Z Hamidi Beheshti; H Alimohammadian; M Talebian; A Shahidi; M.R Ghassemi
Abstract
Geomagnetism is one of the most applied techniques of geophysics in geology. Today, this method is applied in different disciplines such as Magnetic fabric of rocks, palaeomagnetism and environmental magnetism. Each of these magnetic methods is suitable for a particular lithology. Therefore, the potential ...
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Geomagnetism is one of the most applied techniques of geophysics in geology. Today, this method is applied in different disciplines such as Magnetic fabric of rocks, palaeomagnetism and environmental magnetism. Each of these magnetic methods is suitable for a particular lithology. Therefore, the potential of sandstones of Shemshak Group, in central AlborzMountain range were examined to record their response to the application of some conventional magnetic methods. This rock unit is formed during two major tectonic events of early and middle Cimmerian and has great coverage not only in the study area but also in a vast majority of Iran. The total number of 135 oriented core samples was taken from 18 stations. In this study combination of magnetic and petrographic data are examined. The result of magnetic mineralogy analysis of all samples (except samples from stations 10 and 15) show irreversibility i.e. minerals such as hematite and magnetite, have formed during the heating stage. The results of thermal demagnetization analysis showed that ferromagnetic minerals present in the samples (except samples from stations 10, 14 and 15) have demagnetized below 400 °C and by proceeding heating, samples show zigzag pattern or show abnormal increase in susceptibility. This indicates that, the magnetic minerals are formed during heating in one stage and due to unstability lose its susceptibility in other stage. The magnetic susceptibility (Km) vary from 200-400×10-6 SI for more stations of study area, indicating abundance of paramagnetic mineral in this rock unit. From petrography point of view all the sandstone samples are classified as arenites and ratio of quartz to lithic fragment and feldspar is relatively low, which may indicate low mineral maturity. Comparison of magnetic mineralogy and thermal demagnetization data for two stations S10 and S15 show that there is an inverse potential relationship between amount of stable ferromagnetic mineral in a sample and amount of its alteration during heating stage. The results of this study reveal the poor nature of Shemshak Group sandstones for palaeomagnetic studies due to their low mineral maturity and water percolation which increase the possibility of acquisition of chemical remanent magnetization (CRM). The high sedimentation rate of sandstones cause magnetic inclination and declination error, and low ferromagnetic mineral fraction in samples, make them unsuitable to record magnetic directionsduring formation of rocks. However, abundance of paramagnetic minerals such as biotite in these rocks, proves their applicability for magnetic fabric studies.
A Aslani; H Alimohammadian; J Ghalamghash; H Nazari
Abstract
The studied locality covers an area of 120 km2, located at south and southwest of the Hamedan city. The porphyritic granite is the dominant volume of this intrusive body. This granitoid is divided into the granites (monzogranites, syenogranites and alkali feldspar granites) and granodiorites from petrographic ...
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The studied locality covers an area of 120 km2, located at south and southwest of the Hamedan city. The porphyritic granite is the dominant volume of this intrusive body. This granitoid is divided into the granites (monzogranites, syenogranites and alkali feldspar granites) and granodiorites from petrographic point of view. The major part of the study area have magmatic-sub-magmatic fabric and few of the samples show high temperature solid state deformation with presence of chess-board quartz, bending and kink band of biotite and mineral size decreasing from microstructural point of view. The total numbers of 450 oriented core samples were collected from 107 stations, to study the anisotropy of magnetic susceptibility. The total average susceptibility for all samples were 114-314 µSI and the mean susceptibility (Km) of granodiorites was 457.5 µSI, and it ranges from 282-633 µSI, which is higher in compare to granites. This difference is due to the presence of magnetite grains in granodiorites, which has been proved by heavy mineral analysis. The magnetite comprises 0.01-0.025 % of total volume in these rocks. The Km normally varies from 14-514 µSI and indicates the paramagnetic type of granites. The P values vary from 0.72-7.4 %. The T values in most of the samples is positive (T>0) and show planer nature of most of the intrusive body. The high degree of dipping of magnetic foliation and lineation at the central part of the intrusive body, which decreases with increasing distance from center of the intrusive body, may indicate the location of feeding. This characterization can be seen at two other places addition to the center. On the basis of magnetic parameter and other factors such as dominant transpression tectonic regime in this area, one can say that the ascending magma was due existing of stretching environment with NW-SE trend, and this can be proved by direction of magnetic lineations at the same direction. The differences in the direction of NE-SW, which some of the studied sites show, may be due to rotation of magnetic body during injection and emplacement in response to prevailing tectonic forces in studied area and viscosity of magma.
M Asadi; H Alimohammadian; M Oviesy Moakher; N Mirzaei
Abstract
Palaeomagnetic analysis has been applied worldwide on active faults for decades. The palaeomagnetic investigation on the Sahneh Fault, at middle part of the Zagros Main Recent fault, is the main objective of this research. The length of the Sahneh fault, which is about 100 km in study area, cuts the ...
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Palaeomagnetic analysis has been applied worldwide on active faults for decades. The palaeomagnetic investigation on the Sahneh Fault, at middle part of the Zagros Main Recent fault, is the main objective of this research. The length of the Sahneh fault, which is about 100 km in study area, cuts the gabbroic blocks exposed on the both sides with NW-SE trend and connects the Morvarid fault in the NW to the Nahavand fault in the SE of the study area. Tectonically, the mechanism of the Sahneh fault is high angle reverse with dextral strike slip component, and is compatible with the earthquakes focal mechanism solution, movement of the Arabian plate towards the Central Iran and the results of palaeomagnetic data. The paleomagnetic analysis results are based on the drilled oriented samples of 17 selected sites along and on both sides of the Sahneh fault. Nine to eighteen oriented samples were collected from each site. The conducted paleomagnetic analysis includes measurement of NRM, magnetic mineralogy (high temperature), and thermal/ AF demagnetizing. The declination, inclination and ChRM directions of each site separated from the overprint directions by means of the thermal demagnetization method. The mean direction of ChRM and VGP for each site is determined using statistics and palaeomagnetic analysis. For 7 sites the ChRM mean direction is calculated. The obtained mean ChRM directions then compared with the reference palaeomagnetic pole position of the ophioliths of Central Iran, and the sense of rotation were determined for all sites. The dextral strike-slip Movement of the Morvarid and Nahavand faults imposed a compressional and shear stress components on the Sahneh fault, resulted in the formation of transpressional stress regime in the study area. Under this tectonic stress regime, the Riedel of shear systematic fractures may be helpful to interpret the palaeomagnetic data. According to this model, the whole synthetic shear fractures (P, R, D) caused clockwise rotation and the antithetic shear fractures (Ŕ) caused counterclockwise rotation in gabbroic blocks.
