Analysis of deep geological structures by variety of Curie point Depth in Birjand area, east of IRAN

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1 Analysis of deep geological structures by variety of Curie point Depth in Birjand area, east of IRAN MOHAMMAD MAHDI KHATIB, HESAM YAZDANPENAH AND MOHAMAD HOSSEIN ZARRINKOUB Department of geology University of Birjand Po. Box :79 IRAN Abstract: - We examined the geology structures of the crust across complex deformation zones in Birjand (E Iran, N to N and E to E, ca. 600 km2) using the Curie Point Depth (CPD) estimates of the tectonic state of the crust with the seismic activity to provide insights for spatial limits of brittle failure in this region. The CPD estimates of Birjand area from 10.3 to 16 km below the surface. Birjand has one regions of shallow CPD. The shallow CPD region in the Bagheran Mountain in southern part of the study area is caused by upper crustal thinning and swallowing of high conductivity lower crust. In this area where hydrothermal reservoirs are located, CPD significantly shallows. Key-Words: - Birjand, Curie Point Depth (CPD), thermal structure, aeromagnetic data 1 Introduction From an analysis of the crustal magnetic field it is possible to make an estimate of the depth below which no magnetic sources exist. This depth extent of magnetic sources has become synonymous with the depth to the Curie temperature though sometimes it may represent a petrologic boundary [1]. Where the Curie depth correlates with an inferred velocity or density boundary, it is likely to reflect the change in composition; however, where it does not coincide with a velocity or density boundary, it may be interpreted as the Curie temperature isotherm [2]. As magnetite with a Curie temperature of 580 C is believed to be the dominant magnetic mineral in the deep crust within the continental region [3], one can assume that this Curie temperature represents the temperature of 580 C. Estimating depth to Curie temperature on a regional scale from long wavelength magnetic anomalies requires that large areas of survey data be used for the calculations. There is still no consensus on the minimum survey area required to arrive at a reliable estimate of the Curie isotherm depth [4]. Estimates of depth to the Curie temperature can provide valuable insights in the assessment of geothermal energy, calculation of thermal conductivity and tectonic/geodynamic evolution. The depth to the top of magnetic layer is often named as magnetic basement which does not necessarily correspond to geological basement where we have crystalline rocks. Sometimes magnetic basement simply is related to volcanic rocks. All sedimentary rocks are considered nunmagnetic. Therefore, the depth to magnetic basement can give us an estimate of the volume of the sedimentary basins in Birjand area. In this paper we utilize the aeromagnetic data over Birjand (E Iran) to calculate the Curie isotherm depths. The aeromagnetic data of Iran was surveyed by Aero-service Company (Houston, Texas) under the auspices of the Geological Survey of Iran during The data was collected along flight lines with average line spacing of 7.5 km. the survey was done mostly for constant barometric flight heights. This data by 1 and 1 km grid of aeromagnetic map of Birjand was produced using a bidirectional interpolation scheme and filtering all wavelengths smaller than 15 km (fig. 1). ISBN:

2 Fig.1, The Curie point depth map of Birjand area. The depths are considered relative to the ground surface. The aeromagnetic map of Iran correlates well the majority of the geological structures (fig. 2). In this paper, we present the first Curie Point Depth (CPD) map of Birjand area and then correlation by geological structure s tectonic regime and mountain building processes. 2 Geological setting This area is composite of ophiolitic and related oceanic volcanic and sedimentary rocks, and metamorphic rocks derived from them. They occur as block-against-block geological terranes or as blocks in tectonic mélange. Several late cretaceous adakitic granitoid bodies (ca Ma) have intruded into ophiolite mélange complex in north part of Sistan suture zone, in east of Iran. It has concluded that the Neotethys Ocean between the Lut and Afghan continental blocks should have closed before ca. 86 Ma, when the adakitic granodiorites started emplaced in the suture zone as a result of the Lut-Afghan continental suturing/collision. These bodies are mainly tonalite, quartz diorite and granodiorite with thermal effects on their host rocks. The compressional tectonic regime, later, switched to extensional so that eventually led to the voluminous volcanism in eastern Iran, which ascribe to an orogenic collapse associated with delimitation of thickened lithospheric root. This suite of rocks is dominated by (trachy) andesites and (trachy)dacites with minor basaltic andesites, rhyolites, diorites and granites [5]. This extensional regime succeeded to cause furthermore lithospheric thinning and asthenospheric upwelling that gave rise to the ISBN:

3 Recent Researches in Environmental and Geological Sciences fractures and faults are planes of tensile or shear failure at microscopic to regional scales in brittle rock. These faults and fractures are developed mostly in competent rocks within the earth crust. In case of fractures, they are usually developed when the stress applied exceeds the elastic limit of the rock [8]. These two deformations are of great importance in crustal fluid distributions and control. generation of intraplate basalts from the middle Miocene to Quaternary in eastern Iran. Fig.3 The Curie point depth map of Birjand area. The depth 500m. Fig. 5 The Curie point depth map of Birjand area. The depth 2500m. Fig. 4 The Curie point depth map of Birjand area. The depth 1500m. 3 Discussion The roles of faults and fractures on crustal fluids have been of major interest in earth sciences, including geology, seismology, hydrogeology and petroleum geology [6]. The static and dynamic effects of different stress on rock often produce change in rock mass such as fractures, faults and in general permeability which in turn control the flow of fluids in the earth crust(fig.3). According to [7], ISBN: Fig. 6 The Curie point depth map of Birjand area. The depth 3500m. The movement of crustal fluids (in this case, hydrothermal) to the surface from the reservoir rock depends of the pressure, temperature and most 477

4 importantly the presence of active faults and fractures in the subsurface which are extended to the surface (Fig. 4). Magnetic is a geophysical technique that measures the magnetic field intensity of the Earth. It is capable of mapping subsurface structures such as faults and lithology. The aim of magnetic survey is to determine or measure the local magnetic contributions to the total magnetic field. This method is performed both on the ground and in the air. Association of geological formation with magnetic minerals makes it possible for mapping with magnetic field data. According to [9], lithology controls magnetic properties through mineralogy, and sharp variation in rock properties generally coincides with litho-logical contacts(fig. 5). Generally, igneous and metamorphic rocks show significant magnetic properties while sedimentary rock is non-magnetic [10]. Existence of faults and fractures in the geologic unites creates magnetic variation and can cause anomaly in magnetic measurements. In general, the presence of fluid within the faults and fractures would reduce or have no magnetic response. The subsurface structures geometry can be constructed from magnetic profiles data using various inversion processes. The anomaly due to the near surface and deep source can be enhanced using vertical derivative and upward continuation respectively. Depth to magnetic sources and geometry of the structures can be automatically estimated from Euler s de convolution method as applied in this research. Magnetic method was used in this study to map the subsurface structures in the Birjand area, East Iran(fig.6). A portion of the aeromagnetic anomaly map of Birjand, from 32º 28' to 33º 33' N and 58º 30' to 59º 34' E has been analyzed to understand the tectonics of the region. The magnetization of rocks is dependent on the composition (amount of magnetic minerals, primarily magnetite) and temperature. At depth, the composition of the crust can sometimes change such that deeper rocks are magnetite poor or the temperature can increase to the point at which rocks lose their magnetization, called the Curie temperature. Magnetic data, from which the effect of the main field and external current systems are removed, contains information down to the depth where rocks lose their magnetization either due to compositional or temperature changes. Analyzing the long wavelength part of the magnetic data can provide information about this depth. Several methods have been used to estimate the depth at which rocks lose their magnetization from the azimuthally averaged Fourier spectra of the magnetic data as discussed by [6]; for example the centroid method ([11]; [12]; [3]), the spectral peak method ([13]; [7]; [14]; [9]), the power law corrections ([14]; [15]) etc. For noisy data the spectral method may be the only way to determine the depth as the other direct methods will have problems dealing with white noise [16]. The Curie depth of different magnetic minerals can be defined only from the geothermal gradient or from geomagnetic depth estimation. In the latter case it is possible to determine the depth of the deepest magnetic sources in the given area, without any knowledge about the nature of these sources. We use spectral analysis of aeromagnetic data to estimate the depths to the top and bottom of magnetic sources. In this paper, we assume that crustal magnetization is fractal with a fractal dimension of 2:1. It is calculated using the magnetic field of volcanic outcrops. To calculate Curie Point Depth map of Birjand area, we divide the aeromagnetic map into 25*30 squares, each 4*4 km in size. There is 75% overlap between two adjacent squares. In each window, part, depths to the top and the bottom of the magnetic sources are calculated from radials averaged log of power spectrum. The 4 by 4 km window is a reasonable choice because CPD in Birjand area is always below 16 km. 4 Conclusion The CPD estimates of Birjand area from 10.3 to 16 km below the surface. Birjand has one regions of shallow CPD. The shallow CPD region in the Bagheran Mountain in southern part of the study area is caused by upper crustal thinning and shallowing of high conductivity lower crust. In this area where hydrothermal reservoirs are located, CPD significantly shallows. References: [1] Langel, R.A., Hinze, W.J, The Magnetic Field of the Lithosphere: The Satellite perspective, Cambridge Univ. Press,UK, 1998 [2] Beardsmore, G.R., Cull, J.P, Crustal heat flow. A guide to measurements and modelling, Cambridge Univ. Press, New York, USA, [3] Frost, B.R., Shive, P.N, Magnetic mineralogy of the lower continental crust, J. Geophys. Res, Vol.91, 1986, pp ISBN:

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