Suzanne E. Smrekar

Venus tectonics: An overview of Magellan observations

The nearly global radar imaging and altimetry measurements of the surface of Venus obtained by the Magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet. Many areas of Venus record a superposition of different episodes of deformation and volcanism. This deformation is manifested both in areally distributed strain of modest magnitude, such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions, as well as in zones of concentrated lithospheric extension and shortening. The common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes. Ridge belts and mountain belts, which have characteristic widths and spacings of hundreds of kilometers, represent successive degrees of lithospheric shortening and crustal thickening. The mountain belts of Venus, as on Earth, show widespread evidence for lateral extension both during and following active crustal compression. Venus displays two principal geometrical variations on lithospheric extension: the quasi-circular coronae (75–2600 km diameter) and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers. Both are sites of significant volcanic flux, but horizontal displacements may be limited to only a few tens of kilometers. Few large-offset strike slip faults have been observed, but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening. Several large-scale tectonic features have extremely steep topographic slopes (in excess of 20°–30°) over a 10-km horizontal scale; because of the tendency for such slopes to relax by ductile flow in the middle to lower crust, such regions are likely to be tectonically active. In general, the preserved record of global tectonics of Venus does not resemble oceanic plate tectonics on Earth, wherein large, rigid plates are separated by narrow zones of deformation along plate boundaries. Rather tectonic strain on Venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers. These characteristics are shared with actively deforming continental regions on Earth. The styles and scales of tectonic deformation on Venus may be consequences of three differences from the Earth: (1) The absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well-preserved. (2) A high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller-scale deformational features. (3) A strong coupling of mantle convection to the upper mantle portion of the lithosphere, probably because Venus lacks a mantle low-viscosity zone, leads to crustal stress fields that are coherent over large distances. The lack of a global system of tectonic plates on Venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on Earth.

Recent hotspot volcanism on venus from VIRTIS emissivity data

Hotspots on Venus The surface of Venus shows clear signs of volcanism, but are there active volcanoes on Venus today? The answer to this question will bear on our understanding of the planets climate evolution and interior dynamics. Using surface thermal emissivity data returned by the Venus Express spacecraft, Smrekar et al. (p. 605, published online 8 April) looked at three hotspots on Venus. These places were identified by analogy with terrestrial hotspots like Hawaii, which are believed to overlie mantle plumes and to be the most likely sites for current volcanic activity. Lava flows at the three hotspots have anomalously high thermal emissions when compared with their surroundings. Low emissivity is generally interpreted as the result of surface alteration by the corrosive atmosphere of Venus. High emissivity implies that not much alteration took place and thus that the hotspots must represent recently active volcanoes younger than 2.5 million years. Satellite observations suggest that Venus is a geologically active planet. The questions of whether Venus is geologically active and how the planet has resurfaced over the past billion years have major implications for interior dynamics and climate change. Nine “hotspots”—areas analogous to Hawaii, with volcanism, broad topographic rises, and large positive gravity anomalies suggesting mantle plumes at depth—have been identified as possibly active. This study used variations in the thermal emissivity of the surface observed by the Visible and Infrared Thermal Imaging Spectrometer on the European Space Agency’s Venus Express spacecraft to identify compositional differences in lava flows at three hotspots. The anomalies are interpreted as a lack of surface weathering. We estimate the flows to be younger than 2.5 million years and probably much younger, about 250,000 years or less, indicating that Venus is actively resurfacing.

Massive CO2 Ice Deposits Sequestered in the South Polar Layered Deposits of Mars

Radar measurements reveal a substantial buried deposit of carbon dioxide in the south pole of Mars. Shallow Radar soundings from the Mars Reconnaissance Orbiter reveal a buried deposit of carbon dioxide (CO2) ice within the south polar layered deposits of Mars with a volume of 9500 to 12,500 cubic kilometers, about 30 times that previously estimated for the south pole residual cap. The deposit occurs within a stratigraphic unit that is uniquely marked by collapse features and other evidence of interior CO2 volatile release. If released into the atmosphere at times of high obliquity, the CO2 reservoir would increase the atmospheric mass by up to 80%, leading to more frequent and intense dust storms and to more regions where liquid water could persist without boiling.

Magellan observations of Alpha Regio: Implications for formation of complex ridged terrains on Venus

Magellan images of Alpha Regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain. We examine the complex ridged terrain of Alpha Regio, using morphology and crosscutting relationships between structures to derive a sequence of tectonic events. Structures include broad (∼10–20 km wide) linear and arcuate ridges, fine-scale (<3 km wide) ridges, linear disruption zones (LDZs) up to several kilometers wide, and numerous grabens (∼5 km wide) and associated scarps and troughs. Based on their morphology, we interpret the broad and fine-scale ridges as compressional structures, possibly folds. LDZs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric. Graben and associated structures are interpreted as extensional features. They crosscut ridges and LDZs and thus appear to be the youngest structures in Alpha Regio. This sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain (and highlands on Venus in general): a hotspot model and a coldspot model. The presence of compressional features along and parallel to the margins of Alpha Regio and the lack of any high-elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling. Mantle downwelling appears to be the most likely mode of formation of the upland of Alpha Regio and is likely to be important in other highland regions, such as Ovda and Thetis regiones, which are also dominated by complex ridged terrain.

Large topographic rises on Venus: Implications for mantle upwelling

Topographic rises on Venus have been identified that are interpreted to be the surface manifestation of mantle upwellings. These features are classified into groups based on their dominant morphology. Atla and Beta Regiones are classified as rift-dominated, Dione, western Eistla, Bell, and Imdr Regiones as volcano-dominated, and Themis, eastern Eistla, and central Eistla Regiones as corona-dominated. At several topographic rises, geologic indicators were identified that may provide evidence of uplifted topography (e.g., volcanic flow features trending upslope). We assessed the minimum contribution of volcanic construction to the topography of each rise, which in general represents less than 5% of the volume of the rise, similar to the volumes of edifices at terrestrial hotspot swells. The total melt volume at each rise is approximated to be 104-106 km3. The variations in morphology, topography, and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling. Instead, the morphologic variations between the three classes of topographic rises are interpreted to indicate the varying influences of lithospheric structure, plume characteristics, and regional tectonic environment. Within each class, variations in topography, gravity, and amount of volcanism may be indicative of differing stages of evolution. The similarity between swell and volcanic volumes for terrestrial and Venusian hotspots implies comparable time-integrated plume strengths for individual upwellings on the two planets.

PULSED INFLATION OF PAHOEHOE LAVA FLOWS : IMPLICATIONS FOR FLOOD BASALT EMPLACEMENT

Abstract Dilated fractures in Hawaiian pahoehoe lava flows contain three zones that show the kinematics of inflation. The upper columnar zone forms through thermal contraction prior to inflation, the middle planar zone reflects inflation-induced tension, and the lower banded zone contains evidence of brittle and ductile deformation. The formation of the lower banded zone requires varying strain rates during fracture propagation and is best explained by a model where small pulses of lava inject beneath the cooled flow crust through a network of preferred pathways. We demonstrate via simple models of pipe flow that this inflation mechanism is incapable of producing areally extensive continental flood basalts on Earth, although it may explain related features on large Martian volcanoes.

The interaction of mantle plumes with surface thermal and chemical boundary layers: Applications to hotspots on Venus

Large volcanic swells on Venus are believed to be a manifestation of mantle upwelling, or hotspots. The study of these regions provides important information on the interior of the planet. Numerical experiments are carried out to examine the interaction of mantle plumes with the thermal lithosphere and a layer of depleted mantle, a product of pressure-release melting, using an axisymmetric finite element and finite difference code that incorporates temperature-dependent viscosity and pressure-release melting. The lithosphere is defined as a high-viscosity lid; plumes are initiated and maintained by a prescribed temperature at the base of the computational domain. The topographic uplift, the geoid-to-topography ratio, and the volume of pressure-release melt are compared to estimated values for possible hotspots on Venus to constrain the properties of plumes and the lithosphere. The effects of lithospheric thickness, depleted layer viscosity and thickness, mantle temperature, and plume temperature and duration on the surface observables are predicted. Models with a thermal lithospheric thickness of approximately 100–150 km are consistent with observations, assuming a mantle temperature of 1300°C, a maximum plume temperature of approximately 1500°C, and mantle plume durations of 150–250 m.y. To be consistent with estimated volumes of volcanics at Venusian hotspots, a significantly thinner thermal lithosphere requires a much cooler mantle; a thicker lithosphere requires a hotter mantle. Models with a depleted layer thickness of 100–250 km, in addition to a 100-km thick thermal lithosphere, predict the range of parameters found on Venus. Interpretation of large volcanic swells on Venus based on the evolutionary sequence predicted here implies that Beta, Atla, Western Eistla, and Imdr Regiones overlie either active or recently active mantle plumes. The geoid-to-topography ratio at Beta Regio is much larger than values found at other hotspots and may indicate that it is in an early stage of evolution or that the chemical or thermal boundary layers are significantly thicker. Bell, Dione, and Themis Regiones appear to represent very late stage, possibly now extinct, hotspots. An estimate of the total plume buoyancy flux for Venus is much lower than the value obtained for Earth. The small areal distribution of volcanism is consistent with a low level of ongoing resurfacing at a few active hotspots.

Density of Mars' south polar layered deposits.

Both poles of Mars are hidden beneath caps of layered ice. We calculated the density of the south polar layered deposits by combining the gravity field obtained from initial results of radio tracking of the Mars Reconnaissance Orbiter with existing surface topography from the Mars Orbiter Laser Altimeter on the Mars Global Surveyor spacecraft and basal topography from the Mars Advanced Radar for Subsurface and Ionospheric Sounding on the Mars Express spacecraft. The results indicate a best-fit density of 1220 kilograms per cubic meter, which is consistent with water ice that has ∼15% admixed dust. The results demonstrate that the deposits are probably composed of relatively clean water ice and also refine the martian surface-water inventory.

Gravitational spreading of high terrain in Ishtar Terra, Venus

Magellan altimetry measurements indicate that the mountain belts and plateau margins of Ishtar Terra have topographic slopes in the range 2–30°. Magellan radar images reveal that numerous sets of narrow, closely spaced troughs and lineations, which we interpret as graben and normal faults, are located in many of these regions of high slope. The orientation of many of these graben sets perpendicular to the downslope direction suggests that they formed as a result of gravitational spreading. In several of the mountain belts, the extensional structures are parallel to the apparent direction of shortening, consistent with the interpretation that gravitational spreading occurred while mountain building was still active. To explore the implications of this spreading for the tectonic evolution of Ishtar Terra, we model the process by means of a finite element algorithm for viscoelastic deformation in a vertical section of the crust at the margin of a plateau or broad mountain range. Flow in the crust is assumed to be governed by a nonlinear, depth-dependent rheology. We predict brittle failure and relaxation of topographic relief as functions of time, for ranges of crustal thickness (10–30 km) and thermal gradient (5–25 K/km) and for observed ranges in topographic elevation (1–6 km) and slope (1–30°). This parameter range predicts a large spread in the time scales for initial failure and significant relaxation of relief. For a crustal thickness greater than 10 km, a thermal gradient of 15 K/km or more, and average values of relief (3 km) and slope (3°), the topography relaxes to 25% of its original height in less than 10 m.y. Although initial failure, in the form of normal faulting on the highlands and margin and occasional shallow thrusting in the lowlands, is predicted to occur much earlier, values of horizontal surface strain that are likely to be observable (∼1%) do not accumulate until significant relaxation of relief begins. Given the predicted rate of topographic relaxation, either the crust of Ishtar Terra must be anomalous in some way (either much stronger than predicted by standard flow laws or very thin or cold) or the topographic relief and high slopes have been actively built and maintained until times significantly younger than the average crater retention age for Venus of 500 Ma. The large apparent depth of compensation, the abundance of volcanism, and the uncertainty in the local age favor the view that mountain building in Ishtar Terra has occurred until times at least as recent as 10 Ma.

Styles of deformation in Ishtar Terra and their implications

Ishtar Terra, the highest region on Venus, appears to have characteristics of both plume uplifts and convergent belts. Magellan imagery over longitudes 330°–30°E indicates a great variety of tectonic and volcanic activity, with large variations within distances of only a few 100 km. The most prominent terrain types are the volcanic plains of Lakshmi and the mountain belts of Maxwell, Freyja, and Danu. The belts appear to have marked variations in age. There are also extensive regions of tessera in both the upland and outboard plateaus, some rather featureless smooth scarps, flanking basins of complex extensional tectonics, and regions of gravitational or impact modification. Parts of Ishtar are the locations of contemporary vigorous tectonics and past extensive volcanism. Ishtar appears to be the consequence of a history of several 100 m.y., in which there have been marked changes in kinematic patterns and in which activity at any stage has been strongly influenced by the past. Ishtar demonstrates three general properties of Venus: (1) erosional degradation is absent, leading to preservation of patterns resulting from past activity; (2) many surface features are the responses of a competent layer less than 10 km thick to flows of 100 km or broader scale; and (3) these broader scale flows are controlled mainly by heterogeneities in the mantle. Ishtar Terra does not appear to be the result of a compression conveyed by an Earthlike lithosphere. But there is still doubt as to whether Ishtar is predominantly the consequence of a mantle upflow or downflow. Upflow is favored by the extensive volcanic plain of Lakshmi and the high geoid: topography ratio; downflow is favored by the intense deformation of the mountain belts and the absence of major rifts. Both could be occurring, or have recently occurred, with Lakshmi the most likely locus of upflow and Maxwell the main locus of downflow. But doubts about the causes of Ishtar will probably never be resolved without circularization of the Magellan orbit to obtain a more detailed gravity field.