PALEOSTRESS ANALYSIS OF POST-ALLEGHANIAN BRITTLE FAULTS FROM AN EXPOSURE IN THE PUTNAM-NASHOBA TERRANE, EASTERN CONNECTICUT

New construction in Lisbon, CT, has exposed large and conveniently accessible roadcuts with a dense and heterogeneous collection of faults. The roadcuts, which are just west of the Tatnic fault, are well-layered gneiss of the Tatnic Hill Formation in the Putnam-Nashoba terrane. Fault-slip data from 90 faults were collected in order to understand the post-Alleghanian brittle deformation.

Fault-slip data were separated into phases using the program T-Tecto and arranged chronologically based upon age relations identified from fault surfaces in the field. Phase 1 consists of a conjugate set of ~NW-SE-striking normal faults coupled with a conjugate set of ~N-S-striking sinistral strike-slip faults and ~E-W-striking dextral strike-slip faults. Phase 2 consists of ~NW-SE-striking strike-slip faults, interpreted as reactivated surfaces from the normal faults of phase 1. Phase 3 lacks a conjugate set and consists of faults that are most likely reactivated surfaces. Each conjugate set from phase 1 was evaluated separately because drag folds observed in the field suggest the strike-slip faults (renamed phase 1b) postdate the normal faults (renamed phase 1a). Phase 1a displays a ~NE-SW σ3, phase 1b displays a ~NW-SE σ1, phase 2 displays a ~N-S σ1, and phase 3 displays a ~ENE-WSW σ1.

The stress orientations are interpreted as related to Mesozoic rifting and subsequent development of the passive margin. Phase 1 stress orientations indicate a change from NE-SW (phase 1a) extension to NW-SE (phase 1b) compression. Phase 1b possibly developed synchronously with structural inversion of the rift basins. Main-phase rifting with NW-SE extension is not seen, suggesting that phase 1a is the transition from rifting to structural inversion. Phase 2 stress orientations indicate N-S compression and are consistent with other observations in New England. Phase 3 stress orientations indicate ENE-WSW compression possibly characterized by the present-day state of stress. Previous work has seen structural inversion in synrift strata in the Fundy rift basin and southeastern United States characterized by NE-striking reverse faults; however, we see structural inversion in basement characterized by conjugate strike-slip faults.