Development Of Subduction Zones Research Articles

A simple force balance model of subduction initiation

SUMMARY The initiation and development of subduction zones are associated with substantial stress changes both within plates and at plate boundaries. We formulate a simple analytical model based on the force balance equation of a subduction zone, and validate it with numerical calculations of highly non-linear, coupled thermomechanical system. With two kinds of boundary conditions with either fixed velocity or fixed force in the far-field, we quantitatively analyse the role of each component in the force balance equation, including slab pull, interplate friction, plate bending and basal traction, on the kinematics and stress state of a subducting plate. Based on the numerical and analytical models, we discuss the evolution of plate curvature, the role of plastic yielding and elasticity, and how different factors affect the timing of subduction initiation. We demonstrate with the presence of plastic yielding for a plate of thickness, H, that the bending force is proportional to H2, instead of H3 as previously thought. Although elasticity increases the force required to start nucleating subduction it does not substantially change the total work required to initiate a subduction zone when the yielding stress is small. The analytical model provides an excellent fit to the total work and time to initiate subduction and the force and velocity as a function of convergence and time. Plate convergence and weakening rate during nucleation are the dominant factors influencing the force balance of the plate, and 200 km of plate convergence is typically required to bring a nascent subduction zone into a self-sustaining state. The closed-form solution now provides a framework to better interpret even more complex, time-dependent systems in three dimensions.

Toward an integrated model of geological evolution for NE Brazil-NW Africa: The Borborema Province and its connections to the Trans-Saharan (Benino-Nigerian and Tuareg shields) and Central African orogens

Both the Borborema Province of NE Brazil and the geological provinces of NW Africa (the Trans-Saharan Orogen consisted of the Tuareg and Benino-Nigerian shields and the Central African Orogen of Cameroon, Chad, and Central African Republic) are complex geological regions with superposition of distinct deformational, metamorphic and magmatic events and final structural configuration during the Brasiliano/Pan-African Orogeny (ca. 625-510 Ma). These provinces represent the site of major mountain building processes in the Ediacaran/Cambrian transition that culminated in the amalgamation of West Gondwana after the collision of the West African-São Luís, São Francisco-Congo, and Saharan paleocontinents. In the last years, discovery and characterization of key tectonic units such as ophiolites, eclogites, HP/UHP rocks, and both oceanic and continental magmatic arcs are helping to clarify these processes and propose tectonic models for the geological evolution of NE Brazil-NW Africa. Connections of the marginal belts that frame these provinces, bordering the eastern margin of the West African-São Luís Craton (Médio Coreaú-Dahomeyides-Gourma-West Tuareg Shield) and the northern margin of the São Francisco-Congo Craton (Rio Preto-Riacho do Pontal-Sergipano-Yaoundé-Central African) are progressively better constrained, while correlations within the interior, highly reworked and sectioned portions of both the Borborema Province, the Benino-Nigerian Shield, the Central and East Tuareg Shield, Western Cameroon, and Adamawa-Yadé domains are more complicated and demand further investigation. Some of the questions of prime importance in this context are the continuation or not of the 1000-920 Ma Cariris Velhos Belt of NE Brazil into NW Africa, and if the basement-dominated North Borborema/Benino-Nigerian (NOBO-BENI) and Alto Pajeú-Alto Moxotó-Rio Capibaribe-Pernambuco-Alagoas/Adamawa-Yade (APAMCAPAY) domains could represent major decratonized blocks (such as LATEA in the Central Tuareg Shield), perhaps developed due to hyperextension and detachment of a Greater São Francisco-Congo paleocontinent northern margin. In this case, the Goiás-Pharusian and Transnordestino-Central African oceanic realms along with restricted internal oceans such as the hypothetical Piancó-Alto Brígida/Western Cameroon (PAB-WECA) Seaway probably separated these ancient paleocontinental blocks during the Neoproterozoic. The development of subduction zones and the docking of Neoproterozoic juvenile terranes welded the hyperextended Archean/Paleoproterozoic lithospheric fragments together and they became squeezed and reworked in between the major cratonic landmasses during the Brasiliano/Pan-African Orogeny. The quest for the sites of ancient oceans and continents that once composed NE Brazil and NW Africa goes on and tentative scenarios will surely benefit from novel geological, isotopic, and geochronological data put forward in the near future.

Relative impact of mantle densification and eclogitization of slabs on subduction dynamics: A numerical thermodynamic/thermokinematic investigation of metamorphic density evolution

Understanding the relationships between density and spatio-thermal variations at convergent plate boundaries is important for deciphering the present-day dynamics and evolution of subduction zones. In particular, the interaction between densification due to mineralogical phase transitions and slab pull forces is subject to ongoing investigations. We have developed a two-dimensional subduction zone model that is based on thermodynamic equilibrium assemblage calculations and includes the effects of melting processes on the density distribution in the lithosphere. Our model calculates the “metamorphic density” of rocks as a function of pressure, temperature and chemical composition in a subduction zone down to 250km. We have used this model to show how the hydration, dehydration, partial melting and fractionation processes of rocks all influence the metamorphic density and greatly depend on the temperature field within the subduction system. These processes are largely neglected by other approaches that reproduce the density distribution within this complex tectonic setting. Our model demonstrates that the initiation of eclogitization (i.e., when crustal rocks reach higher densities than the ambient mantle) of the slab is not the only significant process that makes the descending slab denser and generates the slab pull force. Instead, the densification of the lithospheric mantle of the sinking slab starts earlier than eclogitization and contributes significantly to slab pull in the early stages of subduction. Accordingly, the complex metamorphic structure of the slab and the mantle wedge has an important impact on the development of subduction zones.

Internal stress in block media as a factor responsible for interface strain activity. Estimation of excess tectonic stress

The paper studies the mechanisms of strain localization at the interfaces of structural elements in quasi-2D block-structured (plate) systems. The model plate system is the ice cover of the Lake Baikal. The emphasis in the paper is on analyzing the relationship between intraplate strains and interplate displacements. It is shown that the activation of convergent interplate displacements and the development of subduction zones in the ice cover are due to an increase in tensile intraplate stress, and this is consistent with the main mechanisms of tectonic processes in the lithosphere. Analysis of the stress distribution allows the conclusion that the deformation mechanisms in the block-structured ice cover fit the concept of plate tectonics. The obtained results make it possible to estimate the characteristic tectonic stress (2–10% of the plate strength) at which convergent interplate displacements are activated. It is found that in individual regions of the plate system, anomalously high internal stress (up to 20–30% of the plate strength) can be reached resulting in fragmentation of structural blocks and involvement of the thus formed interfaces in deformation. It is demonstrated that with the so high internal stress, the deformation at the interplate boundaries is far from typical, and this can be considered as a precursor of coming dynamic seismogenic events.

Evolution of paleoproterozoic magmatism: Geology, geochemistry, and isotopic constraints

Evolution of tectono-magmatic processes in the Paleoproterozoic is divisible into four stages each controlled by its own type of endogenic activity. The critical moment in geological history was at the third stage onset 2.05 Ga ago, when the Archean style of evolution gave way to subsequent one. Magmas of siliceous high- Mg (boninite-like) series (SHMS) dominant during the first stage (2.5-2.3 Ga) had the mixed mantle-crust gen- esis. They originated in highly depleted ultramafic reservoir of asthenospheric mantle being contaminated by the Archean crustal material during ascent to the surface. Typical of the SHMS magmas were the high content of silica, Al, Mg, Cr and elevated concentrations of Ni, Co, Cu, V, PGE and incompatible elements, LREE included, while concentrations of Fe, Ti, Nb and alkalies, especially K, were at a relatively low level. Magma- tism of K-granitoid type was of a limited significance. The second stage, especially its second substage (2.3- 2.05 Ga), was distinct because of mass eruptions of picritic and basaltic magmas enriched in Fe, Ti, Mn, P and incompatible elements, LREE in particular, which also had elevated Cr, Ni, Co, Cu and Ba concentration being relatively depleted in Mg and Al. This change in geochemistry of magmatism was independent of tectonic pro- cesses, which retained the former style. The third stage (2.05-1.8 Ga) was marked by opening of first oceans (Jormua, Purtuniq and other ophiolites) and by formation of orogens comparable with Phanerozoic orogens that was associated with development of subduction zones and back-arc basins with relevant magmatism. High Mg, Fe, Ti, Cr, Ni, Cu, V but low Th concentrations were typical of intense picrite-basaltic volcanism marking onset of the third stage. Magmas of suprasubduction genesis had considerably higher Si, Al, P, Zn, Th concentrations and very low e‡e/Al 2 O 3 ratios. Development of orogens came to the end 1.82-1.80 Ga ago. Appearance of giant belts of intraplate, usually silicic high-K volcanism juxtaposed on stabilized Paleoproterozoic orogens with abnormally thick crust was confined to the fourth stage spanning besides the initial Mesoproterozoic (1.8- 1.5 Ga). Anorthosite-rapakivi granitoid batholiths, which originated above mantle plume heads, corresponded to intermediate magma chambers of relevant magmatic systems. As is suggested, juvenile basaltic melts of this stage retained within the crust provoking a large-scale melting of sialic material. The high K, Ti, Zn, Pb, Zr and elevated Be, Sn, Y, Nb, Rb, F, W, Mo, Li and U contents characterized geochemistry of magmas, which origi- nated at the fourth stage

Late Ordovician stratigraphy, zircon provenance and tectonics, Lachlan Fold Belt, southeastern Australia

Ordovician quartz turbidites of the Lachlan Fold Belt in southeastern Australia accumulated in a marginal sea and overlapped an adjoining island arc (Molong volcanic province) developed adjacent to eastern Gondwana. The turbidite succession in the Shoalhaven River Gorge, in the southern highlands of New South Wales, has abundant outcrop and graptolite sites. The succession consists of, from the base up, a unit of mainly thick‐bedded turbidites (undifferentiated Adaminaby Group), a unit with conspicuous bedded chert (Numeralla Chert), a unit with common thin‐bedded turbidites (Bumballa Formation (new name)) and a unit of black shale (Warbisco Shale). Coarse to very coarse sandstone in the Bumballa Formation is rich in quartz and similar to sandstone in the undifferentiated Adaminaby Group. Detrital zircons from sandstone in the Bumballa Formation, and from sandstone at a similar stratigraphic level from the upper Adaminaby Group of the Genoa River area in eastern Victoria, include grains as young as 453–473 Ma, slightly older than the stratigraphic ages.The dominant detrital ages are in the interval 500–700 Ma (Pacific Gondwana component) with a lessor concentration of Grenville ages (1000–1300 Ma). This pattern resembles other Ordovician sandstones from the Lachlan Fold Belt and also occurs in Triassic sandstones and Quaternary sands from eastern Australia. The Upper Ordovician succession is predominantly fine grained, which reflects reduced clastic inputs from the source in the Middle Cambrian to earliest Ordovician Ross‐Delamerian Fold Belts that developed along the eastern active margin of Gondwana. Development of subduction zones in the Late Ordovician marginal sea are considered to be mainly responsible for the diversion of sediment and the resulting reduction in the supply of terrigenous sand to the island arc and eastern part of the marginal sea.

Plate tectonics, plate moving mechanisms and rifting

Abstract Analysis of a sequence of palaeo-reconstructions of the distribution of continents in the Western Hemisphere suggests that frictional forces exerted on the base of the lithosphere by the slowly convecting sub-lithospheric upper mantle play an important role as a driving mechanism of plate movements. With such a scenario, slab-pull and roll-back, ridge-push and deviatoric tensional stresses, related to upwellings of the asthenosphere as well as to lithospheric over-thickening in orogenic belts, can be considered as secondary, albeit important, plate moving forces. The present stress state of the globe and circumstantial geological evidence suggest that major compressional stresses can be transmitted over great distances through continental and oceanic lithosphere. Assembly of major continental masses (like Pangea) probably has an insulating effect on upper-mantle convection systems, causing their decay and reorganization. Development of new asthenospheric upwelling systems under mega-continents causes their break-up by development of deviatoric tensional stresses in the lithosphere and by exerting drag forces on its base. Extension of the lithosphere, culminating in its failure, is followed by passive advection of asthenospheric material into the space opening between the diverging plates to which it is accreted as oceanic lithosphere. At the same time developing ridge-push forces contribute to plate divergence. However, activity along seafloor spreading axes can terminate abruptly if far-field compressional stresses impede further divergence of the respective plates. This may explain the nearly contemporaneous decay of seafloor spreading axes in often distant areas during periods of plate boundary reorganization. Assuming a finite globe, the generation of new oceanic lithosphere at seafloor spreading axes has to be compensated for elsewhere by the subduction of commensurate amounts of oceanic lithosphere and/or shortening of continental crust and subduction of lower crustal material and the subcrustal lithosphere. Plate interaction, driven by mantle convection systems and their changes, ridge-push and slab-pull and roll-back, plays probably an all-important role in the development of intra-continental rift systems, the opening of new oceanic basins and the inception and development of subduction zones. Mantle plumes rising from the deep mantle do not appear to play a significant role in the development of intra-continental rifts though they probably contribute to a weakening of the lithosphere. The relative importance of the various processes contributing to the movement of lithospheric plates differs probably during the assembly and break up of Pangea-type mega-continents.

Tectonic controls on initial continental rifting and the evolution of young ocean basins—a planetary perspective

A review and comparison of planetary tectonic literature and terrestrial interpretations of both oceanic and continental tectonics suggests that the Earth, in common with other terrestrial planets, may have developed a global tectonic grid very early in its history. This grid, still preserved in ancient continental crustal fragments, is maintained by both vertical and lateral fracture propagation. It influences both the direction of initial continental rifting and the location and orientation of sea-floor fracture zones in a developing young ocean basin. Sea-floor spreading directions must continue to be controlled by the direction of easiest slip along major intercontinental wrench faults; the Atlantic Ocean appears to be still in this stage. Development of subduction zones around the margin of an ocean basin permits greater freedom in spreading direction. At this stage, as represented by the Pacific Ocean, ridge jumps and changes in spreading direction may occur as the spreading geometry readjusts to more closely reflect stresses associated with underlying convection cells. At subduction zones, rejuvenation tectonics still operate; thick, cold oceanic crust may impose its tectonic grain on the relatively thin, over-riding edge of the continental plate; but young, thin, and hot oceanic crust is forced to conform to the tectonic grain of thick, cold continental crust. Examples of both effects can be seen along the eastern margin of the Pacific. It is concluded that the structure and evolution of young ocean basins is strongly influenced by ancient fracture patterns in the adjacent continental blocks. Although greater freedom for readjustment develops as an ocean basin matures, the continued need to move, or interact with, major blocks of continental lithosphere continues to impose severe limitations on directions and rates of spreading until subduction zones decouple oceanic and continental lithosphere. Both oceanic and continental tectonics can best be understood in terms of mutual interactions between plate tectonics, rejuvenation tectonics, and continental remnants of an original ancient global-scale tectonic grid.

Plate Motions relative to the Deep Mantle and the Development of Subduction Zones

THE development of subduction zones may depend on the motion of the two converging lithospheric plates relative to the underlying deep mantle. It is likely that the plates move much more rapidly than the underlying mantle1–4, the return flow being diffused through a large region1,5. The deep mantle is thus nearly stationary and the horizontal motion of sinking plates that extend to depths of 700 km must be significantly restrained.