Subduction and exhumation mechanisms of ultra-high and high-pressure oceanic and continental crust at Makbal (Tianshan, Kazakhstan and Kyrgyzstan)

2014 ◽  
Vol 32 (8) ◽  
pp. 861-884 ◽  
Author(s):  
M. Meyer ◽  
R. Klemd ◽  
E. Hegner ◽  
D. Konopelko
Keyword(s):  
High Pressure ◽  
Continental Crust ◽  
Oceanic And Continental Crust

scholarly journals Up the down escalator: the exhumation of (ultra)-high pressure terranes during on-going subduction

2012 ◽  
Vol 4 (1) ◽  
pp. 745-781 ◽  
Author(s):  
C. J. Warren
Keyword(s):  
High Pressure ◽  
Subduction Zone ◽  
Continental Crust ◽  
Subduction Zones ◽  
Crustal Material ◽  
Time And Space ◽  
Ultra High Pressure ◽  
Tectonic Environment ◽  
Tectonic Style ◽  
Weak Material

Abstract. The exhumation of high and ultra-high pressure rocks is ubiquitous in Phanerozoic orogens created during continental collisions, and is common in many ocean-ocean and ocean-continent subduction zone environments. Three different tectonic environments have previously been reported, which exhume deeply buried material by different mechanisms and at different rates. However it is becoming increasingly clear that no single mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. In order for buoyant continental crust to subduct, it must remain attached to a stronger and denser substrate, but in order to exhume, it must detach (and therefore at least locally weaken) and be initially buoyant. Denser oceanic crust subducts more readily than more buoyant continental crust but exhumation must be assisted by entrainment within more buoyant and weak material such as serpentinite or driven by the exhumation of structurally lower continental crustal material. Weakening mechanisms responsible for the detachment of crust at depth include strain, hydration, melting, grain size reduction and the development of foliation. These may act locally or may act on the bulk of the subducted material. Metamorphic reactions, metastability and the composition of the subducted crust all affect buoyancy and overall strength. Subduction zones change in style both in time and space, and exhumation mechanisms change to reflect the tectonic style and overall force regime within the subduction zone. Exhumation events may be transient and occur only once in a particular subduction zone or orogen, or may be more continuous or occur multiple times.

French and Belgian Uplands

2005 ◽  
Author(s):  
Bernard Etlicher
Keyword(s):  
Continental Crust ◽  
Rift Valley ◽  
Sedimentary Basins ◽  
Shear Zones ◽  
Metamorphic Complex ◽  
French Massif Central ◽  
Massif Central ◽  
Montagne Noire ◽  
Oceanic And Continental Crust ◽  
Moderate Elevation

The French Uplands were built by the Hercynian orogenesis. The French Massif Central occupies one-sixth of the area of France and shows various landscapes. It is the highest upland, 1,886 m at the Sancy, and the most complex. The Vosges massif is a small massif, quite similar to the Schwarzwald in Germany, from which it is separated by the Rhine Rift Valley. Near the border of France, Belgium, and Germany, the Ardennes upland has a very moderate elevation. The largest part of this massif lies in Belgium. Though Brittany is partly made up of igneous and metamorphic rocks, it cannot be truly considered as an upland; in the main parts of Brittany, altitudes are lower than in the Parisian basin. Similarities of the landscape in the French and Belgian Uplands derive from two major events: the Oligocene rifting event and the Alpine tectonic phase. The Vosges and the Massif Central are located on the collision zone of the Variscan orogen. In contrast, the Ardennes is in a marginal position where primary sediments cover the igneous basement. Four main periods are defined during the Hercynian orogenesis (Bard et al. 1980; Autran 1984; Ledru et al. 1989; Faure et al. 1997). The early Variscan period corresponds to a subduction of oceanic and continental crust and a highpressure metamorphism (450–400 Ma) The medio- Variscan period corresponds to a continent–continent collision of the chain (400–340 Ma). Metamorphism under middle pressure conditions took place and controlled the formation of many granite plutons: e.g. red granites (granites rouges), porphyroid granite, and granodiorite incorporated in a metamorphic complex basement of various rocks. The neo-Variscan period (340–320 Ma) is characterized by a strong folding event: transcurrent shear zones affected the units of the previous periods and the first sedimentary basins appeared. At the end of this period, late-Variscan (330–280 Ma), autochthonous granites crystallized under low-pressure conditions related to a post-collision thinning of the crust. Velay and Montagne Noire granites are the main massifs generated by this event. Sediment deposition in tectonic basins during Carboniferous and Permian times occurred in the Massif Central and the Vosges: facies are sandstone (Vosges), shale, coal, and sandstone in several Stephanian basins of the Massif Central, with red shale and clay ‘Rougier’ in the south-western part of the Massif Central.

scholarly journals No evidence for high-pressure melting of Earth’s crust in the Archean

2019 ◽  
Vol 10 (1) ◽  
Author(s):  
Robert H. Smithies ◽  
Yongjun Lu ◽  
Tim E. Johnson ◽  
Christopher L. Kirkland ◽  
Kevin F. Cassidy ◽  
...  
Keyword(s):  
High Pressure ◽  
Continental Crust ◽  
Small Proportion ◽  
Lithospheric Mantle ◽  
Plate Tectonics ◽  
Crustal Evolution ◽  
Average Thickness ◽  
Dominant Group ◽  
Chemical Signatures ◽  
Pressure Melting

AbstractMuch of the present-day volume of Earth’s continental crust had formed by the end of the Archean Eon, 2.5 billion years ago, through the conversion of basaltic (mafic) crust into sodic granite of tonalite, trondhjemite and granodiorite (TTG) composition. Distinctive chemical signatures in a small proportion of these rocks, the so-called high-pressure TTG, are interpreted to indicate partial melting of hydrated crust at pressures above 1.5 GPa (>50 km depth), pressures typically not reached in post-Archean continental crust. These interpretations significantly influence views on early crustal evolution and the onset of plate tectonics. Here we show that high-pressure TTG did not form through melting of crust, but through fractionation of melts derived from metasomatically enriched lithospheric mantle. Although the remaining, and dominant, group of Archean TTG did form through melting of hydrated mafic crust, there is no evidence that this occurred at depths significantly greater than the ~40 km average thickness of modern continental crust.

Experimental study of the effect of stress on α → β quartz transformation at lower continental crust pressure and temperature conditions

2020 ◽  
Author(s):  
Arefeh Moarefvand ◽  
Julien Gasc ◽  
Julien Fauconnier ◽  
Damien Deldicque ◽  
Loic Labrousse ◽  
...  
Keyword(s):  
High Pressure ◽  
Phase Transitions ◽  
Continental Crust ◽  
Elastic Properties ◽  
Deviatoric Stress ◽  
P Wave ◽  
Time Shift ◽  
Monitoring Method ◽  
Lower Continental Crust ◽  
Temperature Conditions

<p>Based on experimental observations, there have been claims that deviatoric stresses may trigger high pressure phase transitions below their equilibrium transition pressures. This implies that the phase assemblages observed in exhumed rocks may reflect stresses induced by tectonic overpressure rather than mere lithostatic pressure, thus resulting in overestimated maximum depths of burial. Despite the numerous studies that have addressed whether mean or principal stress may trigger polymorphic phase changes, the case is still not completely clear. The aim of this study is therefore to investigate the role of deviatoric stress on phase transitions at high PT conditions. In this study, we investigated the α-β transition of quartz, which is one of the most common mineral of the Earth’s crust. This transition has a particular importance for the lower continental crust because of the significantly different elastic properties of the two polymorphs. The α-β quartz transition is also a good experimental candidate because of its displacive and quasi-instantaneous nature.</p><p>A series of experiments was performed with a new high pressure Griggs-type apparatus equipped with ultrasonic monitoring, at the ENS Paris. Cored rock samples of Arkansas Novaculite (mean grain size of 5.6 mm) were subjected to pressure and temperature conditions of 0.5-1.5 GPa and ~ 850 °C. The deviatoric stress was increased to cross the transition while keeping the temperature constant. Two p-wave transducers were used on top and bottom of the assembly as transmitter and receiver to measure travel times across the assembly. The quartz a-b transition was directly observed by a time-shift of the p-wave arrival in the order of 10 ns. The mechanical data clearly show that the phase transformation is controlled by mean stress. The quartz α-β transition induces a softening behavior on our sample because of the volume change induced by the reaction. According to the elastic properties of α and β quartz, the variation of p wave velocity for the quartz α-β transition is in the order of 10 %. The present active monitoring method allowed us to detect variations smaller than 5 %, which can be explained by a partial transformation due to local stress heterogeneities in the sample, since microscopic stress at the grain scale can be different than the macroscopic stress that we measure.</p>

Zircon Hf-O isotope evidence for recycled oceanic and continental crust in the sources of alkaline rocks

Geology ◽  
2017 ◽  
Vol 45 (5) ◽  
pp. 407-410 ◽  
Author(s):  
Yu-Sheng Zhu ◽  
Jin-Hui Yang ◽  
Jin-Feng Sun ◽  
Hao Wang
Keyword(s):  
Continental Crust ◽  
Alkaline Rocks ◽  
Isotope Evidence ◽  
O Isotope ◽  
Oceanic And Continental Crust

A Discussion on the evolution of the Precambrian crust - Geochemistry of granulite facies rocks and problems of their origin

1973 ◽  
Vol 273 (1235) ◽  
pp. 429-442 ◽  
Keyword(s):  
High Pressure ◽  
Continental Crust ◽  
Major Element ◽  
Granulite Facies ◽  
Sr Isotope ◽  
Average Surface ◽  
Magmatic Rocks ◽  
Mineral Associations ◽  
High Pressure Granulite ◽  
Critical Mineral

Occurrences of granulite facies rocks are widespread in continental regions where they mostly are parts of stable shield areas. Granulite facies terrains are classified as low-, medium- or high-pressure terrains on the basis of critical mineral associations. Special interest is attached to the medium- and highpressure terrains, as they are representative of the deepest crustal levels available for study in any areal extent on the surface, and may give information about the composition of the lower continental crust. Granulite facies terrains are mainly composed of metamorphic and metasomatic rocks, but magmatic rocks with primary igneous textures interpreted as formed by crystallization of magmas under granulite facies conditions are frequent in some areas. Examples of such rocks are anorthosites, gabbros and mangerites. The low-pressure—high-temperature granulite facies rocks are chemically indistinguishable from the amphibolite facies gneisses with which they characteristically occur. It is therefore important to make a distinction between these and the higher pressure types. The medium- to high-pressure granulite facies terrains are characterized by a less ‘acidic’ average major element compositions, and significant depletions in Rb, Cs, Th and U compared with average surface shield compositions. Available data also indicate low initial Sr isotope ratios, even in the gneissic types. In the author’s opinion the important problem associated with granulite facies rocks is not that of their origin, but rather of their importance as constituents of the continental crust, and how they attained their present chemistry.

scholarly journals Andesitic Magmatism on a Differentiated Asteroid

2021 ◽  
Author(s):  
Robert Nicklas ◽  
James Day ◽  
Kathryn Gardner-Vandy ◽  
Arya Udry
Keyword(s):  
Partial Melting ◽  
Continental Crust ◽  
Parent Body ◽  
Volatile Components ◽  
Highly Siderophile Elements ◽  
The Past ◽  
Metal Silicate ◽  
Oceanic And Continental Crust ◽  
Siderophile Elements ◽  
Melt Composition

Abstract The Earth differs from other terrestrial planets in having a substantial silica-rich continental crust with a bulk andesitic composition1. The compositional dichotomy between oceanic and continental crust is likely related to water-rich subduction processes2. Over the past decade, the discovery of meteorites with andesitic bulk compositions have demonstrated that continental-crust like compositions can be attained through partial melting of chondritic protoliths3,4,5. Here we show that a newly identified achondrite meteorite, Erg Chech (EC) 002, is a high-Mg andesite but that, unlike previous andesitic achondrites has strongly fractionated and low abundances of the highly siderophile elements (HSE), reminiscent of Earth’s upper continental crust6. The major and HSE composition of EC 002 can be explained if its asteroid parent body underwent metal-silicate equilibrium prior to silicate partial melting without losing significant volatile components. The chemistry of pyroxene grains in EC 002 suggests it approximates a parental melt composition, which cannot be produced by partial melting of pre-existing basaltic lithologies, but more likely requires a metal-free chondritic source. Erg Chech 002 likely formed by ~ 15% melting of the mantle of an alkali-undepleted differentiated asteroid. The discovery of EC 002 shows that extensive silicate differentiation after metal-silicate equilibration was already occurring in the first two million years of solar system history7, and that andesitic crustal compositions do not always require water-rich subduction processes to be produced.

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