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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 REFERENCES CITED Annen, C., and Sparks, R.S.J., 2002, Effects of repetitive emplacement of basaltic intrusions on thermal evolution and melt generation in the crust: Earth and Planetary Science Letters, v. 203, p. 937–955, doi: 10.1016/S0012– 821X(02)00929–9. Ballance, P.F., Ablaev, A.G., Pushchin, I.K., Pletnev, S.P., Birylina, M.G., Itaya, T., Follas, H.A., and Gibson, G.W., 1999, Morphology and history of the Kermadec trench-arc-backarc basinremnant arc system at 30 to 32°S: Geophysical profile, microfossil and K-Ar data: Marine Geology, v. 159, p. 35–62, doi: 10.1016/ S0025–3227(98)00206–0. Bellier, O., Bellon, H., Sebrier, M., Sutanto, and Maury, R.C., 1999, K-Ar age of the Ranau tuffs; implications for the Ranau Caldera emplacement and slip-partitioning in Sumatra (Indonesia): Tectonophysics, v. 312, p. 347–359. 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Siebert, L., and Simkin, T., 2002, Volcanoes of the world: An illustrated catalog of Holocene volcanoes and their eruptions: Smithsonian Institution Global Volcanism Program Digital Information Series, GVP-3, www. volcano.si.edu (August 2005–January 2008). Taylor, S.R., and McLennan, S.M., 1995, The geochemical evolution of the continental crust: Reviews of Geophysics, v. 33, p. 241–265, doi: 10.1029/95RG00262. Walker, G.P.L., 1993, Basaltic-volcano systems, in Prichard, H.M., et al., eds., Magnetic processes and plate tectonics: Geological Society of London Special Publication 76, p. 3–38. Manuscript received 20 January 2008 Revised manuscript received 23 April 2008 Manuscript accepted 25 April 2008 Printed in USA GEOLOGY, August 2008