THE NORUMBEGA FAULT SYSTEM (NFS): LONG-LIVED TRANSCURRENT FAULTING IN EASTERN AND SOUTH-CENTRAL MAINE

The NFS is traced continuously for 450 km from Casco Bay in mid-coastal Maine to central New Brunswick, but its overall length, from the Gulf of St. Lawrence to Long Island Sound, is greater than that of the San Andreas fault. Differential uplift along the NFS in Maine exposes rocks sheared at sub-greenschist to amphibolite facies conditions, representing depths of ~3 to ~18 km. The high-grade segment in south-central Maine is characterized by a 30-km wide zone of distributed strain with narrow zones (< 1km) of more intense strain, both resulting from dextral shear. The supracrustal segment in eastern Maine is ~40 km wide with highest strain partitioned into three major strands separated by broad zones of less intense, widely distributed strain. Seismic reflection imaging reveals that least one of the strands penetrates and offsets the Moho. Despite the different T/P conditions, initial dextral strike-slip deformation was ductile in both deep and shallow areas and began at 380 Ma, tens of millions of years after onset of the Acadian orogeny. Sporadic reactivation of individual faults occurred several times. The narrow dextral ductile high-strain zones in the deeper segment formed in the Carboniferous but all post-380 Ma deformation in the shallow segment was brittle. Small-scale structures indicate that shallow reactivation included dextral, sinistral, and oblique-slip motion, reflecting both transpression and transtension. Ar-Ar and apatite fission track studies suggest that portions of the NFS were reactivated as normal faults in the Mesozoic. The tectonic significance of the NFS remains problematic although it probably records adjustments associated with major Acadian and Alleghanian plate collisions. However, despite its extent and longevity, estimates of offset following post-Acadian plutonism are remarkably low. Pluton provenance studies suggest that the NFS may separate cryptic crustal blocks at depth but it does not appear to do so at the presently exposed surface.