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.

There are few periods in the history of science that compare to the explosion of knowledge from robotic and manned exploration of the bodies of our solar system over the last 50 years. In this golden age of planetary exploration, hundreds of thousands of detailed images of the terrestrial planets, the outer planets and their icy satellites, and many asteroids and comets have been obtained by manned and unmanned spacecraft. In the near future, spacecraft already in flight will complete surveys of our innermost planet, Mercury, and provide the first high-resolution images of outermost Pluto.

In the pursuit of understanding the origins and geologic evolution of the solid bodies in the solar system, many similarities and differences have emerged in the processes that shaped their landscapes. One of the most fundamental of these processes is tectonics. The number and diversity of tectonic landforms is truly remarkable. The investigation of these tectonic landforms has stimulated an equally diverse range of studies, from the characterization and modeling of individual classes of tectonic landforms to the assessment of regional and global tectonic systems. These investigations expose the complex interplay between the forces that act on planetary crusts, both internal and external, and the mechanical properties of crustal material.

Over the past several decades, planetary tectonics has become an important component at geoscience and planetary science meetings, conferences, and workshops worldwide.

Summary

Tectonic landforms on the Moon predominantly occur on the nearside, associated directly with the lunar maria. Basin-localized lunar tectonics is expressed by two landforms: wrinkle ridges, and linear and arcuate rilles or troughs. Wrinkle ridges are complex morphologic landforms found in mare basalts, interpreted to be contractional tectonic landforms formed by thrust faulting and folding. Linear and arcuate rilles are long, narrow troughs, interpreted to be graben formed by extension, deforming both mare basalts at basin margins and the highlands adjacent to the basins. In contrast to basin-localized tectonics, landforms of the nearside are the more broadly distributed lobate scarps. Lobate scarps on the Moon are relatively small-scale asymmetric landforms that are often segmented with lobate margins. These landforms are the surface expression of thrust faults and are the dominant tectonic feature on the lunar farside. Crater density ages indicate that crustal extension associated with lunar maria ceased at ~3.6 Ga. Crustal shortening in the maria, however, continued to as recently as ~1.2 Ga. The cessation of extension may have resulted from the superposition of compressional stresses from global contraction on flexural extensional stress due to loading from the mare basalts. The lobate scarps formed less than 1 Ga and appear to be among the youngest endogenic features on the Moon. The presence of young lobate scarp thrust faults supports late-stage compression of the lunar crust.