Postglacial deformation history of sackungen on the northern slope of Pic d'Encampadana, Andorra

Slopes undergoing deep-seated gravitational deformation create surface landforms that, if trenched and dated, yield critical data on slope kinematics. The northern slope of Pic d'Encampadana descends steeply 800 m down into the glaciated Valira d'Orient of Andorra and has long been known for its numerous antislope scarps (sackungen), indicative of deep-seated gravitational slope deformation (DSGSD). We excavated three trenches across antislope scarps and their adjacent troughs to date the deformation structures and to infer slope kinematics. Trough deposits have been disrupted by normal and reverse faulting, with the largest fault zone underlying the antislope scarp and dipping into the slope at 40°-80°. Trenches in the troughs expose fining-upward sequences of strata from which we interpret several cycles of: (1) episodic downdropping along graben faults and deepening of the topographic trough, (2) filling the trough with marginal colluvial and axial sag-pond sediments, followed by (3) a hiatus of soil profile development. The average time between deformation events in any one trench in the past 12–15 ka is about 3–3.5 ka. Deformation seems to have begun first in the lower part of the northern slope (1935 m elevation) about 15.3 ka, and had spread to the upper parts of the slope (2320 m) by 11.6 ka. Deformation in the lower slope ceased after 5.5 ka, but continued higher on the slope into the mid-late Holocene. The youngest deformation dated (1.4–1.6 ka) occurred in the highest trench on the slope. This pattern of younger deformation with increasing elevation has been documented at several other sites and has been ascribed to an upslope-migrating extensional stress field which originated at the base of the slope from glacial oversteepening and end-glacial debuttressing.