The Filabusi greenstone belt (FGB), Zimbabwe craton, has been geologically remapped relatively recently but its regional tectonic setting and subsurface structure have, until now, remained unresolved. This paper prese...The Filabusi greenstone belt (FGB), Zimbabwe craton, has been geologically remapped relatively recently but its regional tectonic setting and subsurface structure have, until now, remained unresolved. This paper presents gravity and aeromagnetic studies that have been undertaken to provide this important information, and also extend mapping to areas of poor exposure. Several new NNW-trending dykes and structures cutting across the greenstone belt have been revealed, as well as a major extension of one of the metakomatiitic-BIF units, the Shamba Range. ESE-trending dykes identified in the southeast appear on a regional scale to be part of the giant Okavango dyke swarm in northern Botswana. An ~3 km wide NNE-striking magnetic low occurs over the Irisvale-Lancaster shear zone (ILSZ) on the extreme west of the FGB where it roughly marks the boundary with the Bulawayo greenstone belt. Magnetic anomaly trends over ultramafic schists are consistent with strike-slip movement along the ILSZ, and together with the gravity anomalies, support northeasterly directed detachment of the adjacent Fort Rixon belt from the Bulawayo-Filabusi belt. The Bouguer gravity anomaly map shows that the FGB is characterised by a well defined positive anomaly up to 37 mGal, whose symmetry and extent confirm the postulated synclinal structure of the belt. Isolated oval shaped small gravity lows generally correlate with sub-/out-cropping K-rich post-volcanic granite plutons. 2.5D gravity models along three profiles across the greenstone belt show a simple “basin shape” with a possible maximum depth extent of only 4.5 km, compared to an estimated stratigraphic thickness of about 9.0 km. This suggests a truncation at shallow depth of the structurally repeated lithologies. Gravity data and models support the proposed FGB model;deposition of volcanics in an extensional, structurally determined, evolving basin. This autochthonous setting is consistent with other greenstone belts in the Zimbabwe craton and other parts of the world.展开更多
The gravity and magnetic survey lines of about 13,500 km were carried out in the centraland northern parts of the South China Sea from 1977 to 1978. The results obtained showthat the Bouguer gravity and magnetic anoma...The gravity and magnetic survey lines of about 13,500 km were carried out in the centraland northern parts of the South China Sea from 1977 to 1978. The results obtained showthat the Bouguer gravity and magnetic anomalies have a tendency to increase gradually theirvalues from the northern continental shelf, through the slope, to the central abyssal basin of theSouth China Sea. The change in free-air gravity anomaly values coincides to a certain degreewith the undulation of the sea-bottom topography. The primary factor determining regionalvariation of the Bouguer gravity anomayl values is the Moho depth. The main factor deter-mining the magnetic anomly values is the nature of the basement rock. The high magnetieand Bouguer gravity anomaly values observed in some fault basin areas are inferred to becaused by draping the basic and ultrabasic magma extruding along the faults on the basementof the metamorphic rock,or by intrusion of the same magma into the basement.展开更多
文摘The Filabusi greenstone belt (FGB), Zimbabwe craton, has been geologically remapped relatively recently but its regional tectonic setting and subsurface structure have, until now, remained unresolved. This paper presents gravity and aeromagnetic studies that have been undertaken to provide this important information, and also extend mapping to areas of poor exposure. Several new NNW-trending dykes and structures cutting across the greenstone belt have been revealed, as well as a major extension of one of the metakomatiitic-BIF units, the Shamba Range. ESE-trending dykes identified in the southeast appear on a regional scale to be part of the giant Okavango dyke swarm in northern Botswana. An ~3 km wide NNE-striking magnetic low occurs over the Irisvale-Lancaster shear zone (ILSZ) on the extreme west of the FGB where it roughly marks the boundary with the Bulawayo greenstone belt. Magnetic anomaly trends over ultramafic schists are consistent with strike-slip movement along the ILSZ, and together with the gravity anomalies, support northeasterly directed detachment of the adjacent Fort Rixon belt from the Bulawayo-Filabusi belt. The Bouguer gravity anomaly map shows that the FGB is characterised by a well defined positive anomaly up to 37 mGal, whose symmetry and extent confirm the postulated synclinal structure of the belt. Isolated oval shaped small gravity lows generally correlate with sub-/out-cropping K-rich post-volcanic granite plutons. 2.5D gravity models along three profiles across the greenstone belt show a simple “basin shape” with a possible maximum depth extent of only 4.5 km, compared to an estimated stratigraphic thickness of about 9.0 km. This suggests a truncation at shallow depth of the structurally repeated lithologies. Gravity data and models support the proposed FGB model;deposition of volcanics in an extensional, structurally determined, evolving basin. This autochthonous setting is consistent with other greenstone belts in the Zimbabwe craton and other parts of the world.
文摘The gravity and magnetic survey lines of about 13,500 km were carried out in the centraland northern parts of the South China Sea from 1977 to 1978. The results obtained showthat the Bouguer gravity and magnetic anomalies have a tendency to increase gradually theirvalues from the northern continental shelf, through the slope, to the central abyssal basin of theSouth China Sea. The change in free-air gravity anomaly values coincides to a certain degreewith the undulation of the sea-bottom topography. The primary factor determining regionalvariation of the Bouguer gravity anomayl values is the Moho depth. The main factor deter-mining the magnetic anomly values is the nature of the basement rock. The high magnetieand Bouguer gravity anomaly values observed in some fault basin areas are inferred to becaused by draping the basic and ultrabasic magma extruding along the faults on the basementof the metamorphic rock,or by intrusion of the same magma into the basement.
