2.1.1. McKenzie's Stretching Model for Basin Formation

McKenzie's [1] lithosphere-stretching model was the first to quantitatively tie basin development to mantle dynamics. In this model the crust and its underlying lithosphere were assumed sutured together and stretched by tensional tectonic forces such that their original thickness (individually and in sum) was reduced by a factor β−1, and the width of any portion of the crust/lithosphere was increased by β. The temperature at the base of the lithosphere/top of the asthenosphere was considered to be 1350 ◦C, and the lithosphere and any ocean water load on it was considered to be in isostatic equilibrium (e.g., the crust/lithosphere floated on the underlying asthenospheric mantle). The consequence of the stretching was an immediate increase in heat flow, an immediate subsidence or uplift depending on the thickness of the crust, and an ensuing slow subsidence as the thinned lithosphere lost heat and grew back to its original thickness. For typical crust and mantle density and thermal expansion, ocean water loading, and complete oceanization (β large) of a portion of continent, the initial subsidence is ~4.2 km if the crust is 35 km thick, 0 km if it is 15 km thick, and −3.2 km (3.2 km of uplift) if the crust is initially 0 km thick. The thermal subsidence over the next ~60 million years is 3.2 km. The heat flow as a function of time can be calculated from the stretching factor, and the maturation of organic material in the sediments can be computed from this time-temperature history. Sediment loading causes additional isostatic subsidence, but this can also be taken into account.

The McKenzie stretching model proved a very useful baseline for analyzing when buried organic material in a basin might generate hydrocarbons and fill structural traps. It spawned a vast literature investigating the cooling effects of pore fluid movement, modeling the thermal conductivity of sediments, taking into account the impact of differential stretching of the crust and underlying lithosphere, accounting for the effects of gradual stretching and crustal flexure, the cooling effects of rapid sediment deposition, heating by sill intrusion, etc. There proved to be so many factors that needed to be taken into account that most exploration companies chose to use maturity indicators to calibrate the heat flow history in their basins rather than try to predict it from the ground up. However, where from-first-principle predictions can be made they are very informative. An example is given in what follows.
