Consumption of Oceanic Plates – Review of the Subduction Concept

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Fig24MarianaSubduction first of all it means recycling, i.e. recycling of crustal fragments into the asthenosphere. When oceanic crustal fragments are rising above baselevel, we speak about obduction processes, whilst in case crustal fragments are indeed melted into the asthenosphere we may speak about subduction processes.

According to the GSST theory, kinematic plate fragments are in constant motion, showing certain relative plate velocity contrasts. Under constraints of the Coriolis force, hence, due to their accumulated momentum, microplates tend to rather develop transpressional orogenic build-ups on their longer edges than forming subduction zones during their constant eastward drift. As a consequence, oceanic plate fragments instead of being recycled are rather preserved, being obducted into orogenic complexes, where they are forming suture zones. In other words, the presence of ophiolites in orogenic complexes is the proof of obduction processes, and only indirectly can be inferred the presence of subduction zones from mass balance considerations. Hence, it is not surprising that the peri-Tethys area is full of obducted ophiolite units; starting with the Vardar, Transylvanides, Dinarides, dozens of Asian (e.g. Anatolian) obducted ophiolites in the geological record are all product of transpressional strike-slip tectonics.

Because of global kinematic constraints, subduction may happen only perpendicularly to the principal plate drifting vector or more frequently, as a function of the principal drifting vector of kinematic chains. This means, that real subduction may only form roughly in N-S direction. Basically there are only two such kind of convergent plate boundary lineaments on the Earth, where plate recycling could happen: 1) at the western Pacific plate margin, 2) and at the eastern Pacific plate margin.

In section 3, we have already reviewed the main geological, geophysical, geodetic observations made regarding the evolution of the Japan Island. All these data are proving the significance of shear deformation in northeast Japan, where the intensity of fault activities is very pronounced, earthquake epicenters indicating the presence of E-W directed fault planes. Clustering of earthquake epicenters in stress nodes is generally present all along the Western Pacific subduction zones, from the Sumatra Arc to the Philippine subduction zone. Pacific plate velocity measurements might change with time as new GPS stations will be installed in the oceanic domain, and if the motion of smaller crustal units is also going to be integrated. The origin of the Mariana Trench and the Bonin Ridge is linked to the same strike-slip displacement, which was made along the irregular Hawaii North Mp/ Bonin Nanoplate boundary, in S to N direction. This irregular plate boundary in turn is the result of another plate velocity contrast, which is characterizing the Bonin Np–Hawaii North Mp and the North Philippine–Hawaii South Mp kinematic chains.

Therefore, structural features of the Japan Island and the whole southeastern Asia island-belt, suggest that instead of real subduction we are facing various transpressional space management processes, best described with the term of plate-overriding, i.e a mix of obduction, lateral escape, plate buckling mechanisms.

From a kinematic perspective, real subduction, where recycling of crustal fragments occurs it could only happen in the western margin of the American continents. Here, all the kinematic prerequisites of subduction are present: 1) the motor of subduction is present, and it can be identified in the Coriolis force, which is transferred and exploited by the momentum of plates, 2) there is an obvious density, hence a momentum contrast, between the oceanic and continental crustal fragments.

Momentum contrasts between plate fragments can play a significant role in the initiation of both oceanic spreading and subduction processes, in case the Earth suffers angular acceleration. Whether the angular acceleration of planets is a common planetary phenomenon or not, it is out of our knowledge, but certainly accidental larger meteoric impacts may affect the spinning velocity of the Earth.

Published in: Kovács, J.Sz., 2015 (in press), Elements of Global Strike-Slip Tectonics: a Quasi-Neotectonic Analysis, Journal of Global Strike-Slip Tectonics, v1., Szekler Academic Press, Sfintu Gheorghe.