THE NATURE OF OROGENESIS

Current theory divides the mountain-building process into a single "constructional" phase followed by a collapse or "destructional" phase. Rather, we suggest, the evolution of an orogenic belt involves repeated inversion cycles, with major metamorphic and/or magmatic events marking the most significant tectonic mode switches. Some mountain belts are subject to "pull-push" cycles, when the crust and/or lithosphere is first stretched and then shortened. Other mountain belts form during "push-pull" cycles, and the reverse situation applies. Mountain belts that involve "pull-push" cycles display bimodal magmatism and hightemperature lowpressure metamorphism that culminates at the end of the extensional phase. Mountain belts that involve "push-pull" cycles display episodes of high-pressure low-temperature metamorphism that take place at the end of each shortening phase, while the extensional shear zones that will subsequently dismember the orogen begin to form.

Mountain belts are traditionally considered to be the result of slow convergence of lithospheric plates, and the result of overall shortening of the crust and lithosphere. Our current theories for mountain-building processes do not acknowledge a significant role for extensional tectonism, except during the late stage collapse or destruction of mountain belts, while extensional shear zones and detachment faults dismember the previously constructed orogen. Extension during the early stages of orogenesis is thought to be restricted to local buoyancy or channel flow driven phenomena. Yet field observations show that it is major extensional shear zones that draw once deeply buried rocks from the crustal roots of the mountain belt back towards the surface. The sheer scale and the overall attitude of these areally extensive structures means that they could only form in response to overall horizontal stretching of the Earth's crust. This implies that mountain belts do not collapse. Rather they are torn apart by lithosphere scale extension. This can be explained by “roll-back” of subducting slabs adjacent to the orogenic belt. No other as yet postulated mechanism provides a coherent explanation of the observed phenomena.