Supraglacial debris supply in the cuerpo de hombre paleoglacier (spanish central system): reconstruction and interpretation of a rock avalanche event

Abstract

During the deglaciation stages of the last glacial period a rock avalanche took place on the glacier that occupied the upper sector of the uerpo de ombre alley (ierra de éjar). The material displaced during the avalanche fell onto the ice, was transported by the glacier and later deposited as supraglacial ablation till. The cause of the avalanche was the decompression of the valley slopes after they were freed from the glacier ice (stress relaxation). Reconstruction of the ice masses has been carried out to quantify the stress relaxation that produced the collapse.

The rock avalanche took place on a lithologically homogeneous slope with a dense fracture network. The avalanche left a 0.4-ha scar on the slope with a volume of displaced material of 623 ± 15 × 10³ m³. The deposit is an accumulation of large, angular, heterometric boulders (1–100 m³ in volume) with a coarse pebble‐size matrix. The avalanche can be explained as a relaxation process. This implies rock decompression once the glacier retreat left the wall ice free (debuttressing). Calculations show that the avalanche took place where the decompression stresses were highest (130–170-kPa).

In the panish entral ystem paleoglaciers the largest accumulation of morainic deposits occurred after the glacial maximum and the earliest stages of the ice retreat. The process described here is used as an example to formulate a hypothesis that the largest accumulations of tills were formed in relation to enhanced slope dynamics once some glacier retreat had occurred.

Key words:

• paleoglaciers
• debuttressing
• moraines
• rock avalanches
• paraglacial
• Spanish Central System

Acknowledgements

This work has been supported by the Ministry of Science and Innovation of Spain (project CGL2008‐03396/BTE) and the Education and Science Department of Regional Administration of Castilla‐La Mancha (Spain, Project PII1I09‐0138‐6113). We also wish to acknowledge the help and assistance to: Sierra de Gredos Natural Park (JCYL) and Local Administration of Candelario Village. We thank Carlos García Royo for the oblique aerial pictures contribution.