Modelling the thermal perturbation of the continental crust 
after intraplating of thick granitoid sheets: a comparison with 
the crustal sections in Calabria (Italy)

Thick granitoid sheets represent a considerable percentage of Palaeozoic crustal sections exposed in Calabria. High thermal gradients are recorded in upper and lower crustal regional metamorphic rocks lying at the roof and base of the granitoids. Ages of peak metamorphism and emplacement of granitoids are mostly overlapping, suggesting a connection between magma intrusion and low-pressure metamorphism. To analyse this relationship, thermal perturbation following granitoid emplacement has been modelled. The simulation indicates that, in the upper crust, the thermal perturbation is short-lived. In contrast, in the lower crust temperatures greater than 700°C are maintained for 12 Ma, explaining granulite formation, anatexis and the following nearly isobaric cooling. An even longer perturbation can be achieved introducing the effect of mantle lithosphere thinning into the model.

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Crustal structure of the Valhalla complex, British Columbia, from Lithoprobe seismic-reflection and potential-field data

Canadian Journal of Earth Sciences ◽

10.1139/e90-109 ◽

1990 ◽

Vol 27 (8) ◽

pp. 1048-1060 ◽

Cited By ~ 9

Author(s):

David W. S. Eaton ◽

Frederick A. Cook

Keyword(s):

Heat Flux ◽

North American ◽

Seismic Reflection ◽

Surface Heat Flux ◽

Field Data ◽

High Surface ◽

Surface Heat ◽

Upper Crust ◽

Potential Field Data ◽

Lower Crustal

The Valhalla complex, situated in the Omineca crystalline belt in southeastern British Columbia, is a Cordilleran metamorphic core complex bordering the suture zone between Quesnellia and North American rocks. The region is tectonically interposed between a convergent plate margin along Canada's west coast and the stable North American craton, and is characterized by a crustal thickness of ~ 35 km, high surface heat flux, and elevated lower crustal electrical conductivity. In this study, Lithoprobe deep-crustal seismic-reflection data, potential-field data, and geological constraints have been used to gain a better understanding of crustal structure in the vicinity of the Valhalla complex. Analysis of Bouguer gravity and total-field aeromagnetic data indicates that mafic oceanic rocks and various syn- and post-accretionary granitoid plutonic rocks are not major constituents of the upper crust underlying the complex. The seismic data reveal a moderately reflective upper crust and image several fault zones, including a very high amplitude, west-dipping reflection that is interpreted as a significant Late Cretaceous or Paleocene thrust fault. The fault-zone reflectivity may be related to compositional heterogeneity and (or) seismic anisotropy associated with mylonites. The lower crust appears to be nonreflective, in contrast with other areas of high surface heat flux and elevated lower crustal conductivity. Taken together, the various data show that the Valhalla complex is likely cored by North American metasedimentary rocks and reveal features related to the Jurassic to Paleocene compressional fabric, which has been largely overprinted at the surface by subsequent Eocene extension.

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The Role of Lower Crustal Rheology in Lithospheric Delamination During Orogeny

Frontiers in Earth Science ◽

10.3389/feart.2021.755519 ◽

2021 ◽

Vol 9 ◽

Author(s):

Lin Chen

Keyword(s):

Lower Crust ◽

Numerical Models ◽

The Tibetan Plateau ◽

Mantle Lithosphere ◽

Laboratory Studies ◽

Surface Uplift ◽

Continental Lower Crust ◽

Lower Crustal ◽

Flow Laws

The continental lower crust is an important composition- and strength-jump layer in the lithosphere. Laboratory studies show its strength varies greatly due to a wide variety of composition. How the lower crust rheology influences the collisional orogeny remains poorly understood. Here I investigate the role of the lower crust rheology in the evolution of an orogen subject to horizontal shortening using 2D numerical models. A range of lower crustal flow laws from laboratory studies are tested to examine their effects on the styles of the accommodation of convergence. Three distinct styles are observed: 1) downwelling and subsequent delamination of orogen lithosphere mantle as a coherent slab; 2) localized thickening of orogen lithosphere; and 3) underthrusting of peripheral strong lithospheres below the orogen. Delamination occurs only if the orogen lower crust rheology is represented by the weak end-member of flow laws. The delamination is followed by partial melting of the lower crust and punctuated surface uplift confined to the orogen central region. For a moderately or extremely strong orogen lower crust, topography highs only develop on both sides of the orogen. In the Tibetan plateau, the crust has been doubly thickened but the underlying mantle lithosphere is highly heterogeneous. I suggest that the subvertical high-velocity mantle structures, as observed in southern and western Tibet, may exemplify localized delamination of the mantle lithosphere due to rheological weakening of the Tibetan lower crust.

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Thermodynamic constraints on assimilation of silicic crust by primitive magmas

10.5194/egusphere-egu21-1251 ◽

2021 ◽

Author(s):

Jussi S Heinonen ◽

Frank J Spera ◽

Wendy A Bohrson

Keyword(s):

Phase Equilibria ◽

Lower Crust ◽

Primitive Mantle ◽

Upper Crust ◽

Basaltic Composition ◽

Primitive Magma ◽

Partial Melts ◽

Thermodynamic Limits ◽

Melt Composition ◽

Lower Crustal

Some studies on basaltic and more primitive rocks suggest that their parental magmas have assimilated more than 50 wt.% (relative to the initial uncontaminated magma) of crustal silicate wallrock. But what are the thermodynamic limits for assimilation by primitive magmas? This question has been considered for over a century, first by N.L. Bowen and many others since then. Here we pursue this question quantitatively using a freely available thermodynamic tool for phase equilibria modeling of open magmatic systems &#8212; the Magma Chamber Simulator (MCS; https://mcs.geol.ucsb.edu).In the models, komatiitic, picritic, and basaltic magmas of various ages and from different tectonic settings assimilate progressive partial melts of average lower, middle, and upper crust. In order to pursue the maximum limits of assimilation constrained by phase equilibria and energetics, the mass of wallrock in the simulations was set at twice that of the initially pristine primitive magmas. In addition, the initial temperature of wallrock was set close to its solidus at a given pressure. Such conditions would approximate a rift setting with tabular chambers and high magma input causing concomitant crustal heating and steep geotherms.Our results indicate that it is difficult for any primitive magma to assimilate more than 20&#8722;30 wt.% of upper crust before evolving to intermediate/felsic compositions. However, if assimilant is lower crust, typical komatiitic magmas can assimilate more than their own weight (range of 59&#8722;102 wt.%) and retain a basaltic composition. Even picritic magmas, more relevant to modern intraplate settings, have a thermodynamic potential to assimilate 28&#8722;49 wt.% of lower crust before evolving into intermediate/felsic compositions.These findings have important implications for petrogenesis of magmas. The parental melt composition and the assimilant heavily influence both how much assimilation is energetically possible in primitive magmas and the final magma composition. The fact that primitive mantle melts have potential to partially melt and assimilate significant fractions of (lower) crust may have fundamental importance for how trans-Moho magmatic systems evolve and how crustal growth is accomplished. Examples include generation of siliceous high-magnesium basalts in the Precambrian and anorogenic anorthosite-mangerite-charnockite-granite complexes with geochemical evidence of considerable geochemical overprint from (lower) crustal sources.

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The nature and origin of cratons constrained by their surface geology

Geological Society of America Bulletin ◽

10.1130/b36079.1 ◽

2021 ◽

Author(s):

A.M. Celal Şengör ◽

Nalan Lom ◽

Ali Polat

Keyword(s):

North China ◽

Plate Boundary ◽

Crustal Thickening ◽

Metamorphic Rocks ◽

Upper Crust ◽

Subsequent Removal ◽

Total Removal ◽

Common Misconception ◽

Resistance To Deformation ◽

Surface Geology

To the memory of Nicholas John (Nick) Archibald (1951−2014), master of cratonic geology. Cratons, defined by their resistance to deformation, are guardians of crustal and lithospheric material over billion-year time scales. Archean and Proterozoic rocks can be found in many places on earth, but not all of them represent cratonic areas. Some of these old terrains, inappropriately termed “cratons” by some, have been parts of mobile belts and have experienced widespread deformations in response to mantle-plume-generated thermal weakening, uplift and consequent extension and/or various plate boundary deformations well into the Phanerozoic. It is a common misconception that cratons consist only of metamorphosed crystalline rocks at their surface, as shown by the indiscriminate designation of them by many as “shields.” Our compilation shows that this conviction is not completely true. Some recent models argue that craton formation results from crustal thickening caused by shortening and subsequent removal of the upper crust by erosion. This process would expose a high-grade metamorphic crust at the surface, but greenschist-grade metamorphic rocks and even unmetamorphosed supracrustal sedimentary rocks are widespread on some cratonic surfaces today, showing that craton formation does not require total removal of the upper crust. Instead, the granulitization of the roots of arcs may have been responsible for weighing down the collided and thickened pieces and keeping their top surfaces usually near sea level. In this study, we review the nature and origin of cratons on four well-studied examples. The Superior Province (the Canadian Shield), the Barberton Mountain (Kaapvaal province, South Africa), and the Yilgarn province (Western Australia) show the diversity of rocks with different origin and metamorphic degree at their surface. These fairly extensive examples are chosen because they are typical. It would have been impractical to review the entire extant cratonic surfaces on earth today. We chose the inappropriately named North China “Craton” to discuss the requirements to be classified as a craton.

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Elevated magma fluxes deliver high-Cu magmas to the upper crust

Geology ◽

10.1130/g47562.1 ◽

2020 ◽

Vol 48 (10) ◽

pp. 957-960

Author(s):

Daniel Cox ◽

Sebastian F.L. Watt ◽

Frances E. Jenner ◽

Alan R. Hastie ◽

Samantha J. Hammond ◽

...

Keyword(s):

Small Proportion ◽

Ore Deposits ◽

Crustal Evolution ◽

Upper Crust ◽

Convergent Margins ◽

Ore Genesis ◽

Porphyry Cu ◽

Show Evidence ◽

Lower Crustal ◽

Garnet Fractionation

Abstract Porphyry Cu-Au ore deposits are globally associated with convergent margins. However, controls on the processing and distribution of the chalcophile elements (e.g., Cu) during convergent margin magmatism remain disputed. Here, we show that magmas feeding many Chilean stratovolcanoes fractionate sulfides with a high-Cu/Ag ratio early in their crustal evolution. These magmas show evidence of lower-crustal garnet and amphibole crystallization, and their degree of sulfide fractionation and Cu depletion increase with both crustal thickness and the extent of garnet fractionation. However, samples from a small proportion of volcanoes with elevated eruptive fluxes depart from this Cu-depleting trend, instead erupting Cu-rich magmas. This implies that at these atypical sites, elevated magma productivity and crustal throughput, potentially facilitated by “pathways” exploiting major crustal fault systems, enable rapid magma transit, avoiding lower-crustal Cu-depleting sulfide fractionation and potentially playing an important role in porphyry ore genesis.

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Intrusion of UHP metamorphic rocks into the upper crust of Kyrgyzian Tien-Shan: P-T path and metamorphic age of the Makbal Complex

Journal of Mineralogical and Petrological Sciences ◽

10.2465/jmps.071025 ◽

2010 ◽

Vol 105 (5) ◽

pp. 233-250 ◽

Cited By ~ 21

Author(s):

Michio TAGIRI ◽

Shingo TAKIGUCHI ◽

Chika ISHIDA ◽

Takaaki NOGUCHI ◽

Makoto KIMURA ◽

...

Keyword(s):

Tien Shan ◽

Metamorphic Rocks ◽

Upper Crust ◽

T Path

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Lower-crustal rheology and thermal gradient in the Taiwan orogenic belt illuminated by the 1999 Chi-Chi earthquake

Science Advances ◽

10.1126/sciadv.aav3287 ◽

2019 ◽

Vol 5 (2) ◽

pp. eaav3287 ◽

Cited By ~ 12

Author(s):

Chi-Hsien Tang ◽

Ya-Ju Hsu ◽

Sylvain Barbot ◽

James D. P. Moore ◽

Wu-Lung Chang

Keyword(s):

Tectonic Evolution ◽

Laboratory Data ◽

Orogenic Belt ◽

Thermal Gradients ◽

Short Term ◽

Crustal Strength ◽

Seismic Cycles ◽

Natural Settings ◽

Stress And Strain Rate ◽

Lower Crustal

The strength of the lithosphere controls tectonic evolution and seismic cycles, but how rocks deform under stress in their natural settings is usually unclear. We constrain the rheological properties beneath the Taiwan orogenic belt using the stress perturbation following the 1999 Chi-Chi earthquake and fourteen-year postseismic geodetic observations. The evolution of stress and strain rate in the lower crust is best explained by a power-law Burgers rheology with rapid increases in effective viscosities from ~1017to ~1019Pa s within a year. The short-term modulation of the lower-crustal strength during the seismic cycle may alter the energy budget of mountain building. Incorporating the laboratory data and associated uncertainties, inferred thermal gradients suggest an eastward increase from 19.5±2.5°C/km in the Coastal Plain to 32±3°C/km in the Central Range. Geodetic observations may bridge the gap between laboratory and lithospheric scales to investigate crustal rheology and tectonic evolution.

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Medium-high-pressure metamorphic rocks of the Tuhua Orogen, western New Zealand, as lower crustal analogues of the Lachlan Fold Belt, SE Australia

Tectonophysics ◽

10.1016/0040-1951(92)90194-b ◽

1992 ◽

Vol 214 (1-4) ◽

pp. 145-157 ◽

Cited By ~ 22

Author(s):

G.M. Gibson

Keyword(s):

High Pressure ◽

New Zealand ◽

Metamorphic Rocks ◽

Fold Belt ◽

Lachlan Fold Belt ◽

Lower Crustal

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DENSITIES OF METAMORPHIC ROCKS

Geophysics ◽

10.1190/1.1440205 ◽

1971 ◽

Vol 36 (4) ◽

pp. 690-694 ◽

Cited By ~ 32

Author(s):

Scott B. Smithson

Keyword(s):

Metamorphic Rock ◽

Granite Gneiss ◽

Biotite Granite ◽

Metamorphic Rocks ◽

Upper Crust ◽

Rock Density ◽

Rock Types ◽

Crystalline Crust ◽

The Mean ◽

Few Data

Although metamorphic rocks comprise a large part of the crystalline crust, relatively few data concerning metamorphic rock densities are available. In this paper, we present rock densities from seven different metamorphic terrains. Mean densities for rock types range from [Formula: see text] for biotite granite gneiss to [Formula: see text] for diopside granofels. Mean rock densities for metamorphic terrains range from 2.70 to [Formula: see text]. Rock density may decrease in the lower part of the upper crust. Most mean rock densities for metamorphic terrains fall between 2.70 and [Formula: see text]; the mean density of [Formula: see text] commonly used for the upper crystalline crust is too low.

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