Thermally weakened lithospheric foundering on Venus, analogue evidence consistent with a stagnant to plutonic squishy-lid transition

Introduction
Venus has long been interpreted as a stagnant-lid planet, yet its surface preserves a widespread record of tectonic deformation, including more than 500 trench-bearing coronae and evidence for ongoing deformation at several of them [1,3].  This inventory is difficult to reconcile with a fully stagnant lithosphere. Recent numerical models suggest that Venus may be evolving from a stagnant-lid regime toward a plutonic squishy-lid regime [3,4], in which a thermally weakened lid recycles through localized foundering rather than through plate boundaries. How this transition is expressed mechanically at the surface, especially where the lithosphere has been thermally softened, remains unclear.

Here, using scaled three-dimensional analogue experiments performed at the Planetary Analogue Laboratory, University of Freiburg, we test how a thermally weakened lithosphere founders above a thickened thermal boundary layer. The experimental stratigraphy consists of an upper crust of dry quartz sand with an internal friction angle of ϕ = 25°, representing the brittle uppermost lithosphere, overlying lower-crustal and mantle-lithospheric layers made of polydimethylsiloxane (PDMS), plasticine mixtures that reproduce the viscous strength of the deeper ductile lid. A pure PDMS layer represents the asthenosphere. Together, these layers form an analogue stagnant lid with brittle and viscous strength overlying a weaker interior. Localized foundering is initiated by a hemispherical density anomaly at the base of the lower crust. The viscosity scaling factor is 2.75 × 10^-17, so that an experimental lithosphere viscosity of about 10³ Pa s corresponds to a Venus-equivalent viscosity of about 10¹⁹ Pa s. This value is two to three orders of magnitude lower than estimates for cold, dry lithosphere and represents a thermally softened state expected where long-lived mantle plumes heat the base of the lid over geological timescales [6]. Internal deformation is resolved by dual-synchronous particle image velocimetry, while surface deformation is reconstructed by continuous stereo photogrammetry at a vertical resolution of 0.05 cm.

Results
Above the imposed basal density anomaly, gravitational instability first develops as a localized drip with a head and a trailing tail. As descent proceeds, the tail no longer remains axisymmetric. It progressively flattens and elongates into a sheet-like downwelling whose long axis exceeds its short axis by more than a factor of four. A comparable geometry has been documented in three-dimensional mantle convection simulations with yield-stress rheology [7] and has also been proposed as a recycling mode for the early Earth before the establishment of plate tectonics [8]. Above each foundering event, the lid bends but does not fragment. The surface develops the characteristic corona morphology, including a central depression, an annular topographic rise, and a deep arcuate trench. The trench forms by viscous flexure of the intact lid above the descending sheet, rather than by plate-boundary subduction. Morphological screening of Venus coronae catalogued from Magellan radar and gravity data identifies candidates that match successive experimental stages, including Artemis, Atahensik, and Nightingale coronae.

Implications
These experiments do not represent the global state of Venus. Instead, they isolate the mechanical response of a lithosphere whose viscosity has been reduced to about 10¹⁹ Pa s. In this regime, the lid yields viscously to underlying gravitational instabilities rather than failing by brittle plate-boundary processes. The thickened thermal boundary layer descends as a sheet, the overlying lid bends to accommodate removal, and a corona with an arcuate trench is preserved at the surface. This mechanism offers an alternative to plume-induced subduction [2] for the formation of arcuate trenches around large coronae. Combined with the global distribution of trench-bearing coronae and recent evidence for ongoing volcanic and tectonic activity, the results are consistent with Venus being in transition from a fully stagnant lid toward a plutonic squishy-lid mode of mantle heat loss. Thermally softened regions above long-lived mantle plumes are the most likely sites where this transition should be expressed at the surface. Scaled analogue experiments, therefore, capture the mechanical signature of local lithospheric foundering during a possible squishy-lid transition on Venus. Foundering of a thermally softened lithosphere provides a single plate-independent mechanism for trench-bearing coronae and offers testable predictions for upcoming VERITAS, EnVision, and DAVINCI observations.

Acknowledgements. This work was supported by the Freiburg Institute for Advanced Studies (FRIAS) under Grant 2100574001F1058.

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