Early thermal profiles and lithospheric strength of Ganymede from extensional tectonic features

Abstract

Estimates of the brittle lithosphere thickness derived from the width and spacing of extensional tectonic features, coupled with lithospheric strength envelopes (brittle and ductile yield stress versus depth) appropriate for ice, allow the quantitative determination of early thermal profiles and lithospheric strength and stability on Ganymede. Furrows and grooves indicate brittle lithospheric thicknesses of 5–10 and 2–5 km, respectively, assuming that their spacing is controlled by an extensional instability or that their width is controlled by the intersection depth of their bounding faults. Plots of the brittle and ductile yield stress versus depth for the icy lithosphere of Ganymede show a linear increase in brittle strength with depth to a maximum at the brittle-ductile transition, followed by an exponential decrease in ductile yield stress with depth. Because the depth to the brittle-ductile transition depends primarily on the thermal gradient, the thickness of the brittle lithosphere can be used to calculate early thermal profiles of 1.5–6 and 4.5–20°/km during the formation of the furrows and grooves, respectively. Lithospheric strength, the integral of the yield stress versus depth curve, varied from 30–125 GPa m when the furrows formed to 5–30 GPa m when the grooves formed, which correspond to maximum yield stresses of 6–11 and 2.5–6 MPa, respectively. These results indicate that the thermal gradient and lithospheric strength varied laterally by as much as a factor of 5 and that Ganymede cooled in a highly inhomogeneous manner with significant lateral thermal anomalies. Finally, this analysis provides a straightforward explanation for the stability of large remnants of cratered terrain such as Galileo Regio that had a low thermal gradient and strong lithosphere in contrast to small remnants of cratered terrain that were fractured and broken up by grooved terrain as a result of higher thermal gradients and weaker lithospheres.