Survival of Archean cratonal lithosphere

Abstract

[1] I use thermal convection models to search for combinations of physical parameters that are compatible with the results of xenolith studies on the history and present thermal structure of cratonal lithosphere. The cratonal lithosphere above $180 km depth formed in the Archean and remained stable until recently sampled. The mantle adiabat cooled $150 K over this time. The temperature change across the rheologically active boundary layer at the lithospheric base is <300 K over a depth range of several tens of kilometers. Modern cratonal lithospheric thicknesses are relatively uniform, $225 km, much thicker than old oceanic lithosphere. Cratonal lithosphere is now in quasi-steady state with conductive heat flow to the surface in balance with heat supplied to the base of the lithosphere by convection driven by local temperature contrasts within the rheologically active boundary layer. This heat flow and the lithosphere thickness changed little after the cratonal lithosphere stabilized. One possibility is that chemically buoyant lithosphere forms a conductive lid above the convecting normal mantle. To survive, the chemical lithosphere also needs to be more viscous than normal mantle. A reasonable situation has chemical lithosphere a factor of 20 more viscous than normal mantle with weakly temperature-dependent viscosity (a factor of e over 100 K) from 0.2 Â 10 20 Pa s along the modern mantle adiabat. However, a chemical lid is unnecessary for lithospheric thickness to change slowly over time. For example, the base of the lithosphere may be stable if the viscosity at its base (along an adiabat) decreases rapidly with depth.

Temperature and rheological properties of the mantle beneath the North American craton from an analysis of heat flux and seismic data

“…One may envision two different controls on the stable lithosphere, which is defined in opposition to the thin unstable thermal boundary layer that lies beneath it. The lithosphere may be identical to the chemically depleted layer, such that stability is due to both buoyancy and enhanced viscosity [Jordan, 1975;Pollack, 1986;Sleep, 2003a]. Alternatively, it may be thicker than the chemically depleted layer and may include a sublayer made of oceanic-type mantle that is too cold and viscous to become unstable [Sleep and Jellinek, 2008].…”

“…Such lithosphere cannot be thinned because of an imbalance between the convective and conductive heat fluxes. Thinning may be achieved by mechanical erosion, as envisaged by Sleep [2003a], but this is an entirely different mechanism which involves other physical controls. For such lithosphere, as noted by Sleep and Jellinek [2008], the temperature difference in the convective basal boundary layer may be smaller than the rheological value (equation (9)).…”

Long lasting epeirogenic uplift from mantle plumes and the origin of the Southern African Plateau

“…The scaling relationships are mostly developed in appendix A, and we apply them to an idealized case of plume impingement on thick cratonic lithosphere that has a flat base. We then explicitly solve (1), (2), and (3) numerically as a time-dependent boundary value problem in two dimensions following Andrews [1972], Sleep [2002], and Sleep [2003aSleep [ , 2003b (appendix A), and use the results together with the scaling relationships to address the issue of long term plateau uplift in southern Africa.…”

“…We concentrate on what can be done tractably in two-dimensions, and therefore focus on the fate of the plume material once ponded, along with the history of uplift. Sleep [2003aSleep [ , 2003b has used the same numerical model to investigate the long term rheological behavior of cratonic lithosphere and the details of the geotherm at the base of cratons following the ponding of plume material.…”

“…However, for simplicity we assume linear (Newtonian) viscosity and that viscosity depends on composition and temperature but not depth. Following Doin et al [1997] and Sleep [2003a], we assume a function of the form…”