Abstract

Abstract We built a viscoelastic model earth that is appropriate to study the rate of vertical motion and shear stress accumulation driven by lithospheric density anomalies. The lateral extension of these heterogeneities is allowed to vary in the lateral direction to simulate tectonic transport. This model accounts for the effects of sphericity and discontinuities of tectonic phases. Our approach is different from the one followed by Quinlan and Beaumont (1984) and Jordan (1981) in which attention was drawn to the total deflection of the lithosphere beneath the overthrust. We provide the explicit expression for the inverse of the fundamental matrix entering the solution of the system for ordinary differential equations that control the time dependent deformation of an incompressible, viscoelastic solid in spherical symmetry. We found that sphericity plays a crucial role for vertical motions induced by elongated lithospheric anomalies of hundreds of kilometers. When the model is applied to the study of vertical deformation due to the overthrusting of the Apennines in the Mediterranean region, we obtained a lithospheric viscosity ranging between 10 22 –10 23 Pa·s, with the subsidence of the plate underlying the active front of the overthrust around 0.5 – 1.0 mm/year. This is consistent with the amount of sedimentation in the Adriatic foredeep. We analyzed the effects of discontinuities in the tectonic phases on vertical movements in collision zones. We discovered that the rate of vertical motion is a very sensitive function of lithospheric viscosity, horizontal velocity of overthrusting and timing of tectonic phases. We thus found a strong correlation between horizontal and vertical motions: this result suggests that altimetric geodetic surveying along levelling lines of a few hundred kilometers can constrain the tectonic style of a geologic region. Crustal deformation data obtained from satellite missions will provide another set of information on the horizontal motions that are needed in our modelling. Wegener/Medlas project will furnish, in the future, geodetic data on the relative velocities between different plates in the Mediterranean region (Wilson et al. , 1985). Analysis of the stress field induced by an overthrusting load shows that principal stress differences of a few bars can be accumulated on a time scale of 10 2 –10 3 year. These findings are consistent with the stress drop and recurrence times of the earthquakes in the northern Apennines. We conclude that lateral density variations are less effective in triggering earthquakes when comparison is made with other mechanisms, such as transcurrent motions along active plate margins. Discontinuity of tectonic phases plays an important role in controlling the amount of shear stress due to density anomalies.

Full Text
Published version (Free)

Talk to us

Join us for a 30 min session where you can share your feedback and ask us any queries you have

Schedule a call