..Dr. Jean-Emmanuel MARTELAT.. |
|
Carte
gravimétrique de Madagascar
tiré de Pili,
É., Ricard, Y., Lardeaux, J.M, Sheppard S.M.F,
1997 Lithospheric shear zones
and mantle-crust connections: Tectonophysics (280, 15-29)
Lithospheric shear zones and mantle-crust
connections
É. Pili1*1, Y. Ricard1, J.-M.
Lardeaux1,2, S.M.F. Sheppard1
1 Laboratoire de Science de la Terre -URA
CNRS 726, ENS Lyon and UCB Lyon I - École Normale
Supérieure de Lyon - F-69007 Lyon - France
2 Laboratoire de Pétrologie et Tectonique - Université
de Lyon - F-69100 Villeurbanne - France
* to whom correspondence should be addressed.
pili@geologie.ens-lyon.fr fax (33) 72 72 86 77
Abstract :
A crustal-scale ductile shear zone network
in the Precambrian granulite facies crust of Madagascar is examined
to determine the nature of the connections between the mantle and
lower crust. Based on three independent data sets - field and
satellite mapping, C- and O-isotope geochemistry and gravimetry -
this crust is divided into three zones: (1) outside of shear zones,
(2) minor shear zones that are <140 km long and 7 km wide, and (3)
major shear zones that are >350 km long (up to 1000 km) and 20-35
km wide. The mantle is uplifted by about 10 km beneath the major
shear zones. The major shear zones are rooted in and are inferred to
be controlled by the mantle; they directly tapped mantle-derived CO2.
The small-scale minor shear zones were controlled by crustal
processes and focused crustally-derived H2O-rich±CO2 fluids. The
regular distribution of the shear zones on a crustal scale is in
agreement with models of buckling of the continental lithosphere in a
compressional context. The propagation of these mechanical
instabilities promoted and channeled fluid flow. These major
Pan-African shear zones thinned the crust and were reactivated during
the subsequent drifting of Madagascar and opening of the Indian Ocean
during Jurassic to Cretaceous times. They also controlled many of the
brittle fault zones in the overlying sedimentary basins.
Mantle-rooted large-scale shear zones are inferred to be a general
feature of cratonic areas reactivated by shear zone systems.
Full-color
version of the Bouguer Anomaly Map of
Madagascar
Because of the high price of a color plate in Tectonophysics, we have
only published a grayscale map. Please feel free to download this
color version (GraviMada.gif, 558 Ko) to improve your reading of the
above-titled manuscript.