2. Mineralogy – Petrology – Geochemistry - SWISS GEOSCIENCE ...

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Symposium 1: Structural Geology, Tectonics and Geodynamics
1.18
Tectono-thermal evolution of the Atlas system (SW Morocco), insights
from low-temperature thermochronology and Raman spectroscopy on
carbonaceous material
Ruiz Geoffrey1, Negro Francois2, Babault Julien3, Frizon de Lamotte Dominique4, Stuart Fin5, Stockli Dani6, Foeken
Jurgen7, Sebti Samira8, Saddiqi Omar8, Di Nicola L.5 & Thomsen Tonny9
1IMG, Uni. Lausanne, CH-1015 (geoffrey.ruiz@unil.ch)
2CHYN, CH-2009 Neuchâtel
3UAB, Spain
4Uni. Cergy-Pontoise, France
5SUERC-Glasgow, UK
6Uni. Kansas, USA
7VU Amsterdam, NL
8Uni. Casablanca, Morocco
9Uni. Stockholm, Sweden
In Morocco, the High and Middle Atlas of Morocco are intra-continental fold-thrust belts situated in the southern foreland
of the Rif orogen (Fig. 1). It is a key natural laboratory because it 1) is the southern and westernmost expression of Alpine-
Himalayan orogeny, and 2) possibly encompasses Pre-Cambrian to recent evolution of the region. Phases of shortening and
exhumation of this orogen remain however ill constrained and the few available quantitative data do not allow the
present-day high topography (over 4000m) to be explained.
In order to put constrains on the recent orogenic growth of the Atlas system, we investigated the temperature-time history
of rocks combining extensive low-temperature thermochronological analysis (Fission tracks and (U-Th)/He on zircon and
apatite) and peak temperature estimation by Raman spectroscopy of carbonaceous material (RSCM) and U-Pb ages.
The target area is a NE-SW oriented transect crossing the different structural segments of the western Atlas away from
present-day fault systems (Fig. 1). Results are much contrasted from one domain to the other. Pre-Cambrian bedrocks from
the Anti-Atlas domain yield old Fission-Track ages on zircon (340-300 Ma), apatite (180-120 Ma) but also U-Th/He (150-50
Ma) still on apatite.
These datasets are interpreted, with the help of thermal modelling, to record passive margin up and down movements
during the break-up of the Pangea. U-Th/He pair dating on both apatite (80-55°C) and zircons (200-160°C) minerals are
much younger in the High-Atlas once the Tizi N’Test Fault System (TNT), or SAF (Fig. 1) passed to the north, ranging between
~35-5 Ma and 85-30 Ma respectively. Similarly, maximum peak temperatures vary across the TNT, with maximum
temperatures of 500-450°C in the axial zone and less than 250-200°C in the Souss plain to the south. Once all datasets
combined, they indicate that uplift occurred in the Axial Zone in the Oligocene due to tectonic inversion and crustal shortening
and remain constant since allowing a 6-7 km thick pile to be eroded.
Low-thermochronological analyses have also been performed on Cretaceous deposits in the region. Results indicate that
these deposits have been reset to temperatures greater than 80°C. This suggests that a post Cretaceous sedimentary pile
of at least 3 km in thickness is missing, and as a result that a 3-4 km thick pile of substratum have been eroded in the
Axial Zone. Our extensive thermochronological dataset provide for the first time constraints that evidence heterogeneous
exhumation history across and along the chain.
All these constraints are put together with structural, geochemical and geophysical informations to discuss the recent
tectono-thermal evolution of the Atlas system in the frame of the Africa-Europe convergence and thinning of the lithosphere.
Swiss Geoscience Meeting 2011
Platform Geosciences, Swiss Academy of Science, SCNAT