Obduction: no longer a puzzle? (O:NLAP) – O:NLAP

Obduction corresponds to one of plate tectonics oddities, whereby dense, oceanic rocks (ophiolites) are presumably ‘thrust’ on top of light, continental ones (Coleman, 1971). The Peri-Arabic obduction corresponded to a spectacular, almost synchronous thrust movement along thousands of km from Turkey to Oman (in c. 5-10 Ma), and across several hundreds of km! Such a large-scale geodynamic process necessarily implies significant changes in mantle and subduction dynamics.
Obducted ophiolites, which represent our only oceanic record before c. 170 Ma, have been reported from most convergent belts, yet obduction processes are still poorly understood. Indeed, as noted by Nicolas (1989), “ophiolite emplacement is a vast topic covered by an abundant literature and a profusion of models, often poorly supported by facts”. Similarly, we are still left with contrasting interpretations as to why obduction pulses are recorded throughout geological time (e.g., Late Jurassic, Upper Cretaceous).

If obduction today is still enigmatic, it is chiefly because the following questions remain fundamentally unanswered:
- How can dense ophiolitic rocks be emplaced on top of light continental ones?
- Why are large-scale obductions so rare?
- Does obduction require specific geodynamic processes?

Recently, however, several researchers involved in this proposal have reexamined obduction processes for the large-scale ophiolites (Lagabrielle et al., 2005 ; Agard et al., 2007; Yamato et al., 2007; Lagabrielle and Chauvet, 2008) and their findings suggest that obduction:
(1) is not a mysterious process but results from some large scale, normal subduction process, (2) is coeval with some kind of rapid uplift/motion of continental crust beneath,
(3) does not involve necessary significant thrusting and compression in the oceanic lithosphere.

We thus envision obduction as a combination of oceanic and continental subduction requiring specific boundary conditions, rather than as a specific geodynamic process of its own - if further demonstrated, this would be the end of the ‘obduction’ paradigm. We will combine the most recent numerical and petrological techniques and geophysical data to further assess these interpretations and fully understand the nature of obduction.

One may in turn wonder: is obduction rare only because the set of necessary boundary conditions is seldom realized, and/or what are the conditions needed to preserve obduction ? We herein encompass obduction from the triggering and initiation (birth) to the emplacement of a fragment of oceanic lithosphere on top of a continent (death), focusing on:
- constraining the physical conditions required for obduction to develop/nucleate,
- assessing the relative influence of key, controlling parameters through data processing and modelling,
- demonstrating that the emplacement of dense, oceanic material on continental lithosphere does not require specific tectonics.

Understanding and parameterizing obduction would be an important scientific breakthrough, useful to all scientists working on lithosphere/mantle dynamics: this is why this project brings together specialists from various complementary fields with unprecedented high-resolution techniques (e.g., combination of high-precision thermobarometry and P-T-t path determination with fully coupled thermodynamic and mechanical numerical modelling ; high-precision crustal-scale constraints provided by geophysical, igneous and sedimentary data).