Interactions of plutons and detachments: a comparison of Aegean and Tyrrhenian granitoids

Abstract. Back-arc extension superimposed on mountain belts leads to distributed normal faults and shear zones interacting with magma emplacement within the crust. The composition of granitic magmas emplaced at this stage often involves a large component of crustal melting. The Miocene Aegean granitoids were emplaced in metamorphic core complexes (MCCs) below crustal-scale low-angle normal faults and ductile shear zones. Intrusion processes interact with extension and shear along detachments, from the hot magmatic flow within the pluton root zone to the colder ductile and brittle deformation below and along the detachment. A comparison of the Aegean plutons with the island of Elba MCC in the back-arc region of the Apennine subduction shows that these processes are characteristic of pluton–detachment interactions in general. We discuss a conceptual emplacement model, tested by numerical models. Mafic injections within the partially molten lower crust above the hot asthenosphere trigger the ascent within the core of the MCC of felsic magmas, controlled by the strain localization on persistent crustal-scale shear zones at the top that guide the ascent until the brittle ductile transition. Once the system definitely enters the brittle regime, the detachment and the upper crust are intruded, while new detachments migrate upward and in the direction of shearing.

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Interactions of plutons and detachments, comparison of Aegean and Tyrrhenian granitoids

10.5194/se-2021-11 ◽

2021 ◽

Author(s):

Laurent Jolivet ◽

Laurent Arbaret ◽

Laetitia Le Pourhiet ◽

Florent Cheval-Garabedian ◽

Vincent Roche ◽

...

Keyword(s):

Root Zone ◽

Numerical Models ◽

Shear Zones ◽

Normal Faults ◽

Metamorphic Core Complexes ◽

Kinematic Evolution ◽

Brittle Ductile Transition ◽

Mountain Belts ◽

Back Arc ◽

Metamorphic Core

Abstract. Back-arc extension superimposed on mountain belts leads to distributed normal faults and shear zones, interacting with magma emplacement in the crust. The composition of granitic magmas emplaced at this stage often involves a component of crustal melting. The Miocene Aegean granitoids were emplaced in metamorphic core complexes (MCC) below crustal-scale low-angle extensional shear zones and normal faults. Intrusion in such contexts interacts with extension and shear along detachments, from the hot magmatic flow within the pluton root zone to the colder ductile and brittle deformation along the detachment. A comparison of the Aegean plutons with the Elba Island MCC in the back-arc region of the Apennines subduction shows that these processes are characteristic of pluton-detachment interactions in general and we discuss a conceptual emplacement scenario, tested by numerical models. Mafic injections within the partially molten lower crust above the hot asthenosphere trigger the ascent within the core of the MCC of felsic magmas, controlled by the strain localization on persistent crustal scale shear zones at the top that guide the ascent until the brittle ductile transition is reached during exhumation. Once the system definitely enters the brittle regime, the detachment and the upper crust are intruded while new detachments migrate upward and in the direction of shearing. Numerical models reproduce the geometry and the kinematic evolution deduced from field observations.

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Tectonic significance of synextensional ductile shear zones within the Early Miocene Alaçamdağ granites, northwestern Turkey

Geological Magazine ◽

10.1017/s0016756809990719 ◽

2009 ◽

Vol 147 (4) ◽

pp. 611-637 ◽

Cited By ~ 33

Author(s):

FUAT ERKÜL

Keyword(s):

Hanging Wall ◽

Shear Zones ◽

Early Miocene ◽

Aegean Region ◽

Menderes Massif ◽

Metamorphic Core Complexes ◽

Ductile Shear Zones ◽

Northwestern Turkey ◽

Ductile Shear ◽

Metamorphic Core

AbstractSynextensional granitoids may have significant structural features leading to the understanding of the evolution of extended orogenic belts. One of the highly extended regions, the Aegean region, includes a number of metamorphic core complexes and synextensional granitoids that developed following the Alpine collisional events. The Alaçamdağ area in northwestern Turkey is one of the key areas where Miocene granites crop out along the boundary of various tectonic units. Structural data from the Early Miocene Alaçamdağ granites demonstrated two different deformation patterns that may provide insights into the development of granitic intrusions and metamorphic core complexes. (1) Steeply dipping ductile shear zones caused emplacement of syn-tectonic granite stocks; they include kinematic indicators of a sinistral top-to-the-SW displacement. This zone has also juxtaposed the İzmir–Ankara Zone and the Menderes Massif in the west and east, respectively. (2) Gently dipping ductile shear zones have developed within the granitic stocks that intruded the schists of the Menderes Massif on the structurally lower parts. Kinematic data from the foliated granites indicate a top-to-the-NE displacement, which can be correlated with the direction of the hanging-wall movement documented from the Simav and Kazdağ metamorphic core complexes. The gently dipping shear zones indicate the presence of a detachment fault between the Menderes Massif and the structurally overlying İzmir–Ankara Zone. Mesoscopic- to map-scale folds in the shallow-dipping shear zones of the Alaçamdağ area were interpreted to have been caused by coupling between NE–SW stretching and the accompanying NW–SE shortening of ductilely deformed crust during Early Miocene times. One of the NE-trending shear zones fed by granitic magmas was interpreted to form the northeastern part of a sinistral wrench corridor which caused differential stretching between the Cycladic and the Menderes massifs. This crustal-scale wrench corridor, the İzmir–Balıkesir transfer zone, may have controlled the asymmetrical and symmetrical extensions in the orogenic domains. The combination of the retreat of the Aegean subduction zone and the lateral slab segmentation leading to the sinistral oblique-slip tearing within the Eurasian upper plate appears to be a plausible mechanism for the development of such extensive NE-trending shear zones in the Aegean region.

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Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

10.5194/se-2016-135 ◽

2016 ◽

Author(s):

Frances J. Cooper ◽

John P. Platt ◽

Whitney M. Behr

Keyword(s):

High Strain ◽

Shear Zones ◽

Ductile Deformation ◽

Lower Zone ◽

Middle Crust ◽

Metamorphic Core Complexes ◽

Brittle Ductile Transition ◽

Ductile Shear ◽

Metamorphic Core ◽

Whipple Mountains

Abstract. High strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada) suggest the presence of two distinct rheological transitions in the middle crust. (1) The brittle-ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear. (2) The localized-distributed transition or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths. In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. In some examples (e.g. the Whipple Mountains) the lower zone is spatially distinct from the detachment, although high-strain rocks from the lower zone were subsequently exhumed along the detachment. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization.

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Rheological transitions in the middle crust: insights from Cordilleran metamorphic core complexes

Solid Earth ◽

10.5194/se-8-199-2017 ◽

2017 ◽

Vol 8 (1) ◽

pp. 199-215 ◽

Cited By ~ 12

Author(s):

Frances J. Cooper ◽

John P. Platt ◽

Whitney M. Behr

Keyword(s):

High Strain ◽

High Stress ◽

Shear Zones ◽

Ductile Deformation ◽

Middle Crust ◽

Metamorphic Core Complexes ◽

Brittle Ductile Transition ◽

Ductile Shear ◽

Metamorphic Core ◽

Whipple Mountains

Abstract. High-strain mylonitic rocks in Cordilleran metamorphic core complexes reflect ductile deformation in the middle crust, but in many examples it is unclear how these mylonites relate to the brittle detachments that overlie them. Field observations, microstructural analyses, and thermobarometric data from the footwalls of three metamorphic core complexes in the Basin and Range Province, USA (the Whipple Mountains, California; the northern Snake Range, Nevada; and Ruby Mountains–East Humboldt Range, Nevada), suggest the presence of two distinct rheological transitions in the middle crust: (1) the brittle–ductile transition (BDT), which depends on thermal gradient and tectonic regime, and marks the switch from discrete brittle faulting and cataclasis to continuous, but still localized, ductile shear, and (2) the localized–distributed transition, or LDT, a deeper, dominantly temperature-dependent transition, which marks the switch from localized ductile shear to distributed ductile flow. In this model, brittle normal faults in the upper crust persist as ductile shear zones below the BDT in the middle crust, and sole into the subhorizontal LDT at greater depths.In metamorphic core complexes, the presence of these two distinct rheological transitions results in the development of two zones of ductile deformation: a relatively narrow zone of high-stress mylonite that is spatially and genetically related to the brittle detachment, underlain by a broader zone of high-strain, relatively low-stress rock that formed in the middle crust below the LDT, and in some cases before the detachment was initiated. The two zones show distinct microstructural assemblages, reflecting different conditions of temperature and stress during deformation, and contain superposed sequences of microstructures reflecting progressive exhumation, cooling, and strain localization. The LDT is not always exhumed, or it may be obscured by later deformation, but in the Whipple Mountains, it can be directly observed where high-strain mylonites captured from the middle crust depart from the brittle detachment along a mylonitic front.

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Neogene polyphase deformation related to the Alboran Basin evolution: new insights for the Beni Bousera massif (Internal Rif, Morocco)

BSGF - Earth Sciences Bulletin ◽

10.1051/bsgf/2020008 ◽

2020 ◽

Vol 191 ◽

pp. 10

Author(s):

Asmae El Bakili ◽

Michel Corsini ◽

Ahmed Chalouan ◽

Philippe Münch ◽

Adrien Romagny ◽

...

Keyword(s):

Shear Zone ◽

Shear Zones ◽

Normal Faults ◽

Basin Evolution ◽

Polyphase Deformation ◽

Ductile Shear Zones ◽

Rif Belt ◽

Alboran Basin ◽

Back Arc ◽

Internal Domain

Located in the Internal domain of the Rif belt, the Beni Bousera massif is characterized by a stack of peridotites and crustal metamorphic units. The massif is intruded by granitic dykes and affected by several normal ductile shear zones. Structural, petrological and 40Ar–39Ar dating analyses performed on these two elements highlight that (1) the granitic dykes are emplaced within major N70° to N140° trending normal faults and shear zones, resulted from an NNE-SSW extension (2) the Aaraben fault in its NE part is characterized by N70° to N150° trending ductile normal shear zones, resulted from a nearly N-S extension and (3) the age of this extensional event is comprised between 22 and 20 Ma. Available paleomagnetic data allow a restoration of the initial orientation of extension, which was nearly E-W contemporary with the Alboran Basin opening in back-arc context, during the Early Miocene. At the onset of the extension, the peridotites were somehow lying upon a partially melted continental crust, and exhumed during this event by the Aaraben Normal Shear Zone. Afterward, the Alboran Domain suffered several compressional events.

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Late Ottawan orogenic collapse of the Adirondacks in the Grenville province of New York State (USA): Integrated petrologic, geochronologic, and structural analysis of the Diana Complex in the southern Carthage-Colton mylonite zone

Geosphere ◽

10.1130/ges02155.1 ◽

2020 ◽

Vol 16 (3) ◽

pp. 844-874

Author(s):

Graham B. Baird

Keyword(s):

New York ◽

Pure Shear ◽

New York State ◽

Shear Zones ◽

Core Complex ◽

Grenville Province ◽

York State ◽

Mylonite Zone ◽

Ductile Shear Zones ◽

Metamorphic Core

Abstract Crustal-scale shear zones can be highly important but complicated orogenic structures, therefore they must be studied in detail along their entire length. The Carthage-Colton mylonite zone (CCMZ) is one such shear zone in the northwestern Adirondacks of northern New York State (USA), part of the Mesoproterozoic Grenville province. The southern CCMZ is contained within the Diana Complex, and geochemistry and U-Pb zircon geochronology demonstrate that the Diana Complex is expansive and collectively crystallized at 1164.3 ± 6.2 Ma. Major ductile structures within the CCMZ and Diana Complex include a northwest-dipping penetrative regional mylonitic foliation with north-trending lineation that bisects a conjugate set of mesoscale ductile shear zones. These ductile structures formed from the same 1060–1050 Ma pure shear transitioning to a top-to-the-SSE shearing event at ∼700 °C. Other important structures include a ductile fault and breccia zones. The ductile fault formed immediately following the major ductile structures, while the breccia zones may have formed at ca. 945 Ma in greenschist facies conditions. Two models can explain the studied structures and other regional observations. Model 1 postulates that the CCMZ is an Ottawan orogeny (1090–1035 Ma) thrust, which was later reactivated locally as a tectonic collapse structure. Model 2, the preferred model, postulates that the CCMZ initially formed as a subhorizontal mid-crustal mylonite zone during collapse of the Ottawan orogen. With continued collapse, a metamorphic core complex formed and the CCMZ was rotated into is current orientation and overprinted with other structures.

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The kinematic vorticity analysis of ductile shear zones of Ambaji Granulite, NW India and its tectonic implications

10.5194/egusphere-egu2020-7397 ◽

2020 ◽

Author(s):

Sudheer Kumar Tiwari ◽

Anouk Beniest ◽

Tapas Kumar Biswal

Keyword(s):

High Temperature ◽

Low Temperature ◽

Lateral Displacement ◽

Pure Shear ◽

Tectonic Setting ◽

Shear Zones ◽

Normal Faults ◽

Temperature Phase ◽

Brittle Ductile Transition ◽

Deformation Phase

The Neoproterozoic (834 &#8211; 778 Ma) Ambaji granulite witnessed four deformation phases (D1- D4), of which the D2 deformation phase was most significant for the exhumation of granulites in the ductile regime. We performed a field study to investigate the tectonic evolution of the D2 deformation phase and investigated the deformation evolution of the ductile extrusion of the Ambaji granulite by estimating the vorticity of flow (Wm) with the Rigid Grain Net and strain ratio/orientation techniques.During the D2 deformation phase, the S1 fabric was folded by F2 folds that are coaxial with the F1 folds. The F2 folds were produced in response to NW-SE compression. Because the large shear zones are oriented parallel to the axial plane of the F2 folds, they likely formed simultaneously during the D2 deformation phase. Compression during the D2 deformation phase accommodated most of the exhumation of the granulite along the shear zones. D2 shearing was constrained between 834&#8239;&#177;&#8239;7 to 778&#8239;&#177;&#8239;8&#8239;Ma (Monazite ages).The shear zones evolved from a high temperature (>700&#8239;&#176;C) thrust-slip shearing event in the lower-middle crust to a low temperature (450&#8239;&#176;C) retrograde sinistral shearing event at the brittle-ductile-transition (BDT). The&#160;Wm&#160;estimates of 0.32&#8211;0.40 and 0.60 coincide with the high temperature event and suggests pure shear dominated deformation. The low temperature phase coincides with&#160;Wm&#160;estimates of 0.64&#8211;0.87 and ~1.0, implying two flow regimes. The shear zone was first affected by general non-coaxial deformation and gradually became dominated by simple shearing.We interpreted that the high temperature event happened in a compressive tectonic regime, which led to horizontal shortening and vertical displacement of the granulite to the BDT. The low temperature event occurred in a transpressive tectonic setting that caused the lateral displacement of the granulite body at BDT depth. The Wm values indicate a non-steady strain during the exhumation of granulite. From the BDT to surface, the Ambaji granulite exhumed through the NW-SE directed extension for normal faults via brittle exhumation through crustal extension and thinning.

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Migration of deformation, subsidence, and basin formation in the SW Pannonian Basin (central Europe) and the change to contractional deformation

10.5194/egusphere-egu21-13300 ◽

2021 ◽

Author(s):

László Fodor ◽

Attila Balázs ◽

Gábor Csillag ◽

István Dunkl ◽

Gábor Héja ◽

...

Keyword(s):

Pannonian Basin ◽

Numerical Models ◽

Hanging Wall ◽

Shear Zones ◽

Normal Faults ◽

Rift System ◽

Distal Margin ◽

Late Middle Miocene ◽

Basin Scale ◽

Basin Formation

The Pannonian Basin is a continental extensional basin system with various depocentres within the Alpine&#8211;Carpathian&#8211;Dinaridic orogenic belt. Along the western basin margin, exhumation along the Rechnitz, Pohorje, Kozjak, and Baj&#225;n detachments resulted in cooling of diverse crustal segments of the Alpine nappe stack (Koralpe-W&#246;lz and Penninic nappes); the process is constrained by variable thermochronological data between ~25&#8211;23 to ~15 Ma. Rapid subsidence in supradetachment sub-basins indicates the onset of sedimentation in the late Early Miocene (Ottnangian? or Karpatian, from ~19 or 17.2 Ma). In addition to extensional structures, strike-slip faults mostly accommodated differential extension between domains marked by large low-angle normal faults. Branches of the Mid-Hungarian Shear Zone (MHZ) also played the role of transfer faults, although shear-zones perpendicular to extension also occurred locally.During this period, the distal margin of the large tilted block in the hanging wall of the detachment system, the pre-Miocene rocks of the Transdanubian Range (TR) experienced surface exposure, karstification, and terrestrial sedimentation. The situation changed after ~15&#8211;14.5 Ma when faulting, subsidence, and basin formation shifted north-eastward. Migration of normal faulting resulted in fault-controlled basin subsidence within the TR which lasted until ~8 Ma.3D thermo-mechanical lithospheric and basin-scale numerical models predict similar spatial migration of the depocenters from the orogenic margin towards the basin center. The reason for this migration is found in the interaction of deep Earth and surface processes. A lithospheric and smaller crustal-scale weak zones inherited from a preceding orogenic structure localize initial deformation, while their redistribution controls asymmetric extension accompanied by the upraising of the asthenopshere and flexure of the lithosphere. Models suggest ~4&#8211;5 Myr delay of the onset of sedimentation after the onset of crustal extension and ~150&#8211;200 km of shift in depocenters during ~12 Myr. These modeling results agree well with our robust structural and chronological data on basin migration.Simultaneously with or shortly after depocenter migration, the southern part of the former rift system, mostly near the MHZ, underwent ~N&#8211;S shortening; the basin fill was folded and the boundary normal faults were inverted. The style of deformation changed from pure contraction to transpression. The Baj&#225;n detachment could be slightly folded, although its synformal shape could also be considered a detachment corrugation. Deformation was dated to ~15&#8211;14 Ma (middle Badenian) in certain sub-basins while in other sub-basins deformation seems to be continuous throughout the late Middle Miocene from ~15 Ma to ~11.6 Ma.Another contractional pulse occurred in the earliest Late Miocene, between ~11.6 and ~9.7 Ma while the western part of the TR was still affected by extensional faulting and subsidence. All these contractional deformations can be linked to the much larger fold-and-thrust belt that extends from the Southern and Julian Alps through the Sava folds region in Slovenia. Contraction is still active, as indicated by recent earthquakes in Croatia.Mol Ltd. largely supported the research. The research is supported by the scientific grant NKFI OTKA 134873 and the Slovenian Research Agency (research core funding No. P1-0195).

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Development of C- shear bands in brittle-ductile shear zones: Insights from analogue and numerical models.

10.5194/egusphere-egu2020-960 ◽

2020 ◽

Author(s):

Arnab Roy ◽

Nandan Roy ◽

Puspendu Saha ◽

Nibir Mandal

Keyword(s):

Shear Zone ◽

Numerical Models ◽

Shear Bands ◽

Shear Zones ◽

Rheological Parameters ◽

Comprehensive Understanding ◽

Experimental Conditions ◽

Ductile Shear Zones ◽

Analogue Models ◽

Ductile Shear

Development of brittle and brittle-ductile shear zones involve partitioning of large shear strains in bands, called C-shear bands (C-SB) nearly parallel to the shear zone boundaries. Our present work aims to provide a comprehensive understanding of the rheological factors in controlling such SB growth in meter scale natural brittle- ductile shear zones observed in in Singbhum and Chotonagpur mobile belts.&#160; The shear zones show C- SB at an angle of 0&#176;- 5&#176; with the shear zone boundary. We used analogue models, based on Coulomb and Viscoplastic rheology to reproduce them in experimental conditions.These models produce dominantly Riedel (R) shear bands. We show a transition from R-shearing in conjugate to single sets at angles of ~15o by changing model materials. However, none of the analogue models produced C-SB, as observed in the field. To reconcile the experimental and field findings, numeral models have been used to better constrain the geometrical and rheological parameters. We simulate model shear zones replicating those observed in the field, which display two distinct zones: drag zone where the viscous strains dominate &#160;and the core zone, where both viscous and plastic strains come into play. &#160;Numerical model results suggest the formation of &#160;C- SB for a specific rheological condition. We also show varying shear band patterns as a function of the thickness ratio between drag and core zones.

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The ephemeral development of C' shear bands

10.5194/egusphere-egu21-752 ◽

2021 ◽

Author(s):

Melanie Finch ◽

Paul Bons ◽

Florian Steinbach ◽

Albert Griera ◽

Maria-Gema Llorens ◽

...

Keyword(s):

Strain Rate ◽

Numerical Models ◽

Shear Bands ◽

Shear Zones ◽

Stress And Strain ◽

High Mechanical Strength ◽

Weak Phase ◽

Ductile Shear Zones ◽

Stress And Strain Rate ◽

Stress And Strain Distribution

C' shear bands are common structures in ductile shear zones but their development is poorly understood. They occur in rocks with a high mechanical strength contrast so we used numerical models of viscoplastic deformation to study the effect of the proportion of weak phase and the phase strength contrast on C' shear band development. We employed simple shear to a finite strain of 18 in 900 steps and recorded the microstructure, stress and strain distribution at each step. We found that C' shear bands form in models with &#8805;5% weak phase when there is a moderate or high phase strength contrast, and they occur in all models with weak phase proportions &#8805;15%. Contrary to previous research, we find that C' shear bands form when layers of weak phase parallel to the shear zone boundary rotate forwards. This occurs due to mechanical instabilities that are a result of heterogeneous distributions of stress and strain rate. C' shear bands form on planes of low strain rate and stress, not in sites of maximum strain rate as has previously been suggested. C' shear bands are ephemeral and they either rotate backwards to the C plane once they are inactive or rotate into the field of shortening and thicken to form X- and triangle- shaped structures.

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