Ductile Deformation Zones Research Articles

Fluid-Induced Metamorphism and Deformation at the Eastern Boundary of the Sveconorwegian Province

Abstract The Sveconorwegian orogen in Scandinavia and the Grenville orogen in Canada are both remnants of large and hot orogens that formed part of the supercontinent Rodinia around 1 billion years ago. Formerly deeply buried portions of crust in these orogens are exposed and offer insights into the tectonic dynamics of the basement within large orogens. The Eastern Segment of the Sveconorwegian Province hosts a ~ 30 000 km2 crustal portion that was buried to c. 40 km depth at a late stage of the orogeny, 980–960 Ma ago, and is bound towards the foreland in the east by a ~ 25 km wide zone of step anastomosing deformation, the Frontal wedge. This zone represents the outermost ductile deformation that developed within the crystalline basement in the orogen. We investigated a heterogeneously deformed and recrystallised syenodiorite with the aim to understand the character of the deformation-related metamorphism within the Frontal wedge. Field relations, microtextures, and mineral reactions show that the metamorphic recrystallisation was governed by hydrous fluid infiltration along the ductile deformation zones. Equilibrium was attained on a millimetre scale only and metamorphic recrystallisation was dependent on the introduction of hydrous fluid. The metamorphism reached high-pressure epidote-amphibolite-facies; geothermobarometric estimates suggest 540°C to 600°C and 9 to 12 kbar. Metamorphic zircon formed during the breakdown of Zr-bearing igneous phases, primarily baddeleyite. SIMS U–Pb analyses of igneous zircon and baddeleyite date the igneous crystallisation of the syenodiorite at 1230 ± 6 Ma. Metamorphic zircon grains are <20 μm and too small for precise dating, but yielded ages around 1 Ga. Collectively, the metamorphic data indicate that subvertical movements along steep planes within the Frontal wedge allowed for the regional-scale tectonic burial to ~40 km depth of the Eastern Segment to the west. Some of the same steep deformation structures were re-utilised as discrete movement planes during later exhumation.

The effect of strain on the crystallinity of carbonaceous matter: Application of Raman spectroscopy to deformation experiments

Raman spectroscopy, applied to the carbonaceous matter (CM) present in fault zones, is a potentially powerful tool to aid in the reconstruction of slip conditions, provided the respective effect of heating and strain can be disentangled. For this purpose, we have carried out static heating and deformation experiments on low-grade sediments containing disseminated CM particles and tracked the evolution of CM crystalline ordering (crystallinity) using Raman spectroscopy. Heating at high pressure (200 MPa), at varying temperatures (500 to 800 °C) and durations (3 to 1080 h) resulted in an increased crystallinity, which was observed by an increase in the intensity ratio of the Disordered Band over the Graphite Band (Intensity Ratio R1 parameter) of the Raman spectra. The experimental relationship between R1, T and duration was compiled into a law of evolution, which was then used to design the deformation experiments. Experiments were performed in a Paterson rig at 150 MPa or in a Griggs-type rig at 1 GPa. We observed two types of deformation microstructures: (1) purely brittle ones, such as breccia along slip planes and (2) ductile ones, such as foliated layers, resulting from grain reorientation and comminution along with dissolution precipitation. In all deformation microstructures, an increase in R1 is visible and interpreted to be the result of higher crystallinity of carbonaceous particles. Such increase is negligible in breccia, but is significant, up to 40%, in ductile deformation zones. The slow strain rates prevailing in ductile deformation prevented any local temperature increase, therefore the increase in crystallinity of the CM can be solely attributed to deformation. Thus, in natural fault zone, the increased crystallinity of CM reported in the literature and interpreted to be the result of frictional heating during earthquakes might rather be the result of strain localization without transient temperature increase.

Coupled Control of Thermal Maturation and Microscopic Deformation on the Pore Structure Evolution of High-Maturity Coal-Bearing Shale

Affected by the global low-carbon strategy, high-maturity shale, as one of the major sources of shale gas resources, exhibits important energy value and economic benefits. However, the microstructure evolution of high-maturity shales under thermal metamorphism and microscopic deformation remains unclear. In this study, we selected 20 high-maturity coal-bearing shales from the western Guizhou to reveal the evolution of shale microstructure during thermal maturation and the effects of microscopic deformation on pore distribution. The maximum pyrolysis temperature (Tmax) and vitrinite reflectance index (Ro) values (averaging 578 °C and 3.08%, respectively) suggest that the Upper Permian Leping shale reaches the dry gas generation stage, and the average hydrogen index (HI) and carbon isotope of kerogen (δ13C) values indicate that all samples have already undergone intense hydrocarbon generation. Brittle minerals represented by quartz have negative relationships with the pore volume (PV) and surface area (SA), and ductile clay minerals significantly improve the PV and average pore size (AP). Layered clay minerals, especially illite/smectite (I/S), are more conducive to the development of nanopores in a shale matrix. Both H/C and O/C ratios are positively correlated with δ13Corg values, suggesting that the variation in organic elements in kerogen is closely related to carbon isotope fractionation. With the enhancement of thermal evolution, the correlation between AP and Tmax is negative at the first stage and then positive at the second stage. At the high maturation stage, the accumulation of gas leads to the expansion of the organic matter surface and the formation of gas pores, resulting in a rapid increase in AP. The distribution of the pore–fracture system is closely related to the microscopic deformation. Compared with the contact zone of ductile deformation, more microfractures can be observed in the contact zone between brittle deformation and ductile deformation.

Structural and kinematic analysis of the Cuonadong dome, southern Tibet, China: Implications for middle-crust deformation

The Cuonadong Dome is located at the eastern part of the North Himalayan gneiss domes. Structural and kinematic analysis in the middle unit of the Cuonadong Dome provide significant new insights into the structural deformation of the middle crust in the Northern Himalaya. Field observations, microscopic kinematic and EBSD analyses, and deformation temperature estimates from the mid-crustal rocks of the Cuonadong Dome show high-strain D2 (top-to-N) kinematic fabrics with a downward-to-north progression from dominantly top-to-north shear in the garnet zone, to solely top-to-north shear in the kyanite/sillimanite zone. A combination of mineral assemblages, microstructures, and quartz crystal preferred orientation patterns indicates deformation temperatures associated with D2 kinematic fabrics increase with structural depth from ∼450 °C in garnet zone to > 630 °C in the kyanite/sillimanite zone. These values broadly overlap temperature range deduced from metamorphic petrography (450–630 °C). Microtextural relations indicate that peak metamorphism occurred post-D1 and pre-D2 deformation. Combined with the ∼1000-m-thick ductile deformation zone, the interpreted deformation temperatures yielded an average thermal field gradient of ∼ 180 °C/km over 1.05 km of structural thickness. Based on the geochronologic data, midcrustal D2 extensional deformation in the Cuonadong Dome initiated as early as early Oligocene (32 Ma) and ended during the middle Miocene (∼14 Ma). Comparative studies of D2 ductile deformation in midcrustal rocks from other North Himalayan gneiss domes and the South Tibetan Detachment System indicate that D2 shear zone in the Cuonadong Dome represent the northern part of the South Tibetan Detachment System, and highlight temporal and spatial variation in midcrustal D2 deformation patterns over the southern Tibet during the Himalayan orogeny.

Internal deformation of the Gorda plate and its tectonic significance within the Cascadia subduction zone

In this review paper, we compile observations that have been made over decades of research on the Cascadia convergent boundary and re-examine the deformation using structural kinematic models, thereby providing new insights. Deformation in the subducting Juan de Fuca slab and the overriding North American plate is primarily accommodated by fracturing. We show that the along-strike deformation patterns are a function of fracture density and activity, which influences coupling with the overriding North American plate and the underlying mantle wedge. We analyze the deformation within a fracture kinematics framework and show that the orientations of the fractures and motion history follow expected empirical fracture mechanics models and are used to provide a more detailed kinematic history of the Cascadia convergent boundary. We focus on the Gorda plate of the Juan de Fuca subducting slab and show that it is intensely fractured with curved magnetic anomalies such that the brittle deformation is collectively expressed macroscopically as a deforming continuum. The Gorda plate has changed shape via NS shortening and clockwise rotation without losing cohesion at the plate-scale, which suggests that the fractures form an interlocking network to accommodate ductile deformation via cataclastic flow. In the forearc, the fracture and velocity patterns suggest that deformation is accommodated by a continental-scale right-lateral shear zone, trending 320°/140° and is assisted via crustal-scale cataclastic flow. We suggest that ETS activity is strongly correlated to deformation in the over-riding plate. The Gorda plate is also used to better understand larger-scale plate dynamics. We demonstrate that the Gorda plate acts as a ductile deformation zone to assist global plate mobility. Additionally, details of the magnetic lineation patterns in the Gorda plate indicate that slab pull overtakes plate rotation post-subduction. This suggests that plate behavior is strongly influenced by its internal deformation and coupling with the mantle post-subduction.

ГЕОЛОГИЧЕСКАЯ ПОЗИЦИЯ, МИНЕРАЛЫ РЕДКИХ И БЛАГОРОДНЫХ МЕТАЛЛОВ В РУДАХ МЕДНО-НИКЕЛЕВОГО МЕСТОРОЖДЕНИЯ КУН-МАНЬЁ (ЮГО-ВОСТОЧНОЕ ОБРАМЛЕНИЕ СИБИРСКОЙ ПЛАТФОРМЫ)

In this paper we present new data on geology of the Kun-Manie copper-nickel-PGE sulfide deposit of the Kun-Manie ore field located on the eastern periphery of the Pristanovoy orogen in the southeast of the Siberian Platform, as well as a detailed description of ores and basic minerals and the distribution of PGE mineralization in ores of the deposit. The ore bodies are spatially associated with sills of the Late Paleoproterozoic Kun-Manie gabbronorite-websterite-lherzolite complex of 1.76–1.69 Ga. It has been established that the outlines of disseminated Cu-Ni ores and ore-bearing sills of mafic-ultramafic rocks match almost exactly. Breccia and massive ores are confined to schists of the upper and lower parts of the ore shoots. Patchy disseminated and vein-disseminated mineralization is concentrated in zones of ductile deformation and along contacts between rock layers of different competence. The main ore minerals are pyrrhotite, pentlandite and chalcopyrite; less common are pyrite, marcasite, cubanite, bornite, chalcocite, sulfoarsenides (gersdorfite, glaucodot), arsenides (nicolite) and tellurides (melonite) of nickel. Magnetite and ilmenite occur in small amount. PGE mineralization is represented by arsenides (sperrylite), sulfoarsenides (irarsite, holingworthite), tellurides (Pd-melonite, merenskite, kotulskite), and tellurobismuthides (moncheite, michenerite). Arsenides, sulfoarsenides and tellurides of PGE, Ni, Ag and Bi, native gold and silver were found in vein-disseminated, patchy disseminated and breccia Cu-Ni ores. The concentration of a considerable part of the main sulfide minerals and the productivity of PGE mineralization are assumed to be due not only to complex processes of magmatic differentiation, but also to sulfurization of magma differentiates in zones of ductile (tectonic) deformation under the influence of recrystallized fluids. The productivity of PGE mineralization characteristically increases with increasing fluid concentrations of S, As, Te, Bi, and Sb.

The Precambrian of Gotland, a key for understanding the Proterozoic evolution in southern Fennoscandia

Gotland is an island in the central part of the Baltic Sea, with up to 800 m Palaeozoic sedimentary rocks on top of a Precambrian basement belonging to the East European Craton. In this contribution, two major ductile deformation zones (Lickershamn - Östergarn and Lilla Karlsö-Ronehamn) are recognized and connected with the Vingåker-Nyköping and Linköping-Loftahammar Deformation Zones in the adjacent Fennoscandian Shield. These deformation zones constitute the borders between three main Precambrian segments that are correlated with crustal units within the Fennoscandian Shield and concealed parts of the East European Craton east of the Baltic Sea.The Fårö-Northern Gotland segment is dominated by continental Jotnian sandstones and Svecofennian metasedimentary rocks, separated from each other by a fault and an associated dolerite dyke. The metasediments show a specific provenance pattern with 3.29 Ga, 2.95–2.63 Ga and 2.11–1.96 Ga sources, devoid of < 1.90 Ga detrital zircons. The Alby granite truncates the metasedimentary rocks of the Fårö-Northern Gotland segment and is part of the 1.58 Ga Riga rapakivi batholith. The Central Gotland segment is dominated by metabasalts and 1.90–1.88 Ga granitoids and is correlated with the Tiveden and Valdemarsvik areas in the Fennoscandian Shield. The amphibolites in the Southern Gotland segment are correlated with 1.87–1.86 Ga metabasalts, intercalated with the Västervik quartzites in the Fennoscandian Shield. They were intruded by TIB 0 granitoids, recognized at Frigsarve and correlated with the Askersund-Loftahammar granitoids in the Fennoscandian Shield. The TIB 0 granitoids on Gotland can also be followed to the east, first to the E6-1 offshore drill hole, 30 km west of the Latvian coast and further east into southwestern Latvia and western Lithuania as a major component in the Mid-Lithuanian Domain. The Kvarne granitoid on southernmost Gotland is correlated with the TIB 1a generation in the Fennoscandian Shield and with granitoids within the Mid-Lithuanian Domain.1.48 Ga small stitching plutons on southern Gotland penetrate the TIB 0 and 1a granitoids and are correlated with the Götemar and Karlshamn plutons in the Fennoscandian Shield and several plutons in western and southern Lithuania.

Fluid Accumulation, Migration and Anaerobic Oxidation of Methane Along a Major Splay Fault at the Hikurangi Subduction Margin (New Zealand): A Magnetic Approach

AbstractUnderstanding the locus of fluid flow along thrust and splay faults is important to understand the hydraulic properties of accretionary systems and fault mechanics. Here, we use rock magnetic techniques in combination with backscattered electron imaging to depict the locus of enhanced magnetic mineral alteration within the Pāpaku fault, an active splay fault of the subduction interface at the northern Hikurangi Margin. The Pāpaku fault was cored at Site U1518 during Expedition 375 of the International Ocean Discovery Program and we report room temperature magnetic parameters, complemented by first‐order reversal and thermomagnetic curves in the depth interval 250–400 m below seafloor (mbsf). The ∼60‐m wide Pāpaku fault zone comprises two main slip zones, referred to as the upper main brittle (304–321 mbsf) and lower subsidiary (351–361 mbsf) fault zones, and an intervening zoned, termed the lower ductile deformation zone. Two narrow zones, at the top of the main brittle fault zone, and one in a sand‐rich interval above the subsidiary fault zone, experienced enhanced magnetic mineral diagenesis, which resulted in the recrystallization of ferrimagnetic greigite to paramagnetic pyrite. We propose that secondary magnetic mineral diagenesis was driven by anaerobic methane oxidation within these intervals, which occurs in the presence of methane and sulfate. We relate the observed changes to the fault parallel transport of fluids which is restricted to two damage zones. Overlying compacted and clay‐rich sediments likely act as a barrier to upward advective flow through the fault zone and into the hanging wall.

Petrographic characteristics and zircon UPb geochronology of granitogneiss rocks in the Chu Lai - Kham Duc area (Quang Nam province)

The early Palaeozoic granitogneiss association in the Chu Lai - Kham Duc area (Quang Nam) is a large area of hundreds of km2, along southern of the East – West ductile deformation zones (Tam Ky – Phuoc Son fault zone), which is studied in detail in different geologic maps scales by the geologists, which is named Chu Lai complex. The five samples studied in detail are composed mainly of granitogneiss and biotite gneiss from the Chu Lai - Kham Duc area. The samples were crushed and large zircons were extracted. The in-situ zircon U–Pb geochronology was conducted on five samples (60 zircons in total) of age between 444 Ma and 426 Ma. These ages indicated the prolonged magmatic – tectonic period between the late Ordovician and middle Silurian in Kon Tum massif.

Cretaceous igneous-related mineralisation in the Reefton Goldfield, New Zealand

ABSTRACTThe Reefton Goldfield is well known for its orogenic style of Au mineralisation although the timing remains largely unconstrained. Here, a first unequivocal occurrence of igneous-related mineralisation is documented and it is suggested to place a lower age constraint on when orogenic Au mineralisation occurred. Emplacement of the 118.5 ± 1.3 Ma (U-Pb zircon) Blackwater granitoid dike into Early Paleozoic Greenland Group caused contact metamorphism up to hornblende hornfels facies temperatures (>500°C) at low pressures (<3.75 kbar). A hydrothermal convective system that developed during cooling led to propylitic and phyllic alteration associated with the precipitation in order of decreasing abundance of: pyrite, chalcopyrite, molybdenite, arsenopyrite-galena-sphalerite-millerite. Quartz veins and stockworks containing combinations of pyrite, chalcopyrite and molybdenite are common within several hundred metres of the dike. This igneous-related mineralisation overprints Greenland Group metasedimentary rocks displaying evidence for a phase of ductile deformation and associated with the occurrence of pyrite and ankerite-cored siderite porphyroclasts, chalcopyrite and cobaltite. Au-in-pyrite trace element data indicate that these pyrite porphyroclasts are more enriched in Au than the pyrite associated with dike emplacement. Quartz lodes containing Au occur within zones of ductile deformation elsewhere in the goldfield, and therefore Early Cretaceous emplacement of the Blackwater dike provides a lower age constraint for the ductile mineralisation event.

Cooling and deformation history of the Çataldağ Metamorphic Core Complex (NW Turkey)

The Çataldağ Metamorphic Core Complex (ÇMCC) consists of the Latest Eocene-Early Oligocene (34–32 Ma) Turfaldag gneiss-migmatite complex (TGMC) and an Early Miocene (21 Ma) Çataldağ synkinematic pluton (ÇSP) which were exhumed as a dome-shaped core complex in the footwall of the Çataldağ Detachment Fault Zone (ÇDFZ) in the Early Miocene. Microstructural features of the Çataldağ Metamorphic Core Complex (ÇMCC) and a two-feldspar geothermometry were studied to understand the cooling and deformation mechanisms. Microtectonic analyses of the TGMC and ÇSP show that the quartz, feldspar, and mica minerals underwent continuous deformation from ductile to brittle conditions during cooling of the ÇMCC. Two main deformation zones were determined according to temperature and strain intensity: a ductile deformation zone (DZ) within the central zones of the TGMC and ÇSP, and a mylonitic zone (MZ) within the peripheral zones of the TGMC and ÇSP (close to the ÇDFZ). Within the ductile zone, K-feldspar displays microcline twinning, myrmekite along the K-feldspar megacryst and flame-shaped perthite, and quartz displays chessboard extinction, grain boundary migration, and sub-grain rotation recrystallisation. These microstructures indicate that deformation temperature reached about 600 °C. Under high strain intensity along the ÇDFZ, the DZ microstructures turn to MZ microstructures. In the MZ, protomylonitic gneiss and mylonitic schists show distinct foliation and their K-feldspar and micas display C-S structures. The feldspars show bulging recrystallisation, a feldspar-fish structure, and domino-type microfractures, while the quartz displays ribbon structures, indicating ductile-to-brittle deformation with temperatures ranging from 500 °C to <250 °C. Two-feldspar geothermometry yields deformation temperatures in the DZ of 501–588 °C (an average of 544 °C for the ÇSP and an average of 517 °C for the TGMC) consistent with the temperature estimated by the microstructural analysis. The mylonitic zone has lower deformation temperature values of 430–557 °C (an average of 484 °C for ÇSP and an average of 436 °C for TGMC). Thermochronology data display TGMC was cooled slowly (<50 °C/my) throughout the Eo-Oligocene and rapidly (>500 °C/my) along the ÇDFZ in the Early Miocene (21 Ma). The shear zone-controlled ÇSP was emplaced into shallow levels of the Anatolian extending crust, deformed progressively (ductile-to-brittle) along the ÇDFZ, and cooled rapidly (>500 °C/my) in the Early Miocene (21 Ma). Together, the microstructure, two-feldspar geothermometry and thermochronology data indicate that the TGMC and ÇSP, which were emplaced into different crustal levels and in different periods, suffered continuous ductile-to-brittle deformation and were exhumed together along the ÇDFZ during the Early Miocene under the N-S extensional regime in western Anatolia.

Cu and Mo re-enrichment during ductile deformation: A case study of the Yuleken porphyry Cu deposit, Eastern Junggar, NW China

Porphyry Cu deposits can be strongly deformed during subsequent tectonic evolution. To date, research pertaining to the deformation and the enrichment of mineralization in porphyry deposits has suffered from a paucity of proof. Here, we describe the diagnostic petrologic and geochemical signatures of the Yuleken porphyry Cu deposit from stages of metallogenic overprinting. The rocks studied in this work are from deformation-metallogenic structures that have (1) a distinguished metallogenic and tectonic evolution, (2) a three-layer, spatial, deformation-metallogenic structure, (3) distinguishably different fluid components within protomylonite, mylonite, and ultramylonite zones, and (4) variational geochemical characteristics of coexisting chalcopyrite and magnetite.Integrated sulfide and oxide mineral investigations pertaining to the mineral deformation and the trace element variation of the minerals chalcopyrite and magnetite in the Yuleken porphyry Cu deposit indicated that Cu and Mo can be enriched during ductile deformation through physical (analogous geochemical characteristics of chalcopyrite from different stages) and chemical migrations. The elemental variation of magnetite (magnetites from deformed stages have higher contents of Ti, V, Co, Mn, and Ga than those from other stages) and the variation of the fluid components within the protomylonite, mylonite, and ultramylonite zones reveal that the migration and precipitation of Cu, Mo, and Fe were influenced by oxygen fugacity, fluid components, and temperature variation. In conclusion, the arc-related porphyry deposits were able to become involved in the ductile deformation zone, inducing areas of Cu and Mo enrichment in theory. Units subjected to ductile deformation may have great potential for porphyry Cu (-Mo) deposit formation under the influence of magmatism.

Transport-related mylonitic ductile deformation and shape change of alluvial gold, southern New Zealand

Gold is a malleable metal, and detrital gold particles deform via internal distortion. The shapes of gold particles are commonly used to estimate transport distances from sources, but the mechanisms of internal gold deformation leading to shape changes are poorly understood because of subsequent recrystallisation of the gold in situ in placer deposits, which creates a rim zone around the particles, with undeformed >10μm grains. This paper describes samples from southern New Zealand in which grain size reduction (to submicrometer scale) and mylonitic textures have resulted from internal ductile deformation. These textures have been preserved without subsequent recrystallisation after deposition in late Pleistocene-Holocene alluvial fan placers. These mylonitic textures were imposed by transport-related deformation on recrystallised rims that were derived from previous stages of fluvial transportation and deposition. This latest stage of fluvial transport and deformation has produced numerous elongated gold smears that are typically 100μm long and 10–20μm wide. These smears are the principal agents for transport-induced changes in particle shape in the studied placers. Focused ion beam (FIB) sectioning through these deformed zones combined with scanning electron microscopic (SEM) imaging show that the interior of the gold particles has undergone grain size reduction (to ~500nm) and extensive folding with development of a ductile deformation fabric that resembles textures typical of mylonites in silicate rocks. Relict pods of the pre-existing recrystallised rim zone are floating in this ductile deformation zone and these pods are irregular in shape and discontinuous in three dimensions. Micrometer scale biologically-mediated deposition from groundwater of overgrowth gold on particle surfaces occurs at all stages of placer formation, and some of this overgrowth gold has been incorporated into deformation zones. These examples provide a rare view into the nature of the physical processes that accommodate gold particle shape change during sedimentary transport.

The Numba ductile deformation zone (northwest Cameroon): A geometric analysis of folds based on the Fold Profiler method

The Numba ductile deformation zone (NDDZ) is characterised by folds recorded during the three deformation phases that affected the banded amphibole gneiss. Fold-shape analyses using the program Fold Profiler with the aim to show the importance of folding events in the structural analysis of the NDDZ and its contribution to the Pan-African orogeny in central Africa have been made. Classical field method, conic sections method and Ramsay’s fold classification method were applied to (i) have the general orientation of folds, (ii) analyze the fold shapes and (iii) classify the geometry of the folded bands. Fold axes in banded amphibole gneiss plunge moderately (\(<\)15\(^{\circ }\)) towards the NNE or SSW. The morphology of F\(_{1}\), F\(_{2}\) and F\(_{3}\) folds in the study area clearly points to (i) Z-shape folds with SE vergence and (ii) a dextral sense of shear motion. Conic section method reveals two dominant families: F\(_{1}\) and F\(_{3}\) folds belong to parabolic shape folds, while F\(_{2}\) folds belong to parabolic shape and hyperbolic shape folds. Ramsay’s scheme emphasizes class 1C (for F\(_{1}\), F\(_{2}\) and F\(_{3}\) folds) and class 3 (for F\(_{2}\) folds) as main fold classes. The co-existence of the various fold shapes can be explained by (i) the structuration of the banded gneiss, (ii) the folding mechanisms that associate shear with a non-least compressive or flattening component in a ductile shear zone and (iii) the change in rheological properties of the band during the period of fold formation. These data allow us to conclude that the Numba region underwent ductile dextral shear and can be integrated (i) in a correlation model with the Central Cameroon Shear Zone (CCSZ) and associated syn-kinematic intrusions and (ii) into the tectonic model of Pan-African belt of central Africa in Cameroon.

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

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.

Seismic anisotropy of the lithosphere and asthenosphere beneath southern Madagascar from teleseismic shear wave splitting analysis and waveform modeling

AbstractMadagascar occupies a key position in the assembly and breakup of the supercontinent Gondwana. It has been used in numerous geological studies to reconstruct its original position within Gondwana and to derive plate kinematics. Seismological observations in Madagascar to date have been sparse. Using a temporary, dense seismic profile across southern Madagascar, we present the first published study of seismic anisotropy from shear wave splitting analyses of teleseismic phases. The splitting parameters obtained show significant small‐scale variation of fast polarization directions and delay times across the profile, with fast polarization rotating from NW in the center to NE in the east and west of the profile. The delay times range between 0.4 and 1.5 s. A joint inversion of waveforms at each station is applied to derive hypothetical one‐layer splitting parameters. We use finite‐difference, full‐waveform modeling to test several hypotheses about the origin and extent of seismic anisotropy. Our observations can be explained by asthenospheric anisotropy with a fast polarization direction of 50°, approximately parallel to the absolute plate motion direction, in combination with blocks of crustal anisotropy. Predictions of seismic anisotropy as inferred from global mantle flow models or global anisotropic surface wave tomography are not in agreement with the observations. Small‐scale variations of splitting parameters require significant crustal anisotropy. Considering the complex geology of Madagascar, we interpret the change in fast‐axis directions as a ~150 km wide zone of ductile deformation in the crust as a result of the intense reworking of lithospheric material during the Pan‐African orogeny. This fossil anisotropic pattern is underlain by asthenospheric anisotropy induced by plate motion.

High-frequency vibration effects on hole entrance chipping in rotary ultrasonic drilling of BK7 glass

This present investigation exhibited some fundamental information about the influence of the high-frequency vibration on the hole entrance chipping formation involved in rotary ultrasonic drilling (RUD) of BK7 glass process. The entrance chipping morphologies, produced with and without ultrasonic, were observed and evaluated with respect to the fracture mechanics of brittle material. Giving consideration to the variation characteristics of the plastic deformation region in the interior material induced by the specific kinematics principles of the abrasive, the ultrasonic effects on the chipping formation mechanisms were investigated by assessing the groove morphologies obtained in the scratching experiment utilizing the formation mechanisms of the lateral cracking. Furthermore, the formal confirmatory tests with and without ultrasonic were performed to validate these chipping formation mechanisms. It was found that the plastic deformed region reached its maximum at the trajectory bottom. Moreover, the propagation of the lateral cracking initially nucleated at the bottom of the ductile deformation zone resulted in the formation of the entrance chipping in formal RUD process. The slight deformation of the material at the two terminals of each groove produced with ultrasonic would provide the inhibitory effects to the further extending of the lateral cracks, which would shrink with the increased spindle speed, and the inhibitory effect dominated in determining the improvement effects on the hole entrance quality. Additionally, a theoretical relationship between the nucleation depth and the propagation length of the lateral cracking was developed for the conventional drilling (CD) process.

The Ductile Deformation Characteristics of Caledonian Intracontinental Orogeny in the Northeastern Jiangshan‐Shaoxing Tectonic Zone: Insights from Magnetic Fabric Study and Its Geodynamic Implication

AbstractThe Jiangshan‐Shaoxing tectonic zone was the northeastern boundary between the Yangtze Block and the Cathaysia Block during the Neoproterozoic and was an intracontinental erogenic belt during late of the early Paleozoic. In this tectonic zone, there develops a lot of mylonite underwent strong ductile deformation and schist, gneiss, and amphibolite with medium and high grade metamorphism which was formed during the late of early Paleozoic. The research of geometry and kinematic of ductile deformation in Jiangshan‐Shaoxing tectonic zone is very important to reveal the tectonic process of intracontinental orogeny. This paper uses the anisotropy of magnetic susceptibility (AMS) to determine the ductile deformation geometry and kinematic of Jiangshan‐Shaoxing tectonic zone combing with the field survey. In this study, 190 specimens of 19 locations and 221 specimens of 23 locations from Wangjiazhai section and Lipu‐Sizhai section were analyzed. The magnetic foliation over magnetic lineation in both Wangjiazhai and Lipu‐Sizhai sections together with the field observations indicated a compressional deformation pattern. 3 and 4 strong ductile deformation zones can be established in the Wangjiazhai section and the Lipu‐Sizhai section, respectively. According to the magnetic fabric and petro‐fabric studies, the Northeastern Jiangshan‐Shaoxing tectonic zone suffered two kinds of deformation patterns during the late early Paleozoic, i.e., the thrusting deformation followed by sinistral shear deformation.

Tectonic interleaving along the Main Central Thrust, Sikkim Himalaya

Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c . 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an ϵ Nd(0) signature of −18.3 to −12.1, and is intruded by c . 800 Ma and c . 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c . 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an ϵ Nd(0) signature of −27.7 to −23.4, intruded by c . 1830 Ma granites. The data reveal a c . 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones. Supplementary material: Zircon U–Pb geochronological data, whole-rock Sm–Nd isotopic data, sample locations, photomicrographs of sample thin sections, zircon CL images, and detailed analytical conditions are available at www.geolsoc.org.uk/SUP18704 .

Contrasting magma emplacement mechanisms within the Rogart igneous complex, NW Scotland, record the switch from regional contraction to strike-slip during the Caledonian orogeny

AbstractThe Rogart igneous complex is unique within the northern Scottish Caledonides because it comprises an apparent continuum of magma types that records a progressive change in emplacement mechanisms related to large-scale tectonic controls. Syn-D2 leucogranites and late-D2 quartz monzodiorites were emplaced during crustal thickening and focused within the broad zone of ductile deformation associated with the Naver Thrust. In contrast, emplacement of the post-D2 composite central pluton was controlled by development of a steeply dipping dextral shear zone along the Loch Shin Line, interpreted as an anti-Riedel shear within the Great Glen Fault system. The mantle-derived nature of the late-to-post-D2 melts implies that the Naver Thrust and the Loch Shin Line were both crustal-scale structures along which magmas were channelled during deformation. A U–Pb zircon age of 425±1.5 Ma for the outer component of the central pluton provides an upper limit on regional deformation and metamorphism within host Moine rocks. These findings are consistent with the view that a fundamental change in tectonic regime occurred in the Scottish Caledonides at c. 425 Ma, corresponding to the switch from regional thrusting that resulted from the collision of Baltica and Laurentia, to the development of the orogen-parallel Great Glen Fault system.