Vol. 55, No. 1, 2020

Geophysica Vol. 55, No. 1, 2020

Abstract

The Arctic Ocean is a high latitude, ice covered ocean in its upper part strongly stratified in salinity and the warm Atlantic water entering through Fram Strait and the Barents Sea is isolated from surface driven mechanical and thermohaline forcing. This allows other mixing processes such as double-diffusive convection to become important and it has been argued that interleaving, driven by double-diffusive convection, may increase the exchange of Atlantic water between the boundary current and the interior of the basins. Although intrusions are present practically everywhere in the Arctic Ocean their formation has been more difficult to observe. Here it is postulated that intrusions are created almost instantaneously at narrow fronts, where waters with different mixing histories come into contact, especially at the strongest front in the Arctic Ocean, formed at the Kara Sea slope as the two Atlantic inflow branches again meet. A conceptual model is used to describe the formation of intrusions. Two water masses separated by a narrow front are assumed to have the same vertical density, but different salinity and temperature stratifications. The thickness of the intrusions is estimated by requiring that the temperatures are equilibrated in vertical boundary layers between the water masses. This implies upward motion on the cold side, downward on the warm side. The motions are antisymmetric across the front and stop when the waters have attained the same temperature. The water on the cold side is then less dense than that on the warm side and the waters in the boundary layer start to interleave, opening paths across the front for the main water masses. This gives the maximum vertical displacement, and it is unlikely that it is attained before the boundary layers either go unstable or become affected by external disturbances. The double-diffusive transports across the created diffusive and salt finger interfaces homogenize and change the density of the intrusions, driving the cross-frontal spreading. The situation when both heat and salt are initially stably stratified is examined in detail. If the stability at the interfaces separating the intrusions is weak, the transport across the diffusive interface dominates, cold intrusions rise and the salt finger interfaces might overturn. When the stability increases the transport across the salt finger interface becomes the largest and cold intrusions should sink. It is assumed that such change in slope does not take place. The intrusions then cease to expand, become fossil and are transported with the mean circulation around the Arctic Ocean.

Abstract

This study entails the detailed analyses of high-resolution gravimetry dataset using enhancement techniques for characterising and delineating the locations, edges/boundaries, trends, and depths of lithostructural features around Precambrian basement complex of Igabi region, Northwestern Nigeria with a view of evaluating the structural architectures that harbor mineralization in the study area. The analyzed results of the bouguer anomaly and residual maps of study area showed the distribution of the gravity anomalies and magnitudes of the concealed structures based on the observed low to very high gravity anomalies. Bouguer anomalies around Igabi area ranged between -67.77 to -53.34 mGal reflecting the density variations within bedrock. The upward continued bouguer anomaly maps at distance 500 m, 1 km, 2 km, 3 km, and 4 km revealed the variations of the deep-seated basement rocks, the structures and the concealed anomalous bodies with general regional trends in NW-SE, E-W, and NE-SW directions. The bouguer analytic signal and its superimposed maps further revealed that areas with low amplitude signals may be associated with migmatites, schists, less dense felsic rocks (porphyritic granites) and fractures, and areas of high amplitude signals may be associated with denser biotite granitic and gneissic rocks. In addition, the second vertical derivative and tilt derivative maps clearly revealed the density of shallow basement rocks and near circular closures anomalies associated with fractures within the granitic rocks. Spectral analysis suggests depth to gravity sources range between 0.3 km and 0.67 km for shallow, 0.90 km to 0.97 km for intermediate and 1.5 km to 1.86 km for deep sources while Euler sources depths ranged from <1392.3 m to >2059 m. Based on the calculated bouguer anomalies such as variation in rocks densities, different structures and varying trends of litho-structures in with subsurface depth may have suggested intense deformation of the Basement rocks with varying tectonic framework in the study area over time.