6th Colloquium on historical earthquakes & paleoseismology studies

Relative tectonic activity in the Colca catchment revealed by geomorphic indices, Central Andes

24 Oct 2018, 16:30

20m

Han-sur-Lesse

Oral Preference Session 2

Speaker

Dr Krzysztof Gaidzik (Department of Fundamental Geology, Faculty of Earth Sciences, University of Silesia)

Description

Geomorphic indices are widely used to quantitatively appraise relative tectonic activity. Extensive, uncomplicated and nearly unlimited access to digital elevation models and GIS software essentially sustained the worldwide usage of those indices. Even though they can provide valuable information on active tectonics from landforms, the obtained data should be always corroborated in the field. Morphological diversity of the Colca basin in Central Andes has been evaluated as a possible effect of relative spatio-temporal variations in tectonic activity. This area is located in the forearc of the Nazca-South America subduction zone. High seismicity here results from megathrust earthquakes, numerous shallow earthquakes triggered by active crustal faults and volcanic seismicity linked to two active volcanoes (Sabancaya and Mismi) located in the Colca basin. In this study we used both approaches, i.e. selected geomorphic indices and geomorphic observations from the field. Fieldworks were dedicated mainly to 1) identify and measure landforms, 2) collect structural data, 3) verify bedrock lithology, and 4) confirm knickpoints location and their type, etc. Whereas for the analysis of geomorphic indices we used those that proved to be a signal of relative changes in forearc tectonic deformation, i.e. topographic swath profiles, river profile, stream-length gradient index (SL), minimum bulk erosion. For their calculation we used a 30-meter resolution digital elevation model from the Shuttle Radar Topography Mission (DEM SRTM), and digitized topographic maps. The longitudinal profile of the Colca River has a convex shape in the upper reaches and concave shape in the lower reaches with abrupt break (knick point) in the slope that do not correlate with changes in bedrock lithology. Anomaly high values of stream length gradient index and minimum bulk erosion occur in the intermediate part of the basin, suggesting higher erosion in this section of the basin. This agrees with and is supported by field observations, as it coincides with the deepest section of the river basin known as the Colca Canyon. Lack of correlation between observed pattern and variations in bedrock lithology suggest tectonic forcing. Local deviation in calculated indices, as well as observed in the field deformations of landforms are the most probably the results of the activity of regional crustal faults, particularly W-striking extensional ones, as indicates by structural data.