Kinematic modeling of refold structures: can all end-members exist in nature?
Abstract
In general, modern structural geology textbooks distinguish between four end-member types of three-dimensional refold structures (Type 0-3), which are characterized by two-dimensional interference patterns. Based on the kinematic assumptions of this classification, we show by simple kinematic modeling that six different end-members of three-dimensional refold structures do exist. These end-members have orthogonal and/or parallel fold axes and normals to axial planes and can be described by a reduced direction cosines matrix. Theoretically all of these six end-members should have geologic significance but two types have not been recognized in previous studies of refolded rocks. In agreement with the traditional classification, Type 1-3 comprise refold structures showing dome-and-basin, mushroom and convergent-divergent patterns on two-dimensional orthogonal sections. These three refold structures have end-member counterparts Type 01-03, which are simply derived by 90° rotation of the superposed fold around its fold axis. Currently, no simple two-dimensional diagram exists for plotting refold structures based on the spatial relationship between the first and superposed fold orientations. The reduced direction cosines matrix can be used to quantify the angles between the fold axes and the normals to the axial planes of two fold generations in a simple ternary plot. This direction cosine triangle plot illustrates the six end-members of refold structures as well as their transitional types. Abundant natural examples of Type 1-3 refold structures have been reported from the field, but a clear documentation of natural Type 01-03 refolds is still missing so far. This lack raises the question whether these structures are mechanically unlikely to develop or if they are difficult to spot in the field.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....2286G