One of the main achievements in the Earth science is the development of plate tectonics theory in the 20th century. It successfully explains the generation and extinction of oceanic plate from mid-oceanic ridge to subduction zone. The discovery of coesite, a high-pressure polymorph of quartz, which is tiny but indicator mineral of ultrahigh-pressure (UHP) metamorphism, from the continental crust rocks in the 1980s, demonstrated that the low-density continental crust can be subducted into the high-density mantle. This is one revolution of plate tectonics theory, which opened a new research area of continental deep subduction and UHP metamorphism.
"How the low-density continental crust can be subducted into the high-density mantle? This became one important problem in solid Earth science." Said Yi-Xiang Chen, the leading author of the paper, who is a professor from the University of Science and Technology of China located in Hefei, eastern China.
The deep subduction of continental crust is traditionally considered to be driven by the proximal force of slab pull produced by the subduction of pre-existing oceanic plate. This interpretation requires that the continental crust subduction is preceded by the closure of a mature and large oceanic basin. However, is the pull of previously subducted oceanic plate necessary for continental deep subduction? Whether the continental crust boarding to a small ocean basin can also undergo deep subduction? If so, what is the driving force? This is a frontier problem in the field of continental dynamics.
To constrain the above problem, Chen and co-authors investigated the Sesia Zone in the Western Alps. This zone represents a crustal fragment of a rifted, hyper-extended continental margin that did not come into contact with any large ocean basins prior to subduction. Thus, it provides an ideal natural laboratory for exploring the geodynamics of continental deep subduction.
The study led by Chen's team found for the first time the presence of the UHP metamorphic mineral coesite in the Sesia Zone (Fig. 1), which unambiguously indicates that the studied continental crustal rocks have undergone UHP metamorphism. Through thermodynamic phase equilibrium simulations, they determined that the UHP metamorphism occurs at 2.8-3.3 GPa (1 GPa = 109 Pa, about 10,000 atmospheric pressure). These results indicate that part of the crustal rocks in this area were once subducted to a mantle depth of about 80-120 km.
"It is tremendously difficult to find the coesite inclusion in minerals." Chen explains. They produced lots of thin sections, firstly observed by microscope, and then carried out numerous analyses by laser Raman spectroscopy, a method that can effectively identify whether the coesite occurs. "It is an amazing moment when I see the typical peak that shows the occurrence of coesite. I am very excited." Chen Said.
Using zircon uranium-lead isotope dating and trace element analysis, they further constrained the UHP age to be 76 million years ago. This is significantly earlier than the timing of UHP metamorphism for the oceanic rocks in the Piemont-Liguria Ocean (about 44 million years ago; Figure 2). Therefore, the deep subduction of the Sesia Zone could not have been caused by the pull force produced by the subduction of this oceanic plate.
Based on the tectono-metamorphic-magmatic record of the Sesia Zone at that time, it is clear that there is absence of any mature, large ocean basins closely associated with the continental deep subduction. Thus, slab pull cannot be the main driving force. Instead, considering of the plate tectonic reconstruction of the Sesia Zone, the deep subduction of the continental crust is more likely to be the distal push produced by ocean floor spreading or active continental rifting during convergence between Africa plate and European plate.
"Our results not only provide new constraints on the tectonic evolution of the Western Alps, a classical orogenic belt that has a long study history of more than 200 years, but also provide new insights into the geodynamic mechanism of continental deep subduction in the rifted continental margins." Chen said.
This study will inspire further discussion on the geodynamic mechanism of continental deep subduction in the future. This includes: (1) Which kind of rifted continental margins that undergo deep subduction is caused by distal push rather than proximal slab pull? (2) If the rifted continental margin is also in contact with a mature oceanic basin, what is the dominant driving force for its deep subduction? (3) Can we simulate by means of geodynamical modeling the dynamic picture and mechanism of continental deep subduction at the passive continental margin formed after the breakup of supercontinent? (4) Can we constrain the geometrical and geological structure of paleo-convergent continental margins and then precisely reveal the geodynamic process of continental collision? Future studies may give us the answer.
