seismic hazard of the region and highlight that catastrophic river course changes due to sudden uplift in this low-relief region are an underestimated hazard. •We have identified four previously unknown active faults in the Bolivian Amazon.•These faults suggest that here neotectonics are driven by the interplay between the forebulge and the Fitzcarrald Arch.•Faults and tilting blocks control the evolution of the fluvial landscape of the Bolivian Amazon.•Seismic risk in the Bolivian Amazon is higher than previously thought.