Mapping Cross-Cutting Joints in Cape Ann to Establish Past Stress Orientations

Abstract

The Cape Ann Granite, part of the Cape Ann Plutonic Complex, ranges from 431 to 424 Ma. It is a massive, medium to coarse grained alkali granite. Previous studies indicate that the Cape Ann Granite intruded prior to the beginning of the Acadian Orogeny, when Avalonia accreted to Laurentia. In Gloucester and Rockport, Massachusetts, the Cape Ann Granite is cross cut by multiple faults and joints. Our study focuses on mapping and recording joint orientations in the Cape Ann Granite. These are then subdivided into joint sets based on their orientation after plotting them on a rose diagram. Based on our understanding of the spatial relationship between newly developed joints and the principal stress axes σ1 (maximum principal stress), σ2 (intermediate principal stress) and σ3 (minimum principal stress), inferences can be made about the orientation of the paleo-stresses necessary to create the observed patterns of joints. Additionally, the relative timing of various joint sets can be investigated by looking at relative cross cutting relationships. In fact, joints do not cross-cut older joints but rather terminate along pre-existing joints.