A new perspective on the significance of the Ranotsara shear zone in Madagascar |
| |
Authors: | Guido Schreurs Jörg Giese Alfons Berger Edwin Gnos |
| |
Institution: | 1. Institute of Geological Sciences, University of Bern, Baltzerstrasse 1-3, 3012, Bern, Switzerland 4. Institut für Geologie und Pal?ontologie, Westf?lische Wilhelms-Universit?t Münster, Corrensstr. 24, 48149, Munster, Germany 2. Institut for Geografi og Geologi, ?ster Voldgade 10, 1350, Copenhagen, Denmark 3. Museum d’Histoire Naturelle, 1 Route de Malagnou, 1211, Geneve 6, Switzerland
|
| |
Abstract: | The Ranotsara shear zone in Madagascar has been considered in previous studies to be a >350-km-long, intracrustal strike-slip
shear zone of Precambrian/Cambrian age. Because of its oblique strike to the east and west coast of Madagascar, the Ranotsara
shear zone has been correlated with shear zones in southern India and eastern Africa in Gondwana reconstructions. Our assessment
using remote sensing data and field-based investigations, however, reveals that what previously has been interpreted as the
Ranotsara shear zone is in fact a composite structure with a ductile deflection zone confined to its central segment and prominent
NW–SE trending brittle faulting along most of its length. We therefore prefer the more neutral term “Ranotsara Zone”. Lithologies,
tectonic foliations, and axial trace trajectories of major folds can be followed from south to north across most of the Ranotsara
Zone and show only a marked deflection along its central segment. The ductile deflection zone is interpreted as a result of
E–W indentation of the Antananarivo Block into the less rigid, predominantly metasedimentary rocks of the Southwestern Madagascar
Block during a late phase of the Neoproterozoic/Cambrian East African Orogeny (c. 550–520 Ma). The Ranotsara Zone shows significant
NW–SE striking brittle faulting that reactivates part of the NW–SE striking ductile structures in the flexure zone, but also
extends along strike toward the NW and toward the SE. Brittle reactivation of ductile structures along the central segment
of the Ranotsara Zone, confirmed by apatite-fission track results, may have led to the formation of a shallow Neogene basin
underlying the Ranotsara plain. The present-day drainage pattern suggests on-going normal fault activity along the central
segment. The Ranotsara Zone is not a megascale intracrustal strike-slip shear zone that crosscuts the entire basement of southern
Madagascar. It can therefore not be used as a piercing point in Gondwana reconstructions. |
| |
Keywords: | |
本文献已被 SpringerLink 等数据库收录! |
|