To test the volcanic and fluvial hypotheses for the origin of the rafted plate terrain observed in the vicinity of Athabasca Valles (5 degrees N, 150 degrees E, Central Elysium Planitia), we investigated the subsurface radar echo from the Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) 5-MHz band data over this area. The backscattered signal losses were compared to finite difference time domain (FDTD) simulations of those arising from three hypothetical geoelectrical subsurface models, which differed in their assumed composition (percentage basalt versus ice) and assumed mode of origin (fluvial discharge/"frozen sea,'' mudflow, and low-viscosity lavas). The dielectric values used in these models are derived from laboratory measurements of Mars analog materials under Mars-like conditions. FDTD simulations suggest that if the near-surface environment is ice-rich, it will result in an average loss rate of 0.053 dB/m for massive ice (having less than 1% of suspended particulates) and 0.065 dB/m for a mudflow (consisting of a 50/50 mixture of ice and basaltic dust). Whereas the losses associated with a lava flow model increase to 0.19 dB/m. In comparison, the actual signal losses experienced by MARSIS within this region were on the order of 0.18 dB/m within the first 160 m beneath the surface. This suggests that propagation characteristics of Athabasca's near-subsurface are more consistent with a volcanic rather than a fluvial or mudflow origin of the rafted plate terrain, although limitations on radar sounding depth in this region cannot rule out the possibility of more deeply buried massive ice deposits.