The opening of the South Atlantic between 140 and 90 m. y. B. P.
occurred about two poles of rotation. The initial pole of rotation was
maintained until Africa and South America were completely separated.
The subsequent removal of restraints imposed by the pre-existing
structure of Africa and South America on the early spreading
direction permitted a northward migration of the pole of rotation
and concomitant reorientation of spreading direction,
The NNE trending Cameroon-Gabon and southern Angola coastlines
which offset the generally SSE trend of the west African margin
between 5°N and the Walvis Ridge are proposed as initial transform
offsets of the South Atlantic proto-rift. Location of these initial
offsets was controlled by lineations of the Precambrian/Early
Paleozoic belts of thermotectonic activity which occur between older,
stable cratonic nuclei of Gondwana. Shorter offsets of the continental
margin south of the Walvis Ridge and the two major offsets of the
west African coastline north of the Walvis Ridge define a pole of
rotation at 5°N, 26°w for the initial South Atlantic opening.
A set of magnetic anomaly lineations near the continental
margins of Angola and southwest Africa is described and named the
Benguela sequence. These anomalies were formed during the initial
phase of spreading and are displaced right-laterally almost 1000
kilometers across an extension of the continental offset along the
southern Angola margin. This offset is named the Benguela Fracture
Zone. The Benguela anomalies are correlated with anomalies of the
Lynch sequence in the western North Atlantic. The change in
direction between the two pre-Cenozoic phases of South Atlantic
spreading is dated at roughly 120 m. y. B. P. based upon an extrapolation
of the ages of anomalies in the Lynch sequence to the time of
reorientation in the South Atlantic. Formation of the South Atlantic
quiet zones occurred by sea-floor spreading about a pole of rotation
at 21. 5°N, 14. 0°W during the second phase of opening.
The present structure of the Walvis Ridge is controlled by
nearly orthogonal NE and NW trending faults shown by seismic
reflection profiling. The NE trending set of faults approximate small
circles of the pole of rotation at 21. 5°N, 14. 0°W and appear to offset
the Walvis Ridge topography in a right-lateral sense. Reorientation
of spreading would have produced extension across the Benguela
Fracture Zone; development of short offset spreading centers along
the Benguela Fracture Zone during this reorientation is proposed to
explain the right-lateral offsets of the Walvis Ridge topography.
Lack of geophysical information on the lower crustal structure
prevents a direct explanation of the present elevation of the Walvis
Ridge. However the Walvis Ridge is probably underlain by a low
density root produced by alteration of the lower crust and upper
mantle materials beneath the Benguela Fracture Zone which began
during the spreading reorientation. Asymmetric spreading over the
Walvis Ridge may have permitted the zone of crustal accretion to
remain near the older Benguela Fracture Zone long enough to allow
the creation of an anomalously broad low-density root which is
responsible for the uplift of the Walvis Ridge. / Graduation date: 1974
| Identifer | oai:union.ndltd.org:ORGSU/oai:ir.library.oregonstate.edu:1957/28308 |
| Date | 20 December 1973 |
| Creators | Baumgartner, Timothy Robert |
| Contributors | van Andel, Tjeerd H. |
| Source Sets | Oregon State University |
| Language | en_US |
| Detected Language | English |
| Type | Thesis/Dissertation |
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