Geophysical prospection on a complex rock glacier system - Morenas Coloradas revisited (Cordon del Plata, Mendoza, Argentina)

JOACHIM GÖTZ; MARKUS KEUSCHNIG; INGO HARTMEYER; DARIO TROMBOTTO LIAUDAT; LOTHAR SCHROTT

Salzburg, Austria

Jornada; 3. Mitteleuropäischen Geomorphologietagung; 2008

Arbeitsgruppe Geomorphologie und Umweltsysteme, University of Salzburg

Ground penetrating radar (GPR) and electric resistivity tomography (ERT) surveys were carried out complementary on the rock glacier in Morenas Coloradas (Argentina) in February 2008. In order to detect changes in active layer thicknesses and rock glacier activity, some of the profiles followed former 1D electrical resistivity soundings of Barsch & King (1989). The Morenas Coloradas catchment is located approx. 60 km west of Mendoza between 32°54’ - 33°01’ S and 69°27’ - 69°15’ W and is characterised by semiarid climate. It comprises three glacierised subcatchments and several adjacent cirques covering an area of approx. 54 km² between 2500 and 5750 m a.s.l. Lower limits of discontinuous and continuous permafrost are located at around 3600 m and 4500 m a.s.l., respectively. After Barsch (1996) the Morenas Coloradas rock glacier can be described as a multilobe, multiunit, multiroot and multipart rock glacier. It’s a complex, transitional landform composed of a high altitude glaciated zone to a debris covered glacier with thermokarst phenomena followed by various active rock glacier lobes and subsequently, inactive and relict lobes in the lower sections. The total length of the landform complex is about 5 km. Investigations using GPR and ERT were carried out at three different locations, namely at Balcon 1 (3550 m a.s.l.), at Balcon 2 (3740 m a.s.l.) and within the thermokarst area (3810 m a.s.l.). Both methods delivered similar results of the subsurface situation, verifying each other very well. Compared to primary investigations almost 20 years ago, variations of permafrost and active layer conditions become apparent. At the lower part of the rock glacier at Balcon 1, ERT and GPR indicate isolated permafrost lenses with low ice contents. This formerly active lobe with resistivities up to 30 - 50 KWm shifted to an inactive one, indicating permafrost degradation with only residual permafrost lenses and comparatively smaller values of up to 20 – 27.5 KWm. At Balcon 2, ERT delivers increased resistivities of almost 1 MWm and GPR indicates a continuous strong reflection in a depth of 2.5 to 4 m clearly reflecting the permafrost table. Within the past 20 years, only a slight decrease in resistivity values can be observed. The resistivity values recorded are typical for frozen ground with high ice content. Geomorphologic and -morphometric field evidences (e.g. steep terminal front) at Balcon 2 and the high ice content indicate that this part of the rock glacier is still active. The slight decrease of resistivity values within 20 years reflects a probable warming of permafrost, but no permafrost degradation is observable at this altitude. Within the thermokarst area massive ground ice with similar resistivity values of up to 10 MWm could be observed even today but in a deeper layer of approx. 8 - 10 m below surface compared to approx. 2 m in 1989.