Pre-existing Shear Research Articles

Abstract 5513: Shear Stress Independent Formation of Collaterals in Porcine Coronaries

Objectives: To test the hypothesis that intracardiac collateral connections do not result from observable (resolution 40 microns) preexisting vessels and shear stress modulation is not required for collateral formation. Methods: An LCX ameroid constrictor was placed in 6 pigs and the LCX exposed (sham) in 2. After six weeks the hearts were processed and sliced at 40 μ m thickness (3000 slices) in a fluorescence imaging cryomicrotome. High resolution images (2000 * 2000 pixels) were captured, yielding a 3D reconstruction (40 μ m3 voxels) of the vasculature. With custom algorithms, collaterals were detected between (Bridge) and within (Mid) LCX, LAD and RCA. Results: In shams, collaterals were absent. In stimulated hearts 192 collaterals, median ô111 μ m (22–1049), were detected, of which 65% endocardial (p= 0.004). Figure 1 shows a typical case (local opacity altered for visibility). Only 64% of all connections were bridging the LCX to LAD or RCA regions. Mid collaterals (n=24), were found in non-LCX regions with no apparent perturbation of shear stress. Mid collaterals (median 76 μ m, 23–1049) were smaller than bridge (137 μ m, 22–990, Mann-Whitney U p=0.047). Conclusion: Since mid collaterals are parallel to normal vasculature, the novel finding of these vessels contra-indicates a dominant role of shear stress in generating and maintaining collaterals. Higher flow rates are to be expected in Bridge versus Mid connections supporting the hypothesis that shear stress plays a role in outward remodeling. Hence, the detection of substantial numbers of collaterals in regions without flow limitation, suggests additional pathways contributing to collateral growth.

Deformation kinetics of Zr-based bulk metallic glasses—Temperature and strain rate influences on shear banding

At low homologous temperatures metallic glasses exhibit inhomogeneous flow behaviour, which is associated with narrow shear banding. Based on the width of the shear bands and the time-dependent heat conduction, we show here that this process is not fully adiabatic at low strain rates, even though temperatures are sufficiently high to create a drop in viscosity within either a new or a pre-existing shear band. Evaluation of the deformation kinetics at cryogenic temperatures suggests an increase in viscosity within the shear band, although temperatures are still sufficiently high to cause localised melting at fracture. In addition, a change from serrated to non-serrated flow can be observed if the temperature is lowered below a critical value. This macroscopic change in the flow behaviour is directly related to a change in the strain rate sensitivity from negative to positive values, suggesting a clear change in deformation behaviour. We propose a refined shear dilatation model to explain the experimental findings.

Late Palaeozoic deformation of post-Variscan lower crust: shear zone widening due to strain localization during retrograde shearing

The Santa Lucia Shear Zone (SLSZ, Corsica) is a granulite-facies Permian shear zone that developed after the emplacement of a deep-seated gabbroic intrusion. New structural data shows that the SLSZ results from the juxtaposition of three spatially distinct mylonite belts, which are the product of the interaction between magmatism, metamorphism and shearing over a temperature range from ~800 to ~400°C. During the earlier high-grade deformation stage, which was accompanied by decompression from ~7 to ~5 kb at ~800°C, the SLSZ has accommodated high finite strain on a shear zone ≥1 km wide. Strain became increasingly localized as temperature decreased, but rather than reactivating pre-existing shear zones as commonly expected, younger mylonites expanded into previously unsheared rock, extending the total width of the shear zone. The zonation of different fabrics across the SLSZ suggests that pre-existing compositional and grain size heterogeneities in the starting material played a key role in governing superposed generations of shear zones.

Mechanisms of deformation localization at the tips of shear fractures: Findings from analogue experiments and field evidence

Using analogue experiments on polymethylmethaacrylate (PMMA) models, we investigated the process of deformation localization at the tips of preexisting planar shear cracks. Experiments show that this can take place in any of the following four principal mechanisms. Mechanism A: Brittle deformation is the dominant process and forms a pair of long tensile fractures at the crack tips. The tensile fractures propagate along the compression direction and transgress the entire model thickness, causing model failure at a small bulk strain (3%). Mechanism B: It involves both brittle and ductile (plastic) strain localization, where the tensile fractures grow to a limited length and incipient ductile zones appear at the tips. Mechanism C: Deformation localization is characterized by an association of macroscale shear bands and short, opened‐out tensile fissures (cf. wingfractures). Mechanism D: Ductile strain localizes in the form of a pair of shear bands at each tip. Fracture failure does not occur in this case. The transition from Mechanism A to Mechanism D is a continuous phenomenon in the experimental conditions, which we show as a function of initial crack angle (α angle between the crack and the far‐field compression direction) and crack length (l). Mechanism A tends to be replaced by Mechanism D with decreasing α (60° to 20°) and/or l. Using a finite element method (FEM), we calculated the maximum principal tensile stress (σ1max) and the maximum second stress invariant (I2max) of the stress field in the neighborhood of a sliding crack within a linearly elastic medium and analyzed the brittle‐ductile transitions observed in physical experiments. The calculations show that σ1max is directly proportional to l and attains a peak value for α = 45°, promoting Mechanism A. On the other hand, I2max occurs at α < 45°, favoring nucleation of ductile shear bands (Mechanism D). When α and l are increased simultaneously, σ1max takes its peak value at α = 60°. This analysis explains the dominance of Mechanism A for α > 45° in physical models with simultaneously varying crack length and orientation. We also demonstrate probable interactions between plastic strain localization and tensile fracturing at the crack tips. FEM results indicate that a plastic zone lowers the magnitude of tensile stress concentration at wing cracks and thereby dampen their growth when α < 45°. We finally complement our study with field examples.

The Benagerie Shear Zone: 1100 Myr of reactivation history and control over continental lithospheric deformation

A newly identified northwest–southeast oriented, deeply-rooted, steep to vertical, large-scale structural system within the Proterozoic Curnamona Province, Australia, which we term the “Benagerie Shear Zone”, is imaged in regional magnetic and gravity datasets. In this study, we use a combination of field analysis and quantitative geophysical methods, to establish a 1100 Myr history of activity along the Benagerie Shear Zone during which the location of younger geological structures are influenced by the pre-existing shear zone. This deformational system is interpreted to have 1) aided ascent and emplacement of the ca. 1600 Ma Ninnerie (magmatic) Supersuite; 2) controlled the loci of nucleation of normal faults during rifting and continental breakup at ca. 800 Ma; and 3) influenced the development of fold structures as well as acting as a plane co-linear to the rotation axis of pre-existing normal faults such that they were steepened and reactivated as strike slip structures during the ca. 500 Ma Delamerian Orogeny. We interpret that the Benagerie Shear Zone has not undergone uni-directional propagation during its evolution but rather through reactivation was a primary influence on controlling the nucleation of Neoproterozoic rift faults, thereby playing a major role in accommodating strain over a significant period of the evolution of the Curnamona Province. This study demonstrates that crustal-scale shear zones can evolve over hundreds of millions of years, have strike-lengths of hundreds to thousands of kilometers, and have vastly different surface expressions along strike.

The rock mechanics of kimberlite volcanic pipe excavation

The rock mechanics theory for deformation of open pit and underground mining excavations can be used to better understand aspects of the growth and geometry of kimberlite volcanic pipes. Large scale rock mass behaviour around an excavation, such as a volcanic pipe, is dependent on the rock mass strength, the in-situ rock stress conditions and the excavation geometry. Rock mass strength is empirically derived from the intact rock strength and quantification of the shear strength, frequency and orientation of rock discontinuities. Tensile slope or sidewall failure typically occurs in shallow level (~ 0–1000 m) conditions in which pre-existing structures shear due to gravity-driven forces. The orientation of the pre-existing structures provides an important control on the size, shape and position of the rock mass failure. Slope failures are shown to influence the development of many kimberlite volcanic craters and the distribution of layered volcaniclastic facies in the crater. Explosions or removal of key-blocks in a pipe sidewall can cause undercutting and collapse of the sidewall, with the rock unravelling along pre-existing structures towards surface. Many lithic-rich breccias commonly found on the margins of kimberlite pipes are interpreted to form in this way. At intermediate (> 1000 m) to deep (> 2500 m) depths, the increasing compressive stress can cause fractures to develop around an excavation. Stress-induced fractures should cause scaling of a volcanic pipe's sidewalls and expansion perpendicular to the maximum component of compressive stress. A larger horizontal tectonic stress ratio and a lower internal pipe pressure will promote pipe sidewall fracturing. Volcanic pipe expansion will also be increased in rate by (i) mechanical sidewall erosion by flowing magma and/or pyroclasts, (ii) by failure on large intersecting fault or dyke structures, and (iii) by reduction in the country rock mass strength by volcanic explosions. The final pipe shape and distribution of some internal facies is therefore a consequence of the dominant rock mechanical failure processes in the pipe sidewalls. The type of failure is dependant on the rock mass strength, geological structures, stress and depth below surface.

A Large Diameter Triaxial Apparatus to Measure Pore Pressure and Displacements on a Pre-existing Shear Zone/Plane

Abstract The development of an apparatus to measure pore pressure and displacements on a pre-existing shear zone/plane during triaxial testing is presented. The apparatus includes a newly developed 405-mm diameter triaxial cell, a miniature pore pressure transducer for monitoring shear-induced pore pressures, and a displacement transducer for measuring shear displacements. Tests were performed by mounting the pore pressure transducer onto the face of a pre-existing shear plane in the specimen and installing the displacement transducer inside the triaxial cell to monitor local deformations. The results reveal that pore pressures measured on the shear plane and the base of specimens of Athabasca clay, Highvale mudstone, and Fort McMurray highly weathered limestone are identical at a shear displacement rate equal or less than approximately 18 mm/day. This implies that when the shear displacement rate is less than about 18 mm/day for rocks/soils that have a permeability greater than 10−8∼10−9cm/s, the pore pressure obtained from in situinstrumentation, which may not be set exactly on the shear plane, can be used as the shear zone pore pressure.

Structural and provenance analyses of a newly defined major fault: The Homyeong Fault as a northern boundary fault of the Cretaceous Nagdong Trough, South Korea

The Homyeong Fault on the Yecheon Shear Zone is newly defined and described in the northwestern part of North Gyeongsang Province, South Korea. In the study area, the basement rocks consist, of the Precambrian rocks and Jurassic granitoid, and are dextrally deformed by the Jurassic Yecheon Shear Zone to the north of the Cretaceous Gyeongsang Supergroup in a ductile manner. These basement rocks are covered with unconformity by the sindong Group, the lowest part of the Gyeongsang Supergroup. The Homyeong Fault runs on the Yecheon Shear Zone, and is marked by cataclasite that contains mylonite fragments originating from the mylonitic rocks of the Yecheon Shear Zone and that shows sinistral sense of shear with normal-fault component dipping basinward, which indicates reactivation of the Yecheon Shear Zone. The conglomerates of the northern part of the Sindong Group usually contain the gravels of granite, quartzite, blackish quartzite, mylonite, ultramylonite, and mudstone. Petrological similarity between these gravels and the basement rocks suggests that most of the gravels were very likely derived from the Yecheon Shear Zone and its vicinity. Considering the Korean Cretaceous basins mostly formed by sinistral reactivation of the pre-existing shear zones, it can be judged that the reactivation of the Yecheon Shear Zone caused the basement rocks nearby the Homyeong Fault, or the Yecheon Shear Zone and its vicinity, to be deposited as the gravels to the Nagdong Trough where the Sindong Group was filled on the south of the fault. This gives new proposals that the Homyeong Fault had played a role as a northern boundary fault of the Nagdong Trough and also that the inversion tectonics from Jurassic dextral to Cretaceous sinistral motions, observed in the eastern margin of Asia, had an effect on the formation of the Nagdong Trough.

Plastic flow of a nanocrystalline/amorphous Al 90Fe 5Gd 5 composite formed by rolling

Mechanically induced nanocrystallization has attracted much interest and is expected to be an alternative route for synthesizing nanocrystalline/amorphous matrix composites. However, little research has been performed on the mechanical behavior of these composites. Using nanoindentation, we have investigated plastic flow of a nanocrystalline/amorphous Al 90Fe 5Gd 5 composite produced by rolling at room temperature. The nanocrystalline/amorphous matrix composite exhibits less-obvious serrations than the as-spun amorphous alloy. Using high-resolution transmission electron microscopy and atomic force microscopy, we observed the microstructure and morphology of indents. It is believed that the predominant deformation mechanism in the nanocrystalline/amorphous composite is the propagation of preexisting shear bands, in contrast to nucleation and propagation in the as-spun alloy.

Acadian deformation in the shallow crust: an example from the Siluro-Devonian Arisaig Group, Avalon terrane, mainland Nova Scotia

The Silurian – Early Devonian Arisaig Group of the Avalon terrane in northern mainland Nova Scotia consists mainly of thinly bedded sandstones, siltstones, and shales deposited in a near shore environment. These strata were deformed in the middle Devonian to form regional northeast- to NNE-trending folds and record deformation processes in the shallow crust during the Acadian orogeny, one of the most regionally extensive orogenic events in the Canadian Appalachians. Structural features in the Arisaig Group are consistent with fold propagation associated with thrust fault geometry and coeval local extension recorded by a set of conjugate normal faults. Many outcrop-scale folds have sheared limbs and show evidence of a complex progressive deformation. Folding was predominantly accomplished by bulk rotation and flattening above thrust fault tips. Early structures (D1–D2) produced regional cylindrical folds, whereas later (D3a, D3b, D3c) structures produced conical folds. D1–D3 fold orientations show high variability, but are consistent with progressive deformation related to reactivation and coeval dextral strike-slip movement along the Hollow Fault. The style of deformation is compatible with models in which strain is partitioned into preexisting shear zones in the basement, with folds in the overlying Arisaig Group initiated above the tips of upward-propagating thrusts as second-order structures related to movement along those shear zones. Taken together, these data indicate that fold mechanisms and geometry in the shallow crust during the Acadian orogeny in mainland Nova Scotia may be related to dextral strike-slip along major faults in the basement and co-genetic upward-propagating thrusts that rotated and flattened overlying strata.

Cross-Barrier Flow during Orographic Precipitation Events: Results from MAP and IMPROVE

Abstract Ground-based and airborne Doppler radar data collected during passage of frontal rainstorms over the European Alps and Cascade Mountains of Oregon are found to exhibit characteristic cross-barrier flow and precipitation patterns. A stably stratified layer of blocked (or partially blocked) low-momentum flow below mountain-crest level is separated from strong cross-barrier flow aloft by a concentrated layer of vertical wind shear. This shear layer slopes upward toward the mountain crest and persists for several hours as the storm passes over the mountain range. This pattern is remarkably similar from case to case and from one mountain range to the other. The orographically enhanced precipitation in these cases exhibits stratiform structure, marked by a radar bright band, which often drops to a lower height (indicating locally cooler conditions) immediately adjacent to the windward mountain slopes. A terrain-induced gravity wave produces strong downslope flow and spillover of precipitation onto the lee side of the barrier. An elevated reflectivity maximum appears above the mountain crest and extends a short distance (generally 20–40 km) upstream, apparently as a result of gravity wave lifting. While baroclinically induced shear may contribute to the observed pattern, two-dimensional idealized simulations indicate that, in the presence of sufficient upstream static stability, orographic effects alone are sufficient to support development of the upward-sloping shear layer on the windward side of the barrier. Furthermore, idealized simulations show that if there is preexisting vertical shear in the incident upstream flow, this shear is strengthened by the orography and that surface friction may also enhance the strength of the shear layer. The repeatable sheared velocity pattern identified here is apparently an essential feature of cases in which stably stratified low-level flow associated with a baroclinic system impinges upon a sufficiently tall mountain range. Since the layer of orographically generated shear promotes turbulence, which has been shown to aid precipitation growth over the windward slopes, these stable flow dynamics represent a potentially important mechanism for orographic precipitation enhancement in association with the passage of frontal systems over mountainous terrain.

Drained Shear Strength Parameters for Analysis of Landslides

This paper presents recommendations for selecting the type and magnitude of drained shear strength parameters for analysis of landslides. In particular, the importance, existence, and use of the cohesion shear strength parameter is reviewed. For slope stability analyses, it is recommended that the shear strength be modeled using a stress dependent failure envelope or a friction angle that corresponds to the average effective normal stress acting on the slip surface passing through that particular material instead of using a combination of cohesion and friction angle to represent soil shear strength. Other recommendations for stability analyses include using an effective stress cohesion of zero for residual and fully softened strength situations. To facilitate selection of shear strength parameters for landslide analyses, empirical relationships for the drained residual and fully softened strengths are updated from the previous empirical relationships presented by Stark and Eid. Finally, the paper presents torsional ring shear test results that indicate that pre-existing shear surfaces exhibit self-healing that results in increased shear resistance. The magnitude of healing appears to increase with increasing soil plasticity, and this increase could have implications for the size, timing, and cost of landslide remediation.

A large landslide in Zigui County, Three Gorges area

On 13 July 2003, a landslide with a volume of approximately 20 × 106 m3 occurred on the left bank of the Qinggan River, Zigui County, Hubei Province, China. As a result, 14 people died and 10 people are missing. A landslide dam was formed, blocking the Qinggan River. A channelized diversion was constructed for prevention of upstream flooding and damage downstream caused by dam breaching. The landslide was a typical translational rockslide in weathered shale and sandstone, a block of which slipped down along the bedding plane. It is estimated that the landslide was caused by the combined effect of the following factors: (i) a bedding plane between incompetent weathered shale and competent sandstone that daylights at or is shallowly buried at the bottom of the Qinggan River; (ii) a thin layer of clayey soil acting as the slip plane that is likely a preexisting shear surface; (iii) excavation of the shale as raw material for brick fabricating on the slope; and (iv) a combination of prolonged rainfall and reservoir impounding.Key words: landslide, landslide dam, rainfall, impounding, Three Gorges area.

Delineation of subsurface Proterozoic Unkar and Chuar Group sedimentary basins in northern Arizona using gravity and magnetics: Implications for hydrocarbon source potential

Subsurface occurrences of Mesoproterozoic Unkar Group (∼1.25–1.1 Ga) and Neoproterozoic Chuar Group (∼0.9–0.7 Ga) rocks have been delineated in regions east and southeast of the Grand Canyon of northern Arizona. Gravity and aeromagnetic data, combined with gravity modeling, an analysis of seismic velocities, geologic cross sections, and geologic structures, have identified two major Proterozoic extensional regimes in the region: (1) a Mesoproterozoic northwest to southeast trend indicating northeast-southwest–directed extension throughout the study area and (2) a Neoproterozoic, mostly north-south–directed trend indicating predominant east to west extension, evident mostly in northern regions of the study area. A more ancient, northeast-southwest–aligned system of basement shear zones has also been identified. These shear zones correlate with many of the exposed major fault systems of the region such as the Bright Angel and Mesa Butte fault systems. Unkar Group graben and half-graben formation was constrained by the preexisting northeast-directed shear zones. Chuar Group basin formation was influenced by the reactivation of northwest to southeast Unkar Group basin structures, the more ancient northeast-trending shear zones, and by Neoproterozoic age north- to south-oriented extension. This superimposed network of deep-seated normal faults led to the development of a complex array of surface structures during the Laramide. By delineating Proterozoic basin structures in the region, this study helps to delineate the probable subsurface occurrences of hydrocarbon source rocks of the Walcott Member of the Neoproterozoic Chuar Group. John M. Seeley is a research geologist and assistant director for the Pan-American Center for Earth and Environmental Studies at the University of Texas at El Paso. He received his B.S. (1985) and M.S. (1990) degrees from Texas A&I University and his Ph.D. (1999) from the University of Texas at El Paso. His research interests include depositional systems, sequence stratigraphy, gravity and magnetics, and tectonics.G. Randy Keller holds the L. A. Nelson Professorship. He is also chief scientist of University of Texas at El Paso's NASA Earth Science Research Center. His research interests stress the geological applications of geophysics, and he is particularly interested in the integration of diverse data sets and the use of information technology. He is a longtime member of the AAPG, Society of Exploration Geophysicists, Geological Society of America, American Geophysical Union, and the European Association of Exploration Geophysicists.

A granite?gabbro complex from Madagascar: constraints on melting of the lower crust

The Ranomandry Complex is a Neoproterozoic, nested intrusion from central Madagascar composed of a gabbroic core within a coeval peraluminous granite ring intruding pelitic metasediments. Although xenocryst entrainment and magma mixing have both contributed to marginal phases of the granite, the primary melt is characterised by steep LREE/HREE ratios and negligible, or slightly positive, Eu anomalies. Both isotopic and trace element systematics preclude an origin from either partial melting of the metapelitic country rock or from fractional crystallisation of the gabbroic magma. However, trace-element modelling suggests an origin from the dehydration melting of a plagioclase-poor, garnet-bearing metagreywacke at temperatures of 850–900 °C and at lower crustal pressures (>10 kbar). Melting of an enriched subcontinental mantle generated gabbroic magmas that caused advective heating and anatexis at the base of thickened continental crust during their ascent. Transport of the resulting granite magma was facilitated by the pre-existing plumbing system that overcame thermal and mechanical problems associated with both diapirism and self-propagating dykes as mechanisms for long-distance transport of granite magmas. The nested geometry of the intrusions is an indication of a structurally homogeneous lower crust that contains no pre-existing shear zones or fault systems.

Size effect law and fracture mechanics of the triggering of dry snow slab avalanches

A size effect law for fracture triggering in dry snow slabs of high enough length‐to‐thickness ratio is formulated, based on simplified one‐dimensional analysis by equivalent linear elastic fracture mechanics. Viscoelastic effects during fracture are neglected. The derived law, which is analogous to Bažant's energetic size effect law developed for concrete and later for sea ice, fiber composites, rocks, and ceramics, is shown to agree with two‐dimensional finite element analysis of mode II cohesive crack model with a finite residual shear stress. Fitting the proposed size effect law to fracture data for various slab thicknesses permits identifying the material fracture parameters. The value of preexisting shear stress in a thin weak zone of finite length is shown to have significant effect. There exists a certain critical snow depth, depending on the preexisting stress value, below which the size effect disappears. Practical applications require considering that the material properties (particularly the mode II fracture toughness or fracture energy) at the snow slab base are not constant but depend strongly on the slab thickness. This means that one must distinguish the material size effect from the structural size effect, and the combined size effect law must be obtained by introducing into the structural size effect law dependence of its parameters on snow thickness. The thickness dependence of these parameters can be obtained by matching the combined law to avalanche observations. Matching Perla's field data on 116 avalanches suggests that the mode II fracture toughness is approximately proportional to 1.8 power of snow thickness.

Crustal architecture and evolution of the Eastern Ghats Belt and adjacent regions of India

Abstract Extending along the east coast of peninsular India, the Eastern Ghats expose a deep section through a composite orogenic belt that once formed part of the Proterozoic mobile belt system within East Antarctica and East India. The critical evaluation of the existing geological and isotopic data strongly suggests that this orogenic belt includes not only the granulite facies Eastern Ghats Belt but also the Nellore-Khammam Schist Belt and lower grade units at the southern margin of the Singhbhum Craton. The present authors propose its subdivision into four crustal provinces with widely different geological evolutions. The Rengali and Jeypore Provinces formed at the margin of the Bhandara Craton in the Late-Archaean. In the Krishna Province, volcanosedimentary rocks equivalent to the Cuddapah Supergroup accumulated, probably on the Dharwar Craton in the Palaeoproterozoic, and the major tectonometamorphic event took place between 1.67 and 1.55 Ga, subsequent to a short-lived igneous activity. The Eastern Ghats Province, which shows considerable similarities with the Rayner Province of East Antarctica, was strongly affected by pervasive deformation, high-grade metamorphism and crustal-derived magmatism between 1.1 and 0.9 Ga, which extensively modified the crustal structure of present eastern peninsular India. Neoproterozoic and Early Phanerozoic tectonothermal activities were largely restricted to pre-existing shear zones, but the present configuration of the composite orogenic belt may have been achieved only during the Pan-African Orogeny.

A plane slide that occurred during construction of a national expressway in Chongqing, SW China

This paper presents a case study of a major landslide that occurred during the construction of a national expressway in Chongqing, southwestern China. The landslide was a typical plane failure associated with steep cutting in weathered mudstone and sandstone. The weathered mudstone block slipped down along the bedding interface of the mudstone and sandstone and formed an open gap that measured some 6 to 8 m high, by 2 to 7 m wide and 53 m long in the hillside slope. This study referred that the landslide was caused by the combined effect of the following factors: (a) an unsupported high and steep cutting in weathered mudstone; (b) an exposed bedding interface between the incompetent weathered mudstone and the competent sandstone that daylights within the cut slope; (c) the mudstone and sandstone bedding interface as a pre-existing folding tectonic shear plane of low shear strength; and (d) the possible presence of a ground water table in the weathered mudstone perched on the relative impervious sandstone bedding surface. The case study highlights the importance of engineering geologicalmapping during and immediately after the formation of a new cut slope. It also emphasises the importance of identification of the pre-existing tectonic shear planes along the mudstone and sandstone bedding interfaces in similar geological terrain.

Nature, age, and tectonic setting of granitoid-hosted, orogenic gold deposits of the Jiaodong Peninsula, eastern North China craton, China

The Jiaodong gold province, along the southeastern margin of the North China craton, has combined production and resources of >900 tonnes of gold. More than 95% of the gold in this province is hosted in Mesozoic granitoids, which intruded Late Archean metamorphic basement rocks episodically at ca. 160–150 and 130–126 Ma. The deposits of the Jiaodong Peninsula form one of the largest provinces of granitoid-hosted lode-gold deposits in the world. The Jiaodong gold deposits occur as massive auriferous quartz veins hosted in subsidiary second- or third-order faults, and as disseminated- and stockwork-style mineralization hosted in alteration zones along major regional-scale faults. Mineralization took place during brittle reactivation (D3) of pre-existing D2 ductile shear zones. Wall rock alteration is dominated by K-feldspar and sericite, with also sulfide minerals and extensive silicification closest to the orebodies. The alteration was accompanied by the introduction of volatiles, Fe, Mg, K, Rb, Ag, As, Au, Bi, and Sb. Enrichment of base metals in the gold lodes is generally low, although locally significant. Gold deposition took place at temperatures of 250 to 350 °C for most gold deposits, and the ore fluids are CO2-bearing and have calculated salinities of 6–13 wt% NaCl equiv. Deposits from the Zhao–Ye, Xixia, and Muping–Rushan belts have similar, if not identical, structural controls, alteration patterns, ore mineral assemblages, and stable isotope signatures. Lead isotope data indicate that the ore-stage leads were buffered by their host rocks. The granitoid-hosted gold deposits in the Zhao–Ye belt are most radiogenic, the greenstone-hosted deposits in the Xixia belt is the least radiogenic and the granitoid-hosted deposits in the Muping–Rushan belt have intermediate compositions. Gold mineralization in the Zhao–Ye, Xixia, and Muping–Rushan belts in the Jiaodong gold province appears to be broadly contemporaneously at ca. 130–120 Ma, as constrained by SHRIMP U–Pb zircon dating on host rocks and a post-gold dike, as well as by Ar–Ar, Rb–Sr, and K–Ar dating of hydrothermal alteration minerals and ores. This Early Cretaceous age is about 100 million years younger than the peak of ultra-high pressure metamorphism in the Qinling–Dabie–Sulu orogen between the North China and Yangtze cratons, and about 500 km south of the gold province. However, it overlaps with the extension and exhumation stage of the collisional orogeny, and the subduction of the Pacific plate beneath the Eurasian continental crust. Whereas the majority of the Jiaodong gold deposits occur on the margin of the North China craton, some also occur in the suture zone between the two cratons. They are significantly landward relative to the subduction zone of the Pacific Plate compared with typical orogenic lode-gold deposits globally. Lode-gold deposits of ca. 130–120 Ma occurred on both sides of the Tan-Lu fault zone. Available data favor a temporal link of gold mineralization with Pacific Plate subduction.

Late Oligocene volcanism and extension in the southern Red Sea Hills, Sudan

The temporal and spatial relationships between Cenozoic extension in the Red Sea and flood basalt volcanism in the Ethiopian plateau region were poorly understood along the Red Sea Hills region of Sudan. Rift basins in the Red Sea area developed in terranes accreted in Pan-African time, and it is believed that some of these ancient sutures controlled later Red Sea rifting. The objectives of field, remote sensing and geochronological studies of the southern Red Sea Hills near the Sudan–Eritrea border were to constrain the geometry and kinematics of extension, the timing of volcanism, and the structural relationship between Cenozoic and Pan-African faults. Basaltic volcanism began at about 31 Ma in the Derudeb region, coincident with the widespread flood basaltic volcanism in Ethiopia and Yemen associated with the Afar plume, suggesting a causal link. Felsic lavas erupted onto a faulted erosional surface at about 29 Ma, based on 40 Ar/ 39 Ar step heating results on feldspar separates. The original form of these basins has been modified by subsequent faulting, Red Sea rift flank uplift, and erosion. Structural patterns indicate that extension increases eastward toward the Red Sea, with stratal dips of up to 35° in the Adar Ribad region, but most of this fault block rotation post-dates the rhyolite eruptions. The location of the Odi and Adar Ribad basins, and the reactivation of segments of Pan-African shear zones of the Baraka suture suggest that the development of these basins was influenced by pre-existing crustal shear zones, although some faults and dykes cross-cut other basement structures.