Abstract
Greatly increased efforts in the mineralogical sciences, especially in metamorphic and igneous petrology as well as mineral physics, will be needed to support investigations, planned and underway, of the processes that form and modify the continental crust. These efforts will require large teams of earth scientists and will utilize methods such as superdeep drill holes, reflection and refraction profiles, long transects, and studies of deep xenolith suites. In order to justify the large costs of such projects, they must result in a significantly enhanced understanding of crustal processes. The increased knowledge about geologic relations at depth, which is the key to a more comprehensive understanding of crustal processes, is further refined when it is combined with improved methods in geochronology, geochemistry, and geothermobarometry.
Drawing upon participation in the Quebec-Maine-Gulf of Maine Global Geoscience Transect, in which a large amount of detailed data about geologic relations throughout the crust has been obtained from seismic reflection and refraction profiles combined with digitized geologic, gravity, and magnetic data, I offer several speculations. These speculations are about processes along the buried southeastern edge of the Middle Proterozoic Grenville province overridden by Taconian and Acadian thrusts, about Mesozoic crustal extension in Maine, and about the amount and sources of sediments in the Central Maine synclinorium.
Although the Grenvillian crust averages 40 km in thickness, the crust in Maine that includes the southeastern edge of the Grenville province is only about 40 to 42 km thick despite once having been buried by at least 25 and possibly as much as 35 km of Paleozoic thrust sheets. The present crustal thickness has resulted from several crustal processes that operated at different periods of time. Grenvillian crust was thinned by extension to about 20 km before it broke up in Late Proterozoic and Early Cambrian time to form an ocean basin. Loading of the remaining continental edge (Grenvillian crust with its slope-rise cover sequence) by Taconian thrust sheets formed overthickened crust that rose isostatically by thermal recovery and erosion during the Silurian and Early Devonian. Crustal thickening during the Acadian orogeny was concentrated along the thickest Taconian crustal section; in both orogenies this locus of overthickened crust lay above the rifted edge of the old Grenvillian crust. The Acadian thickening resulted in partial melting to yield garnet rhyolites in Maine and hinged isostatic rebound in New Hampshire due to thermal recovery.
The crust thins from about 41 km in western Maine to about 34 km in the Gulf of Maine. Despite earlier expectations, the crust varies little in thickness when passing beneath the inferred boundaries of the principal Paleozoic tectonostratigraphic terranes. The thinning actually takes place in steplike regions about 15 km wide where the Moho rises 3 or 4 km. The steplike regions lie beneath highly laminated crust that forms by ductile extension of deep crust beneath old faults that were extended in the shallow and middle crust to become listric at depth; overall crustal extension is 15 to 20%. Thus the Moho in the region was formed in the Mesozoic and does not significantly reflect Paleozoic accretionary processes. A wide range of igneous, metamorphic, and geophysical features can now be systematically related to extension, failure, and re-formation of the deep continental lithosphere during the early Mesozoic.
The volume of Silurian and Devonian metasedimentary rocks now present in the Central Maine synclinorium can be estimated from seismic reflection and refraction data to be approximately 870 km3 per kilometer of length along strike. At least as much again is inferred to have been present in the Devonian from the pressures estimated from Acadian metamorphic assemblages in the region. Thus crustal thickness after the Acadian orogeny could have been 46 to as much as 51 km. The sources for such a huge volume of sediment could not have been just to the north and west. A possible source to the south and east could be rocks like those now found beneath the southeastern border of the synclinorium; these rocks were metamorphosed at midcrustal depths in the Middle Ordovician, and they could have been the temporal equivalent to the Taconian orogen along the northwest boundary of the Central Maine synclinorium.
