Introduction to orogens

Orogens and orogenesis

Orogens before the plate-tectonic revolution

We review older ideas about orogens because they have left their mark in the terminology of mountain belts.

Zonal subdivision of orogens

Work within mountain ranges led to the recognition that sedimentary successions were very varied. In the Mesozoic of the Alps, different facies were found at the same times in adjacent areas. Deep water belts were separated by shallow-water belts. Hence the Alps were divided into a series of zones - sometimes called isopic (same facies) zones.

Internal and external zones

A traverse across the Alps shows a succession of zones. In the W and N, the Mesozoic rocks are mostly shallow-water limestones and deeper water pelagic sediments. These zones are referred to a external zones.

South and east of a line called the Penninic Front, there are mixed successions of mafic volcanic rocks and sediments, together with mafic-ultramafic associations called ophiolites. These zones are called internal zones.

In the Alps, most of the big thrust sheets appear to have been transported from SE to NW, from the internal zones toward the external zones.

http://en.wikipedia.org/wiki/Geology_of_the_Alps#mediaviewer/File:Alps_geology_map_en.jpg

This internal-external zonal subdivision was extended to other orogens, notably the Appalachians and the Cordillera in North America. In the Appalachians the thrust sheets appear to have moved west relative to the stable continental craton. In the Cordillera they appear to have been thrust east.

In both the Appalachians and the Cordillera this terminology can be a little confusing: the "internal" zones are located on the edges of North America, and the "external" zones are on the side of the orogen away from the ocean!

Geosynclines

In the early history of stratigraphy, in the 19th century, it was realized that stratigraphic successions in mountain ranges were often much thicker, or of much deeper water facies, than equivalent successions on undeformed continental cratons.

This gave rise to the idea that mountain belts originated in extended linear subsiding troughs, which were termed geosynclines.

The geosyncline theory was non-actualistic. 19th and early 20th century geologists had difficulty pointing to modern geosynclines, although deep ocean trenches were suggested as a candidate.

Paired geosynclines

Examination of the characteristics of internal and external zones led to the idea that geosynclines had a paired structure of parallel troughs.

The external miogeosyncline (meaning 'moderate geosyncline') contained mainly non-volcanic sedimentary successions.

The more internal eugeosyncline (meaning 'good geosyncline') contained volcanics and thick successions of immature sediments like wackes.

The two were separated with a geanticline - though in many orogens, direct evidence for the geanticline was hard to find.

How plate tectonics killed the geosyncline model

In the 1960s, as plate tectonics was just getting going, Robert Deitz went looking for modern geosynclines. He suggested that what we now call passive margins, like the eastern seaboard of N America and the Gulf of Mexico, were modern analogues for geosynclines. He suggested that the median geanticlines were absent, and that the geosynclines were actually wedge-shaped accumulations of strata.

Hence he removed the 'syn' from both terms, defining miogeocline and eugeocline.

Subsequently, it was realized that passive margins were too poor in mafic igneous rocks to be equivalents of eugeo-anything. Rocks formerly interpreted as eugeosynclinal were re-interpreted as products of subduction - trenches, forearc basins, and island arcs, transported much longer distances than ever envisaged by the pioneers of geosyncline theory.

Plate tectonic interpretations of orogens

Convergent tectonic environments

From a plate tectonic perspective, we can envisage four related tectonic environments where shortening may take place.

Oceanic arc-trench systems

These are situations where a subduction zone separates two oceanic plates.

Continental margin arc-trench systems and cordilleran orogens

In other places, a subduction zone is located at an active continental margin, with the continent on the plate that is not being subducted (the upper plate). Some component of shortening affects the margin, which is therefore typically an orogenic belt that coincides generally with a volcanic arc. Such orogens may undergo deformation over a protracted period of time (it's difficult to recognize separate orogenic episodes. They are described as 'Andean' or 'Cordilleran' orogens

Continent-arc collision

If, instead, there is a continent on the subducted plate (the lower plate), but the upper plate, with the volcanic arc, is oceanic, then a continent-arc collision results.

Continent-arc collision is taking place at present day in Taiwan and at the north edge of the Australian plate, where Australia is colliding with the Banda arc

Sketch of an ancient (and rather complicated) arc-continent collision in the development of the Appalachians

Continent-continent collision

Finally, continent-continent collision may occur. Typically this will involve at least one active continental margin.

The best modern example is the Himalayas

Cycles in tectonics

J. Tuzo Wilson was probably the first author to clearly propose a link between plate movement and orogenesis in a 1966 paper entitled "Did the Atlantic close and the reopen?". Subsequently, the cycle of ocean closing, orogenesis, rifting, and formation of a new ocean became known as the Wilson cycle.

More recently, it has been realized that supercontinents existed many times during Earth history. Pangea (~300 Ma) was only the most recent of a series of perhaps 4 or 5 major supercontinents that have existed. Probably the best documented is Rodinia (~1 Ga); Rodinia assembly was responsible for the building of the Grenville Orogen of North America.

Terranes

Many of the distantly transported units that have found their way into orogens are now (since the 1980s) characterized as terranes. (Notice that in this specialized use the word is spelled differently from the common word terrain - as in terrain science, glaciated terrain, all-terrain vehicle etc).

A terrane is a fault-bounded unit within an orogen that for at least part of its history had no stratigraphic links with the adjacent units.

In some orogens, units previously described as zones are now alternatively called terranes because they have no stratigraphy in common with the neighbouring zones. Hence in the Appalachians we can speak alternatively of the Gander zone or the Gander terrane: either is correct.

On the other hand, Other orogenic zones do have clear stratigraphic links with their neighbours, and cannot be characterized as terranes. In the Appalachians the Humber zone is clearly attached to the Canadian shield (Laurentia) and so cannot be called a terrane.

Allochthons and autochthons

In many orogens it is common for most of the thrust sheets to have been displaced in roughly the same direction - from the internal zones towards the external.

Notice that we are talking in relative terms here: it's conventional in orogen geology to think about how higher slices have moved relative to lower.

Notice that this frame of reference is completely arbitrary. We could equally regard the highest thrust sheet as fixed, and think of the foreland as being stuffed underneath it by plate movement! However, the foreland-based frame of reference is deeply rooted (no pun intended) in structural terminology.

Nappes: In the Alps, allochthonous units are referred to as Nappes. The French word nappe literally means a tablecloth, and the word is applied equally to sheet-like units of rock that may be

In the Anglophone literature, the word is most usually used for a recumbent fold, especially one with a thinned or mylonitic overturned limb. However, it is important to remember that the word can be used for a range of allochthonous structures.