04_GSAmemoir_streepey

Geological Society of AmericaMemoir 197

2004

Exhumation of a collisional orogen: A perspective from the North American Grenville Province

Margaret M. Streepey*Department of Geological Sciences, Florida State University, Tallahassee, Florida 32306-4100, USA

Carolina Lithgow-BertelloniBen A. van der Pluijm

Eric J. EsseneDepartment of Geological Science, University of Michigan, Ann Arbor, Michigan 48109-1063, USA

Jerry F. MagloughlinDepartment of Earth Resources, Colorado State University, Fort Collins, Colorado 80523-1482, USA

ABSTRACTCombined structural and geochronologic research in the southernmost portion of

the contiguous Grenville Province of North America (Ontario and New York State)show protracted periods of extension after the last episode of contraction. TheGrenville Province in this area is characterized by synorogenic extension at ca. 1040Ma, supported by U-Pb data on titanites and 40Ar-39Ar data on hornblendes, followedby regional extension occurring along crustal-scale shear zones between 945 and 780Ma, as recorded by 40Ar-39Ar analysis of hornblende, biotite, and K-feldspar. By ca.780 Ma the southern portion of the Grenville Province, from Ontario to the Adiron-dack Highlands, underwent uplift as a uniform block. Tectonic hypotheses haveinvoked various driving mechanisms to explain the transition from compression toextension; however, such explanations are thus far geodynamically unconstrained.Numerical models indicate that mechanisms such as gravitational collapse and man-tle delamination act over timescales that cannot explain a protracted 300 m.y. exten-sional history that is contemporaneous with ongoing uplift of the Grenville Province.Rather, the presence of a plume upwelling underneath the Laurentian margin, com-bined with changes in regional stress directions, permitted the observed uplift andextension in the Grenville Province during this time. The uplift history, while on aslightly different timescale from those of most plume models, is similar to that seen inmodels of uplift and extension caused by the interaction of a plume with the base ofthe lithosphere. Some of the protracted extension likely reflects the contribution of far-field effects, possibly caused by tectonic activity in other cratons within the Rodiniansupercontinent, effectively changing the stress distributions in the Grenville Provinceof northeastern North America.

Keywords: Grenville, Rodinia, extension

391

*E-mail: [email protected].

Streepey, M.M., Lithgow-Bertelloni, C., van der Pluijm, B.A., Essene, E.J., and Magloughlin, J.F., 2004, Exhumation of a collisional orogen: A perspective fromthe North American Grenville Province, in Tollo, R.P., Corriveau, L., McLelland, J., and Bartholomew, M.J., eds., Proterozoic tectonic evolution of the Grenvilleorogen in North America: Boulder, Colorado, Geological Society of America Memoir 197, p. 391410. For permission to copy, contact [email protected]. 2004 Geological Society of America.

INTRODUCTION

Central to many questions in structural geology and tecton-ics regarding the evolution of orogens is how crust overthick-ened by continental collisions is modified and stabilized after anorogenic event. To understand how the crust evolves after oro-genesis, it is necessary to study ancient mountain belts, the deepcores of which are exposed at the surface today in high-grademetamorphic terranes. Because results of studies of the tempo-ral evolution of such areas give insight into the time and ratesinvolved in crustal stabilization, these results can be used bothto study the general problem of crustal stabilization and to pre-dict the deep behavior of young orogenic belts.

The Grenville Province in northeastern North America isan outstanding, well-studied example of an exposed, deeplyeroded, ancient mountain system. The province is affected by aca. 1.0- to 1.3 billion-year-old set of orogenic events, seen incratonic blocks worldwide, and culminating in the formation ofthe supercontinent Rodinia (Hoffman, 1991; Dalziel, 1997).One of the best continuous exposures of Grenville-aged rocksis in northeastern North America between Labrador, Canada,and New York state, where Grenville deformation is thought tohave occurred in an arc-continent collision at ca. 1.31.2 Gaand a continent-continent collision at ca. 1.11.05 Ga (Mooreand Thompson, 1980; Easton, 1992; Rivers, 1997; Davidson,1998; Carr et al., 2000; McLelland et al., 2001). The terrane ischaracterized by slices of crust that are separated by ductileshear zones in which more of the deformation is concentrated,some of which record normal motion overprinting an earliercontractional history. The colliding craton causing continent-continent collision in this segment of Rodinia is not known, asthe proposed collision with Amazonia has recently been ques-tioned by paleomagnetic evidence (Tohver et al., 2002). Earlyrifting attempts are recorded in some of the blocks of Rodinia(Li et al., 1999; Karlstrom et al., 2000; Dalziel and Soper, 2001;Tack et al., 2001; Timmins et al., 2001). Most major riftingevents involving the eastern Laurentian margin (present-daycoordinates) appear to have occurred in the late Neoprotero-zoic. Rifting in this region resulted in the opening of the Iape-tus Ocean, which has been dated in the north at ca. 600 Ma(Torsvik et al., 1996; Svenningsen, 2001) and in the south at570550 Ma (Torsvik et al., 1996). However, with documentedpulses of rifting having occurred in Baltica, Congo, China, andthe southwestern United States from ca. 900 Ma to ca. 700 Ma,any extensional activity in the eastern Laurentian block, pres-ent-day northeastern North America, during this period mayreflect initial stages of Rodinias breakup (Li et al., 1999; Tan-ner and Bluck, 1999; Streepey et al., 2000; Dalziel and Soper,2001; Timmins et al., 2001). Well-exposed Grenville structuresin North America provide strong constraints on the nature ofextensional activity in the area and also, when compared toextensional activity in other Rodinian blocks that occurred dur-ing roughly the same period, on the processes that control thebreakup of supercontinents. The driving mechanism(s) for

extension in the Laurentian part of the Grenville orogen is theprimary focus of this contribution.

Geologic Setting

One of the continuous exposures of rocks that showsGrenville-aged deformation occurs in North America. The east-ern edge of the belt abuts the edge of the Appalachian thrust frontand is bounded to the west by the Archean Superior Province andother Archean and Proterozoic provinces. Because of the later-ally continuous nature of this belt, it offers an excellent opportu-nity to study lithotectonic relationships in the orogen.

The Grenville Province is composed of lithotectonicallydistinct blocks representing the autochthonous terrains of theLaurentian craton as well as allochthonous blocks accreted tothe Laurentian margin during Grenville orogenesis (Easton,1992; Rivers, 1997; Davidson, 1998; Hanmer et al., 2000; Fig.1, inset). These blocks are separated by major crustal-scale shearzones and contain distinct, smaller domains that are also sepa-rated by major ductile shear zones (e.g., Davidson, 1984; Eas-ton, 1992). A significant amount of strain recorded by theserocks is concentrated into these zones of deformation, whichprovide the key to unraveling the tectonic history of the region.In many cases, these shear zones appear to be multiply active,with the latest episode of deformation recording extension, orappearing to record extensional activity, synchronous toGrenville-aged contractional pulses (Mezger et al., 1991b; Cul-shaw et al., 1994; Busch et al., 1997; Martignole and Reynolds,1997; Ketchum et al., 1998; Streepey et al., 2001). The currentstructural expression of the region is of an extensional terrain,and the challenge then lies in determining both the magnitude,timing, and origin of extension as well as the earlier, contrac-tional history of the area.

In this paper, we focus on the eastern Metasedimentary Beltof the Grenville Province and its boundary with the adjacentGranulite Terrane (Fig. 1). This area spans southeastern Ontarioand northwestern New York state. The Metasedimentary Belt isone of three major crustal slices that comprise the GrenvilleProvince in this region (Fig. 1). It lies between the Gneiss Beltand the Granulite Terrane and contains variably metamorphosed(greenschist to granulite-facies) metasediments, metagranitoids,and metavolcanic rocks (Easton, 1992).

The Metasedimentary Belt contains several small shearzones that juxtapose lithologically and geochronologically dis-tinct domains. These shear zones within the MetasedimentaryBelt dip to the southeast, and the two major boundaries, the Ban-croft shear zone and the Robertson Lake shear zone, show lateextensional motion. The Carthage-Colton shear zone is locatedat the eastern edge of the Metasedimentary Belt, and separatesit from the Granulite Terrane of the Adirondack Highlands. Thisshear zone also shows a late extensional history, but dips shal-lowly to the northwest, creating a grabenlike geometry betweenthe Robertson Lake shear zone and the Carthage-Colton shearzone (Fig. 1).

392 M.M. Streepey et al.

Studies of the deformation histories of these shear zonesrequire a multidisciplinary approach, with emphasis placed onthe field relationships, peak metamorphic pressures and tem-peratures, and the corresponding geochronologic data that con-strain the cooling and exhumation history. Because most of therocks have experienced more than one phase of deformation andmetamorphism, structural relationships in the field can be com-plex, and field analysis alone is not enough to completely con-strain the significance of these boundaries.

This study presents a synthesis of geochronologic informa-tion combined with structural analysis and thermobarometricdata to describe the kinematics of the uplift or exhumation his-tory of a segment of the Grenville Province in northeasternNorth America. A summary of ages is given, adding to regionalcompilations (Cosca et al., 1991, 1992, 1995; Mezger et al.,1991a, 1992, 1993; van der Pluijm et al., 1994). In addition, new40Ar-39Ar ages from amphiboles in the Adirondack Lowlandsand the Adirondack Highlands are presented and further con-strain the geologic history of the area.

Whereas the combination of structural, petrologic, andgeochronologic information is critical to constructing a kine-matic model of the evolution of the region, it does not give a geo-dynamic picture of the development of the late stages ofmodification and stabilization of overthickened crust. Thisinformation allows us to develop reasonable geologic hypothe-ses about timing of late, postorogenic extension and the natureof motion between blocks of crust, but it does not explain thephysical processes behind the evolution. In addition, geochrono-logic data are restricted to lithologies and assemblages that con-tain minerals with the appropriate elements for radiogenicdating. In areas where the appropriate assemblages are not avail-

able, the geochronologic results are limited or incomplete andcannot provide a full, detailed cooling history of the rocks.

In order to develop a more geodynamically complete pic-ture of the exhumation history of the Grenville Province, wehave developed a two-dimensional numerical model of a slice ofcrust representing this region. The structures assigned to themodel are taken directly from field studies in the region, and therheologies are assigned based on existing literature (Ranalli,1995). The numerical models explore possible driving mecha-nisms for the observed phenomenon of extension in this oro-genic belt. From geochronologic and structural information, thetimescales involved in the transition from compression to exten-sion have been evaluated and have placed constraints on theamount of displacement across shear zones. Although how thisorogenic belt extends following collision is known, why itextends is less evident. It remains uncertain whether extensioncan be attributed to a single mechanism, such as gravitationalcollapse, or whether it requires a combination of mechanisms,such as mantle delamination in addition to changes in far-fieldstresses. Whereas numerical models cannot provide constraintsthat uniquely solve this problem, they give insights as to whetheror not proposed mechanisms can act in a way that fits fieldobservations over the period of time dictated by geochronologicconstraints.

GEOCHRONOLOGIC SUMMARY

In studies of ancient metamorphic terranes, motion alongductile shear zones can often be delineated with a combinationof ages that yield information on the timing of latest metamor-phism and ages that record the cooling or exhumation history of

Exhumation of a collisional orogen 393

Figure 1. Generalized map of the Meta-sedimentary Belt (MB) of the GrenvilleProvince (Ontario and New York). Themap shows the Metasedimentary Belt inbetween the Gneiss Belt (GB) and theGranulite Terrane (GT). The Bancroftshear zone (BSZ), Robertson Lake shearzone (RLSZ), and Carthage-Coltonshear zone (CCSZ) are shown in theirmost current expression as normalfaults. Other shear zones shown are theMetasedimentary Belt Boundary Zone(MBBZ) and the Sharbot Lake shearzone. The inset map shows the GrenvilleProvince of northeastern North Americawith the Grenville Front tectonic zone(GFTZ) as it abuts the Archean SuperiorProvince. Other abbreviations: MTMorin terrane; LSZLabelle shearzone. After Streepey et al. (2001).

the terrane (e.g., van der Pluijm et al., 1994). In such studies itis critical to constrain the pressure-temperature (P-T) conditionsof metamorphism in order to determine whether geochronologicages are ages of cooling from a peak metamorphic event orgrowth ages. Minerals yield cooling ages if the peak conditionsof metamorphism are higher than the closure temperatures of theminerals and growth ages if the minerals can be shown to havegrown during metamorphism but at conditions below their clo-sure temperatures. Therefore, in order to best interpret geo-chronologic data in the eastern portion of the MetasedimentaryBelt, it was necessary to initially assess the metamorphic con-ditions of the terrane.

Figure 2, A and B, shows temperature and pressure maps ofthe Metasedimentary Belt from Streepey et al. (1997; thermo-barometric data from references therein). Metamorphism in thearea reached upper-amphibolite to granulite-facies metamor-phism. Maximum temperatures in the study area from just westof the Robertson Lake shear zone to just east of the Carthage-Colton shear zone ranged from 600 to 650 C in and around theRobertson Lake shear zone and increased to the east to 700750C in and around the Carthage-Colton shear zone. Pressureswere 600 to 800 MPa over the region.

In order to best interpret radiometric ages from polymeta-morphic terranes, it is essential not only to have quantitative

394 M.M. Streepey et al.

Pressures (MPa)

< 600

600-700

700-800

> 800

0 30 km

.

Temperatures (C)

>750

700-750

650-700

600-650

550-600

500-550


assessments of P-T conditions but also to have accurate closuretemperatures for minerals used in analysis and some constraintson the diffusion mechanisms active in isotopic resetting. For thisstudy we consider volume diffusion in grains to be the primarymechanism of isotopic resetting. In addition, published andwidely used closure temperatures for the minerals titanite, horn-blende, biotite, and K-feldspar are considered appropriate for thisstudy of a slowly cooled, regionally metamorphosed terrane(titanite: 600700 C, Mezger et al., 1991a, Scott and St-Onge,1995; hornblende: 480500 C, McDougall and Harrison, 1999;biotite: 300 C, McDougall and Harrison, 1999; K-feldspar: 150to 300 C, Zeitler, 1987, McDougall and Harrison, 1999, Loveraet al., 1991). Because peak temperatures of regional metamor-phism are close to or generally exceed the closure temperature oftitanite in the U-Pb system, the U-Pb ages of titanite constraineither the timing of latest metamorphism or cooling ages veryclose to the timing of peak metamorphism in this area. The 40Ar-39Ar ages of hornblende, biotite, and K-feldspar, which havelower closure temperatures than titanite, constrain most of thecooling history of the study area and are considered to have closedto the K-Ar system at some time after peak metamorphism.

The U-Pb ages from zircon, garnet, monazite, and titanitehave been determined for the Metasedimentary Belt in numer-ous studies (Mezger et al., 1993; Corfu and Easton, 1995, 1997;Perhsson et al., 1996; Wasteneys et al., 1999; Corriveau and vanBreemen, 2000; McLelland et al., 2001). We provide a briefsummary of the metamorphic ages of the Metasedimentary Belt;the reader is referred to reviews by Rivers (1997), Carr et al.(2000), Hanmer et al. (2000), and McLelland et al. (2001) fordetailed descriptions of the early metamorphic history of theMetasedimentary Belt. The Metasedimentary Belt yields infor-mation on two major periods of Grenville-aged orogenesis thatrepresent an arc accretion event from ca. 13001190 Ma (theElzevirian orogeny), culminating in a continent-continent colli-sion at ca. 10801020 Ma (the Ottawan orogeny). These eventsrepresent two episodes of contraction, possibly separated byextensional events, which are recorded by magmatic activity andemplacement of large anorthosite complexes during these peri-ods (McLelland et al., 1988; Davidson, 1995). The RobertsonLake shear zone and the Carthage-Colton shear zone separaterocks showing a marked discontinuity in metamorphic age, andso are fundamental boundaries separating blocks with distinctmetamorphic histories. From the Metasedimentary Belt bound-ary thrust zone east to the Robertson Lake shear zone (encom-passing the Bancroft and Elzevir terranes), metamorphicminerals record geochronological evidence of metamorphismduring the latest contractional event during the Ottawan orogeny.

The Frontenac terrane (including the Adirondack Low-lands) from east of the Robertson Lake shear zone to theCarthage-Colton shear zone shows evidence of metamorphismrelated to the earlier Elzevirian orogeny. It does not, however,appear to record metamorphic ages related to the Ottawanorogeny, indicating that this terrane was either laterally sepa-rated from the Elzevir terrane during this period or was at shal-

low crustal levels (Mezger et al., 1993; Streepey et al., 2000,2001). However, some investigators have proposed that at leastportions of the Adirondack Lowlands may have been deformedduring the Ottawan orogeny (Wasteneys et al., 1999), and moredetailed isotopic work may be useful in resolving the extent andnature of Ottawan deformation in the Lowlands. East of theCarthage-Colton shear zone, peak metamorphism in the Adiron-dack Highlands occurred during the Ottawan orogeny, althoughthere is some evidence that this terrane was also deformed dur-ing the Elzevirian orogeny (McLelland et al., 1988; McLellandand Chiarenzelli, 1989; Kusky and Lowring, 2001). In this scenario, the Frontenac terrane, bounded by the east-dippingRobertson Lake shear zone and the west-dipping Carthage-Colton shear zone, is a block of crust that has been largely protected from the Ottawan pulse of orogenesis, while theAdirondack Highlands and the Elzevir terrane recorded meta-morphic ages during this period.

The cooling history of the terranes immediately adjacent tothese two shear zones is discussed in order to evaluate their sig-nificance as postorogenic extensional structures. The ages com-piled in this study include ages determined from regional U-Pband 40Ar-39Ar geochronologic data (Busch and van der Pluijm,1996; Busch et al., 1996b, 1997; Streepey et al., 2000, 2001,2002, and new results) to provide a complete cooling history forthe eastern half of the Metasedimentary Belt. Sample locationsand their corresponding ages are given in Table 1. The U-Pb agesof titanite give the timing of early deformation, thought to havesome transpressive component along both the east-dippingRobertson Lake shear zone and the west-dipping Carthage-Colton shear zone. The 40Ar-39Ar ages of hornblendes, biotites,and K-feldspars, combined with structural analysis of shearzones, record the timing of later extensional motion along bothshear zones. New 40Ar-39Ar ages of hornblendes further detailthe cooling history of the crust adjacent to the Carthage-Coltonshear zone in northwest New York state.

Robertson Lake Shear Zone

The Robertson Lake shear zone is a multiply active, east-dipping shear zone separating the eastern Elzevir terrane (Maz-inaw domain) and the western Frontenac terrane (Sharbot Lakedomain) within the Metasedimentary Belt (Fig. 1; Easton,1988). The latest episode of motion recorded along this zone isdown-to-the-east, as shown by shear-sense indicators includingS-C, CC fabrics, with these crystal plastic structures crosscutby brittle fabrics delineating an uplift history during deforma-tion (Busch and van der Pluijm, 1996). Its early history recordsa transpressive event at ca. 1030 Ma, indicating imbrication viasinistral transpression of the Mazinaw and the Sharbot Lakedomains (Busch et al., 1997). 40Ar-39Ar cooling ages of horn-blendes and micas in the Robertson Lake shear zone show amarked difference across the zone. Combined with the structuralinformation and the nature of the offset, extensional motion hadto have occurred to juxtapose the crustal blocks on either side of

TAB

LE 1

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PLE

LOCA

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NSH

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ende

Biot

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mpl

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age

Dom

ain

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Coo

rdin

ates

UTM

Coo

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ates

From

Bus

ch e

t al.(

1996

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orth

ing

East

ing

RVL1

18A

1011

Ma

n.d

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arbo

t Lak

e49

7122

537

2600

RVL2

0A12

05 M

an.d

.Sh

arbo

t Lak

e49

7305

036

7025

CDN5

894

7 M

an.d

.El

zevi

r/Maz

inaw

4968

970

3560

50M

TG72

952

Ma

n.d

.El

zevi

r/Maz

inaw

4957

060

3553

00

From

Bus

ch a

nd v

an d

er P

luijm

(199

6)N

orth

ing

East

ing

RVL1

18B

n.d

.96

9 M

aSh

arbo

t Lak

e49

7122

537

2600

LVT1

30B

n.d

.90

1 M

aEl

zevi

r/Maz

inaw

4990

750

3619

20

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9 M

aLo

wla

nds

1 m

i.SW

of P

ierre

pont

on

Rt.2

.Out

crop

on

E sid

e of

road

.

From

Stre

epey

et a

l.(20

01)

Latit

ude

Long

itude

RWS-

110

00 M

an.d

.W

ithin

CCS

ZN

orth

44

deg.

, 22

min

.W

est

75

deg.

, 10

min

.RW

S-3c

999

Ma

n.d

.W

ithin

CCS

ZN

orth

44

deg.

, 22

min

.W

est

75

deg.

, 10

min

.EA

198

1 M

an.d

.W

ithin

CCS

ZN

orth

44

deg.

, 22

min

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est

75

deg.

, 10

min

.D

H98

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8 M

an.d

.W

ithin

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orth

44

deg.

, 22

min

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est

75

deg.

, 10

min

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H-1

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Ma

n.d

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ithin

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orth

44

deg.

, 22

min

.W

est

75

deg.

, 10

min

.CR

7-DH

198

9 M

an.d

.W

ithin

CCS

ZD

ana

Hill

outc

rop:

On

N sid

e of

Dan

a Hi

ll Rd.

CR1-

DH1

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Ma

n.d

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ithin

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ZD

ana

Hill

outc

rop:

On

N sid

e of

Dan

a Hi

ll Rd.

CR3-

8794

1 M

an.d

.W

ithin

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8 km

N o

f int

erse

ctio

n be

tween D

ana

Hill

Rd.

and

Rt.8

7, o

n Rt

.87.

CR2-

A294

8 M

an.d

.W

ithin

CCS

ZD

irect

ly ac

ross

Rt.

87 fr

om C

R3-8

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H2-

810

12 M

an.d

.W

ithin

CCS

ZD

irect

ly ac

ross

Rt.

87 fr

om C

R3-8

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6-A4

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Ma

n.d

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ithin

CCS

ZD

irect

ly ac

ross

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87 fr

om C

R3-8

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499

8 M

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.W

ithin

CCS

ZD

irect

ly ac

ross

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87 fr

om C

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9-A4

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Ma

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ithin

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ZD

irect

ly ac

ross

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87 fr

om C

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87-5

b10

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ithin

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irect

ly ac

ross

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87 fr

om C

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7.n.d

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dat

a;CC

SZ

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age-

Colto

n sh

ear z

one;U

TM

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rsal T

ransv

ers

e M

erca

tor.

the shear zone (Busch et al., 1996b; Fig. 3). The timing of thetransition from compression to extension can be somewhat con-strained by the age of the latest transpressive event in the regionat ca. 1030 Ma (Busch et al., 1997), but cannot be directly deter-mined. The termination of extension across the Robertson Lakeshear zone cannot be constrained from 40Ar-39Ar analyses ofhornblende and biotite, as both show differences of ca. 70100Ma across the zone. Results from the analysis of K-feldspar inthe region suggest that the entire block was uplifting uniformlyby 780 Ma, suggesting termination of extension between 900and 780 Ma (Streepey et al., 2002).

Ages taken from Busch and van der Pluijm (1996), Buschet al. (1996b, 1997), and Streepey et al. (2002) are compiled inFigure 3. These data constrain the cooling history of the terranefrom immediately after orogenesis in the Robertson Lake shearzone area to the time at which the terrane was uplifting as a uni-form block. In addition, the geometry of normal fault motionand the amount of displacement along the shear zone are evident(Busch et al., 1996a). The U-Pb ages from titanites documentthe difference in metamorphic ages between the Mazinaw andthe Sharbot Lake domains. As the closure temperature of titan-ite is ca. 600700 C (Mezger et al., 1991a; Scott and St-Onge,1995), titanite ages give the timing of metamorphism or a cool-ing age that is very close to the age of metamorphism in theamphibolite to granulite-facies rocks of the Elzevir and theFrontenac terranes. Metamorphism in the Sharbot Lake domain(the hangingwall block of the Robertson Lake shear zone)occurred at ca. 1140 to 1170 Ma (Mezger et al., 1993; Corfu andEaston, 1995; Busch et al., 1997). These ages are generally sim-ilar to those across the entire width of the Frontenac terrane(Mezger et al., 1993). However, the youngest metamorphic agesin the Mazinaw domain (the footwall block of the RobertsonLake shear zone) range from ca. 1010 Ma to 1050 Ma (Mezgeret al., 1993; Busch et al., 1997), showing a ca. 100-m.y. differ-ence in the timing of the latest metamorphic event across thisboundary. Transpressional activity is interpreted to have causedjuxtaposition through imbrication of the two terranes at ca. 1030Ma (Busch et al., 1997).

The 40Ar-39Ar cooling ages of hornblendes, micas, and K-feldspars constrain the post-titanite cooling and unroofing his-tory of the blocks of crust adjacent to the Robertson Lake shearzone and constrain the timing and nature of postorogenic exten-sion along this zone. Hornblende ages for rocks in the vicinityof the Robertson Lake shear zone are shown in Figure 3. Theoffset in ages shown by the titanite U-Pb geochronology is alsoshown by the cooling ages of hornblende, with hornblende agesof ca. 1010 Ma in the Sharbot Lake domain (hangingwall),which are at least 60 m.y. older than rocks immediately acrossthe Robertson Lake shear zone in the Mazinaw domain, wherehornblende ages are ca. 950 Ma (Busch and van der Pluijm,1996; Busch et al., 1996a). Biotite, which closes to the K-Ar sys-tem at ca. 300 C, continues to show an offset across the Robert-son Lake shear zone, with biotites in the Sharbot Lake domainrecording ages of 970 Ma that are at least 70 m.y. older than the

900 Ma biotite ages in the Mazinaw domain (Busch and van derPluijm, 1996). At the time of biotite closure, rocks were at fairlyshallow crustal depths of 10 to 12 km assuming an averagegeothermal gradient. Because of the offsets in cooling ages ofthe rocks that are presently exposed at the surface crustal level,it is clear that extensional activity did not terminate across theRobertson Lake shear zone until sometime after 900 Ma.

The 40Ar-39Ar ages of K-feldspars give some informationon the termination of extension along the Robertson Lake shearzone, but do not completely constrain it. Unlike hornblendes ormicas, which are considered to have one diffusion domain andtherefore a single closure temperature, K-feldspars are thoughtto have multiple diffusion domains and therefore multiple clo-sure temperatures (Lovera et al., 1991). Analysis of K-feldsparsgives, instead of a single age, a temperature-time path for thegrain. Thermal modeling of K-feldspar spectra from the Mazi-naw and the Sharbot Lake domains show that the two domainswere juxtaposed by at least 780 Ma, meaning that the termina-tion of extension across the Robertson Lake shear zone musthave occurred between 900 Ma and 780 Ma (Streepey et al.,2002). Thus, postorogenic extension across the Robertson Lakeshear zone terminated between 140 and 260 m.y. after the finalexpression of contractional tectonics in the area at ca. 1040 Ma.

CARTHAGE-COLTON SHEAR ZONE

The Carthage-Colton shear zone separates the eastern Fron-tenac terrane (Adirondack Lowlands) from the Granulite Ter-rane (Adirondack Highlands; Fig. 1). It is 150 km east of the Robertson Lake shear zone and dips to the west, toward theRobertson Lake shear zone. With this geometry, the Frontenacterrane is a grabenlike block bounded by two shear zones dip-ping toward one another. Upper amphibolite-facies marbles and other metasediments dominate the Adirondack Lowlandslithologies, whereas the Adirondack Highlands are comprisedpredominantly of granulite-facies metaigneous assemblages.The Carthage-Colton shear zone crops out as a zone of intensedeformation between the two terranes, although the exact loca-tion of the boundary has been debated (Geraghty et al., 1981).

The Carthage-Colton shear zone had a long history of activ-ity over the duration of the Grenville orogenic cycle. TheAdirondack Lowlands and Highlands both appear to have beenaffected by the ca. 1190 Ma arc-continent collision at the end ofthe Elzevirian orogeny (Mezger et al., 1991a, 1992; Wasteneyset al., 1999; Kusky and Lowring, 2001). However, only theAdirondack Highlands appear to have been pervasively meta-morphosed by the granulite-facies Ottawan orogeny, which hasbeen dated at 10901040 Ma (McLelland et al., 1996). Theentire Frontenac terrane, from east of the Robertson Lake shearzone to just west of the Carthage-Colton shear zone, appears tohave escaped widespread thermal metamorphism and resettingof isotopic ages from this pervasive deformational event, eitherby being at shallower crustal levels during that period or bybeing laterally separated.


Figu

re 3

. M

aps

show

ing

horn

blen

de,

biot

ite,

and

K-

feld

spar

40A

r-39 A

r age

s acr

oss t

he R

ober

tson

Lake

she

arzo

ne

(RLS

Z) a

nd t

he C

artha

ge-C

olton

she

ar zo

ne(C

CSZ)

. Acr

oss t

he R

LSZ,

the a

ges a

re fr

om B

usch

et al

.(19

96),

Busch

and v

an d

er P

luijm

(199

6), an

d Stre

epey

et a

l. (20

02). H

ornble

nde a

ges a

re ita

licize

d. A

cros

s the

CCSZ

, age

s are

from

Stre

epey

et a

l. (20

00, 2

001,

2002

)an

d ne

w r

esults

. Pai

rs s

epar

ated

by

a co

mm

a in

dica

teho

rnbl

ende

and

bio

tite

ages

from

the

sam

e sa

mpl

e. T

heag

e dist

ributio

ns ac

ross

the R

LSZ

indi

cate

mot

ion

alon

gth

e sh

ear z

one

afte

r ca.

900

Ma

and

befo

re c

a. 78

0 M

a.Th

e ag

e di

strib

utio

ns a

cros

s th

e CC

SZ in

dica

te d

efor

-m

atio

n be

twee

n ca

. 950

and

930

Ma,

mos

t lik

ely

at c

a.94

5 M

a. D

H-1

fie

ld o

utcr

op n

ame;

GB

Gne

iss B

elt;

GFT

ZG

renv

ille

Fron

t tec

toni

c zo

ne; G

TG

ranu

lite

Terr

ane;

MB

M

etas

edim

enta

ry B

elt;

MBB

ZM

eta-

sedi

men

tary

Bel

t Bou

ndar

y Zon

e; M

SZ

Moo

rton s

hear

zone;

MT

Mor

in t

erra

ne;

RW

field

out

crop

nam

e;SL

SZ

Shar

bot L

ake

shea

r zon

e.


McLelland et al. (1996) proposed that the Carthage-Coltonshear zone was the locus for extensional collapse of the Ottawanorogen at ca. 1050 Ma, which resulted in exhumation of thehigh-grade core of the Adirondack Highlands, presumably whilethe orogen was still under a contractional stress field and as adirect result of the orogenic event, either through gravitationalcollapse or through mantle delamination beneath the orogen.Streepey et al. (2001) suggested that the Carthage-Colton shearzone was involved in transpressive deformation similar to thatdocumented across the Robertson Lake shear zone at ca. 1040Ma. It is clear that the Carthage-Colton shear zone was activeduring or immediately after the latest episode of Grenville con-traction. In addition, cooling ages from hornblendes and biotitesshow that the Carthage-Colton shear zone was reactivated in anextensional regime similar to that observed along the RobertsonLake shear zone.

We present sixteen new 40Ar-39Ar hornblende analysesfrom the University of Michigan Radiogenic Isotope Laboratorycombined with published 40Ar-39Ar hornblende and biotite agesnear and along the Carthage-Colton shear zone (Streepey et al.,2000, 2001). Standard operating procedures for the collection ofhornblende and biotite analyses in this laboratory are describedin detail in Streepey et al. (2000). Our results are shown in Fig-ure 3, and isotopic data are presented in Table 2.1 Plateaus weredefined as occurring where 50% or more of the total 39Ar wasreleased in three or more consecutive steps and where the agesof the steps overlapped at the 2 level of error.

The hornblende ages in the Adirondack Lowlands of theFrontenac terrane show that these locations within this slice ofcrust reached 500 C at ca. 1036 Ma (Fig. 3; Table 2). Horn-blende ages across the Carthage-Colton shear zone are 947983Ma. The offset in ages indicates active movement along theCarthage-Colton shear zone after hornblendes closed to the K-Ar system, or after 950 Ma. The nature of the offset, combinedwith the regional fabrics, indicates that this motion must havebeen extensional (Heyn, 1990; Streepey et al., 2000). Somehornblendes in the Dana Hill metagabbro tightly cluster at 945Ma. Though there is some textural complexity in these samples,with young ages coming from a variety of veins and other tex-tures, these ages fit well within the regional framework of exten-sion (bracketed by regional hornblende and biotite 40Ar-39Arages) and indicate that this was likely a time of deformationalong the Carthage-Colton shear zone (Streepey et al., 2001).Biotite 40Ar-39Ar ages are ca. 900930 Ma on both sides of theshear zone, indicating that this block of crust was uniformlyuplifting by this time (Streepey et al., 2000). K-feldspar ages aresimilar to those found in the Robertson Lake shear zone area,further supporting the idea that the entire Metasedimentary Belt

was uplifting as a uniform block by ca. 780 Ma (Heizler andHarrison, 1998; Streepey et al., 2002).

NUMERICAL MODELING

Geochronology paired with structural geology shows thatextensional motion took place along a large segment of theMetasedimentary Belt well after the Grenville contractionalorogeny, during a period that was considered to be relatively qui-escent. Application of 40Ar-39Ar and U-Pb geochronologydetails the kinematics and timing of this transition and theamount and nature of extensional deformation that occurredafter it, but gives few constraints on the mechanisms that pro-duced a regional extensional event during this period.

Extension occurred at least 100 m.y. after the latest con-tractional event in the Grenville Province of New York andsoutheastern Ontario. Given this timescale, it is difficult to makea causal link between orogenesis and extension. Whereas sev-eral attempts have been made to create a Himalayan analog tothe Grenville Province (e.g., Windley, 1986), a component ofmajor extension in the latter is clearly postorogenic, so thetimescales for the two are different. In Tibet, for example, grav-itational potential energy due to the elevated topography of theplateau played a major role in driving collapse (e.g., Shen et al.,2001). However, in the Grenville Province the duration and ter-mination of this extensional event were so much later than com-pression that gravitational collapse became an ineffectivedriving mechanism. Earlier synorogenic extensional events (asproposed by McLelland et al., 2001, and references therein) atca. 1050 Ma were different in nature and almost certainly can beascribed to processes such as gravitational collapse, mantledelamination, or some combination of these driving mecha-nisms. The exhumation of the Grenville orogen was not solelythe result of erosional and isostatic processes, but was due atleast in part to active extension along shear zones, which led touplift of the region. Geochronologic data have allowed con-struction of a kinematic model of denudation and unroofing ofmidcrustal levels of the orogenic belt. When uplift is discussedin the context of this paper, we refer to the exhumation of thecore of the Grenville Province and do not constrain a measureof the paleotopographic surface.

A model investigation has been made of the evolution of ablock of overthickened crust, with three common driving mech-anisms proposed to explain postorogenic extension. We haveevaluated the driving forces necessary to generate uplift andextension that match the kinematics of deformation in theGrenville Province as documented through field and laboratorystudies. Two critical field observations in this area that must beexplained are the continuous regional uplift during the time ofextension and the time lag in termination of extensional motionalong the Robertson Lake shear zone versus the Carthage-Colton shear zone detailed in the geochronology (see earlier andFig. 3).

1GSA Data Repository item 2004059, Appendix, hornblende spectra, is avail-able on request from Documents Secretary, GSA, P.O. Box 9140, Boulder, CO80301-9140, USA, [email protected], or at www.geosociety.org/pubs/ft2004.htm.

TAB

LE 2

.HO

RNBL

ENDE

ARG

ON

DATA

Pow

er

39Ar

frac

39Ar

mol

40Ar

/39Ar

37Ar

/39Ar

36Ar

/39Ar

40Ar

*/39 A

r K%

40Ar

atm

osCa

/KAg

e (M

a)1

err

or

(Ma)

A112

-140

00.

1797

71.

33E-

1582

.671

663.

6246

90.

0103

479

.615

13.

6972

36.

6508

110

442

401

0.12

911

9.53

E-16

82.1

3706

3.89

560.

0005

081

.989

30.

1799

07.

1478

910

682

402

0.04

571

3.37

E-16

80.9

9802

3.88

507

0.

0002

581

.070

60.

0896

17.

1285

710

593

420

0.12

908

9.52

E-16

80.8

0290

3.97

513

0.

0001

080

.831

80.

0357

67.

2938

210

562

440

0.04

952

3.65

E-16

79.8

0750

3.82

902

0.

0005

079

.956

40.

1865

77.

0257

210

483

460

0.06

496

4.79

E-16

80.3

2021

3.94

636

0.

0006

980

.524

70.

2545

97.

2410

310

533

480

0.01

616

1.19

E-16

77.8

2671

3.64

937

0.

0024

778

.557

60.

9391

36.

6960

910

349

560

0.08

421

6.21

E-16

79.8

6897

3.63

473

0.00

025

79.7

954

0.09

212

6.66

923

1046

364

00.

1007

67.

43E-

1680

.251

004.

0008

10.

0000

480

.238

70.

0153

27.

3409

410

503

720

0.03

772

2.78

E-16

79.2

4898

3.52

251

0.00

293

78.3

825

1.09

337

6.46

332

1032

680

00.

0386

32.

85E-

1681

.537

494.

1369

10.

0009

381

.261

50.

3384

97.

5906

610

615

880

0.01

055

7.78

E-17

80.5

4386

4.17

951

0.00

409

79.3

344

1.50

162

7.66

883

1041

1410

000.

0642

74.

74E-

1680

.992

303.

9166

20.

0007

480

.772

70.

2711

47.

1864

610

563

1040

0.02

004

1.48

E-16

78.7

2645

3.90

584

0.

0017

379

.236

80.

6482

67.

1666

810

408

4800

0.02

954

2.18

E-16

80.0

8239

3.91

963

0.

0017

380

.594

40.

6393

57.

1919

810

546

J va

lue

9.84

667E

-03

1.

3818

1E-0

5To

tal 3

9 K v

ol =

2.12

529E

-10

CCNT

P/G

Tota

l gas

age

=10

51.9

7

1.39

196

Ma

A114

-236

00.

0087

16.

42E-

1743

7.13

157

4.65

082

1.19

085

85.2

341

80.5

015

8.53

361

1091

5640

00.

0035

72.

63E-

1784

.831

182.

6434

10.

0696

764

.245

224

.267

04.

8502

987

771

440

0.01

623

1.20

E-16

78.7

9309

2.49

256

0.05

687

61.9

881

21.3

280

4.57

350

853

1748

00.

0342

22.

53E-

1675

.257

002.

5539

30.

0073

973

.072

22.

9031

24.

6861

197

08

520

0.07

799

5.75

E-16

76.4

3780

2.69

612

0.00

273

75.6

316

1.05

471

4.94

701

996

456

00.

1064

27.

85E-

1676

.703

642.

7159

80.

0010

276

.403

20.

3916

94.

9834

510

043

600

0.19

840

1.46

E-15

76.4

4511

2.70

967

0.00

100

76.1

492

0.38

709

4.97

187

1001

264

00.

0650

44.

80E-

1675

.725

332.

7073

30.

0001

775

.675

50.

0658

04.

9675

899

74

680

0.05

337

3.94

E-16

74.3

6980

2.69

303

0.00

144

73.9

429

0.57

402

4.94

134

979

472

00.

1441

91.

06E-

1575

.233

562.

7618

20.

0004

575

.101

80.

1751

35.

0675

699

13

760

0.00

258

1.90

E-17

74.3

0312

2.82

691

0.00

236

73.6

062

0.93

794

5.18

699

976

5784

00.

2090

91.

54E-

1575

.251

752.

7319

80.

0007

275

.037

70.

2844

55.

0128

199

02

880

0.04

428

3.27

E-16

75.1

3989

2.76

550.

0000

575

.123

80.

0214

25.

0743

199

16

2000

0.03

130

2.31

E-16

77.0

6652

3.00

146

0.00

076

76.8

428

0.29

030

5.50

727

1008

650

000.

0046

23.

41E-

1780

.452

623.

7787

40.

0309

371

.312

811

.360

56.

9334

795

240

J va

lue

9.74

593E

-03

1.

6958

8E-0

5To

tal 3

9 K v

ol =

1.50

293E

-10

CCNT

P/G

Tota

l gas

age

=99

2.40

3

1.77

968

Ma

A117

-1R

400

0.06

297

4.65

E-16

113.

3235

55.

0468

90.

0467

599

.509

312

.190

19.

2603

512

263

440

0.13

070

9.64

E-16

74.1

2110

4.74

351

0.00

057

73.9

514

0.22

895

8.70

369

982

348

00.

1721

21.

27E-

1574

.658

904.

7491

30.

0006

074

.480

70.

2386

98.

7140

987

252

00.

1083

47.

99E-

1673

.440

854.

7365

10.

0015

572

.981

60.

6253

48.

6908

497

23

560

0.08

989

6.63

E-16

71.9

3891

4.77

110.

0019

771

.356

30.

8098

78.

7543

195

54

600

0.05

587

4.12

E-16

71.2

9738

4.84

565

0.00

070

71.0

905

0.29

017

8.89

110

952

564

00.

0470

13.

47E-

1670

.750

444.

8344

80.

0025

969

.985

21.

0816

18.

8706

194

14

680

0.02

548

1.88

E-16

70.3

0706

4.75

113

0.00

502

68.8

222

2.11

197

8.71

767

929

476

00.

0808

65.

97E-

1674

.379

644.

8044

40.

0020

573

.775

20.

8126

58.

8154

998

03

880

0.04

753

3.51

E-16

72.8

9371

4.88

858

0.00

407

71.6

920

1.64

858

8.96

987

959

610

000.

1361

51.

00E-

1573

.203

745.

3166

30.

0002

573

.130

00.

1007

49.

7552

897

32

2000

0.04

257

3.14

E-16

73.6

1771

5.22

413

0.00

348

72.5

902

1.39

574

9.58

556

968

448

000.

0005

33.

93E-

1898

.697

215.

9871

10.

0895

072

.250

526

.795

810

.985

5296

439

0J

valu

e9.

7801

9E-0

3

1.65

238E

-05

Tota

l 39 K

vol =

1.44

533E

-10

CCNT

P/G

Tota

l gas

age

=98

7.86

9

1.61

561

Ma

A124

-1R

360

0.03

019

2.23

E-16

125.

3363

64.

2134

60.

0474

311

1.32

2011

.181

47.

7311

213

385

400

0.17

033

1.26

E-15

80.5

0973

3.98

635

0.00

127

80.1

340

0.46

669

7.31

440

1051

240

10.

1096

88.

09E-

1680

.733

033.

8988

20.

0014

880

.296

00.

5413

37.

1538

010

533

402

0.04

250

3.14

E-16

81.2

5107

3.85

999

0.00

243

80.5

319

0.88

512

7.08

255

1055

542

00.

0126

39.

32E-

1779

.317

043.

9584

50.

0144

275

.055

85.

3724

17.

2632

110

0014

460

0.21

815

1.61

E-15

81.2

0917

3.91

409

0.00

086

80.9

539

0.31

434

7.18

182

1060

246

10.

0809

95.

98E-

1681

.154

363.

8917

30.

0017

680

.635

70.

6391

17.

1407

910

563

480

0.10

678

7.88

E-16

81.5

2008

3.83

570.

0004

181

.398

60.

1490

27.

0379

810

642

520

0.04

500

3.32

E-16

80.2

3726

3.91

163

0.00

240

79.5

267

0.88

558

7.17

730

1045

456

00.

0218

31.

61E-

1679

.204

674.

0385

80.

0024

978

.468

10.

9299

57.

4102

410

358

600

0.00

773

5.70

E-17

77.3

9229

4.28

706

0.00

899

74.7

364

3.43

173

7.86

617

997

2264

00.

0116

38.

58E-

1778

.559

424.

5639

70.

0019

277

.991

60.

7227

98.

3742

610

3014

720

0.03

076

2.27

E-16

79.9

5281

4.34

586

0.

0001

079

.981

70.

0361

47.

9740

610

506

800

0.02

660

1.96

E-16

80.7

8376

4.17

887

0.00

348

79.7

551

1.27

335

7.66

765

1048

812

000.

0568

54.

19E-

1681

.004

874.

0396

0.00

053

80.8

471

0.19

477

7.41

211

1058

440

000.

0283

72.

09E-

1681

.852

684.

0990

70.

0008

582

.104

80.

3080

27.

5212

310

716

J va

lue

9.87

089E

-03

1.

2059

4E-0

5To

tal 3

9 K v

ol =

2.58

136E

-10

CCNT

P/G

Tota

l gas

age

=10

63.4

3

1.33

794

Ma

A125

-140

00.

2539

01.

87E-

1584

.451

713.

9048

50.

0064

782

.540

22.

2634

37.

1648

610

562

401

0.07

938

5.86

E-16

84.0

4833

3.75

689

0.00

251

83.3

075

0.88

143

6.89

338

1063

444

00.

1303

89.

62E-

1683

.997

653.

7257

50.

0013

683

.594

60.

4798

36.

8362

410

663

480

0.10

856

8.01

E-16

83.7

6073

3.70

631

0.00

069

83.5

557

0.24

478

6.80

057

1065

352

00.

2233

21.

65E-

1584

.004

993.

8024

0.00

102

83.7

027

0.35

984

6.97

688

1067

256

00.

0858

36.

33E-

1685

.409

033.

8426

20.

0009

885

.119

20.

3393

57.

0506

810

804

600

0.00

564

4.16

E-17

77.5

5969

4.87

365

0.00

266

76.7

725

1.01

495

8.94

248

999

3468

00.

0078

35.

77E-

1784

.759

364.

4357

30.

0130

388

.609

74.

5426

78.

1389

511

1329

820

0.01

984

1.46

E-16

83.3

0916

4.05

837

0.

0082

585

.746

82.

9260

27.

4465

510

8614

1000

0.01

869

1.38

E-16

83.4

4706

4.02

577

0.

0084

085

.929

52.

9748

77.

3867

310

8814

4800

0.06

665

4.92

E-16

83.6

6405

4.09

005

0.00

077

83.4

373

0.27

103

7.50

468

1064

4J

valu

e9.

6330

2E-0

3

1.87

146E

-05

Tota

l 39 K

vol=

1.26

569E

-10

CCNT

P/G

Tota

l gas

age

=10

65.0

8

1.92

921

Ma

A128

-136

00.

0200

31.

48E-

1610

3.37

114

0.86

044

0.05

595

86.8

393

15.9

927

1.57

879

1114

538

00.

0097

67.

20E-

1745

.937

180.

5593

70.

0011

845

.587

20.

7618

61.

0263

666

814

420

0.00

899

6.63

E-17

85.6

7338

1.20

975

0.00

381

84.5

478

1.31

382.

2197

210

9213

(conti

nued

)

TAB

LE 2

.Co

ntin

ued

Pow

er

39Ar

frac

39Ar

mol

40Ar

/39Ar

37Ar

/39Ar

36Ar

/39Ar

40Ar

*/39 A

r K%

40Ar

atm

osCa

/KAg

e (M

a)1

err

or

(Ma)

460

0.01

472

1.09

E-16

95.1

5639

2.10

311

0.00

229

94.4

805

0.71

029

3.85

892

1186

1150

00.

0614

54.

53E-

1683

.472

604.

6535

10.

0006

783

.275

50.

2361

38.

5385

510

802

520

0.16

436

1.21

E-15

78.3

9011

4.93

948

0.00

060

78.2

127

0.22

632

9.06

327

1029

153

00.

0604

04.

46E-

1678

.093

544.

6488

90.

0006

277

.910

10.

2349

08.

5300

710

262

540

0.02

521

1.86

E-16

71.3

4198

3.67

143

0.00

056

71.1

764

0.23

209

6.73

657

957

556

00.

0405

92.

99E-

1675

.791

474.

5035

10.

0000

275

.786

10.

0070

98.

2633

210

053

620

0.10

396

7.67

E-16

76.9

1635

4.80

667

0.00

048

76.7

755

0.18

312

8.81

958

1015

266

00.

1161

18.

57E-

1678

.493

214.

9517

40.

0005

578

.329

70.

2083

19.

0857

610

312

720

0.11

533

8.51

E-16

79.9

5906

5.11

697

0.00

065

79.7

680

0.23

895

9.38

894

1045

278

00.

0867

66.

40E-

1680

.744

144.

8914

40.

0006

580

.551

10.

2390

88.

9751

210

532

1000

0.06

995

5.16

E-16

80.5

6078

4.96

834

0.00

099

80.2

677

0.36

380

9.11

622

1050

240

000.

1023

97.

55E-

1666

.014

293.

4490

60.

0013

465

.619

60.

5978

86.

3285

589

91

J va

lue

9.83

709E

-03

1.

4414

8E-0

5To

tal 3

9 K v

ol =

2.63

518E

-10

CCNT

P/G

Ttot

al g

as a

ge =

1022

.11

1.

2886

7 M

a

A129

-136

00.

0233

61.

72E-

1697

.701

206.

7107

0.05

614

81.1

131

16.9

784

12.3

1321

1044

1150

00.

5648

54.

17E-

1584

.981

446.

6316

10.

0005

384

.823

80.

1855

012

.168

0910

792

540

0.09

031

6.66

E-16

84.3

7694

6.50

400.

0012

084

.731

70.

4204

511

.933

9410

794

580

0.10

879

8.03

E-16

85.2

8425

6.53

173

0.00

166

84.7

932

0.57

578

11.9

8483

1079

362

00.

0209

31.

54E-

1685

.023

556.

4932

30.

0009

184

.755

50.

3152

711

.914

1810

7913

1000

0.12

513

9.23

E-16

82.4

3000

6.78

991

0.00

165

81.9

417

0.59

238

12.4

5855

1052

320

000.

0635

14.

69E-

1684

.959

447.

4724

80.

0002

984

.872

30.

1025

713

.710

9710

804

4800

0.00

313

2.31

E-17

79.7

7944

6.68

513

0.

0074

581

.981

12.

7596

812

.266

2910

5288

J va

lue

9.65

675E

-03

1.

7966

5E-0

5To

tal 3

9 K V

ol=

1.17

160E

-10

CCNT

P/G

Tota

l gas

age

=10

75.0

1

2.10

352

Ma

A133

-140

00.

1046

67.

72E-

1678

.384

833.

1879

30.

0064

176

.490

92.

4161

95.

8494

110

012

440

0.28

683

2.12

E-15

73.0

6987

3.12

962

0.00

103

72.7

641

0.41

847

5.74

242

963

144

10.

0877

26.

47E-

1673

.686

013.

1416

70.

0012

773

.310

90.

5090

75.

7645

396

83

460

0.02

532

1.87

E-16

73.4

5835

3.18

647

0.00

229

72.7

827

0.91

977

5.84

673

963

850

00.

0496

33.

66E-

1674

.458

303.

2227

60.

0010

274

.156

10.

4058

65.

9133

297

76

540

0.11

704

8.64

E-16

74.8

1581

3.20

948

0.00

134

74.4

197

0.52

945

5.88

895

980

358

00.

0497

13.

67E-

1674

.777

193.

1368

40.

0023

474

.085

50.

9250

5.75

567

976

562

00.

0113

08.

34E-

1771

.758

093.

1344

80.

0105

168

.653

24.

3268

85.

7513

492

018

700

0.12

656

9.34

E-16

73.3

8544

3.21

055

0.00

100

73.0

899

0.40

273

5.89

092

966

278

00.

0451

33.

33E-

1674

.219

913.

2652

70.

0003

974

.105

20.

1545

65.

9913

297

64

880

0.04

618

3.41

E-16

74.4

9200

3.20

771

0.00

037

74.3

841

0.14

485

5.88

571

979

410

000.

0364

52.

69E-

1673

.883

803.

8105

70.

0025

474

.635

71.

0176

86.

9918

798

26

1200

0.00

925

6.83

E-17

74.9

2031

3.98

344

0.00

117

74.5

737

0.46

264

7.30

906

981

2950

000.

0042

33.

12E-

1774

.502

663.

6995

40.

0114

977

.897

54.

5566

76.

7881

510

1533

J va

lue

9.69

066E

-03

1.

7372

2E-0

5To

tal 3

9 K v

ol =

1.89

216E

-10

CCNT

P/G

Tota

l gas

age

=97

2.99

5

1.63

359

Ma

A134

-136

00.

0580

64.

28E-

1688

.659

323.

1155

30.

0243

481

.468

28.

1109

65.

7165

710

613

380

0.08

990

6.63

E-16

74.6

7086

2.89

451

0.00

067

74.4

739

0.26

378

5.31

103

991

240

00.

1350

69.

97E-

1672

.878

022.

9391

10.

0003

672

.771

60.

1460

25.

3928

697

32

420

0.13

788

1.02

E-15

72.0

7033

2.91

703

0.

0000

272

.076

60.

0087

15.

3523

596

62

430

0.07

872

5.81

E-16

72.9

9311

2.94

646

0.00

062

72.8

086

0.25

278

5.40

635

973

244

00.

0658

74.

86E-

1672

.615

722.

9357

60.

0007

572

.393

40.

3061

75.

3867

296

93

460

0.08

062

5.95

E-16

72.7

2679

2.93

459

0.00

137

72.3

216

0.55

714

5.38

457

968

248

00.

0572

84.

23E-

1672

.604

682.

9346

90.

0019

172

.040

80.

7766

45.

3847

596

64

500

0.06

910

5.10

E-16

73.4

6718

2.96

128

0.00

186

72.9

177

0.74

792

5.43

354

975

252

00.

0362

32.

67E-

1672

.741

532.

8976

30.

0025

172

.000

91.

0181

65.

3167

596

54

560

0.02

905

2.14

E-16

72.4

6758

2.99

739

0.00

192

71.8

994

0.78

405

5.49

980

964

560

00.

0191

01.

41E-

1670

.952

023.

0573

50.

0021

070

.331

00.

8752

75.

6098

294

86

680

0.03

939

2.91

E-16

72.0

1860

3.25

967

0.00

347

70.9

928

1.42

436

5.98

105

955

376

00.

0351

42.

59E-

1671

.750

963.

2029

80.

0018

571

.204

80.

7611

85.

8770

395

74

840

0.01

835

1.35

E-16

72.6

9965

3.60

576

0.00

535

71.1

194

2.17

367

6.61

607

956

640

000.

0502

63.

71E-

1672

.237

524.

2873

20.

0024

271

.522

50.

9898

27.

8666

496

04

J va

lue

9.82

456E

-03

1.

5083

7E-0

5To

tal 3

9 K v

ol =

3.12

587E

-10

CCNT

P/G

Tota

l gas

age

=97

4.68

0

1.34

455

Ma

A135

-136

00.

0048

03.

54E-

1718

9.06

855

3.53

975

0.38

769

74.5

070

60.5

926

6.49

495

996

2942

00.

0321

52.

37E-

1697

.219

902.

8304

70.

0082

294

.792

02.

4973

35.

1935

211

936

460

0.10

836

8.00

E-16

94.1

6484

2.76

105

0.00

110

93.8

398

0.34

519

5.06

615

1184

252

00.

2854

92.

11E-

1592

.272

922.

8059

90.

0009

691

.988

50.

3082

45.

1486

111

671

540

0.17

551

1.30

E-15

91.5

9555

2.83

532

0.00

085

91.3

458

0.27

267

5.20

242

1161

256

00.

0738

25.

45E-

1693

.826

662.

8451

20.

0023

193

.143

40.

7282

15.

2204

011

783

620

0.14

952

1.10

E-15

91.6

0731

2.84

698

0.00

091

91.3

373

0.29

475

5.22

382

1161

266

00.

0696

25.

14E-

1691

.920

432.

8621

70.

0007

692

.144

90.

2442

05.

2516

911

683

720

0.03

631

2.68

E-16

91.5

3615

2.89

577

0.

0005

991

.710

50.

1904

75.

3133

411

646

840

0.04

446

3.28

E-16

92.4

5542

2.94

274

0.00

034

92.3

555

0.10

808

5.39

952

1170

540

000.

0199

61.

47E-

1692

.069

613.

0966

80.

0057

890

.363

01.

8536

15.

6819

811

529

J va

lue

9.88

469E

-03

1.

1115

2E-0

5To

tal 3

9 K v

ol=

1.54

303E

-10

CCNT

P/G

Tota

l gas

age

=11

67.3

6

1.24

451

Ma

A136

-136

00.

0290

2.14

E-16

107.

5147

33.

1368

40.

0598

089

.843

516

.436

15.

7556

711

498

400

0.08

390

6.19

E-16

70.2

5276

3.13

401

0.00

122

69.8

912

0.51

466

5.75

048

950

442

00.

1658

91.

22E-

1569

.971

593.

0752

50.

0003

669

.866

60.

1500

55.

6426

695

02

440

0.23

971.

77E-

1569

.016

903.

1408

10.

0003

368

.918

50.

1425

85.

7629

593

91

450

0.07

365

5.43

E-16

68.4

6276

3.10

302

0.00

155

68.0

044

0.66

950

5.69

361

930

548

00.

0462

63.

41E-

1668

.160

773.

1885

30.

0012

267

.800

90.

5279

85.

8505

192

85

520

0.08

498

6.27

E-16

67.3

9505

3.13

191

0.00

063

67.2

088

0.27

636

5.74

662

921

256

00.

0204

41.

51E-

1667

.855

533.

2227

0.01

155

64.4

438

5.02

793

5.91

321

891

2058

00.

0132

39.

76E-

1767

.226

983.

0830

40.

0031

368

.153

11.

3776

5.65

695

931

1564

00.

0340

62.

51E-

1670

.514

163.

1958

0.00

225

69.8

502

0.94

159

5.86

385

949

780

00.

1283

79.

47E-

1668

.488

293.

1414

50.

0009

168

.220

00.

3917

45.

7641

393

22

880

0.02

252

1.66

E-16

67.3

3619

3.24

219

0.00

315

66.4

059

1.38

156

5.94

897

913

9

(conti

nued

)

TAB

LE 2

.Co

ntin

ued

Pow

er

39Ar

frac

39Ar

mol

40Ar

/39Ar

37Ar

/39Ar

36Ar

/39Ar

40Ar

*/39 A

r K%

40Ar

atm

osCa

/KAg

e (M

a)1

err

or

(Ma)

4000

0.05

804.

28E-

1669

.015

213.

2216

70.

0006

168

.834

60.

2617

5.91

132

939

4J

valu

e9.

9144

5E-0

3

1.20

341E

-05

Tota

l 39 K

vol =

2.21

028E

-10

CCNT

P/G

Tota

l gas

age

=94

3.29

9

1.37

251

Ma

A137

-236

00.

0743

45.

49E-

1679

.545

282.

6555

0.00

766

77.2

828

2.84

427

4.87

248

1010

338

00.

1824

21.

35E-

1576

.643

712.

5629

30.

0005

476

.485

50.

2064

24.

7026

210

022

400

0.29

765

2.20

E-15

77.5

8981

2.57

025

0.00

031

77.4

988

0.11

730

4.71

606

1012

640

10.

0246

21.

82E-

1676

.839

482.

4938

10.

0041

975

.601

21.

6115

14.

5758

099

37

440

0.15

378

1.13

E-15

76.7

6473

2.56

553

0.00

078

76.5

340

0.30

057

4.70

739

1002

146

00.

1135

68.

38E-

1676

.796

232.

5509

70.

0011

776

.451

60.

4487

64.

6806

810

023

480

0.03

633

2.68

E-16

77.3

7406

2.59

891

0.00

191

76.8

109

0.72

784

4.76

864

1005

350

00.

0162

81.

20E-

1676

.301

152.

4661

0.00

684

74.2

795

2.64

957

4.52

495

980

956

00.

0283

62.

09E-

1676

.613

912.

5863

90.

0049

375

.157

11.

9014

94.

7456

798

96

800

0.05

065

3.74

E-16

76.8

3979

2.70

719

0.00

161

76.3

642

0.61

893

4.96

732

1001

440

000.

0220

31.

63E-

1677

.064

613.

4832

20.

0060

375

.281

82.

3134

6.39

123

990

7J

valu

e9.

7086

2E-0

3

1.72

082E

-05

Tota

l 39 K

vol =

2.77

185E

-10

CCNT

P/G

Tota

l gas

age=

1004

.41

2.

3128

3 M

a

A138

-136

00.

0034

12.

51E-

1780

.221

033.

8620

10.

0742

758

.272

827

.359

77.

0862

681

238

400

0.01

336

9.86

E-17

75.3

8258

2.98

881

0.00

238

74.6

796

0.93

255

5.48

406

987

1142

00.

1304

29.

62E-

1674

.158

962.

9600

10.

0004

574

.026

70.

1783

55.

4312

198

02

440

0.10

992

8.11

E-16

74.4

5032

2.96

696

0.00

062

74.2

657

0.24

798

5.44

396

983

246

00.

1022

67.

55E-

1674

.445

562.

9893

30.

0003

974

.561

20.

1553

45.

4850

198

62

480

0.08

788

6.48

E-16

74.5

8835

3.00

933

0.00

059

74.4

137

0.23

416

5.52

171

984

350

00.

1166

48.

61E-

1674

.504

272.

9973

10.

0009

274

.233

10.

3639

75.

4996

598

32

520

0.07

957

5.87

E-16

74.4

4747

2.97

613

0.

0000

374

.455

60.

0109

35.

4607

998

53

560

0.20

707

1.53

E-15

74.3

3688

3.00

375

0.00

039

74.2

214

0.15

535

5.51

147

982

158

00.

0200

11.

48E-

1673

.735

132.

9842

70.

0000

773

.756

20.

0285

85.

4757

297

87

640

0.08

036

5.93

E-16

74.6

8295

2.98

614

0.00

071

74.4

746

0.27

898

5.47

916

985

140

000.

0491

03.

62E-

1674

.240

163.

0346

0.00

125

73.8

720

0.49

591

5.56

807

979

4J

valu

e9.

7523

8E-0

3

1.69

034E

-05

Tota

l 39 K

vol =

2.45

945E

-10

CCNT

P/G

Tota

l gas

age

=98

2.38

0

1.49

725

Ma

A140

-136

00.

0481

13.

55E-

1664

.058

671.

5539

10.

0082

661

.617

83.

8103

72.

8512

185

64

400

0.15

487

1.14

E-15

70.5

3351

2.27

729

0.00

118

70.1

852

0.49

382

4.17

851

949

142

00.

0720

35.

31E-

1670

.739

172.

2903

40.

0004

070

.619

70.

1688

94.

2024

695

32

440

0.12

733

9.40

E-16

70.6

7584

2.34

235

0.00

044

70.5

457

0.18

414

4.29

789

952

346

00.

0507

63.

75E-

1670

.538

672.

2823

10.

0000

170

.541

80.

0044

44.

1877

295

23

500

0.03

876

2.86

E-16

70.6

7347

2.29

519

0.00

156

70.2

130

0.65

155

4.21

136

949

6

540

0.10

871

8.02

E-16

72.4

5699

2.38

892

0.00

278

71.6

342

1.13

555

4.38

334

964

260

00.

1307

29.

65E-

1671

.079

792.

3918

80.

0005

170

.928

50.

2128

54.

3887

795

63

640

0.09

072

6.69

E-16

71.1

0540

2.37

214

0.00

069

70.9

027

0.28

507

4.35

255

956

372

00.

0792

05.

84E-

1670

.815

592.

3225

0.00

092

70.5

424

0.38

577

4.26

147

952

310

000.

0668

84.

93E-

1668

.269

672.

1322

10.

0004

368

.143

20.

1852

53.

9123

192

73

4000

0.03

192

2.35

E-16

66.5

2505

1.88

886

0.00

066

66.3

294

0.29

411

3.46

580

908

6J

valu

e9.

8580

6E-0

3

1.30

213E

-05

Tota

l 39 K

vol =

2.00

423E

-10

CCNT

P/G

Tota

l gas

age

=94

6.27

5

1.27

061

Ma

A142

-134

00.

1064

57.

85E-

1686

.984

282.

6341

90.

0301

278

.084

410

.231

64.

8333

810

232

360

0.00

028

2.07

E-18

83.8

7368

1.

9578

90.

2916

32.

3040

110

2.74

703.

5924

641

717

440

0.00

059

4.38

E-18

87.4

3051

3.62

755

0.

0908

211

4.26

8030

.695

86.

6560

613

5215

846

00.

0018

31.

35E-

1780

.862

333.

1329

20.

0345

770

.646

112

.634

15.

7484

894

765

500

0.00

211

1.55

E-17

76.3

35

04_GSAmemoir_streepey

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