Near-surface in situ stress. 3. Correlation with microcrack fabric within the New Hampshire granites.

We studied the correlation between near-surface in situ stress and the preferred orientation of microcracks at two quarries in the Milford granite and one quarry in the Conway granite, New Hampshire. The orientation of in situ stress was determined by overcoring doorstoppers and from the strike of induced borehole fractures produced by a pressurized packer. The preferred orientation of vertical microcracks was determined using ultrasonic measurements to determine P wave velocity V_p on core and in situ and from thin section analyses. In situ V_p anisotropy was determined from interborehole travel time data. At all three sites, directions of maximum compressive stress sigma ₁ and induced borehole fractures are aligned with the preferred orientation of open microcracks determained from core V_p and thin section data. An analysis of the microcracking sequence within each pluton suggests 1) that the quarry grain resulted from cooling-induced thermal stresses and 2) a method of distinguishing the paleostress axes at the time of cooling from contemporary stress. -Authors Lamont-Doherty Geol. Obs., Columbia Univ, Palisades, New York, USA. English