Continental extension strongly affects the topography of continents by shaping elongated rift valleys, referred to as rift basins in the geological record. A strong implication of the formation of these basins is a two-step subsidence history: an initial rapid subsidence during extension, and a second, slower subsidence once active extension has ceased. Although this observation was made already long ago, its explanation remained unknown until McKenzie came in with the key: if the crust is stretched and thinned, the underlying mantle should do so too, by upwelling its lower limit – the lithosphere-asthenosphere boundary.
A simple model of crustal extension from (Selley et al., 2015)
McKenzie describes and tests the simple approach, of thinning the mantle by the same factor as the crust. This results in a two-stage history evolution of sedimentary basins that he validates with an analytical model coupling thermicity and isostasy. First, horizontal extension thins the lithosphere, i.e. the crust and the mantle, in an homogeneous way. This stage produces a rapidly subsiding faulted basin, but also an upward motion of the (thermal) lower limit of the lithosphere. Second, the lithosphere cools down: whereas the crust-mantle boundary stays where it was, the lower limit re-equilibrates. This causes a second, slow subsidence event, without faulting. With this, McKenzie presents a simple but elegant model in which extension is responsible for both heating of lithosphere and crustal thinning necessary to produce a sedimentary basin (Cochran, 1983). And voilà! He then applied his model to a few, yet well-known basins (Great Basin, the Aegean, the North Sea and the Michigan basin) with pretty good success.
Commonly considered as the mathematical basis for stretching in forward modeling of extensional basins, McKenzie 1978’s paper was regarded as a milestone for research on the thermal condition of extensional systems. The model enables a large number of basin infill simulations addressing the interplay between thermal subsidence, sediment loading, and eustatic sea-level changes.
The model, given its simplicity, suffers from a few drawbacks. (1) Extension is considered to occur instantaneously, in disagreement with the time span of several millions of years observed (Cochran, 1983). (2) The heat is treated in its simplest way, by assuming heat conduction only with a basal heat flow from the asthenosphere (Waples, 2010). However, heating resulting from sources within the lithosphere, notably by radiogenic decay of U, Th and K (that are highly concentrated in the upper crust), may represent four fifths of Earth’s surface heat flow (Hacker et al., 2015). Despite these few caveats, the model does not lose its lustre; it keeps being satisfactory for most rift examples worldwide, both in terms of subsidence history, heat flux and thermal evolution of rift basins.
Pan Luo, Benoît Petri, Patricia Cadenas Martínez and the TS Must Read team
References
Cochran, J.R., 1983. Effects of finite rifting times on the development of sedimentary basins. Earth and Planetary Science Letters, 66, pp.289-302. https://doi.org/10.1016/0012-821X(83)90142-5
Jarvis, G.T. and McKenzie, D.P., 1980. Sedimentary basin formation with finite extension rates. Earth and Planetary science letters, 48(1), pp.42-52. https://doi.org/10.1016/0012-821X(80)90168-5
Hacker, B.R., Kelemen, P.B., Behn, M.D., 2015. Continental Lower Crust. Annu. Rev. Earth Planet. Sci. 43, 167–205. https://doi.org/10.1146/annurev-earth-050212-124117
McKenzie, D., 1978. Some remarks on the development of sedimentary basins. Earth and Planetary science letters, 40, 25 – 32, https://doi.org/10.1016/0012-821X(78)90071-7
Waples, D.W., 2001. A New Model for Heat Flow in Extensional Basins: Radiogenic Heat, Asthenospheric Heat, and the McKenzie Model. Natural Resources Research 10, 227–238. https://doi.org/10.1023/A:1012521309181
Selley, R.C., Sonnenberg, S.A., 2015. Sedimentary Basins and Petroleum Systems, in: Elements of Petroleum Geology. Elsevier, pp. 377–426. https://doi.org/10.1016/B978-0-12-386031-6.00008-4
