Sequential deformation of plains at the margins of Alpha Regio, Venus: Implications for tessera formation

Ms. Gilmore et Jw. Head, Sequential deformation of plains at the margins of Alpha Regio, Venus: Implications for tessera formation, METEORIT PL, 35(4), 2000, pp. 667-687
Citations number
Categorie Soggetti
Earth Sciences
Journal title
ISSN journal
1086-9379 → ACNP
Year of publication
667 - 687
SICI code
The boundaries between the highly deformed tessera terrain and adjacent vol canic plains are primarily those of embayment, where the tessera are strati graphically older than the plains. Previous studies show that <3% of these boundaries display evidence of tectonic tilting after the emplacement of th e plains. One of these unusual boundaries is the western margin of Alpha Re gio tessera, a zone similar to 100 km in width that separates the plains fr om the interior structures of Alpha. This zone is characterized by margin p arallel, fine-scale (1-5 km) fractures, graben, and ridges that truncate an d postdate the broad-scale (10-30 km) ridges and troughs of the interior of Alpha. The western margin is embayed by several volcanic plains units that are progressively tilted and deformed by graben with closer proximity to A lpha Regio. The earliest deformation of the plains consists of northeast-tr ending graben similar to 1 km in width that are similar in morphology and s pacing to graben that deform intratessera plains and plains at the eastern boundary of Alpha. Northwest-trending graben then formed over an interval m arked by the emplacement of two additional plains units; their similarity t o northwest-trending structures emanating from Eve corona and the Lada Terr a rift suggests a possible genetic relationship. The tilting of the plains adjacent to western Alpha implies relative vertical movement of the margin, either uplift of tessera or downwarping of plains subsequent to the format ion and relaxation of the interior of Alpha Regio. Subsidence of plains at this locale is supported by the presence of a basin to the west of Alpha su rrounded by a fracture belt contiguous with western Alpha. Thus, the fractu res and deformation at the western boundary of Alpha may be related to the formation of a basin to the west of Alpha with some influence from the nort hernmost extension of the Lada Terra rift. Such a basin is not present at a section along the eastern boundary of Alpha Regio, where the origin of til ted plains remains equivocal. We conclude that the deformation along the we stern margin of Alpha Regio is not directly related to the process of tesse ra formation but is an example of tessera modification and is consistent wi th the stratigraphic position of tessera as the oldest unit observed on Ven us.