Abstract EGU 2004

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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.