Stress and timing relationships of a fault‐related, paleohydrothermal system in central New Hampshire: Record of a Mesozoic stress change in New England?

Abstract

A mineral assemblage comprised of chalcedony, drusy quartz, and very fine grained iron hydroxide (limonite), with or without blocky calcite (QFC), coats joint surfaces, fills fractures, and commonly occurs as pressure shadow encrustations on brittle fault surfaces in central New Hampshire but only occurs sporadically in other areas of New England. Locally, QFC includes silicified breccias of highly altered host and euhedral quartz crystal fragments suspended in a matrix of chalcedony and limonite. One hundred and four brittle faults from central New Hampshire, mineralized with QFC, can be separated into two families. Set N is composed of primarily north–northeast to northeast trending, steeply dipping normal faults (n=61), and set S is composed of north‐northwest to north‐northeast trending, steeply dipping, primarily right‐lateral strike‐ to normal oblique‐slip faults (n=43). These two sets of faults are thought to record two paleostress conditions. The extension direction for both tensors plunges gently east‐southeast, but the compression direction is near vertical for set N and trends roughly northeast, plunging 30° for set S. Fault‐slip inversion suggests that set N faults are best fit with σ1 most deviatoric (σ2 ≈ σ3). Set S is best fit with σ3 most deviatoric (σ1 ≈ σ2) and low values for fault plane coefficient of friction and/or high fluid pressure. QFC occurs in the 158 Ma Belknap Mountain Complex of the White Mountain Magma Series and is thought to have developed at relatively shallow crustal levels based on the assemblage, nature of fluid inclusions, and association of QFC as brittle fault and fracture filling mineralization. Occurrence of QFC in the 158 Ma Belknap Mountain complex, paleostress implications of QFC‐mineralized brittle faults, and range and character of fault fabrics in the Belknap units suggests that QFC developed around the time of Belknap intrusion and cooling and that Late Jurassic to Early Cretaceous is the approximate time at which a regional change in paleostress conditions occurred in New England. This age and nature of the paleostress change is in accord with a change postulated by de Boer et al. (1988).