Curie point depth from spectral analysis of aeromagnetic data forgeothermal reconnaissance in Afghanistan
H. Saibi a, *, E. Aboud b, c, J. Gottsmann d
a Department of Earth Resources Engineering, Faculty of Engineering, Kyushu University, Fukuoka, Japanb Geohazards Research Center, King Abdulaziz University, Jeddah, Saudi Arabiac National Research Institute of Astronomy and Geophysics, Helwan, Egyptd University of Bristol, Department of Earth Sciences, Bristol, UK
a r t i c l e i n f o
Article history:Received 10 June 2015Received in revised form17 July 2015Accepted 19 July 2015Available online 21 July 2015
The geologic setting of Afghanistan has the potential to contain significant mineral, petroleum andgeothermal resources. However, much of the country's potential remains unknown due to limitedexploration surveys. Here, we present countrywide aeromagnetic data to estimate the Curie point depth(CPD) and to evaluate the geothermal exploration potential.
CPD is an isothermal surface at which magnetic minerals lose their magnetization and as such outlinesan isotherm of about 580 �C. We use spectral analysis on the aeromagnetic data to estimate the CPDspatial distribution and compare our findings with known geothermal fields in the western part ofAfghanistan.
The results outline four regions with geothermal potential: 1) regions of shallow Curie point depths(~16e21 km) are located in the Helmand basin. 2) regions of intermediate depths (~21e27 km) arelocated in the southern Helmand basin and the Baluchistan area. 3) Regions of great depths (~25e35 km)are located in the Farad block. 4) Regions of greatest depths (~35e40 km) are located in the western partof the northern Afghanistan platform. The deduced thermal structure in western Afghanistan relates tothe collision of the Eurasian and Indian plates, while the shallow CPDs are related to crustal thinning.This study also shows that the geothermal systems are associated with complex magmatic and tectonicassociation of major intrusions and fault systems. Our results imply geothermal gradients ranging from14 �C/km to 36 �C/km and heat-flow values ranging from 36 to 90 mW/m2 for the study area.
To study Earth's shallow thermal structure, direct and indirectmethods can be applied. Direct methods consist of temperaturemeasurements taken in boreholes (from few meters to several ki-lometers depth). A major limitation of this approach is the limitedavailability of suitable and thermally well-equilibrated boreholes.Indirect methods using for example geophysical data to derive theCurie point depth (CPD) (Okubo and Matsunaga, 1994) provide amuch wider coverage of temperature estimates. However, suchmethods have several constraints and limitations such as the depthresolution, which depends on the survey dimension, and un-certainties related to complex subsurface geology.
(H. Saibi).
In this study, we apply spectral analysis on aeromagnetic dataover the western portion of Afghanistan in order to estimate theCPD. Ferromagnetic materials lose their magnetism above the Curietemperature (580
�C for magnetite, Dunlop and €Ozdemir, 2001)
because the thermal energy is sufficient to maintain a randomalignment of the magnetic moments of the iron minerals (Dunlopand €Ozdemir, 2001). The bottom of magnetic sources may notgenerally correspond to a Curie temperature isotherm, but mayinstead correspond to a lithologic contact. For instance, if themantle is devoid magnetic minerals and if the Moho is shallowerthan the Curie temperature isotherm, the base of magnetic sourcesestimated with aeromagnetic data should correspond to the Mohoand not the Curie temperature isotherm as suggested byWasilewski et al. (1979). Therefore, using the bottom of themagnetized crust as a proxy we can map the Curie isotherm.Changes in the thickness of the magnetized crust can be explainedas changes in Curie temperature, but only if the Moho is above the
H. Saibi et al. / Journal of African Earth Sciences 111 (2015) 92e99 93
Curie temperature isotherm.A number of studies of CPDs have been performed in various
parts of the globe. Here we highlight some of them. Bhattacharyyaand Leu (1975) analyzed the CPD for geothermal reconnaissance atYellowstone National Park with shallow CPD values ranging from 4to 6 km depth associated with the central part of Yellowstonecaldera. Okubo et al. (1985) estimated the CPD (6.5e15 km depth)of Kyushu Island (Japan) from aeromagnetic data and comparedthem with the setting of volcanic centres. They found that shallowCPDs are located in areas of current geothermal exploitation andfuture prospects. Espinosa-Carde~na and Campos-Enriquez (2008)determined the CPDs (between 14 and 17 km depth) with anelevated geothermal gradient and heat flow of the Cerro Prietogeothermal field in Baja California (Mexico) using spectral analysisof aeromagnetic data. Rajaram et al. (2009) mapped the CPDs(23e39 km depth) of the Indian subcontinent. A shallower CPDwhere shallow CPD is associated with the mobile belts whiledeeper a CPD delineates the cratons, Bouligand et al. (2009) map-ped the CPDs of Western United States (from 4 to 30 km depth)using a fractal model for crustal magnetization, where shallow aCPD is related to the Yellowstone hot spot and Cascade Arc. Usingground magnetic data, Aboud et al. (2011) mapped the CPD of SinaiPeninsula (Egypt) (CPDs from 15 to 25 km) to highlight geothermalprospects and to show that CPDs becomemore shallow towards thetip of the peninsula. De Ritis et al. (2013) detected a shallow-seatedheat source in the central Aeolian Ridge in Italy from CPD analysisusing aeromagnetic data with CPDs ranging from 2 to 3 km belowSalina and Vulcano Islands.
The objectives of this study are to estimate the CPD usingspectral analysis applied to aeromagnetic anomalies, and to providegeothermal gradient and heat flow maps for western Afghanistan.The results may be useful for future geothermal exploitation.
2. Geological and geothermal settings
The color shaded relief map of Afghanistan and the surroundingarea in Fig. 1 shows large areas of mountainous terrain especially inthe central and northeastern parts of the country. A broad lowlandarea is situated in the Helmand province in southwesternAfghanistan.
Afghanistan has a complicated geology. The oldest rocks arePrecambrian and succeeded by rocks from the Paleozoic up to theQuaternary (Fig. 2). Tapponnier et al. (1981) studied the tectonicevolution of Afghanistan since the Permian in relation to accretionof fragments of Gondwana to the margin of Laurasia. The Cimme-rian Orogeny affected the study area by two different collisions,which brought first the Farad block against the Tajic block, suc-ceeded by the Helmand block against the Farad block. The Heratfault (Hari Rod) represents the suture line of this collision, and thePanjao Suture records the line of the second collision that wasended by early Cretaceous. In the late Mesozoic the two blocksPamir and West Nuristan were accreted to Eurasia. These twoblocks with the Farad, Helmand, and Tajic blocks are all known asthe Afghan block. The Kandahar volcanic arc developed as a resultof subduction of the Indian plate beneath the Eurasian plate, withthe formation of a complex magmatic and volcanic rock suites.Igneous activity was, however, not restricted to this region only, butcan also be found in younger alkaline intrusions of Oligocene ageand basaltic extrusions in the Farad block and the sedimentarybasins along the Herat fault.
In the early Cenozoic, the Himalayan Orogeny affected theAfghan block by reactivating the blocks boundaries along with theHerat fault (HR) and the Chaman fault (CH).
The Kabul block is a fragment of continental crust, separatedfrom the Indian and Afghan blocks by oceanic crust, which got
caught up in the collision and was accreted to the edge of theAfghan Block before final collision with India (Schindler, 2002;Wheeler et al., 2005).
3. Methodology
The dataset used in this study is derived from 2006 to 2008aeromagnetic surveys provided by U.S. Geological Survey and theNaval Research Laboratory (NRL) (Ashan et al., 2007; Shenwaryet al., 2011). Details on aeromagnetic datasets used in this studyand data processing are presented by USGS (2011). The gridelevation is 5000 m above the terrain. Five base station magne-tometers were used during the survey. In order to correct theairborne magnetic data for time-varying anomalies, a weightedaverage of data from the five base stations was used to predict thetime-varying field at the aircraft. In order to facilitate the inter-pretation and analysis of aeromagnetic anomalies which areinfluenced by the orientation of the magnetic field and of themagnetization, the map of total magnetic intensity is gridded byminimum curvature method (with a grid size of 1000 m) and thenreduced to the pole (RTP) usingmagnetic inclination of 48.74� and adeclination of 2.01�. The total intensity magnetic anomalies rangefrom �168 nT to 650 nT (Fig. 3).
To estimate the CPD we applied the method of spectral analysisto the observed aeromagnetic data in Afghanistan. Tanaka et al.(1999) assumed that the magnetic layer extends infinitely in allhorizontal directions. The depth to the top of themagnetic source ishence much smaller than the magnetic source's horizontal scale Asa result the layer's magnetization M (x, y) is a random function of xand y.
Okubo et al. (1985) developed an algorithm to estimate the basaldepth frommagnetic data by using a 2-D modeling method for thecalculation of the depth to the base for a single window. Then, thealgorithm calculates the depth to the centroid (Z0) and to the top(Zt) of the magnetic source from the slope of radially averagedpower spectrum of the magnetic anomaly.
Blakely (1995) presented the power-density spectra of the total-field anomaly FDT
FDT�kx; ky
� ¼ FM�kx; ky
�� F�kx; ky
�; (1a)
F�kx; ky
� ¼ 4p2C2mjQmj2���Qf
���2e�2jkjZt�1� e�jkjðZb�ZtÞ
�2; (1b)
FM is power-density spectra of the magnetization, Cm is a pro-portionality constant, Qm and Qf are factors for magnetization di-rection and geomagnetic field direction, and Zt and Zb are top andbasal depth of magnetic source, respectively.
The above equation can be simplified by noting that all terms,except jQmj2 and
���Qf
���2 are radially symmetric. Moreover, the radialaverage of Qm andQf are constant. IfM (x, y) is completely randomand uncorrelated,FM (kx, ky) is a constant. Hence, the radial averageof FDT is:
FDT ðjkjÞ ¼ Ae�2jkjZt�1� e�jkjðZb�ZtÞ
�2; (2)
where A is a constant and k is a wavenumber. For wavelengths lessthan about twice the thickness of the layer, Eq. (2) can be simplifiedas:
lnhFDT ðjkjÞ1=2
izlnB� jkjZt ; (3)
where B is a constant.Eq. (2) can be-rewritten as:
Fig. 1. Color shaded relief map of Afghanistan topography including tectonic block names and abbreviations of major faults. CH, Chaman; CB, Central Badakhsan; DS, Darafshan; DM,Dosi Mirzavalan; GA, Gardez; HR, Hari Rod; HM, Helmand; HV, Henjvan; KR, Kaj Rod; KO, Konar; MO, Mokur; PM, Paghman; PJ, Panjshir; QA, Qarghanaw; SP, Spinghar. The el-evations range from 0 to 7761 m. (For interpretation of the references to color in this figure caption, the reader is referred to the web version of this article.)
Fig. 2. Geological map showing rock units in Afghanistan and the locations of major faults (modified from Mihalasky et al., 2007), overlaid on the shaded relief (SRTM) map. Rosediagram shows the direction of the geological faults, striking mainly in NEeSW and ENE-WSW directions.
H. Saibi et al. / Journal of African Earth Sciences 111 (2015) 92e9994
where C is a constant. At long wavelength, Eq. (4) can be written as:
FDTðjkjÞ1=2 ¼ Ce�jkjZ0�e�jkjð�dÞ � e�jkjðdÞ
�zCe�jkjZ02jkjd; (5)
where 2d is the thickness of the magnetic source. Eq. (5) can bepresented in another form as:
Fig. 3. Reduced-to-pole magnetic map of Afghanistan, overlaid on the shaded relief (SRTM) map. The yellow text shows the major geological and morphological divisions inconnection with the magnetic anomalies. Light yellow circles represent hot springs. (For interpretation of the references to color in this figure caption, the reader is referred to theweb version of this article.)
H. Saibi et al. / Journal of African Earth Sciences 111 (2015) 92e99 95
lnnh
FDT ðjkjÞ1=2i.
jkjozlnD� jkjZ0; (6)
where D is a constant.By fitting a straight line through the high and low wavenumber
parts from the radially average power spectrum of ln½FDT ðjkjÞ1=2�and ½FDT ðjkjÞ1=2�=jkj; Zt and Z0 can be estimated. Finally, the basaldepth of the magnetic source (Okubo et al., 1985; Tanaka et al.,1999) is:
Zb ¼ 2Z0 � Zt ; (7)
The geothermal gradient (dT/dz) between the Earth's surfaceand the CPD (Zb) can be defined by Eq. (8) (Tanaka et al., 1999;Stampolidis et al., 2005; Maden, 2010):
dT=dz ¼ 580 �C=Zb; (8)
In addition, the geothermal gradient can be associated to theheat flow q by using Eq. (9) (Turcotte and Schubert, 1982; Tanakaet al., 1999) and assuming no radioactive source:
q ¼ l580�CZb
: (9)
where l is the coefficient of thermal conductivity.Data windowing was used to calculate the radial power spec-
trum in order to estimate the CPD from a 2D magnetic map. Thedepth resolution in such calculations is constrained to the width ofthe aeromagnetic window (L), whereby the maximum CPD depthestimation is limited to L/2p (Shuey et al., 1977).
The study area is bounded by longitudes 61�300e66�300E andlatitudes 30�e35�300N. The aeromagnetic data grid was dividedinto 15 windows (A, B, C, D, …, and O), each having a size of200 � 200 km.
Fig. 4 shows two examples of power spectrum of the aero-magnetic data from two sub-regions “N” and “O”.
In each window, both power spectrum and scaled-frequencypower spectrum were calculated in order to calculate the depthto top (Zt) and centroid (Z0) depths of magnetic sources. From therelation of Zb ¼ 2Z0�Zt, the curie depth point (Zb) was computed,following the Okubo et al. (1985) procedure. The results are listed inTable 1.
4. Results
4.1. Curie point depth estimates
The resultant CPD (Zb) map of central and western Afghanistan(Fig. 5) shows the CPD depths ranging from 16 km to 40 km.
We identify four broad regions of different CPD depths, whichcan be associated with different tectonic building blocks ofAfghanistan.
1) Circular region of shallow Curie point depths (~16e21 kmdepth) located in the Helmand basin along the Kandahar arc.
2) Regions of intermediate depths (~21e27 km) are located in thesouthern Helmand basin.
3) Regions of great depths (~25e35 km) are located in the Faradblock.
4) Regions of greatest depths (~35e40 km) are located in thewestern part of the northern Afghanistan platform.
We also observe a general deepening of the CPD in all directionsfrom the shallow CPDs in the Helmand basin.
4.2. Geotherms and heat flow estimates
The calculated geothermal gradients (Fig. 6) from Eq. (8) give
Fig. 4. Radial power spectrum for two sub-regions “N” in (a) and “O” in (b).
Table 1Values of the top, centroid, and basal depths of the magnetic sources including error estimation for each parameter.
Sub-region Zt km (top depth) Z0 km (centroid depth) Zb ¼ 2Z0�Zt km (basal depth)
H. Saibi et al. / Journal of African Earth Sciences 111 (2015) 92e9996
values of between 14 �C/km and 36 �C/km in the study area. Areasof shallow CPDs are generally associated with geothermal gradienthigher than 30 �C/km. Using Fourier's Law, we calculated the heat-flow from the CPD estimates for the Curie-point for magnetite of580 �C (Haggerty, 1978) and using an average thermal conductivityof l ¼ 2.5 W/m �C (Stacey, 1977). We derive heat-flow values in the
Fig. 5. The CPD map of western Afghanistan
range of between 36 and 90 mW/m2 (Fig. 7).
5. Discussion
The shallowest CPDs are associated with the basin structures inthe central western part of Afghanistan (Helmand region), whereas
and the main major faults in the region.
Fig. 6. Geothermal gradient map of the study area.
Fig. 7. Heat flow map of the study area.
H. Saibi et al. / Journal of African Earth Sciences 111 (2015) 92e99 97
intermediate to deep CPDs (20e40 km depth) are mainly observedtowards the north-western part of the country. The deep-seatedCPDs may delineate the collision zone between the Indian andEurasian plates.
The shallow and intermediate CPDs delineated by our studymayrepresent areas of greatest geothermal potential in western
Afghanistan and wewill now focus the discussion of the findings inthe context of known hydrothermal activity in that part ofAfghanistan with potential implications for geothermalexploration.
Most surface geothermal manifestations in the country areassociated with the collision tectonics of the Eurasian and Indian
H. Saibi et al. / Journal of African Earth Sciences 111 (2015) 92e9998
plates along the HeratePanjshir geosuture. Manifestations areparticularly exposed along the resultant neotectonic regionsgenerated by these collisions in addition to the contemporaneousPaleo-Neo-Quaternary volcanism (Tapponnier et al., 1981).
In many cases, the location of hot springs is closely related to theprovenance of mafic to acidic intrusive and volcanic rocks along themain suture zone extending along a general NEeSW strike (Shareqet al., 1980; Musazai, 1994). The shallowest CPDs are mapped at thesouth-western portion of the suture zone in the Helmand areawhich comprises an area of crustal thinning and basin formationover accreted terranes. This area hosts the Helmand-ArghandabGeothermal Field (HAGF), which is characterized byCO2enitrogen-bearing thermal waters. Most of the hydro-geothermal activities are related to the HelmandeArghandabgranitoid massifs. The deep-seated ChamaneMokur (CHeMO inFig. 1) fault system is the main structural component in thisgeothermal field, which includes areas such as Mokur, Helmand,and Tirin-Azhdar. The majority of geothermal manifestations are inthe vicinities of the CHeMO fault system, rich in alkali and rareearth elements (Saba et al., 2004).
The western parts of the shallow and intermediate depth CPDsencompass the Farahrud Geothermal Field (FGF) to the south of thecity of Farah. The thermal springs in the Farahrud structuraldepression are associated with a fracture system related to thePasaband deep-seated fault. The hot waters are rich in bicarbonate,calcium, and silica (Saba et al., 2004). Further south intermediatedepth CPDs appear to underlie the Baluchistan Geothermal Field(BGF), which is known for many hydrothermal manifestations inthe volcanic successions of Chagai (Baluchistan). Their geothermalactivities are associated with carbonatitic post-volcanic processesand fluids are characterized mainly by brines rich in CO2, calcium,and chloride (Saba et al., 2004).
Based on our analysis of the distribution of shallow CPDs andassociated high geotherms, the Helmand-Arghandab GeothermalField appears to contain the highest potential for future geothermalenergy exploitation followed by the Farahrud and Baluchistangeothermal fields. All shallow CPDs identified in the study are inareas of neotectonic activity. It is hence reasonable to assume thatone of the main controlling factors in the formation of shallow-seated geothermal systems are major fault systems as identifiedin Fig. 1. Such faults zone creates permeable pathways throughwhich geothermal fluidsmaymigrate effectively (Caine and Forster,1999).
6. Conclusions
Power spectral method was applied to aeromagnetic data fromwestern Afghanistan with the purpose of estimating depths to theCurie isotherm in the study area. In general, the shallowest CPDscan be associated with thinned crust and recent magmatic activity,while deep CPDs are associated with cooler thickened crust.
Generally speaking, the geothermal potential in Afghanistan, isto-date largely unknown. Apart from few initial-stage geothermalexploration projects, which started in 1969 (Saba et al., 2004), thereis an absence of dedicated drilling programs or advanced technol-ogies for geothermal reservoir characterizations. Therefore ourstudy provides an unprecedented opportunity to explore thegeothermal potential of western Afghanistan.
Main findings of this study can be summarized as follows:
- The estimated CPD from aeromagnetic data from westernAfghanistan are ranging from 16 to 40 km. Deep CPDs(30e40 km) have been identified to the north of the main suturezone in the oldest part of the collision zone between the Indianand Eurasian plates. Shallower CPDs of <30 km depth in the
western and southwestern part of Afghanistan could beexplained by crustal thinning and may represent promisingregions for geothermal exploration.
- The calculated geothermal gradients from the relationship be-tween CPD and Curie temperature (580 �C) give values between14 �C/km and 36 �C/km.
- The deduced heat flow values in western Afghanistan rangebetween 36 and 90 mW/m2.
Acknowledgments
The authors are grateful to Dr. Daud Shah Saba (Governor ofHerat Province, Afghanistan) for geothermal data. J.G. acknowl-edges support from the Royal Society (UF090006).
Appendix A. Supplementary data
Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jafrearsci.2015.07.019.
References
Aboud, E., Salem, A., Mekkawi, M., 2011. Curie depth map for Sinai Peninsula, Egyptdeduced from the analysis of magnetic data. Tectonophysics 506, 46e54. http://dx.doi.org/10.1016/j.tecto.2011.04.010.
Bhattacharyya, B.K., Leu, L.K., 1975. Analysis of magnetic anomalies over Yellow-stone National Park: mapping the Curie point depth isothermal surface forgeothermal reconnaissance. J. Geophys. Res. 80, 4461e4465. http://dx.doi.org/10.1029/JB080i032p04461.
Blakely, R.J., 1995. Potential Theory in Gravity and Magnetic Applications. Cam-bridge University Press, London.
Bouligand, C., Glen, J.M.G., Blakely, R.J., 2009. Mapping Curie temperature depth inthe western United States with a fractal model for crustal magnetization.J. Geophys. Res. 114 (B11104), 1e25. http://dx.doi.org/10.1029/2009JB006494.
Caine, J.S., Forster, C.B., 1999. Fault zone architecture and fluid flow: insights fromfield data and numerical modeling. In: Haneberg, W.C., Mozley, P.S., Moore, J.C.,Goodwin, L.B. (Eds.), Faults and Subsurface Fluid Flow in the Shallow Crust.American Geophysical Union, Washington, D. C. http://dx.doi.org/10.1029/GM113p0101.
De Ritis, R., Ravat, D., Ventura, G., Chiappini, M., 2013. Curie isotherm depth fromaeromagnetic data constraining shallow heat source depths in the centralaeolian ridge (southern Tyrrhenian Sea, Italy). Bull. Volcanol. 75 (710), 11. http://dx.doi.org/10.1007/s00445-013-0710-9.
Espinosa-Carde~na, J.M., Campos-Enriquez, J.O., 2008. Curie point depth fromspectral analysis of aeromagnetic data from Cerro Prieto geothermal area, BajaCalifornia, Mexico. J. Volcanol. Geotherm. Res. 176, 601e609. http://dx.doi.org/10.1016/j.jvolgeores.2008.04.014.
Haggerty, S.E., 1978. Mineralogical constraints on Curie isotherm in deep crustalmagnetic anomalies. Geophys. Res. Lett. 5 (2), 105e109. http://dx.doi.org/10.1029/GL005i002p00105.
Maden, N., 2010. Curie-point depth from spectral analysis of magnetic data inErciyes stratovolcano (central Turkey). Pure Appl. Geophys. 167 (3), 349e358.http://dx.doi.org/10.1007/s00024-009-0017-0.
Mihalasky, M.J., Doebrich, J.L., Wahl, R.W., Ludington, S.D., Orris, G.J., Bliss, J.D.,Sutphin, D.M., Schruben, P.G., Bolm, K.S., Hubbard, B.E., Mars, J.C., Peters, S.G.,Wandrey, C.J., Chirico, Pete, 2007. Geographic information system (GIS) toaccompany the non-fuel mineral resource assessment of Afghanistan, appendix1. In: Peters, S.G., Ludington, S.D., Orris, G.J., Sutphin, D.M., Bliss, J.D., Rytuba, J.J.(Eds.), U.S. Geological Survey-Afghanistan Ministry of Mines Joint MineralResource Assessment Team, Preliminary Non-Fuel Mineral Resource Assess-ment of Afghanistan. US Geological Survey Open-File Report, 2007-1214(version 1), two discs.
Musazai, A., 1994. Research on Ultrabasites of Afghanistan and their IndustrialMineralization: Research Monograph. Kabul Polytechnic Inst., Afghanistan,p. 127 (in Persian).
Okubo, Y., Matsunaga, T., 1994. Curie point depth in northeast Japan and its cor-relation with regional thermal structure and seismicity. J. Geophys. Res. ISSN:0148-0227 99 (B11), 22363e22371. http://dx.doi.org/10.1029/94JB01336.
Okubo, Y., Graf, R.J., Hansen, R.O., Ogowa, K., Tsu, H., 1985. Curie point depth of theisland of Kyushu and surrounding areas, Japan. Geophysics 50, 481e494.
Stacey, F.D., 1977. Physics of the Earth, second ed. Wiley, New York.
Stampolidis, A., Kane, I., Tsokas, G.N., Tsourlos, P., 2005. Curie point depths ofAlbania inferred from ground total field magnetic data. Surv. Geophys. 26 (4),461e480. http://dx.doi.org/10.1007/s10712-005-7886-2.
Tanaka, A., Okubo, Y., Matsubayashi, O., 1999. Curie point depth based on spectrumanalysis of the magnetic anomaly data in east and southeast Asia. Tectono-physics 306, 461e470.
Tapponnier, M., Mattauer, M., Proust, F., Cassaigneau, C., 1981. Mesozoic ophiolites,sutures, and large-scale tectonic movements in Afghanistan. Earth Planet. Sci.Lett. 52, 355e371.
Turcotte, D.L., Schubert, G., 1982. Geodynamics. Cambridge University Press, NewYork.
U.S. Geological Survey, 2011. Aeromagnetic Surveys in Afghanistan: an UpdatedWebsite for Distribution of Data. Open-File Report 2011e1055.
Wasilewski, P.J., Thomas, H.H., Mayhew, M.A., 1979. The moho as a magneticboundary. Geophys. Res. Lett. 6 (7), 541e544.
Wheeler, R., Bufe, C.G., Johnson, M.L., Dart, R.L., 2005. Seismotectonic Map ofAfghanistan, with Annotated Bibliography. U.S.G.S. Open-File Report2005e1264.
