How do the tectonic processes governing the Earth’s surface impact the dynamics of the deep mantle? A new study illustrates the importance of subduction zones during major tectonic cycles.
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Seismology is always teaching us more about the architecture and composition of our planet. It is the study of seismic wave propagation which made it possible to establish the internal model of the Earth and the characterization of its various envelopes. Today, investigations are becoming more and more precise. Thus, scientists have discovered that the coat was far from homogeneous. Variations of speed inside the mantle testify to the presence of heterogeneities which can be linked to different processes: rise of a hot mantle plume, plunge of a cold plate at the level of a zone of subduction…
In the deepest part of the mantle, at the interface with the outer core, scientists discovered a few years ago the presence of abnormally slow speed zones. The origin of these low speed zones is still highly debated. Some scientists believe that they would be intimately linked to the evolution of Earth’s crust and suspect that they could influence the speed of tectonic plates. But how can thermochemical heterogeneities buried more than 2,800 km deep impact surface tectonics? What chain of processes connects these two fundamentally different environments? These questions are essential to understanding the evolutionary dynamics of the Earth over the long term.
The enigmatic origin of low-velocity zones in the lower mantle
As seismological studies progressed, the researchers refined the characterization of these low-velocity zones. It appears that these heterogeneities cover 20 to 30% of the surface marking the transition between the mantle and the outer core. Of them anomalies major ones have been discovered: one is located under the southern part of Africa and the other under the Pacific.
The African anomaly is also particularly large with about 1,000 km wide and about 1,200 km high. The wave speed data suggest that these areas are characterized by a different mineralogical composition than the surrounding mantle. This chemical composition could also be inherited from the composition of the primitive Earth before the process of differentiation. Alternatively, it could contain material from subducted oceanic crusts.
Tectonic plates and deep mantle: a single dynamic system
The Earth’s surface is characterized by movements continual tectonic plates. It is these movements which give birth to the architecture of the continents and which tear them apart by the rifting process or group them together during collisions. The Earth has thus seen its continental configuration constantly evolve over millions of years, passing alternately from masses continental aggregated into a single supercontinent to a completely exploded configuration, as is the case now.
However, geodynamic models show that the tectonic plates on the surface of the Earth and the deep mantle would be part of a single dynamic system. However, it is not easy to conceive how the base of the mantle can be affected by modifications in the organization of the plates. In this context, the slabs of ancient oceanic plates that have subduced and plunged deep into the mantle could be the link uniting the surface and interior processes of the Earth.
The importance of subduction zones
Researchers from the University of Wollongong (Australia) have therefore taken an interest in the evolution of these thermochemical structures present in the deep mantle over the last billion years, in relation to the evolution of tectonic plates. During this period, three supercontinents were formed: Rodinia (between 900 and 800 million years ago), Pannotia (between 620 and 600 million years ago) and the well-known pangea (between 320 and 200 million years).
During these phases of regrouping of the continental masses, the Earth is thus divided in two: a hemisphere is occupied by a supercontinent, and the other is occupied by a super ocean. The limit between these two domains is generally marked by the presence of subduction zones on the continental rim which “swallow” the ocean crust. Geodynamic models show that the slabs (the plunging oceanic plate) of these circum-continental subduction zones penetrate very deeply into the mantle.
The arrival of this crustal material with a temperature and composition different from those of the mantle would cause significant disturbances at the level of the core-mantle interface. The anomalous zones located under the supercontinent would thus find themselves cut up and partitioned by the arrival of the slabs. Conversely, the results of the modelization show that the thermochemical structure located under the superocean would be little affected.
An influence on the generation and mobility of hot spots
In addition, it appears that the modification of the thermochemical structure at the base of the mantle can lead to the rise of plumes mantle at the level of the highest areas, thus giving rise to hot spots. This architecture can in particular be modified by surface tectonic reorganizations and rearrangements of subduction zones, leading to migration of mantle plumes.
The results of the study, published in Scientific Reports, show how surface tectonic processes are connected to deep Earth dynamics. Tectonic cycles, via subduction zones, modify and model the thermochemical architectures located at the base of the mantle which, themselves, generate hot spots affecting the earth’s surface.
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