THE IMPORTANCE OF DIFFERENTIATING DUCTILE SLICKENSIDE STRIATIONS FROM STRETCHING LINEATIONS IN KINEMATIC RECONSTRUCTIONS

High-strain zones are common structures in deformed terranes, and kinematic reconstruction and tectonic interpretation of the terranes to a large degree depend on understanding their kinematics.

A key task in the kinematic interpretation of many high-strain zones is to determine their shear directions. Traditionally this is done using stretching lineations (Ls) on S-surfaces (except in true L-tectonites), with the assumption that they indicate the ‘transport' (more precisely ‘shear') direction, especially where the strain is high. However, this long-held assumption is based on the simple shear model and recent developments in the study of high-strain zones show that it is not generally applicable to general high-strain zones such as transpressional and transtentional zones. This is especially true in high-strain zones with a triclinic kinematic framework where the relationship between stretching lineations and shear directions is more complex. On the other hand, ductile slickenside striations (Lc), commonly present on C-surfaces, form parallel to the local shear direction and are a reliable indicator of the shear direction. Therefore Lc may be very useful for kinematic interpretation of high-strain zones and it is important to differentiate Lc from Ls.

Although both S- and C-surfaces are commonly present in high-strain zones, Lc data are much less commonly reported than Ls data. Our experience is that Lc are much more common than reported in the literature. It is very likely that many field geologists do not differentiate the two types of lineations, because of either a lack of appreciation of the importance or the difficulty of doing so. Both types of lineations may have been simply reported as “stretching lineations”.

In this presentation, we first review the recent development in the study of high-strain zones, emphasizing the common deviation of Ls from the shear direction. We then discuss how to distinguish Lc from Ls. Finally, we present the description of a natural high-strain zone and show how differentiating the two lineations can lead to recognition of new phenomena as well as kinematic insights into a high-strain zone.