Monday, February 23, 2009

Paper: Formation of Mountains on Io

The second Io paper posted Saturday on the journal Icarus's Articles in Press page is titled, "Formation of mountains on Io: Variable volcanism and thermal stresses". The authors for this paper are Michelle Kirchoff and Bill McKinnon from the Lunar and Planetary Institute in Houston and Washington University in St. Louis. This paper takes a look at the geophysics behind the formation of mountains on Io and why there is a global scale anti-correlation between mountain and volcanic paterae. In a nutshell, they find that variations in volcanic activity can vary the level of thermal stresses in Io's lithosphere, which in concert with subsidence stress (a compressive stress that increases with depth resulting from the high resurfacing rate), leads to the formation of thrust faulting and mountains if that volcanic activity decreases.

Crustal subsidence stress from volcanic resurfacing is one of the dominant compressive stressors on the Ionian lithosphere, but subsidence stress is insufficient to produce thrust faults that reach the surface as each time a fault is formed and moves, the subsidence stress global is that much more reduced. In order for a mountain or a cluster of mountains to be formed at a particular location, a "focusing" mechanism is required. Three hypotheses have been put forth for these mechanisms:
  • Jaeger et al. 2003 suggested that mantle plumes impinging on the base of the lithosphere could locally increase the compressive stress at a particular location, resulting in thrust faulting and mountain formation. Kirchoff and McKinnon call this hypothesis "plume-modified subsidence."
  • Tackley et al. 2001 suggested that global mantle upwellings and downwellings produce tensile and compressive stresses in the crust. Upwellings results in tensile (extentional) stress on the lithosphere due to crustal thinning and stretching, producing increased volcanic activity. Downwellings result in compressive stress on the lithosphere, producing increased mountain formation. This model was developed in response to the obsevation that while some paterae abut mountains and thus there maybe some local correlations between mountains and volcanic pits, globally these features are anti-correlated. The authors of this paper call this hypothesis "Convection-modified subsidence".
  • McKinnon et al. 2001 suggested that decreases in volcanic activity on a regional scale (and thus decreasing the transport of heat from the interior to the surface), could result in heat building up at the base of the lithosphere, causing it to melt. The increase in thermal stresses caused by this melting result in the propagation of thrust faults closer to the surface and would thus support mountain formation (also known as orogenesis). Kirchoff and McKinnon call this hypothesis "thermal-stress-modified subsidence".
The authors examined the viability of the "thermal-stress-modified subsidence" hypothesis by modeling temperature and stress levels within the lithosphere, first assuming a steady-state resurfacing rate (which, according to the O'Reilly and Davies 1981 heat pipe model, is directly related to heat flow; the advective escape of heat from Io's interior). They then increased or decreased that resurfacing rate by 50% and 100% to see how this affected the temperature and stress levels. Kirchoff and McKinnon looked at both cases where the asthenospheric heating rate and resurfacing rate are coupled and uncoupled. In the coupled case, a decrease in resurfacing rate is the result of a decrease in the local asthenospheric heating rate. In the uncoupled case, the local asthenospheric heating rate stays the same while the amount of heat that is advected to the surface (through volcanism) is varied.
  • In the coupled case, the thickness of the lithosphere is maintained. Temperatures in the lithosphere increase, causing the region in compressive failure (where stresses surpass Byerlee's Rule) to become larger and more shallow. Eventually the temperatures and stress levels reach a steady state.
  • In the uncoupled case, decreasing the resurfacing rate increased heating at the base of the lithosphere, leading it to melt and increasing temperatures throughout the lithosphere (though convection all the way to the surface remains negligible). Over 0.5-1 million years, the crust thins. The resulting thermal stress decreases the depth at which thrust faults could form and propagate. The more the resurfacing rate is reduced, the greater the effect. Increasing the resurfacing rate causes cooling at the base of the lithosphere, thickening it. No steady state is achieved, eventually something has to give, like an increase in volcanic activity.
  • The authors also looked at a case where the lithosphere is 50 km thick, instead of 25 km. This just increases the time it takes for the lithosphere to thin enough to bring the brittle compressive zone close enough to the surface to support orogenesis.
Kirchoff and McKinnon ignored the effects of warm intrusions (which are likely important in formation of paterae) and assume that old lava flows completely cool before the next is laid down and the presence of a partially molten asthenosphere. The thermal effects of the first two are likely negligible as they don't seem to be a major component of the heat flow (heat flow estimates at longer and shorter wavelengths are consistent). The third is consistent with most modern tidal heating models for Io given volcano distribution. They also found that changing crustal rheologies (dry basalt, peridotite, or "komatiite") doesn't greatly change the modeling results. Eruption temperature estimates are consistent with dry basalt with greater iron and magnesium (olivine) content.

The authors then compared their results to the other models. For the plume-modified subsidence hypothesis, stress caused by an impinging mantle plume was not significant compared to crustal stresses. Because tidal heating is focused in the asthenosphere (or upper mantle), heat "plumes" should be downwellings, not upwellings, according to Tackley et al. 2001. For the convection-modified subsidence hypothesis, compressive stress on the lithosphere from mantle downwellings are too small to focus mountain formation. Maximum lithospheric stress are only a few kPa, compared to hundreds of MPa needed for compressive failure.

Finally, the authors examine a potential local cycle of mountain and volcano formation. First, greater volcanic activity increases the resurfacing rate (and thus the subsidence rate). This causes greater compression at depth, which constrains the deep conduits between volcanoes and their deep magma reservoirs, lowering the level of volcanic activity. This does nothing to the level of asthenospheric heat and temperatures increase throughout the lithosphere, leading to melting at its base. This melting causes thermal stresses to propagate to shallower depths, causing compressive failure between 10-20 km, where thrust faults could propagate to the surface, forming mountains. The formation of these faults and mountains relieves the subsidence stresses and thermal stresses in the lithosphere, leading to extension. This then allows magma to ascend to the surface through newly opened conduits as well as form batholiths which can helps deform mountains, such as by fracturing them. Increasing volcanic activity leads to a thickening of the crust back to its normal level. Unlike the view from McKinnon et al. 2001, volcanism does not need to complete shut down for this model to work.

One potential consequence of this cycle is that areas with high numbers of mountain should be beginning to increase their level of volcanic activity, perhaps increase the number of active paterae that abut mountains compared to areas with more paterae but fewer mountains.

Link: Formation of mountains on Io: Variable volcanism and thermal stresses []

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