ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00718.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00719.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTMylonitic gneisses of the Bulgarian and Greek Rhodope were deformed under medium pressure‐type metamorphism. The kinematic information contained in these gneisses shows that shear‐deformation occurred during development of a nappe complex. Lithologies and metamorphic histories allow a lower (footwall) and an upper (hanging wall) terrane to be distinguished that define a crustal‐scale duplex. As oceanic crust is involved, collision between two continental units with subsequent crustal thickening is inferred. The blocks would be Moesia to the north, and the Lower‐Rhodope promontory to the south, which collided in the Mesozoic to early Cenozoic. The nappe complex is characterized by south to southwestward (foreland directed) piling‐up and is associated with both coeval and subsequent extension. The late extension is associated with the establishment of a high temperature‐low pressure metamorphic gradient and plutonism that predates, but makes a transition to, the lithospheric extension of the

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00720.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe study of the directional dependence of seismic velocities (seismic anisotropy) promises more refined insight into mineral composition and physical properties of the crystalline crust than conventional deep seismic refraction or reflection profiles providing average values of P‐and S‐wave velocities. The alignment of specific minerals by ductile rock deformation, for instance, causes specific types of seismic anisotropy which can be identified by appropriate field measurements.Vice versa, the determination of anisotropy can help to discriminate between different rock candidates in the deep crust. Seismic field measurements at the Continental Deep Drilling Site (KTB, S Germany) are shown as an example that anisotropy has to be considered in crustal studies. At the KTB, the dependence of seismic velocity on the direction of wave propagationin situwas found to be compatible with the texture, composition and fracture density of drilled crustal ro

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00721.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTIn recent years, there has been a growing awareness of the importance of local site effects in earthquake damage. A number of studies of recent destructive earthquakes have illustrated the relative contribution of enhanced ground shaking due to unconsolidated sediment layers.Among the different methods used to estimate local site response, the spectral ratio of shaking at a sedimentary site with respect to a bedrock reference site, has been successfuly applied in different geological environments. In this study, a technique recently proposed by Nakamura (1989) is used to evaluate site response using spectral ratios of horizontal vs. vertical components of earthquake recordings from a temporary ocean bottom seismograph (OBS) network in the northern North Sea and a permanent OBS at Oseberg oil field. Comparison with results obtained from the standard spectral ratios, indicate that the method is applicable also to subsea conditions, and the estimates obtained in this study indicate similar amplification factors to those obtained previously from analytical techniques. The ambient noise data on the other hand, gave unstable results, probably due to different noise characteristics in the marine environment. The results obtained on the earthquake data, however, provide an encouraging alternative to previously used analytical techniques for estimating local site response.

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00722.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Amirante ridge/trough complex developed along the Late Cretaceous transform boundary that separated the Seychelles/India and Madagascar/Somali Basin plates. Motion between these plates was complex, comprising sinistral N‐S strike‐slip movement coupled and coeval with counter‐clockwise rotation induced when seafloor spreading developed in only the southern portion of the transform. The overall morphology of the complex comprises a series of arcuate ridge and trough segments. These segments were successively produced by tectonic and volcanic activity within the zone of migrating plate contact adjacent to the rotational pivot where compression was replaced by extension along the transform boundary. In the extensional regime to the south of this contact zone the Mascarene oceanic basin developed, whilst in the compressional zone further north island arcs developed and the ophiolites of Baluchistan were obducted from the Somali Basin onto the leading edge of the Seychelles/India

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00723.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Palaeoproterozoic Lapland Granulite Belt is a seismically reflective and electrically conductive sequence of deep crustal (6–9 kbar) rocks in the northern Fennoscandian Shield. It is composed of garnet‐sillimanite gneisses (khondalites) and pyroxene granulites (enderbites) which in certain thrust sheets form about 500 m thick interlayers. The structure was formed by the intrusion of intermediate to basic magmas into turbiditic sedimentary rocks under granulite facies metamorphism accompanied by shearing of the deep crust about 1.93–1.90 Gyr ago (Gal. Granulites were upthrust 1.90–1.87 Ga and the belt was divided by crustal scale duplexing into four structural units whose layered structure was preserved. The thrust structures are recognized by the repetition of lithological ensembles and by discordant structural patterns well distinguishable in airborne magnetic and electromagnetic data. Thrusting gave rise to clockwise pressure‐temperature evolution of the belt. However, some basic rocks possibly record an isobaric cooling path. The low bulk resistivity of the belt (200–1000 Ωm) is caused by interconnected graphite and subordinate sulphides in shear zones. On the basis of carbon isotope ratios this graphite is derived mostly from sedimentary organic carbon. The seismic reflectivity of the belt may be caused by velocity and density differences between pyroxene granulites and khondalites, as well as by

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00724.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTGloborotalia puncticulataandGloborotalia margaritaeare critical species that define internationally recognized planktonic foraminiferal biozones in the Pliocene. These biozones are defined from stratotype sections on Sicily and their fauna are commonly considered to have been introduced to the Mediterranean after an influx of Atlantic water that terminated the late Miocene desiccation of the basin. Herein new discoveries of these species in rocks that predate the late Miocene desiccation are described. These data are supported by magneto‐ and lithostratigraphy that have been integrated at a single continuous section. Not only do these discoveries question the existing foraminiferal biozone stratigraphy, they also suggest new models for the dispersal of planktonic species. It is proposed thatGloborotalia puncticulataand perhaps evenGloborotalia margaritaeevolved in the Mediterranean during earliest Messinian times (during or before chron C3Bn1n) and dispersed into the Atlantic. This suggests that a marine connection remained between the two sea areas until at least chron C3An.1n. Using the existing geomagnetic polarity time scale, these occurrences are some 2 Myr earlier than previously recorded in the Mediterranean. The distribution ofG. margaritaeand G.puncticulatain Mediterranean sections is likely to reflect palaeoenvironment or the preservation of deposits rather than the absolute age of the sediment

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00725.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTThe Umbria‐Marche‐Sabina foreland fold and thrust belt (Northern Apennines, Italy) provides excellent test‐cases for the hypothesis of ancient syndepositional structural features controlling thrust ramp development. The sedimentary cover, Late Triassic to Miocene in age, is made of platform and pelagic carbonates, whose deposition was controlled by significant synsedimentary extension. Normal faulting, mainly during the Jurassic and the Late Cretaceous‐Palaeogene, determined sensible lateral thickness variations within the relative sequences. By late Miocene the sedimentary cover was detached from its basement along a mainly evaporitic horizon, and was deformed by means of eastward‐verging folds and thrusts.In order to locate the points where thrust ramps branch‐off the basal detachment, both line‐length and equal‐area techniques were used in the construction of a balanced cross‐section through three major fault‐related folds in southeastern Umbria. The nucleation of thrust ramps was controlled by the occurrence of Jurassic and Cretaceous‐Palaeogene synsedimentary normal faults. These interrupted the lateral continuity of the evaporitic unit (the Late Triassic Anidriti di Burano Fm.) at the base of the sedimentary cover, and acted as obstacles to the eastward propagation of the thrust system, giving rise to major folds which originated from tip‐line folding processes.Therefore, the inferred relationships between ancient normal faults and late thrusts indicate that synsedimentary tectonic structures and the related lateral stratigraphic variations can be envisaged as mechanically important perturbations, which effectively control the nucleation and deve

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00726.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY

摘要:

ABSTRACTStudies on Cretaceous pelagic limestones have shown a close correspondence between putative sea‐level curves and the carbon‐isotope profile, suggesting a possible relationship between the two phenomena. Such a relationship could relate to shelf‐sea area governing the global burial rate of organic carbon which, in turn, controls the13C/12C ratio of dissolved inorganic carbon in the oceanic reservoir. In order to see whether or not this relationship might be of more general applicability, the major mid‐Oxfordian (Jurassic) transgression is documented to zonal level and appropriate carbon‐isotope data are presented. Although the applicability of ammonite zonation to the mid Oxfordian is problematic on a regional scale, a clear δ13C excursion is present in thetrunsversariurnZone, which is coincident with regional overstep and/or evidence of shoreline retreat and/or evidence of bathymetric deepening in Europe and elsewhere. The use of the carbon‐isotope curve as a proxy for shelf‐sea area or relative sea level is worthy of furth

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1996.tb00727.x

出版商:Blackwell Publishing Ltd    

年代:1996

数据来源: WILEY