Rock Shear Research Articles

Geoinformatic Modeling of Groundwater Resource Mapping of Shear Zone Regions- A Case Study on Attur Valley, Tamil Nadu, India

The general tendency of mapping groundwater resource using remote sensing and Geographic Information System (GIS) techniques involve assigning higher weightage to geomorphology. But this cannot be used as a thumb rule everywhere, especially an area where many ductile and brittle zones are prevalent. The influence of texture and structure of sheared rocks might play a control over retaining and permitting groundwater to flow. Attur valley is characterized by the presence of many shear zones and faults and hence the rocks are highly fissile within the shear zones. The present study tries to establish a new ranking and weightage scheme and hence a new spatial model for groundwater resource mapping in shear zone area like Attur Valley. This spatial model can be verified with field data such as water level data, pump test and resistivity data.

Tectonic evolution of the Rehamna metamorphic dome (Morocco) in the context of the Alleghanian-Variscan orogeny

Structural and 40Ar/39Ar geochronological investigations of the Rehamna Massif (Meseta, Moroccan Variscan belt) provide new constraints on the tectonic evolution of the Alleghanian-Variscan orogen during the Upper Paleozoic. Three main tectonic events have been recognized: (1) Southward thrusting of an Ordovician sequence over the Proterozoic basement, its Cambrian sedimentary cover, and the overlying Devono-Carboniferous basin. This event caused subhorizontal shearing and prograde Barrovian metamorphism of the buried rocks. (2) Continuous shortening resulting in the development of a synconvergent extrusion of metamorphosed units to form a dome elongated E-W. This was responsible for synconvergent detachment of the Ordovician upper crustal sequence. The timing of these two episodes is constrained to 310-295 Ma by cooling and metamorphic amphibole and mica ages (3) A NW-WNW convergence in a direction orthogonal to the previous one and characterized by the accretion of the Rehamna dome to the continental basement in the east. Based on 40Ar/39Ar cooling ages from a syntectonic granitoid and its host rocks and metamorphic 40Ar/39Ar ages from greenschist facies mylonite, the timing of this event falls between 295 and 280 Ma. The end of the Variscan orogeny in the Moroccan Meseta is constrained by the 40Ar/39Ar cooling age of a posttectonic pluton dated at ~275 Ma. The tectonic events highlighted in Morocco coincide with the late Variscan-Alleghanian tectonic evolution of southern Europe and North America and can be correlated with the global reorganization of plates that accompanied suturing of Pangaea at around 295 Ma.

Hitherto unknown shear rupture mechanism as a source of instability in intact hard rocks at highly confined compression

Today, frictional shear resistance along pre-existing faults is considered to be the lower limit on rock shear strength for confined conditions corresponding to the seismogenic layer. This paper introduces a recently identified shear rupture mechanism providing a paradoxical feature of hard rocks — the possibility of shear rupture propagation through the highly confined intact rock mass at shear stress levels significantly less than frictional strength. In the new mechanism, the rock failure associated with consecutive creation of small slabs (known as ‘domino-blocks’) from the intact rock in the rupture tip is driven by a fan-shaped domino structure representing the rupture head. The fan-head combines such unique features as: extremely low shear resistance, self-sustaining stress intensification, and self-unbalancing conditions. Due to this the failure process caused by the mechanism is very dynamic and violent. This makes it impossible to directly observe and study the mechanism and can explain why the mechanism has not been detected before. This paper provides physical motivation for the mechanism, based upon side effects accompanying the failure process. Physical and mathematical models of the mechanism presented in the paper explain unique and paradoxical features of the mechanism. The new shear rupture mechanism allows a novel point of view for understanding the nature of spontaneous failure processes in hard rocks including earthquakes. • Physical motivation of a fan-shaped shear rupture mechanism is presented. • It creates faults in intact rocks at shear stresses below the frictional strength. • It makes rock failure inevitably spontaneous and violent. • Physical and mathematical models of the mechanism are developed.

Early Paleozoic depositional environment and intraplate tectono-magmatism in the Cathaysia Block (South China): Evidence from stratigraphic, structural, geochemical and geochronological investigations

The early Paleozoic geological evolution of the South China Craton composed of the Yangtze and Cathaysia Blocks has been the focus of long debate. The Cathaysia block has been central to the controversy regarding convergent margin versus intraplate environment in the early Paleozoic. In order to address the early Paleozoic evolution of Cathaysia, we undertook a systematic study of the stratigraphic sequences, deformational features and geochronology of magmatic event. Our results show that (1) during the early Paleozoic, the Jiangnan domain of the SE Yangtze block was characterized by a carbonate platform and the Cathaysia block by a graptolite-facies clastic rock assemblage, (2) in the Cathaysia block, a littoral-neritic depositional environment prevailed in Cambrian whereas a neritic-bathyal setting dominated during the early-middle Ordovician, and (3) the Late Ordovician depositional sequence in Cathaysia witnessed a period of transition from neritic-bathyal to littoral-land environment, marking the initial uplift process. Paleo-current measurements on the crossbeds revealed northwestward and westward transport directions, suggesting a source area to the east-southeast. All samples collected from the Cambrian-Ordovician strata show similar chemical characteristics; they have negative ε~Nd~(*t*) values (−9.7 to −13.7) and two-stage εNd(t) model ages at *ca.*2.04 to 2.36 Ga. This suggests that the early Paleozoic rocks were derived from the eroded Paleoproterozoic basement, and little or no mantle component was identified. During the Silurian, the Cathaysia block underwent strong folding, thrusting, weak metamorphism and large-scale anatexis accompanied by granitoid emplacement, building the South China Fold Belt. The maximum shortening is estimated at 67 percent. A kinematic analysis of the ductile sheared rocks revealed a fan-shape thrust pattern, with top-to-the southeast in the southeastern and top-to-the northwest in the northwestern Cathaysia block. Zircon U-Pb dating of four granitic plutons yielded ^206^Pb/^238^U ages of 435 ± 4 Ma, 424 ± 5 Ma, 428 ± 3 Ma and 427 ± 2 Ma. All the zircon εHf(t) values are negative (−6 to −9) and show a peak of two-stage Hf model ages around 1.9 Ga, indicating that the Silurian granitic magma was derived from the recycling of Paleoproterozoic basement. Major features of the early Paleozoic South China Fold Belt include the lack of early Paleozoic ophiolites and volcanic rocks, the absence of coeval HP-type blueschists, and the absence of mantle-derived juvenile magmatic rocks. Consequently, a subduction-collision-type orogeny is excluded. The magmatism most probably took place in an intraplate tectonic setting with little or no input of mantle components. We therefore conclude that the South China Fold Belt was an intraplate orogen, and is possibly related to the global early Paleozoic continental assembly.

Structure of pseudotachylyte vein systems as a key to co-seismic rupture dynamics: the case of Gavilgarh–Tan Shear Zone, central India

The secondary fractures associated with a major pseudotachylyte-bearing fault vein in the sheared aplitic granitoid of the Proterozoic Gavilgarh–Tan Shear Zone in central India are mapped at the outcrop scale. The fracture maps help to identify at least three different types of co-seismic ruptures, e.g., X–X′, T1 and T2, which characterize sinistral-sense shearing of rocks, confined between two sinistral strike-slip faults slipping at seismic rate. From the asymmetric distribution of tensile fractures around the sinistral-sense fault vein, the direction of seismic rupture propagation is predicted to have occurred from west-southwest to east-northeast, during an ancient (Ordovician?) earthquake. Calculations of approximate co-seismic displacement on the faults and seismic moment (M 0) of the earthquake are attempted, following the methods proposed by earlier workers. These estimates broadly agree to the findings from other studied fault zones (e.g., Gole Larghe Fault zone, Italian Alps). This study supports the proposition by some researchers that important seismological information can be extracted from tectonic pseudotachylytes of all ages, provided they are not reworked by subsequent tectonic activity.

Mathematical Model of Fan-Head Shear Rupture Mechanism

Today frictional shear resistance along pre-existing ruptures (faults) is considered as the lower limit on rock shear strength for confined conditions. The paper proposes a mathematical model of recently identified shear rupture mechanism which can provide propagation of faults through the highly confined intact rock mass at shear stress levels significantly less than frictional strength of pre-existing faults. The model demonstrates that due to the self-unbalancing structure of the rupture head, representing the core of this mechanism, the failure process caused by the mechanism is always spontaneous and violent. It allows a novel point of view for understanding the nature of spontaneous failure processes including earthquakes.

High-T–low-P thermal anomalies superposed on biotite-grade rocks, Wongwibinda Metamorphic Complex, southern New England Orogen, Australia: heat advection by aqueous fluid?

The Wongwibinda Metamorphic Complex is characterised by ∼400 km2 of biotite-grade rocks with irregular, km-scale zones of cordierite-bearing high-T rock (T > 550°C). Cordierite-bearing rocks include different textures: (i) cordierite–K-feldspar–spotted hornfels; (ii) sheared cordierite–K-feldspar–augen schists; and (iii) migmatites with or without comparatively coarse garnet poikiloblasts. Integrated petrography, mineral chemistry and mineral equilibria modelling indicate metamorphic peak conditions in samples of hornfels of approximately P ≤ 250 MPa and T = 570–620°C. Two samples of sheared rock preserve peak conditions of approximately T = 670–690°C or T < 600°C and P ≤ 200 MPa, consistent with rapid cooling or variable thermal structure in the Glen Mohr Shear Zone. Electron microprobe U–Th–Pb monazite data indicate a date of 291.5 ± 1.8 Ma for the shear zone, indistinguishable from the age of Hillgrove Plutonic Suite granitoids in the complex. Monazite ages are complex in the hornfels samples that were likely, on the basis of structural relationships, metamorphosed early in the history of the Wongwibinda Metamorphic Complex (ca 300 Ma). Previous monazite dating of migmatite samples (ca 297 Ma) suggests that the metamorphic cycle was short-lived, lasting less than ca 10 m.y. Metamorphic field gradients are <15–23°C km−1 across much of the complex but locally steep (>50–100°C km−1) around the high-T rocks. Rejection of most heat sources leaves regional conductive heating as the most plausible explanation for the smooth, broad and shallow metamorphic field gradient in the biotite-grade rocks. Another mechanism of heat transfer is required to explain the local, steep metamorphic field gradients and development of high-T cordierite-bearing rocks. The spatial association of quartzite units centred within two cordierite-bearing high-T domains and an increased abundance of quartz veins above the cordierite isograd suggests heat advection by aqueous fluid locally perturbed the broad conductive heating. Fluid was channelled within shear zones and locally infiltrated nearby rocks. Variable fluid flux and strain have produced the range of different textures in cordierite-bearing rocks of the Wongwibinda Metamorphic Complex.

Strength characteristics of transversely isotropic rock materials

For rock materials, a transversely isotropic failure criterion established through the extended Lade-Duncan failure criterion incorporating an anisotropic state scalar parameter, which is a joint invariant of deviatoric microstructure fabric tensor and normalized deviatoric stress tensor, is verified with the results of triaxial compressive data on Tournemire shale. For torsional shear mode with , rock shear strengths decrease with increasing until the rock shear strength approaches minimum value at , and after this point, the rock shear strengths increase as increases further. For the torsional shear mode with b > 0.75, rock shear strengths are almost constant for , but it increases with increase in afterwards. The rock shear strength variation against agrees with shear strength changing tendency of heavily OCR natural London Clays tested before. Prediction results show that the transversely isotropic failure criterion proposed in the paper is reasonable.

Failure mechanism and stability control of a large section of very soft roadway surrounding rock shear slip

The measured data and simulation test phenomenon of surrounding rock deformation and failure at the project site indicate that shear failure which firstly occurs in surrounding rock, block slip and second shear failure are the root cause of deformation and damage of supporting structure of the surrounding rock at a large scale. We derived limit load of surrounding rock shear slip failure and reasonable support resistance of given load by means of shear slip line field theory, discussed the main factors which influence the limit load of surrounding rock. Shear slip line field and limit load of circular tunnel surrounding rock were obtained by means of physical simulation test, which agreed well with the theoretical analysis results. Based on the theoretical analysis and physical simulation test, the cause deformation and failure at large scale of Xinshanghai No. 1 coal mine big section ingate was analyzed, and the shear failure resistance and block slip in surrounding rock were proposed as the core technical supporting ideas. Proper range of supporting resistance which came from calculation was suggested. The support scheme which is mainly composed of large grouting anchor, sprayed anchor net support technique and full-face grille concrete finally ended the dilemma of repeated failure and mending of ingate and created critical conditions for smooth production in the coal mine.

Reliability-based design of rock slopes — A new perspective on design robustness

Traditional reliability-based rock slope designs, in which the lowest-cost design is selected from all designs meeting target reliability requirements, are often sensitive to variations in noise factors such as rock shear properties. Consequently, a design that was initially judged acceptable may not satisfy reliability requirements if the variation of rock properties has been greatly underestimated. The authors present a Robust Geotechnical Design (RGD) approach for purposes of addressing this dilemma, by considering the robustness explicitly in the design process. In the context of rock slope design, this proposed RGD approach aims to make the response (i.e., failure probability) of a rock slope system insensitive to, or robust against, the variation of rock shear properties by adjusting design parameters (i.e., such as slope angle and height). Compared to traditional reliability-based design, the RGD approach adds design robustness as one of its design objectives. Thus, multi-objective optimization, considering both cost and robustness, is needed to select optimal designs in the acceptable design space where safety is guaranteed by a constraint on the reliability. In this paper, the concept of the Pareto Front, a collection of optimal designs that reflect the trade-off between cost and robustness, is implemented in the RGD approach. The proposed RGD approach is demonstrated with an example of a rock slope design.

Mineralogy and microchemistry of the Egat gold deposit, South Eastern Desert of Egypt

Gold-bearing quartz lodes from the Egat gold mine, South Eastern Desert of Egypt, are associated with pervasively silicified, highly sheared ophiolitic metagabbro and island-arc metavolcanic rocks. The mineralized quartz veins and related alteration haloes are controlled by NNW-trending shear/fault zones. Microscopic and electron probe microanalyses (EPMA) data of the ore and gangue minerals reveal that fine-grained auriferous sulfarsenides represent early high-temperature (355–382 °C) phases, with formation conditions as (fS2 = −10, and fO2 around −31). A late, low-temperature (302–333 °C) assemblage includes coarse pyrite, arsenopyrite, and free-milling gold grains (88–91 wt.% Au), with formation conditions as (fS2 = −8 and fO2 around −30). Gold was impounded within early sulfarsenides, while free-milling gold blebs occur along microfractures in quartz veins and as inclusions in late sulfides. Infiltration of hydrothermal fluids under brittle–ductile shear conditions led to mobilization of refractory Au from early sulfarsenide phases and reprecipitated free gold, simultaneous with silicification of the host rocks. The positive correlation between Au and As favors and verifies the use of As as the best pathfinder for gold targets, along the NNW-trending shear zones.

Geochemistry of lepidolitic granitoids from the Mungutiyn Tsagaan Durulj occurrence (central Mongolia)

We studied the geologic position, geodynamic setting, petrology, and geochemistry of veined lepidolitic granitoids from the Mungutiyn Tsagaan Durulj (MTD) occurrence (central Mongolia), found within the area of Mesozoic intraplate rare-metal magmatism. It has been established that their trace-element enrichment resulted from the intense effect of fluids rich in F, K, Li, Rb, Cs, Sn, Be, and W, which arrived from a deep magma chamber of rare-metal granitic melts, on leucogranites with originally weak rare-metal mineralization. Very high contents of F, rare alkali metals, Sn, Be, and W, characteristic of MTD granitoids, are close only to those in greisens of rare-metal granites and topaz-lepidolite-albitic pegmatites. The difference from the greisens in each case might be due to the features of the original rocks. The difference between the greisenized MTD leucogranites and the topaz-lepidolite-albitic pegmatites is more radical: Along with evident petrographic distinctions, it includes an evolution trend toward the albite norm decrease, not typical of Li–F igneous rocks; rock shearing and gneissosity, which must have contributed to their chemical transformation according to this trend; and stably lower contents of Nb and Ta (trace elements which usually accumulate during crystallization fractionation of F–Li granitic melts and are poorly soluble in magmatic fluids). The greisenized MTD granitoids are not only high-grade rare-metal ores of Li, Rb, F, and Sn but are also regarded as an indicator of a deep concealed pluton of rare-metal granites.

Tectonics of the Middle Triassic intracontinental Xuefengshan Belt, South China: new insights from structural and chronological constraints on the basal décollement zone

In orogenic belts, a basal decollement zone often develops at depth to accommodate the shortening due to folding and thrusting of the sedimentary cover. In the Early Mesozoic intracontinental Xuefengshan Belt of South China, such a decollement zone is exposed in the core of anticlines formed by the emplacement of the late-orogenic granitic plutons. Our detailed, multi-scale structural analysis documents a synmetamorphic ductile deformation. In the basal decollement, the Neoproterozoic pelite and sandstone, and the intruding Early Paleozoic granites were deformed and metamorphosed into mylonites and orthogneiss, respectively. The metamorphic foliation contains a NW–SE stretching lineation associated with top-to-the-NW kinematic indicators. The ductile shearing of these high-strained rocks can be correlated with NW-verging folds and thrusts recognized in the Neoproterozoic to Early Triassic sedimentary cover. Monazite U–Th–Pbtot chemical dating, and zircon SIMS U–Pb dating provide age constraints of the ductile shearing between 243 and 226 Ma, and late-orogenic granite emplacement around 235–215 Ma. In agreement with recent geochronological data, these new results show that the Xuefengshan Belt is an Early Mesozoic orogen dominated by the NW-directed shearing and thrusting. At the southeastern boundary of the Xuefengshan Belt, the Chenzhou-Linwu fault separates the Early Mesozoic domain to the NW from the Early Paleozoic domain to the SE. The tectonic architecture of this belt was possibly originated from the continental underthrusting to the SE of the South China block in response to northwest-directed subduction of the Paleo-Pacific plate.

Development of shear zone-related lozenges in foliated rocks

Tectonic lozenges are elongate bodies bounded by relatively more deformed rocks. The focus of this study is on the 2-D structure of tectonic lozenges developed during ductile shear in rocks with a pre-existing mechanical anisotropy. On the basis of a detailed analysis of shear zones in foliated rocks from the Cap de Creus area (Variscan of the eastern Pyrenees), five mechanisms to explain the development of different types of lozenges in foliated rocks are suggested. These mechanisms are explained on the basis of the orientation of the previous foliation relative to the bulk shearing direction. It is shown that the prevailing mechanism does not majorly depend on the bulk kinematics but on the angular relationship between the pre-existing foliation and the bulk kinematic axes, and on shear zone interaction. This has implications on the use of lozenge shapes in tectonic interpretations. The fact that there is a wide range of initial orientations, propagation modes and coalescence types implies that the final lozenge geometry is not univocally related neither to the type of strain nor to the kinematic regime.

Structural Plane of Rock Mass Strength Characteristics and its Impact on Groundwater

The strength characteristic of discontinuity is one of the most important mechanical characteristics of rock mass, and its strength dominates the strength and deformation of joint rock mass. The underground water has an important influence on the strength of discontinuity. In this paper, the strength theory of joint rock mass was summarized and analyzed; further, two kinds of failure mode of joint rock mass which are joint shear failure and rock shear failure were proof. Considering the impact of underground water, the effective stress principle was introduced, and base on it, the joint shear failure strength rule of the saturated rock mass was built. When the influence of underground water on joint strength is studied, the shear strength formula has important theoretical and realistic significance.

Fault-related rocks: deciphering the structural–metamorphic evolution of an accretionary wedge in a collisional belt, NE Sicily

The Alpine chain exposed in the Western Mediterranean area represents a front several kilometres in width, dismembered by more recent tectonics and by opening of the Tyrrhenian Basin. In most exposures of this mountain belt, relics of older metamorphic rocks occur. The deformational sequence of events may be revealed by the recognition of metamorphic records associated with different structures. Within a tract of the Alpine front cropping out in the Peloritani Mountains (NE Sicily), we distinguished two metamorphic complexes characterized by different tectonometamorphic histories. Their present tectonic juxtaposition is a cataclastic thrust linked to the recent Africa-verging Sicilian–Maghrebian fold-and-thrust belt. The Lower Complex is characterized by Hercynian metamorphism (P > 0.2 GPa and T ≈ 350°C) exclusively. It essentially consists of very low-grade metapelites and metavolcanic rocks overlain by an unmetamorphosed sedimentary cover. The Upper Complex, comprising different tectonic slices, consists of medium- to high-grade Hercynian metamorphic rocks (P = 0.3–0.8 GPa and T up to 650°C) with Alpine metamorphic overprint (T > 250°C) affecting also the Mesozoic–Cenozoic cover. Lithotypes, structures, and inferred P–T conditions of investigated rocks suggest the existence of an Alpine accretionary wedge during the Cretaceous deformational collision. Within the Upper Complex, a polyphase Palaeogene mylonitic horizon involving rocks belonging to different tectonic slices fully preserves the tectonometamorphic evolution. For this reason, we focused our attention on these sheared rocks in order to reconstruct the entire tectonic history of this geologically complex area. Our new basic model allows the complex structure of the nappe-pile edifice of the Peloritani Mountains to be simplified, casting new light on the tectonic evolution of this key sector of the southern Calabrian-Peloritani Orogen.

The nature of the Ailao Shan–Red River (ASRR) shear zone: Constraints from structural, microstructural and fabric analyses of metamorphic rocks from the Diancang Shan, Ailao Shan and Day Nui Con Voi massifs

The structural geology, timing of shearing, and tectonic implications of the ASRR shear zone, one of the most striking lineaments in Southeast Asia, have been the topics of extensive studies over the past few decades. The Xuelong Shan (XLS), Diancang Shan (DCS), Ailao Shan (ALS) and Day Nui Con Voi (DNCV) metamorphic massifs along the shear zone have preserved important information on its structural and tectonic evolution. Our field structural analysis, detailed microstructural and fabric analysis, as well as the quartz, sillimanite and garnet fabric studies of the sheared rocks from the massifs demonstrate the dominant roles of three deformation episodes during Cenozoic tectonic evolution in the shear zone. Among the contrasting structural and microstructural associations in the shear zone, D2 structures, which were formed at the brittle to ductile transition during large-scale left-lateral shearing in the second deformation episode, predominate over the structural styles of the other two deformation episodes. Discrete micro-shear zones with intensive grain size reduction compose the characteristic structural style of D2 deformation. In addition, several types of folds (early shearing folds, F21, and late-shearing folds, F22) were formed in the sheared rocks, including discrete to distributed mylonitic foliation, stretching lineation and shear fabrics (e.g., mica fish, domino structures, as well as sigma and delta fabrics). A sequence of microstructures from syn-kinematic magmatic flow, high-temperature solid-state deformation, to brittle–ductile shearing is well-preserved in the syn-kinematic leucocratic intrusions. Deformation structures from the first episode (D1) are characterized by F1 folds and distributed foliations (S1) in rocks due to pure shearing at high temperatures. They are preserved in weakly sheared (D2) rocks along the eastern margin of the ALS belt or in certain low-strain tectonic enclaves within the shear zone. Furthermore, semi-brittle deformation structures, such as hot striae and discrete retrogression zones, are attributed to normal-slip shearing in the third deformation episode (D3), which was probably locally active, along the eastern flank of the DCS range, for example.There are four quartz c-axis fabric patterns in the mylonitic rocks, including type A point maxima, type B Y point maxima with crossed girdles superimposition, type C quadrant maxima, as well as type D point and quadrant maxima combination. They are consistent with microscopic observations of microstructures of high-temperature pure shearing, low-temperature simple shearing and their superimposition. Integrated microstructural analysis and fabric thermometer studies provide information on both high temperature (up to 750°C) and dominant low-temperature (300–600°C) deformations of quartz grains in different rock types. Sillimanite and garnet fabrics, especially the latter, were primarily formed at the peak metamorphism during high-temperature pure shearing.The above structural, microstructural and fabric associations were generated in the tectonic framework of the Indian-Eurasian collision. The low-temperature microstructures and fabrics are attributed to left-lateral shearing along the ASRR shear zone from 27 to 21Ma during the southeastward extrusion of the Indochina block, which postdated high-temperature deformation at the peak metamorphism during the collision.

Using zircon U–Pb ages and trace element chemistry to constrain the timing of metamorphic events, pegmatite dike emplacement, and shearing in the southern Parry Sound domain, Grenville Province, Canada

The integration of zircon U–Pb ages and trace element chemistry with structural and petrologic relations from a range of sample types provides important temporal constraints on the tectono-metamorphic evolution of the southern Parry Sound domain (PSD), Ontario, Canada, and the processes attending development of the underlying Twelve Mile Bay shear zone (TMBSZ). Intact granulites preserve ca. 1145 Ma ages, slightly younger than those farther to the north in the interior PSD, but similar to that of the underlying granulite-to-amphibolite facies Parry Sound shear zone. Weaker zircon HREE enrichment in sheared and retrogressed samples with partially resorbed garnet porphyroblasts (containing abundant zircon inclusions) suggests that the 1128–1143 Ma ages common to sheared rocks in the southern PSD record an earlier phase of metamorphism and deformation, and not the timing of shear zone development. Deformed pegmatite dikes bounded by sheared and retrogressed wall rocks from across the southern interior PSD consistently record ca. 1100 Ma ages, indicating synchronous pegmatite emplacement across the transect and constraining the bounding amphibolite-facies shear zones to ≤1100 Ma. TMBSZ samples retain evidence for (1) the older ( ca. 1145 Ma) high-grade metamorphic event, (2) pegmatite emplacement and shear deformation at ca. 1100 Ma, and (3) a later ( ca. 1070 Ma) shearing event. Combined with published structural and petrologic data, these ages indicate that pegmatite emplacement and shearing in the TMBSZ was synchronous with that in the southern interior PSD; further confirming published models for TMBSZ development. Additionally, the data suggest that the Twelve Mile Bay assemblage (within the TMBSZ) experienced high-grade metamorphism synchronously with the interior PSD, and may therefore be correlative with the basal PSD to the north.

The timing of strike-slip shear along the Ranong and Khlong Marui faults, Thailand

[1] The timing of shear along many important strike-slip faults in Southeast Asia, such as the Ailao Shan-Red River, Mae Ping and Three Pagodas faults, is poorly understood. We present 40Ar/39Ar, U-Pb SHRIMP and microstructural data from the Ranong and Khlong Marui faults of Thailand to show that they experienced a major period of ductile dextral shear during the middle Eocene (48–40 Ma, centered on 44 Ma) which followed two phases of dextral shear along the Ranong Fault, before the Late Cretaceous (>81 Ma) and between the late Paleocene and early Eocene (59–49 Ma). Many of the sheared rocks were part of a pre-kinematic crystalline basement complex, which partially melted and was intruded by Late Cretaceous (81–71 Ma) and early Eocene (48 Ma) tin-bearing granites. Middle Eocene dextral shear at temperatures of ∼300–500°C formed extensive mylonite belts through these rocks and was synchronous with granitoid vein emplacement. Dextral shear along the Ranong and Khlong Marui faults occurred at the same time as sinistral shear along the Mae Ping and Three Pagodas faults of northern Thailand, a result of India-Burma coupling in advance of India-Asia collision. In the late Eocene (<37 Ma) the Ranong and Khlong Marui faults were reactivated as curved sinistral branches of the Mae Ping and Three Pagodas faults, which were accommodating lateral extrusion during India-Asia collision and Himalayan orogenesis.

A Method of Data Processing for Determining Shear Strength Parameters of Rock

One of the main factors that influence the analytic results of slope stability is the determining of mechanical parameters. When the least square method is adopted as the method for determining the rock shear strength parameters, its precondition that the observation errors exist only in one observation variable is always being neglected. By analysis, the calculating of shear strength parameters is divided into three kind of situations, and a new approach to calculate the shear strength parameters is obtained according to the fact that the quadratic sum of distance between line and points in minimum, and the calculating formulae and applying scopes of three different calculating methods are also put forward. A calculating case shows that there are obvious differences among different calculating methods for determining the strength parameters, and it is necessary to select the calculating method based on the observation errors.