Standard models for a warm, wet early Mars require a significant CO 2 ‐H 2 O atmosphere in the past. The source for these phases is assumed to be volcanic degassing. However, no consistent, dynamical models exist relating volcanic degassing to evolving mantle temperatures. Here we use a range of thermal, geophysical, geological, and petrological constraints from Mars to constrain mantle convection model simulations of Mars' post‐Noachian stagnant lid evolution. We develop a methodology to self‐consistently calculate melt extraction from the mantle source region. Using a dike‐propagation algorithm, we can calculate the rate of volcanism and rate of volcanic degassing from these simulations and compare them with estimates for Mars. We find that Martian melt production rates are satisfied by a 200‐km thick lithosphere (surface heat flow 25 ± 5 mW/m 3 ) for an intermediate Martian solidus. Core‐mantle temperatures cannot exceed ∼1850°C from geodynamo constraints, and the enrichment of heat‐producing elements into the crust is unlikely to exceed 25–50%. For hotter Martian mantle temperatures in the past, we find an evolution in rates of volcanism from >0.17 km 3 /yr for the early Hesperian to ∼1 × 10 −4 km 3 /yr at present, consistent with geological evidence. During this same interval, CO 2 flux would have declined from 8.8 × 10 7 to 6.7 × 10 6 kg/yr. If the early Hesperian supported a dense (>1 bar) atmosphere, this implies that the average loss rate of CO 2 from the atmosphere was 15 times greater than the maximum influx rate during this time.
Cratons are areas of continental lithosphere that exhibit long-term stability against deformation. Seismic evidence suggests that cratonic lithosphere may have formed via thrust stacking of proto-cratonic lithosphere. We conducted numerical simulations and scaling analysis to test this hypothesis, as well as to elucidate mechanisms for stabilization. We found that formation of cratonic lithosphere via thrust stacking is most viable for buoyant and viscous lithosphere that is thin and/or possesses low effective friction coefficients. These conditions lead to low integrated yield strength within proto-cratonic lithosphere that allows it to fail in response to convection-generated stresses. Specifically, formation via thrust stacking is viable for lithosphere with chemical to thermal buoyancy ratios of B = 0.75-1.5, viscosity contrasts between the lithosphere and convective mantle of AT) > 10 2 , and friction coefficients of μ = 0.05-0.1. Preservation depends on the balance between the chemical lithosphere's integrated yield and convection-generated stresses. The physical process of thrust stacking generates a thickened cratonic root. This provides a higher integrated yield stress within cratons, which is more conducive to stability subsequent to formation. Increased friction coefficient values, due to dehydration, can also provide higher integrated yield stresses within cratons. To provide long-term stability, integrated yield stresses must be great enough to offset future mantle convection-generated stresses, which can increase with time as the mantle viscosity increases due to cooling. Thin or rehydrated cratonic lithosphere may not provide stability against the increasing convective stresses, thus providing an explanation as to why some cratons are not long-lived.
The original version of the Supplementary Information associated with this Article contained an error in Supplementary Figure 4 in which the colours on the maps rendered incorrectly. The HTML has been updated to include a corrected version of the Supplementary Information.
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