Abstract The Appalachians are a deeply eroded Paleozoic mountain chain (Fig. 1) characterized by Philip King (1970, p. 437) as the most elegant mountain chain on Earth. He recognized that the orderly linearity of the chain is deceiving, and that the Appalachians are full of guile, arousing geologic controversies as acrimonious as any in our science.

Abstract This chapter deals with the evolution of the various parts of the Appalachian orogen in the United States prior to the onset of the formative deformational episodes of the orogen in the early Paleozoic. The general discussion that follows is intended to add perspective to the more detailed discussions within each section.

Abstract Until very recently, most geologists were conditioned to seek the effects of three major events–the Taconian, the Acadian, and the Alleghanian–within the Appalachian orogen. Things are not that simple, however, as the importance of older deformations is increasingly being recognized. Although this chapter is concerned primarily with the Taconic orogen (sensu stricto), two older deformational events are considered herein. These events are the Blountian and Penobscottian orogenies. The Penobscottian event has been recognized for some time (Neuman, 1967; Hall, 1969, 1970), but its importance in Appalachian geology has only recently become apparent by work in northern Maine (Osberg, 1983; Boone and others, 1984) and the Potomac Valley of Virginia and Maryland (Drake and Lyttle, 1981; Drake, 1987). In Maine, the Penobscottian can only be dated as pre-late Ibexian (pre-Arenigian), whereas in the Potomac Valley it is thought to be of late Middle Cambrian to early Late Cambrian (Dresbachian) age. Neither syn- nor post-orogenic sediments are recognized that could have resulted from the Penobscottian deformation. On the contrary, the Blountian event is recognized because of its syn-and post-orogenic sediment wedge, but deformational features related to the event have not as yet been recognized in the Blountian hinterland, although isotopic dating within the Blue Ridge is permissive of deformation at this time. The Blountian orogeny has been recognized for many years (Kay, 1942), and Rodgers (1953) has termed it the Blountian phase of the Taconicorogeny. In my opinion, it was a separate tectonic event that was completed prior to the Taconic (sensu stricto), as its uppermost molasse is overlain by distal Taconian syn- and post-orogenic deposits. It goes without saying that the effects of the Penobscottian and Blountian orogenies are difficult to recognize and separate from those of the Taconic orogeny. For this reason, the effects of the earlier events will be discussed with those of the Taconic where they are believed to be present.

Abstract An orogenic cycle (Wilson, 1963) in simplest form may be of long duration, and it includes rifting, subduction, and final closure by which an ocean basin is initiated and ultimately destroyed. In real orogens the cycle must be more complicated, being punctuated, possibly at several intervals, by such things as cessation of subduction, collision of microplates, or by obduction. The Acadian orogeny probably represents such a punctuation in an orogenic cycle that lasted through Paleozoic time. Where the Acadian orogeny is recognized in the Appalachians, it was in some places preceded by earlier punctuations (Penobscottian, Taconian), and was followed by a final collisional event (Alleghanian). In the context of the Acadian orogeny, only those geologic features that have a causal relationship to the Acadian will be considered in this chapter.

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.

Abstract Post-Paleozoic tectonic activity in the Appalachian orogen (including the concealed basement of the passive margin) is primarily a consequence of the breakup of Pangaea and the opening of the Atlantic Ocean. It embraces a major tectonic cycle that is marked by Late Triassic-Early Jurassic rifting of the Alleghanian-Variscan orogen and by Middle Jurassic to Recent drifting of the newly forming passive margin.

Abstract Paleontology is essential for the determination of world paleogeography, including that of ancient mobile belts such as the Appalachians. In this chapter we review the contributions of paleontology to interpretations of the Paleozoic paleogeographic evolution of the Appalachians–particularly that part of the Appalachians in the United States outboard of the early Paleozoic miogeocline.

Abstract This chapter reviews the geophysical data in the U.S. Appalachians–including gravitational and magnetic fields, refraction and reflection seismology, terrestrial heat flow, and electrical properties. An even treatment of the various kinds of geophysical data is neither attempted nor justified. Instead, emphasis and bias are placed primarily on geophysical data that have been useful for the interpretation of the tectonic history of the orogen; of these, seismic reflection data have had the greatest impact on the development and testing of tectonic models in the U.S. Appalachians and elsewhere because of their greater resolution.

Abstract Previous chapters have documented the polyphase character of Appalachian orogenesis, and have outlined many difficulties in resolving overprinting relationships. A problem along the length of the orogen is to determine the extent to which the tectonothermal record within eastern crystalline terranes developed concomitantly with the late Paleozoic westward vergent folds and thrusts that characterize deformed portions of the foreland. Except for a few locations in southeastern New England, overstepping late Paleozoic cover sequences are absent, and establishing the chronology of superimposed tectonothermal events must be largely based on collaborative field and geochronological investigations. Resolution of the extent and nature of superposed metamorphic events may best be gained by comparing a set of systematically determined argon mineral ages with radiometric dates provided by other more refractory isotopic systems such as Rb-Sr whole-rock and/or U-Pb zircon. This chapter will discuss how argon mineral ages may be used to more clearly resolve the timing and regional extent of late Paleozoic tectonothermal events within U.S. portions of the Appalachian orogen. A brief review of argon dating methods and the problems inherent in these methods is presented initially. This is followed by a review and interpretation of available data from several representative areas. For simplicity of presentation, the discussion is arbitrarily divided into sections dealing with northern and then southern portions of the orogen. All radiometric ages presented have been calculated on the basis of the decay constants and isotopic abundance ratios listed by Steiger and Jäger (1977).

Abstract Paleozoic rocks exposed in the Appalachian Mountains are bordered on the east and south by the Atlantic and Gulf Coastal Plains, and rocks of the Paleozoic orogenic belt may be traced beneath the cover of post-orogenic Mesozoic-Cenozoic Coastal Plain strata (Plate 6). Large-scale lithotectonic components of the exposed Appalachian orogen include the foreland fold-thrust belt and internal metamorphic belts (collectively called the Appalachian Piedmont), which locally contain internal basement massifs. External basement massifs and discontinuous belts of low-grade metamorphic rocks are distributed along the boundary between the fold-thrust belt and the Piedmont. Accreted terranes are identified in the internal metamorphic belts.

Abstract The Appalachian Highlands, with the inclusion of the Appalachian Plateaus, form a major geomorphic region that comprises approximately one tenth of the area of United States, and also includes part of Canada. The U.S. boundaries of the Appalachian Highlands were delineated in 1930 by a committee of geomorphologists headed by N. M. Fenneman. The work was published as the Map of Physical Divisions of the United States, distributed by the U.S. Geological Survey in 1946. The area described in this chapter is divided into six provinces as shown in Figure 1.

Abstract Students of mineral deposits have generally bypassed the Appalachians in preference to studying deposits of the western United States. A “western outlook” in economic geology is not surprising. For the mining and exploration geologist, the western United States has traditionally represented a wide-open land with excellent rock exposure, no glacial cover or saprolite, and vast acreages of public land available for prospecting. Furthermore, many economic geologists have felt that virtually all important deposits of the Appalachians were discovered and thoroughly studied long ago. Hence, a generation or two of economic geologists, with the exception of a few hardy academic, government, and exploration geologists, have given short shrift to the study of Appalachian mineral deposits.

Abstract The Appalachian basin is an elongate asymmetric synclinorium that extends from Lake Ontario southwestwardfor 1600 km through New York, Pennsylvania, Ohio, WestVirginia, Virginia, eastern Kentucky, Tennessee, and Georgia to Alabama (Fig. 1). The basin consists of Paleozoic strata ranging from 600 to 900 m thick on its west flank, along the Cincinnati arch, to more than 13,700 m thick on its east side in central Pennsylvania adjacent to an allochthonous metamorphic terrain. As defined herein, the basin-filling rocks underlie most of the central and southern parts of the Appalachian mountain chain. To the northeast in New England, coal-bearing strata of Pennsylvanian age underlie approximately370 sq km in the Narragansett Basin.

Abstract The preceeding chapters in this book constitute syntheses of our knowledge of the tectonic history of the U.S. Appalachians following their inception as a rifted and passive margin after the Grenville orogeny to the present state of decay. The purpose of this chapter is to summarize the evidence for the major events of the Paleozoic and later history of the Appalachians, to bring to light similarities and differences between along-strike segments, and to explore aspects of Appalachian history from the perspective of the tectonic map (Plate 1) that may not have been brought out in the previous chapters. This, as all syntheses, represents only a progress report whose total complexion may change with the appearance of new data. Details of stratigraphy and structure were outlined in previous chapters.

Abstract Late Paleozoic orogenic structures exposed in the Appalachian Mountains of Alabama and in the Ouachita Mountains of Arkansas extend from opposite directions beneath a cover of post-orogenic Mesozoic-Cenozoic strata in the Mississippi Embayment of the Gulf Coastal Plain (Fig. 1; Plates 6, 9). Although the physiographic expressions of the Appalachian and Ouachita Mountains end at the edge of the Coastal Plain, the orogenic belt neither ends nor changes abruptly along strike at the present onlap limit of Coastal Plain strata. Instead, as shown by data from deep wells and geophysical surveys, a continuous belt of Paleozoic orogenic structures extends beneath the post-orogenic Coastal Plain cover (Fig. 1; Plates 6, 9). Nevertheless, both the Paleozoic stratigraphic sequence and details of structural style exposed in the Ouachita Mountains contrast strongly with those in the nearest Appalachian outcrops, and projection of structural strike from the outcrops does not lead to a simple connection of structures beneath the Coastal Plain cover.

Abstract The name “Ouachita orogen” or “Ouachita orogenic belt” applies to the belt of deformed Paleozoic rocks flanking the southern margin of the North American craton. It is about 2,100 km in length, extending from the subsurface of Mississippi to the Marathon region of Texas; about 80 percent of this distance lies in the subsurface, buried beneath the Mesozoic and Tertiary sediments composing the Gulf Coastal Plain. The two major areas of outcrop of the orogenic belt are in the Ouachita Mountains of Arkansas and Oklahoma and in the Marathon region of west Texas (Fig. 1). The map trace of the Ouachita Orogen, from Mississippi to north Texas, defines a broad arcuate salient (Thomas, 1977a) extending into the North American continent. From north Texas the trace continues southward, passing to the east and southeast of the Llano Uplift of central Texas, where it forms a recess before bending abruptly northwestward to the Marathon region. Southward, from the Marathon region, small, widely scattered outcrops show that the Ouachita Orogen extends at least 450 km into Mexico (Handschy and others, 1987); from there, its southward or westward projection is conjectural

Abstract The Paleozoic rocks of the Ouachita Mountains (westcentral Arkansas and southeastern Oklahoma) consist primarily of sparsely fossiliferous deep-water deposits. Until recently, graptolites have provided most of the biostratigraphic evidence for the correlation of these rocks with sections in other parts of North America. Conodonts have been known from the Arkansas Novaculite for over 60 years, but their presence in limestones within the dominantly shaly Ordovician part of the sequence was demonstrated only recently (Repetski and Ethington, 1977). Shelly fossils are almost unknown from the pre-orogenic rocks of the Ouachita Mountains, although fragments of brachiopods, bryozoans, and trilobites have been found in acid residues of limestones thatwere processed for conodonts.