Topographic reflection of the Socompa debris avalanche,Chile |
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| Authors: | Karim Kelfoun Tim Druitt Benjamin van Wyk de Vries Marie-Noëlle Guilbaud |
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| Institution: | (1) Laboratoire Magmas et Volcans, OPGC, UMR Clermont Université-CNRS-IRD, 5 rue Kessler, 63038 Clermont-Ferrand, France;(2) Present address: Instituto Geofísico, Escuela Politécnica Nacional, Casilla 1701-2759, Quito, Ecuador;(3) Present address: Instituto de Geofísica, Universidad Nacional Autónoma de México, Cuidad Universitaria, 04510 México D.F., México |
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| Abstract: | One of the most remarkable features of the exceptionally well preserved 26 km3 Socompa debris avalanche deposit is the evidence for topographically driven secondary flow. The avalanche formed by sector
collapse of Socompa stratovolcano and spread 40 km across a pre-existing basin, forming a sheet of ∼50 m average thickness.
As the avalanche impinged on the western and northern margins of the basin, it was reflected back, forming a secondary flow
that continued to travel 15 km down a gentle slope at an oblique angle to the primary flow, the front of the return wave being
preserved frozen on the surface of the deposit as a prominent escarpment. Satellite images, aerial photos, digital elevation
models and field observations were used to reconstruct the sequence of events during avalanche emplacement, and in particular
during secondary flow. The avalanche sheet was divided into distinct terrane groups, each believed to have experienced a particular
strain history during emplacement. Evidence for avalanche reflection includes clearly recognizable secondary slide masses,
sub-parallel sets of curvilinear shear zones, headwall scarps separating the (primary) levée from the secondary terranes,
extensional jigsaw breakup of surface lithologies during return flow, and cross cutting, or deflection, of primary flow fabrics
by secondary terranes. Reflection off the basin margin took place in an essentially continuous manner, most major return motions
being simultaneous with, or shortly following, primary flow. The secondary flow occurred as a wave that swept obliquely across
the primary avalanche direction, remobilizing the primary material, which was first compressed, then stretched, as it passed
over and rearward of the wave front. As return flow occurred, surface lithologies were rifted in a brittle manner, and the
slabs were sheared pervasively as they glided and rotated back into the basin; some sank into the more fluidal interior of
the avalanche, which drained out into a prominent distal lobe. Extension by factors of up to 1.8 took place during return
flow. Secondary flow took place on slopes of only a few degrees, and the distal lobe flowed 8 km on a slope of ∼1°. Overall
the avalanche is inferred to have slid into place as a fast-moving sheet of fragmental rock debris, with a leading edge and
crust with near-normal friction and an almost frictionless, fluidal interior and base. The avalanche emplacement history deduced
from field evidence is consistent with the results of a previously published numerical model of the Socompa avalanche. |
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| Keywords: | Socompa Debris avalanche Reflection Secondary flow High mobility |
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