Impact of arc-continent collision on the conditions of burial and exhumation of UHP/LT rocks: experimental and numerical modelling

A 2-D physical and finite-element numerical modelling of arc continent collision was performed to study the deformation and failure of the overriding lithosphere, The experimental technique allowed us to model the whole subduction/collision process from oceanic subduction to deep subduction of the continental crust. With the numerical approach we have modelled the deformation of the overriding plate only through initial stages of its failure and studied the influence of different parameters on this process. The results obtained by both techniques are coherent and mutually complementary. They show that the failure of the overriding plate is physically quite plausible or even inevitable during subduction. The conditions for such a failure (the weakening of this plate) are prepared during oceanic subduction. The weakening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates, and due to back-arc spreading. In oceanic subduction zones with a compressional regime (no back-arc opening, thick and strong backarc lithosphere), the weakest zone is volcanic arc area. When weakening becomes sufficient during subduction, the lithosphere fails in this area. The failure occurs along a fault dipping under the arc in either of two possible directions and results either in subduction reversal or subduction of the fore-arc. Almost half of the presently active subduction zones are characterised by a tensional subduction regime with back-arc spreading. In such subduction zones, the weakest place is not the volcanic are but the back-arc spreading centre. When a subduction regime changes from tensional to compressional, failure occurs in the vicinity of the extinct spreading centre. This process can occur during oceanic subduction again along either a trench-vergent or trenchward-dipping fault, but the formation of a trench-verging, fault is most likely. In this latter case, which is a principal subject of our study, the failure is followed by partial subduction of the arc plate. Complete subduction occurs during arc-continent collision (subduction of the continental margin) when tectonic compression of the lithosphere increases rapidly and becomes sufficient to push the arc plate into the mantle. The arc itself can be subducted completely or be partially or completely scraped-off and accreted. A deeply subducted material (including continental margin) is preserved at relatively low temperatures between the lithospheric mantle and the ''cold'' subducted arc plate to about 150-km depth. Subduction of the are plate is a major phenomenon, which affects all processes associated with continental subduction from deep burial and HP/LT metamorphism to exhumation of subducted material. Does this process occur in nature? Future investigations will allow us to answer this question. In this paper, we analyse the conditions of emplacement of a very young oceanic lithosphere (Samail ophiolite) on the continental crust in Oman in the late Cretaceous and argue that this lithosphere formed in a back-arc basin. It reached and overthrust the Arabian continent after complete subduction of the are plate. (C) 2001 Elsevier Science B.V. All rights reserved.