As w ith v olcanic activity, stru ctu ral fab rics id e n tifie d w ith in G uinevere and Sedna P ianitiae im ply a n o n -d irectio n al rath e r than d ire c tio n a l n atu re to d efo rm atio n al p ro ce sses. T he o b se rv atio n s c o n tra d ic t th o se c ite d in the g lo b al s tra tig ra p h ie m o d el o f Basilevsky and Head.
Tesserae
B ased on em baym ent observations, tessera is com m only cited as the oldest local unit w hen m apping on Venus (e.g. B i n d s c h a d l e r an d Head, 1989; Iva n o v an d B asilevsky, 1993; Greeley et al., 1994;
Copp and Guest, 1995, 1997; Tapper and Guest, 1997). W ithin the g lo b al stra tig ra p h y m o d el, te sse ra is u se d as a g lo b al tim e- stratig rap h ic m arker (the Fortunian) and is co n sid ered the resu lt o f a global period o f deform ation called tesseratio n {Ivanov a n d H e a d , 1995, 1996; B asilevsky and Head, 1995a, 1995b). H ow ever, tesserae are defined by th eir stru ctu ral c h arac teristics w hich are not necessarily an absolute age criterion.
By w ay o f a terre stria l analogy, b efo re ab so lu te d atin g o f ro ck s w as p o ssib le , re la tiv e ageing w as e sta b lish e d u sin g the p rin cip les o f su p erp o sitio n and cro ss-cu ttin g relatio n s, sim ilar to m ap p in g tech n iq u es used w ith M ag ellan d ata today. C ry sta llin e m etam o rp h ic ro ck s w ere id e n tifie d as th e o ld e st m a te ria ls at separate localities and thus m apped as one age. From m ore careful stra tig ra p h ie stu d ies (and later, the ad v en t o f a b so lu te d atin g m ethods) it w as show n that m etam orphic ro ck s span the w hole stratigraphie age o f the Earth, and o f course, are currently form ing today {Hansen a n d Willis, 1996). A sim ilar situ atio n should be considered w ith tesserae on Venus. For exam ple, there is no way o f know ing the age relatio n betw een M anzan-G urm e tesserae and tesserae elsewhere on the planet.
Ridge belts
W ithin the global stratigraphy m odel, d efo rm atio n al p ro cesses resp o n sib le from the p roduction of rid g e b elts are co n sid ered by B a sile v sk y and H ead to be c o n fin ed to the L a v in ia P e rio d , preceding the R usulka regional plains form ing period. R idge belts w ith in th e stu d y a re a do n o t co n fo rm to th is s tra tig ra p h ie re s tric tio n . E x am p le s b o th p re and p o std a te re g io n a l p la in s m aterials. Z orile D orsa (Figure 3.19) is confined to lin eated and m o ttled p lain s m aterials, w hile the rid g e b e lt lo cated at 38.5^ N, 3 2 9. 5 0 E, (Figure 3.23) postdates regional and hom ogeneous plains materials. Hence, the formation of ridge belts has been repetitive.
Wrinkle ridges
A n o th er global co m p ressio n al ev en t p ro p o sed by B a s i l e v s k y a n d H e a d (1995a, 1995b), H e a d a n d B a s i le v s k y (1 9 9 6 ), and
B a s ile v s k y et al. (1 9 9 7 ) is re p r e s e n te d by w rin k le rid g e stru c tu re s, w hich are claim ed to be c o n fin e d to the R u su lk a P e rio d . As w ith rid g e b e lts, o b se rv a tio n s fro m u n its w ith in G uinevere and Sedna P ian itiae suggest m ore than one p erio d o f w rin k le rid g e fo rm atio n (F igures 3.15 and 3.16). L in eated and m o ttled p lain s m ateria ls co n tain a set o f o ld er w rin k le rid g es w hich do n o t deform the younger regional plains {Copp and Guest,
1995); hence a period o f w rinkle ridge form ation took place p rio r to regional plains emplacement, before the Rusulka Period.
F low m a te ria ls b elo n g in g to the A tla P e rio d w h ich are superim posed on the reg io n al plains are co n sid ered n o t to have e x p e rie n c e d w rin k le rid g e d e fo rm a tio n { B a sile v s k y a n d H e a d
1995a, 1995b; Basilevsky et al., 1996, 1997). Y et w rinkle ridges defo rm flow u n its o f N eago F lu ctu s (F ig u res 3.38 and 3.3 9 ), m aterials to the NE o f an unnam ed corona at 9.5^ N, 315.05® B, and flows at 04®S, 311®E, which originate from a central volcano. O ther w ork has also show n that w rinkle ridge form ation w as not g lo b a lly c o n te m p o ra n e o u s . M c G i l l (1993) fo u n d th at w rin k le rid g es have d ifferen t d istrib u tio n and in te rse c tio n p a ttern s on a g lo b al scale. T his o b serv atio n su ggests d iffe re n t reg io n a l stress regimes in the crust occurred at different times.
Coronae
P erh ap s the g re a te st (in co rrec t) a ssu m p tio n o f th e g lo b al stra tig ra p h ie m o d el rela te s to the fo rm a tio n o f c o ro n a e and asso ciated structure and volcanism . R ecall from C h ap ter 2 (2.3) th at an ‘id e a lise d ’ sequence of corona d ev elo p m en t is su g g ested by B asilev sk y and H ead, w ith the v ario u s stages o f fo rm atio n (rep resen ted by the sam e structure at each corona) o ccu rrin g at the sam e tim e th roughout the history o f V enus. O b serv atio n s o f coronae w ithin the V31 m apping quadrangle tell a very d ifferen t story and show that coronae can have a pro tracted history; stages o f corona form ation are not specific to any p erio d o f geo lo g ical time (Chapter 4).