Rock walls distribution and Holocene evolution in a mid-latitude mountain range (the Romanian Carpathians)

Abstract : Rock walls in high mountain areas are the expression of long-term slopes response (10 3-10 5 years) to tectonics, weathering and denudation and a major source of sediment and hazard. Mountain rock walls (RW) characteristics and evolution at mountain-range scale is rarely discussed in the literature. Using a database of 791 RW mapped in the Romanian Carpathians, we present their distribution and morphometry in respect to lithological class, structural features and topography and relate them to post-Younger Dryas (Holocene) rock slope failure chronology. Morphometric data indicate that metamorphic and igneous RW (linked to a great extent to glacial valleys and cirques headwalls) are usually restricted to the highest sectors of the mountain slopes, are characterized by reduced relative heights and have an asymmetrical distribution, being common on the North-exposed slopes but extremely rare on the South. Statistical analysis results show the high significance of structural and tectonic control on RW distribution in sedimentary units which imposes the predominance of West and North orientations and RW dimensions up to a degree higher than in other lithologies. Based on 38 10 Be surface exposure ages obtained on metric boulders from the Southern and Eastern Carpathians, we hypothesise that metamorphic and igneous RW in the formerly glaciated Carpathian valleys were significantly shaped during Early Holocene (before 9 ka) by rock slope failures events that followed the deglaciation of the highest cirques and the intense RW permafrost degradation. We associate the long-term imprints of frost weathering to the significant North/South RW and rock glaciers distribution asymmetry, also identified in other mid-latitude mountain sites with similar topographic constraints.