How (not) to recognize a mid-crustal channel from outcrop patterns

Mid-crustal channel flow has been hypothesised to be responsible both for the Greater and Lesser Himalayan Sequences (the Miocene Himalayan channel theory), and for the present east- and northward movement and extension of the Tibetan upper crust (the Tibetan middle crustal channel flow theory). As processes within the crust cannot be directly observed, various studies have attempted to validate mid-crustal channel flow by using indirect approaches, including outcrop patterns and other field data from Himalayas, Tibet, and exposed older orogenic roots. The results have been highly debated because arguments can be made that the internal structure of a channel and, therefore, the outcrop patterns of a palaeo-mid-crustal channel are not unique. This paper investigates what types of structural patterns may be produced within a mid-crustal channel, and discusses why they can be difficult, if not impossible, to distinguish from outcrop patterns produced by other mechanisms. A new example from the exposed middle crust of southern Finland is also discussed in this context. While outcrop structural patterns must indeed agree with other potential results that may infer a mid-crustal channel, the inverse is not necessarily true: one cannot infer a mid-crustal channel based on outcrop patterns alone, due to the non-unique nature of the patterns.