Direct Inversion Method of Fault Slip Analysis in Bedrock Exposures to Reconstruct the Chronology for Tectonic Events in the Southwestern White Mountain Region of New Hampshire, USA

The orientation and relative magnitudes of paleo tectonic stresses in the western central region of the White Mountains of New Hampshire is reconstructed using the direct inversion method of fault slip analysis on 1-10 m long fracture surfaces exposed on a series of road cuts along Interstate 93, just east of the Hubbard Brook Experimental Forest in North Woodstock, NH. In this method, the direction and sense of offset of paleo tectonically induced slips on fracture surfaces is measured using slickenside indicators. Fracture orientation, slip direction and sense of offset are input into a computer program which groups fracture slips direction and sense of offset into syntectonic events and derives the orientation and relative magnitude of principal stresses (1 ,2 ,3). That group of fractures is then removed, and the process of grouping is repeated to establish another event. Relative chronology of events is established by superposition of slip indicators. The direct inversion method yields five tectonic events that impacted the White Mountain region over the last 390 Ma. Event 1 (~390-375 MYA) is compressional, reverse fault type with 1 at 1300 and 3 vertical. Event 2 (~375-325 MYA) is extensional, normal fault type with 1 vertical and 3 switching between 330 and 2900. Event 3 (~335-260 MYA) is compressional, reverse/strike slip fault type with 1 switching between 2390 and 830 and 3 switching between vertical and 3510. Event 4 (~190-95 MYA) switches between compressional and extensional, reverse/strike slip fault types, with 1 switching from 3300 to 1880 and 3 switching from vertical to 930. Event 5 (15 MYA to present) is strike-slip with 1 at 1300 and 3 at 40o. This study shows that most of the fractures are (~390-95 MYA) and that the current tectonic stress regime (15 MYA-Present) is the most recent to reactivate fractures in the region. We find no reactivation of fractures due to the multiple Quaternary glacial loading and unloading (2.58 MYA-Present).