Rock glacier activity and distribution in the semi-arid Andes of Chile and Argentina detected from dInSAR
Résumé
Rock glaciers in the semi-arid Central Andes ofChile and Argentina (approx. 28-34°S) are wide-spread elements of the high mountain landscape butthey still need to be studied more thoroughly. Thepresence and activity of this type of ice-rich and creep-ing permafrost has potential consequences for the hy-drological functioning of the watersheds as well asfor the stability of the debris slopes (Trombotto etal, 1997; 1999). Indeed, some cases of rock glacierdestabilization observed in this region (Iribarren, 2010;Bodin and Iribarren, 2012) suggest that the geomor-phological processes related to rock glacier dynamicsmay be changing because of warming conditions intothe ground (Trombotto& Borzotta, 2009). Neverthe-less, up to now, knowledge on rock glacier kinematicsin the Central Andes was restricted to a few sites(Apaloo et al., 2012; UGP-UC, 2011; Arenson et al.,2010) where study and monitoring efforts took placesince generally less than 10 years. On the other side,ongoing human activities in high altitude areas, likemining, are increasingly impacting rock glaciers (Bren-ning, 2008; Brenning and Azócar, 2010), which hasled to substantial efforts from public authorities toevaluate the cryospheric resources in their respectiveterritories. For that purpose, in both Chile and Ar-gentina, inventories of rock glaciers have recently beenreleased (UGP-UC, 2011; IANIGLA, 2010) based onaerial and satellite imagery. Morphological features in-dicatives of the presence of ice-rich permafrost, mainlyrock glaciers, have therefore been mapped. The assess-ment of the activity status of those landforms, whichis of crucial importance for further studying theirsensitivity to climate- and human-induced impacts,is generally done thanks to expert knowledge. Basedon visual interpretation of optical imagery, sometimescombined with in situ observations, this approachinduces subjectivity-related biases and a limited reli-ability of the analysis.To overcome those limitations, and under certainconditions (e.g. orbital geometry, weather conditions,slopes orientation), the space-borne imagery methodcalled differential radar interferometry (d-InSAR) canbe used to produce raster maps of surface changesover large areas and for periods covering a few daysto some months. In our work, we used 17 TerraSAR-X images acquired between April 2014 and January2015 over two zones of the Central Andes (Cordóndel Plata: 33°S; El Tapado: 30°S) to generate 15interferograms (8 in descending mode and 7 in as-cending mode) with a pixel size of 10 m, at 11, 22and 33-day intervals and a baseline of the SAR pairsranging from a few meters to some hundreds of meters.The topographic fringes were removed using the 90-mSRTM global DEM and a smoothing filter has beenapplied to the generated interferometric signal. Thed-InSAR dataset has been locally validated thanksto in situ dGPS measurements (Tapado rock glaciersite) performed between 2010 and 2014. We were ableto evidence several areas of distinct velocity patterns,depending on the glacio-geomorphological activity(glacier/debris-covered glacier/rock glacier) for whicha clear d-InSAR signal could be associated, as alsoobserved in other high mountain sites in the Alpswhere such approach has been previously used.The dataset covers a surface of almost 5100 km2,with a median elevation of 4120 m asl (25-75%quantiles = 3464-4606 m asl) and several hundreds offresh rock glaciers inventoried. We present here a syn-thetic overview of the regional rock glacier’s activitybased on the interpretation of the TSX interferograms.The kinematic characteristics of some interesting rockglaciers is further presented and discussed, especiallyin terms of the correspondence between geomorpho-logical observations and d-InSAR signal, as well asfuture research directions