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 coll ision was performed to study the deformation and failure of the overriding lithosphere, The experimental technique allowed us to model the whole subdu ction/collision process from oceanic subduction to deep subduction of the c ontinental crust. With the numerical approach we have modelled the deformat ion 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 s how that the failure of the overriding plate is physically quite plausible or even inevitable during subduction. The conditions for such a failure (th e weakening of this plate) are prepared during oceanic subduction. The weak ening occurs due to the interaction between the subducting lithosphere and the asthenosphere in the mantle corner between the two plates, and due to b ack-arc spreading. In oceanic subduction zones with a compressional regime (no back-arc opening, thick and strong backarc lithosphere), the weakest zo ne is volcanic arc area. When weakening becomes sufficient during subductio n, the lithosphere fails in this area. The failure occurs along a fault dip ping 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 pr esently active subduction zones are characterised by a tensional subduction regime with back-arc spreading. In such subduction zones, the weakest plac e is not the volcanic are but the back-arc spreading centre. When a subduct ion regime changes from tensional to compressional, failure occurs in the v icinity of the extinct spreading centre. This process can occur during ocea nic subduction again along either a trench-vergent or trenchward-dipping fa ult, but the formation of a trench-verging, fault is most likely. In this l atter case, which is a principal subject of our study, the failure is follo wed by partial subduction of the arc plate. Complete subduction occurs duri ng arc-continent collision (subduction of the continental margin) when tect onic compression of the lithosphere increases rapidly and becomes sufficien t to push the arc plate into the mantle. The arc itself can be subducted co mpletely or be partially or completely scraped-off and accreted. A deeply s ubducted material (including continental margin) is preserved at relatively low temperatures between the lithospheric mantle and the ''cold'' subducte d arc plate to about 150-km depth. Subduction of the are plate is a major p henomenon, which affects all processes associated with continental subducti on from deep burial and HP/LT metamorphism to exhumation of subducted mater ial. Does this process occur in nature? Future investigations will allow us to answer this question. In this paper, we analyse the conditions of empla cement of a very young oceanic lithosphere (Samail ophiolite) on the contin ental 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.