A comparison of crustal and upper mantle features in fennoscandia and the rhenish shield, two areas of recent uplift

Abstract

Abstract The creep properties of the asthenosphere and the lithosphere are considered important parameters in controlling vertical and horizontal movements. The boundary between lithosphere and asthenosphere is not well defined, whereas a relatively abrupt chemical boundary like the Mohorovicic-discontinuity (= Moho) can be well traced by seismic methods. In the Fennoscandian area weak reflected waves P m P but strong “head”-waves P n were detected in contrast to the observations within the Rhenish shield where strong P m P and sometimes intracrustal reflections, but only very weak P n -waves are observed. In general, high temperatures in quartz-bearing rocks result in a low-velocity lower crust. A low velocity should increase the reflection coefficient at the Moho considerably — producing strong P m P-waves. A high temperature also produces a negative velocity gradient below the Moho, leading to weak or missing P n -waves, as observed in the Rhenish shield. In Fennoscandia stress patterns in the uppermost part of the crust indicate a strong compressive stress from the Atlantic Ocean to the continent. Creep movements from the lower viscosity and lower density oceanic asthenosphere pushing against and intruding into the continental asthenosphere seem to be responsible for: (1) the above mentioned compressive stresses in the crust; (2) the asymmetric form of the uplift; (3) having scraped off a probable former root of the Caledonides; (4) the long wavelength gravity anomaly; and (5) for the decreasing travel time residuals from northwest to southeast within the area of the Fennoscandian shield. On the other hand, the smaller Rhenish shield seems to be mainly affected by compressive stresses from the direction of the Alps and by some stresses connected with the opening of the lower Rhinegraben. It is believed that here creep movements in the asthenosphere also produced: (1) the above mentioned stresses in the crust; (2) the recent uplift of the Rhenish Massif; (3) the elevation of the crust—mantle boundary, especially in the south; and (4) the related gravity high in the eastern part of the Rhenish Massif. The relatively small area of uplift of the Rhenish Massif compared to that of the Fennoscandian shield may reflect a creep activity at a shallower level in agreement with the higher temperature and lower viscosities in these depth ranges.+