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

Diserahkan: 8 Februari 2010 / Diterima: 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

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*Pengarang untuk surat-menyurat; email: ahmedsaheel423@yahoo.com