ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00521.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe present study is based on the interpretation of more than 1300 km of 16 kJ sparker seismic profiles recorded in July 1990, during the Cruise T‐41 of the Geological Institute of Urbino. The investigated area extends along the 41st parallel in the central Tyrrhenian Sea between the northern Sardinian margin to the west and the Latium–Campanian margin to the east. This zone, situated on continental crust, marks the boundary between the northern Tyrrhenian and the southern Tyrrhenian domains.A kinematic reconstruction is presented, based on the age‐dating of the recognized structures (i.e. normal faults, reverse faults, anticline and flower structures). The evolution of the ‘41st parallel zone’ can be described in terms of polyphase tectonics characterized by different orientations of the stress field during time. The direction of the normal fault‐trends, turned clockwise, striking NE–SW in the late Tortonian–Messinian, E–W in the early Pliocene, NNW–SSE in the late Pliocene and N–S during the Quaternary. The concurrence of compressional and strike‐slip deformations suggests oblique shear motions across the 41st parallel. The occurrence of late Pliocene–Quaternary tectonic activity in the northern Tyrrhenian Sea, locally characterized by inversion tectonics, suggests active mechanisms (intraplate compression?) superimposed on

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00522.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTIn the internal part of the Umbro‐Marchean‐Romagnan Apennines, the foredeep clastic wedge constituting the Neogene part of the sedimentary cover is completely detached from the underlying Mesozoic–Palaeogene succession. The resulting (Umbro‐Romagnan) parautochthon consists of tectonostratigraphic units with a general geometry of broad synclinal blocks separated by narrow faulted anticlines.Thrust‐related structures observed in the field require thrust ramp propagation to have occurred within already folded rocks; therefore, they cannot be restored using simple fault‐bend fold or fault‐propagation folding models. Evidence for a passive fold origin in the studied rocks suggests that an early detachment folding episode preceded ramp propagation. The latter was facilitated by the enhanced thickness of incompetent material in the cores of detachment anticlines, which became the preferential sites where thrust ramps cut up‐section. Depending on the trajectory of such thrust ramps, different types of fault‐related structures could develop. Hanging‐wall anticlines which give way to monoclinal structures higher up in the section are associated with listric thrust ramps, whereas hanging wall monoclines approximately parallel to the underlying fault surface are associated with straight‐trajectory ramps.This kinematic evolution, which occurred partly during syn‐depositional compression, also accounts for the observed lithofacies distribution. The latter reflects an early differentiation of the foredeep trough into sub‐basins that are progressively younger towards the foreland. The detachment anticlines that originally bounded such sub‐basins were the site of later thrust propagation, leading to a tectonic juxtaposition of different tectonostratigraphic units consisting of broad NW

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00523.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThe significance of mafic enclaves as indicators of magma mixing processes between juxtaposed felsic and mafic magmas is evaluated from a rheological point of view. A qualitative model for explaining strain and morphological changes in the enclaves has been developed on the basis of the catastrophe theory.Mafic enclaves in intrusive granitoids commonly behave as physical systems that can be described using a cusp catastrophe model. Their behaviour is characterized by bimodality, divergence and sudden changes, which are properties typical of this model. Accordingly, the presence of mafic enclaves showing variable strain and morphology within the same granitoid intrusion would be indicarive of mutual interaction and mingling between mafic and felsic magmas. Due to the characteristics of these processes, it is not possible to establish unambiguous age relationships between the two magmatic components.

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00524.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTThree Palaeozoic sequences belonging to three different basins crop out in the Calabrian–Peloritan Arc. Their age covers the time span from middle (?) Cambrian to early Carboniferous. The sequences comprise terrigenous, volcanic and carbonate rocks, and show low‐grade metamorphism. The basement is of pre‐middle Cambrian age, crops out in the Calabrian–Peloritan Arc, and was metamorphosed prior to the opening to the Palaeozoic basins.The Palaeozoic basins existed no later than the middle Carboniferous, with inferred maximum crustal extension in the Cambro‐Ordovician. By Devonian time, the tectonic regime was compressional overall with middle Devonian island‐arc type volcanic activity that continued until the closure of the basins. Approximately 330 Myr, the Palaeozoic sequences experienced low‐pressure greenschist facies metamorphism and continuing subduction controlled the Variscan tectonogenesis with 280 Myr island‐arc type intrusive magmas. Subduction ceased and late‐stage 280–270 Myr granitic magmas were emplaced during continental collision.The crustal sector carrying the Palaeozoic basins is interpreted as comprising fragments of an active continental margin, one of the several microplates, located at the southern margin of the Euro‐Asia continent which faced a large (at least 1500 km

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00525.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTA sedimentary sequence overlying a granite pluton near Ishkarwaz (upper Yarkhun valley, Chitral, Pakistan; Karakorum Microplate) contains abundant, but poorly preserved, acritarchs probably referable to the late early Arenig‐early late Arenig interval. The palynological assemblages of Karakorum show a marked similarity to the cold water Peri‐Gondwana assemblages; i.e. to those of Li Jun'sArbusculidium‐Coryphidium‐Striatotheca‘Mediterranean’ Bioprovince. Biogeographical and geological comparisons suggest that, before the accretion of Cimmerian microplates to the Eurasian continent, the Karakorum Microplate was located along the northern margin of Gondwana in a latitude intermediate between the Mediterranean region and South China (Yangtz

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00526.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTIn the past, the doorstopper method has been used successfully for determiningin situtectonic stresses. Here an example is given about the application of the borehole slotter in the domain of tectonic stress determinations, which is an innovative strain relief technique without overcoring. Data are presented that test the reliability of the strain measurements by the borehole slotter under well‐known stress conditions, that indicate the reproducibility of borehole slotter strain data compared with those from doorstopper measurements, and that demonstrate the utility of the borehole slotter even for the determination of weak tectonic strains. In northern Switzerland the borehole slotter was used in a horizontal borehole under lithostatic loading conditions only, in a rock slope area with very heterogeneous stress conditions, and in a vertical borehole where tectonic stress should be dominant. The results show that the borehole slotter is a reliable technique, which allows precise determination ofin situstrains. In addition, borehole slotter measurements are less time‐consuming and therefore less costly than most other stress‐measuring techniques—an important aspect in a future of reduced research budgets.SummaryThe borehole slotter is a strain relief technique, using a recoverable strain sensor that need not be glued to the rock surface and need not be overcored. It is quicker to use than most other stress or strain measuring techniques, especially in comparison to standard overcoring techniques (Fig. 14).The borehole slotter supplied convincing results with respect to principal stress orientations, in good agreement with:(i) principal stress orientations related to lithostatic stress conditions (Dittingen 2 site),(ii) palaeostress orientations during the main stage of Jura folding only 13 to 7 Ma (Laufen 1 site),(iii) the results of neighboring door‐stopper measurements (Dittingen 1 site).The borehole slotter tests do not supply stress magnitudes directly. Thus, the elastic properties of the rock mass have to be determined independently. Stress ratios from borehole dotter tests in Dittingen 2 are realistic.In conclusion, the borehole slotter is a fully developed probe to measurein situstrains in boreholes. It allows the reliable determination of a large number of data within a short time even in weakly stressed rocks. This makes the borehole slotter a powerful tool for the determination of contemporary regional tectonic stre

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00527.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

摘要:

ABSTRACTRegolith occurs at a number of localities along the northern margin of the early Proterozoic Pechenga Greenstone Belt marking a profound stratigraphic unconformity at the base of the 10 km thick Petsamo Supergroup. The age of regolith formation can be bracketed between 2.453 ± 42 Ga (Sm‐Nd) and 2.330 ± 36 Ga (Rb‐Sr). Regolith is developed on a varied substrate of Archaean rocks including early Proterozoic layered gabbros (2.45 Ga). The best example of the regolith occurs in the Pasvik valley at Brattli, Norway. Analogues are widespread on the Baltic Shield at the base of early Proterozoic ‘greenstone belts’ showing a low degree of chemical weathering and carbonatisation. Fluviatile deposits overlying the Brattli regolith preserve unweathered pyrite‐magnetite boulders and terrigenous pyrite grains. These features are considered to relate an arid or semi‐arid palaeoenvironment, possibly with some deficiency of oxygen in the atmosphere‐hydrosphere system. A systematic and well‐preserved regolith or palaeosaprolite at the base of the Petsamo Supergroup is proof that the northern boundary of the Pechenga Greenstone Belt is a fundamental first‐order unconformity. This places constraints on geotectonic modelling of this early Pro

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00528.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00529.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY

ISSN:0954-4879    

DOI:10.1111/j.1365-3121.1994.tb00530.x

出版商:Blackwell Publishing Ltd    

年代:1994

数据来源: WILEY