A Discussion on natural strain and geological structure - Archaean deformation patterns in southern Africa

1976 ◽  
Vol 283 (1312) ◽  
pp. 313-331 ◽  
Keyword(s):  
Shear Zones ◽  
Geological Structure ◽  
Mobile Belt ◽  
Deformation Pattern ◽  
Strain Measurements ◽  
Regional Deformation ◽  
Greenstone Belts ◽  
Extension Direction ◽  
Orogenic Belts ◽  
Limpopo Belt

Strain measurements have been made to help quantify the intensity of deformation and amount of displacement across Archaean greenstone belts in Rhodesia and Botswana and across the gneisses of the Limpopo mobile belt. The area has been divided into three domains based on the orientation of the finite strain fabric and the orientation of the maximum extension direction in associated shear zones. The domains are considered to have different movement patterns and to be similar to small orogenic belts. Early deformation within the greenstone belts accompanied the intrusion of the diaipric granites, but there was also bulk translation and rotation of greenstone belt and gneiss leading to imbrication of the stratigraphic pile and the formation of large nappes of overturned rock. This was followed by regional phases of deformation which affected all the greenstone belts and the gneisses of the Limpopo belt. Detailed strain measurements show a variation in amount of shortening during this phase, from under 30 % across the Shabani-Bellingwe belt in central Rhodesia, to over 60 % across the Tati and Matsitama belts in northern Botswana. Many local variations in intensity of deformation occur within large ductile shear zones and deviations from plane strain may be partly due to such rotational deformation. The regional deformation pattern suggests that there was movement of the Rhodesian craton approximately 200 km to the southwest relative to the gneisses of the Limpopo belt, producing a dominantly flattening deformation in the southwest of Rhodesia, but dominantly simple shear with a nearly horizontal sinistral movement, in the southeast.

scholarly journals The perigranitic W-Au Salau deposit (Pyrenees, France): polyphase genesis of a late Variscan intrusion related deposit

2021 ◽  
Vol 192 ◽  
pp. 22
Author(s):  
Thomas Poitrenaud ◽  
Éric Marcoux ◽  
Romain Augier ◽  
Marc Poujol
Keyword(s):  
3D Modeling ◽  
Main Part ◽  
Isotopic Analysis ◽  
Shear Zones ◽  
Coarse Grained ◽  
Axial Zone ◽  
The Past ◽  
Regional Deformation ◽  
Deposit Type ◽  
Ore Types

A field study combined with a laboratory study and 3D modeling have been performed in order to decipher the genesis of the Salau deposit W-Au mineralization (Pyrenees, France), one of the most important for tungsten in Europe. Results show the existence of two superimposed ore types, emplaced ca. 10 km depth and within decreasing temperature conditions: a calcic silicates skarn with rare scheelite and disseminated sulphides followed by a mineralized breccia with massive sulphides (pyrrhotite and chalcopyrite dominant), coarse-grained scheelite and gold, representing the main part of the ore mined in the past. This breccia is localized in ductile-brittle shear-zones which crosscut the granodiorite. U/Pb dating on zircon, apatite and scheelite, previously realized, confirmed this polyphase evolution. These two types of mineralization, linked to the emplacement of two successive intrusions as confirmed by sulphur isotopic analysis, granodioritic then leucogranitic, can be classified as belonging to the Intrusion-Related Gold Deposit type (IRGD). The emplacement of the high-grade gold and scheelite breccia was initiated by the progressive localization of the regional deformation in the Axial Zone of the Pyrenees during the Permian within E-W dextral-reverse faults.

scholarly journals Looking beyond kinematics: 3D thermo-mechanical modelling reveals the dynamic of transform margins

2021 ◽  
Author(s):  
Anthony Jourdon ◽  
Charlie Kergaravat ◽  
Guillaume Duclaux ◽  
Caroline Huguen
Keyword(s):  
Boundary Conditions ◽  
Strain Localization ◽  
Numerical Models ◽  
Local Stress ◽  
Shear Zones ◽  
Strike Slip ◽  
Convergent Margins ◽  
Mechanical Modelling ◽  
Kinematic Models ◽  
Extension Direction

Abstract. Transform margins represent ~30 % of the non-convergent margins worldwide. Their formation and evolution have long been addressed through kinematic models that do not account for the mechanical behaviour of the lithosphere. In this study, we use high resolution 3D numerical thermo-mechanical modelling to simulate and investigate the evolution of the intra-continental strain localization under oblique extension. The obliquity is set through velocity boundary conditions that range from 15° (high obliquity) to 75° (low obliquity) every 15° for strong and weak lower continental crust rheologies. Numerical models show that the formation of localized strike-slip shear zones leading to transform continental margins always follows a thinning phase during which the lithosphere is thermally and mechanically weakened. For low (75°) to intermediate (45°) obliquity cases, the strike-slip faults are not parallel to the extension direction but form an angle of 20° to 40° with the plates' motion while for higher obliquities (30° to 15°) the strike-slip faults develop parallel to the extension direction. Numerical models also show that during the thinning of the lithosphere, the stress and strain re-orient while boundary conditions are kept constant. This evolution, due to the weakening of the lithosphere, leads to a strain localization process in three major phases: (1) strain initiates in a rigid plate where structures are sub-perpendicular to the extension direction; (2) distributed deformation with local stress field variations and formation of transtensional and strike-slip structures; (3) formation of highly localized plates boundaries stopping the intra-continental deformation. Our results call for a thorough re-evaluation of the kinematic approach to studying transform margins.

scholarly journals Outline of the Nagssugtoqidian mobile belt of East Greenland

1979 ◽  
Vol 89 ◽  
pp. 9-18
Author(s):  
D Bridgwater ◽  
J.S Myers
Keyword(s):  
Shear Zones ◽  
Granulite Facies ◽  
Tectonic Activity ◽  
Mobile Belt ◽  
East Greenland ◽  
The North ◽  
Marginal Areas ◽  
High Level ◽  
North And South ◽  
Wide Zone

The Nagssugtoqidian mobile belt is a 240 km wide zone of deformation and plutonic activity which cuts across the Archaean craton of East Greenland. The belt was established 2600 m.y. ago by the formation of vertical E-W shear zones and the syntectonic intrusion of basic dykes. Tectonic activity along the E-W shear zones was followed by the emplacement of tonalitic intrusions, the Blokken gneisses, 2350 m.y. ago in the central parts of the mobile belt. The emplacement of the Blokken gneisses was accompanied and followed by further emplacement of basic dykes. These are synplutonic in the centre of the mobile belt but are emplaced into more rigid crust in the marginal areas of the belt and in the Archaean craton to the north and south. During a second major tectonic and thermal episode circa 1900 m.y. ago, the region was deformed by thrusting from the north. In the southem part of the mobile belt the earlier steep shear zones are cut by shear zones dipping gently northwards in which rocks are downgraded to greenschist facies. The grade of metamorphism increases northwards and shear zones are replaced by open folds with axial surfaces which dip gently northwards. The increasing ductility in the centre of and northem part of the belt is associated with the intrusion of charnockitic plutons and their granulite facies aureoles. Regional uplift occurred before the intrusion of high level post-tectonic plutons of diorite and granite 1550 m.y. ago.

Petrologic and thermal constraints on the origin of leucogranites in collisional orogens

2004 ◽  
Vol 95 (1-2) ◽  
pp. 73-85 ◽  
Author(s):  
Peter I. Nabelek ◽  
Mian Liu
Keyword(s):  
Partial Melting ◽  
Shear Zones ◽  
Source Rocks ◽  
Radioactive Isotopes ◽  
The Usa ◽  
Appalachian Orogen ◽  
Shear Heating ◽  
Heating Model ◽  
Orogenic Belts ◽  
Collisional Orogens

ABSTRACTLeucogranites are typical products of collisional orogenies. They are found in orogenic terranes of different ages, including the Proterozoic Trans-Hudson orogen, as exemplified in the Black Hills, South Dakota, and the Appalachian orogen in Maine, both in the USA, and the ongoing Himalayan orogen. Characteristics of these collisional leucogranites show that they were derived from predominantly pelitic sources at the veining stages of deformation and metamorphism in upper plates of thickened crusts. Once generated, the leucogranite magmas ascended as dykes and were emplaced within shallower parts of their source sequences. In these orogenic belts, there was a strong connection between deformation, metamorphism and granite generation. However, the heat sources needed for partial melting of the source rocks remain controversial. Lack of evidence for significant intrusion of mafic magmas necessary to cause melting of upper plate source rocks suggests that leucogranite generation in collisional orogens is mainly a crustal process.The present authors evaluate five types of thermal models which have previously been proposed for generating leucogranites in collisional orogens. The first, a thickened crust with exponentially decaying distribution of heat-producing radioactive isotopes with depth, has been shown to be insufficient for heating the upper crust to melting conditions. Four other models capable of raising the crustal temperatures sufficiently to initiate partial melting of metapelites in thickened crust include: (1) thick sequences of sedimentary rocks with high amounts of internal radioactive heat production; (2) decompression melting; (3) thinning of mantle lithosphere; and (4) shear-heating. The authors show that, for reasonable boundary conditions, shear-heating along crustal-scale shear zones is the most viable process to induce melting in upper plates of collisional orogens where pelitic source lithologies are usually located. The shear-heating model directly links partial melting to the deformation and metamorphism that typically precede leucogranite genera

scholarly journals Eburnean deformation pattern of Burkina Faso and the tectonic significance of shear zones in the West African craton

2020 ◽  
Vol 191 ◽  
pp. 2 ◽  
Author(s):  
Dominique Chardon ◽  
Ousmane Bamba ◽  
Kalidou Traoré
Keyword(s):  
Burkina Faso ◽  
Shear Zone ◽  
Regional Scale ◽  
Kinematic Model ◽  
Shear Zones ◽  
West African ◽  
Deformation Pattern ◽  
The West ◽  
West African Craton ◽  
Regional Flow

Shear zones of the Paleoproterozoic Eburnean accretionary Orogen (West African craton) are investigated by means of large-scale structural mapping. Regional scale (10-100 km) mapping was based on the aeromagnetic survey of Burkina Faso and craton-scale (1000 km) mapping on a compilation of fabric data. At both scales, shear zones are arranged as an anastomosed transpressional network that accommodated distributed shortening and lateral flow of the orogenic lithosphere between the converging Kénéma-Man and Congo Archean provinces. Structural interference patterns at both scales were due to three-dimensional partitioning of progressive transpressional deformation and interactions among shear zones that absorbed heterogeneities in the regional flow patterns while maintaining the connectivity of the shear zone network. Such orogen-scale kinematic patterns call for caution in using the deformation phase approach without considering the “bigger structural picture” and interpreting displacement history of individual shear zones in terms of plate kinematics. The West African shear zone pattern is linked to that of the Guiana shield through a new transatlantic correlation to produce an integrated kinematic model of the Eburnean-Transamazonian orogen.

A terrane interpretation of the Archaean Limpopo Belt

1993 ◽  
Vol 130 (6) ◽  
pp. 755-765 ◽  
Author(s):  
H. R. Rollinson
Keyword(s):  
Shear Zones ◽  
Kaapvaal Craton ◽  
Late Archaean ◽  
The North ◽  
Zimbabwe Craton ◽  
Marginal Zones ◽  
Limpopo Belt

AbstractThe Limpopo Belt is a zone of thickened Archaean crust whose origin is currently explained by a late Archaean continent-continent collision between the Kaapvaal and Zimbabwe cratons. This review shows that the two cratons have fundamentally different geological histories and that the Zimbabwe Craton was unlikely to have behaved as a stable ‘cratonic’ block at the time of the Limpopo Belt collision. The geological histories of the Zimbabwe Craton, the North Marginal, Central and South Marginal zones of the Limpopo Belt and the Kaapvaal Craton are shown to be sufficiently different from one another to warrant their consideration as discrete terranes. The boundaries between the five units outlined above are all major shear zones, further supporting a terrane model for the Limpopo Belt. The five units were all intruded by late- to syn-tectonic granites c.2.6 Ga, constraining the accretion event to c. 2.6 Ga.

scholarly journals Kinematics, deformation partitioning and late Variscan magmatism in the Agly massif, Eastern Pyrenees, France

2020 ◽  
Vol 191 ◽  
pp. 15 ◽  
Author(s):  
Jonas Vanardois ◽  
Pierre Trap ◽  
Philippe Goncalves ◽  
Didier Marquer ◽  
Josselin Gremmel ◽  
...  
Keyword(s):  
Cross Sections ◽  
Shear Zones ◽  
Geological Mapping ◽  
Deformation Pattern ◽  
Detailed Structural Analysis ◽  
Eastern Pyrenees ◽  
Structural Work ◽  
Tectonic Forces ◽  
Orogenic Plateau ◽  
Extensional Collapse

In order to constrain the finite deformation pattern of the Variscan basement of the Agly massif, a detailed structural analysis over the whole Agly massif was performed. Our investigation combined geological mapping, reappraisal of published and unpublished data completed with our own structural work. Results are provided in the form of new tectonic maps and series of regional cross-sections through the Agly massif. At variance from previous studies, we identified three deformation fabrics named D1, D2 and D3. The D1 deformation is only relictual and characterized by a broadly northwest-southeast striking and eastward dipping foliation without any clear mineral and stretching lineation direction. D1 might be attributed to thickening of the Variscan crust in a possible orogenic plateau edge position. The D2 deformation is a heterogeneous non-coaxial deformation, affecting the whole massif, that produced a shallowly dipping S2 foliation, and an anastomosed network of C2 shear zones that accommodated vertical thinning and N20 directed extension. D2 is coeval with LP-HT metamorphism and plutonism at ca. 315–295 Ma. D2 corresponds to the extensional collapse of the partially molten orogenic crust in a global dextral strike-slip at the scale of the whole Variscan belt. The D2 fabrics are folded and steepened along a D3 east-west trending corridor, called Tournefort Deformation Zone (TDZ), where the Saint-Arnac and Tournefort intrusives and surrounding rocks share the same NE-SW to E-W subvertical S3 foliation. Along the D3 corridor, the asymmetrical schistosity pattern and kinematic criteria suggest a D3 dextral kinematics. The D3 deformation is a record of E-W striking dextral shearing that facilitated and localized the ascent and emplacement of the diorite and granitic sheet-shaped plutons. D3 outlasted D2 and turned compressional-dominated in response to the closure of the Ibero-Armorican arc in a transpressional regime. The progressive switch from D2 thinning to D3 transpression is attributed to the lessening of gravitational forces at an advanced stage of extensional collapse that became overcome by ongoing compressional tectonic forces at the southern edge of the Variscan orogenic plateau.

40Ar/39Ar Incremental Release Ages of Biotite and Hornblende from Pre-Kenoran Gneisses between the Matagami-Chibougamau and Frotet-Troilus Greenstone Belts, Quebec

1974 ◽  
Vol 11 (11) ◽  
pp. 1586-1593 ◽  
Author(s):  
R. D. Dallmeyer
Keyword(s):  
High Temperature ◽  
Tectonic Evolution ◽  
Superior Province ◽  
High Grade ◽  
Continuous Increase ◽  
Thermal Event ◽  
Greenstone Belts ◽  
Orogenic Belts ◽  
Gas Fractions

Biotite and hornblende from high-grade, granitic gneisses exposed between the Matagami-Chibougamau and Frotet-Troilus greenstone belts in Quebec have been affected by Kenoran metamorphism. Biotites record total gas 40Ar/39Ar ages of 2308 ± 30 m.y. and 2338 ± 30 m.y. Incrementally released gas fractions yield similar plateau ages, suggesting that the biotites have been totally degassed as a result of the thermal event. The ages are interpreted as reflecting the time of post-metamorphic cooling when radiogenic 40Ar began to be retained within biotite. Hornblendes record total gas 40Ar/39Ar ages of 2517 ± 40 m.y. and 2610 ± 40 m.y. Incrementally released gas fractions show a wide deviation from the total gas ages, with a continuous increase in age from low to high temperature release fractions. This lack of correlation suggests that the hornblendes have been only partially degassed by Kenoran metamorphism. However, lack of a high-temperature release plateau indicates that original meramorphic crystallization was older than the ages recorded by the highest temperature release fractions (2599 ± 40 and 2801 ± 40 m.y.). Recognition of an older sialic terrain between these greenstone belts supports recent models proposed for the tectonic evolution of the supracrustal orogenic belts in the Superior Province.

scholarly journals Kinematics of the South Atlantic rift

Solid Earth ◽  
2013 ◽  
Vol 4 (2) ◽  
pp. 215-253 ◽  
Author(s):  
C. Heine ◽  
J. Zoethout ◽  
R. D. Müller
Keyword(s):  
South America ◽  
Kinematic Model ◽  
Shear Zones ◽  
South Atlantic ◽  
West African ◽  
Passive Margin ◽  
Equatorial Atlantic ◽  
The South ◽  
Break Up ◽  
Extension Direction

Abstract. The South Atlantic rift basin evolved as a branch of a large Jurassic–Cretaceous intraplate rift zone between the African and South American plates during the final break-up of western Gondwana. While the relative motions between South America and Africa for post-break-up times are well resolved, many issues pertaining to the fit reconstruction and particularly the relation between kinematics and lithosphere dynamics during pre-break-up remain unclear in currently published plate models. We have compiled and assimilated data from these intraplated rifts and constructed a revised plate kinematic model for the pre-break-up evolution of the South Atlantic. Based on structural restoration of the conjugate South Atlantic margins and intracontinental rift basins in Africa and South America, we achieve a tight-fit reconstruction which eliminates the need for previously inferred large intracontinental shear zones, in particular in Patagonian South America. By quantitatively accounting for crustal deformation in the Central and West African Rift Zones, we have been able to indirectly construct the kinematic history of the pre-break-up evolution of the conjugate west African–Brazilian margins. Our model suggests a causal link between changes in extension direction and velocity during continental extension and the generation of marginal structures such as the enigmatic pre-salt sag basin and the São Paulo High. We model an initial E–W-directed extension between South America and Africa (fixed in present-day position) at very low extensional velocities from 140 Ma until late Hauterivian times (≈126 Ma) when rift activity along in the equatorial Atlantic domain started to increase significantly. During this initial ≈14 Myr-long stretching episode the pre-salt basin width on the conjugate Brazilian and west African margins is generated. An intermediate stage between ≈126 Ma and base Aptian is characterised by strain localisation, rapid lithospheric weakening in the equatorial Atlantic domain, resulting in both progressively increasing extensional velocities as well as a significant rotation of the extension direction to NE–SW. From base Aptian onwards diachronous lithospheric break-up occurred along the central South Atlantic rift, first in the Sergipe–Alagoas/Rio Muni margin segment in the northernmost South Atlantic. Final break-up between South America and Africa occurred in the conjugate Santos–Benguela margin segment at around 113 Ma and in the equatorial Atlantic domain between the Ghanaian Ridge and the Piauí-Ceará margin at 103 Ma. We conclude that such a multi-velocity, multi-directional rift history exerts primary control on the evolution of these conjugate passive-margin systems and can explain the first-order tectonic structures along the South Atlantic and possibly other passive margins.

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