Quaternary tectonic activity in the transition area between the Central
and Patagonian Andes is closely associated with an anomalous cluster
of rockslides: 19 rockslides with volumes up to 4 x 10(9) m(3) developed
in plateau basalts. We divided them into two groups: (A) rockslides
related to neotectonic activity and (B) rockslides not related to
neotectonic activity. Thirteen rockslides, with a total volume of
similar to 10 km(3), which lie on either folds or faults, have been
displaced parallel to the structures and perpendicular to the valley-axis,
and they exhibit headscarps several kilometers-away from the valley
axis. Most of them are larger than 10(9) m(3), and are generally
of rock avalanche type with a high degree of crushing of rocks, although
local relief in some cases does not exceed 200 m. Nine rockslides
with a total volume of 8.9 km(3) are related to folds, while four
with a total volume of 1.3 km(3) are related to faults. The six rockslides
not related to neotectonic activity have a total-volume of 0.25 km(3)
(of which the largest one accounts for 0.17 km(3)), and are rotational
slides and block topples with a low degree of rock fragmentation,
although local relief is up to 400 m. The He-3 and Ne-21 surface
exposure ages for six of these slides, as well as relative age assessment
based on stratigraphic relation with glacial deposits and the drainage
development on the rockslide deposit, suggest that the rockslide
ages spread rather randomly between pre-glacial and mid Holocene,
discarding climatic conditions as a common triggering factor. The
absence of structures that can represent ideal sliding planes shows
that rock fracturing due to neotectonic activity is a major conditioning
factor for failures and that the magnitude of landslides is strongly
controlled by the type of deformation.