Neo-tectonic activity and indications for fault-melt interaction along Aphrodite Terra´s Chasmata system, Venus.

Introduction

Venus’ surface is densely populated with fractures, faults, and shear zone networks of different dimensions. This study aims to pursue a detailed investigation on large-scale shear zones of Eastern Aphrodite Terra, a high relief region on Venus that expands along the equator. The area  is radar-bright due to the presence of high fracture densities. The key feature of Eastern Aphrodite Terra is an interconnected network of straight and deep ENE-WSW to E-W trending chasmata (Fig. 1a), and large coronae, which are surrounded by deep arcuate troughs. The origin of the chasmata system is a subject of scientific debate. Herrick et al. (1989) and others proposed that chasmata are rifts, that were formed above a dynamic mantle upwelling system. Others suggested that neither divergent nor convergent tectonics occur along the troughs of Eastern Aphrodite Terra, but vertical adjustments dominate that are related to mantle up-welling and down-welling (e.g., Hansen and Phillips, 1993). In contrast, the arc-shaped and asymmetric troughs that surround the large coronae in this region were explained by convergent tectonics (e.g., Sandwell and Schubert 1992; Kenkmann et al. 2024).

Fig. 1. Diana Chasma South. A) Elevation map of Diana Chasma and adjacent area. The region of interest ROI is shown in B as original SAR data and in C) as inverted SAR data. D) Detail region of interest (ROI) of C) with location of the shear zone including dip angle and strike of the fault plane and altitude information. E) Geological map with mapped fractures, smooth patches and rugged terrain at the fault terrace. The trace of the profile is indicated. F) Fracture density map. G) Vertically exaggerated topographic profile. Note that the dip of the fault is not vertically exaggerated. H) inverted SAR of area shown in f). Hw and fw indicates hanging wall and footwall.

Observations and Measurements

• The equatorial chasmata system of Eastern Aphrodite Terra is among those regions on Venus that has the strongest relief (Fig. 1a). Most of the valleys are asymmetric in cross-section with a steeper slope of up to 40 ° inclination and a gentle slope of roughly 5° (Fig. 1g).
• We mapped distinct large-scale shear zones along the chasmata (Fig. 1c-e) and the arcuate troughs surrounding coronae in that region. The lengths of the mapped faults range between 218 km and 706 km. The faults always intersect the steep chasmata slopes.
• The faults dip at angles of 25°-37°, opposite to the slopes (Fig. 1d). Faults are strongly localized and associated with damage zones of intense fracturing (Fig. 1e-f). The fault planes themselves are partly exposed and form distinct terraces along the steep slopes (Fig. 1g).
• In close vicinity to the shear zones are patchy, small-scale hills of delicate rugged terrain, which are surrounded by radar smooth, pristine, pond like halos that partly show flow features and cover exposed parts of the shear zones (Fig. 1e, h).
• Elevation corrected radar emissivity data of the hanging wall and footwall of the shear zones, as well as of the associated rugged terrain and radar smooth planes mostly differ from each other.
• Locally, small-scale and fresh landslide deposits emanating from the steep hanging wall of the shear zone occur in the vicinity of the shear zones.

Interpretation

The occurrence of the shear zones along the steep slopes of the asymmetric chasmata suggests that they were formed by thrusting. The exposure of their fault planes, however, indicates that they got later reactivated as normal faults (Fig. 1g). The immediate proximity of landslides with shear zones points to a seismic trigger of the mass movements. The rugged and patchy hills surrounding by smooth halos are interpreted as fault breccia mixed with a fluid-like phase extruded from the faults. When the material reached the surface the low viscous material dispersed fluidly around the blocky rugged material and covered the local relief. We propose that the low viscous material is melt since a fluid phase is unlikely to occur at the present conditions on Venus. Emissivity data indicates that the extruded possible melt is compositionally different from the rugged material and from the hanging wall and footwall lithologies of the shear zones.

The melts may originate from relatively shallow crustal depths from nearby volcanic edifices and corona interiors. The shear zones serve as ascent paths for melts, lower the friction coefficients on the shear planes and with this contribute to deformation localization. Alternatively, we discuss a possible formation by frictional melting in the environment on Venus. The morphological distinctiveness and freshness of the shear zones, which are not cut by fractures, and their systematic association with delicate melt ponds and coarse breccia indicates that the shear zones are very young tectonic features that have not been geologically overprinted in any way.

References

Hansen, V. L., & Phillips, R. J. (1993). Tectonics and volcanism of east Aphrodite Terra, Venus: No subduction, no spreading. Science, 260(5107), 526–530.

Herrick, R. R., Bills, B. G., & Hall, S. A. (1989). Variations in effective compensation depth across Aphrodite Terra, Venus. Geophysical Research Letters, 16(6), 543–546

Kenkmann, T., Karagoz, O., & Veitengruber, A. (2024). Structural analysis and evolution of large Venusian coronae: Insights from low-angle faults at coronae rims. Planetary and Space Science, 250, 105955.

Sandwell, D. T., & Schubert, G. (1992). Evidence for retrograde lithospheric subduction on Venus. Science, 257, 766–770.