Goochland Terrane Research Articles

The stratigraphy and stratigraphic nomenclature of the Goochland Terrane in the piedmont province of east-central Virginia

The Goochland terrane is a structurally isolated crustal block in the eastern Piedmont of Virginia. It is composed of the previously named State Farm Gneiss, Montpelier Anorthosite, Sabot Amphibolite, and Maidens Gneiss, but also includes the Scotchtown Gneiss, Teman Gneiss, and Old Bandana Gneiss which are formally named and defined herein. The eastern part of the Goochland terrane is antiformal and cored by Mesoproterozoic rocks (the State Farm Gneiss and the Montpelier Anorthosite). These basement units are overlain by a late Neoproterozoic to early Paleozoic (Ediacaran to Early Cambrian) saprolitic, metavolcanic, and metasedimentary sequence that sequentially includes the Scotchtown Gneiss, Sabot Amphibolite and Maidens Gneiss. The western part of the terrane is synformal and includes in its core two additional units that overlie the Maidens Gneiss: the Teman Gneiss and the Old Bandana Gneiss. Based on mineralogy and zircon grain morphology, the protoliths of the Maidens, Teman, and Old Bandana gneisses were predominantly sedimentary rocks. The protoliths of the Teman Gneiss and Old Bandana Gneiss were deposited unconformably upon the protolith of the Maidens Gneiss. The eastern and western parts of the Goochland terrane are separated by the Dabneys fault, which has considerable east-side-up vertical offset and possibly also significant transverse displacement. Correlation of the upper part of the Goochland terrane (Teman and Old Bandana gneisses) with the Setters and Cockeysville gneisses in the Baltimore region suggests that the Goochland terrane was left about 135 miles (ca. 220 km) southwest of its original North American location, which was to the east of Baltimore, Maryland. This displacement was caused by the oblique collision of the eastern North American continent with the western edge of the Gondwanan craton during the later Carboniferous (Pennsylvanian) Period.

Five hundred million years of punctuated addition of juvenile crust during extension in the Goochland Terrane, central Appalachian Piedmont Province

ABSTRACTThe Goochland Terrane is an enigmatic crustal block in the Appalachian Piedmont Province of central Virginia, USA. Sparse exposures of terminal Mesoproterozoic and late Neoproterozoic igneous rocks in the central Goochland Terrane offer the opportunity to investigate both the continental affinity of the terrane during the Proterozoic Eon and the timing and mechanisms of crustal growth. We apply multiple geochemical tools to these rocks: tectonic discrimination using whole-rock major and trace element abundances; whole-rock Sm-Nd isotopes; O, U-Pb, and Lu-Hf isotope analyses of spots in zircon; and measurement of O isotopes in multi-grain quartz separates. Eruption of the Sabot Amphibolite protolith is difficult to date, but we tentatively assign an age of 552 ± 11 Ma. Goochland Terrane continental crust first separated from the mantle prior to ca. 1050–1010 Ma intrusion of the Montpelier Anorthosite and the State Farm Gneiss protolith. The granitic magma that became the State Farm Gneiss protolith could have been derived entirely from partial melting of this initial Goochland Terrane crust. In contrast, the magmas that became the Montpelier Anorthosite, Neoproterozoic granitoid, and the Sabot Amphibolite were mixtures of mantle melt and preexisting Goochland Terrane crust. This production of juvenile continental crust occurred during continental extension and, eventually, rifting. The timing and compositions of terminal Mesoproterozoic magmatism in the Goochland Terrane closely match those in the nearby Blue Ridge Province. Although the compositions of the Neoproterozoic magmas in the two regions are similar, intrusion and possibly eruption occurred about 10 M.y. later in the Goochland Terrane.

Northward extension of carolina slate belt stratigraphy and structure, south-central virginia: Results from geologic mapping

Geologic mapping in south-central Virginia demonstrates that the stratigraphy and structure of the Carolina slate belt extend northward across a steep thermal gradient into upper amphibolite-facies correlative gneiss and schist. The Neoproterozoic greenschist-facies Hyco, Aaron, and Virgilina Formations were traced northward from their type localities near Virgilina, Virginia, along a simple, upright, northeast-trending isoclinal syncline. This syncline is called the Dryburg syncline and is a northern extension of the more complex Virgilina synclinorium. Progressively higher-grade equivalents of the Hyco and Aaron Formations were mapped northward along the axial trace of the refolded and westwardly-overturned Dryburg syncline through the Keysville and Green Bay 7.5-minute quadrangles, and across the northern end of the Carolina slate belt as interpreted on previous geologic maps. Hyco rocks, including felsic metatuff, metawacke, and amphibolite, become gneisses upgrade with areas of local anatexis and the segregation of granitic melt into leucosomes with biotite selvages. Phyllite of the Aaron Formation becomes garnet-bearing mica schist. Aaron Formation rocks disconformably overlie the primarily felsic volcanic and volcaniclastic rocks of the Hyco Formation as evidenced by repeated truncation of internal contacts within the Hyco on both limbs of the Dryburg syncline at the Aaron-Hyco contact. East-northeast-trending isograds, defined successively by the first appearance of garnet, then kyanite ± staurolite in sufficiently aluminous rocks, are superposed on the stratigraphic units and synclinal structure at moderate to high angles to strike. The textural distinction between gneisses and identifiable sedimentary structures occurs near the kyanite ± staurolite-in isograd. Development of the steep thermal gradient and regional penetrative fabric is interpreted to result from emplacement of the Goochland terrane adjacent to the northern end of the slate belt during Alleghanian orogenesis. This mapping study indicates that the Carolina slate belt does not terminate on the north against through-going faults or rest on higher-grade basement as previously suggested.

Northern ancestry for the Goochland terrane as a displaced fragment of Laurentia: Comment and Reply: COMMENT

The recent article by [Bartholomew and Tollo (2004)][1] concludes that the enigmatic Goochland terrane is a displaced Laurentian fragment. In their model, the Goochland terrane originated in the New York promontory, rifted away from Laurentia during the formation of Iapetus, and was subsequently

Northern ancestry for the Goochland terrane as a displaced fragment of Laurentia: Comment and Reply: REPLY

We welcome the opportunity to discuss further our ideas regarding translational tectonics along the Iapetan margin of Laurentia. We look forward to seeing published papers on the Alleghanian history of the Goochland terrane based on recent abstracts of our colleagues from William and Mary, but

Northern ancestry for the Goochland terrane as a displaced fragment of Laurentia

The ancestral position of the Goochland terrane, an isolated block containing Mesoproterozoic crust in the Appalachian Piedmont of eastern Virginia, during Grenvillian orogenesis has direct bearing on the breakup of Rodinia. Ages, lithology, geochemistry, and Pb and Nd isotope compositions of the Montpelier Anorthosite and State Farm Gneiss of the Goochland terrane allow correlation with the Grenvillian Roseland Anorthosite and associated igneous suites of the Blue Ridge farther west in Virginia. In contrast, extensive metapelites, which underwent Devonian metamorphism and intrusion, and amphibolites of the Goochland terrane lack equivalents in the Blue Ridge. Dextral slip faults, which bound the Goochland terrane, as well as dextral faults farther north, suggest large-scale translation prior to late Paleozoic reaccretion. Ages of extension-related A-type granitoids within the Virginia Blue Ridge (735-680 Ma) suggest physical separation from compositionally similar granitoids in the Goochland terrane (630-588 Ma), Reading Prong (602 Ma), and Manhattan Prong (563 Ma). Pre-Iapetan restoration of the Goochland terrane northeastward of the Blue Ridge and outboard of the Reading Prong accounts for its affinities to the Blue Ridge (unique anorthosites), Manhattan Prong (abundant amphibolites), and Reading Prong (ca. 600 Ma magmatism). Thus similar to the translation of Madagascar out of Africa, the Goochland terrane was rifted from Laurentia, marooned in oceanic crust, and then dextrally translated southward ∼500 km prior to late Paleozoic reaccretion. Documentation of large-scale dextral translation of eastern Laurentia relative to its fragments and other cratons during the Neoproterozoic and Paleozoic may assist in locating far-traveled pieces of the Grenville orogen for reconstruction of Rodinia.

Geochronology of the Mesoproterozoic State Farm gneiss and associated Neoproterozoic granitoids, Goochland terrane, Virginia

Research Article| August 01, 2003 Geochronology of the Mesoproterozoic State Farm gneiss and associated Neoproterozoic granitoids, Goochland terrane, Virginia Brent E. Owens; Brent E. Owens 1Department of Geology, College of William and Mary, Williamsburg, Virginia 23187, USA Search for other works by this author on: GSW Google Scholar Robert D. Tucker Robert D. Tucker 1Department of Geology, College of William and Mary, Williamsburg, Virginia 23187, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Brent E. Owens 1Department of Geology, College of William and Mary, Williamsburg, Virginia 23187, USA Robert D. Tucker 1Department of Geology, College of William and Mary, Williamsburg, Virginia 23187, USA Publisher: Geological Society of America Received: 14 Sep 2002 Revision Received: 15 Jan 2003 Accepted: 07 Feb 2003 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2003) 115 (8): 972–982. https://doi.org/10.1130/B25258.1 Article history Received: 14 Sep 2002 Revision Received: 15 Jan 2003 Accepted: 07 Feb 2003 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Brent E. Owens, Robert D. Tucker; Geochronology of the Mesoproterozoic State Farm gneiss and associated Neoproterozoic granitoids, Goochland terrane, Virginia. GSA Bulletin 2003;; 115 (8): 972–982. doi: https://doi.org/10.1130/B25258.1 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 SocietyGSA Bulletin Search Advanced Search Abstract The Goochland terrane is an isolated block of Mesoproterozoic crust in the Piedmont Province of central Virginia. We report U-Pb zircon dates and whole-rock major and trace element data for the State Farm gneiss, one of the main units in the terrane, and additional results for a newly recognized suite of Neoproterozoic granitoids that intrude the gneiss. The State Farm gneiss ranges in bulk composition from quartz monzodiorite to granite, and three samples yield U-Pb zircon dates of 1046 +7/–6 Ma, 1039 +18/–11 Ma, and 1023 ± 10 Ma. We interpret these dates as igneous crystallization ages, which indicate a maximum emplacement interval of ca. 1057–1013 Ma for these samples. Neoproterozoic granitoids (including the Fine Creek Mills and Flat Rock granites) are more alkaline than the State Farm gneiss and display all compositional characteristics of A-type granites (e.g., high Fe/Mg, Ga, Ga/Al, Nb, Zn, Y, Zr). U-Pb zircon analyses from five separate bodies indicate crystallization ages from ca. 654 to 588 Ma, but all results are complicated by Mesoproterozoic (ca. 1 Ga) inheritance, coupled, in some cases, with secondary Pb loss. We interpret the ages and compositions of the younger rocks to reflect Neoproterozoic rifting of the Goochland terrane, but its location during rifting is uncertain. The terrane may have been separated from Laurentia during the Neoproterozoic breakup of Rodinia and later reattached. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Nd and Sr isotopic constraints on the source of Alleghanian granites in the Raleigh metamorphic belt and Eastern slate belt, southern Appalachians, U.S.A.

Pennsylvanian-Permian granites that intrude the Raleigh metamorphic belt and the Eastern slate belt of the southern Appalachian orogen account for ∼ 40% of the total volume of granite produced during the Alleghanian orogeny. We report major- and trace-element concentrations and initial Nd and Sr isotopic compositions from 31 samples of Allegahanian granites that intrude these belts to further constrain the nature of the sources of the granites. Nd isotopic compositions of three Raleigh metamorphic belt orthogneisses and two Eastern slate belt metavolcanic rocks are also presented to further constrain the nature of the crust in this region of the orogen. Major- and trace-element concentrations of Raleigh metamorphic belt and Eastern slate belt Alleghanian granites are similar for every element examined. Major-element concentrations of the Alleghanian granites indicate that they are high K 2O (3–5 wt%), slightly peraluminous [Al 2O 3/(CaO + Na 2O + K 2O) = 0.96–1.12]. Trace-element compositions of the Alleghanian granites are similar to those of granites produced in both volcanic arc and continental collision settings. Initial Nd isotopic compositions of Raleigh metamorphic belt granites range from ε Nd(300 Ma) −4.4 to +3.0 and initial 87Sr/ 86Sr ratios from 0.703 to 0.708. Eastern slate belt granites have ϕ Nd(300Ma) between −2.7 and +2.2, and initial 87Sr/ 86 Sr ratios range between 0.703 and 0.706. Depleted mantle model ages of granites from both belts range from 600 to 1070 Ma. Gneisses from the Raleigh metamorphic belt and volcanic rocks of the Eastern slate belt have ε Nd(300 Ma) values of −4.4 to +4.4, similar to those of the Alleghanian granites, suggesting that anatexis of the crust in these belts could have produced the granites. The neodymium isotopic compositions of the gneisses from the Raleigh metamorphic belt are significantly more radiogenic than that of Grenville crust. The granites that intrude the Raleigh metamorphic belt also show no evidence of interaction with Grenvillian crust, suggesting that neither the surface rocks nor unexposed portions of the Raleigh metamorphic belt are predominantly composed of Grenville crust. Thus in contrast to previous suggestions, the Raleigh metamorphic belt is significantly different than the adjoining Goochland terrane, but is similar to the nearby Carolina terrane.

Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia

Research Article| November 01, 1996 Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia John N. Aleinikoff; John N. Aleinikoff 1U.S. Geological Survey, Box 25096, M.S. 963, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar J. Wright Horton, Jr.; J. Wright Horton, Jr. 2U.S. Geological Survey, 926 National Center, Reston, Virginia 20192 Search for other works by this author on: GSW Google Scholar Marianne Walter Marianne Walter 1U.S. Geological Survey, Box 25096, M.S. 963, Denver, Colorado 80225 Search for other works by this author on: GSW Google Scholar Author and Article Information John N. Aleinikoff 1U.S. Geological Survey, Box 25096, M.S. 963, Denver, Colorado 80225 J. Wright Horton, Jr. 2U.S. Geological Survey, 926 National Center, Reston, Virginia 20192 Marianne Walter 1U.S. Geological Survey, Box 25096, M.S. 963, Denver, Colorado 80225 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1996) 108 (11): 1481–1491. https://doi.org/10.1130/0016-7606(1996)108<1481:MPAFTM>2.3.CO;2 Article history First Online: 01 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 John N. Aleinikoff, J. Wright Horton, Marianne Walter; Middle Proterozoic age for the Montpelier Anorthosite, Goochland terrane, eastern Piedmont, Virginia. GSA Bulletin 1996;; 108 (11): 1481–1491. doi: https://doi.org/10.1130/0016-7606(1996)108<1481:MPAFTM>2.3.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 SocietyGSA Bulletin Search Advanced Search Abstract Uranium-lead dating of zircons from the Montpelier Anorthosite confirms previous interpretations, based on equivocal evidence, that the Goochland terrane in the eastern Piedmont of Virginia contains Grenvillian basement rocks of Middle Proterozoic age. A very few prismatic, elongate, euhedral zircons, which contain 12–29 ppm uranium, are interpreted to be igneous in origin. The vast majority of zircons are more equant, subangular to anhedral, contain 38–52 ppm uranium, and are interpreted to be metamorphic in origin. One fraction of elongate zircon, and four fragments of a very large zircon (occurring in a nelsonite segregation) yield an upper intercept age of 1045 ± 10 Ma, interpreted as the time of anorthosite crystallization. Irregularly shaped metamorphic zircons are dated at 1011 ± 2 Ma (weighted average of the 207Pb/206Pb ages). The U-Pb isotopic systematics of metamorphic titanite were reset during the Alleghanian orogeny at 297 ± 5 Ma. These data provide a minimum age for gneisses of the Goochland terrane that are intruded by the anorthosite. Middle Proterozoic basement rocks of the Goochland terrane may be correlative with those in the Shenandoah massif of the Blue Ridge tectonic province, as suggested by similarities between the Montpelier Anorthosite and the Roseland anorthosite. Although the areal extent of Middle Proterozoic basement and basement-cover relations in the eastern Piedmont remain unresolved, results of this investigation indicate that the Goochland terrane is an internal massif of Laurentian crust rather than an exotic accreted terrane. This content is PDF only. Please click on the PDF icon to access. 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.

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.

Simultaneous determination of hypocenter location and crustal structure in the Virginia seismic region

Simultaneous inversions for one‐ (1‐D) and three‐dimensional (3‐D) seismic structure and hypocenters (SSH) for both the central and western Virginia seismic regions are performed, using direct and refracted phases from about 60 earthquakes. Optimal inversion models are computed by employing regularization techniques of Tikhonov and of Backus and Gilbert. Improvements for the residual sum squared, of up to 70% are obtained for the 3‐D models and somewhat less for the 1‐D models. For the central Virginia seismic region the SSH 1‐D velocity models show a strong negative bias in the deduced Pn velocity which could only be alleviated after a ray‐tracing travel time correction for the Pn phases due to the east‐west downdipping Moho has been applied. A 4° Moho dip turns out to be optimal for best explaining the observed travel time residuals and returning a realistic Pn velocity of 8.2–8.3 km/s. However, the deduced velocity in the lower crust is only ∼ 6.35 km/s, which is lower than that predicted from gravity modeling but can be explained in terms of a lower crustal petrology of anorthosite‐rich granulite. A more detailed analysis of the lower crust shows that the hypothesized high‐velocity layer at the bottom of the crust (Pratt et al., 1988) is not clearly identifiable by the present travel time data and so can neither be corroborated nor refuted. Many of the 1‐D crustal results are further substantiated by the 3‐D model, which also provides a reduced lower crustal velocity under the Goochland Terrane. Positive velocity anomalies, which are found in the upper and middle crust under the Blue Ridge/Piedmont boundary, correlate qualitatively with the high‐density, thrust‐faulted slab deduced by Pratt et al. (1988) from gravity modeling. The vertical movements of the relocated hypocenters in the central region are of O(2–5 km) and are such as to further widen the aseismic gap under the Goochland Terrane. In the eastern section of the seismic zone (at the Piedmont/Atlantic Coastal Plain boundary) the downward focal shifts at the lower seismic boundary place events well into the Grenville basement, indicating a lowering of the brittle‐ductile transition boundary. For the western Virginia seismic region the SSH 1‐D velocity models result in a gradual increase in velocity with depth. Significant shifts in the hypocentral relocations are obtained which indicate a further deepening of the brittle‐ductile transition. This and results of thermomechanical creep models favor a quartz‐poor, diorite rich, middle and lower crust in the western region. The 3‐D velocity model of the western region shows a variety of features which can be related to the geology and the tectonics of the Valley and Ridge province. In the surficial layer a velocity low in the Mississippian‐Permian molasse of the West‐Virginian Valley and Ridge province and a velocity high in the thicker Cambrian‐Ordovician carbonate thrust and fault sheets of the Giles county area of the Valley and Ridge are obtained. In the lower crust, higher velocities are detected in the western Valley and Ridge province, which correlates well with the thickening of the crust in the western section of the Virginian Appalachian.

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

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.

A geophysical study of the Earth's crust in central Virginia: Implications for Appalachian crustal structure

A regional seismic reflection line (I‐64) across the Virginia Piedmont has provided a stacked section suitable for an integrated interpretation of geophysical data in the region. A highly reflective upper crust, an allochthonous Blue Ridge Province, underlying thrust sheets including the Blue Ridge master decollement, and a basal decollement at a depth of about 9 km (3 s) are confirmed on the seismic data. Immediately east of the Blue Ridge Province, Appalachian structures plunge to as much as 12 km (4 s) depth. The Evington Group, Hardware terrane, and Chopawamsic metavolcanic rocks (Carolina terrane) crop out in the Piedmont Province, and numerous eastward dipping reflections originate from these rocks in the subsurface. These eastward dipping reflectors overlie a gently west dipping (10°–15°), highly reflective zone that varies in depth from 1.5 s (4.5 km) beneath the Goochland terrane to 4 s (12 km) beneath the rocks of the Evington Group. Some of the overlying eastward dipping reflections apparently root in this zone. The zone may include decollement surfaces along which the overlying rocks were transported. Relatively few reflections originate from within autochthonous Grenville basement at the western end of the profile. The Goochland granulite terrane is interpreted to be a westward thrust nappe structure that has overridden a portion of the Chopawamsic metavolcanic rocks. A broad zone of east dipping (20°–45°) reflections bounds the Goochland terrane on the east. These reflections may originate from deformation zones and continue to Moho depths. They appear to be correlative with similar events seen on other Appalachian lines. The pervasiveness of the zone of east dipping events on other seismic reflection lines and the continuity of the adjacent Piedmont gravity high suggest continuity of crustal features along the length of the Appalachians. A major conclusion of this study is that crustal thinning is responsible for the main components of the gravity field in Virginia, that is, the Appalachian gravity gradient and the Piedmont gravity high. The crust thins from about 52 km beneath the Appalachian mountains to about 35 km beneath Richmond, Virginia, and then rethickens by up to 10 km beneath the zone of east dipping reflections (mylonites?) east of Richmond. The I‐64 seismic data also contain a sequence of reflections at about 9–12 s, indicative of lower crustal layering; the base of this zone of reflections coincides almost exactly with the Mohorovicic discontinuity interpreted from earlier refraction work. The layering extends about 70 km west from Richmond, Virginia, and is interpreted as a lower crustal transition zone that is believed to persist across most of Virginia.

Tectonic evolution of the easternmost Piedmont, North Carolina

The northeastern North Carolina Piedmont is made up of a basement sequence of unknown age (possibly a southern extension of the Grenville-age Goochland terrane of eastern Virginia), structurally overlain by a volcanogenic sequence of probable late Precambrian to early Paleozoic age (an eastward continuation of lithologies occurring in the Carolina slate belt). A model is proposed in which the basement and volcanogenic sequences are separated by a pre-Alleghanian [Taconic(?)] decollement. Northwestward and westward vergence of associated folds of the upper plate suggests that the volcanogenic sequence was thrust from the southeast or east over the basement terrane. This deformation occurred during a greenschist-grade (M 1 ) metamorphic event. Alleghanian granites (313 to 285 Ma) intrude both the basement and volcanogenic sequences, clearly cutting the inferred decollement in some areas. This Alleghanian plutonism was accompanied by regional heating to produce the upper greenschist- to amphibolite-grade (M 2 ) assemblages of the Raleigh belt. The Raleigh belt remained hot until late Alleghanian compression produced regional-scale (F 3 ) folds and D 3 mylonite zones with the resultant uplift and cooling of this belt by 250–240 Ma.