The lower Paleozoic rocks that extend from northwestern Newfoundland, through the Gaspé Peninsula, the south shore of the St. Lawrence River as far west as Quebec City, the Champlain Valley, western New England, eastern New York, and north-central New Jersey to southeastern Pennsylvania were deformed markedly by the Ordovician Taconic orogeny. This belt is bordered to the north and west by the little-disturbed foreland; in Canada the boundary includes Logan’s Line. To the south and east, the identity of this belt is lost in rocks that have been more severely deformed and metamorphosed by later, principally Acadian, orogeny. Rocks of this identifiable Taconic orogenic belt are here termed the Taconides. During Cambrian and much of Early Ordovician time, sedimentation within the Taconide belt was arranged in parallel zones: to the northwest, on the craton, was a shelf environment of shallow subtidal, intertidal, and supratidal carbonate deposition. Southeast of this shelf across a steep slope and an abrupt facies change, that probably reflects a sharp increase in water depth, was an area of clastic sedimentation; between these two zones was a zone of carbonate-clast slump conglomerates. The zone of clastic sediments, called the transitional zone, in turn passed seaward into one of typical eugeosynclinal sedimentation where the rocks are poor in carbonate, but rich in volcanic components. Another major facies change is preserved within the area of the shelf sequence deposition. Depending on the geographic location, this second facies change is late Early Ordovician to Middle Ordovician in age; it is marked by the regional unconformable overlap of a syntectonic black-mantling shale sequence on the carbonate rocks of the shelf sequence and older units. The shelf and mantling shale sequences are divisible into two tectonic zones: a foreland to the northwest, and a zone of deformed autochthonous rocks to the southeast, in which the intensity of deformation and metamorphism increases to the southeast. Rocks of the transitional sedimentary facies are even more intensely deformed, and over wide areas have been moved bodily northward and westward for long distances. These moved rocks are preserved in three forms: (1) klippe, now surrounded entirely by rocks of the shelf or mantling shale sequence across structural contacts; (2) allochthonous rocks only partly surrounded by rocks of the shelf or mantling shale sequence across contacts; (3) allochthons now eroded to a mere structural stump, but whose former extension and large movement are recorded by distinctive syntectonic sedimentary rocks. To the first category belong the Hare Bay and Humber Arm klippen of Newfoundland, small klippen in the Quebec City area, the Taconic klippe in New England and New York, and small scattered areas of allochthonous sedimentary rocks as well as a large klippe of Precambrian crystalline rocks in New Jersey and Pennsylvania. To the second category belong the rocks immediately southeast of Logan’s Line, from the tip of Gaspé Peninsula to the Vermont-Quebec border. To the third category belong the rocks of the Hinesburg thrust in northern Vermont, where large-scale Ordovician movement at the surface level is recorded in the wildflysch-type sedimentary rocks exposed along the Lake Champlain shore. South and east of the zone of allochthons and of the deformed shelf sequence is a zone of structurally high ground, part of the axial region of the composite Berkshire–Green Mountain–Sutton Mountain–Notre Dame Mountain–Shickshock Range–Indian Head Range–northern Long Range anti-clinoria. Where the structural relief is especially great or where epeirogenic uplift has caused erosion to reach sufficient depth, the zone is marked by Precambrian basement rocks; these Precambrian rocks approximately mark the southeast limit of basement rocks of 1 (±) b.y. (billion years) age and appear to be the edge of a lower Paleozoic craton. Coincident with the zone of structural highs is a zone of Bouguer gravity highs; the coincidence extends from northwestern Newfoundland through the Gulf of St. Lawrence, as far south as the north end of the Berkshire massif. From here south to Long Island Sound, the gravity ridge is displaced east of the structural ridge. It passes through the Coastal Plain deposits and reappears in the area of the Glenarm Series in Maryland. Northwest of the gravity ridge is a coextensive zone of Bouguer gravity troughs. The troughs follow the zone of deformed shelf sequence; where the allochthons occur, the troughs coincide with these features. Lower to Middle Ordovician ultramafic rocks occur near the western boundary of the eugeosynclinal facies, in a narrow belt parallel with and just east of the zone of structural highs. From Gaspé Peninsula southwest, these ultramafic rocks are apparently strictly intrusive. In Newfoundland, however, intrusive ultramafic bodies may be genetically related to an apparently extrusive ultramafic-mafic ophiolite complex preserved in the allochthons. I suggest that the process leading to the locations of the structural, igneous, and gravity features was the interaction of an oceanic segment of the crust with the adjoining craton. The location of this junction of crustal segments originally determined the location of the sedimentary facies junction between the shelf and basin sequences; compressive plunging of the oceanic crust under the craton caused rafting of the lighter cratonal margin, thus accounting for the structural uplift of the outermost (southeasternmost) known belt of 1-b.y.-old Precambrian basement rocks through much of the length of the Taconides. Farther into the craton (west and north), the compressive forces caused a gentle downwarp of the crust, leading to the subsidence of the former shelf area and, therefore, to a bathymetric reversal. The reversal allowed a black mantling shale sequence, whose sediments were derived in large part from the uplifted cratonal margin to the east, to be deposited over the former shelf area; continued uplift of the cratonal margin and subsidence of the former shelf area led eventually to wholesale emplacement of allochthons by gravity sliding of rocks of the transitional facies off the uplifted cratonal margin into the basin that was the former shelf area. Continued compression in the Taconides after the initial submarine gravity sliding led to northwestward thrusting of consolidated rocks, including Precambrian crystalline rocks of the uplifted cratonal margin, in a more deep-seated environment, probably in Late Ordovician or Early Silurian time. Regional metamorphism accompanied this last stage of diastrophism. The underthrusting of oceanic crust, in a process that probably involved the upper mantle as well, was accompanied by intrusion of the ultramafic bodies and the extrusion of siliceous, mafic, and ultramafic rocks on the surface. These igneous rocks are preserved today mainly in the eugeosynclinal sequence formerly deposited on the oceanic crust, but they are found also among the gravity slides and as volcanic ash in the shelf sequence. The addition of a mass of relatively dense oceanic material under the margin of the craton, as well as the concomitant introduction of dense intrusive rocks, resulted in the belt of positive Bouguer gravity anomalies. Where the gravity ridge is southeast of rather than coinciding with the belt of Precambrian rocks in the Taconide zone, the Precambrian rocks have undergone large lateral transport toward the craton. Compared to the Taconic orogeny, the Acadian orogeny in the northern Appalachian region was of wider regional extent, developed larger systems of nappes, led to more intense regional metamorphism, and was accompanied by larger scale plutonism. Despite these facts, however, the Taconic orogeny appears to have defined structural trends in the lower Paleozoic rocks that effectively controlled structural evolution of the northern Appalachian orogen during later Paleozoic orogenies, including the Acadian orogeny.

Abstract We present a tectonic evolution of Mexico over the past 600 m.y., with focus on the position of southern margin of Proterozoic North America in Mexico, the Phanerozoic events that shaped this margin, and the accretion of terranes that built Mexico out from that margin to its present configuration. The evolution of Mexico is a peculiar and difficult problem in tectonics. Whereas the kinematic history of the region of Mexico, for example, the end of Proterozoic continental breakup, the collision of North and South America in the late Paleozoic, the drifting apart of those two continents in the Mesozoic, and the motions of plates in the Pacific basin relative to North America in Cretaceous and Cenozoic time, is reasonably well known, the kinematics of terranes now in the region is poorly understood. In large part, this disparity is due to the extraordinary extent and volume of Cenozoic volcanism. Although such volcanism informs us about Cenozoic convergence, it cautions that the crust of Mexico has probably undergone major modification in Cenozoic time. Moreover, the volcanic cover limits greatly the direct access to older rocks and structures necessary to develop the story of an evolution that is well resolved in space and time. A tectonic synthesis of Mexico thus requires more assumptions than for most mountainous regions. Our model of tectonic evolution of Mexico attempts to satisfy both far-field and sparse near-field kinematic histories and existing geologic and geophysical data. This chapter is a companion to an in-depth synthesis of data, identification

Abstract The continental margin transect program evolved through a desire to define the specific characteristics of North America’s modern continental margins and to compare the modern margins with Paleozoic examples, now found within the continent. Thus, the features of modern margins can be used to locate and elucidate the structures of ancient analogues. Conversely, the exposed rocks of ancient margins provide an insight into the evolution and deep structures of modern examples. Ocean-continental transitions, rifting mechanisms, development of sedimentary basins, processes of continental breakup, and ancestral controls are all features that warrant comparison between modern and ancient examples. The Atlantic coast has the best-known passive margin of North America. Inboard of it is the Appalachian Mountain chain, which records a clear history of late Precambrian rifting and the passive development of a Paleozoic continental margin. Transects across both these modern and ancient examples provide information on scale and detail that is useful for comparisons. The modern continental margin of eastern Canada (Fig. 1a, 1b) has a wider variety of ages and styles than that of its eastern United States counterpart. Segments of the margin southeast of Nova Scotia and east of Labrador are typical of rifted margins, though of different ages. Between them lies the tortuous segments of the northern and eastern Grand Banks where rifting and subsidence occurred over a broad area of the continental shelf. In contrast, the southern boundary of the Grand Banks is a transform segment. Rocks of the Appalachian Paleozoic margin are best preserved and exposed

ABSTRACT The amalgamation of Laurentia’s Archean provinces ca. 1830 Ma was followed by ~700 m.y. of accretionary orogenesis along its active southeastern margin, marked by subduction of oceanic lithosphere, formation of arcs and back-arcs, and episodic accretion. This prolonged period of active-margin tectonic processes, spanning the late Paleoproterozoic and Mesoproterozoic eras, resulted in major accretionary crustal growth and was terminated by closure of the Unimos Ocean (new name). Ocean closure was associated with rapid motion of Laurentia toward the equator and resulted in continental collision that led to profound reworking of much of the accreted Proterozoic crust during the ca. 1090–980 Ma Grenvillian orogeny. The Grenvillian orogeny resulted in formation of a large, hot, long-duration orogen with a substantial orogenic plateau that underwent extensional orogenic collapse before rejuvenation and formation of the Grenville Front tectonic zone. The Grenvillian orogeny also caused the termination and inversion of the Midcontinent Rift, which, had it continued, would likely have split Laurentia into distinct continental blocks. Voluminous mafic magmatic activity in the Midcontinent Rift ca. 1108–1090 Ma was contemporaneous with magmatism in the Southwestern Laurentia large igneous province. We discuss a potential link between prolonged subduction of oceanic lithosphere beneath southeast Laurentia in the Mesoproterozoic and the initiation of this voluminous mafic magmatism. In this hypothesis, subducted water in dense, hydrous Mg-silicates transported to the bottom of the upper mantle led to hydration and increased buoyancy, resulting in upwelling, decompression melting, and intraplate magmatism. Coeval collisional orogenesis in several continents, including Amazonia and Kalahari, ties the Grenvillian orogeny to the amalgamation of multiple Proterozoic continents in the supercontinent Rodinia. These orogenic events collectively constituted a major turning point in both Laurentian and global tectonics. The ensuing paleogeographic configuration, and that which followed during Rodinia’s extended breakup, set the stage for Earth system evolution through the Neoproterozoic Era.

In considering the tectonic evolution of the Acadian orogen in New England and Maritime Canada, account should be taken of the fact that from the Early Cambrian through the Mississippian the Coastal Acadia marine faunas of eastern North America are very distinctive biogeographically from those occurring to the west and northwest within the Northern Appalachians and adjacent parts of the continent. The limited amount of boundary mixing can be largely ascribed to dispersible larval stages of a few taxa. This biogeographic situation suggests that the surface-current circulation pattern that maintained this type of reproductive isolation and biogeographic integrity from the Early Cambrian through the Mississippian implies a certain level of geographic remoteness as well, although specific distances cannot be derived from such data. Paleogeographies and plate tectonic concepts need to be consistent with the available biogeographic information. Early-through-Late Cambrian biogeographic units have been recognized, since early in this century, in the Acadian orogen and adjacent regions. Baltic Realm (= Atlantic Realm, = Acado-Baltic Realm) faunas are restricted to the coastal regions of New England (Boston region), the Maritimes (St. John, New Brunswick), Nova Scotia, and eastern Newfoundland. On the other hand, Laurentian Realm (= Pacific Realm) faunas are restricted to a belt that extends from western Newfoundland down the valley of the St. Lawrence River and through Vermont and eastern New York. Ordovician biogeography is less well documented, with earliest Ordovician (Tremadocian) Baltic Realm-type faunas also occurring in eastern Newfoundland, Nova Scotia, and Coastal Acadia, whereas on the North American Platform to the west, Laurentian Realm-type faunas occur. Baltic Realm faunas of the later Early Ordovician (Arenigian) are known in eastern Newfoundland. In central Newfoundland, the few shelly faunas are largely of the Laurentian Realm. There is no useful Middle to Upper Ordovician (upper Arenigian-Ashgillian) fauna from Coastal Acadia, whereas fauna of that age-span on the northwestern edge of the Northern Appalachians is strictly Laurentian Realm in character. During this time interval in central Newfoundland, central and northern New Brunswick, and northern Maine, the few shelly faunas presently available are largely of the Laurentian Realm, with a Baltic admixture in some cases, although more study is required for a definitive statement on this matter. In the latest Ashgillian (Hirnantian) there is at least one good occurrence of the globally very extensive cold water, Gondwana (=Malvinokaffric) Realm Hirnantia fauna in eastern Gaspe. Silurian European Province faunas occur in Coastal Acadia, including parts of mainland Nova Scotia, southern New Brunswick, coastal Maine, and the Boston area. North American Province Silurian occurs farther to the west and northwest, including northern New Brunswick, northern Maine, and the Connecticut River Valley region, as well as in areas farther to the west. Marine Devonian units of Old World Realm type occur in Nova Scotia, coastal Maine, and southern New Brunswick. Eastern Americas Realm faunas, on the other hand, are present to the west and northwest in central and northern New Brunswick, northern Maine, eastern Quebec, and New Hampshire. Mississippian marine European Province faunas are present in Newfoundland, Nova Scotia, and coastal New Brunswick. By contrast, both North American and Southeastern Province faunas are known well to the west of Greater Acadia in the Southern Appalachians and the Mid-Continent region. Later Paleozoic European and North American nonmarine biota are all of Euramerian Province type; that is, there is no evidence for provincialism within the Northern Appalachians. These biogeographic data are of concern in tectonic analysis, particularly of the Acadian orogen, because belts yielding biogeographically similar organisms are unlikely to have been geographically remote from each other, and vice versa. Examples of boundary biogeographic mixing also indicate greater proximity—such examples are known in a few areas within the Northern Appalachians, particularly in eastern Quebec for the Early and Middle Devonian and in Nova Scotia for the Early Devonian. Early Devonian mixing is also present in northern Maine and adjacent New Brunswick. Boundary biogeographic mixing is also known in central Newfoundland and a few locales to the southwest in northern Maine and New Brunswick for the Middle and later Ordovician. The timing of the Acadian orogeny deduced from datable fossils preserved above and below the post-Acadian unconformity is Middle Devonian, with a Givetian date for the maximum being most likely. This statement applies to all parts of the Northern Appalachians except for Newfoundland, where datable beds overlying Acadian-deformed strata are scarce.