https://doi.org/10.1130/G48568.1 Manuscript received 25 October 2020 Revised manuscript received 4 January 2021 Manuscript accepted 5 January 2021 © 2021 The Authors. Gold Open Access: This paper is published under the terms of the CC-BY license. Published online 4 March 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 1 Department of Earth Sciences, National Taiwan Normal University, Taipei 11677, Taiwan 2 Institute of Earth Sciences, Academia Sinica, Taipei 11529, Taiwan 3 Department of Geosciences, National Taiwan University, Taipei 10617, Taiwan 4 Department of Geology, University of Malaya, 50603 Kuala Lumpur, Malaysia 5 Department 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 Geological Society of America | GEOLOGY | Volume 49 | Number 6 | www.gsapubs.org 713 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/49/6/713/5323282/g48568.1.pdf by guest on 29 September 2021 AB 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.