Abstract:

Tieshan in southeast Hubei is the second place in the world where the  trapezoidal boudinage was discovered. Using trapezoidal boudins with similar shape and locating in the same layer in this area as studying objects, this study acquired the distribution of true differential strain(ε-1－ε-2), stratum thickness ratio(S), kinematic vorticity number(Wk)and major long axe of finite strain ellipse(λ1)through finite strain measurement at different positions of its matrix with inertia moment ellipse method, then studied the data combined with its morphological and petrologic features. The results shows that the thickness ratio(S)of shear zone in the direction perpendicular to its shear direction tends to be negatively related to differential strain. This study also proved that trapezoidal boudinages in this location were formed due to comprehensive effects of difference in thickness between the upper and lower matrix layers, pure shear with extension parallel to strata and with contraction perpendicular to strata in most parts of matrix layers, and simple shear distributed mainly in partial matrix. Besides, the hydrothermal flow concentrated relatively in hornfel layers near wedgeshaped cracks also played a significant role. Characteristic features during the formation of trapezoidal boudinages similar in shape and continuous in competent layer manifest in two aspects, namely, on the one hand, a large difference in thickness between matrix layers, and on the other hand, the combined action of the sustained pure shear with extension parallel to strata and with contraction perpendicular to strata in most parts of matrix layers, and the simple shear concentrated locally in matrix layers. The above results indicate that this sort of trapezoidal boudinages is a good rheological indicator of rocks.

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