Transtensional Basins and Their Hydrocarbon Systems
Transtensional basins involve extension oblique to the basin margins. The strike-slip displacement component can be partitioned in the basin in several ways: in elongate domains of wrench-dominated faulting; by fault block rotations about vertical axes; by successive generations of faults in new orientations. Likely examples of regional-scale transtensional basins include Bohai, Sirte, Nam Con Son, the Northumberland Basin, the Oslo Graben, the Norwegian margin, the Atlas basins and, possibly, the West Siberian Basin. This list includes prolific hydrocarbon producers. Active examples of rifts with an oblique component are the Gulf of Aqaba and the Baikal Rift.
There are specific reasons why transtensional basins are both favourable and unfavourable for hydrocarbon generation and accumulation. Isolation and anoxia are favoured by the typically low length to width ratios of transtensional basins, which in turn favours the deposition of source rocks. Fault block rotations and reorganizations of fault systems lead to frequent changes in drainage patterns, that can cause complex syn-rift stratigraphies. Unfortunately, this does not favour the accumulation of large volumes of good quality reservoir sands. Oblique rift systems are associated with potential traps not found in other rifts. Examples include: footwall fault blocks bounded by normal faults on four sides; combined strike-slip and normal fault block traps; en echelon fault systems; folds created during the transtensional strain. At a smaller scale, reservoir faulting patterns developed under 3-D strain are more complex than plane strain Andersonian patterns.
AAPG Search and Discovery Article #90039©2005 AAPG Calgary, Alberta, June 16-19, 2005