ON THE ORIGIN OF OROGENS

Our current understanding of orogenic tectonics comes from a variety of sources including conceptual models based largely on geological observations, physical models based on analogue materials, and numerical models based on continuum mechanics. While each approach has its merits we argue that, in order to understand how orogens really “work”, a quantitative approach is essential. In particular, do the processes envisaged in conceptual tectonic models really happen as suggested? Can the diverse array of tectonic features observed in modern and ancient orogenic belts be reconciled in the context of “working” numerical models that are consistent with both the underlying physics and first-order geological constraints?

We present a simple conceptual framework for orogenesis in terms of the progression from small-cold to large-hot orogenic systems, and use forward numerical models to test simple hypotheses corresponding to specific stages along the temperature-magnitude (TM) spectrum. Model results are analysed in terms of the P (pro-wedge) – U (uplifted plug) – R (retro-wedge) – C (subduction conduit) tectonic elements, with O (orogenic plateau) replacing U in large-hot orogens. A "working" model must produce internally consistent predictions that are also compatible with geological observations including crustal structure, metamorphism, and geochronology.

We apply this approach to two contrasting orogenic systems with characteristic tectonic features that are currently the subject of debate. In the case of the Alps, an orogen that lies part way along the TM spectrum, we focus on the formation and exhumation of ultra-high-pressure metamorphic rocks using a model that also reproduces the first-order tectonic elements of the Alpine system. In the case of the Himalayan-Tibetan system, an end-member large-hot orogen, we focus on the transitions in space and time from external critical wedges to internal ductile flow. We demonstrate that the “wedge vs channel” debate is a false dichotomy - each plays an important role at different stages of orogenic evolution, and the two regimes must coexist and interact at plateau margins. Finally, we explore the implications of this work for orogenic tectonics in general.