Floor‐fractured impact craters on Venus: Implications for igneous crater modification and local magmatism

Abstract

Regional tectonism and volcanism affect crater modification and crater loss on Venus, but a comparison of Venusian craters to lunar floor‐fractured craters suggests that a third style of more localized, crater‐controlled magmatism also may occur on Venus. Based on lunar models for such magmatism, Venusian crustal conditions should generally favor crater‐filling volcanism over crater‐centered floor fracturing. Nevertheless, three craters on Venus strongly resemble extensively modified craters on the Moon where deformation can be attributed to failure over large crater‐centered intrusions. Models for crater modification over igneous intrusions indicate typical magmatic pressures beneath these three craters of ∼200–300 bars and intrusion depths of the order of 1–6 km. All three craters also share common settings and low elevations, whereas craters embayed by regional volcanism preferentially occur at much higher elevations on Venus. We suggest that the style of igneous crater modification on Venus thus may be elevation dependent, with crater‐centered intrusions primarily occurring at low elevations on Venus. This interpretation is consistent with theoretically predicted variations in magmatic neutral buoyancy depth as a function of atmospheric pressure suggested by other authors.

Internal crater modification on Venus: Recognizing crater‐centered volcanism by changes in floor morphometry and floor brightness

“…Indeed, it has been noted previously that mare-floored and floor-fractured craters on the Moon show many striking similarities [Schultz, 1976]. It has also been proposed that magmatic conditions on Venus generally favor crater-centered eruptions over crater-centered intrusions [Wichman and Schultz, 1995] Volcanism does not explain all of the crater modification trends, however. First, the bright-floored craters show distinct differences in morphometry from the parabolic ejecta craters, suggesting that other processes can affect floor or crater rim diameters on Venus.…”

“…Since crater floor-fracturing seems to reflect deformation over crater-centered magmatic intrusions [Schultz, 1976;Wichman and Schultz, 1995], these similarities can be attributed simply to the effects of common controls on two closely related styles of crater modification. Indeed, it has been noted previously that mare-floored and floor-fractured craters on the Moon show many striking similarities [Schultz, 1976].…”

“…Therefore crater-centered volcanism could vary significantly with elevation. Based on the inferred variation of magma depths with altitude [Head and Wilson, 1992], crater floor volcanism is most likely at low to intermediate elevations on Venus (places where crater depths nearly equal the estimated magma depths [Wichman and Schultz, 1995]). At higher elevations the magmas are likely to be too deep (>3-4 km [Head and Wilson, 1992]) to interact directly with most crater depths (<1-2 km, [Sharpton, 1994]).…”

“…Very few craters show clear, crosscutting evidence for younger volcanism or tectonism (only -•3% and 12%, respectively, [Strom et al, 1994[Strom et al, , 1995 Schultz 1993Schultz , 1995, but these seem to be extremely rare, probably due to differences in magma buoyancy and crustal structure [Wichman and Schultz, 1995].…”

Volcanism and tectonism across the inner solar system: an overview

“…This observation indicates that shallow buoyancy zones do not occur on the Moon, and that lavas did not extrude in continuing sequences of short-duration, low-volume eruptions from shallow reservoirs. However, there is evidence that in a few locations magma may have stalled near the surface to form shallow sills or laccoliths, as possibly indicated by the formation of floor-fractured craters (Schultz 1976;Wichman & Schultz 1995) (see below).…”