Sains Ma1aysiana 25(4) 1996: 31-45 Sains Bumi/ Earth Sciences
Zon Sesar Alur Lebey Utara Semenanjung Malaysia (Alur Lebey fault zone in North Peninsular Malaysia)
Zaiton Harun Jabatan Geologi, Universiti Kebangsaan Malaysia 43600 UKM Bangi Selangor D.E. Malaysia
ABSTRAK
Zon sesar Alur Lebey yang selari dengan sebahagian Sungai Lebey, tersingkap di km 221.5 dari Kota Bharu di lebuhraya Timur-Barat. Panjang singkapan keratan di timur jalan di sekitar 155m, manakala di barat jalan pula 105m. Morfologi sesar ditunjukkan oleh barisan susuh bukit beifaset di sepanjang lembah. Di lapangan dan di peta geologi, sesar ini ditafsirkan sebagai sempadan di antara metavolkano di baratnya dan metaklas di timurnya. Lanjutannya ke utara ditafsirkan bahawa sesar tersebut memotong sepanjang sempadan di antara pelit bersilika di barat dan rijang serta pelit bersilika di timur. Sungguhpun gerakan mendatar ke kiri terpamer dalam milonit, namun pergerakan mendatar ke kanan boleh dicerap dalam zon tersebut. Dalam area Lansat 3 bertarikh 10 Januari 1979 lineamen berjurus 355° yang mewakili zon sesar Alur Lebey dapat dikesan sepanjang 55 Ian dari Lebuhraya Timur-Barat hingga ke pertengahan bahagian timur Sungai Pattani di Thailand. Singkapan sesar tersebut terdiri daripada milonit separuh terluluhawa yang mengandungi kekanta kuarza dan tuf. Selain berbentuk kekanta, kuarza juga berbentuk koma, dan ramping-dan-ampul. Klas yang berbentuk tersebut digunakan untuk menentukan hala pergerakan dalam zon sesar. Umumnya milonit menjurus ke utara dengan kemiringannya euram hingga tegak. Satah-satah sesar mendatar memo tong foliasi milonit pada sudut tirus atauJdan ada pula yang selari dengannya. Selain sesar mendatar terdapat pula sesar songsang yang kemiringannya landai hingga sederhana ke barat. Gerakan umumnya menyongsang ke timur-tenggara. Foliasi milonit yang menjurus ke utara mengandungi tanda-tanda pergerakan ke kiri dan ke kanan, manakala foliasi yang menjurus ke baratlaut mengandungi hanya pergerakan mendatar ke kiri. Sebilangan lipatan yang berkait-rapat dengan sesar menunjam ke utara-baratlaut, selatan-baratdaya dan ke arah timur dieerap di dalam zon sesar tersebut. Gabungan eerapan lapangan dan analisis struktur akhirnya sampai kepada pentafsiran sejarah canggaan yang berlaku di dalam zon sesar tersebut. Empat sistem tegasan boleh disimpulkan. Sistem tegasan maksimum (mampatan) tersebut ialah .utara-timurlaut, timurtenggara, dan yang termuda utara-timurlaut. Canggaan tertua dicirikan oleh pergerakan dekstral di sepanjang zon milonit yang mengandungi kekanta tuf dan kuarza yang asimetri berjurus 355° hingga 05°. Pergerakan sinistral di sepanjang milonit yang berjurus 354° dan 326° merupakan bukti daripada sistem tegasan timur-tenggara. kedua-dua episod canggaan mungkin mewakili penyesaran aseismos yang berlaku dalam sekitaran tekanan mengepung yang tinggi. Namun demikian perulangan tindakan sistem tegasan timur-tenggara menyebabkan pergerakan sinistral di sepanjang satah sesar berjurus baratlaut dan utara, penyesaran songsang di atas satah berjurus selatan hingga baratdaya dan lipatan seret. Pergerakan dekstral di sepanjang satah berjurus utara dan baratdaya merupakan episod canggaan yang termuda. Dua episod yang terakhir mungkin mewakili canggan rapuh akibat daripada kedudukan cetek jasad batuan sekurang-kurangnya di sepanjang segmen sesar tersebut.
ABSTRACT
The Alur Lebey fault zone, which is paralled to part of Sungai Lebey, is exposed along the East-West Highway at km 221.5 to Kota Bharu. The outcrop is about 155 m wide on the east and 105 m wide on the west side of the road. Fault morphology can be recognised from the faceted hill spurs along the valley. The fault zone has been interpreted as a lithological contact between metavolcanic in the west and metaclastic in the east. The northern extension of the fault was interpreted as the contact between silicous pelite in the west, and chert and siliceous pelite in the east. Even though left lateral motion is exhibited within the zone, evidences of right lateral motion were also observed. On Landsat 3 image, a lineament in the direction of 355° representing the Alur Lebey fault zone could be traced for some 55 km from the East-West Highway to the middle of east Pattani River in Thailand. The outcrop at the East-West Highway comprises medium indurated mylonite containing lenses of quartz and tuff. Other than asymmetric lenses, quartz also occurs in the form of a comma, and pinch-and-swell structures. These forms of clasts are used to determine the sense of movements along the fault. In general, steeply to vertically dipping mylonite strikes towards the north. Fault planes with horizontal senses of motion cut the foliotion at an acute angle. Some of these fault planes are parallel to the foliation. There are also reverse fault planes with medium to gentle dips to the west. General motions of the fault were tOwards the east southeast. The mylonite foliation which strikes north contains evidences of both left and right lateral motions, whereas the northwest striking foliation exhibits left lateral motion. Several fault-associated folds plunging towards the north northwest, south southwest and east were observed in the fault zone. The combination of field observations and structural analysis are used to arrive at the interpretation on the history of the fault zone. Four systems of compressions (maximum principal stresses) can be deduced from the study. these are the north-northeast, east-southeast, east-southeast, and the youngest north-northeast directions. The oldest deformation is characterised by dextral movement along the mylonite zone. the zone contains asymmetric lenses of tuff and quartz striking 355° to 05°. The sinistral movement along mylonite fooliation striking 354° and 326° is the evidence of the east-southest compressive stress system. Both deformation episodes could represent aseismic faultings under fairly high confining pressure. However the recurrance of the east-southest stress system could have caused the sinistral movement along the northwest and north striking fault planes, the reverse faulting on south to southwest planes and the resulting drag folds. The dextral movement along the north and northwest planes is the youngest deformation episode. The final two episodes could represent brittle deformation resulting from the shallow position of the rock body at least along that particular fault segment.
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