THE HAASTTSE-BAAD CONCENTRIC RING GRABEN COMPLEX, NORTHERN APHRODITE TERRA (VENUS): A VALHALLA-TYPE RING STRUCTURE

Concentric ring graben define a >1000 km diameter feature that cuts ribbon-tessera terrain (RTT) of Haastte-Baad Tessera, northern Aphrodite Terra, Venus. The composite structure, informally the Haasttse-baad ring complex (HBRC), includes a series of partly-preserved, well-defined concentric, spaced (10-50 km), nested, arcuate troughs that cut local RTT structures. The RTT fabric is marked by short- to medium-wavelength folds (1-6 km), and orthogonal ribbon structures (1-3 km) and spaced graben complexes (~10+ km). Compared to RTT structures, the concentric graben are curved in planform, marked by singular fault scarps, and have larger length:width ratios. The HBRC is well-defined to the south (4:00 to 8:30 in analog clock framework) where it cuts RTT of Haasttse-baad Tessera, but poorly defined to the north, where RTT only occurs as isolated lowland kipukas. In the south the HBRC is divisible into two parts. The SW part sits at lower elevation with more late flooding, has wider and more closely spaced concentric graben. The SE part has narrower more widely spaced concentric graben—consistent with these graben cutting a thicker layer; a regionally extensive suite of late-formed NNW-trending graben also dissect the SE part. In both areas the concentric graben postdate the RTT fabric, but predate regional NNW-graben formation and late flooding.

Models for concentric ring graben structures include endogenic magmatic and exogenic bolide impact processes. The HBRC differs in morphology, size, and predicted strain with endogenic features, but is similar to Valhalla-type ring structures in morphology (e.g., Callanish & Tyre craters, Europa) proposed to have formed by bolide impact on targets with 3 rheological layers—an upper thin elastic layer and a lower strong substrate, separated by a low-viscosity layer. The number, spacing and morphology of concentric graben reflect elastic layer strength and crater diameter.

We propose that the HBRC represents a newly recognized Venus impact feature—a Valhalla-type ring structure formed on rheologically layered RTT, composed of a thin elastic layer above a low-viscosity layer underlain by a strong substrate. At >1000 km diameter the HBRC would be Venus’ largest impact feature. Other partial concentric structures within RTT may represent similarly formed impact structures.