Abstract

At Riverton, Virginia, in the Athens limestone are conjugate sets of en echelon tension fractures which demonstrate that division of the strained rock mass into wedge-shaped units precedes shearing.

The lower Ordovician argillaceous Athens limestone at Riverton, Virginia, exhibits pinnate tension fractures aligned, en echelon, in fairly straight rows. Some rows contain as many as 50 fractures. The rows of conjugate sets trend east-west and northwest-southeast. The shearing couple origin of echelon fractures suggests that the trend of each row may be considered as the trace of a plane of incipient shearing. Some shear fractures parallel the trend of a row. The direction of greatest shortening (the greatest stress axis) bisects the acute angle between the conjugate incipient shear planes.

The axis of elongation of the incipient shear planes is essentially horizontal, while the axis of elongation of the folds and thrust faults of the area is essentially vertical. However, the axes of shortening practically coincide, which suggests that the direction of greatest stress was essentially the same.

Two hypotheses are advanced: (1) There were two periods of compressive stress, each accompanied by a different direction of elongation. A modification of this hypothesis postulates that the similarity in orientation of the least strain axes at Riverton is a local coincidence and that a regional survey might show a change in the direction of these axes. (2) As in the deformation of a prism in Daubrée's classic experiment, two sets of conjugate shears developed simultaneously as the result of one compressive stress. Direction of elongation for one set of shears, however, was normal to that for the other.

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