Gravity data as a tool for delineating subsurface geology of Ariana

Gravity data as a tool for delineating subsurface
geology of Ariana region (Diapirs zone, Tunisia)
B. Sarsar Naouali, I. Hamdi Nasr, A.Amiri, A. Chaqui, M. H. Inoubli
Faculté des Sciences de Tunis, Université El Mana
Summary
Detailed gravity data were used to investigate and characterize subsurface geology in the Ariana region.
The integration of geological data and computed gravity fields (Bouguer anomaly, upward continuation
and Enhanced Horizontal Derivative) help recognizing the subsurface heterogeneities beneath the
masking homogeneous quaternary interval. The substratum is made of aligned heavy anomalies which
consist of two orthogonal structures. The first one corresponds to northwest directed elevated zone,
limited by N140 bearing buried fault system. The second one is directed northeast and well expressed in
surface geology. This structural architecture is inscribed in a compressive regime associated to important
strike-slip fault displacements in Northeast Tunisia. The overall stress-strain pair is integrated in the
regional as well as the global tectonic regime induced by the combined African and European plate
movements.
Key-words: Bouguer anomaly, upward continuation, enhanced horizontal derivative, distensive
structure, graben, compressive structure, Strike-slip fault.
Introduction
The North African margin is characterized by thick Meso-Cainozoic deposits (Burollet, 1956; Crampon,
1971; vendeville et jackson, 1992). Due to successive compressive periods, this interval was heavily
folded and faulted, leading to complex geologic structures (Burolet, 1951-1956; Castany, 1956;
Crampon, 1971; Rouvier, 1971; Perthuisot, 1978; Ben Ayed, 1986, Chihi, 1995; Dlala, 1995; Malki,
1997; Kacem, 2004, Hamdi et al., 2009). This complexity is emphasized by sedimentation processes
within basins, which are closely governed by the spatio-temporal migration of subsidence. The
geodynamic induced back-erosion of blocks, molassic deposits, thickness reduction and local gaps
during the Meso –Cainozoic geological period (Alouani et al., 1990; Alouani et al.,1992).
Ariana belongs to this domain; it is occupied by quaternary deposits, except two small Mesozoic
outcrops represented by J. Amar and J. Nahli. As already pointed out by many authors, the sector has
been considered as predicament area, divided into structures with separate evolution. Hence, three
models were proposed to explain its geologic complexity (Pini, 1971; Truillet et al., 1980; Richert, 1980;
Devolvé et al., 1981; Ben Ayed, 1986; Alouani et al., 1990; Kacem, 2004).
This paper aims to clarify the structural organization of the area in order to propose a comprehensive
structural sketch integrating all available surface and subsurface geology together with gravity data.
Gravity data
High resolution gravity data were collected by the Office Nationale des Mines (ONM) of Tunisia. 662
measurements were acquired approximating a grid of 1x1km2, over an area of 640 km2. The complete
Bouguer anomaly map was generated using Geosoft/Oasis software. It indicates values ranging from 5 to
36mGal (Fig.1-b-). The map points out mainly two distinctive areas: a high response area contrasting
with a low response one.
The high response zone is characterized by two extended anomalies, located at central Est and central
West part of the map; these anomalies exhibit orthogonal direction trending NE and NW. The map
expresses another high gravity response on its southwestern part.
The negative zone is surrounding the previously defined positive area. The same trending is expressed
by those anomalies. They are located within areas occupied by quaternary sediments to the north as well
as by Cainozoic series at the center and to the East.
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The anomaly high expressed by the NE direction corresponds to a structural orientation described in
Tunisia, in general, and in the studied area, in particular, as indicated by J. Nahli anticline. The second
“high”, well expressed in the map and occupying an important areal proportion, develops a strange
structural orientation. This particular distribution urged us to look for its origin in order to better
understand the subsurface geology of the area.
Upward continuation
The separation of Bouguer anomaly into couple of anomalies (regional and residual) is performed
through upward continuation. Furthermore, this method is efficient for depth estimation for the
expressed anomalies through successive upward continuations (Gupt and Ramani, 1980; Blakely and
Simpson, 1986, Peter and McGrath, 1991; Hearst et al., 2001). Upward continuation at 30 km elevation
results in the smoothest map. The later shows a regional northwest oriented gradient.
(a)
(b)
PLAIN ZONE
Quaternary
Miocene
Cretaceous
J.NAHLI
Jurassic
Her.
SAROULA
J.AMMAR
Triasic
ARIANA’S
GRABEN
Fig.1: (a) Geological and DEM map (b) Bouguer anomaly map of Ariana region.
L2
F2 N
L1
F1
Fig.2: The Enhanced Horizontal Derivative
gravity gradient map of Ariana region.
Gravity lineaments
H orst structures
compression coner
S tructural lineaments
Graben stuctures
direction contraint
Fig. 3: New structural sketch of Ariana region
Data Enhancement
In order to characterize the lateral boundaries related to the main gravity anomalies, the Enhanced
Horizontal Derivative is used (Rapolla et al., 2002; Gordon et al., 2008). The horizontal derivative of the
sum of vertical derivatives tends to enhance the signal over the source boundaries and to suppress
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spurious maxima due to the single derivatives of different order. Source boundaries are therefore defined
by considering the location of maxima of the EHD function (Fedi and Florio, 2001).
This technique shows new gravity lineaments (Fig.2). It helps identifying structures orientation and
organization. These lineaments have firstly emphasized the existence of geologic elements confirmed by
authors: N70, N160 and N180 directions which are expressed by surface geology in Ain Souisi (located
in the East of J.Ammar), in Kef El nsour as well as in J.Nahli (Devolvé, 1980; Richert, 1981; Ben Ayed,
1986). The other detected buried elements which are masked by the thick quaternary deposits are
directed N60, N140 and N180. These structural lineaments are tightly associated to distensive structures
trending N120 evidenced in Northeastern Tunisia by Kacem (2004).
Discussions and Conclusion
The compilation of the detailed gravity analysis confronted to geological data, helps clarifying the
organization of the structural features in Ariana region. Our results confirm, on one hand, the presence of
some directions and structures that are observed in outcrops. They reveal in the other hand, the existence
of buried structures not expressed by surface geology. The structural architecture indicates that the main
directions result from major strike slip fault movements (Fig.3). This pattern can be integrated in
compressive deformation regime marked by a conjugated dextral and sinister system induced by the
african and european plate convergence.
The sinistral movement along the N140 directed faults (F1 on fig.3), and the dextral movement along the
N60 (F2 on fig.3) are guided by the compressive regime during the Tortonian age (Ben Ayed, 1994).
Their relative displacements generate a compressional corner. This result is expressed on surface
geology through northwestern extremity of J. Goufda (Alouani et al., 1992). The NW directed Atlasic
phase generates, also, distensive structures, well illustrated by grabens mainly mainly taking place in
central Tunisia (Chihi, 1995). In the area of study, the observed NW directed alignments (L1 and L2,
Fig. 3) would correspond to distensive fault system bordering an elevated zone. Based on reflection
seismic interpretation, Kacem (2004) indicated this type of occurrence.
The tectonic mechanism explaining the described surface and subsurface structural model in the area of
study is part of the regional model of Northern Tunisia ( Richert, 1971; Richert, 1980; Ben Ayed, 1994,
Dlala, 1995) which joins the global mechanism of the western Mediterranean as illustrated by Renzo
(2004), Goes (2004) and Guiraud (2005).
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