RIFT BASINS AND SUPRADETACHMENT BASINS - INTRACONTINENTAL EXTENSIONALEND-MEMBERS

Two end-members characterize a continuum of continental extensional te ctonism: rift settings and highly extended terrains. These different s tyles result in and are recorded by different extensional basins. Intr acontinental rifts (e.g. East Africa, Lake Baikal) usually occur in th ermally equilibrated crust of normal thickness. Rift settings commonly display alkali to tholeiitic magmatism, steeply dipping (45-60 degree s) bounding faults, slip rates < 1 mm yr(-1) and low-magnitude extensi on (10-25%). Total extension typically requires > 25 Myr. The fault an d sub-basin geometry which dominates depositional style is a half-grab en bounded by a steeply dipping normal fault. Associated basins are de ep (6-10 km), and sedimentation is predominantly axial- or hangingwall -derived. Asymmetric subsidence localizes depocentres along the active basin-bounding scarp. Highly extended continental terrains (e.g. Colo rado River extensional corridor, the Cyclade Islands) represent a diff erent tectonic end-member. They form in back-are regions where the cru st has undergone dramatic thickening before extension, and usually rea ctivate recently deformed crust. Volcanism is typically calc-alkalic, and 80-90% of total extension requires much less time (< 10 Myr). Boun ding faults are commonly active at shallow dips (15-35 degrees); slip rates (commonly > 2 mm yr(-1)) and bulk extension (often > 100%) are h igh. The differences in extension magnitude and rate, volcanism, heat flow, and structural style suggest basin evolution will differ with te ctonic setting. Supradetachment basins, or basins formed in highly ext ended terrains, have predominantly long, transverse drainage networks derived from the breakaway footwall. Depocentres are distal (10-20 km) to the main bounding fault. Basin fill is relatively thin (typically 1-3 km), probably due to rapid uplift of the tectonically and erosiona lly denuded footwall. Sedimentation rates are high (similar to 1 m kyr (-1)) and interrupted by substantial unconformities. In arid and semi- arid regions, fluvial systems are poorly developed and alluvial fans d ominated by mass-wasting (debris-flow, rock-avalanche breccias, glide blocks) represent a significant proportion (30-50%) of basin fill. The key parameters for comparing supradetachment to rift systems are exte nsion rate and amount, which are functions of other factors like crust al thickness, thermal state of the lithosphere and tectonic environmen t. Changes in these parameters over time appear to result in changes t o basin systematics.