scholarly journals Fracturing and crystal plastic behaviour of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

Solid Earth ◽  
2019 ◽  
Vol 10 (5) ◽  
pp. 1635-1649 ◽  
Author(s):  
Friedrich Hawemann ◽  
Neil Mancktelow ◽  
Sebastian Wex ◽  
Giorgio Pennacchioni ◽  
Alfredo Camacho
Keyword(s):  
Plastic Deformation ◽  
Continental Crust ◽  
High Stress ◽  
Shear Zones ◽  
Granulite Facies ◽  
Lower Continental Crust ◽  
Granulite Facies Metamorphism ◽  
High Calcium ◽  
Central Australia ◽  
Lower Crustal

Abstract. Garnet is a high-strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade crystal plastic deformation, except under relatively high-temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal plastic deformation. Granulite facies metamorphism at ∼1200 Ma generally dehydrated the rocks and produced millimetre-sized garnets in peraluminous gneisses. A later ∼550 Ma overprint under sub-eclogitic conditions (600–700 ∘C, 1.1–1.3 GPa) developed mylonitic shear zones and abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. In the mylonites, granulite facies garnet porphyroclasts are enriched in calcium along rims and fractures. However, these rims are locally narrower than otherwise comparable rims along original grain boundaries, indicating the contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal plastic deformation, which coincides with areas of enhanced diffusion, usually along zones of crystal lattice distortion and dislocation walls associated with subgrain rotation recrystallization. The fracturing of garnet under dry lower-crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte. Highlights. Garnet is deformed by fracturing and crystal plasticity under dry lower-crustal conditions. Ca diffusion profiles indicate multiple generations of fracturing. Diffusion is promoted along zones of higher dislocation density. Fracturing indicates transient high-stress (seismic) events in the lower continental crust.

scholarly journals Fracturing and crystal plastic behavior of garnet under seismic stress in the dry lower continental crust (Musgrave Ranges, Central Australia)

2019 ◽  
Author(s):  
Friedrich Hawemann ◽  
Neil Mancktelow ◽  
Sebastian Wex ◽  
Giorgio Pennacchioni ◽  
Alfredo Camacho
Keyword(s):  
Plastic Deformation ◽  
High Strength ◽  
Calcium Content ◽  
Shear Zones ◽  
Granulite Facies ◽  
Plastic Behavior ◽  
Granulite Facies Metamorphism ◽  
High Calcium ◽  
Central Australia ◽  
Lower Crustal

Abstract. Garnet is a high strength mineral compared to other common minerals such as quartz and feldspar in the felsic crust. In felsic mylonites, garnet typically occurs as porphyroclasts that mostly evade deformation, except under relatively high temperature conditions. The microstructure of granulite facies garnet in felsic lower-crustal rocks of the Musgrave Ranges (Central Australia) records both fracturing and crystal-plastic deformation. Granulite facies metamorphism at ~ 1200 Ma generally dehydrated the rocks and produced mm-sized garnets in peraluminous gneisses. A later ~ 550 Ma overprint under sub-eclogitic conditions (600–700 °C, 1.1–1.3 GPa) developed shear zones and with abundant pseudotachylyte, coeval with the neocrystallization of fine-grained, high-calcium garnet. The granulitic fractured garnet porphyroclasts in mylonites show high calcium content along rims and fractures. However, in certain cases, these rims are narrower than equivalent rims along original grain boundaries, indicating contemporaneous diffusion and fracturing of garnet. The fractured garnets exhibit internal crystal-plastic deformation, that coincide with areas of enhanced diffusion, usually along zones of crystal lattice distortions and dislocation walls and by subgrain rotation recrystallization. Fracturing of garnet under dry lower crustal conditions, in an otherwise viscously flowing matrix, requires transient high differential stress, most likely related to seismic rupture, consistent with the coeval development of abundant pseudotachylyte.

scholarly journals Metamorphism of the Sierra de Maz and implications for the tectonic evolution of the MARA terrane

Geosphere ◽  
2021 ◽  
Author(s):  
Andrew Tholt ◽  
Sean R. Mulcahy ◽  
William C. McClelland ◽  
Sarah M. Roeske ◽  
Vinícius T. Meira ◽  
...  
Keyword(s):  
Tectonic Evolution ◽  
Shear Zones ◽  
Granulite Facies ◽  
Metamorphic Petrology ◽  
Granulite Facies Metamorphism ◽  
Northwest Argentina ◽  
Sufficient Detail ◽  
Ductile Shear Zones ◽  
Gondwana Margin ◽  
Lower Crustal

The Mesoproterozoic MARA terrane of western South America is a composite igneous-metamorphic complex that is important for Paleozoic paleogeographic reconstructions and the relative positions of Laurentia and Gondwana. The magmatic and detrital records of the MARA terrane are consistent with a Laurentian origin; however, the metamorphic and deformation records lack sufficient detail to constrain the correlation of units within the MARA terrane and the timing and mechanisms of accretion to the Gondwana margin. Combined regional mapping, metamorphic petrology, and garnet and monazite geochronology from the Sierra de Maz of northwest Argentina sug- gest that the region preserves four distinct litho-tectonic units of varying age and metamorphic conditions that are separated by middle- to lower-crustal ductile shear zones. The Zaino and Maz Complexes preserve Barrovian metamorphism and ages that are distinct from other units within the region. The Zaino and Maz Complexes both record metamorphism ca. 430–410 Ma and show no evidence of the regional Famatinian orogeny (ca. 490–455 Ma). In addition, the Maz Complex records an earlier granulite facies event at ca. 1.2 Ga. The Taco and Ramaditas Complexes, in contrast, experienced medium- and low-pressure upper amphibolite to granulite facies metamorphism, respectively, between ca. 470–460 Ma and were later deformed at ca. 440–420 Ma. The Maz shear zone that bounds the Zaino and Maz Complexes records sinistral oblique to sinistral deformation between ca. 430–410 Ma. The data suggest that at least some units in the MARA terrane were accreted by translation, and the Gondwana margin of northwest Argentina transitioned from a dominantly convergent margin to a highly oblique margin in the Silurian.

Early Archaean continental crust in the Eastern Ghats granulite belt, India: isotopic evidence from a charnockite suite

2001 ◽  
Vol 138 (5) ◽  
pp. 609-618 ◽  
Author(s):  
S. BHATTACHARYA ◽  
RAJIB KAR ◽  
S. MISRA ◽  
W. TEIXEIRA
Keyword(s):  
Continental Crust ◽  
Granulite Facies ◽  
Crustal Thickening ◽  
Eastern Ghats ◽  
Mobile Belt ◽  
Granulite Facies Metamorphism ◽  
Granulite Belt ◽  
Facies Metamorphism ◽  
Geochemical Studies ◽  
Lower Crustal

The Eastern Ghats granulite belt of India has traditionally been described as a Proterozoic mobile belt, with probable Archaean protoliths. However, recent findings suggest that synkinematic development of granulites took place in a compressional tectonic regime and that granulite facies metamorphism resulted from crustal thickening. The field, petrological and geochemical studies of a charnockite massif of tonalitic to trondhjemitic composition, and associated rocks, document granulite facies metamorphism and dehydration partial melting of basic rocks at lower crustal depths, with garnet granulite residues exposed as cognate xenoliths within the charnockite massif. The melting and generation of the charnockite suite under granulite facies conditions have been dated c. 3.0 Ga by Sm–Nd and Rb–Sr whole rock systematics and Pb–Pb zircon dating. Sm–Nd model dates between 3.4 and 3.5 Ga and negative epsilon values provide evidence of early Archaean continental crust in this high-grade terrain.

Exhumation of high-grade terranes—a review

1992 ◽  
Vol 29 (4) ◽  
pp. 737-745 ◽  
Author(s):  
Jacques Martignole
Keyword(s):  
High Temperature ◽  
Shear Zones ◽  
Granulite Facies ◽  
High Grade ◽  
Granulite Facies Metamorphism ◽  
Magmatic Underplating ◽  
Show Evidence ◽  
Juvenile Crust ◽  
Uplift And Erosion ◽  
Lower Crustal

High-grade (granulite-facies) terranes are brought to the surface by a combination of uplift and erosion (exhumation). The reported mechanisms and durations of exhumation are variable and depend partly on the mode of formation of a given high-grade terrane. In this paper, we consider the case of granulite-facies conditions that are attained (i) in juvenile crust, in the roots of magmatic arcs (e.g., Kohistan, Fiordland), (ii) around deep-seated high-temperature plutonic complexes, and (iii) in the lower parts of thickened continental crust. In the case of the roots of magmatic arcs, Phanerozoic examples suggest that they are exhumed along shallow-dipping contraction faults or shear zones that developed during continental obduction in a convergent tectonic regime. This process is not fundamentally different from processes leading to the exhumation of high-pressure (blueschist, eclogite) terranes. In contrast, deep-seated high-temperature plutonic complexes are thermostructural domes, analogous to the lower levels of core complexes, which may also have contributed to the uprise of high-grade terranes. Such domes should be sought for around anorthositic or mafic plutons, where their ascent may also have been favoured by continental extension. These modes of exhumation are compatible with a monocyclic evolution. However, many high-grade terranes show evidence of a polycyclic evolution and, in such cases, the nature of the thermal perturbation responsible for granulite-facies metamorphism is still debated. Thermal modelling based on heat conduction in collision orogens shows that granulites cannot form at mid-cristal levels, namely those exposed after isostatically driven denudation. Thus, magmatic underplating and crustal extension have been suggested as causes of steepened geotherms. Underplating (or intraplating) supplies the heat and thickens the crust from below. Postcollisional extension has also been considered as a mechanism providing a heat pulse emanating from the asthenosphere, probably after the "detachment" of a relatively cold thermal boundary layer. Finally, isolated crustal-scale intracratonic thrusting may favour the rise of intermediate to lower crustal wedges (e.g., the Kapuskasing wedge, uplifted prior to the trans-Hudson collision).

Fluid flow vs. scale of shear zones in the lower continental crust and the granulite paradox

Geology ◽  
1997 ◽  
Vol 25 (1) ◽  
pp. 15 ◽  
Author(s):  
Eric Pili ◽  
Simon M. F. Sheppard ◽  
Jean-Marc Lardeaux ◽  
Jean-Emmanuel Martelat ◽  
Christian Nicollet
Keyword(s):  
Fluid Flow ◽  
Continental Crust ◽  
Shear Zones ◽  
Lower Continental Crust
Sign in / Sign up

Export Citation Format

Share Document