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It Has Long Been Known to Geologists That Below The BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA VOL. 43. PP. 353-382, 4 FIGS. JUNE 30. 1932 INSTJLAR ABCS, FOBEDEEPS, AND GEOSYNCLINAL SEAS OF THE ASIATIC COAST * BY ANDREW C. LAWSON (Read before the Geological Society December 29, 1931) CONTENTS Page Constitution of the earth’s crust.................................................................................. 353 Sima and sial............................................................................................................ 353 Layered structure of the crust..............................................................................356 Derivation of sial from sima................................................................................. 357 Heterogeneities and irregularities of the sial.............................................................. 360 Fragmentation of continents.........................................................................................362 Isostatic rejuvenation..................................................................................................... 364 Thickness of sial and its constituent layers........ ...................................................364 Determination of the thickness...........................................................................364 Relation of thickness to continental relief......................................................... 365 Underthrust due to landward creep of sima..............................................................366 Origin of arcs....................................................................................................................367 Continental rocks of insular arcs..................................................................................368 Earlier recognition of underthrusting..........................................................................369 Tectonic effects of underthrust.....................................................................................370 General discussion..................................................................................................370 Foredeeps in front of the arcs...............................................................................370 Isostatic balance of foredeeps.......................................................................................371 Geosynclinal seas behind the arcs................................................................................373 Substitution of heavier rock for lighter below the seas........................................... 374 Malaysian arc.................................................................................................................. 377 Bonin-Marianne-Pelew arc............................................................................................379 C onstitution of t h e E a r t h ’s Cr u st SIMA AND SIAL It has long been known to geologists that below the granitic rocks, which are very widely, spread over the continental surfaces, there is an even more widely spread, and probably universal, layer of basaltic rock. The evi­ dence on which this knowledge is based consists of (1) the innumerable dikes of diabase which cut the granites in all parts of the world, and (2) the universality of basaltic volcanoes. The dikes, such for example as are so common in the Archean terranes of Canada, are narrow, usually not * Manuscript received by the Secretary of the Society December 19, 1931. (353) XXIII—B d l l . G e o l . S o c. A m ., V o l. 43, 1932 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/43/2/353/3414943/BUL43_2-0353.pdf by guest on 25 September 2021 354 A. C. LAWSON----INSULAR ARCS, FOREDEEPS AND SEAS exceeding 150 feet in width. Individual dikes can rarely be followed for more than a few miles; but zones of dikes continue in the same general course for scores of miles. The dikes occupy vertical gashes in the gran­ ites, which are probably large-scale shrinkage cracks due to loss of volume by cooling. The dikes were in all probability at one time associated with surface manifestations of vulcanism which have been swept away by erosion. • The volcanoes of the world, both of the past and the present, are preponderatingly basaltic. The supplies of basaltic lava which they have emitted at the land surface can only have come up through vents travers­ ing the general granitic foundation of the continents. The vast outpour­ ings of the Deccan, Columbia River Plateau, Paraguay, Abyssinia, the north Atlantic, and the Karroo, testify to the extent and homogeneity of the subgranitic basalt. It is this layer which Suess named the sima as dis­ tinguished from the more superficial and lighter rocks of the crust which he called the sial. Geologists have also been cognizant of the fact that while these two layers, the granitic and basaltic, or sial and sima, certainly exist in the continental areas, there is no evidence of the presence of the sial beneath the great oceanic basins. The only known rocks in the latter are the basalts of the volcanic islands; and while the volcanoes of the land have ejected many fragments of the sial torn from their vents, those of the ocean have, so far as is known, produced no fragments of the sial included either in their lavas or in their ejecta. Comes now seismology, the youngest of the geological sciences, to tell us more of the sial and the sima. When stress difference in the elastic, relatively rigid crust exceeds the strength of the crust the stress is relieved by sudden rupture, or by sudden slipping on an old rupture, overcoming frictional resistance. In either case the sudden discharge of elastic stress generates two waves which are propagated through the earth with unequal velocities even in the same medium. In the swifter or longitudinal waves the movement is in the direction of propagation, while in the slower or deformational waves the movement is transverse to that direction; but each of these waves has its own velocity varying with the density and elasticity of the rocks through which it is propagated. The time of arrival of the waves is automatically recorded on delicate seismographs at many stations, so that their velocity can be measured. These waves are subject, moreover, to reflection and refraction at the contact surfaces of different kinds of rocks; and their path is usually not straight, owing to the intervention of such contact surfaces, as well as to a gradation of density and elasticity within the same rock mass. Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/43/2/353/3414943/BUL43_2-0353.pdf by guest on 25 September 2021 CONSTITUTION OF THE EARTH’S CRUST Figure 1.—Eastern and southeastern Coast of Asia Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/43/2/353/3414943/BUL43_2-0353.pdf by guest on 25 September 2021 356 A. C. LAWSON----INSULAR ARCS, FOREDEEPS AND SEAS LAYERED STRUCTURE OF THE CRUST Now seismologists have observed that seismic surface waves which reach their stations along paths under the Pacific have a higher velocity than similar waves under the continents. The difference in velocity corre­ sponds to that between propagation in basalt and in granite; and they have confidently inferred that there is no granite layer beneath the main basin of the Pacific, but that its waters rests directly on the surface of the sima. This conclusion is significant from the point of view of the geologist in his ¿peculations as to the constitution of the earth’s crust and its his­ tory as inferred from its make-up. Similarly from a study of the veloci­ ties and paths of waves under the continents seismologists have reached the conclusion that the sial rests upon the sima at moderate depths. The sima of the oceanic regions appears to consist of at least two layers: a basalt of small thickness resting on a heavier rock, peridotite, of great thickness; and the same condition probably prevails beneath the sial of the continent. The sial is probably also twofold in constitution; but for the continents there is not unanimity of opinion as to the nature and relations of the crustal layers. Jeffreys 1 recognizes a surface layer of granite, a deep layer of peridotite (dunite) and an intermediate layer of glassy basalt or tachylyte, of the same composition as the sima of the oceanic regions but differing in its physical state. Gutenberg 2 recognizes four layers distinguishable in central Europe by the difference in velocity of seismic waves. Mohorovicic 3 has a heterogeneous sial composed of granite, granodiorite, and syenite succeeded in depth by a layer of diorite and gabbro (basalt) resting on pyroxenite and peridotite at 60 kilometers. Adams and Williamson4 conclude that there is an outer layer 60 kilo­ meters thick of granite grading down into something more basic than gabbro (basalt) and resting on a shell of peridotite (dunite) extending in depth to 1,600 kilometers. Daly5 advocates a sial of granite and granodiorite underlain by basalt part of which is in a glassy state. In the present paper the view is expressed that the sial comprises a surface layer of granite and a lower of diorite, and that the sima beneath it is similar to that under the ocean, consisting of basalt resting on peridotite (dun- 1 The earth, 2d ed.. 1929, p. 116. 2 Von B. Gutenberg: Der Aufbau dei- Erdkruste in Europa. Geol. Rundschau. Bd. 19, 1928. Heft 6. 3 S. Mohorovicic : Ueber Nahbeben and iiber die Konstitution des Erd—und Mondin- nern. Gerland Beitrag. z. Geophys. 17, 1927, pp. 180-231. * The composition
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