A continental structural junction: Evidence of shallow crustal discontinuities deduced from the gravity data over the Niger – Benue troughs (confluence) area, Nigeria

The anomaly signature from gravity survey over the confluence area of the lower Niger and lower Benue basins and adjacent regions show the existence of three sets of lineaments trending NW-, N-S, and NE-. These lineaments flank the sides of broad axial en echelon gravity highs. They also coincide with the Cretaceous sediments/crystalline Basement boundaries along the margins of the two Troughs and along the course of the River Niger to the south of Lokoja. Two of the sets of lineaments matching the boundaries of the Niger (NW-SE) and the Benue (NE-SW) troughs, intersect each other at about 90¢ª angle and both terminate in the confluence area in a less prominent N-S trend of the Basement Complex extending south of the Lokoja area thus forming a structural crosscutting junction. Interpretations of these lineaments suggest that those trending NW- form the structural boundaries of the Niger Basin while those trending NE- form the structural boundary of the Benue Basin and correlate with the oceanic fracture zones of the Romanche, Chain, and Charcot within the study area. These linear structures that probably controlled the opening of the Troughs were earlier reactivated by intrusions of the Paleozoic and Mesozoic ages. The results also suggest that this structural junction area is associated with intrabasement basic or mantle derived intrusive ridges at a depth extent of 2.23-7.00 km beneath the northern margin and within the southern margin, depth extent 3.54-7.00 km, of the Lower Benue Trough. These interpretations suggest that the three armed Y- shaped shallow crustal rifting started around the area, was basement controlled and most probably originated from deep source(s) of dynamic cycle(s) under the two basins. This further suggests a common origin for the two Troughs of the Niger and Benue.Keywords: Gravity, Continental-Oceanic Fracture Zones, Niger-Benue Confluence