A quantitative M easure of the Amount of M aterials Associated with an Identifiable Source

From the suite o f units id en tified w ith in the study area, a q u a n titativ e m easure o f the percentage o f p lain s m aterials w hich can be attrib u ted to a volcanic or tectonic source w as calculated. To achieve this, the study area (G uinevere and Sedna Planitiae) C2 p h o to p ro d u cts w ere d iv id ed into 33 10® by 10® boxes. The total area o f the 33 boxes is 32.41x10^ km^ and represents 7% o f the

venusian surface, an area sim ilar to th at analysed by B a s i l e v s k y a nd H e a d (1995a, 1995b, 1996, 1997) and B asilevsky et al. (1997) in tbeir global stratigraphie m odel. In each box an estim ate o f the percentage o f m aterials w hich could be attributed to a source was noted. T he categories for sources were: edifices and edifice fields, in te rm e d ia te v o lc a n o e s (w ith a m ean d ia m e te r a p p ro x im a te ly <200 km ), larg e volcanoes, coronae, and g reat flo w s asso ciated w ith e x te n sio n a l defo rm atio n . F o r each box, the p e rc en ta g e o f m a te ria ls h id d e n by im p act c ra te r m a te ria ls and by stru c tu ra l m o d ific a tio n w as estim ated . A ll m ateria ls w hich co u ld n o t be c o n fid e n tly traced to a source w ere sum m ed and rec o rd ed as unid en tified flow m aterials (UFM ). The use o f full reso lu tio n data and sy n th e tic stereo h e lp e d to aid so u rce id e n tific a tio n . T he re su lts o f the study are p resen ted in T ab les 3.2 and 3.3 and graphically in Figure 3.41.

N e g le c tin g m a te ria ls o b sc u re d by im p a c t m a te ria ls and structures, it is show n that approxim ately 40% o f m aterials w ithin the study area can be traced to a source. The greatest contribution com es from sm all edifices and ed ifice fields, fo llo w ed by larg er volcanoes (both large shields, such as S if and G ula and sm aller central volcanoes) and coronae. The percentage here is considered to be an u n d e re stim a te since som e m ateria ls had w ell d efin ed flow m orphologies but could not be confidently traced to a source, for instance where a flow has later materials superimposed on it.

C learly the statistics o f this area m ay not be rep resen tativ e o f V enus as a w hole. T his fact m ay be illu stra te d w ith m aterials a ttrib u te d to coronae. W hile the p lain s o f G uinevere and Sedna contain som e coronae, Stofan et al. (1997) state that coronae m ost com m only occur along chasm ata system s. It is probable that in the v icin ity o f chasm ata, the degree o f plains m aterials w hich can be attrib u ted to coronae w ill be greater. T his theory is supported by a p relim in ary in v estig atio n o f the U lfrun R egio w hich contains a g reater n u m b er o f coronae. W ithin 18 10® by 10® boxes, 30% of m ateria ls in the U lfru n area (~ 150N, 225®E) o rig in a te fro m coronae (B rian, per. com m s.) and 63% o f observed m aterials are

Box No.

Lat/ Long

iCM 2VC 3LS 4e f Coronae SEx+GF %TR Data

Gap 7UFM 1 -10 350 10 10 0 15 10 0 7 15 33 2 -10 340 3 0 0 40 0 0 3 17 37 3 -10 330 3 0 0 20 0 0 1 25 51 4 -10 320 0 0 0 0 0 0 0 100 0 5 -10 310 5 0 35 5 0 0 1 10 44 6 00 350 15 20 0 5 10 0 5 3 42 7 00 340 12 0 5 10 0 0 3 15 55 8 00 330 5 7 0 10 0 0 3 25 50 9 00 320 3 0 0 0 0 0 0 80 17 10 00 310 0 0 35 0 25 0 3 25 7 11 10 350 3 0 15 10 0 0 2 3 67 12 10 340 2 5 10 10 12 0 3 10 52 13 10 330 5 0 0 5 25 0 7 3 55 14 10 320 30 0 0 0 0 0 0 30 40 15 10 310 5 0 0 50 25 0 0 5 20 16 20 350 3 10 40 3 10 0 8 5 21 17 20 340 15 15 5 10 10 0 15 15 15 18 20 330 10 0 0 25 0 0 5 3 47 19 20 320 3 10 0 12 0 0 5 30 50 20 20 310 0 0 0 28 18 0 5 0 49 21 30 350 8 0 0 20 0 0 10 5 57 22 30 340 1 10 0 30 0 0 6 15 38 23 30 330 1 0 0 30 0 0 20 0 49 24 30 320 10 0 0 5 10 0 7 28 40 25 30 310 2 10 0 7 30 0 15 3 33 26 40 350 1 0 0 10 8 15 17 2 48 27 40 340 1 0 0 5 0 45 3 10 36 28 40 330 0 5 0 30 0 1 7 2 55 29 40 320 1 0 0 25 0 0 2 30 42 30 40 310 3 15 0 20 7 0 10 1 44 31 40 300 0 0 10 20 5 20 20 2 23 32 40 290 4 0 30 7 0 30 5 0 24 33 40 280 0 0 40 8 0 25 10 0 17

^ Crater materials - percentage of area within box obscured by craters and associated materials. Underlying materials cannot be identified.

^ Central volcanoes with a mean radial flow field of approximately 100 km

^ Central volcanoes with a mean radial flow field greater than 100 km (e.g. Sif and Gula Montes). ^Edifice Fields and edifices, edifices smaller than approximately 10 km.

^ Great flows (as defined by Lancaster et al. 1995), associated with extensional structures. ^ Structures - Percentage corresponds to sum of all forms structure within box

^ Unidentified flow materials - materials to which a source could not be confidently attributed. Lat. long, corresponds to lower right band comer of each box

Table 3.2 Percentages of materials which can he associated vrith a source from 33 10^ hy 10*^ boxes covering Guinevere and Sedna Planitiae. The percentage of structure, data gap and crater materials are also given.

Crater Materials Volcanic Centres Large Shields Edifice Fields Coronae ^Extension + GF

Structure Data Gap 2UFM

Total % of materials in the 33 boxes

4.88 3.55 6.82 14.24 6.21 4.12 6.12 15.67 38.12

% after data gap subtraction

5.79 4.20 8.08 16.89 7.37 4.89 7.26 45.20

Total surface of 33 boxes (10^ km

32.408 32.408 32.408 32.408 32.408 32.408 32.408 32.408 32.408

Total surface area of materials in 33 boxes (10^ km

1.876 1.361 2.618 5.474 2.388 1.585 2.353 14.648

^ Great flows (as defined by Lancaster et al. 1995), associated with extensional structures. ^ Unidentified flow materials - materials to which a source could not be confidently attributed.

Table 3.3 Quantitative measure o f materials within Guinevere and Sedna Planitiae calculated as a percentage and surface area. Surface areas for each hex latitude were calculated using spherical trigonometry.

U)

1 0 0 75-

I

I

Figure 3.41 A shows the percentage of unidentified flow materials in each of the 33 boxes. B shows the cumulative total of fiow materials attributed to a volcanic landform seen in the 33 boxes.

seen to o rig in ate from a volcanic landform . H ence here, coronae are im p o rtan t sources o f p lain s m ateria ls. T h ere is still som e d eb ate co n cern in g a d istin c tio n b etw een v o lcan o es and co ro n ae

{Stofan et al., 1992, 1997) and so the percentage associated w ith each landform m ay depend on in d iv id u al in terp retatio n . H ow ever, the qualitative study is generally useful in accounting fo r the bulk o f m aterials w hich have an id en tified origin. The sig n ifican ce of these observations regarding stratigraphy are discussed below.

3.6 Stratigraphie Observations from Guinevere and Sedna