Determination of the Curie Point Depth and Heat Flow From Magsat Data of Western Anatolia

12 Pages·2006·0.44 MB·English

JOURNAL OF BALKAN GEOPHYSICAL SOCIETY, Vol.8, No 4, November 2005, p.149-160

Determination of the Curie Point Depth and Heat Flow From Magsat Data
of Western Anatolia

Müjgan Şalk1, Oya Pamukçu1, Ilknur Kaftan1

1Dokuz Eylul University, Faculty of Engineering, Departmant of Geophysics
35160 Buca – Izmir / TURKEY
Corresponding author. Tel : [phone] ; fax: [phone]
[email protected]

Abstract: Temperature in the earth is one of the most important parameters in models for the
constitution and active tectonics of the crust. However this subject is most poorly known.
A boundary condition on temperature at depth in the continental crust can be principle to
obtained by mapping the curie isotherm where it forms the base of magnetic crust. At the
Curie temperature, a substance loses cause to magnetic polarization. Consequently, it may be
possible to locate a point on the isothermal surface by determining the depth to the bottom of
a polarized rock mass. If enough depths can be determined, an isothermal surface at the
Curie temperature can be defined. An approach to map the magnetic crustal thickness using
satellite magnetic anomaly data could be described.
In this study, the moving windows power spectrum and other power spectrum methods
based on spectrum analysis have been applied to the total component Magsat data of the
Western Anatolian Region and Curie depths were determined. Then, heat flow values were
determined with regard to the region by using the depths and taking into consideration of
heat conductivity (2 W/m0C).
In conclusion, the results of both methods were compared and interpreted for the
aforementioned region. Moreover, the heat flow values derived from the Curie depths, which
has found through the application spectral analysis were interpreted with the tectonic
structure whether are in association. Additionally, prominent geothermal fields in the region
were investigated together with their computed heat flow values.
Keywords: Curie depth, Magsat, Heat flow, Western Anatolia.

Introduction Magnetic anomalies at the satellite
altitude can be explained by a thick
Geocentric X (North), Y (East) and magnetic layer with relatively lower
Z (Radial inward) components of susceptibility contrast or by a thin layer
geomagnetic field and total intensity with higher susceptibility contrast.
together with the geocentric latitude, Furthermore, a given magnetic anomaly
longitude and radial distance are measured can be interpreted in terms of different
from the Magsat spacecraft. Magsat maps models. For example, the anomaly can be
were prepared by subtracting the external interpreted due to lateral variations in
and main field from Magsat data. Total temperature inside on an otherwise
component Magsat maps of western uniform layer, suggesting that the magnetic
Turkey were prepared using Magsat data anomalies are due to variations in depth to
obtained from the British Geological the Curie isotherm within the layer
Survey covering the area between 25° - (Mayhew, 1982). The anomalies can also
32° E longitudes and 36° - 41° N latitudes. be interpreted as being due to lateral
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Müjgan Şalk, Oya Pamukçu, Ilknur Kaftan

variations in the magnetic mineral content The results of both methods applied
and in the rock type of the magnetic layer to total component Magsat data that have a
or by a combination of these factors. sampling distance of 2 km are parallel to
In a total component Magsat tectonic activity, volcanics and geothermal
magnetic anomaly map, magnetic fields in the region.
anomalies are irregularly distributed over However, when the moving
western Turkey (Şalk, 1994). Positive window power spectrum was compared to
magnetic anomalies are observed along the the spectrum analysis, the obtained Curie
Izmir-Ankara ophiolite zone, in the North depths were shallower and the heat flow
Aegean and in the Massifs of western values were higher. When these results
Turkey. These anomalies can be related to were compared to the heat flow values
the intrusives of magmatic and granitic formed by Tezcan and Turgay (1989), the
masses. results of the spectrum analysis seem to be
In this study, total component more consistent. The reason is that the
Magsat maps were used to compute the average depth of structure is found by
Curie depth using the moving power using the moving window power spectrum,
spectrum (Spector and Grant, 1970) and but the basal depths of the magnetic
the spectrum analysis (Tanaka et al., sources were estimated from the spectrum
1999). analysis.
Theoretical studies about the The obtained results are consistent
Curie depth have been conducted by with the previous studies regarding the
several researchers (e.g. Vacquier and region (e.g. Aydın et al., 2005)
Affleck, 1941; Serson and Honnoford,
1957; Alldredge and Van Voorhis, 1961;
Battacharrya and Morley, 1965; Hisarlı, Geology of Western Turkey
1996; Tsokas et al., 1998; Tanaka et al.,
1999; Stampolidis and Tsokas, 2002; The tectonic gross feature of Turkey
Pamukçu, 2004; Dolmaz et al., 2005; is shown in Fig 1.
Cathrine Fox Maule et al., 2005; Aydın et
al., 2005)

Figure 1. Main tectonic structures of Turkey (Koçyiğit, 1984).
150
Determination of The Curie Point Depth and Heat Flow
The Aegean and Anatolian plates These volcanics are essentially calc-
cover the western and central parts of the alkaline in nature, although volumetrically
country. minor shoshonitic-alkaline volcanics
Many of the natural phenomena, overlap with the calc-alkaline ones in both
which lie within the scope of geophysics, space and time. Calc-alkaline volcanics
are related with the earth’s heat. Some of range in composition from basaltic
these can be described as follows: all andesite to rhyolite. However, they are
physical properties of rocks, the existence dominated by andesites and dasites.
of many minerals, geothermal areas, Shoshonitic volcanics (Afyon, Denizli,
internal mantle movements as a result of Bodrum, Kos, Patmos) and the alkaline
plate tectonics and geomagnetic field of volcanics with sodic nature (Ezine, Urla)
the earth due to the induce are upper Miocene and younger in age and
electrokinematic currents produced within are comprised of dominantly basaltic
the melted outer core. All these cause compositions (Innocenti et al., 1982;
increase of temperature with depth. Turkey Keller, 1983; Fytikas et al., 1984; Ercan et
is within an area of high heat flow. al., 1985; Savaşçın and Güleç, 1990). In
Western Turkey is one of the most western Turkey, Quaternary volcanics are
seismically active and rapidly extending observed in Kula and are represented by
regions in the world. As a result, it has a lava flows, cones and craters.
long history of large earthquakes,
magmatic and volcanic activity and high Method
heat flow. The Neogene geological Moving Windows Power Spectrum
evolution of Western Turkey involves a
widespread magmatism producing The application of the power
intrusive as well as extrusive rocks. spectrum to potential field data and the
Three main types of magmatic identification of anomalies by this method
rock groups can be readily distinguished in has been studied by Bhattacharyya (1965-
the region. The first group is an 1966), Spector and Bhattacharyya (1966).
intermediate to the felsic variety of the The depths relating to these anomalies
volcanic series were referred as andesitic have been determined by Spector and
suite. The second group consists Grant (1970) and these consequently have
predominantly of the felsic varieties were been used by many researchers afterwards.
referred as the granitic suite. The third The relation between the power
group is represented by the locally spectrum and the structure causing
developed basic lavas. The basic rocks anomaly was expressed using equation 1.
were referred as the basaltic suite (Yılmaz, p
∑
1989). S(w) = f p(w,αp,αp,.....,αp)exp(−2wh)
1 2 n
The volcanics in western Turkey p=1
(1)
contain both calc-alkaline and alkaline. In
where S(w) is the power spectrum, α is the
the region of western Turkey and the
structure parameter, h is the depth, f is an
Aegean, volcanism has been active since
anomalous function. The accuracy of the
the Eocene with varying intensity and
spectrum estimation, which is a statistical
distribution in both space and time. Lava
approach, is possible with the smallest
flows and pyroclastics cover extensive
values of variance and mean square errors.
areas from the present Hellenic Arc in the
In this practice, the data were divided
south towards the Black Sea in the north.
equally with the help of a window
Domes, lava flows and agglomerates with
function, the values at the same frequency
minor tuffs and ignimbrite sheets cover
were added and the arithmetical average
extensive areas in western Turkey as well
was found. This operation is defined by the
as in the northern and central islands.
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Müjgan Şalk, Oya Pamukçu, Ilknur Kaftan

following equation (Jenkis and Watts, Here, S (w), is the power spectrum
r
1968); belonging to each section and R is the
1 R number of sections. The first spectrum was
∑
S(w) = S (w)
R r obtained by Cianciara and Marcak (1976).
r=1
If the approach given in equation (2) was

substituted in equation (1), the equation (3)
(2)
is obtained.
1 R P
S ( w ) = ∑ ∑ f p ( w ,α pr ,α pr ,....... α pr ) exp( − 2 wh )
R r 1 2 n
r =1 p =1
(3) 1993). The measurements show
geographically irregular distribution, they
In the equation (3) remain insufficient to define the regional
f p(w,αpr,α pr,.....,α pr) = cpr = constant thermal structures since the measured heat
r 1 2 n
flow values reflect local thermal
anomalies. Therefore, Curie point
C = ∑R 1 ∑P cpr determination based on the spectrum
R analysis of magnetic anomaly data is
r=1 p=1
significantly important. By this method,

the depth at which is the magnetite passes
neccessary transforms are achieved, the
from a ferromagnetic state to paramagnetic
relation could be given by equation 4.
state under the effect of increasing

temperature, is determined. The lower
S = C.exp(−2wh)
limit depth of such a magnetic source is
(4)
taken as the Curie point depth.
If the logarithm is taken on both
The method is similar to the
sides in equation (4), the average depth
practical work performed by Spector and
depending to the structures causing
Grant (1970), depending on the
anomaly is found with equation (5).
determination by lower limit using the
ln(w )−ln(w )
h = İ+1 İ i = 1, 2,........ middle and upper depths. Blakely (1995),
İ
2(w −w )
İ+1 İ noted that in the case that a layer is
(5) presumed to extend infinitely in the
Benefiting from equation (5), horizontal direction, the depth of the
structural depth is determined for every magnetic source from the surface remains
window of the anomaly, which was very minor when compared to the
separated into windows. The structural horizontal scale. In this situation, he
depths computed by means of moving described the magnetization M (x, y) as a
windows spectrum method are assigned to random function of x and y axes. The
the mid-point of the x-axis, which they power-density spectra of the total field
belong to. Thus, structural depth is anomaly are introduced as follows:
obtained more in-to-depth along a profile. ΦΦΦΦ (k ,k ) ====ΦΦΦΦ (k ,k ).F(k ,k )
∆∆∆∆T x y M x y x y

F(k ,k ) ==== 4ππππ2C2 ΘΘΘΘ 2 ΘΘΘΘ 2e−−−−2kd(1−−−−e−−−−kt)2
x y m m f
Theoretical Model Based on Spectrum
(6)
Analysis
where, d is the depth of the upper surface

and t is the thickness of the structure. Here,
The heat flow is the primary
the Ф is the power-density spectra, C is
M M
observable quantity to determine
the proportionality constant. Θ and Θ
M f
temperature variations. Many heat flow
stand for the direction of magnetization
observations are derived from drillings on
and geomagnetic field, respectively. As
land and probes underwater. (Pollack et al.,
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Determination of The Curie Point Depth and Heat Flow

│Θ │2 and │Θ│2 are radially If the equation is arranged for short
M f
symmetrical, the relation between them wavelengths; equation 11 was obtained:
can be simplified. Equation (6) was Ln(G(k)1/2)=lnC−kh
1
investigated by many authors (e.g. Tanaka
(11)
et al., 1999) in diverse forms and basal
where C is a constant. From the upper
depth of the magnetic source was found.
depth of the structure is computed from
In this study, one dimensional
equation (11). Additionally, assuming that
spectrum analysis of a vertical magnetic
h is the mid-point of the structure,
0
sheet is investigated and using the
equation (10) is rearranged into the form
approaches of Tanaka et al. (1999), the
given below:
basal depth of the magnetic source was
G(k)1/2 =De−kh0(e−k(h1−h0) −e−k(h2−h0))
found theoretically.
The vertical component of the (12)
vertical finite magnetic sheet is given with
the equation (7): (Grant and West, 1965). where D is a constant. The expression of
equation (12) at long wavelengths could be
 
xsinθ−h cosθ xsinθ−h cosθ
∆Z=A 2 − 1  expressed as follows:
 x2 +h22 x2 +h12  G(k)1/2 =De−kh0(e−k(−d) −e−k(d))≈De−kh02kd
(7)
(13)

where 2d is the thickness of the magnetic
If the polarization angle of the
source. In order to determine the mid-point
magnet is at a position of θ= 900, the
of the structure, the equation given below
vertical component will be equal to the
is rearranged:
total component ( ∆Z=F). So, the equation
1 / 2
ln( G ( k ) / k ) = ln E − kh
(7) turns to the equation (8): 0
(14)
 
 x x 
∆F=A − 
x2 +h22 x2 +h12 Benefiting from the mid-point and upper
(8) limit depths of the source, the lower depth
(base depth) is found from the following
where the upper and the lower depths of equation (Tanaka et al., 1999):
vertical magnetic sheet are h1 and h2, h2=2h0-h1
respectively. The amplitude spectrum is (15)
explained by equation 9: As a result, by using the approaches
( )
Φ(k)=Be−kh1 −e−kh2 of Blakely (1995) and Tanaka et al. (1999),
the same results were obtained with one-
(9)
dimensional power spectrum.

In order to exemplify this situation;
where B is a constant. If a similar approach
the power spectra applied to the vertical
to the study of Tanaka et al. (1999) is
magnetic sheet with upper depth of h = 6
applied, the power spectrum is derived as 1
km and a lower depth of h = 16 km
follows: 2
through equations (11) and (14), are
( )2
G(k)=Be−2kh11−e−k(h2−h1) depicted in Fig. 2.
(10)

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Müjgan Şalk, Oya Pamukçu, Ilknur Kaftan

10
8
6
4
2
0
0 0.1 0.2 0.3 0.4 0.5
k (2π/km)

4
2
0
0 0.1 0.2 0.3 0.4 0.5
k (2π/km)

Figure 2. Power spectra belonging theoretical data and estimated depths.

From this figure it was found that dT
q ====k 
the upper depth is approximately 5 km
 dz 
while the mid-point depth was 10 km. By

using the approach given in equation (10),
(16)
the lower depth was determined as 15 km.
The unit is µkal/cm2s (HFU) in
The depth value obtained here is also
c.g.s. system and mWm-2 in SI system. In
called as the Curie point depth. From the
equation (16); k is the heat conductivity, T
Curie point depths, the heat flow could be
is the average Curie depth and z is the
obtained by taking into consideration the
Curie depth of the domain.
area:

154
Determination of The Curie Point Depth and Heat Flow

Applications Anatolia between the latitudes of 360-410
N and longitudes of 250-320 E (Fig. 3).
Magsat magnetic data was sampled
at intervals of 2 km for the area in western

Figure 3. Map of total component Magsat magnetic data of West Anatolian
(contour interval is 0.6 nT)

Ten cross-sections, which were 500 to the each cross-section within five
km length and taken in the direction of N-S windows. Furthermore the average
on the anomaly map, are given in Fig. 3. structure depths were determined. The
Then, one dimensional moving windows computed depths were mapped as probable
power spectrum method has been applied Curie depths in Fig. 4.

400
14
12
10
8
6
370 4
km
0 100 200 km
260 320
Figure 4. Map of Curie depth regarding West Anatolia by means of equation (5)
(contour interval is 1 km).
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Müjgan Şalk, Oya Pamukçu, Ilknur Kaftan

Using the computed depth values, W/m0C , the heat flow map belonging to
the Curie temperature was found as 560 0C western Anatolia has been constructed by
for the studied domain. Provided that the means of equation (16) (Fig. 5)
heat conductivity is accepted as k=2
0
40
230
190
150
110
0
37 70
0 100 200 km mW/m2
260 320

Figure 5. Heat flow map of West Anatolia computed from Curie depths in Fig. 4
(contour interval is 20 mW/m2).

In the second stage of the study, spectrum analysis (Fig. 6). Additionally,
fifty eight cross-sections taken on the the Curie temperature was found to be 560
anomaly map are given in Fig. 3. The 0C and the heat flow map was plotted for
depths obtained from the equations (11) the same region by using equation (16)
and (14) were substituted in equation (15). (Fig.7).
Curie depths were calculated with

400
26
22
18
14
10
0
37
6
km
0 100 200 km
260 310
Figure 6. Curie depth map obtained from spectral analysis (contour interval is 1 km).

156
Determination of The Curie Point Depth and Heat Flow

0
40 Canakkale Bursa
Kutahya
Balikesir
140
Manisa Usak
Afyon
120
Izmir
100
Denizli
Aydin 80
Mugla 60
0
37
40
mW / m2
0 100 200 km
260 310

Figure 7. Heat flow map of West Anatolia computed from Curie depths in Fig. 6 and Main
geothermal fields (contour interval is 10 mW/m2).

Discussions and Conclusions found, followed by the determination of
Curie depths and the heat flow map by
All the current literature state that using the power spectrum method that was
the Curie point depth is greatly dependent developed by Spector and Grant (1970)
upon geological conditions. Curie point (Figs. 4 and 5). In the next step, the
depths are shallower than 10 km for spectrum analysis developed by Tanaka et
volcanic and geothermal fields, between al. (1999) has been conducted leading to
15-25 km for island arcs and ridges, and the Curie depths and heat flow maps (Figs.
deeper than 20 km in plateaus and trenches 6 and 7).
(Tanaka et al., 1999). Generally, the units The consequences of the practical
that comprise high heat flow values work indicate that the Curie depth and heat
correspond to volcanic and metamorphic flow values computed in the first approach
regions since these two units have high are different than those found in the
heat conductivities. Additionally, second. The heat flow values that were
tectonically active regions affect the Curie obtained in the second stage are
depth and heat flow. compatible with the heat flow map that
Two power spectrum methods was drawn by Tezcan and Turgay (1989)
have been compared in this manuscript. based on the geological-geophysical
First of all, the mid-point depths have been studies and well base temperatures (Fig. 8).
157
Müjgan Şalk, Oya Pamukçu, Ilknur Kaftan

90
42
30
5
0
80 50
70
90
GREECE
80 150 100
40 90 1215009 0
80
90
80
AEGEAN
SEA TURKEY
38
60 125 100
80 80
70
60
50
90 125 403020
10
32
24 26 28 30

Figure 8. Map of heat flow including the study area of Tezcan and Turgay (1989).

In the light of these results, the magmatic mass that has not completed its
spectrum analysis method yields better cooling yet in association with relevant
results in the calculations of Curie depth young volcanism and a faulted structure.
and heat flow when compared to mid-point The progress of geothermal fields in
depth determination. western Anatolia is at the same time
It is a commonly known fact that correlated with the active tectonic nature of
shallower Curie depths create negative the region. Generally the structural features
magnetization. Similarly, in our study, it that are considered to occur due to N-S
was recorded low magnetic anomalies extensions. This result could be explained
overlap with shallow Curie depths for both by the horst and grabens which lead a
methods (Fig. 3), where high heat flow significant geothermal effectivity by the
values were obtained (Figs. 4 and 7). convection of rain and ground water
The main tectonic elements formed toward the surface. This situation supports
by Koçyiğit (1984) as in Fig 1 was the conclusion that Curie depth is shallow
compared to the computed Curie depth and where the heat flow is high. Especially, the
heat flow values. Especially, in the areas shallow Curie point depths observed in the
with the increasing heat flow correspond to study area correspond to recent geological
extension areas and continuation of features such as the grabens of the
trenches in the continent. Menderes Massif. Additionally, shallow
Additionally, the obtained Curie Curie point and high heat flow values is
depths and heat flows are consistent with significant especially in the Aegean Sea
the young volcanics and metamorphics around İzmir. This region is near
existing in the region. The heat source of a Seferihisar-İzmir. Seferihisar is known to
geothermal field is governed by a deep
158