Fetheddine Melki

Erratum to: Neogene sediment deformations and tectonic features of northeastern Tunisia: evidence for paleoseismicity

The Neogene stratigraphic series is characterized by predominant clayey facies alternated by other sand layers. The outcrop and subsurface studies show varied and complex styles of deformations and lead to relate the structures to paleoseismic events. The seismicity of eastern onshore and offshore Tunisian margin follows the master fault corridors oriented globally N–S, E–W, and NW–SE that correspond to the bordering faults of grabens and syncline corridors and associated faulted drag fold structures oriented NE–SW. Epicenters of magnitudes between 3 and 5 are located along these border fault corridors. The Neogene strata record brittle structures, including numerous and deep faults and fractures with straight and high-angle dipping planes. The structuring of NE–SW en echelon folds and synclines inside and outside NW–SE and E–W right lateral and N–S and NE–SW left lateral tectonic corridors indicates the strike-slip type of bordering faults and their seismogenic nature. Wrench fault movements that induce mud and salt diapirs, mud volcanoes, and intrusive ascensions are related to seismic shocks. Seismic waves caused by activity along one, or most likely, several tectonic structures would have propagated throughout the Quaternary cover producing seismites. The similarity of deposits, structuring, and seismites between the Tunis-Bizerte to the North and Hammamet-Mahdia to the South accredits the hypothesis that the seismic episodes might have affected sedimentation patterns along the Sahalian large geographic area. The paleoseismic events in northeastern Tunisia might be related to tectonic fault reactivations through time. This hypothesis is consistent with the geomorphologic context of the study area, characterized by several morphostructural lineaments with strong control on the sediment distribution, as well as uplifted and subsiding terrains. The estimated magnitude of the seismic events and the great regional tectonically affected areas demonstrate that the northeastern Tunisia experienced stress through the last geological episodes of its evolution. This Neogene kinematic reconstruction highlights the neotectonic system inducing the actual seismicity on this margin. Therefore, there is a straight relationship between deepseated faults and seismicity.

Neogene sediment deformations and tectonic features of northeastern Tunisia: evidence for paleoseismicity@@@تغيرات رواسب العهد النيوجيني وميزات التكتونية في شمال شرق البلاد التونسية: علاقتها بالأحداث الزلزالية القديمة

The Neogene stratigraphic series is characterized by predominant clayey facies alternated by other sand layers. The outcrop and subsurface studies show varied and complex styles of deformations and lead to relate the structures to paleoseismic events. The seismicity of eastern onshore and offshore Tunisian margin follows the master fault corridors oriented globally N–S, E–W, and NW–SE that correspond to the bordering faults of grabens and syncline corridors and associated faulted drag fold structures oriented NE–SW. Epicenters of magnitudes between 3 and 5 are located along these border fault corridors. The Neogene strata record brittle structures, including numerous and deep faults and fractures with straight and high-angle dipping planes. The structuring of NE–SW en echelon folds and synclines inside and outside NW–SE and E–W right lateral and N–S and NE–SW left lateral tectonic corridors indicates the strike-slip type of bordering faults and their seismogenic nature. Wrench fault movements that induce mud and salt diapirs, mud volcanoes, and intrusive ascensions are related to seismic shocks. Seismic waves caused by activity along one, or most likely, several tectonic structures would have propagated throughout the Quaternary cover producing seismites. The similarity of deposits, structuring, and seismites between the Tunis-Bizerte to the North and Hammamet-Mahdia to the South accredits the hypothesis that the seismic episodes might have affected sedimentation patterns along the Sahalian large geographic area. The paleoseismic events in northeastern Tunisia might be related to tectonic fault reactivations through time. This hypothesis is consistent with the geomorphologic context of the study area, characterized by several morphostructural lineaments with strong control on the sediment distribution, as well as uplifted and subsiding terrains. The estimated magnitude of the seismic events and the great regional tectonically affected areas demonstrate that the northeastern Tunisia experienced stress through the last geological episodes of its evolution. This Neogene kinematic reconstruction highlights the neotectonic system inducing the actual seismicity on this margin. Therefore, there is a straight relationship between deepseated faults and seismicity.الخلاصةتتميز ستراتيغرافيا العهد النيوجيني (Neogene) بطبقات طينية سميكة تتخللها طبقات أخرى من الرمل. وتظهر الدراسات الميدانية و تحت سطح الأرض (جيوفيزيائية) أنماط متنوعة ومعقدة من التشوهات، تؤدي إلى ربطها بأحداث زلزالية قديمة. تبدو الزلازل في شرق البلاد قد اتبعت ممرات الصدوع الرئيسية والمتجهة عموما شمال-جنوب، شرق-غرب و شمال غرب-جنوب شرق توافقا مع الصدوع المتاخمة للهياكل المنخفضة و المقعرة (grabens) وما يرتبط بها من طيات ذات اتجاه شمال شرق-جنوب غرب. ويتراوح مركز الزلزال بين 3 و 5 درجات على مقياس ريشترRichter))، على امتداد ممرات الصدوع. تظهر طبقات النيوجين هياكل هشة تحتوي على صدوع عديدة وعميقة ذات زاويا مرتفعة. كما أن هيكلة الطيات و المنخفضات المتجهة شمال شرق-جنوب غرب داخل وخارج الممرات التكتونية ذات التحرك الجانبي الأيمن والمتجهة شمال غرب-جنوب شرق و شرق-غرب والأخرى ذات التحرك الجانبي اليساري والمتجهة شمال-جنوب و شمال شرق-جنوب غرب، تشير إلى نوعية الصدوع و طبيعة السيسموجينية (seismogenic) المرافقة لها. إن التحركات الجانبية للصدوع والتي رافقتها هياكل نتيجة تحركات و صعود الطين (mud diapirs) والملح (salt diapirs) والبراكين، قد ارتبطت بالصدمات الزلزالية. و يبدو أن الموجات الناجمة عن نشاط واحد أو العديد من الهياكل التكتونية قد انتشرت عبر طبقات العصر الرباعي منتجة بذالك العديد من السايسميت (seismites). إن تشابه الترسبات والهياكل و السايسميت بين جهتي تونس-بنزرت من الشمال والحمامات-المهدية من الجنوب، يدعم فرضية أن الحلقات الزلزالية قد أثرت على أنماط الترسيب على مساحة جغرافية واسعة من الساحل التونسي. تبدو الأحداث الزلزالية القديمة (paleoseismic events) في شمال شرق البلاد التونسية ذات صلة بعودة النشاط التكتونى للصدوع (tectonic fault reactivations) عبر الزمن. و تتلاءم هذه الفرضية مع الإطار الجيومورفولوجي (geomorphologic) لمنطقة الدراسة و الذي يتميز بعدة عناصر مورفو بنيوية (morphostructural) تتحكم في توزيع الرواسب والتضاريس المرتفعة و المنخفضة. إن حجم الأحداث الزلزالية وأهمية التكتونية الإقليمية تثبت أن شمال شرق البلاد التونسية قد تعرض لضغوط خلال الحلقات الجيولوجية الأخيرة من تطوره. وتبين نماذج التغير الديناميكي للعهد النيوجيني أهمية التكتونية الحديثة (neotectonic) في تحفيز الأحداث الزلزالية في الوقت الحالي. لذالك توجد علاقة مباشرة بين توزيع الصد وع العميقة و نوعيتها وتحركاتها عبر الزمن، و الأحداث الزلزالية المسجلة.

Tectonosedimentary evidence in the Tunisian Atlas, Bou Arada Trough: insights for the geodynamic evolution and Africa–Eurasia plate convergence

The Bou Arada Trough is an east–west-oriented structure located 80 km SW of Tunis, characterizing the central Tunisian Atlas. This trough is filled by a thick Quaternary sand and clay series and is bordered by complex systems of folds generally trending NE–SW. Contacts between the Bou Arada Trough and the neighbouring folds are accommodated by NE–SW- and NW–SE-oriented faults. In contrast to the other troughs of the Tunisian Atlas, which are related to the Pliocene–Quaternary orogenic period, the geodynamic evolution of the Bou Arada Trough began in the Maastrichtian and has continued until the present day. Structural, tectonosedimentary and seismic data analyses are undertaken in the study area to better understand the evolutionary scenario of this trough. The results obtained show that the Bou Arada Trough is fragmented into three NW–SE-oriented sub-basins and records a continuous history of downthrow. Indeed, during extensional to transtensional regimes, this trough has evolved in response to the two networks of perpendicular fractures whereas during compressive to transpressive periods, the collapse of the Bou Arada Trough has been induced by a pull-apart mechanism using the same network of faults but with a strike-slip movement. The Bou Arada Trough thus preserves a record of the convergence between the European and African plates since the Maastrichtian.

Tectonique synsedimentaire d'age cretace superieur en Tunisie nord orientale; blocs bascules et reorganisation des aires de subsidence

Structural analysis of late Cretaceous sequences from the northeastern Tunisian Atlas, led to conclude on an active basin floor instability. Regional tectonics resulted in tilted blocks with a subsidence reorganization, since the Campanian time. These structural movements are controlled both by N140 and N100-120 trending faults. The Turonian-Coniacian and Santonian sequences display lateral thickness and facies variation, due to tectonic activity at that time. During Campanian-Maastrichtian, a reorganization of the main subsidence areas occurred, the early Senonian basins, have been sealed and closed and new half graben basins developed on area which constituted previously palaeohigh structures. These syndepositional deformations are characterized by frequent slumps, synsedimentary tilting materials, sealed normal faults and progressive low angle unconformities. These tilted blocks combined to a subsidence axis migration were induced by a NE-SW trending extensional regime. This extension which affects the Tunisian margin during the Upper Cretaceous, is related to the Tethyan and Mesogean rifting phase which resulted from the combined movements of the African and European plates.