Extensional vs. compressional deformation in the Central High Atlas salt province: a paleomagnetic approach

Abstract

In this paper we address the problem of the distinction between diapiric, salt-driven and compressional structures, using the outstanding example of the Central High Atlas (Morocco). A remagnetized component carried by magnetite has been isolated in 32 new paleomagnetic sites. It is characterized by: maximum unblocking temperatures around 450 °C, syn-folding behavior and normal polarity.

These 33 mean paleomagnetic directions were analyzed together with other 68 from published works around the study area to construct a robust paleomagnetic dataset along a cross-section perpendicular to the main structures. The remagnetization direction (n: 100, Dec: 332.2°, Inc: 34.5°, η: 6.2°, ξ: 2.0°, A/n: 6.427°) and the paleo-dip of beds (the attitude of the beds at the remagnetization occurrence) were calculated through small circle methods. The remagnetization can be dated as ca. 100 Ma. Because of its occurrence between the extensional and compressional periods, this remagnetization offers the possibility of restore the basin to its pre-inversion geometry.

Comparison between present-day and pre-inversion structure allows discriminating three different evolutionary patterns: (i) thrusted and welded salt-walls mainly structured during the extensional stage (Ikkou ridge) with steep limbs close to the salt-wall core. (ii) Jurassic salt-walls with weaker deformation, restricted to the areas adjacent to the structure (Tadaghmamt and Timedouine); in this case, Cenozoic compression is limited to welding of the salt-walls and buttressing of the sedimentary sequences against faults. (iii) salt-rollers gently initiated during the Jurassic (Toumliline diapir), thrusted during the Cenozoic compression. Results show the importance of salt tectonics both during extension and compression, as well as the control of the compressional features by the inherited extensional structures. The performed restorations prove that paleomagnetism is a useful, independent tool to obtain palinspastic restorations and to separate, and quantify, the imprint generated during the basinal stage from the inversional features.