NEOGENE MUD DIAPIRISM, STRIKE-SLIP TECTONICS AND SEISMICITY
IN NORTHEASTERN TUNISIAN MARGIN
Mourad Bédir*, Hakim Gabtni, Hamida Saidane and Sami Khomsi
Georesource Laboratory. Institut National de Recherche Scientifique et Technique, BP 95, 2050 Hammam-Lif, Tunisie
* Mourad.bedir@inrst.rnrt.tn
Abstract
Subsurface seismic and surface geologic studies conducted on northeastern Neogene series of Mediterranean Tunisian margin had
permitted to highlight mud diapir, mud volcanoes and intrusives related to lithostatic pressions of Plio-Quaternary thick deposits and
transtensive and transpressive movements of North-South and East-West ?ower fault corridors. Along the corridor fault borders took place
historic and recent seismicity with deepseated faults epicentre location. The instrumental seismic magnitudes vary between 3 and 6 Richter
degrees with some historic catastrophic events. This major seismic epicenter distributions seems to be related to strike-slip fault corridors
where occurs a mudkinesis movements.
Key Words : Mud Diapirs, Strike-slip, Seismicity, Tunisia.
Rapp. Comm. int. Mer Médit., 37,2004
8
Introduction
Geologic and geophysic studies carried out on the Neogene
deposits of the Eastern margin of Tunisia (1-5) and the north regions
by seismic sequence stratigraphy and tectonics had permitted to reco-
gnize sequence and basin structuring accompanied by mud diapirs
and intrusive structures with seismicity activities of deepseated faults
(6, 7, 2, 3, 8). Several petroleum and high resolution seismic re?exion
profils, petroleum well and historic and instrumental seismicity data
covering the Sahel, Kéchabta and Medjerda Onshore and gulf of
Hammamet and Pelagian sea Offshore zones had been used for the
Neogene horizon correlations, isochrone and isopach mapping,
seismic stratigraphy and seismic tectonic analyses.
Lithostratigraphic characteristics
In Northeastern Tunisia,Neogene deposits are composed by several
hundred meters of alternating packages of clays, marls, Lignites and
sandstones from Miocene, Pliocene and Quaternary series. These de-
posits show important changes in lithologic facies and thicknesses.
They are very thick in the gulf of Hammamet, the Sahel (7), the Pela-
gian sea (4) and in the Northwest Medjerda and Kechabta basins (9,
10). Particularly, Miocene and Pliocene clays are very thick in the
Eastern margin of the Cap Bon, the Sahel and the pelagian zones
comprising Miocene sand Pliocene sandstone turbidite sequences
outcropping in the Jebel Abderrahman (2) and in the coastal Nabeul
and Hammamet regions (11).
Tectonic framework and kinematics
The Northeastern margin is structured by North-South and Est-
West subsurface transtensive and transpressive strike-slip ?ower fault
corridors that limit platform, graben, syncline and fold structures (6,
7). These corridors have been reactivated by Langhian-Serravallian
distensive strains and Tortonian-Messinian and Quaternary
compressive stress. Neogene sequence deposits are distributed around
these structures where their thiknesses is very important in synde-
positional subsiding zones of graben and synclines (7, 2). The main
tectonic structures that constitute the background and the guide of the
Neogene sequence deposit distribution. Vertical and lateral claykine-
sis movements are attested by depocenter migrations and inversions
along fault corridors.
Mud diapirs, volcanoes and sequence structuring
Diapiric and intrusive seismic structures seems to be strongly
associated to the faults that limit the graben, syncline and platform
zones. Evidence of mud volcanoes structures is showed in the Sahel
offshore Pelagian sea by high resolution seismic profils. The mud
material come from Miocene sequences marked by Lower Miocene
decollement level of Langhian limestone Ain Ghrab Formation. These
features had been recognized in small outcrop scale in the Sahel and
in the North Atlassic Miocene deposits of Medjerda and Kéchabta
zones (3) in the Upper Miocene sequences showing metric to
decametric lenticular intrusive shales and sands along kilometric
strike-slip faults (1, 3, 11) and micro-sand and micro-clay volcanoes
seismits. Seismic sequence deposits show around mud diapirs ?anks
high angle downlap prograding system tracts at the base, overlapped
by aggradational and retrogradational onlap and toplap erosional
unconformities and pinch outs.
Seismicity and basin modeling
The seismicity of eastern onshore and offshore Tunisian margin
follow the master fault corridors oriented North-South, East-West and
associated faulted dragfold structures oriented Northeast-southwest.
The great seismic richter magnitudes situated between three and five
richter degrees of epicenters are located along these fault corridors.
This distribution relay subsurface mud diapirs, mud volcanoes,
intrusives and fold structures. Therefore, there is a straight relation-
ship between deepseated faults, seismicity and mud diapir structures.
As a combination of these results, new structural, sequence strati-
graphic and seismicity origin, basin model highlights mud diapirs dis-
tribution guided by the bordering faults of grabens, folds and syncline
corridors.
The structuring of Northeast-Southwest en echelon folds and
synclines inside and outside Est-West right lateral and North-South
left lateral tectonic corridors indicates the strike-slip type of bordering
faults and their seismogenic nature. Wrench fault movements induce
mud diapirs and intrusive ascensions. This Neogene kinematic recons-
truction highlights the neotectonic system inducing the actual seismi-
city on this margin.
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