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Abstract: Even though it is a well-established fact that the Earth is currently in a plate-tectonics mode, the question on how to “break” lithospheric plates and initiate subduction remains a matter of debate. Here we focus on shear heating as a potential mechanism to cause lithospheric shear localization and subsequent subduction initiation in oceanic plates. It is shown that shear heating under some conditions (i) facilitates the formation of a lithospheric-scale shear zone and (ii) is capable of stabilizing a lithospheric-scale shear zone, thus creating the necessary condition for subduction initiation to occur. Furthermore, we demonstrate that not only the localization process is of importance, but also the post-localization stage, where rapidly growing convective instabilities might prevent an incipient subduction zone from developing. We develop scaling laws for both the localization and the post-localization stages. In the case of lithospheric localization, the characteristic length scale equals a quarter of the dominant folding wavelength in the lithosphere, thus of the order of the competent layer thickness. This shows that oceanic lithosphere develops an intrinsic perturbation length scale that is largely independent of smaller-scale heterogeneities. We compare the subduction initiation potential of wet olivine and dry olivine rheologies and find that, for Earth-like conditions, a dry olivine rheology is more likely to result in subduction initiation. A large plate age does not always increase the potential for subduction initiation, as it increases the potential for convective instabilities to occur. Instead, an optimal plate age exists for shear heating induced subduction initiation, which is around 40Ma for a wet olivine rheology. [Copyright &y& Elsevier]

The development of opposing continental subduction zones remains scantly explored in three dimensions. The Hindu Kush–Pamir orogenic system at the western end of the Himalayan orogen provides a rare example of continental collision linked to two opposing intra-continental subduction zones. The subducted plates feature a peculiar 3D geometry consisting of two distinct lithospheric fragments with different polarities, subduction angles and slab-curvatures beneath the Hindu Kush and Pamir, respectively. Using 3D geodynamic modeling, we simulate possible development of two opposing continental subduction zones to understand the dynamic evolution of the Hindu Kush–Pamir orogenic system. Our geodynamic model reproduces the major tectonic elements observed: (1) the deeper subduction depth, the steeper dip angle and the southward offset of the Hindu Kush subduction zone relative to the Pamir naturally occur if convergence direction of the subducting Indian plate and dip-direction of the Hindu Kush subduction zone match. (2) The formation of the highly asymmetrically curved Pamir region and the south-dipping subduction is promoted by the initial geometry of the indenting Indian lithosphere together with the existence of a major strike-slip fault on the eastern margin of the Pamir region. (3) Subduction of only the lower continental crust during continental collision can occur if the coupling between upper and lower crusts is weak enough to allow a separation of these two components, and that (4) the subduction of mainly lower crust then facilitates that conditions for intermediate-depth seismicity can be reached. (5) The secondary tectonic features modeled here such as strike-slip-fault growth, north–northwest striking extension zone, and lateral flow of the thickened ductile upper crust are comparable to the current tectonics of the region. (6) Model results are further compared to the potentially similar orogenic system, i.e., the Alpine orogen, in terms of the curved Western Alpine arc and the two opposing subducted slabs beneath the Alps and the Dinarides. [ABSTRACT FROM AUTHOR]

It remains unclear why – in contrast to all other known silicate planets – the Earth's geodynamic evolution is still characterized by the motion of discrete plates at its surface. While the onset of deep subduction is synonymous with the beginning of ocean-plate tectonics (at some point >800 Ma), the initiation of new subduction zones is likely the key to maintaining ocean-plate tectonics on the Earth more recently.Despite its importance in maintaining plate tectonics, the process to form new subduction zones on a planet with ongoing plate tectonics remains enigmatic. This is possibly due to the incomplete or missing and geographically discontinuous geologic evidence, as well as the long timescales and the numerous physical processes involved in forming new plate boundaries, thus requiring strongly inter-disciplinary studies to understand this phenomenon.In a dedicated and collaborative effort, we gathered experts in different fields across the Earth Sciences to provide and collect presently available data on recent subduction zone initiation (SZI) events and build a cross-disciplinary Subduction Zone Initiation Database. This novel database covers recent SZI events currently with a focus on the Circum-Pacific realm, and will be made openly available. The SZI Database is a product of the YoungCEED initiative, which aims to bridge the various subfields of the Earth Sciences and foster new ideas by supporting young scientists. The evidence collected within the SZI Database provides new insight to the long-standing questions about how, where and when new subduction zones have initiated on the Earth in the past 100 Myr. [ABSTRACT FROM AUTHOR]