A new perspective on the significance of the Ranotsara shear zone in Madagascar

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.

Numerical Modeling of Mantle Flow Beneath Madagascar to Constrain Upper Mantle Rheology Beneath Continental Regions

“…It crosscuts 400 km of the Malagasy Precambrian basement. Although the Ranotsara shear zone is typically considered as an intracrustal mega strike‐slip shear zone with a sinistral sense of shear (e.g., de Wit et al, ), Schreurs et al () argue that the Ranotsara shear zone is a composite structure with ductile deflection in the central zone based on remote sensing and geologic observations. When Madagascar attained its current position with respect to Africa in the middle Miocene, lithospheric scale E‐W extension began with the reactivation of N‐S striking faults (Bertil & Regnoult, ) that formed rift basins such as the Alaotra‐Ankay Rift (Figure ).…”

“…Madagascar (Figure ) is an ideal natural laboratory to study the sources of anisotropy beneath continental regions and the rheological implications for the lithosphere‐asthenosphere system because (1) active volcanism is minimal or absent across most of the continental island, thus limiting the effect of melt lenses (Michon, ), (2) there are well‐exposed tectonic fabrics for comparison (Collins & Windley, ; Schreurs et al, ), and (3) numerous geological and geophysical observations provide evidence of present‐day tectonic activities potentially linked to viscous coupling to asthenospheric flow (Bertil & Regnoult, ; Kusky et al, ; Rambolamanana et al, ; Rindraharisaona et al, ).…”

“…Madagascar (Figure 1) is an ideal natural laboratory to study the sources of anisotropy beneath continental regions and the rheological implications for the lithosphere-asthenosphere system because (1) active volcanism is minimal or absent across most of the continental island, thus limiting the effect of melt lenses (Michon, 2016), (2) there are well-exposed tectonic fabrics for comparison (Collins & Windley, 2002;Schreurs et al, 2010), and (3) numerous geological and geophysical observations provide evidence of present-day tectonic activities potentially linked to viscous coupling to asthenospheric flow (Bertil & Regnoult, 1998;Kusky et al, 2010;Rambolamanana et al, 1997;Rindraharisaona et al, 2013). Reiss et al (2016) produced the first SKS splitting observations along the SELASOMA seismic profile (SEismological signatures in the Lithosphere/Asthenosphere System Of Southern Madagascar) that spans southern Madagascar in a NE-SW orientation, while Ramirez et al (2018) provided additional SKS splitting measurements across the entire continental island (Figure 1).…”

Garnet crystal plasticity in the continental crust, new example from south Madagascar

“…S1 and development of a new vertical foliation, S2 (second regional fabric). The rock formations affected by vertical foliation bear a new N-S L2 horizontal stretching lineation sometimes sub-parallel to an intersection lineation (Martelat et al, 2000; D3 deformation phase of de Wit et al, 2001;Schreurs et al, 2009). A vertical high-grade shear zone network represents the D2 deformation pattern: Ihosy, Zazafotsy, Lamboany and Beraketa shear zones (Rolin, 1991;Martelat et al, 2000;Randrianasolo, 2009).…”

Crustal Radial Anisotropy and Linkage to Geodynamic Processes: A Study Based on Seismic Ambient Noise in Southern Madagascar

“…All these shear zones feature tight to isoclinal folds and are highly flattened crustal‐scale zones resulting from the Pan‐African collision. The Ranotsara zone shows ductile sinistral deflection confined to its central segment and prominent NW‐SE trending brittle faulting along most of its length (Schreurs et al, ). The shear zones appear to be rooted in a zone of broadly distributed deformation in the crust or mantle (Reiss et al, ).…”