Mid-Miocene Volcanic Migration in the Westernmost Sunda Arc Induced by India-Eurasia Collision Yu-Ming Lai1*, Sun-Lin Chung2,3*, Azman A

https://doi.org/10.1130/G48568.1

Manuscript received 25 October 2020 Revised manuscript received 4 January 2021 Manuscript accepted 5 January 2021

Mid-Miocene volcanic migration in the westernmost Sunda arc induced by India-Eurasia collision Yu-Ming Lai1*, Sun-Lin Chung2,3*, Azman A. Ghani4, Sayed Murtadha5, Hao-Yang Lee2 and Mei-Fei Chu3 1Department of Earth Sciences, National Taiwan Normal University, Taipei 11677, Taiwan 2Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan 3Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan 4Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia 5Department of Geology, Syiah Kuala University, Banda Aceh 23111, Sumatra, Indonesia

ABSTRACT zone, however, has attracted little attention. The The migration of arc magmatism that is a fundamental aspect of plate tectonics may reflect Toba caldera that occurs in the transition area the complex interaction between subduction zone processes and regional tectonics. Here we (Fig. 1A) has been a research focus because of report new observations on volcanic migration from northwestern Sumatra, in the westernmost its super-eruptions and environmental impacts Sunda arc, characterized by an oblique convergent boundary between the Indo-Australian (see Chesner [2012] for a review). and Eurasian plates. Our study indicates that in northwestern Sumatra, volcanism ceased at As synthesized by Barber et al. (2005), K-Ar 15–10 Ma on the southern coast and reignited to form a suite of active volcanoes that erupt ages obtained mainly by Bellon et al. (2004), exclusively to the north of the trench-parallel Sumatran fault. Younger volcanic rocks from the along with sparse age and geochemical data re-

north are markedly more enriched in K2O and other highly incompatible elements, delineating ported by earlier studies, indicate that magmatism a geochemical variation over space and time similar to that in Java and reflecting an increase in Sumatra can be divided into several pre-Ceno- in the Benioff zone depth. We relate this mid-Miocene volcanic migration in northwestern zoic stages and five subsequent stages active from Sumatra to the far-field effect of propagating extrusion tectonics driven by the India-Eurasia the Paleocene to recent. Magmatic stages from collision. The extrusion caused regional deformation southward through Myanmar to north- pre-Cenozoic to recent may be affiliated with western Sumatra and thus transformed the oblique subduction into a dextral motion–governed a change in the subduction system that started plate boundary. This tectonic transformation, associated with opening of the Andaman Sea, is operating in the eastern portion of the Paleo-Te- suggested to be responsible for the volcanic migration in northwestern Sumatra. thys and persists to the modern Indo-Australian system in Southeast Asia (Hall, 2012; Metcalfe, INTRODUCTION we attribute to the interplay between oblique 2013; Zhang et al., 2019; Li et al., 2020). There is considerable evidence that arc mag- subduction and regional tectonics, with an em- Sumatra consists of three geologic units: matism is constant in neither time nor space (Pa- phasis on the far-field role of the India-Eurasia namely, from southwest to northeast, the Woyla terson and Ducea, 2015). Arc magma migration, collision in Southeast Asia. terrane, the West Sumatra block, and the East consequently, may reflect changes in conver- Sumatra block (Barber et al., 2005). The Woyla gence rate, subduction geometry, the depth of BACKGROUND terrane is an intra-oceanic arc complex formed slab dehydration or the extent of partial melting, The convergent movement of the Indo-Aus- in eastern Tethys and accreted to its present lo- and/or their combined effect (Karlstrom et al., tralian plate beneath the Eurasian plate is respon- cation in the Early Cretaceous (Hall, 2012; Ad- 2014). The West Pacific and Sunda subduction sible for the Sunda subduction zone (Fig. 1). vokaat et al., 2018). West Sumatra was conven- zones, circum–East and Southeast Asia (Fig. 1), Whereas the subduction in Java is nearly per- tionally correlated with Cathaysia, whereas East in particular, have interacted with major tecton- pendicular to the trench, that in Sumatra is highly Sumatra was thought to be a part of Sibumasu, ic events such as continental deformation and oblique, resulting in the trench-parallel, strike- which belongs to East Gondwana (Metcalfe, propagating extrusion owing to the collision slip Sumatran fault system (Malod et al., 1995; 2013). Zhang et al. (2018), however, used new of India into Eurasia (Tapponnier et al., 1982; McCaffrey et al., 2000). Running the length of detrital zircon evidence to argue that divides Schellart et al., 2019). We report our finding of Sumatra, this transform fault extends northward Sibumasu and integrates West and East Sumatra volcanic migration in the middle Miocene in into the spreading center of the Andaman Sea to correlate with the West Burma block, or part northwestern Sumatra as part of the outcome of (Curray et al., 1979). Active volcanoes typically of the Irrawaddy block, a pre-Cenozoic tectonic a systematic investigation on the magmatism of occur near the fault (on either side), ∼100 km element renamed by Ridd (2016). the island (Lai et al., 2019; Zhang et al., 2019; above the Benioff zone, except those from north- Li et al., 2020) (Fig. 1). A geochemical change western Sumatra that formed exclusively to the SAMPLES AND METHODS is associated with the volcanic migration, which north of the fault system (Fig. 1). Such a volcanic We collected a total of 23 basalt and andesite “offset”, first noticed by Page et al. (1979) and samples from northwestern Sumatra (Fig. 1B), *E-mails: [email protected]; [email protected] attributed to the changing angle of the Benioff including 6 from the Tertiary volcanicsalong the

CITATION: Lai, Y.-M., et al., 2021, Mid-Miocene volcanic migration in the westernmost Sunda arc induced by India-Eurasia collision: Geology, v. 49, p. 713–717, https:// doi.org/10.1130/G48568.1

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Figure 1. (A) Simplified tectonic map of the Sunda arc and adjacent regions in Southeast Asia. (B) Sample and volcanic map of northwestern Sumatra, after Barber et al. (2005). More detailed magmatic and age data are summarized in Table S1 and Figure S1 (see footnote 1). SV— Seulawah Agam volcano; GV—Geureudong volcano; TV—Tertiary volcanics.

southern coast, 7 from Seulawah Agam, and 10 of detrital zircons (Fig. 2A) from sample sites in 2019), including young detrital zircon ages from from Geureudong, the latter two of which are prin- northern Sumatra (Fig. 1A) are also presented here back-arc basins and riverbanks (Fig. 1A; Table cipal volcanoes that formed north of the Sumatran (Zhang et al., 2018, 2019; Table S3). S3). These age data indicate a migration of the fault. In situ zircon U-Pb age dating of six samples volcanic arc between 15 and 5 Ma. More specifi- was carried out using laser ablation–inductively RESULTS AND DISCUSSION cally, in northwestern Sumatra, arc volcanism coupled plasma mass spectrometry (LA-ICPMS) Volcanic Migration in Space and Time previously occurred along the southern coast as at the Department of Geosciences, National Tai- The two samples of Tertiary volcanics a linear extension of the volcanic front in cen- wan University (see Table S1 in the Supplemental from the southern coast gave early Miocene tral and southeastern Sumatra (Fig. S1), where Material1). Whole-rock major and trace element mean 206Pb/238U ages at 16.5 ± 0.5 Ma (sample Quaternary volcanic centers were limited to the determinations for all studied samples, along with 13SU01) and 20.1 ± 0.3 Ma (sample 13SU04) southern coast along the Sumatran fault (Page Sr-Nd isotopic analyses of selected samples, were (Figs. 2B and 2C). Magmatic zircon separates et al., 1979; Barber et al., 2005). In this portion of performed at the same institution (Table S2, and from both samples are mostly euhedral to sub- the arc, volcanism ceased south of the Sumatran supplemental text in the Supplemental Materi- hedral, with those from basaltic andesite sample fault at ca. 15–10 Ma and then resumed at ca.

al). Note that relevant zircon age data from our 13SU01 (SiO2 = 53.6 wt%) having lower ura- 10–5 Ma to the north of the fault, forming a suite counterpart analyses of Cenozoic arc volcanism nium (80–471 ppm) than those from andesite of active volcanoes. The Toba caldera complex

from the entire Sumatra island (Lai et al., 2019), sample 13SU04 (SiO2 = 58.5 wt%; U = 687– developed in the volcanic “offset” or transitional including those from Toba and adjacent areas, are 1742 ppm) (Table S1). In contrast, zircon sepa- area between the southern and northern portions summarized in Figure S1. Additional U-Pb ages rates from four other andesite samples from the of the Cenozoic volcanic arc (Fig. S1). We note Seulawah Agam and Geureudong volcanoes are that despite the abundance of Quaternary ages all too young (<0.3 Ma) to be dated precisely by from Toba in the age histogram (Fig. 2A), vol- 1Supplemental Material. Analytical methods, the LA-ICPMS method (Lai et al., 2019; Fig. S1). canic reinitiation north of the Sumatran fault at ages and geochemical data. Please visit https:// doi.org/10.1130/GEOL.S.14046875 to access Their ages are therefore referred to as Quaternary. ca. 10–5 Ma is documented by a late Miocene the supplemental material, and contact editing@ Figure 2A synthesizes all available age data sandstone that yielded U-Pb ages of ca. 10–5 Ma geosociety.org with any questions. from northwestern Sumatra (Fig. S1; Lai et al., on 48 of 94 detrital zircons (sample 13SU26;

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Figure 2. (A) Volcanic age A histogram of northwestern Sumatra from the Mio- cene. Compiled K-Ar age data are listed in Figure S1 (see footnote 1), and detrital zircon age data are in Table S3. (B,C) Con- cordia plots for magmatic C zircon U-Pb age results of two volcanic samples from southern part of northwestern Sumatra. MSWD—mean squared weighted deviation.

Table S3). These detrital zircons are presumably diagram (Fig. 3B). Note that nearly all the Sunda (Paterson and Ducea, 2015; Zhang et al., 2019). sourced from nearby volcanic deposits. arc volcanics, except for Muria highly potas- In northwestern Sumatra, we consider two prin- sic rocks, exhibit a “juvenile” isotopic nature cipal tectonic controls on termination of the arc

Associated Change in Magma Composition marked by positive εNd values (Fig. 3B). volcanism at ca. 15–10 Ma along the southern The volcanic migration in northwest- coast. One possible cause is the resistance of ern Sumatra is associated with changes in Causes of Volcanic Cessation in the South slab subduction beneath northern Sumatra, magma composition. The samples studied Island arcs commonly undergo magmatic where ca. 35–50 Ma oceanic lithospheric asso- vary from basalt to andesite in composition flare-ups and lulls relating to regional tectonics ciated with the Wharton Fossil Ridge (Whittaker

(SiO2 = 48.2–62.4 wt%;Table S2), with K2O contents increasing from south (Tertiary volcanics, 0.30–1.26 wt%; low-K tholeiitic series) A to north (Seulawah Agam, 0.83–1.71 wt%; Geu- reudong, 1.68–2.84 wt%; high-K calc-alkaline series) (Fig. 3A). Associated geochemical variations are depicted by the rare earth element (REE) patterns and multi-element spidergrams (Fig. S2), with all samples showing marked de- Figure 3. (A) K2O versus pletion in high field strength elements (Ti, Nb, SiO2 plot of three volcanic and Ta) and enrichment in large ion lithophile suites in this study (GV— elements (LILEs; Cs, Rb, Ba, and Sr), typical Geureudong volcano; SV—Seulawah Agam of subduction-related magmas. volcano; TV—Tertiary vol- The compositional change from south to canics). Comparison data north, moreover, is correlated with Benioff zone (small circles) are from depths (Fig. 1B). Early Miocene volcanics to GEOROC (http://georoc. the south crop out ∼80 km above the present mpch-mainz.gwdg.de/ georoc/), and from Luhr subducting slab, whereas the active volcanoes and Haldar (2006) for in the north (Seulawah Agam and Geureudong) Barren Island. (B) Corre- erupted from ∼120 km and ∼180 km depths. lation diagram of Nd and According to the tectonic setting of this area, B Sr isotopic ratios. Data sources include: Suma- we argue this variation can be compared to the tra (this study; Fig. S1 [see K-h correlation (enrichment of K and other footnote 1]), Toba-related LILEs as a function of depth [h]) in Java, the volcanics (Chesner, 2012; eastern Sunda subduction zone (Fig. 1A). A Fig. S1), Java (Turner “widened” magmatic arc occurs here, with the and Foden, 2001), Muria (Edwards et al., 1991), volcanic front ∼100 km above the Benioff zone, and Barren Island (Luhr consisting of a calc-alkaline main arc (Whitford and Haldar, 2006). et al., 1981) and more K-rich alkaline rocks in the back-arc, such as Muria volcano (Edwards et al., 1991) situated ∼300 km above the subducting slab (Hall and Spakman, 2015). Com- parison of these two magma suites from Sumatra

and Java is illustrated using a K2O versus SiO2 plot (Fig. 3A) and Sr-Nd isotopic correlation

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Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/6/713/5323282/g48568.1.pdf by guest on 29 September 2021 et al., 2007; Jacob et al., 2014) is now being subducted (Fig. 1). In contrast, east of the Investiga- tor fracture zone, older Cretaceous-age crust is currently subducting beneath central and southern Sumatra (Jacob et al., 2014). Consequently, beneath northern Sumatra, younger and more buoyant oceanic lithosphere resists subduction, and this manifests as complexly folded subducted lithosphere in the mantle transition zone, as imaged by tomographic studies (Pesicek et al., 2008; Hall and Spakman, 2015). The other, and arguably more significant, control is progressive transition of the tectonic setting from oblique subduction to predominantly strike- slip movement that we argue to have begun during Miocene times as a result of the far-field effect of propagating block extrusion driven by the India-Eurasia collision (Tapponnier et al., 1982). We propose a conceptual model (Fig. 4) to illus- trate how such propagating extrusion may have interacted with regional tectonics in the western part of Southeast Asia, confined by dextral movement of the Sagaing-Sumatran fault system, initiating at ca. 20–15 Ma on the Sagaing fault (Bertrand et al., 1999) and ca. 23–15 Ma on the Sumatran fault (Curray et al., 1979). The propagating extrusion that caused crustal deformation A B from the eastern Himalayas southward through the West Burma block to northwestern Sumatra Figure 4. Conceptual model depicting the far-field effect of propagating extrusion caused by would have progressively transformed the oblique the India-Eurasia collision on tectonic evolution from Burma to Sumatra and volcanic migration in northwestern Sumatra, characterized by mid-Miocene transition of the tectonic setting subduction there into a dextral motion–governed from oblique subduction in the westernmost Sunda arc system (A) to dominance of strike-slip plate boundary in response to the continuing In- motion along the trench associated with the opening of the Andaman Sea (B). Red dashed dian collision (Figs. 4A and 4B). As noted specifi- arrows beside the subduction zone show the transition in directions parallel and perpendicular cally in the Burmese-Popa arc (Rao and Kalpna, to the shear direction. Red dashed lines show the faults just starting to change directions. Sibusima (Sibumasu excluding Sumatra) is from Zhang et al. (2018). TV—Tertiary volcanics; 2005) or more widely over this highly oblique SV—Seulawah Agam volcano; GV—Geureudong volcano; NSB—North Sumatra Basin. section of the Indo-Australian convergent zone (Richards et al., 2007), the stress field in the upper part (<90 km) of the subducted lithosphere subduction of the Investigator fracture zone erating depths (∼100 km), despite the complex is governed by horizontal plate motion, albeit its (Fig. 4B). The slab tear, if indeed responsible folding of the slab in the mantle transition zone lower part (>90 km) is still controlled by gravi- for Toba super-eruptions from a gigantic silicic (Pesicek et al., 2008; Hall and Spakman, 2015). tational loading of the slab. We therefore suggest magma reservoir in the upper continental crust Therefore, an interpretation of local subduction such a tectonic transition from oblique conver- (Chesner, 2012), however, would have been way forcing for the volcanic migration is unfavored. gence to dextral motion of the subducted slab too restricted to also account for the volcanic We relate the volcanic renewal also to the ex- to have played a controlling role in the magma- reignition in northwestern Sumatra across its trusion tectonics that played a role in the opening tism all the way from the West Burma block to northern coast and extending into the southeast- of the Andaman Sea, a late Cenozoic pull-apart northwestern Sumatra during the middle Miocene ern part of the Andaman Sea (Fig. 1). Notably, basin formed by dextral shear of the Sagaing-Su- (Fig. 4A), not only resulting in the volcanic gap our northwestern Sumatran samples have Sr-Nd matran fault system (Curray et al., 1979). While and/or migration observed from the Popa arc (Lee isotopic compositions (Fig. 3B) that are distinct the Central Andaman Basin has been the site of et al., 2016), but also terminating the volcanism from those of any Toba-related volcanics, also active seafloor spreading since the early Pliocene along the southern coast in northwestern Sumatra. not supportive of a link with the slab tear for the (ca. 4 Ma) or as early as the middle Miocene volcanic reinitiation. (Morley, 2017), its adjacent regions are mostly Causes of Volcanic Reinitiation in the North Resistant subduction of the Wharton Fos- extended continental crust resulting from the in- Renewal of magmatism requires thermal per- sil Ridge in the west of Toba may have led to terplay between the western Sunda back-arc ex- turbation in the source region, similar to that change in the subduction angle and/or rate, thus tension and the differential motion of India with proposed for the Burmese-Popa arc where vol- causing not only a magmatic lull but also migra- respect to Southeast Asia (Curray, 2005; Morley canism renewed in the Quaternary (<1 Ma) due tion of the volcanic arc with changing compo- and Alvey, 2015). Such an extensional regime, in to onset of a transtensional regime triggered by sition. If so, the northward volcanic migration particular with the seafloor spreading, could have roll-back of the sinking slab (Lee et al., 2016). could be an indication of subduction flattening in sufficiently enhanced the regional geotherm to The cause of Toba super-eruptions, for another northwestern Sumatra, owing to presence of the initiate magma generation from the trench side example, has been accepted by most workers younger and thus more buoyant Wharton Fossil (Narcondam and Barren Islands) to the back-arc (Page et al., 1979; Barber et al., 2005; Hall and Ridge. However, this is not the case because no side on and off northwestern Sumatra (Fig. 4B). Spakman, 2015) to have been extra heat from apparent shift of the Benioff zone contours to the The geochemical and Sr-Nd isotopic constraints mantle upflow through a slab tear confined by west and east of Toba is observed at magma-gen- enable us to argue that, relative to high-Al ba-

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