IdDCcu-3214 - NTRS - NASA

IdDCcu-3214 LARGE

VOLCANIC

RISES

SUZANNE

ity"

E. SMREKAR

Jet Propulsion

._,

Laboratory

WALTER Lunar

ON VENUS

S. KIEFER

and Planetary

Institute

and ELLEN

R. STOFAN

Jet Propulsion Venus

Large

study

the behavior

of

rises on Venus

of mantle and large

previously

University

volcanic

manifestation volcanism,

II,

upwelling,

positive

on the basis

gravity

hotspots,

Arizona

have been

of the lithosphere

known

forces.

eastern

Eisria,

Magellan

radar,

gravity

and topography

data.

for these degree Eisria

regions

and order Regio.

lithospheric signature models

Dione,

from

spherical

et al. (1996)

and manrie

behavior

of the hotspots, with Earth-like

field,

parameters

lithosphere

recognized

Eisria,

Imdr).

New

estimates

are 15-40

find a value have

been

with most studies

100 Pan. Models

of the thermal

western

harmonic

nine regions

coronae-dominated

65 to 260 km.

Phillips

of approximately thickness

(Aria, Beta),

Eistla

and Themis,

These

than previously

(Bell, ranges

Imdr,

result

Regiones, have

(central

focusing

in estimates

new

probable

identified

in the

a wider

to be 300 krn or more.

Themis),

using

the 90

and 25 km at western Numerous

the gravity

models

thinning

additional

of

and topography Convective

of the thermal

or thermal Without

been

of compensation

thickness,

on Atla or Beta Regiones.

lid convection

Eisria,

depths

km at Bell Regio,

of

and have

and eastern

of the elastic

of the thickness

range

hotspots,

The apparent

to

to the four

five

been

exhibit

abundant

opportunity

In addition

for venusian

to interpret

rises,

an important

of 30 km at Aria Regio.

proposed

of stagnant

or the surface

topographic

to mantle

Eisria,

or volcano-dominated

of their broad

and western

1997

as hotspots,

in response

central

as rift-dominated

interpreted

press,

offer

Dione,

classified

in

Hotspots

Aria, Bell, Beta,

characteristics

Press,

anomalies.

hotspots,

volcano-tectonic

Laboratory

lithosphere

infer

the

constraints,

any

of the model

fits are equally

with the volcanic based

valid.

and tectonic

on estimates

characteristics

of the elastic

1996).

The advantage

provide

an explanation

last half

billion

The thinner

lithosphere

of the hotspots.

thickness

of larger

thermal

a/so support

estimates

for the apparently

estimates

Estimates

a relatively lithospheric

modest

of geologic

consistent

of the thermal

thin lithosphere

of the thermal levels

are most

thickness activity

gradient

(Phillips

et al.

is that they on Venus

over the

years.

_TRODUCTION

Broad

regional

were identified in Pioneer

Venus,

large gravity

and radar hotspots

positive

data, which

lithosphere, Malin

gravity

1983;

topographic

McGill

et al. (1981)

of evidence

define

hotspots,

anomalies

for a broad

to indicate

upwelling

Smrekar

and Phillips

1990).

topographic shape,

(phillips

Malin

1983,

1987),

Dione

associated

and Malin 1984 CampbeU Regio

Senske

highs

(Keddie

1984;

et al. 1984), and Head

et al. 1992; Grimm

thermal

et al. 1992),

(Basilevsky

1995), eastern,

central,

and Phillips

2

1992;

in Pioneer

at the base rises

Venus

and

on their include:

(McGill

Atla

et al. 1981;

and Janle

1987;

Janle

and western

Eistla

Regio

Imdr

The

of the

(Phillips

based

1994),

to

and volcanism.

of compensation,

McGill

on

Venus

identified

Beta Regio

Bell Regio

of Pioneer

upwellings.

volcanic

found

rise

of mantle

to be hotspots depth

volcanic

rifting,

and

and were

was analogous

anomalies

for large

1) interpreted

and apparent Senske

swell,

as Beta Regio

deep

et al. 1981),

that Beta Regio

edifices

et al. 1984)

the largest

manifestation

volcanic

Campbell

on the basis

topographic

hypothesis

(see Figure

volcanism,

1983,

upwelling

or tile surface

over such features

were interpreted

Beta Regio,

argued

with large

1977;

1982; McGill

region.

of mantle

the mantle

Regio

et al. 1991a;

data.

and Malin

et al. 1980; Schaber

furthered

Major

1003 km in diameter,

radar data (Saunders

to be a result

because

characteristics

over

in the equatorial

was f'trst interpreted

terrestrial

rises,

data (Masursky

be concentrated

topography

These

in Earth-based

Venus

to generally

topographic

Regio

et al. (Senske (Stofan

et

al. 1995),andThemisRegio(Stofanet al. 1992).Otherareasexhibit somebut not all of the characteristicsof hotspots.Ulfrun Regiois anelongatedrisewith a seriesof volcanic peaksto the northeastof Atla Regiointerpretedasa hotspotby BasilevskyandHead(1988),but it hasa relatively shallowcompensationdepth(SmrekarandPhillips 1991).Asteria Regio,to the west of Beta Regio,hasvolcanicedificesanda largedepthof compensation,but it is a topographic plateauratherthanadome(SmrekarandPhillips 1990). Riseson Venusexhibit greatvariationsin their morphologicandtopographic characteristics(Stofanet al. 1989;Bindschadleret al. 1992;Senskeet al. 1992),as well astheir gravity signatures(SmrekarandPhillips 1991;Smrekar1994;Simonset al. 1994,Stofanet al. 1995). Many topographicrisesexhibit abundantvolcanism,generallyin the form of large shield volcanoesandlavaflood plains(Headet al. 1992). Otherrises,suchaswesternEistla, alsoare characterizedby small( _) le3 0

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