A new model for the initiation, crustal architecture, and extinction of pull-apart basins

Abstract:

We present a new model for the origin, crustal architecture, and evolution of pull-apart basins. The model is based on results of three-dimensional upper crustal numerical models of deformation, field observations, and fault theory, and answers many of the outstanding questions related to these rifts. In our model, geometric differences between pull-apart basins are inherited from the initial geometry of the strike-slip fault step which results from early geometry of the strike-slip fault system. As strike-slip motion accumulates, pull-apart basins are stationary with respect to underlying basement and the fault tips may propagate beyond the rift basin. Our model predicts that the sediment source areas may thus migrate over time. This implies that, although pull-apart basins lengthen over time, lengthening is accommodated by extension within the pull-apart basin, rather than formation of new faults outside of the rift zone. In this aspect pull-apart basins behave as narrow rifts: with increasing strike-slip the basins deepen but there is no significant younging outward. We explain why pull-apart basins do not go through previously proposed geometric evolutionary stages, which has not been documented in nature. Field studies predict that pull-apart basins become extinct when an active basin-crossing fault forms; this is the most likely fate of pull-apart basins, because strike-slip systems tend to straighten. The model predicts what the favorable step-dimensions are for the formation of such a fault system, and those for which a pull-apart basin may further develop into a short seafloor-spreading ridge. The model also shows that rift shoulder uplift is enhanced if the strike-slip rate is larger than the fault-propagation rate. Crustal compression then contributes to uplift of the rift flanks.