Evidence for post-Acadian through Alleghanian deformation in eastern Maine: multiple brittle reactivation of the Norumbega Fault system

Detailed mapping in eastern and east-central Maine reveals a complex history of multiple brittle reactivations along the Norumbega fault system (NFS) following initial late Acadian (380 Ma) ductile shearing. Three distinct episodes of brittle faulting are now recognized, each characterized by a unique combination of fault kinematics and dynamics. These three events record a previously poorly-known part of northern Appalachian history — the transition from Acadian stress regimes related to one set of plate collisions to Alleghanian stresses associated with very different plate interactions. Although NFS ductile strain was partitioned into three major high-strain zones in eastern Maine, only the central Waite zone experienced significant post-Acadian brittle deformation. The Waite zone is therefore considered the master fault for the brittle reactivations. All post-Acadian redbed exposures are located in the Waite zone, including two new fault-bounded slivers discovered during this study. The first two brittle faulting episodes predated deposition of the post-Acadian redbeds and both involved dextral strike-slip motion although episode 1 also had a significant dip-slip component. Episode 2 was responsible for the formation of the post-Acadian (Carboniferous?) redbed basins, possibly through a pull-apart mechanism. Brittle episodes 1 and 2 probably occurred in late Devonian through Mississippian times. Oblique (reverse-sinistral) faulting during episode 3 deformed the redbeds and is attributed to the Permian Alleghanian orogeny. Paleostress analyses based on abundant outcrop-scale conjugate faults suggest that the maximum principal stress (఼1) for the three brittle episodes rotated counterclockwise from a WNW — ESE orientation during episode 1 to NE — SW for episode 2 and changed to N-S for episode 3. RESUME La cartographie detaillee de l'Est et du centre-est du Maine revele un passe complexe de retablissements fragiles multiples le long du systeme de failles de Norumbega (SFN) a la suite du cisaillement ductile initial de l'Acadien tardif (380 Ma). On reconnait maintenant trois episodes distincts de formation de failles fragiles, caracterises chacun par une combinaison unique de la cinematique et de la dynamique des failles. Ces trois episodes ont enregistre une tranche auparavant mal connue du passe du nord des Appalaches : la transition des regimes de contrainte acadiens apparentes a une serie de collisions de plaques, aux contraintes alleghaniennes associees a des interactions entre plaques tres differentes. Meme si la deformation ductile du SFN a ete segmentee en trois zones principales a forte contrainte dans l'Est du Maine, seule la zone centrale de Waite a subi une deformation fragile prononcee apres l'Acadien. La zone de Waite est par consequent consideree comme la faille maitresse des retablissements fragiles. Tous les affleurements de couches rouges ulterieurs a l'Acadien se situent dans la zone de Waite, notamment deux nouvelles tranches delimitees par des failles decouvertes au cours de la presente etude. Les deux premiers episodes de formation de failles cassantes ont precede la sedimentation des couches rouges ulterieures a l'Acadien et ont tous deux comporte un coulissage dextre, bien que le premier episode ait en plus presente un rejet incline marque. Le second episode a entraine la formation des bassins de couches rouges ulterieurs a l'Acadien (Carbonifere?), possiblement en vertu d'un phenomene d'extension. Les deux premiers episodes fragiles sont probablement survenus au cours du Devonien tardif a l'epoque du Mississippien. La formation de failles diagonales (inverses-senestres) au cours du troisieme episode a deforme les couches rouges; on l'attribue a l'orogenese alleghanienne du Permien. Des analyses des paleocontraintes basees sur les failles conjuguees d'affleurement abondantes permettent de supposer que la contrainte principale maximale (఼1) des trois episodes fragiles a pivote dans le sens antihoraire, passant d'une orientation ONO-ESE au cours du premier episode, au NE—SO au cours du deuxieme episode, puis au N-S au cours du troisieme episode.