U.S. DEPARTMENT OF THE INTERIOR TO ACCOMPANY MAP I-2329 U.S. GEOLOGICAL SURVEY GLOBAL GEOSCIENCE TRANSECT 8: QUEBEC-MAINE-GULF OF MAINE TRANSECT, SOUTHEASTERN CANADA, NORTHEASTERN UNITED STATES OF AMERICA Principal compilers D.B. Stewart,1 B.E. Wright,1 J.D. Unger,1 J.D. Phillips,! and D.R. Hutchinson1 With contributions by J.H. Luetgert,l W.A. Bothner,2 K.D. Klitgord,l L.M. Liberty,1 and Carl Spencer3 INTRODUCTION thick crust includes Early Devonian garnet rhyolites extruded after fractional melting of felsic rocks at depths of greater than 50 km Transects produced by the North American Continent-Ocean (Stewart, 1989), a hinge zone (Hatch and others, 1983) between Transect Program (Speed and others, 1982; U.S. Geodynamics a thin and a very thick (-10 km) sedimentary prism deposited in Committee, 1989) describe the complex orogen formed along the a successor basin (the Central Maine synclinorium) that formed Late Proterozoic to Early Cambrian rifted margin of the Laurentian after the late Early and Middle Ordovician (Taconian) collisional craton, including continental rocks of the Grenville province, which episode, and kyanite-bearing assemblages indicating the burial of themselves had been earlier formed by a Middle Proterozoic even the youngest of these sediments to mid-crustal depths. Direct continent-continent collision. Continental accretion was followed geophysical evidence today indicates that thick continental crust by continental separation and the formation of a passive continen­ remaining from Paleozoic accretion is lacking. However, strong tal margin. This 900-km-long transect crosses the entire Appala­ seismic reflectors at the base of allochthonous sheets can be traced chian orogen and the passive continental margin from the craton to to depths of approximately 25 km, and 10 km or more of Atlantic oceanic crust. It is supported by abundant seismic reflec­ additional allochthonous sheets may initially have been present but tion and refraction data that were gathered specifically for this have since been eroded. It is reasonable to infer that the underlying transect, in part, to resolve questions posed by the earlier transects continental crust must have been depressed by such thick tectonic across the Atlantic passive continental margin. Thus, it is a loading. However, most stratigraphic evidence for a foreland basin "second-generation" transect. This transect has been accepted in in the Ordovician has been buried under later thrust sheets, and the Global Geoscience Transects Project (CC-7) of the Inter­ evidence for a foreland basin related to the Acadian orogeny is Union Commission on the Lithosphere, International Council of lacking in the region of this transect. Scientific Unions (Monger, 1986) as Transect 8. As shown by abundant seismic reflection and refraction data, the The Quebec-Maine-Gulf of Maine transect provides excellent present Appalachian crust was shaped principally by late Paleozoic data that can be used to understand the processes of continental to Early Jurassic extension. This extension began with the forma­ accretion and separation. An uncertain number, possibly ten or tion of Mississippian and Pennsylvanian transtensional rift basins more, of tectonostratigraphic terranes of predominantly continental and continued with the development of a 400-km-wide province of affinity were accreted to the craton, as shown by the transect. In rift basins of Triassic to Early Jurassic age that was accompanied addition, at least two oceanic terranes also were accreted. Although across a wider region by the intrusion of numerous dike swarms some terranes had been joined together to form composite ter­ and widespread normal faulting. Extension reversed the throw on ranes before being accreted (Boone and Boudette, 1989, describe many inclined Paleozoic thrust faults. The new seismic data suggest an example), in general, these terranes were accreted episodically that during extension the lower crust and Moho concurrently to the southeastern (current geographic direction) part of the acquired their present shape and internal features by ductile shear craton by thrust and (or) strike-slip faulting at successively younger and recrystallization. Many of the faults bounding rift basins times during the Paleozoic. The terranes differ principally in the become listric into the top of the lower crust (Stewart and Luetgert, nature of their Late Proterozoic to middle Paleozoic history and 1990) and, in some places, are paired with a rise of Moho by as stratigraphy (Keppie, 1989) and paleontology (Neuman and oth­ much as 4 km over a distance of 15 km. Numerous but discontin­ ers, 1989). Accretion in the part of the orogen shown on this uous subhorizontal reflectors in the lower crust indicate that during transect took place during multiple collisional or transpressive extension laminar packets of contrasting lithologies were formed episodes in the early and middle Paleozoic. and simultaneously through-going older faults were obliterated. A very thick continental crust was produced during these The strong seismic reflections from the allochthons in the upper episodes. Geologic evidence that indicates the formation of this crust die out at depths of 20 to 25 km into the uppermost newly reformed lower crust. The top of the lower crust is a gradational zone as much as 4 km thick where seismic velocity increases from Manuscript approved for publication November 18, 1991. about 6.4 to 6.8 km/s; such a large increase in velocity must 1U.S. Geological Survey. correspond to a decrease in quartz and potassium feldspar con­ 2 University of New Hamphire. tents. The extended region has newly reformed lower crust and a 3 Geological Survey of Canada. well-defined reflection and refraction Moho, which contrasts with the indistinct Moho beneath Grenvillian crust to the northwest. transect graphic and pamphlet are listed as compilers. This group Extension ·led to rupture of the continental crust and formation of was substantially augmented for specific tasks by· the scientists the present Atlantic Ocean, which was followed by deposition of listed as contributors. Many of the contributors have or will publish sedimentary rocks upon the passive continental margin. their work in greater detail elsewhere; most published work is cited The Quebec-Maine-Gulf of Maine transect demonstrates that the in this report. Most field data were collected in 1983-1985, record of Paleozoic orogenies in this part of the orogen is preserved although some geophysical data were collected as recently as almost exclusively in the presently brittle upper crust. Paleozoic 1988. In addition, we obtained important data, advice, or other terrane boundaries only can be identified to the top of the lower valuable assistance from W.A. Anderson, M.E. Loiselle, and R.G. crust, which appears to pass smoothly beneath all terranes. Any Marvinney of the Maine Geological Survey; Phyliss Charlesworth, genetic relation of the lower crust to the overlying crust becomes John Glynn, Alan Green, Patrick Morel-a-l'Huissier, W.H. Poole, increasingly uncertain as the amount of extension increases pro­ and M.D. Thomas of the Geological Survey of Canada; G.M. gressively southeastward toward the locus of oceanic rupture. The Boone of Syracuse University; R.A. Phinney and Christel Bottcher lower crust was attenuated and extended southeastward from its of Princeton University; W.E. Doll of Colby College; S.L. Klern­ original position as it was ductilely deformed. The high proportion perer, then at Cornell University; C.V. Guidotti and P.H. Osberg of of the upper crust that now consists of exposed granitic rocks was the University of Maine-Orono; A.M. Hussey II of Bowdoin formed by partial melting of lower crust in the Paleozoic. Given any College; C.E. Mann of Stanford University; and R.A. Ayuso, reasonable figure for melt extracted, indications are that most of Richard Goldsmith, and A.M. Trehu of the U.S. Geological Survey. the lower crust was partially melted then. The bulk composition of Joshua Comenetz and Sara Minnich of the U.S. Geological Survey the lower crust must have been changed by gravitational ascen­ helped to prepare the transect graphic. dance of magmas, resulting in depletion of quartz and alkali feldspar. For these reasons we have shown the lower crust as a discrete geologic entity on our interpretive cross section. SOURCES OF DATA USED This transect is a cartographic product that is an internally Extensive new compilations of regional magnetic and gravity consistent generalized representation of the data in an underlying data on land and at sea were recently prepared for much of the digital geographic information system (GIS) where the map units transect region (Macnab and others, 1990; D.R. Hutchinson, are shown with the detail present in the original 1:250,000- and unpub. data). Included were gravity and magnetic data collected 1:500,000-scale maps (Globensky, 19.87; Klitgord and others, by transect participants in support of this transect. Recently pub­ 1988, plate 2C; Osberg and others, 1985; St.-Julien and Slivitsky, lished geologic maps for Maine (Osberg and others, 1985) and 1985; Weed and others, 1974). Map units were assigned attributes southeastern Quebec (St.-Julien and Slivitsky, 1985; Globensky, according to a scheme devised by Wright and Stewart (1990). 1987) were used (see small-scale index map for coverages). The Geologic data preserved in the GIS that cannot be shown on maps regional compilations were prepared at scales larger than the at a scale as small as 1:1,000,000 include the specific names, transect, mostly at 1:500,000 or 1:250,000, so only a minor part of symbolic abbreviations, and lithologies of stratified formations and each regional compilation was used. Digitizing all these regional their members; greater detail about intrusive rock names, compo­ data sets was one of the most time-consuming aspects of transect sitions, and mineralogies; and more complete details concerning preparation. All data were converted to the Lambert Conformal age assigl)ments and their ranges, if any.