Late-stage Deformation Research Articles

Orientation dependence of twinning and strain hardening behaviour of a high manganese twinning induced plasticity steel with polycrystalline structure

The twinning induced plasticity steel 0.6 C–18 Mn–1.5 Al (wt.%) employed in this study exhibited a strength of about 1GPa, combined with a large uniform elongation of over 60% at moderate strain rates. This excellent combination of tensile mechanical properties was shown to be a result of complex dynamic strain-induced microstructural reactions, including dislocation glide, dislocation dissociation, stacking fault formation, dynamic recovery, mechanical twinning and dynamic strain ageing. The contribution of each of these processes was discussed with respect to the strain hardening behaviour. An important observation was that the propensity for mechanical twinning in a particular grain strongly depends on its crystallographic orientation and is clearly correlated with the magnitude of the corresponding Taylor factor. The current results reveal that the development of partial dislocations and stacking faults at an early stage of straining and the consequent interaction of gliding dislocations with stacking fault interfaces enhances the strain hardening rate. Subsequent or concurrent initiation of mechanical twins also contributes to strain hardening through a dynamic Hall–Petch effect limiting the dislocation mean free path and consequently enhancing dislocation storage. Dynamic strain ageing (DSA) took place at a late stage of deformation, adding to strain hardening. DSA is a factor reducing ductility by promoting strain localization.

Testing the transpression hypothesis in the western part of the Cheyenne belt, Medicine Bow Mountains, southeastern Wyoming

A system of subvertical, northeast-striking shear zones collectively called the Cheyenne belt comprises the fundamental boundary between the Archean Wyoming province and the Paleoproterozoic Colorado province in the Medicine Bow Mountains of southeastern Wyoming. These shear zones are generally interpreted as a mid-crustal thrust system rotated to its present-day subvertical orientation during late-stage, orogen-scale folding. However, the map geometry, presence of large domains of S tectonites, and vertical mineral lineations closely match numerical simulations of transpressional shear zones. This transpression hypothesis is tested using two detailed case studies that integrate detailed geologic mapping, kinematic analyses of S and L-S tectonites, quartz crystallographic fabric analyses, and deformation mechanism analyses of shear zones in the western part of the Medicine Bow Mountains. These data point toward general shear deformation that accommodated two coeval deformation components: (1) southeast-side-up, dip-slip or southeast-side-up/dextral, oblique-slip motion and (2) foliation-normal shortening. According to the broadest definition, these are transpressional shear zones. However, the strike-slip component is small, and these data do not require any significant oblique plate motion during the formation of the Cheyenne belt shear zones. The lack of evidence for overprinting of initial southeast-side-up fabrics by horizontal shortening, the near parallelism of the subvertical shear zones throughout the upper and middle crust, and the presence of subvertical mafic dike swarms in Archean basement rocks immediately adjacent to the Cheyenne belt all indicate that the Cheyenne belt shear zones were not rotated into their present-day subvertical orientations during late-stage deformation. Instead, we interpret the shear zones as a ∼1,750-Ma steeply dipping stretching fault system between the penetratively deformed young, hot, rheologically weak rocks of the Colorado province and the old, cold, rheologically strong Archean rocks north of the belt.

Multiple accretion at the eastern margin of the Rio de la Plata craton: the prolonged Brasiliano orogeny in southernmost Brazil

The Neoproterozoic-Eoplalaeozoic Brasiliano orogeny at the eastern margin of the Rio de la Plata craton in southernmost Brazil and Uruguay comprises a complex tectonic history over 300 million years. The southern Brazilian Shield consists of a number of tectono-stratigraphic units and terranes. The Sao Gabriel block in the west is characterized by c.760–690 Ma supracrustal rocks and calc-alkaline orthogneisses including relics of older, c. 880 Ma old igneous rocks. Both igneous and metasedimentary rocks have positive eNd(t) values and Neoproterozoic TDM model ages; they formed in magmatic arc settings with only minor input of older crustal sources. A trondhjemite from the Sao Gabriel block intruding dioritc and tonalitic gneisses during the late stages of deformation (D3) yield an U–Pb zircon age (LA-ICP-MS) of 701 ± 10 Ma giving the approximate minimum age of the Sao Gabriel accretionary event. The Encantadas block further east, containing the supracrustal Porongos belt and the Pelotas batholith, is in contrast characterized by reworking of Neoarchean to Palaeoproterozoic crust. The 789 ± 7 Ma zircon age of a metarhyolite intercalated with the metasedimentary succession of the Porongos belt provides a time marker for the basin formation. Zircons of a sample from tonalitic gneisses, constituting the Palaeoproterozoic basement of the Porongos belt, form a cluster at 2,234 ± 28 Ma, interpreted as the tonalite crystallization age. Zircon rims show ages of 2,100–2,000 Ma interpreted as related to a Palaeoproterozoic metamorphic event. The Porongos basin formed on thinned continental crust in an extensional or transtensional regime between c. 800–700 Ma. The absence of input from Neoproterozoic juvenile sources into the Porongos basin strongly indicates that the Encantadas and Sao Gabriel blocks were separated terranes that became juxtaposed next to each other during the Brasiliano accretional events. The tectonic evolution comprises two episodes of magmatic arc accretion to the eastern margin of the Rio de la Plata craton, (i) accretion of an intra-oceanic arc at c. 880 Ma (Passinho event) and (ii) accretion of the 760–700 Ma Cambai/Vila Nova magmatic arc (Sao Gabriel event). The latter event also includes the collision of the Encantadas block with the Rio de la Plata craton to the west. Collision and crustal thickening was followed by sinistral shear along SW–NE-trending orogen-parallel crustal-scale shear zones that can be traced from southern Brazil to Uruguay and have been active between 660 and 590 Ma. Voluminous granitic magmatism in the Pelotas batholith spatially related to shear zones is interpreted as late- to post-orogenic magmatism, possibly assisted by lithospheric delamination. It marks the transition to the post-orogenic molasse stage. Localized deformation by reactivation of preexisting shear zones continued until c. 530 Ma and can be assigned to final stages of the amalgamation of West Gondwana.

Strengthening and toughening mechanisms in Mg–Zn–Y alloy with a long period stacking ordered structure

The deformation behavior and corresponding microstructure evolution of a Mg 97Zn 1Y 2 (at.%) alloy with a long period stacking ordered (LPSO) structure subjected to hot compression were investigated. The peak stress at 573 K was about 190 MPa, and no macroscopic fracture took place up to a strain of about 60%. The mechanisms responsible for the mechanical performance of the Mg 97Zn 1Y 2 (at.%) alloy are discussed based on microstructural investigations using various electron microscopy techniques. The high strength at elevated temperature could be attributed to synergetic strengthening refinement of the LPSO via kinking and a limited fraction of dynamical recrystallization. Microcracks nucleated at the interfaces in the sandwich structure composed of LPSO and nanometer thick Mg slices could weaken the alloy at late stages of deformation, but their propagation could be limited within the individual kink band where the microcracks nucleated, which could ensure the capability of the alloy to resist premature or catastrophic fracture. Furthermore, lack of deformation twins in Mg grains effectively reduced the potential nucleation sites for cracks, which should be another reason for the good ductility of the alloy. These findings may provide or evoke insights into methods for optimizing the mechanical properties of Mg alloys.

Fractal analysis of the evolution of a fracture network in a granite outcrop, SE Korea

Kinematical and fractal analyses were conducted on a fracture network in a well-exposed granite outcrop in SE Korea. The objective of this study is to examine the temporal and spatial evolution of the fracture network. From the orientation and abutting relationships of fracture sets, six fracturing events and their relative ages were established, several of which included strike-slip reactivations of earlier formed fractures. These events may be correlated with the Cenozoic to Recent evolution of the Yangsan Fault and surrounding areas. 2-D box counting analyses were performed on maps of the fracture network at the six stages of its evolution. For the earliest event D = 1.11, and represents a clustered development of fractures. Event 2 leads to a large increase of D = 1.51, with subsequent events producing a gradual increase up to 1.55 after the final event. This stabilization of the fractal dimension involved a change from the widespread development of new fractures (event 2) to the greater importance of reactivation of existing fractures, with only localized development of new fractures, usually as tip and linkage damage around pre-existing fractures. Fracture density was greatest for the event 2 fractures and increased only gradually after that. This pattern is similar to that of the fractal dimension and a good linear correlation was found between these two parameters. Although fractal dimensions and density show little change during the late stages of deformation, the fracture connectivity continued to increase due to the formation of local secondary fractures developed during reactivation of earlier formed fractures.

New U–Pb and 40Ar/ 39Ar ages from the northern margin of the Barberton greenstone belt, South Africa: Implications for the formation of Mesoarchaean gold deposits

Gold mineralization in the Mesoarchaean New Consort gold mine in the Barberton greenstone belt, South Africa, is situated in the immediate hanging wall of an exhumed, composite granitoid gneiss dome known as the Stentor Pluton. The granitoid-greenstone contact is characterized by a contact-parallel high-strain zone that separates the mid-crustal gneisses from the greenschist facies greenstone belt. In contrast to the lower greenschist facies deposits in the central parts of the greenstone belt, the mineralized shear zones at New Consort are hosted by distinctly higher grade metamorphic rocks, ranging from upper greenschist to upper amphibolite facies conditions. A distinct change of ore and alteration assemblages with increasing metamorphic grade points to a temporal and spatial relationship between gold mineralization and the juxtaposition of the Stentor gneiss dome against the overlying greenstone belt. Examination of the structural and petrologic evolution of the mineralized high-strain zone, combined with new U–Pb and Ar–Ar data and previously published ages, indicates a multistage tectono-metamorphic evolution. Early high-T, medium-P metamorphism (ca. 600–700 °C and 5 ± 1 kbar) in the structurally deeper parts of the mine is interpreted to be related to an early period of orogen-parallel (NE–SW) extension and associated detachment shearing during the main (D 2) accretionary event in the centre of the greenstone belt, between ca. 3260 and 3225 Ma. Recent work has shown that these early high-T fabrics were reactivated at ca. 3105 Ma during the subsequent D 3 tectonism and associated emplacement of large potassic granite batholiths. The gold mineralization was coeval with NE–SW extensional exhumation and solid-state doming of the Stentor gneiss dome during the subsequent D 4 deformation. This late-stage deformation and the gold mineralization postdate the emplacement of the D 3-related potassic granites by more than 50 million years, as indicated by (1) a U–Pb age of 3046 ± 27 Ma for a retrograde titanite generation in a D 2-related amphibolite (sample 113), and (2) a 3027 ± 7.5 Ma age for hydrothermal titanite from the so-called footwall lens mineralization, which temperature has been estimated at 590 ± 40 °C. This high-T event is also recorded by a “corrected” 3040 ± 4 Ma hornblende Ar–Ar age from the same sample, suggesting that cooling of the Stentor gneiss dome shortly followed the exhumation. A considerably younger Ar–Ar age (2966 ± 4 Ma) has been obtained for an amphibolite from the contact-parallel high-strain zone, pointing to non-uniform cooling following the mineralization. The gold mineralization was largely contemporaneous with the emplacement of NNW trending, syn-kinematic granitic pegmatites and strain localization along the pegmatite swarm at ca. 3030–3040 Ma. The mineralization is also considerably younger than the gold mineralization in the greenschist facies gold deposits in the Barberton greenstone belt, which has previously been dated at ca. 3080 Ma. In conjunction with previously published ages, the data indicate the Au mineralization in the Barberton greenstone belt to have occurred during, at least, two mineralization episodes at ca. 3080 and 3040 Ma. The main phase of Au mineralization was introduced during the end of an either protracted or episodic structural and thermal evolution of the granitoid-greenstone terrain that may have lasted some 200 Ma.

Structure, timing and morphological signature of hidden reverse basement faults in the Fars Arc of the Zagros (Iran)

Abstract In the Zagros Fold–Thrust Belt (ZFTB) of Iran it is firmly established that the basement is involved in the deformation. The strongest line of evidence for this assertion comes from the relatively intense mid-crustal seismic activity. On one hand, the main basement structures such as the Main Zagros Fault (MZT) and High Zagros Fault (HZF) reach the surface and are therefore well identified. On the other hand, basement faults south of the HZF are hidden by sedimentary cover and their location is uncertain. In the Eastern Zagros, basement control on surface structures occurred only at a late stage of the tectonic evolution. In other words, the current thick-skinned style of Zagros deformation succeeded a more general thin-skinned phase of orogeny. This chronology is particularly well illustrated by spectacular interference patterns, in which early detachment folds are cut by late oblique basement faults. We present a combined morphological and structural analysis of such structures, and we explore their impact on the river network. We confirm that basement involvement occurred at a late stage of deformation and we show that thick-skinned deformation progressively propagated towards the foreland. An overview of basement steps throughout the Zagros based on published cross-sections allows us to conclude that although basement deformation is concentrated on two major faults in the Central Zagros, it is distributed on several segmented faults in the Fars Arc. This segmentation increases to the SE towards the so-called Oman line and the transition to the Makran accretionary prism.

Reply to comment on 'Tectono‐sedimentary evolution of lower to middle Miocene half‐graben basins related to an extensional detachment fault (western Crete, Greece)'

In their comment, van Hinsbergen et al. (2008) question the validity of the tectono-sedimentary model proposed by Seidel et al. (2007) for the Topolia type breccia deposits in western Crete. Apart from various sedimentological aspects that could probably be sorted out in the field, they argue that the proposed time brackets of c. 20–15 Ma for the formation of the Topolia breccia do not reflect the onset of extensional deformation in the overriding plate of the Hellenic subduction zone after collision. Instead, they propose that the onset of extensional deformation is reflected by the formation of a late Miocene (c. 12–11 Ma) sedimentary basin unconformably overlying the Topolia type breccia deposits. Notwithstanding the fact that the age of the breccia is otherwise not well constrained apart from being deposited prior to the exposure of the highpressure metamorphic units in western Crete, we do not concur with this view. Whatever van Hinsbergen et al. (2008) mean with terms syn-orogenic and post-orogenic in the present context, they claim that the onset of crustal scale, post-orogenic N–S extension is dated by the oldest Cretan supradetachment basin. They also state that this basin formed well after the deposition of the oldest Neogene sedimentary unit in western Crete – the Topolia breccia . First, it should be noted that Crete represents a narrow horst structure developed in the forearc of the active Hellenic subduction zone since the Pliocene. When dealing with crustal scale processes, the highly incomplete Neogene sedimentary record on the island is just one limited source of information. Moreover, the buoyant escape model proposed by Thomson et al. (1998, 1999) should not be misunderstood to invoke extrusion from a subduction channel , as quoted by van Hinsbergen et al. (2008), which may rather be a potential cause of the marked uplift of Crete since the Pliocene (Meier et al., 2007). In the buoyant escape model, the partly subducted microcontinent is proposed to return towards the Earth surface by filling the space created by continuous roll back as a more or less coherent slice, becoming exhumed beneath an extensional detachment fault (e.g. Fassoulas et al., 1994; Kilias et al., 1994; Jolivet et al., 1996). This process probably causes extensional deformation in the hangingwall to the detachment, which is represented by the unmetamorphic upper units on Crete; extensional deformation continues because of roll back into the present. Any record of normal faulting in the upper plate to the extensional detachment does not allow an unequivocal distinction to be made between the buoyant escape stage (Thomson et al., 1999) and a subsequent stage of more coaxial crustal extension (Thomson et al., 1999; Rahl et al., 2005), stages possibly referred to as syn-orogenic and post-orogenic by van Hinsbergen et al. (2008). The structural record of the high-pressure metamorphic Phyllite–Quartzite Unit shows an episode of intense extensional deformation in the brittle field (e.g. Seidel et al., 2005), which took place between c. 20 and 15 Ma, according to fission track thermochronometry (Thomson et al., 1998, 1999) and at a depth of less than about 10 km (Kuster and Stockhert, 1997). This extensional deformation therefore happened after the major part of rapid exhumation from more than 30 km depth, when the high-pressure metamorphic Phyllite–Quartzite Unit already resided in the upper crust. We suspect that this marked extensional deformation of the Phyllite–Quartzite Unit should have had dramatic effects also in the higher levels of the upper crust and consequently at the surface. We therefore envisage that the formation of fault scarps, which are required to produce the fault-bound proximal breccia deposits with a minimum thickness of several hundred meters, preserved in western Crete, could be contemporaneous with the buoyant escape stage as well as with this later stage of extensional deformation. The breccia deposits, as all other units of the upper plate to the extensional detachment, were juxtaposed with the high-pressure metamorphic Phyllite–Quartzite Unit along normal faults during ongoing extension. In our simple structural model (Seidel et al., 2007), we propose that the normal fault presently bounding the Topolia breccia towards the south could have evolved by progressive deformation (or by repeated reactivation) from the fault along which the breccia was deposited on the hangingwall block, the rocks of the Phyllite– Quartzite unit forming the footwall block being juxtaposed with the breccia at a late stage of deformation. The proposed half-graben geometry corresponds to that observed in other extensional tectonic settings (e.g. Davis and Coney, 1979; Wernicke, 1981; Davis et al., 1986) with a more complete structural record and is motivated by the clast size and roundness trends observed in the Topolia area. We feel that the interpretation in terms of a major half-graben bounding fault with progressive exhumation of the footwall block is consistent with Correspondence: Dr Markus Seidel, Am Lindbruch 29, D-41470 Neuss, Germany. E-mail: markus.seidel@gmx.net

In situ TEM straining of single crystal Au films on polyimide: Change of deformation mechanisms at the nanoscale

In situ transmission electron microscopy straining experiments were performed on 40, 60, 80 and 160 nm thick single crystalline Au films on polyimide substrates. A transition in deformation mechanisms was observed with decreasing film thickness: the 160 nm thick film deforms predominantly by perfect dislocations while thinner films deform mainly by partial dislocations separated by stacking faults. In contrast to the 160 nm thick film, interfacial dislocation segments are rarely laid down by threading dislocations for the thinner films. At the late stages of deformation in the thicker Au films prior to fracture, dislocations start to glide on the (0 0 1) planes (cube-glide) near the interface with the polymer substrate. The impact of size-dependent dislocation mechanisms on thin film plasticity is addressed.

On the Serrated Behavior during Plastic Deformation of a Zr-Based Bulk Metallic Glass

The uniaxial compression tests were conducted on an amorphous Zr-based alloy at room temperature. The results show that this alloy exhibited the typical features of metallic glasses and the stress-strain curve is extensively serrated within the plastic regime. The analysis reveals that for all the shear band events, the maximum stresses remained approximately constant during the course of plastic deformation, and the minimum stresses began to decrease when the deformation enters the late stage. This phenomenon is probably associated with the different shear-banding behaviors within various deformation stages. Calculations find that the stress accumulation sections in the stress-strain curve are parallel to each other, indicating that such reloading processes are elastic in nature. Therefore, there are elastic and plastic sections in the stress-strain curve. Further analysis discloses that the plastic strain component carried by each shear band event increases with the applied strain within the late deformation stage.

Enhanced deformation mechanisms by anisotropic plasticity in polycrystalline Mg alloys at room temperature

This article presents room-temperature deformation mechanisms in polycrystalline Mg alloys. Dislocation slip of basal 〈a〉 and prismatic 〈a〉 types are shown to occur nearly at the same ease when the basal planes are tilted in such a way that the Schmid-factor ratio (equivalent to the critically resolved shear stress (CRSS) ratio) of prismatic 〈a〉 to basal 〈a〉 slip is larger than a value ranging from 1.5 to 2.0, depending on the initial texture distribution and grain size. Grain-boundary sliding (GBS) also occurs at room temperature up to 8 pct of total strain, enhanced by plastic anisotropy as well as by the increasing number of grain-boundary dislocations. Twinning plays an important role in both flow and fracture behaviors. Twins are induced mostly by stress concentrations caused by the anisotropic nature of dislocation slip. Twins can be classified into two types based on their shape: a wide lenticular type and a narrow banded type. The wide twins are $$\{ 10\bar 12\} $$ twins appearing in the early stage of deformation and accompany little change of surface height. The narrow twins are $$\{ 10\bar 11\} $$ or $$\{ 30\bar 32\} $$ appearing in the late stage of deformation and accompany a substantial change in surface height. The formation of the narrow twins seems to give rise to highly localized shear deformation within the twin, leading to strain incompatibility and to final failure.

Structural and metamorphic evolution of the Camughera – Moncucco, Antrona and Monte Rosa units southwest of the Simplon line,Western Alps

This structural and petrological study examines relationships between Alpine deformation and metamorphism in the Camughera-Moncucco, Antrona and northeastern Monte Rosa units, and it correlates major late stage deformation structures, such as the Vanzone antiform and Simplon normal fault. D1/D2 deformation and related top-N or top-NW thrusting started under high-pressure conditions (12.5–16 kbar) at relatively high temperatures (c. 620–700°C). Petrological and structural data, together with published radiometric data, suggest that top-SE shearing within the structural top of the high-pressure units in the upper Penninic Alps is coeval with top-N or top-NW thrusting at their structural base. This suggests differential ascent of high-pressure units relative to the surrounding units during nappe stacking and associated crustal shortening. Barrovian metamorphism in the upper Penninic Alps is related to a first phase of backfolding, active between c. 35 Ma and c. 29-26 Ma ago (D3). D3 involves dextral shearing, combined with top-WSW shearing and orogen-parallel extension. Unroofing by orogen-parallel extension delays cooling during decompression and leads to isothermal decompression after the high-pressure stage. Towards deeper structural levels in the east, D3 deformation becomes progressively younger and prevailed at increasingly higher temperatures. Hence, compared to the higher structural levels further west, the brittle-ductile transition occurs later in the Ossola valley. Masera synform and Brevettola antiform form an open fold pair and represent the eastern continuation of the major Vanzone antiform (D4). Normal faulting across the Simplon line overlaps in time with the formation of these major D4 backfolds. Displacement along the Simplon normal fault decreases to insignificant values towards the southeast. Contemporaneous dextral shearing, however, occurs within the southern limbs of the Vanzone and Brevettola antiforms.

A kinematic evolutionary model for the Penninic sector of the central Ligurian Alps

The nappe pile presently cropping out in the central sector of the Ligurian Alps, is represented by some principal groups of tectonic units. Starting from the foreland, the outer and lower, weakly metamorphic (up to 0.3 GPa) Brianconnais units support the high-pressure (up to 1.3 GPa) ensemble of inner Brianconnais nappes, in turn overridden by the Prepiedmont units, sourced from the European continental margin. Prepiedmont units form two superposed groups. The lower is composed only of a pre-Namurian basement (Alpine metamorphism up to 0.6 GPa); and the upper is mainly composed of a slightly metamorphic (greenschist facies) post-Namurian cover. At the top lie the high-pressure metamorphosed (up to 0.8 GPa in the sector here considered) ophiolitic units. The group of the non-metamorphic Helminthoid Flysch nappes (original stratigraphic cover of the ophiolitic units) has travelled the greatest distance and is presently mainly set onto the outer part of the chain. Only events up to the stacking of the nappe pile are discussed, disregarding late-stage deformation. As the examined sector is located at a considerable distance from the collisional zone, late processes did not change the overall order of superposition formerly acquired. The model proposes the development of two major, subhorizontal detachment surfaces. The first, shallower one confines at the base a very thin-skinned set of nappes, nearly totally made up of Prepiedmont sedimentary covers that are bounded at their top by the Helminthoid Flysch units. Both these groups underwent a mainly horizontal outwards transport. In contrast, the underlying Prepiedmont crust and the adjoining Brianconnais inner sector (separated by the second, deeper major detachment surface) were progressively dragged into the subduction zone under the ophiolitic units and duplexes were generated. Exhumation of the metamorphic units occurred along the subduction channel, as did stacking of the nappe pile.

Late tectonic evolution of the Northern Apennines: the role of contractional tectonics in the exhumation of the tuscan units

Structural analysis performed in the continental tuscan units of the Northern Apennines (Italy) highlight the presence of transversal and parallel structures, with respect to the main trend of the belt (NW-SE), affecting the tectonic nappe pile. These structures, following the main synmetamorphic phases associated to isoclinal folds with axial plane foliations, are upright folds, brittle and brittle-ductile shear zones. Nearly upright folds are related to NW-SE and NE-SW directions of shortening. The two perpendicular directions of shortening, associated with a contraction setting, induced the vertical growth of the metamorphic domes enhancing the process of exhumation of the metamorphites. The presence of later NE-verging brittle-ductile shear zones crosscutting both systems of folds testifies that contraction long lasted during the orogenesis. Fluid inclusion analyses, performed in syntectonic veins coeval with NW-SE trending upright folds, point out that, after the first tectonic phase, pressure values decrease supporting that this tectonic unit was exhuming during the development of upright folds. Post-collisional extensional tectonics gave rise to collapse folds and low- and high-angle normal faults in the latest stages of deformation.

Deformation in Moffat Shale detachment zones in the western part of the Scottish Southern Uplands

A study of the décollement zones in the Moffat Shale Group in the Ordovician Northern Belt of the Southern Uplands of Scotland reveals a progressive sequence of deformation and increased channelization of fluid flow. The study concentrates on exposures of imbricated Moffat Shale on the western coast of the Rhins of Galloway. Initial deformation occurred in partially lithified sediments and involved stratal disruption and shearing of the shales. Deformation then became more localized in narrower fault zones characterized by polyphase hydrothermal fluid flow/veining events. Deformation continued after vein formation, resulting in the development of low-temperature crystal plastic microstructures and further veining. Late-stage deformation is recorded as a pressure solution event possibly reflecting the cessation of slip on these faults as the slice became accreted. Most deformation can be ascribed to SE-directed thrusting and incorporation of the individual sheets into the Southern Uplands thrust stack. Later sinistral shear deformation, not observed in overlying turbidites, is also localized in these fault zones. The study reveals the likely structures formed at levels of an accretionary prism deforming under diagenetic to low-grade metamorphic conditions.

Structural analysis of faults related to a heterogeneous stress history: reconstruction of a dismembered gold deposit, Stawell, western Lachlan Fold Belt, Australia

In the internationally significant Victorian goldfields a complex system of faults dismembers the 5 million ounce Magdala gold deposit. These faults represent a combination of neoformed faults and inherited faults that reflect deformation associated with stress tensors of variable orientation and stress shape ratio ( φ). The fault geometry is strongly controlled by the pre-existing rheology. Faults have propagated around the flanks of an antiformal basalt dome, along earlier ductile cleavages and the margins of porphyry dykes. Many of the faults do not have Andersonian geometries and there is no correlation between the orientation of the faults and the palaeostress directions. Much of the faulting is associated with the emplacement of porphyry dykes, additional gold mineralisation related to plutonism and late-stage deformation post-dating the intrusion of the Stawell pluton. Systematic mapping of extension veins associated with faults, striations and conjugate joint sets allowed the construction of a revised and more robust history of brittle deformation. This successfully predicted the offset direction of the currently mined Magdala ore body beneath the studied system of faults. The use of extension veins was a critical aspect of the analysis. If striations on the fault surfaces had solely been used, the offset direction of the new Golden Gift orebody would not have been correctly ascertained. The palaeostress history was delineated via use of compression and tension dihedra, stress inversion of slip data and calculation of theoretical resolved shear stress for faults with orientations similar to those mapped. The calculation of theoretical resolved shear stress directions highlights the importance that the intermediate stress has on the slip direction for faults whose pole does not lie in the plane containing σ 1 and σ 3.

Development of extension faults on the oblique thrust ramp hanging wall: example from the Tethys Himalaya

Inhomogeneous strain at late stages of deformation of modelling clay models with frontal and oblique thrust ramp geometries results in formation of extension faults on the hanging wall. The extension faults trend nearly parallel to the axis of maximum compression and orthogonal to the trend of the frontal ramp. Faults with similar geometries occur in the Upper Satluj River Basin of the Tethys Himachal Himalaya. The experimental results help in understanding the present day normal faulting and related active tectonics of the area.

A unified kinematic model for the evolution of detachment folds

Detachment folds represent a major structural element in a number of fold belts. They are common in the Jura Mountains, the Zagros fold belt, the Central Appalachian fold belt, the Wyoming fold-belt, the Brooks Range, the Parry Islands fold belt, and parts of the SubAndean belt. These structures form in stratigraphic packages with high competency contrasts among units. The competent upper units exhibit parallel fold geometries, whereas the weak lower unit displays disharmonic folding and significant penetrative deformation. Two distinct geometric types, disharmonic detachment folds, and lift-off folds have been recognized. However, these structures commonly represent different stages in the progressive evolution of detachment folds. The structures first form by symmetric or asymmetric folding, with the fold wavelength controlled by the thickness of the dominant units. Volumetric constraints require sinking of units in the synclines, and movement of the ductile unit from the synclines to the anticlines. Continuing deformation results in increasing fold amplitudes and tighter geometries resulting from both limb segment rotation and hinge migration. Initially, limb rotation occurs primarily by flexural slip folding, but in the late stages of deformation, the rotation may involve significant internal deformation of units between locked hinges. The folds eventually assume tight isoclinal geometries resembling lift-off folds. Variations in the geometry of detachment fold geometry, such as fold asymmetry, significant faulting, and fold associated with multiple detachments, are related to variations in the mechanical stratigraphy and pre-existing structure.

Ascent and emplacement of buoyant magma bodies in brittle‐ductile upper crust

The emplacement of silicic magma bodies in the upper crust may be controlled by density (such that there is no buoyancy to drive further ascent) or temperature (such that surrounding rocks are too cold to deform significantly over geological timescales). Evidence for the latter control is provided by negative gravity anomalies over many granitic plutons. Conditions of diapir ascent and emplacement in this case are studied with a numerical model for deformation and heat transport allowing for ductile, elastic and brittle behavior. A large‐strain formulation is used to solve for temperature, stress, strain, and strain rate fields as a function of time for a range of diapir sizes, density contrasts, and background geotherms. The method allows for large viscosity contrasts of more than 6 orders of magnitude and determines the dominant deformation mechanism depending on the local values of temperature, strain, and strain rate. Emplacement depth and final deformation characteristics depend on diapir size and buoyancy. Small diapirs (less than about 5 km in diameter) cannot reach shallow crustal levels and do not involve brittle deformation. In the ductile regime the diapir flattens significantly upon emplacement due to stiff roof rocks and to the free surface above. Late stage deformation proceeds by horizontal spreading, with little upward displacement of roof rocks and is likely to be interpreted as “ballooning.” Large diapirs (more than about 5 km in diameter) rapidly rise to shallow depths (1–5 km) and induce brittle faulting in the overlying rocks. In this regime, buoyancy forces may lead to faulting in roof rocks. In this case, late stage ascent proceeds by vertical intrusion of a plug of smaller horizontal dimensions than the main body. Buoyant diapirs keep on rising after solidification, long after the relatively short‐lived high‐temperature magmatic stage. This may account for some phases of late caldera resurgence in extinct volcanic systems.

GEOLOGIA E GEOQUÍMICA DAS ROCHAS METAULTRAMÁFICAS DA ANTIFORME CAPANÉ, SUÍTE METAMÓRFICA PORONGOS, RS

The metaultramafic rocks from the Capané Antiform were mapped in detail and their field, petrographic and geochemical characteristics are described. They consist of serpentinites with and without relic texture, subordinated talc schists, chlorite schists, and antophyllite-bearing schists. These rocks are tectonically hosted by a supracrustal sequence consisting of felsic to mafic calkalcaline to tholeiitic metavolcanic and metassedimentary rocks. Major, trace and rare earth element geochemistry suggest that the metaultramafic rocks are a basal part of a disrupted ophiolite. This is consistent with the subduction environment suggested by the metavolcanic rocks and explains both the tectonic emplacement of the serpentinites and the associated rodingites and deeply metassomatized mafic rocks. The mylonitic deformation created two superimposed foliations (S 1 and S 2 ) gently folded during the late deformation stages. The association of antophyllite+olivine in relics of S 1 point to a metamorphic peak in the medium amphibolite facies, under a pressure lower than 6 kbar, while the transformation of antophyllite to clorite in relics of S 2 resulted from retrograde metamorphism.