Ned A. Ostenso

Geopshysical Studies in the Greenland Sea

Magnetic, gravity, and seismic sub–bottom profiles are used to contribute to the structural interpretation of the Greenland Sea and environs. The most anomalous feature of the magnetic survey is the subdued magnetic expression of the Mid–Atlantic Ridge north of the Jan Mayen fracture zone. Over the median rift valley magnetic anomalies are an order of magnitude smaller than those recorded in the Atlantic Ocean. A suggested explanation, supported by seismic data, is that crustal tension decreases northward along the ridge. The inferred structural consequence is that the tensional mid-oceanic ridge merges into the compressional Verkhoyansk Range beneath the Arctic Ocean. Gravity data suggest an abrupt increase in crustal thickness under the Greenland continental shelf. Maximum seismic penetration into the sediments of the ocean floor was 1.2 sec. Due to masking by strong bottom reflection multiples, poor sub–bottom resolution was obtained on the continental shelf. The steep and jagged continental slope off Kronprins Christian Land, the occurrence of seamounts, and the proximity of earthquake epicenters support the thesis that the Lena Trough is the medial rift of a northern extension of the Mid–Atlantic Ridge. The sub–bottom profile oft the east coast of Greenland reveals three widely separated faults. These correlate with projections of known tectonic features and suggest: (1) The thrust fault of the main Caledonian orogeny of northeastern Greenland extends to a depth in excess of 3 km and continues into the ocean floor at least 85 km from the coast; (2) the Jan Mayen fracture zone, which offsets the Mid–Atlantic Ridge, extends into Greenland at approximately 72° N.; and (3) the Caledonian fold belt may correlate directly between eastern Greenland and northern Britain.

A Gravity Survey of the Chukchi Sea Region, and Its Bearing on Westward Extension of Structures in Northern Alaska

Gravity data are used to support the conclusion that the Brooks Range structure is continuous across the Chukchi Sea with the Chukchi-Anadyr fold belt. A morphological reconstruction of this section of the mobile belt that is consistent with known geologic and structural similarities, recorded sub-bottom seismic profiles and observed gravity follows: Moderate Tertiary folding produced a dry-land intercontinental connection that persisted until about a million years ago when erosion finally reduced the low mountain barrier to a near peneplain. Subsequently, the sea shelf has been modified by numerous changes in sea level and deposition of a thin, often patchy veneer of sediments. Scanty gravity data suggest that the Brooks Range structure is not continuous with the Franklin geosyncline to the east. The Chukchi Shelf is now in isostatic equilibrium. A Bouguer gravity low over Herald Island and Herald Reef suggests that they are formed by a single large granitic pluton. Small gravity highs off Lisburne Peninsula and south of Wrangel Island may be caused by basic or intermediate extrusives of Cretaceous to early Cenozoic age. The Tigara uplift produces a relative gravity high across the end of the Lisburne Peninsula.

Aeromagnetic Evidence for Origin of Arctic Ocean Basin: ABSTRACT

Approximately 147,000 km of low-level (450 m) aeromagnetic tracks were flown over the Arctic Ocean and adjacent Greenland and Norwegian Seas. From these data inferences could be made about the geologic structure and evolution of the Arctic Ocean basin. The Alpha and Nansen Ridges produce magnetic profiles that show axial symmetry and correlate with profiles in the North Atlantic that cross the Reykjanes Ridge and profiles in the Norwegian Sea that cross Mohns Ridge. A quantitative attempt has been made to verify these correlations, which infer that the Alpha Cordillera became inactive 40 m.y. ago, when the locus of rifting shifted to the Nansen Cordillera. The lack of magnetic disturbances associated with the Lomonosov Ridge is interpreted to be a section of the former Eu asian continental margin that was translated into the Arctic basin by sea-floor spreading along the Nansen Cordillera axis. Within the Canada basin there is a thickening of sediments from the Asia continental margin toward the Canada Arctic Archipelago. Sediment thickness in the Makarov basin is estimated to be 1-1½ km. There appears to be only about ½ km of sediment covering the younger Fram and Nautilus basins. The absence of large magnetic anomalies over these basins can be explained by a 10-km elevation of the Curie isotherm. End_of_Article - Last_Page 2512------------

Geophysical Studies in Barents and Kara Seas: ABSTRACT

A reconnaissance geophysical survey of the Barents and Kara Seas, conducted from icebreakers in 1965 and 1966, consisted of about 10,000 line km of bathymetric-magnetic data, 57 gravity measurements, and 2 short end-to-end seismic refraction profiles. Magnetic data do not support a continuation of Caledonian or Precambrian basement structures under the Barents Sea at shallow depth. A broad 400 ^ggr anomaly due to a source about 20 km below sea level strikes NNW between Norway and Bear Island. Magnetized basement approaches the sea floor only near Bear Island and 74.5°N, 33°E. Free-air anomalies in the north-central Barents Sea are within 20 mgals of zero. Kara Sea magnetic anomalies parallel, but do not connect, tectonic structures of Taimyr and Novaya Zemlya. M gnetic basement is deep in the southwestern Kara Sea, but shoals northeast of a line connecting northern Novaya Zemlya with the Yenisey estuary. Anomalies with amplitudes greater than 500 ^ggr occur End_Page 2511------------------------------ only within 100 km of the Precambrian Izvestiy Tsik Islands. The East Novaya Zemlya trench is slightly under compensated, having a median free-air gravity of -9 mgals and extremes of -33 and +11. Two refraction measurements in the eastern Barents Sea yielded 400 m unconsolidated sediments over a 3.1 km/sec basement and 700-1,100 m of 2.8 km/sec sediment over a 4.1 km/sec basement. The basement is probably Paleozoic sandstone or shale. End_of_Article - Last_Page 2512------------