Implications for the Ranau Caldera Emplacement and Slip-Partitioning in Sumatra (Indonesia)
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ELSEVIER Tectonophysics 312 (1999) 347±359 www.elsevier.com/locate/tecto K±Ar age of the Ranau Tuffs: implications for the Ranau caldera emplacement and slip-partitioning in Sumatra (Indonesia) Olivier Bellier a,Ł,Herve Bellon b, Michel SeÂbrier a,Sutantob,c,ReneÂC.Mauryb a UMR CNRS 8616, `ORSAYTERRE', BaÃtiment 509, Universite Paris-Sud, 91405 Orsay Cedex, France b UMR CNRS 6538, `Domaines OceÂaniques', Universite de Bretagne Occidentale, 6 Avenue Le Gorgeu, B.P. 809, 29285 Brest, France c Jurusan Gologi, UPN Veteran, Jl. Lingkan Utara, 55281 Yogyarta, Indonesia Received 2 December 1998; accepted 7 June 1999 Abstract The Sumatran subduction is an example of oblique convergence which is partitioned into a component normal to the plate boundary and a wrench component taken up by strike-slip deformation within the overriding plate. Indeed, off Sumatra, the approximately NNE-trending convergence is mechanically accommodated both by subduction processes and strike-slip deformation partly localised on the Great Sumatran dextral Fault (GSF). The GSF parallels the trench and follows approximately the magmatic arc, where major calderas are installed. The Ranau caldera is one of those located along the GSF in south Sumatra. A Ranau Tuff sample yielded 40K±40Ar ages of 0:55 š 0:15 Ma for its separated feldspars, which places the major Ranau caldera collapse between 0.7 and 0.4 Ma, a period of paroxysmal calderic activity along the Sumatran Arc. Geomorphic features affecting the Ranau Tuff and offset by the GSF yield a long-term dextral slip rate of 5:5 š 1:9mm=yr at 5ëS. Consequently, south Sumatra represents an intermediate case between complete slip-partitioning and purely oblique thrusting, where the leading edge is characterised by a low convergence obliquity (<20ë) accommodated by strike-slip deformation in the overriding plate. This demonstrates that even for low obliquity, slip-partitioning can exist. 1999 Elsevier Science B.V. All rights reserved. Keywords: Ranau caldera; K±Ar tuff age; Sunda subduction; oblique convergence; slip-partitioning; Great Sumatran Fault 1. Introduction Ranau caldera produced one of the major widespread ignimbritic tuffs in Sumatra, the emplacement age of The Ranau caldera is one of the major calderas which was unconstrained. located along the Great Sumatran Fault (GSF) in Sumatra Island provides one of the best exam- south Sumatra (location on Fig. 1). Its shape and ples of oblique convergence with partition of plate wide size have been interpreted as resulting from convergence into a component normal to the plate the geometric evolution of the GSF segmentation boundary and a wrench component accommodated (Bellier and SeÂbrier, 1994). It collapsed within an by strike-slip deformation within the overriding plate along-strike GSF pull-apart presently inactive. The mainly localised along the GSF (e.g., Fitch, 1972; Jarrard, 1986). Global modelling along the south- Ł Corresponding author. Fax: C33-1-6019-1446; E-mail: ern segment predicts a convergence obliquity that [email protected] increases from 0ë at the Sunda Strait to about 20ëE, 0040-1951/99/$ ± see front matter 1999 Elsevier Science B.V. All rights reserved. PII: S0040-1951(99)00198-5 348 O. Bellier et al. / Tectonophysics 312 (1999) 347±359 Fig. 1. Sumatran geodynamic setting showing slip rates along the Great Sumatran Fault (GSF). The MentawaõÈ Fault (MF) and the GSF are reported after Diament et al. (1992) and Beaudouin et al. (1995), respectively, while the back-arc transpressional belts are from Detourbet (1995). The approximate location of the major volcanoes are after Sutanto (1997). Mean plate convergence rate and orientation are after Tregoning et al. (1994). Box at 5ëS points to the studied zone. Other slip rates in boxes are given after Bellier and SeÂbrier (1995), while the NW-trending opening rates of the Andaman Sea back-arc basin and Sunda Strait are after Curray et al. (1979) and Diament et al. (1990), respectively. O. Bellier et al. / Tectonophysics 312 (1999) 347±359 349 with an about 10 mm=yr shear rate at 5ëS (Baroux et dicts a convergence obliquity that increases from 0ë al., 1998). In addition, several geologic and geomor- at about 6ëS to about 20ë at about 3ëS, while along phic features affecting the Ranau Tuffs are displaced the northern segment, obliquity is of the order of by the GSF (Bellier et al., 1991; Bellier and SeÂbrier, 25 š 5ë (Baroux et al., 1998), the largest northward 1994). Using these offsets, the long-term horizontal increase in convergence obliquity occurring between slip rate of the southern GSF has been estimated to about 6ëS and 2ëN (Bellier and SeÂbrier, 1995). 6 š 4mm=yr (Bellier et al., 1991). Due to the lack Oblique thrust slips deduced from earthquake fo- of a reliable age of the Ranau Tuffs, the deduced slip cal mechanisms along the Sumatran subduction show rate was inaccurate. In contrast, GPS measurements oblique slips which indicate that part of the trench- realised at ca. 200 km further north of the Ranau parallel convergence component is taken up by the caldera gave a far-®eld dextral slip rate of about subduction (Fig. 3). Thus, off Sumatra, the conver- 25 š 7mm=yr (e.g., Duquesnoy, 1997), thus, with gence obliquity is accommodated by both strike-slip a large discrepancy with respect to the geologically deformation on the GSF and subduction processes determined slip rate. (e.g., McCaffrey, 1991). The northward increase in We present in this paper a new 40K±40Ar age from convergence obliquity along the Sumatran margin the Ranau Tuffs related with the paroxysmal eruption led McCaffrey (1991) to propose a northwestward associated with the ultimate episode of the caldera increase of the fore-arc motion relative to the upper evolution which produced the largest collapse. The plate that would stretch the fore-arc and produce a aims of the current study are: (1) to deduce the age variable slip rate along the GSF. In order to constrain of the major collapse event of the Ranau caldera; (2) the GSF dextral slip rate, detailed analysis of the to constrain the geologically determined long-term active fault zone on high-resolution SPOT images, GSF slip rate in south Sumatra calculated from the further constrained by ®eld studies, has been con- geomorphic offsets affecting the Ranau Tuffs; and ducted (Bellier and SeÂbrier, 1994, 1995). Even if the (3) to discuss slip-partitioning processes for a low slip rate estimates for the GSF are not precise, the convergence obliquity as along the south Sumatran results are accurate enough to show an along GSF subduction. strike northward slip rate increase from 6š4mm=yr, at 5ëS, to 23 š 2mm=yr, at 2ë100N (Fig. 1). For cen- tral Sumatra, estimates con®rm this increase from 2. Geodynamics and oblique convergence a slip rate of 11 š 5mm=yr, at about 3ë±4.5ëS, to partitioning of the Sumatran subduction 17 š 6mm=yr, at 0.5ë±1ëN. In addition, Natawidjaja and Sieh (1994) calculated a slip rate of 12 mm=yr, The 1650-km-long, NW-trending, dextral GSF is at 0.5ëS. parallel to the Sunda Trench and connects from north Both geologically and geodetically determined to south, the Andaman Sea back-arc basin to the slip rates are similar for the northern-central GSF, but Sunda Strait extensional area (Fig. 1) (Hamilton, they could be signi®cantly different for the southern 1979). The GSF approximately follows the Sumatra GSF. Indeed, four sites from a wide geodetic network calc-alkaline magmatic arc that acts as a mechani- (GEODYSSEA project, see Wilson et al., 1998) sug- cally weak zone. Because the volcanic arc is directly gest for the GSF domain horizontal far-®eld shear above the asthenospheric wedge, the GSF is viewed rates of about 20 š 7mm=yr, at about 2.5ëN, and as a lithospheric-scale fault (Fig. 2). 25 š 7mm=yr, at about 3.5ëS (Duquesnoy, 1997; Along the Sunda Trench, the Indo±Australian Walpersdorf, 1997). This latter value is inconsistent Plate is subducting under the Eurasian Plate with with the geologically determined ca. 6 š 4mm=yr a mean convergence rate of 67 š 7mm=yr in a direc- slip rate at 5ëS, 200 km further south. Conversely, tion of N11 š 4ëE (Tregoning et al., 1994). As the two local networks around the central (lat. 0ë450S) mean azimuth of the Sumatran Trench, northwest of and the southern (lat. 5ëS) GSF reported horizon- the Sunda Strait is N140ëE, the convergence is thus tal near-®eld slip rates of 28 š 3mm=yr (Duques- signi®cantly oblique off Sumatra (Fig. 1). Along the noy et al., 1999) and about 10 mm=yr (Duquesnoy southern segment, plate kinematics modelling pre- et al., 1996), respectively. Nevertheless, this last 350 O. Bellier et al. / Tectonophysics 312 (1999) 347±359 O. Bellier et al. / Tectonophysics 312 (1999) 347±359 351 value is inferred from a speculation concerning the ing on the overriding plate increases in some man- 1994 Liwa earthquake rupture with respect to the ner with increasing convergence obliquity. However, inter-seismic=co-seismic deformations. south of Sumatra between 6ë and 7ëS of latitude, Geological and geophysical studies revealed sig- earthquake slip vectors, of about N25ë š 3ëE are sig- ni®cant distributed right-lateral transpressional de- ni®cantly de¯ected away from the plate convergence formation across the arc domains contributing to vector of N11ë š 4ëE and from the trench nor- accommodation of the slip rate variation along the mal azimuth of N42ëE, the trench azimuth being of GSF and to the convergence obliquity (e.g., Bellier N132ëE, at these latitudes. Taking into account these and SeÂbrier, 1995). Indeed, oceanic cruises revealed previous values and the GPS deduced plate conver- in the fore-arc area the existence of the Mentawai gence of about 67 š 7mm=yr (Tregoning et al., Fault (MF, Figs.