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 The first composite magnetic-anomaly map of the conterminous United States and adjacent offshore areas has been published at a color-contour interval of 200 gammas and at the scale and projection of other national geologic and geophysical maps for easy comparison. This map, despite the inconsistent characteristics of the surveys from which it was compiled, is useful in providing a regional framework for the interpretation of magnetic studies of limited areas, in selecting areas for more detailed magnetic investigations, and in studying the distribution and character of regional geologic features. The map has a wide variation of magnetic-anomaly patterns, trends, and types, thus reflecting the diversity of the geologic terranes of the United States. In general, the anomaly pattern east of the Cordillera in the craton and in the Appalachian Mountains consists of more and greater intensity anomalies. The muted nature of the anomalies of much of the Cordillera is a result of several factors but appears to be primarily related to a decreased crustal magnetization caused by an abnormally shallow Curie isotherm. The anomalies of the Appalachian Mountains and the Cordilleran system primarily reflect the major structural patterns of the orogens, but important exceptions occur, such as those associated with rocks underlying thrust sheets in the Appalachian Mountains and westerly-striking anomaly trends in the Cordillera, which are correlated with igneous intrusives, faults, and mineral deposits. The buried southern and eastern edges of the Pre-cambrian craton are indicated by changes in the magnetic anomalies and their dominant trends. Within the central United States, numerous regional magnetic-anomaly provinces are observed that reflect the long, complex history of the Precambrian basement rocks of the craton. These provinces are transected by conspicuous, intense, long, generally linear anomalies that originate from mafic extrusive or shallow intrusive igneous bodies within failed rifts, such as the Midcontinent rift system, the Southern Oklahoma aulacogen, and the Reelfoot rift buried beneath the Mississippi embayment. These are only a few of the many interesting regional geologic features that are observed on the composite magnetic-anomaly map of the United States.

The evolution of the middle Paleozoic subduction in the South Armorican Massif (southern Brittany, France) is compared with that of the Petite Sole-Cordoba fault (western Spain and Portugal). This comparison suggests that this fault acted as a transform fault during Ordovician and Devonian times. The gravity and magnetic data compiled on both sides of the North Atlantic indicate that the southern end of the Cordoba fault links with a 2,400-km-long mafic body which extended from the South Portuguese zone to New England (U.S.A.) before the opening of the present Atlantic Ocean. The mafic body may be considered as the suture of the Theic Ocean. However, the closing of the South Armorican and Theic oceans did not occur at the same time; the South Armorican subduction ended in Late Devonian time, whereas the Theic closed in early Carboniferous time. This diachronous evolution could be explained by the existence of an Iberian (Spanish) microplate.

Paleomagnetic data have become available in the past decade that enable us to constrain better the motions of the continental blocks involved in the Paleozoic assembly of Pangea. Drawing upon various aspects of earlier models for this assembly, this paper briefly outlines a new model that is fully compatible with the currently available paleomagnetic poles. In this new model, the last two major collisions are between Gondwana and the northern continents in the Carboniferous and between the more northerly cratonic blocks of Asia (Siberia and Kazakhstan) and the combined Atlantic-bordering continents in the Permian. Earlier, the northern continents (the North American craton, the Baltic Shield-Russian Platform, and Hercynian Europe, herein called Armorica) assembled. The timing of this coalescing of the northern continents cannot be precisely identified by the available paleo-poles, but the orogenic belts that mark the collision zones formed during the time between the Late Ordovician (Taconic orogeny) and the Middle Devonian (Acadian orogeny), that is, between 440 and 380 Ma. It should be noted, furthermore, that the northern continents assembled in a configuration different from that of the fit by Bullard and coworkers; the paleomagnetic data argue for a Carboniferous megashear between the North American craton, on the one hand, and the Baltic Shield, Russian platform and Armorica, on the other hand.

All available gravity and magnetic data for Newfoundland, the British Isles, and their adjacent continental margins are compiled on an early Mesozoic, pre-continental drift reconstruction of the North Atlantic. These data support geological correlation between the southern termination of the Caledonides and the northern termination of the Appalachians. Geological zones southeast of the postulated early Paleozoic suture in both Newfoundland and the British Isles have distinctive geophysical signatures, and these may be extrapolated to locations at the edge of the continental shelf that were in close proximity in early Mesozoic time. Geological zones northwest of the suture also have associated distinctive geophysical anomalies, but their correlations on the early Mesozoic reconstruction are more tenuous because (1) off Ireland the zones converge and trend toward a low-angle intersection with the continental margin, (2) off Newfoundland the zones diverge and trend perpendicular to the margin, and (3) the Mesozoic location of Rockall Plateau, the intervening continental fragment, is poorly defined.

Since the advent of plate tectonics, the widely accepted model for the development of the Appalachian orogen has involved the opening and closing of a late Precambrian-Paleozoic Iapetus Ocean. Only a few of a growing number of geologically distinctive terranes are easily explained by this model. Vestiges of Iapetus are nowhere coupled to the ancient North American margin. Furthermore, it cannot be demonstrated that any of the extensive Appalachian terranes, now east of the Iapetus tract or its suture, were once connected to the North American miogeocline. All are therefore suspect. The major suspect terranes of the Appalachian orogen are in most respects analogous to previously recognized zones or tectonic lithofacies belts. In the north, these are the Dunnage, Gander, Avalon, and Meguma terranes. In the south, they include easterly parts of the Blue Ridge, the Piedmont, Slate Beit, and the geophysically distinctive Brunswick and Tallahassee-Suwannee terranes beneath the Atlantic Coastal Plain. Most of these are composite and include smaller terranes of uncertain paleogeography. Taconic allochthons are included because they fit the definition of suspect terranes. Stratigraphic and sedimentologic analyses indicate that the Appalachian orogen built up during three major Paleozoic accretionary events. Their timing coincides with the times of structural, metamorphic, and plutonic effects assigned to the Taconian, Acadian, and Alleghanian orogenies. Accretion of the Appalachian orogen progressed from the North American miogeocline outward. The boundaries of the earliest accreted western terranes are marked by melange zones and ophiolite complexes, implying head-on collisions. Later boundaries between eastern terranes are steep mylonite zones and brittle faults, implying oblique movements. The suspect terrane concept, first developed for the North American Cordilleran, provides new insights into the evolution of the Appalachian orogen and solves several enigmas. It is a surgically clean analytical approach and a superior framework in which to view the anatomy of any complex orogen.

Recent discoveries in the North American Cordillera of composite exotic terranes that had become accreted to the Cordillera during its evolution require reexamination of the older Appalachian mountain systems for evidence of possibly similar history. In the New England segment of the Appalachian orogen, the three Paleozoic orogenies (Taconian, Acadian, Alleghanian) must be separately examined. Evidence for Taconian orogeny supplies the best support for subduction processes at the margin of a continent-ocean plate junction. Definition of ancestral North America prior to the completion of that subduction process is the starting point for a search of Taconian exotic terranes. On the basis of such criteria as age of basement, occurrence of in-place ophiolite, melange, blueschist, continental-margin facies, and island-arc rocks, this margin is proposed to be best preserved in northern Maine, where it runs from the Jim Pond-Boil Mountain ophiolite south of the Chain Lakes massif northeast to the Elmtree ophiolite in New Brunswick. Rocks of the Weeksboro-Lunksoos Lake and Miramichi anticlinoria are southeast of this boundary. In Maine, this boundary, which was the trace of a subduction zone, was marked by a residual marine basin in Late Ordovician and Early Silurian time. No Taconian accreted terrane has been detected on the North American craton side except for the Chain Lakes massif, which is suggested to be an obducted allochthon derived from the opposite side of Iapetus Ocean; this opposite side is labeled “Craton X” and is otherwise largely unknown. The Merrimack synclinorium is interpreted to have formed on Craton X. Acadian orogeny probably resulted from a continent-continent collision. The nature and extent of the Silurian and Devonian flysh sequences demand basins of deposition much larger than present geologic relations allow; these sequences may or may not be in mutual sedimentary contact, and may not have been even before their deformation and metamorphism. This fact and the anomalous paleomagnetic pole position for the Merrimack synclinorium suggest possible large-scale tectonic transport during the Acadian orogeny. In that sense, the terrane now occupied by the synclinorium may be exotic, both because its basement was originally Craton X and because the Taconian suture may have been disrupted by younger longitudinal transport of unknown extent. The coastal belt of Rhode Island, Massachusetts, and Maine contains rocks in distinct lithotectonic blocks. These blocks are best defined in northeast Massachusetts and around Penobscot Bay in Maine, where they are mutually separated and also separated from the Acadian version of North America by large faults. These blocks appear to be exotic; they may have arrived at their present locations since the peak of the Acadian orogeny and thus have been largely unaffected by it. This coastal belt includes the Avalonian terrane; it may have been emplaced during latest Acadian to early Alleghanian deformations. If the Avalonian terrane did arrive late, then it could not have constituted Craton X during the Taconian event. The three Paleozoic orogenies led to three types of accreted terranes: (1) Taconian, thrust allochthons directly attributable to subduction-induced collision during the closing of Iapetus Ocean; (2) Acadian, continent-continent collision and possible large concomitant transcurrent displacement; (3) Alleghanian, oblique-slip high-angle faulting, the concomitant formation of a sedimentary basin having no immediately identifiable sediment source, and the formation of a microplate collage. For ancient mountain belts, the detection of microplate accretion is at best difficult. The use of a combination of geological, geochemical, and geophysical methods is necessary. Sedimentologic analysis may furnish the best clue to the arrival of new terranes; criteria to detect root zones of transcurrent faults are needed. Geochemical study may lead to definition of discrete blocks and the nature of sutures between them. Geophysical data are generally corroborative rather than definitive; even paleomagnetic data need geologic confirmation and are best used to sniff out suspect land and eventually to define the extent of motion. The hard middle part of establishing an exotic terrane must remain a geologic task.

Regional seismic reflection studies in the New England and southern Appalachians by COCORP and in Québec by the Ministére des Richesses Naturelles have provided critical subsurface geological information. The data clearly show considerable horizontal transport of off-shelf metasediments over coeval, relatively undeformed, lower Paleozoic shelf and miogeoclinal rocks. In the southern Appalachians, long distance (>200 km) transport of thin crystalline thrust sheets can be shown as well. The COCORP data from the Green Mountains of Vermont and a USGS seismic study in the Grandfather Mountain window of North Carolina and Tennessee appear to indicate that Precambrian (ca. 1.0 b.y.) Grenville basement in those areas is allochthonous and underlain either by shelf sediments or detachment horizons. In Québec, allochthonous basinal facies clastics are still preserved over a major anticlinorial structure, and extensive exposures of Precambrian basement are not found in an internal position in this part of the Appalachian mountain belt. The Bouguer gravity gradient in the central and southern Appalachians and the gravity high in New England and Québec are interpreted to mark a fundamental crustal density change at depth along the mountain chain, perhaps representing a preserved transition from continent to ocean. We infer, in part from the distribution of surface rock units with respect to the locus of the gravity gradient, that allochthonous off-shelf rocks may have been transported farther in the southern than in the northern Appalachians. Perhaps this is true for allochthonous Grenville basement as well, although the question cannot be unequivocally answered at this time. The seismic data suggest that highly deformed rocks exposed in the Appalachian chain are part of a relatively thin, composite allochthon presently confined to high structural levels and that the deeper part of the crust may constitute a largely undeformed ancient continental margin, perhaps including a transition from continental to rift stage or oceanic crust. Surface geologic relationships similar to those described for the Appalachians exist in a number of other mountain belts, and a modern analog for the subsurface structure of the frontal part of the Appalachians is present in the Banda arc of Indonesia. Regional deep crustal seismic surveys are clearly needed in other ancient deformed mountain belts and their active modern analogs.