ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00548.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe formation of two orthogonal sets of extension joints either crossing or abutting each other is a typical product of brittle deformation. Such systems of joints, with the two joint sets being geologically coeval, have been called a fracture grid‐lock. The two sets are of common genesis and thus a unique remote stress field can be inferred. This interpretation causes some perplexity if the two joint sets are purely extension fractures and formed perpendicular to the least principal stress. In the present paper a conceptual model to explain the origin and the evolution of such systems is proposed. In a volume of rock undergoing a tensional and uniform remote stress state, caused for example by a tectonic regime, two horizontal and negative (i.e. tensional) stress rates are assumed to exist. When the tensile strength of the rock is locally reached, failure occurs perpendicular to the least principal stress. Then, that direction locally experiences a positive stress drop due to the stress release. For this reason, the stress field, retaining the same principal directions, is locally distorted by a swap between the σ3and the σ2components in a volume of rock surrounding the fracture. As a consequence of the persisting remote stress rates, when elastic failure conditions are newly accumulated, a second fracture forms and propagates perpendicular to the previous one. Repeated failure events, stress‐drops and stress swaps eventually generate a fracture grid‐lock. The whole process is also described with a simplified analytical model by applying elasticity

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00549.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTIn 140 metres of Maastrichtian White Chalk (nannofossil chalk) exposed near Hemmoor, NW Germany, values of87Sr/86Sr increase from 0.707760 in theBelemnella sumensisZone (Lower Maastrichtian) at the base of the section (‐54.5 m; referred to 0 m at a prominent marl, M900) to 0.707821 in theBelemnella baltica/danicaZone (Upper Maastrichtian) at the top of the section (+84.5 m). A plateau in87Sr/86Sr occurs between ‐5m and +50m in the section, probably as a result of a very high rate of sedimentation in this interval. A belemnite and associated nannofossil chalk have similar87Sr/86Sr values, suggesting that there has been little diagenetic alteration of the87Sr/86Sr ratios in the chalk, which therefore preserves its original87Sr/86Sr. Comparison of87Sr/86Sr and nannofossil zonations for sequences at Bidart, France, and DSDP Sites reveals discordance and so possible diachronism of the basal boundaries of nannofossil Zones CC25B and CC

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00550.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe orientation of axes of sigmoidal, staircase or spiral inclusion trails within porphyroblasts provides an indicator of the direction of movement during deformation that is synchronous with metamorphism. A simple technique is presented in detail to find this axis in 3D by radially sectioning a horizontal slab cut from an oriented sample. When viewed from one direction, the switch in asymmetry of the porphyroblast inclusion trails in these sections defines the trend of the axis. Further radial sectioning of a vertical slab cut parallel to this trend determines the plunge of this axis.This technique is independent of the model adopted for the formation of sigmoidal or spiral‐shaped inclusion trails and can be used to evaluate the mechanism by which they form. It can also be used to evaluate theories of folding and orogenesis. Measurements of spiral and sigmoid axes in garnet porphyroblasts from the European Alps show that they reflect the movement of the African Plate relative to Europe better than linear indicators preserved within the same rock

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00551.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTIn order to better identify the mineral phase which controls the rheology of the transition zone (between 410 and 660 km depth) transmission electron microscopy observations were made on several coexisting spinel‐garnet assemblies: alkremite xenolith; pyrope‐rich – MgO:1.1Al2O3spinel assembly deformed at 1173K, 800 MPa in a Griggs apparatus; (Mg,Fe)3(Al,Mg,Si)2Si3O12majorite – (Mg,Fe)2SiO4spinel assembly synthesized in a laser heated diamond anvil cell. It was found that garnet crystals systematically remain undeformed while spinel crystals are plastically deformed. These results are in accord with the assumption that the rheology of majorite is stronger than the rheology of spinel, in the conditions of the transiti

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00552.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTA stratigraphic analysis of late Palaeocene sands of the Søgne Basin and the western part of the Norwegian–Danish Basin shows that the sand bodies are of differenct ages. The geographic distribution of the sand deposits shows that they are related to underlying Mesozoic structures suggesting a controlling effect of Tertiary tectonics on the deposition of sand during the late Palaeocene. However, the structural setting of various sand bodies varies from reactivation of older faults and reactivation of salt structures. The local character of the structures active during the late Palaeocene introduces minor depressions with no lateral connection. The sand bodies, which are interpreted as having been deposited in these depressions, are thus in general separate bodies with no lateral connection. The Fennoscandian shield and eroded Mesozoic sediments along the Fennoscandian Border Zone are suggested as source area for the late Palaeocene sand deposi

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00553.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTResolution and reliability estimates of results obtained by seismic tomography strongly depend on the reference model. Inadequate initial reference models may severely distort tomographic images or introduce artefacts that lead to misinterpretations of the results. Reference models are usually obtained by means ofa priorinear‐surface geological information or by geophysical information derived by controlled‐source seismology.Starting from the idea that a reference model must approximate the weighted average of data selected for the three‐dimensional (3D) inversion, one‐dimensional (1D) model for Northwestern Italy is derived that is able to minimize mean of RMS of a set of well‐locatable earthquakes, by computing a solution of the coupled hypocentre 1D velocity problem.Such a model, termed the Minimum 1D model, can be used both as an initial reference model for 3D inversion and as a reference velocity model for high‐quality routine earthqua

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00554.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

摘要:

ABSTRACTThe magnetic properties of the late Quaternary hemipelagic sediment from piston core BS77‐15 taken in the Tyrrhenian Sea were investigated and the results compared with palaeoclimatic records. The magnetic mineralogy of the sediment was studied and the magnetic carriers were identified by their spectra of coercivity and unblocking temperature. Investigations of rock magnetic parameters such as the Natural Remanent Magnetization (NRM), the low‐field magnetic susceptibility χ, the frequency‐dependent susceptibility χfd, the acquisition of Anhysteretic Magnetization (ARM) and Isothermal Magnetization (IRM) and the Median Destructive Field (MDF) have established the downcore variations in terms of magnetic mineralogy, grain size and coercivity in the sediments. The rock magnetic parameters were compared with an alternative climatic record from the same core based on the faunal association of planktonic foraminifera. A significant correlation was found between faunal climatic record and χfd. Together with the magnetic properties of the sediment this suggests that a palaeoclimatic signal is recorded by a fine‐grained low‐coercivity fraction of the magne

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00555.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00556.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1995.tb00557.x

出版商:Blackwell Publishing Ltd    

年代:1995

数据来源: WILEY