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09-12-2007

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Geologist crushing rocks!

Nanga Parbat

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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.

 

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