No abstract

The Norumbega fault zone has been traced from southern New Brunswick to Merrymeeting Bay in south-central Maine. In south-central Maine the zone contains a complex record of narrow dextral high-strain shear zones set in a 20-40-km-wide zone of weaker dextral strain. Numerous brittle faults showing normal and reverse dip-slip and left-lateral strike-slip motion are also in this area. Age assignments for the various types of movement in the Norumbega fault zone in the Casco Bay-Merrymeeting Bay area range from middle and late Paleozoic to Mesozoic. The broad zone of weaker dextral shear is of Middle to Late Devonian age and coincident with the latest stage of high-grade metamorphism late in the Acadian orogeny. High-strain zones are of probable Permian age followed by brittle faults of Mesozoic age. In the Casco Bay area, the Norumbega fault zone widens, and narrow high-strain zones, represented by the Flying Point, South Harpswell, and Broad Cove faults, and the Macworth phyllonite splay out as curvilinear strands trending south and southwest toward large granitic to intermediate plutons of Devonian to Carboniferous age (Biddeford, Webhannet, Lyman and Exeter). The plutons are on strike and mostly older than the Norumbega fault zone high-strain segments, but show neither significant shear fabric nor mappable contact offsets. Several high-strain zones have been recognized in New Hampshire. The Portsmouth and Great Commons faults are along and within the Rye (mylonite) complex, and the Nannie Island and Calef faults are within the Merrimack Group. Outcrop evidence for connecting these faults in southeastern New Hampshire with those of the Casco Bay area is sparse; potential connections are constrained to corridors between plutons and to offshore areas. Possible onshore correlations include projections of the Broad Cove and Flying Point faults to the Nannie Island and Calef fault zones of southeastern New Hampshire via a corridor between the Biddeford and Lyman plutons, and west of the Webhannet pluton. Additional strain of the Norumbega fault zone may be more broadly distributed through phyllonitic rocks of the Macworth and Eliot Formations of the Merrimack Group, and the mylonites of the Rye complex in New Hampshire. Offshore, curvilinear structural trends indicated by regional aeromagnetic anomalies suggest a connection between rocks of the Casco Bay sequence and the Rye complex, and thus potentially the South Harpswell and Portsmouth faults. Farther offshore, parallel curvilinear aeromagnetic anomalies are interpreted to correspond to rocks of the Fredericton trough and sequences to the east and to the belt of Coastal Maine plutons. The anomaly patterns are sigmoidal, consistent with rock sequences deformed within a large dextral shear zone, the boundaries of which are interpreted to be Norumbega

Zircon and monazite were extracted from ϳ5 to 10 kg rock samples by routine mineral separation processes, including crushing, pulverizing, Wilfley table concentration of heavy minerals, and purification of mineral fractions using magnetic separation and methylene iodide. Detrital zircons were sprinkled onto double-stick tape to obtain relatively unbiased samples. Individual grains of monazite and titanite were handpicked under a binocular microscope. All samples were mounted in epoxy, ground to nearly half-thickness, and polished using 6 m and 1 m diamond suspension. All grains were digitally imaged in both transmitted and reflected light to identify areas free from inclusions, cracks, and other imperfections. Using the scanning electron microscope, zircons were imaged by cathodoluminescence (CL) to distinguish cores and overgrowths; monazites and titanites were imaged by back-scattered electrons (BSE).U-Pb ages of zircon and monazite were determined using the USGS/Stanford sensitive high resolution ion microprobe-reverse geometry (SHRIMP-RG) at Stanford University, following the methods described in Williams (1998); titanite was analyzed on SHRIMP II at the Research School of Earth Sciences, Australian National University. The area excavated by the primary oxygen ion beam was about 25 to 30 m in diameter for zircon and titanite, and about 15 to 20 m for monazite. 206 Pb/ 238 U ages were corrected for instrument fractionation using zircon standard R33 (419 Ϯ 1 Ma; Black and others, 2004), monazite standard 44069 (424 Ϯ 1 Ma; Aleinikoff and others, 2006) or titanite standard BLR-1 (1047 Ϯ 1 Ma, Aleinikoff and others, 2007). Uranium concentrations are believed to be accurate to Ϯ 20 percent. Raw data were reduced using Squid 1 (Ludwig, 2001) and plotted using Isoplot 3 (Ludwig, 2003). For detrital zircons from metasedimentary rocks, U-Pb data are plotted on Tera-Wasserburg concordia diagrams to display degree of discordance. Analyses of detrital zircons that are less than 10 percent discordant (table 1) are used to construct relative probability age plots. Specifically, we use the 206 Pb/ 238 U age for zircons younger than about 1200 Ma, and 207 Pb/ 206 Pb ages older than about 1200 Ma. A well-known phenomenon first documented by Williams and Hergt (2000) is that SHRIMP analyses of zircons containing Ͼϳ2500 ppm uranium can result in 206 Pb/ 238 U ages that are too old, presumably due to a change in instrument fractionation (that is, a calibration shift). Data from such high-U concentration, reversely discordant metamorphic zircons with 206

40Ar/39Ar analyses of hornblendes from a 120 km long NE-trending transect within the high-grade rocks of the Casco Bay Group provide information regarding the timing of thermal events and subsequent cooling history of this region. In the north, plateau ages of 368-372 Ma record the time of cooling through argon closure temperatures (500°C) following Acadian metamorphism. In the central portion of the transect, hornblendes display younger ages (350-323 Ma) and release spectra which show evidence of a Late Paleozoic thermal overprint. To the south, significantly lower plateau ages are recorded (270-290 Ma). These lower ages date either the time of a Late Paleozoic thermal event or the time of cooling following a prolonged period of burial at deep crustal levels. The former interpretation is favored on the basis of the disturbed release spectra in the central portion of the transect and the fact that ages over the 60 km long southern portion of the transect are relatively constant at 280+10 Ma. If a Late Paleozoic thermal event occurred in this region it was evidently not accompanied by large amounts of deformation as structural evidence for such an event is lacking. The exact nature of the Late Paleozoic resetting of hornblendes in this portion of the Casco Bay Group is not clear; however, it may be related to the events which lead to granite emplacement 275 Ma ago in the Massabesic Gneiss Complex and the 272-282 Ma ages for monazites from the nearby Sebago Batholith. Des analyses 40Ar/39Ar, effectuees sur des hornblendes prelevees sur une traverse NE de 120 km de longueur au sein des roches metamorphiques de haut degre du Groupe de Casco Bay, ont foumi des donnees sur Page des episodes thermiques et l'histoire du refroidissement qui leur succeda dans ce Secteur. Au nord, des ages-plateaux de 368 a 372 Ma datent le refroidissement via les temperatures de fermeture de P argon (500°C) suivant le metamorphisme acadien. Dans la portion centrale dela traverse, les hornblendes exhibentdes ages pi us jeunes (350 a 325 Ma) etdes spectres de diffusion qui portent la trace d 'une surimpression thermique tardi-paleozoi'que. Vers le sud, on enregistre des ages-plateaux bien plus jeunes (270 a 290 Ma). Ces demiers datent soit un episode thermique tardi-paleozoi'que, soit un refroidissement apres un enfouissement prolonge profondement dans la croute. On favorise la premiere interpretation a cause des spectres de diffusion deranges que montre la portion centrale de la traverse et parce que les ages obtenus au long des 60 km de la portion sud de la traverse sont relativement constants a 280±10 Ma. Si cette region a subi un episode thermique tardi-paleozoi'que, ce ne fut assurement pas de pair avec une deformation importante puisque la structure ne foumit aucun indice d 'un tel evenement. La nature exacte de la remise a zero tardi-paleozoi'que des hornblendes dans cette portion du Groupe de Casco B ay n 'estpas apparente. On pourrait cependant larelier aux evenements a l'origine de l'emplacement de granite dans le Complexe de Gneis...

Whole-rock Rb–Sr and U–Pb zircon age measurements on intrusive rocks in southwestern Maine indicate igneous activity at 400, 340, and 320 Ma. These plutonic rocks were emplaced into deformed Ordovician to Devonian(?) (and perhaps Hadrynian) rocks of the Shapleigh and Merrimack Groups. Folding of the Shapleigh and Merrimack Group rocks is interpreted to have occurred during the Acadian event, or before. The [Formula: see text] age of the Webhannet pluton in southwestern Maine sets a minimum time for Acadian deformation in this region.The 320 Ma age of the Lyman two-mica granite pluton of this study coupled with the reported ages of the Milford two-mica granite (275 Ma) and Lake Sunapee two-mica granite (323 Ma) of New Hampshire suggests a spectrum of Hercynian igneous activity in northern New England similar to that of the well established Hercynian intrusive events in the southern Appalachians and western Europe.