L-TECTONITES: WHAT DO THEY TELL US ABOUT DEFORMATION HISTORY?

Relations between discrete and/or anastomosing zones of finite strain, and what they as a system may illustrate about strain accommodation during transpression/transtension have been well documented. However, unlike L-S and S>L tectonite formation, we have yet to appreciate how L>S and L-tectonites (L>>S) develop in these systems, or what these apparent constrictional structures indicate about structural processes active in the orogenic system. L>S or L-tectonite fabric localities have become recognized more commonly in the orogenic rock record of the middle and lower parts of the crust. Indeed these fabrics are found in all rock types that have plastically deformed at different metamorphic conditions in orogens formed during the Proterozoic (e.g., the Grenvillian) through the present (e.g., the Himalayan). Therefore, it has become critical to account for these structures as they apparently record a fundamental accommodation of strain in the plastic zone, and it is clear these structures are important to our understanding of basic orogenic processes.

The first step is to document and compare objectively the details of the fabrics formed in varied rock types at different temperatures in different orogens using structural and petrographic analyses. From these we have devised a simple frame and three basic types of L-tectonites. Some are the apparent result of the intersection of two or more planar fabrics (type 1). However, at the hand-specimen scale most L>S fabrics show evidence of a polydeformation history where different L>S fabrics developed sequentially in the same rock at different temperatures (type 2), either prograde or retrograde (or both), and either punctuated or progressively (or both). These observations suggest the formation process proceeds at all plastic conditions of the strain history. At larger scales, some L-tectonites form zones from several meters to kilometers wide, and either anastomose or terminate in discrete structures (type 3). In some orogens these zones are part of the main structure.

All of these observations beg new questions regarding the formation of these structures which may now be possible using the fundamental observations as a base for understanding simple deformation histories these fabrics have recorded. We may then combine these to understand the more complex histories.