TECTONIC CONTINUOUS CASTING ON EARTH, VENUS, AND PLUTO

Continuous casting is a process whereby a fluid medium solidifies as it is extruded from a mold, with heat loss across a slip surface. The process is widely used for steel production with continuous linear forms. In magmatic continuous casting magma solidifies as it is extruded from a vent or fissure or is uncovered as it flows out from beneath a solid crust. An excellent example is a “squeeze-up” at Bonito flow, Sunset Crater (northeast of Flagstaff) where magma from the interior of a lava flow was extruded through the overlying crust as it cracked open.

Deep-sea core complexes formed by sea-floor spreading are the most obvious examples of tectonic continuous casting. These core complexes reflect a transition from symmetric, magma-dominated extension to asymmetric extension with extrusion of new crust, including mylonitic peridotite, from beneath moderately to gently dipping normal faults at sea-floor spreading centers. The 7000 km² Godzilla core complex in the Philippine Sea includes numerous grooves along its 125 km length that are parallel to nearby transform faults. One 5-km-wide groove is 60-km long (the Godzilla megagroove). Deep-sea core complex grooves have been interpreted as giant fault grooves, or “megamullions.”

Continental core complexes are interpreted as exhumed normal-fault footwalls. The 50-km length of the antiformal Harcuvar Mountains in western Arizona, and the 30-40 km length of several others, indicates groove genesis partly by non-brittle processes as continental normal faults are generally not seismic at depths greater than 10-15 km. Continuous casting is likely in these settings but structural studies have not so far been able to determine if plastic shaping of footwalls actually occurred.

Tectonic extension within the interior of Artemis Corona, Venus, parallel to shortening on its southeastern flank, appears to have produced a grooved surface that resembles grooved core complexes on Earth and is similarly inferred to have originated by continuous casting.

A 550 km rift trough on Pluto, photographed in 2015 by NASA’s New Horizons spacecraft and named Sleipnir Fossa, includes a 100 km section that looks like an arched, normal-fault footwall with grooves that plunge into the axis of the trough. These grooves are 1-4 km wide and 5-15 km long, comparable in scale to deep-ocean core complexes on Earth.