Characterizing the southwestern extent of the Norumbega fault system, a mid-Paleozoic crustal-scale strike-slip fault system in the New England Appalachians

Abstract

The >300 km long, northeast-trending dextral transpressive Norumbega fault system in Maine and New Brunswick is defined by a tens of kilometers wide zone of multiple dextral shear zones. Based on published 40Ar/39Ar hornblende data and U-Pb zircon and monazite ages, dextral shear along the fault system initiated by ~380 Ma. The location and nature of the southwestern termination of the Norumbega fault system was investigated in this study. Previously, no fault system in New Hampshire or eastern Massachusetts has been recognized with an orientation, timing, and motion sense consistent with those of the NFS. Detailed structural mapping was carried out along topographic lineaments and mapped shear zones in New Hampshire and Massachusetts to test whether the NFS extends into those areas. The Rye Complex in southeastern coastal NH was the only location investigated that presented field characteristics consistent with the NFS. The Nannie Island shear zone in southeastern NH is along strike with the NFS, but contains both sinistral and dextral shear sense indicators and has a lower metamorphic grade. The dextral ENE-trending Shirley mylonite zone in central-eastern MA is similar to the NFS, but the ENE trend is not consistent with that of the NFS, and no connecting shear zone between the Shirley mylonite zone and NFS has been found. U-Pb monazite laser ablation inductively coupled mass spectrometry (LA-ICPMS) dates dextral deformation along the NFS and its potential extensions to the southwest. The four sample locations dated were the NFS in ME, the Rye Complex, the Nannie Island shear zone, and the Shirley mylonite zone. The NFS contained monazite ages of ~390-370 Ma related to dextral deformation. The Rye Complex yielded ages of ~430 to ~370-360 Ma, ~380 Ma and ~370 Ma, consistent with NFS deformation and earlier Acadian metamorphism that started at ~430 Ma. The Nannie Island shear zone produced monazite populations of ~441, ~700, ~900-800, and ~1775 Ma, which are all older than the late Silurian or earliest Devonian age of the rock and, therefore, interpreted as detrital. The Shirley mylonite zone in MA yielded an age of ~380 Ma which is consistent with the age of deformation along the NFS. Thus, the Rye Complex is the only location investigated in this study that matches the NFS in shear zone orientation, deformation style, shear sense, and age of deformation, and therefore could be the southernmost extension of the NFS. The NFS has been demonstrated not to extend farther to the southwest.