POSSIBLE SOUTHWESTWARD CONTINUATION OF THE NORUMBEGA FAULT SYSTEM BENEATH THE SOUTHERN NEW ENGLAND FOLD-THRUST BELT

The alignment, steep northwest dips, and linearity of the Norumbega Fault System (NFS) and Higganum dike system (HDS) and the fact that the HDS crosscuts Paleozoic terranes in southern New England suggest that the NFS may continue southwestward beneath the fold-thrust belt of Massachusetts and Connecticut. According to this hypothesis, southeast-directed thrusting during Pennsylvanian time would have beheaded the NFS there, concealing the deeper part of the fault beneath the fold-thrust belt. The beheaded brittle part of the NFS eventually would have been eroded away as it was thrust upward and southeastward several tens of kilometers. Reactivation of the concealed NFS at depth since the Alleghanian could have fractured the overlying fold-thrust belt. The only such feature that crosscuts the fold-thrust belt along this trend is the approximately 200-km-long Higganum dike system (HDS), which consists of Early Jurassic (about 201 Ma), left-stepping en echelon tholeiitic basalt dikes up to 150 m wide and dipping about 55°-85°NW. Sinistral (transtensional) strike-slip reactivation of the NFS beneath the fold-thrust belt by WNW-ESE-oriented crustal extension during the Early Jurassic fractured the overlying fold-thrust belt, thus providing a conduit for the HDS intrusives. Although the amount of sinistral offset was small along the HDS-related fractures, this is consistent with studies of the NFS that indicate that no significant lateral movement has occurred on the NFS since the Paleozoic.