Grenville age of basement rocks in Cape May NJ well: new evidence for Laurentian crust in u.s....

10260 Elsevier Geod Ms 232 Revise February 1999

\PERGAMON Geodynamics 16 "0888# 512Ð522

9153Ð2696:88:, ! see front matter Þ 0888 Elsevier Science Ltd[ All rights reserved[PII] S9153Ð2696"87#99904Ð4

Grenville age of basement rocks in Cape May NJ well]New evidence for Laurentian crust in U[S[ Atlantic Coastal

Plain basement Chesapeake terraneRobert E[ Sheridana�\ Timothy J[ Maguirea\

Mark D[ Feigensona\ Lina C[ Patinoa\ Richard A[ Volkertb

a Department of Geological Sciences\ Rutgers University\ New Brunswick\ NJ 97892\ U[S[A[b New Jersey Geological Survey\ CN316\ Trenton\ NJ 97514\ U[S[A[

Received 4 September 0886^ accepted 01 March 0887

Abstract

The Chesapeake terrane of the U[S[ mid!Atlantic Coastal Plain basement is bounded on the northwest bythe Salisbury positive gravity and magnetic anomaly and extends to the southeast as far as the Atlanticcoast[ It underlies the Coastal Plain of Virginia\ Maryland\ Delaware and southern New Jersey[ Rubidium:Strontium dating of the Chesapeake terrane basement yields an age of 0[91429[925Ga[ This age is typicalof Grenville province rocks of the Middle to Late Proterozoic Laurentian continent[ The basement lithologiesare similar to some exposed Grenville!age rocks of the Appalachians[ The TiO1 and Zr:P1O4 composition ofthe metagabbro from the Chesapeake terrane basement is overlapped by those of the Proterozoic ma_cdikes in the New Jersey Highlands[ These new _ndings support the interpretation that Laurentian basementextends southeast as far as the continental shelf in the U[S[ mid!Atlantic region[ The subcrop of Laurentiancrust under the mid!Atlantic Coastal Plain implies unroo_ng by erosion of the younger Carolina "Avalon#supracrustal terrane[ Dextral!transpression fault duplexes may have caused excessive uplift in the SalisburyEmbayment area during the Alleghanian orogeny[ This extra uplift in the Salisbury area may have causedthe subsequent greater subsidence of the Coastal Plain basement in the embayment[ Þ 0888 Elsevier ScienceLtd[ All rights reserved[

0[ Introduction

The nature of the Chesapeake terrane in the mid!Atlantic Coastal Plain basement has beencontroversial in recent years "Lefort and Max\ 0880^ Sheridan et al[\ 0880^ Glover et al[\ 0881^Volkert et al[\ 0885#[ A corollary to this controversy is the di}erence in interpretations of the

� Corresponding author[

R[E[ Sheridan et al[ : Geodynamics 16 "0888# 512Ð522513

Salisbury positive gravity and magnetic anomaly "Fig[ 0#\ which is an arcuate\ lobate anomalybordering the Chesapeake terrane on the northwest from Virginia to New Jersey "Klitgord et al[\0877#[ The di}erent possible interpretations have important bearing on the tectonic developmentof the Appalachian orogenic belt and on the crustal and mantle behavior in the rifting and breakupof the Mesozoic Atlantic continental margin "Sheridan et al[\ 0882^ Rankin\ 0883#[

Bobyarchick "0870# and Lefort and Max "0880# note that the Chesapeake terrane is adjacent tothe Reguibat uplift of Africa in reconstructions of North America and Africa during Permian[ TheReguibat uplift is a promontory in the African margin made up of Archean to Lower Proterozoic"0[5Ð2[9 Ga# metamorphic rocks[ Lefort and Max "0880# suggest that the Reguibat promontoryserved as an indenter in the North American:African collision in the Alleghanian orogeny\ creatingthe Salisbury anomaly as ma_c and ultrama_c rocks within the Alleghanian suture[ In the indentermodel\ African Archean to Lower Proterozoic rocks were left on North America as the Chesapeaketerrane with the formation of the central North Atlantic Ocean[ The arcuate trace of the Alleghaniansuture was caused by the indenter collision\ not by the original arcuate shape of the Laurentianmargin[

In contrast\ Glover et al[ "0881# and Pratt et al[ "0877# based on the I!53 seismic line "Fig[ 0#\interpret the Salisbury anomaly as a through!the!crust Taconic suture bordering the Carolina"Avalon# terrane to the southeast[ Such a situation conforms with the Glover et al[ "0881# claimthat the Goochland terrane "Fig[ 0# is Laurentian crust rather than an exotic accreted terrane\ acontention now supported further by additional Grenville ages for Goochland rocks "Aleiniko}et al[\ 0885#[ In this interpretation\ the Chesapeake terrane would include Upper Proterozoic "599Ð549 Ma# plutonic rocks and Upper Proterozoic to Lower Paleozoic metavolcanic and meta!sedimentary rocks[ The Alleghanian suture\ if it exists\ would be o}shore or on the African plate[

Based on seismic re~ection and gravity data in the southern New Jersey Coastal Plain nearBuena\ NJ "Fig[ 0#\ Sheridan et al[ "0880# interpret the cause of the Salisbury anomaly to berootless imbricate thrust slices of ma_c and ultrama_c Taconic suture rocks[ The ma_c rocks donot extend to the southeast into the Chesapeake terrane\ which is explained by the presence of anAlleghanian antiformal nappe bringing up Grenville!age "899Ð0299 Ma# Laurentian crust "Fig[ 1#[Glover et al[ "0881# interpreted a similar antiformal nappe to bring up the Goochland terraneGrenville!age Laurentian crust and to transport the Taconic suture rocks upward[ In this interpret!ation\ Laurentian crust would extend under the Chesapeake terrane as far southeast as the mid!Atlantic continental shelf[ The synformal inliers of Taconic suture rocks would follow an arcuatetrace\ such as the Salisbury anomaly "Fig[ 0#\ because the Alleghanian thrusts align with pre!existing weaknesses in the ori`inal accurate shape of the Laurentian margin[

The EDGE project deep crustal seismic re~ection data on the U[S[ Mid!Atlantic continentalmargin o} Virginia reveal a major detachment thrust dipping southeast "Sheridan et al[\ 0882#[The rocks overlying the detachment project landward into metavolcanic\ metasedimentary\ andplutonic rocks identi_ed as the Carolina "Avalon# terrane based on lithologies and age dates[Sheridan et al[ "0882# interpret this detachment thrust as the Taconic suture between the Carolina"Avalon# terrane and Laurentia^ they project Laurentian crust to subcrop under the Coastal Plainin the Chesapeake terrane[

Rankin "0883# favors the interpretation of Lefort and Max "0880# that the Salisbury anomaly isthe Alleghanian suture\ and that the Chesapeake terrane includes the Archean to Lower ProterozoicAfrican rocks left behind in the Mesozoic breakup of Africa and North America[ He also likes the

R[E[ Sheridan et al[ : Geodynamics 16 "0888# 512Ð522 514

Fig[ 0[ Location map of Mid Atlantic continental margin showing New Jersey Coastal Plain relative to exposedAppalachian Piedmont[ Also shown are Piedmont and Salisbury gravity anomalies^ Buena and I!53 seismic lines^Newark\ Gettysburg\ Culpeper\ and Richmond rift basins^ East Carolina slate belt\ and Chesapeake "hachured# andGoochland terranes^ and East Coast Magnetic anomaly[ The Cape MayÐAnchor Gas Dickinson well "AGD# ž islocated\ as is the limit of Laurentian crust from "Rankin\ 0883# xxxxxx[


Fig[ 1[ Diagrammatic cross section across the New Jersey Coastal Plain from the Valley and Ridge province near theDelaware Water Gap to near Cape May "AÐA0\ Fig[ 0#[ Hachured crust represents transported Taconic suture rocksand metavolcanic and metasediments of Carolina "Avalon# terrane[ Blocky crust is Lower Paleozoic limestones\ shales\and sandstones[ Note that the position in the air of the transported Taconic suture at the southeast end of the sectionnear the Cape MayÐAnchor Gas Dickinson well implies that Grenville age\ Laurentian crust exists this far southeast"from Sheridan et al[ "0880##[ Also shown are the Bouguer gravity anomaly from Bonini "0854# and Sheridan et al["0880#\ and total _eld aeromagnetic data from Zietz and Gilbert "0870#[

indenter hypothesis for the curvature of the mid!Atlantic Appalachians[ According to Rankin|s"Rankin\ 0883# terrane map of the Appalachians\ this interpretation requires the edge of Laurentiancrust to be landward of the Salisbury anomaly "Fig[ 0#[

With all these alternative crustal ages "Archean to Lower Proterozoic\ Middle Proterozoic\ LateProterozoic to Early Paleozoic# for the Chesapeake terrane at the edge of North America\ thepotential di}erent rheologies and structures would in~uence the behavior of the crust and mantlein the Mesozoic rifting and breakup of the Atlantic[ Di}erent age crusts would have di}erentthermal histories^ deep through!to!mantle sutures would provide di}erent crustal weaknesses thanshallow\ transported rootless suture rocks[ Resolving the di}erent models for the Chesapeaketerrane would have impact on the understanding of the rifting process on the U[S[ mid!Atlanticcontinental margin[


To test these alternative interpretations for the Chesapeake terrane crust\ the basement rocks ofthe Anchor Gas Dickinson well in Cape May\ NJ were analyzed "Figs 0Ð1#[

1[ Anchor Gas Dickinson well

Located at 27> 46?19ý N and 63> 46?99ý W at an elevation of 3[16 m "03 ft# and a derrick ~oorelevation of 5[60 m "11 ft#\ the well reached total depth at 0837 m "5277 ft# after penetrating 8[34 m"20 ft# of basement "Daniels\ 0874#[ Cuttings of the basement are available from samples at 0841\0842\ and 0842[3 m "df# "5393\ 5395\ and 5396 ft"df##[

In all the samples\ four types of basement mineral and rock fragments consistently make upapproximately 64) of the volume[ These are]

0[ Fine!grained rock containing hornblende and plagioclase identi_ed by Volkert et al[ "0885# asmetagabbro\

1[ Medium grained hornblende\2[ Medium!grained quartz fragments with inclusions of biotite and3[ Medium!grained quartzofeldspathic gneiss or granitoid containing biotite and:or muscovite[

The metagabbro is somewhat more abundant than the quartzofeldspathic gneiss or granitoidand the mineral fragments are minor[ The rock and mineral fragments generally have cleavage\and the metagabbro and gneiss both have mineral foliations[ The metagabbro is grayish!green togreenish!black where fresh and pale green in color where weathered with milky!white lineatedplagioclase crystals[ This petrology can be interpreted as two lithologies consisting of a medium!grained feldsparÐquartzÐbiotiteÐmuscoviteÐgneiss or granitoid interlayered with\ or intruded by\a _ne!grained metagabbro[

Geophysically\ the Anchor Gas Dickinson "AGD# well is an area of low Bouguer gravityanomalies "−09 to −19 mGal# and comparatively low magnetic anomalies "249 nT# relative toan overall magnetic low region "−199 to −399 nT with respect to the Salisbury anomaly# "Fig[ 1#[These anomalies indicate that the metagabbro is only a small portion of the more regional basementlithology\ of which the feldsparÐquartzÐbiotiteÐmuscoviteÐgneiss or granitoid may be morerepresentative[

The geochemistry of the metagabbro indicates that it is moderately fractionated\ enriched inTiO1 with values comparable to within!plate tholeiitic basalts "Volkert et al[\ 0885# "Fig[ 2#[ Volkertet al[ "0885# show that the TiO1 versus Zr:P1O4 composition of immobile elements discriminatesmetabasalts from Laurentian and accreted exotic terranes "Fig[ 2#[ Major! and trace!elementabundances of the metagabbro are similar to Late Proterozoic ma_c dikes that intrude MiddleProterozoic rocks of the Appalachians\ such as the Bakersville Gabbro dikes "Goldberg et al[\0875# and Catoctin Formation dikes "Reed and Morgan\ 0860#\ as well as ma_c dikes of the NewJersey Highlands "Fig[ 1# "Volkert and Pu}er\ 0884# and New York Hudson Highlands "Ratcli}e\0876#[ The geochemical similarities of these widespread ma_c dikes that intruded Laurentian crustimply a common Proterozoic magma source and fractionation\ or several similar ma_c magmascontaminated by a similar crust[ The _ne!grained texture and inferred limited areal extent suggeststhat the AGD metagabbro may be a dike[ The quartzofeldspathic gneiss or granitoid is similar to


Fig[ 2[ Plot of Proterozoic ma_c rocks from New Jersey compared with other north!central Appalachian ma_c rockson a TiO1 versus Zr:P1O4 diagram "after Volkert et al[ "0885##[ Dividing line is between alkalic "A# and tholeiitic "T#rocks[ Note the compositional similarity of the Anchor Gas Dickinson well sample to other New Jersey rocks[ HighTiO1 _elds are] solid\ New Jersey Highlands diabase dikes "Volkert and Pu}er\ 0884#^ short dashed\ Trenton Prongdiabase and amphibolite "Volkert and Drake\ 0882#^ dotted\ diabase dikes and amphibolite from West Chester Prong\Avondale anticline\ and east of the Wilmington complex "M[ E[ Wagner\ unpub[ data#[ Low TiO1 _elds are] doubledashed\ Chopawamsic Formation basalt "Pavlides\ 0870#^ single dot\ Gilpins Falls Member of James Run Formation"Higgins and Conant\ 0889#^ triple dot\ Ta River amphibolite "Pavlides\ 0870#[

some sodic rocks exposed in the New Jersey Piedmont "Figs 0Ð1# which Volkert and Drake "0882#interpret as being Middle Proterozoic in age[

2[ Age of basement

Fresh mineral and rock fragments were picked from the cuttings^ approximately 14 fragmentseach of the metagabbro\ hornblende\ and quartz and feldspar gneiss with biotite inclusions wereseparated[ Minerals and rock fragments from the deepest available sample at 0836 m "5274 ft# wereselected to sample as far from the 0827 m "5246 ft# unconformable contact as possible[ Also\ thesample at 0836 m "5274 ft# was analyzed for major! and trace!element geochemistry "Volkert et al[\0885#[ Rubidium and strontium concentrations were measured using an inductive!coupled plasmamass spectrometer "ICPÐMS#^ Strontium isotopes\ Sr75\ Sr76\ Sr77\ were then measured relatively


Table 0Rubidium and Strontium Measurements for the Anchor Gas Dickinson Well\ Cape May\ NJ\ from Sample AGD 5396at 0836 m "5274 ft[# depth

Measured Grainspecimen size Color Rock or mineral Rb76:Sr75 Sr76:Sr75

0 _ne gray green\ white speckles metagabbro 9[090 9[6028801 _ne gray green\ white speckles metagabbro 9[005 9[6032132 medium black hornblende 9[033 9[6036163 medium gray\ black speckles feldsparÐbiotite gneiss 0[962 9[617206

on a thermal ionization mass spectrometer "TIMS#[ Combining the two measurements\ theRb76:Sr75 and Sr76:Sr75 ratios were determined for each sample "Table 0#[

The samples selected for geochronology from drill cuttings represent mineral aggregates ratherthan puri_ed mineral separates[ For example\ the feldspar gneiss fraction contains numerousbiotite inclusions within feldspar and quartz[ By employing these larger sample aggregates\ thepossible e}ects of contamination and open system behavior are eliminated[ The analytical pro!cedure included sample preparation by open beaker digestion\ in the following sequence[ First\099 milligrams of sample powder "9[0 g# were treated with HF\ HF¦HClO3¦HNO2\ HClO3\HClO3\ 19) HNO2\ 19) HNO2\ each time to incipient dryness[ The sample was then taken up in1[4 ml of 19) HNO2 and diluted to 4 ml to make a stock solution[ This stock solution was thensplit into two aliquots\ one for ICPÐMS "element concentration# and one for TIMS "isotope ratios#[The ICPÐMS solution was further diluted by a factor of 29 with 4) HNO2 and analyzed by ICPÐMS for Rb and Sr concentration following the procedure of Patino et al[ "0886#[ The TIMSsolution was evaporated to dryness\ taken up in 0 ml of 0[4 N HCl\ and loaded onto a conventionalcation!exchange column for Sr puri_cation[ Sr isotope ratios were determined in a VG SectorTIMS and reported as measured^ Sr standard NBS 876 was measured at Sr76:Sr75 of 9[609144 "39analyses\ 1s�9[999905#\ normalized to Sr75:Sr77�9[0083[ By these combined methods\ RbÐSrconcentration and Sr isotopic ratios were determined on precisely the same sample[

The rubidium:strontium isochron for the measured samples gives an age of 0[91429[925 Ga\using the IUGS Rb76 decay constant of 0[31×09−00:yr and the regression technique of York "0856#"Fig[ 3#[ Point number 3 on the isochron "Fig[ 3# was eliminated from the regression to test linearity\and an age of 0[06829[916 Ga was obtained[ Both ages are Middle Proterozoic and Grenville andthat is signi_cant in the Appalachians[ Errors given are one standard deviation uncertainty[ Withinthe 84) con_dence limit "two standard deviations# the ages of the two lines are indistinguishableand little can be said about the apparent age di}erence[ The relatively low values of Rb76:Sr75 forthe minerals are more compatible with a sodic\ somewhat more ma_c composition of the AGDbasement than a felsic\ potassium feldspar!rich composition[

The 0[914 Ga age is within the age range "899Ð0299 Ma# of the Middle Proterozoic rocks of theNew Jersey Highlands "Fig[ 1# and Laurentian crust of the Grenville province[ This age probablyrecords a metamorphic event since specimens from both AGD lithologies plot on the sameisochron[ As in several other cases in the Appalachians\ the Grenville age of the AGD basementindicates that Rb and Sr were not mobile in the subsequent Paleozoic orogenies[ This must have


Fig[ 3[ Rubidium:strontium isochron for the mineral and rock specimens from the basement of the Cape MayÐAnchorGas Dickinson "AGD# well[ The low Rb76:Sr75 ratios are typical for potassium feldspar!poor rocks[ Specimen numbersrefer to Table 0[ Errors given are one standard deviation uncertainty[

been true for the Rb:Sr age of 0[92029[936 Ga reported for the Goochland terrane "Fig[ 0# "Gloveret al[\ 0881#\ in comparison with the 0[93429[909 and 0[90029[991 Ga ages determined by U:Pbfor Goochland rocks "Aleiniko} et al[\ 0885#[ The RbÐSr samples were from the Goochlandterrane within 04 km of a Pennsylvanian age granite^ the granitic intrusion apparently had littleremobilization e}ect[ Projection of the Goochland terrane rocks into the NJ Coastal Plain basementbrings them close to the AGD well "Fig[ 1#[ If the Rb and Sr of the Goochland terrane rocks werenot remobilized in the Paleozoic orogenies\ it is understandable that the Rb and Sr of the AGDbasement rocks were not as well[

The AGD quartzofeldspathic gneiss or granitoid is similar to the lithology of the nearestGrenville massifs\ as is the AGD metagabbro[ Other Proterozoic ma_c rocks in the Grenvilleprovince occur as TiO1!rich dikes suggesting intrusion through a thick continental crust withmoderate fractionation "Ratcli}e\ 0876\ Volkert et al[\ 0885#[ These within!plate intrusions aredistinguishable by immobile elements from known Paleozoic exotic\ accreted terrane basalts\ suchas the Chopawamsic "Pavlides\ 0870# and Gilpins Falls basalts "Higgins and Conant\ 0889# "Fig[2#[ The Paleozoic Appalachian accreted terrane basalts are from shallow primitive mantle sourceslow in TiO1 "Volkert et al[\ 0885#[ These accreted terrane basalts occur as pillow basalts associatedwith marine metasediments[

3[ Discussion

The age determination and geochemical correlation of the AGD basement support the interpret!ation that the Chesapeake terrane is made up of Laurentian Grenville!age crust that is within an


antiformal nappe cut by a later out!of!sequence thrust and displaced upward with transport of theoverlying Taconic suture "Sheridan et al[\ 0880^ Sheridan et al[\ 0882# "Fig[ 1#[ This nappe is similarto the Goochland terrane nappe of Virginia "Glover et al[\ 0881# in that high!angle Alleghanianthrust faults carried the older Laurentian basement up through the overlying allochthonousCarolina "Avalon# terrane[ The breached Carolina "Avalon# terrane was eroded from the upliftedAppalachian Mountains\ exposing the core of the nappes\ and leaving the synformal inliers of theCarolina "Avalon# terrane and Taconic suture rocks[ Suture rocks include ophiolitic fragments ofma_c and ultrama_c crust mixed with metasediments\ metavolcanics and melange[ The synformalinliers of the more ma_c Taconic suture rocks "Fig[ 1# cause the Piedmont and Salisbury positivegravity and magnetic anomalies "Fig[ 0#[

The exposure of the Laurentian crust in the Chesapeake terrane contrasts with the Appalachianterranes along strike[ To the north and south the Laurentian basement plunges beneath thePaleozoic metavolcanic and metasedimentary rocks of oceanic and islandÐarc origins\ such asoccurs in the northwest part of the Central Maine terrane "Rankin\ 0883#[ During the Alleghanianorogeny\ the elevation of the Laurentian basement was highest in the area of the mid!AtlanticCoastal Plain[

One explanation for the excessive uplift of Laurentian basement is possibly in the dextral!transpression phase of deformation of the Appalachians in the Alleghanian orogeny "Gates et al[\0875^ Valentino et al[\ 0883#[ A component of dextral wrench faulting along the high!angle part ofAlleghanian thrusts could have created an imbrication of dextral transpression!faults into atranspression!fault duplex[ This is especially true in the mid!Atlantic region where the New Yorkand Virginia promontories may have restrained dextral transport of more inboard thrust slices[The dextral!transpression fault duplex was compressed and uplifted\ and the blocks rose to higherelevations than along strike to north or south[

Because the Appalachian core zone was consequently higher in the mid!Atlantic region\ moreerosion and denudation occurred exposing Laurentian basement[ This erosion was the source ofthe thick and coarse!grained Mauch ChunkÐPottsville clastic wedge in Pennsylvania "Hatcher etal[\ 0878#[ The excess denudation resulted in a thinner crust\ which subsequently subsided to agreater degree under the Chesapeake terrane[ This created the slightly deeper Coastal Plainbasement under the Salisbury embayment "Fig[ 0#[

While Mesozoic rifting further contributed to thinning the crust during that extensional phase\the rift basins\ such as the Newark Basin "Fig[ 0#\ occur in the Piedmont and under the inner andmiddle Coastal Plain "Fig[ 1#\ and there is no exceptional rifting evident in the Chesapeake terrane[It appears that the Mesozoic rifting did not preferentially thin the Chesapeake terrane crustcompared to the inner Coastal Plain crust[ Consequently\ the extra thinning of the Chesapeaketerrane crust by the Late Paleozoic transpressional event is proposed[

4[ Conclusions

0[ The Chesapeake terrane of the U[S[ mid!Atlantic Coastal Plain basement contains Grenvilleage rocks of Laurentian crust[

1[ Breaching of the Carolina "Avalon# allochthonous rocks on the Laurentian crust by Alleghanian


faulting\ coupled with transpressional uplift and erosion\ led to the exposure of the Laurentianrocks in PennsylvanianÐPermian[

2[ Excess crustal thinning of the Chesapeake terrane by erosion led to slightly greater subsidencein the Mesozoic breakup and drifting of the Atlantic margin causing the Salisbury Embayment[

Acknowledgements

We wish to thank Richard Benson of the Delaware Geological Survey for subsamples of theAnchor Gas Dickinson well basement samples\ and Gene Hall of the Rutgers University Depart!ment of Chemistry for use of the ICPÐMS[

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Grenville age of basement rocks in Cape May NJ well: new evidence for Laurentian crust in u.s....

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