Overpressure during indentation and the origin of ultra-high-pressure rocks in the Alps

Ultra-high-pressure (UHP) rocks in the European Alps, which record metamorphic pressures of >30 kbar, are commonly interpreted to indicate burial of crustal rocks to depths of >100 km. Tectonic models that explain such high pressures assume that rocks were taken to great depths by subduction of an oceanic Tethyan lithosphere and were subsequently decompressed during exhumation. The paradox of Alpine geodynamics is that plate reconstructions and geological data indicate that Alpine oceans were limited to narrow basins that never developed into true oceanic domains. Therefore, these basins would have been unable of generating mature subduction zones capable of driving events of deep burial and exhumation, which are used to explain the origin of the UHP rocks. Here we show an alternative model, whereby UHP metamorphism results from the build up of overpressure during indentation. We consider the concept of contained plasticity in contact mechanics (Johnson, K.L., Contact Mechanics. 2nd ed. 1989, Cambridge: Cambridge University Press. 452.), and demonstrate the possibility of significant overpressures during continental collision and associated vertical thickening. The highest overpressures are reached when the collisional orogeny shows significant strike-slip faulting (see Figure 1). Our results resolve the paradox of the origin of UHP rocks in the Alps, showing that their metamorphism could have occurred at considerably shallower depths (by more than a factor of 3-4) than previously suggested. Figure 1 shows over/underpressures calculated from Sibson's approach (Nature 249, 542-544,1974). Our new contribution is to show that tectonic overpressures can be boosted significantly in strike slip faultiing.