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Although many intact rock types can be very strong, a critical confining pressure can eventually be reached in triaxial testing, such that the Mohr shear strength envelope becomes horizontal. This critical state has recently been better defined, and correct curvature or correct deviation from linear Mohr–Coulomb (MC) has finally been found.
Folds in ductile shear zones are common structures that have a variety of origins. These can be pre-existing folds that become modified or folds developed during the shearing event. Among the syn-shearing folds, a second subdivision is based on the relative age of the folded surface, which can be pre-existing or newly formed during the shearing event. In each of the three categories final fold geometry and orientation show complex relationships with the kinematic frame. The final fold geometry depends on the vorticity within the shear zone, the rheology and the initial orientation of the folded surface relative to the kinematic framework. It follows that folds are complex structures, difficult to use as kinematic indicators. However, in shear zones where undeformed wall rocks with pre-shear structures are accessible and where kinematics can be well established, folds can provide a valuable natural means to understand the initiation and evolution of structures under non-coaxial regimes. We point to the need of discriminating among different plausible categories, based on the nature of the folded surface and on the inherent structural features of each category. <...>
For decades, the base of the Hettangian (the Triassic-Jurassic system boundary) in the terrestrial succession of the Newark Supergroup of eastern North America has been placed incorrectly at a stratigraphic level that coincides with a modest palynological turnover immediately beneath the lowest basalt flow of the Central Atlantic Magmatic Province (CAMP), e.g. the Orange Mt. Basalt in the Newark basin or the North Mountain Basalt in the Fundy basin (see review in Whiteside, et al., 2007). According to numerous studies (reviewed in Whiteside, et al., 2007),
The Paleo-Mesoproterozoic Xiong’er volcanic rocks along the southern margin of the North China Craton are lithologically and geochemically similar to those formed in subduction-related, continental margin volcanic arcs. The volcanic rocks are primarily composed of basaltic andesites and andesites, with minor dacites and dacitic rhyolites. Traditionally, the Xiong’er volcanic rocks have been divided from lower to upper into the Xushan, Jidanping and Majiahe Formations, but the ages of volcanic rocks in these formations have not been well constrained, which has hindered further understanding the tectonic significance oftheXiong’er volcanicbeltatthe southern marginofthe NorthChina Craton.
In previous years, the genesis of uniquely large mineral deposits became one of the most urgent geological problems because of the key role played by giant deposits: accounting for <5 % of the currently developed mineral deposits, these deposits satisfy 85 % the world’s consumption of mineral resources. The development of these deposits should provide a basis for the stable progression of our civilization (Rundkvist and Kravchenko 1996 ). Another important aspect in the analysis of super-large mineral deposits is their genesis because these deposits are, in fact, giant geochemical anomalies in the Earth’s crust (e.g., the PGE concentrations in the Noril’sk ores are six to seven orders of magnitude higher than those in the clarkes) <...>
The Hattu schist belt is an emerging gold-producing ore district in the western part of the Archean Karelian Province of the Fennoscandian Shield. The belt consists of 2.76 to 2.70 Ga tonalite, granodiorite, and leucogranite intruded into a mafic-felsic epiclastic-volcanic sequence of an only slightly older age. Complex and successive folding, shearing, and hydrothermal processes affected these rocks prior to the lower amphibolites facies peak metamorphism (550° ± 50°C; 3–5 kbar) at ca. 2.70 Ga. Orogenic gold deposits are hosted by the highly strained zones that developed during the Archean deformation of the belt. However, previous K-Ar and Rb-Sr geochronological studies indicated that a second tectonothermal overprint affected the Hattu schist belt between 1.7 and 1.8 Ga during the Svecofennian orogeny.
Силчестер: меняющийся облик римского города. Интеграция геофизики и археологии - результаты картографического проекта Силчестера 2005-10 гг.
Silchester (Calleva Atrebatum) is a Roman town in Hampshire, in central southern Britain. Unlike many the site was abandoned in the post-Roman period leaving it as a largely green-field site now. The Amphitheatre, Town Wall and some Outer Defences all remain visible for the visitor to see. The town has been highly influential in the development of Romano-British archaeology due to the large-scale excavation by the Society of Antiquaries in 1890-1908 developed an overall plan of the site.
This chapter reviews the geological occurrences, structures, and phase transitions of the low-pressure silica polymorphs-quartz, tridymite, and cristobalite. All these phases experience displacive transformations that involve structural contraction with decreased temperature, and research over the past three decades has sought out the mechanisms that control these transitions. The passage from ~- to a-quartz is associated with an intermediate phase that is stable over a 1.3°C temperature interval. X-ray diffraction and transmission electron microscopy have revealed that this phase consists of Dauphine microtwins that are incommensurately modulated. Meteoritic and synthetic tridymite experience a series of structural alterations with decreasing temperature during which the symmetry changes from hexagonal (HP) to orthorhombic (OC, OS, and OP) to monoclinic (MC). Phase transition behavior in terrestrial tridymite (PO-n and MX-1) is more complex, probably due to a greater degree of structural disorder. The transformation from cubic ~- cristobalite to tetragonal a-cristobalite is marked by a high spontaneous strain and a large hysteresis in the transition temperature. The three high-temperature polymorphs-s-bquartz, HP-tridymite, and ~-cristobalite-exhibit evidence for dynamical disorder, but the nature of the atomic oscillations in these phases remains an active area of investigation. <...>
Концепции корреляции с высоким разрешением возраста и фаций
As the search for oil and gas becomes more sophisticated and producing basins and fields become more intensely developed, geoscientists need correspondingly more accurate techniques for stratigraphic analysis. To achieve this accuracy, companies are shooting higher-resolution seismic lines, acquiring 3-D seismic surveys over fields, and coring more to quantify reservoir properties.
