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Earth and Planetary Science Letters 128 (1994) 341-355
3He evidence for a wide zone of active mantle melting beneath
the Central Andes
L. Hoke
a,
D.R. Hilton
b,1
S.H. Lamb a, K. Hammerschmidt b, H. Friedrichsen
a Department of Earth Sciences, University of Oxford, Parks Road, Oxford OX1 3PR, UK
b Institut fiir Mineralogie, FR Geochemie, Freie Universitiit Berlin, Boltzmannstra~e 18-20, D-14195 Berlin, Germany
Received 20 May 1994; accepted 24 August 1994
Abstract
We report results of a regional survey of helium isotopes measured in water and gas samples in volcanic
sulfataras and geothermal springs from the Central Andes of northern Chile and Bolivia between the latitudes 15°S
and 23°S. The highest 3 H e / 4 H e ratios (reported as R / R A ratios: R = sample 3 H e / 4 H e , R A = air 3 H e / 4 H e ) are
associated with the active volcanic arc of the Western Cordillera (0.92 < R / R A < 5.52) and approach ratios found at
other convergent margins in the circum-Pacific region. A significant 3He component is also present in fluid and gas
samples from the high Altiplano plateau (0.48 < R / R A < 3.56) and the Eastern Cordillera (0.03 < R / R A < 1.2), up
to 300 km east of the active arc and more than 300 km above the subducting slab. This wide zone of 3He anomalies
is delineated both to the east and the west by regions with low 3 H e / 4 H e ratios ( < 0.2RA), typical of radiogenic
helium production in the crust. Studies of the regional groundwater regime suggest that the wide zone of elevated
3 H e / 4 H e values away from the active volcanic arc is unlikely to be caused by lateral and shallow transport of
magmatic helium and there is no evidence for significant crustal sources of 3He. The high 3 H e / 4 H e ratios are
interpreted as reflecting degassing of volatiles from mantle-derived magmas emplaced over an area 400 km wide
beneath and into crust up to 75 km thick. The subducting slab is at depths of 100-350 km in this region. In the west,
underneath the active volcanic arc, mantle melting is probably largely controlled by mantle hydration and
dehydration and the helium isotope data can be used to delineate the extent of the asthenospheric mantle wedge at
depth. In contrast, mantle melting behind the arc, beneath the Altiplano and Eastern Cordillera, may be a result of
convective removal of the base of the lithosphere. The sharp cut-off in the mantle helium signal in the east is
interpreted as marking the western edge of thick and relatively cold lithosphere, devoid of mantle melts, which could
transport mantle volatiles towards the surface. This may coincide with the limit of underthrusting of the Brazilian
shield beneath the eastern margin of the Central Andes.
I. Introduction
C o n s i d e r a b l e c o n t r o v e r s y s u r r o u n d s t h e form a t i o n a n d uplift of a r e a s of u n u s u a l l y thick
1 Present address: Vrije Universiteit Amsterdam, Faculty of
Earth Sciences, De Boelelaan 1085, 1081 HV Amsterdam,
The Netherlands.
c o n t i n e n t a l crust ( < 75 km) in active p l a t e conv e r g e n c e zones, such as the high p l a t e a u regions
of the C e n t r a l A n d e s a n d Tibet. T h e d e b a t e is
a b o u t t h e m e c h a n i s m s o f b o t h crustal t h i c k e n i n g
a n d uplift. F o r instance, crustal t h i c k e n i n g a n d
uplift m a y b e p r e d o m i n a n t l y a result of h o m o g e n e o u s crustal s h o r t e n i n g , o r s o m e o t h e r m e c h a nism, such as t h e a d d i t i o n of s u b s t a n t i a l v o l u m e s
0012-821X/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved
SSDI 0 0 1 2 - 8 2 1 X ( 9 4 ) 0 0 1 8 6 - 3
L. Hoke et al. / E a r t h and Planetary Science Letters 128 (1994) 341-355
342
of new material to the base of the crust. Dynamic
models of such regions predict that parts of the
mantle lithosphere exert an important control on
the overall pattern of deformation [1-6]. Of particular interest is the idea that convective removal
of the base of the thickened lithosphere may
cause surface uplift [3,6] in addition to that caused
by crustal thickening. The thermal consequences
of convective removal of the lower (cold) litho-
70:'W
I
sphere and replacement with (hot) asthenosphere
should be melting at depths of less than ~ 120
km [7]. This model has been used to explain
volcanism with mantle characteristics in both the
Tibetan plateau [6-8] and the Puna region of
northwest Argentina [9,10]. Segregating and ascending mantle melts would be expected to transport mantle volatiles into the crust, degassing
during emplacement. Geothermal and deeply cir-
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(a)
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A[ ucam a /
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66'
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Fig. 1. ( a ) G e o l o g i c a l m a p of the Central A n d e s of Bolivia and northern Chile showing the localities of 70 g e o t h e r m a l and sulfatara
sample sites listed in Table 1. T h e pre-Tertiary and C e n o z o i c outcrop pattern and all active v o l c a n o e s along the arc of the W e s t e r n
Cordillera are also shown.
L. Hoke et al. / E a r t h and Planetary Science Letters 128 (1994) 341-355
69°W
(b)
18°S
68°W
(c)
343
68°w
67°W
66°W
17°S
19°S
18°S
17°S
16°S
19°S
20°S
18°S
17°S
20°S
21 °S
19°S
18°S
21 °S
22°S
19°S
68%V
67°W
67°W
66 °w
65 °W
Fig. 1. (b) Helium isotope ratios for sample sites along the volcanic arc and adjacent areas, including the Precordillera in the west
and the Altiplano in the east (see box in a). (c) Helium isotope ratios of the sample sites in the Eastern Cordillera and along the
transition zone between the Eastern Cordillera and the Altiplano in the west (see box in a). L F = the Miocene-Pliocene Los
Friales ignimbrite. Darker shading = the pre-Tertiary outcrop pattern; lighter shading = the Cenozoic outcrop pattern.
eulating groundwaters would finally transport
these volatiles to the surface. It is in this context
that the present study was undertaken. The presence of an unequivocal tracer of mantle volatiles
in regions of thick crust, such as 3He, would be a
clear and unambiguous indication of mantle melting on a regional scale and of the mode of both
lithospheric deformation and crustal thickening.
The advantage of using helium is that its isotopic
ratio ( 3 H e / 4 H e ) in mantle-derived materials is
distinct and about three orders of magnitude
greater than helium produced by nucleogenic/radioactive processes in the crust.
It has been known for some time that mantle
degassing associated with the formation of new
ocean crust is accompanied by helium with a
3 H e / 4 H e ratio approximately eight times that of
air [11-14]. High ratios have been found in actively spreading back-arc basins [15,16] and also
in arc-related environments in the circum-Pacific
region [17]. 3 H e / a H e ratios greater than that of
air have also been observed in the continents, in
areas of thinned lithosphere, especially where
there is recent volcanism, such as in the lower
Rhine graben and western Pannonian basin [1922]. In all these cases the results have been interpreted in the context of mantle melting as a
consequence of upwelling and adiabatic decompression of the hot asthenospheric mantle induced by lithospheric extension. To date, however, there have been few helium isotope surveys
of areas of unusually thick continental crust in
zones of active continental shortening. In this
study we present the results of a detailed investi-
Table 1
List of water and gas samples from sulfataras, geothermal and mineral water springs in the Central Andes of Bolivia and northern
Chile, showing location, general characteristics and He isotope analyses
(1)
Locality
Sample
Let.
Long.
(2)
A l t i t u d e Sample
(m) a.sJ. t y p e
Precordillere
- Northern
Chile
Las C u e v a s
1 8 ° 0 9 ' 69026 .
3950
Jurase
18013 ` 69031 '
4060
Berenguella
19009 . 69012 .
3740
Pucholtisa
19025 . 68058 .
4150
Chusmisa
19042 . 69011 .
3500
MamiSa
20004 . 69013 `
2890
Volcanic arc o Western Cordillera
- Bolivia and
Sajama
18°04 ' 68°58 '
4205
Rio J u n t h u m a
18006 ' 69002 '
4410
Churiguyaya
18021 ' 69011 '
4420
Surirs-4
18055 ` 68039 '
4260
S u r i r e -2
1 8 ° 5 5 ' 68039 '
4260
T o d o s Santos
19003 ' 68051 '
4060
Caico
19008 ' 69005 '
4025
V o l c a n Isluga
19010 ' 68050 '
5070
Enquelca
19014 ' 68047 '
3960
Lirima
19051 ' 6 8 ° 5 4 '
3900
Canto
19054 ' 68037 '
3780
Abra de N a p p a
20°32 ' 68°34 '
3730
Mine C o n c e p t i o n
20033 ' 68032 '
3920
Empexa
20035 ` 68°21 '
3900
Irrutupuncu
2 0 ° 4 3 ' 68039 '
3800
Volcan Irrutupuncu
20044 ' 68°32 '
4950
Volcan O l c a
2 0 ° 5 7 ' 68028 '
5320
Volcan Ollague
21018 ' 68°11 '
5510
El Tatio
2 2 ° 1 9 ' 68000 '
4160
22042 ' 68000 '
3470
Puritans
Altiplano
high plateau Bofivia
Viscachani
17010 ' 67058 '
3900
Obrajes
17049 ' 66059 '
3790
Capachos
1 7 ° 5 4 ' 67003 '
3740
Machacamarca
18°09 ' 67002 '
3700
18023 ' 66056 '
3700
Poopo
Pazna
18035 ' 66055 '
3750
B e l c h de A n d a m a r c a
18053 ' 67044 '
3665
Challapata
18055 ' 6 6 ° 4 7 '
3360
Castilluma
19008 ' 66043 '
3870
19014 ' 68023 '
3653
Coipasa-2
19043 ' 66033 '
4315
Machicao
Rio Mulatos
19043 ' 66046 '
3820
Tornave
20°01 ' 6 6 ° 3 4 '
3950
L a g u n a Pastos G r a n d e
21°37 ' 67°50 '
4380
L a g u n e Colorado
22010 ' 67047 '
4280
Sol de M a n a n a
22025 ' 67046 .
4700
Eastern
Cordillera
- Bolivia
Mina Matilde
15°46 ' 68°58 '
2890
Urmiri / L a Paz
1 6 ° 5 7 ' 67057 '
3480
Kami
1 7 ° 2 5 ' 66051 '
3020
Caba~os
17031 ' 66020 '
2470
Cayacayani
17°33 ' 66012 '
2550
Pojo
17051 ' 64046 '
2000
Colcha
17°51 ' 6 6 ° 2 4 '
2775
A q u a s Calientes, O r u r o
17053 ' 66037 °
3520
Julo G r a n d s
18°03 ' 6 5 ° 4 9 '
2110
Huanui
18°12 ' 6 6 ° 5 2 '
9940
Ventilla
18024 ' 66037 '
3830
Catavi
18°24 ' 66°35 '
3690
Uncia
18°26 ' 6 6 ° 3 3 '
3680
Urmiri
18°34 ' 66°52 '
3740
Chomiri
18°36 ' 6 6 ° 3 8 ,
3680
Laluni
18040 ' 6 6 ° 2 3 '
3750
Salines
18°52 , 6 6 ° 0 7 ,
3540
Estancia C o m p a n s i a
18°54 ' 65°19 ,
2850
Yurimata
18°56 ' 65°41 '
3360
Tinquipaya
19°14 ' 65°47 '
3100
Miraflores
19°27 ' 65°42 '
3400
Miraflores 2
19°27 ' 65°48 '
3280
Tarapaya
19028 ' 6 5 ° 4 3 '
3350
Don Diego
19°30 ' 65°36 '
3550
Chaqui
1 9 ° 3 7 ` 65034 '
3620
J a t u n Mayu
19°54 ' 65°43 '
3200
Caiza
2 0 ° 0 4 ' 65040 '
3070
Pukara
20°06 . 66°19 '
3695
(3)
Temp.
° C
s/w
31°C
s/g
65°C
s/w
60°C
f/g
860C
s/w
48°C
s/w
55°C
northern
Chile
s/w
43°C
s/g
88°C
s/w
46°C
s/g
68°C
s/g
73°C
s/w
33°C
s/w
68°C
fs/g
92°C
s/w
31°C
s/w
68°C
s/g
45°C
s/w
34°C
fs/g
>90°C
S/W
36°C
s/w
56°C
ss/g
400°C
ss/g
85°C
fs/g
>120°C
s/w
76°C
s/w
35°C
(4)
pH
(5)
R/RA
(6)
X
(7)
[Hs]C
~ccSTP/g
6-6.5
6
5
6
5
5,5
1.11
0.95
0.42
1.22
0.18
0.095
2.45
266,99
22,93
1.27
91.05
37.72
0.37
6-6.5
6
7
5.5
5.5
6-6.5
6-6.5
N.D.
0.23
N.D.
108
70
0.33
2.68
333
16.91
21.09
27.93
0.84
6.51
1.23
2.14
N.D.
42.45
15.4
37.72
2.55
0.02
5
4.5
1.42
1.82
0.92
5.03
4.73
3.4
3.63
5.51
2.75
0.36
2.45
1.06
3.01
1.68
2.35
4.96
1.75
1.8
1.71
1.02
6
5.5
7-7.5
6
6
3
s/9
s/w
s/w
s/w
s/w
s/w
s/g
s/g
s/g
s/g
s/g
s/g
s/g
s/w
s/g
s/g
36°C
79°C
51°C
50°C
77°C
57°C
13°C
38°C
57°C
110C
52°C
14°C
57°C
32°C
23°C
85°C
6.5-7
6
7.5
6.5
7
6.5
6
7.5
7
6
7
6-6.5
7-7.5
6-6.5
6.5
6
0.48
0.56
0.65
1,9
0.55
0.62
1.93
1.15
1.38
2.4t
3.5
1.72
2.87
3.52
2.34
2.21
3063
10.41
388.85
15.34
252.62
159.51
98.58
5.71
128.02
5.16
1.57
0.89
23.21
66.27
118
76.68
s/g
s/g
f/g
s/w
s/g
s/w
s/g
f/g
s/w
s/w
s/w
s/w
s/g
s/w
s/w
f/g
s/8
s/g
s/w
f/g
s/w
s/w
s/9
s/w
s/w
s/9
f/g
s/g
55°C
75°C
86°C
48°C
30°C
34°C
60°C
79°C
30°C
39°C
72°C
62°C
40°C
61°C
70°C
92°C
65°C
45°C
43°C
63°C
66°C
63°C
57°C
35°C
69°C
62°C
89°C
49°C
5.5
7.5
7.5
6.5
6.5
5
7
8
6.5
6.5
6.5
7
7
6.5-7
7
7
7
7
7
7
6.5
6
7-7.5
6.5-7
7
7,5
7.5
7-7.5
0.37
0.26
0.52
0.13
0.065
0.045
0.13
0.49
0.2
0.98
0.87
1.03
0.95
0.72
1.18
1.14
1.19
0.19
0.84
0.55
0.52
0.55
0.57
0.35
0.4
0.13
0.2
0.76
99.27
3.4
0.55
206.1
N.D.
20.1
24.7
0.52
1.03
2.76
171.15
4.2
30.88
97.2
N.D.
N.D.
28.96
30.74
1.04
433.4
225.2
5.82
78.73
7.74
32.11
12.19
26.22
9.93
0.21
0.93
4.62
3.66
0.07
0.27
0.15
0.03
0.01
0.008
0.05
0.13
6.28
0.009
1.73
1.15
13.75
12.62
29.51
0.42
0.62
0.71
0.03
0.89
0.26
0.004
1.88
1.44
0.41
(7)
RC/RA
1.19
0.95
0.39
(>1.22)
0.17
0.078
(8)
% mantle He
14.3
11.3
4.3
(>14.7)
1.5
0.3
1.42
(>1.82)
0.92
5.07
4.78
(>3.4)
5.19
5.52
2.86
0.33
2.5
(>1.06)
3.37
(>1.68)
3.53
4.96
1.77
1.86
1.73
1.03
68.8
35.4
3.5
30.8
(>12.7)
41.8
(>20.5)
43.8
61.8
21.5
22.8
21.1
12.3
0.48
0.51
0.65
1.96
0.65
O.62
1.94
1.18
1.38
2.75
(>3.5)
(>1.72)
2.95
3.56
2.35
2.23
5.4
5.7
7.6
24
7.6
7.2
23.8
14.2
16.7
34
(>43.4)
(>21.0)
36.5
44.2
28.9
27.4
0.36
(>0.26)
(>0.52)
0.13
0.065
0.045
0.07
(>0.49)
(>0.2)
0.03
0.87
1.04
0.95
0.72
1.18
1.14
1.2
0.16
(>0.84)
0.55
0.52
0.49
0.56
0.28
0.38
0.05
0.17
0.73
3.9
(>2.6)
(>5.9)
1
<1
<1
<1
(>5.5)
(>1.9)
1.4
10.3
12.5
11.3
8.4
14.2
13.7
14.5
1.4
(>9.94)
6.3
5.9
5.5
6.4
2.5
4.2
0
1.5
8.6
17.2
(>8.6)
10.9
63.1
59.5
(>42.1)
64.7
L. Hoke et al. / Earth and Planetary Science Letters 128 (1994) 341-355
gation of the helium isotope characteristics of
volcanic gases and geothermal springs from the
Central Andes. The work follows on from the
reconnaissance survey of Hilton et al. [18] which
reported high ratios in the high plateau region
(Puna) of northwestern Argentina. Here, we
specifically consider the problem of using 3He
data to map mantle melt additions to the base of
the continental crust, in both the active arc and
adjacent regions behind the arc, and we examine
the implications of this data for the evolution of
such regions of thickened crust.
2. Geological background to the Central Andes
The Central Andes form a high ( < 6500 m)
and wide ( < 900 km) mountainous region which
slopes off towards the Pacific Ocean in the west
and the Amazon basin in the east. It can be
divided into a number of distinctive physiographic and geological provinces (Fig. la). There
is the high plateau region (Altiplano) in the central part of the mountain range which extends
roughly 1200 km along strike, is up to 250 km
wide and has an average elevation of 3800 m and
is underlain by crust up to 75 km thick [23,24,51].
The Altiplano is the second largest high plateau
on Earth after Tibet. The active volcanic arc
345
(Western Cordillera) bounds the plateau in the
west, where M i o c e n e - Q u a t e r n a r y composite volcanoes reach elevations < 6 5 0 0 m [31]. The
Chilean Precordillera lies to the west of the arc
and was the site of the active arc in the Cretaceous and early Tertiary. The eastern edge of the
Altiplano is bounded by the Eastern Cordillera,
which rises abruptly to heights of over 6000 m in
the Cordillera Real and passes into the Subandean ranges and Amazon basin further east.
The great thickness of the Andean crust is
largely the result of plate convergence between
the oceanic Nazca plate and the South American
continental plate which has taken place since the
Cretaceous. At present, there is ca. 85 m m / y r of
plate convergence at the latitude of the Central
Andes [25], of which 65-75 m m / y r is accommodated near the interface with the downgoing slab
[26]. The remaining convergence is absorbed at
the surface by underthrusting of the Brazilian
shield in the Subandean zone. However, prior to
ca. 5 Ma, surface compressional deformation was
active in the Altiplano and Eastern Cordillera
[27]. The Altiplano acted as a major intermontane basin for much of the Tertiary, accumulating
< 10 km of continental sedimentary and volcanoclastic sequences, which were folded prior to a
Lower Miocene unconformity, with further compressional deformation continuing up to 5 Ma
Notes to Table 1:
He isotope analyses are shown as air normalised ( R / R A ) and air corrected ( R C / R A ) ratios and also as percentage mantle
component of He.
a Sample names refer to the nearest locality, in most cases a village or estancia (1:250,000 a n d / o r 1:50 000 scale Carta Nacional
Bolivia and the 1:250,000 Carta Terrestre de Chile).
2 Sample type: s = geothermal spring; f = fumarole; fs = flank sulfatara; ss = summit sulfatara; w = water sample; g - gas sample.
3 Maximum temperature measured at the source of the geothermal spring.
4 pH of surface water above source.
5 The measured 3 H e / 4 H e ratio (R) is normalised to the atmospheric helium isotope ratio R A (1.4 × 10-6).
6 X represents the sample ( H e / N e ) ratio normalised to the calculated air ( H e / N e ) ratio at the estimated altitude of recharge of
approximately 4000 m (air 4He/ZONe = 0.421), multiplied by 1.322, which is the ratio of the Bunsen solubility coefficients of Ne to
He at the estimated recharge temperature of 0°C.
7 The measured ( R / R A) ratio and helium concentration (water samples only) are corrected for any air-dissolved helium, assuming
all the measured neon is of atmospheric origin using R c / R A = [ ( R / R A × X ) - 1 ] / ( X - 1) and [He] c = [He] . . . . . . . . o ( X - 1 ) / X
after Craig et al. [52]. In the cases where X < 2, the air correction is large. In all these cases, the measured R / R A is considered a
minimum estimate of the true R c / R n ratio, which is shown in brackets.
8 Calculated percentage mantle He values, based on the assumption that mantle-derived He has R c / R A - 8 and the measured
production ratio of average continental crust in the Central Andes is 0.05.
346
L. Hoke et al. / Earth and Planetary Science Letters 128 (1994) 341-355
ago. Volcanic sequences younger than 5 Ma are
only gently tilted and rest with marked angular
unconformity on these older sequences [27].
3. Sampling and experimental techniques
A total of 70 geothermal sites were sampled
from the Central Andes of Bolivia and northern
Chile for helium isotope analyses (Table 1; Figs.
1, 3). Most sites occur in two distinct zones: one
follows the active volcanic arc, along the international border between Chile and Bolivia, and the
other covers the eastern edge of the Altiplano at
•1
1
10
100
1000
10
10000
MORB
2
.1
R~=O
"0]
........
.1
I
1
~CRUST
t~/RA=0.02
........
10
Samples: , ~ Arc Sulfatara
X
•
100
1000
Altiplano
10000
[] Eastern
Cordillera
the transition to the Eastern Cordillera. Other
samples come from the Altiplano, as well as from
the Eastern Cordillera of Bolivia (as far east as
the Subandean foothills) and the Chilean Precordillera ( < 100 km west of the active volcanic
arc). All samples of gas, condensate or water
were collected in duplicate and stored in copper
tubes sealed by cold welding [28]. Prolonged
flushing of the sampling apparatus minimized the
effects of air contamination.
Laboratory techniques adopted for the present
study closely follow those described previously
[18,29]. Fluids and gases were processed on a
separate glass extraction line, which allowed isolation of the non-condensable gas fraction (containing the helium) and collection in glass breakseals. Helium isotope analyses were carried out
using a MAP215E rare gas mass spectrometer
with the h e l i u m / n e o n ratio monitored in all samples to correct for any air helium contamination
[18]. Helium results are reported as R c / R A ratios, where the measured 3 H e / 4 H e ratio (R) is
corrected for air contamination (R c) and normalised to the 3 H e / 4 H e ratio of air (R A) (Table
1). For the most part, the air correction resulted
in minor ( < 10%) changes to the measured
3 H e / m i l e ratio. However, where the H e / N e ratio approached that of air ( X < 2), the correction
procedure was not applied and the measured
value was considered the best (minimum) estimate of the true 3 H e / 4 H e ratio.
Q Arc Geothermal O Precordillera
Fig. 2. Diagram showing measured, air-normalised helium
R / R A values, plotted against X (Table 1) of the water and
gas samples analysed in this study. This illustrates three
principal sources for the helium measured in the sample: (1)
crustal, (2) mantle and (3) air saturated water (ASW) sources.
Most samples plot between the two mixing trajectories defined by the mixing of ASW with a mantle source (MORB
helium, R / R A = 8) or a crustal source ( R / R A = 0.02). For
samples which fall outside these trajectories the air correction
(Table 1) results in negative R c / R A values. These samples
have probably lost helium relative to neon, in addition to air
contamination (cf. [50]. For samples with X < 2, the air correction was not applied (shown in brackets in Table 1) and the
measured value was considered the best (minimum) estimate
of the true 3 H e / 4 H e ratio. In general, the samples from the
volcanic arc and the Altiplano show significant mantle-derived
components (1 < R / R A < 5.5) compared to most of the samples from the Eastern Cordillera and the Chilean Precordillera. ( R / R A < 1).
4. Results
Helium isotope results in the present study are
given in Table 1 and Figs. lb,c and 3. In the
absence of other plausible sources of helium (see
discussion), all samples can be represented by a
simple 3-component e n d - m e m b e r mixture of
mantle-derived ( ~ 8RA), crustal ( < 0.1R A) and
atmospheric helium (1R A) (Fig. 2). In the following presentation only the air-corrected ratios are
discussed. The results span a range from a high
of 5.5R A at Isluga volcano, in the active arc, to
typical crustal values of 0.078R a at Mamifia, in
the Precordillera, and < 0.05R A at Pojo and
Cayacayani in the Eastern Cordillera. If we as-
L. Hoke et al. / Earth and Planetary Science Letters 128 (1994) 341-355
sume that pure radiogenic helium produced in
the crust has an R c / R A < 0.1, then the general
picture which emerges is one in which mantle-derived helium is present within a ~ 400 km wide
zone across the Central Andes. The mantle component is most prominent in the active arc and
Altiplano and diminishes in the Precordillera and
Eastern Cordillera. In detail, the helium isotope
distribution in the various provinces of the Central Andes can be described as follows.
4.1. Precordillera and Western Cordillera
The highest 3 H e / 4 H e ratios measured in this
study come from the presently active volcanic arc,
although considerable variation exists between
individual volcanoes (Fig. lb). Two of the highest
ratios are found within 28 km of each other at
Isluga volcano (5.5R A) and the geothermal field
at Surire (5.1RA). Assuming that pure mantle-derived helium is characterised by 8RA, then close
to 70% of the helium at Isluga is of mantle
derivation. Both to the north and south of these
localities, 3 H e / 4 H e ratios are more variable and,
for the most part, considerably lower. For example, there is a large variation in four closely
spaced volcanoes about 120 km to the south:
sulfatara gas samples from both Mina Conception
(on the flank of Cerro Caiti) and the summit of
Irrutupuncu volcano have R c / R a ratios of 3.4
and 5.0, respectively, whereas gases from Olca
and Ollague volcanoes, between 25 km and 45 km
further south again, have 3 H e / 4 H e ratios of
1.8R a and 1.9R A, respectively. This lack of geographic control on the 3 H e / 4 H e ratios confirms
previous observations [18] further south in the
same arc.
Away from the active volcanoes, geothermal
water samples also have a large range in 3 H e / 4 H e
values. In addition to Surire, Caico, situated 26
km from Isluga volcano, has a high proportion of
mantle-derived helium (5.2RA, equivalent to 65%
mantle helium). In contrast, predominantly radiogenic values are found at Churiguyaya (0.9RA),
Abra de N a p p a ( > 1.1R A) and Puritana (I.0RA).
There is little indication that 3 H e / 4 H e ratios of
the gases and waters are controlled simply by
contamination with radiogenic helium during flow
347
to the surface. The highest values are found
25-30 km away from volcano summits and not on
the flanks or at the base of the volcanoes, indicating that prolonged subsurface flow may not necessarily corrupt geothermal 3 H e / 4 H e values; indeed, at Canto, located some 80 km south of
Isluga volcano, a persistent mantle signal (2.5R A)
is still evident. However, in some circumstances
this may not always be the case. For example,
lower values on flanks or at the base of volcanoes
compared to the summit values may reflect addition of a radiogenic crustal helium after degassing
of the magmatic body; a geothermal water sample
taken from the geothermal field at Enquelca, at
the base of the Isluga volcano, had a R c / R A
ratio of 2.9, whereas the near-summit sulfatara
gas sample had a 3 H e / 4 H e ratio of 5.5R a.
Towards the west, in the Precordillera, the
mantle helium component decreases abruptly to
an essentially pure radiogenic crustal helium isotope ratio of 0.078R a in the geothermal area of
Mamifia, located approximately 80 km west of the
active volcanic arc. Low R c / R A values are also
found in the Chilean Precordillera north of
Mamifia, Chusmisa (0.17 R A) and Berenguella
(0.39R A) (Fig. lb). At Lirima, which is situated in
a region where the active volcanoes are unusually
widely spaced (Pica gap [30,31]), a low R c / R A
value of 0.33R a has been measured.
4.2. Altiplano
Sixteen samples have been analysed from the
high plateau region of the Altiplano (Fig. lb,c).
They too show a wide range of helium isotope
ratios, varying from 0.48R A (Viscachani) in the
north to 3.6R A (Laguna Pastos Grande) in the
south; equivalent to between 5 and 44% mantlederived helium. Most of the samples from the
northern part of the Altiplano have low 3 H e / 4 H e
ratios (Table 1), with the highest values concentrated in the southern, central and eastern regions. For example, Machicao and Tomave have
R c / R A ratios of > 3 . 5 and 3.0, respectively,
equivalent to between 36% and 44% mantle-derived helium. These samples are located about
220 km east of the active volcanic arc, to the east
of the Salar de Uyuni and at the southwestern
L. Hoke et al. /Earth and Planetary Science Letters 128 (1994) 341-355
348
edge of the Los Frailes volcanic field. This volcanic field is the largest composite eruptive field
in the Eastern Cordillera, consisting mainly of
high-potassium acidic to intermediate ashflow
tufts, which cover an area of 8500 km 2 [32]. The
main phase of eruption occurred during the
Miocene, but ignimbrites only a few million years
old have been recorded [27,32]. The significant
mantle-derived component of helium in hot
springs in this area suggests current melt produc-
7{}°W
68°W
P, cgion wilh clcvalcd
14°S
-3 l
tion at depth, despite the fact that no recenl
basaltic lava flows are found on the surface.
4.3. Eastern Cordillera
Eastern Cordillera geothermal springs have
3 H e / 4 H e ratios of 1.2 > R c / R A > 0.03, equivalent to between ~ 0% and 14% mantle-derived
helium (Fig. lc). The highest values are concentrated east of Lake Poopo in one of Bolivia's
66°W
~4°W
', "1-",", "," ,"," ,", "l
I~'(I/RA: O < {).2
n~lnllc Ic c(}mponcnl
P.cgi{m of sn~fll h~L~allic
andcsile cones (< 5.5Ma)
Active Volcan{}
.
0.3-1.4
Q 1.5-2.75
(1~ 2.8-4.4
•
4.5-5.5
BRAZH,1AN
SHIE1A)
,
x
.'
x
/
\
/
,
z
x
/
~
/
,
/
,~
."
x
~
l / / I l l . c / /
. . . . .
%/%/N/~
',",'7
16°S
16 °
;anta
i:mz
-
18{'I
18os
20os
/
22 ~
22°S
I
7(I°W
6B°W
66°W
64°W
Fig. 3. Sketch m a p of the Central A n d e s showing sample distribution and their R c / R A ratios. Elevated ( R c / R A > 0.2) mantle
helium isotope signatures define a ~ 350 km wide zone bounded in the east and west by samples characterised by radiogenic
helium. T h e high 3tte/4I~Ie ratios are interpreted as reflecting degassing of volatiles from mantle-derived magmas at depth. D e p t h
contours of the Benioff zone are also shown [38]. The Benioff zone is at a depth of 100-120 km beneath the active volcanic arc and
300 km beneath the transition to pure radiogenic helium isotope ratios in the east. Also shown is the region of small basaltic
andesite cinder cones (younger than 5.5 Ma) scattered across the Altiplano. Two S W - N E trending lines define the helium isotope
profiles in the northern (Fig. 5) and southern central Altiplano (Fig. 4). All data points up to 100 km either side of the lines were
used in the construction of the profiles in Figs. 4 and 5.
L. Hoke et al. / Earth and Planetary Science Letters 128 (1994) 341-355
most intensely worked tin mining areas, situated
between the extinct Morocacala volcanic field in
the north and the Los Frailes volcanic field in the
south. R c / R A values become more radiogenic to
the east and west. Notably, the region with
R c / R A > 1 is rich in shallow, intrusive granitic
and acidic volcanic rocks of Tertiary age, which
cut Palaeozoic, low grade sedimentary sequences.
The marked 3He signature in geothermal waters
amongst extensive outcrops of acidic rocks, with
relatively high radioelement contents, further emphasizes the degassing of mantle-derived volatiles
in this region.
The eastern margin of the zone of elevated
3 H e / 4 H e ratios in the Eastern Cordillera can be
defined approximately by a line which separates
localities with R c / R A ratios > 0.2 from those
with R c / R a v a l u e s < 0.2 (Figs. lc and 3). In the
north, along the eastern shore of Lake Titicaca
and in the La Paz area, this boundary is close to
the transition zone between the Altiplano and
the Eastern Cordillera; further south it is well
within the Eastern Cordillera and follows a trajectory from Urmiri in the north, via the western
side of the Cochabamba basin, to Sucre and
Caiza in the south. Therefore, to the east of this
line radiogenic helium isotope ratios predominate
(Fig. 3); for example, at the Pojo hot spring
(0.045R A) and around the Cochabamba basin at
Cabafios (0.13R A) and Cayacayani (0.065RA).
5. Discussion
Helium isotope results described above indicate that mantle-derived helium is widespread
throughout the active arc and Altiplano region of
the Central Andes. While the observation that
mantle degassing through arc-related volcanism
has been made both for the Western Cordillera
[18] and other Pacific rim localities [17], the high
3 H e / 4 H e ratios in the thick crust of the Altiplano and also in the western part of the Eastern
Cordillera are more surprising. An important
consideration in this respect is the possibility that
some of the arc-related mantle 3He may have
been transported laterally by shallow aquifer systems, thereby distorting the regional 3He picture.
349
This may be the case on a local scale ( < 10 km),
for instance, along the volcanic arc where some
variation in the helium isotope signature might be
related to mixing with radiogenic helium present
in shallow aquifer systems. However: (1) marked
differences in stable isotope (O and H) signatures
of closely spaced geothermal systems [Hoke, unpublished data]; and (2) the presence of high
3 H e / 4 H e ratios in areas crossed by numerous
watersheds; suggest that, on a regional scale, the
transport of helium in geothermal fluids is predominantly vertical rather than lateral.
For the most part, 3 H e / a H e ratios from the
volcanically active regions measured in this study
(0.92 < R c / R A < 5.52) are lower than those reported for volcanic gases from other Pacific rim
arcs (5 < R c / R A < 8 )
[17] but span approximately the same range of values measured in the
southern portion of the same Western Cordillera
and in the southern volcanic zone of central Chile
[18]. This wide range was interpreted as reflecting
the addition of crustal radiogenic helium to mantle-derived melts through the processes of magma
aging or waUrock interaction - - with the possibility also of 4He addition [18] in zones of magma
assimilation at the crust/mantle transition zone
(MASH zones [33]).
5.1. Li-rich salars: a crustal source of 3He?
Three of the sample sites in this study are
close to saline and alkaline lakes and salars in the
central and southern part of the Bolivian Altiplano. These lakes and salars are known for their
high lithium concentrations and, as such, are a
potential source of crustal 3He, due to the
6Li(n,a)3H-3He reaction. Risacher and Fritz [34]
estimate an average of ~ 150 ppm Li in the
Uyuni salar for a total mass of salt crust and
brine of 66 × 10 9 t. We calculate the total production of 3He over the lifetime of the salar
( ~ 10 000 yr) to be 4.2 × 10 11 ccSTP/g, assuming a 3He production ratio of 6.1 × 10 - 6 atoms
g-~ s-1 ppm Li-1 [35], calculated for an altitude
of 3650 m, a latitude of 21°S and an attenuation
factor of 0.5 at depth in the salar. This is less
than the measured 3He concentration of 6.93 ×
10 - l l c c S T P / g H 2 0 for Laguna Pastos Grande,
350
L. Hoke et al. /Earth and Planetary Science Letters 128 (1994) 341-355
even in the unlikely case of complete retention of
the highly mobile 3He daughter product during
the entire life of the salar. The measured lithium
concentrations are much less in the geothermal
waters collected for helium isotopes than in the
brines (Laguna Coipasa = 9.42 ppm, Laguna Pastos Grande = 0.78 ppm and Laguna Colorado =
0.46 ppm, [36]), indicating that the waters themselves are a less likely source of crustal 3He.
Additional evidence against this crustal source of
3He is afforded by phenocryst phases of nearby
basaltic lava flows. The crushing of olivines from
the very young ( < 0.5 Ma) Nekke Kkota Maar
[Hoke, unpublished K / A r ages], located near the
southeastern corner of Salar de Coipasa [37]
yielded a 3 H e / 4 H e ratio of 6.9R A [Hoke and
Hilton, unpublished data], further emphasizing
the mantle origin for the high 3 H e / 4 H e ratios
and contemporaneous volcanism in this region of
thickened crust.
5.2. Mantle melting in the Central Andes
This study has documented 3 H e / 4 H e ratios
significantly above atmospheric values, not only
in volcanic gases and geothermal sites associated
with the active volcanic arc but also in a vast
region extending up to 300 km behind the arc, in
the Altiplano high plateau region and western
part of the Eastern Cordillera. In the absence of
significant crustal sources of 3He, the most likely
origin for this 3He is mantle melts present at
depth. We suggest that this mantle helium signature maps an extensive area of active mantle
melting and concomitant subsurface basalt addition to the Andean crust. This is also suggested
by the presence of small young basaltic andesite
volcanic centres ( < 5.5 Ma) [Hoke unpublished
K / A r ages] in the Altiplano (Fig. 3), mainly concentrated in the area between the Coipasa and
Uyuni salars [42]. However, the helium isotope
results from geothermal springs show clearly that
outgassing of mantle-derived helium is not only
restricted to these regions of young basaltic volcanism, b u t occurs over a much wider area, extending right across the Altiplano and well into
the Eastern Cordillera (Figs. 3-5). This supports
the notion that helium isotopes are a very sensi-
SW
SOUTHERN CENTRAL ALTIPLANO
NE
100
10
L)
.1
.01
.001
km 0
Samples:
100
200
Q Water
•
300
/~ Sulfatara
400
500
o Gas
Olivine phenocrysts - Nekke Khota Maar
Fig. 4. NE-trending helium isotope profile across the southern
central Bolivian Altiplano showing the variation in the helium
isotopic ratios measured in geothermal springs and sulfataras
(see Fig. 3 for location and construction). Also shown is the
3 H e / 4 H e ratio of 6.9 R A [Hoke and Hilton, unpublished
data] measured in fluid inclusions in olivines from the < 0.5
Ma Nekke Kkota Maar, located near the southeastern corner
of Salar de Coipasa. The elevated R c / R A mantle-derived
signature is clear within a 350 km wide zone.
tive tracer of mantle degassing, even in areas
where there is no active volcanism [19-22].
Arc volcanism
The narrow region of active volcanism in the
Central Andes coincides with the anticipated
melting zone at a depth of 100-120 km, where
the mantle wedge undergoes the transition from
amphibolite to eclogite facies metamorphism and
hydration and partial melt formation in the overlying mantle column [39,40] occur. The long Tertiary history of arc volcanism suggests that there
has been a steady and focused supply of magma
at depth [30]. In addition, andesitic magmas have
erupted at temperatures of ll00°C or higher
[Sparks, pers. commun.]. Thus, melting in the
zone of induced flow in the hot asthenospheric
mantle wedge is the most plausible origin of both
sustained arc volcanism and the measured 3He
[17,39]. In this case, the depth of the Benioff zone
must be greater than the depth for the lithosphere-asthenosphere boundary. It should be
noted that this implies that the base of the lithosphere beneath the arc is at a much shallower
L. Hoke et al. / Earth and Planetary Science Letters 128 (1994) 341-355
depth than in the region much further east beneath the Brazilian shield, where it is expected to
be at a depth much greater than 200 km [23].
The high 3 H e / 4 H e ratios of the active arc
decrease towards the west in a relatively short
distance to radiogenic crustal values in the
Mamifia (0.078R A) and Chusmisa (0.17R A)
geothermal areas in the Chilean Precordillera. If
the elevated helium signatures are due to the
presence of mantle melts at depth below the
sampling sites then these low ratios suggest that
the westernmost limit of the asthenospheric man-
351
tle wedge lies between these localities and up to
80 km west of the arc. To the east, however,
where the Benioff zone is at greater depths, the
mantle and the subducting slab are considered
too dry to trigger melt formation [39,40] and
segregation [41]. Therefore, a mechanism different from subduction-induced fluid fluxing and
melting aLvve the slab and beneath the active
volcanic arc has to be considered to explain the
presence of mantle melts at depth beneath both
the Altiplano and western parts of the Eastern
Cordillera.
ACTIVE MANTLE MELTING
100
8
100
42
m
12
,d
~"
Z
.<
•
<1
"
3
1
0.1
9
10
<
~
1
.1
~
O
-
-
-
0
GAS SAMPLE
•
WATERSAIVlPLE
A
SULFATARA
MAN'ILE MELT
-- r / - / ) GENERATION
BRAZILIAN
SHIELD
.01
TRENCH
RN
_LLERA
.001
100
11
200
300
400
km
0
200
400
600
800
1000
Fig. 5. Lithospheric scale S W - N E cross-section through the Central Andes with superimposed helium isotope profile from the
northern central Altiplano (Fig. 3). The position of the subducted Nazca plate [24] and the Andean crustal structure are also shown
[51]. There is a wide 3He anomaly, which is defined as R c / R A values greater than 0.2. This is assumed to be a consequence of
mantle melting at depth, which releases primordial mantle helium. The superimposed helium isotope profile from the northern
central Altiplano is a projection of R c / R A values on to a S W - N E transect (see Fig. 3 for location and construction of profile),
illustrating the smooth variation in the helium isotope ratios as a function of distance across the Andes. Note the inferred
southwestern limit of the asthenospheric wedge, which is taken to coincide with the southwestern limit of the 3He anomaly. The
lithosphere is inferred to be thin beneath the wide zone of 3He anomalies, where the crust is also thickest ( < 75 km). The
northeastern limit of the 3He anomaly is taken to mark the western edge of thick and relatively cold lithosphere of the Brazilian
shield, which has been underthrust beneath the Subandes and part of the Eastern Cordillera.
352
L. Hoke et al. / Earth and Planetary Science Letters 128 (1994) 341-355
Mantle melting behind the A r c
The presence of mantle-derived helium beneath the Altiplano and western part of the eastern Cordillera and also young ( < 5.5 Ma), small,
olivine-bearing basaltic volcanic cones scattered
across a limited area of the central Altiplano
[37,42] (Fig. 3), suggest regional mantle melting
since the Late Miocene, postdating the main
phase of crustal shortening in the Altiplano [27].
The initiation of this minor basaltic volcanism
was most probably preceded by a change in the
thermal structure of the thickened lithosphere.
Such a change could be brought about by a
gradual heating of the thickened lithosphere or
by more rapid convective removal of the base of
the thickened, or actively thickening, lithosphere
and its replacement by hot asthenosphere. Numerical models show that gradual heating by
thermal diffusion takes place on a time scale of
several tens to hundreds of million years, whereas
the latter can take less than 10 million years [4].
Given that most of the crustal thickening in the
Central Andes appears to have occurred in the
past 30 Ma [27] and that the present phase of
mafic volcanism is relatively young ( ~ 5 Ma), the
thermal diffusion model seems unlikely. However, convective erosion of a relatively cold and
dense, thickened lithospheric root [4] and its replacement with relatively hot and less dense upwelling asthenosphere, can explain both mafic
melt generation and active mantle melting, in a
300 km wide zone behind the arc, and the formation of the Altiplano high plateau area during the
Cenozoic.
Com, ecti~'e remo~'al o f the base o f the lithosphere
The model of Houseman et al. [4] is based on
the gravitational and thermal instability of a
growing lithospheric root during lithospheric
shortening, which could induce local convection
and lead to detachment of the lithospheric root.
This may have happened beneath the Tibetan
plateau after considerable lithospheric thickening
[3,6]. However, unlike the Tibetan plateau, there
is a well-defined subducting slab beneath the
Central Andes. Subduction will also induce local
mantle convection, which could interact with, and
drive detachment of, the overlying lithospheric
root, although this will depend critically on the
overall geometry. In addition, convective removal
of the base of the lithosphere could occur asymmetrically, with erosion of the base of the lithosphere progressively moving away from the arc.
Thus, in the Central Andes, removal or erosion
of a lithospheric mantle root could have occurred
gradually during lithospheric thickening rather
than catastrophically after thickening, as has been
suggested for the Tibetan plateau [6]. Detachment of the lithospheric root will lead to surface
uplift in isostatic response to the replacement of
the dense root with buoyant hot asthenosphere
[6]. Thus, the rate of detachment should be clearly
reflected in the surface uplift history. Fissiontrack data suggest accelerated relative uplift of
the Bolivian Altiplano and an increase in the rate
of surface denudation on the eastern margin of
the Altiplano in the Cordillera Real during the
Pliocene-Pleistocene [43]. Also, the presence of
young normal faulting in the Altiplano of southern Peru [44,45] could suggest collapse, as a consequence of a Pliocene-Pleistocene increase in
surface height, greater than that required by
crustal thickening alone [3], and caused by the
detachment of a thickened lithospheric root. Normal faulting in the Puna region of northwestern
Argentina could be a consequence of the same
process.
The maximum lithospheric thickness beneath
the active volcanic arc is constrained by geophysical evidence to be above the Benioff zone, which
is at a depth of ~ 120 km (Fig. 3). It is suggested
here that this zone of thin lithosphere ( < 120 km
thick) extends beneath the Altiplano and western
part of the Eastern Cordillera as far as can be
mapped with a mantle-derived contribution to
the helium isotope signal (Figs. 3-5). The transition to pure crustal helium isotope ratios in the
east delineates the change to a thicker lithosphere ( > 200 km, Fig. 5), presumably where the
Brazilian shield has underthrust the Eastern
Cordillera. This lithospheric structure is similar
to that proposed by Isacks [23], but is based on
independent evidence. However, Whitman et al.
[46] have used seismic energy attenuation data to
infer a different lithospheric structure for this
part of the Bolivian Andes. It remains to be
L. Hoke et aL / Earth and Planetary Science Letters 128 (1994) 341-355
tested whether our proposed lithospheric structure, based on the helium results (Fig. 5), is
consistent with this data.
Temperatures at the base of the lithosphere
beneath continental crust of normal thickness
approach, but apparently do not reach, the dry
solidus [47]. However, McKenzie [48] suggests
that small volumes of volatile-rich melt are being
continually generated in the underlying and convecting asthenospheric mantle, which could rise
up and freeze above the thermal boundary layer
in the overlying lithosphere, creating over time a
volatile-rich metasomatic layer. This could be
remelted by a slight increase in temperature,
caused by convective removal of the underlying
thermal boundary layer and juxtaposition of hot
asthenosphere with this metasomatic layer [49].
The first melts would be expected to be enriched
in potassium and should also contain a mantle
helium isotope signature. The major element
chemistry of the young basaltic andesites of the
central Altiplano [Hoke and Entenmann, unpublished data] and the helium isotope results both
support this scenario.
6. Concluding remarks
This study has documented a wide region in
which gas and water samples from volcanic and
geothermal systems contain an elevated 3He component compared to crustally derived radiogenic
helium. In the absence of any plausible crustal
sources, this is interpreted as the effect of a
mantle-derived helium component ( < 70% mantle helium), released during mantle melting at
depth.
The elevated 3He signature in the volcanic arc
is interpreted as being a result of melting in the
mantle wedge between the subducting slab and
the overlying lithosphere. The 3He signature here
cannot be explained by melting of the subducting
slab [17]. However, dehydration of the downgoing
slab may hydrate the overlying mantle wedge,
which will melt where temperatures are greater
than ~ ll00°C.
The mechanism invoked to explain melting
beneath the volcanic arc cannot explain mantle
353
melting in the wide region behind the arc mapped
with 3 H e / 4 H e > 0.2. We argue that mantle melting here is a consequence of the thermal effects
of convective removal of the lower part of the
thickening lithosphere and its replacement with
hot asthenosphere, which melts a volatile-rich
lithospheric mantle. There are several lines of
evidence in support of this for the Central Andes:
(1) high 3 H e / a H e ratios < 6.9 R A in a region
extending ~ 300 km east of the active arc, where
the crust is < 75 km thick; (2) the timing of the
onset of minor potassium-rich basaltic volcanism
at ~ 5 Ma, postdating a major phase of crustal
thickening in the Altiplano between ~ 30 and 5
Ma; and (3) normal faulting in the northernmost
and highest part of the Altiplano in southern
Peru and also in the Puna of northwestern Argentina, which may be a consequence of surface
uplift greater than that required by crustal thickening alone. The uplift history of this area as a
consequence of convective removal of a lithospheric root will depend on the rate of removal.
This may have happened gradually during crustal
and lithospheric thickening since ca. 30 Ma, or
more rapidly just before conditions for mantle
melting were reached. Mantle melting has continued to the present, with mantle-derived helium
still outgassing in a region which is much greater
in extent than that suggested by the distribution
of young basaltic volcanic centres alone and extends well into the Eastern Cordillera.
Acknowledgements
This research was supported by a British
Petroleum field research grant held by Prof. J.
Dewey (University of Oxford) and a Royal Society research grant held by Simon Lamb. L. Hoke
acknowledges the FWF Austria for a Schr6dinger
Fellowship and the British Council joint BritishGerman Research programme for laboratory support. Laboratory expenses were covered by the
Freie Universit~it Berlin and we thank M. Feth
for analytical assistance. Lorcan Kennan, Jiirgen
Entenmann, Gerhard W6rner, Susan Aitcheson,
Christine Guldi, Martin Shepley, Carlos Fernandez and Hamish Campbell are acknowledged for
354
L. Hoke et al. /Earth and Planetary Science Letters 128 (1994) 341-355
field assistance. We are grateful to Peter Molnar,
John Platt, John Dewey, Gerhard W6rner and
Erika Griesshaber for critical and helpful reviews.[UC]
[16]
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