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Incision amplified buckling: the Oxaya Antiform, northern
Chile
G. Zeilinger(1), F. Schlunegger(1), G. Simpson(2)
(1) Geological Institute, University of Berne, (2) Geological
Institute, ETH Zurich
The Western Escarpment of the Andes of Northern Chile is
characterized by the presence of gently folded surfaces (or pediplains) tens
to hundreds of km2 wide, and >1500 m deep valleys that dissect these
surfaces. The most prominent structure is the Oxaya Anticline, dissected by
the Lluta- and Azapa Valley. We will argue that the formation of this
anticline was enhanced due to fluvial incision.
The Oxaya Anticline is formed by Mesozoic basement and a
Tertiary series of fluvio-volcanoclastics. These Tertiary sediments cover a
pre-existing relief defined by the top of the Mesozoic. The fold axis of the
anticline corresponds with today’s maximum elevation of the unconformity
between the Mesozoic basement and the Tertiary unit, which clearly reflects
the anticline geometry. The spatial distribution of convergent and
extensional structures with minor displacement and the sub parallel
strike-orientation of the anticline to the Andes suggest buckling as
predominant tectonic process. The anticline developed supposedly as a
response to crustal shortening in the plate overriding the Farallon–Nazca
plate, between the trench and the Western Cordillera.
Cross-cutting relationships between the Oxaya Anticline
and dated sediments imply that its formation was initiated by crustal
shortening before at least 25 Ma in the Mesozoic basement. At this time up
to 1500 m of coarse-grained sediments started to accumulate over some 40 km
E–W distance between the Coastal Cordillera and the western slope of the
Andes. Further significant growth of the anticline occurred later than 20 Ma
(age of Oxaya-Formation, which is open folded). The maximum growth, however,
was supposedly reached at ca. 8 Ma as indicated by the Lluta collapse (one
of the largest paleo-landslides in the world), and the inversion of the
drainage system at the eastern flanks of the Oxaya Anticline. This period of
enhanced buckling coincides with the time interval, when rates of
down-cutting in the Lluta- and Azapa-valleys started to accelerate.
The conclusion is that the incision of these valleys
potentially has had a major influence on deformation of the surrounding
Oxaya Anticline. Theoretical models indicate that this is the case if
incision occurs at the same time the crust is deforming plastically in
response to regional compression. In this case, incision amplifies
background deformation at relatively small scale leading to the formation of
non-cylindrical folds with culminations coinciding with river incision,
which, in turn, is consistent with the geological data.