James W. Skehan

Significance of fossiliferous Middle Cambrian rocks of Rhode Island to the history of the Avalonian microcontinent

Middle Cambrian trilobites of Acado-Baltic affinities have been found in southern Narragansett Bay, Rhode Island, in phyllites previously mapped as part of the Pennsylvanian stratigraphy of the Narragansett Basin. The trilobite-bearing phyllites form the basal unit of an approximately 1-km-thick sequence that has undergone four episodes of folding and cleavage formation. Three different trilobites are represented. Badulesia tenera (Hartt), which gives a diagnostic medial Middle Cambrian age, is also known from New Brunswick, eastern Newfoundland, southern Germany, northern Spain, and eastern Turkey. This species in northern Spain characterizes a subzone within the Middle Cambrian Badulesia zone correlating approximately with the lower part of the Paradoxides paradoxissimus zone of northern Europe. Nearby in Newport, Rhode Island, a sequence of maroon and green clastic sediments rests unconformably on Precambrian igneous rocks and sediments. If these should be proven to be Lower Cambrian, this succession in southern Narragansett Bay would be the most complete Cambrian succession yet recognized in southern New England and possibly a nearly complete record of sedimentation for this area for the entire Cambrian Period. These well-exposed rocks are part of a distinctive succession of upper Precambrian and lower Paleozoic rocks and are interpreted as a fragment of the Avalonian platform or microcontinent, closely related to rocks of the eastern margin of the northern Appalachians and the western margin of western Europe.

Geologic profile across Southeastern New England

Abstract This progress report describes major geological features of three segments of a profile across southern New England: (1) the Clinton—Boston, (2) the Boston—Narragansett Basins, and (3) the Narragansett Bay cross sections. Segment (1) is a traverse across four tectonic blocks from the Merrimack synclinorium across the Clinton—Newbury and Bloody Bluff fault zones and across the southwestern tip of the Boston Basin. The Clinton—Newbury and Bloody Bluff fault zones together may define the suture between the North American plate and the Avalonian terrain of the Eur-African plate. The Avalonian plate may have been overridden by the North American or vice versa, these two faults in any case mark the northwestern and southeastern boundaries of crustal underplating. These tectonic features probably formed during the Acadian, although their present geometry was defined during the Alleghanian orogeny. The Boston and Narragansett basins, segments 2 and 3 are within the Avalonian platform. The age of the cover rocks of the former is uncertain but arguments for a Late Precambrian—Cambrian age are presented; those of the Narragansett Basin are fossiliferous Pennsylvanian. The northwestern part of the Avalonian platform is characterized by SE-directed, NW-dipping, thrusts and highangle reverse faults; the southeastern part of the platform, on the other hand, is characterized by NW-directed, SE-dipping faults. In the Narragansett Basin the Alleghanian (Variscan) orogeny is a major orogenic event consisting of the following elements: (1) isoclinal folding and refolding associated with thrusting; (2) upper amphibolite facies Barrovian regional metamorphism; and (3) intrusion of probably anatectic granites.

Late Paleozoic Orogenies in western Africa and eastern North America: The diachronous closure of Theic Ocean

The Carboniferous evolution in western Morocco, southeastern Canada, and New England was dominated by intracontinental processes: the subsidence of pull-apart sedimentary basins, and their subsequent deformation during middle Carboniferous to Early Permian times, with the development of autochthonous structures and mainly low- to medium-grade metamorphism. On the other hand, the structural style of the late Carboniferous deformation in the southern Appalachians, and to a substantial degree also in New England, that is related to a continental collision is characterized by deep-seated northwest-vergent thrusts and medium- to high-grade metamorphism. The Mauritanides structures are symmetrically east-vergent thrusts. This pattern is explained by the diachronous closure of the Theic ocean that separated the Avalon and Carolina microcontinental blocks from paleo-Gondwana. The closure of the Theic ocean occurred during Devonian times between the northern Appalachians and the northwestern edge of paleo-Gondwana (Morocco), whereas it was achieved only at the end of Carboniferous times between the southern Appalachians and western Africa (Mauritania-Senegal). The Late Devonian to early Carboniferous counterclockwise rotation of Gondwana induced the opening of the pull-apart basins of Canada, New England, and Morocco as well as, later, the oblique collision in the southern Appalachians and the generalized late Paleozoic dextral transcurrent faulting along the northern edge of paleo-Gondwana.

Relationship Between Precambrian and Lower Paleozoic Rocks of Southeastern New England and Other North Atlantic Avalonian Terrains

The eastern margin of the Appalachian orogen of eastern North America, as well as parts of western Europe and northwestern Africa, is comprised of Late Precambrian strata and igneous intrusions known collectively as the Avalonian terrain. This terrain is defined, in part, by Late Precambrian volcanic and sedimentary rocks, and by mafic to intermediate plutons cut by a series of granitic plutons dated from 595 to 650 Ma. In part, also, it is defined by Cambrian sedimentary cover bearing an Acado-Baltic trilobite fauna, and also Cambrian to Ordovician volcanics. The basement and cover rocks are cut by anorogenic alkaline plutons ranging from Ordovician to Devonian in age. In southeastern New England, all of the above mentioned rocks are present, except for Ordovician sedimentary rocks, but they are deeply eroded and are overlain by Upper Carboniferous fluvial basin sediments. The Late Precambrian rocks were deformed in the Avalonian orogeny; the Upper Carboniferous rocks were deformed during the polyphase Alleghanian-Variscan orogeny; and the Lower Paleozoic strata were mildly deformed at an as yet undetermined time between the above orogenic events, possibly during a Cadomian III orogenic episode. The Avalon Platform records Avalonian orogenic events of Late Precambrian age related to Cadomian II of western Europe and northwestern Africa. The mild deformation of the Lower Paleozoic rocks probably records an episode of orogenesis in nearby Gondwanaland related to deformational events of from Caledonian (or Taconian ?) to Post-Acadian time. This terrain also preserves a rich record of the Variscan-Alleghanian orogeny.

Assembly and dispersal of supercontinents: The view from Avalon

Abstract Recent models for the post-750 Ma Rodinian supercontinent dispersal (e.g. Hoffman, 1991) envision that cratons margined by Grenvillian belts, were reorganized before ca 540 Ma to form the Gondwanan supercontinent. Laurentia and Baltica distanced themselves from Gondwana by moving out of the Rodinian cratonal cluster. West Gondwana, of which Avalon was a part during the late Proterozoic to Cambrian cratonal assembly, consisted mainly of Africa and South America. The main geological evidence is presented for: (1) a transition from continental platform conditions to those of a subduction-related volcanic arc regime in Late Proterozoic time during the dispersal of the Rodinian supercontinent, and the resulting assembly of the Gondwanan supercontinent; and (2) a second transition that marked a reversal from the volcanic arc regime to marine platformal environments by early Cambrian time. Evidence for progressive instability of the continental shelf margining the Rodinian supercontinent is contained in late Proterozoic olistostromes, mylonite zones, calc-alkaline magmatism, and arc-derived clastic rocks, some being glacigenic, during three phases of the Avalonian orogeny. By early Cambrian time the reversal from a tectonically unstable volcanic arc regime to more stable platformal conditions took place as Avalon, Armorica and related microcontinental blocks rifted from Gondwana. These Gondwanan fragments sequentially come into collision, first with each other and Baltica, and then with Laurentia in Mid to Late Paleozoic time as Pangaea was being assembled.

Changing tectonic environments of the Avalon superterrane and the Nashoba terrane in Massachusetts

Abstract The paper is composed to test the hypothesis (Skehan and Rast, 1992) that the Nashoba zone terrane of southern New England is basement to the Avalon, and that both zones are remnant blocks of the Avalonian volcanic-plutonic arc. These blocks preserve evidence for long strike-slip fault zone(s) within the volcanic arc trending parallel to an associated subduction zone, as is the case in modern arcs. We propose that the “exotic” or “enigmatic” Nashoba Formation mantling the gneissose basement core complex of the Nashoba terrane can be generally correlated with the easterly Late Proterozoic Avalonian formations that antedate the formation of the Avalonian volcanic-plutonic arc. A related objective is to evaluate the geodynamic model accounting for the anticlinorial uplift of the core complex of the Nashoba block to its present position west of the Avalon zone and southeast of the Merrimack trough. Keys to verifying these propositions are: (1) the ongoing mapping of the extensive mylonite zones of the Boston Avalon zone into the Nashoba zone; (2) the role of Taconian age tectonics in the emplacement of Ordovician-Silurian plutons in the Nashoba and the Massabesic blocks; (3) the existing basis for isotopic dating of mylonites. Additionally, a key to understanding is the role of the Alleghanian orogeny. This study has implications for understanding correlations among several Nashoba-like basement inliers of eastern New England, and also similar Late Proterozoic complexes in the Avalonian, Cadomian, and Pan-African terranes.

Major geological features and lateral variation of crustal structure in southern New England

Abstract Rg dispersion studies in southern New England (SNE) provide a basis for comparing lateral variation in crustal structure with geological features. Rg waves reveal lateral variations at depths ranging from near the surface down to a few kilometers. The similarity between Rg results and those of other geophysical studies suggests that variations revealed by Rg studies penetrate into the middle of the crust, if not deeper. The Waterbury Dispersion Region (WDR), named after a local geological structure, is characterized by the highest Rg group velocities in SNE (∼2.9–3.1 km/s at a period of 1 s). These higher velocities may be due to rocks enriched in oceanic type crust and/or granulite facies metamorphic rocks. The Hartford and New Haven dispersion regions (HDR and NHDR) comprise the northern and southern parts of the Mesozoic Hartford Rift Basin, respectively. The HDR is characterized by the lowest group velocities in SNE (∼2.1–2.4 km/s at a period of 1 s). These lower velocities appear to be related to relatively thick Pleistocene glacial sediments overlying a thick sequence of Mesozoic sedimentary rocks. Higher velocities in the NHDR are attributed to a thinner cover of glacial sediments and a thinner package of sedimentary rocks. These younger rocks overlie metamorphic basement which is, on average, closer to the surface in the NHDR than in the HDR. The Bronson-Avalon Dispersion Region (BADR), east of the Hartford Rift Basin, represents several terranes that are geologically diverse but, at the scale of this study, not differentiated by Rg dispersion. The seismic structure beneath the BADR appears to be quite homogeneous; group velocities in this region range from about 2.6 to 2.8 km/s at a period of 1 s. The BADR is interpreted to consist of stacked layers of gently dipping to nearly flat-lying thrust sheets. The Torrington Dispersion Region (TDR) lies north of the WDR and west of the HDR and has group velocities similar to those of the BADR.

AVALONIAN (PAN-AFRICAN) MYLONITIC DEFORMATION WEST OF BOSTON, U.S.A.

Abstract West of Boston, Mass., Castle and others (1976) recognized an up to 5km wide, possibly folded, NE-SW trending Burlington Mylonite Zone. We have extended mapping south into Natick and Framington quadrangles, and supplemented it by fixing local directions of tectonic motion, which are more variable than reported by Goldstein (1989). In Natick the mylonite zone is partly migmatized and converted into blastomylonites, forming the lithodemic Rice Gneiss and is intersected by the Dedham Granite dated ca 630 Ma. The granite also invades deformed, folded, and commonly mylonitized Westboro Quartzite. Thus mylonitization, folding, and formation of migmatitic blastomylonites are all earlier than ca 630 Ma, and can collectively be attributed to the main phase of the Avalonian orogeny that in Africa is referred to as the Pan-African I. The sense of movements in the Rice Gneiss is generally sinistral strike-slip with a NE-SW trend of foliation. Other local mylonites have more variable directions of motion. A narrower E-W zone of mylonitization has been recognized by Grimes (M.S. thesis 1993, Boston College) and named the Nobscot Shear Zone. It affects the Milford Granite, also about 630 Ma in age, while similar but narrow shear zones affect other local granites including the Dedham. These zones, dipping steeply north and including the Nobscot, are less intensely mylonitized and are not associated with migmatites. Their age is not known, but since they affect only Precambrian rocks, they are assumed to be late Proterozoic. We attribute these zones to the second stage of the Avalonian or the Pan-African II. The older rocks west of Boston are widely affected by numerous brittle faults. These are all of unknown age, but probably Phanerozoic. The most significant brittle fault in the Burlington area is the mid to late Paleozoic Bloody Bluff Fault. We do not associate large scale mylonitization with that fault, because the mylonites are commonly cut by undeformed or little deformed Siluro-Devonian gabbro-diorites.

SHEAR ZONES WITHIN THE BLOODY BLUFF FAULT ZONE: ANALYSIS AND TECTONIC IMPLICATIONS

Abstract The Bloody Bluff fault zone, which divides the New England Avalon zone and Nashoba zone, contains at least two shear zones that are within Avalonian rocks. The Rice Road shear zone (sinistral, strike-slip) affects the Westboro Formation and is intruded by the 630 Ma Dedham Granite. The Rice Road shear zone, and equivalent pre-granite mylonites appearing in drill cores, parallel the terrane boundary, and may have controlled the later mylonitization. The Nobscot shear zone (dextral, strike-slip) is a prograde shear zone cutting a granite assumed to be related to the surrounding 630 Ma plutons. Similar shear zones have been seen cutting Late Proterozoic plutons in the New England Avalon zone, and represent a series of en echelon strike-slip shears. The Burlington mylonite zone (shear sense equivocal) is part of the terrane boundary. This is a retrograde shear zone that forms the southeastern border of the Wolfpen lens, a lenticular body of sheared and altered metamorphic and intrusive rock that has been assumed to be part of the New England Avalon zone. Microstructural characteristics indicate that the Burlington mylonite zone was active after the Nobscot shear zone. In particular, quartz in the Nobscot shear zone was dynamically recrystallized by a combination of grain boundary migration and rotation recrystallization processes, thought to occur during shearing at upper-greenschist conditions. In contrast, quartz in the Burlington mylonite zone was recrystallized predominantly by rotation recrystallization, indicating lower-greenschist, retrograde, deformation. The two shear zones are too close for these differences to be a result of a simple thermal field gradient. While mineral assemblages in most of the study area indicate no metamorphic grade higher than upper-greenschist temperatures, the Wolfpen lens contains amphibolites with assemblages formed at temperatures above the oligoclase isograd, indicating mid-amphibolite facies metamorphism. As metamorphic contrast is one of the key features differentiating the Nashoba zone from the New England Avalon zone, the Wolfpen lens cannot be assumed to be part of Avalon. It may be a small block of rocks of intermediate grade between the two terranes.