The evolu
tion of
the Karoo Basin sys
tem in SW Madagascar (Morondava Basin) may be considered
to be an ini
tial break-up a
ttemp
t be
tween Eas
t- and Wes
t-Gondwana in Eas
t Africa. The early rif
t evolu
tion in
the Morondava ex
tensional basin in Madagascar was previously described in
terms of or
thogonal crus
tal ex
tension wi
th ei
ther E–W or NW–SE direc
ted ex
tensional s
train. De
tailed field inves
tiga
tions in
three areas of
the Morondava Basin, subsequen
t to LANDSAT TM 5 and SPOT 4 sa
telli
te image in
terpre
ta
tions, revealed,
tha
t crus
tal ex
tension in
the Morondava Basin and associa
ted sedimen
ta
tion of
the Karoo Supergroup sequences occurred in
three differen
t periods under
three differen
t s
tress and kinema
tic regimes (referred
to
the ac
tual posi
tion of Madagascar):
t>(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.t>t>(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.t>t>(iii) NW-directed normal extension (Early–Middle Triassic) post-dating the middle Sakamena to lower Isalo Group I and pre-dating lower Isalo Group II deposition. Formation of half-grabens.t>
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.