Kinematic evolution of the Morondava rift basin of SW Madagascar - from wrench tectonics to normal extension

The evolution of the Karoo Basin system in SW Madagascar (Morondava Basin) may be considered to be an initial break-Lip attempt between East- and West-Gondwana in East Africa. The early rift evolution in the Morondava extensional basin in Madagascar was previously described in terms of orthogonal crustal extension with either E-W or NW-SE directed extensional strain. Detailed field investigations in three areas of the Morondava Basin, subsequent to LANDSAT TM 5 and SPOT 4 satellite image interpretations. revealed, that crustal extension in the Morondava Basin and associated sedimentation of the Karoo Supergroup sequences occurred in three different periods under three different stress and kinematic regimes (referred to the actual position of Madagascar): (i) Sinistral strike-slip movement (Early-?Late Permian), post-dating lower Sakoa Group deposition, and syn-depositional with middle Sakoa Group sedimentation. Formation of N-trending pull-apart basins. (ii) Sinistral strike-slip movement (post-latest Permian), post-dating lower Sakamena Group sediments and syn-depositional normal faulting (latest Permian). Formation of transtensional basins. (iii) NW-directed normal extension (Early-Middle Triassic) post-dating the middle Sakamena to lower Isalo Group I and predating lower Isalo Group II deposition. Formation of half-grabens. Throughout the Permian period, strike-slip deformation, triggered by approximately N-S oriented compressive intraplate stresses, resulted in the formation of relatively limited pull-apart basins. At around the Permian-Triassic transition, the stress system gradually developed towards transtension with the consequence of significant widening of the depositional system. From the Early Triassic onwards, the stress system was purely tensional with widespread normal faulting prevailing, resulting in the increasing formation of half-graben systems, characterised by orthogonal extensional strain. (C) 2004 Elsevier Ltd. All rights reserved.