It’s possible that I shall make an ass of myself. But in that case one can always get out of it with a little dialectic. I have, of course, so worded my proposition as to be right either way (K.Marx, Letter to F.Engels on the Indian Mutiny)
Monday, December 11, 2023
Evidence that ancient Tethys Ocean influenced Earth’s past environments
IN THE CONTEXT OF THE “TETHYS ONE-WAY TRAIN” (LONG-TERM CYCLICAL NORTHWARD BREAKUP-DRIFTING OF GONDWANA CONTINENTAL FRAGMENTS), INCREASE IN LOW-LATITUDE CONTINENTAL AREA LEADS TO THE DECREASE IN GLOBAL TEMPERATURE; THE SUBDUCTION OF OCEANIC PLATES ENRICHING IN ORGANIC-RICH SEDIMENTS AFFECTS THE ARC MAGMA OXYGEN FUGACITY.
This study, led by geologist Bo Wan from the Chinese Academy of Sciences' Institute of Geology and Geophysics, connects the Tethyan plate tectonic movements to changes in Earth’s surface environment. The work reveals Earth’s multi-sphere interactions.
The story begins from the Tethyan tectonic evolution, represented by the opening and closure of the succession of the Proto-, Paleo-, and Neo-Tethys oceans. These three oceans opening and closing allowed continental fragments of Gondwana to drift northward one after the other. Such latitudinal movement of continents will change the sea-land distribution in the warm and humid tropical region. How Earth’s surface environments respond to such sea-land changes in the tropics is still poorly known, according to the authors.
First, the authors have found a temporal linkage between significant increases in continental area at low latitudes and global surface cooling effects during the past 500 million years. The authors then go further beyond temporal connections to causal linkages between the two surprisingly connected phenomena.
Bo Wan explains it in this way: “Low-latitude regions receive more solar energy influx on Earth’s surface than high latitude areas. Therefore, an increase of landmass in low-latitude regions attenuates the net energy absorption by the Earth’s surface, consequently impeding the conduction and convection of absorbed energy toward the poles. The eventual result is a decrease in global surface temperature.”
Following such logic, the authors used subduction-driven plate tectonics in the Tethys realm to explain significant ice ages over the past few hundreds of millions of years.
Second, the authors also found that the surface environmental changes can affect the mineral resources in the Tethyan realm.
The tropical regions are ideal for the flourishing of marine plankton species and therefore the generation of organic-rich sediment. Massive biological debris is therefore deposited on continental margins when a continent drifts across the tropics. This creates favorable conditions for subsequent hydrocarbon and reservoir formation. Moreover, northward subduction of organic-rich sediments during the closure of the Tethyan oceans could result in the generation of mafic arc magmas with low oxygen fugacity. This chemical environment helps the mineralization of reduced-type ore deposits such as tungsten, tin, and lithium.
In sum, subduction-driven plate tectonics in the Tethys realm changes the distribution of oceans and landmass, subsequently affecting the balance and distribution of solar energy across Earth’s surface. These changes trigger consequential environmental shifts which in turn, impact the composition of rocks and minerals along the Eurasian margin due to subduction. Altogether, the Tethyan realm and its history is an ideal natural laboratory for comprehending the processes and changes of the entire Earth’s system.
See the article:
Wan B, Wu F, Zhu R. 2023. The influence of Tethyan evolution on changes of the Earth’s past environment. Science China Earth Sciences, 66(12):2653‒2665, https://doi.org/10.1007/s11430-023-1185-3
(A) THE PROTO-TETHYS TECTONIC DOMAIN IN WESTERN CHINA. (B) THE PALEO-TETHYS TECTONIC DOMAIN IN EASTERN CHINA. “SUB”, “COL”, “EXH”, AND “P-CO” DENOTE SUBDUCTION, COLLISION, EXHUMATION, AND POSTCOLLISIONAL STAGES, RESPECTIVELY. “C” AND “O” REPRESENT CONTINENTAL AND OCEANIC SUBDUCTION, RESPECTIVELY.
This synthesis study is led by Prof. Yong-Fei Zheng at University of Science and Technology of China. It focuses on the thermal and tectonic evolution of regional metamorphism at convergent continental margins based on a systematic outline of metamorphic temperature (T), pressure (P) and time (t) information on high-grade metamorphic rocks along the Central China Orogenic System (CCOS). The CCOS includes the Proto-Tethys tectonic domain in western China and the Paleo-Tethys tectonic domain in eastern China, which were produced by the closure of the Proto-Tethys and Paleo-Tethys oceans, respectively. As metamorphic rocks along the CCOS were produced at different stages from continental subduction to post-collisional extension, they are robust recorders of metamorphic thermobaric information at the convergent continental margins, and bear great significance to reconstruct their thermal and tectonic evolution.
Metamorphic T/P ratios and corresponding geothermal gradients for high-grade metamorphic rocks from both Proto-Tethys and Paleo-Tethys tectonic domains shows an increasing trend with age. It appears that these high-grade metamorphic rocks experienced three types of regional metamorphism in terms of their metamorphic T/P ratios: (1) an early stage of low T/P Alpine-type blueschist- to eclogite-facies high-P to ultrahigh-P metamorphism; (2) a middle stage of medium T/P Barrovian-type medium-P amphibolite to high-P granulite-facies metamorphism; and (3) a late stage of the high T/P Buchan-type low P amphibolite to MP granulite-facies metamorphism. For the ages of Alpine-type, Barrovian-type, and Buchan-type metamorphisms, they occurred at 500 to 400 Ma in the Paleozoic for the Proto-Tethys tectonic domain and 250 to 120 Ma in the Mesozoic for the Paleo-Tethys tectonic domain.
Convergent continental margins are characterized by the change of geothermal gradients during their tectonic evolution from dynamic compression to extension, giving rise to metamorphic rocks with different T/P ratios. In the stage of oceanic subduction, low geothermal gradients make cold subduction to produces low T/P Alpine-type metamorphic rocks. In the continental subduction/collision stage, the deep subduction of continental crust took place at low geothermal gradients but its collisional thickening occurred at moderate geothermal gradients. Therefore, this stage would produce both low T/P Alpine-type and moderate T/P Barrovian-type metamorphic rocks. In the post-collisional stage, the thickened continental lithosphere was thinned due to its gravitational or rheological instability, inducing asthenospheric upwelling and leading the thinned lithosphere to high geothermal gradients for high T/P Buchan-type metamorphism.
Based on the P-T information on and spatiotemporal relationships between the metamorphic rocks in the different orogens along the CCOS, the tectonic evolution of the Proto-Tethys and Paleo-Tethys tectonic domains can be reconstructed as follows. The continental collision/subduction in the Proto-Tethys orogenic system would occur either at 500–490 Ma in the Altyn, North Qinling and North Tongbai orogens, or at 450–430 Ma in the North Qaidam and East Kunlun orogens, and continental rifting would occur either at 460–450 Ma in the Altyn, North Qinling and North Tongbai orogens or at 410–400 Ma in the North Qaidam and East Kunlun orogens. On the other hand, continental subduction/collision in the Paleo-Tethys orogenic system would occur at 250–220 Ma, and continental rifting would occur at 140–120 Ma.
Although timescales of either continental deep subduction or hard collision are restricted to 10-30 Myr, time intervals between the low T/P Alpine-type metamorphism and the high T/P Buchan-type metamorphism are as short as about 40–60 Myr for the Proto-Tethys tectonic domain but as long as about 110 Myr for the Paleo-Tethys tectonic domain. These similarities and differences indicate that the high-grade metamorphic rocks in the two different Tethys tectonic domains record the tectonic transition of convergent continental margins from cold subduction through warm collision or exhumation to hot rifting. This is associated with the metamorphic transformation from the early Alpine-type facies series to the late Barrovian-type facies series, and then superimposition by Buchan-type facies series.
See the article:
Zhang Q Q, Gao X Y, Chen R X, Zheng Y F. 2023. Metamorphic evolution of the East Tethys tectonic domain and its tectonic implications. Science China Earth Science, 66(12): 2686–2711; https://doi.org/10.1007/s11430-023-1209-6
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