The Open Geology Journal

2010, 4 : 105-116
Published online 2010 December 31. DOI: 10.2174/1874262901004010105
Publisher ID: TOGEOJ-4-105

Indentation Model Using Gravity Anomaly and Anisotropy of Magnetic Susceptibility: Consequences on Nappe Rotations in the Chélif Basin, Algeria

Tahar Aïfa and Mouloud Idres
Géosciences-Rennes, CNRS UMR6118, Université de Rennes 1, Bat.15, Campus de Beaulieu, 35042 Rennes Cedex, France.

ABSTRACT

An anisotropy of magnetic susceptibility (AMS) study was conducted on 57 specimens of Tortonian to Pliocene sandstones and completed by an interpretation of the gravity anomaly map of the Chélif Basin (Algeria) in order to confirm the indentation model of the basement and its consequences on the rotations of the nappes.

Inversion of residual gravity data, obtained from 428 measurements, using the Parker-Oldenburg algorithm helped to investigate the undulation of the basement and to evidence the main geological features. Gravity modeling along five NW-SE and SW-NE oriented cross-sections did not allow us to detect any vertical offset of T1 and T2 evidenced contacts, but confirms the T1 dextral and T2 sinistral shears.

Rock magnetic investigations show that the corrected anisotropy degree P’ ranges from 1.03 to 1.38, except for the El- Abadia (EA) site where it reaches 1.82. The main magnetic fabric is either oblate or prolate, except for two sites (OS and OU) where the oblate shape prevails. The main magnetic carrier is likely magnetite; other minerals such as goethite and sulphides were evidenced through transformation into magnetite after heating.

Due to successive remagnetizations, the rotation amounts deduced from AMS measurements are higher than those deduced from remanence and follow the same direction. The Oum-Drou (OU) site, facing the indenter, did not rotate significantly (~2°) which is in agreement with both magnetic lineation and remanent magnetization and consistent with the indentation model. West of the T1 dextral strike-slip fault, evidenced by gravity, a clockwise rotation of ~25° is recorded. Far to the east, the rotation value decreases as far as the Mdjaïa (MD) site, where a clockwise rotation of ~10° is recorded.

Finally, the deepened compressional basin corresponds with the direction of shortening deduced from neotectonics, gravity and AMS.

Keywords:

Gravity, Parker-Oldenburg inversion, AMS, modeling, Rotation, Chélif Basin.