Plates And Plate Margins

The combination of the concept of transform faults with the hypothesis of sea floor spreading led to the construction of the theory of plate tectonics. In this theory the lithosphere is divided into an interlocking network of blocks termed plates. The boundaries of plates can take three forms (Isacks et al., 1968).

1 Ocean ridges (accretive or constructive plate margins) mark boundaries where plates are diverging. Magma and depleted mantle upwell between the separating plates, giving rise to new oceanic lithosphere. The divergent motion of the plates is frequently perpendicular to the strike of the boundary, although this is not always the case and is not a geometric necessity. In the Pacific it appears to be an intrinsic characteristic of spreading whenever a steady direction has been established for some time (Menard & Atwater, 1968).

2 Trenches (destructive plate margins) mark boundaries where two plates are converging by the mechanism of the oceanic lithosphere of one of the plates being thrust under the other, eventually to become resorbed into the sub-lithospheric mantle. Since the Earth is not expanding significantly (Section 12.3), the rate oflithospheric destruction at trenches must be virtually the same as the rate of creation at ocean ridges. Also included in this category are Himalayan-type orogens caused by the collision of two continental plates (Section 10.1), where continued compressional deformation may be occurring. The direction ofmotion of the underthrusting plate need not be at right angles to the trench, that is, oblique subduction can occur.

3 Transform faults (conservative plate margins) are marked by tangential motions, in which adjacent plates in relative motion undergo neither destruction nor construction. The relative motion is usually parallel to the fault. There are, however, transform faults that possess a sinuous trace, and on the bends of these faults relatively small regions of extension and compression are created (Section 8.2). For the time being such structural elements are ignored.

Within the basic theory of plate tectonics plates are considered to be internally rigid, and to act as extremely efficient stress guides. A stress applied to one margin of a plate is transmitted to its opposite margin with no deformation of the plate interior. Deformation, then, only takes place at plate margins. This behavior is rather surprising when it is appreciated that plates are typically only about 100 km thick but may be many thousands of kilometers in width. When plate behavior is examined in more detail, however, it is recognized that there are many locations where intra-plate deformation occurs (Gordon & Stein, 1992; Gordon, 1998, 2000), especially within the continental crust (Section 2.10.5). Zones of extension within continental rifts may be many hundreds of kilometers wide (Section 7.3). Continental transforms are more complex than oceanic varieties (Section 8.1). Orogenic belts are characterized by extensive thrust faulting, movements along large strike-slip fault zones, and extensional deformation that occur deep within continental interiors (Section 10.4.3). Within oceanic areas there also are regions of crustal extension and accretion in the backarc basins that are located on the landward sides of many destructive plate margins (Section 9.10).

Plates are mechanically decoupled from each other, although plate margins are in intimate contact. A block diagram illustrating schematically the different types of plate boundaries is presented in Fig. 5.1.

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