GRAVITY OVER CORONAE AND CHASMATA ON VENUS

The global spherical harmonic model of Venus' gravity field MGNP60FSAA P, with horizontal resolution of about 600 km, shows that most coronae have little or no signature in the gravity field. Nevertheless, some coronae and some segments of chasmata are associated with distinct pos itive gravity anomalies. No corona has been found to have a negative g ravity anomaly. The spatial coincidence of the gravity highs over four closely spaced 300- to 400-km-diameter coronae in Eastern Eistla Regi o with the structures themselves is remarkable and argues for a near-s urface or lithospheric origin of the gravity signals over such relativ ely small features. Apparent depths of compensation (ADCs) of the prom inent gravity anomalies at Artemis, Latona, and Heng-o Coronae are abo ut 150 to 200 km. The geoid/topography ratios (GTRs) at Artemis, Laton a, and Heng-o Coronae lie in the range 32 to 35 m km(-1). The large AD Cs and GTRs of Artemis, Latona, and Heng-o Coronae are consistent with topographically related gravity and a thick Venus lithosphere or shal lowly compensated topography and deep positive mass anomalies due to s ubduction or underthrusting at these coronae. At arcuate segments of H ecate and Parga Chasmata ADCs are about 125 to 150 km, while those at Fatua Corona, four coronae in Eastern Eistla Regio, and an arcuate seg ment of Western Parga Chasma are about 75 km. The GTRs at Fatua Corona , the four coronae in eastern Eistla Regio, and the arcuate segments o f Hecate, Parga, and Western Parga Chasmata are about 12 to 21 m km(-1 ). The ADCs and GTRs of these coronae and arcuate chasmata segments ar e generally too large to reflect compensation by crustal thickness var iations. Instead, they suggest compensation by thermally induced thick ness variations in a moderately thick (approximate to 100 km) lithosph ere. Alternatively, the gravity signals at these sites could originate from deep positive mass anomalies due to subduction or underthrusting . Weighted linear least squares fits to GTR vs h (long-wavelength topo graphy) data from Heng-o and Fatua Coronae, the four coronae in easter n Eistla Regio, and the arcuate segments of Hecate, Parga, and western Parga Chasmata are consistent with compensation by thermally induced thickness variations of a dense lithosphere above a less dense mantle; the fits imply an average lithosphere thickness of about 180 km and a n excess lithospheric density of about 0.5 to 0.7%. Gravity anomalies at the arcuate segments of Dali and Diana Chasmata that form Latona Co rona, at Artemis Chasma, and other arcuate segments of Parga and Hecat e Chasmata occur on the concave sides of the arcs. By analogy with gra vity anomalies of similar horizontal scale (600 km-several thousand ki lometers) on the concave sides of terrestrial subduction zone arcs, wh ich are due in large part to subducted lithosphere, it is inferred tha t the gravity anomalies on Venus are consistent with retrograde subduc tion at Artemis Chasma, along the northern and southern margins of Lat ona Corona, and elsewhere along Parga and Hecate Chasmata. (C) 1994 Ac ademic Press, Inc.