4-DIMENSIONAL MODEL OF DIACHRONOUS CHEVRON FOLDING IN AN ACCRETIONARY PRISM SETTING

Diachronous harmonic chevron folding is a common phenomenon in exhumed accretionary prism rocks, but is difficult to explain mechanically. Chevron folds are generally modeled as synchronous structures that initiate as superposed wave trains in which a dominant wavelength amplifies with time; the fold train is present from the start. We present an alternative model for diachronous chevron formation by buttressing against a rigid block. This model has been developed for the Puncoviscana Formation and related turbiditic rocks in central Argentina, and may best fit the tectonic setting of an accretionary prism, although it can be applicable in other settings as well such as shortening of a pull-apart basin. In our model, material passes through a quasi-steady state brittle front and forms chevrons, with associated thrusts and other minor structures. As the chevrons tighten and amplify, their lower levels will pass through a quasi-steady state ductile front. With further prism growth, folds move down through the ductile front and the deformation mechanisms will change accordingly, so that brittle deformation mechanisms operate near to surface while ductile deformation ensues at depth along the axial plane of a single fold.

We suggest that the tectonic setting of an accretionary prism demands a progressive deformation model, with "fronts" of both brittle and ductile deformation through which stratigraphy is progressively fed like a factory conveyer belt. Furthermore, there should be a spatial connection between the brittle front, where chevron folds are currently forming and the ductile front, where previously folded rocks develop a schistosity and a steep extensional fabric. We propose that the formation of folds and fabrics in accretionary prism sedimentary rocks will be diachronous and therefore strain restoration techniques must reflect that diachroneity. Our 4-dimensional view of the progressive deformation and metamorphism of a typical accretionary prism is likely applicable to many other similar tectonic settings.