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Seismic refraction and continuous reflection measurements were made in the southern Barents Sea. On the basis of these investigations and earlier data from the area, the total thickness of sediments and thickness of the semiconsolidated section have been mapped. The main part of the investigated area consists of Mesozoic and Paleozoic sediments, while the areas near the shelf edge show a westward increasing wedge of Cenozoic deposits above the same sediments found to the east. Along the Norwegian coast and in the areas southwest of Bjornoya a rapid increase of sediments is noted, while a much more gentle westward growth is found in the main part of the Barents shelf. The data suggest the southern flank of Bjornoyrenna is an area of structural deformation. Structures that can be associated with salt diapirs are recorded in a limited area on Tromsoflaket.

Three-dimensional modelling of gravity data shows that the Dutsen-Wai ring complex of the Nigerian Younger Granite Province can be a high-level, thin, tabular body with subsurface extension of felsic material, dominantly biotite granite, well beyond the outcrop limits of the complex in all directions. This geometry is consistent with emplacement of the body by near-surface lateral spreading of an intrusive diapir. Emplacement by this mechanism creates horizontally directed tensile stresses in the overburden which leads to the formation of ring fractures above the spreading intrusive.

In the Cretaceous flysch of the Northern Pyrenees, polymict conglomerates interbedded in the flysch are described from the vicinity of Orio (near San Sebastian). These contain components derived from rocks of all periods from Paleozoic to Lower Cretaceous. During Late Maastrichtian times a diapir, originated in Keupez evaporites, penetrated the deep-sea Cretaceous flysch which was still undergoing deposition. This resulted in the expulsion of large quantities of diapiric mass. After the solution of the evaporites the pebbles, which had been dragged along with the diapiric masses, slumped into a neighbouring marginal depression where they were redeposited. The reddish strata of Late Maastrichtian and Danian age in this region are interpreted as being out-thrust and transported Keuper shales. It will be shown that the other breccias and conglomerates described by Feuille and Mathey (1972) from the Late Cretaceous of the Basque Pyrenees probably have the same origin. The comprehensive name “Vascongadian Diapir Zone” is suggested for the diapir field on the northern edge of the Basque sedimentary trough.

High-resolution sub-bottom (3.5 kHz) traverses of the southwestern portion of the Balearic Abyssal Plain between North Africa and the Balearic block show an irregular topography where rates of deformation appear to locally exceed those of sedimentation. The basin is underlain by salt of Messinian age, and diapirism has disturbed post-Messinian sediments. The 3.5 kHz records, augmented by 30,000 Joule seismic reflection profiles, indicate that this diapirism is continuing at present and is even now significantly deforming the uppermost sediment sequences. A high concentration of structural features including doming and the development of rim synclines within the 35–60 m of section penetrated are associated with diapirism. Over 70% of the domes have penetrated the surface in the studied area; of these, most display a relief of 10 m or more. In some instances, domes apparently act as dams behind which sediment is ponded. Non-faulted relief averages 3–4 m/km, notable for an abyssal plain surface. Frequently, the uppermost Holocene strata are offset several meters as a result of normal faults and grabens. This concentration of sea-floor irregularities necessarily modifies the present-day sediment dispersal pattern and rules out the uniform flow of turbidity currents and lateral continuity of sedimentation units across this basin plain.

The Rhodesian Archaean Craton consists of greenstone schist belts tightly folded between gneiss domes. The Selukwe Schist Belt, situated near the centre of the craton, consists of mainly Sebakwian Group greenstones, ultra-mafics and sediments, older than intrusive 2900 m.y. tonalities. It appears to be part of an imbricated and overturned lower limb of a large recumbent fold, resting allochthonously on a gneissic basement. Four main tectonic slices, separated by mylonite shear zones were recognized. The inverted succession together with the shear zones were refolded during two subsequent tectonic episodes, before being arched and tilted by intrusion of the tonalite diapir in the southeast.

New facts relating to the geology of the Paxos and Ionian zones, the outermost zones of the Hellenide structure of western Greece, are reported. In Leukas island, a shaly bituminous dolomite formation has been found to occur below the Cardita guembeli limestones which had previously been thought to be the oldest in Greece. In the eastern part of Cephallonia island there is evidence of transition between the Paxos and Ionian zones, represented principally by the extension of the limestone facies through the Oligocene and the presence of lower Miocene Flysch. The age of the gypsum found in the discordant upper Tertiary deposits is a problem; from their structural position and the nature of the associated limestone breccias, they could be Triassic or even Permian emplaced in two stages by a sort of diapirism.

A detailed survey by Texas Instruments Incorporated under contract to the United States Naval Oceanographic Office in the vicinity of the Southeast Newfoundland Ridge suggests that this ridge is structurally controlled and related to the northern extent of the Jurassic Atlantic basin. Possible salt diapirs were found in the Sohm Abyssal Plain and lower continental rise. Evidence of formerly greater bottom current activity in the region is suggested by the presence of buried abyssal anti-dunes.

In recent years, the term "train' has become increasingly important in descriptive structural geology. Expressions like "train facies' or 'strain fabric' (formerly Veformation fabric') have been introduced (Halasen 197 1, Clifford 1972), which are exclusively related to tectonic flow, and these are used with increasing frequency, while use of the term strain is comparatively uncommon in papers on non-diastrophic deformation or flow of lava and magma. If this trend' continues 'geologic strain' will become synonymous with 'tectonic flow'. It need hardly be pointed out that strain is not a structural property but rather a general concept of theoretical mechanics, and that it is not the only method of specifying a coptinuous deforn~ation (minus translation) It has become clear, however, that the strain concept, rather than alternative methods, will be an important tool for the modcrn structural geologist (cf. Ramsay 1969). Clifford's ( 1972) study is essentially an application of the strain theory to the wall rock deformation of a granitic batholith. The paper employs such engineering terms as 'pure shear' and 'simple shear', as well as tectonic expressions related to strain. These technical terms have been unequivocally defined in standard books of theoretical mechanics, and/or in recent textbooks of structural geology (Table 1 ). Clifford ( 1972) does not use 'pure shear' and other expressions in their standard sense, neither does he re-define his technical vocabulary. The reader is forced to deduce from the general context which meaning is to be attached to the various terms (Table 1 ). Clifford ( 1972) prescnts convincing evidence that the triaxial strain of the wall rocks is markedly heterogeneous, i.e. that the magnitude of horizontal compressive strain increases toward the contact of the batholith. Me then claims that this inhomogcneous deformation corresponds closely to irrotational strain (labelled 'pure shear in three dimensions', see Table I ) , and attributes it largely to axially symmetric inflakion of thc uiaconsolidatcd batholith. Of gcomctric necessity, most heterogeneous strains involve large components of rigid rotation, i.e. the local strain is generally rotational (Ramsay 1969, p. 52) . Figure 1 illustrates schematically what heterogencous strains might be expected around uniformly expanding circular diapirs. The cross-sectional strain (normal to the axis of diapirs) must be irrotational (Fig. la ) , but large components of rigid rotation occur in other sections (Fig. l b ) . These components will tend to be greatest where geometry of the granite contact is approximately cylindrical. Inflation of noncircular diapirs will generally lead to rotational crosssectional strain in the sul-rounding rocks. Clifford ( B 972) considers two general models of radial horizontal compression of the supracrustal wall rocks, ( 1 ) by emplacement of a conical granitic mass, and (2) by late-stage horizontal "inflation' of a batholith. He asserts that the former model is characterized by biaxial strains (labelled 'constrictive', Table 1 ), and that the latter model involves an early phase of subvertical simple shear parallel to the contact. Figure 1 b illustrates that subverti

In eastern Africa (Barberton, E. Transvaal; Rhodesia; Kenya and Uganda) sequences more than 3 b.y. old of submarine volcanics and vulcanogenic sediments (‘greenstones’) appear to have accumulated on still older gneissic crust of continental type. No primitive oceanic crust has been found. The granites which were intruded in enormous volumes into these ancient greenstones do not differ from those which were intruded in later periods. Some of this granite probably came from the mantle but much of it represents reactivated pre-existing crust which rose as mantled gneiss domes and diapirs. In the Pan-African (0.6 b.y.) domain of eastern Africa, granite plutons are rare in the high-grade granulites and gneisses of Tanzania and Kenya but become increasingly abundant northward in the lower-grade and higher-level rocks of the eastern Sudan, Egypt and Saudi Arabia. Their distribution suggests that they may have been extracted from the lower parts of the crust. The volume of the African continental crust appears to have increased appreciably even during the last 1.5 b.y. This is deduced from the elevation of veneers of little-disturbed ancient sediments which rest on the old cratons.

Submarine slides, upper Albian and upper Cretaceous fault diapir, post-Lutetian (Eocene) decollement and imbricate structures

This paper, written at the request of the Societe Minerais et Metaux, summarizes the unpublished 400-page report (1928) on the Corocoro copper-mining region of Bolivia by the late L. Crussard, with modifications from recent observations. The country rocks of the mining district, exposed in a large diapiric anticline, are principally the Oligocene (?) Ramos series of incompetent lagoonal shaly sandstones with intercalated gypsum, rock salt, and gypsiferous clays, and ore beds of competent lacustrine and fluviatile sandstones. The copper deposits, surrounded by alteration haloes, occur selectively in the most permeable sandstones within 150 meters of the Corocoro fault. Chalcocite is the principal copper mineral in the ore beds, and native copper characterizes the Ramos series as well as the deeper zone. The gangue is principally gypsum. Structurally, the district is on one flank of a diapiric anticline whose incompetent Ramos core was squeezed out by disharmonic movement into a recumbent position upon the competent beds of the limb. A thick bed of shale served as the mattress for the movement, and differential movement sufficed to slickenside the shale enough to simulate a major fault. Epigenetic solutions rose along channels in the vertical side of the diapir to deposit copper.

As a result of petroleum exploration east and south of Massawa, Ethiopia, a review of local geologic features indicates the structural aspects of this distinct bend of alinements along the Red Sea coast. The Buri Peninsula holds a horst bounded on the west by a deep faulted corridor. To the east and northeast of the Buri high trend the Dahlak salt basin occurs, where deep faults seem to have initiated salt diapirs. Aeromagnetic information suggests a deepening of the magnetic basement toward the southwest where over 5000 m of sedimentary section is expected to occur opposite a shallow faulted belt near the coast. Conclusions: (1) a narrow passage extending south of the Gulf of Zula once connected the Red Sea with the Danakil depression, (2) a positive Precambrian trend on the Buri Peninsula continues the Danakil horst to the northwest until it plunges into the Bay of Massawa, and (3) results of deep exploratory drilling provide data for recognition of the extensive Dahlak salt basin.

The western Gulf of Mexico sea floor contains numerous linear, parallel, anticlinal features that comprise the Mexican ridges province. The development of this ridge province essentially began during the Mesozoic with evaporite deposition in the gulf basin. These evaporites were subsequently buried by a thick sediment cover. Increasing overburden pressure and tectonic activity produced salt flowage on an irregular basement and thereby triggered salt anticline formation, the primary cause of ridge development. The ridges first developed during late Mesozoic or early Tertiary as salt anticlines and diapirs beneath the overburden. By Early Pliocene, salt tectonism had produced incipient surface ridges that effectively stemmed the seaward transport of terrigenous sediments. The ridges apparently are still undergoing uplift. Ridge trends are predominantly north-south but are not consistent and clearly have been influenced by basement uplifts. We present here a hypothesis for the development and trends of the Mexican ridges based on extensive geophysical data to show that the ridges are due to salt tectonism or flowage and the influence by basement highs.

Jurassic block in Albian-Cenomanian (Cretaceous) nappe partially overlain by Senonian microbreccia, lower Miocene autochthonous? substratum, three-stage emplacement of Jurassic block, upper Jurassic deformation, Triassic diapirism, upper Cretaceous decollement, Miocene imbricate structures

Late Precambrian-Early Paleozoic and Middle Paleozoic oceanic basins are reconstructed in the orogenic belt of Central Asia. These basins coincide with eugeosynclinal ophiolites which can be considered as remnants of a former oceanic floor. As these ophiolites are of decreasing age from the edges of the eugeosynclinal zones toward their centres, one can assume the creation of an ancient oceanic basin by a sea-floor spreading process. Restored oceanic basins can be considered as analogues of present marginal and inland seas. The distinct structural-magmatic zonal pattern came into existence during the generation of a given oceanic basin. This pattern appears to be governed by the ancient Benioff zone along which energy and light lithophile substance ascended as mantle diapir.

The Beltana zinc‐lead deposit, the largest of several such small deposits in the Beltana area, owes its formation to intense localised marginal faulting and brecciation of Ajax Dolomite at its contact with the Beltana Diapir. Considerable frictional heat was generated by the upward movement of this diapir. The enormous stresses set up in it squeezed out hot interstitial saline mineralising fluids into the brecciated dolomite footwall to form initially a prominent lead arsenate‐rich horizon containing hedyphane, finnemanite, vanadinite and several minor components. Sporadic, but significant manganese mineralisation also occurred during this phase resulting in the precipitation of coronadite and hollandite. During the final and most prominent stage in the paragenetic sequence, the mineralising fluids became more siliceous and zinciferous leading to extensive replacement of earlier arsenates and precipitation of willemite. Late‐stage alteration is largely confined to the earlier arsenate‐rich zone, ...

The mathematical theory of the dynamics of multilayered systems in the field of gravity when applied to geological and global structures results in a system of simultaneous equations that relate the rates of deflective movement of the several interfaces with the viscosities, densities, and layer thicknesses of the various layers and with the wavelength of the deflections. In this system of equations, a matrix of coupling coefficients controls the interrelation among the deflections on the various interfaces. The eigenvalues of the coupling matrix determine the dynamic evolution of the system in the sense that the dominant wavelengths are determined by the maximal values of the numerically largest eigenvalue, and the ratio between the amplitudes of the deflections approaches rapidly the ratio between the components of the eigenvector that belongs to the optimal eigenvalue. Five geodynamic problems are treated: (1) a salt sheet buried below more dense sediments, (2) a low-velocity (low density) layer situated in the upper mantle, (3) the gneiss granite basement in orogens, (4) sediments overlain by a sheet of submarine basalt, and (5) a low-density stratum buried deep in the mantle. Dominant wavelength and velocity of amplitude growth of these structures are determined quantitatively by the computerized mathematical model. Rate of rise varies from 0.02 cm year−1 to several meters per year in the unlike systems. Magma generation by pressure-release fusion in the ascending diapirs is suggested as a realistic possibility. That the rising diapirs can effect continental drift and other global-scale motion in the earth's crust is demonstrated by experimental models run in a centrifuge.

Intrusive sedimentary breccias in Archaeocyatha limestone (Cambrian), diapirism, faults, hydrogeologic controls, Ordovician Delemarian Orogeny mesothermal ores, enrichment, mobility of ions, evolution, changes

Study of the contact zones of the Suzette diapir in Vaucluse and the Propiac diapir in Drome, France, shows that the emplacement of these Triassic blocks took place between the Miocene and Pliocene.

The continental margin of northwest Africa has developed through the Mesozoic and Tertiary by prograding sedimentation on a subsiding basement. Eocene phosphorites were deposited in offshore continuations of shallow Early Tertiary gulfs in the region of A1 Jadida and of Essaouira. These phosphorites now crop out on the outer shelf seaward of outcropping Cretaceous strata. Oligocene earth movements gave rise more or less to the present shelf-slope configuration. Subsequent subsidence notably continued only in the northern part of the Aaiun Basin and the Souss Trough. The major locus of post-Oligocene deposition has been the continental slope and probably the continental rise. Parts of the uppermost slope off the southern Spanish Sahara and the High Atlas Chain are tectonically oversteepened and essentially non-depositional environments. A transgressive phosphatic Miocene deposit thinly mantles parts of the Moroccan shelf although much appears to have been removed by Pleistocene erosion. Restricted subsidence in the northern Aaiun Basin is represented by the development of Lower Pliocene phosphorites. The existence of salt diapirs within the continental margin is inferred locally.

The basaltic lavas erupted throughout the Mojave Desert are basanites (SiO2 5%, and K2O>1.5%), alkali-olivine basalts (SiO2=46–48%; ne=0–5%; and K2O=1.0–1.5%), and low-alumina, sub-alkaline basalts (SiO2=48–51%; ne=0; K2O

Gosses Bluff, west of Alice Springs, Northern Territory, comprises a roughly circular rim of steeply dipping sandstone, locally overturned with downward facing folds, surrounding a topographically lower core of steeply dipping faulted sandstone, shale, and limestone. Abundant shattercones occur both in outcrop and to depths of 1,000 m. The structure lies on an’ anticlinal trend‐Structural, gravity, seismic and drill hole data indicate that the structure continues to some depth. Shattercones also occur outside the rim of Gosses Bluff. At a nearby prominence, Mt Pyroclast, they occur with peculiar breccias and devitrified glassy rocks, some of which may be of volcanic or impact fusion origin. Gosses Bluff has previously been regarded as a diapir caused by salt intrusion. The evidence now available suggests that it is a crypto‐explosion structure, resulting either from deep‐seated explosive volcanic activity, or from meteorite impact, probably at some time during the Mesozoic Era.

The alkaline ultrabasic rooks (damkjernite) of the Fen alkaline complex contain abundant rounded inclusions of spinel lherzolite, closely similar in composition and mineralogy to the nodules found in many undersaturated basaltic rocks. Clinopyroxene compositions indicate crystallization at P=10–13 KB, T=1200–1250° C. A model involving partial melting in a rising diapir of mantle peridotite allows interpretation of the nodules either as mantle fragments or as cognate cumulates, and suggests that crustal thinning in the Oslo area possibly began as early as 500–600 m.y. ago. The presence of the nodules indicates that the damkjernite did not pause for intracrustal differentiation during its rapid ascent to the surface. The crustal contamination suggested by recent Sr-isotope studies may have occurred during differentiation at the base of the continental crust.

Pliocene intrusions in Eocene, Oligocene, and Pliocene volcanic-sedimentary rocks of the eastern Nakhichevan A. S. S.R. are of three principal types: 1) a single central cutting massif; 2) central massif with conformable apophyses; 3) conformable apophyses almost camouflaging a broken up central massif. All three are explained by differences in the morphology and tectonics of the enclosing rocks. Similarities and differences between minor intrusions and salt, gypsum, and clay diapir phenomena are discussed. ‐ ‐ Auth.

Summary The composite diapir of Ardara, County Donegal, foreibly displaced contact-altered Dalradian rocks of its envelope by folding, thrusting and shearing. The intrusion consists of a granodiorite core surrounded by a mantle of course tonalite. The outer part of the core is made up, in the north, of contaminated quartz-diorite and, in the south, of less contaminated granodiorite. These rocks, and the tonalite, are well foliated. The central part of the core is composed of uncontaminated and unfoliated biotite-granodiorite The emplacement of the body is believed to have begun with the movement of granodiorite magma along a pre-existing thrust-plane. The magma came in contact with basic rocks of an older complex and mobilized part of this complex, sweeping it upwards as a plastic skin which consolidated to form the tonalite mantle. Magma contaminated with basic material accumulated within this mantle to form the quartz-diorite and the slightly contaminated southern granodiorite. The outer part of the body cooled and developed radial fractures. A new upwelling of essentially uncontaminated granodioritic magma pierced the still molten core of the pluton, causing remobilization and further distortion of the envelope. Radial fractures, later followed by tangential fractures, were again formed. At a late stage, the solidifying pluton rose vertically, with resultant shearing of its envelope.

Seismic reflection studies in the Laurentian Channel have located a structure, which may be a diapiric salt intrusion, at 45° 12′ N, 57° 07′ W. It occurs over the eastern end of a part of the region of low, free-air gravity anomalies, which extends from eastern Nova Scotia. It is reasonable to suppose that the structure is a diapiric salt intrusion; evaporites have been reported in one drill hole to the east, on the Grand Banks. If so, this confirms the interpretation of the gravity anomalies by Loncarevic and Ewing. A reflection profile across the whole channel does not support the view that the present form of this part of it is of structural origin.

Recent borings along a north-south line in the Belus region, Landes, France, show that a Triassic diapir is wedged into the core of the Belus fold. The upper Cretaceous and Tertiary formations of the Belus-Saint-Lon region were subjected to four stages of tectonic activity. Injection of the diapir structures probably took place during Aquitanian crustal movements (the last of the four stages).

Sampling and seismic profiling in the Tofino Basin west of Vancouver Island show there is a thick sequence of Tertiary rocks ranging in age from late Eocene to Pliocene. The rocks are mainly mudstones containing abundant foraminifera indicating a bathyal depositional environment throughout most of the Tertiary. Subsequent uplift has exposed the deep water sediments on the shelf over much of the area. Eocene-Oligocene sediments occur in a belt along the inner shelf, while Miocene and Pliocene rocks lie seaward of this. Pliocene rocks form a regressive sequence overlapping the older Tertiary, with the greatest thickness in the south.At least two major periods of deformation resulted in faulting, folding, and diapirism on the continental shelf. Deformational patterns show a marked change from north to south. North of Brooks Peninsula sediments are undeformed by folding but are truncated by faulting along the steep continental slope. The Kyuquot Uplift south of Brooks Peninsula exposes Eocene-Oligocene sediments across the shelf. Farther south Mio-Pliocene sediments unconformably overlie the uplift. Folding increases southward culminating in an area of diapirism off Nootka Sound. Elongate diapirs trend parallel or subparallel to the coastline.Tectonic features on the shelf and slope appear to be related to present and earlier configurations of nearby offshore spreading centers, plates, and transform faults. Crustal plate movements may have been responsible for the observed shelf and slope deformations.

The present development of exploration in Gabon and the latest successes obtained by drilling have led the authors to publish a historical account of the geological and geophysical research concerning two particularly typical salt domes: those of N'Daminze and Ozouri. Both of them are located in the western basin of Gabon: the first one in the zone of the savannahs, the second being a part of a series of salt domes in the delta of Ogooue. The western basin of Gabon forms, as a whole, a huge westward-inclined monocline and is characterized by a general phenomenon of diapirism the type of which, in the delta of Ogooue, greatly differs from that of the savannahs zone. Because of the scarcity of outcrops in these zones geophysical exploration has been of valuable help.

The Upper Tertiary geosynclinal beds of the East Coast region of the North Island of New Zealand rest on an incompetent substratum of Lower Tertiary marls and bentonitic rocks and show, in the structures described, evidence of decollement. A Lower Tertiary outcrop of 3½ square miles represents an intrusion through some 10,000 ft of the Upper Tertiary succession and lies close to a sharp structural subsidence of similar area. The processes of formation of these structures are considered and it is concluded that at least 8 cubic miles of Lower Tertiary material were transferred from beneath the subsidence, which coUapsed in consequence, to be extruded near by to surface through the overlying beds.

The study of tectonics of salt and of diapirs in particular is an important problem in the mining field as well as in that of petroleum exploration. This paper describes surveys that were carried out, using the method of the rectangle of resistivities. After stating the principle of the method, the way it is carried out and its advantages, three examples are discussed: two in the Alsace plain (Hettenschlag-Oberentzen and Blodelsheim), the third in Baden-Wurtemberg (Weinstetten-Heitersheim). Geological conditions are practically the same (resistive alluvia, very conductive marls and resistive salt series); the depth at which the top of the salt was found varied from 100 meters to more than 1000 meters. Maps of resistivities (with current electrode spacing of 6000 meters) and graphs of a few electrical soundings are presented, together with the results obtained under the form of isobath maps of the top of the salt series. The surveys lasted 26 months and covered 171 square kilometers. The work presented corresponds to three sections totalling 93 square kilometers and was chosen because of the diversity of the tectonics. Four drill holes have been carried out since then and have confirmed the results quite well. The method may be extended to the survey of the tectonics of a salt series or of a resistive substratum, for instance a limestone formation, that are deeper than the above cases (2000 meters or more), using current electrode spacing of 20 kilometers. Surveys of this type have already been realized in the field.

Two modern geodynamic models on the Alps are tested: plate tectonics (mechanical effects of colliding lithospheric mega-units), and mantle diapirism (mechanical and geochemical effects of ultra-lowvelocity bodies, rising from the low-velocity layer or asthenosphere). Verification occurs by means of comparing the expectations of these models (their ‘prognoses’) with the great wealth of available ‘diagnostic’ facts on the geology and geophysics of the Alps. It appears that the picture of the Alpine structural evolution, drafted by plate tectonics, is inadequate to describe the observed reality, whereas the idea of mantle diapirism, combined with crustal corrosion (geochemical oceanization) and gravity tectonics, provides a functionally correct model, that logically and coherently explains the entire Alpine cycle of orogeny. In the concluding remarks comparisons are made with the formation of island arcs in the western Pacific, the origin of the basin-and-range topography and volcanicity along the eastern margin of the Pacific, as well as the Sunda arc of Indonesia.

Remapping of parts of the Galway granite has shown that over most of its outcrop it is a homogeneous adamellite. A steeply dipping belt of basic rocks borders the intrusion on the north where there is a reaction zone between adamellite and basic rock. A similar but gently dipping belt of basic rocks overlies the granite to the west. In the south, where there is no reaction zone, the granite intrudes and hornfelses a group of pillow lavas, greywackes, and conglomerates. The structural features of the northern border indicate that the granite has been emplaced as a diapir and radial fractures in the north-east border further indicate that the granite and its basic envelope moved upward and outward together into the Connemara schists.

The application of fluid dynamics to layered systems with one or several flexible boundaries affected by gravity is reviewed. The theoretical equations are verified from models studied by the finite-element method and by experimental tests with liquid models. The rate of growth and geometry of the models studied by the finite-element technique are shown to be very sensitive to different boundary conditions at the top and bottom surfaces of the model. Diagrams of rate of amplitude growth and distributions of principle and normal stresses are presented. Plexiglass boxes filled with two immiscible fluids of known properties were used to test the spacing between domes and rate of growth of domes. The buoyant rise starts as gentle ridges which later split into individual domes. The velocity of rise increases until the domes reach half of the height of the model box. From then on the rise decreases and the material spreads laterally below the roof.

Subsequent to post-Lutetian pre-Miocene folding of the thick series of Aptian (Cretaceous) to Lutetian (Eocene) formations of the Tebessa-Ouenza region, Algeria, subsidence troughs were formed, in which movement continued into the Quaternary. Emplacement of Triassic diapirs took place along fractures produced during dislocation.

The eastern margins of the Gulf of Mexico are areas of carbonate buildup. They are represented by the south Florida and Yucatan platforms which are similar in respect to topography, sediment type, seismic velocities, depth to equivalent age horizons, and bordering buried reefs. The existence of the Lower Cretaceous reefs is of particular importance as they have been the major factor in controlling the sedimentary history during the Mesozoic and Cenozoic in the eastern Gulf of Mexico. The areal extent of the Lower Cretaceous reef is best known on the Florida Platform but its existence on the edge of the Yucatan Platform suggests the possibility of a Lower Cretaceous reef complex that nearly encircles the Gulf. In this case these buried offshore reefs would be part of a system including the Golden Lane (Faja de Oro) of eastern Mexico and the well known oil producing Lower Cretaceous reef trends of Texas and Louisiana. Reef material recovered from topographic highs in the Straits of Florida suggests that the bordering reefs of the west Florida Platform continue to northern Cuba. These similarities between the Yucatan and Florida platforms allude to a geographical connection between these areas in the geologic past. If this is true, the mode of separation of the platforms suggested by seismic reflection data is erosion (controlled mainly by the Gulf Stream) and faulting. The western margins of the Gulf of Mexico are characterized by thick deposits of terrigenous elastics. The main structural element is in the north; the Gulf Coast geosyncline. Structure within the geosyncline is very complex, structural mobility being caused by the presence of extensive buried Triassic-Jurassic salt. Similar conditions exist in the southwestern Gulf (Bay of Campeche) where salt structures dominate the topography. Although there is no conclusive proof, there is much evidence that indicates the north-south trending ridges of the western Gulf of Mexico are also related to buried Mesozic salt. If these folds off eastern Mexico prove to be salt anticlines at depth, the post-Paleozic development of the western margins of the Gulf of Mexico becomes clear; 1. (1) during Mesozoic, salt was deposited on the western margins of the Gulf of Mexico; 2. (2) the salt was covered by pelagic and continental derived sediments; 3. (3) the salt was folded into a series of linear ridges parallel with the seaward extent of the salt; and 4. (4) as sedimentation over the anticlines continued the salt became more mobile, more complex structures developed and diapirism became the dominant mechanism. Both phases three (3) and four (4) are evident in the western Gulf. The linear features on the central portion of the eastern Mexican slope are examples of the relatively undeformed salt anticlines and the structures of the Bay of Campeche represent an intermediate stage where the ridge outlines can still be detected but secondary growth features are prevalent. The continental slope off Texas and Louisiana represents the final stage of development i.e., there is no preferred alignment of structures and diapirism predominates. The crustal structure of the central basin is quasi-oceanic. Earthquake seismology studies and gravity measurements both indicate that the oceanic type crust extends some distance inland on the northern margin of the gulf coastal plain. The northern extent of the oceanic crust and the great thickness of unconsolidated sediments present across the central basin suggest a long history of filling. This is consistent with a hypothesis which considers the Gulf of Mexico as an old ocean basin which is being flooded by sediments. The effect of the filling has been to decrease the water depth in the basin and to increase the depth to the Mohorovicic discontinuity. In terms of the new global tectonics, this concept of the Gulf representing a remnant of an old ocean with the characteristics mentioned above, it appears that the Gulf of Mexico moved passively with the North American continent.

Layered ultrabasic and basic intrusive rocks occur in three structural environments in the Lewisian of the North-West Highlands of Scotland and 41 such layered masses, or groups of masses, in the area between Loch Laxford and Loch Torridon are recognized and described. Their igneous origin is indicated by the common mineralogical banding, the occurrence of sedimentation features, particularly current-and wedge-bedding, and by the association, in banded sequence, of ultrabasic, basic, and anorthositic rocks which are similar in chemical composition to corresponding rock-types in other layered igneous complexes. One group of metaperidotites and metagabbroic rocks has undergone coeval, or near-coeval, granulite-facies metamorphism and folding, probably after an earlier period of disruption. Subsequently, about 2400 m.y. ago, further disruption and amphibolitization took place. No evidence of the later events is shown by most of these masses, but they have moved tectonically, as units, by slip around their margins during subsequent episodes of deformation. The masses are usually in the form of massive metaperidotite lenses generally concordant with the banding of the adjacent gneisses or granulites, although a few serpentinite masses now have the form of tectonic diapirs. Low-grade retrogressive metamorphism in zones of steep reverse faulting affected some of the masses. Another group of metaperidotites has been deformed by isoclinal folding on axes trending nw-se with steeply dipping axial planes so that the mineralogical banding of the ultrabasic bodies is now vertical. Subsequent hydrous metamorphism, sometimes with the introduction of potassium, has caused marginal alteration of many of these masses to talc-trem-olite-carbonate-serpentine rocks, sometimes with biotite and muscovite. Complete minera-logical reconstitution to this low-temperature hydrous assemblage has taken place in some cases, but relict banding remains. After this episode, which was about 1500 m.y. ago, the rocks were deformed by a series of box-like folds, with steeply plunging axes, causing local deflexion of the banding from its nw-se trend. A small group of banded picrites and gabbros has intrusive relations to these gneisses and shows subsequent retrogression with the uralitization of the pyroxenes.

Two genetic models have been compared for a tectonic pattern typical of undisturbed remnants of Archaean crust. A model implying simple thermal convection in a granodioritic crustal layer is found to have advantages over the classic model assuming the gregarious batholiths of Rhodesia to be granite diapirs.

Summary The Troodos igneous complex of Cyprus is an ophiolite body composed of ultramafics, gabbros, dolerites and pillow lavas. Seismic velocities measured in these rocks were expected to be similar to- those in the layers under the deep ocean. Seismic experiments made in the field in individual rock types revealed only low velocities typical of deep sea layer 2. We suggest that the low velocities are due to high porosity in the shattered rocks. Geophysical field work was carried out during March 1971, in order to measure the seismic velocities at outcrop of the rocks of the Troodos igneous complex. The aim of the experiment was to compare the velocities obtained with those of the oceanic crust already established by marine seismic refraction work. Financial support for the expedition came from I.B.M. (U.K.) Ltd grants to the University of Cambridge. A preliminary account of the results has already appeared (Matthews etal. 1971). The Troodos Massif is believed to be a fragment of Mesozoic oceanic crust which has been thrust up on land during the northerly movement of the African plate towards Europe (Gass & Masson-Smith 1963; Gass 1968). The Massif occupies the central and south-eastern part of the island (Fig. 1); the ultramafics form the domed central circular outcrop around the summit of Mt Olympus at 6403ft, and are pierced by a serpentinite diapir. The rocks belong to an igneous suite characteristic of ophiolite complexes, and comparable petrologically with rocks dredged from the deep ocean floor (Moores & Vine 1971). The plutonic members range from dunites, harzburgites and pyroxenites to olivine gabbros, gabbros and quartz diorite and are ringed by the Sheeted Complex which is a swarm of steeply dipping altered basic dykes, without intervening screens, possibly formed by sea floor spreading. The Pillow Lavas occupy the outer margin of the oval outcrop pattern, and are divided into Upper Pillow Lavas with few dykes and Lower Pillow Lavas with abundant basic dykes, separated by an unconformity recognizable in only a few places. Moores and Vine classify the rocks into 3 units: Pillow Lavas, Sheeted Complex and Plutonic Complex, and these are used for the purposes of the following discussion rather than the more complicated scheme of the Cyprus Geological Survey. They suggest that a correlation may be made with rocks of the oceanic crust and upper mantle. Experimental work was carried out after careful site selection, using the geological maps of the Survey memoirs (see Appendix), and the detailed field knowledge of Drs I. G. Gass and F. J. Vine. With three exceptions all the lines were shot using the

Introduction. Several knolls discovered in 1953 during Vema cruise 3 [Ewing, Ericson, and Heezen, 1958] rising to a few hundred meters above the Sigsbee abyssal plain in the Gulf of Mexico were investigated by continuous seismic reflection in 1961 during Vema cruise 17 [Ewing, Worzel, and Ewing 1962] and found to be accompanied by a large number of buried structures. The entire group was judged to be a field of diapirs, called the Sigsbee knolls and domes, resulting from inversion of density in a thick accumulation of (largely) turbidite sediments covering the oceanic crust of the basin floor. It was reported that no large gravity or magnetic anomalies were associated with these features.

Several quarries southwest of Paris, near the confluence of the Marne with the Seine, display uplifts of the Ludian Eocene marls. Despite their small size, they are termed diapirs in that they tilt and pierce the overlying strata. The Bonneuil quarry shows six diapirs, varying in shape and orientation. They all display steep slopes, convex summits, and severe tilting of overlying Quaternary gravel beds. Some are isolated and others form complexes representing high points of single uplifts. The Trioux a Chelles quarry has 12 diapirs, several cut in vertical section. In addition to being pierced by marl diapirs, the overlying Quaternary gravel beds of both quarries are injected with thin marly veinlike structures. The complex form of the diapirs, interpenetration of the injections and accommodation of overlying strata to uplift suggest that the deformations were produced when the marls were in a softened state.

After a brief recapitulation of the mathematical theory of the dynamics of viscous layers with contrasted density in the field of gravity, the instability of a buoyant stratum buried deep in the mantle is discussed. Such layers are particularly interesting from the point of view of global tectonics because of the large dominant wavelength which they sometimes develop. Under certain conditions the wavelength is several thousand times the thickness of the layer. For layers only a few km thick then, their wavy ascent may cause crustal disturbances on the scale of oceans and continents, contributing to continental drift and the opening-up of oceans. The rate of rise calculated varies from 0.002 cm/y to no less than 247 cm/y. An episodic character of tectonic evolution is predicted because the rate of ascent of a layer increases faster than linearly with its thickness. The buoyant layer is imagined to grow in thickness by an endothermic and exospatic (volume increasing) process such as e.g. the change from spinel-type (Mg, Fe)2SiO4 to the olivine-type, due to radioactive heating. Not until the thickness of the layer has reached a minimum value will it start rising with a sensible speed. As the layer thickens further, the rate of rise increases fast and the wavy pattern of the layer amplifies rapidly, developing into independent diapirs which ascend through the mantle and cause tectonic movements in the crust. After the energy of the buoyant diapirs has been exhausted, a period of dynamic quiescence follows until enough radioactive heat develops to produce a new buoyant layer with the minimum thickness. Experimental models imitating the rise of a low-density layer in the mantle are presented, and so are models imitating the spreading of a low-density pillow (mushroom) underneath the Mid-Atlantic Ridge, and the consequent drifting apart of the New and the Old worlds. Fusion and other phase changes due to pressure relaxation in a rising diapir in the mantle is discussed. This is found to be a realistic magma-producing process capable of melting a major fraction of the rising crystalline body.

The finite-element method of analysis is described, and its use in solving finite-deformation and stress problems is demonstrated. A plane-stress analysis of boudinage structure suggests explanations of the barrel shape of boudins and the deposition of mobile chemical elements between boudins. A study of the change in the fold geometry of laterally compressed layers shows that simultaneous, finite buckling and layer-shortening is a plausible explanation of the formation of similar folds. An application of the method to the growth of diapirs of finite size in a two-layer system illustrates theoretically the shapes and normal stress distributions in diapirs at different stages of growth.

Primary carbonate‐bearing lamprophyric rocks have been recognised in the Walloway Diapir, South Australia. Their petrography and trace‐element geochemistry indicates that they have carbonatitic affinities. The rocks are magnetic and can be detected by ground magnetometer surveys.

The large excesses of formation pressures, over and far above the hydrostatic pressure, often recognized in deep oil wells in the Apsheron-Baku district, and their interpretative analysis suggest that such excesses are indeed the prerequisites of (a) the development of diapir, cryptodiapir, and other radically dislocated trap structures and (b) the primary migration of the hydrocarbons. Further analysis suggests that traps formed prior to the beginning of the pressure discharge are particularly promising targets in exploration for oil and gas.

Detailed field and micropaleontologic investigations of outcrops along a north-south section across the Tell region, Algeria, from the eastern Ouarsenis massif through the Cheliff basin to the Miliana massif, show that the zone of complex structures is overthrust relative to the zone in which Cretaceous and overlying Miocene formations have been subjected to broad folding and in which Triassic diapirs are absent.