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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. 391–410. 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. 1.3–1.2 Gaand a continent-continent collision at ca. 1.1–1.05 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 at570–550 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 Rodinia’s 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: MT—Morin terrane; LSZ—Labelle 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 700–750°C 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
<500
0 30 km
78˚W
N
CC
SZ
SLSZ
77˚W
MBBZ
45˚N
N
SLSZ
78˚W
45˚N
MBBZ
RLS
Z
77˚W
BSZ
CC
SZ
RLS
Z
MS
Z
BSZ
MS
Figure 2. Regional thermobarometricgradients in the Metasedimentary Belt.(A) Contoured temperatures. The lowesttemperatures are less than 500 °C in theElzevir terrane, and the highest temper-atures are granulite facies (700 °C andabove) in the Gneiss Belt, the Frontenacterrane, and the Adirondack Highlands.(B) Contoured pressures. Pressures gen-erally follow the pattern of temperaturesand indicate regional metamorphism inthe region. Pressures over most of thearea are 700–800 MPa. BSZ—Bancroftshear zone; CCSZ—Carthage-Coltonshear zone; MBBZ—MetasedimentaryBelt Boundary Zone; RLSZ—Robert-son Lake shear zone; SLSZ—SharbotLake shear zone.
A
B
Exhumation of a collisional orogen 395
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: 600–700 °C, Mezger et al., 1991a, Scott and St-Onge,1995; hornblende: 480–500 °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. 1300–1190 Ma (theElzevirian orogeny), culminating in a continent-continent colli-sion at ca. 1080–1020 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, C–C′ 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.S
AM
PL
E L
OC
AT
ION
S
Hor
nble
nde
Bio
tite
Sam
ple
age
age
Dom
ain
Loca
tion
UT
M C
oord
inat
esU
TM
Coo
rdin
ates
From
Bus
ch e
t al.
(199
6)N
orth
ing
Eas
ting
RV
L118
A10
11 M
an.
d.S
harb
ot L
ake
4971
225
3726
00R
VL2
0A12
05 M
an.
d.S
harb
ot L
ake
4973
050
3670
25C
DN
5894
7 M
an.
d.E
lzev
ir/M
azin
aw49
6897
035
6050
MT
G72
952
Ma
n.d.
Elz
evir/
Maz
inaw
4957
060
3553
00
From
Bus
ch a
nd v
an d
er P
luijm
(19
96)
Nor
thin
gE
astin
gR
VL1
18B
n.d.
969
Ma
Sha
rbot
Lak
e49
7122
537
2600
LVT
130B
n.d.
901
Ma
Elz
evir/
Maz
inaw
4990
750
3619
20
From
Str
eepe
y et
al.
(200
0) a
nd u
npub
lishe
d da
taA
102
990
Ma
n.d.
Low
land
sP
oppl
e H
ill F
orm
atio
n, n
ear
Rus
sell.
A11
210
55 M
a94
0 M
aLo
wla
nds
Nea
r G
ouve
rneu
r, H
wy
58/8
12, N
Jct
.of P
opla
r H
ill R
d.A
114
990
Ma
n.d.
Low
land
sA
long
Gra
ss R
iver
, Hw
y 17
, W s
ide
of r
oad,
1.5
km
NW
of R
usse
ll.A
117
967
Ma
n.d.
Hig
hlan
ds0.
5 m
i.E
of R
t.27
and
Bro
user
’s C
orne
r.A
124
1043
Ma
n.d.
Low
land
sE
mor
yvill
e R
d., c
a.0.
5 m
i.N
W o
f pow
er p
lant
, ca
0.25
mi.
W o
f tur
noff
to T
alcv
ille.
A12
510
66 M
an.
d.Lo
wla
nds
Em
oryv
ille
Rd.
, E o
f Hai
lsbo
ro, s
mal
l out
crop
nor
th o
f roa
d, N
W o
f hou
se, 1
.8 m
i.E
of b
ridge
, 2.1
mi.
from
Isla
nd B
ranc
h R
d.A
128
1020
Ma
964
Ma
Low
land
sTr
out L
ake
Rd.
, N o
f Edw
ards
, alo
ng W
sid
e of
Tro
ut L
ake,
S s
ide,
W o
f 2 is
land
s, W
sid
e of
roa
d.C
ut a
bout
40
m lo
ng.
A12
910
79 M
a92
4 M
aLo
wla
nds
S.o
f Tro
ut L
ake,
Rt.
19 to
Edw
ards
, ca.
due
W o
f wes
tern
mos
t ext
rem
e of
Ced
ar L
ake.
A13
397
5 M
an.
d.H
ighl
ands
Cre
st o
f Whi
te’s
Hill
, Whi
te H
ill R
d., S
E o
f Par
ishv
ille
abou
t 3 m
i.at
aba
ndon
ed lo
okou
t.15
0′on
sm
all t
rail
to th
e W
.A
134
972
Ma
n.d.
Hig
hlan
dsV
ery
low
out
crop
s al
ong
both
sid
es o
f the
hig
hway
.S s
ide
of W
hite
Hill
, abo
ut 1
mi.
S o
f loo
kout
.Out
crop
on
Whi
te H
ill R
d.,
with
in a
mph
ibol
ite.
A13
511
65 M
an.
d.H
ighl
ands
Just
N o
f Ste
rling
Rd.
, nea
r po
wer
line
, abo
ut 1
50fr
om ju
nctio
n w
ith J
oe In
dian
Rd.
A13
694
3 M
a93
7 M
aH
ighl
ands
Sm
all o
utcr
op o
n Jo
e In
dian
Rd.
at th
e cr
est o
f a s
mal
l hill
.Loc
ated
at t
he N
70°W
, 60°
N s
ymbo
l on
Leon
ard
and
Bud
ding
ton’
s m
ap.
A13
710
05 M
an.
d.H
ighl
ands
Rt.
56, a
bout
3 m
i.S
of S
tark
.W s
ide
of h
wy,
roa
dcut
of a
mph
ibol
ite m
igm
atite
s.A
138
983
Ma
n.d.
Hig
hlan
dsS
mal
l out
crop
on
E s
ide
of H
wy
56, a
bout
1 m
i.S
of 1
37, a
bout
4 m
i.S
of S
tark
.A
140
947
Ma
n.d.
Hig
hlan
dsH
wy
3, ju
st E
of P
itcai
rn.E
xpos
ures
on
emba
nkm
ent N
of t
he r
oad,
at c
rest
of h
ill.
A14
295
3 M
a91
9 M
aH
ighl
ands
Sm
all o
utcr
op o
n th
e E
sid
e of
Mud
Lak
e R
d.at
the
junc
tion
of M
ud L
ake
and
Brig
gs R
d.(S
E c
orne
r of
inte
rsec
tion)
.A
145
935
Ma
n.d.
Hig
hlan
dsA
bout
2 m
i.E
of T
exas
on
Texa
s R
d., 6
mi.
from
Hw
y 81
2.H
M95
n.d.
915
Ma
Low
land
sN
W o
f Rus
sell
on R
t.17
in D
evil’
s E
lbow
, 2 m
i.S
of H
erm
on.S
ampl
e fr
om E
sid
e of
out
crop
.S
E59
6-49
n.d.
895
Ma
Hig
hlan
dsO
n R
t.87
, N o
f Dan
a H
ill R
d., 1
.9 m
i.S
of W
hipp
orw
ill C
orne
rs.O
utcr
op o
n N
W s
ide
of r
oad.
LB59
6-31
bn.
d.92
5 M
aLo
wla
nds
4.5
mi.
from
inte
rsec
tion
of H
wy
3 an
d H
wy
812,
on
812
S o
f Bal
mat
.Sam
ple
from
E s
ide
of o
utcr
op.
LB93
n.d.
904
Ma
Low
land
sO
utcr
op ju
st N
of L
B59
6-31
b.C
N59
6-56
n.d.
924
Ma
Hig
hlan
ds0.
2 m
i.S
of j
unct
ion
of H
anso
n R
d.an
d O
rebe
d R
d.on
Ore
bed
Rd.
, nea
r C
olto
n.O
utcr
op o
n W
sid
e of
roa
d.P
P59
6-60
n.d.
899
Ma
Low
land
s1
mi.
SW
of P
ierr
epon
t on
Rt.
2.O
utcr
op o
n E
sid
e of
roa
d.
From
Str
eepe
y et
al.
(200
1)La
titud
eLo
ngitu
deR
WS
-110
00 M
an.
d.W
ithin
CC
SZ
Nor
th 4
4 de
g., 2
2 m
in.
Wes
t 75
deg.
, 10
min
.R
WS
-3c
999
Ma
n.d.
With
in C
CS
ZN
orth
44
deg.
, 22
min
.W
est 7
5 de
g., 1
0 m
in.
EA
198
1 M
an.
d.W
ithin
CC
SZ
Nor
th 4
4 de
g., 2
2 m
in.
Wes
t 75
deg.
, 10
min
.D
H98
-199
8 M
an.
d.W
ithin
CC
SZ
Nor
th 4
4 de
g., 2
2 m
in.
Wes
t 75
deg.
, 10
min
.R
WH
-197
3 M
an.
d.W
ithin
CC
SZ
Nor
th 4
4 de
g., 2
2 m
in.
Wes
t 75
deg.
, 10
min
.C
R7-
DH
198
9 M
an.
d.W
ithin
CC
SZ
Dan
a H
ill o
utcr
op:O
n N
sid
e of
Dan
a H
ill R
d.C
R1-
DH
198
0 M
an.
d.W
ithin
CC
SZ
Dan
a H
ill o
utcr
op:O
n N
sid
e of
Dan
a H
ill R
d.C
R3-
8794
1 M
an.
d.W
ithin
CC
SZ
1.8
km N
of i
nter
sect
ion
betw
een
Dan
a H
ill R
d.an
d R
t.87
, on
Rt.
87.
CR
2-A
294
8 M
an.
d.W
ithin
CC
SZ
Dire
ctly
acr
oss
Rt.
87 fr
om C
R3-
87.
DH
2-8
1012
Ma
n.d.
With
in C
CS
ZD
irect
ly a
cros
s R
t.87
from
CR
3-87
.C
R6-
A4
1005
Ma
n.d.
With
in C
CS
ZD
irect
ly a
cros
s R
t.87
from
CR
3-87
.C
R12
-A4
998
Ma
n.d.
With
in C
CS
ZD
irect
ly a
cros
s R
t.87
from
CR
3-87
.C
R9-
A4
944
Ma
n.d.
With
in C
CS
ZD
irect
ly a
cros
s R
t.87
from
CR
3-87
.H
87-5
b10
19 M
an.
d.W
ithin
CC
SZ
Dire
ctly
acr
oss
Rt.
87 fr
om C
R3-
87.
n.d.
—no
dat
a;C
CS
Z—
Car
thag
e-C
olto
n sh
ear
zone
;UT
M—
Uni
vers
al T
rans
vers
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. 70–100Ma 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. 600–700 °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 1090–1040 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.
Exhumation of a collisional orogen 397
Figu
re 3
. M
aps
show
ing
horn
blen
de,
biot
ite,
and
K-
feld
spar
40A
r-39A
r age
s ac
ross
the
Rob
erts
on L
ake
shea
rzo
ne
(RL
SZ)
and
the
Car
thag
e-C
olto
n sh
ear
zone
(CC
SZ).
Acr
oss
the
RL
SZ, t
he a
ges
are
from
Bus
ch e
t al.
(199
6), B
usch
and
van
der
Plu
ijm (
1996
), a
nd S
tree
pey
et a
l. (2
002)
. Hor
nble
nde
ages
are
ital
iciz
ed. A
cros
s th
eC
CSZ
, age
s ar
e fr
om S
tree
pey
et a
l. (2
000,
200
1, 2
002)
and
new
res
ults
. Pa
irs
sepa
rate
d by
a c
omm
a in
dica
teho
rnbl
ende
and
bio
tite
ages
fro
m th
e sa
me
sam
ple.
The
age
dist
ribu
tions
acr
oss
the
RL
SZ in
dica
te m
otio
n al
ong
the
shea
r zo
ne a
fter
ca.
900
Ma
and
befo
re c
a. 7
80 M
a.T
he a
ge d
istr
ibut
ions
acr
oss
the
CC
SZ i
ndic
ate
defo
r-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
—fi
eld
outc
rop
nam
e; G
B—
Gne
iss
Bel
t;G
FTZ
—G
renv
ille
Fron
t te
cton
ic z
one;
GT
—G
ranu
lite
Terr
ane;
MB
—M
etas
edim
enta
ry B
elt;
MB
BZ
—M
eta-
sedi
men
tary
Bel
t Bou
ndar
y Z
one;
MSZ
—M
oort
on sh
ear
zone
; M
T—
Mor
in t
erra
ne;
RW
—fi
eld
outc
rop
nam
e;SL
SZ—
Shar
bot L
ake
shea
r zo
ne.
Exhumation of a collisional orogen 399
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 2σ 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 ∼947–983Ma. 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. 900–930 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.H
OR
NB
LE
ND
E A
RG
ON
DA
TA
Pow
er39A
r fr
ac39A
r m
ol40A
r/39A
r37A
r/39A
r36A
r/39A
r40A
r*/3
9A
r K%
40A
r at
mos
Ca/
KA
ge (
Ma)
1σer
ror
(Ma)
A11
2-1
400
0.17
977
1.33
E-1
582
.671
663.
6246
90.
0103
479
.615
13.
6972
36.
6508
110
442
401
0.12
911
9.53
E-1
682
.137
063.
8956
0.00
050
81.9
893
0.17
990
7.14
789
1068
240
20.
0457
13.
37E
-16
80.9
9802
3.88
507
–0.0
0025
81.0
706
–0.0
8961
7.12
857
1059
342
00.
1290
89.
52E
-16
80.8
0290
3.97
513
–0.0
0010
80.8
318
–0.0
3576
7.29
382
1056
244
00.
0495
23.
65E
-16
79.8
0750
3.82
902
–0.0
0050
79.9
564
–0.1
8657
7.02
572
1048
346
00.
0649
64.
79E
-16
80.3
2021
3.94
636
–0.0
0069
80.5
247
–0.2
5459
7.24
103
1053
348
00.
0161
61.
19E
-16
77.8
2671
3.64
937
–0.0
0247
78.5
576
–0.9
3913
6.69
609
1034
956
00.
0842
16.
21E
-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
-16
80.2
5100
4.00
081
0.00
004
80.2
387
0.01
532
7.34
094
1050
372
00.
0377
22.
78E
-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
-16
81.5
3749
4.13
691
0.00
093
81.2
615
0.33
849
7.59
066
1061
588
00.
0105
57.
78E
-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
-16
80.9
9230
3.91
662
0.00
074
80.7
727
0.27
114
7.18
646
1056
310
400.
0200
41.
48E
-16
78.7
2645
3.90
584
–0.0
0173
79.2
368
–0.6
4826
7.16
668
1040
848
000.
0295
42.
18E
-16
80.0
8239
3.91
963
–0.0
0173
80.5
944
–0.6
3935
7.19
198
1054
6J
valu
e9.
8466
7E-0
3 ±
1.38
181E
-05
Tota
l 39K
vol
=2.
1252
9E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
1051
.97
±1.
3919
6 M
a
A11
4-2
360
0.00
871
6.42
E-1
743
7.13
157
4.65
082
1.19
085
85.2
341
80.5
015
8.53
361
1091
5640
00.
0035
72.
63E
-17
84.8
3118
2.64
341
0.06
967
64.2
452
24.2
670
4.85
029
877
7144
00.
0162
31.
20E
-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
-16
75.2
5700
2.55
393
0.00
739
73.0
722
2.90
312
4.68
611
970
852
00.
0779
95.
75E
-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
-16
76.7
0364
2.71
598
0.00
102
76.4
032
0.39
169
4.98
345
1004
360
00.
1984
01.
46E
-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
-16
75.7
2533
2.70
733
0.00
017
75.6
755
0.06
580
4.96
758
997
468
00.
0533
73.
94E
-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
-15
75.2
3356
2.76
182
0.00
045
75.1
018
0.17
513
5.06
756
991
376
00.
0025
81.
90E
-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
-15
75.2
5175
2.73
198
0.00
072
75.0
377
0.28
445
5.01
281
990
288
00.
0442
83.
27E
-16
75.1
3989
2.76
550.
0000
575
.123
80.
0214
25.
0743
199
16
2000
0.03
130
2.31
E-1
677
.066
523.
0014
60.
0007
676
.842
80.
2903
05.
5072
710
086
5000
0.00
462
3.41
E-1
780
.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
9K
vol
=1.
5029
3E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
992.
403
±1.
7796
8 M
a
A11
7-1R
400
0.06
297
4.65
E-1
611
3.32
355
5.04
689
0.04
675
99.5
093
12.1
901
9.26
035
1226
344
00.
1307
09.
64E
-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
-15
74.6
5890
4.74
913
0.00
060
74.4
807
0.23
869
8.71
4098
72
520
0.10
834
7.99
E-1
673
.440
854.
7365
10.
0015
572
.981
60.
6253
48.
6908
497
23
560
0.08
989
6.63
E-1
671
.938
914.
7711
0.00
197
71.3
563
0.80
987
8.75
431
955
460
00.
0558
74.
12E
-16
71.2
9738
4.84
565
0.00
070
71.0
905
0.29
017
8.89
110
952
5
640
0.04
701
3.47
E-1
670
.750
444.
8344
80.
0025
969
.985
21.
0816
18.
8706
194
14
680
0.02
548
1.88
E-1
670
.307
064.
7511
30.
0050
268
.822
22.
1119
78.
7176
792
94
760
0.08
086
5.97
E-1
674
.379
644.
8044
40.
0020
573
.775
20.
8126
58.
8154
998
03
880
0.04
753
3.51
E-1
672
.893
714.
8885
80.
0040
771
.692
01.
6485
88.
9698
795
96
1000
0.13
615
1.00
E-1
573
.203
745.
3166
30.
0002
573
.130
00.
1007
49.
7552
897
32
2000
0.04
257
3.14
E-1
673
.617
715.
2241
30.
0034
872
.590
21.
3957
49.
5855
696
84
4800
0.00
053
3.93
E-1
898
.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 39K
vol
=1.
4453
3E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
987.
869
±1.
6156
1 M
a
A12
4-1R
360
0.03
019
2.23
E-1
612
5.33
636
4.21
346
0.04
743
111.
3220
11.1
814
7.73
112
1338
540
00.
1703
31.
26E
-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
-16
80.7
3303
3.89
882
0.00
148
80.2
960
0.54
133
7.15
380
1053
340
20.
0425
03.
14E
-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
-17
79.3
1704
3.95
845
0.01
442
75.0
558
5.37
241
7.26
321
1000
1446
00.
2181
51.
61E
-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
-16
81.1
5436
3.89
173
0.00
176
80.6
357
0.63
911
7.14
079
1056
348
00.
1067
87.
88E
-16
81.5
2008
3.83
570.
0004
181
.398
60.
1490
27.
0379
810
642
520
0.04
500
3.32
E-1
680
.237
263.
9116
30.
0024
079
.526
70.
8855
87.
1773
010
454
560
0.02
183
1.61
E-1
679
.204
674.
0385
80.
0024
978
.468
10.
9299
57.
4102
410
358
600
0.00
773
5.70
E-1
777
.392
294.
2870
60.
0089
974
.736
43.
4317
37.
8661
799
722
640
0.01
163
8.58
E-1
778
.559
424.
5639
70.
0019
277
.991
60.
7227
98.
3742
610
3014
720
0.03
076
2.27
E-1
679
.952
814.
3458
6–0
.000
1079
.981
7–0
.036
147.
9740
610
506
800
0.02
660
1.96
E-1
680
.783
764.
1788
70.
0034
879
.755
11.
2733
57.
6676
510
488
1200
0.05
685
4.19
E-1
681
.004
874.
0396
0.00
053
80.8
471
0.19
477
7.41
211
1058
440
000.
0283
72.
09E
-16
81.8
5268
4.09
907
–0.0
0085
82.1
048
–0.3
0802
7.52
123
1071
6J
valu
e9.
8708
9E-0
3 ±
1.20
594E
-05
Tota
l 39K
vol
=2.
5813
6E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
1063
.43
±1.
3379
4 M
a
A12
5-1
400
0.25
390
1.87
E-1
584
.451
713.
9048
50.
0064
782
.540
22.
2634
37.
1648
610
562
401
0.07
938
5.86
E-1
684
.048
333.
7568
90.
0025
183
.307
50.
8814
36.
8933
810
634
440
0.13
038
9.62
E-1
683
.997
653.
7257
50.
0013
683
.594
60.
4798
36.
8362
410
663
480
0.10
856
8.01
E-1
683
.760
733.
7063
10.
0006
983
.555
70.
2447
86.
8005
710
653
520
0.22
332
1.65
E-1
584
.004
993.
8024
0.00
102
83.7
027
0.35
984
6.97
688
1067
256
00.
0858
36.
33E
-16
85.4
0903
3.84
262
0.00
098
85.1
192
0.33
935
7.05
068
1080
460
00.
0056
44.
16E
-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
-17
84.7
5936
4.43
573
–0.0
1303
88.6
097
–4.5
4267
8.13
895
1113
2982
00.
0198
41.
46E
-16
83.3
0916
4.05
837
–0.0
0825
85.7
468
–2.9
2602
7.44
655
1086
1410
000.
0186
91.
38E
-16
83.4
4706
4.02
577
–0.0
0840
85.9
295
–2.9
7487
7.38
673
1088
1448
000.
0666
54.
92E
-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 39K
vol
=1.
2656
9E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
1065
.08
±1.
9292
1 M
a
A12
8-1
360
0.02
003
1.48
E-1
610
3.37
114
0.86
044
0.05
595
86.8
393
15.9
927
1.57
879
1114
538
00.
0097
67.
20E
-17
45.9
3718
0.55
937
0.00
118
45.5
872
0.76
186
1.02
636
668
1442
00.
0089
96.
63E
-17
85.6
7338
1.20
975
0.00
381
84.5
478
1.31
382.
2197
210
9213
(con
tinue
d)
TAB
LE
2.
Co
nti
nued
Pow
er39A
r fr
ac39A
r m
ol40A
r/39A
r37A
r/39A
r36A
r/39A
r40A
r*/3
9A
r K%
40A
r at
mos
Ca/
KA
ge (
Ma)
1σer
ror
(Ma)
460
0.01
472
1.09
E-1
695
.156
392.
1031
10.
0022
994
.480
50.
7102
93.
8589
211
8611
500
0.06
145
4.53
E-1
683
.472
604.
6535
10.
0006
783
.275
50.
2361
38.
5385
510
802
520
0.16
436
1.21
E-1
578
.390
114.
9394
80.
0006
078
.212
70.
2263
29.
0632
710
291
530
0.06
040
4.46
E-1
678
.093
544.
6488
90.
0006
277
.910
10.
2349
08.
5300
710
262
540
0.02
521
1.86
E-1
671
.341
983.
6714
30.
0005
671
.176
40.
2320
96.
7365
795
75
560
0.04
059
2.99
E-1
675
.791
474.
5035
10.
0000
275
.786
10.
0070
98.
2633
210
053
620
0.10
396
7.67
E-1
676
.916
354.
8066
70.
0004
876
.775
50.
1831
28.
8195
810
152
660
0.11
611
8.57
E-1
678
.493
214.
9517
40.
0005
578
.329
70.
2083
19.
0857
610
312
720
0.11
533
8.51
E-1
679
.959
065.
1169
70.
0006
579
.768
00.
2389
59.
3889
410
452
780
0.08
676
6.40
E-1
680
.744
144.
8914
40.
0006
580
.551
10.
2390
88.
9751
210
532
1000
0.06
995
5.16
E-1
680
.560
784.
9683
40.
0009
980
.267
70.
3638
09.
1162
210
502
4000
0.10
239
7.55
E-1
666
.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
9K
vol
=2.
6351
8E-1
0 C
CN
TP
/GT
tota
l gas
age
=10
22.1
1 ±
1.28
867
Ma
A12
9-1
360
0.02
336
1.72
E-1
697
.701
206.
7107
0.05
614
81.1
131
16.9
784
12.3
1321
1044
1150
00.
5648
54.
17E
-15
84.9
8144
6.63
161
0.00
053
84.8
238
0.18
550
12.1
6809
1079
254
00.
0903
16.
66E
-16
84.3
7694
6.50
40–0
.001
2084
.731
7–0
.420
4511
.933
9410
794
580
0.10
879
8.03
E-1
685
.284
256.
5317
30.
0016
684
.793
20.
5757
811
.984
8310
793
620
0.02
093
1.54
E-1
685
.023
556.
4932
30.
0009
184
.755
50.
3152
711
.914
1810
7913
1000
0.12
513
9.23
E-1
682
.430
006.
7899
10.
0016
581
.941
70.
5923
812
.458
5510
523
2000
0.06
351
4.69
E-1
684
.959
447.
4724
80.
0002
984
.872
30.
1025
713
.710
9710
804
4800
0.00
313
2.31
E-1
779
.779
446.
6851
3–0
.007
4581
.981
1–2
.759
6812
.266
2910
5288
J va
lue
9.65
675E
-03
±1.
7966
5E-0
5To
tal 3
9K
Vol
=1.
1716
0E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
1075
.01
±2.
1035
2 M
a
A13
3-1
400
0.10
466
7.72
E-1
678
.384
833.
1879
30.
0064
176
.490
92.
4161
95.
8494
110
012
440
0.28
683
2.12
E-1
573
.069
873.
1296
20.
0010
372
.764
10.
4184
75.
7424
296
31
441
0.08
772
6.47
E-1
673
.686
013.
1416
70.
0012
773
.310
90.
5090
75.
7645
396
83
460
0.02
532
1.87
E-1
673
.458
353.
1864
70.
0022
972
.782
70.
9197
75.
8467
396
38
500
0.04
963
3.66
E-1
674
.458
303.
2227
60.
0010
274
.156
10.
4058
65.
9133
297
76
540
0.11
704
8.64
E-1
674
.815
813.
2094
80.
0013
474
.419
70.
5294
55.
8889
598
03
580
0.04
971
3.67
E-1
674
.777
193.
1368
40.
0023
474
.085
50.
9250
5.75
567
976
562
00.
0113
08.
34E
-17
71.7
5809
3.13
448
0.01
051
68.6
532
4.32
688
5.75
134
920
1870
00.
1265
69.
34E
-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
-16
74.2
1991
3.26
527
0.00
039
74.1
052
0.15
456
5.99
132
976
488
00.
0461
83.
41E
-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
-16
73.8
8380
3.81
057
–0.0
0254
74.6
357
–1.0
1768
6.99
187
982
612
000.
0092
56.
83E
-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
-17
74.5
0266
3.69
954
–0.0
1149
77.8
975
–4.5
5667
6.78
815
1015
33J
valu
e9.
6906
6E-0
3 ±
1.73
722E
-05
Tota
l 39K
vol
=1.
8921
6E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
972.
995
±1.
6335
9 M
a
A13
4-1
360
0.05
806
4.28
E-1
688
.659
323.
1155
30.
0243
481
.468
28.
1109
65.
7165
710
613
380
0.08
990
6.63
E-1
674
.670
862.
8945
10.
0006
774
.473
90.
2637
85.
3110
399
12
400
0.13
506
9.97
E-1
672
.878
022.
9391
10.
0003
672
.771
60.
1460
25.
3928
697
32
420
0.13
788
1.02
E-1
572
.070
332.
9170
3–0
.000
0272
.076
6–0
.008
715.
3523
596
62
430
0.07
872
5.81
E-1
672
.993
112.
9464
60.
0006
272
.808
60.
2527
85.
4063
597
32
440
0.06
587
4.86
E-1
672
.615
722.
9357
60.
0007
572
.393
40.
3061
75.
3867
296
93
460
0.08
062
5.95
E-1
672
.726
792.
9345
90.
0013
772
.321
60.
5571
45.
3845
796
82
480
0.05
728
4.23
E-1
672
.604
682.
9346
90.
0019
172
.040
80.
7766
45.
3847
596
64
500
0.06
910
5.10
E-1
673
.467
182.
9612
80.
0018
672
.917
70.
7479
25.
4335
497
52
520
0.03
623
2.67
E-1
672
.741
532.
8976
30.
0025
172
.000
91.
0181
65.
3167
596
54
560
0.02
905
2.14
E-1
672
.467
582.
9973
90.
0019
271
.899
40.
7840
55.
4998
096
45
600
0.01
910
1.41
E-1
670
.952
023.
0573
50.
0021
070
.331
00.
8752
75.
6098
294
86
680
0.03
939
2.91
E-1
672
.018
603.
2596
70.
0034
770
.992
81.
4243
65.
9810
595
53
760
0.03
514
2.59
E-1
671
.750
963.
2029
80.
0018
571
.204
80.
7611
85.
8770
395
74
840
0.01
835
1.35
E-1
672
.699
653.
6057
60.
0053
571
.119
42.
1736
76.
6160
795
66
4000
0.05
026
3.71
E-1
672
.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
9K
vol
=3.
1258
7E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
974.
680
±1.
3445
5 M
a
A13
5-1
360
0.00
480
3.54
E-1
718
9.06
855
3.53
975
0.38
769
74.5
070
60.5
926
6.49
495
996
2942
00.
0321
52.
37E
-16
97.2
1990
2.83
047
0.00
822
94.7
920
2.49
733
5.19
352
1193
646
00.
1083
68.
00E
-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
-15
92.2
7292
2.80
599
0.00
096
91.9
885
0.30
824
5.14
861
1167
154
00.
1755
11.
30E
-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
-16
93.8
2666
2.84
512
0.00
231
93.1
434
0.72
821
5.22
040
1178
362
00.
1495
21.
10E
-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
-16
91.9
2043
2.86
217
–0.0
0076
92.1
449
–0.2
4420
5.25
169
1168
372
00.
0363
12.
68E
-16
91.5
3615
2.89
577
–0.0
0059
91.7
105
–0.1
9047
5.31
334
1164
684
00.
0444
63.
28E
-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
-16
92.0
6961
3.09
668
0.00
578
90.3
630
1.85
361
5.68
198
1152
9J
valu
e9.
8846
9E-0
3 ±
1.11
152E
-05
Tota
l 39K
vol
=1.
5430
3E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
1167
.36
±1.
2445
1 M
a
A13
6-1
360
0.02
902.
14E
-16
107.
5147
33.
1368
40.
0598
089
.843
516
.436
15.
7556
711
498
400
0.08
390
6.19
E-1
670
.252
763.
1340
10.
0012
269
.891
20.
5146
65.
7504
895
04
420
0.16
589
1.22
E-1
569
.971
593.
0752
50.
0003
669
.866
60.
1500
55.
6426
695
02
440
0.23
971.
77E
-15
69.0
1690
3.14
081
0.00
033
68.9
185
0.14
258
5.76
295
939
145
00.
0736
55.
43E
-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
-16
68.1
6077
3.18
853
0.00
122
67.8
009
0.52
798
5.85
051
928
552
00.
0849
86.
27E
-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
-16
67.8
5553
3.22
270.
0115
564
.443
85.
0279
35.
9132
189
120
580
0.01
323
9.76
E-1
767
.226
983.
0830
4–0
.003
1368
.153
1–1
.377
65.
6569
593
115
640
0.03
406
2.51
E-1
670
.514
163.
1958
0.00
225
69.8
502
0.94
159
5.86
385
949
780
00.
1283
79.
47E
-16
68.4
8829
3.14
145
0.00
091
68.2
200
0.39
174
5.76
413
932
288
00.
0225
21.
66E
-16
67.3
3619
3.24
219
0.00
315
66.4
059
1.38
156
5.94
897
913
9
(con
tinue
d)
TAB
LE
2.
Co
nti
nued
Pow
er39A
r fr
ac39A
r m
ol40A
r/39A
r37A
r/39A
r36A
r/39A
r40A
r*/3
9A
r K%
40A
r at
mos
Ca/
KA
ge (
Ma)
1σer
ror
(Ma)
4000
0.05
804.
28E
-16
69.0
1521
3.22
167
0.00
061
68.8
346
0.26
175.
9113
293
94
J va
lue
9.91
445E
-03
±1.
2034
1E-0
5To
tal 3
9K
vol
=2.
2102
8E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
943.
299
±1.
3725
1 M
a
A13
7-2
360
0.07
434
5.49
E-1
679
.545
282.
6555
0.00
766
77.2
828
2.84
427
4.87
248
1010
338
00.
1824
21.
35E
-15
76.6
4371
2.56
293
0.00
054
76.4
855
0.20
642
4.70
262
1002
240
00.
2976
52.
20E
-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
-16
76.8
3948
2.49
381
0.00
419
75.6
012
1.61
151
4.57
580
993
744
00.
1537
81.
13E
-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
-16
76.7
9623
2.55
097
0.00
117
76.4
516
0.44
876
4.68
068
1002
348
00.
0363
32.
68E
-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
-16
76.3
0115
2.46
610.
0068
474
.279
52.
6495
74.
5249
598
09
560
0.02
836
2.09
E-1
676
.613
912.
5863
90.
0049
375
.157
11.
9014
94.
7456
798
96
800
0.05
065
3.74
E-1
676
.839
792.
7071
90.
0016
176
.364
20.
6189
34.
9673
210
014
4000
0.02
203
1.63
E-1
677
.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 39K
vol
=2.
7718
5E-1
0 C
CN
TP
/GTo
tal g
asag
e=10
04.4
1 ±
2.31
283
Ma
A13
8-1
360
0.00
341
2.51
E-1
780
.221
033.
8620
10.
0742
758
.272
827
.359
77.
0862
681
238
400
0.01
336
9.86
E-1
775
.382
582.
9888
10.
0023
874
.679
60.
9325
55.
4840
698
711
420
0.13
042
9.62
E-1
674
.158
962.
9600
10.
0004
574
.026
70.
1783
55.
4312
198
02
440
0.10
992
8.11
E-1
674
.450
322.
9669
60.
0006
274
.265
70.
2479
85.
4439
698
32
460
0.10
226
7.55
E-1
674
.445
562.
9893
3–0
.000
3974
.561
2–0
.155
345.
4850
198
62
480
0.08
788
6.48
E-1
674
.588
353.
0093
30.
0005
974
.413
70.
2341
65.
5217
198
43
500
0.11
664
8.61
E-1
674
.504
272.
9973
10.
0009
274
.233
10.
3639
75.
4996
598
32
520
0.07
957
5.87
E-1
674
.447
472.
9761
3–0
.000
0374
.455
6–0
.010
935.
4607
998
53
560
0.20
707
1.53
E-1
574
.336
883.
0037
50.
0003
974
.221
40.
1553
55.
5114
798
21
580
0.02
001
1.48
E-1
673
.735
132.
9842
7–0
.000
0773
.756
2–0
.028
585.
4757
297
87
640
0.08
036
5.93
E-1
674
.682
952.
9861
40.
0007
174
.474
60.
2789
85.
4791
698
51
4000
0.04
910
3.62
E-1
674
.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 39K
vol
=2.
4594
5E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
982.
380
±1.
4972
5 M
a
A14
0-1
360
0.04
811
3.55
E-1
664
.058
671.
5539
10.
0082
661
.617
83.
8103
72.
8512
185
64
400
0.15
487
1.14
E-1
570
.533
512.
2772
90.
0011
870
.185
20.
4938
24.
1785
194
91
420
0.07
203
5.31
E-1
670
.739
172.
2903
40.
0004
070
.619
70.
1688
94.
2024
695
32
440
0.12
733
9.40
E-1
670
.675
842.
3423
50.
0004
470
.545
70.
1841
44.
2978
995
23
460
0.05
076
3.75
E-1
670
.538
672.
2823
1–0
.000
0170
.541
8–0
.004
444.
1877
295
23
500
0.03
876
2.86
E-1
670
.673
472.
2951
90.
0015
670
.213
00.
6515
54.
2113
694
96
540
0.10
871
8.02
E-1
672
.456
992.
3889
20.
0027
871
.634
21.
1355
54.
3833
496
42
600
0.13
072
9.65
E-1
671
.079
792.
3918
80.
0005
170
.928
50.
2128
54.
3887
795
63
640
0.09
072
6.69
E-1
671
.105
402.
3721
40.
0006
970
.902
70.
2850
74.
3525
595
63
720
0.07
920
5.84
E-1
670
.815
592.
3225
0.00
092
70.5
424
0.38
577
4.26
147
952
310
000.
0668
84.
93E
-16
68.2
6967
2.13
221
0.00
043
68.1
432
0.18
525
3.91
231
927
340
000.
0319
22.
35E
-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 39K
vol
=2.
0042
3E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
946.
275
±1.
2706
1 M
a
A14
2-1
340
0.10
645
7.85
E-1
686
.984
282.
6341
90.
0301
278
.084
410
.231
64.
8333
810
232
360
0.00
028
2.07
E-1
883
.873
68–1
.957
890.
2916
3–2
.304
0110
2.74
70–3
.592
46–4
171
744
00.
0005
94.
38E
-18
87.4
3051
3.62
755
–0.0
9082
114.
2680
–30.
6958
6.65
606
1352
158
460
0.00
183
1.35
E-1
780
.862
333.
1329
20.
0345
770
.646
112
.634
15.
7484
894
765
500
0.00
211
1.55
E-1
776
.335
582.
8843
20.
0150
871
.878
75.
8385
35.
2923
396
065
600
0.00
280
2.07
E-1
775
.587
473.
2046
80.
0497
460
.888
219
.446
75.
8801
584
233
660
0.15
283
1.13
E-1
573
.799
892.
5902
80.
0018
373
.258
50.
7335
94.
7528
197
41
680
0.02
791
2.06
E-1
671
.044
722.
6204
30.
0032
470
.086
51.
3487
64.
8081
394
14
700
0.02
705
2.00
E-1
672
.027
602.
6001
20.
0025
771
.267
91.
0547
44.
7708
695
36
720
0.11
147
8.22
E-1
671
.326
442.
5652
70.
0011
270
.995
70.
4637
04.
7069
295
02
740
0.04
120
3.04
E-1
671
.010
692.
5882
0.00
227
70.3
400
0.94
449
4.74
899
944
380
00.
0401
12.
96E
-16
71.3
3549
2.55
555
0.00
076
71.1
102
0.31
581
4.68
908
952
388
00.
0621
14.
58E
-16
71.9
6983
2.61
321
0.00
125
71.6
005
0.51
317
4.79
488
957
392
00.
0616
04.
55E
-16
71.6
6352
2.55
238
0.00
195
71.0
886
0.80
225
4.68
327
951
310
000.
0833
76.
15E
-16
70.8
1080
2.57
033
0.00
128
70.4
329
0.53
367
4.71
620
945
210
800.
1778
81.
31E
-15
70.9
2116
2.57
601
0.00
037
70.8
111
0.15
518
4.72
662
948
111
600.
0803
45.
93E
-16
70.8
7883
2.58
045
0.00
103
70.5
750
0.42
866
4.73
477
946
240
000.
0200
91.
48E
-16
70.3
4298
2.60
776
0.00
529
68.7
793
2.22
294
4.78
488
927
7J
valu
e9.
7680
8E-0
3 ±
1.67
233E
-05
Tota
l 39K
vol
=2.
6380
2E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
959.
956
±1.
4351
7 M
a
A14
5-1
360
0.00
415
3.06
E-1
739
4.93
547
2.59
78–0
.001
4139
5.35
20–0
.105
474.
7666
128
7327
420
0.03
778
2.79
E-1
669
.469
582.
5739
3–0
.003
5570
.518
0–1
.509
184.
7228
195
57
480
0.16
300
1.20
E-1
568
.419
972.
6550
8–0
.000
4068
.537
7–0
.172
074.
8717
193
52
490
0.18
985
1.40
E-1
568
.424
562.
6565
9–0
.000
5168
.576
6–0
.222
204.
8744
893
51
500
0.01
403
1.04
E-1
668
.090
592.
5561
–0.0
0679
70.0
984
–2.9
4874
4.69
009
951
2458
00.
0737
35.
44E
-16
68.2
5978
2.66
78–0
.002
8069
.086
3–1
.210
844.
8950
594
03
640
0.09
774
7.21
E-1
668
.437
302.
6323
20.
0004
068
.319
20.
1725
74.
8299
493
21
680
0.04
941
3.65
E-1
668
.326
192.
6462
80.
0004
168
.205
30.
1769
34.
8555
693
12
720
0.07
108
5.24
E-1
667
.628
252.
6015
80.
0004
867
.486
40.
2097
44.
7735
492
32
920
0.05
431
4.01
E-1
667
.942
762.
6388
30.
0007
567
.722
20.
3246
34.
8418
992
62
1000
0.17
403
1.28
E-1
568
.099
872.
7811
30.
0004
067
.981
60.
1736
85.
1029
992
91
4000
0.07
089
5.23
E-1
667
.828
032.
7744
4–0
.000
2367
.895
1–0
.098
895.
0907
292
84
J va
lue
9.90
225E
-03
±1.
0785
9E-0
5To
tal 3
9K
vol
=3.
2160
1E-1
0 C
CN
TP
/GTo
tal g
as a
ge =
947.
267
±1.
1159
2 M
a
Model Setup
Postorogenic extension is a feature commonly observed inmountain belts, regardless of the age of the orogen (Dewey,1988). Physical mechanisms proposed to explain this phenom-enon include (a) gravitational collapse; (b) changes in the stressregime, perhaps induced by changes in plate motion that causeeither reduced rates of convergence or a transition to extensionalstresses; (c) mantle-induced uplift and extension, including theeffects of mantle plumes or mantle delamination and slab break-off (e.g., Mareschal, 1994; Burg and Ford, 1997). Gravitationalcollapse acts on all mountain belts during orogenesis and tendsto occur shortly afterward, but the timescales and maximumamount of extension produced by gravitational collapse suggestthat it is unlikely to have been the primary driving force behindthe observed extension.
Changes in the stress regime acting in the region, referredto in this paper as far-field stresses, are likely to have acted overlarge areas beyond the core of the orogen and were thereforehomogeneous across the relatively narrow region between thetwo shear zones. In other words, while far-field stresses couldhave acted over long periods of time and led to the requiredamount of extension and exhumation, they cannot explain theasymmetric uplift observed in the Grenville Province.
Slab break-off and mantle delamination are often invokedas mechanisms acting in mountain belts (Sacks and Secor, 1990;Platt and England, 1994). Both are suggested as inducing upliftand exhumation of orogens by replacing cold mantle with hot-ter asthenospheric mantle. However, both require an initialperiod of subsidence while the slab is in the process of breakingoff (and even thereafter), or as the mantle delaminates. This isthe required dynamic response of the crust to sinking material.Both mechanisms should also lead initially and locally to com-pression in the region immediately above the delamination. Ourstructural, geochronological, and kinematic constraints on theGrenville suggest this was not the case there. Even if the coldand hot mantle had been as much as a thousand degrees apart intemperature, this would have led to only 3% of the difference indensity, and hence would have had only an isostatic effect on theorder of 2 km, which is less than the observed amount of upliftseen in the Grenville orogen. Furthermore, isostatic uplift actsover short timescales and cannot alone explain the protractedperiod of exhumation.
A mantle plume will initially generate uplift, via dynamictopography, on the order of 1–2 km (Lithgow-Bertelloni and Sil-ver, 1998; Panasyuk and Hager, 2000). It can also lead to exten-sion, as it induces stresses on the overlying lithosphere as wellas adding excess gravitational potential energy due to the addedtopography. However, the uplift caused by the plume will ceaseonce the head reaches the bottom of the lithosphere.
Given that each individual mechanisms seems unable toexplain all geological observations, our study has focused on theimpact of a combination of mechanisms on the deformation anduplift of an overthickened crustal block. The block contains
shear zones and is subject to different types of stresses acting inthe presence of gravity, such as symmetric extensional stressesthat stretch the block and vertical stresses that simulate the pres-ence of an upwelling or downwelling mantle. The latter is anapproximation of the dynamical effects of mantle convection,particularly of the deformation of the lithosphere subject to suchnormal stresses. We have not included isostatic effects on topog-raphy that would have been derived from the density contrastbetween the upwelling or downwelling and the normal mantle.We refer to them as bottom heating sources for convenience.
We have solved the equations of motion for an incompress-ible elastic-plastic material via the finite element method, usingthe general-purpose finite element code ABAQUS (version 6.2;Hibbit et al., 2000). We considered two-dimensional models ofan overthickened block of crust with a geometry similar to thatof the Grenville Province in this region at the peak of orogenesis.The dimensions of the block measure 300 km in length, with adepth of 60 km. In this block we site weak zones in positionsthat are determined by the present-day surface positions of theRobertson Lake shear zone and the Carthage-Colton shear zone(Fig. 4).
We have chosen a simple elastic-plastic rheology, with val-ues determined experimentally or taken from the literature (e.g.,Ranalli, 1995). For this study we did not address heterogeneitiesin rheology as a function of depth (or temperature), but main-tained a homogenous crustal material outside the weak zones(the parameters defining crust and weak zones are given in Table3). Boundary conditions were no-slip in the vertical directionand free-slip in the horizontal direction and at the surface. Rota-tion in and out of the plane of the page was not allowed. Theblock was subjected to extensional side loads and vertical trac-tions that simulated the presence of a plume or a delaminatingslab. The time evolution from the latest Grenville compressionto the final periods of extension when the crust must have beenuplifting as a uniform block was 300 m.y. After a 300-m.y. timeevolution, the predicted stresses and displacements were com-pared to field observations.
Our block of crustal material was subjected to a two-sidedextensional load of 100 MPa. The bottom of the block was sub-jected to a concentrated vertical stress ranging from 10 to 50MPa, acting upward to simulate a plume or downward to simu-late a delaminating mantle. A gravitational body force wasapplied over the block for the duration of the analysis. Themodel was then allowed to evolve over a 300-m.y. period.
Results
Models using only far-field stresses simulated by applyinga two-sided 100 MPa load to the block show the requiredamounts of extension but do not explain the temporal differ-ences between zones or the uplift of the block. Models thatinclude only plume stresses explain the uplift of the block butdo not generate local stresses around weak zones that are highenough to induce significant amounts of normal faulting. The
406 M.M. Streepey et al.
Exhumation of a collisional orogen 407
same behavior is observed if delamination is simulated. How-ever, plume and delamination stresses are different in one impor-tant respect. To have generated the asymmetric uplift observedin the region, delamination must have acted locally on the west-ern rather than the eastern edge (by the Robertson Lake shearzone) of the region, since their dynamic effect would have pulleddown the crust. Any uplift generated by this mechanism wouldhave required that the mantle previously attached to the crust bereplaced by hotter asthenospheric mantle and that the delami-nating mantle sink far enough into the mantle to no longer exertstress on the lithosphere. The isostatic uplift from hotter mantlewould have acted on timescales comparable to those of post-glacial rebound. Models that include only gravitational energyimposed on the block of crust with sited weak zones do not showthe appropriate amount of extension. However, the total magni-tude of gravitationally driven extension should have changedwith the inclusion of paleotopography. The timescales overwhich it would have acted, however, would have remained thesame. We suggest that the most likely physical scenario requiresthe presence of far-field stresses that induced periodic extension
for 250 m.y. and plume stresses that induced the requiredamount of asymmetric uplift observed in the region. The tilt pro-duced in the Frontenac or Adirondack Lowlands regionsmatches the tilt inferred by Dahl et al. (2001) from regionalhornblende ages.
In this combined model, the results show a 300-m.y. periodof uplift and extension, with the greatest uplift occurring underneath the Adirondack region, matching observed geo-chronologic results that show extensional motion along theCarthage-Colton shear zone due to rapid uplift of the Adiron-dack Highlands. Plots of contoured deviatoric stresses (Fig. 4,B) show high concentrations of stresses beneath the Carthage-Colton shear zone. The surface around the Robertson Lake shearzone and the Carthage-Colton shear zone regions dropped tovery low stresses as the weak zones activated as faults. All strainwas confined to the weak zones, indicating that they became theloci of permanent deformation in the region. As expected, shearstresses were highest in the regions immediately adjacent tofaults.
DISCUSSION
Two-dimensional finite-element models that simulate theGrenville Province at the peak of orogenesis may be used toevaluate the temporal and spatial characteristics of uplift andextension in the region. Postorogenic extension along theCarthage-Colton shear zone occurred at ca. 945 Ma, accordingto the argon ages of hornblende within the Carthage-Colton
Figure 4. (A) Setup of the finite elementmodel. Dimensions are given, as areboundary conditions and loads duringanalysis. (B) Contour map of resultingstress distributions. Highest stresses areseen around shear zones, with loweststresses occurring within the blockbounded by shear zones. Model parame-ters are given in Table 3. CCSZ—Carthage-Colton shear zone; RLSZ—RobertsonLake shear zone.
TABLE 3. MODEL PARAMETERS
Crust Weak zone
Young’s modulus (GPa) 80 50Poisson’s ratio 0.25 0.25Density (kg/m3) 2650 2650Yield stress (MPa) 150 15
408 M.M. Streepey et al.
shear zone (Streepey et al., 2001). The time of extension acrossthe Robertson Lake shear zone is less precisely known, but isbracketed between 900 Ma and 780 Ma, as documented by 40Ar-39Ar analyses of biotite and K-feldspar in the hangingwall andfootwall blocks of the Robertson Lake shear zone. Figure 5 showsthe combined kinematic and dynamic model of the postorogenicextensional history of the Metasedimentary Belt. Sometime after1045 and before 945 Ma, the entire region underwent a transitionfrom a contractional regime to an extensional environment, caus-ing normal motion along the Robertson Lake shear zone and theCarthage-Colton shear zone. Extension terminated along theCarthage-Colton shear zone at ca. 945 Ma, as shown by argonages of hornblende that are offset across the zone and biotite agesthat are not offset along the Carthage-Colton shear zone (Fig. 3),but did not cease along the Robertson Lake shear zone until some-time after 900 Ma. Since extension occurred so much later thancontraction, uplift of the crustal blocks that comprise this portionof the Grenville cannot be explained by protracted orogenesis.This differs significantly from the case in southern Asia, whichhas often been proposed as an analogue to the Grenville Province.In the Himalayan region, India is still underthrusting Asia, lead-ing to continued uplift of the Tibetan plateau even while theplateau is actively extending, probably due to gravitational insta-bilities or flow in the lower crust (e.g., Shen et al., 2001).
One viable mechanism for generating uplift while alsocausing active extension is heating from below. This heat sourcecan take several forms, including the formation of a plume, man-tle delamination, or slab break-off, all responsible for replace-ment of crust by hot asthenospheric mantle. In addition, it haslong been postulated that the existence of a supercontinent cre-ates a blanket over the mantle (e.g., Gurnis, 1988). With heatproduced in the mantle unable to escape via spreading centers,
thermal instabilities under the supercontinent can lead toregional uplift and extension.
Results elsewhere in Rodinia indicate that extension is wide-spread. Bottom-heated sources commonly generate large amountsof magma, creating mafic igneous provinces or massive dikeswarms. Whereas dike swarms in the Adirondack region aremuch younger, on the order of 580 Ma (Geraghty et al., 1979),abundant dike swarms at ca. 780 Ma occur in the western UnitedStates (Park et al., 1995). Extensional activity in the Baltica craton has been suggested at ca. 870 Ma (Dalziel and Soper,2001). In addition, ages of extension have been suggested at ca. 920 Ma along the western edge of the Congo craton (Tack et al., 2001). Mafic dike swarms in the South China block have been dated at ca. 820 Ma. Collectively these results supportthe idea of widespread extensional activity in the Late Mesopro-terozoic or Early Neoproterozoic, before the onset of large-scalerifting of the Rodinian supercontinent along this side of the Laurentian margin. It is possible that other evidence of magma-tism in the North American Grenville Province during this period has been obscured by Phanerozoic tectonic activity in theAppalachian orogen.
Finally, bottom heating alone does not result in the patternof geology seen at the surface today. It would have been neces-sary to apply a force to the block, approximately equal to a plate-driving force, to generate both the amounts of uplift and theamounts of extensional displacement documented in the field(Fig. 4). Therefore, we surmise that an initial instability in themantle triggered an adjustment of the plates while Rodiniaremained a coherent supercontinent. This adjustment, or changein far-field stress, would have triggered initial stages of breakupof the supercontinent, generating failed rifts or weakening thecrust, in advance of later breakup.
Figure 5. Kinematic and dynamic modelof regional extension in the GrenvilleProvince of southeastern Ontario andnorthwestern New York. Between 1040and 945 Ma, the entire region shiftedfrom a compressional stress regime to an extensional regime. Extension alongthe Carthage-Colton shear zone (CCSZ)ended at 945 Ma, while extension acrossthe Robertson Lake shear zone (RLSZ)did not terminate until after 900 Ma.
Exhumation of a collisional orogen 409
ACKNOWLEDGMENTS
This research was funded by the David and Lucille PackardFoundation (CLB), National Science Foundation (NSF) grantEAR-9980551 (CLB), NSF grant EAR-9627911 (BvdP), anNSF postdoctoral fellowship (JFM), the Geological Society ofAmerica (MMS), and the University of Michigan F. Scott TurnerFoundation (MMS). We thank Chris Hall and Marcus Johnsonfor their generous assistance in the Radiogenic Isotope Geo-chemistry Laboratory at the University of Michigan. KlausMezger and Larry Ruff are thanked for helpful discussions.
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