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A preliminary model for the evolution of small extensional or pull-apart basins is presented. The very rapid subsidence, sediment accumulation, and hydrocarbon maturation observed in many basins of this type are explained using a McKenzie-type approach. Lateral heat loss is shown to be a critical factor in controlling the rate of heat loss in basins of finite width. In a simple two stage model where stretching and cooling are assumed to occur as separate processes, more than one-third of the total thermal subsidence occurs in the first 200,000 yr of cooling for a basin 30 km (20 mi) across. This allows for the accumulation of over 2 km (6,500 ft) of sediment. Because the time required for a 10-km (6-mi) long block to be stretched to 30 km (20 mi) is substantially greater han 200,000 yr, much of the cooling and subsidence must take place during stretching. This simultaneous stretching and cooling is approximated by alternating short periods of stretching and cooling.
The resulting model is applied to the development of the basins associated with the San Andreas fault in southern California. These basins have recently (Miocene to Present) undergone rapid (up to 6 km, 4 mi) subsidence and sediment accumulation as well as rapid maturation of hydrocarbons. They appear to have been initiated in an extensional regime along irregularities in the strike-slip motion of the fault, even though some of the basins have been modified by subsequent compression. These basins are therefore excellent candidates for testing the proposed model.
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