Rheological and petrological implications for a stagnant lid regime on Venus

Abstract

a b s t r a c tVenus is physically similar to Earth but with no oceans and a hot dense atmosphere. Its near-random distribution of impact craters led to the inferences of episodic global resurfacing and a stagnant lid regime, and imply that it is not currently able to lose proportionately as much heat as Earth. This paper shows that a CO 2 -induced asthenosphere and decoupling of the mantle lid from the crust, caused by the elevated surface temperature, enables lid rejuvenation. Global hypsography implies a rate of 4 Á 0 7 0 Á 5 km² a À 1 and an implied heat loss rate of 32 Á 8 7 3 Á 6 TW, $ 90% of a scaled Earth-like rate of heat loss of 36 TW. Estimates of the rate of lid rejuvenation by plume activity -0 Á 07 to 0 Á 09 km² a À 1 -imply that ten times the number of observed plumes are required to equal this rate of heat loss. However, lid rejuvenation by convection allows Venus to maintain a stable tectonic regime, with subcrustal horizontal extension (half-spreading) rates of between 25 and 50 mm a À 1 determined from fits to topographic profiles across the principal rift systems. While the surface is largely detached from these processes, the association of rifting and other processes does imply that Venus is geologically active at the present day.

The distribution of volcanism in the Beta‐Atla‐Themis region of Venus: Its relationship to rifting and implications for global tectonic regimes

“…Relationships confirmed by this work are also consistent with the evolution from a stagnant lid regime to the hypothesized subcrustal spreading regime of Ghail []. The transition from widespread to more localized activity could result from the concentration of plumes in response to magma‐assisted rifting, a thickening lithosphere, and crustal detachment of the subcrustal lid.…”

“…First of all, the coronoids, with the exception of type 2 coronae, all show a strong association with rifts (Figures bi–biii), especially at Parga and Hecate Chasmata, which is consistent with the subcrustal lid rejuvenation model [ Ghail , ]. However, this association is less significant when only the “young” rifts are considered (Figure bi–biii), consistent with corona formation events being broadly coincident with phases of older rifting.…”

“…The underlying causes of these volcanic features are thought to be the interaction of subsurface mantle plumes or diapirs for the former [ Head et al , ; Janes et al , ; Squyres et al , ; Stofan et al , ] and magma migration to the surface for the latter [ Head and Wilson , ; Head et al , ; Crumpler and Aubele , ]. The dispersed nature of this initial phase of Venus volcanism could result from many widely dispersed small plumes impinging on the base of a globally relatively thin lithosphere in an earlier “mobile lid” regime [ Moresi and Solomatov , ] or the initiation of subcrustal lid extension [ Ghail , ], in either case causing buoyant uplift of the lithosphere. The crust then responds by gravitational collapse/flow, allowing magma to propagate up into the rifts.…”

“…Cartoons to illustrate a proposed Venusian global tectonic regime (a) before and (b) after the onset of subcrustal spreading. In the model of Ghail [], a detachment layer at the base of the crust and a CO 2 ‐induced “petrological” asthenosphere facilitate spreading and recycling of the mantle “lid.” Regions of upwelling result in lid rejuvenation and rift formation, whereas regions of downwelling may be responsible for stress‐related features such as the wrinkle ridges found to affect the regional plains occurring at localized areas of compression.…”

“…In this regime, it is assumed that there is no low‐viscosity asthenosphere, but instead, elastic, “rigid” mantle lithosphere passes with increasing depth to high‐viscosity convecting mantle, transitioning when a rheological boundary is crossed [ Solomatov and Moresi , ]. Ghail [] recently proposed a subcrustal spreading model in which a petrological transition at the crust‐mantle boundary within the “stagnant lid” lithosphere may allow these two lithospheric components to become separated at a detachment horizon. Regions of upwelling and downwelling mantle may then drive a form of plate tectonics beneath the crust, rejuvinating the lithosphere.…”