An intrusive cryomagmatic origin for northern radial labyrinth terrains on Titan and implications for the presence of crustal clathrates

Abstract

Titan's labyrinth terrains are an organic-rich, topographically elevated, highly dissected and puzzling geomorphic unit. How these features came to be composed of organics and remain elevated may hold clues about Titan's complicated history, and in particular the dynamics and composition of Titan's crust. One subtype of labyrinth terrains, the radially networked labyrinth terrains, is found in Titan's mid-latitudes. They are dome-shaped with radial drainage patterns and appear to be a clustering of uplifted, organic-rich dissected plateaux. We use scaling relationships to determine whether they formed as elevated surfaces that were uplifted by solid-state diapirs or cryomagmatic laccoliths at depth. Based on the large, variable spacing between features, we find it unlikely that they formed via density-driven diapirism. Instead, their dimensions suggest that they are cryomagmatic intrusions that formed near the most prominent rheological contrast in the ice shell, the brittle–ductile transition (BDT). At that location, we argue that intrusions spread horizontally and inflate, forming large cryomagmatic laccoliths (upturned, large saucer-shaped sills). The intrusions would flex the overlying lithosphere and surficial sedimentary layers (likely undifferentiated plains), resulting in prominent domed features that are susceptible to erosion and incision by methane rain and wind. This process then leads to the highly dissected, dome-shaped labyrinth terrains. To determine whether the laccoliths formed near Titan's BDT, we calculate lithospheric strength envelopes for a pure water ice shell with and without an insulating methane clathrate crust using two conductive heat flows: 4 and 7 mW/m2. The plausible range for the BDT for an ice shell with a 1 km thick methane clathrate crust is 12–34 km for the 4 mW/m2 heat flow. This agrees well with the expected intrusion depths of the laccoliths (21–28 km) associated with a cluster of radial labyrinths in Titan's northern mid-latitudes, as derived from a scaling relationship relating intrusion depth to dome width. We find that methane clathrate thicknesses of 2 km or greater result in a BDT that is generally too shallow (3–24 km) to match our observations. If we consider the higher heat flow, these BDTs are even shallower. Although methane clathrate is known to be stronger than ice, its low thermal conductivity significantly raises the underlying ice shell's temperature, which results in a significantly thinner lithosphere that is not able to support these large plateaux. We conclude that in the location of this radial labyrinth terrain cluster there may be intrusive cryomagmatic activity approximately 21 km deep within the water ice shell, with a putative surface methane clathrate crust, if it exists atop a conductive ice shell, that is constrained to be <2 km thick.