USE OF PALEOSTRESS INVERSION TECHNIQUES TO EVALUATE TECTONIC PROCESSES AT VOLCANIC PASSIVE MARGINS, EARLY MESOZOIC HARTFORD BASIN, CONNECTICUT AND MASSACHUSETTS, USA

Contractional structures have been recognized in the early Mesozoic rift basins of the eastern North American rift system since the late 19th century. Their origin, however, remains enigmatic. The contractional structures formed after deposition of the synrift strata and, in the southeastern United States, before intrusion of the 201 Ma igneous dikes of the central Atlantic magmatic province (CAMP). These observations, in combination with the generally northwest strike of CAMP dikes in the southeastern United States, led Withjack and coworkers to propose that the contractional structures formed in response to active asthenospheric upwelling at the incipient spreading center, effectively resulting in an enhanced ridge push force.

The enhanced ridge push hypothesis predicts a phase of postrift compression in which the maximum compressive stress axis is subparallel to the minimum compressive stress axis that prevailed during rifting and is testable through the techniques of paleostress inversion. The main challenge lies in separating heterogeneous fault-slip data into homogeneous subsets. Much is known, however, about the stress history of the eastern North American margin, and so we use two approaches to test the hypothesis: a blind inversion in which no geological information is used and a guided inversion in which we systematically remove fault-slip data that are consistent with known states of stress. We apply these approaches to fault-slip data from the Hartford basin, which is on the volcanic part of the eastern North American margin and is, therefore, expected to have experienced a phase of postrift compression related to active asthenospheric upwelling during breakup. Both approaches yield a phase of conjugate strike-slip faults with the expected orientation of the maximum compressive stress axis, supporting the enhanced ridge push hypothesis for the origin of contractional structures in rift basins of the eastern North American rift system.