BRITTLE DEFORMATION WITHIN THE EASTERN NORTH AMERICAN VOLCANIC MARGIN: PALEOSTRESS INVERSION OF FAULTS IN THE HARTFORD BASIN

The Hartford basin is an early Mesozoic, fault-bounded half-graben that formed during the breakup of Pangea. Along with other eastern North American rift basins, it has incurred significant post-rift deformation. Previous authors have identified three phases of deformation in the Hartford basin: WNW-ESE extension (rifting), NNE-SSW shortening (basin-parallel inversion), and E-W shortening (consistent with the current state of stress on the eastern North American margin). More recent work from other rift basins suggests a fourth phase of deformation possibly associated with massive volcanic activity shortly after cessation of rifting. Paleostress directions inferred from CAMP dike orientations and large-scale compressive structures in the southern rift basins reflect a NW-SE shortening phase that is seemingly absent from the Hartford, Newark, and Fundy basins of the central segment. If this compressional phase is consistent along volcanic passive margins, it should be recorded in the synrift deposits of the central basins. Our preliminary dataset shows a significant number of faults that are inconsistent with the previous three-phase model of WNW-ESE rifting, NNESSW compression, and E-W compression. A paleostress inversion of these incompatible faults reveals a distinct conjugate pattern of strike-slip faults that are indicative of NW-SE compression. This phase is consistent with the basin inversion seen in the southern rift basins. The presence of a NW-SE compressional stage is also consistent with the hypothesis that depicts massive volcanic upwelling that acts to shorten the upper crust after rifting. If this model is consistent along the eastern North American margin, it is likely applicable to volcanic margins around the world.