Stages Of Ductile Deformation Research Articles

Strain rate influence on fracture development in experimental ductile multilayers

The far-field strain rate is a crucial parameter that controls the transition between brittle and ductile deformation. We have used analogue experiments to study the strain rate influence on the development of brittle fractures in a ductile composite material. Plasticine multilayer models were deformed under coaxial boundary conditions at three different strain rates to analyse the transition from non-localised deformation to the development of a brittle fracture network that accommodates part of the deformation. The results show that tension cracks and voids are the first macroscopic structures that nucleate after an early stage of ductile deformation. Coalescence and collapse of these structures lead to the development of brittle shear fractures. The evolution of fracture orientations, lengths and displacements was systematically analysed. The ratio of the accumulated fracture displacement vs. fracture length ( d max / L) depends not only on the total deformation, but also on the strain rate at which the system is deformed. The accumulated displacement with respect to fracture length increases with strain rate.

Local shear zone pattern and bulk deformation in the Gran Paradiso metagranite (NW Italian Alps)

The Gran Paradiso nappe of the northwestern Alps mostly consists of augen gneisses derived from the Alpine deformation of Permian granitoids. The regional foliation of the augen gneisses developed at lower amphibolite facies conditions and is associated with a top-to-west sense of shear. The granitoid protolith is preserved in the kilometre-scale low-strain domain of the Piantonetto Valley and mainly consists of a porphyritic metagranite including joints, leucocratic dykes and biotite-rich schlieren. In this low-strain domain, the Alpine deformation is mainly localized in discrete ductile shear zones within weakly foliated metagranite. The shear zones mostly dip towards S–SE in a shallow (shear zones 1) to steep inclination (shear zones 2). The shear zones show typical features that can be explained by reactivation of pre-existing joints and planar compositional heterogeneities. Palaeostress and strain analysis indicate that shear zones and the metagranite foliation both formed in the presence of a strong component of flattening. The kinematics of individual shear zones depends on the orientation of the original heterogeneities (acting as nucleation planes) and by partitioning of strain components at the kilometre-scale with concentration of the flattening component to the Piantonetto low-strain domain. The strain geometry and the kinematics of individual shear zones within Piantonetto are not directly connected to the top-to-west sense of tectonic transport observed elsewhere in the Gran Paradiso nappe. However, the bulk stress ellipsoid reconstructed for the incipient shear zone network within very weakly deformed granites is oriented consistently with the bulk direction of tectonic transport within the Gran Paradiso massif. We conclude that the shear zone network of the Piantonetto Valley is representative of the incipient stages of ductile deformation of a granite nappe. Even if its architecture is determined by the arrangement of pre-existing structural and compositional heterogeneities, aspects of the large-scale bulk strain can be derived from this local shear zone pattern.

Deciphering a multistage history affecting U–Cu(–Fe) mineralization in the Singhbhum Shear Zone, eastern India, using pyrite textures and compositions in the Turamdih U–Cu(–Fe) deposit

The ∼200-km-long intensely deformed Singhbhum Shear Zone (SSZ) in eastern India hosts India’s largest U and Cu deposits and related Fe mineralization. The SSZ separates an Archaean cratonic nucleus to the south from a Mesoproterozoic fold belt in the North and has a complex geologic history that obscures the origin of the contained iron-oxide-rich mineral deposits. This study investigates aspects of the history of mineralization in the SSZ by utilizing new petrographic and electron microprobe observations of pyrite textures and zoning in the Turamdih U–Cu(–Fe) deposit. Mineralization at Turamdih is hosted in intensively deformed quartz–chlorite schist. Sulfides and oxides include, in inferred order of development: (a) magmatic Fe(–Ti–Cr) oxide and Fe–Cu(–Ni) sulfide minerals inferred to be magmatic (?) in origin; followed by (b) uranium, Fe-oxide, and Fe–Cu(–Co) sulfide minerals that predate most or all ductile deformation, and are inferred to be of hydrothermal origin; and (c) Fe–Cu sulfides that were generated during and postdating ductile deformation. These features are associated with the formation of three compositionally and texturally distinct pyrites. Pyrite (type-A), typically in globular–semiglobular composite inclusions of pyrite plus chalcopyrite in magnetite, is characterized by very high Ni content (up to 30,700 ppm) and low Co to Ni ratios (0.01–0.61). The textural and compositional characteristics of associated chalcopyrite and rare pyrrhotite suggest that this pyrite could be linked to the magmatic event via selective replacement of magmatic pyrrhotite. Alternatively, this pyrite and associated sulfide inclusions might be cogenetic with hydrothermal Fe-oxide. Type-B pyrite that forms elongate grains and irregular relics and cores of pyrite with high Co contents (up to 23,630 ppm) and high Co to Ni ratios (7.2–140.9) are interpreted to be related to hydrothermal mineralization predating ductile deformation. A third generation of pyrite (type C) with low Co, low Ni, and moderate Co to Ni ratios (0.19–13.93) formed during and postdating the ductile deformation stage overgrowing, replacing, and surrounding type-B pyrite. The textural evolution of pyrite parallels the tectonometamorphic evolution of the shear zone demonstrating grain elongation during progressive ductile deformation and prograde metamorphism, annealing at the peak metamorphic condition, porphyroblastic growth at the retrograde path and cataclasis following porphyroblastic growth. Compositional characteristics of hydrothermal pyrite and available geological information suggest that the U–Cu(–Fe) deposit at Turamdih might be a variant of the Fe oxide (–Cu–U–rare earth elements) family of deposits.

Mechanical quality of bedrock with increasing ductile deformation

Most of the rocks in Precambrian shield areas have experienced a complex structural and metamorphic evolution, processes which have a strong influence on bedrock quality. The properties vary on both a local and a regional scale. It is highly beneficial to know the variations in detail when exploiting the rocks for industrial purposes. The main objective of the investigation was to study the variation of rock mechanical properties in an originally more or less isotropic rock at various stages of ductile deformation. The rocks investigated were Paleoproterozoic and with ages of ca. 1.80–1.88 Ga, and the areas chosen for sampling were situated north-east of Lake Vänern (Kristinehamn; 10 samples), south of the city of Eskilstuna, South Central Sweden (5 samples) and south of Ödeshög, near Lake Vättern in southern Sweden (7 samples). The 12 samples from the latter two areas are described in this investigation, while the 10 samples from Kristinehamn have been published earlier (Göransson et al., 2004). A comprehensive study of various parameters of importance for bedrock quality has been performed on all samples, e.g. studded tyre test (STT) and Los Angeles test (LA), uniaxial compressive strength (UCS), ultrasonic velocity, perimeter measurements of mineral phases, and petrographic and chemical analyses. The weakly deformed and massive (more or less isotropic) rocks show a tendency towards better properties of abrasion (STT) than the strongly deformed rocks and this can also be said for UCS, reflecting the greater ability of rocks to split along foliation planes. This is not entirely unambiguous, as the more deformed rocks, such as the mylonites, may have varying properties. This depends on the combined effects of, e.g. grain size, recrystallisation and foliation. However, the brittleness (LA) shows somewhat better values with increased deformation. This may depend on higher amounts of dark minerals, as their existence does not affect this test as much as in the case of abrasion tests. The perimeter values of the mineral phases display generally higher values, i.e. grain boundaries for the more strongly deformed rocks are more complicated. However, the values for the investigated mylonites may vary between low and high. The lower value may be due to dynamic recrystallisation and the creation of triple points (static recrystallisation) making the rock weaker. Besides, the development of a strong foliation may decrease rock strength despite the usually finer grain size. The results show that it is extremely important to consider all possible variations of bedrock before classification and exploitation, as the bedrock material in fact is highly heterogeneous.

Zircon geochronology and compositional record of late- to post-kinematic granitoids associated with the Bavarian Pfahl zone (Bavarian Forest)

The northwest-striking Pfahl zone, Bavarian Forest, is a mylonitic shear zone that is associated with brittle-ductile deformation fabrics and a conspicuous hydrothermal quartz mineralization. Two granites from this shear zone yield U–Pb and Pb–Pb evaporation ages between 321–329 Ma and two granodiorites give concordant 238U–206Pb and 235U–207Pb ages of 325±3 Ma and 326±3 Ma, respectively. Zircon populations of the granitoids show sub-types clustering around S20, S10 (granite) and S22 to S24 (granodiorite) testifying different magma affinity. Compositional and isotopic characteristics indicate that the granites and granodiorites were coeval melts, but not differentiates of a single parent magma. The granodiorites were derived from a source with higher time-integrated 87Rb/86Sr and lower 147Sm/144Nd ratios than the granites. One granite body is transected by the shear zone but the main mass of the granite is largely undeformed. This finding suggests that granite intrusion predates the final stage of ductile deformation along the Pfahl shear zone.

The variation of bedrock quality with increasing ductile deformation

Most of the rocks in Precambrian shield areas have experienced a complex structural and metamorphic evolution, processes which have a strong influence on the bedrock quality. This may vary both on a local and regional scale. The main object of this investigation was to study the variation of the rock mechanical properties at various stages of ductile deformation of an originally isotropic rock. Ten samples from five different localities northeast of Lake Vanern (south central Sweden) were tested, including studded tyre and LA tests, uniaxial compressive strength (UCS), ultrasonic velocity, electrical resistivity, Swedish impact value and Swedish flakiness index as well as petrographic and chemical analyses. This study highlights the importance of considering all possible variations in bedrock before classification, and of undertaking regional investigations.

Kinematic vorticity and strain rate patterns associated with ductile extrusion in the Chelmos Shear Zone (External Hellenides, Greece)

Kinematic vorticity, strain rate, finite strain and structural data have been used to describe the heterogeneous nature of ductile deformation in an extruding crustal-scale shear zone, the Chelmos Shear Zone, (CSZ) of the External Hellenides. The CSZ has a curved shape, which is marked by a frontal-flat and a steep zone. The spatial variation in the kinematic vorticity number is strongly correlated with the geometry and the structural depth of the shear zone. The pure shear component of deformation decreases from back to front and from top to bottom. The ductile thinning normal to the shear zone's boundaries is higher in the steep zone and decreases westwards. Quartz grain size and strain rate in the CSZ is distributed into two approximately homogeneous domains. The higher strain rate is recorded in the steep zone and the base of the frontal-flat zone with lower rates in the upper parts of the frontal-flat zone. Based on mesoscopic structural data and the spatial distribution of the shear strain rate, the existence of a west-dipping back-flow zone in the boarder zone between these two domains is inferred. The overall pattern of deformation in the CSZ implies that tectonic extrusion from the steep zone occurred at the final stages of ductile deformation and was associated with the formation of the backflow zone in the front of the shear zone. Ductile deformation of rocks probably started close to simple shear on a planar low-angle shear zone, progressively became penetrative, and more general shear occurred as the CSZ steepened up. This progressive increase in pure shearing may have occurred in a heterogeneous manner and was higher close to both the eastern margin of the steep zone and the top of the frontal-flat zone. Solid-state extrusion in the CSZ was also associated with a heterogeneous and progressive increase in strain rate.

Mechanical twinning of jadeite – an indication of synseismic loading beneath the brittle–plastic transition

The orientation distribution of mechanically twinned jadeite crystals in a metagranite from the Sesia Zone, Western Alps, is evaluated to derive the orientation of the principal stress directions for a prominent stage of ductile deformation at ca. 300 to 350 °C that has been interpreted to reflect synseismic loading and postseismic creep by Kuster and Stockhert (1999). It is tested whether the orientation distribution can be used to obtain information on the magnitude of differential stress if the critical shear stress for twinning is known. Assuming that the critical shear stress of 140 MPa determined by Kolle and Blacic (1982) for mechanical twinning of clinopyroxene in the (100) [001] system holds true for jadeite, and assuming a homogeneous stress field as a first approximation, differential stresses of the order of 1–2 GPa are inferred for metagranites with ca. 15% jadeite, and of 0.5 GPa for jadeitite with 80% of jadeite. Notwithstanding the uncertainty for the critical resolved shear stress for jadeite, these unexpectedly high values are suspected to be due to a combination of the following effects: (1) an inhomogeneous stress field in the polyphase material with curved stress trajectories, (2) stress concentration in jadeite due to load transfer from the quartz matrix, and (3) minor reorientation of the jadeite crystals in the flowing quartz matrix. Hence, absolute values of differential stress cannot be derived from the orientation distribution of twinned jadeite in polyphase rocks. Notwithstanding the failure in deriving reliable absolute values, a short period with exceptionally high stress must have occurred, and is fully consistent with the microstructural record of other minerals. It is proposed that the mechanical twinning of jadeite was caused by a short-lived stress peak and flow at laboratory strain rates related to quasi-instantaneous loading of the uppermost level of the ductile lower crust during brittle failure of the brittle upper crust in a major seismic event.

Determination of magnetic anisotropy and a ca 1.2 Ga palaeomagnetic pole from the Bremer Bay area, Albany Mobile Belt, Western Australia

Granulite facies tonalitic gneiss, mafic granulite and late metadolerite dykes from Bremer Bay in the Mesoproterozoic Albany Mobile Belt yield palaeomagnetic remanence that were acquired between ca 1.2 Ga and 1.1 Ga. A well‐constrained pole (66.6°N, 303.7°E) fits the ca 1.2 Ga part of the Precambrian Australian apparent polar wander path. This pole is in agreement with the high‐latitude position of Australia at ca 1.2–1.1 Ga shown on some Rodinia reconstructions. More data are required before any significance can be attributed to a second, poorly defined pole (41.8°S, 243.7°E) that falls at some distance from the ca 0.8 Ga part of the Australian apparent polar wander path. Magnetic anisotropy measurements from all samples except late granite dykes indicate northeast‐southwest elongation (i.e. parallel to the local trend of the orogenic belt) and northwest‐southeast contraction. This is in agreement with the orientation of principal strain axes deduced from structures formed during late stages of ductile deformation. The mean magnetic fabric lineation (long axis of the strain ellipsoid) is subparallel to a mineral elongation lineation and the axes of late upright to inclined folds. Short axes of the strain ellipsoid determined from magnetic fabric measurements are in a similar orientation to poles to the axial surfaces of these folds and to the associated cleavage. This mean shortening axis bisects late conjugate ductile shear zones that overprint the folds. This study has shown that structurally complex high‐grade gneisses and intrusive rocks with variable timing relationships may yield meaningful palaeomagnetic results for late stages of metamorphism. Magnetic anisotropy analysis is also seen to be a valuable tool in providing principal strain directions for late ductile deformation.

Ductile transpression and localization of deformation along tectonic boundaries in the Caledonides of northwestern Ireland

Orogenesis is increasingly interpreted in terms of strain focusing, localization and partitioning processes. Such heterogeneous deformation is considered a consequence of the tectonic framework, with pre-existing structural and stratigraphic variability providing inherent zones of crustal weakness. Detailed structural investigation of Neoproterozoic Dalradian metasediments in the Glencolumbkille region, northwest Ireland, enables patterns of reworking and strain localization to be assessed in terms of four overprinting ductile deformation episodes. A well-defined and intricate Dalradian stratigraphy provides readily distinguishable markers which not only focus deformation along marked rheological boundaries, but also aid in the definition and identification of resultant geometries. Overall structural and stratigraphic relationships show that whilst D1 was not associated with major structures, D2 is related to north–northeast directed folding and ductile thrusting resulting in a major phase of crustal thickening and almandine-amphibolite facies metamorphism. Structures generated during D2 deformation subsequently became the locus of intense D3 strain and were reactivated in an oblique sense associated with south or southwest directed translations. Local overprinting relationships clearly demonstrate S2 fabrics being transposed by S3 resulting in a composite foliation over large areas. Similarly, the L2 mineral lineation is abruptly transposed by L3 over relatively small distances indicating high D3 strain gradients and the susceptibility of lineations to reworking. The final stage of ductile deformation (D4) which was increasingly localized and focused into earlier (D2−D3) high strain zones, is marked by a pronounced phase of sinistral transpression associated with clockwise cleavage and minor fold transection of northwest verging upright folds. Sinistral shear is strongly partitioned in to the steep limbs of mesoscopic F4 folds. The detailed investigation of structures generated within such multiply deformed and reworked zones provides evidence of both the kinematic and tectonic evolution of regional deformation systems.

Use of pyrite microfabric as a key to tectono-thermal evolution of massive sulphide deposits – an example from Deri, southern Rajasthan, India

AbstractPyrite is an ubiquitous constituent of the Proterozoic massive sulphide deposit at Deri, in the South Delhi Fold Belt of southern Rajasthan. Preserved pyrite microfabrics in the Zn-Pb-Cu sulphide ores of Deri reveal a polyphase growth history of the iron sulphide and enable the tectono-thermal evolution of the deposit to be reconstructed.Primary sedimentary features in Deri pyrites are preserved as compositional banding. Regional metamorphism from mid-greenschist to low amphibolite facies is recorded by various microtextures of pyrite. Trails of fine grained pyrite inclusions within hornblende porphyroblasts define S1-schistosity. Pyrite boudins aligned parallel to S1 mark the brittle–ductile transformation of pyrite during the earliest deformation in the region. Isoclinal to tight folds (F1 and F2) in pyrite layers relate to a ductile deformation stage during progressive regional metamorphism. Peak metamorphic conditions around 550°C, an estimation supported by garnet–biotite thermometry, resulted in annealing of pyrite grains, while porphyroblastic growth of pyrite (up to 900 µm) took place along the retrogressive path. Brittle deformation of pyrite and growth of irregular pyritic mass around such fractured porphyroblasts characterize the waning phase of regional metamorphism. A subsequent phase of stress-free, thermal metamorphism is recorded in the decussate and rosette textures of arsenopyrite prisms replacing irregular pyritic mass. Annealing of such patchy pyrite provides information regarding the temperature conditions during this episode of thermal metamorphism which is consistent with the hornblendehornfels facies metamorphism interpreted from magnetite–ilmenite geothermometry (550°C) and sphalerite geobarometry (3.5 kbar). A mild cataclastic deformation during the penultimate phase produced microfaults in twinned arsenopyrite prisms.

Temporal Association of Ductile Deformation and Granitic Plutonism: Rb-Sr and $$^{40}Ar-^{39}Ar$$ Isotopic Evidence from Roof Pendants above the Central Bohemian Pluton, Czech Republic

The Staré Sedlo gneiss complex occurs as a roof pendant in the early Carboniferous Central Bohemian Pluton. Mylonitic tonalitic gneiss and amphibolite in the complex share a common foliation, which is discordant to the margins of the granitoid masses of the enclosing pluton. Hornblende and biotite-whole-rock, biotite-plagioclase Rb-Sr isotopic data give closely corresponding early Carboniferous cooling ages for hornblende (500°C) and biotite (300°C) in amphibolite and mylonitic tonalitic gneiss, respectively. Ages of , , and (biotite-whole-rock and biotite-plagioclase Rb-Sr) and , , and (hornblende ) are interpreted as representing times of closure of isotopie systems associated with rapid cooling during the final stages of ductile deformation and metamorphism in a thrust regime. They overlap the time of emplacement of at least early intrusions that show some foliation, and possibly much of the Central Bohemian Pluton (including the old Blatná granodiorite) whose gross shape and emplacement are interpreted to have been controlled by a steeply disposed ramp(s) in a major listric thrust zone.

Distribution of REE and other trace elements in the Hetai gold deposit of South China: implications for evolution of an auriferous shear zone

The Hetai gold deposit is contained within a shear zone which is hosted by metavolcano-sedimentary units of the Late Proterozoic Yunkai Group. The shear zone, striking northeast and dipping northwest, is over 3 km long and has a thickness of about 20–50 m. The ore occurs in the shear zone as a series of lenticular and vein-type bodies. The auriferous shear zone has undergone three stages of evolution: the early stage was characterized by ductile deformation when auriferous mylonite containing less than 1 ppm Au was formed; the intermediate stage took place in brittle-ductile environments during which the lenticular ores with about 10–100 oom Au were formed; the last stage was developed during brittle deformation resulting in emplacement of auriferous quartz veins with grades of 1–5 ppm Au. REE pattern analysis demonstrates that no significant modification of lanthanide distribution occurred during the ductile deformation stage of the shear zone evolution, which resulted in the formation of mylonite and ultramylonite, although important mineralogical and chemical changes of the original metamorphic rocks have been observed. In contrast, extensive variations of REE patterns occurred in the hydrothermally infiltrated rocks which are products of the brittle-ductile and the brittle deformation stages of the shear zone evolution. This confirms that REE behavior during dynamometamorphism seems to be similar to that during metamorphism and hydrothermal alteration, which is determined both by fluid/rock ratios and by REE patterns and abundances in reacting fluids and rocks.