Mesoproterozoic metamorphic rocks exposed in the eastern Llano uplift, central Texas, were involved in a ca. 1150–1116 Ma Grenville-age orogenic event along the southern margin of Laurentia. Collision of the exotic Coal Creek arc and a southerly continental block with Laurentia tectonically telescoped and stacked three distinct lithotectonic domains. A major ductile shear zone forms the contact between the Coal Creek ensimatic arc terrane, the southwesternmost domain, and rocks with Laurentian affinities to the north. Directly north of the arc boundary, supracrustal rocks of the Packsaddle domain represent basinal sedimentary and volcanic rocks deposited along the southern Laurentian margin. Farther north, granitic gneisses of the Valley Spring domain, which consist of both plutonic and supracrustal rocks, represent a Laurentian continental-margin arc emplaced beneath the Packsaddle domain along a shear zone. Recent mapping in the northeastern uplift shows that the Valley Spring domain records a polyphase deformational history equivalent to that observed in the Packsaddle domain of the southeastern uplift. Early deformation occurred under uppermost amphibolite-facies metamorphic conditions and was accompanied by partial melting and formation of foliation-parallel leucosomes, consistent with deformation deeper in the orogenic pile. Mylonitic rocks in the shear zone separating these two domains show thrusting to the northeast, similar to that shown by the shear zone separating the Coal Creek arc from the Packsaddle domain. The Valley Spring domain igneous complex adjacent to the shear zone is highly attenuated into thin sheets in this zone. The Grenville event in the eastern uplift is characterized by polyphase ductile deformation synchronous with upper amphibolite-facies dynamothermal metamorphism. Deformation progressed from northeast-directed ductile thrusting and folding (D 1 , D 2 ), which accommodated collision-related crustal thickening and contraction, to polyphase regional-scale folding (D 3 , D 4 , D 5 ), which accommodated continued collision-related north- to northeast-directed contraction. The kinematics of the deformation in the eastern uplift are consistent with the northeastward collision of an exotic arc terrane and a southern continental block with the generally east-trending Laurentian margin. No evidence of transcurrent motion along the margin has been observed.

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Abstract This chapter describes the Grenville orogen as it is preserved in areas of outcrop as well as in the subsurface in the United States, Late Proterozoic continental rifting that fragmented that orogen, and Precambrian rocks within terranes accreted to the rifted eastern and southern margins of Laurentia (earliest Paleozoic North America). The accretion of terranes to the eastern and southern margins of Laurentia formed the Paleozoic Appalachian (Caledonide)-Ouachita orogen. Outliers of the Grenville orogen, variously deformed by Paleozoic orogenies, crop out within the western part of the Appalachian orogen. Although protoliths as old as Archean have been identified along the northwestern margin of the Grenville orogen in Canada, as far as we know, no rocks in the areas covered by this chapter are older than Middle Proterozoic. Some rocks, however, indicate ties to older source areas. Quartzites from the Adirondack Lowlands contain detrital zircons with minimum ages of 1.65 Ga, 1.8 Ga, and 1.95 Ga (McLelland and others, 1988a). Recent work by Dunning and Cousineau (1990) and Olszewski and others (1990) on rocks of the Chain Lakes block, Maine-Quebec, and correlatives to the north in the Canadian Appalachians has shown that some zircons, possibly detrital, in diamic-tite are as old as 2.8 Ga. Sinha and Bartholomew (1984) report a discordia intercept of 1.87 ± 0.2 Ga for zircons, probably detrital, from layered gneiss of the Blue Ridge Grenvillian outlier in Virginia. Zartman and Hermes (1987) report a Late Archean inheritance in zircon from Permian granites in the southeastern New England Avalonian terrane; they attribute this to the under-plating of the Avalonian microplate by Archean crustal components, possibly of Africa, in the late Paleozoic during the collision of Gondwanaland with Avalonia.

Abstract The Alleghanian orogeny is the most pervasive event to affect the central and southern Appalachians. This is the event referred to in the older literature as the “Appalachian revolution,” and is the mountain-building event most associate with the Appalachian chain. The Alleghanian orogen includes a foreland belt of folds and thrust faults that propagated into sedimentary rocks of the North American craton in the western part of the southern and central Appalachians, and to the east, an internally complex belt of allochthonous mostly pre-Alleghanian metamorphic rocks (Fig. 1; Plate 1). A line of rootless external Precambrian basement massifs is near and approximately parallel with the cratonward limit of early to middle Paleozoic metamorphic rocks, and the metamorphic belt contains several internal massifs of Precambrian basement rocks, mostly in windows. A more internal zone of Alleghanian amphibolite-facies metamorphism appears in both the southern and New England Appalachians.

An erosional hiatus over almost the entire area between Pennsylvania and western New Brunswick suggests that the region was mountainous from the Middle Devonian Acadian orogeny through Pennsylvanian time. Of seven basins or deposits of southeast-era New England, the ages of three (Narragansett, Norfolk, and Worcester) are florally determined as Westphalian B (Middle Pennsylvanian) to Stephanian B or C (Late Pennsylvanian); three lack flora but are of inferred Carboniferous age (North Scituate, Woonsocket, and Pin Hill), and one is of possibly Carboniferous age (Sturbridge). The first three are characterized by flora suggesting a tropical or subtropical climate and by alluvial fan facies deposited in an intermontane basin. Four of these basins or deposits lie in the Avalon Terrane, three just west of the Nashoba Terrane, but none has been recognized in the intermediate Nashoba Terrane. These basin deposits can be correlated with similar deposits in Atlantic Canada. Tectonic effects of the Alleghanian orogeny are many and diverse, resulting in important tectonic controls on the formation and evolution of the coal basins. Grabens surrounded by uplands were formed by extension or strike-slip fault-related extension and were filled with Carboniferous sediments during the earliest Alleghanian orogenic episode. These sediments along with the basement complex, were multiply deformed during Permo-Carboniferous Alleghanian orogenic episodes, which involved folding, thrust faulting, plutonism, regional metamorphism, and strike-slip faulting. Metamorphism throughout the outcrop areas ranges from anchizone to K-spar zone in the Narragansett Basin; anchizone to possibly lower greenschist in the Norfolk Basin; and below the almandine zone in the “Worcester Coal Mine” deposit. Important effects of the tectonism are the widespread anthracitization and tectonic thickening of the low-sulfur and high-ash coals.