Abyssal Fractures

or lineaments, narrow, linearly extended zones of discontinuity in rock which pierce the earth’s crust and penetrate the earth’s mantle. They can measure hundreds and thousands of kilometers in length and as much as 700 km in depth, and their width varies from several hundred meters to a few dozen kilometers. Abyssal fractures break up the earth’s crust into blocks that are noted for the nature of their movements and structure. They develop over long intervals of geological time (hundreds of millions, sometimes more than 1 billion years), and, as a major type of discontinuity of the earth’s crust, they determine the boundaries of the earth’s main structural elements. The first abyssal fractures originated at the beginning of the Proterozoic (about 2.5 billion years ago). They were identified as a special category in the 1940’s as a result of the work of such scientists as A. P. Karpinskii, V. A. Obruchev, and I. G. Kuznetsov in the USSR and of H. Cloos, R. Sonder, and H. Stille abroad. A comprehensive definition of the term “abyssal fracture” was proposed by A. V. Peive in 1945. The science of abyssal fractures has become an independent branch of geotectonics.

Abyssal fractures act as zones of heightened permeability of the earth’s crust and upper mantle, as a result of which magmatic centers arise within them (primary centers in the mantle or asthenosphere, secondary ones in the crust) and magmatic activity is concentrated there. Volcanic belts, belts where ultrabasic magma intrudes (alpine-type ultrabasites), plutons of granitoids, and ore fields are associated with abyssal fractures. The boundaries of continents, seas and oceans, and highlands are often associated with abyssal fractures. The composition, facies, and thickness of sediments are different on different sides of abyssal fractures.

Abyssal fractures are identified and studied mainly by geophysical methods, especially with the aid of deep seismic soundings.

The focuses of earthquakes are associated with the surfaces of abyssal fractures; study of the distribution of these focuses provides information on the depth to which fracturing occurs and on the dip of fracture surfaces, including those beyond the reach of seismic soundings. Based on seismological data abyssal fractures are divided into three groups: those dying out in the topmost parts of the mantle (those above the asthenosphere), those reaching depths of 100-300 km (those below the asthenosphere), and those reaching depths of 400-700 km (those in the middle mantle). The most widespread abyssal fractures are those of the first group (normal abyssal fractures). Abyssal fractures of the second and third groups occur only in shifting geosynclinal belts, and moreover, abyssal fractures of the third group (super-deep abyssal fractures) are found exclusively on the Pacific Rim.

Abyssal fractures are subdivided into four classes in terms of the nature of dominant displacements (A. V. Peive, V. E. Khain, A. I. Suvorov): (1) deep faults, (2) deep separations, (3) deep displacements, and (4) deep thrusts. Abyssal fractures of the fault type are numerous in geosynclines (at the stage in which they subside), in platforms, and on the periphery of young oceans—the Atlantic and Indian. Separations form structures of the rift type—the Baikal rift, Rhenish rift, East African rifts, and the rifts of the central oceanic ridges. They are formed where there is lateral separation, and they are accompanied by basalt flows. (In the oceans they are also accompanied by the intrusion of ultrabasites.) Deep displacements are observed in various geostructural regions, both in oceans and on continents, but they develop mostly in specific geological epochs (in geosynclines, in orogenic epochs). In relation to the strike of the shifting belts they are longitudinal, transverse, or diagonal. Deep thrusts are developed in the internal zones of geosynclinal belts and along their periphery (the ring of fractures around the Pacific Ocean). Their activity occurs during periods of orogeny.

There is a distinct pattern in the distribution of abyssal fractures over the earth’s surface: two systems of fractures in mutually perpendicular directions dominate—an orthogonal system parallel to the meridians and parallels and a system diagonal to the meridians and parallels (northwest to southeast and southwest to northeast). Some researchers differentiate one more (north-northwest to south-southeast and south-southwest to north-northeast), or two more systems (in addition, west-northwest to east-southeast and west-southwest to east-northeast). The origin of this regmatic (after Sonder) planetary grid of fractures is usually related to the tensions that arise when the speed of the earth’s rotation changes and that cause changes in its shape (increased or decreased polar flattening).