Tectonic landforms
Circumferential / radiating / linear lineament system
Bundles and belts of long linear features (troughs, ridges) may form in several different patterns.
There are radial (see the figure), circumferential (see the plate) and linear lineament systems.
Circumferential lineaments (Ernst et al. 2001, 2003) are troughs and fractures that curve around a round feature, most often a corona and sometimes a caldera.
Radial lineaments may be narrow fractures, wider troughs or rift valleys, or dykes (and, rarely, ridges). They radiate from a magmatic-tectonic feature. Collapse pit craters and chains of pits, or their merged troughs, may also be linked to these linear features, as may lava channels, rilles and small shield volcanoes.
There are two main theories of their origin. In the tectonic view, doming above a rising mantle diapir causes fractures, and these create a smaller share of them. In the magmatic view, they combine fractures, eruptions and collapses linked to dykes that spread radially from a shallow magma chamber. The fractures keep spreading as long as the magma pressure in the dyke is high enough (Grosfils and Head 1994).
The long, straight bundles of linear lineament systems sometimes cross one another (Ernst et al. 2003). Fracture belt zones and gridded plains are of this kind. They form either through extension of a thin surface layer or above deeper dykes.
