Margin Features Research Articles

Eolian deposits and dunes on Mars

A complex of coalescing ridges and diverse marginal features observed in Mariner 9 pictures of the Hellespontus region of Mars is identified as a dune mass. Many other possible dune features are recognized in mid-latitudes and polar latitudes, and regional differences in the appearance of cratered terrains are attributed to modification by dune deposits. The morphology and temporal behavior of some Martian albedo features can be explained by dune-forming saltation processes. Eolian activity seems to be widespread, if not ubiquitous, on Mars and appears to produce major regional variations in the surface environment.

Flake Tectonics and Continental Collision

The concept of crustal flakes can explain several of the features of complex plate margins.

Marginal offsets, fracture zones, and the early opening of the North Atlantic

A simplified model is proposed that can explain the evolution of marginal offsets, ridges, and fracture zones. Submarine features of the equatorial Atlantic margins are examined and are found to be in reasonable agreement with the model. The results reaffirm the suggestion that the South Atlantic opened in two stages beginning about 140 m.y. and 80 m.y. ago. The pole of rotation describing the relative motion of the South American and African plates changed radically when the constraints imposed by adjacent continental blocks were relaxed.

Origin of the Bay of Fundy, an interpretation from sub-bottom profiles

It has long been debated whether the Bay of Fundy is derived from glacial or fluvial erosion of the underlying Acadian Triassic Basin. The recent acquisition of over 1,100 kilometers of sub-bottom profiles provides considerable data pertinent to this problem. Features indicative of glacial erosion are widespread. The bedrock floor of the Bay, generally mantled with ten m or less of drift, is exceedingly flat, with gradients of two meters per kilometer and less. Where freshly exposed by tidal scour on the shore, this surface is a striated glacial pavement. Longitudinal submarine troughs on the floor of the Minas Channel, Chignecto Bay, and at the mouth of the Bay of Fundy are still blocked with glacial drift, and were formed in part by longitudinal ice movement. However, the orientation of striae at the Bay's margins and of submarine morainal segments and boulder fields suggest transverse movements. A drift-buried, bedrock river channel is displaced south of the Bay's bathymetric axis; if an ice margin feature, it outlines a former ice lobe that extended southeast across the Bay from New Brunswick. The discovery of a Lower Cretaceous valley filling marginal to the Bay suggests that the present drainage system of Nova Scotia may be an exhumed Mesozoic system. If so, the Bay of Fundy, while strongly modified in the Pleistocene, was probably initiated by Mesozoic subaerial erosion.

Ice margin features in the Julianehåb District, South Greenland

The present report deals with the extension of the ice margin deposits in the Julianehåb district, South Greenland. An attempt is made to establish a Holocene chronology for the ice margin deposits within the region on the basis of their association with raised marine shorelines, combined with a determination of fluctuations of the glaciation limits of the individual stages in Holocene times. As the Narssarssuaq region in the north-eastern part of the district contains numerous extensive ice deposits, this region is treated in more detail than the other localities within the district. On the basis outlined above, it is attempted to establish a relative chronology for the ice margin deposits in the Narssarssuaq region. The remaining deposits within the district are then tentatively incorporated in the chronological scheme for the Narssarssuaq region. After the deglaciation of the district during the last phases of the Wisconsin, four periods of stagnation or readvance of the glacier lobes and the ice caps (four "stages") seem to have given rise to the formation of ice deposits. The earliest of these stages is the Niaqornakasik stage (older Dryas??), succeeded by the Tunugdliarfik stage (probably younger Dryas), the Narssarssuaq stage (probably Roman time), and the maximum extension of the ice in historie times (ca. 1750-1900 A.D.). The variation in the volume of the ice coverings (the inland ice and the Julianehåb ice cap) during the period from the Tunugdliarfik stage to the present day is studied. The superficial conditions of the ice covering above an altitude of ca. 1,700 m do not seem to have altered much since the Tunugdliarfik stage. Finally, deposits from former ice-dammed lakes in the Narssarssuaq region are treated. All the deposits from such lakes found here seem to show that all the lakes at the glacier front had a maximum height of the water level of 120-150 m. This is in accordance with J.W. Glen's theory of subglacial outbursts of ice-dammed lakes.

II.—Volcanic Neck Emplacement and Subsidence Structures at Dunbar, South-East Scotland

SynopsisStructures in and around volcanic necks at Dunbar are interpreted as belonging to two different stages in volcanism. Some, which are related to an early neck emplacement stage, include small ring structures—probably incipient (crypto-volcanic) necks—filled with marginally flow-banded breccias, dykes of similarly margined unbedded tuff, and tuff-penetrated wall rocks. Among the last are sandstones veined along planes of minor fracture by very fine sand.Other structures belong to a later stage during which bedded tuffs, that had originally accumulated sub-ærially, subsided into the necks in a process here likened to caldera formation. In one large neck the bedding is relatively undisturbed, but in smaller necks there are high inward dips from the margins and collapse structures, sometimes accompanied by reconstitution of the clastics, at the centres. Diagnostic features of subsidence margins are anastomozing veins of carbonate, locally accompanied by hæmatite, and these appear to be related to dykes of flow-banded sandstone and sandy tuff intruded along fractures initiated inside the necks during subsidence. Other structures described are ring-fracture, with or without gouge, and down-drag of wall-rocks: these suggest a solution to the space problem posed by caldera mechanism.

On the Growth of Continents by Plastic Flow Under Gravity

Summary A coherent explanation of many major geological phenomena follows from the hypothesis that the strength of basement rocks at shallow levels is very small for long-continued stresses, and that gradual plastic flow occurs widely in a superficial layer of the crust. A likely mechanism of flow in a layer 2–3 km thick is that of solution and re-crystallization. By an adaptation of Nye's theory of flow in ice-sheets it is shown that the high-standing continental masses are approximately in equilibrium for a yield point of order 106dyn/cm2. Any increase in the average height of a continent by volcanic or orogenic activity causes the continents to grow in area and thus to encroach upon neighbouring oceans. The pattern of growth is in general asymmetrical and changes with time; recent rates of radial expansion are estimated to have ranged up to the order of 0.1 cm/yr. Such a process of growth by seaward flow of a superficial plastic layer is compatible with many actual features of continental margins; submarine canyons may have been formed by subaerial erosion and later carried across the coastline and submerged. Continental shields are regions that have remained unaffected by flow through several geological eras. The configuration of the slip-line field in a plastic sheet suggests a mechanism of geological faulting by shear fracture induced by plastic flow. Rift valleys and tilted blocks are thus seen as essentially shallow phenomena. Thrust sheets and insular aprons are attributed to extrusion at the base of plastically subsiding mountains and sea-mounts. The fundamental postulate of stress tectonics, that the crust is acted upon by horizontal forces originating in the mantle, becomes largely redundant. Disturbances projected into the substratum as a result of superficial flow are linked with orogenic, volcanic and seismic activity in a complex of cause and effect. It is suggested that mid-oceanic elevations develop by deformation of the substratum, due to progressive loading at the adjacent margins as the continents expand.

Interpretation of a vertical field magnetic survey in North-East England

Summary A vertical field magnetic survey of Durham and South Northumberland supplements the gravity survey described in the previous paper. The field survey and its reduction are described and some of the problems of interpretation are mentioned. A negative magnetic anomaly over the Alston Block is interpreted as another manifestation of a concealed granite under Weardale. Similar anomalies occur over exposed mineralized granites elsewhere, and arise from granite with low polarization existing in country rocks with higher polarization. The magnetic minima are displaced to the south-east of the corresponding gravity minima, and it is thought that this is caused by permanent polarization directed westwards in the country rocks. A violent positive anomaly lies immediately to the north at Blanchland and is tentatively interpreted as a marginal feature of the assumed granite. A magnetic anomaly of Caledonian strike in Northumberland probably marks the southern limit of low-polarization basement rocks. This may well represent the south-eastern limit of the sub-Carboniferous outcrop of the Southern Uplands Upper Ordovician and Silurian rocks.

Book Reviews

Two years ago attention was directed to Prof. L. Kober's view that folded mountain-chains are marginal features of a geosynclinal “orogen” nipped between two mutually apprgaching masses of “kratogen” in the depths (NATURE, vol. 108, October 20,1921, p. 236). The present work embodies a lucid review of the researches of the last forty years in the Alpine region, which is intimately known to the author from the Pennines to the Transylvanian wall. Through all details, however, he maintains his outlook on the world at large. In neat diagrams he shows how a dual structure is traceable in the western United States, in the Caledonian orogen of Scotland and Scandinavia, and. in the axis of Japan. The floor of the Tethys channel (Fig. 2) has been squeezed up here and there to form mountain bulges from Andalusia to Sumatra, over a distance of 14,000 km. In the Alpine region only, a one-sided character has been imparted to the mountain- mass, and this is due to the fact that the southern marginal range, the Dunaric, has been moved north ward until part of it overlies the east Alpine sheet. In agreement with H. Roothaan (1918), Prof. Kober (p. 252) places the beginning of Alpine overfolding in Cretaceous times, and the main movements in the Oligocene period. To quote the final words of this stimulating volume, “noch manche Ratsel bergen die Alpen.”

The Marginal Features of a Basic Dyke at Peldar Tor, Charnwood Forest

In the most westerly working of Peldar Tor Quarry, in the northwest district of Charnwood Forest, a basic dyke is intruded into the porphyroid. Professor T. G. Bonney1 briefly described the dyke in 1915. The intrusion shows a remarkable laminated margin, and the present paper is chiefly directed to a consideration of this feature.

Marginal Glacial Drainage Features in the Finger Lake Region

A study of a considerable number of the channels or scourways formed by streams associated with the Pleistocene ice sheet in the southern Finger Lake region of New York has served to bring out many features of more than local interest and importance, both as to the broader phases of glaciation in this part of New York State, and as to the value of a study of such channels as an aid in working out the glacial geology of a region. This study has a distinct bearing on the interglacial problem in that several of these channels give conclusive evidence of more than one stage of glaciation, while one points strongly to three or more such stages with corresponding interglacial epochs, some of which seem to have been considerably longer than post-glacial time. Other channels are so situated as to make possible a fairly accurate estimate of the slope of the ice margin along the valley tongues. Still another furnishes proof of an extensive sinking of the surface consequent upon the melting-out of a large block of buried ice-a phenomenon, the importance of which seems not to be fully appreciated. The Finger Lake region, lying as it does along the belt of the great recessional moraines of the Wisconsin ice sheet, is especially favorable for the study of marginal glacial drainage features. The long halts of the glacier, while the moraines were building, gave ample time for the associated streams to carve for themselves distinct channels