Article,
Formation of grooved terrain on Ganymede: Extensional instability mediated by cold, superplastic creep
Icarus, 154 (2): 321--336 (2001)
Abstract
The morphology of Ganymede's bright terrain is dominated at multikilometer
wavelengths by sets of subparallel, linear-to-sinuous ridges and
troughs, collectively known as grooves. It has been often suggested
that the grooves are the result of a necking-type extensional instability,
but the detailed analysis of Herrick and Stevenson (1990, Icarus
85, 191-204) concluded that such instabilities were not sufficiently
strong to explain the observed topographic relief. We have reexamined
this model, wherein an instability arises when a strong plastic or
brittle layer and underlying viscous or ductile substrate undergo
extension, by incorporating the recently determined flow laws for
superplastic creep of cold ice at low stresses. The behavior of these
flow laws is closer to Newtonian than to earlier determined, high-stress
flow laws. The power-law creep of very cold ice at high stresses
has also been remeasured to be very stress dependent, which decreases
its influence with respect to other flow mechanisms at low stresses.
In addition, because of a dimmer, younger Sun and potentially higher
albedos of recently emplaced bright terrain material, surface temperatures
in bright terrain were probably lower during the time of groove genesis
than they are today. Both incorporation of closer-to-Newtonian ductile
flow laws and lower temperatures serve to better decouple the strong,
deforming layer from the substrate, resulting in the development
of stronger instabilities. Consequently, we find that it is possible
to generate sufficiently strong instabilities at topographic wavelengths
consistent with the grooved terrain topography of Ganymede and at
geologically plausible strain rates (similar to10(-16) to 10(-14)
s(-1) for specific examples). With sufficient strain, it is even
possible to form grooves in dark terrain. The required thermal gradients
for groove formation in either bright or dark terrain are one-to-a-few
x 10 K km(-1), which require exceptional heat flows with respect
to the steady-state radiogenic background. (C) 2001 Elsevier Science
(USA).
Tags
- comets;
- conditions;
- dark
- ecliptic
- evolution;
- features;
- galilean
- high-resolution;
- ice;
- lithosphere;
- planetary
- satellites;
- tectonic
- terrain;
- thermal
- topography
