Fault-melt interaction and its implications for Venusian Tectonic regimes in Aphrodite Terra, Venus

Venus is believed to be deformed in a stagnant-lid, episodic-lid, or a plutonic-squishy-lid regime with mantle convection occurring beneath a unified lithosphere [1-3]. Its surface age, estimated at 240–800 Ma from impact crater records [4], suggests either a catastrophic resurfacing event involving rapid lithospheric recycling [5] or continuous, regionally tectonic and volcanic processes [6]. Here we raise a key question how strain localization occurs on Venus: do Venusian faults show evidence of multi-stage activation capable of leading to large-scale lithospheric deformation, and is it possible to use this to unravel the tectonic history of Venus?

To address this, we focused our investigation on the equatorial chasmata system in Eastern Aphrodite Terra, Venus, whose origin continues to be a subject of scientific debate. This study documents that the troughs consistently exhibit asymmetric cross-sectional profiles, with steeper slopes intersected by large-scale faults trending subparallel to the trough axis. These shear zones dip at low angles and occasionally form terraces along the slope profile, exposing sections of the shear planes. The shear planes are radar-smooth and exhibit radar emissivities distinct from the adjacent hanging wall and footwall. We propose that these fault planes be coated with melt films, which in some cases display flow features along downslope trajectories.

The formation of these melt films is explored in the context of frictional melting during co-seismic faulting. Frictional melting may be enhanced on Venus due to its elevated ambient temperatures and the likely water-free, mafic composition of its rocks. However, multi-incremental friction-induced melting is unlikely to result in significant strain localization, and the volume of melt generated even under Venusian conditions is insufficient to be resolved in the available SAR imagery. Instead, we hypothesize that the fault planes act as conduits for transporting magma from shallow subsurface reservoirs to the surface. Volcanic centers and edifices near the steep chasmata slopes and within corona interiors are potential sources for shallow subsurface melt reservoirs. Melt veneers along the fault planes may reduce friction coefficients, facilitating normal faulting at shallow dip angles.

The overall morphology of the troughs suggests that the faults were initially formed as thrust faults and later reactivated. Evidence of their youthfulness is provided by fresh hummocky landslide deposits originating from the steep hanging wall scarps, which partially obscure the exposed fault planes. They were likely triggered by fault-induced seismicity, suggesting that faulting on Venus is seismogenic. Seismic moments for the studied shear zones have been calculated to support fault activation.

References

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