A new model for brittle failure at depth involving high-pressure metamorphism

2020 ◽  
Author(s):  
Philippe Yamato ◽  
Marie Baïsset
Keyword(s):  
High Pressure ◽  
Host Rock ◽  
Continuous Process ◽  
Shear Zones ◽  
Brittle Deformation ◽  
Local Pressure ◽  
Intermediate Depth ◽  
Crustal Rocks ◽  
High Pressure Metamorphism ◽  
Convergence Zones

<p>Intermediate-depth earthquakes are registered in convergence zones where crustal rocks are expected to deform by ductile flow. This paradox is also evidenced in exhumed crustal rocks where brittle structures (e.g., pseudotachylytes and breccias) associated to high-pressure metamorphism have been documented. If the link between brittle deformation and metamorphic reactions appears obvious today, the mechanism involved is still a burning issue. We propose that the initial heterogeneity of rocks, by itself, is sufficient to trigger both metamorphic reaction and brittle deformation. Based on a mechanically consistent dynamic model, we show that local pressure variations due to pre-existing heterogeneities can be high enough to reach the thermodynamic conditions required for reaction initiation. Brittle behaviour is then controlled by the strength difference between the untransformed host rock and its reaction product. This continuous process also explains the higher pressures recorded in eclogite facies rocks of ductile shear zones compared to their brittle host rock. Our results, constraint by natural data, have therefore significant implications for intermediate-depth seismicity.</p>

The occurrence and timing of high-pressure metamorphism on Margarita Island, Venezuela: a constraint on Caribbean-South America interaction

2009 ◽  
Vol 328 (1) ◽  
pp. 705-741 ◽  
Author(s):  
Walter V. Maresch ◽  
Rolf Kluge ◽  
Albrecht Baumann ◽  
James L. Pindell ◽  
Gabriela Krückhans-Lueder ◽  
...  
Keyword(s):  
High Pressure ◽  
South America ◽  
High Pressure Metamorphism

Late Caledonian transpression and the structural controls on pluton construction; new insights from the Omey Pluton, western Ireland

2015 ◽  
Vol 106 (1) ◽  
pp. 11-28 ◽  
Author(s):  
William McCarthy ◽  
R. John Reavy ◽  
Carl T. Stevenson ◽  
Michael S. Petronis
Keyword(s):  
Spatial Distribution ◽  
Host Rock ◽  
Shear Zones ◽  
Magma Ascent ◽  
Structural Controls ◽  
Rock Structure ◽  
Granite Complex ◽  
Caledonian Orogeny ◽  
Western Ireland

ABSTRACTThe Galway Granite Complex is unique among the British and Irish Caledonian granitoid terranes, as it records punctuated phases of magmatism from ∼425–380 Ma throughout the latest phase of the Caledonian Orogeny. Remapping of the Omey Pluton, the oldest member of this suite, has constrained the spatial distribution and contact relationships of the pluton's three main facies relative to the nature of the host rock structure. The external contacts of the pluton are mostly concordant to the limbs and hinge of the Connemara Antiform. New AMS data show that a subtle concentric outward dipping foliation is present, and this is interpreted to reflect pluton inflation during continued magma ingress. Combined field, petrographic and AMS data show that two sets of shear zones (NNW–SSE and ENE–WSW) cross-cut the concentric foliation, and that these structures were active during the construction of the pluton. We show that regional sinistral transpression at ∼420 Ma would have caused dilation along the intersection of these two fault sets, and suggest that this facilitated centralised magma ascent. Lateral emplacement was controlled by the symmetry of the Connemara Antiform to ultimately produce a discordant phacolith. We propose that regional sinistral transpression at ∼420 Ma influenced the siting of smaller intrusions over NNW–SSE faults, and that the later onset of regional transtension caused larger volumes of magma to intrude along the E–W Skird Rocks Fault at ∼400 Ma.

Under Pressure: High-Pressure Metamorphism in the Alps

Elements ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 17-22 ◽  
Author(s):  
Lucie Tajčmanová ◽  
Paola Manzotti ◽  
Matteo Alvaro
Keyword(s):  
High Pressure ◽  
Structural Data ◽  
Metamorphic Rocks ◽  
Lithostatic Pressure ◽  
Mineral Inclusion ◽  
Crustal Material ◽  
Local Pressure ◽  
Ultra High Pressure ◽  
Mechanical Models ◽  
The Alps

The mechanisms attending the burial of crustal material and its exhumation before and during the Alpine orogeny are controversial. New mechanical models propose local pressure perturbations deviating from lithostatic pressure as a possible mechanism for creating (ultra-)high-pressure rocks in the Alps. These models challenge the assumption that metamorphic pressure can be used as a measure of depth, in this case implying deep subduction of metamorphic rocks beneath the Alpine orogen. We summarize petro-logical, geochronological and structural data to assess two fundamentally distinct mechanisms of forming (ultra-)high-pressure rocks: deep subduction; or anomalous, non-lithostatic pressure variation. Furthermore, we explore mineral-inclusion barometry to assess the relationship between pressure and depth in metamorphic rocks.

scholarly journals Timing of Paleozoic Exhumation and Deformation of the High-Pressure Vestgӧtabreen Complex at the Motalafjella Nunatak, Svalbard

Minerals ◽  
2020 ◽  
Vol 10 (2) ◽  
pp. 125 ◽  
Author(s):  
Christopher J. Barnes ◽  
Katarzyna Walczak ◽  
Emilie Janots ◽  
David Schneider ◽  
Jarosław Majka
Keyword(s):  
High Pressure ◽  
Tectonic Evolution ◽  
Shear Zones ◽  
White Mica ◽  
Strike Slip ◽  
Structural Studies ◽  
Metamorphic Overprint ◽  
Subaerial Exposure ◽  
Facies Metamorphism

The Vestgӧtabreen Complex exposed in the Southwestern Caledonian Basement Province of Svalbard comprises two Caledonian high-pressure units. In situ white mica 40Ar/39Ar and monazite Th-U-total Pb geochronology has resolved the timing of the tectonic evolution of the complex. Cooling of the Upper Unit during exhumation occurred at 476 ± 2 Ma, shortly after eclogite-facies metamorphism. The two units were juxtaposed at 454 ± 6 Ma. This was followed by subaerial exposure and deposition of Bullbreen Group sediments. A 430–400 Ma late Caledonian phase of thrusting associated with major sinistral shearing throughout Svalbard deformed both the complex and the overlying sediments. This phase of thrusting is prominently recorded in the Lower Unit, and is associated with a pervasive greenschist-facies metamorphic overprint of high-pressure lithologies. A c. 365–344 Ma geochronological record may represent an Ellesmerian tectonothermal overprint. Altogether, the geochronological evolution of the Vestgӧtabreen Complex, with previous petrological and structural studies, suggests that it may be a correlative to the high-pressure Tsäkkok Lens in the Scandinavian Caledonides. It is suggested that the Vestgӧtabreen Complex escaped to the periphery of the orogen along the sinistral strike-slip shear zones prior to, or during the initial stages of continental collision between Baltica and Laurentia.

Macro- and microstructural analysis of the Zhujiafang ductile shear zone, Hengshan Complex: Tectonic nature and geodynamic implications of the evolution of Trans−North China orogen

2020 ◽  
Author(s):  
Lingchao He ◽  
Jian Zhang ◽  
Guochun Zhao ◽  
Changqing Yin ◽  
Jiahui Qian ◽  
...  
Keyword(s):  
Shear Zone ◽  
North China ◽  
Shear Zones ◽  
Crustal Thickening ◽  
Slip Systems ◽  
High Grade ◽  
Ductile Shear Zone ◽  
Crustal Rocks ◽  
Ductile Shear ◽  
Lower Crustal

In worldwide orogenic belts, crustal-scale ductile shear zones are important tectonic channels along which the orogenic root (i.e., high-grade metamorphic lower-crustal rocks) commonly experienced a relatively quick exhumation or uplift process. However, their tectonic nature and geodynamic processes are poorly constrained. In the Trans−North China orogen, the crustal-scale Zhujiafang ductile shear zone represents a major tectonic boundary separating the upper and lower crusts of the orogen. Its tectonic nature, structural features, and timing provide vital information into understanding this issue. Detailed field observations showed that the Zhujiafang ductile shear zone experienced polyphase deformation. Variable macro- and microscopic kinematic indicators are extensively preserved in the highly sheared tonalite-trondhjemite-granodiorite (TTG) and supracrustal rock assemblages and indicate an obvious dextral strike-slip and dip-slip sense of shear. Electron backscattered diffraction (EBSD) was utilized to further determine the crystallographic preferred orientation (CPO) of typical rock-forming minerals, including hornblende, quartz, and feldspar. EBSD results indicate that the hornblendes are characterized by (100) <001> and (110) <001> slip systems, whereas quartz grains are dominated by prism <a> and prism <c> slip systems, suggesting an approximate shear condition of 650−700 °C. This result is consistent with traditional thermobarometry pressure-temperature calculations implemented on the same mineral assemblages. Combined with previously reported metamorphic data in the Trans−North China orogen, we suggest that the Zhujiafang supracrustal rocks were initially buried down to ∼30 km depth, where high differential stress triggered the large-scale ductile shear between the upper and lower crusts. The high-grade lower-crustal rocks were consequently exhumed upwards along the shear zone, synchronous with extensive isothermal decompression metamorphism. The timing of peak collision-related crustal thickening was further constrained by the ca. 1930 Ma metamorphic zircon ages, whereas a subsequent exhumation event was manifested by ca. 1860 Ma syntectonic granitic veins and the available Ar-Ar ages of the region. The Zhujiafang ductile shear zone thus essentially record an integrated geodynamic process of initial collision, crustal thickening, and exhumation involved in formation of the Trans−North China orogen at 1.9−1.8 Ga.

Deformation textures in pyrite from the Vangorda Pb-Zn-Ag deposit, Yukon, Canada

1993 ◽  
Vol 57 (386) ◽  
pp. 55-66 ◽  
Author(s):  
D. Brown ◽  
K. R. McClay
Keyword(s):  
Grain Boundary ◽  
Boundary Migration ◽  
Shear Zones ◽  
Grain Boundary Sliding ◽  
Ductile Deformation ◽  
Mining District ◽  
Strain Partitioning ◽  
Triple Junctions ◽  
Brittle Deformation ◽  
Deformation Textures

AbstractThe Vangorda Pb-Zn-Ag orebody is a 7.1 M tonne, polydeformed stratiform massive sulphide deposit in the Anvil mining district, Yukon, Canada. Five sulphide lithofacies have been identified within the desposit with a typical mineralogy of pyrite, sphalerite, galena, and barite. Pyrrhotite-sphaleritemagnetite assembalges are locally developed. Etched polished sections of massive pyrite ores display relict primary depositional pyrite textures such as colloform growth zoning and spheroidal/framboidal features. A wide variety of brittle deformation, ductile deformation, and annealing textures have been identified. Brittle deformation textures include thin zones of intense cataclasis, grain indentation and axial cracking, and grain boundary sliding features. Ductile deformation textures include strong preferred grain shape orientations, dislocation textures, grain boundary migration, dynamic recrystallisation and pressure solution textures. Post deformational annealing has produced grain growth with lobate grain boundaries, 120° triple junctions and idioblastic pyrite porphyroblasts. The distribution of deformation textures within the Vangorda orebody suggests strong strain partitioning along fold limbs and fault/shear zones, it is postulated that focussed fluid flow in these zones had significant effects on the deformation of these pyritic ores.

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