Second Stage Of Deformation Research Articles

Inversion of extensional basins parallel and oblique to their boundaries: inferences from analogue models and field observations from the Dolomites Indenter, European eastern Southern Alps

Abstract. Polyphase deformation of continental crust is analysed through physical analogue models for settings wherein platform–basin geometries at passive continental margins are subject to subsequent shortening and orogenesis. In a first stage, segmentation of the brittle and brittle–ductile models into basins and platforms is achieved by extension. Basins are partly filled with brittle material to allow for a strength difference between basin and platform realms, simulating relatively weaker, incompetent deposits of grabens surrounded by competent pre-rift basement or carbonate platform rock, respectively. In a second stage of deformation, contraction parallel and oblique (10 to 20∘) to the basin axes has been applied, leading to the inversion of basins formed earlier. The experiments show that strength contrasts across platform–basin transitions control the localisation and overall style of compressional deformation, irrespective of the nature of the basal décollement (frictional versus viscous), the rheology of the basin fill, or changing platform–basin thickness ratios. Orientations of thrust faults change laterally across inherited platform–basin transitions throughout all experiments; higher obliquity of basin inversion leads to stronger alignment of thrust curvature with the orientation of pre-existing rift axes. At individual thrust faults, variations in the strike of thrust fronts are accompanied by changes in the shortening direction during incremental phases of deformation. Reactivation of normal faults occurs in oblique basin inversion settings only, favourably at platform–basin transitions where the normal faults face the shortening direction. The amount and style of fault reactivation depend on the material used. Our experiments are relevant for natural cases such as the Dolomites Indenter of the eastern Southern Alps, underlining the importance of inherited geologic features for the subsequent shortening geometries. Field structural data from the western segment of the Belluno thrust of the Valsugana fault system support predicted variations of thrust fault orientation and a lateral change in shortening direction (from SSW to SSE along-strike) along one single fault. Based on our modelling results, we suggest that this variability of thrust fault orientation and shortening directions, controlled by inherited structures, is consistent with strain partitioning during a single phase of deformation and does not necessarily reflect different deformation phases.

Study on intermediate annealing process of thermal mechanical treatment of 7185 alloy with two stage deformation

This study investigated the effect of intermediate annealing (IA) during a special thermo-mechanical treatment on the microstructure and properties of 7185 Al alloy. The results show that after IA, many sub-structures are formed in the alloy and many particles are precipitated. The particle size increases and the width of the dislocation wall becomes narrower with increasing IA time and temperature. After the second stage of deformation, the dislocations are ordered again and generate micro shear bands (MSBs). The final T6 treated alloy under IA2 process (holding time for 40 min at 400 °C) has good plasticity and corrosion resistance. The tensile strength is 523 MPa, the yield strength is 458 MPa, the elongation is 17.6%, the depth of intergranular corrosion layer is 15.6 μm, and the corrosion grade after 6 and 12 h exfoliation corrosion is N, PC.

Effect of a special thermo-mechanical treatment process on microstructure and properties of 7185 alloy

A special thermo-mechanical treatment (ST) process involving two stages of deformation was studied, and the influence mechanism for the first stage of deformation (pre-deformation) on the microstructure evolution and properties of 7185 alloy was researched in detail. The results showed that increasing the amount of pre-deformation could increase the number of spherical precipitates, significantly increase the dislocation density, and form dislocation walls and dislocation cells. The medium-temperature precipitation (MTP) could transform the high-density dislocation cells into ordered dislocation arrays and form many substructures and sub-crystals. After the second stage of deformation (final hot rolling, FHR), the number of precipitates increased again. The sub-crystals were regularly arranged and microscopic shear bands formed. With a weakening of the pinning force, the boundary of the banded microstructure composed of high-density dislocations became straighter. After solution treatment, the movement of low angle grain boundaries (LAGBs) and sub-grain boundaries was blocked due to particle pinning and accumulated strain, which significantly promoted the transition from LAGBs to high angle grain boundaries, thus achieving grain refinement. Therefore, the maximum pre-deformation process (ST3: pre-deformation (250 °C, 60%) + MTP (400 °C, 40 min) + FHR (400 °C, 50%)) could significantly improve the plasticity and corrosion resistance of 7185 alloys.

Partial Auxeticity of Laterally Compressed Carbon Nanotube Bundles

Carbon nanotubes (CNTs) have attracted increasing attention because of their enormous potential in various technologies. Herein, the evolution of the structure and elastic properties of a CNT bundle under compression in uniaxial and biaxial regimes is analyzed using a chain model with a reduced number of degrees of freedom. The compression stress–strain curves consist of four stages, each of which is characterized by a specific structure and deformation mechanism. In the first stage, all CNTs have the same cross section; in the second stage, the translational symmetry is preserved in the system, but with a doubled translational cell; in the third stage, CNT collapse takes place, leading to the loss of the translational symmetry; the fourth stage begins when all CNTs collapse. Elastic constants are calculated for the CNT bundle under uniaxial and biaxial compression during the first two stages. In all loading schemes, during the second stage of deformation, the CNT bundle exhibits partial auxetic properties. The results obtained contribute to the fundamental knowledge for the design of carbon nanomaterials with enhanced properties.

Experimental investigation of the permanent deformation and void ratio attenuation of porous asphalt mixtures with hydraulic effects

To investigate the quantitative influence of the material gradation characteristics, saturation degree and loading frequency on the permanent deformation of porous asphalt (PA) mixtures under temperature, loading, and hydraulic coupling conditions, a hydraulic coupling repeated loading test is conducted for five types of PA mixtures with different nominal maximum aggregate sizes, void ratios, saturation degrees and load frequency inputs. The test results indicate that the PA-20 mixture has better anti-deformation performance than the PA-13 and PA-16 mixtures. A larger void ratio corresponds to weaker anti-permanent deformation performance. When the initial void ratio is 24%, the flow number prominently decreases to approximately less than 11% of the initial void ratio of 16%. The results of the loading test with water storage show that the saturation state has a more significant impact on the permanent deformation of PA-13 mixtures than PA-16 and PA-20. When the frequency increases, the cumulative microstrain of the mixture greatly decreases, and the influence degree of frequency on the deformation becomes increasingly obvious when the void ratio increases due to the effect of hydraulic coupling. To easily evaluate the void ratio attenuation of PA, a uniform expression is proposed to characterize the three-stage strain growth, which shows that the permanent deformation curve exhibits nonlinear growth in the second stage of deformation. Then, the minimum permanent deformation growth rate is presented to quantitatively evaluate the deformation resistance performance. Overall, a higher saturation degree of the lower loading frequency corresponds to a smaller PDGRmin. A formula is presented to analyse the effect of permanent deformation on the change in void ratio: the loading frequency, saturation degree and initial void ratio significantly affect the attenuation of the AV ratio. Specifically, the coupling of frequency and water storage slows the decrease in air void ratio.

Tectonic evolution of the hinterland of the Zagros Orogen revealed from the deformation of the Golpaygan Metamorphic Complex, Iran

In the southwestern part of the Central Iran zone, the Golpaygan Metamorphic Complex (GMC) was evolved during the Zagros orogeny. This complex is composed of different metamorphic rocks including schist, marble, slate, gneiss and amphibolite. The metamorphic complex is overlain by a cover sequence including Permian carbonates, Jurassic clastic rocks, Cretaceous carbonates and Eocene clastic rocks. The internal structure of the GMC is characterized by four deformational stages. The first stage (D1) developed contemporaneously with the prograde regional metamorphism and resulted in formation of the first generation folds and prevailing axial plane foliation. The second stage of deformation (D2) was accompanied by deformation partitioning and formation of contractional shear zones. These structures were overprinted by different scale folds of the third stage (D3). The fourth stage of deformation (D4) is characterized by generation of the extensional shear zones which gradually transformed into normal faults. D1 and accompanied metamorphism may have resulted from steep subduction of the Neotethys Ocean beneath the active margin of the Central Iran zone during Jurassic times. D2–D3 structures and retrograde metamorphism may have been formed by a regional shortening and transpression deformation event which resulted in exposure of the metamorphic rocks in shallower levels during Cretaceous flat slab subduction. This condition lasted until Paleocene time when the slab roll-back caused extension and thinning of the overriding Central Iran plate. The subsequent decompression and regional extension resulted in intrusion of magmatic rocks and generation of D4 which formed the GMC and related magmatism in Paleocene-Eocene time.

Microstructure, forming limit diagram, and strain distribution of pre-strained DP-IF steel tailor–welded blank for auto body application

In the present study, tailor-welded blanks (TWBs) of dissimilar material combination were fabricated by laser welding of interstitial-free (IF) and dual-phase (DP) steels using 2.4-kW power and 4 m/min scan speed. Subsequently, TWBs of as-received sheet materials and IF steels were pre-strained up to 20% major strain in the deformed specimens through an equi-biaxial pre-straining setup. It was found that highly non-uniform strain distribution with nearly plane strain deformation mode was induced in the pre-strained TWBs, whereas an equi-biaxial strain was recorded for IF monolithic blank. Microhardness profiles and the effect of weld zone on the microstructural and mechanical properties of the as-received and pre-strained TWBs were studied. Further, the forming limit diagrams (e-FLDs) of as-received TWB and IF steel were experimentally evaluated. The e-FLD of pre-strained TWBs was experimentally determined, and e-FLD of the pre-strained IF material was estimated using the Yld89 anisotropy plasticity model with the Hollomon hardening law. Subsequently, all these respective e-FLDs were implemented as damage models in the FE simulations for predicting the limiting dome height (LDH) of as-received and pre-strained TWBs. It was observed that the error in LDH prediction of pre-strained TWB domes was within 9.1% when the estimated e-FLD of the pre-strained IF material was used as a damage model. The FE-predicted strain distributions and weld line movements of TWBs after the second stage of deformation were also successfully validated with the experimental data.

Structural doming tectonics: intracontinental deformation of the SW margin of the Yangtze Block, China

The southwest margin of the Yangtze Block underwent multiple tectonic events including the collision between the Yangtze and the South China Plate, Caledonian folding and uplift of South China, subduction of the western Pacific Plate, and Cenozoic continental escape during the Indo–Asia collision. The structural framework and tectonic evolution of the area are currently unclear. Through field petrologic and structural studies, microstructures within the deformed units, and estimates of the temperature of deformation, we constrain the conditions of deformation and further classify at least four-stage deformation. The first stage involved northeastward thrusting that produced folds, mylonites, and dynamically recrystallized quartz and calcite. Electron backscatter diffraction data indicate that deformation during this event occurred at < 400 °C, corresponding to a depth of ~ 10 km. The second stage of deformation involved uplift, extension, and the generation of a domal structure. During the third event, high-angle normal faults formed within the cover sequence that surrounds the dome. The fourth stage of deformation occurred in the Cenozoic and involved motion along sinistral and dextral strike–slip faults, which cut structures that formed during the previous stages of deformation. The first deformation event was related to Triassic N–S collision with the Indochina block; the second and third stages resulted from far-field compression related to westward subduction of the paleo-Pacific plate; and the fourth event was related to displacement along the Ailaoshan Red-River strike–slip fault. The dome in the study area represents a transitional structure between a metamorphic core complex and a gneissic dome, which we refer to as structural doming.

Structural characterization of the Katebasu gold deposit, Xinjiang, China: Tectonic correlation with the amalgamation of the western Tianshan

The western Tianshan orogen is one of the world’s most important gold provinces. It hosts numerous world-class gold deposits along the southern and northern margins of the Middle Tianshan. Most deposits occur within large-scale fault zones and/or ductile-brittle deformation zones that were formed during the Paleozoic evolution of the Paleo-Asian Ocean, thereby indicating a correlation between orogenic tectonics and the gold mineralization. However, the correlation is often poorly constrained and remains controversially discussed. The Katebasu gold deposit in Xinjiang, China, is located on the northern margin of Middle Tianshan and is hosted by secondary fault zones to the south of the North Nalati Fault. The main period of gold mineralization is suggested to be the Late Carboniferous, which is broadly coeval with the late Paleozoic amalgamation of the western Tianshan. Therefore, the Katebasu gold deposit provides an excellent opportunity to search for possible correlations between structural processes at a deposit scale and orogenic processes at a regional scale to better understand the tectonic controls on large-scale gold mineralization in the western Tianshan. In this paper, we report detailed structural mapping and field investigations on the fault geometries and kinematics of the Katebasu gold deposit that permit inversion of the tectonic stress field. Our structural data, combined with results of previous studies of the regional tectonics, allow us to reconstruct the structural history of the Katebasu gold deposit, to look for possible correlations with the orogenic processes in the western Tianshan and to determine their control on gold mineralization. The first stage of deformation (D1) was top-to-the-north thrusting that was caused by a roughly N–S compression and is represented by ENE–WSW-striking thrust faults. D1 was likely related to the closure of the South Tianshan Ocean and the resulting collision between the Tarim craton and Middle Tianshan–Yili block. The second stage of deformation (D2) was dextral strike-slip motion along the ENE–WSW-striking faults that was caused by the WNW–ESE stress field and that corresponds to Permian dextral wrenching in the Tianshan orogen. Subsequent NE–SW compression led to the third deformation event (D3) that mainly formed NW–SE-striking reverse faults and NNE–SSW-striking dextral faults.

Necking and fracture limit analyses of different pre-strained sheet materials in polar effective plastic strain locus using Yld2000-2d yield model

This paper aims to estimate and analyze both the necking and fracture based path-independent forming limit criteria during two-stage forming of anisotropic sheet materials. In this context, in-house experimental setups were fabricated to impart pre-strain along three distinct strain paths viz. equi-biaxial, plane strain and uniaxial in different automotive grade sheet materials namely EDD, AA5052 and DP600. Uniaxial tensile and stack compression tests were conducted, and the test results were used to evaluate the eight anisotropic coefficients of Yld2000-2d yield model. The predicted evolutions of the yield loci and both the strength and plastic strain ratio (r-value) directionalities of the pre-strained materials were calibrated with experimental data. In the second stage of deformation, all pre-strained sheet materials were stretch formed up to the onset of necking and further until the initiation of fracture. Correspondingly, the forming limit diagram (ε-FLD) and fracture forming limit diagram (ε-FFLD) were experimentally evaluated. The necking and fracture limit of as-received materials was theoretically predicted incorporating calibrated Yld2000-2d yield model in the geometrical imperfection based Marciniak-Kuczynski (MK) model and ductile fracture based Bao-Wierzbicki (BW) damage model, respectively. It is noteworthy that the dynamic shifting of pre-strained ε-FLDs and ε-FFLDs has been recorded in principal strain space (ε1, ε2) depending on the type and amount of pre-strain. Hence, the polar effective plastic strain (PEPS) based necking (PEPS-FLD) and fracture (PEPS-FFLD) limits were estimated using Yld2000-2d anisotropy plasticity theory, and it was observed that the shift of the limiting strains was restricted successfully. It was found that the maximum absolute error during necking and fracture dome height prediction of pre-strained materials was restricted up to 5% and 6%, respectively, when PEPS based failure limits were used as damage model in FE simulation.

Relation between thermal effects and structural changes under deformation of thermoplastics

The article reports thermal effects accompanying deformation of widely used thermoplastics: PE of different density, highly isotactic and stereodefective PP, PA6, PC. A number of structural methods (X-ray diffraction at large and small angles, DSC, IR spectroscopy, microscopy, volume measurements) were used to establish the factors underlying generation of heat under tension. In all the polymers studied, the greatest temperature and structural changes are observed in the second stage of deformation, when the neck is being formed. For polyolefins the temperature jump at the transition to the neck was found to correlate with the degree of crystallinity. Analysis of structural data showed that for these polymers the most important heat sources are the following structural processes: decrease in entropy of polymer chains due to orientation, appearance and growth of cavities, strain-induced crystallization of a part of amorphous phase.In the PEs, studied in this work, the crystallographic modification does not change under plastic deformation. In highly isotactic PP a part of initial α-crystals lose three-dimensional ordering and transform into the mesomorphic form during fragmentation of lamellae. In PA6 necking triggers transition from a less ordered γ-phase to a more stable α-modification, which proceeds in parallel with the crystallization of a part of the amorphous phase in the α-form. When the glassy amorphous PC is deformed, phase transformations, crystallization and pore formation do not occur. In this polymer, processes of internal friction are supposed to be the main cause of heat generation during deformation.

Intra-continental transpression and gneiss doming in an obliquely convergent regime in SE Asia

The Tengchong terrane comprises a sequence of linear dome-like zones, cored by granite and migmatitic layers. These cores are mantled by predominantly gneiss and subordinate schist sequences, decreasing in deformation intensity outward from extensive mylonitization to weak mylonitization. The pre-doming deformation was characterized by the formation of large-scale top-to-the-east shearing (D1) in the gneiss terrane, locally preserved flat-lying foliation (S1), weak folding (F1) and emplacement of the Mangbang granite during the Cretaceous (114-104 Ma). The second stage of deformation (D2) consisted of map-scale east-verging folds (F2, dome amplification) and minor lateral strike-slip shear zones between the anticlines in the gneiss and migmatitic sequences. Extensive partial melting and emplacement of 67-30 Ma synkinematic granitoid bodies/veins occurred, leading to the emplacement of wedges of granite into the easterly directed F2 fold cores. These wedges formed kilometer-scale granitoid domes. The post-doming D3 deformation with transpression recorded strain partitioning with simple shear-dominated high-strain zones along the Gaoligong and Nabang dextral lateral strike-slip shear zones (active during 30-11 Ma). Late transtensional deformation (D4) during cooling of the entire terrane involved the localized low-temperature Gaoligong west and east detachment faults that controlled the late exhumation of the Gaoligong metamorphic zone (since 10 Ma). Our structural observations, combined with previous studies, suggest that this style of doming is a representative type of intra-continental deformation in the Cenozoic during the oblique India-Asia collision. The actual dome shapes reflect formation of antiforms during compression-dominated transpression, prior to localized strike-slip shearing, in the accommodation belt around the Eastern Himalayan Syntaxis. Vertical exhumation of crustal material by contractional doming played an important role in absorbing the vast majority of the internal deformation of crustal fragments during oblique collision.

Structural controls on carbonate-hosted Pb–Zn mineralization in the Dongmozhazhua deposit, central Tibet

Fault zones control the locations of many ore deposits, but the ore-forming processes in such fault zones are poorly understood. We have studied the deformation and ore textures associated with fault zones that controlled the lead–zinc mineralization of the Dongmozhazhua deposit, central Tibet, ∼100km southwest of Yushu City. Geological mapping shows that the structural framework of the Dongmozhazhua area is defined by NW–SE-trending reverse faults and superposed folds that indicate at least two stages of deformation. The first stage is characterized by tight nearly E–W-striking folds that formed during the closure of the Jinshajiang Paleo-Tethyan Ocean in the Triassic. The second stage of deformation produced NW–SE-trending reverse faults and related structures of the Fenghuoshan–Nangqian fold-and-thrust belt associated with India–Asia collision in the late Eocene to Oligocene. Scanline surveys along the ore-controlling fault zones show an internal structure that comprises a damage zone, a breccia zone with clasts that have become rounded, and a breccia zone with lenticular clasts, and this complex architecture was formed during at least two compressional substages of deformation. The Pb–Zn mineralization in the Dongmozhazhua area occurs exclusively close to NW–SE-trending reverse fault zones. Microtextural observations reveal that mineralization occurred as veinlets and disseminated blebs in limestone clasts, and as continuous bands and cements in fractured rocks. Cataclastic sulfide grains also can be seen in the matrix of some fault zones. The types of mineralization differ with structural position. The fillings of the ore-bearing veinlets typify the products of hydraulic fracture and both types of mineralization took place concurrently with regional contraction. We consider, therefore, that the ore-bearing fluids in the Dongmozhazhua deposit were concentrated in fault zones during regional compression and that the ore minerals were precipitated during hydraulic fracturing of host rocks. Subsequent fault activity pulverized some pre-existing sulfide material into cataclastic grains in the matrix of a tectonic breccia that developed in the same faults.

Разработка научных основ и оптимизация технологических режимов нового способа протяжки литых слитков

The paper considers an idea of realization of alternating deformation in forging cast ingots and determines a rational shape of forging tools. The results of the analysis of the strained state of metal during compression in heads with a profiled surface and in smooth heads (at the second stage of deformation) are presented. The use of new tools provides the formation the metal structure in first passes by means of additional alternating deformation without a significant change of the cross-section shape of the billet. Alternating deformation increases the accumulated strain without changing the cross-section shape of the billet, stimulates the formation of the polygonized subgranular structure and refines the grain structure. Results of the strained state analysis of metal during forging in profiled and flat heads are presented. The degree of the accumulated strain in particles in the deformation zone is calculated for six points along the height of the section of the head hollow in order to estimate strain inhomogeneity in the ingot height. The accumulated strain for a specific particle is determined from the problem solution for a flat deformed state under isothermal conditions by means of Deform software. In layers adjacent to the contact zone the strain is minimal since there are areas of hindered deformation (sticking areas), the size of these areas decreases towards the depth of the metal; at the beginning of plastic deformation a free surface appears which is not under the working tool. Ways to control the technological process of forging are found for increasing homogeneity of strained state in the ingot section and stimulating the formation of the polygonized substructure during compression and the ultradisperse structure after metadynamic recrystallization during a pause.

Paleoproterozoic granulites from the Xinghe graphite mine, North China Craton: Geology, zircon U–Pb geochronology and implications for the timing of deformation, mineralization and metamorphism

The Xinghe graphite deposit is located in the north-central part of the North China Craton (NCC) where Archean gray gneiss, metasedimentary rocks (khondalites) and mafic (high-pressure) granulites constitute the dominant rock types. In this study, we report results of detailed geological and structural mapping around the Xinghe graphite mine. We identify three stages of deformation (D1–D3) and the graphite mineralization is correlated with D1 or between D1 and D2. The first stage of deformation represented by rootless intrafolial folds whose axial surface cuts across the regional penetrative gneissosity (S2) of D2, was constrained at ca. 1.95–1.90Ga by U–Pb age of zircon grains from a mafic high-pressure granulite. The second stage of deformation is represented by tight folds with formation of regional penetrative foliation and strong ductile shearing, when garnet broke down to plagioclase+pyroxene. This stage is dated as ca 1.85Ga, from U–Pb analysis of zircons in leucosome from a migmatitic gneiss. The third stage of deformation is represented by large scale folding, which probably formed between 1.85 and 1.80Ga, as constrained from the ca. 1.80Ga emplacement age of an undeformed pegmatite that cuts across the host metasediments. Prior to the high-pressure granulite metamorphism, voluminous ca. 2.15–2.0Ga granites were emplaced within the khondalites. The graphitization postdates the emplacement of these granites as deduced from the presence of flaky graphites in the metagranites. The protolith of the mafic high-pressure granulite dike that intruded the khondalite at ca. 2.0Ga was also metamorphosed at 1.95–1.90Ga, and is part of the exploited graphite ore body. We therefore constrain the timing of graphite mineralization to be between ca. 1.95 and 1.85Ga. Our geological and geochronological data indicate that all the rock units in the Xinghe graphite mine were subjected to high-pressure granulite facies metamorphism at ca. 1.95–1.90Ga.

Metadynamic recrystallization behavior of AZ61 magnesium alloy

Metadynamic recrystallization (MDRX) behavior of AZ61 magnesium alloy and its effects on flow behavior and microstructure evolution have been investigated in this study. Towards this end, a set of double-hit hot compression tests was conducted under strain rate of 0.1s−1 at 400°C. To differentiate the static and metadynamic recrystallization dominant strain regions, the first stage of deformation was carried out up to the different pre-strains with a constant inter-pass annealing time of 200s. The results indicated that the MDRX is predominant recrystallization mechanism where the pre-strains are higher than 0.35. Furthermore, to investigate the influence of MDRX on subsequent flow behavior and the related microstructure, an elaborated inter-pass annealing treatment was executed employing a range of inter-pass annealing time (2–500s). The results show that the progress of MDRX leads to an increase in the flow stress as well as the rate of work hardening encountered in the subsequent deformation. Additionally, the microstructural examinations confirm that the observed hardening phenomenon is a consequence of grain growth evolved from MDRX and its direct effect on the onset of dynamic recrystallization at the second stage of deformation.

Weakening processes in thrust faults: insights from the Monte Perdido thrust fault (southern Pyrenees, Spain)

AbstractThe Monte Perdido thrust fault (southern Pyrenees) consists of a 6‐m‐thick interval of intensely deformed clay‐bearing rocks. The fault zone is affected by a pervasive pressure solution seam and numerous shear surfaces. Calcite extensional‐shear veins are present along the shear surfaces. The angular relationships between the two structures indicate that shear surfaces developed at a high angle (70°) to the local principal maximum stress axis σ1. Two main stages of deformation are present. The first stage corresponds to the development of calcite shear veins by a combination of shear surface reactivation and extensional mode I rupture. The second stage of deformation corresponds to chlorite precipitation along the previously reactivated shear surfaces. The pore fluid factor λ computed for the two deformation episodes indicates high fluid pressures during the Monte Perdido thrust activity. During the first stage of deformation, the reactivation of the shear surface was facilitated by a suprahydrostatic fluid pressure with a pore fluid factor λ equal to 0.89. For the second stage, the fluid pressure remained still high (with a λ value ranging between 0.77 and 0.84) even with the presence of weak chlorite along the shear surfaces. Furthermore, evidence of hydrostatic fluid pressure during calcite cement precipitation supports that incremental shear surface reactivations are correlated with cyclic fluid pressure fluctuations consistent with a fault‐valve model.

Geometry and folding history of the Baraboo Syncline: Implications for the Mazatzal Orogeny in the north-central U.S.

The Baraboo Syncline in south-central Wisconsin formed during the Mazatzal Orogeny. It is a doubly plunging, asymmetric syncline. Internal structures, such as chocolate-tablet style boudinage and tension gashes, indicate stretching parallel to the fold axis, and preclude folding by plane-strain models. Both the fold geometry and the internal deformation within the fold limbs suggest a history of (1) buckling that varied along the fold axis resulting in doubly plunging folds and stretching along the fold axis followed by (2) continued buckling combined with top-to-the-south simple shearing that may have varied along the fold axis. The second stage of deformation could be the result of the onset of an overriding thrust package. The deformation history is consistent with a typical south-verging fold forming in a fold and thrust belt that inherited its geometry from a north-dipping subduction zone.

Duplex at the lateral tip of a thrust fault: the “La Cagalière” example (NE Pyrenees, France)

A duplex located at the lateral tip of a thrust fault is identified and this paper aims to understand why and how such a structure developed at this particular place. The “La Cagalière” duplex is a well-exposed example of a second-order structure developed firstly as an “autochthonous roof duplex” at the front of the Lagrasse ramp-related fold (NE Pyrenees, France). During a second stage of deformation, an out-of-sequence thrusting allows the emplacement of the overturned forelimb of Lagrasse fold as an allochthonous roof above the duplex. Using geometric analysis, magnetic fabric and structural measurements, it is shown that the “La Cagalière” duplex developed as a response to the progressive inhibition of the propagation of the Lagrasse fold frontal thrust. This study provides the first natural example of a duplex acting as a “stopping structure” at a thrust tip.

Thrust tectonics in the North Patagonian Massif (Argentina): Implications for a Patagonia plate

In the northeastern segment of the North Patagonian Massif (southern Argentina), S to SW directed thrusting affected a Late Proterozoic to Cambrian phyllite succession and the Silurian to Lower Devonian Sierra Grande formation. Thrust tectonics was partly combined with the formation of mylonites and followed by the intrusion of the Navarrete granodiorite. Mylonites of the Tardugno granitoids are attributed to a ductile shear horizon in the subsurface. As second stage of deformation after cessation of thrusting, ∼NW‐SE compression led to the formation of widely distributed fold structures around NE‐SW trending axes as well as SE directed reverse faults. It was followed by the late kinematic intrusion of a porphyric granite as final pulse of the Navarrete Intrusive Complex. Local folding around ∼N‐S to NW‐SE trending axes represents the final stage of the compressive deformational history. Thrust tectonics plus subsequent stages of deformation can be assigned to the Late Paleozoic time interval, probably the Permian. They are seen in conjunction with the formation of granite mylonites within the North Patagonian Massif, the Cerro de los Viejos granite mylonite of the La Pampa Province, and the compressive structures in the Sierras Australes fold‐and‐thrust belt north of the boundary between Patagonia and Gondwana South America. Also, the lack of Early Paleozoic (Famatinian) deformation in this part of the North Patagonian Massif and the inferred boundary of the Early Paleozoic Cuyania (Precordillera) Terrane north of Patagonia support the interpretation that extra‐Andean Patagonia collided with Gondwana South America during the Gondwanide orogeny. It is possible that Patagonia was part of a plate and that the long Gondwanide fold belt represented its collision zone.