Abstract
The Um Had area, central Eastern Desert, Egypt shows a regional stretching in the NW–SE and a contraction in the NE–SW direction. Major NW–SE folds, small recumbent folds, and local thrusts and reverse faults were recognized. Complicated relation between folds and boudinage was identified. This stretching amount ranges from 1.282 to 1.309. Earlier coaxial and later non-coaxial strains were inferred. The change from axial to non-coaxial stress regime was gradual and the latter was associated with minor clockwise and anticlockwise rotation of structural elements. During the non-coaxial strain, strain fringes were formed as a consequence of the high circulation of fluids in low temperature and high pressure conditions. Superimposed strain fringes indicating right- and left-lateral senses of movement were recognized. At least three generations of fringes were recognized, implying three stages of non-coaxial stretching. Each generation has about 15 increments which show irregular strain gradient and intensity over the different increments. Eastwards, the strain increments became mature and westwards, the finite strain increases. The strongest finite strain was found in a narrow belt delimiting the basement rocks on the west and underlying the Phanerozoic sediments. Chocolate-tablet structure was recorded and indicates later multidirectional tension. Not all Nubia Sandstone exposures are overlying the basement rocks and some are separated by NW–SE normal faults. Major NW–SE normal faults are cutting basement rocks of different ages.
Abstract
أحمدمصر – جامعة جنوب الوادي - كلية العلوم – قسم الجيولوجيامنطقة أم حاد ، وسط الصحراء الشرقية ، مصر ، تعتبر مثال لدراسة طبيعة الحد الغربي لصخور القاع المكشوفة. هذه المنطقة بها استطالة عامة في اتجاه شمال غرب وانضغاط في اتجاه شمال شرق، وقد أمكن التعرف على طيات كبيرة ومتوسطة الحجم لها محاور تتوجه شمال غرب ، وطيات أخرى نائمة recumbent وكذلك فوالق دسرية ومعكوسة، كما تم رصد علاقات معقدة بين البوديناج boudinage والطيات ، وتم استخدام البوديناج لحساب قيمة الاستطالة والتي تراوحت بين 1.282 إلى 1.309. أمكن كذلك استنباط أن المنطقة تعرضت في وقت مبكر إلى انفعال محوري axial strain تلاه آخر غير محوري non-coaxial strain وقد حدث ذلك تدريجيا. أثناء الانفعال الغير محوري نشأت أهداب fringe في الأماكن البعيدة عن تأثير الانفعال الانضغاطي وفي اتجاه الاستطالة وذلك نتيجة لحركة الموائع fluids في درجة حرارة منخفضة وضغط مرتفع ، وقد أمكن التعرف على أهداب متراكبة superimposed دلت على حركات يمينية ويسارية متعاقبة. تم أيضا التعرف على ثلاث مراحل على الأقل من الأجيال generations الخاصة بالأهداب والتي تدل على مراحل متعاقبة من الاستطالة ، وقد تم احتساب على الأقل 15 مرحلة من الانفعال المتزايد increments ولكل منها شدته ، وهذا بدوره يدل على عدم الانتظام في قيمة الإجهاد والانفعال الذي تعرضت له الصخور. ناحية الشرق نجد أن الانفعال المتزايد incremental strain يصبح أكثر نضوجا ، بينما ناحية الغرب نجد أن الانفعال النهائي finite strain يصبح أكثر شدة ، وقد وجد اكبر انفعال في حزام ضيق يحد صخور القاع من الغرب مباشرة أسفل صخور الفانيروزويك الرسوبية. تم أيضا التعرف على ظاهرة تركيب قطع الشيكولاته chocolate-tablet structure والتي دلت على أن المنطقة قد تعرضت في أوقات لاحقة إلى شد من جميع الاتجاهات، كما تم أيضا ملاحظة أن الحجر الرملي النوبي يعلو مباشرة صخور القاع ، وفي بعض المواقع مفصولين بفوالق عادية تتوجه شمال غرب ، كم تم قياس فوالق عادية داخل صخور القاع تأخذ اتجاهات عديدة.
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Acknowledgments
I thank the AJGS reviewers for revising and enhancing the manuscript. Prof. Dr. Samir El Gaby and Prof. Dr. Moustafa Youssef (Assiut University) are thanked for discussion of the results and his valuable comments and Dr. Colin Ferguson and Dr. Geoff Lloyd (Leeds University, UK) for the availability of the Strain reversal software via internet.
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Akawy, A. Structural elements and incremental strain history of the basement rocks of Um Had area, central Eastern Desert, Egypt. Arab J Geosci 2, 285–300 (2009). https://doi.org/10.1007/s12517-008-0029-6
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DOI: https://doi.org/10.1007/s12517-008-0029-6