Lakshmi Planum, a broad smooth plain located in western Ishtar Terra and containing two large oval depressions (Colette and Sacajawea), has been interpreted as a highland plain of volcanic origin. Lakshmi is situated 3 to 5 km above the mean planetary radius and is surrounded on all sides by bands of mountains interpreted to be of compressional tectonic origin. Four primary characteristics distinguish Lakshmi from other volcanic regions known on the planet, such as Beta Regio: (1) high altitude, (2) plateau-like nature, (3) the presence of very large, low volcanic constructs with distinctive central calderas, and (4) its compressional tectonic surroundings. Building on the previous work of Pronin, the objective is to establish the detailed nature of the volcanic deposits on Lakshmi, interpret eruption styles and conditions, sketch out an eruption history, and determine the relationship between volcanism and the tectonic environment of the region

Head, James W.

Roberts, Kari M.

English

NASA Technical Reports Server

N90-12444
37
LAKSHMI PLA_: A DISTINCTIVE HIGHLAND VOLCANIC PROVINCE; Karl M. Roberts and James W.
Head, Dept. of Geological Sciences, Brown University, Providence, R. I. 02912
Inu'oductiort- Lakshmi Planum, a broad smooth plain located in western Ishtar Terra and containing two large oval
depressions (Colette and Sacajawea). has been interpreted as a highland plain of volcanic origin. 1"5 Lakshrni is situated 3-5
km above the mean planetary radius and is sun-ounded on all sides by bands of mountains interpreted to be of compressional
tectonic origin (Fig. 1). 5-7 Four primary characteristics distinguish Lakshmi from other volcanic regions known on the
planet, such as Beta Regio: 1) high altitude, 2) plateau-like nature, 3) the presence of very large, low volcanic constructs
with distinctive central calderas (Colette and Sacajawea). and 4) its compressional tectonic surroundings. Building on the
previous work of Pronin, 5 the purpose of this study is to establish the detailed nature of the volcanic deposits on Lakshmi,
interpret eruption styles and conditions, sketch out an eruption history, and determine the relationship between volcanism and
the tectonic environment of the region. The following is revised and reprinted from a previous abstract, 13 intended as a
review for the Flagstaff Venus Geoscience Workshop. A more detailed, thorough discussion and interpretation of features and
units on Lakshrni Planum may be found in a paper to be submitted to the Journal of Geophysical Research, entitled
"Characterization and Interpretation of Lakshmi Planum, Venus: A Distinctive Highland Volcanic Province."
Observations and Interor.etations- On the basis of our detailed mapping we have compiled a province map (Fig I.) that
illustrates some of the basic characteristics and relationships in Lakshmi Planum. Ielaior shields/f.aldc.Els: Two major caldera
structures (Colette,C; Sacajawea; S) and their circumferential low-shield-forming flow deposits dominate the region. Colette is
130 x 180 kin, elongated in a N-S direction, is approximately 1-2 km deep 4'5 and is surrounded by an extensive radiating
system of flows having an average width of 15 kin, and lengths of 100-300 km. The shield structure surrounding Colette is
about 500 x 700 km in dimension and descends about 1 km from the rim to the surrounding plains. Sacajawea is a 200 x 120
km oval-shaped depression elongated in a SW-NE direction, approximately 1.5-2 km deep, and lacks the distinct radial lobate
flow patterns of Colette, although mottled deposits surrounding Sacajawea, and distinct from the undivided plains, have been
mapped extending about 300 kin from the center of Sacajawea. The Sacajawea shield structure (defined by the caldera and the
surrounding deposits) is very low (less than 1 km from the rim to the surrounding plains). On the basis of the relative
crispness and distinctiveness of the flows and structures comprising the Colette shield, it is interpreted to be younger than
Sacajawea. 4,5 Elsewhere, we have described the characteristics and relationships of Colette and Sacajawea calderas. 8
A wide range of additional smactures interpreted to be volcanic source vents have been mapped including: 1) domes and
cones, which range from 1-50 km in diameter and include small cones (<10 km diameter) scattered throughout the region;
small domes (10-15 km in diameter) sometimes containing summit depressions and apparently preferentially located in
association with strucutral features (e.g., inside Colette, along a rift associated with Sacajawea, and within the Ridged Terrain);
and low shields, up to 75 km across, almost indiscemable topographically, and containing a summit pit; 2) Diffuse halo (Fig.
1, [1]); SE from Colette is a large, dark semicircular feature about 50 km in diameter surrounded by an elongated halo of
diffuse radar-bright deposits that are apparently superimposed on the more distinct flow deposits of Colette. The diffuse
character, the lack of distinctive lobate patterns, the elongation and the superposition of the deposits suggest that they may
be of pyroclastic origin; 3) Vent Complex (Fig. 1, [2]); flanking Sacajawea to the S-SW is a very broad (-200 km diameter)
radar-bright feature that lacks distinctive topographic expression in the currently available altimetry. It appears to be a
localized center of volcanism, characterized by very mottled lobate flow deposits which become more continuous towards a
central region containing numerous volcanic craters and domes.
Plains units dominate the surface of Lakshmi and are interpr ted to be volcanic on the basis of their embayment
relationships, their flatness and uniform albedo, and their association with volcanic source vents. Undivided plains units (PU)
are characterized as smooth tow-albedo plains in which individual flow features are not seen in either Venera 15/16 or Arecibo
data. They cover a large part of Lakshmi, and may be derived from the major shields, from the numerous domes and cones, or
fTom presently buried sources. Grooved plairls (PG) occur in a single patch about 150 km to the north of Sacajawea and are
typified by very finely spaced furrows or grooves arranged in sweeping curvilinear sets trending generally N-S but frequently
curving to the NE or NW. This system of faults 5 appears to be extensional in origin, and is embayed by Sacajawea deposits
and undivided plains. Ridged Plains (PR) are defined by faint bright lineaments on plains material arranged in subparallel sets
that follow the borders of the planum or trend S-SE across it. On the basis of the ridge-like n:_ture of some of these bands,
and their association with the compressional deformation in the surrounding orogenic belts, we have interpreted them to be of
compressional origin. The parallelism of many of these ridged units with the adjacent orogenic belts, and the general tilting
of the ridged plains away from the mountains, strongly suggest that _e plains have been involved in at least the latter
phases of deformation producing the orogenic belts.
Structural features and units occur within the plains, as well as dominating the surrounding mountains. Ridged Terrain
(RT) is characterized by a very rough-textured system of ridges and grooves and is concentrated in east central Lakshmi
where, in at least one occurrence, the ridges and grooves are arranged in rhomboidal sets with the small angle subtending
about 35 ° and the bisectrix oriented about N35°W. Virtually all other units are superposed on or embay RT, and we thus
concur with Pronin 5 that this unit is representative of an episode of deformation early in the history Lakshmi. Adjoining
Sacajawea to the SE is a system of linear features interpreted by Pronin 5 to be fault scarps with characteristics similar to
graben, at least one of which contains a volcanic dome. This may be a flanking rift zone similar to those occurring on
https://ntrs.nasa.gov/search.jsp?R=19900003128 2020-03-20T00:59:02+00:00Z
38
LAKSHMI PLAN_'M: Roberts. K.M., and Head, J.W.
Hawaii. In addition, a series of domes and cones appear to be arrayed in a preferred orientation, trending E-SE along a line
connecting Colette nnd Saeajawea.
1) _: The range of deposits and structures mapped in Lakshmi Planum indicates that the region is dominated
by at lea,st three styles of volcanism: a) centralized effusive, very large low shield structures (>500 kin) with numerous long
flows and extremely large calderas 8 (100-200 kin); b) distributed effusive, with a wide range of source vents most typically
forming cones and domes in the 1-50 km diameter size range; although dispersed throughout Lskshmi, many are localized
along structural trends; on the basis of their radar characteristics, most mapped effusive deposits appear to be relatively
smooth at scales of decimeters to meters; c) possible pvroclastic, represented by the Diffuse Halo; if further mapping
confirms a pyroclasfic origin, this would imply the presence of volalile-rich magmas on Venus. 9
2) Sequence and Geolozic Histo_: Formation of the Ridged Terrain was followed by emplacement of plains which were
subsequently deformed to produce the Grooved Plains N of Sneajawea. Sacajawea was formed and its related deposits embayed
the Grooved Plains and Ridged Plains. Although brighter than those of Sn_ajawen, the deposits of the Vent Complex appear
truncated by the large caldera; we thus interpret it to be older thnn Sacajawea. Colette and its associated flows were formed
subsequent to Sacajawea. but it is not known if activity in the two structures overlaps in time. The Diffuse Halo appears to
postdate Colette. Ridged Plains appear to have formed throughout the history of Lakshroi, app_tly deforming in response
to eompressional deformation and regional tilting in the adjacent mountain ranges. Ivanov et al 1o have argued that the radar
brightness (decimeter to meter scale roughness) associated with fresh impact crater haloes is lost by a smoothing process after
about 120-250 my. If this is true, and bright units on Lakshrai have a similar roughness, then this may imply that at least
some of the volcanism occurred relatively recently.
3) Relation to Tectonic Defonnatlon: There is abundant evidence for the synchronicity of volcanism and tectonism
(elongation of calderas, rift zones adjacent to caideras, association of domes and cones with preexisting structure, Grooved
Plains, Ridged Plains of various ages and orientations, etc.). Further analysis is required to determine the sequence of
deformation in the adjacent orogenic belts and its link to Lakshmi volcanic history, but it tentatively appears that the center
of volcanism has migrated from east to west during the history of Lakshmi. 4) Distinctiveness of Lakshmi Planum: Our
studies further emphasize the unique nature of _hmi Planum in terms of its eompressional tectonic environment, altitude,
and presence of large low shields with extremely large calderas. No other such volcanic province has yet been identified on
Venus.
5) OriRin of Lakshmi Planum: On the basis of the presence of distributed regional compressional dcformation surrounding
Lakshmi Pleura, 6-7,1 !-12 the sense of tectonic transport in toward Lakshmi from the no_ 11 and the e,ast, 12 the evidence
for crustal thickening in FTeyja and Maxwell Montes in excess of several tens of kilometers, 11-12 and the topographic
elevation of the plateau itself, we interpret Lakshmi Planum to be the locus of convergence and crustal thickening, and the
volcanic activity there to be linked to melting associated with processes of convergence and crustal thickening. This model
is in conta'ast to that of Pronin, 5 who attributes the volcanism to a large hot spot upwelling below Lakshmi. and spreading
laterally to cause the surrounding deformation.
References: (1) H. Masursky, et al. (1980) JGR, 85. 8232; (2)V. Barsukov, et aL (1986) JGR, 91, D378; (3)A.
Basilevsky, et al. (1986) JGR, 91. D399; (4) E. Stofan et al. (1987) Earth, Moon., 38, 183; (5) A. Pronin (1986)
Geotectonlea, 4_,26 (in Russian); (6) Campbell et aL (1983) Science, 221. 644; (7) Crumpler et al. (1986) Geology,
]._4,1031; (8) Magee and Head (1988) Lunar Planet. Scl., XIX, 711; (9) Head and Wilson (1986) JGR, 91, 9407; (10) B.
Ivanov, et al., (1986) JGR. 91. 12413; (11) Head (1988) Lunar Planet. Scl., XIX. 467; (12) Vorder Bruegge and Head
(1988) Lunar Planet. Sci.._[, 1220, (13) Magee'_d Head (1988) Lunar Planet. Sci., xrx. 713.
330 u oo
6o!
-'- ' :, ; ,j '/If, I _;(_;,=:',,V, ,-':w,_', ,_ " • ;
,,, .... _, _ _' '_t_'_¢_L.._ ;_,, ,.,
_'; ; ,,.._',_f_J,'_ _ \ \ b_.'- ._._;:2;_'_"'.""::=V_.,
• -..'-_ //.:M._¢). o_uo m
320 ° 355 °
P. PLAINS (PU - UNDIVIDED, PR - RIDGED, PLI - GROOVED) C - COLETr E
RT - RIDGED TERRAIN S - S,t_AJAWF.A
65 °
50 °
Figure 1. Province map of
Lakshmi Planum. See text for
explanation. Lines outside of
Lakshmi Planurn indicate
structural trends in adjacent
deformed terrain.
ORIC;NAL P._GE Is
OF i_@OR QUALITY


Lakshmi Planum: A distinctive highland volcanic province

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