Print this pageSequence Variability in Rift Basins: Controls and Products
GAWTHORPE, ROB; IAN SHARP, JOHN UNDERHILL and SANJEEV GUPTA
Within rift basins, rates of subsidence/uplift and the style of defommation show marked spatial and temporal variability around propagating nommal faults. Such variations exert an important control on accommodation, physiography and sediment supply, and in tum affect sequence development.
Sequence variability in rift basins can be linked to fault zone evolution. Two-dimensional dip sections across normal fault zones display two main styles of deformation during fault growth: initial monoclinal growth folding above a buried fault tip, followed by a phase of more localized displacement once the fault breaks surface. The folding phase leads to stratal convergence towards the fault zone, with marked truncation along downshift surfaces. Tightening of the monocline leads to progressive rotation of sequences. Low rates of subsidence during the folding phase mean that accommodation is dominated by sea-level change.
In contrast, once the fault breaks surface, development of the monocline stops and higher rates of subsidence become localized against the fault zone. As a result, stratal surfaces diverge towards the fault zone and stacking patterns are predominantly aggradational. During this phase of evolution, high subsidence rates significantly modify the effects of sea-level fall.
As fault displacement increases, so does fault length. Thus, in the third dimension, folding and associated convergent stratal packages form at fault tips, whereas divergent stratal packages develop towards the centrer of fault segments were the fault has already broken surface. The implications for sequence development are clear - major along-strike variations in sequence evolution should be expected.