Paleomagnetic studies and estimation of geomagnetic paleointensity at the Early/Middle Riphean boundary in rocks of the Salmi Formation (North Ladoga area)

The territory of Karelia ( Baltic Shield) is virtually not represented in the global paleomagnetic database for the Lower Riphean time interval ( 1650 - 1350 Ma). As regards the paleointensity H-an, the huge interval 1 - 2 Ga in length is represented in the global paleointensity database by only eight determinations concentrated in the interval 1 - 1.35 Ga. The paper presents results of paleomagnetic studies of volcanic and subvolcanic rocks composing the Early Riphean Salmi Formation, which outcrops in the valley of the lower Tulemaioki River in the northern coast area of Lake Ladoga. Results of the study indicate that, in the Early Riphean time, the East European craton was located in the tropical region of the Southern Hemisphere between 15 degrees S and 40 degrees S. The inferred value of H-an is close to the lower boundary of the interval (1.36 - 11.56) x 10(22) A m(2), encompassing previously published intensity values of the paleofield 1 - 1.35 Ga; this supports the hypothesis on the existence of long intervals of a lower field in the period in question [Maquoin et al., 2003].