Task 5: Modelling of accelerated plate tectonics on a ridge following a major earthquake in a transform shear zone: inferences on the rheological structure below Iceland

Start: March 1997 (month 13)
End: February 1998 (month 24)
Responsible partner: UBLG.DF

Andrea Antonioli, Maurizio Bonafede, Antonio Piersanti, Eleonora Rivalta and Giorgio Spada

Magma ascent through a mid-oceanic ridge can be modelled as a tensile crack within which the overpressure is determined by magma buoyancy. Explicit analytic solutions are given for the elementary dislocation problem in the most simple layered medium, made up of two welded half spaces characterized by different elastic parameters. Particularly interesting appears to be the case of a crack cutting across the boundary between the two media. In this case a further singularity appears in the kernel of the integral equation, which is connected with the presence of the boundary surface. The problem can be solved by splitting the crack into two interacting open cracks. The application of a constant overpressure within the crack is found to produce drastically different stress regimes in neighbouring regions located on opposite sides of the interface; this feature may provide a straightforward explanation for the episodic reversal (from sinistral to dextral) of strike-slip mechanisms observed in the SISZ. If the rheological discontinuity between the lithosphere and the asthenosphere is considered, model results predict a much larger horizontal flow in the asthenosphere than is accomplished by motion of lithospheric plates. Furthermore, the stress field near the transition depth is strongly controlled by differential shear flow in the asthenosphere, thus yielding a simple explanation for the different stress regimes prevailing in the seismogenic zones of Iceland.

  

Figure: See text for discussion.

  

Figure: See text for discussion.

Figure 34 shows graphically a result of great interest in interpreting Iceland seismicity: if a vertical dyke 4 km long cuts across the interface between the half-space z>0, with rigidity 10 times greater than the half-space z<0, the horizontal compression in the stiffer medium may be much higher than magma overpressure (5 MPa) in the dyke. This component of stress is dominant in generating a significant deviatoric stress. Figure 35 shows the deviatoric component of the stress field generated in a layered medium, compared with results pertinent to a homogeneous medium. Since the asthenosphere has much lower long-term rigidity than the brittle crust above it, these results explain in a straightforward way the compressive stress transients inferred from shear wave splitting in connection with magma injection episodes along the rift zone.

Results are presented in two papers [,]. A further paper, devoted to the study of stress induced by dyke injection over rheological discontinuities, is to be submitted soon. Preliminary results on this topic were shown at the PRENLAB workshop in Paris [].