Joint interpretation of AMT, gravity and magnetic surveys data along the regional profile across the Pasha-Ladoga rift zone (NW Russia).

Abstract:

Complex investigations using AMT soundings, gravity and magnetic methods have been realized along a regional profile in NW of Russia. These studies were intended on the structure investigation of Pasha-Ladoga graben which is the part of wide Proterozoic rift system (Lower and Middle Riphean in Russian time chart). This system is prevalent on East-European (Russian) platform and Fennoscandian shield. The established unconformity-type Karku uranium deposit is situated in the northern part of Pasha-Ladoga graben. The placer diamonds discoveries are also known in this territory, that is why the Riphean rift structure can be considered as promising for the prospecting of kimberlite pipes.

The investigated area is the Proterozoic sedimentary basin. Riphean terrigenous and effusive rocks unconformably overly Archaean-Proterozoic granite gneiss basement. Vendian- (Lower) Palaeozoic sediments are bedded at the upper part and cover the Pasha-Ladoga graben. The rift zone boundaries have been defined earlier using the aero-gravity data where the graben position has been linked to the gravity minimum. The thickness of Riphean sediments in the graben central part was previously estimated as 3 km.

As the result of joint data interpretation of AMT soundings, gravity and magnetic methods it has been decided that the gravity minimum corresponds to the Archaean rapakivi granites situated in the basement. They are characterized both by heightened resistivity and low density. According to AMT data, graben boundaries have been shifted to 10 km in SW direction comparing to the previously detected ones. There has been also determined the thickness of Riphean sediments and it has been found that the depth of Pasha-Ladoga graben does not exceed 1.5 km. In the basement structure, some blocks of basic rocks have been detected. They are characterized by heightened resistivity and density. Local conductive anomalies which are connected with graphitized fault zones have been marked out. They coincide with magnetic anomalies. The described complex investigations thus allowed us to obtain improved information about the deep structure of the Pasha-Ladoga graben SW part.