Northeastern Section - 53rd Annual Meeting - 2018

Paper No. 56-4
Presentation Time: 8:00 AM-12:00 PM


GENTRY, Emilie N., Geology and Geological Engineering, Colorado School of Mines, 1500 Illinois St., Golden, CO 80401, KUIPER, Yvette D., Department of Geology and Geological Engineering, Colorado School of Mines, 1516 Illinois Street, Golden, CO 80401, HOLM-DENOMA, Christopher S., U.S. Geological Survey, Central Mineral and Environmental Resources Science Center, Denver Federal Center, Denver, CO 80225 and HEPBURN, J. Christopher, Earth and Environmental Sciences, Boston College, Chestnut Hill, MA 02467-3809

The NE-trending >300 km long, middle Paleozoic, crustal-scale, right-lateral transpressive Norumbega fault system (NFS) is present in Maine (ME) and southern New Brunswick. Northeasterly-trending shear zones in New Hampshire (NH) and topographic lineaments in NH and Massachusetts (MA) were investigated, through field mapping and in situ monazite U-Pb Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS), to test whether they may be part of the NFS and to characterize the southwestern termination of the NFS.

NNE-trending lineaments in eastern MA did not show evidence for dextral shear and are not interpreted as part of the NFS. The Calef shear zone in NH follows the southern projection of the NFS, but does not show evidence for dextral movement. The orientation of the Nannie Island shear zone in NH is similar to the NFS, and displays both sinistral and dextral shear sense indicators. The Wekepeke fault in MA and the Campbell Hill fault in NH are brittle and do not preserve dextral shear sense. The east-trending Shirley Mylonite zone in MA has a different orientation than the NFS, but shows dextral shear sense indicators. The Rye Complex in NH contains mylonite with a NW-dipping foliation and north-trending sub-horizontal lineation, with dextral shear sense indicators, similar to the NFS.

Monazite in selected samples was located using automated mineralogy and imaged using backscattered electron microscopy to elucidate its textural relationships. Monazite ages from the Appleton Ridge and Sandhill Corner segments of the NFS in ME are ~390-359 Ma, which may signify a continuous period of shear, or reactivation. This age is consistent with existing interpretations based on 40Ar/39Ar hornblende and U-Pb zircon and monazite ages that dextral shear along the NFS had initiated by 380 Ma. Structural observations and ~403-380 Ma monazite ages from the Rye Complex indicate it may have been affected by the NFS. Concordant >~460 Ma monazite data from the Nannie Island shear zone are older than the host rock and interpreted as detrital. In MA, dextral shear along the Shirley Mylonite zone occurred at ~365 Ma, but its orientation is not consistent with that of the NFS. Thus, structural and U-Pb monazite results indicate that the deformation in the Rye Complex is the only shearing south of ME that may be consistent with the NFS.