Effects of the superposition of compaction and tectonic strain during folding of a multilayer sequence—model and observations

Abstract

Sedimentary rocks deformed at shallow crustal depth can show a wide variety of planar, linear and mixed planar-linear fabrics depending on their structural position within folds. A model has been developed which tries to explain this variation in fabric development in terms of the strain states that it represents. We analyse the finite strain states that may develop during compaction and subsequent buckle folding of sedimentary multilayers composed of a regular alternation of competent and incompetent beds. The geometric properties of the multilayer during buckling have been analysed from a finite element model characterized by: linear viscous material properties, a viscosity contrast μ 1/ μ 2 = 40, and a ratio of incompetent to competent layer thickness d 2/ d 1 = 1. Sequential matrix multiplication allows the determination of the finite strain states for all stages of fold development. Finite strain states predicted from the model appear to be in good agreement with observations of fabric development in naturally deformed rocks from the Southern Apennines. During progressive deformation in the model, the finite strain ellipsoid in the limb regions of folded incompetent layers is consistently oblate, and the XY plane lies at a small angle to bedding as a consequence of previous compaction. Natural cleavage consistently lies at a low angle to bedding in folded argillites from the Southern Apennines and it can be interpreted to represent the XY plane of the total finite strain ellipsoid in strongly compacted sediments, and not that of tectonic deformation alone. In the hinge regions of open folds, finite strains predicted in the incompetent layers of the model are of prolate type, and they become oblate as the interlimb angle decreases; the presence of a ‘true’ pencil structure in open folds and of a spaced cleavage associated with a ‘regular’ pencil structure in more close folds, often noted in the case study of the Southern Apennines, most probably reflects this change from prolate to oblate strain in the hinge zones as folding proceeds.