mahnaz rezaeian

Structurally, the Tarom Metallogenic Zone is located in an NW-SE trending anticline with 50 km wave-lenght, consists of the Karaj formation equivalent units and Neogene red beds. This anticline was created during the Alborz orogen formation- related phase has multiple fractures with NW-SE, NE-SW, and E-W trend. Detailed mesoscopic and microscopic investigations of the existing fractures for studying their propagations during folding, including their relative age, sequential deformation of the fractures in various beds, distribution of the fractures, and their changes during the following geological events, were performed many times. Comparison of various fracture patterns in folded layers not only illuminates the brittle deformation successions but also resulted in the detection of the main fold formation mechanism of the studied area. Based on the existing evidence at limbs of the anticline including 1- The existence of parasitic folds in soft layers, 2- Creation of layer- parallel shearing related strata-bound fractures in soft layers on various scales. Dyke intrusion into pre-existing fractures at fold limbs, preferentially has been happened in soft beds, especially in the Karaj formation equivalents pencil shales. While at the hinge zone, they have been created in hard layers, for example, andesite and basalt. The flexural slip mechanism is proposed for the Tarom Anticline.

Tabriz city with 1.6 m population is located in the NW Iran. Estimation of empirical attenuation relations for the region is a key for realistic seismic hazard assessments. Recently, Motaghi et al. (2016) have employed microseismicity of the region to estimate an attenuation relationship for the NW Iran. Their regression has left significant amplitude residuals at different stations where neighboring stations have similar average residuals confirming important lateral structural variations ignored in the attenuation estimations. For instance, stations located around the NTF have systematic negative residuals consistent with location of a thick sedimentary basin in the region. Such systematic residual patterns have been reported before in Canada and North America by Atkinson (2004) or in Alborz Mountains, north Iran, by Motaghi and Ghods (2012). These observations have inspired us to conduct 2-D amplitude residual tomography similar to widely used local travel time tomography (e.g., Rawlinson and Sambridge, 2003). We assume that attenuation variations are only affected by anelastic coefficient variations generated by change of rock properties. Thus, we have discretized the study region and considered anelastic attenuation coefficient (inverse of quality factor) as an unknown parameter for the inversion procedure. 

Methodology and Approaches:

We formulized a linear relationship between the amplitude residual (as datum) and anelastic coefficient variation (as unknown parameter), and then, we analyzed seismograms recorded from 943 local earthquakes with magnitudes between 1.6 and 5.2, and azimuthal gap less than 250o . The data were gathered by 35 seismic stations located in the NW Iran to carry out the inversion. We used a weighted damped least squares approach (e.g. Menke, 1989) in which weight matrices for the data and model parameters were used. The weights for the data come from the signal to noise ratio calculated for each signal. The weights for the model parameters come from the number of rays per block. This helps to exclude model parameters associated with blocks not crossed by rays. An optimal damping value of 23 was selected from the trade-off curve between the total residuals and the weighted model variances. The inversion procedure estimated variations of intrinsic attenuation coefficient relative to the average value (0.0012). We converted the obtained values into change of quality factor, ΔQ, which was more usual in the literature.

Results and Conclusions

Our tomogram for the NW Iran showed a lower quality anomaly for Neogene sedimentary basin around the North Tabriz Fault in contact with high quality Cretaceous volcanic and Cambrian metamorphic basement outcrops in the north of the region. Such clear consistency between our tomogram and lithology variations at the surface disappeared in the eastern part of the tomogram where deep (depth = 20–45 km) events occurring in the oceanic-like crust of the South Caspian Basin are observed. Comparing our eastern anomalies with a teleseismic tomography across the same region, we found that low and high quality patches in eastern part of our tomogram are probably generated by thermal effects of Sabalan volcano and oceanic like crust of the South Caspian Basin, respectively. Good consistency between our results and previously reported features confirms that the amplitude residuals are proper data set to detect structural heterogeneities responsible for large amplitude variations in active seismic regions.

The aeromagnetic data of Iran was surveyed by Aeroservice Company (Houston, Texas) under auspicious of Geological Survey of Iran during 1974-1977. The survey was done with a two-engine airplane using a cesium vapor magnetometer with a sensitivity of 0.02 gama. The data was collected along flight lines with average line spacing of 7.5 km over 62 flight blocks mostly with a constant barometric flight height. A barometric elevation range of 1000-3600 m was used (Yousefi and Feridberg, 1977) which translated to about a 500-1000 m height from the Earth surface. Saleh (2006) used the raw data of the 62 blocks and produced an aeromagnetic composite map of Iran. The raw data had already been corrected for daily variations of the geomagnetic field. To produce a composite grid of the aeromagnetic map of Iran from the original ratherraw data, Saleh (006) first implemented the detailed leveling and micro-leveling proceduresfor each block. The fully leveled blocks were stitched initially to eight geol gically coherentlarger blocks and finally the larger blocks were stitched. The final 1 km by 1 km grid of theaeromagnetic map of Iran was produced from the combined data set using a bidirectionalinterpolation scheme. Magnetic anomalies in Iran latitudes do not correlate directly with their corresponding causative magnetic bodies because the direction of the geomagnetic field and magnetization are not normal to the Earth surface. The asymmetry between the magnetic anomalies and their causative magnetic bodies increases from north to south of Iran. The deviation could reach to tens of kilometers for aeromagnetic anomalies located in the south of Iran. For geological interpretation purposes, it is very desirable to derive aeromagnetic anomalies that are positioned over their causative magnetic bodies, quite similar to that expected from gravity anomalies, or an induced magnetic body located in the North Pole. Baranov and Naudy (1964) introduced a procedure called reduction-to-the-pole (the standard RTP method) which converts magnetic anomalies in mid-latitudes to that produced by the magnetic bodies having vertical magnetization, and lying at the north geomagnetic pole. The standard RTP is only valid for regions in which the direction of the geomagnetic field is almost constant. Therefore, the standard RTP method is not applicable to produce an RTP map of the aeromagnetic field of Iran. The RTP methods which allow for variations of geomagnetic field are called differential reduction-to-pole methods (DRTP). In this study, the revised aeromagnetic map of Iran (Saleh, 2008) was reduced to the pole considering the variations of inclination and declination of the geomagnetic field over Iran.The new aeromagnetic map was produced using the differential reduction to the pole (DRTP) method developed by Arkani-Hamed (1988). The DRTP operator shifts aeromagnetic anomalies in different geographical latitudes to the top of their causativesources, thus facilitating an easier geological interpretation of the magnetic anomalies. Wefirst applied the DRTP method to the synthetic magnetic anomalies of three identical spheres lying in north, centre and south of Iran assuming induced magnetization for the spheres. We found that the DRTP method did not show any instability; thus it is appropriate to use in Iran. The DRTP aeromagnetic map of Iran showed significantly a better correlation between the magnetic anomalies and the boundaries of the main tectono-stratigraphic units of Iran (e.g. Alborz, Jazmurain Depression), volcanoes (e.g. Sabalan Mountain) and major active faults (e.g. Tabriz and Doruneh faults).