Major Strike-slip Shear Zone Research Articles

The structural geometry and metamorphic evolution of the Umm Gheig shear belt, Central Eastern Desert, Egypt: implications for exhumation of Sibai core complex during oblique transpression

Field and structural studies supported by remote sensing and thermobarometeric data are attempted to decipher the deformation and metamorphic histories of Umm Gheig shear belt which represents the eastern supracrustal rocks of the Sibai core complex in the Central Eastern Desert of Egypt. Umm Gheig shear belt is build up of three major structural domains (Abu Lassaf, Umm Luseifa, and Kab Ahmed) formed by four deformation events (D1–D4) during the late Cryogenian–Ediacaran (650–542 Ma). The border between these domains is controlled by major thrusts and sinistral-reverse shear zones. Abu Lassaf shear belt includes NW-oriented major folds, serpentinite klippes, and oppositely dipping bidirectional thrust forming pop-up structures. Umm Luseifa shear belt is marked by a major doubly plunging, curvilinear, asymmetrical anticline, series of fault bounded thrust slices, and bordered by two major NE- and SW-verging thrusts. Kab Ahmed shear belt includes the highly strained rocks and expresses as an imbricate NE-verging thrusts forming a simple piggyback imbricate thrust system with a unidirectional sense of thrust propagation to the NE. Presence of D3-related horsetail splays that grouped into a major of NW-trending sinistral shear zones in the Umm Gheig shear belt gives additional evidence for the formation and exhumation of Sibai core complex in oblique island arc collision zones and includes lateral extrusion in a transpressional collision zone along major strike-slip shear zones. The rocks of the Umm Gheig shear belt are subjected to prograde low-medium-grade regional metamorphism in the range of greenschist to lower amphibolite facies (350–620 °C at 2–6 kb). The structures in Umm Gheig shear belt are the products of an E–W shortening phase accompanied by compressional due to oblique shortening of the Arabian–Nubian Sheild between East and West Gondwana.

Iron mineralization associated with a major strike–slip shear zone: Radiometric and oxygen isotope evidence from the Mengku deposit, NW China

Some skarn-associated gold deposits display close spatial relations with shear zones in ancient orogens; however to date no skarn-associated iron mineralization has been genetically linked to a shear zone. To address this problem, we conducted a systematic study of the Mengku iron deposit in the Chinese Altai, Northwest China. All orebodies of the deposit are enclosed by skarn and stratabound by volcaniclastic rocks. LA–ICP-MS dating of zircon from a meta-rhyolitic volcaniclastic (MK19) yielded a weighted 206Pb/ 238U age of 404 ± 5 Ma. We interpret this date as the formation time of volcanic rocks in the Mengku area, which is consistent with the age of strata hosting nearby VMS deposits (400–413 Ma). Zircon from the mineralized skarn (MK13) are considered to be hydrothermal in origin and yield a weighted 206Pb/ 238U age of 250 ± 2 Ma, which we interpret to represent the time of skarn formation based on the petrographic relationship between hydrothermal zircon and skarn garnet. This age is much younger than that determined from previous studies, younger than the regional igneous and metamorphism activities (early Permian), but synchronous with the movement of the nearby regional Erqis shear zone and Abagong Fault (late Permian to Triassic). The oxygen isotope values of the hydrothermal zircon are 2.3‰ to 3.0‰, consistent with the δ 18O of garnet (1.2‰ to 3.2‰) in the skarn, but markedly different from those of zircon from igneous rocks in the mine area (MK1, 2, 3, 19; 6.3‰ to 8.0‰). The fluid source has a strong meteoric water signature as indicated by the oxygen isotope values of garnet and hydrothermal zircon, which are very different from those of the granite and volcanic rocks in the mine area. On the basis of field, chronological and geochemical evidence, we suggest that the Mengku is not a volcanogenic massive sulfide deposit, but is related to the Erqis strike–slip shear zone and Abagong Fault, which acted as fluid conduits. The upwelling of hot mantle in the Permian to Triassic supplied the heat, and thereby caused material to dehydrate and mixed with meteoric water that give rise to low-oxygen isotope fluids similar to the common genetic model for orogenic lode gold deposits.

Ediacaran high-pressure collision metamorphism and tectonics of the southern Ribeira Belt (SE Brazil): Evidence for terrane accretion and dispersion during Gondwana assembly

The Curitiba Terrane represents a major segment of the southern Ribeira Belt (SE Brazil), which was derived from the collision between the São Francisco, Congo, Paranapanema and Luís Alves Cratons during the Neoproterozoic (Brasiliano/Pan-African Orogeny). The tectonic setting and the metamorphic records of two major juxtaposed units from the Curitiba Terrane, a Neoproterozoic shallow continental-shelf metasedimentary assemblage (Turvo-Cajati Formation) and an Archaean to Paleoproterozoic TTG-type orthogneiss assemblage (Atuba Complex), were investigated. Migmatitic paragneisses from the Turvo-Cajati Formation underwent a deep collision metamorphism. Conventional geothermobarometry and petrological modelling in the system NCKFMASHTi indicate peak metamorphic conditions between 670 and 810 °C at 9.5–12 kbar. Metamorphic paths calculated from zoned garnet and plagioclase using the Gibbs method of differential thermodynamics indicate distinct evolution for two major groups of migmatites from the Turvo-Cajati Formation: (i) kyanite migmatites evolved from low-temperature eclogite to high-pressure granulite facies conditions following near isobaric heating; (ii) sillimanite migmatites underwent near isothermal decompression and apparently evolved from high-temperature eclogite facies conditions. Chemical dating of monazite indicates that the peak metamorphism of the Turvo-Cajati Formation occurred at 589 ± 12 Ma, followed by a greenschist facies metamorphic overprint at 579 ± 8 Ma related with late transcurrent shear zones. 40Ar– 39Ar biotite ages indicate that the Turvo-Cajati Formation cooled below 250–300 °C at 555 ± 4 Ma. P– T data and petrological evidence of rocks from the Atuba Complex suggest a retrograde metamorphic path with cooling from 730 to 630–650 °C at 6–7 kbar. Available K–Ar and 40Ar– 39Ar data indicate that the Atuba Complex had cooled to below 300–500 °C between ca. 590 and 580 Ma. Geochronological data indicate that the main metamorphism of the Turvo-Cajati Formation and the Atuba Complex are coeval, but very contrasting metamorphic signatures reflect formation in different parts of a collisional suture. The integration of structural and petrological data indicates that the structural pattern of the Curitiba Terrane is related to Ediacaran westward directioned nappes during the late- to postmetamorphic period. This is concomitant with a main, crustal-scale, strike-slip regime, dominant throughout the Ribeira Belt. The nappe stack was later deformed by cylindrical folds with E–W trending sub-horizontal axes parallel to the synthrusting stretching lineation and was dismembered and dispersed by late sinistral strike-slip shear zones. The late tectonic assembly of the Ribeira Belt was controlled by significant postcollision terrane dispersion along major strike-slip shear zones.

Syntectonic crustal melting and high-grade metamorphism in a transpressional regime, Variscan Massif Central, France

Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike–slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 ± 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike–slip shear zones. We interpret U/Pb monazite ages of 315 ± 4 Ma for the gneisses and 316 ± 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks. Some granites emplaced in the strike–slip shear zone are bounded at their roof by low dip normal faults that strike N–S, perpendicular to the E–W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen.

Structure of late Variscan Millevaches leucogranite massif in the French Massif Central: AMS and gravity modelling results

In the Limousin area, Variscan leucogranitic plutons are spatially associated with normal faults and major strike-slip shear zones that are a continuation of the South Armorican shear zone. Our study focuses on the large N–S-trending Millevaches granitic massif (Massif Central, France), and intends to highlight, through gravity modelling, structural and anisotropy of magnetic susceptibility (AMS), the massif structure at depth and to discuss the mode of emplacement of granites within a strike-slip tectonic context. The mica subfabric suggests that the magnetic foliations display a general NW–SE sub-horizontal pattern on both sides of the N–S Pradines dextral wrench fault zone that deforms the core of the massif on 5 km width. The magnetic lineation trend exhibits a sigmoïdal pattern, N–S in the Pradines fault zone and NW–SE on both sides of it, which are consistent with a dextral wrench component. The horizontal magnetic foliations and lineations are consistent with the thin granite laccolith model. There is no significant imprint of the extensional Variscan belt collapse on the internal fabric of Millevaches granites than the tectonic dextral transcurrent movement prevailing in this area.

Structural and tectonic evolution of the Umm Gheig/El-Shush region, central Eastern Desert of Egypt

The rocks of the Wadi Umm Gheig/El-Shush area in the central Eastern Desert of Egypt form part of the Nubian Shield, a component of the Neoproterozoic Pan-African Orogeny. The rocks have been divided into three units: (i) low-grade metamorphosed rocks, which consist of metavolcanic rocks interleaved with slices of ophiolitic melange; (ii) high-grade metamorphic rocks, which consist of syn-tectonic granitoids; and (iii) post-tectonic granites, which intrude into both the low- and high-grade rocks. Three distinct tectonic and two magmatic events have been deduced from the structural analysis of the area. These are listed in chronological order: (i) the formation of the major D 1 sinistral strike-slip El-Shush Shear Zone, which occurs within the granitoid rocks (six individual granitoid bodies, all now intensely sheared, are thought to have been intruded into the active El-Shush Shear Zone); (ii) the emplacement of the metavolcanic rocks over the granitoid rocks by major D 2 thrusting along a low angle dip-slip shear zone; (iii) upright open folding of the rocks during D 3; and (iv) the intrusion of late stage granites, which are virtually undeformed. A model for the tectonic evolution of the study area is proposed. It is argued that the major strike-slip shear zone of the region is possibly related to island-arc accretion and that the various granitoid rocks were intruded along this active strike-slip zone. During the collision associated with island-arc accretion, ophiolite sheets were interleaved with the volcanic rocks and together were thrust over the granitoid basement. Field observations show that an important episode of folding occurred after thrusting and that the folds possess an axial planar fracture cleavage. They are, therefore, the result of a sub-horizontal tectonic compression rather than of diapirism as previously suggested. Thus, the domal structures of the Eastern Desert may be the result of folding rather than diapirism.

Thermochronometry and microstructures of quartz—a comparison with experimental flow laws and predictions on the temperature of the brittle–plastic transition

A gradient in quartz microfabrics across a major strike-slip shear zone (with a minor vertical component), active during the Oligocene in the Eastern Alps (Alto Adige, Italy), is correlated with new zircon fission track thermochronometric data and available Rb–Sr biotite ages to constrain the depth/temperature range of the recorded rheologic regimes. Distributed deformation in the semibrittle regime (i.e. beneath the brittle–ductile transition) is effective near the closure temperature for fission tracks in zircon (which we estimate as 280±30°C), with high-stress dislocation creep of quartz, microcracking, and pressure solution being active simultaneously. Steady state dislocation creep of quartz at moderate stress in the fully plastic regime is effective at temperatures above the closure temperature for the Rb–Sr and K–Ar systems of biotite (ca. 310±30°C) and below that for the K–Ar system of white mica (ca. 350±50°C). For the inferred temperatures and correlated flow stresses derived by paleopiezometry, the majority of available experimental flow laws for wet quartzite predict strain rates on the order of 10 −13–10 −14 s −1, consistent with the geological constraints. This finding supports the validity of the extrapolation of experimental flow laws to natural strain rates.

Contrasting mechanism of crustal growth: Geodynamic evolution of the Paleoproterozoic granite–greenstone belts of French Guiana

The Paleoproterozoic granite–greenstone belts of French Guiana formed during a period of major crustal growth at the scale of the Earth. Two contrasting mechanisms of crustal growth have been identified by recent structural, metamorphic, geochronological and geochemical investigations in the Cayenne–Régina region, which is part of the northern granite–greenstone belt of French Guiana. Following the formation of oceanic crust, which is characterized by a tholeiitic volcanic sequence dated at 2174±7 Ma, the early stage of crustal generation was marked by successive building of calc-alkaline plutonic–volcanic complexes, with the formation of the Ile de Cayenne complex dated at 2144±6 Ma to the north, and of the Central Guiana complex dated at 2115±7 Ma to the south. This first period of crustal generation by accumulation of mantle-derived magmas was followed by a period of crustal recycling and tectonic accretion. Sediments derived from the granite–greenstone belts were deposited in large marginal basins, such as the Orapu basin, separating the plutonic–volcanic complexes. Convergence between the newly formed crustal blocks caused deformation during the Transamazonian orogeny of the Orapu marginal basin caught in between the Ile de Cayenne and Central Guiana complexes. Oblique convergence is further characterized by the development of en-echelon pull-apart basins along the North Guiana Trough sinistral strike-slip shear zones, affecting the edges of the Orapu marginal basin. The lithological succession in these basins is typical of foreland basins, and the metamorphic and structural evolution of the Kaw and Régina basins in the Cayenne–Régina region indicate burial to depth up to 20 km in a context of heterogeneous transpression. The geochemical signature of small syntectonic granitic plutons dated at 2093±8 Ma and 2083±8 Ma emplaced along the major strike-slip shear zones is consistent with crustal thickening and partial melting of an enriched crustal source. These features suggest that the last stage of the Transamazonian orogeny was marked by oblique convergence, collision and crustal thickening in a similar way to modern orogens.

Landsat MSS imagery of a Lower Cretaceous regional dyke swarm, Damaraland, Namibia: a precursor to the splitting of western Gondwana : Lord J., Oliver G.J.H. & Soulsby J.A., International Journal of Remote Sensing, 1996, 17/15 (2945–2954)

The rocks of the Wadi Umm Gheig/El-Shush area in the central Eastern Desert of Egypt form part of the Nubian Shield, a component of the Neoproterozoic Pan-African Orogeny. The rocks have been divided into three units: (i) low-grade metamorphosed rocks, which consist of metavolcanic rocks interleaved with slices of ophiolitic melange; (ii) high-grade metamorphic rocks, which consist of syn-tectonic granitoids; and (iii) post-tectonic granites, which intrude into both the low- and high-grade rocks.Three distinct tectonic and two magmatic events have been deduced from the structural analysis of the area. These are listed in chronological order: (i) the formation of the major D1 sinistral strike-slip El-Shush Shear Zone, which occurs within the granitoid rocks (six individual granitoid bodies, all now intensely sheared, are thought to have been intruded into the active El-Shush Shear Zone); (ii) the emplacement of the metavolcanic rocks over the granitoid rocks by major D2 thrusting along a low angle dip-slip shear zone; (iii) upright open folding of the rocks during D3; and (iv) the intrusion of late stage granites, which are virtually undeformed.A model for the tectonic evolution of the study area is proposed. It is argued that the major strike-slip shear zone of the region is possibly related to island-arc accretion and that the various granitoid rocks were intruded along this active strike-slip zone. During the collision associated with island-arc accretion, ophiolite sheets were interleaved with the volcanic rocks and together were thrust over the granitoid basement. Field observations show that an important episode of folding occurred after thrusting and that the folds possess an axial planar fracture cleavage. They are, therefore, the result of a sub-horizontal tectonic compression rather than of diapirism as previously suggested. Thus, the domal structures of the Eastern Desert may be the result of folding rather than diapirism.

Progressive localization of deformation during exhumation of a major strike-slip shear zone: Norumbega fault zone, south-central Maine, USA

Detailed structural studies along with 40Ar/39Ar thermochronology provide important constraints on the temporal,spatial and structural evolution of a major orogen-parallel strike-slip shear zone, the Norumbega fault zone in Maine. Detailed structural analysis in south-central Maine reveals two very different styles of dextral noncoaxial deformation, a wide zone (>25 km) of heterogeneously distributed ductile shear structures, and a relatively narrow zone (∼1 km) of lower greenschist facies high-strain mylonitization. 40Ar/39Ar thermochronology indicates that these two zones of deformation developed during a prolonged period of regional exhumation following Middle Devonian amphibolite facies metamorphism. The wide zone of dextral shear is interpreted to reflect a major episode of moderate temperature (post-amphibolite facies metamorphism but pre-regional cooling below 320°C) Late Devonian to Early Carboniferous transcurrent tectonism. The narrow zone of dextral shear represents both a younger (latest Carboniferous) and significantly cooler (<320°C) deformational event occurring at much higher structural levels. A general model of increasingly narrow, but more highly focused noncoaxial deformation during progressive regional exhumation is supported by the observations.

Magmatic and solid state deformation partitioning in the Ox Mountains granodiorite

AbstractThe Ox Mountains granodiorite (western Ireland) is situated along the Fair Head—Clew Bay line, a major Caledonian structure. The pluton was emplaced synchronously with respect to deformation on a major strike-slip shear zone system and experienced a pervasive episode of sinistral shear. Remnants of magmatic state deformation such as phenocryst alignments and tiling fabrics are present. An early phase of strain partitioning produced sinistral offsets on internal contacts. These structures were overprinted by a solid state fabric characterized by crystal plastic deformation in feldspar and the operation of recovery processes during quartz deformation indicating temperatures in the range 450–500 °C. A sinistral shear band fabric is ubiquitous and its formation was enhanced by myrmekite production leading to the formation of fine-grained feldspar and quartz. Late deformation partitioning during further pluton cooling produced a conjugate shear zone system containing mylonites. Deformation within the Ox Mountains granodiorite provides important constraints on the nature of late Caledonian age mid-crustal deformation along the Highland Boundary fault. The pluton also provides exceptional examples of the range of structures that may form in a syntectonic pluton.

High-temperature metamorphism in a major strike-slip shear zone: the Ailao Shan—Red River, People's Republic of China

Petrographic and thermobarometric analysis provides constraints on thePT path of the mylonitic gneisses of the left-lateral Ailao Shan-Red River shear zone which has accommodated the lateral extrusion of Indochina during the Tertiary. Two different paragenesis, P1 and P2, are coeval with this deformation and correspond respectively to the amphibolite and greenschist facies. Microprobe analysis reveals that P1 garnets bear a chemical zonation from core to rim. This zonation indicates a temperature increase during garnet growth. Conditions of formation of garnet rims (P1b), which are estimated using biotite-garnet and plagioclase-garnet thermobarometers, are close to the granitic solidus (710 ± 70°C and4.5 ± 1.5 kbar). P2 conditions are estimated to be approximately 500°C and < 3.8 kbar. Both P1b and P2 conditions correspond to much higher temperatures than expected for their depths in the continental crust, suggesting a perturbed geothermal gradient during strike-slip deformation along the Ailao Shan-Red River shear zone. Thermochronology results suggest that cooling between P1b and P2 was fast ( ≈ 100°C/Ma) and may have been associated with significant uplift. Uplift during the left-lateral shearing may have resulted from a slight reverse or, more probably normal, component of movement along the strike-slip fault. A simple numerical model suggests that the high temperatures in the shear zone at the time of deformation may be explained by shear heating in the more competent upper mantle and by advection of this heat along the shear zone by ascent of magmas and/or fluids. In this hypothesis, the medium-pressure and temperature schists bounding the mylonitic gneisses to the southwest previously interpreted as resulting from collision-related metamorphism result instead from ‘contact’ metamorphism of the shear zone at mid-crustal depths.