Taher Zouaghi

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.

The Chotts Fold Belt of Southern Tunisia, North African Margin: Structural Pattern, Evolution, and Regional Geodynamic Implications

At the North of the old African continent, craton and shields having more than two billion years, Tunisia, Algeria and northern Morocco underwent a complex geodynamic and structural evolution during the Mesozoic and Cenozoic times (Dercourt et al., 1985; Bouillin, 1986; Frizon de Lamotte et al., 2009). This evolution resulted in the development of varied paleogeographic fields, in relation with the Tethyan and Atlantic movements. Its end led to the genesis of the North-African alpine orogen (Dercourt et al., 1985; Martinez et al., 1990) formed by the Maghrebid and Atlassic domains (Fig. 1). Tunisia occupies the eastern part of this orogen, located at the north of a large Saharan platform, developed on the stable African craton, not deformed during the alpine cycle and bounded by a major structural lineament « South Atlassic fault » composed of complex overlapping folds trending NE-SW, E-W and NW-SE (Caire,1971; Zargouni, 1985; Turki, 1988; Zouari et al., 1990; Ben Ayed, 1993; Boukadi, 1994; Bedir, 1995; Bouaziz, 1995; Zouari, 1995; Bouaziz et al., 1999, 2002; Abbes, 2004; Zouaghi et al., 2005a, b, 2011; Ouali, 2007; Melki et al, 2010). Structures of the North African margin were usually subject of discussion. This domain could be considered as a passive margin, close to the oceanic opening, characterized by a strong subsidence marked by accumulations of prograding deposits (Dercourt et al., 1985; Biju-Duval et al., 1976). For others, it is a transform margin related to displacements of the African plate compared to the Eurasian plate. These movements generated opening of the Paleo-Tethys (Arthaud and Thomas, 1977). The Africa-Europe relative motions would be at the origin of the recent ocean floor spreading of the Mediterranean (Taponnier, 1977; Reading, 1980; Olivet et al., 1982; Alvarez et al., 1984; Ricou, 1994). The study area belongs to the North African margin and the northern edge of the Saharan platform. Studies undertaken on Paleo-Tethys show the development of deformed and subsiding zones between the cratonic blocks and the basins (Caire, 1974; Arthaud and Thomas, 1977; Aubouin and Debelmas, 1980; Bernoulli and Lemoine, 1980; Durand-Delga and Fonrbote, 1980; Bousquet and Philip, 1981; Dercourt et al., 1992). The geodynamic

Radiolarian and planktic foraminifera biostratigraphy of the Early Albian organic rich beds of Fahdene Formation, Northern Tunisia

The lower part of Fahdene Formation outcropping in northeastern Tunisia is represented by pelagic sequences and characterized by an important siliciclastic fraction that includes radiolarian and organic-rich beds of Allam Member. Litho-biostratigraphic analysis based on planktic foraminifera and radiolaria shows that deposition of organic-rich beds of Allam Member is confined to the Microhedbergella rischi Zone through the lower part of Ticinella primula Zone. Age diagnostic radiolaria recovered from these beds has been identified to constrain a direct age of black shale deposition. The assemblages can be correlated with the radiolarian biochronozone U.A.10–11 indicating an Early Albian to early Middle Albian in age. Radiolarian assemblages composed of species characteristic of U.A.10 biochronozone (A. montisserei, D. gracilis) are associated with radiolarian taxa belonging to Costata Zone (U.A.9 Pseudoeucyrtis hanni and Thanarla pseudodecora), which can be correlated with Dercourt Member in the Ionian Zone and Paquier level of the Vocontian Basin (southeast of France). Biostratigraphic and abundance curves analysis has demonstrated significant diversification of cryptocephalic Nassellaria and Archaeodictyomitrae, probably adapted to eutrophic conditions that characterized the Aptian-Albian transition. Ecological conditions may have governed the stratigraphic range of many cosmopolitan taxa (i.e., Pseudodictyomitrae lodogaensis) compared with stratigraphic distributions schemes reported from other domains. The timing of black shales deposition was diachronic due to local geodynamic conditions and upwelling currents distribution. The Allam black shales are correlative with the oceanic anoxic event OAE1b which is characterized by the widespread of supraregionnally organic-rich beds in the Mediterranean Tethys basins across the Aptian-Albian transition.

Seismic Reflection Contribution to the Study of the Jerid Complexe Terminal Aquifer (Tunisia)

According to Kilan (1931) the term « Continental Terminal » concerns the sandy and clayey continental Formations dated Miocene-Pliocene. In 1966, Bel & Demargne highlighted a vertical communication between the aquifer contained in these Formations and the Eocene, Senonian and Turonian aquifers. Consequently, the Continetal Terminal is redefined as a multi-layered aquifer which extends from the Late Cretaceous to the Miocene –Pliocene. Frequently, this hydrogeological system is designated by the term «Complexe Terminal» proposed by Bel & Cuche in 1969.