2.0 Tectonic Features

Many features recognized from Magellan images are reminiscent of tectonic structures on Earth . Some of these features cover regions 1,000's of square kilometres, whereas other features are narrow linear structures, that appear to be related to fratures, grabens and perhaps dykes . A classification and nomenclature of Venusian physiographic features and how they may be related to tectonism has been presented by Head et al. (1992) . Some of these features are described below:

Large tectonic forms (100 to 1,000 km) are:

Tessera (tesserae, pl.) : a terrain network consisting of two or more directions of anastamosing linear ridges and troughs. They are tectonically complex terrains, thought to be largely compressional in origin and may represent the oldest part of the crust.
Chasma (chasmate, pl.) : a broad trench or linear zone consisting of a parallel arrangement of troughs or valleys bounded by fault scarps. These zones are interpreted to reflect extensional tectonics.
Mons (Montes, pl.) : large highland provinces are termed montes ( e.g. , Maxwell Montes, Danu Montes, Akna Montes and Freyja Montes), descriptive of their mountain range-like appearance.

others features likely to be associated with volcanic and/or intrusive activity are:

Corona (coronae, pl.) : (mean diameter = 250 km ) are large circular structures whose circumference is defined by an elevated ring-like zone consisting of compressional ridges and extensional troughs. The interior part often shows evidence of volcanic activity and early fracturing. Radial graben-like structures may extend beyond the outer diameter. Topographically, the interior may be raised or depressed relative to the surrounding terrain. Corona structures likely represent the surface expression of mantle upwelling. They range from 75 to over 2000-km across and occur in groups, in chains or as isolated structures (Stofan et al., 1992). More information about named coronae and their distribution on Venus can be seen using the interactive corona atlas . More than 360 have been recognized, of which about half have been formally assigned names and 1/4 of them are presently available in this database.
Arachnoid(s) : (mean diameter = 115 km ) are circular to elliptical structures that consist of a central dome or depression, surrounded by an extensive network of radial and concentric linear features. These locally developed structures may represent the surface expression of faulting and dyke emplacment associated with magma injection at shallow depths. About 259 have been found on Venus.
Nova (novae, pl.) : (mean diameter = 190 km ) are similar to arachnoids, but are dominated by radial structures. Novae are generally centered on a domal uplift and may possibly represent the early stages of corona formation. Over 50 have been identified.

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