Small planitiae on Venus: Tensional or compressional tectonics?

Abstract

The two small planitiae south of Fortuna Tessera define distinct crustal units not very unlike to small crustal plates or subplates. The mobile transformation zone between Fortuna Tessera and Allat Planitia is caused by colliding crustal plates and evidently indicates the mobilization of the planitia unit foiled by the parquet terrain. Ridges parallel to this zone and in dextral ridge groups on planitia support the idea of the main N(W)-S(E) compression. Allat Planitia has been pushed approximately from the south and southeast against Fortuna Tessera which, in contrary, has spreaded to the southeast. Within the smaller planitia there are two conjugate ridge sets and a third ridge set parallel to the parquet border. The crossing ridge sets favour the existence of a compressional NW-SE force, as do the N-S directed ridges of the middle planitia area. At least three tectonic phases within Allat Planitia can be found. The main compression was in N(W)-S(E) direction. Prominent right-handed en echelon ridge groups and long parallel ridges of the northern planitia area indicate this comrpessional environment as well as the transformation zone against Fortuna Tessera. Short dome-like ridges indicate the tension gash opening during a NW-SE compression phase. An E-W (or NWW-SEE) compression resulted in the formation of the long linear wrinkle ridge-like N-S structures on Allat Planitia. The NW-SE compression, which has caused the formation of the dextral, E-W oriented major fault, was then the youngest of the main tectonic phases involved within the area studied.