Near-orthogonal foliation development in orogens: meaningless complexity, or reflection of fundamental dynamic processes?

Orogenic belts are geometrically complex owing to repeated deformation. Within this complexity, there is evidence that may suggest a common pattern of sequential steeply dipping and gently dipping foliations. Seven possible explanations are presented for the sequential development of these foliations, which can probably be reduced to four of general importance: (1) the passage of thrust sheets over flats and ramps; (2) switching of the maximum and minimum compressive stress orientations during orogenesis; (3) reversal of structural development owing to strong rheological anisotropy; and (4) back-rotation of crenulation hinges during crenulation cleavage development. It is suggested that all four of these reflect fundamental dynamic processes at work to build an orogen, and therefore that sequentially overprinted steeply and gently dipping foliations also reflect such fundamental processes. This illustrates that detailed geometrical analysis at the meso- and microscales can provide valuable input to dynamic models of orogenesis.