Paleostress Inversions of Post-Triassic Brittle Faults in Paleozoic Accreted Terranes and the Early Jurassic Higganum Dike, Eastern Connecticut

The geological development of a passive margin begins with the extension and attenuation of the lithosphere and ultimately leads to seafloor spreading. This is commonly referred to as the rift-drift tectonic model. Geological observations and tectonic modeling recognize the process to be significantly more complex than described. Reverse faults and folds in early Mesozoic rift basins of eastern North America are evidence for a compressional event occurring sometime around the rift-drift transition. While most research studies have concentrated on structures in synrift strata, we concentrate on brittle structures found in crystalline bedrock. Our study focuses on eastern Connecticut and includes fault-slip data from various Paleozoic accreted terranes and the early Jurassic Higganum dike. This allows us to restrict our analysis to Mesozoic rifting and subsequent development of the passive margin. Paleostress inversion of the fault-slip data combined with age relations observed in the field produces four tectonic phases. At the regional-scale, phase 1 consists primarily of normal faults striking in a variety of directions but mainly ~NE-SW and displays a NW-SE σ3. At one site, however, phase 1 consists of a conjugate set of ~NW-SE striking normal faults and displays a NE-SW σ3. Faults in phase 2 have a variety of orientations but are dominated by a conjugate set of ~N-S and ~E-W-striking strike-slip faults and displays a NW-SE σ1. Phase 3 consists of a conjugate set of ~NW-SE and ~NE-SW-striking strike-slip faults and displays a N-S σ1. Phase 4 consists of a conjugate set of ~NE-SW and ~WNW-ESE-striking strike-slip faults and displays an ENE-WSW σ1. Phase 1 stress orientations are characteristic of main-stage rifting. Phase 2 faults may have developed synchronously with structural inversion of the rift basins. The local evidence in phase 1 for NE-SW extension is suggested to be the transition from rifting to postrift inversion or may indicate flattening strain during rifting. Phase 3 faults are consistent with widespread observations for N-S compression in the region. Phase 4 stress orientations are parallel to the present-day state of stress. Our stress inversions for eastern Connecticut identify four tectonic phases and are consistent with other studies in eastern North America of the post-Triassic states of stress.