*Manuscript Click here to view linked References
1 Early Cretaceous origin of the Woyla Arc (Sumatra, 2 Indonesia) on the Australian plate
3
4 Eldert L. Advokaat*1, Mayke L.M. Bongers1, Alfend Rudyawan2, Marcelle K. BouDagher-
5 Fadel3, Cor G. Langereis1, Douwe J.J. van Hinsbergen1
6
7 1Department of Earth Sciences, Utrecht University, 3584 CS Utrecht, The Netherlands
8 2Geology Study Program, Institut Teknologi Bandung, Bandung, Jawa Barat 40132,
9 Indonesia
10 3Department of Earth Sciences, University College London, London WC1E 6BT, United
11 Kingdom
12
13 *corresponding author: [email protected]
14 for: EPSL
15
16
1 17 Abstract
18 Key to understanding the plate kinematic evolution of the Neotethys oceanic domain
19 that existed between the Gondwana-derived Indian and Australian continents in the
20 south, and Eurasia in the north, is the reconstruction of oceanic plates that are now
21 entirely lost to subduction. Relics of these oceanic plates exist in the form of ophiolites
22 and island arcs accreted to the orogen that stretches from Tibet and the Himalayas to SE
23 Asia that formed the southern margin of Sundaland. The intra-oceanic Woyla Arc
24 thrusted over western Sundaland – the Eurasian core of SE Asia – in the mid-Cretaceous.
25 The Woyla Arc was previously interpreted to have formed above a west-dipping
26 subduction zone in the Early Cretaceous, synchronous with east-dipping subduction
27 below Sundaland. The oceanic ‘Ngalau Plate’ between the Woyla Arc and Sundaland was
28 lost to subduction. We present paleomagnetic results from Lower Cretaceous limestones
29 and volcaniclastic rocks of the Woyla Arc, Middle Jurassic radiolarian cherts of the
30 intervening Ngalau Plate, and Upper Jurassic–Lower Cretaceous detrital sediments of
31 the Sundaland margin. Our results suggest that the Woyla Arc was formed around
32 equatorial latitudes and only underwent an eastward longitudinal motion relative to
33 Sundaland. This is consistent with a scenario where the Woyla Arc was formed on the
34 edge of the Australian plate. We propose a reconstruction where the Ngalau Plate
35 formed a triangular oceanic basin between the N-S trending Woyla Arc and the NW-SE
36 trending Sundaland margin to account for the absence of accreted arc rocks in the
37 Himalayas. As consequence of this triangular geometry, accretion of the Woyla Arc to
38 the western Sundaland margin was diachronous, accommodated by a southward
39 migrating triple junction. Continuing convergence of the Australia relative to Eurasia
40 was accommodated by subduction polarity reversal behind the Woyla Arc, possibly
41 recorded by Cretaceous ophiolites in the Indo-Burman Ranges and the Andaman-
42 Nicobar Islands.
2 43
44 1 Introduction
45 Plate kinematic reconstructions are first and foremost based on marine magnetic
46 anomalies and fracture zones of the modern oceans, aided by geologic and
47 paleomagnetic data from the stable continents (Seton et al., 2012; e.g. Torsvik et al.,
48 2012). Quantifying the motions of former oceanic plates and their intra-oceanic
49 boundaries that were subsequently lost to subduction, however, is notoriously difficult,
50 and the resolution of plate kinematic restorations thus decreases back in time. If such
51 former intra-oceanic plate boundaries were subduction zones, relics may be available in
52 the geologic record in the form of accreted and deformed island arcs and ophiolites that
53 may allow constraining intra-oceanic plate boundary evolution in deep geologic time.
54 The vast Neotethys oceanic domain existed between the Australian and Indian
55 continents in the south, and Eurasia in the north. Convergence and collision between
56 these continents and the plates that hosted them led to the formation and complex
57 deformation of the orogen that stretches from Tibet and the Himalaya in the northwest
58 to SE Asia in the southeast (Figure 1) and little is known about the plates that may have
59 existed within the Neotethys ocean that have now been subducted.
60 In the central part of this orogen, on the island of Sumatra, lies the accreted Upper
61 Jurassic–Lower Cretaceous Woyla Arc which may shed light on the past intra-oceanic
62 plate configurations (Figure 1). This arc is interpreted as a former intra-oceanic
63 subduction-related arc that lies thrust onto the continental West Sumatra Block in the
64 mid-Cretaceous (e.g. Barber, 2000; Barber et al., 2005; Wajzer et al., 1991). The West
65 Sumatra Block is thought to have been part of the Eurasian core of SE Asia – Sundaland –
66 since the Late Triassic–Early Jurassic (Barber et al., 2005; Barber and Crow, 2009;
67 Metcalfe, 2013, 1996). Because the continental margin of Sundaland hosts arc magmatic
3 68 rocks of the same time interval as the Woyla Arc, Barber et al. (2005) suggested that the
69 Woyla Arc was associated with a subduction plate boundary that was not the Eurasian
70 plate boundary. From this it follows that at least one - presumably oceanic - plate must
71 have existed within the eastern Neotethys surrounded by Eurasia, Australia, and
72 possibly India (e.g. Hall, 2012).
73 In this study, we attempt at a kinematic reconstruction of this plate. To this end, we
74 collected new paleomagnetic data from Upper Jurassic-Lower Cretaceous clastic
75 sediments of West Sumatra and from volcaniclastics and limestones of the Woyla Arc.
76 We use these to first test whether the West Sumatra Block was part of Sundaland. We
77 then develop simplest-case plate kinematic scenarios in which the Woyla arc is assumed
78 to have been part of the Indian, Australian, or Tethyan Himalayan plates based on
79 previous reconstructions (Seton et al., 2012; van Hinsbergen et al., 2018), placed in a
80 paleomagnetic reference frame (Torsvik et al., 2012), and test these against our new
81 paleomagnetic data. Finally, we provide a plate kinematic scenario for the eastern
82 Neotethys that may explain the origin and arrest of the Woyla Arc, and the formation of
83 the modern plate boundary along western and southern Sundaland.
84
85 2 Geologic setting
86 Sundaland is the core of SE Asia and consists of multiple continental blocks and
87 volcanic arcs, bounded by suture zones representing remnants of closed ocean basins
88 (e.g. Hall, 2012; Metcalfe, 2013, 1996). The core of Sundaland consists of the Indochina-
89 East Malaya, Sibumasu, West-Sumatra, West Burma, and SW Borneo blocks that
90 amalgamated against the South China Block in the north during the Paleozoic to Late
91 Mesozoic (e.g. Hall, 2012; Metcalfe, 2013, 1996). The continental terranes are thought to
92 have separated from the northern margin of eastern Gondwana, opening new oceans to
4 93 their south and closing older ones to their north upon their northward flight towards
94 Sundaland (e.g. Metcalfe, 2013, 1996). The Woyla Arc in Sumatra is the latest major
95 crustal block that accrete to Sundaland in the Late Cretaceous (e.g. Barber, 2000; Hall,
96 2012; Metcalfe, 2013, 1996) (Figure 1). Rocks of the Mawgyi Nappe were accreted to the
97 West Burma Block to the north, which was in pre-Neogene time likely 500-1000 km
98 farther south than today relative to Sumatra (e.g. Mitchell, 1993 and references therein;
99 Van Hinsbergen et al., 2011), have been suggested as equivalents to the Woyla Arc
100 (Barber and Crow, 2009; Mitchell, 1993). Hall (2012) interpreted the Woyla arc to be
101 contiguous with an intra-oceanic subduction zone (Incertus Arc) that would include the
102 Lower Cretaceous ophiolites found in the Indus-Yarlung suture of southern Tibet. These
103 ophiolites have previously been interpreted to have formed at the equator (Abrajevitch
104 et al., 2005), which would support such a scenario. Recent paleomagnetic and sediment
105 provenance data from the Indus-Yarlung ophiolites, however, demonstrated that these
106 formed in the forearc of southern Tibet at a latitude of ~16°N and that previous low-
107 latitude interpretations were likely an artefact from unrecognized compaction-induced
108 inclination shallowing (Huang et al., 2015b). No accreted Lower Cretaceous arc rocks
109 that may be equivalent to the Woyla Arc are known from the accretionary prisms below
110 the Tibetan ophiolites, and there is no evidence that the Woyla arc continues to the
111 longitude of Tibet.
112 The present-day margin of Sundaland in the Sumatra region is characterized by
113 oblique subduction of the Indian-Australian plate accommodated by the Sunda trench
114 (Figure 2a). In the Pliocene–Pleistocene this oblique subduction became partitioned
115 over the trench and the right-lateral Sumatran Fault System (SFS) that cuts through all
116 rock units along the strike of Sumatra and has an estimated displacement of 50–150 km
117 (Barber, 2000; Barber et al., 2005). The geology of Sumatra is dominated by a Cenozoic
118 volcanic arc, with active volcanoes built on pre-Cenozoic basement (Figure 3). This
119 basement is mainly exposed in the Barisan Mountains along the SFS (Barber, 2000).
5 120 The pre-Cenozoic basement of Sumatra is divided into four units: The East Sumatra
121 Block which is part of Sibumasu, the Medial Sumatra Tectonic Zone that separates
122 Sibumasu from the West Sumatra block, and the Upper Jurassic – Lower Cretaceous
123 Woyla Group (Figure 2a) (Cameron et al., 1980). The Woyla Group and correlatives
124 comprise rocks of the Woyla Arc, an oceanic accretionary assemblage, and a continental-
125 arc assemblage (Barber, 2000; Cameron et al., 1980). The Woyla Group is found in
126 northern Sumatra near Banda Aceh and Natal, and similar units to the southeast with
127 the same lithologies and age ranges, including near Padang, in the Gumai and Garba
128 Mountains, and near Bandar Lampung are correlated with it (Figure 2) (Barber, 2000).
129 The Woyla Group is folded, presumably related to the collision of the Woyla Arc with the
130 West Sumatra Block sometime in the mid-Cretaceous (Barber, 2000).
131 The Woyla volcanic arc assemblage consists mainly of basaltic to andesitic volcanic
132 rocks that include xenoliths of radiolarian chert, dykes, and volcaniclastic sandstones
133 lacking quartz, and shales (Barber, 2000; Wajzer et al., 1991). Sparse radiometric dating
134 of the volcanic rocks yielded K–Ar ages of 122–105 Ma (Gafoer et al., 1992; Koning,
135 1985). In addition, Upper Jurassic–Lower Cretaceous limestones with volcanic detritus
136 associated with the basaltic to andesitic volcanic rocks (Bennett et al., 1981; Gafoer et
137 al., 1992; Yancey and Alif, 1977) are interpreted as fringing reefs built on volcanic
138 edifices (Barber, 2000; Cameron et al., 1980; Wajzer et al., 1991). The volcanic arc
139 assemblage and interlayered sedimentary rocks are nowhere associated with continent-
140 derived rocks and are thus interpreted as remnants of a Late Jurassic - Early Cretaceous,
141 intra-oceanic volcanic arc (Barber et al., 2005). The only previous paleomagnetic study
142 on limestones of the Woyla Group was performed by Haile (1979), who reported a
143 paleolatitude of 26°S, but also indicated that these rocks were remagnetised. Taking
144 these data at face value, Metcalfe (1996) interpreted that the Woyla Arc originated near
145 the northern margin of Eastern Gondwana in the southern hemisphere.
6 146 The oceanic accretionary assemblage contains internally deformed serpentinites,
147 pillow basalts, cherts, volcanic breccia, gabbros and red shales separated by faults. This
148 assemblage is interpreted as imbricated units derived from now-subducted ocean floor
149 (Barber, 2000).
150 The continental-arc assemblage consists of Middle Jurassic–Lower Cretaceous
151 quartzitic and calcareous sandstones and shales, intruded by Jurassic–Cretaceous
152 granites. This assemblage is partially metamorphosed, with metamorphic grades
153 increasing westwards up to amphibolite grade conditions (Suwarna et al., 1994).
154 Metamorphic rocks yielded K–Ar ages of 125–95 Ma (Andi Mangga et al., 1994; Koning,
155 1985). This assemblage is interpreted to have formed at a subduction zone along the
156 continental West Sumatra margin (Barber, 2000).
157 To account for synchronous magmatism at the intra-oceanic Woyla Arc and the West
158 Sumatra active continental margin, and the juxtaposition of unmetamorphosed units of
159 the Woyla Arc to metamorphosed units of the West Sumatra margin, Barber et al. (2005)
160 suggested that the Woyla Arc formed above a SW-dipping (in modern coordinates)
161 subduction zone, and the West Sumatra arc above a synchronous NE-dipping
162 subduction zone. Upon mid-Cretaceous closure of the intervening oceanic lithosphere,
163 the Woyla Arc was thrust over the West Sumatra margin (Barber et al., 2005) (Figure 4).
164 3 Methods
165 3.1 Paleomagnetism 166 Rock samples from sedimentary rocks of the Woyla Group were obtained from 13
167 sites (Figure 2, Table 1) using a petrol-powered drill with a drill bit having an inside
168 diameter of 25 mm. We sampled intervals varying between 2 m to 70 m per site, enough
169 to average out paleosecular variation (see parameter Table 1 and explanation below).
7 170 Typically one core sample was collected per exposed bed. The cores were oriented in
171 the field using a Brunton magnetic compass with an inclinometer attached. In cases
172 where hand specimens were taken, cores were drilled normal to the orientation plane.
173 The cores were cut into subsamples of 22 mm length using a double-blade circular saw.
174 Laboratory analyses were performed at the paleomagnetic laboratory Fort
175 Hoofddijk at Utrecht University, The Netherlands. The natural remanent magnetisation
176 (NRM) of samples was measured on a 2G DC-SQUID magnetometer and further
177 investigated using thermal as well as alternating field (AF) stepwise demagnetisation.
178 Stepwise thermal demagnetisation was carried out using 20–100 °C increment up to
179 660 °C (or until complete demagnetisation). Samples analysed by AF demagnetisation
180 were heated to 150°C to remove goethite or possible stress induced by weathering
181 (Velzen and Zijderveld, 1995), and then subjected to the following field strengths (in
182 mT): 0, 4, 8, 12, 16, 20, 25, 30, 35, 40, 45, 50, 60, 70, (80, 90, 100).
183 Statistical analysis and interpretation were performed using the online, platform
184 independent portal Paleomagnetism.org (Koymans et al., 2016). Demagnetisation
185 diagrams are plotted as orthogonal vector diagrams (Zijderveld, 1967). Interpretation of
186 demagnetisation diagrams was performed by interpreting a characteristic remanent
187 magnetisation (ChRM) for components decaying towards the origin following an eigen
188 vector approach (Kirschvink, 1980). We applied a 45° cut-off to the virtual geomagnetic
189 pole (VGP) distribution of a set of directions (Deenen et al., 2011; following Johnson et
190 al., 2008). This is an arbitrary fixed angle cut-off meant to remove outliers due to
191 excursions or transitional directions, or to remove outliers due to (assumed, possible)
192 errors in sampling, orientation or measurement. Mean directions were determined
193 using Fisher (1953) statistics while directional statistics were derived from the
194 corresponding VGP distribution (Deenen et al., 2011), and errors in declination (ΔDx)
195 and inclination (ΔIx) were calculated from the cone of confidence (A95) of the mean VGP
8 196 following Butler (Butler, 1992). We applied the reliability criteria of Deenen et al.
197 (2011) by determining A95 of the VGP distribution, and calculate the N-dependent
198 values of A95min and A95max. Values plotting within this envelope may be
199 straightforwardly explained by paleosecular variation (PSV). Values of A95>A95max may
200 indicate additional sources of scatter, while values of A95 201 dispersion (high K-values, as with individual lava flows) and likely underrepresent PSV. 202 All methods used are available on Paleomagnetism.org. In the online supplementary 203 information, we provide all the demagnetisation results and interpretations as *.dir files 204 that are easily imported into Paleomagnetism.org. Similarly, all statistical results 205 including custom made apparent polar wander paths (APWPs) are provided as an 206 *.pmag file. 207 208 3.2 Biostratigraphy 209 Benthic and planktonic foraminifera and algae of five samples from limestones of 210 the Woyla Arc were dated based on BouDagher-Fadel (2015, 2008) relative to the 211 biostratigraphic timescale of Gradstein et al. (2012). The faunal assemblage and the age 212 range are reported in the Supplementary Material. 213 214 4 Results 215 Samples were collected at thirteen sites in sedimentary rocks and volcaniclastic 216 rocks of the Woyla Arc, the oceanic accretionary assemblage and of the West Sumatra 217 Block (Figure 2) at the sparse locations where these rocks are exposed. Representative 218 Zijderveld diagrams are shown in Figure 4. The interpreted ChRM directions are shown 219 in Figure 5. The statistical properties of each site are shown in Table 1. 9 220 221 4.1 Northern Sumatra 222 We sampled limestones associated with the arc assemblage of the Woyla Group at 223 four sites along the road from Banda Aceh to Lamno (Figure 2b). Sites BA1 and BA2 224 correspond to the Raba Limestone Formation (Bennett et al., 1981) and sites BA3 and 225 BA4 to the Lamno Limestone Formation (Bennett et al., 1981). The Lamno Limestone 226 Formation yielded faunal assemblages with a Late Jurassic to Early Cretaceous age range 227 and the Raba Limestone is correlated to have a similar age range (Bennett et al., 1981). 228 229 4.1.1 Raba Limestone Formation 230 At site BA1, we sampled a ~14 m thick interval of dark-grey, thin-bedded (~10 cm) 231 limestones exposed in an active quarry. Thin section analysis revealed that the 232 limestone is completely recrystallized. Twenty-one samples were demagnetised with AF 233 demagnetisation and 16 with thermal demagnetisation. Intensities after the first heating 234 step of 80 °C are below 70 µA/m and samples have a poor magnetic signal, characterized 235 by chaotic demagnetisation behaviour (Figure 4a). Some AF data show higher intensities 236 (~1000 µA/m) but the component does not decay towards the origin at the highest AF 237 field (100 mT) (Figure 4b), indicating the presence of a high-coercive mineral. Because 238 of the lack of interpretable directions, we discard this site. 239 At site BA2 we sampled a ~12 m thick interval of dark-grey thin-bedded limestone. 240 Thin section analysis revealed that the limestone is completely recrystallized. Thirty- 241 nine samples were demagnetised by AF and 25 samples thermally. The intensity of the 242 samples is low (~1000-2000 µA/m) with some stronger samples ranging to 8000 µA/m. 243 At low temperatures/alternating fields, Zijderveld diagrams reveal a direction 244 coinciding with the recent field, i.e. the geocentric axial dipole (GAD) field at the present 10 245 latitude. At higher temperatures and fields there is a linear decay toward the origin from 246 300 °C/35 mT, with full demagnetisation at 480 °C suggesting titanomagnetite as 247 dominant carrier (Figure 4c). One sample yielded no interpretable direction. The tilt- 248 corrected ChRM is D=81.5±1.9°, I=-14.1±3.5° and the in-situ ChRM is D=82.7±1.9°, 249 I=17.2±3.5° (Figure 5a, Table 1). The in-situ A95 (1.9°) is smaller than A95min (2.3°), 250 indicating that PSV may have been insufficiently sampled, or is partly averaged within 251 samples. 252 253 4.1.2 Lamno Limestone Formation 254 At site BA3 we sampled a 50 m thick interval of dark-grey, thick-bedded limestones. 255 We refined a previously assigned Late Jurassic to Early Cretaceous age range (Bennett et 256 al., 1981) to the Aptian (126.3–113.0 Ma) (Supplementary Material). Fifteen samples 257 were measured with AF demagnetisation and 13 by thermal demagnetisation. Initial 258 intensities are relatively low (~1000 µ/m). Zijderveld diagrams reveal a direction 259 coinciding with the recent field at low temperatures/intensities. The linear decay 260 towards the origin from 300 °C with full demagnetisation at 450 °C (Figure 4d), suggests 261 that the magnetisation is carried by titanomagnetite. Two directions are of reversed 262 polarity, antipodal to the normal cluster, possibly recording the early Aptian M0r event 263 (Gradstein et al., 2012). Four directions were rejected after 45° cut-off (Figure 5b). The 264 tilt-corrected ChRM is D=35.2±2.9°, I=6.2±5.8° and the in-situ ChRM is D=35.5±3.2°, I=- 265 5.8±6.3° (Table 1), with the tilt-corrected A95 (2.9°) being smaller than A95min (3.4°), 266 suggesting PSV may have been partly averaged within samples. 267 At site BA4 we sampled a ~70 m thick interval of thick-bedded limestones exposed 268 along a road east of Lamno leading towards the mountains inland. Thin section analysis 269 revealed that the limestone is partly recrystallized. We refined a previously assigned 270 Late Jurassic to Early Cretaceous age range (Bennett et al., 1981) to the Albian (113.0– 11 271 100.5 Ma) (Supplementary Material). Twenty samples were measured by AF 272 demagnetisation and 15 by thermal demagnetisation. Zijderveld diagrams show a linear 273 decay to the origin from 330 °C, with full demagnetisation at 450 °C, suggesting 274 titanomagnetite as dominant carrier (Figure 4e). No samples were rejected after 45° 275 cut-off (Figure 5c). The tilt-corrected ChRM is D=6.8±3.7°, I=-8.2±7.2° and the in-situ 276 ChRM is D=4.8±3.9°, I=27.9±6.3° (Table 1), with A95 between A95min and A95max. 277 278 4.2 Central Sumatra 279 Rock units in Middle Sumatra correlated to the Woyla Group are exposed near 280 Padang and in the Rawas Mountains (Figure 2a, c). The Woyla Arc assemblage was 281 sampled at site PA4 in the Lubuk Peraku Limestone (McCarthy et al., 2001) that is 282 overlain by the Golok Tuff (McCarthy et al., 2001). The oceanic assemblage was sampled 283 at site PA1 in the Ngalau Chert, which occurs as imbricated thrust slices between 284 limestones (McCarthy et al., 2001). Sedimentary rocks of the West Sumatra continental 285 margin were sampled at four sites (PA2 and PA3 in the Siguntur Formation (Rosidi et al., 286 1976) and sites RW1 and RW2 in the Penata Formation (Suwarna et al., 1994)). 287 288 4.2.1 Lubuk Peraku Limestone 289 At site PA4 we collected five hand samples from a 3 m thick interval of massive 290 bedded limestones exposed along the Lubuk Peraku River at the village of Indarung 291 (Figure 2c). We refined a previously assigned Late Jurassic to Early Cretaceous age 292 range (Yancey and Alif, 1977) to early Aptian–early Albian (Supplementary Material). 293 From these samples, we derived core samples in the lab, of which 28 samples were 294 thermally demagnetised and 7 with AF. Initial intensities of most samples are ~1500 295 µA/m, with a few up to 5000 µA/m. The Zijderveld diagrams reveal a component with a 12 296 linear decay to the origin from 350 °C, with full demagnetisation at 500 °C suggesting 297 titanomagnetite as dominant carrier (Figure 4f). One sample was rejected after 45° cut- 298 off (Figure 5d). The tilt-corrected ChRM is D=342.3±6.5°, I=-54.4±5.4° and the in-situ 299 ChRM is D=23.9±2.9°, I=-5.4±5.7° (Table 1). We observe that the in-situ A95 is equal to 300 A95min (2.9°) indicating that PSV was insufficiently sampled. 301 302 4.2.2 Ngalau Chert 303 At the abandoned Ngalau Quarry in Indarung (Site PA1; Figure 2c), we collected five 304 oriented hand samples from a 10 m thick interval of Middle Jurassic (Aalenian–lower 305 Bajocian; McCarthy et al., 2001) bedded radiolarian chert belonging to the oceanic 306 assemblage. From these samples, we derived core samples in the lab, of which we 307 derived directions from 12 thermally and 5 AF demagnetised samples. Initial intensities 308 are very low (<300 µA/m). Most samples show a clear component coinciding with the 309 recent field, followed by a linear decay to the origin from 400 °C up to temperatures of 310 570 °C, suggesting magnetite as the dominant carrier (Figure 4g).. The tilt-corrected 311 ChRM is D=60.6±5.5°, I=-4.6±10.9° and the in-situ ChRM is D=57.3±6.2°, I=27.5±10.0° 312 (Figure 5e, Table 1), with A95 between A95min and A95max. 313 314 4.2.3 West Sumatra continental margin 315 At site PA2, exposed at a waterfall near the road from Padang to Siguntur (Figure 316 2c), we sampled a 4 m thick interval of shales of the Siguntur Formation. Rosidi et al. 317 (1976) reported an age range of Late Jurassic–Early Cretaceous, which we were unable 318 to refine further. We demagnetised 9 samples thermally, and 9 by AF. Initial intensities 319 are typically below 500 µA/m. Zijderveld diagrams show noisy demagnetisation 320 behaviour, but overall decay towards the origin with full demagnetisation at 470–570 13 321 °C, suggesting magnetite as dominant carrier (Figure 4h). One sample yielded no 322 interpretable direction. No directions were rejected after 45° cut-off (Figure 5f). The tilt- 323 corrected ChRM is D=343.1±7.1°, I=20.2±12.8° and the in-situ ChRM is D=344.8±6.2°, I=- 324 5.1±12.4° (Table 1), with A95 between A95min and A95max. 325 At site PA3, three cores yielded six samples that were collected from a 2 m thick 326 interval of quarzitic sandstones of the Siguntur Formation, exposed along the road from 327 Padang to Siguntur (Figure 2c). Three samples were demagnetised by AF and the other 328 three thermally. Initial intensities are ~1000-1500 µA/m. Thermal demagnetisation 329 diagrams reveal three components, of which the low-temperature component resembles 330 the recent field (Figure 4i). The high temperature component decays linearly to the 331 origin between 430 °C and 540 °C, suggesting (titano)magnetite as the dominant carrier. 332 All samples are of normal polarity. One sample yielded no interpretable direction. No 333 directions were rejected after 45° cut-off (Figure 5g). The tilt-corrected ChRM is 334 D=35.6±9.8°, I=47.2±10.2° and the in-situ ChRM is D=42.5±6.4°, I=21.0±11.4° (Table 1). 335 The in-situ A95 is equal to A95min (6.3°), suggesting that PSV was insufficiently sampled. 336 This is in line with the high K value, which is typical for spot readings of the field. We 337 therefore discard this site from further analysis. 338 Along the road from Bangko to Lake Kerinci, we sampled sites RW1 and RW2 in 339 exposures of the Peneta Formation that was previously dated at Middle Jurassic – Early 340 Cretaceous (Suwarna et al., 1994), whereas Beauvais et al. (1988) reported a Late 341 Jurassic age for the limestone members of the formation. 342 At site RW1, we sampled a ~2 m thick interval of calcareous sandstones and 343 siltstones of the Mersip Limestone Member. Twelve samples were demagnetised 344 thermally and five by AF. The intensity is very low (~100 µA/m) and demagnetisation 345 diagrams are chaotic (Figure 4j). No ChRM directions were interpreted and site RW1 346 was discarded from further analysis. 14 347 At site RW2 we sampled a 50 m thick interval of red tuffaceous shales. A total of 43 348 samples were demagnetised thermally and 29 with AF. Initial intensities are high 349 (~20000 µA/m). demagnetisation behaviour varies strongly between AF and thermally 350 demagnetised samples. AF demagnetisation diagrams reveal a tight cluster at high 351 intensities, without complete demagnetisation (Figure 4l), pointing to a high coercivity 352 mineral. Thermal demagnetisation led to gradual decay towards the origin at 353 temperatures well above 600°C (Figure 4k). We thus interpret the dominant magnetic 354 carrier as hematite. The direction indicated by the AF cluster yields a similar direction 355 as the component demagnetizing towards the origin using thermal demagnetisation and 356 both were used to interpret the ChRM direction. One sample yielded an anomalous 357 direction and was therefore discarded. The tilt-corrected ChRM is D=348.0±2.1°, 358 I=35.6±2.9° and the in-situ ChRM is D=353.2±1.6°, I=-17.1±3.0° (Figure 5h, Table 1). The 359 in-situ A95 (1.6°) is smaller than A95min (2.2°). 360 361 4.3 Southern Sumatra 362 We sampled volcaniclastic sedimentary rocks of the Saling Formation (Gafoer et al., 363 1992) belonging to the Woyla Arc assemblage in the Gumai Mountains (site GM1), 364 cherts belonging to the oceanic assemblage exposed in the Garba Mountains (site GB1) 365 and turbiditic sandstones exposed near the city of Bandar Lampung belonging to the 366 West Sumatra Block continental arc assemblage (site BL1) (Figure 2a). 367 368 4.3.1 Saling Formation 369 At site GM1, we collected eight samples from a ~4 m wide exposure of volcaniclastic 370 sedimentary rocks of the Saling Formation (Gafoer et al., 1992) along the Serampo River 371 in the Gumai Mountains. The Saling Formation is intruded by a diorite dyke with a K–Ar 15 372 age of 116 ±3 Ma (Aptian), interpreted to be coeval with the volcaniclastic sediments 373 (Gafoer et al., 1992). Samples were thermally demagnetised to 270 °C and further 374 demagnetised by AF. Initial intensities are <500 µA/m. The Zijderveld diagrams reveal a 375 linear decay to the origin (Figure 4m). Six samples are of reversed polarity, possibly 376 recording the early Aptian M0r event (Gradstein et al., 2012). After inverting these 377 reversed directions to normal polarity, the tilt-corrected ChRM is D=16.0±13.9°, 378 I=17.6±25.5° and the in-situ ChRM is D=37.8±16.2°, I=15.9±30.3° (Figure 5i, Table 1), 379 with A95 between A95min and A95max. 380 381 4.3.2 Garba Formation 382 At site GB1 we collected five hand samples from a 2 m thick interval of bituminous 383 shales of the Situlanglang chert member of the Garba Formation (Gafoer et al., 1994) 384 exposed along a tributary of the Kumering River near the Garba Mountains (Figure 2a). 385 The chert member did not yield age-diagnostic fossils (Barber, 2000). A minimum age 386 for the Garba Formation comes from the cross-cutting composite Garba Pluton, which 387 yielded K–Ar ages of 117–79 Ma (Gafoer et al., 1994; McCourt et al., 1996). From the 388 hand samples, 15 core samples were derived in the laboratory. Due to the high organic 389 matter contents, samples were only demagnetised thermally to 270 °C, and 390 subsequently demagnetised by AF. Initial intensities were ~300 µA/m. Most 391 demagnetisation diagrams revealed two components, whereby the higher coercivity 392 component decays approximately towards the origin (Figure 4n). Samples that were not 393 subjected to thermal demagnetisation reveal a high coercive component that is not 394 removed by alternating field demagnetisation. This component is likely carried by 395 goethite. One samples yielded an anomalous direction. The tilt-corrected ChRM is 396 D=258.5±8.1°, I=-42.2±9.8° and the in-situ ChRM is D=311.9±6.7°, I=-27.4±10.9° (Figure 397 5j, Table 1), with A95 between A95min and A95max. 16 398 399 4.3.3 Menanga Formation 400 At site BL1, along the Menanga River near Bandar Lampung, we sampled a 4 m wide 401 exposure of a sequence of thin turbiditic sandstones and siltstones of the Menanga 402 Formation (Andi Mangga et al., 1994). Previous studies dated the limestones 403 interbedded in the Menanga formation as Aptian–Albian (Andi Mangga et al., 1994; 404 Zwierzycki, 1932), which we adopt as age range for the turbiditic sequence. Eighteen 405 samples were demagnetised thermally, and 10 by AF. Intensities of the samples are high, 406 ranging from 1.5 to 200 mA/m. Zijderveld diagrams reveal two types of 407 demagnetisation behaviour. One group of samples show linear demagnetisation towards 408 the origin up to temperatures of 570 °C. The second group of samples shows three 409 components, with linear decay to the origin from 370–570 °C (Figure 4o), suggesting 410 magnetite as dominant magnetic carrier. Two samples yielded no interpretable 411 directions. Three samples yielded reversed directions antipodal to the normal cluster. 412 After converting these directions to normal polarity, no samples were rejected after 45° 413 cut-off for in-situ directions, but one sample was rejected after 45° cut-off and tilt- 414 correction (Fig 4k). The tilt-corrected ChRM is D=11.5±11.2°, I=53.3±9.6° and the in-situ 415 ChRM is D=28.8±6.6°, I=-14.7±12.4° (Table 1), with A95 between A95min and A95max. 416 417 5 Discussion 418 5.1 Paleolatitude reconstruction of the Woyla Arc and West Sumatra 419 Block 420 Plate tectonic reconstructions suggest that the West Sumatra Block accreted to 421 Sundaland in the Late Triassic–Early Jurassic (Barber et al., 2005; Barber and Crow, 17 422 2009; Metcalfe, 2013). We therefore compare our paleomagnetic results from the West 423 Sumatra Block with the global APWP (GAPWaP) of Torsvik et al. (2012) rotated into the 424 coordinates of Sundaland. To this end, we use the tool on paleomagnetism.org described 425 in Li et al. (2017) that allows calculating the global GAPWaP into the coordinates of a 426 restored block when Euler poles of that block relative to South Africa are provided. We 427 use the estimated Euler poles of Sundaland of Advokaat et al. (under review) (Figure 428 7a), which show no relative rotation to Eurasia and the Atlantic plate circuit of Torsvik 429 et al. (2012) and Seton et al. (2012). The tilt-corrected inclination of site PA2 indeed 430 coincides with the APWP of Sundaland during the Late Jurassic (Figure 5f, 6a), but also 431 the in-situ inclination overlaps with the APWP of Sundaland in the Late Jurassic–Early 432 Cretaceous. As we do not have independent confirmation of the primary nature of the 433 magnetisation of this site, it is also possible that this site was remagnetised during the 434 mid-Cretaceous collision of the Woyla Arc with the West Sumatra margin. The tilt- 435 corrected inclinations of sites PA3, RW2 and BL1 are higher than predicted by the 436 APWP. However, we have no independent confirmation of the primary nature of the 437 magnetisation of these sites. Because the in-situ inclination of PA3 and BL1 overlaps 438 with the Sundaland APWP, we suspect that these sites may have been remagnetised. The 439 in-situ inclination of site RW2 is below the Sundaland APWP, but given that its 440 magnetisation is carried by hematite, which may form in laterite as alteration product of 441 magnetite, we suspect this site may have been remagnetised and was subsequently 442 further tilted during later deformation. 443 Four out of six sites from the Woyla Arc assemblage (BA2, BA3, BA4, GM1) provide tilt- 444 corrected inclinations that suggest near-equatorial latitudes (Figure 5a-c, 5i). Because 445 sites BA2 and BA4 are recrystallized and their in-situ inclination coincides with the 446 Sundaland APWP, we suspect these sites were remagnetised during the mid-Cretaceous 447 collision of the Woyla Arc with the West Sumatra margin. Site PA4 has a high tilt- 448 corrected inclination, but because the in-situ inclination coincides with the Sundaland 18 449 APWP, we suspect that also this site was remagnetised during the mid-Cretaceous 450 collision of the Woyla Arc with the West Sumatran margin. 451 Finally, we examine sites PA1 and GB1 obtained from oceanic assemblage rocks 452 exposed between the Woyla Arc and the West Sumatra Block. These rocks were thus 453 likely derived from the once intervening ocean basin that was consumed by subduction 454 below the Woyla Arc and the West Sumatra Block (Figure 3). Here, we call this 455 conceptual ocean the Ngalau Ocean, after the Ngalau Chert. No fold test was possible for 456 sites PA1 and GB1 and we have thus no independent confirmation of primary 457 magnetisation. The tilt-corrected inclination (-4.6±10.9°) of site PA1 indicates a near- 458 equatorial paleolatitude, but has a high in-situ inclination of 27.5±10.0° that would 459 suggest a paleolatitude of λ=14.6° N or S (λmin=9.0°, λmax=21.0°). Tilting likely occurred 460 upon or after accretion of the cherts of this site to the Woyla Arc, or to the West Sumatra 461 Block. Neither of these has been at a latitude of 14.6°N or S since the Cretaceous, and it is 462 thus unlikely that the magnetisation post-dates the tilting. Conversely, site GB1 has a 463 high tilt-corrected inclination (-42.2±9.8°) which would suggest a paleolatitude of 464 λ=28.4°N or S (λmin=20.7°, λmax=38.1°). If the magnetisation is primary, this would 465 suggest a ~3000 km paleolatitudinal plate motion within the Ngalau ocean basin 466 between sites PA1 and GB1 since the Late Jurassic. We consider this unlikely. On the 467 other hand, the in-situ inclination of site GB1 is -21.0±11.4°. If the magnetisation 468 postdates tilting it should then have been acquired at a paleolatitude of 10.9°N or S 469 (λmin=4.8°, λmax=17.6°), which is within range of both the Woyla Arc and the West 470 Sumatra Block during collision in the mid-Cretaceous. We thus tentatively interpret the 471 data to suggest that the rocks derived from the Ngalau Ocean were also at equatorial 472 latitudes, with site PA1 carrying a primary magnetisation, and site GB1 carrying a post- 473 tilt magnetisation. 19 474 In summary, our sites that likely carry a primary magnetisation suggest that the 475 Woyla Arc remained at a near-equatorial latitudes in the Early Cretaceous. All other 476 sites, including those from the conceptual Ngalau Ocean, either carry magnetisations 477 that in tectonic coordinates also show equatorial latitudes, or that have equatorial 478 inclinations in in-situ coordinates, which may be explained by syn-collisional 479 remagnetisation. Throughout the Cretaceous, the West Sumatra Block, to which the 480 Woyla Arc accreted in the mid-Cretaceous, was also at equatorial latitudes, suggesting 481 that the Woyla Arc did not undergo significant N-S directed motions during its approach 482 to Sundaland. 483 484 5.2 Plate kinematic reconstruction: introducing the Ngalau Plate 485 The structure and composition of Sumatra led Barber et al. (2005) to suggest that 486 the Woyla Arc formed on oceanic lithosphere that was separated from Eurasia by two 487 subduction zones that consumed oceanic lithosphere (what we here call the Ngalau 488 Ocean), which subducted westward below the Woyla Arc and eastward below 489 Sundaland. We now test whether the lithosphere on which the Woyla Arc was situated 490 may have been part of one of the major surrounding plates, or if not, whether yet 491 another plate needs to be invoked to explain the paleolatitudes of the Woyla Arc. 492 First, Hall (2012) and Metcalfe (2013) suggested that the Woyla Arc may have 493 formed on lithosphere of the Indian plate, to account for the low paleolatitude of Woyla 494 rocks reported by Haile (1979). Second, van Hinsbergen et al. (2012) and Huang et al. 495 (2015a) argued that a plate that carried a Tibetan Himalayan microcontinent broke off 496 India and underwent a northward flight relative to the main Indian continent between 497 Early and latest Cretaceous time. This model aims to account for low, near-northern 498 Indian paleolatitudes derived from Lower Cretaceous, Triassic, and Ordovician rocks of 20 499 the Tibetan Himalaya suggesting a separation of <800 km (consistent with West- 500 Australian margin reconstructions of Gibbons et al. (2012), but a >2000 km wide 501 separation at the time of early Eocene collision as required by paleomagnetic data of 502 southern Tibet. If the Woyla Arc formed on the Indian plate in the Early Cretaceous, it 503 would have undergone this northward flight. We test this scenario using the recent 504 Euler poles for the Tibetan Himalaya relative to India of van Hinsbergen et al. (2018; 505 2011) and Li et al (2017). Third, we test whether the Woyla Arc may have formed on the 506 Australian plate as suggested by Metcalfe (1996). We constructed these scenarios using 507 the online platform paleomagnetism.org (Koymans et al., 2016), whereby we plot our 508 data against the GAPWaP (Torsvik et al., 2012) in coordinates of India, the conceptual 509 Tibetan Himalaya plate, or Australia. Only tilt-corrected inclinations of the sites with 510 inferred primary magnetisations are plotted. In all scenarios we assume a ~95 Ma age of 511 Woyla-Sundaland collision, corresponding to the youngest K–Ar age from metamorphic 512 rocks from the West Sumatra margin (Koning, 1985). 513 If the Woyla Arc was formed on the edge of the conceptual Tibetan Himalayan plate, 514 it would have undergone a ~40° northward latitudinal motion between ~130 and 95 Ma 515 (Figure 7b). The predicted APWP is clearly inconsistent with our data from the Woyla 516 Arc (Figure 6b). If no Tibetan Himalayan plate ever formed (e.g. Ingalls et al., 2016), 517 which would require that all paleomagnetic data from the Tibetan Himalaya are 518 unreliable and 1000s of km of Indian lithosphere subducted without leaving a trace (van 519 Hinsbergen et al., 2018; Van Hinsbergen et al., 2012), the Woyla Arc may have formed 520 on India. In this scenario, the Woyla Arc would have experienced a ~25° northward 521 latitudinal motion between 130 and 95 Ma (Figure 7b), which predicts our data better, 522 but is still inconsistent with site GM1 (Figure 6b). Finally, if the Woyla Arc formed on the 523 Australian plate it would have moved eastwards relative to Eurasia and Sundaland, over 524 a distance of ~1700 km, and remained at near-equatorial latitudes between 130 and 95 525 Ma (Figure 7). The predicted APWP is consistent with our data (Figure 6b). Hence, we 21 526 propose that the Woyla Arc has likely formed on the Australian plate, and that there is 527 no kinematic requirement to invoke a more complex scenario in which the Woyla Arc 528 formed on a plate independent from Australia, India, and the Tibetan Himalaya. 529 If the Woyla Arc formed on the Australian plate, the India-Australia plate boundary 530 would have been located to the west of the Woyla Arc, and we may explore how a three- 531 plate system of Australia-Ngalau-Eurasia (Sundaland) may have logically evolved prior 532 to, during, and after Woyla Arc-Sundaland collision. Prior to the formation of the Woyla 533 Arc, ~E-W convergence of the East Gondwana Plate – to which both Australia and India 534 belonged – relative to Sundaland was accommodated by a NE-dipping subduction zone 535 under the West Sumatra continental margin, as shown by a belt of Jurassic–Cretaceous 536 arc plutons (McCourt et al., 1996). At some stage at or prior to ~130–122 Ma, a 537 ~westward intra-oceanic subduction initiated below the Australian plate above which 538 the Woyla Arc formed (Figure 7a). It is unclear why this subduction zone formed, but it 539 may relate to the ~130 Ma breakup of East Gondwana into Antarctica, Australia, and 540 India (e.g. Gibbons et al., 2012; Seton et al., 2012). Also the cause for localizing the Woyla 541 trench remains open for speculation, but Barber et al. (2005) suggested westward 542 subduction below the Woyla Arc may have initiated along a north–south transform fault. 543 Either way, if we reconstruct the location of the Woyla Arc as part of the Australian 544 plate, assuming a ~95 Ma collision at Sumatra, we restore a trench ~1700 km west of 545 the West-Sumatra block at 130 Ma. We assume that the Woyla trench ended in a triple 546 junction somewhere along the western margin of the West Burma Block (Figure 7a). 547 Equivalents of the Woyla Arc may exist in Myanmar (Barber and Crow, 2009; Mitchell, 548 1993). We are not aware of accreted intra-oceanic arcs of Early Cretaceous age in the 549 Indus-Yarlung suture zone, while there was northward subduction below Tibet since at 550 least 130 Ma (e.g. Guilmette et al., 2009). The Australia-India (or Tibetan Himalaya) 551 plate boundary that also formed around 130 Ma (e.g. Gibbons et al., 2012; Seton et al., 552 2012; Van Hinsbergen et al., 2012) was likely located not far to the west of the Woyla 22 553 Arc, and Tibetan Himalaya-Asia convergence drove south Tibetan subduction. We thus 554 restore a ~N–S striking Woyla trench, at a small angle to the NW-SE striking western 555 Sundaland margin. 556 We thus infer a small, triangular Ngalau plate, caught between a transform fault in 557 the south, and the west-dipping Woyla and east-dipping West Sundaland trenches that 558 merged in a trench-trench-trench triple junction in the north (Figure 7a). This inferred 559 triangular geometry assumes that West Sumatra has not experienced major vertical axis 560 rotations since the Late Jurassic. Previously, large rotations of Sundaland were 561 postulated based on both CW and CCW declinations in rocks from the Malay Peninsula, 562 where Otofuji et al. (2017) interpreted that the Malay Peninsula underwent a regional 563 ~70° CW rotation together with Indochina. This is inconsistent with paleomagnetic 564 results from the Malay Peninsula, which show both CW and CCW declinations (Haile et 565 al., 1983; Richter et al., 1999), and Indochina, which only indicate a 15° CW rotation in 566 Cenozoic time (Li et al., 2017) and the rotations on the Malay peninsula are likely 567 governed by local deformation. Governed by E-W Australia-Sundaland convergence, the 568 Ngalau plate became consumed by subduction and its only relicts are preserved in the 569 accretionary complexes of the Woyla Arc and the West Sumatra margin (McCarthy et al., 570 2001). In the configuration of Figure 7, this convergence would have led to diachronous 571 Woyla-Sundaland collision, younging southward, and associated southward migration of 572 the triple junction. Such a southward migration remains speculative in the absence of 573 hard constraints on the age of collision. Following collision of the Woyla Arc with 574 Sundaland, convergence between Sundaland and Australia continued (Seton et al., 575 2012), and must have been accommodated by renewed subduction (Barber et al., 2005). 576 With the plate kinematic scenario as outlined in Figure 7, the only stable triple junction 577 arises if the post-collisional subduction zone is east-dipping, and necessarily locates 578 west of the Woyla Arc so as to preserve its geological record. Possible records of this 579 subduction polarity reversal following Woyla-Sundaland collision are found in supra- 23 580 subduction ophiolites on the margin of West Burma (127–116 Ma, Liu et al., 2016; Singh 581 et al., 2017) and the Andaman-Nicobar Islands (93.6±1.6 Ma, Sarma et al., 2010). 582 6 Conclusions 583 In this study, we attempted to reconstruct the oceanic plate between the Woyla Arc 584 and the western Sundaland margin. We show that the Woyla Arc formed above a west 585 dipping subduction zone in the Early Cretaceous. Paleomagnetic data from limestones of 586 the Woyla Arc indicate that these were formed and remained at equatorial latitudes. We 587 tested plate kinematic scenarios where the Woyla Arc was part of the Tibetan 588 Himalayan plate, the Indian plate and the Australian plate against paleomagnetic data. 589 Scenarios where the Woyla Arc was part of the Tibetan Himalayan plate or Indian plate 590 predict large latitudinal motions, inconsistent with our paleomagnetic data. Only a 591 scenario where the Woyla Arc is part of the Australian Plate, which predicts ~1700 km 592 eastwards longitudinal motion relative to West Sumatra, is consistent with our 593 paleomagnetic data. 594 We propose a reconstruction where the Ngalau Plate formed a triangular basin 595 between the Woyla Arc and the western Sundaland margin, to account for the absence of 596 accreted arc rocks in the Himalayas. As consequence of this triangular geometry, 597 accretion of the Woyla Arc to the western Sundaland margin was diachronous, 598 accommodated by a southward migrating triple junction. Continuing convergence of the 599 Australia relative to Eurasia was accommodated by subduction polarity reversal behind 600 the Woyla Arc, possibly recorded by Cretaceous ophiolites in the Indo-Burman Ranges 601 and the Andaman-Nicobar Islands. 602 24 603 Acknowledgements 604 ELA and DJJvH acknowledge funding through ERC Starting Grant 306810 (SINK) to 605 DJJvH. DJJvH acknowledges Netherlands Organization for Scientific Research (NWO) 606 Vidi grant 864.11.004. ELA thanks Anthony Barber and Michael Crow for their 607 introduction and discussion of the rocks of the Woyla Group. ELA and MLMB 608 acknowledge the Indonesian government for research permits through RISTEK. ELA, 609 MLMB and AR thank Edo Marshal, Adit Safriadi and Yudi Wandra for their help during 610 sampling in Sumatra. MLMB thanks Dan Palcu for help during paleomagnetic analysis at 611 Paleomagnetic Laboratorium ‘Fort Hoofddijk’. We are greatly indebted to Pierrick 612 Roperch for careful inspection of our data in a previous version of this manuscript, and 613 thank Mathijs Koymans for help with data conversion. We thank Anthony Barber, John 614 Geissman and an anonymous reviewer for their constructive comments. 615 616 Figure captions 617 1. Map of continental and arc fragments in SE Asia, modified from Barber et al. 618 (2005), Metcalfe (2013), and van Hinsbergen et al. (2011) and Li et al. (2017) 619 2. A) Simplified geological map of Sumatra and sample locations, B) Detail map of 620 Aceh, C) Detail map of Padang, modified from Barber (2000) and Barber et al. 621 (2005). 622 3. Cross sections of the Woyla Arc and West Sumatra continental margin, modified 623 from Barber et al. (2005). A) Early Cretaceous, B) mid-Cretaceous to present- 624 day. 625 4. Tilt-corrected Zijderveld diagrams of representative samples 626 5. Equal area projections of interpreted ChRM directions per site in tilt-corrected 627 coordinates and in-situ coordinates 25 628 6. A) Expected paleolatitude of West Sumatra (Sundaland) from Advokaat et al. 629 (submitted) and primary tilt-corrected paleomagnetic data from the West 630 Sumatra continental margin (this study), B) Predicted paleolatitude for the 631 Woyla Arc as part of the Tibetan Himalayan plate (grey), Indian plate (green), 632 and Australian plate (orange) against primary tilt-corrected paleomagnetic data 633 from the Woyla Arc (blue). 634 7. A) Reconstruction at 130 Ma, the triangular Ngalau Plate is bordered to the west 635 and east by two opposing subduction zones, and to the south by a transform 636 fault, B) Reconstruction at 95 Ma: the triangular Ngalau Plate is entirely 637 consumed by subduction. Motion paths between 130–95 Ma in 5 Ma time steps 638 for scenarios where the Woyla Arc is part of Tibetan Himalayan plate (grey), 639 Indian plate (green), and Australian plate (orange), C) Schematic kinematic 640 scenario showing southward triple junction migration. 641 642 Tables 643 1. Statistical parameters of sampling sites. N: total number of interpreted ChRM 644 directions; N45: number of ChRM directions after after 45° cut-off; D±ΔDx: 645 declination and 95% confidence interval following Butler (1992); I±ΔIx; 646 inclination and 95% confidence interval following Butler (1992); K; precision 647 parameter of VGP distribution following Fisher (1953); A95; 95% cone of 648 confidence on VGP distribution; A95min and A95max: N-dependent confidence 649 envelope for A95 values following Deenen et al. (2011); λ, λmin and λmax : 650 paleolatitude, and minimum and maximum paleolatitude. 651 26 652 Supplementary Material 653 1. Table S1: Biostratigraphic ages of the Woyla Arc 654 2. 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Ned. 796 33 Graphical Abstract (for review) EURASIAN PLATE ~130 Ma EURASIAN PLATE ~113 Ma EURASIAN PLATE ~95 Ma triple junction ophiolitesBurma Burma ophiolites migrating triple junction Andaman Sundaland Sundaland Sundaland Woyla Arc Ophiolite Woyla Arc Woyla Arc NGALAU PLATE NGALAU PLATE AUSTRALIAN PLATE AUSTRALIAN PLATE AUSTRALIAN PLATE Figure 1 70°E 80°E 100°E 110°E Click here to download Figure: Fig1_map_revised.pdfQiantang Lhasa 30°N Tibetan Himalayas South China India West Burma Mawgyi Nappe 20°N 20°N Indochina Andaman Ophiolite 10°N 10°N Sundaland Sunda Trench 0° Borneo WoylaWest Arc Sumatra Java 10°S 10°S 70°E 80°E 90°E 100°E 110°E Figure 1 Figure 2 Click here to download Figure: Fig2abc_Sumatra3.pdf 0 25 50 km ° PA4 6° 96° 98° 96 Banda Aceh PADANG Indarung BA1,2,3,4 BA1 BANDA ACEH Lho'nga S Raba F PA1 g.2B BA2 S 1°S -1° 4° BA3 Geumpang Lamno Lhoong Lam Minet PA3 B BA4 G 5° N L Tangse Sikuleh Gume Siguntur 2° Batholith Teunom PA2 Jaleuem C 0 5 TAKENGON10km Gume CALANG Ta keng er Natal iv Bale R la y o W 0° Indian A 0° Accretionary Complex n Ocean u Sunda Trench - undifferentiated B Padang MEULABOH a t PA1,2,3,4 e 4° N g.2C e RW1,2 Bangko F a Babahrot -2° Faults1°15'Lake u -2° Rawas l Kerinci Muaragadang t Woyla Arc Thrusts0 5 Mountains10km Ngalau Ocean Sampling site 100°30' West Sumatra SFS Sumatran Fault System Woyla Suture KL Kla Line GL Geumpang LineGM1 Gumai Cenozoic volcanics and sediments -4° Mountains -4° Meukuek Gneiss Woyla Group and correlatives Garba GB1 0 50 100km Mountains West Sumatra basement TAPAKTUAN Medial Sumatra Tectonic Zone 96° Bandar BL1 SIBUMASU Basement Lampung -6° -6° 0 100 200 300 400 500 km A 96° 98° 100° 102° 104° 106° Figure 3 Click here to download Figure: Fig3_cross_section.pdf Woyla Arc Ngalau Ocean West Sumatra (Neotethys) Sundaland margin Early Cretaceous Arc Accretionary Accretionary Asai-Peneta-Rawas Jurassic–Cretaceous Arc reefs Complex Complex Forearc basin A Accreted Woyla Arc Sundaland intra-oceanic island arc Magmatic Arc West Sumatra SIBUMASU Renewed overthrust onto subduction continental margin MSTZ at Sunda Trench B Figure up/West 4 up/West up/West up/West Click here to download Figure: Fig4_Zijderveld.pdf N 450° C N N 450° C Zijderveld Diagram (PA1.2-1A) Tectonic Coordinates Up/West 300° C 80° C Diagram (BA4.14A) N Zijderveld Diagram (PA2.1A) onic Coordinates Tectonic Coordinates Up/West 0-100 mT Up/West 300° C 120° C Zijderveld Diagram (PA4.4-1A) Tectonic Coordinates A BA1.9A Up/W Best BA1.2B C BA2.11A D BA3.8A up/West up/West up/West up/West Horizontal Projection Vertical Projection Horriizonttall P(ProCjAe)c t#i1on Vertical Projection Horizontal (PCA) #1 Vertical (PCA) #1 Vertical (PCA) #1 orizontal Projection Vertical Projection Paleomagnetism.org (Zijderveld Diagram) Paleomagnetism.org (Zijderveld100° Diagr aCm ) Paleomagnetism.org (Zijderveld Diagram) N 570° C 400° C Diagram (PA3.3A) Zijderveld Diagram (RW2.16B) onic Coor450°dina tCes Horizontal Projection NV ertical Projection Horizontal (PCA) #1 Tectonic Coordinates 570° C N Up/West Vertical (PCA) #1 Up/West 330° C 120° C Zijderveld Diagram (RW1.11PaAleo)magnetism.org (Zijderveld Diagram) Tectonic Coordinates ZijderveldN D iagram (RW2.15A) Tectonic Coordinates Up/West 150° C Up/West 500° C 100° C 400° C 300° C E BA4.14A F PA4.4.1A G PA1.2.1A H PA2.1A up/West up/West up/West up/West Horizontal Projection Vertical Projection Horizontal (PCA) #1 Vertical (PCA) #1 0-100 mT Paleomagnetism.org (Zijderveld Diagram) N N ertical Projection Horizontal (PCA) #1 Horizontal Projection Vertical Projection Horizontal (PCA) #1 Vertical (PCA) #1 600° C 540° CPaleomagnetism.org (Zijderveld Diagram) Paleomagnetism.org (Zijderveld Diagram) 100° C 630° C Horizon t430°al Pr Cojection Vertical Projection Horizontal (PCA) #1 N N VerticZali j(dPCeAr)v #e1ld Diagram (GM1.10A) Tectonic Coordinates Paleomagnetism.org (Zijderveld Diagram) Up/North 100° C Zijderveld Diagram (GB1.4-1A) Zijderveld Diagram (BL1.9A) Tectonic Coordinates Tectonic Coordinates I PA3.3A J RW1.11A Up/West K RW2.15A Up/West L RW2.16B up/North up/West up/West ertical Projection Horizontal (PCA) #1 Horizontal Projection Vertical Projection Horizontal (PCA) #1 Vertical (PCA) #1 Paleomagnetism.org (Zijderveld Diagram) Paleomagnetism.org (Zijderveld Diagram) Horizontal Projection Vertical Projection Horizontal (PCA) #1 Horizontal Projection Vertical Projection W Vertical (PCA) #1 Paleomagnetism.org (Zijderveld Diagram) 70 mT 270° C / 0 mT Paleomagnetism.org (Zijderveld Diagram) 150° C 120° C 45 mT 370° C 70 mT S S N 12 mT O BL1.9A 120° C horizontal projection horizontal component M GM1.10A N GB1.4.1A vertical projection vertical component Figure 5 tilt-corrected in-situ tilt-corrected in-situ Click here to download Figure: Fig5_ChRM.pdf BA2 D=81.5±1.9° BA2 D=82.1±1.9° PA2 D=343.1±7.1° PA2 D=344.8±6.2° A ChRM Directions I=-14.1±3.5° ChRM Directions I=17.2±3.5° F ChRM Directions I=20.2±12.8° ChRM Directions I=-5.1±12.4° 0° N45=63 0° N45=63 0° N45=18 0° N45=18 270° 90° 270° 90° 270° 90° 270° 90° 180° 180° 180° 180° D=35.2±2.9° D=35.5±3.2° D=35.6±9.8° D=42.5±6.4° Accepted Rejected BAM3ean Confidence ∆Dx Accepted Rejected BAM3ean Confidence ∆Dx Accepted Rejected PAM3ean Confidence ∆Dx Accepted Rejected PAM3ean Confidence ∆Dx B ChRM Directions PI=6.2±5.8°aleomagnetism.org (ChRM Directions) ChRM Directions I=-5.8±6.3°Paleomagnetism.org (ChR M DiGrections) ChRM Directions PI=47.2±10.2°aleomagnetism.org (ChRM Direc tions) ChRM Directions PaleomaI=21.0±11.4°gnetism.org (ChRM Directions) 0° N45=24 0° N45=24 0° N45=5 0° N45=5 270° 90° 270° 90° 270° 90° 270° 90° 180° 180° 180° 180° D=6.8±3.7° D=4.8±3.9° D=348.0±2.1° D=353.2±1.6° Accepted Rejected BAM4ean Confidence ∆Dx Accepted Rejected BAM4ean Confidence ∆Dx Accepted Rejected RWM2ean Confidence ∆Dx Accepted Rejected RWM2ean Confidence ∆Dx C ChRM Directions I=-8.2±7.2°Paleomagnetism.org (ChRM Directions) ChRM Directions I=27.9±6.3°Paleomagnetism.org (ChRM DiHrections) ChRM Directions PaleI=35.6±2.9°omagnetism.org (ChRM Direc tions) ChRM Directions PaleomagI=-17.1±3.0°netism.org (ChRM Directions) 0° N45=35 0° N45=35 0° N45=71 0° N45=71 270° 90° 270° 90° 270° 90° 270° 90° 180° 180° 180° 180° D=342.3±6.5° D=23.9±2.9° D=16.0±13.9° D=37.8±16.2° Accepted Rejected PAM4ean Confidence ∆Dx Accepted Rejected PAM4ean Confidence ∆Dx Accepted ReGjecMted1_nMoearnmalConfidence ∆Dx Accepted ReGjecMted1_nMoearnmalConfidence ∆Dx D ChRM Directions I=-54.5±5.4°Paleomagnetism.org (ChRM Directions) ChRM Directions I=-5.4±5.7°Paleomagnetism.org (ChR M DiIrections) ChRM Directions PI=17.6±25.5°aleomagnetism.org (ChRM Direc tions) ChRM Directions PaleomaI=15.9±30.3°gnetism.org (ChRM Directions) 0° N45=34 0° N45=35 0° N45=6 0° N45=6 270° 90° 270° 90° 270° 90° 270° 90° 180° 180° 180° 180° D=60.6±5.5° D=57.3±6.2° D=258.5±8.1° D=311.9±6.7° Accepted Rejected PAM1ean Confidence ∆Dx Accepted Rejected PAM1ean Confidence ∆Dx Accepted Rejected GBM1ean Confidence ∆Dx Accepted Rejected GBM1ean Confidence ∆Dx E ChRM Directions I=-4.6±10.9°Paleomagnetism.org (ChRM Directions) ChRM Directions I=27.5±10.0°Paleomagnetism.org (ChR M DiJrections) ChRM Directions PaI=-42.2±9.8°leomagnetism.org (ChRM Direc tions) ChRM Directions PaleomI=-27.4±10.9°agnetism.org (ChRM Directions) 0° N45=17 0° N45=17 0° N45=14 0° N45=14 270° 90° 270° 90° 270° 90° 270° 90° 180° 180° 180° 180° D=11.5±11.2° D=28.8±6.6° Accepted Rejected Mean Confidence ∆Dx Accepted Rejected Mean Confidence ∆Dx Accepted Rejected BLM1ean Confidence ∆Dx Accepted Rejected BLM1ean Confidence ∆Dx Paleomagnetism.org (ChRM Directions) ChRM Directions Paleomagnetism.org (ChRM Directions) ChRM Directions Paleomagnetism.org (ChRM Directions) Paleomagnetism.org (ChRM DKirections) I=53.3±9.6° I=-14.7±12.4° 0° N45=25 0° N45=26 270° 90° 270° 90° 180° 180° Figure 6 Click here to download Figure: Fig6_palat.pdfExpected Paleolatitude A Latitude: -1.096 Longitude: 100.475 20 15 PA2 10 ) ° ( e d u t i t a 5 l o e l a P 0 West Sumatra APWP -5 -10 175 150 125 100 75 50 25 0 Age (Ma) West Sumatra (Custom APWP) SWestelec tSumatraed Site D (paleomagneticata data) Paleomagnetism.org [Expected Polar wander] - after van Hinsbergen et al., 2015 Expected Paleolatitude B Latitude: 5.172 Longitude: 95.307 40 30 West Sumatra APWP 20 GM1 ) 10 ° ( e d u t i t BA3 a 0 l o Woyla Arc (AUS) APWP e l a P -10 Woyla Arc (IND) APWP -20 Woyla Arc (TH) APWP -30 -40 175 150 125 100 75 50 25 0 Age (Ma) West Sumatra (Custom APWP) Woyla Arc TH (Custom APWP) Woyla Arc IND (Custom APWP) Woyla Arc AUS (Custom APWP) SWoylaelecte Arcd S i(paleomagneticte Data data) Paleomagnetism.org [Expected Polar wander] - after van Hinsbergen et al., 2015 Figure 7 ~130 Ma ~95 Ma Click here to download Figure: Fig7_reconstructions.pdf Indochina 20°N 20°N Indochina West Burma West Burma 10°N 10°N 10°N Andaman Ophiolite Sundaland AUS Sundaland scenario Woyla Arc 0° 0° 0° 0° Woyla Arc Ngalau Plate ~1700 km 10°S 10°S 10°S IND scenario 20°S 20°S 20°S paleomagnetic referenc frame TH paleomagnetic referenc frame A (Torsvik et al., 2012) B scenario (Torsvik et al., 2012) EURASIAN PLATE ~130 Ma EURASIAN PLATE ~113 Ma EURASIAN PLATE ~95 Ma triple junction ophiolitesBurma Burma ophiolites migrating triple junction Andaman Sundaland Sundaland Sundaland Woyla Arc Ophiolite Woyla Arc Woyla Arc NGALAU PLATE NGALAU PLATE AUSTRALIAN PLATE AUSTRALIAN PLATE C AUSTRALIAN PLATE Figure 7 Table 1 Click here to download Table: Table1_pmag_parameters_re-interpreted.xlsx Table 1. Age, GPS position, bedding (strike/dip), means and statistical parameters per sampled site Site Assemblage Age range GPS position Tectonic Coordinates stratigraphic numerical latitude longitude bedding N N 45 D D D x I D I x K A 95 A 95min A 95max λ BA1 Woyla Arc Late Jurassic – Early Cretaceous 163.5–100.5 Ma 5.510520 95.278390 130/70 ------BA2 Woyla Arc Late Jurassic – Early Cretaceous 163.5–100.5 Ma 5.373784 95.256141 310/40 63 63 81.5 1.9 -14.1 3.5 95.6 1.8 2.3 6.0 -7.1 BA3 Woyla Arc Aptian 126.3–113.0 Ma 5.172197 95.307377 185/30 24 28 35.2 2.9 6.2 5.8 102.2 2.9 3.4 11.1 3.1 BA4 Woyla Arc Albian 113.0–100.5 Ma 5.094386 95.366598 290/40 35 35 6.8 3.7 -8.2 7.2 45.1 3.6 2.9 8.7 -4.1 PA4 Woyla Arc early Aptian – early Albian 126.3–105 Ma -1.073028 100.440750 350/30 34 35 342.3 6.5 -54.4 5.4 22.1 5.4 2.9 8.9 -34.9 PA1 Ngalau Ocean Aalenian – early Bajocian 174.1–168.3 Ma -0.955858 100.489290 100/30 17 17 60.6 5.5 -4.6 10.9 43.6 5.5 3.9 13.8 -2.3 PA2 W. Sumatra Late Jurassic – Early Cretaceous 163.5–100.5 Ma -1.095900 100.475000 150/28 18 18 343.1 7.1 20.2 12.8 25.3 7.0 3.8 13.3 10.4 PA3 W. Sumatra Late Jurassic – Early Cretaceous 163.5–100.5 Ma -1.073028 100.440750 328/68 5 5 35.6 9.8 47.2 10.2 80.4 8.6 6.3 29.7 28.4 RW1 W. Sumatra Late Jurassic 163.5–145.0 Ma -2.117255 102.008910 100/48 ------RW2 W. Sumatra Late Jurassic 163.5–145.0 Ma -2.115869 101.979570 110/60 71 71 348.0 2.1 35.6 2.9 73.7 2.0 2.2 5.6 19.7 GM1 Woyla Arc 116±3 Ma 119–113 Ma -3.803093 103.176780 205/65 6 6 16.0 13.9 17.6 25.5 24.9 13.7 5.9 26.5 9.0 GB1 Ngalau Ocean >117 Ma >117 Ma -4.477090 104.167800 275/22 14 14 258.5 8.1 -42.2 9.8 30.0 7.4 4.2 15.6 -24.4 BL1 W. Sumatra Aptian – Albian 126.3–100.5 Ma -5.554367 105.185150 140/78 25 26 11.5 11.2 53.3 9.6 10.8 9.3 3.3 10.8 33.8 Supplementary material 1 Click here to download Supplementary material for online publication only: mmc1.pdf dir files - Raw research data (under CC BY license; see above) Click here to download Raw research data (under CC BY license; see above): dir_files.zip pmag file - Raw research data (under CC BY license; see above) Click here to download Raw research data (under CC BY license; see above): Sumatra+APWP.pmag