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Timing of Volcanism, Sedimentation, Deformation, Metamorphism, and Plutonism
In the Early Silurian, marine clastic sedimentation continued in the Central Maine, East Harpswell, and Fredericton trough(s). By Late Silurian (circa 420-425 Ma) rocks of the eastern part of the Bath map sheet, in the Megunticook and Benner Hill sequences, had been deformed and metamorphosed to amphibolite facies (West and others, 1995). Moreover, many of the post-tectonic plutons in that area have been dated in the same Late Silurian age range (Tucker and others, 2001). The largest of these exposed in the Bath map area is the Spruce Head pluton. Tucker and others (2001) consider this plutonic, metamorphic, and presumably deformational event to be an early phase of the Acadian orogeny.
To the north of the Bath map sheet (Tucker and others, 2001), the Fredericton sequence and Casco Bay Group are intruded by some Late Silurian plutons of the same age as some plutons east of the Sennebec Pond fault. Tucker and others (2001) suggest, however, that the peak metamorphism in the Liberty area is Devonian, whereas peak metamorphism east of the Sennebec Pond fault is Silurian (West and others, 1995). The extent to which the Fredericton and more western sequences may have been deformed and metamorphosed in the Late Silurian has not been established, due primarily to the intensity of the younger, Devonian events.
The Sennebec Pond fault projects into the Bath map sheet along the east side of the Waldoboro pluton. To its west, the Cape Elizabeth Formation in the Small Point area shows clear evidence of two stages of metamorphism (Hussey, 1988). The earlier stage is indicated by the large pseudomorphs of muscovite described above. The later stage is indicated by porphyroblasts of fresh staurolite and large poikiloblasts of fresh andalusite in the same rocks. As discussed above, the Cape Elizabeth Formation at Small Point displays two deformational events. The younger, D2, was accompanied by a metamorphic event, presumably the event that crystallized hornblendes which give Early to Middle Devonian cooling ages. The age of the earlier metamorphism at Small Point is unknown, but it may be of Late Silurian age and correlative with that of the area east of the Sennebec Pond fault. If this is the case, it may be that rocks on both sides of the Sennebec Pond fault experienced the same thermal events, even though they differ in intensity. Thus, the Sennebec Pond fault may not strictly separate two regions of distinct thermal histories. More information is needed on the time of metamorphism to test this hypothesis.
Movement of the Boothbay thrust predates F2 folding and metamorphism, and may be contemporaneous with F1 recumbent folding and cleavage development. The relationship of the Lincoln Sill to the thrust fault is perplexing. It is folded by, and therefore predates, F2 deformation. It predates peak Acadian metamorphism. The 418 Ma zircon age reported by Tucker and others (2001) indicates a time of intrusion of latest Silurian to earliest Devonian. It is tempting to think that the intrusion is in some way related to the large-scale thrusting that formed the Boothbay thrust sheet. However, the uncertainty of the mechanism of its intrusion and the fact that the sill does not occupy the sole of thrust but is slightly below it, in the autochthonous part of the Bucksport Formation, remain unexplained.
West of the Sennebec Pond fault, 40Ar/ 39Ar hornblende cooling ages (Figure 52) indicate that metamorphism occurred during an Early to Middle Devonian phase of the Acadian orogeny (West and others, 1988, 1993). As reported by West and others (1993) the latest hornblende cooling ages of the Casco Bay Group range between 340 and 360 Ma, compatible with regional cooling after Acadian metamorphism.
Middle to Late Devonian ages are more common for intrusive rocks in the Bath map sheet west of the Sennebec Pond fault. The Waldoboro pluton, which intrudes the Bucksport and Cross River Formations, the foliated quartz diorite in Georgetown that intrudes the Casco Bay Group, and pegmatites that intrude the Falmouth-Brunswick sequence have all been dated at Middle to Late Devonian. The 40Ar/39Ar hornblende cooling ages of West and others (1988) imply that the peak, amphibolite facies metamorphism was also Middle to Late Devonian. Thus, migmatization of the Falmouth-Brunswick sequence, and probably also of the Casco Bay Group between the Sennebec Pond and Flying Point faults, is associated with the Middle to Late Devonian phase of the Acadian orogeny.
Rocks of the Falmouth-Brunswick and Central Maine sequences in the northwestern corner of the Bath map sheet have been deformed by folds that are strongly overturned to the west (Figure 35 and Figure 36). Vergences are predominantly to the east on the northwest limb of the major antiform that dominates the outcrop belt of the Falmouth-Brunswick sequence (Figure 37). These folds affect both gneissic foliation of the paleosome and thin foliated pegmatitic stringers of the neosome (Figure 36), thus postdating migmatization. Some pegmatitic stringers and dikes cut these folds (Figure 35) suggesting a second generation of pegmatite injection, possibly related to intrusion of the Permian-age pegmatites of the Topsham area studied by Tomascak and others (1996). Folding of the Falmouth-Brunswick and Central Maine rocks may thus be related to Early Carboniferous transpression that was responsible for dextral shearing of the Norumbega fault system. Ages of undeformed pegmatites in the Gardiner area (Figure 52) on strike to the north of the map area suggest major pegmatite intrusion during the Late Devonian, as a thermal effect of the Acadian orogeny. The rocks of the Gardiner area are also migmatized. Sorting out the ages of migmatization and deformation in the Topsham-Brunswick area requires additional detailed radiometric age studies.
In the Tenants Harbor area (Figure 5), most of the rocks have been affected by retrograde metamorphism (Guidotti, 1979, 1989). Andalusite and staurolite have been replaced by muscovitic pseudomorphs, and irregular biotite clots presumably have replaced an Fe-Mg mineral. The age of the retrograde metamorphism is not known, but the preservation of delicate pseudomorph shapes suggests it was post-tectonic, implying only that it was post-Silurian.
A major thermal event, of Late Pennsylvanian to Permian age (Alleghenian?), affected the rocks of the Falmouth-Brunswick sequence and probably a large area of the Central Maine sequence west of the Flying Point fault (Figure 33). Tomascak and others (1996) report a U-Pb monazite age of 293 ± 2 Ma for the Sebago granite northwest of the Bath map sheet. According to West and others (1988, 1993), 40Ar/39Ar hornblende cooling ages within 60 km east of the Sebago pluton, including the northwest corner of the Bath map sheet, are remarkably consistent, between 283 and 287 Ma (Figure 52). Farther northeast, in the Gardiner-Augusta area, the hornblende cooling ages become abruptly older (approximately 323 Ma), increasing northeast to 372 Ma. The older ages reflect cooling from Acadian metamorphism, but the younger ages are likely related to Late Pennsylvanian-Permian heating associated with the intrusion of the Sebago granite.
A small area in the Brunswick-Topsham vicinity gives somewhat younger ages. Tomascak and Francis (1995) report U-Pb monazite ages for two-mica granite lenses and pegmatites ranging from 268 to 275 Ma. West and others (1993) report two similar hornblende cooling ages in this area, of 266 and 270 Ma, anomalously young compared with the 283-287 Ma ages of the surrounding region. This suggests that the Topsham pegmatite district may represent a localized thermal pulse 15 to 20 m.y. younger than the Sebago granite. Not enough precise age dates are currently available to define this late event.
There is no indication in this area of an Alleghenian compressional event. However, Swanson (1995, 1999b) suggests the possibility of a late Paleozoic transpressional shear event associated with movement on the Norumbega fault system and with the injection of the late pegmatite and granite lenses noted above.
The timing of ductile dextral shearing and subsequent brittle faulting in south-central Maine is discussed by West and others (1993) and most cogently by West (1999). The region of pervasive dextral shear fabric, covering two-thirds of the map area, postdates metamorphism east of the Flying Point fault, inasmuch as porphyroblasts that formed during Acadian metamorphism are commonly sheared and flattened. West (1999) points out that ductility of these rocks probably required temperatures above 320oC, or about the closure temperature for argon diffusion in muscovite. He reports 40Ar/39Ar muscovite cooling ages for the region of the Bath map sheet to range from 350 to 298 Ma, Late Devonian to Early Carboniferous.
Localized dextral high strain zones cut through the region of pervasive dextral strain. West and Lux (1993) report a 40Ar/39Ar age of about 290 Ma (Early Permian) on muscovite that they believe grew or recrystallized during mylonitic deformation on the Sandhill Corner fault, north of the Bath map area. Dextral mylonitic deformation on other strands of the Norumbega fault system, including the Flying Point and South Harpswell faults, may have been active at this time. Tomascak and others (1996) note that intense ductile deformation of the Flying Point fault in the Topsham area predates undeformed pegmatites and granites with ages as old as 275 Ma.
Relative age of brittle normal and possibly left-lateral strike-slip faulting is not well constrained by offset of geological features. The only rocks that are not offset by faults are the basaltic dikes. Of these, the only dated dike is the large, through-going Christmas Cove dike for which West and McHone (1997) report preliminary ages ranging from Late Triassic to Early Jurassic. The mineral cooling ages described above imply that normal faulting along the Flying Point fault occurred after Early Triassic time (West, 1999), and the age of the Christmas Cove dike suggests that movement happened prior to Early Jurassic. Normal fault movement is believed to be a result of crustal extension associated with the initial breakup of Pangea. The regional setting for a left-lateral component of motion on the Cape Elizabeth, The Basin, and Back River faults is not clear.
Last updated on February 1, 2008
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