Nov 10, 1991 - lowest Passaic formations [Witte and Kent, 1989]), the middle. Newark strata .... N2 poles would support a high-latitude Jurassic APW path, as.
JOURNAL
OF GEOPHYSICAL
RESEARCH,
VOL. 96, NO. B12, PAGES 19,569-19,582, NOVEMBER
10, 1991
TectonicImplicationsof a RemagnetizationEvent in the Newark Basin WII I.IAM K. WITrE
AND DENNIS V. KENT
Lamont-DohertyGeologicalObservatory,ColumbiaUniversity,Palisades,New York The Newark basin red bedscontaina secondarymagnetization(the B component)acquiredduring the Middle Jurassicafter the 50-20ø basin-widenorthwesterly dip was impartedto the strataof the basinand after most, if not all, of the limb rotationin the Jacksonwaldsyncline. The B componentmagnetization was mostlikely relatedto the samehydrothermaleventwhichevidentlyremagnetized many of the igneous intrusionsin the basin and reset their K/Ar systemsat 175 Ma. The remagnetizationof the red beds occurredover a few million yearsand was approximately coincidentwith the transitionfrom continental rifting to seafloorspreadingin the adjacentNorth Atlantic. The B componentmagnetizationdirection
yieldsa paleomagnetic poleat 74øN,96øE(K = 63,A95= 2.6ø,N= 50 sites)afterstructural correction for
1/3 of the Jacksonwald folding andnoneof the regionaltilt. This pole supportsrecentevidencefor a highlatitudemodelof Jurassicapparentpolarwanderfor North America.
PREVIOUS WORK
INTRODUCTION
The Newark basinof PennsylvaniaandNew Jerseycontainsa thick lacustrine and fluvial clastic section whose deposition spansat least25 m.y. of the Late Triassic(middleCamian) to earliestJurassic(Hettangian). The naturalremanentmagnetism (NRM) of these red beds consists of a characteristic magnetization(the C component)and an ubiquitoussecondary magnetization (the B component). The high unblocking temperature(>640øC)C componenthasbeenisolatedin studies of the lower Newark strata (the Stockton, Lockatong, and lowest Passaic formations [Witte and Kent, 1989]), the middle Newark stratawith a positivefold test (the PassaicFormation [Witte et al., 1991]), and the upperNewark strata(the Newark extrusivezone [Witte and Kent, 1990]). The lower, middle, and upper Newark studiesyield similar pole positionsafter tilt correction (54øN, 102øE; 56øN, 95øE; and 55øN, 95øE, respectively)which suggesta very slow rate of North American
In addition to the clastic rocks, the Newark basin also includes
closelyrelated suitesof diabaseintrusiverocksand tholeiitic basalt flows. Paleomagnetic results from the igneous rocks associated with the Newark
and other eastern North American
apparentpolar wander (APW) during the Late Triassic and
rift basinshave played an importantrole in the interpretation of the JurassicNorth American APW path. For example, the compilation by Smith and Noltimier [1979] yielded two statisticallydistinct referencepoles, N1 (63øN, 83øE) and N2 (65øN, 103øE), which were thought to correspondwith two igneousevents [Sutter and Smith, 1979] datedat 179 Ma and 195 Ma, respectively (K/At ages corrected for new decay constants). While the N1 and N2 poles have been key paleomagneticreference poles in recent models of Jurassic North American APW [Gordon et al., 1984; May and Butler, 1986], field and radiometricstudiesnow indicatethat igneous activity in at least the Newark and Gettysburgbasins(which were the sourceof manyof the N1 andN2 data)wasconfinedto
earliest
a short
Jurassic.
time
interval
in the earliest
Jurassic
at-200
Ma
[Kodama, 1983; Ratcliffe, 1988; Sutter, 1988; Dunning and Hodych, 1990]. The N 1 pole consistsof resultsfrom the basaltsof the Newark
The pervasiveB componentis invariablyof normalpolarity (northwardwith -+40 ø inclination) and is effectively isolated between 300øC and 600øC [Witte and Kent, 1989, 1990]. The B component direction was well defined in those studies, and Hartford basin extrusive zones and from selected diabase althoughit was not clear whetherthe remagnetization occurred intrusions in the Hartford basin. Biostratigraphic evidence indicatesthat the extrusiveepisodein the Newark andHartford before or after strataltilting. Likewise, as was notedby Witte and Kent [1989], it remains unclear whether the Newark trend basinsbegan very early in the Jurassic[Cornet, 1977; Cornet igneousN2 pole should be tilt corrected. The choice of a and Olsen, 1985], and further analysis has indicated the extrusiveepisodeprobablylastedlessthan 1 m.y. [Olsen and referenceframe for thesepoles has importantconsequences for Fedosh, 1988]. Although the basaltsand the intercalatedred the Middle JurassicNorth AmericanAPW path. The purpose of the present study is to test explicitly the beds of the Newark extrusive zone all yield normal polarity magnetizations, the mean magnetization direction after timing of the acquisition of the B componentrelative to structural tilting and to determine the appropriatereference alternating field (AF) demagnetization of the basalts is frame for the B componentpaleomagnetic pole. Accordingly, significantly steeper than the mean high unblocking we have sampled the red beds in a more complex structural temperature magnetization isolated in the red beds of the settingthan the homoclinalregionswhere we have previously extrusive zone (upper Newark) [Mcintosh et al., 1985; Witte sampled. By comparing the B component magnetizations and Kent, 1990]. We interpreted this disparity as resulting from residual contaminationof a basalt primary magnetization isolated at various stratigraphic levels throughout the by Middle Jurassicor younger overprints [Witte and Kent, geographicextent of the basin we have also endeavoredto 1990]. explore the spatialvariation of the remagnetization. The N2 pole consistsof resultsfrom intrusionsin Maryland, Pennsylvania,New Jersey,Connecticut,and Nova Scotiathat Copyright1991 by the AmericanGeophysical Union. were once thought to have been emplacedat about 179 Ma. However, recent U/Pb zircon dams from the Palisades and
Papernumber91JB01866. 0148-0227/91/91JB-01866
$05.00
Gettysburgsills, two of the major intrusive sheetsfrom which 19,569
19,570
W1TFEAND KENT: NEWARKREMAGNETIZATION
data contributing to the N2 pole were obtained, show that the sills crystallizedat 201 _+I Ma [Dunning and Hodych, 1990], similar to the age of the extrusives. Thesecrystallizationages are consistentwith observationsthat the Palisadessill actually fed some of the basalt flows in the Newark basin [Kodama, 1983; Ratcliffe, 1988] and imply that the intrusive and extrusive rocks were crystallizedin a single igneousepisode.
70 ø to the north and south, and nine sites are from the northern
Sampling
Fold Test
and centralparts of the basinin regionsof northwestdipping homoclinalstrata(Figure 1). Samplingsite locationsare listed by Witte et al. [1991], where the characteristic(C component) magnetizationof these27 sitesis also described. At least five coreswere drilled with a portabledrill at each site exceptone (TGF) where nine oriented hand sampleswere taken. Field Yet, the N2 pole (65øN, 103øE)differs significantly from the orientations were obtained with a magnetic compassand N1 pole (63øN, 83øE, or the depositionallycoeval Hettangian verified in heavily urbanizedzones with a solar compassor upperNewark pole, 55øN, 95øE) and falls on a muchyounger geographic sightings. part of Gordonet al.'s [1984] or May and Butler's[1986] North PALEOMAGNETIC RESULTS American APW paths than the demonstrated crystallizationage The NRM of the 10 cm3 sampleswasmeasured on an SeT of the igneousrocks would indicate. We have interpretedthis and rangedin intensityfrom -0.2 to disparity as evidence that the N2 pole does not representa cryogenicmagnetometer primary thermal remanent magnetization, but rather a ---60mA/m. AF demagnetizationto 100 mT was ineffectivein secondary magnetization most likely acquired during a resolvingcomponentsof magnetizationin the Newark red beds hydrothermal episode that Sutter [1988] suggestedreset the owing to high coercivities, but progressive thermal K/Ar geochronometersin the diabaseintrusionsof the Newark, demagnetization revealed two ancient components of Gettysburg,and Culpepperbasinsat about 175 Ma [Witte and magnetization(Figure 2). Typically between 300øC and at least 600øC, linear demagnetizationtrajectoriesindicatedthe Kent, 1989]. More than half of the site mean virtual geomagneticpoles unblocking of a component of magnetization directed (VGPs) included in the N2 compilationwere obtainedfrom uniformly northwardand moderatelydown (the B component). diabase intrusionsof the Newark and Gettysburgbasins and Above about 640øC, demagnetization trajectories that were corrected for tilt [Beck, 1972]. The B component converged to the origin indicated unblocking of the magnetizationisolatedfrom the lower Newark red bedsyielded characteristicmagnetization,the C component,which occurs in two nearly antipodal populationsdirected northward and a mean pole positionsimilar to the pole from this subsetof N2 southward with shallow inclinations [Witte et al., 1991]. when both are comparedin the same (either geographicor tilt Principal componentanalysis(PCA) [Kirschvink, 1980] was corrected)referenceframe [Witte and Kent, 1989]. The uniform used to estimate the direction of linear demagnetization normal polarities and similar directions of the B and N2 magnetizationssuggestthat both were acquiredas secondary trajectoriesfor each specimen. Only one site, TJK, failed to magnetizationsat aboutthe sametime, mostlikely duringthe producereliable estimatesof the B (or C) componentand the samples displayed very low unblocking temperaturesand 175 Ma hydrothermal event [Sutter, 1988]. The N2 magnetizationshave been incorporatedinto the literature with anomalouslyhigh NRM intensitiesand susceptibilifies.This the assumptionthat they were acquiredbeforetilting; however, site was taken from altered sediments within 50 m of a diabase this assumptionwas not demonstrated by field evidencesuchas intrusionwhich most likely altered the magneticmineralogyof a fold test. The timing of the acquisitionof the B component the red beds. The other26 sitesyieldedat leastthreeestimates secondary magnetization with respect to the tilting of the of the B componentdirection from independentlyoriented basin strata has similarly not been tested. If the sampleswhich were averagedby the methodof Fisher [1953] to give site means (Table 1). The B componentmagnetization remagnetizations occurred before tilting, then the B componentand N2 pole positionswould supporta mid-latitude site mean directionsof these26 sitesyielded an overall mean ø (k = 33, c•95= 5.0ø) in geographic APW pathin the Jurassicsimilarto that suggested by Gordonet directionof 357.40/46.9 and353.90/29.8 ø (k = 17,c•95= 7.2ø)aftera simple al. [1984] or May and Butler [1986]. If the remagnetization coordinates was acquiredafter tilting, however,then the B componentand bedding tilt correction (rotating the bedding to horizontal N2 poles would supporta high-latitudeJurassicAPW path, as about the present strike). The factor of 2 decreasein the suggestedby Irving and Irving [ 1982] andVan Fossenand Kent precision parameter k already suggeststhat the B component was acquiredduring or after folding and tilting. [1990]. Of the 26 middle Newark sites with reliable estimates of B We have previously reported B component site mean directionsfrom 22 sites in the middle Carnian/earlyNorJan componentdirections, 15 were taken on the limbs of the Stockton, Lockatong, and Passaic formations in the central Jacksonwaldsyncline. A PCA fit to the poles to bedding Newark basin [Witte and Kent, 1989] and from four sitesin the observedat the 15 sitesin the Jacksonwaldsynclineyields a Hettangian sedimentary rocks of the extrusive zone in the fold axis plunging+13ø toward an azimuthof 289ø (Figure3). northern Newark basin (Witte and Kent, 1990) (Figure 1). Samplesfrom two siteson the north limb of a small unnamed These 26 sites sampledrelatively homoclinalstrata, dipping anticlineimmediatelyto the north of the Jacksonwald yielded 50-20ø to the northwest,which did not offer sufficientbedding reliable paleomagneticresults,but these sites (TJL and TJM) attitude variation for a fold test. were excluded from further analysisbecauseof the lack of The presentpaperincludesnew resultsfrom 27 sitessampling sufficientstructuralcontrolon plunge. The small, but measurable, plunge of the Jacksonwald middle and late NorJanstrataof the predominantlylacustrine red beds of the PassaicFormation. Most (18) of the sitesare syncline allows us to considerstructurallycorrectingthe B from the Jacksonwaldsyncline and associatedfolds in the componentmagnetizationdirectionsby a two-step method, southernpart of the basin where the beddingis dippingup to which entails a combination of a rotation about the fold axis
W•
AND KENT: NEWARKREMAGNETIZATION
19,571 740OO '
74o30 '
76o00'
75ø30 ' Extrusive Zone
40ø45' 75ø0•• /
76000 '
ß
Perkion•
River•4• 74oOO'
1•Turnpike 75030 ' /
-_-_0 5 10 15 20 25
•40o30 '
75o00 ,
km
Formations
and
Litholooies
North
;America
"•L•.•- Diabase dikes andsills •L•__.• ' Basalt flows L__.J- Passaic, Feltville, Towaco, andBoonton formations(Mostlyred lacustrineclastics)
• •
- LockatongFormation (Mostlygray lacustrineclastics)
Africa
'..• .'.'.:. - Stockton Formation
(mostlyfluvialand alluvialclastics)
Fig. 1. Geologicsketchmapof the Newarkbasinwith generalized formationboundaries.Detailsof the sitelocationsare givenby Witteand Kent [1989, 1990],andWitteet al. [1991]. Inset,presentcontinental coastlines reconstructed to their predriftconfiguration [Bullardet al., 1965]showingthelocations of the easternNorthAmericanrift basinsdiscussed in the text (solid areas):N, Newark basin;G, Gettysburgbasin;C, Culpepperbasin.
until the bedding dip direction is parallel to the fold axis (unfolding)and a rotationabouta horizontalline perpendicular to the fold axis until beddingis horizontal(unplunging). The shallowplungeof the Jacksonwald synclineensuresthat the order in which we apply theserotationsdoesnot significantly alter the result, although such rotations are not generally
timing of the B componentacquisitionrelative to the regional plungeremainsto be addressed. The resultsof this incrementalfold test on the B component stand in marked contrast with results of an incremental
fold test
on the C componentat virtually the samesites [Witte et al., 1991]. A set of 15 site mean C componentdirectionsfrom the commutative. Jacksonwaldsyncline yielded a positive fold test, significant The 15 Jacksonwald syncline B component site mean at the 99% confidencelevel, with the least dispersionachieved directions are significantly less dispersedwithout structural at virtually full unfolding (Figure 3). Thus the C component correction (overall mean = 358.50/49.8ø, k = 27) than after site mean directions were most likely acquired before any unfolding (overall mean = 358.30/32.7ø, k = 12) at the 95% significantfolding and thusvery shortlyafter deposition. confidencelevel. However, an incrementalunfolding test on the B componentsite directionsyields a peak in the precision Tilt Test Nine additional sites from the 1-280 and Raritan regions parameterat an intermediate valueof unfolding(overallmeanat 1/3 unfolding is 359.90/44.0ø, k = 49) that is significantly sampledthe samemiddle and late Norian strataas sampledin greater (at the 99% confidencelevel) than the full unfolding the Jacksonwaldregion but in homoclinalbedswith dips to the correctionprecisionand greater(at the 90% confidencelevel) northwestof about5o-20ø. All nine sitesyieldedat leastthree than the in situ precision(Figure 3). Thus, at face value, the independentlyoriented estimatesof the B component(Table Jacksonwaldfold test indicatesthat the B componentwas not 1). With the 26 B componentsite means from the lower and acquiredprior to foldingat a highdegreeof confidencebut was upper Newark strata [Witte and Kent, 1989, 1990], the mean possibly acquired before the completionof folding. The direction of these 35 homoclinal sites shows no significant
19,572
WRITE AND KENT: NEWARK REMAONETIZATION H
W,Up
W,Up
1 mA/m
S
. .
'
.•:
._-
acksonwald• JN06A
)
oC
NRM
b)
E, Dn
W,Up W,Up :..•.675øC 1 mA/m
S
N
500øC
IT Delaware DA02A River 1
c)
E, Dn
NRM
E Dn
W,Up
W,Up lO mA/m
N -1 mA/m
I Raritan•
TGJ02.1A)
NRM
Dn
e)
NRM
f)
E Dn
Fig.2. Zjiderveld [1967]demagnetograms from six samples fromthe Passaic Formation in the Newarkbasin. Open symbols areprojections of vectorendpoints ontotheverticalnorth-south plane,andsolidsymbols areprojections ontothe horizontalplane,all in the geographic reference frame. B component lineartrajectories are highlightedon the vertical projections by stippledlines. Demagnetograms in Figures2a, 2c, and2e showdemagnetization of samples with normal polaritycharacteristic magnetization, anddemagnetograms in Figures2b,2d and2f showdemagnetization of samples with reversedpolarity characteristicmagnetizations.
WITrE AND KF2qT: NEWARK REMAGNETIZ•TiON
19,573
TABLE 1. B Component Site MeanDirectionsFromMiddleNewarkStrata
Optimal* Structural Correction
Geographic _
Site
n
Strike
Dip
k
•9s
_
D
I
D
Full Tilt Correction
I
D
I
Jacksonwald Region TJA TJB TJC TJD TJE TJF TJG TJH TJI TJJ
5 4 4 4 5 5 5 4 3 4
278 238 254 242 153 148 268 268 29 1 287
74N 20N 25N 30N 35W 37W 28N 28N 65N 50N
TJL•' 5 TIMe' 3
293 293
30N 20N
TJN TJO TJP
108 118 118
20S 27S 25S
5 6 4
25.9 123.0 107.8 407.0 98.2 484.1 216.7 323.6 239.7 20.3 196.1 566.8
15.3 8.3 8.9 4.6 7.8 3.5 5.2 5.1 8.0 20.9 5.5 5.2
009.3 013.4 011.5 005.8 001.7 003.1 350.5 018.4 354.8 344.5 333.2 335.4
63.4 42.1 46.3 49.7 33.2 61.8 57.1 50.8 62.7 57.6 40.9 40.3
009.1 012.8 010.8 005.9 001.7 358.6 353.8 016.7 000.3 351.2 -.-
347.0 33.8
34•i½ 39.4
2.4 15.7 8.5 14.0
345.5 346.5 357.8 354.4
27.8 26.1 56.6 68.8
344.5 345.9 000.2 001.8
35.8 13.0
TSQ
5
268
40N
TJR
4
280
55N
765.1 35.0 82.7 44.3
TGA TCK2 TGD TGE TGF TGJ • TGH TGI
6 4 5 5 5 9 5 4 4
228 228 230 218 236 230 231 222 239
12N 11N 10N 13W 16N 13N 11N 07W 09W
85.6 28.0 96.7 42.9 53.2 26.2 127.6 36.2 12.2
40.4 38.0 39.7 43.3 40.9 70.5 49.4 42.2 42.6 43.4 -.-.-
008.3 004.4 004.0 355.0 334.2 289.2 353.4 012.! 009.1 359.6 345.5 344.3
-10.7 26.8 23.5 23.1 42.8 60.1 29.2 23.9 0.3 11.2 19.0 25.7
35.1 33.0 45.1 51.8
329.6 333.4 358.1 006.5
45.2 42.9 16.6 15.8
41.3 40.8 34.6 41.7 48.0 51.8 37.6 35.6 53.2
355.4 345.4 354.6 351.8 002.8 345.9 358.1 358.1 010.4
32.1 31.3 26.6 33.0 35.8 40.7 29.1 30.8 46.8
336.2 50.0
Raritan River and 1-280 Regions 7.3 17.7 7.8 11.8 10.6 10.2 6.8 15.5 27.4
001.2 349.4 358.2 359.4 012.3 351.3 002.7 001.5 017.9
41.3 40.8 34.6 41.7 48.0 51.8 37.6 35.6 53.2
001.2 349.4 358.2 359.4 012.3 351.3 002.7 001.5 017.9
Middle Newark Mean B Component Direction All
24
359.5
47.2
000.4
43.7
k= 35,
k= 59,
•95= 5.1
•95= 3.9
354.7
k=
30.4
16,
•5 = 7.7
n is numberof samplesfor calculationof sitemeandirection(numberof sitesfor overallmean),k is the
bestestimate of Fisher's precision parameter, 0•95istheradius of the95%confidence circle, D is
declinationin degrees,I is inclinationin degrees. *The optimalstructuralcorrectionconsists of a 34% unfoldingwith no unplunging f.,r the Jacksonwald synclinesitesand no untiltingfor the homoclinalRaritanRiver and 1-280 sites.
tinadequateconstraint on localplungeprevented optimalstructural corrections for thesesites;thesesites were thus excluded from the calculation of the means.
changein dispersionfrom in situ (003.9o/40.5ø, k = 71) to tiltcorrected (358.3o/32.2ø, k = 70) coordinates,reflecting the moreor lessuniformshallowbeddingdips. Thereis a differenceof--25 øbetweenthe azimuthof dip in the homoclinalNewark basin and the azimuth of plunge in the Jacksonwaldsyncline. If the optimally unfolded(by 1/3) B component mean direction from the Jacksonwaldsyncline
indistinguishable beforeany tilt or plungecorrection ('go= 5ø, 'g,= 6ø),whereas themeansarestatistically distinct ('go = 9ø,'g, = 5ø) after untilting/unplunging.Thus the tilt test indicatesthe B component was most likely acquired after tilting and plunging. Similarly, the fold test discussedabove indicates that the B componentwas acquiredafter most,if not all, of the folding.
(359.9o/44.0 ø, k = 49) is unpiunged (eitherby a uniform13ø Variation in RemagnetizationDirection The B componentof magnetizationdisplays a remarkable from the homoclinalsitesas theyare similarlyuntilted(Figure consistencyin directionthroughoutthe Newarkbasin. The 50 4). The homoclinal and Jacksonwald means show no B componentsitesmeandirections,with what we regardas the significant difference in direction before any tilt or plunge optimalstructural correction (one-third unfolding, no untilting corrections (To= 5ø,'go= 6ø,whereTois the observed angular or unplunging), yield an overall mean direction of
unplunging or by unplunging each site until bedding is horizontal),it divergesfrom the B componentmean direction
difference betweenthe meansof thetwo subsets and'gois the 002.8ø/41.6ø (k = 61, {x95= 2.6ø) (Figure 5). If the B critical angle at which we can reject the null hypothesisof a
componentmean direction from middle Newark sites from the
commonmean directionat the 95% confidencelevel [McFadden and Lowes, 1981; McFadden and McElhinny, 1990]) but the
withanoverallmeanof 001.2ø/43.0 ø,k = 77,{x9•= 5.9ø,N = 9)
northernpart of the basin(includingthe Raritanand1-280sites
difference is significant at full untilting/unplunging (To= 10ø, is comparedwith the B componentmeandirectionfrom middle 'g,= 6ø). If thistestis performed on siteVGPswe obtainthe Newark sites from the southern part of the basin (the same
result:
the
means
of
the
two
VGP
subsets
are
Jacksonwaldsynclinewith an overall mean of 359.9o/44.0ø, k
19,574
WRITE AND KENT: NEWARK REMAONETIZATION
I
I
a) I
I
I
I
I
N=15
I
I
I
I
I
I
I
--
ß
!
ß ß
e)
_
50
40
C
I
30 20
lO-!
ol o
2•)
45
6•) ..... 8•I
' I 100
% structural correction
Fig.3. Jacksonwald syncline foldtest. Solid(open)circlesindicate downward (upward) directed directions on equalarea stereographic projections (Figures 3a-3d).Cross withellipse indicates theoverallmeanof sitesandits0t95 envelope. (a) B component sitemeandirections in geographic coordinates. (b) B component sitemeandirections after1]3unfolding about theJacksonwald foldaxis(andnounplunging). (c) B component directions withfull tilt correction (rotation of bedding to horizontalaboutpresentstrike). (d) Polesto beddingat the 15 sitesfromtheJacksonwald syncline; greatcircleindicates least-squares fit to thesebeddingpolesandthe squareindicatesthefold axis(FA). (e) Incremental fold teston the B andC
components at 15 sitesin theJacksonwald syncline.Variation in bestestimates of Fisher's[1953]precision parameter k is plottedagainst percent structural correction whichfor B is unfolding withoutunplunging butfor C is rotation of bedding to horizontalaboutthe presentstrike(discussed by Witteet al., [1991]). Verticallinesindicatethe 95% confidenceintervals on theprecisionparameterfor eachcurve.
= 49, ix05= 5.5ø,N = 15),oneseesno significant difference in
B component magnetization directions from different geographicregionsor different stratigraphiclevelswithin the basin arguesagainst any differential post-remagnetization tectonicrotationsbetweenthe regionswe havesampled.This PerkiomenCreek siteswith an overall mean of 005.0ø/40.4ø, k observationis consistentwith the generalcoherencyof the C = 66, {•05= 3'8ø,N = 22) is compared withtheB componentcomponent directions from the Newark basin [Witte et al., mean direction from upper and middle Newark sites (the 1991]. Furthermore, thegeneralparallelism of theNewarkrift
thesetwo subsetsof B component directions.Likewise,if the B c.omponent mean direction from lower Newark sites (includingthe Delaware River, Pennsylvania Turnpike,and
extrusive zone, Raritan, 1-280, and Jacksonwald sites with an
basin border fault with local Precambrian basement structures
overallmeanof 001.0ø/42.5 ø, k = 59,•z0•= 3.6%N = 28),one [e.g., Ratcliffe and Burton, 1985;Swanson, 1986] argues also sees no significant difference in the two subsetsof B component directions.The lack of significantdifferencein the
againstwholesale rotationof thebasinwith respectto cratonic North America.
WRITE AND !(Y2qT:NEWARKREMAGNETI7•TION
There is, however,a small variationin the uniformlynormal polarity B componentthat is dependenton the polarity of the C componentcharacteristic magnetization.The meanof the B componentsite directions at sites with a normal polarity C
!9,575
component(003.5ø/38.0 ø, k = 77, •x95= 3.3ø,N = 25) is 7ø + 5ø shallower than the mean of the B component site directions at sites with a reversed polarity C component
(001.4ø/45.2 ø, k = 62, •x95= 4.0ø,N = 22). A similarbiasin the B componentwas notedin the lower Newark subset[Witte and
Homoclinal
_
sites
Untilting N - 35
,100%
Jacksonwild N=
1989]
and attributed to a small residual
Variation in RemagnetizationIntensity
The small, but measurable,dependence of the B component directionon the polarity of the C componentimpliesthat there is some overlap in the unblocking spectra of these components. To compare the relationshipbetween the two thermal unblocking spectra, we calculated demagnetization graphs for the B and C magnetization components in representativesamplesby a methodsimilarto that describedby
sites
Unplunging
Kent,
contamination by the C component characteristic magnetization(a phenomenondescribedas "underprinting" by Miller and Kent [1988]). However, any bias of the B componentdirection shouldbe cancelledif the contamination from normal and reversed polarity characteristic magnetizations is similar. Indeed, the preferred mean B component direction (002.8ø/41.7ø, N = 50) differs by less than 1ø from the B componentmeanof the normalandreversed characteristicpolarity subsets(002.5ø/41.6ø, N = 2).
N
--
15
Fig. 4. Lower hemisphereequ•-area stereographic projectionof overall site mean directions and associated confidence circles for B
componentdirections isolated from !5 Jacksonwaldsites and 35 homoclinallydipping Newark sites. The meansof thesetwo subsets a?eplottedbefore(0%) and after (100%) plungeor tilt correctionand are connectedby the tracesof the mean directionswith intermediate untilting/unplungingcorrections. The solid square indicates the overall site meandirectionof 15 Jacksonwald sitesafter a singlestep unfolding of 1/3 of the presentbeddingtilt (using a rotationabout presentstrike). Above 25% tilt correction(light lines) the subset directionsare statisticallydistinctat the 95% confidencelevel.
N
N
N
_t
ee •e ©-•
eee• e i
Geographic coordinates
=
_
-
• -•
•
Optimal Structural
_
correction
-
• '•
Fulltilt correction
i
_
_.,
N = 50 sites
FiB.:5. F.qua]-areastereoõraphic projections of the :50reliableB component sitemeandirectionsin 8eoõraphic coordinates (]eft), after optima]structuralcorrection(middle),andafterfull tilt correction(rishO. The optima]structuralcorrectionwas determinedto consistof 1/3 unfoldingaboutthe fold axis in the Jacksonwald regionandno unplunging/untilting throughout the basin.
i
19,576
WITrE AND KENT: NEWARK REMAGNETIZATION
Zijderveld [1967]. In this application, the vector at each demagnetization step at 300øC and above was resolved into componentsparallel to the PCA estimatesof the B and C magnetization directions. Figure 6 demonstrates this relationship for a sample with a reversed polarity C component;sampleswith normal polarity C componentsgave
components are very similar, however, the C component intensity has a slightly larger range than the B component. Aside from the anomalouslylarge B componentin the Delaware River region (Figure 7b), there seemsto be little systematic variation in the B component intensity throughout the
similar but noisier results.
The lack of any consistent correlation, either direct or inverse,between the B and C componentintensitiespoints to
The analysisconfirmsand furtherquantifieswhat one would already suspect from a qualitative examination of the demagnetizationdiagrams: The B componentis unblocked over a wide temperaturerange from 300øC to about 650øC, whereas the C component is primarily unblocked over a discretetemperaturerangefrom about660øCto 690øC(Figure 6). Thus, the B and C components of magnetization differ not only in their directionsof magnetizationand their maximum unblockingtemperaturesbut also in the characterand width of their unblockingspectra. A slight demagnetization of the C componentover the unblocking temperaturerange of the B componentleads to the underprinting;the virtually complete decay of the B componentbefore the discrete unblocking interval of the C componentindicatesa negligibleoverprint contamination,as suggestedby the positive reversal test on the C component[Witte et al., 1991].
geographicor stratigraphic extentof the basin.
independent origins of the magnetizations, wherethe B
componentmagnetizationcan be regardedmore as an addition or an overprint rather than a replacementor resettingof the preexistingC component. Nevertheless,the high maximum unblockingtemperatures of both the B (up to 650øC)and C (up to 680øC) components suggest that hematite is the predominantmagneticmineral in the red bedsand the carrierof both components.The smoothness of the isothermalremanent magnetization (IRM) acquisition curve, with a lack of saturation below 1.5 T (Figure 8), and the thermal demagnetization of IRM curve [Witte and Kent, 1989] also indicatethat remanenceis carriedby a singlemagneticphase. For example,no evidencefor lower coercivityand unblocking temperaturemagnetite is observed. Thus it seemsthat the range in unblockingtemperaturesobservedin theserocks is We also estimated the absolute intensities of the B and C likely related to variation in somephysicalproperty,suchas componentsfor those sampleswhosedemagnetization curves grain size, of the hematite. Similar to the observations demonstratedboth linear B and C componenttrajectories. reportedby Mclntosh et al. [1985], we observehematiteas a Demagnetizationtrajectoriessuch as shownin Figure 2f were very fine grained (< 5 gm) interstitial pigment and also as not used because significant overlap between the larger (up to 50 I•m) but rare, speculargrains in thin and magnetizationunblockingspectraof the two components made polishedsections. A tentative interpretationfor the different it difficult to accuratelyestimatethe intensityof the B and C unblockingspectraof the B and C components,following the components. The relative intensities of the B and C model of Irving and Opdyke [1965], is that the thermally componentsshow a wide range of variation even within a distributedB componentmagnetizationis associated with finer single site (Figure 7a). In general, the intensitiesof the two grainedhematite,whereasthe thermallydiscreteC component_ 3 I
c
2 W, Up
B
H
S04oo•9so c'.N
'
200
i
300
,
i
,
400
i
,
500
i
600
700
T(øC)
b)
2 I
TJF02A
•acksonwald• •JF02A •J V
E
RM
E,
a)
I 300
•'"•, 400
'
I 500
T(øC)
'
I 600
'
I 700
c)
Fig. 6. Thermaldemagnetization of a representative middleNewarkred bed specimenfrom the PassaicFormation. (a) Zjidervelddemagnetogram indicatingvectorendpointsin geographic coordinates; opensymbolsare projectionsontothe verticalnorth-southplane,and solid symbolsare projections ontothe horizontalplane. (b) Demagnetizationgraphs calculatedfor the B and C components as describedin the text. (c) Histogramsof magnetizationsunblockedin 50øC intervalsfor the B and C components.
WITrE AND KF2qT:NEWARKREMAGNETIZATION
19,577
100
Delaware
10.
}•Porkiomon
Jacksonwald
Extrusive zone, Turnpike Raritan, 1-280 1.0
1.0
b)
0.1
0.1
)0.
0.1
1.0
10
100
C (mA/m)
Fig. 7. Intensity of the B componentremanenceversusthe intensityof the C componentremanence. (a) Component intensitiesfrom 207 samplesof Newark basin red beds,showingthe lack of any systematiccorrelationeven for samples from the samesite (e.g., TGJ, TJN, and TDF). (b) Log-meanand standarddeviationof B and C componentintensities accordingto geographic groupings of sitesin the Newarkbasinas definedin Figure1.
magnetization may be preferentially carried by somewhat coarserhematite grains. Theoreticalsupportfor such a grain size dependenceof unblocking temperatureis provided by
The B componentmagnetizationin the Jacksonwaldsyncline achieves the tightest clustering with at most partial (1/3) unfolding. This implies that this magnetization was not Dodson and McClelland-Brown [ 1980]. acquiredbefore folding but may have been acquiredduring the late stages of folding. An apparently "synfolding" AGE OF REMAGNETIZATION magnetization may in some instances result from strain Folds in the eastern North American Mesozoic rift basins, modification of a pre-folding magnetization[Stamatakosand such as the Jacksonwaldsyncline in the Newark basin, are Kodarna, 1990]. However, the positive fold test on the invariably associatedwith either the normalborderfaults (as is coexistingC componentfrom these same sites, with its peak the case in the Jacksonwaldregion where Mesozoic sediments in precisionvery near full unfolding, leads us to believe that are faulted againstPrecambrianand Paleozoicbasementrocks) penetrativestrain has not significantlymodified the remanent or normal internal faults (in these casesthe hanging wall is magnetizationof these rocks. most commonlyfolded) [Lyttle and Epstein,1987; Olsenet al., The otherpertinenttectonicfeaturein the Newark basinis the 1989]. Each such fold decreasesin amplitude with distance ubiquitous50-20ø dip down toward the borderfaults. This tilt from its associated fault, suggesting that the folding was was probablyacquiredduringsubsidence in the basinas a result directly associatedwith slip on the normal faults. In the of dip slip on normal faults [e.g., White et al., 1986]. Jacksonwald syncline, stratigraphic thickness and facies Assuming that the northwesterlytilt of the homoclinalbeds relations in the cyclic lacustrine sediments indicate that throughoutthe northernand centralpartsof the basinis related folding of the strata was probably initiated penecontemporaneously with deposition and related to subsidence along the border fault [Olsen et al., 1989]. 3 Although the initiation of this folding and the tilting associatedwith subsidenceis probablynearly syndepositional, cessationof folding and tilting is lesstightly constrained. The youngestpreserved strata in the Newark basin (the Boonton Formation of Hettangian age) are folded, but Cretaceouscoastalplain deposits(which overfietilted stratain partsof the Newark basinat the surface)are not deformed. In the offshorerift basinsof the BaltimoreCanyonand Georges Bank regions the oldest sediments above the postrift unconformityare at leastMiddle Jurassicin age [Klitgordet al., -1 / 0 Applied Field 1988], indicating that subsidencein the rift basinswas largely (Tesla) completeby the Middle Jurassic.Thusthe tilting of the Newark strata was certainly completedby the Cretaceousand if the Fig. 8. IRM acquisitioncurve and backfieldexperimentfor a typical structuraldevelopmentof the Newark basin was analogousto red bed specimenfrom the PassaicFormationindicatingthat saturation the offshore rift basins, then the folding and tilting of the IRM is not achievedby 1.5 T and remanentcoercivityis at least0.7 T, propertiesthat are attributedto hematite. Newark stratais constrainedto endby the Middle Jurassic.
19,578
W1TI• AND KENT: NEWARK REMAGNETIZATION
to the west-northwesterlyplunge of the Jacksonwaldsyncline, the statistically significant divergence of the B component mean directions from these areas with increasing tilt and plunge correctionindicatesacquisitionlate in the deformation history of the strata, after all tilting/plungingwas complete and after folding was two thirdsor more complete. The site mean directions of the B componentexpressedas virtual geomagneticpoles (VGPs) yield a meanpaleomagnetic
of the age of the hydrothermalepisodewhich reset the K-Ar systemin the intrusionsto be a good estimateof the age of the B componentremagnetization. REMAGNETIZATION MECHANISM
Owing to the high unblockingtemperatures observedfor the B component (extending upwards to 650øC, Figure 6), thermoviscous acquisition by hematite would demand implausiblyhigh temperatures,of the order 600øC, even if the poleat 74øN,96øE(k = 63,A95= 2.6ø,N = 50) afteroptimal thermal event was sustainedfor an extendedgeologicaltime structural correction. From constraints which are available on interval [Pullaiah et al., 1975; Kent and Miller, 1987]. The the age of folding and the observationthat the B component lack of any evidenceof regional high temperaturealterationof magnetization may be synfolding, the age of this the red beds,exceptin hornfelsin the very immediatevicinity paleomagnetic remagnetization pole is constrainedto the Jurassic.
(
