linite flakes in tightly interlocking, monominer- alic crystals give a texture to flint clay resemb- ling that of a monomineralic igneous rock which crystallised from a magma (e.g. syenite). Flint clay in Missouri is inferred to have originated by crystallisation from a colloidal gel (‘magma’) that had a chemical composition approximating that of kaolinite. The geological environment was a karstic topography dissolved in limestone- dolomite rocks at about sea level. Clay sediment deposited in the karstic basins and sink holes was subsequently dialysed to a relatively stable chemical composition approximately that of kaolinite.

In Australia, large deposits of flint clay in the Sydney Basin originated mainly from volcanic ash as parent rock. Much of it was apparently first altered to smectite (clay) which character- istically has a ‘corn flakes’ texture, and later leached to kaolinite that retained in part the pseudomorphic texture of smectite (Fig. 23). Some of the flint clay, although lithologically similar to Missouri flint clay, is finer in grain size and lacks the ‘books’, or stacks, of crystal flakes common in the Missouri clay.

Mexico possesses some hydrothermal kaolins that are sufficiently resistant to slaking to be used in the unfired state in the manufacture of

refractories, thus qualifying technologically as flint clay. Flint clay in South Africa shows texture similar to that in Missouri. Flint clay occurs also in Israel, in southern France, in Ayrshire, Scotland, and in the USSR where it has been called ‘toasted (bread) clay’.

In summary, kaolin is most diverse in texture, geologic mode and environment of origin, and for uses in human culture. The textures, as observed in SEM, may be directly correlated with the properties and processes of origin and, in turn, related to the crystallography of the minerals in the kaolin family.

Suggestions for further reading Grim, R.E. 1953. Clay Mineralogy (2nd edn.

1968), and Applied Clay Mineralogy (1962), McGraw-Hill.

Keller, W.D. Scanning electron micrographs of kaolins collected from diverse environments of origin. I-IV, Clays and Clay Minerals, v. 24 and 25, 1976 and 1977: also Geological Society ofAmericaBulletin, v. 93, pp. 27-36, 1982.

W . D . Keller is Professor Emeritus of Geology, University of Missouri-Columbia, Columbia, USA.

A new conceDt of mountain building

A

TONY BARBER

Mountain belts are constructed over periods of hundreds of millions ofyears from unrelated fragments carried across the ocean basins from distant sites, to be accreted along continental margins. Continental collision is an incidental rather than a necessary requirement for orogeny . Palaeogeographical reconstructions using the present geographical relationships of the fragments which make up orogenic belts are doomed to failure.

T h e origin of mountain belts and the nature of orogeny (mountain building) are problems cen- tral to tectonics. Dr David Jones and his col- leagues in the United States Geological Survey, the Geological Survey of Canada and the univer- sities have, over the last few years, put forward a new synthesis of what constitutes an orogenic belt and a new mechanism for the formation of mountain chains. In the course of a mapping programme in the Western Cordillera of North America they found that it was possible to de- lineate distinct ‘terranes’. A terrane (a more common, but not universal, American variant spelling of the English ‘terrain’) is a mappable

structural entity which has a stratigraphic se- quence and an igneous, metamorphic and struc- tural history quite distinct from those of adja- cent units. Each terrane is separated from its neighbours by a structural break which may take the form of a normal fault, a reverse fault, a wrench fault, or an overthrust. On crossing this boundary to an adjacent terrane this is also found to have its own distinctive stratigraphic, igneous, metamorphic and structural history.

Using this principle Jones and his colleagues have distinguished at least 50 major terranes making up the Western Cordillera of North America from Mexico to Alaska. The rep-

116IGEOLOGY TODAY Jul-A~g 198.5

resentation of terranes on geological maps is largely a matter of map scale, so that many more separate terranes could be represented on larger scale maps (Fig. 1).

The terrane concept Many of the terranes are composed of rocks of similar ages to those in the North American Craton but are of quite different types or of different facies. Faunas in the rocks of these terranes are also distinct from those of the same age on the craton, belonging to different biogeographical provinces.

Some terranes are recognisably the result of subduction processes (e.g. the Franciscan ter- rane in California), or were formed in a forearc environment (e.g. Great Valley Sequence of California), but others are fragments of volcanic arcs, basaltic plateaus or slivers of continental crust. Occasionally terranes are separated by thin smears of ophiolitic rocks representing rem- nants of disappeared ocean floor. Jones and his colleagues have concluded that all these terranes are allochthonous to the North American conti- nent, having originated in the Pacific Basin, and have been accreted to the western margin of the continent since Triassic times.

Until the Triassic, the western margin of

North America was a passive, Atlantic type of continental margin, with a thick wedge of con- tinental shelf sediments, built out westwards since late Proterozoic times from a basement of Precambrian metamorphic rocks which form the continental shield. In the Triassic this stable situation was disrupted by the development of subduction systems along the western margin of North America, together with the arrival of terranes originating in the Pacific Basin and carried eastwards by movements of the plates occupying the basin. The process of accretion of terranes to the western margin of North America continued throughout the Mesozoic and Terti- ary, and still continues at the present day along those parts of the margin where subduction is still in progress. Continual accretion since Trias- sic times has built up a zone 500 km wide along the western margin of North America to form the Western Cordillera, in volume amounting to about a third of the whole continent.

Accretion under a compressional regime at a convergent plate margin results in the defor- mation of previously accreted materials, with thickening of the crust due to the stacking up of terranes along thrust surfaces. Terranes accreted at the original continental margin have been thrust eastwards over the craton, generating

Fig. 1. Allochthonous terranes in the Western Cordillera of North America added to the western margin of the North American craton since early Mesozoic times. Collision has caused deformation well inside the craton. The dispersed fragments of Wrangellia, with its distinctive stratigraphic sequence, are shown, as well as the direction of movement of presently active transcurrent faults.

Ophiolitic: referring to ophiolite, a distinctive association of igneous and metamorphic rocks that includes basalts, dolerites, gabbros and peridotites.

Allochthonous: a rock, or an associaton of rocks, that was formed elsewhere than in its present position.

GEOLOGY TODA Y Jul-A Ug 1 98511 17

PHILIPPINE SEA PLATE PACIFIC PLATE

Volconlc Subduction Marianas Complex

AT

,,Trench

n

Fig. 2. Seamounts (oceanic volcanoes) carried on the Pacific Plate are shown approaching the subduction zone in the Marianas Trench in this physiographic diagram. One seamount occupies the floor of the trench and is in the process of accretion into the subduction complex which already includes several seamounts scraped off the downgoing Pacific Plate at an earlier stage.

upthrusts of the basement and the formation of a foreland fold and thrust belt from the overlying continental shelf sediments.

Stromatolite: a structure produced by the trapping and binding of sediment by micro- organisms, principally blue-green algae.

Normal: at right angles (i.e. 'head on').

Wrangellia Some of the allochthonous terranes have highly distinctive stratigraphic sequences. One such, called 'Wrangellia' after the Wrangell Moun- tains in Alaska, consists of Permian shallow water carbonates, overlain by deep water de- posits and then by up to 3 km of basaltic volcanics of Triassic age, some pillowed, inter- preted as an oceanic island arc, topped again by Triassic shallow water carbonates including stro- matolites, indicating their origin in a tidal en- vironment in an arid tropical climate. Faunas in the sedimentary parts of the sequence are unre- lated to those of equivalent age in the North

Fig. 3. Oceanic ridges and plateaus carried north-westwards at rates of up to 10 cm yr-' by the movement of the Pacific Plate are destined to collide with subduction complexes from Alaska to the Philippines to form parts of orogenic belts along the margins of the continents.

American craton. Palaeomagnetic determina- tions confirm the climatic evidence that these rocks originated in a tropical environment about 10" from the equator. Known movements of the lithospheric plates occupying the Pacific Basin have been used to track the course of Wrangellia from a position in the neighbourhood of New Guinea in Triassic times to its present position in North America.

Transcurrent faulting Fragments of volcanic arc with associated sedi- ments identical to those of the Wrangellia Mountains have been identified in several ter- ranes dispersed throughout the Western Cordil- lera (Fig. 1). Palaeomagnetic determinations show that all these terranes originated within lo" of latitude of each other, and presumably once constituted a single continuous terrane. These fragments are now separated for distances of up to 2000 km. Evidently, following its accretion to the North American continent, Wrangellia was disrupted and broken into slivers along transcur- rent faults, the fragments being dispersed to reach their present locations.

Transcurrent movement along major faults is an essential component of the tectonic features of the mobile belt which forms the western margin of North America. The San Andreas fault is just one well-known example of an active transcurrent fault which is currently disrupting and dispersing already accreted terranes in the same way that Wrangellia was dispersed in the past.

Major transcurrent faults occur wherever two lithospheric plates are moving in opposite direc- tions parallel to their contact, as in California, or where plates are converging at an oblique angle. In South-east Asia, for example, the Sumatran Fault has been generated by the oblique collision of the Indian Plate with the Sunda Arc, and the Philippine Fault has formed where the Philip- pine Sea Plate converges obliquely with the Philippine Islands. A look at present day sub- duction zones and plate movements on a tectonic map will demonstrate that oblique convergence is much more common than normal convergence at the present time. Even where convergence is at present normal, any change in the direction of plate movement, well-documented from the past, would cause convergence to become obli- que. Major transcurrent faulting is therefore an essential feature of all mobile belts.

Maps of orogenic belts such as the Western Cordillera, show that they are composed of lenticular structural units, with the lenses elon- gated parallel to the trend of the orogenic belt. This characteristic structure is the result of the prevalence of transcurrent faulting which is an intrinsic feature of present day mobile belts and must also have been a feature of all orogenic belts in the past.

1lllGEOLOGY TODAY Jul-Aug 1985

The new concept The new interpretation of the structure of the Western Cordillera of North America and of the origin of the units of which i t is composed has given us a new concept for the nature of the process of mountain building with the formation and development of orogenic belts, and ulti- mately for the origin of the continental crust itself. According to this new hypothesis, orogenic belts are formed by the accretion of allochthonous terranes to a continental margin with the subduction of the oceanic plate upon which they were transported. It is probable that any structure which rises above the general level of the ocean floor, seamounts, oceanic islands, basaltic plateaus, island arcs or continental frag- ments, will be accreted to the continental margin when it reaches a subduction zone, rather than being subducted (Fig. 2 ) .

Physiographic diagrams of the ocean floor, such as those prepared by Heezen L? Tharp, or any detailed bathymetric map of part of the ocean floor, demonstrate how common such structures are. Many are small seamounts or oceanic islands, a few kilometres in extent. It is likely that the arrival of such structures at the

subduction zone has little effect outside their immediate area of impact. They may break up at the surface to form melange and at deeper levels cause compression with thrusting and folding, with perhaps the development of cleavage in previously accreted sediments, as the structure is sheared off the continuously moving plate beneath and is added to the accretionary com- plex .

Some basaltic plateaus in the Pacific Basin are thousands of square kilometres in extent (e.g. Ontong Java Plateau, south-west Pacific), and are securely anchored in the mantle with roots extending to depths of 40 km (Fig. 3). The arrival of such a block will have a much more profound effect on previously accreted material, with folding, thrusting and cleavage develop- ment extending far back into the accretionary complex through the accreted terranes, perhaps to the margins of the original continent itself, before increasing resistance causes the block to be sheared off the downgoing plate. It is easily visualised that such periodic collision events would produce the multiple deformations corn- monly recorded in older orogenic belts.

MC,ange: a body of rock comprising a mixture of fragmentsofmany different sizes embedded in a fragmented and sheared matrix.

1 Ophiollte \ AUSTRALIAN CONTINENTAL SHELF 1 0 .. . Ophiolitir melange . . . .

Foreland fold and thrust bell

Fig. 4. A Palaeogene volcanic arc collided with the northern margin of Australia in northern New Guinea during Miocene times. When the Australian continental shelf arrived at the northern dipping subduction zone the arc was thrust southwards across the Australian margin, generating a foreland fold and thrust belt in the continental shelf sediments and forming a foreland basin in which younger sediments have been deposited. The collision zone is marked by serpentinous melange and the oceanic crust which underlay the arc is represented by a major ophiolite complex. Continued north-eastward movement of Australia since collision has developed a southward dipping subduction zone in the New Guinea Trench. A second volcanic arc is about to be added to the collision complex at the eastern end of the trench.

GEOLOGY TODAY Jul-Aug 19851119

Accretion of a very large block can even have the effect of reversing the direction of subduc- tion. This appears to have taken place in north- ern New Guinea in Miocene times when the northerly moving Australian continent collided with a volcanic arc (Fig. 4). The arc was thrust across the continental margin with the develop- ment of a fold and thrust belt from the sedi- ments on the continental shelf. Northward movement of Australia has continued, and when subduction resumed, the Pacific Plate was sub- ducted southwards beneath the northern margin of New Guinea. The direction of subduction makes no difference to the eventual result, the net effect is that fragments are carried towards and eventually accreted at the continental

margin. The arrival of a block of continental dimen-

sions at an accreting margin has an even more catastrophic effect, seen in the collision of India with the sciuthern margin of Asia during the Tertiary. As a result of this collision, the Hima- layas were formed from previously accreted terranes and the collision had repercussions ex- tending well inside the Asian continent, thousands of kilometres to the north.

Continental collision, which has always played such an important part in theories for the for- mation of orogenic belts, is now seen as just one possibility in a whole range of accretionary events. The arrival of a continent at an ac- cretionary margin is an adventitious event and is

NORTHERN HIGHLANDS r

Mororion 7 5 0 Ma LAUREN TI AN C RATON Grenvillion 1000 Ma p&rmy$rou (Combro-Ordovlcion) z f i w n $pper Proterozolc) Lewisian (Archoeon- Proterozolc)

Gronuhte basement

C W Y I I I I I YN.

Combrion- L. Ordov!cion

b. Proterozoic

;ig. 5. Distinctive structural units make up the British Isles each with its own stratigraphic sequence and gneous, metamorphic and structural history. Each unit is separated from adjacent units by a major structural liscordance many of which are major transcurrent faults, with perhaps thousands of kilometres of differential novement. There is no necessary relationship between adjacent structural units, which represent allochthonous

terranes accreted to the margin of the Laurentian craton and disrupted by transcurrent faulting prior to Devonian times. At the south-east margin the Midland craton, composed of amalgamated fragments of Late Proterozoic volcanic and magmatic arcs, shows no evidence of collision but was emplaced by transcurrent movement along the Church Stretton Fault by Mid-Ordovician times. Abbreviations: Pz, Proterozoic; C, Cambrian; Ord., Ordovician; LORS, UORS, Lower and Upper Old Red Sandstone; Carb., Carboniferous. In the succession, wavy lines are unconformities, the straight line is a thrust.

120IGEOLOGY TODAY Jul-Aug 1985

not essential to the formation of a mountain belt. Collision of a large continental block has the effect of bringing orogeny to an end along that segment against which collision occurred. New- ly accreted material will be added on the far side of the continental block. Meanwhile smaller accretionary events will continue along those segments of the original accretionary margin which were not affected by the arrival of the continent.

Implications for the British Caledonides Concepts derived from the interpretation of the Western Cordillera of North America by Jones and his colleagues have immense implications for the interpretation of older orogenic belts, such as the Caledonides of the British Isles. Through changing tectonic fashions over many years, geologists have attempted to reconstruct the palaeogeography of Lower Palaeozoic times from the fragments preserved in the British Caledonides. The new concept implies that such attempts are doomed to failure.

Terranes now juxtaposed within the British Caledonides are likely to have originated far from their present position and in areas widely separated from each other. The various terranes have been transported perhaps for tens of thousands of kilometres on oceanic plates to be accreted to the south-eastern margin of North America, represented by the Lewisian, with its cover of Cambro-Ordovician shelf sediments, in the North-west Highlands of Scotland.

Even after their accretion these terranes are likely to have been disrupted and transported along transcurrent faults, perhaps for thousands of kilometres along the trend of the accretionary margin, to be brought into contact with quite unrelated terranes.

One of the problems in reconstructing the palaeogeography of the Caledonides of the British Isles (Fig. 5) is that in Britain we have only a narrow strip extending NW-SE across the strike of the orogenic belt. It is commonly found in palaeogeographic reconstructions that the pebbles in conglomerates within one terrane cannot be matched with a source area in adjacent terranes, even where current direction indicators

are unequivocal. The new concept suggests that the provenance of distinctive clam should be sought not in adjacent terranes but perhaps thousands of kilometres along strike either in the Caledonides of Scandinavia or North America.

Attempts at structural correlations through- out an orogenic belt are also likely to be unsuc- cessful if orogeny is not the result of a single catastrophic continental collison, but is due to a series of minor local collisions of varying intensi- ty as successive terranes are accreted. Deforma- tion will take place as a series of random and localised events. Disruption by later transcur- rent faulting will complicate the structural inter- pretation still further. If there was no single continental collision it follows that a search for a unique suture (e.g. the Iapetus Suture) marking the zone of impact of two colliding continents, is also illusory.

Suggestions for further reading Coney, P.J., Jones, D.L. & Monger, J.W.H.

1980. Cordilleran suspect terranes. Nature,

Jones, D.L., Silberling, N.J. & Hillhouse, J. 1977. Wrangellia - A displaced terrane in northwestern North America. Canadian Jour- nal of Earth Sciences, v. 14, pp. 2565-2577.

Jones, D.L., Cox, A., Coney P.J. & Beck, M. 1982. The growth of western North America. Scientific American, v. 247 ( 5 ) , pp. 50-64.

Nur, A. & Ben-Avraham, Z. 1982. Oceanic plateaux, the fragmentation of continents and mountain building. Journal of Geophysical Re- search, v. 87, pp. 3644-3661.

Schermer, E.R., Howell, D.G. & Jones, D.L. 1984. The origin of allochthonous terranes: Perspectives on the growth and shaping of continents. Annual Review of Earth and Planetary Science, v. 12, pp. 107-131.

Silver, E.A. & Smith, R. 1983. Comparison of terrane accretion in modern Southeast Asia and the Mesozoic North American Cordillera. Geology, v. 11, pp. 192-202.

V. 288, pp.329-333.

Tony Barber is Reader in Geology at Chelsea College, University of London.

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