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Enhanced buckling due to
river incision: the Oxaya Antiform, northern Chile
Gerold Zeilinger,
Fritz Schlunegger, Univ. Bern
Guy Simpson, ETH
Zurich
The Western Escarpment of the Andes of Northern Chile
is a tectonically active mountain belt that 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 underlain by Mesozoic basement
and a Tertiary series of fluvio-volcanoclastics. These Tertiary sediments
cover a pre-existing relief defined by the contact with the top of the
Mesozoic. At the surface the hinge line of the anticline fits with today’s
maximum elevation of the unconformity between the Mesozoic basement and the
Tertiary unit. The Oxaya Ignimbrites which are part of the Tertiary
fluvio-volcanoclastics are clearly bend by the anticline geometry. The
spatial distribution of convergent and extensional structures as well as the
sub parallel strike-orientation and the minor displacement, especially at
the western limit of the anticline, suggests buckling as predominant
tectonic process. Indeed, the geometry and the observed structures are best
described as a flexed layer. 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 formation of this structure was initiated by
crustal shortening before at least 25 Ma in the Mesozoic basement, when 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 of this
anticline, however, was supposedly reached at ca. 8 Ma as indicated by the
Lluta collaps (one of the largest palaeo-landslides in the world), and the
inversion of the drainage system at the eastern flanks of this anticline.
This period of enhanced buckling coincides with the time interval when rates
of downcutting in the Lluta- and Azapa-valleys started to accelerate.
The conclusion is that incision of these valleys
potentially have 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.