Lynn Glover

Seismic structure of the U.S. Mid‐Atlantic continental margin

Multichannel and wide-angle seismic data collected off Virginia during the 1990 EDGE Mid-Atlantic seismic experiment provide the most detailed image to date of the continent-ocean transition on the U.S. Atlantic margin. Multichannel data were acquired using a 10,800 in3 (177 L) airgun array and 6-km-long streamer, and coincident wide-angle data were recorded by ten ocean bottom seismic instruments. A velocity model constructed by inversion of wide-angle and vertical-incidence travel times shows strong lateral changes in deep-crustal structure across the margin. Lower-crustal velocities are 6.8 km/s in rifted continental crust, increase to 7.5 km/s beneath the outer continental shelf, and decrease to 7.0 km/s in oceanic crust. Prominent seaward-dipping reflections within basement lie within layers of average velocity 6.3–6.5 km/s, consistent with their interpretation as basalts extruded during rifting. The high-velocity lower crust and seaward-dipping reflections comprise a 100-km-wide, 25-km-thick ocean-continent transition zone that consists almost entirely of mafic igneous material accreted to the margin during continental breakup. The boundary between rifted continental crust and this thick igneous crust is abrupt, occupying only about 20 km of the margin. Appalachian intracrustal reflectivity largely disappears across this boundary as velocity increases from 5.9 km/s to >7.0 km/s, implying that the reflectivity is disrupted by massive intrusion and that very little continental crust persists seaward of the reflective crust. The thick igneous crust is spatially correlated with the East Coast magnetic anomaly, implying that the basalts and underlying intrusives cause the anomaly. The details of the seismic structure and lack of independent evidence for an appropriately located hotspot in the central Atlantic imply that nonplume processes are responsible for the igneous material.

Ages of regional metamorphism and ductile deformation in the central and southern Appalachians

Abstract A summary of available geochronology of deformational fabrics formed at greenschist and higher grades in the central and southern Appalachians is presented as a map showing the ages of regional metamorphism and ductile deformation. The ca. 480-435 Ma Taconic event affected most of the Piedmont and Blue Ridge provinces. The ca. 380-340 Ma Acadian event was milder and was centered in the west central Piedmont and eastern Blue Ridge. The 330-230 Ma Alleghanian event exhibits an abrupt ductile deformation front in the eastern Piedmont and extends an unknown distance eastward below the Atlantic Coastal Plain.

Deep seismic reflection data of EDGE U.S. mid-Atlantic continental-margin experiment: Implications for Appalachian sutures and Mesozoic rifting and magmatic underplating

The EDGE seismic experiment across the Virginia continental margin delineated a Paleozoic suture, buried Appalachian terranes, and Mesozoic rifting and magmatic events. The seismic grid revealed that the Mesozoic Norfolk rift basin exists only in the northern one-third of the previously mapped area. The north-striking listric border fault of the Norfolk basin half-graben parallels seismic laminations in the basement. The Jurassic volcanic wedge pinches out just landward of the Baltimore Canyon trough hinge zone and downlaps on the hummocky oceanic basement under the continental rise. Under the continental slope, the volcanic wedgereaches depths >9 s (20 km). Two distinct intracrustal reflections at 4.0-5.0 s and at 7.0 s TWIT (two-way traveltime) dip southeastward at low angles (∼15°). The Moho reflection is disrupted where it is intersected by the 7.0 s reflection. Northwest of this point the Moho dips landward; seaward it is horizontal. Seaward of this point, the lower-crustal boundary laminations exist in a narrow interval (10.5-11.0 s) and are of strong amplitude. These changes in the Moho and lower crust represent the seaward edge of the Grenville-age North American crust and the landward edge of Jurassic magmatic underplating. A northwest-dipping reflection observed for the first time on the U.S. Atlantic margin may be the top of the Jurassic magmatic- underplating layer; the northwest-dipping reflection truncates the southeast- dipping 7.0 s TWTT reflection. Landward projection of the 7.0 s reflection yields a north-south trace on the postrift unconformity under the center of lower Chesapeake Bay. This trace is near a basement fault between low-grade metamorphic rocks (Carolina slate-Avalonia) on the east and high-grade rocks (Goochland terrane) on the west. This fault boundary and the southeastdipping 7.0 s reflection probably represent the Taconic suture.

Stratigraphy and tectonics of the Virginia–North Carolina Blue Ridge: Evolution of a late Proterozoic–early Paleozoic hinge zone

Late Proterozoic (690-570 Ma) through Ordovician rocks of the Virginia—North Carolina Blue Ridge and Valley and Ridge record the evolution of an eastward-facing passive margin and its subsequent deformation during continent-arc collision (Taconic orogeny). Comparison of Blue Ridge stratigraphy with modern rifts and passive margins suggests that late Proterozoic rifting involved substantial crustal attenuation in the eastern Blue Ridge. The axial zone of the Blue Ridge from central Virginia southward is occupied by a major fault system (Hayesville-Fries-Rock-fish Valley fault). Contrasts between rocks deposited east and west of this fault suggest that it is a reactivated hinge zone, separating highly attenuated continental crust to the east from continental crust of more normal thickness to the west. During rifting, normal faulting and rapid subsidence of attenuating crust in the eastern Blue Ridge produced rift basins in which deep-water clastic and volcanic rocks accumulated (Lynchburg Group, Ashe Formation). Landward of the hinge zone in the western Blue Ridge, mainly terrestrial to shallow-water clastic and volcanic rocks were deposited in a series of subaerial rift basins (Swift Run/Catoctin, Grandfather Mountain, Mount Rogers Formations). The hinge zone not only controlled depositional patterns throughout the rift and passive-margin history, but it was also reactivated during the Taconic collision, when attenuated crust and its cover were thrust westward over the shelf edge. Turbidite-dominated rift sequences of the eastern Blue Ridge are unlike “typical” rift deposits, which are characterized by predominantly subaerial and shallow-water sedimentation. Deep-water rifts may be common on attenuated crust (as in the Bay of Biscay); however, they are poorly known from the stratigraphic record because of their susceptibility to destruction during orogeny.

Deformation and metamorphism in the Hylas zone and adjacent parts of the eastern Piedmont in Virginia

Structural and petrographic studies of the Hylas zone northwest of Richmond, Virginia, reveal that late Paleozoic ductile shearing produced mylonites and ultramylonites from pre-existing biotite gneiss, granite gneiss, and amphibolite in the Goochland complex and from Petersburg(?) granite. Rocks in the Goochland complex west of the Hylas zone underwent at least two deformations (D 1 , D 2 ) prior to ductile shearing in the Hylas zone (D 3 ). D 3 caused a pervasive mylonitic foliation (S c ) and a later widely spaced shear cleavage (S s ) to be formed in rocks in the Hylas zone. Brittle deformation in the form of high-angle faulting and locally intense fracturing (D 4 ) was superimposed on the Hylas zone about 220 m.y. ago and is probably equivalent to the Palisades disturbance of New England. D 4 was associated throughout the Piedmont with synchronous downwarp and dominantly continental sedimentation to form a series of parallel Triassic basins represented in the study area by the Richmond Basin. Northwest-oriented high-angle faults that apparently displace Triassic sedimentary rocks are interpreted to predate the Late Cretaceous. Regional prograde metamorphism of the Goochland complex to amphibolite facies (M 1 ) was inclusive of D 1 and D 2 , because structural elements that compose these deformational events consist primarily of oriented metamorphic minerals. Well-equilibrated microstructures in the gneisses suggest that the M 1 peak persisted past D 2 . M 1 is inferred to have occurred about 340 m.y. ago. D 3 was accompanied by retrograde metamorphism (m 2 ) to greenschist facies in the Hylas zone during late Paleozoic time. The presence of laumontite, quartz, and calcite in brecciated zones suggests that brittle deformation during D 4 occurred under zeolite facies conditions. Cenozoic reverse faulting 90 km along strike of the study area in the Brandywine area of the Coastal Plain in Maryland may indicate a continuation of the Hylas zone to the north. Superposed deformation and intermittent reactivation of zones of instability similar to that of the Hylas zone are reported in the Eastern Piedmont fault system, which extends from Alabama to Virginia. Current stress release beneath the Coastal Plain may be influenced by the instability of the Hylas zone and analogous fault systems throughout the Piedmont.

A tectonics test of the most commonly used geochemical discriminant diagrams and patterns

Abstract During the last two decades many tectonomagmatic geochemical discriminant diagrams were created and are now being widely used in paleotectonic reconstructions. The sources of original data and discriminating quality of fourteen of the most commonly used diagrams are examined herein. Over 200 Jurassic rift basalts, diabases and chilled diabase margins from eastern North America are plotted on these diagrams. The inconsistent results from other regions with known tectonic settings are also discussed. It is found that most diagrams fail to identify continental basalts (CB), the majority of which plot in every field except within-plate basalts (WPB). Because continental basalts do plot in other fields, such as MORB and arc basalts, the discriminating ability of these diagrams is insufficient without additional criteria. Large amounts of chemical data (especially trace- and rare-earth element data) and intensive studies of continental basalts were not available until the late 1980s. Lack of representative data at the time these diagrams were created is a major reason for their failure to identify continental basalts. Moreover, the genesis of continental basalt magma involves a series of complicated processes that may vary from place to place even within the same tectonic province. As a result, the geochemical patterns of continental basalts cover a wide range in compositions of isotopes, trace elements, REE, as well as major and minor elements. It is, therefore, difficult to create a universal chemical diagram that can discriminate continental basalts from all others. Our conclusion is that the reconstruction of paleotectonic settings should not be done by using geochemical data alone. Although this may seem obvious to many, perusal of the literature shows that tectonic setting assignments frequently are based almost entirely on geochemistry. Tectono-stratigraphic analysis based on knowledge of field relations, structure and petrology of epiclastic, volcaniclastic and igneous protoliths is an essential adjunct to geochemistry in determining ancient tectonic environments.

Alleghanian tectono-thermal evolution of the dextral transcurrent Hylas zone, Virginia Piedmont, U.S.A.

Abstract The Hylas zone is the northernmost segment of the Eastern Piedmont fault system and lies in the Grenville Goochland terrane, Virginia. Like the other major segments, the Nutbush Creek zone, the Hollister zone and the Modoc (Irmo) zone, the Hylas zone experienced late Paleozoic dextral transcurrent shearing. Early deformation in the zone was ductile and produced type I and type II S-C mylonites under amphibolite-grade metamorphic conditions. Later deformation was at the brittle-ductile transition at which time feldspar underwent microfaulting and cataclasis while quartz formed ribbons. The feldspar exhibits a strain-dependent sequence of microstructures, including kink-bands, antithetic extensional microfaults (pull-apart), bends in the microfaults by development of transverse fractures and microboudinage. Correlation of mineral mechanical response with a thermal-decay curve based on isotopic mineral ages yields a temporally constrained deformation history. Ductile dextral shearing occurred subsequent to the intrusion of the 330 Ma Petersburg granite and passed into the brittle-ductile transition by approximately 260 Ma. Dextral faulting terminated before 240 Ma. The Appalachian dextral transcurrent faulting event therefore continued through Permian in some areas.

The Edge Experiment and the U.S. East Coast Magnetic Anomaly

The EDGE experiment offshore the U.S. east coast obtained near vertical incidence seismic reflection data, as well as wide angle seismic reflection and seismic refraction data. These results are combined with magnetic total intensity data in order to establish the origin for the East Coast Magnetic Anomaly (ECMA) and to hypothesize about geological events taking place at the time that opening was initiated in the North Atlantic.

Late Paleozoic transcurrent tectonic assembly of the central Appalachian Piedmont

Recent investigations in south-eastern Pennsylvania and northern Maryland have demonstrated a major anastomosing strike-slip shear system. The Pleasant Grove-Huntingdon Valley shear system emerges from beneath the coastal plain cover at Trenton, New Jersey, and extends to the area west of Baltimore, Maryland, where it is overlain by the Culpepper Mesozoic rift basin. The sense of offset across this system is dextral. In the Susquehanna River region and north of the shear zone, the rocks of the Octoraro Formation contain evidence for two metamorphisms and deformations prior to strike-slip shearing, whereas south of the shear zone the Peters Creek Formation contains evidence for only one. The discordance in metamorphic and deformational history across the shear zone suggests the now juxtaposed rocks originated in different parts of the orogen. Although conclusive ages for the strike-slip deformation do not exist at this time, the timing of deformation is loosely constrained where the shear system crosscuts known Taconian structures in the Piedmont. Comparison of deformation style with other regions in the Appalachian suggests the Pleasant Grove-Huntingdon Valley shear system is related to Alleghanian transcurrent tectonics in the Piedmont. Palinspastic reconstruction of the Pleasant Grove-Huntingdon Valley shear system reveals fundamental problems in current tectonic models for the central Appalachian Piedmont. A minimum of 150 km of dextral offset is proposed for the Pleasant Grove-Huntingdon Valley shear system based on reconstruction of the Cambrian-Ordovician shelf edge between northern Maryland and southeastern New York. Displacement of this magnitude can account for the previously proposed tectonic models that portray a failed Iapetan rift block and microcontinent that contains the Baltimore Grenvillian massifs. Even though a history of early orthogonal collision is preserved within discrete structural blocks, transcurrent shearing has greatly influenced the distribution of those blocks. Models not including the strike-slip component of tectonic assembly need serious reconsideration, as evidence grows that the magnitude of orogen-parallel displacement is equal to or larger than the orthogonal component.

The regional extent of the ca. 600 Ma Virgilina deformation: Implications for stratigraphic correlation in the Carolina terrane

We present a regional stratigraphic and structural architecture of the southern Appalachian Carolina terrane, based on new field studies in central North Carolina. The Virgilina deformation is interpreted to be a regional feature. Stratigraphic units include an older (ca. 700 to 600 Ma), calc-alkalic volcanic (Hyco and Virgilina Formations) and intra-arc rift(?) epiclastic (Aaron Formation) sequence, folded and faulted by the Virgilina event. Unconformably overlying the older stratigraphy, there is a younger (ca. 600 to 540 Ma, bimodal volcanic-sedimentary sequence (Uwharrie Formation and Albemarle Group). Some present models imply that the effects of the Virgilina deformation are not wide-spread throughout the Carolina terrane. This misconception arose because the northern slate belt stratigraphy (Hyco, Aaron, and Virgilina Formations) was miscorrelated with central slate belt units (Uwharrie Formation and Albemarle Group). A re-evaluation of the physical stratigraphic similarities and temporal constraints of Ediacarian fauna in these units indicates that the two sequences are not equivalent. Stratigraphic and structural data from the study area document the Virgilina deformation and its associated unconformity in the central North Carolina portion of the Carolina terrane. Stratigraphic and map evidence indicates that the Uwharrie Formation unconformably overlies the Hyco Formation. The unconformity between the older and younger stratigraphic units is a boundary between discordances in lineations, fold axial traces, and fold tightness. The Virgilina deformation appears to be coeval with the Monian and Cadomian events of Europe and with the Pan-African orogeny of west Africa. The stratigraphic and tectonic sequence presented herein provides a terrane characterization for the Carolina terrane that may be of use in reassembling Avalon, if Avalon was ever a single entity.