T hese m aterials are recognised as iso lated sm all p atches w ith a w eak ra d a r b a c k s c a tte r (see A p p e n d ix IV ). T y p ic a lly , the m aterials can be traced to sm all pits (< 650 m in diam eter) seen w ith full reso lu tio n data (Figure 3.35). T he lo n g est dim ension o f the m aterials is usually less than 50 km , how ever m aterials from a num ber o f pits m erge at 4.5^ N, 333.2® E (Figure 3.35). Isolated flo w m a te ria ls show no r e lie f on M a g e lla n im a g e ry . Som e m aterials are topographically controlled by w rinkle ridges, (e.g. at 3 .1 0 N 3 3 2 . 3 0 E). In the V31 q u a d ra n g le, the u n it o v e rlies h o m o g en eo u s and re g io n a l p lain s m ateria ls (F ig u re 3 .3 6 ), and lineated and mottled plains.
05.20 N
04.40 N
332.80 E 50 km 333.60 E
Figure 3.35 Example of the Isolated flow unit (/). The three radar-hright pits situated to the west of the centre of the radar-dark flows are interpreted to be the source. The flows superpose both the Guinevere regional and homogeneous plains. The flow material are interpreted to he relatively smooth owing to the weak radar backscatter, and must have been emplaced relatively rapidly with little surface disruption. The style of volcanism is thought to be analogous to eruptions from basaltic shields observed in Iceland (C1-MIDR.00N333;1 browse).
%
336.40 E
06.00 N
02.10 N
332.05 E 100 km
Figure 3.36 Isolated flow units (/) seen overlying on plains materials of southern Guinevere Planitia. Sati canali is marked in the SE portion of the image. The radar-dark, delta-shaped flow material seen at the western end of Sati is situated at a greater elevation than the canali themselves. The image also shows the backscatter difference between the regional igpr) and homogeneous (gph) plains (C 1-MIDR.00N333; 1 browse).
Interpretation
The sm all iso lated flow m aterials are co n sid ered to be the m a n ife s ta tio n o f lo ca l areas o f m eltin g , p ro d u c in g v o lc a n o e s lacking edifices. Instead, flow m aterials originate from pits w hich lie at or n ear the surrounding su rface level. The m aterials are in te rp re te d as sm ooth, ow ing to a w eak rad ar b a ck sc atter, and h e n c e th e c ru s ts o f th e se flo w s are p ro b a b ly r e la tiv e ly undeform ed. Since w rinkle ridges can control the flow m aterials, it is in ferred th at the m aterials w ere relativ ely flu id w hen erupted. T he low v isco sity o f the erupted m aterial w ould also explain the lack of edifice construction.
3.4.7 Volcanic Centre -vc, and Sif and Gula Materials -s, g
V o lc an ic cen tres (v c ) c o n stitu te m a te ria ls w h ic h o rig in a te from an ed ifice (ty p ically betw een 15 and 40 km in d iam eter), w hich m ay ex ten d up to 200 km from the source (m ean apron ex ten t is app ro x im ately 100 km ). Tw o p rom inent exam ples occur in V19; E vaki tholus (37.510 ]Sf^ 342.17® E) and Toci tholus (29.50® N , 3 5 5 . 2 9 0 E, F ig u re 22). E v ak i and T o ci m a te ria ls are su p e rim p o sed on S edna reg io n a l p lain s m ateria ls. E v ak i has a su m m it w h ic h has a n u m b er o f p its and d o m es. T w o E -W elo n g ated p its (less than 4 km in length) occur on the southern flank. The sum m it is surrounded by flow m aterials w ith a d igitate ap p earan ce, e x ten d in g up to 160 km from the c e n tra l sum m it peak.
T o ci e x h ib its a p ro m in e n t c irc u la r c e n tra l p e a k w ith tw o cen tral depressions, the sm aller (4 km in diam eter) being p artially n ested w ithin the larger (10 km in diam eter). A concentric series o f fra c tu re s w hich m easu re 35 km in d ia m e te r su rro u n d the sum m it. To the SW, a num ber o f fracture sets in tersect to form a polygonal pattern. A finer fabric radiates to the SE from the centre o f the volcano. As m entioned earlier, synthetic stereo d ata show th at flo w s from T oci have been w arped by u p liftin g since the so u th ern flow apron flow s 'uphill'. The w arping is in te rp reted to
be asso ciated the u p lift o f W estern E istla R egio (Stofan et al.,
1995). Flow m aterials extend in a sem i-radial fashion aw ay from the su m m it reach in g up to 180 km (to the SW ) and h av e a rela tiv e ly co n stan t rad ar retu rn only slig h tly g rea ter than Sedna re g io n a l p la in s m a te ria ls. A n u m b er o f v o lc a n ic c e n tre s are c o n tain ed w ith in the lin eated and m o ttled p lain s m ateria ls (see V31 m ap) ty p ified by edifices w ith a sum m it cald era and rad ial flows.
Two larger volcanic centres, S if and G ula M ontes {s and g )
o ccu r w ith in V 31. T he v o lcan o es are d e fin e d by a p ro m in e n t rad ial ex ten sio n o f flow m aterials (w ith a m ean d istan ce g reater than 300 km ) w hich originate from cen tral edifices id en tifie d in to p o g rap h ic data. The flow m aterials o v erlie the reg io n a l p lain s m aterials o f G uinevere and Sedna P lan itia. The tw o large shield v o lca n o es d isp lay com plex eru p tio n h isto rie s re c o rd e d by flow units o f varying rad ar backscatter and m orphology. A full account o f the stratig rap h y and in terp retatio n o f S if and G ula M ontes is presented in Chapter 5.
Interpretation
V o lcan ic cen tres id e n tifie d w ith in the m ap p in g q u a d ra n g le are in te rp re te d as v o lcan o es w ith a c en tral co n d u it. T he m ean diam eter o f the flow aprons associated w ith these volcan o es are m arg in ally g reater than 100 km and hence m ay be called large v o lcan o es according to the c la ssifica tio n o f H ea d et al. (1992). M u ltip le p h a se s o f m ag m a e x tru s io n an d w ith d ra w a l are c o n sid e re d resp o n sib le fo r the cald era s and su m m it stru c tu re s observed at some volcanic centres (e.g. T oci), analogous to caldera formation on Earth.
S if and Gula M ontes are interpreted as large central volcanoes asso ciated w ith the topographic highland o f W estern E istla R egio. W hile the volcanoes are considered to have form ed from volcanic c o n s tru c tio n by the e ffu sio n o f m ag m a c o n ta in e d w ith in a re se rv o ir (C h ap ter 5), the h ig h lan d on w hich they are situ ated (W estern E istla Regio) is referred to as a volcanic rise. Som e o f the to p o g rap h ic exp ressio n o f W estern E istla R egio is co n sid ered the
resu lt o f processes other than volcanic co n stru ctio n , fo r in stan ce th erm al su p p o rt {Senske et al., 1992; Stofan et al., 1995, Figure 3.37).
3.4.8 Coronae
C o ro n a e are d e sc rib e d as d o m in a n tly c irc u la r stru c tu re s consisting o f an annulus o f concentric ridges and/or fractures w ith e ith e r a to p o g ra p h ic a lly p o sitiv e o r n e g a tiv e in te rio r an d a p erip h eral rim and/or trough {Barsukov et al., 1986; H ea d et al.,
1992; Stofan et a l , 1991, 1992, 1997). N ine coronae lie w ithin the a re a d e fin e d by the V 19 and V31 q u a d ra n g le s w ith o th ers occurring elsewhere within Guinevere Planitia.
C o ro n a e c o n ta in e d w ith in the m ap p in g q u a d ra n g le s h av e various units associated w ith them including flow s, volcanoes and ed ifice field s. As coronae are o f co n sid erab le im p o rtan ce in the stratig rap h y and overall geological p ro cesses on V enus, the next c h ap ter has been set aside to p resen t the resu lts o f a study o f coronae from the V31 m apping quadrangle, two o f w hich overlap into V19 (to the north) and one into V43 (to the south). The units asso c iated w ith coronae in the m apping areas are d e scrib e d in detail there.
In te rio r u n its o f co ro n ae in c lu d e v o lc a n ic e d ific e s w ith associated flow m aterials (see H eng-o C orona, V31) edifice fields, som e o f w hich also have associated flow m aterials (e.g. B enten C orona, V 31), and plains m aterials. The age o f in terio r units varies c o n s id e ra b ly (d isc u s se d in C h a p te r 4 ). M an y c o ro n a e h a v e extensive flow m aterials associated w ith them (e.g. B enten C orona, V31) w hich can extend for 1 to 2 corona radii from the rim and/or an n u lu s ( M a g e e -R o b e rts a n d H ea d , 1993; Sto fa n a n d S m reka r,
1 9 96). In m any c a se s, th e flo w m a te ria ls m ay be fu rth e r su b d iv id ed into units based on rad ar ch arac teristics, m orphology, and sp a tia l e x te n t. Such e x te n siv e u n its are im p o rta n t w h en considering the composition and stratigraphy of the plains.
m a jo r v o lc a n o e s c o ro n a e v o lc a n ic rise s la rg e -s c a le m a n tle p lu m e risin g m a n tle d ia p ir
Figure 3.37 Different scales of melting and convection considered responsible for the formation of hot-spot related volcanism on Venus. Large shield volcanoes (left) are associated with large bodies of magma. Large-scale mantle plumes are considered responsible for volcanic rises such as Western Eistla Regio (the site of Sif and Gula). Intermediate between the two are coronae. After Stofan etal. (1992).
Interpretation
T he v o lc a n ic u n its and te c to n ic fe a tu re s a sso c ia te d w ith coronae are in terp reted as the resu lt o f in itial d iap iric upw elling and early stage volcanism , im pingem ent o f the d iap ir ag ain st the underside o f the lithosphere, and final grav itatio n al relax atio n and late stage v o lcan ism as the d iap ir co o ls and lo sses b u o y an cy
{Stofan et aL, 1991, 1997; S q u yres et aL, 1992). A d e ta ile d d e sc rip tio n o f the p ro p o sed th ree stag e m o d el o f fo r co ro n a e v o lu tio n is p re s e n te d in C h a p te r 2 ( 2 .6 .1 ) . R e g a rd in g s tra tig ra p h y , new o b se rv a tio n s fro m c o ro n a e in V31 show co n sid erab le evidence o f a protracted h isto ry ran g in g from before to after lo cal reg io n al p lain s em placem ent. A fu ll trea tm e n t o f corona evolution and stratigraphy is presented in Chapter 4.