Notes on volcanism and structure in Franklin County

Abstract

The basaltic rocks of Franklin County, South Auckland, are divided into two formations, the Franklin Basalts and the Bombay Basalts. The age of the former is Mid-Pleistocene, younger than the Tyrrhenian sea-level of 110 ft, and older than the Monasterian sea-levels of 40 to 80 ft; the latter may be Late Pliocence to Early Pleistocene in age. Faulting of Pleistocene age has been due mainly to collapse along preexisting fractures, following expulsion of lava. Nevertheless, transcurrent faulting shows that some horizontal compression was present. The local principal horizontal stress direction is likely to have been north-north-east during and since the Miocene, but the main period of faulting ceased prior to volcanism.