PULSATING OBLATE AND PROLATE 3-DIMENSIONAL STRAINS

The progressive ductile deformation of competent spherical inclusions is modeled analytically. Results of this study may help to understand better the limitations connected to geological field methods using com petent inclusions for strain analyses. Parameters studied and quantifi ed here are the strain magnitude, the progressive change in inclusion shape, the orientation of the finite strain axes, the frequency of pul sation, and the coupling between the strain ellipticity and viscosity contrast. Competent inclusions develop pulsating apparent strains if t he host material is subjected to a component of simple shear and provi ded time or strain rate is sufficient to complete the strain cycle. Th e disparity between the strain magnitude inferred from competent visco us inclusions and that undergone by the host rock, increases for large r viscosity between them. The pulsation of the inclusion may suggest z ero strain after a strain cycle has been completed, even though strain in the host rock is extremely large. The inclusion will develop pulsa ting oblate strains ifa shortening rate is superposed normal to the pl ane of pulsation. Conversely, pulsating prolate strains occur if an ex tension rate is superposed instead of shortening. Stretching lineation s outlined by deformed competent inclusions within shear zones beneath collapsing nappe sheers may even point perpendicular to the direction of nappe transport. This finding offers an explanation for the occurr ence of mutually perpendicular pebble elongations in nearby locations within the Bygdin conglomerate beneath the Jotun nappe, Norwegian Cale donides.