Structural evolution along the Susa Shear Zone: the role of a first-order shear zone in the exhumation of meta-ophiolite units (Western Alps)

Abstract In the Western Alps, different shear zones acting at different depths have been investigated for explaining multistage exhumation of (U)HP units, and several exhumation models have been proposed for explaining present-day stacking of different tectonometamorphic units. This study aims to reconstruct the tectonic evolution of the Susa Shear Zone (SSZ), a polyphasic first-order shear zone, outcropping in the Susa Valley. The SSZ consists of a thick mylonitic zone, along which units characterized by different Alpine metamorphic P–T peaks are coupled. In the study area, the footwall of the SSZ mostly consists of oceanic units (i.e., Internal Piedmont Zone), which record eclogitic conditions, whereas the hanging wall consists of oceanic units (i.e., External Piedmont Zone), which record blueschist-facies conditions. These tectonic units were deformed during subduction- and exhumation-related Alpine history, throughout four main regional deformation phases (from D1 to D4), and were coupled along the SSZ, wherein two shearing events have been distinguished (T1 and T2). T1 occurred during early exhumation and was characterized by “apparent reverse” Top-to-E kinematics, whereas T2 occurred during late exhumation and was characterized by Top-to-W kinematics. Detailed fieldwork and structural analysis allowed us to describe the main features of the different deformation stages and define the deformation relative timing. As final result, we propose a four-step geodynamic model, focused on the different stages developed along the SSZ, from pre-T1 to syn-T2, showing the geometrical relationships between the tectonic units involved in the exhumation. The model aims at explaining the role of the SSZ in the axial sector of the Western Alps.

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Crystallographic and Seismic Anisotropies of Calcite at Different Depths: A Study Using Quantitative Texture Analysis by Neutron Diffraction

Minerals ◽

10.3390/min10010026 ◽

2019 ◽

Vol 10 (1) ◽

pp. 26 ◽

Cited By ~ 2

Author(s):

Michele Zucali ◽

Daniel Chateigner ◽

and Bachir Ouladdiaf

Keyword(s):

Neutron Diffraction ◽

Texture Analysis ◽

Shear Zone ◽

Shear Zones ◽

Orientation Distribution ◽

Western Alps ◽

S Wave ◽

Wide Variability ◽

Fold Belts ◽

Different Depths

Eight samples of limestones and marbles were studied by neutron diffraction to collect quantitative texture (i.e., crystallographic preferred orientations or CPO) of calcite deforming at different depths in the crust. We studied the different Texture patterns developed in shear zones at different depth and their influence on seismic anisotropies. Samples were collected in the French and Italian Alps, Apennines, and Paleozoic Sardinian basement. They are characterized by isotropic to highly anisotropic (e.g., mylonite shear zone) fabrics. Mylonite limestones occur as shear zone horizons within the Cenozoic Southern Domain in Alpine thrust-and-fold belts (Italy), the Briançonnais domain of the Western Alps (Italy-France border), the Sardinian Paleozoic back-thrusts, or in the Austroalpine intermediate units. The analyzed marbles were collected in the Carrara Marble, in the Austroalpine Units in the Central (Mortirolo) and Western Alps (Valpelline). The temperature and depth of development of fabrics vary from <100 ∘ C, to 800 ∘ C and depth from <10 km to about 30 km, corresponding from upper to lower crust conditions. Quantitative Texture Analysis shows different types of patterns for calcite: random to strongly textured. Textured types may be further separated in orthorhombic and monoclinic (Types A and B), based on the angle defined with the mesoscopic fabrics. Seismic anisotropies were calculated by homogenizing the single-crystal elastic tensor, using the Orientation Distribution Function calculated by Quantitative Texture Analysis. The resulting P- and S-wave anisotropies show a wide variability due to the textural types, temperature and pressure conditions, and dip of the shear planes.

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Role of rheological heterogeneities in vein-ore localization

Canadian Journal of Earth Sciences ◽

10.1139/e93-011 ◽

1993 ◽

Vol 30 (1) ◽

pp. 113-123 ◽

Cited By ~ 2

Author(s):

C. Castaing ◽

D. Cassard ◽

Y. Gros ◽

M. Moisy ◽

J. C. Chabod

Keyword(s):

Lower Order ◽

Shear Zones ◽

Ore Deposits ◽

Second Order ◽

Massif Central ◽

First Order ◽

Ore Localization ◽

Ore Minerals ◽

Partial Destruction

Structural studies of the Saint-Salvy zinc deposit and other Hercynian, veinhosted ore deposits in the French Massif Central and Pyrénées reveal a fourstage evolution of mineralized structures under rheological control: (i) localization of potential mineralized areas, guided by the presence of first-order lithological or structural heterogeneities that caused stress and strain perturbations; (ii) creation of second-order heterogeneities, corresponding to indurated shear zones that acted as rheological discontinuities; (iii) tectonic activation of these second-order heterogeneities, opening voids that allowed circulation of hydrothermal fluids and periodic trapping of ore minerals; (iv) reworking and partial destruction of the mineralized structures, caused by the reactivation of anisotropic surfaces acting as zones of weakness. The interaction between preexisting, first-order heterogeneities and regional shear strain caused instability, which in turn produced second-order and then lower-order heterogeneities. Such progressively smaller heterogeneities induced an increasingly focused, centripetal localization of structural disturbances that enabled hydrothermal fluid channelling. This is the reason that lower-order and late structures preferentially bear economic mineralization.

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Geological and geochemical implications of the genesis of the Qolqoleh orogenic gold mineralisation, Kurdistan Province (Iran)

Geologos ◽

10.1515/logos-2015-0002 ◽

2015 ◽

Vol 21 (1) ◽

pp. 31-57 ◽

Cited By ~ 3

Author(s):

Batoul Taghipour ◽

Farhad Ahmadnejad

Keyword(s):

Shear Zone ◽

Early Stage ◽

Hanging Wall ◽

Regional Metamorphism ◽

Shear Zones ◽

Northwestern Part ◽

Type I ◽

Two Phase ◽

Mean Values ◽

Calcite Veins

Abstract The Qolqoleh gold deposit is located in the northwestern part of the Sanandaj-Sirjan Zone (SSZ), within the NE-SW trending Qolqoleh shear zone. Oligocene granitoids, Cretaceous meta-limestones, schists and metavolcanics are the main lithological units. Chondrite-normalised REE patterns of the ore-hosting metavolcanics indicate REE enrichment relative to hanging wall (chlorite-sericite schist) and footwall (meta-limestone) rocks. The pattern also reflects an enrichment in LREE relative to HREE. It seems that the LREE enrichment is related to the circulation of SO42- and CO2-bearing fluids and regional metamorphism in the Qolqoleh shear zone. Both positive and negative Eu anomalies are observed in shear-zone metavolcanics. These anomalies are related to the degree of plagioclase alteration during gold mineralisation and hydrothermal alteration. In progressing from a metavolcanic protomylonite to an ultramylonite, significant changes occurred in the major/trace element and REE concentration. Utilising an Al-Fe-Ti isocon for the ore-hosting metavolcanics shows that Sc, Y, K, U, P, and M-HREE (except Eu) are relatively unchanged; S, As, Ag, Au, Ca, LOI, Rb and LREE are enriched, and Sr, Ba, Eu, Cr, Co and Ni decrease with an increasing degree of deformation. Based on geochemical features and comparison with other well-known shear zones in the world, the study area is best classified as an Isovolume-Gain (IVG) type shear zone and orogenic type gold mineralisation. Based on the number of phases observed at room temperature and their microthermometric behaviour, three fluid inclusion types have been recognised in quartz-sulphide and quartz-calcite veins: Type I monophase aqueous inclusions, Type II two-phase liquid-vapour (L-V) inclusions which are subdivided into two groups based on the homogenisation temperature (Th): a) L-V inclusions with Th from 205 to 255°C and melting temperature of last ice (Tm) from -3 to -9°C. b) L-V inclusions with higher Th from 335 to 385°C and Tm from -11 to -16°C. Type III three-phase carbonic-liquid inclusions (liquid water-liquid CO2-vapour CO2) with Th of 345-385°C. The mean values of the density of ore-forming fluids, pressure and depth of mineralisation have been calculated to be 0.79-0.96 gr/cm3, 2 kbar and 7 km, respectively. The δ18Owater and δD values of the gold-bearing quartz-sulphide veins vary from 7.2‰ to 8‰ and -40.24‰ to -35.28‰, respectively, which are indicative of an isotopically heavy crustal fluid and likely little involvement of meteoric fluid. The δ18Owater values of the quartz-calcite veins have a range of -5.31‰ to -3.35‰, and the δD values of -95.65‰ to -75.31‰, which are clearly lower than those of early-stage quartz-sulphide-gold veins, and are close to the meteoric water line. Based on comparisons of the D-O isotopic systematics, the Qolqoleh ore-mineralising fluids originated from metamorphic devolatilisation of Cretaceous volcano-sedimentary piles. Devolatilisation of these units occurred either synchronously with, or postdates, the development of penetrative (ductile) structures such as shear zones and during overprinting brittle deformation

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Gold Deposits of the ~15-Moz Ahafo South Camp, Sefwi Granite-Greenstone Belt, Ghana: Insights into the Anatomy of an Orogenic Gold Plumbing System

Economic Geology ◽

10.5382/econgeo.4829 ◽

2021 ◽

Author(s):

Quentin Masurel ◽

Paul Morley ◽

Nicolas Thébaud ◽

Helen McFarlane

Keyword(s):

Rock Mass ◽

Shear Zone ◽

Gold Mineralization ◽

Hanging Wall ◽

Spatial Association ◽

Sedimentary Rocks ◽

Shear Zones ◽

Gold Deposits ◽

Orogenic Gold ◽

Plutonic Rocks

Abstract The ~15-Moz Ahafo South gold camp is located in southwest Ghana, the world’s premier Paleoproterozoic gold subprovince. Major orogenic gold deposits in the camp include Subika, Apensu, Awonsu, and Amoma. These deposits occur along an ~15-km strike length of the Kenyase-Yamfo shear zone, a major tectonostratigraphic boundary juxtaposing metamorphosed volcano-plutonic rocks of the Sefwi belt against metamorphosed volcano-sedimentary rocks of the Sunyani-Comoé basin. In this study, we document the geologic setting, structural geometry, and rheological architecture of the Ahafo South gold deposits based on the integration of field mapping, diamond drill core logging, 3-D geologic modeling, and the geologic interpretation of aeromagnetic data. At the camp scale, the Awonsu, Apensu, and Amoma deposits lie along strike from one another and share similar hanging-wall plutonic rocks and footwall volcano-sedimentary rocks. In contrast, the Subika gold deposit is hosted entirely in hanging-wall plutonic rocks. Steeper-dipping segments (e.g., Apensu, Awonsu, Subika) and right-hand flexures (e.g., Amoma, Apensu) in the Kenyase-Yamfo shear zone and subsidiary structures appear to have represented sites of enhanced damage and fluid flux (i.e., restraining bends). All gold deposits occur within structural domains bounded by discontinuous, low-displacement, sinistral N-striking tear faults oblique to the orogen-parallel Kenyase-Yamfo shear zone. At the deposit scale, ore-related hydrothermal alteration is zoned, with distal chlorite-sericite grading into proximal silica-albite-Fe-carbonate mineral assemblages. Alteration halos are restricted to narrow selvages around quartz-carbonate vein arrays in multiple stacked ore shoots at Subika, whereas these halos extend 30 to 100 m away from the ore zones at Apensu and Awonsu. There is a clear spatial association between shallow-dipping mafic dikes, mafic chonoliths, shear zones, and economic gold mineralization. The abundance of mafic dikes and chonoliths within intermediate to felsic hanging-wall plutonic host rocks provided rheological heterogeneity that favored the formation of enhanced fracture permeability, promoting the tapping of ore fluid(s). Our interpretation is that these stacked shallow-dipping mafic dike arrays also acted as aquitards, impeding upward fluid flow within the wider intrusive rock mass until a failure threshold was episodically reached due to fluid overpressure, resulting in transient fracture-controlled upward propagation of the ore-fluid(s). Our results indicate that high-grade ore shoots at Ahafo South form part of vertically extensive fluid conduit systems that are primarily controlled by the rheological architecture of the rock mass.

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Complex kinematics in a major ductile shear zone, NW Shetland: Evidence of ductile extrusion during Caledonian transpression

10.5194/egusphere-egu21-722 ◽

2021 ◽

Author(s):

Timothy Armitage ◽

Robert Holdsworth ◽

Robin Strachan ◽

Thomas Zach ◽

Diana Alvarez-Ruiz ◽

...

Keyword(s):

Shear Zone ◽

Hanging Wall ◽

Regional Scale ◽

Shear Zones ◽

White Mica ◽

Strike Slip ◽

Amphibolite Facies ◽

Lateral Extrusion ◽

Shear Sense ◽

Ductile Shear

Ductile shear zones are heterogeneous areas of strain localisation which often display variation in strain geometry and combinations of coaxial and non-coaxial deformation. One such heterogeneous shear zone is the c. 2 km thick Uyea Shear Zone (USZ) in northwest Mainland Shetland (UK), which separates variably deformed Neoarchaean orthogneisses in its footwall from Neoproterozoic metasediments in its hanging wall (Fig. a). The USZ is characterised by decimetre-scale layers of dip-slip thrusting and extension, strike-slip sinistral and dextral shear senses and interleaved ultramylonitic coaxially deformed horizons. Within the zones of transition between shear sense layers, mineral lineations swing from foliation down-dip to foliation-parallel in kinematically compatible, anticlockwise/clockwise-rotations on a local and regional scale (Fig. b). Rb-Sr dating of white mica grains via laser ablation indicates a c. 440-425 Ma Caledonian age for dip-slip and strike-slip layers and an 800 Ma Neoproterozoic age for coaxial layers. Quartz opening angles and microstructures suggest an upper-greenschist to lower-amphibolite facies temperature for deformation. We propose that a Neoproterozoic, coaxial event is overprinted by Caledonian sinistral transpression under upper greenschist/lower amphibolite facies conditions. Interleaved kinematics and mineral lineation swings are attributed to result from differential flow rates resulting in vertical and lateral extrusion and indicate regional-scale sinistral transpression during the Caledonian orogeny in NW Shetland. This study highlights the importance of linking geochronology to microstructures in a poly-deformed terrane and is a rare example of a highly heterogeneous shear zone in which both vertical and lateral extrusion occurred during transpression.<img src="https://contentmanager.copernicus.org/fileStorageProxy.php?f=gepj.0cf6ef44e5ff57820599061/sdaolpUECMynit/12UGE&app=m&a=0&c=d96bb6db75eed0739f2a6ee90c9ad8fd&ct=x&pn=gepj.elif&d=1" alt="">

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Geology of the shear-hosted Brookbank gold prospect in the Beardmore–Geraldton belt, Wabigoon subprovince, Ontario

Canadian Journal of Earth Sciences ◽

10.1139/e06-118 ◽

2007 ◽

Vol 44 (7) ◽

pp. 925-946 ◽

Cited By ~ 1

Author(s):

Jerry C DeWolfe ◽

Bruno Lafrance ◽

Greg M Stott

Keyword(s):

Shear Zone ◽

Hanging Wall ◽

Shear Zones ◽

Gold Deposits ◽

Iron Formation ◽

Alteration Zone ◽

Gold Deposition ◽

Low Grade ◽

Metasedimentary Rocks ◽

Polymictic Conglomerate

The Beardmore–Geraldton belt consists of steeply dipping, intercalated panels of metavolcanic and metasedimentary rocks along the southern margin of the granite–greenstone Wabigoon subprovince in the Archean Superior Province, Ontario. It is an important past-producing gold belt that includes classic epigenetic iron-formation-hosted deposits near Geraldton and turbidite-hosted deposits, north of Beardmore. The Brookbank gold prospect belongs to a third group of related gold deposits that formed along dextral shear zones localized at contacts between panels of metasedimentary and metavolcanic rocks. The Brookbank prospect occurs along a steeply dipping shear zone at the contact between footwall polymictic conglomerate and hanging-wall calc-alkaline arc basalt. Early during shearing the basalt acted as a structural and chemical trap that localized brittle deformation, veining, and gold deposition, ankerite–sericite–chlorite–epidote–pyrite alteration, and the replacement of metamorphic magnetite and ilmenite by gold-bearing pyrite. This produced a low grade (≤5 g/t Au) ankerite-rich alteration zone that extends up to 20 m into the hanging-wall basalt. Later during shearing, gold was deposited within higher grade (≤20 g/t Au) quartz–orthoclase–pyrite alteration zones superimposed on the wider ankerite-rich alteration zone. Auriferous quartz–carbonate veins oriented clockwise and counter-clockwise to the shear zone walls are folded and boudinaged, respectively, consistent with dextral slip along the shear zone. A key finding of the study is that different groups of gold deposits in the belt, including epigenetic iron formation gold deposits near Geraldton, formed during post-2690 Ma regional dextral transpression across the belt.

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The role of oroclinal bending in the structural evolution of the Central Anatolian Plateau: evidence of a regional changeover from shortening to extension

Geologica Carpathica ◽

10.2478/v10096-011-0026-7 ◽

2011 ◽

Vol 62 (4) ◽

pp. 345-359 ◽

Cited By ~ 13

Author(s):

Erman Özsayin ◽

Kadir Dirik

Keyword(s):

Fault Zone ◽

Structural Evolution ◽

Shear Zones ◽

Fault Zones ◽

Fault System ◽

Western Boundary ◽

Southeastern Part ◽

Anatolian Plateau ◽

Isparta Angle

The role of oroclinal bending in the structural evolution of the Central Anatolian Plateau: evidence of a regional changeover from shortening to extensionThe NW-SE striking extensional Inönü-Eskişehir Fault System is one of the most important active shear zones in Central Anatolia. This shear zone is comprised of semi-independent fault segments that constitute an integral array of crustal-scale faults that transverse the interior of the Anatolian plateau region. The WNW striking Eskişehir Fault Zone constitutes the western to central part of the system. Toward the southeast, this system splays into three fault zones. The NW striking Ilıca Fault Zone defines the northern branch of this splay. The middle and southern branches are the Yeniceoba and Cihanbeyli Fault Zones, which also constitute the western boundary of the tectonically active extensional Tuzgölü Basin. The Sultanhanı Fault Zone is the southeastern part of the system and also controls the southewestern margin of the Tuzgölü Basin. Structural observations and kinematic analysis of mesoscale faults in the Yeniceoba and Cihanbeyli Fault Zones clearly indicate a two-stage deformation history and kinematic changeover from contraction to extension. N-S compression was responsible for the development of the dextral Yeniceoba Fault Zone. Activity along this structure was superseded by normal faulting driven by NNE-SSW oriented tension that was accompanied by the reactivation of the Yeniceoba Fault Zone and the formation of the Cihanbeyli Fault Zone. The branching of the Inönü-Eskişehir Fault System into three fault zones (aligned with the apex of the Isparta Angle) and the formation of graben and halfgraben in the southeastern part of this system suggest ongoing asymmetric extension in the Anatolian Plateau. This extension is compatible with a clockwise rotation of the area, which may be associated with the eastern sector of the Isparta Angle, an oroclinal structure in the western central part of the plateau. As the initiation of extension in the central to southeastern part of the Inönü-Eskişehir Fault System has similarities with structures associated with the Isparta Angle, there may be a possible relationship between the active deformation and bending of the orocline and adjacent areas.

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Role of the deep crustal scale geometry on Western Alps strain partitioning : Insights from S-wave velocity tomography

10.5194/egusphere-egu21-8928 ◽

2021 ◽

Author(s):

Stéphane Schwartz ◽

Ahmed Nouibat ◽

Yann Rolland ◽

Thierry Dumont ◽

Anne Paul ◽

...

Keyword(s):

Wave Velocity ◽

Shear Zones ◽

Western Alps ◽

Crustal Thickening ◽

Variscan Orogeny ◽

Western Boundary ◽

S Wave ◽

Structural Anisotropy ◽

Velocity Tomography

The recent S-wave velocity tomography undertaken at the scale of the Alps by Nouibat et al. (2021) allows a reappraisal of the deep structure of this mountain belt. These geophysical data highlight the role of crustal geometry in the strain field development observed in the Western Alps. The geophysical imagery shows a standard crustal thickness in the foreland, with slow velocities (<3.6 km.s-1) in the lower crust. The occurrence of a sharp Moho offset of 5-12 km is detected beneath the External Crystalline Massifs (ECMs). The ECMs do not show any significant crustal thickening in their frontal parts (<35 km), except for the Pelvoux ECM (35-40 km). Beneath the internal zones, east of the Penninic Frontal Thrust, the crustal geometry is more complex with the presence of an European continental slab subducting locally deeper than 80 km beneath the Adria plate. This slab is overlain by a high-pressure metamorphic orogenic prism. The lower part, corresponding to the Ivrea gravimetry anomaly, shows seismic signatures of serpentinized mantle (Vs between 3.8 and 4.3 km.s-1) whose upper limit is located at 10 km depth below the Dora Maira internal crystalline massif. This new crustal-scale image can be compared to the current deformation pattern, which appears highly partitioned at the scale of the Alpine arc. The internal zones show a transtensional deformation regime, whose activity is distributed along two major seismic lineaments (the &#8216;Piemontais&#8217; and &#8216;Brian&#231;onnais&#8217; ones). The Alpine European foreland shows a transpressional deformation that is more diffuse and associated with vertical displacements in the ECMs. Beneath the Po plain, the seismic activity is deeper (>40 km), and correlates with a transpressional deformation which is localized along sub-vertical lineaments. The deformation of the orogenic prism appears controlled by a deeper and rigid mantle indenter split in two units by a major subvertical serpentinized structure. The upper unit, which indents horizontally and vertically the crustal orogenic prism, is located between 20 and 45 km depth. The lower unit corresponds to the western boundary of the Adria mantle that pinches directly the European slab. The surface observations and geochronological data suggest that the Moho offstets are superposed on European crustal-scale faults trend inherited from the Variscan orogeny, following the East-Variscan strike-slip system. This structural anisotropy was reactivated during the Alpine orogeny as shear zones in a mainly transpressional regime since about 25-30 Ma, as documented by Ar-Ar data on syn-kinematic mica and U-Pb on monazite. The comparison of current seismicity with the kinematics of exhumed shear zones suggests a continuity of this regime since 25-30 Ma, in response to the Adria plate anticlockwise rotation.

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Structural evolution, metamorphism and melting in the Greater Himalayan Sequence in central-western Nepal

Geological Society London Special Publications ◽

10.1144/sp483.3 ◽

2018 ◽

Vol 483 (1) ◽

pp. 305-323 ◽

Cited By ~ 11

Author(s):

Rodolfo Carosi ◽

Chiara Montomoli ◽

Salvatore Iaccarino ◽

Dario Visonà

Keyword(s):

Hanging Wall ◽

Regional Scale ◽

Structural Evolution ◽

Shear Zones ◽

Geological Mapping ◽

Eastern Himalaya ◽

Main Central Thrust ◽

Tectonic Model ◽

Time Deformation ◽

Greater Himalayan Sequence

AbstractJoining geological mapping, structural analysis, petrology and geochronology allowed the internal architecture of the Greater Himalayan Sequence (GHS) to be unraveled. Several top-to-the-south/SW tectonic–metamorphic discontinuities developed at the regional scale, dividing it into three main units exhumed progressively from the upper to the lower one, starting from c. 40 Ma and lasting for several million years. The activity of shear zones has been constrained and linked to the pressure–temperature–time–deformation (P–T–t–D) evolution of the deformed rocks by the use of petrochronology. Hanging wall and footwall rocks of the shear zones recorded maximum P–T conditions at different times. Above the Main Central Thrust, a cryptic tectonometamorphic discontinuity (the High Himalayan Discontinuity (HHD)) has been recognized in Central-Eastern Himalaya.The older shear zone, that was active at c. 41–28 Ma, triggered the earlier exhumation of the uppermost GHS and allowed the migration of melt, which was produced at peak metamorphic conditions and subsequently produced in abundance at the time of the activation of the HHD. Production of melt continued at low pressure, with nearly isobaric heating leading to the genesis and emplacement of andalusite- and cordierite-bearing granites.The timing of the activation of the shear zones from deeper to upper structural levels fits with an in-sequence shearing tectonic model for the exhumation of the GHS, further affected by out-of-sequence thrusts.

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