Open Session - SWISS GEOSCIENCE MEETINGs

8
Symposium 1: Structural Geology, Tectonics and Geodynamics
the ‘true’ present-day erosion product of the chain are compared with ones from an 'artificial' one we generated and this to
investigate the recent evolution of the eastern Andes. The artificial detrital record was engendered by the combination of
197 individual grain ages we produced from a bedrock profile in the region (Ruiz in press). Interestingly, the 'artificial' sand
express a clear homogeneous AFT signal with a single and pooled AFT age of 4.1 ± 0.1 Ma. This age is identical to the youngest
age population (P1) we extracted from the 'true' sand within the same catchment (4.4 ± 0.4 Ma) and suggest that the ‘true’
dated grains of the P1 population were derived from, if not this one, a region with similar thermal record. Our results are
of main importance because they indicate for the first time that a detrital age population, once statistically individualized
and limitations of the method perfectly excluded, most likely reflects the erosion in a single part of a catchment. In the
eastern Andes of Peru, the older age populations we extracted are probably derived from upper levels within the catchment
that reflect by their presence, but not directly quantify, former denudation. Reversely, the youngest age populations for all
present-day river sands are younger than 6.8 Ma. These data point towards lower levels of the eastern Andes that undergo
rapid denudation and this since recent time (Ruiz in press) because of the preservation of older thermal record.
The approach we developed is innovative and aims to reduce the amount of necessary analysis to constrain long-term denudation
rates in different orogenic settings. It also hosts a methodological aspect by comparing results from direct (bedrock)
and indirect (present-day river sands) thermochronological analyses within the same catchment.
1. 8
The Atlas Mountains: why there? Why now?
Ruiz G.M.H., Negro F., Foeken J., Stuart Babault J., F., Ivy-Ochs S., Kober F., Saddiqi O., Stockli D., Champagnac J-D. & Frizon
de Lamotte D.
The target area is the WSW- ENE oriented intra-continental Atlas chain in Morocco located between the West Africa Craton
and the Betic-Rif system. It is a key natural laboratory encompassing the Pre-Cambrian to recent evolution of the region even
though the stratigraphic record is incomplete. The presence of high surface elevations in both the High-Atlas (>4000m) and
Anti-Atlas (>2500m; Figs. 1 & 2) domains to the south is subject to discussions because there is little quantitative data available
at present. Phases of uplift are thus ill constrained as places where the associated erosion products were accumulated.
To better constrain the orogenic growth of the Atlas chain, we investigated the time-Temperature paths of bedrocks from two
morpho-structural domains that are separated by the narrow Souss Basin, i.e. the High and Anti-Atlas (Fig. 1). Pre-Cambrian
bedrocks from the Anti-Atlas domain yield old thermochronological Fission-Track ages on both zircon (390-300 Ma) and
apatite (180-120 Ma) minerals that are associated with slightly younger Atlantic (U-Th)/He ages on apatite (120-110 Ma, Fig. 2).
There are two possible (probably interfingering) interpretations for the preservation of such old thermal record: a) the Anti-
Atlas was not affected by the 'Alpine' orogeny and (always) remain ‘stable’, or b) it is being affected since recent time but no
level with such record is yet exposed.
Bedrocks from the High-Atlas yielded (U-Th)/He ages on apatite comprised between 4 and 35 Ma (Fig. 1), which is a very different
signal (Alpine?) than the one of the Anti-Atlas (Fig. 2). Age-elevation relationship suggests that denudation increased
towards 1.0 km/my for the Late Miocene . Continental series of Cretaceous age from both Sub-Atlas domains indicate total
resetting to temperatures greater than 80°C. Using a geothermal gradient of 25°C/km, this suggests that a post Cretaceous
sedimentary pile of at least 3 kilometres in thickness is missing, which would be in agreement with sourcing from the High-
Atlas region. The timing of the erosion of this pile is still unknown but it is being constrained by thermal modelling through
Apatite Fission-Track analysis.
Our thermochronological data provide for the first time constraints that evidence heterogeneous exhumation history between
the different structural domains of the chain. Additional analyses using thermochronometers with higher temperature
of closure (e.g. Fission-Track or (U-Th)/He on zircon) are being performed to investigate rates of medium to long-term
processes. Our results are being compared with existing geological records from Morocco but also on the other side of the
Atlantic ocean, to identify the relevant tectonic process that shaped the landscape of this mountain range in the frame of
the Africa-Europe convergence but also, which is novel, opening of the Atlantic ocean.