DEEP PENETRATION OF WRINKLE RIDGES ON VENUS DEDUCED FROM RIDGE SPACING

Patterns of tectonics are one of the few ways to constrain the lithospheric structure of other planets. Wrinkle ridges, which express horizontal shortening, are often grouped in sub-parallel sets with characteristic spacing l. On Venus, l~20 km (Banerdt et al., Venus II, 1997). Building on our study of Martian wrinkle ridges, we assume that the ridge spacing is controlled by a localization instability (Montési and Zuber, LPSC, 2001). The localization instability is a deformation mode of a mechanically-layered lithosphere akin to the more familiar buckling/necking instability, with the difference that the brittle layers have a tendency to abandon a distributed style of deformation in favor of a localized one; the material tends to form major faults or shear zones. This behavior is obtained by considering that the brittle rock strength decreases with strain rate. The localization instability results in regularly-spaced shear zones.

The strain-rate weakening is parameterized by the effective stress exponent, n_e. Based on our analysis of Martian ridges, we use n_e~-16, which is consistent with brittle failure mechanisms. Then, the fault spacing implies that the depth-penetration of the faults underlying wrinkle ridges is H~20 km. That faults penetrate to such depths rules out that the rheology of the lithosphere is controlled by wet diabase, which is ductile below a couple kilometers. Assuming that faulting penetrates to the depth where rocks first become ductile, H helps constraining the geotherm at the time of ridge formation. If the rheology at 20 km depth is controlled by the creep of dry diabase (Mackwell et al., JGR, 1998), the geotherm implied by H~20 km is between 5 and 8 K.km^-1 for strain rates between 10^-18 and 10^-15 s^-1. A geotherm of 5 to 8 K.km^-1 gives a heat flow of 15 to 25 mW.m^-2. For comparison, the lithospheric heat flow beneath the continents on Earth is 13 to 15 mW.m^-2 (Jaupart et al., JGR, 1998). If clinopyroxene, which is a very strong phase when dry (Bystriky and Mackwell, JGR, 2001), is sufficiently abundant to dominate the ductile strength, the geotherm must be of order 18 K.km^-1, probably excessive. Control by olivine rheology would require that the crust is less than 14 km thick, which is unlikely.