SUGGESTED ORIGIN OF THE MOZAMBIQUE CHANNEL
by
G. Flores
(Trans., LXXIII, 1, 1-16)
DISCUSSION
Dr. O. R. van Eeden
I wish to thank the author for his very interesting and important paper. I think he has
put forward a very plausible theory to explain the origin of the Mozambique channel. His
idea of a north-easterly striking dextral transcurrent fault is supporlted by evidence in the
Soutpansberg and Eastern Botswana areas. In the latter area Mason (1969) describes a
dextral transcurrent move~ent of 80 km along the Letlakane fault..
I have analysed the faults in the Soutpansberg and other areas in a paper that is in
preparation and some of my conclusions have a bearing on those of the author. There are
three directions of post-Waterberg but pre-Karroo faulting in the Soutpansberg area. The
one direction, viz. east-north-eas,f, is more or less parallel to the Letlakane fault in Botswana
and the Messina and Dowe-Tokwe faults north of the Soutpansberg. This is a direction
of dextral transcurrent movement. Another direction is north-west south-east and is represented
by, e.g., the Siloam fault in the Soutpansberg and Lechwana fault in Botswana. It is a
direction of sinistral transcurrent movement. These faults indicate that the maximum pressure
was east-west and the minimum pressure north-south when they were formed. Another
direction is north of north-east but the faults of this direction are normal faults with the
down throw side to the east.
In post-Karoo times normal faulting occurred along the transcurrent faults of both
directions. There is no evidence of movement along the faults between the periods of
deformation.
From regional evidence in South Africa it would appear that the transcurrent faults
are about 1100 m.y. old, i.e. of !he same age as the Namaqualand-Natal period of metamorphi~m and granitisation.
The mechanics envisaged by Hardie (1962) for the formation of the Natal fault system
is untenable on theoretical grounds. There is enough evidence in the literature for a view
that the post-Karroo movement along the faults in Natal occurred along older faults as is
the case in the Soutpansberg area. I agree, however, that the post-Karroo movement was the
result of tensional forces during the break up of Gondwanaland but as the transcurrent
faulting is pre-Karroo, it could not have been the cause of the tensional forces. It would
seem that the pre-Karroo continent was fractured by transcurrent faulting (compressional
stress) but during the disruption of Gondwanaland the forces were tensional in the same
area.
I wish to point out that although the author correctly describes the transcurrent movement
as right lateral (dextral), the movement is indicated as being sinistral in Figure 4.
Geological Survey.
Pretoria.
REFERENCES
MASON, R. (1969). Transcurrent dislocation in the Limpopo orogenic belt. Proc. geol. Soc.
Lond., 1655, pp. 93-96.
HARDIE, L. A. (1962). The fault-pattern of coastal Natal: An experimental reproduction.
Trans. geol. Soc. S. A/r., 65, pp. 203-205.
Author's reply to discussion
Reply to Dr. O. R. van Eeden
I am grateful to Dr. O. R. van Eeden for his discussion of my paper.
This also gives me an opportunity to clarify that the paper was submi~ted in June of
1968, accepted by the Society in November of 1968 and actually published only at the end
of 1970 due to a quite extraordinary delay. Many new data on crustal plates and drift
mechanics were made available since the paper was written, which could not be taken into
consideration.
Referring to Dr. van Eeden's discussion:
1. The movement shown on Figure 4 is in agreement with "right lateral" notation.
I quote Moody and Hill (1):
110
2.
3.
4.
5.
6.
DISCUSSIONS
"right lateral indicates clockwise and left lateral indicates counter clockwise separation."
The fault indicates the clockwise separation of Madagascar from the African Continent.
Mason's work confirms that the NE trending transcurrent fault outlined on Figure 4
continues into the Soutpansberg as suggested in my paper. This fault, as shown, is
suggested to have been associated with the N-S fracture which eventually widened into
the Mozambique Channel.
That this N-S fracture system was active throughout the Karroo time is suggested by
the outline of the Ecca-Sakamena (Figure 5), of the Stormberg-Isalo (Figure 6) and of
the Liassic and Lower Cretaceous lava flows (Figure 7).
The present day physiography of the sea bottom, on the other hand (Figure 8) shows a
NE-SW trending escarpment centered at about 15° Sand 45° E whic~ is strongly
suggestive of a fault trace coincident with the trace of our supposed NE extension
of the Soutpansberg transcurrent fault. It is now intersected by the N-S "Mozambique
Fracture Zone" detected by Langseth and Heezen (2).
The above suggests that both the N-S and the NNE fracture have been active continuously
from pre-Karroo time to the present.
To be noted is that the transcurrent fault system discussed by Mason (3) for the
Soutpansberg shows maximum displacement of the ordoc of 30-40 km. The NE projection
of this fault system as shown has seen displacements of the order of hundreds of km.
Once the Madagascar plate was detached from the continent it is entirely possible to
conceive that this plate moved independently to the NE (or Africa to the westward)
fer distances of this order, regardless of the limited initial movement along the Soutpansberg fault area. If so, the great age suggested by Df. van Eeden (1100 m.y.) would
indeed apply to the initial stage (30-40 km displacement) of this very ancient fracture.
Considering the enormous span of time involved in the process of the development of
the Mozambique Channel there appears to be no reason to believe that once the rightlateral movement contributed to the poss;ble initial rifting in the pre-Karroo time (even
as a consequence, if one wishes, of a N-S directed compressional force) the widening
of tile rift at much later times could not be associated with tensional forces generally
directed to the eastward, once Madagascar's plate began to detach itself from the
continent.
If we assume that ,the N-E transcurrent fault and the N-S fraoture system are associated
there appears to be no reason to reject that the tensional pattern of coastal Natal (and
Mozambique) Gould have developed along the major lines proposed by Maud in the
manner discussed by Hardie (4).
REFERENCES
1. MOODY, J. D., and HILL, M. J. (1956). Wrench-fault tectonics. Bull. G.S.A. Vol. 67,
pp. 1207 -1246.
2. LANGSETH, M., and HEEZEN, B. C. (1964). Seismic Reflection Measurements in the
Mozambique Channel. Tr. Am. Geoph. Un. Vol. 45, No. 1.
3. MASON, R. (1969). Transcurrent dislocation in the Limpopo orogenic belt. Proc. Geol.
Soc. London, 1655, pp. 93-96.
4. HARDIE, L. A. (1962). The fault pattern of Coastal Natal. Trans. Geol. Soc. S. Africa, 65,
1, pp. 203, 205.