PALEOSTRESS ANALYSIS OF MESOZOIC FRACTURES AND BASALT DIKES IN TUCKERMAN RAVINE, NEW HAMPSHIRE

The purpose of this research is to measure the fractured bedrock geometry of Tuckerman Ravine in the Presidential Range, New Hampshire, in an effort to determine the paleostress regimes in the Mesozoic during the continental rifting of the supercontinent Pangea. The bedrock of the study area is composed of the early Devonian Littleton Formation that was multiply deformed and metamorphosed in the Devonian Acadian orogeny which then experienced extension during the Mesozoic rift-drift stage of the Atlantic Ocean. Strike and dip of fractures and associated basalt dikes of the region were measured and recorded spatially. Schematic maps and cross-sections of the fractures and basalts were made and later imported into ArcGIS. A rose plot map was also made that shows structural domains of discrete fracture patterns within the region. The main fracture trends of the region were determined using the Kamb contour method. The field data was then compared to a lineament analysis which was performed in ArcGIS on three separate datasets: 1) a black and white air photo taken from Mt. Washington observatory; 2) hillshade maps derived from a 10m DEM with an illumination angle of 315 and 45; and 3) 1m resolution SPOT Earth observation satellite imagery. All three lineaments sets were merged and the duplicates removed. A domain overlap analysis was then made to identify discrete regions within the study area where lineaments and fracture domains have the same azimuth overlap to remove any final discrepancies. There are two dominant fracture trends striking 264.4°and 58.1° with two less dominant trend striking 2.3° and 153.2°. The dips for each fracture trend are 79.1 north, 84.1 southeast, 80.4 east, and 78.7 southwest, respectively. Two basalt dikes were mapped and one is parallel to the 60° fracture set (striking 61.6) while the other does not match any fracture set and strikes 110.0°. The dips for the dikes are 85.4 southeast for the 60 set and 85.4 southwest for the 110.0. A preliminary interpretation is that the four fracture sets yield four different stress fields oriented NW-SE (58.1° set, the oldest?), E-W (264.4° set, intermediate in age), and N-S (2.3° set and 153.2° set, youngest?). The relative ages of the sets are being examined currently and the relationships of these to plate tectonic events in the New England will be the focus of future work.