Geomorphic evidence of active deformation and uplift in a modern continental wedge-top foredeep transition : example of the eastern Ecuadorian Andes.
Abstract
Although markedly narrower than the nearby Peruvian/Bolivian and Colombian Andes, the eastern Ecuadorian Andes appear as a fold and thrust belt adjacent to a continental foredeep represented now by the world largest tropical alluvial megafan debouching into the Amazonian lowland. Very active volcanism, frequent earthquakes, large-scale landslides, and strong fluvial erosion indicate pre-steady state behaviour. Analysis of tectonic structures, drainage, and fluvial terraces, supported by 14C dating, indicate rapid tectonic uplift of the wedge-top : (1) the mighty Pastaza River was diverted two times as a result of thrust-fold growth, (2) thrust-related large-scale landslides controlled the new formed rivers, (3) the drainage shows a brutal change from immature to mature, with a well-defined hinge line representing the probable outer limit to landslide control, (4) although the upper surface of the megafan dated at ~18,000 14C yr BP. probably formed as a result of a cool and arid climate during the LGM, the Holocene strath terraces were results of tectonic activity. Average incision rates were very high (0.4 to 2.8 cm yr-1), increasing from 18,00014C yr. BP. to now. A comparison of these values with fold and thrust fault uplift rates enables us distinguishing between very rapid ‘regional’ uplift and thrust-related ‘local’ incision/uplift relatively moderate except for the foremost thrust-folds. Regional uplift has affected the Subandean Zone and Eastern Cordillera as a whole, and is likely to have been caused by a regional-scale low angle thrust ramp underlying the Subandean and Eastern Cordilleran thrust sheets, and emerging in front of the Subandean Zone.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....5941B