麻豆传媒

The careful measurement and analysis of deformed worm burrows and buckled sand layers and quartz veins occurring in slates of the Cambro-Ordovician Meguma Group of Nova Scotia have permitted the first estimates on the amount of finite strain, expressed as percentage of shortening measured at right angles to the axial-plane slaty cleavage of the terrain, which folded during the Acadian orogeny into north-east trending upright folds.

Worm burrows from Blue Rock, Lunenburg Co., which originally had a circular section, have been deformed into elliptical prisms, whose minor and major axes are in average 30% shorter and 43% longer respectively than their original radius, if the assumption is made that the deformation has not involved loss of volume. This is in agreement with the percentage of shortening of buckled quartzite layers in shale at the same locality: 32%.

Buckled quartz veins, however, which were deformed as more competent bodies in a less viscous shale matrix, were found to have been shortened by a minimum of 42% and a maximum of 63% (average 50%). The difference between the strain of worm burrows and of veins may be due to inhomogeneity in the folding domains, differences in the scale of the domains considered or to loss of volume due to considerable dehydration of the worm burrows and their matrix during formation of the slaty cleavage. It is concluded that the Meguma group metasediments occupied a belt at least twice as wide as its present width, and this does not include the shortening imposed by folding.

The concepts of slaty cleavage and of finite strain in two dimensions of a homogeneous body are analyzed by means of geometrical models and experimentally. Implications for the rotation of vectors during pure shear deformation are briefly discussed in the light of the above data.

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