Summary
The fundamental building blocks of Lake Tanganyika are half-graben that tend to be arcuate or crescent-shaped in plan view. When combined with the asymmetric subsidence of the half-graben, this geometry creates horizontal components of motion which are expressed as shearing and rotation along the ends of the units. The key to understanding rift-basin morphology is recognizing the various ways in which these fundamental units can be linked together in time and space. Although many modes of linking occur, the basic theme is one of alternating half-graben polarities along the strike of the rift. Where adjacent half-graben face in opposite directions and do not overlap with each other, they are usually separated by interbasinal ridges trending oblique to the rift axis. In cases where facing half-graben overlap, the apparent morphology can be that of a full-graben with a central high or a half-graben abutting a structural platform, depending upon the orientation of the observation line relative to the overlap geometry. Mechanically, such full-graben should be viewed as opposing half-graben which hinge away from the central high. Examination of the linking models in Tanganyika has enabled us to derive an empirical model of rifting. The model is extremely useful in understanding the stratigraphy of Tanganyika, and it seems to explain many of the styles and patterns of deposition which are peculiar to rift basins. If the model has general applicability to continental rifts, it can also be useful to those who deal with the mechanics of rifting and the deep structure of passive margins.
Structure and climate have each been proposed as the primary control on sedimentation in continental rifts. Numerous Phanerozoic rifts spanning a wide range of inferred palaeoclimates have consistent relationships between structure and sedimentation, and between structural evolution and stratigraphic sequences. Six principal structural features strongly influence sedimentation patterns in rifts. (1) Topographically high accommodation zones segment rifts into 50–150 km long structural half-graben, each one a discrete depocentre. (2) Reactivation of pre-existing structural elements commonly controls the location and orientation of accommodation zones. Drainage systems outside the rift then may be captured and diverted towards the rift axis, establishing some accommodation zones as input points for sediment. (3) Footwall uplift at main border faults directs most drainage away from young rifts. Along the rift flanks, uplift is minimized at accommodation zones, enhancing their role as sediment input points. (4) Gentle roll-over of topography on flexural margins produces drainage systems that are generally small, and that do not introduce large amounts of sediment into the rift. (5) Breakup of the basin floor into elongate, rift-parallel fault blocks greatly enhances along-axis sediment transport. (6) Development of fault relay zones and transfer faults localizes down-dip sediment movement. These structural controls occur in the present-day low latitude rifts of East Africa, where climate ranges from arid to humid. Similar features have been identified in ancient rifts that are interpreted to span a comparable range in climates. Clearly, climate is important in determining the sedimentary history of continental rifts. The present study indicates that climatic effects, however, are superimposed on the fundamental structural geometries that evolve during rifting.
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