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

mkhatib@birjand.ac.ir , mkhatibm@yahoo.com

Abstract: - We examined the geology structures of the crust across complex deformation zones in Birjand (E

Iran, 32°27′N to 33°28′N and 58°30′E to 59°30′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 nun-

magnetic. 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

1974-1977. 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).

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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. 86-71 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

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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. 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],

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.

Fig. 5 The Curie point depth map of Birjand

area. The depth 2500m.

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

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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.

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Recent Researches in Environmental and Geological Sciences

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ISBN: 978-1-61804-110-4 479