George E. McGill

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.

Tharsis Province of Mars - Geologic sequence, geometry, and a deformation mechanism

Abstract The early history of Mars included two large-scale events of great significance: (1) the lowering and resurfacing of one-third of the crust, followed closely by (2) evolution of the Tharsis bulge. Tharsis development apparently involved two stages: (1) an initial rapid topographic rise accompanied by the development of a vast radial fault system, and (2) an extremely long-lived volcanic stage apparently continuing to the geologic present. A deformational model is proposed whereby a first-order mantle convection cell caused early subcrustal erosion and foundering of the low third of the planet. Underplating and deep intrusion by the eroded materials beneath Tharsis caused isostatic doming. Minor radial gravity motions of surficial layers off the dome produced the radial fault system. The hot underplate eventually affected the surface to cause the very long-lived volcanic second stage. Deep crustal anisotropy associated with the locally NE-trending boundary between the highland two-thirds and the lowland one-third caused the NE elongation of many features of Tharsis.

ORIGIN OF GIANT MARTIAN POLYGONS

Extensive areas of the Martian northern plains in Utopia and Acidalia planitiae are characterized by “polygonal terrane.” Polygonal terrane consists of material cut by complex troughs defining a pattern resembling mudcracks, columnar joints, or frost-wedge polygons on Earth. However, the Martian polygons are orders of magnitude larger than these potential Earth analogues, leading to severe mechanical difficulties for genetic models based on simple analogy arguments. Plate-bending and finite element models indicate that shrinkage of desiccating sediment or cooling volcanics accompanied by differential compaction over buried topography can account for the stresses responsible for polygon troughs as well as the large size of the polygons. Although trough widths and depths relate primarily to shrinkage, the large scale of the polygonal pattern relates to the spacing between topographic elevations on the surface buried beneath polygonal terrane material. Geological relationships favor a sedimentary origin for polygonal terrane material, but our model is not dependent on the specific genesis. Our analysis also suggests that the polygons must have formed at a geologically rapid rate.

Plains tectonism on Venus: The deformation belts of Lavinia Planitia

High-resolution radar images from the Magellan spacecraft have revealed the first details of the morphology of the Lavinia Planitia region of Venus. A number of geologic units can be distinguished, including volcanic plains units with a range of ages. Transecting these plains over much of the Lavinia region are two types of generally orthogonal features that we interpret to be compressional wrinkle ridges and extensional grooves. The dominant tectonic features of Lavinia are broad elevated belts of intense deformation that transect the plains with complex geometry. They are many tens to a few hundred kilometers wide, as much as 1000 km long, and elevated hundreds of meters above the surrounding plains. Two classes of deformation belts are seen in the Lavinia region. “Ridge belts” are composed of parallel ridges, each a few hundred meters in elevation, that we interpret to be folds. Typical fold spacings are 5–10 km. “Fracture belts” are dominated instead by intense faulting, with faults in some instances paired to form narrow grabens. There is also some evidence for modest amounts of horizontal shear distributed across both ridge and fracture belts. Crosscutting relationships among the belts show there to be a range in belt ages. In western Lavinia in particular, many ridge and fracture belts appear to bear a relationship to the much smaller wrinkle ridges and grooves on the surrounding plains: Ridge morphology tends to dominate belts that lie more nearly parallel to local plains wrinkle ridges, and fracture morphology tends to dominate belts that lie more nearly parallel to local plains grooves. We use simple models to explore the formation of ridge and fracture belts. We show that convective motions in the mantle can couple to the crust to cause horizontal stresses of a magnitude sufficient to induce the formation of deformation belts like those observed in Lavinia. We also use the small-scale wavelengths of deformation observed within individual ridge belts to place an approximate lower limit on the venusian thermal gradient in the Lavinia region at the time of deformation.

Wrinkle ridges, stress domains, and kinematics of Venusian plains

Wrinkle ridges are nearly ubiquitous landforms on the plains of Venus. By analogy with similar structures on other planets, venusian wrinkle ridges are inferred to trend normal to the direction of maximum principal compression in the crust, an inference that is verified by geometrical relationships with positive and negative relief features on Venus. Because plains are the dominant terrain on Venus, wrinkle ridges provide an excellent opportunity to determine the orientations of shallow crustal principal stress trajectories over most of the planet. In most places there are two or more sets of wrinkle ridges, and commonly one of these persists over a very large area, defining a regional stress domain. Intersection relationships indicate that these domains differ in age.

Tectonic and geologic evolution of the Espanola Basin, Rio Grande Rift: Structure, rate of extension, and relation to the state of stress in the western United States

Abstract The Espanola basin of the Rio Grande rift began as a broad crustal downwarp in latest Oligocene time. Most of the basin is 2–3 km deep, but localized faulting allowed accumulation of up to 5 km of sedimentary fill in a central sub-basin. The localized early faulting ended before filling of the central Espanola basin was completed about 10 m.y. ago. Movement on faults that define the present western margin of the Espanola basin began ~ 10 m.y. ago. Jemez Mountain volcanism, in the western Espanola basin, also began at about this same time. West tilting of up to 30° occurred due to movement along pervasive N-trending intrabasin faults about 7.5 m.y. ago in conjunction with continued movement along the western border faults. Volcanism continued after this tilting, forming many of the large volcanic constructs of the Jemez Mountains. Regional uplift of the entire northern Rio Grande rift began ~ 7 m.y. ago. Movement on the Pajarito fault zone began about 5 m.y. ago and continues to the present. This fault zone defines the western margin of the velarde graben, a narrow central sub-basin where recent movement has been concentrated. Some volcanism also has occurred within the southern Velarde graben. Total extension across the Espanola basin since ~ 26 m.y. ago is estimated to have been ~ 5.5 km (roughly 10%) or between 3.5 and 8 km assuming high-angle planar faulting. The ~ 0.2 mm/yr averaged long term rate of extension has been separated into three periods of activity: 1. (1) ~ 0.14 mm/yr from 26 to 10 m.y. ago. 2. (2) ~ 0.5 mm/yr from 10 to 5 m.y. ago. 3. (3) ~ 0.14 mm/yr from 5 m.y. ago to present. A change in least principal stress direction from WSW-ENE to WNW-ESE that occurred throughout the western United States about 10 m.y. ago coincides with a roughly 3.5 times increase in the rate of extension, preferential development and movement of N- to NE-trending normal faults, and a few degrees of clockwise rotation of rocks in the western Espanola basin. Similar to the Espanola basin, initial basins of the southern Rio Grande rift were broad downwarps and rifting was greatly accelerated after ~ 10 m.y. ago. Accelerated uplift of the northern Rio Grande rift also occurred at about this time indicating that activity in the entire Rio Grande rift was modulated by this change in extension direction ~ 10 m.y. ago that appears related to Pacific-North American plate interactions. This modulation coupled with major faulting (~ 10 m.y. ago) preceding uplift (~ 7 m.y. ago) in the Espanola basin suggest a passive rifting process for the Rio Grande rift whereby stresses due to plate interactions elsewhere cause faulting in the lithosphere which leads to the development of a “passive” asthenospheric uplift. Furthermore, the roughly 20 m.y. pre-uplift history of sediment accumulation in basins of the central and southern rift, and the inherited character, trend, and geometry of the Rio Grande rift as a whole are also more consistent with a passive rifting process.

Attitude of fractures bounding straight and arcuate lunar rilles

Abstract Many straight and arcuate lunar rilles show a systematic increase in width with increase in elevation. This property may be utilized to measure the average inclinations (dips) of the two fractures bounding such rilles. Measurements on ten rilles indicate dips clustering about 60°, with a possible separate type of rille with nearly vertical bounding fractures. All measurements are consistent with a stress system characterized by vertical maximum compressive stress.

Origin of the Martian Crustal Dichotomy" Evaluating Hypotheses

Abstract Any hypothesis for the origin of the Martian global dichotomy should survive three elementary tests: (1) it must account for the observed plan shape and apparent depth of the Martian northern lowland, (2) it must be physically consistent, and (3) it must be compatible with available geological and geophysical data. At present, there are three contending types of hypotheses for the origin of the dichotomy: creation by some endogenic process or processes, creation by a single mega-impact, or creation by several overlapping large impacts. None of these hypotheses can survive all three tests without the incorporation of additional processes. The endogenic and mega-impact hypotheses require the presence of additional impact basins to explain many of the topographic details of the Martian northern lowland and of the dichotomy boundary, and the mega-impact hypothesis probably also requires extensive primordial erosion. The multiple-impact hypothesis requires an additional process or processes to account for the large portion of the northern lowland that is external to the rims of the basins inferred to be the cause of the lowland. While we believe that aspects of the multiple-impact hypothesis are required to account for some topographic details of the boundary and of the lowland, other processes appear better able to account for the dichotomy as a whole. These could include an early mega-impact, endogenic processes, or some combination of these.

Hotspot evolution and Venusian tectonic style

Because hotspots represent an important manifestation of heat loss on Venus, their geological evolution is of fundamental importance for any attempt to understand Venusian tectonics. Eistla Regio is a ∼7500-km-long, moderately elevated region inferred to overlie one or more large mantle upwellings or hotspots. It also contains many shield volcanoes and coronae believed due to the rise of thermal plumes in the mantle. Central Eistla Regio includes two large volcanoes, Sappho and Anala, and several coronae in close proximity. Detailed mapping in this region results in two conclusions of tectonic significance: (1) Sappho and Anala occur near the intersection of two major extensional deformation zones, and (2) the coronae are older than the large volcanoes. Several of the coronae occur as a chain along Guor Linea, one of the major extensional deformation zones. Stratigraphic relationships indicate that the coronae began forming very soon after the emplacement of the widespread regional plains materials. Thus Central Eistla Regio was the site of a swarm of plumes that first formed coronae and then later formed shield volcanoes. The expected result of such a swarm would be thermal thinning of the elastic lithosphere with time. However, model results, geological observations, and gravity data suggest that the change from coronae to shield volcanoes was accompanied by a thickening of the lithosphere with time. This thickening is interpreted to be the result of global cooling of the lithosphere following the most recent episode of near-global resurfacing. The global cooling must have occurred faster than local heating of the lithosphere due to the impingement of thermal plumes.

Crustal history of north central Arabia Terra, Mars

Detailed geological studies across the highland-lowland boundary in north central Arabia Terra, Mars provide constraints on the age and nature of processes responsible for a complex array of structures and landforms. The area studied is bounded by 27.5° and 47.5°N, 330° and 335°W. Highland basement was formed in Early Noachian and was extensively resurfaced in Late Noachian. Materials underlying highland plateaus were emplaced in Middle Noachian. Erosion responsible for fretted terrane and fretted channels most likely occurred in Early Hesperian but definitely prior to the middle of the Hesperian and during or after Middle Noachian. There is good evidence for the involvement of liquid water in fretted channel formation. Within the area mapped, the presence of fretted terrane correlates with evidence for extensive development of large grabens. All lowland materials were emplaced after the erosion that formed fretted terrane and channels; the earliest of these materials were emplaced in mid-Hesperian, and the youngest were emplaced in Late Amazonian. Morphological evidence supports debris flow or rock glacier processes for the formation of the youngest materials. Debris flow materials on the floors of fretted channels are much too young to be related to channel formation. The entire region exhibits a northerly tilt of ∼0.1°–0.15° that formed after deposition of the oldest lowland materials but before emplacement of Upper Hesperian and younger debris flow/rock glacier materials. The nature and ages of structures and landforms in this region are not consistent with the presence of a Late Noachian to Early Hesperian convergent plate boundary in northern Arabia Terra.