摘要:

ABSTRACTExposed crystalline basement of the Serre Mountains in Calabria presents a tilted block of a nearly complete section of the Hercynian continental lower crust (HCLC). In addition to petrological and structural data from surface mapping, and petrophysical data from the laboratory, a seismic reflection‐refraction experiment was conducted in May 1990. This consisted of a 40 km long N‐S profile crossing the HCLC and of four short transverse profiles, each recorded using 3‐component receivers with an 80 m spacing and explosive sources.The reflectivity of the outcropping lower crustal units is lower than theoretically predicted from the observed compositional layering. A low‐velocity zone, outcropping in the north, and dipping to the south, marks the contact between the HCLC and the underlying Alpine metamorphic units. Below this zone, the deeper crust appears well‐structured by strong and continuous, discrete reflections down to 6.5–8 s t.w.t. (presumably the crust‐mantle boundary at 19–24 km depth) with a dominant dip toward the south.Analysis of refracted‐wave velocities reveals values systematically lower by up to 30% than laboratory data on rock samples or calculated data from modal analysis. This discrepancy can only partly be explained by the effect of microcracks (10%), the underestimation of the amount of leucosomes (2–5%) and the effect of seismic anisotropy (0–5%). The remaining discrepancy must be attributed to large scale alteration of the rocks due to Apennin

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00452.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe CO2atmospheric content has shown large variations over geological times. High contents (up to one order of magnitude more than present‐day values) ultimately correspond to discrete episodes of mantle degassing, either juvenile, or subduction‐related (carbon recycling). A number of arguments (e.g. the continuous volume increase of carbonate‐bearing sediments with time) suggest that, throughout the Earth's history, juvenile CO2has formed a major contribution to the global carbon budget of the Earth.The absence of a direct relationship between major volcanic episodes and the average CO2atmospheric content suggests that volcanoes might not be the only way by which mantle CO2is transported to the surface. It is proposed that large quantities of juvenile CO2could temporarily be stored in the lower continental crust during major episodes of granulite formation. These are primarily caused by magmatic underplating and they result in a vertical accretion of the crust by accumulation of CO2‐bearing, mantle‐derived magmas. Most of the CO2migrates through the crust during post‐metamorphic evolution and isostatic restoration of the normal continental thickness. However, large quantities of CO2can still be present in some areas, notably as high‐density fluids enclose

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00453.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTDeep seismic reflection images from a set of profiles shot in Western Europe have been reviewed and compared, and tentative conclusions have been proposed concerning the evolution of the layered lower crust and the Moho. The disappearance of Variscan mountain roots is related to the set‐up of a new Moho at a typical 30‐km depth and the creation of seismic layering in the lower crust. Deep seismic profiles suggest that these processes resulted, at least in part, from magmatic intrusion, partial crustal melting and metamorphism of deep crustal rocks into eclogite. On the other hand, the layered lower crust is greatly attenuated beneath Cretaceous basins and Tertiary rifts in relation to prominent Moho upwellings. The unusual amplitude of the Moho reflection and the presence of anomalously high seismic velocities in the lowermost crust beneath the Tertiary rifts suggest that the Moho and part of the layering are comparatively young features related to interactions between crust and mantle. Beneath Triassic‐Jurassic basins, the layered lower crust was not affected by the subsidence of the basement, with the whole crustal thinning being entirely concentrated in the upper crust. This indicates that the layered lower crust and the Moho were formed or restored during or after the main rifting phase. Seismic data reveal constraints on the processes that affect the crust‐mantle transition and seem to restore the Moho to its typical depth after any mechanical deformation of the lith

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00454.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe chemical and petrological composition of mafic rocks from the lower continental crust are discussed by comparing mafic granulites and meta‐gabbroic rocks from the Ivrea Zone and the Northern Hessian Depression (NHD) xenolith suite. Both regions contain contrasting types of meta‐mafic lithologies (i) former basaltic rocks with trace element patterns ranging from MORB‐Iike to subduction‐related or intra‐plate tholeütes and (ü) Ca‐and Al‐enriched, plagiodase‐dominated gabbroic rocks showing positive Eu‐anomalies generated by complex deep crustal magmatic processes such as fractionation, accumulation of plagiodase and pyroxene, and crustal contamination. The absence of typical garnet‐omphadte parageneses in these rocks indicates that the eclogite stability field was not reached during Palaeozoic orogenic processes.A compilation of experimentally determined P‐wave velocities and densities for mafic granulites, gabbroic rocks, eclogites and peridotites is used to evaluate key physical properties of lower crustal mafic rocks during crystal thickening caused by continent‐continent collision. In a step‐by‐step scenario it is demonstrated that the position of the seismic Moho (defined as a first‐order velocity discontinuity) and the petrological Moho (defined as the boundary between non‐peridotitic crustal rocks and olivine‐dominated rocks) is not identical for the case that mafic rocks are transformed into edogites at the base of orogenically thickened crust. P‐wave velocities of eclogites largely overlap with those of peridotites, although their densities are significantly higher than common upper mantle rocks.As a consequence, refraction seismic field studies may not detect edogites as crustal rocks. This means that the seismic Moho detected by refraction seismic field studies appears at the upper boundary between edogites and overlying crustal units. Since edogites generally have higher densities than peridotites, they might be recycled into the deeper lithosphere thereby transferring excess Eu into the upper mantle. This process could be a due for understanding the negative Euanomaly in the upper continental crust which is apparently not balanced quantitatively by the ab

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00455.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe Kola Superdeep Drillhole was drilled through 7 km of Proterozoic rocks of the Pechenga structure and 5 km of the Archean basement in the Baltic Shield to 12 km depth. The results of drilling revealed a significant heterogeneity of physical properties of the crystalline rocks and a surprisingly high porosity and water content throughout the section. Seismic wide‐angle reflection data, obtained in the region of the hole, were used to delineate the volcanic‐sedimentary layers of the Pechenga structure and the intersecting faults. These studies revealed also a subhorizontal boundary at a depth of 7 km of uncertain origin. Core sampling implies that this boundary is associated with a change in core pressure, with higher porosity and with velocity anisotropy. A change in rock fissuring and in the fluid regime correlates with a velocity inversion zone reached by the hole at a depth of 4.5

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00456.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

摘要:

ABSTRACTThe site of the ancient Greek city of Helice, which was destroyed by an earthquake in 373bc, has never been found. We propose that it was submerged as the result of seismic slip on a normal fault traceable SE of Egion and that the fault was reactivated in 1881, when Schmidt reported a fresh scarp 2 m high and 13 km in length at the same locality. Earlier movements on the fault are inferred fromLithophagaborings on coastal Sectors of the footwall which yield14C ages representing over 6.5 m of uplift in the last 4880 years.

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00457.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00458.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00459.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00460.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1992.tb00461.x

出版商:Blackwell Publishing Ltd    

年代:1992

数据来源: WILEY