Episodic displacement on a sackung scarp in Benasque Valley (Central Spanish Pyrenees). Paleoseismic record?

Abstract

A sackung scarp has been investigated by trenching in the El Ubago glacial valley (central Spanish Pyrenees). The scarp is located 18 km to the SW of the North Maladeta Fault, which was the source of the Mw 5.3 Vielha earthquake of 1923. Three displacement events have been inferred based on colluvial wedge stratigraphy and fault truncation. Event X at ca. 16.9 ka created the sackung. Events Y and Z have been constrained at 15-8.3 and 8.3-5.3 ka. The timing of event Y partially overlaps with that of a faulting event recognised in a previously investigated sackung at about 2 km. A minimum vertical slip rate of 0.18 mm/yr has been calculated for the sackung. The spatial association of the sackung features in this sector of the Pyrenees with the North Maladeta Fault and the millennial recurrence of the inferred displacement events (5.6 kyr) suggests that the kinematics of the studied sackung has been controlled by seismic activity. Demonstrating in future investigations that the sackung features in the area constitute archives of large paleoearthquake would be of great interest for seismic hazard assessments. They might help to improve the catalogue of paleoearthquakes and might provide information on earthquake recurrence intervals and the age of the MRE.