APPLYING CORDILLERAN TERRANE STYLE ANALYSIS TO UNDERSTANDING THE HIMALAYAN OROGEN

The research works of Clark Blake and other pioneers of terrane analysis in the North American Cordillera demonstrate to us that to assume that relationships between presently juxtaposed terranes have been the same through time is folly. The recognition of the possibility that various ‘terranes' may be exotic relative to their present locations was a major breakthrough in understanding of North America geology. In order to understand the complexities of convergent plate margins considerable attention needs to be applied to stratigraphic and spatial constraints. The records of many geological events are commonly subtle and can only be detected with careful work or, in some cases, sophisticated modern analytical equipment. In the case of the Tibet-Himalayan orogenic system it is becoming increasingly clear that new data challenge conservative and somewhat two-dimensional plate tectonic models developed in the late 1970s and 80s to best explain then available observations. For instance, many ophiolites previously regarded as mid-ocean ridge crust are now seen to preserve a supra-subduction zone signature. The chances for preservation of subducting lithosphere are not great but the presence of these SSZ ophiolites informs us of the former existence of intra-oceanic subduction systems and gives the possibility of recognizing lost plate boundaries. In other areas enthusiastic application of the term ophiolite may have resulted in development of models where sutures zones were mistakenly identified. A somewhat unexplored aspect of the development of the Tibet-Himalayan orogenic system is the likelihood that various tectonostratigraphic units may have experienced lateral translation during or subsequent to their accretion to the Eurasian margin. Detailed observations and new detrital zircon data hint that, at various times, subduction may have been oblique to the margin of Eurasia. Structural disruption associated with the on-going effects of India driving into Asia has resulted in further disruptions to tectonostratigraphy and offset of units across major strike-slip faults. Such dislocations need to be restored so that reconstructions are appropriate to the various time slices to which they apply. In this paper, various models for the tectonic evolution of the Himalaya-Tibet orogenic system are tested in the light of new data.