EL TRABAJO DE CAMPO: LOS RESULTADOS SOBRE EL

H eng-o C orona in southern G uinevere P lan itia (2.0^ N, 355.5® E) is the second largest corona on Venus w ith a m ean diam eter o f 965 km. It w as originally classified as an older, iso lated corona w ith a m edium am ount o f asso c iated v o lcan ism (Stofan et a l ,

1992, 1997). H eng-o extends from the V31 q u ad ran g le into the V43 quadrangle to the south (Greeley et a l, 1994).

H eng-o is approxim ately circular in plan view and has varying am o u n ts o f d e fo rm a tio n around its m arg in (F ig u re s 4 .1 6 and 4 .1 7 ). T he sim p lest larg e-scale rim stru ctu re is on the w estern side. H ere the large-scale rim topography is com posed o f a ridge, w hich has little elevation above the flo o r o f the corona (typically less than 600 m etres) and w hich has a narrow band (typically 12 km across) o f lin eatio n s along the sum m it. No in n er or outer trench is observed here.

The so u th ern topographic rim is a double rid g e and trough (F igures 4.18 and 4.19). The outer ridge d isp lay s an annulus of tw o sets o f in tersectin g graben (Figure 4.20). B oth sets term inate at the sum m it o f the outer rid g e and are n o t seen w ith in the troughs. The double ridge and trough to p o g rap h y and asso ciated cro ss-latticed graben annulus decrease to the w est and NW along the m argin. To the east and SW, fragm ents o f old em bayed graben and ridge structures are observed (Figure 4.17, m arked A and B, and Figure 4.21).

T he n o rth ern rim topography is d em arcated by a p ro m in en t rid g e w ith an in n er and o u ter trough (H eng-o C hasm a, F igures 4 .1 8 , 4.19 and 4 .2 2 ). A n n u lu s d e fo rm a tio n in th e fo rm o f co m p ressio n al rid g es is co n cen trated a p p ro x im ately h a lf w ay up the n o rth facin g flan k o f the ridge. C o n tin u in g up the rid g e, n a rro w er c o n cen tric rid g es are o b serv ed . T he m ain rid g e rises approxim ately 1 km above the local topography. T he crest o f the rid g e displays another fabric w hich is m ade up o f thin lin eatio n s oriented radially to the corona.

n

M

Figure 4.16 Magellan image of Heng-o and Beltis Coronae (B). The topographie rim of Heng- o deforms the local regional plains. An extensive flow fîeld lies to the west and is thought to originate from Beltis Corona (to the NW of Heng-o) and the western annulus of Heng-o. Three volcanic centres of different ages lie within the annulus of Heng-o (portion of Cl-MIDR 00N352).

H e n g - o C h a s m a t a B e l t i s

H e i l m a n

I

©

gpr Regional Plains Materials Wrinkle ridges

3 0 0 k m

Graben

Volcanic Edifices <15 km Volcanic Edifices >15 km Small scale fractures and ridges

Large scaie rim topography: Ridges Large scale rim topography: Troughs Impact Crater

Fig. 4.17, Sketch map of Heng-o and Beltis Coronae. The interior of Heng-o Corona has three distinct volcanic centres. A and B indicate areas of older deformation which may be representative of an older annulus-forming event.

6.08N, 351.82E 3.45N 348.39E iBeltis 6051 300 200 100

I

1

Î

Figure 4.18 Topographic profile of Beltis Corona and the northern and southern rim of Heng-o Coronae. Beltis Corona shows a complex interior topography. The northern rim of Heng-o displays both an outer and inner trough while the southern rim topography of Heng-o shows two inner troughs separtared hy a ridge.

7.80 N -S ) 348.00 E Planetary Radius (km) I 16051.90-6052.18 16051.63-6051.90 .35 - 6051.63 6051.07-6051.35 300 km 4.70 S 1.50 E 6050.79 - 6051.07 6050.52 - 6050.79 6050.24 - 6050.52

Figure 4.19 Topographie map of Heng-o and Beltis Coronae. Topographie profiles are marked by fines. N, S eorespond to the northern and southern seetion of Heng-o Corona and B to Beltis Corona, see Figure 4.18.

I

01.00 s

352.30 E

300 km 358.50 E

05.10 S

Figure 4.20 Image showing the southern margin of Heng-o Corona. The corona shows a marked inner and outer trough with crossing annulus structure which coincides with the outer rim topography. Figure 4.18 shows the topography along a cross-section take through this area (Cl-MIDR ).

rt _» . 'è- ' '"t~ ït , /" '' ■ i; - * . ' V \ -* •'i JW. "» ~ »Xo.^-.-.-Jl^-.-.vJw^- ^ . :* %rl?’^ AW. 00.05 N 00.07 S 350.7 E _{35 km} 351.8 E

Figure 4.21 Two phases of annulus deformation are seen at the SW rim of Heng-o Corona. The inner annulus structures represent an earlier phase of deformation, and have subsequently been embayed. The outer structures are considered to represent a later phase of annulus deformation. It is considered from this observation that coronae may have a protracted history, with more than one phase of annulus formation (C1-MIDR00N352;1 framelet 28).

07.60 N

351.50 E

300 k m 355.25 E

02.90 N

Figure 4.22 The northern rim of Heng-o Corona. The concentric wrinkle ridges form the annulus of Heng-o and are situated on the outer northern slope of the large-scale topographic rim (Heng-o Chasma). The rim topography and annulus deform the regional plains materials, hence the corona is considered relatively young (C1-MIDR.00N352;1 browse).

A num ber o f volcanic centres lie w ithin the rim o f H eng-o, including central volcanoes, and sm all shields and cones. T here are three m ajo r central volcanoes (H eng-o volcanic centres, 1, 2, and 3, labelled h v l , h v 2 and h v 3 . Figure 4.17). H v l is an edifice at lea st 100 km across w ith a 60 km d iam eter caldera deform ed by in te n se N W tren d in g graben. A sso c ia ted w ith th is v o lcan o are clusters o f sm all shields. H v l m aterials are overlain by flow s from

h v 2 (Figure 4.17) w hich itse lf is cut by N N E trending fractu res. T hese fractures form ed after the set w hich trend NW , and cut h v l

m aterials. B oth fracture sets are oriented obliquely to the rim and annulus and do not p arallel reg io n al p lain s structure. H v 3 has a p rom inent apron o f flow m aterials, the distal m argins o f w hich are superimposed on the fractures that cut the volcano hv2.

F rom o b se rv a tio n s, the stra tig ra p h y o f H en g -o in v o lv e s a n um ber o f phases o f volcanism and deform ation, b u t the b u lk o f the co ro n a appears to be com posed o f reg io n al p lain s m aterials. H eng-o volcanic centre 1 is interpreted as the oldest volcanic u n it

{ h v l . Figure 4.17). H v 2 form ed next, follow ed by h v 3 ; each phase o f v o lca n ism p ro b ab ly o v e rla p p ed in tim e. T he tw o b e lts o f d efo rm atio n w hich cut h v l and h v 2 are oblique to and do not cut the rim to p o g rap h y o f H eng-o, hence it is d iffic u lt to e stab lish their relation to Heng-o.

How do the above phases o f volcanism relate to the annulus and topographic rim form ation o f H eng-o? T he large volcano h v l

situ a te d in the SB q u ad ran t o f the c o ro n a a p p a re n tly fo rm ed d uring the early stages o f corona form ation, since it is deform ed by g rab en co nfined w ithin the corona. An age re la tio n b etw een

h v l m aterials and the annulus cannot be established since they do n o t coincide. The SW m argin o f m aterials associated w ith volcano

h v 2 appear to be topographically controlled by the rim o f H eng-o. T he w estern volcano h v 3 , appear to have flow s w hich b ank up against the western ridge, hence postdating its formation.

Relation with surrounding units

H eng-o is rela tiv e ly rare am ong co ro n ae in th at it o ccu rs d ista n t from ch asm ata or large to p o g rap h ic rises {Stofan et al.,

1997). T he re la tio n th a t the rim o f H en g -o h as w ith th e su rro u n d in g m aterials is c ritic a l in e stab lish in g its stra tig ra p h ie p o sitio n . To the north and south it is po ssib le to trace reg io n al G u in ev ere p lain s m aterials (u n it g p r . Figure 4.17) over the rim to p o g rap h y in w ard tow ards the cen tre o f the co ro n a; the rim to p o g rap h y c le arly deform s the p lain s m ateria ls, and h en ce is y o u n g er at these locations. O lder structures proxim al to the m ain rim to p o g ra p h y are c o n ce n tric and are em b ay ed by re g io n a l plain s m aterials (Figure 4.17, A and B, and F igure 4.21). T hese m aterials are in terp reted to rep resen t an e arlier annulus at H eng- o, su g g estin g that, as at Idem -K uva, m ore than one p h ase o f an n u lu s fo rm atio n has o ccu rred . W h ile som e m in o r v o lca n ism asso ciated w ith h v l postdates the regional plains, the age relatio n betw een the plains and h v l is obscured by dense fractu rin g w hich cuts both units.

T he n o rth ern rim topo g rap h y d e scrib e d above term in ates to th e N W at a sm a lle r c o ro n a . A v e ra g in g 375 k ilo m e tre s in d iam ete r, B eltis C orona (p ro v isio n al lA U nam e) is d efin e d by se m i-c irc u la r rim to p o g rap h y in the form o f a rid g e. T he rim to p o g ra p h y lack s annulus d efo rm atio n . T he flo o r o f B e ltis is u n d u latin g (Figure 4.18) and displays a num ber o f d ifferen t styles o f v o lca n ism in clu d in g cald era s and n u m ero u s sm all e d ific e s. V o lcan ic deposits extend from B eltis C orona {bf. Figure 4.17) to the N W m argin o f H eng-o. The flow s postdate the reg io n al plains m a te ria ls and are to p o g rap h ically co n tro lled by the w estern rim to p o g ra p h y o f H eng-o. H ow ever, it is ap p aren t th a t stru c tu re s a sso c ia te d w ith the SW m arg in o f H en g -o hav e cu t the flow m a te ria ls and h e n c e p o s td a te th em . T h is re p re s e n ts fu rth e r evidence for multiple stage annulus formation at Heng-o.

W h ile it is n o t p o ssib le to p rec ise ly e stab lish the e a rlie st p h a se o f the d e v elo p m e n t o f H eng-o, it is a p p a re n t th a t the p re se n t topographic rim and annulus are yo u n g er than p rev io u sly su g g e ste d {S a n d w ell a n d S chubert, 1992). As w ith the o th er

c o ro n a e in th is stu d y , H eng-o has a co m p lex stru c tu ra l and volcanic history rather than representing a discrete event in time.

The sty le o f in te rio r volcanism has rem ain ed c o n stan t o v er tim e, i.e. three phases of volcanism have each produced an edifice w ith associated flow m aterials. This is in co n trast to the evolution in volcanic style seen at other coronae (e.g. Benten Corona).

4.2.5 Silvia C o ro n a

S ilv ia C o ro n a (p ro v isio n al lA U nam e), 12.5^ N, 355.5® E (F igures 4.23 and 4.24) has a m ean diam eter o f 300 km and is ap p ro x im ately circu lar in plan view. T he corona has a p ro m in en t n o rth e rn and so u th ern to p o g rap h ic rim in the fo rm o f rid g e s (Figures 4.25 and 4.26). Low er topographic ridges are observed to the east and west; no internal or external trough is observed. The in te rio r stands h ig h er than the reg io n al p lain s and has a h ig h ly irre g u la r u n d u latin g topography (F igures 4.25 and 4.26). In terio r m aterials have a m ottled appearance in the rad ar im age. W eakly d efin ed c o n cen tric fractu res coincide w ith the n o rth ern and NW rim topography.

In c o n tra st to the o th er co ronae stu d ied h ere, no c o ro n a r e la te d v o lc a n is m is seen su p e rim p o se d on re g io n a l p la in s m a te ria l and little ev id en ce ex ists fo r any in te rio r v o lca n ism , e x c e p t fo r a few sm all (< 10 km ) c ald era s. H o w ev er, it is c o n c e iv a b le th a t an y e a rly s ta g e v o lc a n is m h a s b e e n su p e rim p o sed by the surrounding reg io n a l p lain s. No p ro m in e n t e arly in te rio r deform ation is observed. T he in te rio r is in te rp reted as o ld er p lain s m aterials w hich have been u p lifted p rio r to the fo rm atio n o f the to p o g rap h ic rim by w hich they are defo rm ed . T he reg io n a l p lain s w hich surround the co ro n a em bay the o ld er in te rn a l m a te ria ls and are also d e fo rm e d by rim to p o g ra p h y (F ig u re 4 .2 3 ). A num ber o f w rinkle rid g es w hich are p e rv a siv e th ro u g h o u t the re g io n a l p lain s are seen to cro ss p a rt o f the w estern m arg in o f Silvia and hence are presum ed to have form ed after or contemporaneously with the corona rim (McGill, 1994).

- 13

- i l

300 km

Figure 4.23 Magellan image of Silvia Corona. The interior of the corona is composed of elevated relict plains which have been embayed by the younger regional plains. Little volcanism is associated with the formation of the corona (portion of Cl-MIDR 15N352).