Orogenic tectonism on Io
Journal of Geophysical Research-Planets, 108 (E8):
5093 (2003)Abstract
We catalog 143 Ionian mountains (montes) and mountain-like features
(mensae, tholi, plana, and small peaks) in order to investigate orogenic
tectonism on Io. From this comprehensive list, we select 96 mountains
for which there are sufficient coverage and resolution to discern
spatial relationships with surrounding geologic features. Three of
the 96 mountains are probably volcanoes, 92 appear to be tectonic
massifs, and 1 is ambiguous. Of the 92 tectonic mountains, 38 abut
paterae ( volcanic or volcano-tectonic craters with irregular or
scalloped margins). This juxtaposition is unlikely to be a coincidence
as the probability of it occurring by chance is similar to0.1%. We
propose instead that orogenic faults may act as conduits for magma
ascent, thus fueling patera formation near mountains. As resurfacing
buries a shell of material from Io's surface to the base of the lithosphere,
its effective radius is reduced and it heats up. We calculate the
lithospheric volume change due to subsidence and thermal expansion
as a function of lithospheric thickness. Conservation of volume dictates
that this material must be uplifted at Io's surface. By estimating
the total volume of the mountains, we are able to place a lower limit
of 12 km on Io's lithospheric thickness. We hypothesize that, in
some cases, mountain formation may be facilitated by asthenospheric
diapirs impinging on the base of the lithosphere. The resulting lithospheric
swell could focus the compressive stresses that drive orogenic tectonism.
This model is one of several possible mechanisms for uplifting isolated
mountains such as are observed on Io.