Abstract:

Tieshan in southeast Hubei is the first area where boneshaped boudinage was discovered in Asia. To acquire the distributing diagrams of relating parameters such as true differential strain (1-2), kinematic vorticity（Wk） and orientations of the major axes of the finite strain ellipse, the finite strain measurement at different positions in the matrix of asymmetric boneshaped boudinage in this area was carried out by inertia moment ellipse method. It is shown that the finite strain in matrix of the asymmetric boneshaped boudinage is controlled by its competent layer, namely negatively related to the distance away from the competent layer and to the granularity of minerals in the matrix. The finite strain in matrix also shows larger in its thinned sections than that in thickened sections along direction of its elongation. Other structures around the boudin with similar scale could disrupt the distribution of strain in the study matrix. Besides, the study shows that the asymmetric boneshaped boudinage in this area was formed by simple shear parallel to the layer during early period and by pure shear with extension parallel to the layer and contraction perpendicular to it later. Comparison between asymmetric boneshaped boudinage and asymmetric fishhead boudinage of the same area reveals that the pure shear and simple shear in their matrix are corresponding to the relatively thinned sections and thickened sections respectively, and that their differences in shape are mainly related to the difference in competency of minerals filled in their discontinuous sections, which proves that both sorts of boudinages could serve as rheological indicators of rocks.

Key words: strain measurement, strain analysis, boneshaped boudinage, inertia moment ellipse method, Tieshan in southeast Hubei