Styles of post-subduction collisional orogeny: Influence of convergence velocity, crustal rheology and radiogenic heat production

A number of models of continental collision already exist, but the role of foregoing oceanic plate subduction or variable coupling between plates still remains to be explored. In addition, heat generation by radioactive decay may be quite variable. For example, our geochemical data from low-to-high grade metamorphic rocks of the Lesser and Higher Himalayan sequences reveal measured heat production values on the order 4–5 μW/m3, which is considerably higher than normal inferred upper crustal values ( 700 °C) on top of coherent LP-LT units. The geometry of metamorphic isograds (inverted), shapes of P-T paths at different grades and exhumation rates are similar to those observed in parts of the Himalayan chain, indicating that the models capture the essence of real systems. The model results demonstrate that internal heat production rate is a crucial parameter in determining the timing of slab breakoff, polarity reversal and exhumation of the inverted metamorphic sequence with respect to the timing of partial melting. Late stages of some numerical experiments showed interesting geodynamical evolution of the collisional zone, with the appearance of polarity reversal, roll over of both plates and roll back of the subducting slab. Irrespective of these details, hundreds of km of continental lithosphere are recycled into the mantle in all models, supporting the idea that continental crustal consumption is, indeed, a common and considerable process during collision. This affects inferences about extents of convergence and/or shortening during orogenesis. Moreover, crustal recycling into the deep mantle during collisional orogeny needs to be considered in models of crustal growth and evolution.