FORTY YEARS OF FAULT-SLIP ANALYSIS IN THE HARTFORD–DEERFIELD BASIN: WHAT HAVE WE LEARNED?

Studies analyzing fault-slip data from the Hartford–Deerfield basin have a bimodal distribution in time. Early work in the mid 1970s to early 1990s coincided with early development of computer-based paleostress inversion, and this work was done by hand or with a combination of manual and computer-based approaches. To make the analysis tractable, faults were assumed to have the characteristics of neoformed faults with σ₁ at 30° to the fault and σ₂ in the fault at 90° to the striae. This was assumed for direct determination of principal stress axis orientation or for separation of faults into groups before computer-based determination of principal stress axis orientation. Recent work in the 2010s has taken advantage of improvements in computer-based paleostress inversion, precluding the need to predetermine fault groups or to assume the faults are neoformed.

Our work with a dataset of over 700 faults from the basin shows early work was generally successful at defining the main phases of deformation despite the large proportion of faults that do not conform to the expectations of neoformed faults. Two aspects of the fault-slip data, however, were underappreciated: (1) the radial character of the normal faults and (2) a set of strike-slip faults consistent with a NW–SE σ₁. Paleostress inversion and forward modeling indicate the normal faults consist of faults consistent with a NW–SE σ₃ and faults consistent with a stress ratio of 0, i.e., σ₂ and σ₃ of about equal magnitude. We suggest the latter characteristic may be related to ponding of CAMP magma prior to extrusion and thermal doming of the crust. The strike-slip faults that are consistent with a NW–SE σ₁ are likely related to basin inversion. Our dataset shows the dip direction of these faults is symmetric about the vertical whereas the dip direction of synrift strike-slip faults is biased toward the west. We, therefore, suggest much of the dip of the basin strata was acquired during postrift arching of the region, as has been proposed for the Newark basin on the basis of detailed work addressing the geometry of the basin fill. The fault data indicate arching was a result of horizontal shortening rather than vertical uplift and lend support to the idea that basin inversion was driven by active asthenospheric upwelling along a volcanic passive margin during early seafloor spreading.