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- Routh, Joyanto, 1968-
(författare)
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Metamorphism and Structural Interpretation of the Zanskar Shear Zone, PJW Himalaya, India
- 1993
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Ingår i: Journal of the Geological Society of India. - : Geological society of India. - 0016-7622 .- 0974-6889. ; 41:3, s. 187-198
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Tidskriftsartikel (refereegranskat)abstract
- Herren in 1987, reported normal faults resulted in the telescoping of metamorphic isograds within a 200 meter zone between Sumche Topko to Mulung Topko in the Zanskar Shear Zone NW Himalaya. However petrographic study-of sections obtained from Mulung Topko and surrounding areas (pensila-padarn section) indicate that the rocks belong to the kyanite-sillimanite-starurolite grade only. Extension crenulation cleavage and other shear criteria show an initial NESW movement after which a layer parallel extension occurred and the shear zone developed. Minieralogical assemblage and mapping indicate that the isograds run paralleI to each other before they possibly truncate against the shear zone along the Pensila-Padam section. A probable model has been proposed to explain the features
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| 4. |
- Öhlander, Björn, et al.
(författare)
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Delineation and character of the Archaean-Proterozoic boundary in northern Sweden
- 1993
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Ingår i: Precambrian Research. - 0301-9268 .- 1872-7433. ; 64:1-4, s. 67-84
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Tidskriftsartikel (refereegranskat)abstract
- Before the deposition of a Proterozoic cover and the repeated Proterozoic reworking of the older rocks, the presently exposed Archaean areas in northern Sweden formed part of a coherent craton. In the present study, we have used Sm---Nd isotopic analyses of Proterozoic granitoids and metavolcanics to delineate the Archaean palaeoboundary. In a regional context, the transition from strongly negative εNd(t) values in the northeast to positive values in the southwest is distinct, and approximately defines the border of the old craton. The Archaean palaeoboundary extends in a WNW direction, and is subparallel to the longitudinal axis of the Skellefte sulphide ore district but it is situated ≈ 100 km farther to the north. The ≈ 1.9 Ga old granitoids on the two sides of the palaeoboundary were all formed in compressional environments, but those situated to the north have higher contents of LILE and LREE at similar contents of Si. This indicates that they were generated in an area with thicker crust and supports the location of the Archaean-Proterozoic palaeoboundary. There is no simple correlation between the Archaean palaeoboundary, as defined by the isotopic results, and any of the major fracture systems as interpreted from regional geophysical measurements. Reflection seismic work indicates that juvenile volcanic-arc terrains to the south have been thrust onto the Archaean craton. Possible thrust faults have been identified from aeromagnetic measurements. Rifting of the Archaean craton created a passive margin ≈ 2.0 Ga ago. Spreading shifted to convergence with subduction beneath the Archaean continent ≈ 1.9 Ga ago. Subsequently, the resulting juvenile volcanic arc collided with the old continent, and the Archaean palaeoboundary as existing today was formed by a collision characterized by overthrusting. The boundary then was disturbed by later deformation predominantly along NNE-trending fracture systems.
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| 7. |
- Wang, Yanmin, et al.
(författare)
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Magnetic aggregation in dispersions of mineral ultrafines
- 1995
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Ingår i: Journal of Dispersion Science and Technology. - : Informa UK Limited. - 0193-2691 .- 1532-2351. ; 16:2, s. 137-154
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Tidskriftsartikel (refereegranskat)abstract
- Particle aggregation can be enhanced in two ways; (a) field-induced magnetic moment and (b) magnetic moment due to the remanent magnetisation. Investigation of the magnetic field-induced aggregation of hematite and chromite in aqueous suspension with the use of a laboratory scale electromagnetic solenoid related the aggregation process to particle size and external magnetic field in the natural pH value of the dispersions. This study have shown that hematite ultrafines in a well-dispersed slurry are selectively aggregated with sized magnetite in the absence of aggregating reagents, high shear rates or an external magnetic field.
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| 8. |
- Ayele, Atalay
(författare)
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Seismicity and earthquake source mechanism study in the East African rift
- 1998
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- A new earthquake catalogue of the Horn of Africa has been compiled for the period from 1960 to 1993. The threshold magnitude of this catalogue is found to be 4.5 (mb). Spatial and temporal variation of seismic energy release and b-value are investigated for this catalogue. The b-value is lower in the continental part of the Afro-Arabian rift system (Afar and southern Sudan) than in the southern Red Sea and Gulf of Aden rifts which is in agreement with the increasing fault segmentation from north to south. From the recent spatial distribution of earthquakes in the East African Rift, Danakil & Aisha horsts and the Tanzanian Craton are behaving as prototypes of microplates.Earthquake source parameters in some parts of the East African Rift are studied for both the pre-WWSSN (World-Wide Seismographic Station Network) eras as well as afterwards. Digitized old seismograms from Wiechert-type instruments and bulletins are used to analyze source parameters for three pre-WWSSN events. The old analog seismograms are scanned and digitized using the nXscan software. P-wave first-motion readings and waveform data from short-period, broadband and long-period instruments are employed to investigate the earthquake source mechanisms.A total of 10 earthquakes in the magnitude range from 4.8 (mb) to 6.9 (Mw) are studied for fault mechanisms using P-wave first-motion readings and waveform inversion techniques. The results show both normal and oblique slip deformations with left-lateral strike-slip components. This indicates sinistral motion of the Somalia plate with respect to the Nubia plate, in agreement with the general E-W extension of the East African Rift.
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| 9. |
- Chouliaras, Gerasimos
(författare)
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Seismic quiescence patterns and earthquake prediction research : Testing hypotheses for Greece and China
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The main aim of this investigation is to evaluate the most well known hypotheses and models in earthquake prediction research. For this reason we investigate the "localized" seismic quiescence hypothesis using the Z value griding method as well as the recently introduced Seismolap hypothesis of "extended" seismic quiescence.The results indicate that precursory seismic quiescence has preceded crustal main shocks in China and Greece and also that aseismic quiescence can be attributed to "silent earthquakes " such as creep events in subduction zones. The application of the Z-value method in investigating the recent seismic quiescence regime in Greece has given indications of three areas in Greece where significant quiescence is observed and which also are candidate areas for strong earthquakes according to their previous seismic history.Recently, the Gulf of Corinth in Central Greece has been appointed as an official multiparameter site for earthquake prediction research by the European Council and in this framework, magnetotelluric and seismological investigations revealed the presence of fluids in the Earth's crust that are mainly responsible for the observed seismic and electromagnetic anisotropy which correlates well with the local stress field as revealed from GPS measurements. In addition, the spectral fitting method was used on digital short period data from this region in order toobtain dynamic source parameters from local earthquakes and also to determine scaling relations using two different earthquake source models.
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| 10. |
- Quintero, Ronnie
(författare)
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Seismotectonics and lithospheric structure of Costa Rica
- 1999
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The interaction of different tectonic features on a short-distance scale of 100's of km produces the complexity of the Costa Rica region. The intraplate Caribbean earthquake activity of shallow-depth nature is mainly concentrated along an E-W axis in the central part of the country. All types of faults are presented in this region, although a strike-slip fault system, with NB-SW direction is proposed. Intraplate Caribbean esrthquakes in northwestem Panama are shallow and dominated by thrust fault motion. The seismicity along the Pacific coast is mainly associated with the subduction of the Cocos under theCaribbean plate, with predominant thrust fault mechanisms. The interaction of the Cocos and Nazca plates in southeastem Costa Rica, along the Panama fracture fault system produces events with strike-slip mechanism.A proposed 1D P-velocity model for the region is presented. Relocation of earthquakes of this area made through this ID-velocity model provides a reliable geometry of the Wadati-Benioff zone, with a slab dipping at 45° to a maximum depth of approximately 250 km.Station corrections and P- and S-velocity distribution correlate well with the major upper crust geological features of the area. The P- and S-velocity distribution for depth below 30 km shows a lateral velocity variation, which is associated with the subductionslab.
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