Abstract

In the Piedmont province of southern Lancaster County, Pennsylvania, equal area, lower hemisphere plots of lineations, and poles to foliations and fractures are compared for serpentinite, areas at the schist-serpentinite contact, and schist. Analysis of the resultant patterns indicates that: shear and strike joints are more numerous in serpentinite than in schist; fractures in the serpentinite are more abundant and more varied than in schist; schist-serpentinite contacts are commonly sheared; near these contacts fracture planes and plunging fold axes tend to be steepened or over-turned to the north; cleavage in schist near serpentinite parallels irregularities in the strike of the serpentinite contact; the major, southeastern serpentinite belt transects the strike of regional cleavage in schist at a small angle; and several deformations have affected both schist and serpentinite.

An analysis of the sequential age relationships of serpentinite fractures indicates that two deformational patterns are present that are younger than the final emplacement of the serpentinite.

From a discussion of the patterns, it is concluded that: fracture pattern analysis of serpentinites and the surrounding host rock can be utilized in a study of serpentinite tectonics; based upon differences in homogeneity and anisotropy between schist and serpentinite, serpentinites responded to deformational stresses largely as a brittle material; the serpentinite was more responsive than the schist to local stress variations; the final emplacement of the serpentinite was controlled by some pre-existing structural lineament; the superimposed fracture patterns were locally influenced by the south-dipping attitude of the schist-serpentinite contact; and chrysotile veinlets, banded chromite, pegmatites, and deweylite-filled fractures exhibit preferred orientations.

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