THE THICKNESS OF GANYMEDE'S DARK TERRAIN LITHOSPHERE FROM FURROW FLEXURE

A fundamental issue in understanding the geological history of Jupiter's icy moon Ganymede is its thermal evolution through time. Recently Nimmo et al. (GRL, 29, 10.1029/2001GL013976, 2002) have applied a broken-plate elastic model to rift flank uplift identified aside lanes of grooved terrain in a stereo-derived topographic model, finding an effective elastic thickness of 0.9 and 1.7 km for two study areas. We apply analogous methods to model flexure identified aside ~10 to 20 km wide furrows in the more ancient dark terrain of Galileo Regio. We derive an effective elastic thickness of <=1.2 km and a corresponding heat flux ~100 mW/m^2 for two cross-cutting furrows within the study region. Consistent results are found by modeling the shape of the flexural profile only, or by modeling both the flexural shape and the inferred negative topographic load from furrow faulting. We infer asymmetry in the structure of these furrows based on asymmetry in rim height and a corresponding sense of inferred flexurally induced topography. This suggests formation of the topographically more pronounced side of the furrow as an uplifted footwall block, with associated rollover and antithetic faulting of the hanging wall block with lesser flexural uplift there. A similar effective elastic thickness of ~1 km in both grooved and dark terrains contrasts with previous results in which all furrows and grooves were modeled as fault-bounded graben that intersect at the brittle-ductile transition during the time of deformation. If our results can be extrapolated to dark and grooved terrains as a whole, then similar inferred effective elastic thickness in ancient dark and more recent grooved terrain suggests that either: (1) Ganymede's lithospheric thickness remained essentially constant from the period of dark terrain formation through the period of grooved terrain formation, or (2) Ganymede's heat flux was similar during the periods of dark terrain and grooved terrain deformation but waned in between. The latter hypothesis might be reconciled with dark terrain deformation during an early warm period after satellite accretion, and grooved terrain deformation during a later epoch of heating such as a tidal heating event.