Marc Sbar

Summary Strain relaxation in the Barre Granite and surrounding metasediments in Vermont, was measured by overcoring strain gauge rosettes bonded to outcrop surfaces. The average maximum expansion upon relieving 15.2 cm diameter cores trends N55°W, while the average maximum expansion of 7.6 cm diameter cores coaxial with 15.2 cm cores trends N70°W. The maximum strain relief of the internal overcores

four sites in the Late Cambrian Keeseville member of the Potsdam sandstone indicates that compressive stresses were relieved upon overcoring with an average direction of maximum expansion of N78øW. These nine measurements also indicate that stress can be consistent in orientation over areas of tens of square kilometers. The average of three measurements at one site in the Nicholville member 11 km from the other four sites indicates a maximum expansion oriented N18øE. At several sites, residual strain was recovered when specimens free of boundary loads were overcored. We suggest that the residual strain measured in the Nicholville member may be related to a late Proterozoic (Hadrynian) or Early Cambrian stress field.

Downloaded from gsabulletin.gsapubs.org on 13 July 2009 MARC L. LYNN R. SBAR SYKES Lamont-Doherty Geological Observatory of Columbia University, Palisades, New Yor{ 10964 Contemporary Compressive Stress and Seismicity in Eastern North America: An Example ...