Showing papers on "Archean published in 1970"

Evolution of the Labrador Geosyncline

Abstract: The Labrador “trough” is the preserved portion of an Early Proterozoic (Aphebian) geosyncline, extending from the Grenville Front at Wabush to Payne Bay, over 600 mi farther north-northwest. Archaean gneisses constitute the foreland to the west of the geosyncline, and remetamorphosed Archaean gneisses compose much of its eastern hinterland. It is the purpose of this paper to outline and correlate the essential geological features of the Labrador trough and to propose a working hypothesis on its origin. The basin filling represents two, or possibly three, tectonic cycles, each initiated by an orthoquartzite-limestone sequence (including iron formation in the second and third cycles), and culminating in deposition of shales and of flysch-type deposits in the external zone of the geosyncline, and in intense volcanic activity in the central and internal zones. The paleogeography of the first cycle is briefly examined. A rapidly uplifted area situated east of the geosyncline is the probable source of much clastic material. The basin center migrated westward during the preophiolitic stage, and a geanticline emerged during the early phase of the ophiolitic stage in the center of the geosyncline. Very large volumes of mafic volcanic and plutonic rocks erupted in the eastern and central parts of the geosyncline during the latter part of the tectonic cycles. The eugeosynclinal mafic rocks are mainly K-deficient tholeiites. Spilites and transitional types of basalts occur in subordinate volumes. Serpentinized ultra-basic sills are present. The Aphebian rocks of the Labrador trough were folded during the Hudsonian Orogeny. A tectonic section across the geosyncline is discussed. The basement below the western part of the geosyncline has not been involved in the Hudsonian deformation, whereas the Archaean underlying the central and eastern portions of the geosyncline was folded jointly with its cover. The relations between structural style, depth of burial and metamorphic grade are briefly examined. A late-kinematic and post-kinematic metamorphism of an intermediate pressure series overprinted the internal zones of the geosyncline. Hudsonian granites and migmatites are absent, and possible causes of their absence are discussed.

Continental Drift, II: High-Latitude Evaporite Deposits and Geologic History of Arctic and North Atlantic Oceans

Abstract: High-latitude evaporites-late Proterozoic through Early Permian-are widespread in Canada and Eurasia. Their distribution patterns show that they precipitated from marine waters entering the continents from the Eurasian-Arctic basin and from the North Atlantic, but not from the Canadian-Arctic basin, which is separated from the former by the Proterozoic Lomonosov sill and from the North Pacific by the Archean Bering-Chukotsk Shelf. Late Proterozoic through Devonian evaporites which precipitated from Arctic waters do not extend (except locally) west of the present Rocky Mountains or east of the present Chukotsk-Koryak Ranges. Marine connections between the high-latitude evaporite basins and those of Tethys were minimal. After Devonian time, evaporite depocenters shifted systematically Atlanticward with the formation of the Franz Josef and Faeroes-Greenland sills. High-latitude evaporite deposition was scarce after the Faeroes-Greenland sill formed. Thus the requisite temperature and salinity for late Proter...

Geological and Geophysical Investigations of a Basaltic Layer in Archean Terrain of Eastern India

Abstract: In the Singhbhum district of Bihar (eastern India) there is a prominent band of metamorphosed basaltic rocks (known as Dalma volcanics) in a complex Archean terrain. Its outcrops were interpreted as the core of a regional syncline and, accordingly, a stratigraphic sequence was erected. We question this interpretation and show by both geological and geophysical investigation that this basaltic band does not occupy the core of a syncline but instead forms a sheet in the folded Archean sequence. On this basis, the stratigraphic sequence is revised.

Development of Precambrian Shield in Southwestern Greenland, Labrador, and Baffin Island: ABSTRACT

Abstract: The Precambrian rocks on either side of Davis Strait show a similar pattern of events and can be interpreted as having formed part of a single shield. Eight major stages in the development of this shield are suggested. (1) Formation of an extensive early crust before 3,000 m.y. ago, relicts of which are now preserved as migmatite and high-grade gneiss in the Archean block of eastern Labrador and southwest Greenland. (2) Deposition of greenstone belts between 2,700 and 3,000 m.y. ago. (3) Plutonic activity in the period 2,500-2,900 m.y. ago, affecting both the greenstone belts and the major part of the basement on which they lie. (4) Intrusion of numerous basic dike swarms in the general period of 2,000-2,600 m.y. ago. (5) Lower Proterozoic (Aphebian) geosynclinal rocks we e deposited on the consolidated Archean basement and were involved in an orogeny known in Canada as Hudsonian and in Greenland as Ketilidian and Nagssugtoquidian. (6) Post-orogenic magmatism, particularly prominent in areas affected by Hudsonian metamorphism, extends from southern Greenland through Labrador. This produced chiefly anorthosites, adamellitic granites, monzonites, and norites, that were probably emplaced between 1,400 and 1,700 m.y. ago although the areas in which they occur commonly remained thermally active to 1,200 m.y. ago or later. (7) Emplacement of the post-orogenic rocks was accompanied and followed, in southern Greenland and in parts of Baffin Island, by graben faulting, deposition of molasse sediments, and widespread intrusion of basic dikes. The majority of these ikes are tholeiitic; however, locally (for example, in the Gardar Province of southern Greenland), alkaline and peralkaline intrusions took place 1,100-1,300 m.y. ago. (8) The Archean and Proterozoic rocks in the southern part of the Canadian shield were metamorphosed and tectonically "worked" by the Grenville orogeny about 900-1,100 m.y. ago. The only effect of this orogeny in southern Greenland was a slight updating of earlier rocks from areas close to major faults so that they yield K/Ar ages of about 900-1,000 m.y. End_of_Article - Last_Page 2472------------

Crustal model based on aeromagnetic profiles, gravity studies and wave velocities in rocks and crust

Abstract: Regional airborne magnetic profiles from India and U.S.A. are analyzed. Profiles are i) 130 km offshore Manglore to 60 km offshore Madras (India) along 13th parallel; ii) Washington to San Francisco (U.S.A.): iii) Brownsville (Texas) to Guatemala City (Mexico). Depth to the sources of magnetic anomalies along Manglore-Madras profile and Washington-San Francisco profiles is calculated either by elementary approximation ofSmellie or Prism model method ofVacquieret al. It is significant that depth values for some of the anomalies obtained by these methods are in very good agreement with those based on drilling data. The magnetic pictures along these profiles are compared with Bouguer gravity anomaly maps and it is shown that in almost all cases where magnetic bodies lie below 5 km (approximately) from sea level they are not reflected in gravity maps whereas all the magnetic bodies which are above 5 km (approximately) produce a markable feature in Bouguer gravity anomaly. This indicates that density of material below this level is almost equal to that of normal basic rocks (2.80 gm/cm3) and those above 5 km have a density less than this. Based on these results the top most layer in crust is considered to be metasedimentary including intrusive rocks and below this it is tentatively taken as Quartz-diorite accounting for the quartz rich Archean formations. Curves representing the variation of compressional wave velocity in i) granite; ii) quartz-diorite; iii) gabbro and iv) dunite, with pressure and temperature as reported from measurements in laboratory, are studied in the light of the general variation of P-wave velocity in the earth's crust reported from seismic sounding studies. It is found that a change in composition from metasedimentary zone to quartz diorite at about 5 km below sea level is supported by this study. It is found that further increase in compressional wave velocity in earth's crust can be explained by a compositional change from quartz diorite to gabbro. At certain places an unusual high velocity for compressional wave at the base of the crust is reported. This can be explained by considering that gabbro merges to Dunite in those areas. Based on this crustal model a probable explanation for the origin of granite masses is attempted.

High-Latitude Evaporite Deposits and Geologic History of Arctic and North Atlantic Oceans: ABSTRACT

Abstract: High-latitude evaporites--late Proterozoic through Early Permian--are widespread in Canada, Siberia, and Europe. Their distribution patterns show that they precipitated from marine waters entering the continents from the Eurasian-Arctic basin, not from the Canadian-Arctic basin, which is separated from the former by the Proterozoic Lomonosov sill, and from the North Pacific by the 1,350-km-wide, Archean, Bering-Chukotsk shelf. Late Proterozoic through Devonian evaporites which precipitated from Arctic waters do not extend (except locally) west of the Rocky Mountains or east of the Chukotsk-Koryak Ranges. Marine connections between the high-latitude evaporite basins and those of the Tethys seas were minimal. After Devonian time, evaporite depocenters shifted systematically Atlanticward with the progressive formation of the Franz Josef and Faeroes-Greenland sills. High-latitude evaporite deposits are scarce after formation of the Faeroes-Greenland sill. Thus the requisite temperature and salinity for late Proterozoic-Paleozoic evaporite deposition in high latitudes during evaporite-maximum periods can be attributed only to the existence, and persistence, of the Gulf Stream-North Atlantic Drift system since middle Proterozoic time. No proposed mechanism of continental drift or polar wandering accounts for the high-latitude evaporite-deposition pattern, or for the consistent and progressive Atlanticward shift of evaporite depocenters through time. Continental drift and polar wandering in the Arctic and North Atlantic Ocean areas, if either ever took place, are pre-late Proterozoic events. End_of_Article - Last_Page 2495------------

Interpretation of elastic - strain - recovery measurements near Elliot Lake, Ontario1

Abstract: In situ stresses obtained by measurements of elastic - strain - recovery in quartzose sedimentary rocks near Elliot Lake are interpreted in terms of two stress environments: one stress field induced by mining close to the mine openings and a remanent stress field preserved in the rocks from a time of tectonic deformation in the area. i The research on which we would like to comment concerns prediction of in situ stateof-stress in rock masses around mines. In situ rock stresses are currently being investigated in uranium mines near Elliot Lake, Ontario, by the Mines Branch Mining Research Center, using strain cells cemented to the end of boreholes and measuring elastic - strain - recovery after overcoring. Stress measurements of this type have been carried out in many parts of the world during the last ten years. The most interesting result is that very high horizontal compressive stresses were found in many places, greatly exceeding the values to be expected under conditions of pure gravitational loading. Consequently, several authors have suggested a tectonic origin for these high stresses (among others Hast 1958; Obert 1962; Coates and Grant 1966). To examine the possibility of a tectonic control of high horizontal stresses near Elliot Lake the regional structure was analyzed in detail and measurements in vertical upholes were reexamined with the tectonic fabric of the region in mind. Tectonic deformation of the area probably took place sometime between 1800 and 1200 million years ago (Eisbacher 1969). The Archean basement and the Huronian sedimentary cover were shortened slightly and the granitic basement was uplifted differentially 'Presented at the Symposium on Recent Crustal Movements, Ottawa, Canada, March 17-18, 1969. FIG. 1. Compression axes for the main deformation of the Elliot Lake region. Long arrows represent the major principal tectonic compressive stress axes. Oblique ruling: Archean greenstone and greywacke; V-pattern: mainly Archean granitic rocks; Blank space and dotted pattern: Huronian sedimentary rocks.