Crustal shear zones and thrust belts: their geometry and continuity in Central Africa

The Precambrian orogenic belts of Africa are often defined by ductile shear zones which developed in response to large displacements, and which mark orogenic ‘ fronts ’ between mobile and stable parts of the crust. They are thought to represent the major crustal reflectors seen by seismic reflection profiling in younger orogenic belts. These orogenic fronts are connected by shear zones that transfer displacement or accommodate different displacements, between orogenic segments. Smaller shears within an orogenic belt occur as a result of differential movements. These shear zones are seen to pass from flat-lying to steep structures and may have a thrust or strike-slip sense. They compare with the staircase trajectories characteristic of foreland thrust belts. In common with thrust belts, the geometry of the shear zones can be used to estimate displacement direction, as can regional extensional fabrics developed in the associated high-strain tectonites. Central Africa has been previously described as a complex network of late Proterozoic ‘mobile belts’. The recognition of similar displacements and time equivalence in these belts allows their reinterpretation in terms of a linked thrust and strike-slip shear-zone system. An example is the Damaran, Lufilian, Zambezi and Ukingan system. These orogenic belts share a similar displacement picture and broad time equivalence and were apparently linked in a lower crustal shear zone of continental dimensions. This shear zone system appears to have developed under a single tectonic framework

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Geochronological and kinematic constraints on crustal shortening and escape in a two-sided oblique-slip collisional and magmatic orogen, Paleoproterozoic Taltson magmatic zone, northeastern Alberta

Canadian Journal of Earth Sciences ◽

10.1139/e00-089 ◽

2000 ◽

Vol 37 (11) ◽

pp. 1549-1573 ◽

Cited By ~ 40

Author(s):

Michael R McDonough ◽

Vicki J McNicoll ◽

Ernst M Schetselaar ◽

Timothy W Grover

Keyword(s):

Shear Zone ◽

Shear Zones ◽

Remotely Sensed Data ◽

Kinematic Data ◽

Magmatic Arc ◽

Magmatic Rocks ◽

Ductile Shear Zones ◽

Continental Magmatic Arc ◽

Northeastern Alberta ◽

Lower Crustal

The southern Taltson magmatic zone (south of 60°N) is a composite continental magmatic arc and collisional orogen resulting from the convergence of the Buffalo Head terrane with the Archean Churchill craton. Taltson basement (ca. 3.23.0 Ga and 2.42.14 Ga) and Rutledge River supracrustal gneisses (2.132.09 Ga) were intruded by voluminous I- and S-type magmatic rocks between 1.99 and 1.92 Ga. Taltson magmatic zone was deformed by three ductile shear zones: Leland Lakes, Charles Lake, and Andrew Lake, exhibiting both strike- and dip-lineated mylonitic domains. Kinematic data for shear zones are reported at microscopic, mesoscopic, and macroscopic (remotely sensed data) scale. We present field and UPb isotopic data (zircon and monazite) for magmatic and metamorphic rocks that constrain the timing of granulite to upper amphibolite-grade shearing in the Leland Lakes and Charles Lake (formerly Allan) shear zones to ca. 19381934 Ma. Foreland (easterly) vergent thrusting on the Andrew Lake shear zone is ca. 1932 Ma. Taltson shear zones were overprinted by widespread amphibolite- to greenschist-grade shearing, which is constrained by published 40Ar39Ar and KAr dates on hornblende and muscovite to between ca. 1900 and 1800 Ma. We propose a crustal architecture, resembling a crustal-scale asymmetric flower structure, in which the Charles Lakes shear zone formed the fundamental shear zone of a middle to lower crustal sinistral transpression system that accommodated southward escape of crust in the upper plate of an oblique continental subductioncollision zone, with shortening partitioned into synchronous outwardly vergent thrust systems to the east and west of the main shear zone.

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The problems of the post-Cenomanian tectonic evolution of the central parts of the Sredna Gora Zone. The wrench tectonics – how real is real?

Geologica Balcanica ◽

10.52321/geolbalc.49.2.39 ◽

2020 ◽

Vol 49 (2) ◽

pp. 39-58

Author(s):

Alexandre Kounov ◽

Ianko Gerdjikov

Keyword(s):

Shear Zone ◽

Tectonic Evolution ◽

Shear Zones ◽

Strike Slip ◽

Evidence Based ◽

Strain Partitioning ◽

Substantial Evidence ◽

The North ◽

Thrust Belts ◽

The Moment

The Sredna Gora Zone holds a unique place in the tectonic subdivisions of the Balkanide orogen and its evolution is still a subject of debate. In the last twenty years, the idea of strike-slip-related evolution of the zone has been invoked. However, for the moment, the number of thorough studies where such a scenario is envisaged is limited, and substantial evidence based on detailed fieldwork is still missing. In this article, we discuss some of the major problems of the suggested wrench tectonic concept in the evolution of the central part of the Sredna Gora Zone. These are the character of some major shear zones in the area, to which strike-slip movements are attributed, and the transtension-transpression evolution scenario for the Chelopech and Panagyurishte basins. Despite refuting completely their wrench tectonic-related evolution, we confirm the presence of strike-slip and oblique slip structures cutting the sediments, whereas the time of their activity and role in the deformation of the basin fill are yet to be revealed. Finally, we present a model based on natural examples and analogue modeling, in which the long-lived dextral Maritsa shear zone represents a zone of localized strain partitioning, separating the opposite vergent thrust belts of the Rhodope to the south and the Sredna Gora and Balkan fold-thrust belt to the north, during oblique or possibly orthogonal convergence.

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Major southeast-directed Caledonian thrusting and folding in the Dalradian rocks of mid-Ulster: implications for Caledonian tectonics and mid-crustal shear zones

Geological Magazine ◽

10.1017/s0016756800009882 ◽

1993 ◽

Vol 130 (2) ◽

pp. 233-244 ◽

Cited By ~ 21

Author(s):

G. I. Alsop ◽

D. H. W. Hutton

Keyword(s):

Shear Zone ◽

Shear Zones ◽

Strike Slip ◽

Ductile Shear Zone ◽

Large Area ◽

Apparent Absence ◽

Igneous Complex ◽

Crustal Shear Zones ◽

Surrounding Areas ◽

Dominant Structure

AbstractThe dominant structure controlling the disposition of Dalradian stratigraphy in mid-Ulster has hitherto been regarded as a southeast-facing gently inclined F1 anticline, a gross geometry modelled on, and thought to be a possible correlative of, the Tay Nappe in Scotland. Remapping of the supposedly inverted southern limb of this major fold reveals that much of it is in fact the correct way up. However, a stratigraphie repetition coupled with a reversal in younging does occur in the Sperrin Mountains, much further south than previously realized. This hitherto unrecognized upward southeast-facing isoclinal Sperrin Nappe is, however, a D2 structure, traceable for at least 40 km along strike and responsible for a regional stratigraphie inversion over an area of 300 km2. Following D2, a major 10 km thick D3 ductile shear zone resulted in translation towards the east-southeast. In the south, this deformation carried the Dalradian over Ordovician volcanics of the Tyrone Igneous Complex along the Omagh Thrust. Penecontemporaneity of magmatism with deformation clearly demonstrates that D3 is Caledonian (Arenig-Llanvirn). This deformation correlates with similar southeast-directed Caledonian thrusting in southern Donegal and Connemara. The apparent absence of Dalradian deformation of this age in southwestern Scotland may imply that Caledonian collision of outboard terranes with the miogeoclinal margin was initiated in Ireland and/or subsequent strike-slip has removed the evidence for deformation of this age from southwestern Scotland. The D3 shear zone in the Sperrin Mountains affects a very large volume of psammitic rocks. Within this shear zone the strain is not markedly higher than surrounding areas; however, its existence is demonstrated by the reorientation of mineral lineations over a large area. Such broad zones of only moderate strain may, we believe, be typical of translatory tectonics in areas of the mid-crust where there is little lithological diversity.

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Migmatite and leucogranite in a continental-scale exhumed strike-slip shear zone: Implications for tectonic evolution and initiation of shearing

Geological Society of America Bulletin ◽

10.1130/b35988.1 ◽

2021 ◽

Author(s):

Junyu Li ◽

Shuyun Cao ◽

Xuemei Cheng ◽

Franz Neubauer ◽

Haobo Wang ◽

...

Keyword(s):

Shear Zone ◽

Thermal State ◽

Shear Zones ◽

Strike Slip ◽

Red River ◽

Crustal Anatexis ◽

Plate Convergence ◽

Continental Scale ◽

Geological Processes ◽

Lower Crustal

Plutons within continental strike-slip shear zones bear important geological processes on late-stage plate transpression and continent-continent collision and associated lateral block extrusion. Where, when, and how intrusions and shearing along transpressional strike-slip shear zones respond to plate interactions, however, are often debated. In this study, we investigated migmatite associated leucogranite and pegmatite from the exhumed >1000-km-long Ailao Shan-Red River left-lateral strike-slip shear zone in Southeast Asia that was active during India-Eurasia plate convergence. Most zircons from the migmatites and leucogranitic intrusions present inherited core-rim structure. The depletion of rare earth element patterns and positive Eu anomalies suggest that leucosomes and leucogranites are the result of crustal anatexis. Zircon rims from the foliated migmatites and leucogranites record U-Pb ages of 41−28 Ma, revealing the timing of the Cenozoic crustal anatexis event along this strike-slip shear zone. Ages of the magmatic zircons from the unfoliated pegmatites provide the timing of the termination of a high-temperature tectono-thermal event and ductile left-lateral shearing at 26−23 Ma. The Cenozoic crustal anatexis along the Ailao Shan-Red River strike-slip shear zone indicates that thickened crust underneath the shear zone involved previously subducted crust. We propose that the Cenozoic thermal state has an important effect on the crustal anatexis and thus on the rheological behavior of the lithosphere by thermal weakening, which plays an essential role in localizing the initiation of the deep-seated lower-crustal shear zone.

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Kinematics and Geochronology of Late Paleozoic–Early Mesozoic Ductile Deformation in the Alxa Block, NW China: New Constraints on the Evolution of the Central Asian Orogenic Belt

Lithosphere ◽

10.2113/2021/3365581 ◽

2021 ◽

Vol 2021 (1) ◽

Author(s):

Beihang Zhang ◽

Jin Zhang ◽

Heng Zhao ◽

Junfeng Qu ◽

Yiping Zhang ◽

...

Keyword(s):

Shear Zone ◽

Shear Zones ◽

Strike Slip ◽

Late Paleozoic ◽

Ductile Shear Zone ◽

Early Mesozoic ◽

Central Asian ◽

Ductile Shear Zones ◽

Dextral Shear ◽

Ductile Shear

Abstract Strike-slip faults are widely developed throughout the Central Asian Orogenic Belt (CAOB), one of the largest Phanerozoic accretionary orogenic collages in the world, and may have played a key role in its evolution. Recent studies have shown that a large number of Late Paleozoic–Early Mesozoic ductile shear zones developed along the southern CAOB. This study reports the discovery of a NW–SE striking, approximately 500 km long and up to 2 km wide regional ductile shear zone in the southern Alxa Block, the Southern Alxa Ductile Shear Zone (SADSZ), which is located in the central part of the southern CAOB. The nearly vertical mylonitic foliation and subhorizontal stretching lineation indicate that the SADSZ is a ductile strike-slip shear zone, and various kinematic indicators indicate dextral shearing. The zircon U-Pb ages and the 40Ar/39Ar plateau ages of the muscovite and biotite indicate that the dextral ductile shearing was active during Middle Permian to Middle Triassic (ca. 269–240 Ma). The least horizontal displacement of the SADSZ is constrained between ca. 40 and 50 km. The aeromagnetic data shows that the SADSZ is in structural continuity with the coeval shear zones in the central and northern Alxa Block, and these connected shear zones form a ductile strike-slip duplex in the central part of the southern CAOB. The ductile strike-slip duplex in the Alxa Block, including the SADSZ, connected the dextral ductile shear zones in the western and eastern parts of the southern CAOB to form a 3000 km long E-W trending dextral shear zone, which developed along the southern CAOB during Late Paleozoic to Early Mesozoic. This large-scale dextral shear zone was caused by the eastward migration of the orogenic collages and blocks of the CAOB and indicates a transition from convergence to transcurrent setting of the southern CAOB during Late Paleozoic to Early Mesozoic.

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Chapter 8 Outboard-migrating accretionary orogeny at 1.9–1.8 Ga (Svecokarelian) along a margin to the continent Fennoscandia

Geological Society London Memoirs ◽

10.1144/m50-2019-18 ◽

2020 ◽

Vol 50 (1) ◽

pp. 237-250 ◽

Cited By ~ 8

Author(s):

Michael B. Stephens

Keyword(s):

Base Metal ◽

Regional Scale ◽

Volcanic Rocks ◽

Shear Zones ◽

Strike Slip ◽

Base Metal Sulphide ◽

Metal Sulphide ◽

Time Period ◽

Ductile Shear Zones ◽

Strike Slip Movement

AbstractAn intimate lithostratigraphic and lithodemic connection between syn-orogenic rock masses inside the different lithotectonic units of the 2.0–1.8 Ga (Svecokarelian) orogen, Sweden, is proposed. A repetitive cyclic tectonic evolution occurred during the time period c. 1.91–1.75 Ga, each cycle lasting about 50–55 million years. Volcanic rocks (c. 1.91–1.88 Ga) belonging to the earliest cycle are host to most of the base metal sulphide and Fe oxide deposits inside the orogen. Preservation of relict trails of continental magmatic arcs and intra-arc basins is inferred, with differences in the depth of basin deposition controlling, for example, contrasting types of base metal sulphide deposits along different trails. The segmented geometry of these continental magmatic arcs and intra-arc basins is related to strike-slip movement along ductile shear zones during transpressive events around and after 1.88 Ga; late orogenic folding also disturbed their orientation on a regional scale. A linear northwesterly orogenic trend is suggested prior to this structural overprint, the strike-slip movement being mainly parallel to the orogen. A solely accretionary orogenic model along an active margin to the continent Fennoscandia, without any trace of a terminal continent–continent collision, is preferred. Alternating retreating and advancing subduction modes that migrated progressively outboard and southwestwards in time account for the tectonic cycles.

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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

Geological Society of America Bulletin ◽

10.1130/b35672.1 ◽

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.

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Ductile shear zones – future perspectives

10.5194/egusphere-egu2020-4273 ◽

2020 ◽

Author(s):

Christoph Schrank

Keyword(s):

Shear Zone ◽

Research Effort ◽

Rheological Behaviour ◽

Shear Zones ◽

Ductile Deformation ◽

Field Observations ◽

Ductile Shear Zones ◽

In Situ Experiments ◽

Ductile Shear ◽

Physical Consequences

About 50 years ago, John Ramsay and colleagues established the thorough foundation for the field-scale observational and mathematical description of the structures, deformation, and kinematics in ductile shear zones. Since then, these probably most important instabilities of the ductile lithosphere enjoyed an almost explosive growth in scientific attention. It is perhaps fair to say that this tremendous research effort featured four main themes:&#160;[1] The historic scientific nucleus &#8211; quantification of shear-zone geometry, strain and associated kinematic history from field observations&#160;[2] Qualitative and quantitative analysis of microphysical deformation mechanisms in the field and the laboratory&#160;[3] Shear-zone rheology&#160;[4] The development of physically consistent mathematical models for shear zones, mainly using continuum mechanics.&#160;In concert, these four cornerstones of shear-zone research enabled tremendous progress in our understanding of why and how ductile shear zones form. So, what are some of the outstanding problems?&#160;A truly comprehensive model for ductile shear zones must account for the vast range of length and time scales involved, each easily covering ten orders of magnitude, as well as the associated intimate coupling between thermal, hydraulic, mechanical, and chemical processes. The multi-scale and multi-physics nature of ductile shear zones generates scientific challenges for all four research themes named above. This presentation is dedicated to highlighting exciting challenges in themes 2, and 3 and 4.&#160;In the microanalytical arena [2], the nano-scale is an exciting new frontier, especially when it comes to the interplay between metamorphism and ductile deformation. The nano-frontier can be tackled with new synchrotron methods. I showcase some applications to fossil shear-zone samples and discuss opportunities for in-situ experiments. In the domain of rheology [3], I present some simple experiments with strain-softening materials and field observations that support the notion: transient rheological behaviour is very important for shear localisation. In the modelling domain [4], some recent examples for the intriguing physical consequences predicted by new multi-physics and cross-scale coupling terms in ductile localisation problems are illustrated.

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Evolution of brittle structures in plagioclase-rich rocks at high-grade metamorphic conditions – Linking laboratory results to field observations

10.5194/egusphere-egu2020-8367 ◽

2020 ◽

Author(s):

Sarah Incel ◽

Jörg Renner ◽

Bjørn Jamtveit

Keyword(s):

Structural Evolution ◽

Confining Pressure ◽

Shear Zones ◽

Ductile Shear Zones ◽

Laboratory Results ◽

Experimental Laboratory ◽

Eclogite Facies ◽

Ductile Shear ◽

Host Rocks ◽

Lower Crustal

Plagioclase-rich lower crustal granulites exposed on the Lofoten archipelago, N Norway, display pseudotachylytes, reflecting brittle deformation, as well as ductile shear zones, highlighting plastic deformation. Pristine pseudotachylytes often show no or very little difference in mineral assemblage to their host-rocks that exhibit limited, if any, metamorphic alteration. In contrast, host-rock volumes that developed ductile shear zones exhibit significant hydration towards amphibolite or eclogite-facies assemblages within and near the shear zones. We combine experimental laboratory results and observations from the field to characterize the structural evolution of brittle faults in plagioclase-rich rocks at lower crustal conditions. We performed a series of deformation experiments on intact granulite samples at 2.5 GPa confining pressure,&#160; a strain rate of 5&#215;10-5 s-1,&#160; temperatures of 700 and 900 &#176;C, and total strains of either ~7-8 % or ~33-36 %. Samples were either deformed &#8216;as-is&#8217;, i.e. natural samples without any treatment, or with ~2.5 wt.% H2O added. Striking similarities between the experimental and natural microstructures suggest that the transformation of precursory brittle structures into ductile shear zones at eclogite-facies conditions is most effective when hydrous fluids are available in excess.

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