Allan Ludman

Seismic reflection evidence for the evolution of a transcurrent fault system: The Norumbega fault zone, Maine

A seismic reflection profile in east-central Maine reveals a steeply dipping fault zone that terminates in diffraction hyperbolae that are associated with an offset of the Moho. When combined with data from geologic mapping, the seismic data imply Mesozoic or post-Mesozoic dip-slip reactivation of the Norumbega fault zone, hitherto interpreted as a locus of mid- to late Paleozoic dextral offset. This finding highlights the value of seismic reflection data in determining the history of fault displacement and warns of the complexities that arise in attempts to seismically characterize faults in polygenic tectonic regimes.

Evolution of a transcurrent fault system in shallow crustal metasedimentary rocks: the Norumbega fault zone, eastern Maine

Abstract Differential exhumation in Maine exposes nearly half of the crustal profile of the Norumbega fault zone (NFZ), a major transcurrent fault zone of the Northern Appalachians. A large exposure of turbidites in eastern Maine records a multi-stage deformation history that provides insight into the epizonal evolution of the NFZ. Early deformation (Stage 1) involved episodic bed-parallel faulting with fluids playing a progressively smaller role. Strain was broadly distributed at the exposure but was localized by lithic anisotropy in pelitic layers. Little Stage 1 deformation occurred in the dominant wacke beds (by diffusive mass transfer); most took place in subordinate pelitic horizons by combined diffusive mass transfer and crystal plastic mechanisms. Later (Stage 2) activity was almost entirely brittle, producing isolated NE-trending faults and two deformation zones filled with cataclasite. The latter comprise complexly anastomosing fault strands with multiple slickenline generations indicating episodic faulting. Regional-scale NFZ structures and history mirror those at outcrop scale. Late brittle faulting was superimposed on a broad region that had experienced largely ductile shearing, and the regional-scale brittle deformation was concentrated in three deformation zones characterized by anastomosing fault strands. It is the Stage 2 features that are mapped elsewhere in the Northern Appalachians as the Norumbega fault zone.

Timing of terrane accretion in eastern and east-central Maine

The Norumbega fault zone is often cited as a post-Acadian suture between exotic blocks, even though stratigraphic, structural, and metamorphic data indicate that there is little offset of the Silurian-Devonian strata that the zone cuts in eastern Maine. Similarly, the Kingman fault zone has been shown by gravity and geochemical studies to separate distinct crustal blocks, whereas mapping shows that it lies entirely within a Silurian turbidite package. These conflicts are resolved if the two fault zones represent boundaries between Ordovician or older crustal blocks that had accreted to form a composite terrane prior to deposition of the cover sequences. The faults now mapped within these younger rocks formed by reactivation of the pre-Silurian boundaries during late Acadian time; movement continued until the late Carboniferous. Most of the accretionary history of Maine had thus ended before the Silurian. A complex composite terrane may have formed during Cambrian-Ordovician time that (1) interacted with cratonic North America during the Taconian orogeny and (2) became the “basement” upon which the Silurian and Lower Devonian strata of eastern Maine were deposited.

The Norumbega Fault Zone, Maine: a mid- to shallow-level crustal section within a transcurrent shear zone

From studies of structure, metamorphism, and geochronology we have evidence that the Norumbega Fault Zone represents a transition from mid- to shallow crustal levels in a dextral, transcurrent shear zone within the northern Appalachian Orogen. The Norumbega Fault Zone strikes parallel to the orogen (northeast-southwest), is ~5 to 30 km wide, and is characterized by distributed ductile dextral shear fabrics in the southwestern section with a transition to brittle fabrics toward the northeast. Within the zone of distributed shear, deformation is partitioned into local zones of very high strain. Upright, isoclinal folds are common in areas of high and low strain. Metamorphic grade decreases from amphibolite facies in the southwest to sub-green schist facies in the northeast. 40 Ar/ 39 Ar mineral ages from recrystallized minerals in a high strain zone, regional cooling ages in areas of lower strain, and metamorphic textures are consistent with a polyphase history of deformation. We interpret a younging trend in 40 Ar/ 39 Ar cooling ages toward the northeast, together with the deformational fabrics andmetamorphic features, to represent exhumation of the southwestern section of the Norumbega Fault Zone from mid-crustal levels during the polyphase history of this transcurrent zone. The Norumbega Fault Zone may therefore serve as a model for deformational processes at mid- to shallow crustal levels in active strike-slip systems. n n RESUME n Des etudes de la structure, du metamorphisme et de la geochronologic nous fournissent la preuve que la zone faillee de Norumbega represente une transition des niveaux crustaux moyens a des niveaux peu profonds dans une zone de cisaillement transversal dextrale a linterieur de la partie septentrionale de lorogene appalachien. La zone faillee de Norumbega suit une direction parallele a lorogene (nord-est-sud-ouest); die a cinq a 30 km de largeur; et elle est caracterisec par des structures cisaillees dextrales deformables se ramifiant dans la section sud-ouest avec une transition a des structures cassantes vers le nord-est. A linterieur de la zone de cisaillement ramifie, la deformation est divisee en zones locales de tres forte contrainte. Les plis droits isoclinaux sont courants dans les secteurs de forte et faible contrainte. Lintensite du metamorphisme decroit dun facies a amphibolite dans le sud-ouest a un facies secondaire de schistes verts dans le nord-est. Les periodes minerales 40 Ar/ 39 Ar des mineraux recristallises dans une zone de forte contrainte, les periodes de refroidissement regional dans les secteurs de faible contrainte et les textures metamorphiques correspondent a une orogenese polyphasee. Nous interpretons la tendance de rajeunissement dans les periodes de refroidissement 40 Ar/ 39 Ar vers le nord-est, de meme que les textures de deformation et les caracteristiqucs metamorphiques, comme une exhumation de la section sud-ouest de la zone faillee de Norumbega depuis les niveaux crustaux moyens pendant lhistoire polyphasee de cette zone transversale. La zone faillee de Norumbega pourrait, par consequent, servir de modele pour les processus de deformation aux niveaux variant de moyens a peu profonds dans les systemes a decrochement. n n[Traduit par la redaction]

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.n 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. n 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 (u0c3c1) 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.n n RESUMEn La cartographie detaillee de lEst 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 lAcadien 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.n Meme si la deformation ductile du SFN a ete segmentee en trois zones principales a forte contrainte dans lEst du Maine, seule la zone centrale de Waite a subi une deformation fragile prononcee apres lAcadien. La zone de Waite est par consequent consideree comme la faille maitresse des retablissements fragiles. Tous les affleurements de couches rouges ulterieurs a lAcadien se situent dans la zone de Waite, notamment deux nouvelles tranches delimitees par des failles decouvertes au cours de la presente etude.n Les deux premiers episodes de formation de failles cassantes ont precede la sedimentation des couches rouges ulterieures a lAcadien 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 lAcadien (Carbonifere?), possiblement en vertu dun phenomene dextension. Les deux premiers episodes fragiles sont probablement survenus au cours du Devonien tardif a lepoque du Mississippien. La formation de failles diagonales (inverses-senestres) au cours du troisieme episode a deforme les couches rouges; on lattribue a lorogenese alleghanienne du Permien. Des analyses des paleocontraintes basees sur les failles conjuguees daffleurement abondantes permettent de supposer que la contrainte principale maximale (u0c3c1) des trois episodes fragiles a pivote dans le sens antihoraire, passant dune 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.

Revised stratigraphy of the Cookson Group in eastern Maine and southwestern New Brunswick: an alternate view

Fossil evidence reported by Fyffe and Riva (1990) requires inversion of the stratigraphic succession originally proposed for the St. Croix Terrane (Ludman, 1987). Other suggested revisions, however, are not in accord with structural and sedimentologic data in eastern Maine. Comparison of structural histories indicates that the Pocomoonshine Lake and Digdeguash formations are more appropriately part of the Frederic ton Belt than of the St Croix Terrane as suggested by fyffe and Riva(1990). An alternate view of the stratigraphic and tectonic relationships between the St Croix Terrane and Fredericton Belt includes the following changes: the Pocomoonshine Lake Formation, originally assigned to the St Croix Terrane, is now considered to be the oldest unit of the Fredericton Belt, lying conformably below the Digdeguash Formation; a late Acadian thrust fault is thought to have transported the St Croix Terrane westward over the Fredericton Belt; subsequent high-angle dip-slip and strike-slip faulting have obscured original relationships—these faults now form the boundary between the two terranes in southeastern Maine. n nRESUME n Le temoignage apporte par les fossiles quont signaled Fyffe et Riva (1991) necessite une inversion de la sequence stratigraphique proposee a lorigine pour la Laniere de St Croix (Ludman, 1987). Cependant, les autres revisions, ayant ete suggerees sont en contradiction avec les donnees stiucturales et seclimentologiques en provenance du Maine oriental. La comparaison des antecedents structuraux indique quil est plus adequat dincorporer les formations de Pocomoonshine Lake et de Digdeguash a la Ceinture de Fredericton qua la Laniere de St. Croix comme lont suggere Fyffe et Riva (1991). Une conception de rechange ayant trait aux relations stratigraphiques et tectoniques entre la Laniere de St Croix et la Ceinture de Fredericton englobe les changements suivants: la Formation de Pocomoonshine Lake, rattachee a lorigine a la Laniere de St Croix, est maintenant tenue pour etant la plus vieille unite au sein de la Ceinture de Fredericton, recouverte en concordance par la Formation de Digdeguash; un chevauchement tardi-acadien est envisage comme ayant transporte la Laniere de St Croix vers louest par-dessus la Ceinture de Fredericton; le jeu subsequent de decrochements et de failles a rejet selon leur pendage, tous fortement inclines, a obscurci les rapports originaux—ces failles constituent maintenant la frontiere entre ces deux lanieres dans le sud-est du Maine. n n[Traduit par le journal]

Tectonic significance of black pelites and basalts in the St. Croix Terrane, coastal Maine and New Brunswick

The St Croix Terrane of coastal Maine and adjacent New Brunswick is characterized by Ordovician carbonaceous pelites in the upper part of the stratigraphic section. These pelites are locally interbedded with mafic volcanic rocks. Metamorphism in the terrane varies from the albite-epidote-homfels facies along the New Brunswick-Maine border to greenschist and lower amphibolite facies in southwestern Maine. n n The mafic volcanic rocks are evolved basalts that have trace-element abundances similar to intraplate tholeiites. Their La/ Nb ratios of 1.3 to 1.9 and absolute REE concentrations of 30 times chondrite in the more evolved basalts arc features common to many continental flood basalts. Enrichment in high field-strength elements relative to MORB distinguishes them from volcanic-arc tholeiites. n n Deposition of carbonaceous pelites coincided with the widespread development of oceanic crust in the early Paleozoic. The trace-element and REE patterns of the associated basalts suggest eruption through rifted continental crust marginal to an expanding ocean basin. n n RESUME n La presence de pelites riches en carbonc dans la panic superieure de sa section stratigraphique caracterise la Laniere de St. Croix qui se rencontre au littoral du Maine et dans les regions du Nouveau-Brunswick avoisinantes. Ces pelites sinterlilent localement avec des volcanites mafiques. Le metamorphisme de la laniere va du facies des comeennes a albite et epidote le long de la frontiere entre le Nouveau-Brunswick et le Maine jusquau facies inferieurdes amphibolites dans le sud-ouest du Maine. n nLes volcanites mafiques sont des basaltes evoluws presentant des abondances delenents en traces semblables a celles des tholeiites intraplaques. Leurs rapports La/Nb de 1.3 a 1.9, ainsi que leurs concentrations absolues en T.R. egales a 30 fois celles des chondrites au sein des basaltes plus evolues, sont communes a plusieurs nappes dinondation basaltique continentales. On les distingue des tholeiites d arc insulaire par lew enrichissement en elements a inlensite de champelevee par rapport aux MORB. n n Le depot de pelites riches en carbone coincida avec le developpement largement repandu de croût;e oceanique au debut du Paleozoique. Les spectres de T.R. et delements en traces des basaltes qui leur sont associees suggercnt une eruption a travers une croute continentale a la derive en bordure dun bassin oceanique en expansion. n n[Traduit par le journal]