Summary

Mars is a key intermediate-sized terrestrial planet that has maintained tectonic (and overall geologic) activity throughout its history, and preserved a record in rocks and terrains exposed at the surface. Among the earliest recorded major geologic events was lowering of the northern plains, relative to the southern highlands, possibly by a giant, oblique impact (or endogenic process) that left an elliptical basin with a thinned crust. Sitting on the edge of this global crustal dichotomy is Tharsis, an enormous elevated volcanic and tectonic bulge that rises ~10 km above the datum. It is topped by four giant shield volcanoes, and is surrounded by radial extensional grabens and rifts and concentric compressional wrinkle ridges that together deform the entire western hemisphere and northern plains. Deformation in the eastern hemisphere is more localized in and around large impact basins and volcanic provinces. Extensional structures are dominantly narrow grabens (several kilometers wide) that individually record of order 100 m extension, although larger (100 km wide), deeper rifts are also present. Compressional structures are dominated by wrinkle ridges, interpreted to be folds overlying blind thrust faults that individually record shortening of order 100 m, although larger compressional ridges and lobate scarps (thrust fault scarps) have also been identified. Strike-slip faults are relatively rare and typically form in association with wrinkle ridges or grabens.

Summary

Faults have been identified beyond the Earth on many other planets, satellites, and asteroids in the solar system, with normal and thrust faults being most common. Faults on these bodies exhibit the same attributes of fault geometry, displacement–length scaling, interaction and linkage, topography, and strain accommodation as terrestrial faults, indicating common processes despite differences in environmental conditions, such as planetary gravity, surface temperature, and tectonic driving mechanism. Widespread extensional strain on planetary bodies is manifested as arrays and populations of normal faults and grabens having soft-linked and hard-linked segments and relay structures that are virtually indistinguishable from their Earth-based counterparts. Strike-slip faults on Mars and Europa exhibit classic and diagnostic elements such as rhombohedral push-up ranges in their echelon stepovers and contractional and extensional structures located in their near-tip quadrants. Planetary thrust faults associated with regional contractional strains occur as surface-breaking structures, known as lobate scarps, or as blind faults beneath an anticlinal fold at the surface, known as a wrinkle ridge. Analysis of faults and fault populations can reveal insight into the evolution of planetary surfaces that cannot be gained from other techniques. For example, measurements of fault-plane dip angles provide information on the frictional strength of the faulted lithosphere. The depth of faulting, and potentially, paleogeothermal gradients and seismic moments, can be obtained by analysis of the topographic changes associated with faulting.

Summary

The geocentric realm of tectonics changed with the dawn of robotic exploration of the other bodies of the solar system. A diverse assortment of tectonic landforms has been revealed, some familiar and some with no analogues to terrestrial structural features. In this chapter, we briefly introduce some of the major topics in the book. The chapters review what is known about the tectonics on Mercury, Venus, the Moon, Mars, the outer planet satellites, and asteroids. There are also chapters that describe the mapping and analysis of tectonic features and review our understanding of the strength of planetary lithospheres and fault populations.

Introduction

At the most basic level, tectonics concerns how landforms develop from the deformation of crustal materials. The root of the word “tectonics” is the Greek word “tektos,” meaning builder. The building of tectonic landforms is in response to forces that act on solid planetary crusts and lithospheres. Tectonic landforms in turn provide a wealth of information on the physical processes that have acted on the solid-surface planets and satellites.

Until little more than a century ago, the study of tectonics and tectonic landforms was limited to those on the Earth. This changed in the early 1890s when G. K. Gilbert began to study the lunar surface with a telescope. He described sinuous ridges in the lunar maria and interpreted them to be anticlinal folds (see Watters and Johnson, Chapter 4). This marked the beginning of the scientific investigation of tectonic landforms on planetary surfaces.

Summary

As on Earth, other solid-surfaced planetary bodies in the solar system display landforms produced by tectonic activity, such as faults, folds, and fractures. These features are resolved in spacecraft observations directly or with techniques that extract topographic information from a diverse suite of data types, including radar backscatter and altimetry, visible and near-infrared images, and laser altimetry. Each dataset and technique has its strengths and limitations that govern how to optimally utilize and properly interpret the data and what sizes and aspects of features can be recognized. The ability to identify, discriminate, and map tectonic features also depends on the uniqueness of their form, on the morphologic complexity of the terrain in which the structures occur, and on obscuration of the features by erosion and burial processes. Geologic mapping of tectonic structures is valuable for interpretation of the surface strains and of the geologic histories associated with their formation, leading to possible clues about: (1) the types or sources of stress related to their formation, (2) the mechanical properties of the materials in which they formed, and (3) the evolution of the body's surface and interior where timing relationships can be determined. Formal mapping of tectonic structures has been performed and/or is in progress for Earth's Moon, the planets Mars, Mercury, and Venus, and the satellites of Jupiter (Callisto, Ganymede, Europa, and Io).

Summary

Mercury has a remarkable number of landforms that express widespread deformation of the planet's crustal materials. Deformation on Mercury can be broadly described as either distributed or basin-localized. The distributed deformation on Mercury is dominantly compressional. Crustal shortening is reflected by three landforms, lobate scarps, high-relief ridges, and wrinkle ridges. Lobate scarps are the expression of surface-breaking thrust faults and are widely distributed on Mercury. High-relief ridges are closely related to lobate scarps and appear to be formed by high-angle reverse faults. Wrinkle ridges are landforms that reflect folding and thrust faulting and are found largely in smooth plains material within and exterior to the Caloris basin. The Caloris basin has an array of basin-localized tectonic features. Basin-concentric wrinkle ridges in the interior smooth plains material are very similar to those found in lunar mascon basins. The Caloris basin also has the only clear evidence of broad-scale, extensional deformation. Extension of the interior plains materials is expressed as a complex pattern of basin-radial and basin-concentric graben. The graben crosscut the wrinkle ridges in Caloris, suggesting that they are among the youngest tectonic features on Mercury. The tectonic features have been used to constrain the mechanical and thermal structure of Mercury's crust and lithosphere and to test models for the origin of tectonic stresses. Modeling of lobate scarp thrust faults suggests that the likely depth to the brittle–ductile transition (BDT) is 30 to 40 km.