Destruction Of Continental Crust Research Articles

СУБДУКЦИОННАЯ ЭРОЗИЯ НА КОНВЕРГЕНТНЫХ ОКРАИНАХ ТИХООКЕАНСКОГО ТИПА

The paper presents a review of processes of subduction or tectonic erosion at the Pacific-type convergent margins (PTCM) including definition of “tectonic erosion”, its triggers, driving forces and consequences. We review examples of tectonic erosion at the Circum-Pacific PTCMs and at the fossil PTCMs of the Paleo-Asian Ocean (PAO) currently hosted by the Central-Asian Orogenic Belt (CAOB). Recent geological and stratigraphic studies have shown two types of PTCMs: accreting and eroding. Accreting PTCMs consist of older deposits of accretionary and frontal prisms and grow oceanward, i.e. the trench retreats. Eroding PTCMs are characterized by the destruction of the prism, approaching arc and trench and typically form during shallow-angle and fast subduction of an oceanic slab with oceanic floor topographic highs. The mechanism of tectonic erosion includes destruction of oceanic slab, island arcs, accretionary prism, fore-arc and related prism. Tectonic erosion is a common phenomenon at many Circum-Pacific PTCMs, e.g., in South America, Tonga and Nankai troughs, Alaska. Accretion and subduction of oceanic rises contributes greatly to the processes of formation, transformation and destruction of continental crust at PTCM. The episodes of tectonic erosion can be also reconstructed for an ancient ocean, for example, for the PAO, which evolution and suturing formed the CAOB. Many CAOB foldbelts (Altai, Tienshan, eastern Kazakhstan, Transbaikalia, Mongolia) carry signs of disap-pearance of big volumes of continental crust (arcs). Studying processes responsible not only for the formation of continental crust, but also for the disappearance of big volumes of crustal mate-rial is important for correct evaluation of the nature of intra-continental orogenic belts, e.g., CAOB, and development of reliable tectonic models.

Secular change in lifetime of granitic crust and the continental growth: A new view from detrital zircon ages of sandstones

U-Pb ages of detrital zircons were newly dated for 4 Archean sandstones from the Pilbara craton in Australia, Wyoming craton in North America, and Kaapvaal craton in Africa. By using the present results with previously published data, we compiled the age spectra of detrital zircons for 2.9, 2.6, 2.3, 1.0, and 0.6 Ga sandstones and modern river sands in order to document the secular change in age structure of continental crusts through time. The results demonstrated the following episodes in the history of continental crust: (1) low growth rate of the continents due to the short cycle in production/destruction of granitic crust during the Neoarchean to Paleoproterozoic (2.9–2.3 Ga), (2) net increase in volume of the continents during Paleo- to Mesoproterozoic (2.3–1.0 Ga), and (3) net decrease in volume of the continents during the Neoproterozoic and Phanerozoic (after 1.0 Ga). In the Archean and Paleoproterozoic, the embryonic continents were smaller than the modern continents, probably owing to the relatively rapid production and destruction of continental crust. This is indeed reflected in the heterogeneous crustal age structure of modern continents that usually have relatively small amount of Archean crusts with respect to the post-Archean ones. During the Mesoproterozoic, plural continents amalgamated into larger ones comparable to modern continental blocks in size. Relatively older crusts were preserved in continental interiors, whereas younger crusts were accreted along continental peripheries. In addition to continental arc magmatism, the direct accretion of intra-oceanic island arc around continental peripheries also became important for net continental growth. Since 1.0 Ga, total volume of continents has decreased, and this appears consistent with on-going phenomena along modern active arc-trench system with dominant tectonic erosion and/or arc subduction. Subduction of a huge amount of granitic crusts into the mantle through time is suggested, and this requires re-consideration of the mantle composition and heterogeneity.

The Cenozoic formations in the Peter the Great Gulf, Japan Sea and on its shores: Indicators of a polygenic continent–ocean transition zone

The Cenozoic formations in the Peter the Great Gulf in the Japan Sea and on its shores are part of the activated cover (Triassic to Neogene) of a young platform. They reflect the Cenozoic destruction of continental crust accompanied by active basaltoid magmatism and by transformation of this crust into marginal-sea crust in the near-continental part of the polygenic (Late Cambrian to Cenozoic) Japan-Sea transition zone from continent to ocean. The Cenozoic formations notably include an industrial coal-bearing and a potentially hydrocarbon-bearing group of Paleogene and Early to Middle Miocene formations, as well as a potentially diamond-bearing Late Miocene to Pliocene formation. The existence of these formations shows that continent–ocean transition zones have potential for these mineral resources. The accumulation of industrial coal-bearing Cenozoic formations seems to have occurred in a connected extensive sedimentation basin that was overlain by Paleozoic tectonic blankets, while the evolution of potentially diamond-bearing pipes that are akin to kimberlites occurred during a powerful phase of tectonomagmatic activation.

Yin and yang of continental crust creation and destruction by plate tectonic processes

Earth's continental crust today is both created and destroyed by plate tectonic processes, a balance that is encapsulated by the traditional Chinese concept of yin–yang, whereby dualities act in concert as well as in opposition. Yin–yang conceptualizations of crustal growth and destruction are mostly related to plate tectonics; both occur mostly at subduction zones, by arc magmatic creation and by subduction removal. Crust is also created and destroyed by processes unrelated to plate tectonics, including losses by lower crust foundering and additions at hotspots. At present, creation and destruction of continental crust is either in balance (∼3.2 km3/year, or 3.2 AU) or more crust is being destroyed than created; the uncertainty comes from unknown deep losses of continental crust at collision zones and due to lower crustal foundering. The yin–yang creation–destruction balance changes over a supercontinent cycle, with crustal growth being greatest during supercontinent break-up due to high magmatic flux at new arcs and crustal destruction being greatest during supercontinent amalgamation due to subduction of continental material and increased sediment flux due to orogenic uplift. These conclusions challenge the widely held view that continental crust volume has increased over time due to plate tectonic activity; it is just as likely that this volume has decreased.

The growth and destruction of continental crust during arc–continent collision in the Southern Urals

The Southern Urals of Russia contain a well preserved example of a Paleozoic arc–continent collision in which the Laurussia margin was subducted beneath the Magnitogorsk island arc in the Devonian, providing an ideal field area for studying the possible growth and destruction of the continental crust during this process. High-pressure rocks derived from the leading edge of the continental margin indicate that it was subducted to a depth of between 70 km (eclogite assemblages) and 120 km (micro diamonds). The vast majority of the high-pressure rocks have a sedimentary protolith, with mafic eclogite having been derived from dikes intruding into the sediments. High-pressure rocks derived from the mafic ganulite that currently makes up the middle and lower crust of the Southern Urals do not occur in outcrop, suggesting that much of the subducted margin remained in the upper mantle. However, extensive geological, geochemical, and geophysical data do not unequivocally clarify its subsequent fate. Crustal delamination and foundering into the mantle can be ruled out, and recycling through the volcanic arc system appears to have been minor. While the metamorphic products of subducted continental crust can remain in the mantle for long periods of time, in the Southern Urals it has not been imaged by the available geophysical data sets. A possible explanation for this is that the progressive metamorphism of mafic granulite to eclogite assemblages would have altered its physical properties to those of typical mantle lithologies, making them difficult to detect by geophysical methods. It is estimated that the volume of continental crust that was subducted and lost to the mantle was approximately one third of the volume that was added to the Laurussia margin by the accretion of the Magnitogorsk arc.

Geology of the Hubert Miller seamount, Marie Byrd seamounts province, Amundsen Sea, West Antarctic

This work is dedicated to the results of joint Russian-German geodynamic studies carried out in the West Antarctic (areas of the Amundsen Sea, the Southern Ocean, the Marie Byrd seamounts, and the foot of the continental slope of Marie Byrd Land) during cruises 18/5a and 23/4 of the “R/V Polarstern” in 2001 and 2006, respectively. The material collected on the Hubert Miller seamount (Marie Byrd seamount) attests to the relict continental appearance of the rocks. This suggests the heterogeneity of the Amundsen seafloor and its formation through a spatiotemporal combination of the destruction of continental crust, progressive thalassogenesis (oceanization-taphrogenesis), and rifting, as opposed to a spreading origin. The high postconsolidation mobility during the destruction stage led to the areal dismembering and high permeability of the continental crust, as well as tectonomagmatic activation. The main process during the reworking of the continental crust is its magmatic substitution by mantle-derived basic-ultrabasic material with subsequent formation of a secondary oceanic crust and preservation of relics of the continental crust. The endogenic activity of the Earth was driven by transmagmatic fluids, which were supplied from the liquid core and caused transformation of the Earth’s crust and mantle.

Rock material from the Weddell Sea Floor of the South Ocean

This work is devoted to the results of the joint Russian-German geodynamic research carried out in the Weddell Sea and West Antarctica during cruise ANT-XXII/3 of the R/V Polarstern in 2005. The study of rock samples collected from the sea floor showed that a heterogeneous structure of the Weddell Sea was formed by spatiotemporal combination of the destruction of continental crust, progressive thalassogenesis (oceanization-taphrogenesis), and rifting, as opposed to a spreading origin. High postconsolidation mobility during the destruction stage led to the areal dismembering and high permeability of the continental crust, as well as to tectonomagmatic activation. The main mechanism of reworking of the continental crust is recognized to be the magmatic replacement by basic-ultrabasic mantle material with formation of a secondary oceanic crust and preservation of relics of the continental crust. The Earth’s endogenous activity was driven by transmagmatic fluid flows, which ascended from the melted core and caused transformation of the Earth’s crust and mantle.

Tectonosphere of the Earth: upper mantle and crust interaction

The endogenic geological processes, which include tectonic, magmatic and metamorphic processes, form regular combinations called endogenic regimes. These regimes are: géosynclinal, orogenic, platform, rift, tectonic-magmatic activation (diwa), taphrogenic, plateau-basalt, oceanic basins and mid-oceanic ridges. The endogenic regimes are connected with the peculiarities of the structure, composition and state of the entire tectonosphere, i.e. not only of the crust but of the upper mantle as well. Heat flow is a major factor controlling the type of the regime. The other conditions are the temperature distribution in the tectonosphere and the degree and type of penetrability of the tectonosphere to melts and fluids. There is a certain regular succession of regimes. The structural evolution of the tectonosphere and the transformation of the matter in it are in close relationship. The main trend in the development of the tectonospheric material is directed towards geochemical depletion of the upper mantle by fractioning. At the initial stages, fractioning occurred mostly by degassing, and under these conditions the continental crust was formed, rich in non-compatible elements. At that stage the calc-alkaline magmas prevailed. As the upper mantle was depleted and began to lose its volatiles, the mechanism of fractioning changed: degassing was substituted for selective melting, and in this environment most of the tholeiitic magmas were formed. This change in magma composition and in fractioning mechanism was combined with the destruction of the continental crust and the formation of the oceanic crust. The diwa regime and the rifts were the first steps in the destruction of continental crust. The stages that followed were represented by taphrogenic regimes at various levels. These kinds of regimes were manifested in deep continental and marine depressions, compensated and not compensated by sediments. Taphrogenic regimes are advancing from the east and west onto the Eurasian continent: in the east they form marginal seas and cause subsidence of the eastern parts of the Chinese platform; in the west they produce collapses of the crust in the Mediterranean area. The major crisis occurred between the Palaeozoic and Mesozoic and since that time the process of substitution of the continental crust by the oceanic crust has proceeded over increasingly large territories. The evolution of the tectonosphere, instigated by the changes in its matter, was further complicated by temporal and spatial irregularities in deep heat escape, which caused the alternation of excited and quiescent endogenic regimes (tectonomagmatic periodicity) and their co-existence. The combination of all these phenomena creates the structural inhomogeneity of the Earth's crust at any stage of its history.