Subduction Initiation in Nature and Models: A Review
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Subduction Initiation in Nature and Models
New subduction zones have become a crucial part of the plate tectonics regime. Bending and resistance of the lithosphere are forces that prevent the initiation of subduction. Geologists have formed new subduction zones without paying close attention to force balance. Subduction has led to the creation of weak lines and fractures in the lithosphere, which has resulted in controversies related to forces that cause subduction initiation (SI). Thus, SI involves the transient transfer of materials from the top of the slab to the upper regions. Subduction can single-sided or double-sided. The paper analyses various scenarios that result in the initiation of subduction.
SI is a fundamental process of plate tectonics, and it is also the central theory of modern geology and geophysics. One vital aspect of subduction is that it is one-sided or asymmetric. Thus, it occurs through the sinking of the subducted part while there is a horizontal movement of the overriding plate. One-sided subduction can be formed through a direct asymmetric or gradual amplification of perturbations in plate bending or motion. The geometry of asymmetry subduction is inherently related to the distribution of deformation and strength in the lithosphere. Distinct models of subduction initiation imply that weak zones in the lithosphere can be inherited from the past plate tectonic history. The weak zones can also develop self-consistently due to rheological feedback mechanisms during subduction initiation. Besides, induced SI can also result in the development of weak zones in the lithosphere.
There are several opinions related to the challenges and ease of establishing new subduction zones. Predictions made from simple numerical and analytical models prove that it is not easy to establish new subduction zones. However, observations reveal that there is frequent yet episodic creation of new zones of subduction initiation. Difficulties encountered in recognition and characterization of subduction initiation processes include poor understanding of dynamics and data inaccessibility. The subduction initiation mechanisms can be spontaneous or induced. The induced mechanisms can compression-induced or extension induced. Spontaneous subduction initiation mechanisms include topographic loading at continental margins and small-scale convection at the sub-lithospheric mantle. However, the transference and passive margin collapse, which are subduction initiation types, are supported by natural observations. They reflect less strength of the normal lithosphere, which makes their existence questionable.
Therefore, there has been an introduction of modern approaches to understanding subduction initiation, such as using a 2D visco-elastoplastic model to prove that the fracture zone can be transformed into a self-sustaining subduction zone after convergence of approximately 100km. There is also an increased exploration of how the magma changes due to SI processes. There is a reduction in the tectonic plate’s strength when a slab breaks, with extensive melting occurring close to the trench. However, increased tectonic plate strength results in continuous subduction initiation without spreading the infant arch, and there is minimal melting of magma. 3D plume-induced subduction initiation is a modern approach that enables geologists to monitor melt extraction and upward transport from the plume. The experiments conducted using this model showed that the weakening of the melt’s lithosphere is critical for subduction initiation.
Conclusively, a combination of field studies has improved the progress being made in the establishment of new subduction initiation zones. Increased computation using 2D and 3D models has led to new modes of induced and spontaneous subduction zones.
References
Stern, R. J., & Gerya, T. (2018). Subduction initiation in nature and models: A review. Tectonophysics, 746, 173-198.