Extensional Crenulation Cleavage Research Articles

The Early Mesozoic NE–SW Extensional Model and Exhumation Processes at the Southeastern Margin of the Central Asian Orogenic Belt: Insights from the Strain and Kinematic Vorticity Analysis of the Sonid Zuoqi Ductile Detachment Zone

AbstractThe Sonid Zuoqi ductile detachment zone is located at the southeastern margin of the Central Asian orogenic belt (CAOB), striking EW and dipping to the S. The major rock type of the Sonid Zuoqi ductile detachment zone is mylonite derived from granite. The sequence of mylonite features is: (1) S and C foliations of mylonite, and (2) extensional crenulation cleavage (ecc) or C and the kinematic vorticity (Wk) value changed from 0.70 to 0.95 and from 0.37 to 0.69, respectively; the strain type of the mylonites within the Sonid Zuoqi ductile detachment zone is compressional to planar strain. The strong deformation mylonite and Halatu plutons yielded a zircon U‐Pb age of 244 Ma and a zircon (U‐Th)/He age of 214 Ma, respectively. Based on the strain and kinematic vorticity analysis, together with the zircon U‐Pb and zircon (U‐Th)/He ages and the regional tectonic background, the study area experienced three stage evolution: tangential simple‐shear (244 Ma), simple‐shear‐dominated general shear represented by upper crustal extension (224 Ma) and pure–shear–dominated general shear represented by the Halatu pluton doming (214 Ma), which constrained the early Mesozoic NE–SW crustal extension at the southeastern margin of the CAOB. This NE–SW extension probably originated from the post‐orogenic extensional collapse of the CAOB, subsequent exhumation being controlled by the far afield effects of the closure of the Mongol–Okhotsk belt.

Crustal Melting Evidence in Migmatites of Higher Himalayan Crystallines (HHC) along Bhagirathi, Dhauliganga Valleys, and Sikkim Himalaya, India

Abstract An attempt has been made to evaluate crustal melting evidence through textural studies within a narrow zone of migmatite present in NW Himalaya along Bhagirathi and Dhauliganga valleys and two zones close to MCT and throughout within in the Sikkim Himalayas, NE Himalaya. It appears that partial melting was initiated by muscovite dehydration melting with a positive volume change driving melt segregation and discontinuous crystallization of peritectic biotite in the leucosome. Further, during retrogression due to cooling, a certain amount of melt was consumed. The occurrence of isolated pseudomorphosed melt pockets and lack of euhedral magmatic flow textured feldspar further indicates that the melt fraction was low for the alignment of crystals. The processes of melt segregation and migration could have been limited. The migmatite leucosomes and a small volume of in situ tourmaline-bearing leucogranite along extensional crenulation cleavages and melt pods indicate water-saturated melting of pelitic metasedimentary rocks. The formation of migmatites happened at around 46 Ma, corresponding to a peak metamorphic event due to collisional tectonics of the Himalayan orogeny. The presence of feeder dikes for main tourmaline-bearing leucogranite indicates that the source for the main body could be migmatite which is also supported by the similarity in REE patterns of the main body and in situ tourmaline-bearing leucogranite.

Behavior of extensional crenulation cleavage (C' foliation) during progressive deformation and its effect on shear zone foliation (S foliation)

伸展褶劈理（extensional crenulation cleavage，简称C’面理）作为韧性剪切带内常见的构造面理之一，在递进变形过程中是否旋转以及其如何影响剪切带面理（S面理）是尚未解决的构造地质前沿问题。本文统计了不同剪切带中C’面理、S面理和C面理（糜棱岩面理）的夹角。结果发现，从剪切带边缘向中心随着应变逐渐增大，C’面理与剪切平面（C面理）的夹角（α）呈近正态单峰式分布，表明C’面理在递进变形过程中不旋转。在C’面理发育的情况下，S面理与C面理的夹角（β）随着应变的增加逐渐增大，即S面理在C’面理形成后发生反向旋转，因此原用于计算剪应变的公式：tan 2β=2/γ失效。强弱互层材料的应变分配结果显示，一般剪切条件下，粘性分层与剪切平面的夹角（β）减小到20°时，强硬层的运动学涡度接近于0，即表现为纯剪切，而软弱层表现为近简单剪切；β小于10°时，强硬层发生反向旋转。由此可见，伸展褶劈理是不均匀岩层在递进变形过程中应变分配（strain partitioning）的结果。随着递进变形的进行，S面理与C面理的夹角逐渐减小，强硬层被分配以更多的纯剪切而布丁化，随后内部产生微型剪切条带并扩展形成C’面理，承担应变分配的简单剪切。至此，随着应变分配的完成，C面理上的剪切作用自行停止，因此C’面理不发生旋转，而S面理在C’面理的制约下开始反向旋转。;Extensional crenulation cleavages (C’ foliations) are common structural foliations in ductile shear zones. It is still confusing that the behavior and effects of C’ foliations on shear zone foliations (S foliations) during the progressive deformation. This paper collected and analyzed degrees of the included angles between C’ foliation, S foliation and C foliation (mylonite foliation) in different shear zones. The results show that the degrees of the angle (α) between the C’ foliation and the shear plane (C foliation) presents a nearly normal unimodal distribution with the strain increasing from the edge to the center of the shear zone, which indicates that the C’ foliations do not rotate during progressive deformation. When there are C’ foliations, the angle (β) between S and C foliation becomes larger with the increasing of strain. As a result, S foliation rotates backwards (anticlockwise in dextral shear or clockwise in sinistral shear) after the C’ foliations developed, and the formula used to calculate shear strain: tan 2β=2/γ is invalid. There is a strain partitioning in the layered materials. During general shear, when the angle (β) between the viscous layer and the shear plane decreases to 20°, the kinematic vorticity of layers with high viscosity is close to 0, that is, pure shear, while layers with low viscosity play simple shear. When β is less than 10°, layers with high viscosity rotate away from the shear plane. Thus, extensional crenulation cleavages may result from the strain partitioning during the progressive deformation of heterogeneous layered strata. During the progressive deformation, the angle between the S foliation and the shear plane becomes smaller, and layers with high viscosity are distributed with most pure shear and boudinage, and then the micro shear bands in S foliations expand to C’ foliations. At this moment, the strain is completely repartitioned. The simple shear is partitioned into the slip along C’ foliations, and C’ foliations will not rotate during the progressive deformation. S foliations begin to rotate away from the shear plane because of the slip along the C’ foliations.

Extensional Structures in the Ophiolites from Cuba and China: New Constraints to Slowly‐Spreading Ancient Oceanic Lithosphere

AbstractA group of low‐angle normal faults developed in banded gabbro of Moa Ophiolite, Cuba. The dark gabbro was cut into puddings by several normal faults, while light gabbro was just swelling in layer thickness. In Hongliuhe ophiolite at eastern segment of South Tien Shan Suture Zone in China, the extensional deformation concentrates on fine cumulus gabbro which is typically mylonitized. Abundant structural features were discovered in HLH ophiolite such as S‐C foliation, C’ foliation, extensional crenulation cleavage, small toughness normal fault, low‐angle normal faults and high‐angle normal faults. According to the above tectonic phenomenon from the ophiolite belts in Cuba and China, we will get the conclusion: the maximum principal compressive stress (b1) is vertical to cumulus bedding, and the maximum tensile stress (b3) is paralleling to cumulus bedding. Considering of the above evidence, the extensional tectonic event should developed at mid‐ocean ridge. Due to seafloor spreading, the maximum tensile stress is paralleling to cumulus layer, and extensional tectonic is kept in cumulus gabbro. In this way, normal faults developed in dark gabbro, while brittle‐ductile extensional developed in light gabbro. A large number of domes, folds paralleling to ocean ridge and detachment faults represented by low angle normal fault were discovered near ocean ridge in Indian Ocean and Atlantic Ocean. In this way, materials from deep oceanic lithosphere (e.g. gabbro, mantle peridotite) outcrop at the crust surface of ocean basin. The above evidences from China and Cuba are consistent with extensional tectonic and metamorphic core complex from slowly and super‐slowly spreading Indian Ocean and Atlantic Oceanic lithosphere based on ODP. Therefore, extensional deformation in the ophiolite belt is of significant meaning for clarifying the formation process and mechanism of ancient oceanic basin.

The characteristics of the structure, strain and kinematic vorticity of the Wulian detachment fault zone, Shandong Peninsula, China

As one of the vital tectonic units of the Wulian Metamorphic Core Complex (MCC), the Wulian detachment fault zone (WDFZ), which developed in the Jiaodong peninsula, separates the lower plate of the ultrahigh pressure (UHP) metamorphic rocks in the Sulu orogenic belt from the upper plate of the early Cretaceous Zhucheng basin and the basin basement. The fault zone generally strikes NNE with a dip in the west along the southern portion of the MCC and strikes NE with a dip in the WNW along the northern portion. The fault zone displays a wavy-tile-shaped extension on the plane, principally composed of the fault breccias and mylonite and transits downward to the mylonitic gneiss As a whole, the detachment fault zone shows a top-to-west or a WNW extension. By calculating the harmonic mean, we obtain a Flynn index K of 0.98–2.0, and the mean value is approximately 1.35 in the fault zone. According to the polar Mohr construction, the extensional crenulation cleavage, the RS/θ, and the quartz C-axial fabric methods, we acquire mean kinematic vorticity values of 0.64–0.97, 0.76–0.93, 0.6–0.92, and 0.63–0.98 with mean values of 0.83, 0.80, 0.78 and 0.86, respectively, for mylonite and promylonite. The strain measurement results and the kinematic vorticity values indicate that the WDFZ is a normal ductile shear zone developed in the extensional setting. The kinematic track shows that the kinematic vorticity value decreases gradually from the NW to the SE as a whole. A simple shear dominates in the middle and upper parts of the shear zone, which is reflected by a higher vorticity value (>0.75, up to 0.98), a low thinning rate and a lower K value. In contrast, toward the footwall, the pure shear is increased significantly, showing a lower vorticity value (<0.70, low to 0.64), a relatively high thinning rate and a higher K value. Combined with the geotectonic background, the development and evolution of the WDFZ should respond to lithospheric thinning and the destruction of the North China Craton (NCC). As a result, the WDFZ can be defined as a thinning normal shear zone developed in the extension tectonic setting and the combined result of the simple shear caused by the crust extension and pure shear led by the rapid uplift of the footwall and magmatic upwelling.

Formation of Late Jurassic–Early Cretaceous metamorphic core complexes in Northeast China: insight from a structural study of the Yiwulüshan ductile detachment zone

ABSTRACTLate Jurassic to Early Cretaceous extensional structures are widespread in Northeast China and the eastern part of the Asian continent. The Yiwulüshan ductile detachment shear zone, the master ductile detachment zone of the Yiwulüshan metamorphic core complex (MCC), occurs in the northern North China Craton. This shear zone dips to the northwest on the western flank and to the southeast on the eastern flank of the MCC and consists of brittle faults, breccia, chloritized microbreccia, and ductile (mylonite) zones. The well-developed mylonitic foliation and stretching lineation are characterized by oriented minerals (quartz and feldspar) that exhibit ductile deformation, including undulose extinction, deformation bands, subgrain formations, and recrystallized grains. The shear indicators, such as asymmetric porphyroclasts, S-C fabrics, and shear bands, suggest consistent top-to-the-NW shear on both the western and eastern flanks of the MCC. Strain measurements using feldspar porphyroclasts show that the strain intensity (Es), Lode’s parameter (υ), and Flinn parameter (k) range from 0.46 to 1.03, −0.23 to 0.62, and 0.23 to 1.6, respectively. Kinematic vorticity (Wk) values, estimated by the Polar Mohr diagram and the C′ methods, range from 0.61 to 0.96, with an average of 0.80, for the mylonitic deformation, and from 0.24 to 0.53, with an average of 0.37, for extensional crenulation cleavage (ecc) domains. The kinematic vorticity values, together with the regional structural background, suggest the Yiwulüshan ductile detachment zone experienced simple-shear-dominated general shearing that formed the mylonitic fabric in the early stages of extension. Later in the deformation history, pure-shear-dominated general shearing dominated when ecc developed. Strain measurements and kinematic vorticity estimates indicate that the Yiwulüshan ductile detachment zone is a lengthening-thinning shear zone. It is proposed that simple-shear-dominated general shear developed during crustal extension and that pure-shear-dominated general shear is related to the upward flow of Precambrian basement and Jurassic plutons.

Fluid inclusion and sulfur and oxygen isotope studies on quartz-carbonate-sulfide veins of the Carvoaria Velha deposit, Córrego do Sítio gold lineament, Quadrilátero Ferrífero, Minas Gerais, Brazil

The Córrego do Sítio lineament is defined as a 16-km long, NE-SW-trending ductile shear zones, which controls fourteen gold deposits, including the Carvoaria Velha deposit. The dominant lithotypes of this deposit are metagrewackes with subordinate carbonaceous phyllites lenses of the Archean Rio das Velhas greenstone belt, which host gold mineralization. Metamafic dikes and sills are parallel and crosscut the host metasedimentary sequence. All these rocks exhibit alteration to quartz, carbonate and sericite, besides sulfides and sulfosalts. The main gold mineralization styles at the Carvoaria Velha deposit, and at Córrego do Sítio as a whole, are quartz-carbonate-sulfide±sulfosalt veins of varied distribution. The veins are classified as: V1 veins – quartz-ankerite-pyrite-berthierite-gold – parallel to the main regional foliation Sn,; V2 veins – quartz-ankerite-pyrite – developed at extensional crenulation cleavage Sn+1, and rarely gold mineralized; V3 veins – quartz-ankerite – filling Sn+3 fractures, usually free of sulfides and sulfosalts; V4 veins – quartz-calcite – of restricted occurrence in metamafic dikes and sills. The latter crosscut the metasedimentary sequence, are extensional and display no preferential orientation. The most common ore minerals in all vein types are arsenopyrite, pyrite, berthierite, and pyrrhotite. Microprobe analyses reveal the presence of metallic phases rich in Sb, Pb and Co, such as stibnite, ullmanite, tetrahedrite, galena, cobaltite, which commonly overgrow the sulfides. Fluid inclusion studies trapped in quartz from V1, V2 and V4 veins have identified a H2O-CO2±CH4-NaCl mineralizing fluid that may contain KCl, MgCl2 e FeCl2. The presence of CH4 in fluid inclusions of the V1 and V2 veins reflect interaction of the fluid with the Córrego do Sítio and Santa Quitéria carbon-bearing metapelitic host units.Based on the microthermometric data, the hydrothermal fluid is interpreted to have evolved in at least two stages: i) an early fluid stage, trapped in smoky quartz, of moderate salinity (~8.5eq. wt% NaCl), and minimal trapping temperature of 330±17°C; and ii) a late-stage fluid trapped in recrystallized quartz with lower salinity (~4.6eq. wt% NaCl), and a minimal trapping temperature of 365±33°C. Isotopic values of δ18Ofluid (+7.9 and +13.0‰), Δ33S (+0.3 to +3.5‰) and δ34S (−2.9 to +6.1‰) suggest that indeed the hydrothermal fluid responsible for the gold mineralization at the Carvoaria Velha deposit must have had a metamorphic origin, and interacted with metasedimentary sequences during its ascending path. The addition of CH4 during fluid-rock interaction may have caused some decrease in ƒO2 of the fluid which, as a consequence, destabilized gold-bearing sulfur complexes, liberating S−2 for the formation of Fe sulfides and sulfosalts (arsenopyrite, pyrite, pyrrothite and berthierite, etc.), and outcome gold deposition.

Strain, Kinematic Vorticity and Ductile Thinning along the Detachment Zone of the Yunmeng Shan Metamorphic Core Complex, Beijing, China

Abstract:The Yunmeng Shan metamorphic core complex (MCC) is composed of the lower plate, the upper plate and the detachment zone. The detachment zone consists of ductile shear zone (mylonite zone), chloritized microbreccias zone and the brittle fault plane. The ductile shear zone contains mylonitic rocks, protomylonites, and mylonites. Finite strain measurements of feldspar porphyroclasts from those rocks using the Rf/φ method show that the strain intensities increase from mylonitic rocks (Es=0.66–0.72) to protomylonites (Es=0.66–0.83), and to mylonites (Es=0.71–1.2). The strain type is close to flatten strain. Kinematic vorticity estimated by Polar Mohr diagrams suggest that foliations and lineation of mylonite (0.47&lt;Wk&lt;0.85) record a bulk simple‐dominated general shearing at the initial evolution stage of the Yunmeng Shan MCC's detachment zone; and the extensional crenulation cleavage(ecc) (0.34&lt;Wk&lt;0.77) recorded a bulk pure‐dominated general shearing at the later stage of the evolution. Kinematic vorticity measurements also show that the Yunmeng Shan MCC detachment zone is a result of a combination of simple‐dominated general shearing caused by crustal extension at the early stage and pure‐dominated general shearing caused by MCC uplifting at the late stage. The ductile thinning estimated by finite strain measurements and estimation of Kinematic vorticity ranges from 52% to 82%, which is the minimum thining estimation. Our studies provide new evidence for mechanisms of the Yunmeng Shan MCC detachement zone.

Analysis of thrust shear zones in curve-shaped belts: Deformation mode and timing of the Olevano-Antrodoco-Sibillini thrust (Central/Northern Apennines of Italy)

This work reports the results of our analysis of the brittle-ductile shear zone associated with the Olevano-Antrodoco-Sibillini (OAS) curved-shape thrust in the Central/Northern Apennines of Italy. Its southern sector is characterized by NNW–SSE trending footwall anticlines, which also affect the OAS thrust surface. S tectonites in the NNE–SSW trending sector are crenulated by conjugate extensional surfaces (extensional crenulation cleavage). The NW–SE trending segment of the OAS thrust is characterized by SC tectonites developed along the thrust-related shear zone. Both the SC and S tectonites are consistent with a top-to-the N60–70°E direction of tectonic transport.The SC tectonites are associated with a simple-shear-dominated deformation (Wn = 0.86–0.98). On the other hand, the S tectonites and their related extensional shear planes document two stages of strain: (i) sub-simple shear (Wn = 0.34–0.50, related to OAS thrusting contemporaneous to growing incipient footwall anticlines, and (ii) pure-shear-dominated deformation (Wn = 0.17–0.00), subsequent to the OAS thrusting and caused by the definitive growth of footwall anticlines within an in-sequence deformation context.The present study proposes the analysis of shear zones to: (i) discriminate in-sequence against out-of-sequence evolution, and (ii) use as a tool to constrain the modes and timing of the curved belt's development.

Pattern and kinematic polarity of late Mesozoic extension in continental NE Asia: Perspectives from metamorphic core complexes

Late Mesozoic extension in NE Asia resulted in the development of a large extensional province. Metamorphic core complexes (MCCs) are the major features in this province and have 40Ar/39Ar ages of 130–110 Ma for the mylonites and U‐Pb zircon ages of 150–110 Ma for the integral granitic intrusions. Based on this and previous studies, this paper summarizes major characteristics of these MCCs and recognizes a regional kinematic shear sense. Most MCCs in the Transbaikalia region, Sino‐Mongolia border tract, and the northwest‐central portion of the North China craton (NCC) show a top‐to‐the‐southeast (SE) shear, whereas those in the eastern and southern NCC locally underwent top‐to‐the‐northwest (NW) shear. The three largest basins (Songliao, Huabei and Ordos) in North China are located in the transitional zone between domains of opposing shear sense. We interpret the extension in the Transbaikalia, Sino‐Mongolia tract and northwestern part of the NCC to reflect late‐orogenic collapse of thickened crust following Middle‐Late Jurassic collision along the Okhotsk suture. The southeastward extension is probably controlled by crustal‐scale top‐to‐the‐SE tangential shear. The transition from contraction to extension is marked by detachment faults that nucleated as extensional crenulation cleavage (ecc, i.e., C′) in sub‐horizontal ductile shear zones late in orogenic crustal thickening. The combined effect of gravitational loading and thermal‐uplifting is considered to be the origin of the late‐or post‐orogenic collapse. The top‐to‐the‐NW extension in the NE of the NCC might reflect antithetic sub‐extensional zones or Mesozoic back‐arc extension as a far‐field effect of Cretaceous Pacific plate subduction.

Shear band formation and strain localization on a regional scale: Evidence from anisotropic rocks below a major detachment (Betic Cordilleras, Spain)

Regional-scale deformation taking place in a strongly anisotropic, yet homogeneous metapelitic protolith during an apparently single tectonic event was systematically investigated as a function of the distance to the main tectonic contact (i.e., the Filabres shear zone, a major detachment in the Betic Cordilleras, Spain). The density of C 3′ shear bands (or extensional crenulation cleavage) reworking the earlier S2 schistosity increases exponentially towards the contact, in parallel with the decrease in the size of the shear domains. Systematic variations in angles and shape ratios are also reported. Deformation and age patterns, however, suggest that this spectacular trend at least partly results from a progressive localization of the deformation through time. This fossilized shear strain gradient was thus produced somewhat diachronously. Such shear strain patterns nevertheless provide a mean to constrain the rheological properties for such weak lithologies and a mean to better understand crustal deformation.

Strain and kinematic vorticity analysis of the Louzidian low-angle ductile shear detachment zone in Chifeng, Inner Mongolia, China

The Louzidian low-angle ductile shear detachment zone at the south of Chifeng is a SE-dipping, low-angle normal fault system. It is composed mainly of ductile shear zone, ductile-brittle shear zone and brittle fault zone. The ductile shear zone consists of, from bottom to top, mylonitic rocks, protomylonites and mylonites. Finite strain measurement of feldspar strain markers from those rocks using the R f /ϕ method shows that strain intensities (Es) of the mylonite at core of the ductile shear zone (Es=0.65–0.96) are higher than those of the mylonitic rocks close to the granite intrusions (Es=0.59–0.62) and of the protomylonites at top of the ductile shear zone (Es= 0.47–0.70), and the strain types of the protomylonites and mylonties are elongate strain and plane-flattening strain, respectively. The kinematic vorticity values (W k ) estimated by the Polar Mohr diagram and the Rigid Grain Net range from 0.81 to 0.90 with an average of 0.85 for the protomylonites, and from 0.53 to 0.80 with 0.66 on average for the mylonites; Wk values of the extensional crenulation cleavage, i.e., C′, estimated by C′ method range from 0.63 to 0.37 with an average of 0.50. The angles between the maximum principal stress and shearing direction determined using the Maximum effective moment criterion evolved from 61° to 69° and to 75°, and finally normal to shearing direction. The results of strain and kinematic vorticity measurements suggest that high strain corresponds to low kinematic vorticity. Kinematic vorticity measurements show that the Louzidian low-angle ductile shear detachment zone is a result of a combination of simpledominated general shearing at the early stage and pure-dominated general shearing at the late stage. All these, together with isotope geochronology and regional tectonic background, suggest that the Louzidian ductile shear detachment zone resulted from a combination of crust extension and magma intrusion. The model of simple shear at the early stage and pure shear at the late stage in the formation of metamorphic core complex has probably general significance.

Structural evolution of different tectonic units across the Austroalpine–Penninic boundary in the middle Orco Valley (Western Italian Alps)

A structural study along the Orco Valley, which represents a geological section through the Alpine belt (western Italy), is reported. A Lower Tectonic Element, composed of units with eclogite facies relics (Gran Paradiso and Zermatt–Saas-like Unit) and an Upper Tectonic Element, made of greenschist facies units (Combin-like Unit and Gneiss Minuti Complex of the Sesia–Lanzo Zone) have been distinguished. These two elements show a different main Alpine tectono-metamorphic event (D 1) and are separated by a mylonitic contact (D 2) developed under greenschist facies conditions (Orco Shear Zone). This regional scale shear zone was responsible for the exhumation of the deeper eclogitic units, and has been inferred as the continuation of the Combin Fault south of the Aosta Valley. From D 3 onward, the same tectono-metamorphic evolution was recorded in all units. The following extensional crenulation cleavage (D 4) occurs mostly along a NNE–SSW deformation zone about 1 km thick at the base of the Upper Tectonic Element which leads to a further exhumation of the Lower Tectonic Element.

A importância da tectônica transcorrente no alojamento de granitos pré a sincolisionais na região do vale do médio Rio Doce: o exemplo das suítes graníticas Galiléia e Urucum

The Galileia and Urucum suites, respectivelly, with 594 ± 6 Ma and 584 ± 2 Ma zircon U-Pb ages, are situated in the middle region of the Rio Doce Valley, eastern of Minas Gerais State. They are characterized by elongated batholiths in the NW-SE and N-S trending associated with high angle shear zone of Conselho Pena-Resplendor. The batholiths present solid-state and magmatic flow foliations and a well-developed magmatic lineation. Two deformation phases were described: the first was responsible by the development of a solid-state foliation and magmatic flow lineation in the granitic suites and a schistosity and stretching lineation in the country rocks (Sao Tome schist), and the second was responsible by an extensional crenulation cleavage and other associated structures. Kinematic analysis indicates that the Galileia and Urucum suites were emplaced along of transcurrent structures.

Geometry and kinematics of a landslide surface in tertiary clays from the Duero Basin (Spain)

The Peñalba roto-translational landslide is one of the large landslides that can be observed in the central area of the Duero Basin (Spain). Its present morphology is a semicircular-shaped depression in which more than 6 m of vertical movement have been measured. The erosion of the banks of the Duero River affecting the slope of the Peñalba Hill has incised vertical cliffs where a slip surface outcrops and can be observed. Currently, the incision level of the Duero River is located 22 m below this surface. The slip surface, on the translational zone within a large roto-translational landslide, developed within the Dueñas Facies in a single bed with a high PI (39.0–47.8) and lower carbonate contents, would have behaved as low-shear strength surfaces ( ϕ′ R = 18°–21.8°). At Peñalba, the slip surface and the deformation structures related to it have been exceptionally well preserved. The structures observed are similar to those usually described in shear zones of tectonic origin: an S–C fabric, related to progressive simple shear, and C′ planes, called extensional crenulation cleavage. These similarities suggest that analogous kinematic processes would have taken place in completely different geodynamic environments. In the present work, we have adopted a structural geological–kinematic approach to explain the development of the features in this roto-translational landslide.

The Maximum Effective Moment Criterion (MEMC) and Its Implications in Structural Geology

Abstract The Mohr-Coulomb criterion has been widely used to explain formation of fractures. However, it fails to explain large strain deformation that widely occurs in nature. There is presently a new theory, the MEMC, which is mathematically expressed as Meff = (σ1 — σ3) L · sin 2α sin α)/2, where σ1-σ3 represents the yield strength of the related rock, L is a unit length and α is the angle between σ1 and deformation bands. This criterion demonstrates that the maximum value appears at angles of ±54.7° to σ1 and there is a slight difference in the moment in the range of 55°±10°. The range covers the whole observations available from nature and experiments. Its major implications include: (1) it can be used to determine the stress state when the related deformation features formed; (2) it provides a new approach to determine the Wk of the related ductile shear zone if only the ratio of the vorticity and strain rate remains fixed; (3) It can be used to explain (a) the obtuse angle in the contraction direction of conjugate kink-bands and extensional crenulation cleavages, (b) formation of low-angle normal faults and high-angle reverse faults, (c) lozenge ductile shear zones in basement terranes, (d) some crocodile structures in seismic profiles and (e) detachment folds in foreland basins.

Extensional collapse in the Neoproterozoic Araçuaí orogen, eastern Brazil: a setting for reactivation of asymmetric crenulation cleavage

The Araçuaí orogen of eastern Brazil is one of many Brasiliano/Pan African orogens formed during the Neoproterozoic assembly of Gondwana. Its western edge, bordering the São Francisco craton, is the Serra do Espinhaço fold-thrust belt, in which top-up-to-the-west (reverse-sense) faults, west-verging folds (F 1), and east-dipping spaced to phyllitic cleavage (S 1) developed. We have found that the kinematics of deformation changes markedly at the hinterland margin of this fold-thrust belt. Here, beneath a plateau known as the Chapada Acauã, metadiamictite and fine-grained pelitic schist comprise an east-dipping belt that contains an assemblage of structures indicative of top-down-to-the-east (normal-sense) shear. This assemblage includes a cascade of F 2 folds that refold F 1 folds and verge down the dip of the belt's enveloping surfaces, vertical tension gashes, and top-down-to-the-east rotated clasts. Based on the presence of these structures, we propose that the plateau exposes a regional-scale normal-sense shear zone, here called the Chapada Acauã shear zone (CASZ). Because F 2 folds refold F 1 folds, normal-sense shear in the CASZ occurred subsequent to initial west-verging thrusting. Considering this timing of motion in the CASZ, we suggest that the zone accommodated displacement of the internal zone of the Araçuaí orogen down, relative to its foreland fold-thrust belt, and thus played a role in extensional collapse of the orogen. The CASZ trends parallel to preserved thrusts to the west, and thus may represent an inverted thrust fault. Notably, throughout the CASZ, S 1 schistosity has been overprinted by a pervasive, west-dipping asymmetric crenulation cleavage (S 2). The sigmoid shape of S 1 surfaces in S 2 microlithons require that slip on each S 2 surface was top-down-to-the-west. S 2 cleavage is axial-planar to the down-dip verging F 2 folds. Based on its geometry, we suggest that S 2 cleavage initiated either as an antithetic extensional crenulation cleavage during reverse-sense shear, or as a near vertical asymmetric crenulation cleavage formed during east–west shortening of a preexisting east-dipping schistosity. Subsequent normal-sense shear in the CASZ reactivated this cleavage, causing clockwise rotation of S 2 domains (as viewed looking along-strike to the north), in a manner similar to that of rotational ‘bookshelf faults’. Such movement could have accommodated concomitant vertical flattening of the CASZ during extensional collapse.

The Caldas Novas dome, central Brazil: structural evolution and implications for the evolution of the Neoproterozoic Brası́lia belt

The Caldas Novas dome (Goiaás state, central Brazil) lies in the southern segment of the Neoproterozoic Brası́lia belt (center of the Tocantins Province) between the Goiás magmatic arc and the margin of the ancient São Francisco plate. The core of the dome comprises rocks of the Meso-Neoproterozoic Paranoá group (passive margin psamitic-pelitic sediments and subgreenschist facies) covered by a nappe of the Neoproterozoic Araxá group (backarc basin pelitic-psamitic sediments and volcanics of greenschist facies, bitotite zone). Hot underground waters that emerge along fractures in the Paranoá quartzite and wells in the Araxá schist have made the Caldas Novas dome an international tourist attraction. A recent detailed structural analysis demonstrates that the dome area was affected by a D 1–D 3 Brasiliano cycle progressive deformation in the ∼750–600 Ma interval (published U–Pb and Sm–Nd data). During event D 1, a pervasive layer-parallel foliation developed coeval the regional metamorphism. Event D 2 (intense F 2 isoclinal folding) was responsible for the emplacement of the nappe. D 1 and D 2 record a regime of simple shear (top-to-SE relative regional movement) due to a WNW-ESE subhorizontal compression ( σ 1). Event D 3 records a WSW-ENE compression, during which the dome rose as a large-scale F 3 fold, possibly associated with a duplex structure at depth. During the dome's uplift, the layers slid back and down in all directions, giving way to gravity-slide folds and an extensional crenulation cleavage. A set of brittle fractures and quartz veins constitutes the record of a late-stage D 4 event important for understanding the thermal water reservoir.

Maximum effective moment criterion and the origin of low-angle normal faults

The origin of the low-angle normal faults of metamorphic core complexes has been debated for over two decades. Proponents of Andersonian fault mechanics have long argued that it is mechanically unfeasible for slip to occur along shallowly dipping normal faults. A new theory, named the maximum effective moment criterion, is proposed here for their origin. Using an effective moment approach formulated as M eff= FH, where F is the force tangentially acting on the unit shear boundaries and H represents its arm. The maximum value appears at angles of ±54.7° with the σ 1. Since extensional crenulation cleavages (eccs) and the contractional crenulation cleavages (cccs) occur in conjugate pairs with ∼110° angle between them, it is suggested that they tend to be oriented in the directions of maximum effective moment. The differential stress for formation of the ecc or ccc are less than that for fracturing. The orientations of conjugate eccs depending on relative magnitudes of simple shear versus coaxial strain components. The synthetic ecc set is much better developed than the antithetic set due to anisotropy. Upward propagation of the synthetic ecc set from mid-crustal domains of mylonitization through strain localization and strain softening is considered an effective mechanism for the formation of the low angle-normal faults of metamorphic core complexes.

Apparent shear-band geometry resulting from oblique fold sections

Small-scale shear zones inclined at intermediate angles to an earlier anisotropy are often observed in deformed rocks. They are traditionally described as shear-bands, C-bands, extensional crenulation cleavage or normal kink-bands formed as a result of extension along the anisotropy. Their asymmetries are widely used to describe the large-scale kinematics of deformation and the deformational history of a given area. We demonstrate that when various three-dimensional fold structures are observed on two-dimensional outcrop surfaces or in thin section, they can appear geometrically identical. We have developed a simple technique that allows the geometrical evaluation of any section across a cylindrical fold of arbitrary geometry. The ranges of planar sections on which a fold exhibits shear-band like geometry are presented on a stereographic projection in order to simplify the determination of critical orientations. We demonstrate that for any fold geometry, there are two distinct groups of sections showing shear-band like geometry with opposite ‘senses of shear’ criteria systematically arranged around the axial plane and which are inclined at a high angle to the major anisotropy. We provide a field example from Western Carpathians, where kinematic analysis, mainly based on apparent extensional shear-bands, led to overemphasis of the role of post-orogenic extension on the final structural pattern of the belt.