CALDERAS ON VENUS AND EARTH (II): COMPARISON AND MODELS OF FORMATION. A. S. Krassil-nikov1, 2 and J. W. Head2, 1Vernadsky Institute, 119991, Moscow, Russia, kras@geokhi.ru, 2Department of Geological Science, Brown University, Providence, RI, 02912, USA.

Introduction. Calderas on Venus are described as “circular to elongate depressions not associated with a well-defined edi-fice and are characterized mainly by concentric patterns of enveloping fractures” [1]. They are most common 40-80 km in diameter [1-3]. The "Catalog of Volcanic structures of Venus" [2] includes 97 calderas. We have studied topography and geol-ogy of all calderas and their surroundings by way of detailed geological mapping [3]. Our goals are to compare venusian and terrestrial caldera geology, topography and mechanism of for-mation. Methods. We used previous work for analysis of venu-sian calderas [1-3] and our review in this issue for analysis of terrestrial calderas [4].

Observations. Diameter. Venus: Average diameter of cal-deras from "Catalog" [2] is 67.8 km, standard deviation 36 km. Earth: Quaternary calderas vary widely in size (1.6-over 50 km), 94 % of examples being less than 20 km [5].

Topography. Venus: 53% of calderas are represented by simple depressions, 11% have an irregular flat shape, 6% are depressions with a very low rim (Fig. 1,4), 1% is dome-like, and 29% have topo data not sufficient for analysis. Depth of caldera depressions is usually first hundreds of meters, up to 1 km (Fig. 4). Earth: Not eroded calderas usually have a prominent topog-raphic rim and inner topographic wall (Fig. 2).

Fig. 1. Perspective view of Derceto Corona, caldera on Venus (Fig. 4), made by superposing of SAR images on the elevation model, vertical exaggeration x20, fragment of C1-MIDR.45S011, view is toward SE.

Tectonic structures. Venus: 4 sets of tectonic features are observed [3]: 1) Concentric extensional structures are observed in 63% of calderas; 2) Concentric extensional fracturing and radial compressional structures inside and/or outside of the con-centric fracturing are observed in 29%. In some cases, concen-tric ridges inside depressions are observed; 3) 7% of calderas have concentric and radial extensional fracturing; 4) 1% has radial fracturing only. Most characteristic concentric extensional fracturing of calderas is dense, fracture to fracture spacing is usual 200-300 m and less, and is located in broad belt (30-50 km wide) on the slope of depression. In some cases influence of regional stress on distribution of fracturing were identified (Fig. 4). Earth: Bounding caldera faults are mostly gravitational in nature forming by reservoir subsidence. Ring faults can accom-modate uplift as well as subsidence [6], ring dike formation have connection with faults formation. Ring faults may be verti-cal, inward and/or outward [6,7]. At many calderas regional tectonic trends have influenced the geometry of collapse to varying degree [6,7].

Associated volcanism. Venus: Calderas have no evidences of collapse of one volcano. Secondary shield volcanoes are as-

sociated with 47% of calderas, extensive lava flows with 24%, both are associated with 17% (Fig. 4); 12% show no volcanic activity. Calderas also have related pit craters, pit chains and canali (Fig. 4). Most volcanic activity has a connection with dense concentric fracturing. Earth: see summary in this issue [4].

Fig. 2. Perspective view of Crater-lake caldera (Earth), geologic map [4] is overlain on the elevation model, view toward NNW, caldera diameter is ~10.5x9.5 km, no vertical exaggeration. For legend see [4].

Fig. 3. Perspective view of Yellowstone caldera (Earth), geologic map [8] is overlain on the elevation model, view toward NNE, caldera diame-ter is ~60x42 km, no vertical exaggeration. For legend see [4].

Subsidence depth. Venus: In most cases the caldera de-pression is not totally covered by lava plains [3], and depth of subsidence is no more than 1 km, in few cases 1.5 km (Fig. 4c). Earth: Almost in all cases calderas are filled by lavas and ash deposits (Fig. 2,3), the best solution for total subsidence depths at large calderas (exclude ash deposits) are typically at least 3-4 km, sometimes more [6,7].

Resurgence. Venus: There is no structural evidence for re-surgence [3], only volcanism is observed after caldera subsi-dence (Fig. 4). Earth: Many terrestrial calderas have traces of resurgence; e.g. Yellowstone caldera has two resurgent domes at SW and NE parts of caldera interior (Fig. 3, Fig. 3 in [4]) [9].

Geodynamic position. Venus: Calderas are clustered in Atla-Beta-Themis triangle [1,2,3]. At Venus (one-plate planet) we can correlate caldera location with location of rift systems only [3]. Eighty three percents of calderas are located outside of rifts and/or fracture belts, and only 17% are located inside rifts and/or fracture belts. Earth: Shield calderas are most common at ocean hot spots such as Hawaii, Galapagos, etc. [10]. Smaller shields also occur at subduction zones (Oregon, California) and spreading centers. Most of Crater-lake type calderas occur at subduction zones, and also at extensional zones/rifts, and at both oceanic and continental hot spots [10]. Ash flow calderas are concentrated mostly at subduction zones and rifts [10].

Microsymposium 38, MS052, 2003(A4 format)

CALDERAS ON VENUS AND EARTH (II): A. S. Krassilnikov and J.W. Head

Fig. 4. Derceto Co-rona, caldera at 47°S 20°E (Venus) and geological interpreta-tion of region sur-rounding. a. SAR image (illumination from the left) portion of C1-MIDR.45S011; b. sketch geologic map of area shown in 4a; c. topographic profiles; for perspective view and profile tracks see fig. 3 and fig. 4a

Interpretation and discussion. 1) Average diameter of venusian calderas is more than twice larger than terrestrial, therefore their formation should have connection with evolution of larger magmatic reservoirs than on Earth or be due to other causes. Only a few terrestrial calderas connected with uplift of large magmatic diapirs (as Yellowstone caldera) may be com-pared in size with venusian. 2) Smaller depth of subsidence in venusian calderas is evidence for rather small volume of melt in formation of these calderas (and/or deep position of reservoir) relative to terrestrial structures, whose formation is connected with collapse of rather small magmatic reservoir inside the crust. 3) Both venusian and terrestrial calderas have similar sets of tectonic structures, but concentric fracturing of venusian calderas is much more dense and prominent in more wider area at caldera periphery. Also in venusian calderas tectonic features are observed which are not usual for terrestrial – radial com-pressional structures and sometimes radial extensional fractur-ing. Formation of these tectonic features on Venus is usually described as traces of influence of evolving magmatic diapirs [1,11-13]. 4) Most of volcanic activity of venusian calderas is connected with dense concentric fracturing, which is evidence for transport of melt to the surface through this fracturing, or for possible formation of this fracturing by ring dikes. 5) All venu-sian calderas, opposite to majority of terrestrial, have no con-nection with collapse of volcanic construct. 6) We do not see traces of resurgence in venusian calderas. 7) Most of venusian calderas have no association with rifts. Their spatial distribution with concentration in Atla-Beta-Themis region [1-3] is not dis-tinguished from distribution of coronae and arachnoids, which believed as produced by diapiric uplift [11-13]. 8) Topographi-cal shape and tectonic structures of venusian calderas is evi-dence for downsag subsidence model of these calderas forma-tion (Fig. 4 in [4]). Some calderas have asymmetric topographi-cal profile, which evidence for possible trap-door subsidence [6]. Both of these models are favor by deep magma chambers and/or small eruptive volumes [6].

Summary. Summarizing all observation and interpretation we see evidence that formation of venusian calderas occurs in different conditions than majority of terrestrial. Formation of venusian calderas appears to be related to evolution of large magmatic diapirs and small volume of pressure released melting in diapir head perhaps due to thicker lithosphere than on Earth, similar to arachnoid and coronae on Venus [11-13]. Morphology of sets of tectonic structures, topography, and also volume and style of volcanism may be evidence for formation of concentric fracturing due to downsag or trap-door subsidence and ring dike emplacement from rather small and deep magmatic reservoir on the diapir head. In this case formation of venusian calderas may be mechanically similar with formation of large cauldrons [14] and large ash flow calderas (as Yellowstone) on Earth above large magmatic diapirs [15] without previous formation of large strato- or shield volcano. The difference in style of volcanic eruption is also influenced by low likelihood of explosive erup-tion on Venus [16], that lead to possible replacement of ash deposits in terrestrial calderas (Fig. 2,3, Fig. 2,3 in [4]) by lava flows in venusian volcanic structures (Fig. 1,4).

Acknowledgements. The authors thank A.T. Basilevsky for help

and RFBR grant # 02-05-65068 for support. References. [1] Head J.W. et al. (1992) JGR, 97, 13153-13197. [2]

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