Download Tectonic controls on the nature of large silicic calderas in volcanic arcs

Tectonic controls on the nature of large silicic calderas in volcanic arcs
Gwyneth R. Hughes* and Gail A. Mahood
Department of Geological and Environmental Sciences, Stanford University, 450 Serra Mall, Building 320,
Stanford, California 94305-2115, USA
ABSTRACT
Silicic magma bodies stabilize the continental margin by redistributing low-density material into the upper crust. We examined the tectonic traits of 91 young, arc-related, silicic calderas to test previous assertions that the nature of voluminous silicic arc volcanism depends
on specific tectonic characteristics. We find that caldera occurrence positively correlates with
convergence rate except in arcs with backarc spreading, which have few or no calderas. Calderas located on oceanic and young continental crust are predominantly dacitic, whereas
rhyolitic calderas are mainly located on Mesozoic or older continental crust under extension.
Occurrence does not increase with greater local subduction obliquity or duration of present
arc activity. Our determination of controls on the formation and composition of silicic calderas in volcanic arcs contributes to the study of continental evolution.
Keywords: calderas, volcanism, continental margins, island arcs, Quaternary, rhyolites.
INTRODUCTION
At least since the Archean, silicic magma
bodies have contributed to the growth and evolution of continental crust. New crust at continental margins is constructed by a combination
of island arc accretion and arc magmatism, yet
these processes alone are too mafic to account
for the intermediate nature of upper continental
crust (Rudnick, 1995). Silicic plutons are the
geological sutures that stitch accreted crust to
continents, stabilizing the continental margin
by adding evolved, less dense material to the
upper crust. The study of factors that contribute to large-scale silicic magmatism is therefore essential to understanding how continental
crust at active margins changes and stabilizes
through time. We examine the occurrence and
nature of silicic calderas in modern volcanic
arcs in order to elucidate the tectonic and
crustal parameters that favor accumulation of
silicic magma in arc settings.
METHODS
Based on a literature review, we identified six
tectonic and crustal parameters that have been
proposed as favoring or influencing silicic magmatism in arc settings: (1) trench-normal convergence rate, as it relates to basaltic input from
the mantle; (2) crustal stress regime; (3) crustal
thickness; (4) crustal age and composition;
(5) duration of arc volcanism; and (6) local obliquity of subduction. Given that a volcanic arc can
migrate widely over its lifetime, we recorded
the duration and orientation of subduction in its
present location. We compared these parameters
to the incidence of voluminous silicic magmatism as represented by caldera-forming eruptions (CFEs) in 19 volcanic arcs. The Aleutian
*E-mail: [email protected].
and Central American arcs are divided based
on changes in the underlying crust. We did not
include calderas in arcs with complex settings
(e.g., the Philippines) and in arcs missing from
the compilations by Lallemand et al. (2005) and
Cruciani et al. (2005) of convergent margin data
(e.g., New Zealand and the Aegean).
We defined silicic calderas as having CFEs
with compositions of at least 63 wt% SiO2. For
calderas with more than one CFE, data from
the oldest eruption were used when possible.
We included only calderas younger than 2 Ma,
so that the present tectonic setting could be
assumed to approximate that at the time of caldera formation. Only calderas >5 km in diameter, formed during a catastrophic eruption, were
considered, in order to eliminate features not
associated with sizeable magma bodies. A caldera density was calculated for each arc, equal
to the number of calderas meeting the selection
criteria per 1000 km of volcanic arc length.
After extensive review of more than 200
references (GSA Data Repository item DR11;
Newhall and Dzurisin, 1988; Siebert and Simkin, 2002), we determined that 91 calderas met
these criteria (Fig. 1A; Table DR1). Compositions of CFEs were characterized as dacitic
(63–67 wt% SiO2), rhyodacitic (68–72 wt%
SiO2), or rhyolitic (>72 wt% SiO2). The composition of the individual CFEs and the caldera
densities of the examined arcs were compared to
the enumerated tectonic and crustal parameters.
Trench-normal convergence rates were based
1
GSA Data Repository item 2008152, indexed
bibliography of references used in the compilation
(item DR1) and a spreadsheet of compiled data
(Table DR1), is available online at www.geosociety.
org/pubs/ft2008.htm, or on request from editing@
geosociety.org or Documents Secretary, GSA, P.O.
Box 9140, Boulder, CO 80301, USA.
on the nearest transect(s) reported in Lallemand
et al. (2005) (Fig. 1B). Extension was examined
at two scales. First, in order to characterize the
overall stress regime of the arc, we utilized
the backarc classifications for transects, also
from Lallemand et al. (2005). These values are
based on upper-plate focal mechanisms, and,
for the end members, the presence of backarc
spreading or thrusting. In addition, we classified
the local or intra-arc stress regime surrounding
each caldera as extensional or compressional.
This local stress was determined from published
studies, theses, government reports, and conference proceedings specific to the caldera or the
surrounding volcanic arc, in addition to data in
Newhall and Dzurisin (1988). Crustal thickness
and age, as well as the duration of the volcanic
arc in its present location, were similarly gathered from regional studies. For continental margins, and arcs underlain by pieces of continental
crust (such as Sumatra), the age of the oldest
known underlying basement was recorded and
categorized as Tertiary, Mesozoic, Paleozoic, or
Precambrian. Obliquity of subduction was taken
as the angle between the direction of motion for
the downgoing plate and the trench-normal vector for each segment, as reported in Cruciani
et al. (2005). Calderas were examined and distances were measured using Google Earth and
the Smithsonian Global Volcanism Program
data set (Siebert and Simkin, 2002).
RESULTS
There is a striking positive correlation
between caldera density and trench-normal convergence rate (Fig. 2A). An important exception
to this trend is the Vanuatu arc, which, despite
having a convergence rate of 85 mm/yr, has only
one true silicic caldera. Vanuatu is the only one
of the 19 arcs with calderas that is undergoing
© 2008 The Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or [email protected].
GEOLOGY,
August
2008
Geology,
August
2008;
v. 36; no. 8; p. 627–630; doi: 10.1130/G24796A.1; 3 figures; 2 tables; Data Repository item 2008152.
627
A
135°E
135°W
45°N
0
SCHI1
34°S
B
Diamante
72°W
SCHI2
Atuel
Calabozos
Composition
Rio Colorado
Dacite
Laguna
del Maule
36°
Rhyodacite Rhyolite
Caldera diameter (km)
45°S
SCHI3
100 km
0
= Stratovolcano or shield
<10 10–20 20–30 >30
Figure 1. A. Locations of examined silicic calderas marked by circles coded by diameter and
caldera-forming eruption (CFE) composition. Dark black lines represent trenches. See GSA
Data Repository item DR1 and Table DR1 (see footnote 1) for compilation details. B. Example
of arc segment from southern Andes. Black lines are transects from Lallemand et al. (2005).
Atuel was, for example, assigned the trench-normal convergence rate and backarc stress
regime of transect SCHI1.
backarc spreading, as categorized by Lallemand
et al. (2005) (Table 1). In comparing the histogram of convergence rates for all arc transects
(including those without calderas) with that of
the convergence rates associated with examined
calderas (Fig. 2B), we note that the caldera data
are grouped around a relatively high conver-
628
16
14
A
Caldera density
19
Continental crust
Oceanic crust
Oceanic crust with
backarc spreading
12
18
17
10
16
15
14
8
13
6
4
2
2
3
1
5
4
6
0
10
30
20
40
10
7
8
02
50
11
9
03
60
12
70
80
01
90
Trench-normal convergence rate (mm/yr)
40
Percent of total
Figure 2. A. Trench-normal
convergence rate versus
caldera density. For further discussion of arcs
with backarc spreading, see text. 1—Lesser
Antilles, 2—Cascades,
3—Sumatra, 4—northern
Andes, 5—Izu-Bonin, 6—
Mexico, 7—Alaska Peninsula, 8—Aleutians, 9—
Kermadec, 10—Java-Bali,
11—central Andes, 12—
Vanuatu, 13—southern
Andes, 14—southern Central America, 15—Kurile,
16—northern Ryukyu,
17—northern Central
America, 18—northeast
Japan, 19—Kamchatka,
01—Tonga, 02—Marianas,
03—South Sandwich.
B: Histogram of trenchnormal convergence rates
for all transects from
Lallemand et al. (2005)
and rates assigned to 91
calderas included in this
study.
gence rate. A two-tailed t-test confirmed that the
mean of caldera-related rates is statistically different (p <0.005) from that of all segments.
Our investigation of the importance of stress
regime yields mixed, though interesting, results.
In examining local (or intraarc) stresses regimes,
we find that ~60% of the calderas occur in
regions of local extension (Table 2). Of the calderas with rhyolitic eruptions, however, 77% are
located in extensional regimes. As for backarc
stress regimes (Table 1), which we assume to
characterize the overall stress state of the arc, the
lack of calderas in arcs with the most extension
(backarc spreading) is striking. It is notable that
no calderas that met our criteria are found in the
South Sandwich, Marianas, or Tonga arcs, all of
which are categorized as having backarc spreading. Calderas do, however, occur in segments
with moderate to low backarc extension and in
arcs having compressional or neutral backarcs.
There are approximately equal numbers of
dacitic, rhyodacitic, and rhyolitic calderas (33,
27, and 31, respectively). The silica content of the
CFE generally correlates positively with crustal
thickness (Fig. 3A). Examined dacitic calderas
form exclusively on crust <37 km thick, whereas
rhyolitic calderas appear on crust >25 km
thick. Similarly, more-evolved crust hosts more
silicic CFEs (Fig. 3B). Dacite is the dominant
composition of calderas on oceanic and young
(Tertiary) continental crust, and the proportion
of rhyolitic calderas increases on older continental crust. Because of subduction processes
such as terrane accretion, subduction erosion,
and lithospheric foundering, few arcs are underlain by Precambrian or very thick crust, and so
these observed trends are difficult to assess at
these high end-member values.
Durations of arc volcanism in their present
locations for the 19 arcs ranged from 3 Ma to
65 Ma, with a median age of 23 Ma and a standard deviation of 17 Ma. We found no correlation between caldera density and the duration of
arc volcanism in its present location.
Our investigation of subduction obliquity
(i.e., the angle between the convergence direction and normal to the trench) shows that higher
obliquity does not favor silicic caldera formation, and, in fact, seems to inhibit it; silicic
calderas are not present in arc segments with
obliquity >40°.
B
All rates
Calderas
30
20
10
0
0
10
20
30
40
50
60
70
80
Trench-normal convergence rate (mm/yr)
90
>100
154
DISCUSSION
Convergence Rate
The positive correlation between trenchnormal convergence rate and caldera density
is logical if faster rates are associated with
increased basaltic input from the mantle (Clift
and Vannucchi, 2004). Sustained magmatic flux
is likely to be the most fundamental requirement
for the formation of silicic calderas in arc settings
for several reasons. First, generation of silicic
magmas by either magmatic differentiation or
partial melting requires a significant volume
of mafic magma. The ratio of basaltic magma
required to produce rhyolite varies from ~10:1
for fractional crystallization to ~1:1 for partial
melting of silicic crust during calc-alkaline magmatism (Grunder, 1995). Additionally, frequent
GEOLOGY, August 2008
Extension
2
TABLE 1. BACKARC STRESS REGIMES
Neutral
Compression
1
1
2
TABLE 2. LOCAL STRESS REGIMES OF
CALDERAS BY ERUPTION COMPOSITION
CFE Lithology
Extension
Compression
Rhyolite
24 (77%)
7 (23%)
Rhyodacite
16 (59%)
11 (41%)
Dacite
18 (55%)
15 (45%)
All
58 (64%)
33 (36%)
Note: CFE—caldera-forming eruption.
Reference details are in the GSA Data Repository
(see footnote 1).
3
3
Calderas
1
12
10
27
13
15
13
(1%)
(13%)
(11%)
(30%)
(14%)
(16%)
(14%)
All Transects
19
11
14
33
17
5
14
(17%)
(10%)
(12%)
(29%)
(15%)
(4%)
(12%)
Note: Stress codes from Lallemand et al. (2005). 1 = slight, 2 = moderate, 3 = high.
A
Rhyodacite
Dacite
15
10
5
0
10
20
basaltic inputs over long periods heat the crust,
making it more ductile. This preserves magmatic
pathways, ensuring that magma is repeatedly
supplied to the same crustal location (Walker,
1993), and favoring the storage and evolution of
magma (Jellinek and DePaolo, 2003). Although
our results cannot differentiate between these
processes, our finding that caldera abundance
correlates strongly with convergence rate points
to greater basaltic flux driving the development
of sizeable silicic magma bodies.
Tectonic Stresses
Extension of the crust has been cited as
favoring formation of long-lived silicic magma
chambers (e.g., Hildreth, 1981; Hanson and
Glazner, 1995). Modeling by Jellinek and
DePaolo (2003) demonstrated that extension
favors long-term magma storage over frequent
eruption, allowing the magma chamber to grow
and evolve. We find that extension, whether
local or backarc, is not a requirement for generating silicic calderas. However, the greater
association of higher silica composition CFEs
with local extension (Table 2) suggests that
near-field extension allows for greater magmatic evolution than does compression.
Given these results, our finding that arcs with
backarc spreading lack silicic calderas is at first
sight contradictory. The Vanuatu, Tonga, Marianas, and South Sandwich arcs are all oceanic,
and might be expected to have few silicic calderas
simply because of the lack of felsic crust available for partial melting. Yet, the similarly oceanic
Kermadec, Izu-Bonin, and Kurile arcs have caldera densities appropriate to their trench-normal
GEOLOGY, August 2008
B
Rhyolite
30
40
50
60
70
Number of calderas
20
Number of calderas
Figure 3. Relationship
between composition of
the caldera-forming eruption and nature of underlying crust. A: Histogram
of crustal thicknesses
associated with examined
calderas. B: Histogram
of type and age of crust
underlying examined calderas. Ages refer to crust
that is continental in
nature. Oceanic crust here
is mafic and has no continental character.
15
10
5
Oceanic
Crustal thickness (km)
convergence rates. Our results suggest that in
the presence of backarc spreading, unlike areas
of intra-arc or local extension, the formation of
silicic calderas is inhibited. One possible explanation is that arc volcanism in general wanes during backarc basin development, as suggested by
modeling (Conder et al., 2002) and by the hiatus
in arc volcanism observed at Tonga during formation of the Lau Basin (Ballance et al., 1999).
All four of these arcs are, however, currently volcanically active. Two alternative explanations are
(1) during rifting, mantle basalts are less likely to
interact with crust, and so silicic partial melts do
not form; or (2) rifting and crustal thinning allow
magma to erupt without having evolved. We are
currently examining these arcs in more detail, as
well as the extensional Aegean arc, in order to
clarify the effects of backarc extension on silicic
caldera formation.
Crustal Thickness and Age
We find that the silica content of the CFE
positively correlates with crustal thickness,
empirically confirming previous studies suggesting that magmatism on thicker crust results
in more evolved compositions (e.g., Grunder and
Mahood, 1988; Hildreth and Moorbath, 1988).
This phenomenon could be the result of greater
incorporation of silicic crustal material and/or
increased differentiation during long ascent.
Our result, that calderas located in oceanic arcs
are dominantly dacitic, is not surprising, given
that oceanic crust is relatively thin and mafic.
More interesting is the apparent positive correlation between the silica content of the CFE
and the age of underlying continental crust. This
crust
Tertiary
Mesozoic
Paleozoic
Precambrian
Continental crust
relationship is logical if older continental crust
is more evolved as a result of multiple cycles
of intrusion by fractionated melts, partial melting, and possibly delamination of the mafic
lower crust (Kay and Kay, 1993; Taylor and
McLennan, 1995). If older crust is more silicic,
it can result in more silicic CFEs through two
mechanisms: (1) by its low density inhibiting
the rise of more mafic compositions, allowing
only the more evolved melts to reach the upper
crust; and (2) by providing lithologies that will
partially melt at relatively low temperatures,
increasing the role of crustal assimilation.
Duration of Present Arc
We compare the duration of arc activity in
its current location to caldera density in order
to determine whether protracted arc volcanism
is associated with increased silicic caldera formation, given that silicic magma chambers take
~105–106 yr to form (Annen and Sparks, 2002).
One possibility for the lack of correlation is the
apparent large dependence of caldera density
and CFE composition on convergence rate and
crustal attributes, respectively. Given arcs with
identical convergence rates and crustal properties, differences due to arc duration might be
apparent, but our data set is too varied.
Obliquity
It has been suggested that large-scale transverse faulting associated with oblique subduction favors silicic caldera formation (Bellier
et al., 1999). Our finding that greater subduction
obliquity was not linked to higher caldera occurrence is likely related to the fact that higher
629
subduction obliquity results in a lower trenchnormal convergence rate, thus reducing overall
basaltic input. Transverse faulting is probably
important in localizing silicic centers, but mafic
flux must also be high enough to sustain the
generation of silicic magmas.
CONCLUSIONS
We find that silicic caldera formation is favored
by high convergence rates, and that higher silica
CFE compositions generally occur on relatively
thick, old continental crust under local extension. In addition, we find that silicic calderas do
not generally form in oceanic arcs under backarc
spreading. This study contributes to the general
understanding of where large CFEs take place
in present arc settings, and, specifically, where
rhyolitic eruptions, the most explosive and
potentially destructive CFEs (Self, 2006), are
prone to occur. To the extent that silicic calderas
are the surface expressions of magmatic systems
preserved as plutons and batholiths, understanding the factors that control their abundance and
composition will aid in interpreting the tectonic
settings of ancient arcs. In addition, this study
quantifies the occurrence of silicic caldera formation during normal arc volcanism, and thereby
provides a baseline to which anomalous ignimbrite flare-ups could be compared (e.g., de Silva
and Gosnold, 2007). Work is under way to further examine the spatial and volumetric relationship between silicic calderas and more typical
intermediate to mafic arc volcanism, and how
local modern structures such as fault stepovers
or ancient crustal fault zones determine where
silicic calderas occur within arcs.
ACKNOWLEDGMENTS
We thank D. Pollard, M. Coble, M. Cardiff, and
E.L. Miller for helpful discussions. The manuscript
benefited from constructive reviews by A. Barth,
E. Christiansen, S. de Silva, and A. Grunder. Hughes
is funded by a Graduate Research Fellowship from
the National Science Foundation, United States.
630
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Manuscript received 20 January 2008
Revised manuscript received 23 April 2008
Manuscript accepted 25 April 2008
Printed in USA
GEOLOGY, August 2008