Earth’s continents are slowly moving across the planet’s surface due to plate tectonics, culminating in regions of crustal expansion and collision. In the latter case, high temperatures and pressures lead to the reworking of the crust, affecting its composition, as well as that of the underlying mantle. Furthermore, when two continental plates collide, distinct topographic features are produced, namely mountain ranges, which are surficial manifests of Earth’s thickened crust.
Three such collision zones form the Himalaya-Tibetan Plateau, the European Alps and the Zagros Mountains in Iran, Iraq and Turkey, originating during the Cenozoic (last 66 million years). New research, published in Earth and Planetary Science Letters, has attempted to quantify the amount of continental crust lost to the mantle when two plates collide at each of these boundaries.
To do so, Dr. Ziyi Zhu, Research Fellow at Monash University, Australia, and colleagues developed a theoretical model for the mass/volume balance of continental crust and compared the amount of shortened crust with the crust being vertically thickened, laterally extruded and eroded at the surface.
Simplifying the utility of each of these parameters in the calculations, Dr. Zhu says, “Imagine squeezing a soft chocolate bar: the material compressed (horizontal shortening) forms a pile (vertical thickening).
“Additionally, the crust can move in directions perpendicular to the compression (extrusion) or undergo erosion. If crustal mass is conserved, the mass of the shortened crust should balance with the mass of the thickened crust, along with any crust lost to erosion or lateral extrusion. Any imbalance indicates that the missing crust likely sinks into the mantle.”
The research team identified that at least 30% of continental crust was lost to the mantle during the formation of the Himalaya-Tibetan Plateau and Zagros Mountains (potentially up to 64% for the latter, depending upon the initial crust thickness), while up to 50% of the Alps’ volume may have been destroyed. Importantly, this loss to the mantle had double the destructive effect than that of surface erosion, which is estimated based on the volumes of sediment fans associated with each mountain range.
Detailing the importance of this research, Dr. Zhu states, “Our research quantifies the amount of Earth’s crust lost to the mantle during continental collisions, such as those forming the Himalayas. While it’s widely known that erosion from the massive Himalayan mountains has created Earth’s largest and second largest sedimentary fans (the Bengal and Indus fans), our findings indicate that crustal loss into the mantle is actually twice that of surface erosion.”
Dr. Zhu explains that delamination is the likely mechanism responsible for crustal recycling during the formation of the Himalayan-Tibetan Plateau.
“This process involves the rapid sinking or detachment of the lower continental lithosphere into the mantle, driven by the increased density of eclogites that formed at the base of the mountain roots. Evidence of delamination includes the formation of potassic-adakitic rocks, whose deep-source geochemical signatures suggest they were created by heat from the upwelling hot asthenosphere (a molten portion of Earth’s mantle) following the delamination of the mountain roots.
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“A key consequence of delaminating dense mountain roots is rapid uplift of the mountain range above. Imagine a piece of floating wood with an iron layer attached beneath it, which partially submerges it; if the iron layer detaches and sinks, the wood will pop up to the surface.
“In the Himalayan region, this delamination-induced rapid uplift, corresponding with the ages of potassic-adakitic rocks, aligns with the onset of intensified monsoon rainfall around 22 million years ago. This highlights a connection between deep crustal processes and surface climate change during mountain belt formation by continent-continent collision.”
Specifically, since the onset of the Himalaya-Tibetan Plateau formation approximately 59 million years ago, plate reconstructions suggest India and Asia have converged by ~3,000 km, though only 1,000-2,000 km of this can be attributed to crustal shortening. The remainder is considered to have not been preserved in the rock record but attributed to a portion of continental crust that was subducted or delaminated into the mantle, known as Greater India.
Meanwhile, the European Alps orogeny began forming around 35 million years ago, with half of the crustal volume potentially being destroyed. This loss may be due to the subduction of the lower continental crust into the mantle, as qualitatively described in previous studies.
At the same time, the Zagros Mountains initiated formation with the collision of Arabia and Eurasia. Here, the orogenic loss is attributed to a combination of factors, including the loss of continental margin with the detached oceanic slab, continental subduction and dripping, a process whereby ‘blobs’ of continental crust drip from its base.
“Mountain ranges resulting from continental collisions also formed further back in geological history, especially during the assembly of supercontinents, when massive continental mass came together (for instance, the collision between East and West Gondwana created a vast mountain belt known as the Transgondwanan Supermountains around 500 million years ago),” Dr. Zhu explains.
“Therefore, if similar orogenic loss processes took place, as seen in the Himalaya-Tibetan Plateau, European Alps and Zagros Mountains, substantial amounts of continental materials would have been recycled into the mantle during mountain-building events, ‘contaminating’ the mantle over billions of years throughout past supercontinent cycles.”
More information:
Ziyi Zhu et al, Quantifying the loss of continental crust into the mantle from volume/mass balance calculations in modern collisional mountains, Earth and Planetary Science Letters (2024). DOI: 10.1016/j.epsl.2024.119070
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Himalayas formation may have destroyed at least 30% of continental crust in collision zone (2024, November 28)
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