Sains Malaysiana 40(8)(2011): 853–864

Mapping of Faults in the Libyan Sirte Basin by Magnetic Surveys

(Pemetaan Sesar dalam Lembangan Sirte Libya dengan Survei Magnet)

A.S. Saheel*

Libyan Petroleum Institute-Tripoli-Libya

Abdul Rahim Bin Samsudin & Umar Bin Hamzah

School of Environment and Natural Resource Sciences, Faculty of Science and Technology

Universiti Kebangsaan Malaysia , 43600 Bangi, Selangor, Malaysia

Received: 8 February 2010 / Accepted: 23 September 2010

ABSTRACT

Magnetic surveys were carried out in Farigh area which is located in the eastern part of the Libyan Sirte basin. Interpretation of the onshore magnetic anomaly of this area, suggests that the high total magnetization may be caused by an intrusive body. Analysis of the magnetic power spectra indicates the presence of four sub-anomalies at depths of 340 m, 1400 m, and 2525 m which is probably related to the igneous rocks. The presence of igneous rock as basement at depth of 4740 m was confirmed by drilling. Assuming that all rock magnetization in the area is caused by induction in the present geomagnetic field, it strongly suggests that the causative structure has a remnant magnetization of declination (D) = −16° and inclination (I) = 23°. Based on pseudogravity data, the total horizontal derivative map shows high gradient values in NW-SW trends related to the structures in the eastern part of the Sirte basin. The 3D Euler deconvolution map derived from gravity data clearly indicates the location of igneous body in the study area as well as its tectonic trends and depth, which is estimated at 350 m to 1400 m below the surface. Depth of gravity anomalies at 1400 m to 2525 m is considered as anomalies in between shallow and deep. Anomaly at depth of approximately 4740 m below the surface is interpreted as basement rock. Geologically, the magnetic survey shows that the source of anomaly is a mafic igneous rock of Early Cretaceous age. The study also discovered a left-lateral sheared fault zone along the NW-SE of Hercynian age which was believed to be reactivated during Early Cretaceous.

Keywords: Deconvolution; fault zone; igneous bodies; magnetic survey; rock magnetization

ABSTRAK

Survei magnet telah dijalankan di kawasan Farigh yang terletak di bahagian timur lembangan Sirte di Libya. Pentafsiran yang dibuat ke atas data anomali magnet yang diperoleh di daratan mencadangkan bahawa kemagnetan jumlah yang tinggi mewakili satu jasad intrusi. Analisis data spectral kuasa menunjukkan kehadiran empat subanomali yang terletak pada kedalaman 340, 1400 dan 2525 m yang juga dikaitkan dengan batuan igneus. Kehadiran batuan igneus pada kedalaman 4740 m dibuktikan dengan data penggerudian. Jika sekiranya kesemua kemagnetan batuan dihasilkan secara aruhan medan geomagnet semasa, struktur penyebab kepada anomali magnet yang diukur mempunyai kemagnetan baki bernilai deklinasi (D) = -16° dan inklinasi (I) = 23°. Berdasarkan data pseudograviti, peta terbitan jumlah mengufuk menunjukkan kecerunan yang tinggi pada arah barat laut-tenggara yang dikaitkan dengan struktur di bahagian timur lembangan Sirte. Peta dekonvolusi Euler 3D yang dihasilkan daripada pada data graviti menunjukkan dengan jelas lokasi jasad igneus di kawasan kajian serta tren tektonik dengon kedalaman anomali yang dianggarkan terletak pada kedalaman 350 m dan 1400 m di bawah permukaan bumi. Anomali graviti pada kedalaman 1400 m dan 2525 m dianggap mewakili anomali pada kedalaman pertengahan di antara anomali cetek dan dalam. Anomali yang terletak pada kedalaman 4740 m di bawah aras permukaan ditafsirkan sebagai mewakili batuan dasar. Berdasarkan fakta geologi, survei magnet menunjukkan bahawa punca anomali merupakan batuan igneus jenis mafik yang berusia Kapur Awal. Kajian juga dapat menemukan zon sesar ricih jenis kekiri yang mempunyai tren barat laut – tenggara serta berusia Hercynian yang diaktifkan semula pada zaman Kapur Awal.

Kata kunci: Dekonvolusi; jasad igneus; kemagnetan batuan; survei magnet; zon sesar

REFERENCES

Ahlbrandt, T.S. 2001. The Sirte Basin Province of Libya: Sirte-Zelten Total Petroleum System. U.S. Geological Survey Bulletin 2202-F (Version 1.0)

Bilim, F. 2007. Investigations into the tectonic lineaments and thermal structure of Kutahya-Denizli region, western Anatolia, from using aeromagnetic, gravity and seismological data. Physics of the Earth and Planetary Interiors 165(3-4): 135-146.

Blakely, R.J. & Simpson, R.W. 1986. Locating edges of source bodies from magnetic and gravity anomalies. Geophysics 51(7): 1494-1498.

Bournas, N., Galdeano, A., Hamoudi, M. & Baker, H. 2003. Interpretation of the aeromagnetic map of Eastern Hoggar (Algeria) using the Euler deconvolution, analytic signal and local wavenumber methods. Journal of African Earth Sciences 37(3-4): 191-205.

Chennouf, T., Khattach, D., Milhi, A., Andrieux, P. & Keating, P. 2007. Major structural trends in northeastern Morocco: The contribution of gravimetry. Comptes Rendus – Geoscience 339: 383-395.

Cordell, L. & Grauch, V.J.S. 1985. Mapping basement magnetization zones from aeromagnetic data in the San Juan Basin, New Mexico. In Hinze, W. J., Ed., The utility of regional gravity and magnetic anomaly maps: Society of Exploration Geophysicists: 181-197.

Cordell, L. 1979. Gravimetric expression of graben faulting in Santa Fe Country and the Espanola Basin, New Mexico, Guidebook to the 30th Field Conference, Santa Fe Country, New Mexico Geological Society (Ed), New Mexico Geological Society, Socorro: 59-64.

Corner, B. & Wilsher, W.A. 1989. Structure of the Witwatersrand basin derived from interpretation of the aeromagnetic and gravity data. In Garland, G. D., Ed., Proceedings of Exploration ’87, Third Decennial International Conference on Geophysical and Geochemical Exploration for Minerals and Groundwater: Ontario Geological Survey, Special 3: 532-546.

El-Hawat, A.S., Missallati, A.A, Bezan, A.M. & Taleb, T.M. 1996. The Nubian Sandstone in Sirte Basin and its Correlatives. In: Salem, AJ, El-Hawat, AS and AM Sbeta (Eds), The Geology of Sirt Basin. Amsterdam: Elsevier, p. 3-30.

Fairhead, J.D., Bennet, K.J., Gordon, R.H. & Huang, D. 1994. Euler: Beyond the Black Box. 64th Annual International Meeting, Society of Exploration Geophysicists, Expanded Abstracts, pp. 422-424.

Geosoft Reference Manual 2009. Software for Earth Sciences Geosoft INC., Toronto, Cana.

Goudarzi, G.H. 1980. Structure- Libya. In: Salem, MJ and Buserwil, MI (editors). The Geology of Libya. 3: 879-892. London, UK: Academic Press.

Gras, R. & Thusu, B. 1998. Trap architecture of the Early Cretaceous Sarir Sandstone in the eastern Sirt Basin. In: Macgreor, DS, Moody, RTJ and DD Clark-Lowes (eds), Petroleum Geology of North Africa. Geological Society, London, Special Publication No:132: 317-334.

Guiraud, R. & Bosworth, W. 1997. Senonian basin inversion and rejuvenation of rifting in Africa and Arabia: synthesis and implications for plate scales tectonics. Tectonophysics 282: 39-82.

Hallet, D. 2002. Petroleum Geology of Libya. 487 pp, Elsevier, Amsterdam.

Huang, D., Gubbins, D., Clark, R.A. & Whaler, K.A. 1995. Combined study of Euler’s homogeneity equation for gravity and magnetic field. 57th Conference and Technical Exhibition EAGE, Glasgow, Extended Abstracts, P144.

Klitzsch, E.H. & Squyres, C.H. 1990. Paleozoic and Mesozoic geological history ofnortheastern Africa based upon new interpretation of Nubian Strata. The AAPG Bull. 74: 1203-1211.

Klitzsch, E. 1971. The structural development of part of north Africa since Cambrian time. In: Gray, C(editor), First Symposium on the Geology of Libya. Faculty of Science,University of Libya, pp. 253-262, Tripoli.

Klingele, E.E., Marson I., & Kahle, H.G. 1991. Automatic interpretation of gravity gradiometric data in two dimensions: vertical gradient. Geophysical Prospecting 39: 407-434.

Tawadros, E, Rasul, S.M. & Elzaroug, E. 2001. Petrography and palynology of quartzite in the Sirte Basin, central Libya. Jour. African Earth Sciences 32: 373-390.

Li, C.-F., Zhou, Z., Hao, H., Chen, H., Wang, J., Chen, B. & Wu, J. 2008. Late Mesozoic tectonic structure and evolution along the present-day northeastern South China Sea continental margin. Journal of Asian Earth Sciences 31(4-6): 546-561.

Ma, Z.-J., Gao, X.-L. & Song, Z.-F. 2006. Analysis and tectonic interpretation to the horizontal-gradient map calculated from Bouguer gravity data in the China mainland. Chinese Journal of Geophysics (Acta Geophysica Sinica) 49(1): 106-114.

Marson, I., & Klingele, E.E. 1993. Advantages of using the vertical gradient of gravity for 3-D interpretation. Geophysics 58: 1588-1595.

Phillips, J.D. 1998. Processing and interpretation of aeromagnetic data for the Santa Cruz Basin-Patahonia Mountains area, South-Central Arizona. U.S. Geological Survey Open-File Report 02-98.

Pilkington, M. 2007. Locating geologic contacts with magnitude transforms of magnetic data. Journal of Applied Geophysics 63(2): 80-89.

Reid, A.B. 2003. Short note: Euler magnetic structural index of a thin-bed fault. Geophysics 68: 1255-1256.

Reid, A.B., Allsop, J.M., Granser, H., Millet, A.J. & Somerton, I.W. 1990. Magnetic interpretation in three dimensions using Euler Deconvolution. Geophysics 55: 80-91.

Reid, A.B., FitzGerald D. & McInerney P. 2003. Euler Deconvolution of gravity data. Society of Exploration Geophysicists (SEG), Annual Meeting, 2003, pp. 580-583.

Robertson, 1970. Geological study in the concession 12. Unpblished report.

Shepherd, T., Bamber, J.L. & Ferraccioli, F. 2006. Subglacial geology in Coats Land, East Antarctica, revealed by airborne magnetics and radar sounding. Earth and Planetar Science Letters 244(1-2): 323-335.

Thompson, D.T. 1982. EULDPH – A new technique for making computer-assisted depth estimates from magnetic data. Geophysics 47: 31-37.

Van der Meer, F. & Cloetingh, C. 1993. Intraplate stresses and subsidence history of the Sirte Basin (Libya). Tectonophysics v. 226: 37-58.

Wilsher, W.A. 1987. A structural interpretation of the Witwatersrand basin through the application of the automated depth algorithms to both gravity and aeromagnetic data. Thesis (MSc). University of Witwatersrand. (Unpublished)

*Corresponding author; email: ahmedsaheel423@yahoo.com