Archean subduction

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Archean subduction is a contentious topic involving the possible existence and nature of subduction in the Archean, a geologic eon extending from 4.0-2.5 billion years ago. Until recently there was little evidence unequivocally supporting one side over the other, and in the past many scientists either believed in shallow subduction or its complete non-existence. However, the past two decades have witnessed the potential beginning of a change in geologic understanding as new evidence is increasingly indicative of episodic, non-shallow subduction. [1]

Contents

The importance of Archean subduction

Subduction is the density-driven process by which one tectonic plate moves under another and sinks into the mantle at a convergent boundary. Gravitational pull from dense slabs provides approximately 90% of the driving force for plate tectonics, [2] and consequently subduction is crucial in changing the Earth's layout, guiding its thermal evolution [3] and building its compositional structure. [1] In particular, subduction zones are the primary sites of present-day continental crust formation, [4] another process of modern Earth that has a mysterious past. Furthermore, subduction is the main mechanism by which surface materials enter the deep Earth [5] and is also largely responsible for the formation of ores. [6] Considering the importance of subduction in many geological processes, it is clear that studying its past and present nature is essential to developing our understanding of the Earth as a dynamic system.

The case against Archean subduction

Those who favour non-existent subduction in the Archean point to the well-established model that the Archean Earth was significantly hotter than it is today, which would have affected lithospheric density in such a way as to perhaps prohibit subduction. The higher temperatures of the Archean Earth can be attributed to the release of tremendous amounts of energy from the accretion of Solar System material and subsequent differentiation into core and mantle. [1] This energy, coupled with a greater concentration of heat-producing elements, [7] led to the Earth being 200 K hotter in the Archean than it is today. [3] Assuming seafloor spreading generated oceanic lithosphere in the Archean, higher temperatures led to greater melting of mantle material rising at oceanic spreading centres. [8] This in turn produced thicker oceanic crust and thicker regions of underlying depleted lithospheric mantle. [8] As such, the density of the lithosphere was reduced due to both differentiation of the crust from the mantle and the ensuing relative depletion of the residual mantle in Fe and Al. [9] These expected properties have led to suggestions that oceanic lithosphere was so light that it subducted very shallowly or not at all. [10] Scientists who favour this hypothesis argue that felsic material formed from hydrous partial melting of thickened oceanic crust in the root zones of oceanic plateaus, [11] and not from subduction zones as generally believed.

The case for Archean subduction

Those who favour Archean subduction claim that recent modelling has elucidated the following fundamental features of the Archean, which they argue can be used to describe why subduction was occurring:

1) Mantle temperatures were indeed 200 K hotter than they are today. [9]

2) The oceanic crust was approximately 21 km thick, compared to 7 km thick today. [9]

3) The depth to which the mantle was partially melted was 114 km, compared to 54 km today. [9]

4) Heat flow into the base of the tectonic plates was 1.3-2.0 times higher than it is today. [9]

Mathematical reasoning based on these constraints led to the conclusion that cooling was sufficient to provide a driving force for subduction. [9] In fact, it is thought that the low flexural rigidity of Archean plates perhaps made subduction initiation easier than it is today. [9] On one hand, the lower density of oceanic plates reduced slab pull, but this effect was likely balanced by delamination of low-density crust as well as the passage of thick crust through the eclogite transition. [9] In addition to modelling, geologic evidence has been discovered that further supports the existence of Archean subduction. Many Archean igneous rocks show enrichment of large-ion lithophile elements (LILE) over high-field-strength elements (HFSE), which is a classic subduction signature commonly observed in volcanic arc rocks. [1] Furthermore, the presence of structural thrust belts and paired metamorphic belts are also hallmarks of subduction dynamics and subsequent environmental changes. [1]

While the existence of Archean subduction implies that continental crust likely formed via subduction to an extent, it does not require that subduction was the only way to form continental crust. Thus the continued debate over the origin of continental crust cannot be fully resolved by subduction arguments alone.

Conclusion and future directions

Though the subject of Archean subduction has long been controversial, the emergence of innovative modelling and geologic evidence has begun to sway some of the scientific community toward favouring the existence of non-shallow, episodic subduction. Moving forward, the rheology of early-Earth materials should be emphasized in future research as it is not well understood, and therefore subduction dynamics are poorly constrained. [1] Moreover, the paucity of Archean data requires an even better understanding of the links between the Earth's interior and its surface processes if we plan on gaining additional insight into Archean subduction. [1]

Related Research Articles

<span class="mw-page-title-main">Plate tectonics</span> Movement of Earths lithosphere

Plate tectonics is the scientific theory that Earth's lithosphere comprises a number of large tectonic plates which have been slowly moving since about 3.4 billion years ago. The model builds on the concept of continental drift, an idea developed during the first decades of the 20th century. Plate tectonics came to be accepted by geoscientists after seafloor spreading was validated in the mid-to-late 1960s.

<span class="mw-page-title-main">Lithosphere</span> Outermost shell of a terrestrial-type planet or natural satellite

A lithosphere is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust and the lithospheric mantle, the topmost portion of the upper mantle that behaves elastically on time scales of up to thousands of years or more. The crust and upper mantle are distinguished on the basis of chemistry and mineralogy.

<span class="mw-page-title-main">Subduction</span> A geological process at convergent tectonic plate boundaries where one plate moves under the other

Subduction is a geological process in which the oceanic lithosphere and some continental lithosphere is recycled into the Earth's mantle at convergent boundaries. Where the oceanic lithosphere of a tectonic plate converges with the less dense lithosphere of a second plate, the heavier plate dives beneath the second plate and sinks into the mantle. A region where this process occurs is known as a subduction zone, and its surface expression is known as an arc-trench complex. The process of subduction has created most of the Earth's continental crust. Rates of subduction are typically measured in centimeters per year, with rates of convergence as high as 11 cm/year.

Obduction is a geological process whereby denser oceanic crust is scraped off a descending ocean plate at a convergent plate boundary and thrust on top of an adjacent plate. When oceanic and continental plates converge, normally the denser oceanic crust sinks under the continental crust in the process of subduction. Obduction, which is less common, normally occurs in plate collisions at orogenic belts or back-arc basins.

<span class="mw-page-title-main">Convergent boundary</span> Region of active deformation between colliding tectonic plates

A convergent boundary is an area on Earth where two or more lithospheric plates collide. One plate eventually slides beneath the other, a process known as subduction. The subduction zone can be defined by a plane where many earthquakes occur, called the Wadati–Benioff zone. These collisions happen on scales of millions to tens of millions of years and can lead to volcanism, earthquakes, orogenesis, destruction of lithosphere, and deformation. Convergent boundaries occur between oceanic-oceanic lithosphere, oceanic-continental lithosphere, and continental-continental lithosphere. The geologic features related to convergent boundaries vary depending on crust types.

<span class="mw-page-title-main">Island arc</span> Arc-shaped archipelago formed by intense seismic activity of long chains of active volcanoes

Island arcs are long chains of active volcanoes with intense seismic activity found along convergent tectonic plate boundaries. Most island arcs originate on oceanic crust and have resulted from the descent of the lithosphere into the mantle along the subduction zone. They are the principal way by which continental growth is achieved.

<span class="mw-page-title-main">Craton</span> Old and stable part of the continental lithosphere

A craton is an old and stable part of the continental lithosphere, which consists of Earth's two topmost layers, the crust and the uppermost mantle. Having often survived cycles of merging and rifting of continents, cratons are generally found in the interiors of tectonic plates; the exceptions occur where geologically recent rifting events have separated cratons and created passive margins along their edges. Cratons are characteristically composed of ancient crystalline basement rock, which may be covered by younger sedimentary rock. They have a thick crust and deep lithospheric roots that extend as much as several hundred kilometres into Earth's mantle.

<span class="mw-page-title-main">Continental crust</span> Layer of rock that forms the continents and continental shelves

Continental crust is the layer of igneous, metamorphic, and sedimentary rocks that forms the geological continents and the areas of shallow seabed close to their shores, known as continental shelves. This layer is sometimes called sial because its bulk composition is richer in aluminium silicates (Al-Si) and has a lower density compared to the oceanic crust, called sima which is richer in magnesium silicate (Mg-Si) minerals. Changes in seismic wave velocities have shown that at a certain depth, there is a reasonably sharp contrast between the more felsic upper continental crust and the lower continental crust, which is more mafic in character.

<span class="mw-page-title-main">Earth's crust</span> Earths outer shell of rock

Earth's crust is Earth's thick outer shell of rock, referring to less than 1% of Earth's radius and volume. It is the top component of the lithosphere, a division of Earth's layers that includes the crust and the upper part of the mantle. The lithosphere is broken into tectonic plates whose motion allows heat to escape the interior of the Earth into space.

<span class="mw-page-title-main">North China Craton</span> Continental crustal block in northeast China, Inner Mongolia, the Yellow Sea, and North Korea

The North China Craton is a continental crustal block with one of Earth's most complete and complex records of igneous, sedimentary and metamorphic processes. It is located in northeast China, Inner Mongolia, the Yellow Sea, and North Korea. The term craton designates this as a piece of continent that is stable, buoyant and rigid. Basic properties of the cratonic crust include being thick, relatively cold when compared to other regions, and low density. The North China Craton is an ancient craton, which experienced a long period of stability and fitted the definition of a craton well. However, the North China Craton later experienced destruction of some of its deeper parts (decratonization), which means that this piece of continent is no longer as stable.

<span class="mw-page-title-main">Slab (geology)</span> The portion of a tectonic plate that is being subducted

In geology, the slab is a significant constituent of subduction zones.

Tectonic subsidence is the sinking of the Earth's crust on a large scale, relative to crustal-scale features or the geoid. The movement of crustal plates and accommodation spaces produced by faulting brought about subsidence on a large scale in a variety of environments, including passive margins, aulacogens, fore-arc basins, foreland basins, intercontinental basins and pull-apart basins. Three mechanisms are common in the tectonic environments in which subsidence occurs: extension, cooling and loading.

<span class="mw-page-title-main">Eclogitization</span> The tectonic process in which the dense, high-pressure, metamorphic rock, eclogite, is formed

Eclogitization is the tectonic process in which the high-pressure, metamorphic facies, eclogite, is formed. This leads to an increase in the density of regions of Earth's crust, which leads to changes in plate motion at convergent boundaries.

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

<span class="mw-page-title-main">Subcontinental lithospheric mantle</span>

The subcontinental lithospheric mantle (SCLM) is the uppermost solid part of Earth's mantle associated with the continental lithosphere.

<span class="mw-page-title-main">Tonalite–trondhjemite–granodiorite</span> Intrusive rocks with typical granitic composition

Tonalite–trondhjemite–granodiorite (TTG) rocks are intrusive rocks with typical granitic composition but containing only a small portion of potassium feldspar. Tonalite, trondhjemite, and granodiorite often occur together in geological records, indicating similar petrogenetic processes. Post Archean TTG rocks are present in arc-related batholiths, as well as in ophiolites, while Archean TTG rocks are major components of Archean cratons.

<span class="mw-page-title-main">Earth's crustal evolution</span>

Earth's crustal evolution involves the formation, destruction and renewal of the rocky outer shell at that planet's surface.

Ridge push is a proposed driving force for plate motion in plate tectonics that occurs at mid-ocean ridges as the result of the rigid lithosphere sliding down the hot, raised asthenosphere below mid-ocean ridges. Although it is called ridge push, the term is somewhat misleading; it is actually a body force that acts throughout an ocean plate, not just at the ridge, as a result of gravitational pull. The name comes from earlier models of plate tectonics in which ridge push was primarily ascribed to upwelling magma at mid-ocean ridges pushing or wedging the plates apart.

<span class="mw-page-title-main">Plate theory (volcanism)</span>

The plate theory is a model of volcanism that attributes all volcanic activity on Earth, even that which appears superficially to be anomalous, to the operation of plate tectonics. According to the plate theory, the principal cause of volcanism is extension of the lithosphere. Extension of the lithosphere is a function of the lithospheric stress field. The global distribution of volcanic activity at a given time reflects the contemporaneous lithospheric stress field, and changes in the spatial and temporal distribution of volcanoes reflect changes in the stress field. The main factors governing the evolution of the stress field are:

  1. Changes in the configuration of plate boundaries.
  2. Vertical motions.
  3. Thermal contraction.

Intraplate volcanism is volcanism that takes place away from the margins of tectonic plates. Most volcanic activity takes place on plate margins, and there is broad consensus among geologists that this activity is explained well by the theory of plate tectonics. However, the origins of volcanic activity within plates remains controversial.

References

  1. 1 2 3 4 5 6 7 Van Hunen, J; Moyen JF (2012). "Archean Subductionl: Fact or Fiction?". Annual Review of Earth and Planetary Sciences. 40 (1): 195–219. Bibcode:2012AREPS..40..195V. doi:10.1146/annurev-earth-042711-105255.
  2. ↑ Forsyth, D; Uyeda, S (1975). "On the relative importance of the driving forces of plate motion". Geophysical Journal of the Royal Astronomical Society. 43 (1): 163–200. Bibcode:1975GeoJ...43..163F. doi: 10.1111/j.1365-246X.1975.tb00631.x .
  3. 1 2 Jaupart, C; Labrosse S; Mareschal J-C (2007). "Temperatures, heat and energy in the mantle of the Earth". Treatise on Geophysics: 253–303. Bibcode:2007mady.book..253J.
  4. ↑ Davidson, JP; Arculus, RJ (2006). "The significance of Phanerozoic arc magmatism in generating continental crust". Evolution and Differentiation of Continental Crust: 135–172.
  5. ↑ R̈upke, LH; Morgan JP; Hort M; Connolly JAD (2004). "Serpentine and the subduction zone water cycle". Earth and Planetary Science Letters. 223 (1–2): 17–34. Bibcode:2004E&PSL.223...17R. doi:10.1016/j.epsl.2004.04.018.
  6. ↑ Bierlein, FP; Groves DI; Cawood PA (2009). "Metallogeny of accretionary orogens – the connection between lithospheric processes and metal endowment". Ore Geology Reviews. 36 (4): 282–292. Bibcode:2009OGRv...36..282B. doi:10.1016/j.oregeorev.2009.04.002.
  7. ↑ Leitch, AM (2004). "Archean Plate Tectonics". American Geophysical Union, Spring Meeting.
  8. 1 2 Sleep, NH; Windley BF (1982). "Archean plate tectonics: constraints and inferences". Journal of Geology. 90 (4): 363–379. Bibcode:1982JG.....90..363S. doi:10.1086/628691. S2CID   129466505.
  9. 1 2 3 4 5 6 7 8 Hynes, A (2014). "How feasible was subduction in the Archean?". Canadian Journal of Earth Sciences. 51 (3): 286–296. Bibcode:2014CaJES..51..286H. doi:10.1139/cjes-2013-0111.
  10. ↑ Abbott, DH; Drury R; Smith WHF (1994). "Flat to steep transition in subduction style". Geology. 22 (10): 937–940. Bibcode:1994Geo....22..937A. doi:10.1130/0091-7613(1994)022<0937:ftstis>2.3.co;2.
  11. ↑ Condie, KC (2011). "Did early Archean continental crust form without plate tectonics?". Geological Society of America Fall Meeting. 43 (5).
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