Developmental Model for the Southern Appalachians

Understanding the evolution and destruction of past oceans not only leads to a better understanding of earth history, but permits comparison with extant ocean basins and tectonic processes. This paper reviews the history of the Early Paleozoic circum‐Atlantic oceans by analogy with the Pacific Ocean and Mesozoic Tethys. Rifting and continental separation from 620 to 570 Ma led to the development of passive margins along parts of the northern margin of Gondwana (the western coast of South America); eastern Laurentia (eastern North America, NW Scotland and East Greenland), and western Baltica (western Scandinavia). Meagre paleomagnetic data suggest that western South America and eastern North America could have been joined together to form facing margins after breakup. Although western Baltica is an apparently obvious candidate for the margin facing NW Scotland and East Greenland, the paleomagnetic uncertainties are so large that other fragments could have been positioned there instead. The Iapetus Ocean off northeastern Gondwana was probably a relatively wide Pacific‐type ocean with, during the late Precambrian to early Ordovician, the northern margin of Gondwana as a site of continentward‐dipping subduction zone(s). The 650‐500 Ma arc‐related igneous activity here and the associated deformation gave rise to the Cadomian, ‘Grampian’, Penobscotian, and Famantinian igneous and orogenic events. By 490‐470 Ma, marginal basins had formed along the eastern Laurentian margin as far as NE Scotland, along parts of the northern margin of Gondwana, and off western Baltica, but none are known from the East Greenland margin. These basins closed and parts were emplaced as ophiolites shortly after their formation by processes that, at least in some cases, closely resemble the emplacement of the late Cretaceous Semail ophiolite of Oman. This orogenic phase seems to have involved collision and attempted subduction of the continental margin of Laurentia, Gondwana and Baltica. In Baltica it gave rise to some eclogite facies metamorphism. Marginal basin development may have been preceded by arc formation as early as ca 510 Ma. A double arc system evolved outboard from the eastern Laurentian and western Baltica margins, analogous to some of the arc systems in the present‐day western Pacific. At 480‐470 Ma, there was a second phase of breakup of Gondwana, affecting the active Gondwanan margin. Eastern and Western Avalonia, the Carolina Slate Belt, Piedmont, and other North American exotic continental blocks rifted away from Gondwana. Farther east, Armorica, Aquitainia, Iberia and several European exotic continental blocks also rifted away, though it is unlikely that they all rifted at the same time. Between 460‐430 Ma, peaking at ca 450 Ma, orogenic events involved continuing arc‐continent collision(s). From 435‐400 Ma the remaining parts of the Eastern Iapetus Ocean were destroyed and the collision of Baltica with Laurentia caused the 430‐400 Ma Scandian orogeny, followed by suturing of these continents during the Siluro‐Devonian Acadian orogeny or Late Caledonian orogeny to 380 Ma, leaving a smaller but new ocean south of the fragments that had collided with the Laurentian margin farther south. The Ligerian orogeny 390‐370 Ma collision of Gondwana‐derived Aquitaine‐Cantabrian blocks with Eastern Avalonia‐Baltica and removed the part of the Iapetus south of Baltica.Prior to any orogenic events, the Eastern Iapetus Ocean between Baltica and Laurentia may have resembled the present‐day central Atlantic Ocean between Africa and North America. The ocean appears to have closed asymmetrically, with arcs forming first outboard of the western margin of Baltica while the East Greenland margin was unaffected. The Western Iapetus Ocean between Laurentia and Gondwana also closed asymmetrically with a dual arc system developing off Laurentia and an arc system forming off the northern margin of Gondwana. Like the Pacific Ocean today, the Eastern Iapetus Ocean had a longer and more complex history than the Western Iapetus Ocean: it was already in existence at 560 Ma, probably developed over at least 400 million years, by mid‐Cambrian time was many thousands of kilometres wide at maximum extent, and was associated with a < 30 million year phase of marginal basin formation. In contrast, the Western Iapetus Ocean appears to have been much narrower, shorter lived (probably < 100 million years), and associated with the rifting to form two opposing passive carbonate margins, analogous to the Mesozoic Tethys or the present‐day Mediterranean.

Within the southern and central Appalachians, Grenville-age basement rocks are found in major massifs in the Blue Ridge and Sauratown Mountains anticlinoria and in the vicinity of the Grandfather Mountain window. These massifs are, respectively, Pedlar and Lovingston Massifs in the Blue Ridge anticlinorium, Sauras Massif in the Sauratown Mountains anticlinorium, and Watauga, Globe, and Elk River Massifs near the Grandfather Mountain window. In central Virginia the Lovingston Massif is juxtaposed against the Pedlar Massif, and in northwestern North Carolina-southwestern Virginia, the Elk River Massif is thrust over the Globe and Watauga Massifs, all along faults of the Fries fault system, which includes the Rockfish Valley, Fork Ridge, Devil’s Fork, and Linville Falls faults, as well as the Fries fault per se. The Pedlar Massif is a deeper granulite facies country-rock terrane intruded by charnockite plutonic suites. The Lovingston Massif primarily is a shallower granulite/amphibolite facies terrane intruded by biotite dioritoid plutonic suites containing bodies of charnockite. Country rocks of the Watauga Massif were subjected to metamorphic conditions similar to those of the Lovingston Massif, but were intruded by a plutonic suite of biotite dioritoid, biotite granitoid, and granitoid. The Elk River, Globe, and Sauras Massifs all are terranes metamorphosed to amphibolite facies and intruded by granitoid/dioritoid suites containing some porphyritic biotite dioritoid phases. A suite of late Precambrian (post-Grenville) peralkaline granitoid plutons intruded all of the massifs except the Pedlar. These plutons presumably are related to upper Precambrian volcanic rocks that were associated with a rifting environment and that were later metamorphosed and deformed along with overlying sedimentary rocks to form part of the Appalachian orogenic belt.

Ages of detrital monazite and zircon from alluvium collected from the French Broad River drainage basin, an orogen-crossing main trunk river, and alluvium in first-order tributary streams, provide an unconventional perspective for examining the regional tectonic and metamorphic history of the southern Appalachian orogen (eastern United States). The French Broad River system samples migmatitic Ashe–Tallulah Falls suite paragneisses (with inferred Neoproterozoic clastic protoliths) of the Eastern Blue Ridge and western Inner Piedmont, Mesoproterozoic basement orthogneisses, numerous Paleozoic metaplutonic gneisses, and tectonite equivalents of these lithologies in the Brevard fault zone. Middle Ordovician ages dominate the monazite age spectrum. Monazite from tributaries has a dominant 208 Pb- 232 Th age peak ca. 463 Ma. Monazite from the French Broad River alluvium suite yields a dominant 208 Pb- 232 Th age mode ca. 450 Ma, but differs from the tributaries in having scattered Mesoproterozoic, Silurian–Devonian, and Carboniferous ages. Electron microprobe total Th-U-Pb chemical ages for selected tributary monazite grains also analyzed by ion microprobe reveal additional monazite growth events (i.e., metamorphic reaction) at 480–475 Ma and 445–440 Ma. Tributary and French Broad River zircon age spectra are dominated by Mesoproterozoic and Ordovician grains. Most Ordovician zircon from the French Broad River has Th/U > 0.1 and is most likely derived from the Henderson orthogneiss (447.6 ± 5.4 Ma), the largest pluton in the French Broad River headwaters region. A minor zircon age population at 450 Ma, represented primarily by metamorphic zircon rims with Th/U Ordovician monazite ages and zircon rim ages correspond to Taconian metamorphism in the Eastern Blue Ridge province. The dominant monazite age mode (463 Ma) from tributaries is slightly older than the Ordovician metamorphic zircon age mode (450 Ma), which is accounted for by monazite growth primarily via prograde metamorphic reactions, and zircon growth by melt-forming reactions in migmatites at the thermal peak. The scattered middle to late Paleozoic zircon and monazite ages attest to the lack of significant thermotectonic and magmatic events of that age in the Southern Blue Ridge providing sediment to the French Broad River drainage system. This dearth of ages is consistent with the pattern of nonpenetrative late Paleozoic deformation, retrograde metamorphism, and scattered plutonism northwest of the Brevard fault zone. A reasonable source of Neoproterozoic zircon in alluvium or in Ashe–Tallulah Falls paragneisses is Neoproterozoic rift-related magmatic rocks. The rarity of Mesoproterozoic monazite compared to zircon contrasts with the marked abundance of monazite in Ashe–Tallulah Falls paragneisses, and emphasizes the responsiveness of monazite compared to zircon in regional metamorphism. The abundant Mesoproterozoic zircon was inherited from Mesoproterozoic basement lithologies by Neoproterozoic sediments, preserved through regional metamorphism and three phases of orogenesis, and persists in modern alluvium being shed by the orogen.

The Carolina slate belt includes volcanic and sedimentary rocks of Late Precambrian and Cambrian age, metamorphosed to lower greenschist facies, that extend through the Piedmont from Georgia to Virginia. The segment in central North Carolina (Fig. 1) is probably the best-known part of the belt. Rocks in the Albemarle area are mildly deformed and metamorphosed; the dominant structures are open folds plunging southwest and the regional metamorphism is chlorite and biotite grade. The Albemarle region includes type localities for several stratigraphic units. The localities described here are on the Albemarle (Conley, 1962) and Denton (Stromquist, Choquette, and Sundelius, 1971) 15-minute quadrangles. The guidebook by Stromquist and Conley (1959) marked the beginning of modern studies in the central Carolina slate belt, as they demonstrated that stratigraphy and structure could be worked out on a regional basis. Conley and Bain (1965) applied formation names to the section in the Albemarle area (Fig. 2) and extended this stratigraphic nomenclature through most of the slate belt in North Carolina. In the Albemarle region, a thick, dominantly felsic volcanic unit (Uwharrie Formation) is overlain by a dominantly sedimentary sequence (Albemarle Group). The Morrow Mountain Rhyolite and Badin Greenstone of the Tater Top Group were interpreted to be the uppermost units and to lie with angular unconformity above folded older units (Conley and Bain, 1965). Stromquist and Sundelius (1969) reinterpreted part of the middle and upper stratigraphic sequence. They considered the Tater Tbp Group to be interlayered with other units of the Albemarle Group, rather than

North American Geosynclines--Test of Continental-Drift Theory

Two major autochthonous nappes and a deep synformal structure occur in the Piedmont of northwest South Carolina. The cataclastic Brevard zone, which borders the Blue Ridge geologic belt on the southeast, is associated with the synformal belt of partially cataclastic and retrograded rocks at low amphibolite facies grade. This Non-migmatitic belt contains tight cylindroidal isoclines and tectonic slides with a northwest vergence. Refolded isoclines in the Non-migmatitic belt occur near the Brevard zone as a result of recurring displacements there. Immediately southeast lies the Walhalla nappe, which contains kyanite-grade rocks complexly folded into a northeast-plunging conical system. The migmatite front lies within this nappe and in places is coincident with the major slide boundary between the Walhalla nappe and the underlying synformal Non-migmatitic belt. The major slide that separates the overlying Six Mile nappe from the Walhalla nappe is subhorizontal; however, in the broad root zone of both nappes in the southeast part of the area, northwest dips and steep dips represent the attitudes of s-surfaces in many locations. The autochthonous nappe complex probably formed in a continental rise assemblage subjected to intense thermal effects in the early Paleozoic. Sedimentary and (or) volcanic-volcaniclastic assemblages sited along the continental margin were squeezed as a detachment zone between the northwest-driving Piedmont nappes and the activated craton margin. These assemblages became tectonically related to a detachment zone overlying the rising infrastructural nappes, which is now eroded away with the overlying supra-structure. Annealed textures indicate that metamorphism outlasted penetrative deformation, except along the Brevard zone, which continued as a zone of dislocation while thermal metamorphism waned. Late Paleozoic Alleghanian movements may be recorded as microbreccia zones within the Brevard zone and Inner Piedmont belt. This event was not penetrative; instead, it was one of vertical en bloc uplift primarily centered in the Inner Piedmont, and perhaps it accompanied a weak retrogressive metamorphism.

This fi eld trip highlights the current understanding of the tectonic assemblage of the rocks of the Central Appalachians, which include the Coastal Plain, Piedmont, and Blue Ridge provinces. The age and origin of the rocks, the timing of regional deformation and metamorphism, and the signifi cance of the major faults, provide the framework of the tectonic history which includes the Mesoproterozoic Grenvillian, Ordovician Taconian, Devonian to Mississippian Neoacadian, and Mississippian to Permian Alleghanian orogenies.

The Precambrian terrane of the southern Lake Superior region records a long and complex geologic history.The oldest rock in the region was formed around 3.B to 3.6 Ga and repeated tectonic, igneous, and metamorphic activity continued throughout the Precambrian until about 1.1 Ga.Two contrasting Archean--aged geologic terranes are present.The first is an older southern gneiss terrane, parts of which were produced at least 3. B to 3.6 Ga.This terrane experienced regional metamorphic activity at approx imately 3.1, 2.6, 1. B, and 1. 6 Ga.To the north, a granite-greenstone terrane which constitutes the second and younger group of Archean rock was formed 2.75 to 2. 6 Ga.Archean metamorphism(s) in the gneiss terrane was relatively high grade (P = 5-6 Kb, T = 650-BOO OC), with some localized areas reaching the granulite facies.In contrast, regional metamorphism in the greenstone belts of the granite-greenstone terrane was low grade (P 3 Kb, T < 450 OC).LateArchean metamorphism in the southern gneiss terrane may be related to a collis ion event with the more northern granite-greenstone terrane.Early Proterozoic sedimentation, volcanism, and plutonism were extensive throughout much of the southern Lake Superior region, especially in Wisconsin.Metamorphism associated with the Penokean orogeny of 1.90 -1.BO Ga was wide spread and occurred mainly in the low-pressure to intermediate-to low-pres sure (P < 4 Kb) facies series, except for a possible belt of intermediate--pres sure facies series (P -7.5 Kb) kyanite schist in northern Wisconsin.The high est grade attained was the lower sillimanite zone, but much of the metamorphism reached only into the greenschist facies.Metamorphism at this time was con current with or preceded slightly the major deformational activity.At least two major phases of deformation can be recognized throughout the Lake Superior area.It is proposed that the metamorphism accompanying the Penokean orogeny can be interpreted in a plate-tectonic context.Medium-pressure kyanite schist in northern Wisconsin may represent a suture or mark the site of crustal collision_ Post--Penokean metamorphism around 1. 63 Ga also appears to be widespread, but is of low grade.Interpretation of the 1. 63 Ga metamorphism is complicated by the intrusion of the Wolf River Batholith and related rock (1_5 -1.45 Ga), which possibly produced widespread contact-like metamorphism in central and southern Wisconsin.The tectonic interpretation of the 1. 63 Ga event is uncer tain and widely debated at the present time.A possible, but presently unconfirmed pre-Penokean metamorphism around 2. 4 -2. 1 Ga is also obscured and complicated by younger tectonic and metamorphic events.

Late Proterozoic phyllonite in the granitoid basement terrane of the northern Virginia Blue Ridge

Fluid history of the Blue Ridge anticlinorium in the central Appalachians

A sequence of semi-brittle deformational tectonic events in gently dipping middle Precambrian argillite and graywacke, and late Precambrian sandstone and volcanic rocks on the northwest coast of Lake Superior is interpreted from detailed geologic mapping in the Pigeon Point, Cook County, Minnesota area. The earliest tectonic event was broad, open folding in the middle Precambrian Rove Formation along N 35 °E axes and the development of two sets of joints (North and N 70° E). The second event was eastward trending, high-angle faulting in the Rove Formation and the disconformably overlying late Precambrian Puckwunge sandstone and North Shore Volcanic Group; the south sides moved upward relative to the north sides. These faults and associated joints were the loci of emplacement of 'early mafic' dikes of ilmenite-bearing diabase, which probably correlate with the 'Logan intrusions'. Subsequently, east-northeastward trending olivine diabase sills and dikes were emplaced in the Rove Formation and the North Shore Volcanic Group, and they cross-cut the early mafic dikes and sills. The third event occurred after cessation of igneous activity; two sets of regional joints (N 14 °W and N 53 °E) were formed, apparently as a result of fracturing accompanying initial subsidence of the so-called 'Lake Superior syncline' to the southeast. The fourth event was the development of a zone of cataclastic rock trending N 65 °E from the Pigeon Point area northeastward at least 60 km. To the north of this fracture zone, bedding in the Rove Formation dips 15–25 °SE. Copper and silver mineralization was subsequently emplaced within this fracture zone.The first three tectonic events appear to represent reactivation of structures of early Precambrian age, and appear to control the general outcrop pattern. The fourth event is an entirely Keweenawan feature, and marks the beginning of subsidence of the 'Lake Superior syncline'. It may correlate in time with the deposition of the Copper Harbor Conglomerate, and other late middle Keweenawan events.

Research Article| October 01, 1977 Eastern Piedmont fault system: Speculations on its extent Robert D. Hatcher, Jr.; Robert D. Hatcher, Jr. 1Geology Department, Clemson University, Clemson, South Carolina 29631 Search for other works by this author on: GSW Google Scholar David E. Howell; David E. Howell 2South Carolina State Development Board, Division of Geology, Harbison Forest Road, Columbia, South Carolina 29210 Search for other works by this author on: GSW Google Scholar Pradeep Talwani Pradeep Talwani 3Department of Geology, University of South Carolina, Columbia, South Carolina 29208 Search for other works by this author on: GSW Google Scholar Author and Article Information Robert D. Hatcher, Jr. 1Geology Department, Clemson University, Clemson, South Carolina 29631 David E. Howell 2South Carolina State Development Board, Division of Geology, Harbison Forest Road, Columbia, South Carolina 29210 Pradeep Talwani 3Department of Geology, University of South Carolina, Columbia, South Carolina 29208 Publisher: Geological Society of America Received: 16 Jun 1977 Accepted: 07 Jul 1977 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1977) 5 (10): 636–640. https://doi.org/10.1130/0091-7613(1977)5<636:EPFSSO>2.0.CO;2 Article history Received: 16 Jun 1977 Accepted: 07 Jul 1977 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Robert D. Hatcher, David E. Howell, Pradeep Talwani; Eastern Piedmont fault system: Speculations on its extent. Geology 1977;; 5 (10): 636–640. doi: https://doi.org/10.1130/0091-7613(1977)5<636:EPFSSO>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Geologic mapping, interpretation, and field checking of recent aeromagnetic data suggest the existence of a closely associated series of faults and splays extending from Alabama to Virginia, herein termed the Eastern Piedmont fault system. Characteristic magnetic anomalies were found to be associated with known faults and were used to trace them through covered intervals. The fault system extends northeastward from the Goat Rock fault of Alabama and west-central Georgia, crossing the lower Piedmont of South Carolina, passes beneath a segment of the Coastal Plains in the Carolinas, and then flanks the Raleigh belt in North Carolina and continues into Virginia. From east-central Georgia to Virginia, cataclastic rocks along the faults of the system are bounded to the northwest and southeast by rocks of the Carolina slate belt, forming perhaps the most extensive fault system in eastern North America. Its movement history is similar to that of the Brevard fault: an early ductile mylonitic phase, followed by periods of brittle deformation. We interpret the fault system to have been initiated during collapse of the late Precambrian–early Paleozoic Carolina slate belt island arc. The Paleozoic continental suture probably lies farther east, buried beneath the Coastal Plain. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

PreviousNext No AccessSEG Technical Program Expanded Abstracts 1982Some effects of regional metamorphism and geologic structure on magnetic anomalies over the carolina slate belt near roxboro, North CarolinaAuthors: E.S. RobinsonP. V. PolandL. GloverJ. A. SpeerE.S. RobinsonVirginia Polytech Inst. and State Univ., P. V. PolandVirginia Polytech Inst. and State Univ., L. GloverVirginia Polytech Inst. and State Univ., and J. A. SpeerVirginia Polytech Inst. and State Univ.https://doi.org/10.1190/1.1826920 SectionsAboutPDF/ePub ToolsAdd to favoritesDownload CitationsTrack CitationsPermissions ShareFacebookTwitterLinked InRedditEmail Permalink: https://doi.org/10.1190/1.1826920FiguresReferencesRelatedDetails SEG Technical Program Expanded Abstracts 1982ISSN (print):1052-3812 ISSN (online):1949-4645Copyright: 1982 Pages: 520 publication data© 1982 Copyright © 1982 Society of Exploration GeophysicistsPublisher:Society of Exploration Geophysicists HistoryPublished Online: 24 Mar 2005 CITATION INFORMATION E.S. Robinson, P. V. Poland, L. Glover, III, and J. A. Speer, (1982), "Some effects of regional metamorphism and geologic structure on magnetic anomalies over the carolina slate belt near roxboro, North Carolina," SEG Technical Program Expanded Abstracts : 256-258. https://doi.org/10.1190/1.1826920 Plain-Language Summary PDF DownloadLoading ...

The Rockford Granite is a plutonic suite of synmetamorphic, two-mica granitoids that intrude graphite-bearing, amphibolite-facies metasediments in the eastern Blue Ridge of the Alabama Appalachians. Geochemical classification indicates peraluminous granite and trondhjemite with subordinate granodiorite. The trondhjemites are products of pervasive, infiltrative Na-metasomatism; whereas the granites and granodiorites sufficiently retain primary traits for petrogenetic modelling. A significant proportion of the Rockford Granite magmatic phases may represent refractory restite, expressed as centimeter-size biotite clots, normal oscillatory zoned plagioclase with a subidiomorphic zone-boundary core, and rounded, zoned xenocrystic zircons. Major and trace element modelling indicate that the Rockford Granite could be a product of 40% equilibrium partial melting of a metagraywacke source producing felsic, granitic melt plus a restite assemblage of plagioclase49-quartz31-biotite14-apatite5 5-zircon0 5. The limited geochemical heterogeneity of the Rockford Granite is attributed to progressive restite-melt separation, reflected by systematic collinearity of calculated source and restite compositions in bivariate diagrams. Whole rock oxygen and reduced carbon isotope analyses support the metagraywacke source model for the Rockford Granite and also record regional isotopic exchange with metamorphic fluids from the volumetrically-dominant metapelites. Combined whole-rock oxygen and carbon isotope determinations both constrain granitoid source rock characteristics and measure the extent of C-O-H metamorphic fluid-rock interaction. Quantitative geothermobarometric estimates for the Rockford Granite and surrounding metasediments indicate kyanite-sillimanite facies series regional metamorphism, with a Barrovian geothermal gradient of 30°C/km. The prograde P-T path is abruptly terminated by nearly isothermal decompression, interpreted as due to diapiric intrusion and associated thermal doming. Rockford Granite and migmatite samples fall in the P-T regime where the maximum overlap between muscovite dehydration and minimum melt conditions exist suggesting that the former controlled anatexis. The tectonic setting and depositional environment of the host metasedimentary protoliths are interpreted as associated with opening of the lapetus Ocean. Structurally lowermost are interlayered amphibolites and quartzites, representing synrift volcanism and sedimentation, overlain by thick passive margin pelitic and graywacke deposits. The Rockford Granite contains a Grenville-age (l·0–1·2 Ga) inherited, xenocrystic zircon component interpreted to record crustal recycling via weathering of a Grenville gneiss source to produce metagraywacke protoliths, followed by regional metamorphism and anatexis to yield the granitic magmas. Barrovian regional metamorphism, associated crustal shortening, and anatexis in an Acadian continental-oceanic subduction regime best represents the tectonomagmatic setting of Rockford Granite genesis.

A list of 59 earthquake events felt in North Carolina between 1732 and 1900 has been compiled. It is the most complete such list yet catalogued. Thirty-eight of these events probably had their epicenters within the state or within a few miles of the state line. Much of the activity cannot easily be related to known geologic structures although some of the epicenters fall near the Blue Ridge front and the Brevard zone, near the border faults of the Dan River and Deep River Triassic Basins, and in the Cape Fear Arch region. Magnitudes of the N.C. events were all probably less than 4.5 on the Richter Scale. The maximum intensities experienced were due to the New Madrid, Missouri, earthquakes of 1811–12 and the Charleston, South Carolina, earthquake of 1886. There is a correlation between the growing population of N. C. from 1732 and the number of reported events but the state probably became saturated with respect to seismic detection around 1855 and, hence, the increase in seismic activity after 1870 is probably of genuine geologic origin. Among the most interesting events is the 1874 swarm near Rumbling Bald Mountain in McDowell County when more than 75 earthquakes occurred in two months. Field inspection of the site shows huge relatively recent-looking splits in the mountains there that could have been due to this swarm. The historical record of N. C. is probably not long enough to be a very valid basis for making earthquake risk projections into the future. It may be that the greatest threat to N.C. is large earthquakes outside her borders such as those in the Mississippi Valley and South Carolina. On the otherhand, there is nothing now known that can rule out the possibility that the greatest threat to N.C. is within her own boundaries. This latter possibility becomes a very real and serious concern when one studies the land elevation changes currently in progress in eastern N. C.