Early Mesozoic Rift Basins of Eastern North America and Their Gravity Anomalies

TECTONICS, VOL. 7, NO. 3, PAGES447-462, JUNE 1988

EARLY MESOZOIC RIFF BASINS OF EASTERN NORTH AMERICA AND THEIR GRAVITY ANOMALIES: THE ROLE OF DETACHMENTS DURING EXTENSION RobinE. Bell1, Garry D. Kamer,and Michael S. Steclder

Lamont-DohertyGeological Observatory of ColumbiaUniversity, Palisades, New York

Abstract. Two end-member models of exten- applied. The simple shearmodel fits the outer sion involvingdetachments have been developed. hangingwall anomalyand permits a region of One model incorporatesa fault that solesat mid- lower crustal extension to be mapped. These crustal level overlying a broad region of pure basins contain an abundance of basalt flows and shear in the lower crest. The second, referred to diabase sills despite the lack of evidence for as the simpleshear model, includes a detachment regionalheating or thermalsubsidence, implying continuingthrough the entirecrest and terminating that a source,external to the basin, must exist for in a regionof concentratedextension in the lower this magmaticmaterial. The detachmentfault may crest. Both modelspredict basins with no local- facilitate the movement of the molten mafic ized thermal effect. With the inclusion of flexural material into the rift basins from an offshore isostasy,both models predict footwall uplift whose regionof greaterheating and extension. The addi- amplitudeand wavelengthare controlledby the tion of 2 km of mafic material along the model detachmentgeometry and the lithosphericstrength. detachmentaccounts for the observedinner gravity A gravity anomalyover the hangingwall block high and the lack of a negativegravity anomaly distinguishesthe simple shear model from the across these basins. The match of this modified intracmstaldetachment model. The early Meso- simpleshear model to the observedgravity sug- zoic basins of the eastern North America, believed geststhat the regionof greaterextension seaward to have formed as the result of the normal-slip of the hingezone is the sourcefor the widespread reactivationof a Paleozoic thrust system as the dikes and sills within the basins and the coastal Atlantic opened,are associatedwith distinctive plain and helps explain the geochemical hangingwall gravity highs.These gravity highs, homogeneityof these intrusivesand extrusives the basin geometry,the lack of a thermal sub- along2000 km of the easternseaboard. sidencephase in the rift basins,and the presence of a highlyextended and heatedregion to the east, INTRODUCTION suggestthat the simple shear model may be In recentyears there have been great advances in our understandingof lithosphericrifting. 1Alsoat Department of Geological Sciences, Analysisof basinsubsidence has revealed a major ColumbiaUniversity, New York. thermal contributionto the postrift subsidenceof large extensionalbasins and passive margins Published in 1988 [Sleep,1971; Steckler and Watts, 1978]. Concep- by the AmericanGeophysical Union. tual models [e.g., McKenzie, 1978] helped to define a relationshipbetween lithospheric exten- Papernumber 8T0115. sion and the heatingand crustalthinning in pas- 0278/7407/88/008T-0115510.00 sive rifts, allowingquantitative estimates of verti- Bell et al.' Early MesozoicRift Basins cal motionsto be made. The recognitionof low- extension. To understand these vertical motions, angle normal faults by surfacemapping has added we need to determinethe isostaticresponse of the a new dimensionto our understandingof how the lithosphereduring rifting, a process that is not upper crust extends [Wemicke and Burchfiel, well understood. Virtually all investigatorshave 1982; Wemicke, 1985]. Information on crustal assumedlocal isostasyduring rifting [McKenzie, extensionhas generallycome from seismicrefrac- 1978; Stecklerand Watts, 1978, 1980; Roydenand tion and reflectionprofiling. In particular,seismic Keen, 1980; Sclater and Christie, 1980; Watts et data has: (1) imaged the thinning of the crust al., 1982; Beaumont et al., 1982 ]. under some basinsand passivemargins [e.g., Bar- The state of isostasyplays a critical role in ton, 1986; Large Aperture Seismic Experiment determining the effect of detachmentsduring (LASE), 1986], (2) demonstratedthe importanceof extension. Our currentunderstanding of isostasy detachments in localizing extension, and (3) duringrifting has comeprincipally from continen- shown that small rift basins form within the col- tal margins,large intracratonicbasins and major lapsedhanging wall of normallyreactivated thrusts rift zones. In all of thesecases, a large thermal [British Institutes Reflection Profiling Syndicate perturbationis introducedinto the lithosphere, (BIRPS) and Etude de la Crot•te Continentaleet resultingin isostaticuplift. The thermalinput not Oc6aniquepar R6fiexion et R6fracfion Sismique only producesa time-varyinguplift but also pro- (ECORS), 1986]. Although these observations gressivelymodifies the flexural rigidity. Conse- document the existence of thin crest beneath quently,this paperwill focuson basinswith only extended regions and the complex geometry of minor, local thermal perturbationsin order to extension, they do not address the process by document the effects of flexure during rifting. which the thinning occurs [Allmendinger et al., Such basins exist where the presenceof detach- 1987]. ments indicate that heatingis laterally displaced Direct knowledgeof the processof lithospheric relative to the rift, therebyisolating the uppercru- extension remains restricted to observations of the stal response. Our objectiveis to develop two upper crust. Evidence for deep-level extension end-member rifting models to compare with comesprimarily from seismicactivity within rift known examples of thermally "isolated" basins zones and the exposureof exhumed detachment and to investigatethe distributionof strainthrough surfaces, but the actual distribution of extension the crust,particularly in the lower crustalregions. with depth, and the role of detachments,is poorly The purpose of this paper is to investigatethe constrained. It is these detachments that transmit isostatic state of thermally isolated basins during surface extension down to mid crustal and lower rifting, the distributionof strainin the lower crust, crustal levels. Whatever the role of these detach- and the role of detachmentsin the developmentof mentsis in laterallytransmitting strain, the amount basins. We particularlyinvestigate the Mesozoic of extension observed at the surface of a rift must agedbasins of easternNorth America. be balancedby an equal amount of extensionat depth [Kligfield et al., 1984]. MODELING ASYMMETRIC EXTENSION There are two important models for the distribution of extension at depth beneath rifts. The simplest and most commonly assumedmodel is It has been over 30 years since Vening that of uniform, symmetriclithospheric extension Meinesz first described the flexural effects across [McKenzie, 1978]. This model assumesthat the the Rhine and Africanrifts, which imply a nonlo- lithospherefails by depth-independentpure shear. cal isostaticresponse of the lithosphereto exten- Numerousvariations on this theme have beenpro- sion [Vening Meinesz, 1950; Heiskanenand Ven- posedfor the distributionof extensionwith depth ing Meinesz, 1958]. The African rift lakes are [Royden and Keen, 1980; Hellinger and Sclater, characteristically half graben, associated with 1983; Rowley and Sahagian,1986]. Thesemodels prominent footwall uplift with a magnitude generallyassume that althoughthe distributionof directly proportionalto the basin subsidence[B. extension varies with depth, it remains centered Rosendahl,personal communication,1986]. This beneath the rift. A radically different model is uplift is difficult to explain as a thermal effect that of uniform sensesimple shearthroughout the because of its asymmetricnature and the fre- lithosphere[Wernicke, 1985; Lister et al., 1986]. quency with which it alternatesbetween sides of With this proposed geometry, detachmentscon- the lake. Footwalluplift requiresflexural strength tinue throughout the entire lithosphere. As a of the lithosphereduring rifting. result, the extension and upwelling at depth is Vening Meinesz' original analysisassumed: (1) asymmetricallydistributed with respectto the rift. the existenceof normal planar faults which frac- Asymmetricextension, as implied by the presence ture the entire crust (and by inference the elastic of detachments,should produce different vertical lithosphere),and (2) once the crust is broken, it motions of the lithospherethan that of uniform acts as a pair of independentcantilever beams Bell et al.: Early MesozoicRift Basins

separatedby the fault. The unloadingand loading [a] of the crest on either side of the fault is therefore km considered independently. Because of this ResultantUplift extreme assumption,lower plate (footwall) uplift and upper plate (hanging wall) subsidenceare maximized. The main control on the uplift and subsidencewavelength is the flexural rigidity of Te = 20 km 10 km heave the upper and lower plates. Increasingfault dip tendsto decreasethe footwall block uplift, a vertical fault dip yieldingno uplift. s :! ...... ):i::-•...-:•j•i:•i•:i•HangingWallBlock This classicalanalysis of normal faulting and footwall uplift fails to adequatelyconsider: (1) the 20 possible mechanicalinteraction of the uplifting lower plate and subsidingupper plate, which may • 5• 1•0 1}0 2•0 2}0 3•0 3}0 significantly reduce the deformation of both blocks,(2) that breakingthe crest doesnot neces- kilometers sarily imply flexural failure of the whole litho- Fig. 1. (a) Illustrationof the isostaticadjustments sphere, and (3) that crustal extensionis often of the lithosphereto extension along a listtic characterizedby the utilizationof low-angleintra- detachment. Net isostaticrebound caused by the crustal detachments,which show a general listric extensionfor a Te=20km and heave (total hor- nature, rather than high-angle normal faults. izontal extension)of 10 km. Uplift is inducedin These points must be addressedbefore a realistic both the footwall and hangingwall block. (b) Cru- assessmentcan be made of the importance of stal structureafter extension has occurred along flexureand isostasyduring rifting. the fault, and the hangingwall block has collapsed Zandt and Owens [1980] attemptedto correct but beforethe systemresponds to the hole. Shape for the secondof these deficiencies. They con- of upper plate resultsfrom failure along vertical sideredcomplete decoupling and continuityacross faults. the fault as two possibleend members. Jackson and McKenzie [1983] extendedVening Meinesz' flexural rigidity across the fault zone can be formulationby includingvertical forcesexerted by approximatedby a continuouselastic plate with the upper and lower plate on each other. Because constantflexural rigidity overlying a weak fluid the forceswere not specified,they could only esti- (the flexural ridigity is expressedin terms of T e, mate the ratio of footwall uplift to hanging wall the effective elastic thicknessof the lithosphere, subsidenceand not the actual magnitudes. Jack- and reflectsthe integratedrheological properties of son and McKenzie, as Vening Meinesz, resolved the lithosphere).The examplegiven usesa T e of these forces onto a vertical plane as end-loading 20 km. Other assumptionsfor the flexural rigidity forces on a cantilever beam. The mismatch of the across a fault zone include a reduction of the elas- bendingin the overlappingparts of the footwall tic thicknessby the thicknessof the upperplate (if and hangingwall was still not considered. the upper plate is fragmented),or if the upper To addressall three of the problemsassociated plate has strength,an effectiverigidity correspond- with the original Vening Meinesz approach,we ing to a leaf springcombination of the rigidities of view extensionalong low-anglefaults as a unload- the upper and lower plates, i.e., geometricmean. ing process,i.e., the reboundassociated with the We have found that the difference between these negativeload of the hole producedby the move- assumptionsis important only when the elastic ment of the hanging wall block along a listric thicknessof the lower plate becomes small or fault. In the example given (Figure 1) the shape when the detachmentapproaches the base of the of the hole is generatedby the mechanicalcol- elasticlithosphere. lapse of the hangingwall by a seriesof vertical Isostatic uplift, in addition to producing the faults [Gibbs, 1984; Bosworth et al., 1986]. Other well-knownfootwall uplift, alsoproduces uplift of stylesof upper plate fragmentationmay be con- the hangingwall block. This uplift must deform sidered [e.g., McClay, 1987] by adjusting the the detachmentsurface. The typical abandonment shapeof the upper plate. The heave, or horizontal of the breakawayzone and exposureof the detach- movementalong the fault, is 10 km in this case. ment at the surfaceby antiformaluplift [Wernicke, The final predictedtopography will be the sum of 1981; Spencer, 1984] may be, in part, a natural the flexural responseto upper plate unloading, consequenceof lithosphericstrength during exten- with the shapeof the hole controlledby mechani- sion [W. R. Buck, Flexural rotation of normal cal failure of the hangingwall block. faults, submitted to Tectonics, 1988]. A detach- For simplicity we have assumedthat the ment surfaceis first lockedby hangingwall uplift, •50 Bell et al.: EarlyMesozoic Rift Basins

PredictedTopography (local compensation) and then, following the formation of a new brea- 2- kaway zone, exposed by subsequentfootwall uplift.

-2-

TWO ASYMMETRIC EXTENSION MODELS

-6- INCORPORATING FLEXURE

-8- PredictedGravity (local compensation) In order to estimate the effects of flexure during rifting we considertwo end-memberextension 20 models where the upper crustalrifting does not

10 overlie a local thermalanomaly (Figures 2 and 3). The first examinesthe isostaticresponse to exten- 0 sion across an upper crustal, listtic fault which -10 flattens into a zone of distributed strain in the

-20 lower crest (Figure 2). The thermaluplift associated with this model will have a relativelylong PredictedTopography Te = 30 km wavelength. With this broad thermal uplift, the 2- localizedflexural upwarpingat the surfacerift can be easily recognizedand separated. The entire region encompassingthe rift will have an elevated heat flow, and the observedflexural rigidity will be reduced. The second model examines the isostatic responseof simple shear extension,in which a -8 - single narrow zone of failure cuts through the PredictedGravity Te = 30 km entire crest and lithosphere(Figure 3). In this 20 model the mechanical and flexural behavior of the

10 upper crest is spatially isolatedfrom the thermal effectsdue to the asymmetryof the extension. Thesetwo modelsrepresent end-member cases, -10 and most actual rifts probably incorporateele-

-20 ments of both models. Zones of failure, for instance,commonly are localizednear the surface and becomebroader and more diffusewith depth as the failure mechanismchanges from brittle faultingto ductilecreep. Settingsin which basins similarto our end-membermodels might occur are •0 Footwall Bloc Hanging Wall Block

15 extensionin exceedinglythick, hot, and therefore weak crest, e.g., Tibetan Plateau, or normally reactivated thrust faults with relatively small 3O amountsof extension,e.g., Wessex Basin, southern England. The resultantpatterns of uplift, subsidence,and gravity anomaliesfor the two models are illus-

kilometers tratedin Figures2 and 3. We have calculatedthe results for both flexural and local compensation Fig. 2. Resultsof extensionof the crustinvolving schemes. Local compensation produces no an intracrustal detachment for both local and footwall or hangingwall uplift, and the predicted flexural isostasy.In the uppercrust, extension is basin is shallowerthan for a flexurally compen- accommodatedalong a listtic fault, while in the sated basin produced by the same amount of lower crust the same amount of extension is extension.The locally compensatedsimple shear accommodated in a zone of distributed strain model developsa secondbasin directly over the (shaded). The predicted topographyincludes region of concentratedstrain in the lower crest, results for a sediment-starved(dashed line) and which is identical to the main rift basin. Both sediment-filledbasin (solid line). The gravitywas locally compensatedmodels are characterizedby a calculated for a sediment-filled basin. The flexural 15-mGal low centered over the main rift basin. A resultswere calculatedwith T•30 km while the similar amplitude gravity low is associatedwith locally compensatedcalculations are equivalentto the secondbasin in the locally compensatedsim- Te--0km. ple shearmodel. Bell et al.: Early MesozoicRift Basins 451

PredictedTopography (local compensation) Both modelswith flexural compensationpro- 2 - duce footwall uplift, an asymmetricbasin, and a topographicbulge on the hangingwall side. The footwall uplift, for an elasticthickness of 20 km, a 10-km heave along the fault and a sediment starvedbasin, is about 1 km. The hangingwall

-6- block uplift is 300 m. However, both uplifts are greatlydecreased by sedimentloading. The grav- -8 - ity signatureassociated with these models pri- PredictedGravity (local compensation) marily reflectsthe sum of densitycontrasts across 20 the sediment-crest interface and the crest-mantle interface. 10 The two flexural extensionalmodels (Figures 2 0 and 3) are distinguishedby several predictions.

-10 For example the intracmstaldetachment model is characterizedby a broad uplift due to the zone of -20 distributed strain in the lower crest and mantle and PredictedTopography Te = 30 km a similar shift in the gravity level. Dependingon the breadth of the zone of distributed strain, these 0 - featuresmay or may not be easily detectable. In

-2 contrastthe simple shearmodel developsa positive load due to crustalmaterial being replacedby -4 mantlematerial where the detachmentcuts through

-6 the crest. The flexuralresponse of the lithosphere to the narrow zone of deepcrustal thinning is the -8 - formationof a broad sag basin. The gravity ano- PredictedGravity Te = 30 km maly producedby this deep crustalthinning criti- 2O cally depends on the amount of sediment fill 10 within this basin. If the sag basin is sediment

0 starved,the gravity signal over the basin is dom- inated by the surfaceinterface and is a broad low. -10 Filling the sag basin with sedimentresults in a

-20 distinctive15-mGal high directlycentered over the

0 region of deep crustalthinning. This gravity high

5 on the hangingwall block is an importantindica- 10 ?'C HangingWallBlock tor of the the actuallocation of the region of sub- 15 crustalthinning. However,the gravity signatureis 20 quite insensitiveto the actualshape of the thinned 25 region. For a constant amount of extension 30 FootwallBlock increasingthe lateral extent of the thinnedlower I 3OO crest will decreasethe amplitudeof the hanging wall gravity high slightly. Doubling the lateral kilometers extentof this extendedregion reduces the anomaly Fig. 3. Resultsof crustalextension involving a producedby 1-2 mGal. through-goingsimple shear detachmentfor both A number of factorscontrol the amplitudeand local and flexuralisostasy. In the lower crosssec- the shape of the uplift for the flexural models. tion the dashedline indicatesthe positionof the Filling the half grabenwith sedimentwill dampen plates after extensionbut before collapseof the the uplift acrossthe entire system. The dip of the blocks (solid line). The amount of extension is fault is critical in controllingthe resultanttopogra- the same (heave of 10 km) as presentedfor the phy. Given a constantflexural strengthand heave, intracmstal detachmentmodel in Figure 2. The a through-cuttingplanar fault with no midcrustal predictedtopography includes the predictedverti- detachmentproduces decreasing deflections as the cal motions for a sediment-starved(dashed line) fault steepens[Vening Meinesz, 1950; Jackson and a sediment-filledbasin (solid line). The grav- and McKenzie, 1983]. This contrasts with the ity was calculatedfor a sediment-filledbasin. Note results presentedhere for a fault intersectinga the developmentof a sagbasin within the hanging horizontal detachment at midcrustal levels where a wall block and its associatedhanging wall gravity steeperfault will producegreater uplift and a nar- high for the flexural isostasycase. The flexural rower, deeperbasin. In Figure 1 and all subse- results were calculatedwith Te=30km while the quent modelswe have used a listric-shapedfault locally compensatedresults are equivalent to a soling into a horizontaldetachment at midcrustal T,=0 km. levels. The listric shapehas little influenceon our Bell et al.: Early Mesozoic Rift Basins resultsas a planar fault soling into a detachment, Evidence for the reactivation of ancient thrust as suggestedby Eyidogan and Jackson[1985], faults during the formationof these Mesozoicrift producesvery similar results. The depth of the basins comes from both structural geology and intracmstaldetachment is positively correlatedto seismic reflection data. The structural fabrics the amountof uplift. A deeperdetachment level, within the mylonites along the Musconetcong for a given set of parameters,will displace a thrust system, close to the border fault of the greateramount of crust,produce a larger hole and Newark Basin, record an extensional overprint consequentlygreater uplift. As the elasticstrength [Ratcliffe et al., 1986]. This thrust system has of the lithosphereincreases, the amplitudeof the been traversed by seismic reflection lines and uplift produceddecreases while the area uplifted drilled in a series of boreholes [Ratcliffe et al, becomesbroader. Simultaneously,the main basin 1986; Ratcliffe and Costain, 1985]. The seismic becomes narrower and the sag basin over the reflection studies indicate that the Musconetong region of subcrustalextension becomes less dis- thrust systemis subparallelto the border fault of tinct. the Newark Basin, suggestingthat the basin Although the resultsare dramaticallydifferent geometrywas controlledby preexistingstructures. for flexural versuslocal compensation(Figures 2 The basins represent the preserved western and 3), the models are relatively insensitiveto limit of Mesozoic rifting. Reconstructionsof the variationsin Te; changesin predicteduplift are initial rift configuration indicate that extension similar in amplitudeto the uncertaintiesin prefift took place over an extremely broad area during topography.Other indicators,such as the increase the early stages of rifting. The most highly in the size of the hangingwall high for the simple extended and heated lithospherelies seawardof shear model (Figure 3) are problematicbecause the hinge zone, which marksthe westernmostlimit the region of concentratedextension is generally of the highly stretchedcrest which producedthe poorly constrained. Changingthe distributionof passivemargin sequence, as evidencedby the dis- extensionin the lower crustproduces shifts in the tributionof postriftsedimentation and the location gravity field similar to thoseresulting from varia- of maximum crustalthinning [Watts and Steckler, tionsin T½. Althoughthese models can be usedto 1979; LASE, 1986]. Geochemical evidence sug- identify the existenceof lithosphericstrength, i.e., geststhat a maximumsediment thickness of 2 km flexural versuslocal compensation,the numberof has been removed from these basins [Pratt et al., poorly constrainedvariables makes the accurate 1985; Katz et al., 1988]. This lack of a thick pos- determinationof T½during rifting difficult. trift sediment cover over the Mesozoic basins and their peripheralposition to the continentalmargin EXAMPLES OF ASYMMETRIC RIFT BASINS suggeststhat they were nevergreatly heated. The presenceof uppercrustal extension with no appre- Early MesozoicBasins • Eastern ciable local thermal subsidenceimplies that the North America near-surface extension within these basins was probably accommodatedat depth by extension Extension within old collisional belts is often closer to the main offshore rift basins. The characterizedby the normal reactivationof large geometryof thesebasins therefore appears similar thrust faults [BIRPS and ECORS, 1986; Lake and to the extensionalstyle describedby our simple Karner, 1987]. Such reactivations within an shearmodel (Figure 3). extensional tectonic setting may produce a rift A number of the Mesozoic half grabensare geometrysimilar to that of our simpleshear model associatedwith a distinctive15- to 25-mGal grav- (Figure 3). The early Mesozoic rift systemalong ity anomaly high over the hanging wall block. the east coast of North America consists of a These highs, frequentlylocal culminationssuper- seriesof half grabenbelieved to representthe first imposed on the Appalachian gravity high, are phaseof extensionwhich openedthe centralNorth associatedwith the Newark, Gettysburg, Culpeper Atlantic. These half grabens appear to exist and Riddleville basins (Figures 4 and 6). This within the collapsed hanging wall of normally gravity high is absentin regionswhere no basin reactived Paleozoic thrust faults [Ratcliffe and exists, as is the case southof the Culpeperbasin. Burton, 1985; Swanson,1986]. This rift system, Further, these basins and the associatedgravity containing the sedimentsof the Newark Super- highs cut across the structural trend of the group[Froelich and Olsen, 1984], consistsof more Appalachiansand the trend of the Appalachian than 20 basins along the easternseaboard, some gravity high. This cross-cuttingrelationship is exposedat the surface, such as the Fundy, Hart- important,as it impliesthat the gravity high must ford, Culpeper and Newark basins, and others be associatedwith the basin-formingprocess and covered by coastal plain sediments,such as the is not simply the resultof preexistingcrustal inho- Riddleville Basin and the Long Island Platform mogeneities. basins [Peterson et al., 1984; Hutchinson et al., To highlight the Mesozoic basin gravity 1986;Klitgord and Behrendt,1979]. anomaliesand associ_atedhanging wall highs, the Bell et al.' Early MesozoicRift Basins t•53

-79 ø -78 ø -77 ø -76 ø -75 ø -74 ø -73 ø -72ø 42 ø 42 ø

41 ø 41 ø

40 ø 40 ø

39 ø 39 ø

38 ø 38 ø

37 ø 37 ø

36 ø 36 ø .79 ø .78 ø .77 ø .76 ø _75 ø _74 ø .73 ø .72 ø

Fig. 4. Bouguergravity map of the mid-Atlanticregion of the U.S. east coast. The dark shadingindicates the locationof the Mesozoicbasins (N, Newark;G, Gettysburg;C, Culpeper; H, Hartford).The light shadingpattern indicates the gravityhighs on the hangingwall sideof the basins. The dashedlines are the locationsof the profilesused in this study.

Appalachiangravity high must be removed. The the Appalachiangravity high, no distinct high Appalachian gravity high is associatedwith the remains after the filtering processsubstantiating deep crustalstructure formed during the collisional our filtering procedure(Figure 5). eventsof the Taconicand Acadianorogenies from Several-distinctive features appear in the the Ordovicianthrough the Carboniferous[Karner filtered gravity profiles shown in Figure 6. The and Watts, 1983; Cook 1984]. To remove the first are the gravity highs over the hangingwall effectsof this preexistingstructure from the signa- blocks that were also apparentin the map view ture of the Mesozoic basins, we have treated the (Figure 4). Almost all the profiles have a short- profiles (locations shown in Figure 4) with a wavelength(50 km), 25-mGal high just to the east Gaussianfilter that removeswavelengths greater of the basin edge (inner gravity high), and most than 200 km. Where no basin is associated with also have a secondgravity high further to the east 454 Bell et al.: Early MesozoicRift Basins

discriminatingfeature for the models. Neither the 80- / ß GettysburgBasin intracmstaldetachment model nor the locally com- f ,, ., .• pensatedsimple shearmodel producesa hanging wall gravity high. In fact, the latter model produces a large gravity low over the secondary 40- •__- ,. outergravity high basin. The only model which appearsto match the gravity anomaly across these basins is the flexurallycompensated simple shear model (Figure 0 - 3). This model also closely correlateswith the asymmetricgeometry inferred from the reactiva- inner gravity high tion of the Mustconetcongthrust system. Our modelingof thesebasins is constrainedby 80- b /u .... --. No Basin the basindepth and width, the densityof the infill, the surfacedip of the borderfault, and the gravity

/ anomaly. The modeling results presentedincor- poratea 10-km heavealong the borderfault which 40- r producesa standardbasin, 45 km wide and 6 km deep, approximatelythe mean size of the Newark and Gettysburgbasins. The basin infill for the

0 - modelsis a weightedaverage of thebasalt density (2.9-3.1g/cm 3) andthe sediments (2.4-2.7 g/cmø; [Sumner, 1977]) based upon their relative abundance and locationin the section. The densityof the surroundingcrustal rocks varies from 2.6 to • 50I IIX}I 150I 200I 250I 300I 350I 3.0 g/cm3. Densitiesof 2.6 g/cm3 for thesurface kilometers rocksand 3.3 g/cm 3 for the mantle were used. The border fault dip along the Newark basin ranges Fig. 5. (a) Observed(dotted) and filtered (solid) from 70ø in the north to 30ø in the south [Ratcliffe profilesacross the the GettysburgBasin (profilej, and Burton, 1985]. The model border fault is Figure 4). Shadingshows present exposure of the definedas a curve whosedip decaysexponentially basin. (b) Observed (dotted) and filtered (solid) with depth. All the modelsuse a fault with a sur- profilesacross the Appalachiangravity high south face dip of 45ø and an exponentialdecay of 15 of the CulpeperBasin where no basin is closely km. associatedwith the Appalachian gravity high The remainingfree parametersare the strength (profile 1, Figure 4). of the lithosphere(T,) and the locationof the region of lower crustal extension. As noted above, the lithosphericstrength primarily controls (outer gravity high). The outer gravity high, seen the developmentof a sagbasin and the amplitude distinctlyin profilesa, c, j, and k (Figure 6) has of the gravity anomalyover the region of subcru- an amplitude of 20-mGal (similar to the inner stal thinning. Any sagbasin developed with these gravity high), is slightly broader(up to 100 km Mesozoic basinsis now coveredby coastalplain wide), and sometimesappears to merge into the sediments.Thus the gravity signatureis the only inner gravity high. The merging of these two remainingindicator of T, duringrifting. Assum- highs produceseither a broad double-peakedhigh ing that the lower crustalextension mirrors the as seen in the Central Gettysburg(profile h) or a uppercrust extension, the amplitudeof the outer single high as in the SouthernNewark (profilesf hangingwall gravityanomaly is bestfit by a T, of and g). The secondinteresting feature of these 30 kmo basinsis the lack of a large gravity low associated The gravity signatureof the model is most sen- with the sedimentaryinfill. The gravity low over sitive to the locationof the region of lower crustal the basin reaches a minimum of-8 mGal over the extension. Concentratingthe lower crest exten- Newark and -10 mGal over the Gettysburg. In sion in a narrow region similar in shape to the some cases,such as profiles f and g, there is no basin producesa 20-mGal, 120 km wide gravity low over the basin. high. Figure 7 showsthe gravity anomaly pro- As no evidenceremains of the uplift historyof duced by varying the distancefrom the surface these basins,the gravity anomaliesare the only fault to the subsurfaceregion of extension(D L) key to the isostaticstate of the basinsduring rift- from 50 to 200 km in 50-km increments. At 200 ing. All the models, regardlessof detachment km the subcrustalanomaly is distinct from the geometryor isostaticstate, predict a large 15- to basin anomaly while at 50 km the amplitudeof 20-mGal low centered over the rift basin. The the subcrustalanomaly almost completely cancels distinctive hanging wall gravity high is the the basin low. Bell et al.: Early MesozoicRift Basins 455

Newark

Gettysburg 0

80 8O

4O h it •..._..,,

0 o 80 ,,'%, 80 i ! 4O 4o

0 o

80 /x 80 ß ! ..

t i 4O mgal4o !

0 o 80 8o e

mgal 4O 40

0 o 80 i i i i i i i i o 50 100 150 200 250 300 350

4O kilometers

4O !

! i i i i i i i 0 50 100 150 200 250 300 350

.. kilometers

Fig. 6. Raw Bouguergravity profiles (dashed lines) acrossthe Newark and Gettysburgbasins superimposedon the filteredprofiles (solid lines). The locationof theseprofiles is found in Figure4. The shadingindicates the presentsurface exposure of the basins.

The predictionsof the simpleshear model for a matchesthe model for the majority of the profiles. T e of 30 km and a heave of 10 km are shownin A consistentdiscrepency between the model and Figure 8 overlayingthe profilesfrom the Newark the observed gravity profiles exists across the and Gettysburgbasins. Only the distancefrom the basinsand the inner gravity high. The model fails border fault to the region of concentratedsubsur- to account for some excess mass beneath the basin face extension(Di) was allowed m vary. To fit and the proximalhanging wall. the data, values from 100 km in the southern The goodnessof fit for the outer hangingwall Newark basin to 170 km in the northernGettys- anomaly varies along the basin. The predicted burg basin were needed.The wavelengthand the anomalies fit best in the northern Newark basin amplitude of the outer gravity high closely (profilesa-d) and in the Gettysburgbasins. The 456 Bell et al.: Early MesozoicRift Basins

In conclusion we have found that the Mesozoic basins of eastern North America have distinctive gravity highs over the hangingwall block. This outer gravityhigh can be modeledby the density contrastsalong a detachmentwhich cuts through the entire crest (Figure 3). A detachmentwhich soles at midcrustal levels above a region of broadlydistributed strain does not producea hanging wall gravity high and cannot be used to explain the gravity anomaly across the Newark and Gettysburgbasins. The lithospheremust have -lO 200 krn strengthduring the rifting processto producethis gravity high. The outer gravity high can be used to trace the location of the extension within the -lOø 150km lower crust. This region of concentratedextension generallycorresponds to the highly stretchedcrust o beneaththe continentalmargin. -lO 100 km '::.:•.:..':-

-lO :•..-:: • 50 km Role • Detachmentsas Magma Conduits

I I I I I I I I I 0 50 100 150 200 250 300 350 400 Althoughthe simple shearmodel with flexural compensation(Figure 3) sucessfullypredicts the kilometers outer gravity high, we fail to predictthe gravity signatureacross the basin and the inner gravity Fig. 7. Predicted gravity for the simple shear high. The close proximity of the basins to the detachmentmodel allowing the distancefrom the inner gravity high implies'that a single density border fault to the region of subcrustalextension contrast can explain both the basin and inner (D L) to vary from 200 to 50 km from the border hanging wall gravity signatures. A number of fault. The shading representsthe extent of the possibleexplanations could be proposedfor the predictedbasin. absenceof a large low across the basins. One explanationis that the basementgeometry is more complexthan a simple half grabenbounded by a single border fault. A more complicatedfault deteriorationof the fit in the otherareas probably geometrycould easily rotate blocks of relatively reflectsthe importantcontrol the borderfault dip high-densitybasement close to the surfaceas is has on the resulting basin anomaly. As noted seen in modem extensional enviroments such as above, the dip of the Newark basin border fault the Gulf of Suez. This may be the case in the decreases from a maximum of 70 ø in the north to southernNewark basin (Figure 8, profile f) where 30ø in the south. The threeNewark basin profiles a basementblock is exposedin the center of the which closely match the modeled anomaly are basinbut seemsless likely in the northernNewark thosewhere the borderfault hasa measureddip of where no evidencefor major faulting within the 45ø. The modelspresented in Figure8 incorporate basinis found. A secondexplanation for the lack a detachmentwith a surfacedip of 45ø and an of a gravity anomaly within the basinscould be exponentialdecay of 15 km. high densitytholeiitic dykes and sills. After the For a given set of parametersthe location of regional gradientis removed,local gravity highs the region of lower crustal extensionis the vari- of 5-11 mGal occur over the surfaceexposure of able which is best constrainedby these models. the sills in the southem Newark basin [Sumner, The results from the Newark and Gettysburg 1977]. Profilesf and g (Figure 8) both crosssills basinsimply that this region is further to the east which may explainthe lack of a negativeanomaly of the Gettysburgbasin than the Newark basin. acrossthe basins. Neither of these explanations The model predictsmaximum rangesof 150-170 satisfactorallyaccounts for the small anomalies km to the east of the borderfault for the Gettys- acrossthe northernNewark basin and the Gettys- burg basin and 100-150 km to the east for the burg basinwhere no densematerial is exposedat Newark. The hinge zone is 110-160 km east of the surface. A third solution is to fill the basin the Newark basin border fault while it is 230 km with large amounts of diabase, decreasingthe east of the Gettysburgbasin. The resultsof the average density contrastbetween the basin infill modeling correlate broadly with previously and the surroundingcrustal material. This alterna- mapped limits of stretching[Watts and Steckler, tive requires more than double the presently 1979). mappedoccurrence of igneousmaterial to elevate Bell et al.' Early MesozoicRift Basins 457

-10 a ?• North

0 -10 \ :'..':•:•i."/

-10 mgal•o wark

-10o] -10 • / \/

o o Gettysburg -1o regal-•o

o o -.-'......

-1o

o o -1olO • ...... -•o \ / I I •""• 0' 50' 1•0 150' 2 •0 250' 300' 350 o ;o 100 150' •o 25O' 3•o go

kilometers kilometers

Fig. 8. Resultsfor a simple sheardetachment model (dashedlines) superimposedupon the filtered profiles (solid lines) acrossthe Newark seriesbasins. The distancefrom the border fault to the regionof subcrustalextension (DL) wasallowed to vary. the infill densitysufficiently. No evidencefor this widespreadb•salts and diabases within the basin amount of diabase material has been detected while isolating the Mesozoic basins from the within these basins. As it is difficult to explain major thermal event to the east. The detachment this gravity high with any known densitycontrasts providesa conduitfor the mafic material to move within the basin,it mustresult from a densitycon- from the hinge zone to the basinsthereby poten- trast within the hanging wall block, probably tially explainingthe basinand hangingwall grav- relatedto the basin-formingprocess. ity anomalies. This failure of the model to explain the gravity We have modified the simple sheardetachment high over the basinand the proximalhanging wall model to include the emplacementof a layer of block implies that a massexcess must exist for the mafic material along the detachment. This entire region. This massexcess could be caused modified model takes into account both the load- by preexistingcrustal inhomogeneities such as a ing and gravity effect of such a layer. Loading suite of ophiolitic slivers. Small pieces of producesa slight regional sag that accentuatesthe ultramafic material, such as the Staten Island long-wavelengthnegative gravity effect of the ophiolite,have been identifiedwithin the hanging basin and hanging wall. The gravity anomaly wall block. As these ultramafic bodies have a resultingfrom sucha diabaselayer is a functionof very limited distributionand are often associated the varying density constrastbetween the layer with gravity lows, they cannot account for the and the surroundingcrust. The portion of the gravitypositive across the hangingwall. layer closestto the surfacehas the greatesteffect It is clear that the gravity anomalyacross these primarilydue to the large densitycontrast between basinsrequires a large high-densitybody beneath the diabase and the sediments(Figure 9). The the basin and the inner hangingwall. A second deeperportions of the layer have correspondingly unexplainedcharacteristic of these basins is the smallergravity effectsbecause of the decreasein apparentlack of major thermal subsidencedespite the density contrastbetween the diabaseand the the abundance of diabase within them. An upper and lower crust. The result is a skewed appealing mechanism is to transport the large positive anomaly with a large positive over the quantifiesof melt from the regionof greaterexten- basinand the proximal hangingwall (Figure 9). sion and heating around the hinge zone to the The unconstrained variables for this modified basins. This •rovides a common source for the model are the thickness of the diabase and the 458 Bell et al.: Early MesozoicRift Basins

subcrustalthinning (D O was-allowed to vary.The additionof diabasealong the detachmentdramati- cally improves the model fit to the basin and hanging wall anomalies. The model fits best in

total

0 -10

diabase 10 0 I I -10

sediment-crest 10

mgal-I020 t crest-mantle IO [a] o -1o 1o o , I -1o 250 350

1o kilometers o -lO mgal 20t• 150km D Fig. 9. Schematicof a 2 km thick diabasebody lO L along the detachmentand the individual contribu- o tionsto the total gravityanomaly. -lO20 t• 100km

lO o -lO ,km

positionof the subcrustalextension. Increasing ! the thicknessof the diabase(TD) increasesthe • 50 l•10150 ' 2•0 250' 300I 350I • amplitudeof the inner hangingwall high and kilometers decreasesthe low over the basin(Figure 10a). A diabasethickness of 1.5-2.0km reducesthe grav- [b] ity low over the basin to a value close to the observedwithout requiring any complexbasement 0 ------geometryor any drasticchange of infill density. -10 •ø1------• 0.50.0km km As before, shiftingthe locationof the subcru- lO o • • stal thinning significantlyalters the characterof -!o the resultant gravity anomaly. We illustrate the calculatedgravity anomalyfor a 2 km thick dia- mgallO base,varying the distancefrom the borderfault to -10o TD the outermostregion of subcrustalthinning (D O from 200 to 50 km in Figure 10b. A broad 140 ,oo • km wide anomaly with two peaks resultsfrom a -10 separation of 200 km. As the separation •o • o •• decreases,the two peaksmerge into a singlenar- -10 rower peak as the subcrustalpositive begins to overlap the negativeof the basin. When the sur- 0, 50, I & 150, 2 & 250, 3•, 3•, & face and subsurfaceregions of extension are •1ometers separatedby 50 km, only a smallpositive remains over the basin. Fig. 10. (a) Gravityeffect of allowingthe thick- The range of anomaliesshown in Figure 10b ness of •e •abase layer along the demc•ent correlates closely with the observed anomalies (TD) to v•. (b) Gravity effect for a m•el acrossthe Newark and Gettysburgbasins. Figure inclu•ng a 2 • thick •abase layer, the effectof 11 shows the filtered profiles with best fitting v•ing the positionof the •gion of subcrustal model resultssuperimposed. A Te of 30 km, an extension•om the borderfault (DL) •om 50 to infill densityof 2.6 g/cm3 anda 2 km thickdia- 200 •. •e shading•presents •e extentof the base body were used. Only the location of the pre•cted basin. Bell et al.: Early MesozoicRift Basins 459

o .... -1o,o• ...•a •....•:::•.. North

0 '"-'? -10

o -1o Newark mgalm o

-1o -10 10 i • '"'•...... '"

o -1o mgal4o . . Gettysburg

o o -1o -1o South

o o -1o -1o

0 I 150 200 2 0 300 350

kilometers kilometers

Fig. 11. The resultsof simpleshear detachment model with a 2 km thick diabasebody along the detachment(dashed lines) superimposedupon the filtered profiles (solid lines). Only the distancefrom the borderfault to the regionof subcrustalextension (Di) was allowedto vary. the Gettysburgbasin and the northern Newark Atlanticregion [King, 1961, 1971]. Generally,the basin while the profiles across the southern early Jurassicdikes are found only in crustunder- Newark basin remain problematical. This lain by the major thrusts associatedwith the deterioration of the model fit toward the south Paleozoic collisional orogenies that were later probably reflects the more complex basin reactivatedduring the Mesozoic opening of the geometry,the presenceof sills at the surface,and Atlantic. Transporting material from a single the decreasingdip of the borderfault. offshore source eliminates the need for sufficent If the detachment surface is used as a conduit heat beneath the entire region to produce melts, for mafic material, there are several important therebyexplaining the lack of a substantialther- consequencesfor both the material within the mal subsidencephase within theserift basins.The basins and the dikes intruded into the crust compositionof the Mesozoicdikes in the northeast betweenthe hinge zone and the edge of the Blue requiresa ponttingof the magmawithin the crest Ridge front. The conduitmodel restrictsthe pos- [Weigand and Ragland, 1970] which could sible origin of the dikes and sills to a single representfractionation along the detachment. region, seawardof the hinge zone. Along their full extent, from Nova Scotia to Alabama, the basaltsand diabasesof the basins,were emplaced CONCLUSIONS synchronously[Sutter, 1988] and are geochemi- cally homogeneous[Puffer, 1984]. A single Flexural rifting modelsincorporating an intra- offshore source region for the mafic material crustal detachmentor a simple shear detachment explainsboth the similarityin timing and chemis- predict footwall uplift and basin formation across try along the 2000 km extent of the basins. In rifts with little or no local thermalanomaly. While addition to the mafic material within the basins, these models are sensitive to the differences there are a large number of early Jurassicdikes between local and flexural compensation,they within the hangingwall block. The majority of cannot be used to determine an accurate value of these dikes were emplaced to the east of the T e, the effective elastic thicknessof the litho- Bevard Zone in the SouthernAppalachians and sphere. seaward of the basin border faults in the mid- A flexurally compensatedsimple shear model t•60 Bell et al.: Early MesozoicRift Basins predicts a gravity high over the hanging wall Knuepfer, and J. Oliver, Overview of the while local compensationwould producea gravity COCORP 40 ø N Transect, western United low. This model is applicablewhere major thrusts States: The fabric of an orogenic belt, Geol. have been normally reactivated.The gravity high Soc. Am. Bull., 98, 308-319, 1987. predictedby the flexurally compensatedmodel is Barton, P. J., Comparisonof deep reflection and similar to the outer gravity high over the hanging refraction structures in the Noah Sea, in wall of the Mesozoic basins of eastern North Reflection Seismology:A Global Perspective, America. This gravity high can be usedto locate Geodyn.Ser., vol. 13, edited by M. Barazangi the region of concentratedextension within the and L. Brown, pp. 297-300, AGU, Washington, lower crest. The locationof the region of concen- D.C., 1986. trated extension that results from modeling the Beaumont, C., C. E. Keen, and R. Boutilier, On Mesozoic basins correspondsto the previously the evolutionof rifted continentalmargins: com- mappedlimits of high stretchedcrest beneaththe parison of models and observationsfor Nova continentalmargin. Scotian margin, Geophys. J. R. Astron. Soc., However, the model doesnot fully accountfor 70, 667-715, 1982. the hangingwall gravity anomalyassociated with British Institutes Reflection Profiling Syndicate the Newark and Gettysburg basins. A mass (BIRPS) and Etude de la CrotateContinentale et excessacross the basin and over the hangingwall Oc6aniquepar R6flexionet R6fractionSismique block is suggestedby both the absenceof a (ECORS), Deep seismic reflection profiling significantgravity low over the basin and a resi- between England, France and Ireland, J. Geol. dual positiveon the proximal hangingwall. The Soc. London, 143, 45-52, 1986. additionof a 2 km thick diabaselayer along the Bosworth, W., J. Lambiase, and R. Keisler, A new detachmentimproves the fit of the model by intro- look at Gregory's rift: The structuralstyle of ducing a positive gravity anomalyskewed toward continental rifting, Eos Trans. AGU, 67, the basin. The addition of this excess mass 577-583, 1986. reducesthe basin gravity signatureand enhances Cook, F.A., Towards an understandingof the the hanging wall high without filling the basin southern Appalachian Piedmont crustal with mafic materialor calling upon large, preexist- transition-A multidisciplinaryapproach, Tecto- ing crustal inhomogeneities. The resultant nophysics,109, 77-92, 1984. double-peakedgravity anomaly closely matches Eyidogan, H., and J. A. Jackson,Aseismological the observedhanging wall and basin anomaliesof study of normal faulting in the Demirci, the Newark and Gettysburg basins. A single Alasehir and Gediz earthquakesof 1969-70 in offshoresource explains the synchronousemplace- westernTurkey: implicationsfor the natureand ment and similar geochemicalsignature of the geometry of deformation in the continental mafic material within the basins. The detachment crust,Geophys. J., 81, 569-607, 198-5. act as a conduit for magma and a probableloca- Froelich, A. J., and P. E. Olsen, Newark Super- tion for differentiation. It also helpsto explainthe group, a revision of the Newark Group in presenceof early Jurassicdikes within the hanging eastern North America, U.S. Geol. Surv. Bull., wall block, their general limitation to the east of 1573A, A55-A58, 1984. the Brevard Zone, and the failure of the basin to Gibbs, A.D., Structural evolution of extensional develop a significantthermal subsidencephase basin margins, J. Geol. Soc. London, 141, subsequentto rift basinformation. 609-620, 1984. Heiskanen,W. A., and F. A. Vening Meinesz, The Acknowledgments.This paper greatly benefited Earth and its Gravity Field, 470 pp., McGraw- from discussionsand reviewsby J. Weissel,W. R. Hill, New York, 1958. Buck, W. C. Burton, W. Bosworth, A. Watts, R. Hellinger, S. J., and J. G. Sclater,Some comments Schlische, P. Olsen, B. Coakley, and an on two-layer extensional models for the anonymousreviewer. This researchwas supported evolution of sedimentarybasins, J. 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