Implications for the Ranau Caldera Emplacement and Slip-Partitioning in Sumatra (Indonesia)

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K±Ar age of the Ranau Tuffs: implications for the Ranau caldera emplacement and slip-partitioning in Sumatra (Indonesia)

Olivier Bellier a,Ł,HerveÂ Bellon b, Michel SeÂbrier a,Sutantob,c,ReneÂC.Mauryb a UMR CNRS 8616, `ORSAYTERRE', BaÃtiment 509, UniversiteÂ Paris-Sud, 91405 Orsay Cedex, France b UMR CNRS 6538, `Domaines OceÂaniques', UniversiteÂ 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,

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 central 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 signi®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. 1 and 2) parallel to the trench, 1994), using McCaffrey's relationship (e.g., McCaf- which exhibits evidence of a combined thrust (Karig frey, 1991) we calculated the expected arc-parallel et al., 1980) and strike-slip component (Diament shear acting on the overriding plate to accommodate et al., 1992; Malod et al., 1993). Apart from the the resulting obliquity. It corresponds to a dextral MF, seismic pro®les and shallow seismicity recorded slip rate of 17 š 10 mm=yr that may be consistent using a land±sea seismological network, revealed ac- with both GSF geologically estimated slip rate of tive deformation in the southern part of the fore-arc, 6 š 4mm=yr at 5ëS and GSF geodetically calculated including strike-slip focal mechanisms (Zen, 1993). slip rate of 25 š 7mm=yr at 3.5ëS. The present-day transpressional tectonic regime of In addition, GPS measurements acquired over the fore-arc is con®rmed by GPS measurements southeast Asia (GEODYSSEA) permit to identify a across the Sumatran Arc domain (e.g., Prawirodirdjo rigid Sunda platelet that is rotating clockwise with et al., 1997). However, these recent GPS measure- respect to Eurasia around a pole located south of ments yield evidence for a dominant NE- to NNE- Australia (Chamot-Rooke et al., 1999). As the GSF trending shortening affecting the fore-arc domain is localised along the southeast boundary of this (Fig. 3) with no signi®cant, or a small, component of Sunda block, the northward-increasing velocity of strike-slip displacement along the MF. the GSF contributes to this rotation. Thus, this ro- Transpression in the arc domain also includes de- tation being inconsistent with the GEODYSSEA deformations within the back-arc (Bellier and SeÂbrier, termined slip rate of 25 š 7mm=yr at about 3.5ëS, 1995) (Figs. 1 and 2). Observations of fault zones it is necessary to make a reappraisal of the available oblique to the GSF show that they correspond gen- long-term slip rate on the southern GSF to discuss erally to high dip faults characterised by an oblique- the tectonic processes that account for slip rate vari- slip dextral-reverse component. These faults approx- ation. imately bound transpressional uplifted belts that cut obliquely across the eastern Barisan and reach the back-arc Sumatran basins where they are associ- 3. The Ranau caldera ated with folds within Tertiary sediments (Detourbet, 1995). Along the GSF extensive out¯ows of Quaternary As presented above, the arc-parallel shear act- ignimbritic tuffs result from large eruptions associ-

Fig. 2. SW±NE cross-section sketch across the Sumatran segment of the Sunda Arc subduction, modi®ed after Bellier and SeÂbrier (1994, 1995) and Detourbet (1995). 1 D Cenozoic ®lling of both fore-arc and back-arc basins; 2 D continental crust; 3 D subcrustal lithospheric mantle; 4 D accretionary prism.

Fig. 3. Sketch of the regional geodynamic framework of Sumatra showing the role of the deformation zones within the overriding plate accommodating the convergence off the Sumatra Trench. 1 D GSF approximate horizontal slip-rates; 25, 15 and 5 are in mm=yr; 2 D back-arc transpressional belts; 3 D transpressional MF zone, red arrows represent the average displacement vector azimuth of the western block of MF with respect to Sunda, deduced from GPS measurements (e.g., Prawirodirdjo et al., 1997); 4 D Sunda Trench with the average slip-vector azimuth after McCaffrey (1991) while the mean oblique convergence is after Tregoning et al. (1994). 352 O. Bellier et al. / Tectonophysics 312 (1999) 347±359 O. Bellier et al. / Tectonophysics 312 (1999) 347±359 353 ated with huge calderas. Three major out¯ows are development of a new fault across the central part known in the vicinity of the calderas, from north to of the pull-apart basin. This fault corresponds to the south: the Toba, the Padang-Singkarak and the Pa- conspicuous active NW-trending strike-slip fault that sumah-Lampung out¯ows, the latter surrounding the cross-cuts the Ranau Lake and horizontally displaces Ranau caldera (Bellier and SeÂbrier, 1994). recent geomorphic features. The Ranau caldera has a 12 km wide and 16.5 km long rectangular shape (Fig. 4) and results from the relationships between the GSF tectonic evolu- 4. The Ranau Tuffs: age and lithology tion and the Sumatran calc-alkaline arc volcanism. Indeed, Bellier and SeÂbrier (1994) reported that huge We present new 40K±40Ar age data for one rhy- volcanic calderas are tectonically controlled by the olitic sample collected along the Way Robok River GSF and collapsed within pull-apart grabens where from the upper northern river bank escarpment at bounding faults act as substitutes for the ring frac- about 5 m from the top. Sampling is done where tures of usual circular calderas. The anomalous shape Ranau Tuffs are thick, morphologically young and (elliptical to rectangular) and large size of these belong to the youngest Ranau caldera ash-¯ow tuffs, calderas may be independent of those of the un- emitted during the ultimate episode of the Ranau derlying magma chamber because it is essentially caldera evolution, i.e., during the paroxysmal tuff related to the shape of the pull-apart (Bellier and eruption which produced the major collapse. SeÂbrier, 1994). The banks of the Ranau Lake generally correspond to degraded caldera borders even if 4.1. Sample description and geochemistry the southeastern edge of the Ranau caldera is partly covered by volcanic ¯ows that originated from a This quartz-free rhyolitic pumice contains ca. 5% resurgent volcanic cone in the southeastern part of modal phenocrysts set into a highly vesicular glassy the caldera. groundmass. The phenocrysts include, by order of The Ranau caldera evolution can be summarised decreasing abundance, strongly zoned feldspar, less as follows (Bellier and SeÂbrier, 1994). A pull-apart than 1 modal % biotite and rare titanomagnetite. The basin formed within a wide releasing step-over along corresponding ICP±AES chemical analysis is shown the strike-slip fault system with normal faults bound- in Table 1, together with that of a Toba Tuff sample ing the future location of the Ranau caldera. In this collected and studied by Sutanto (1997). Its com- rectangular 12 ð 16:5 km step-over of crustal weak- position matches that of island-arc medium-K calc- ness zone, volcanic and geothermal activities took alkaline rhyolites (Gill, 1981). It is characterised by place. This was followed by the formation of minor very low contents in compatible transition elements, calderas in the step-over. Increase of volcanic ac- a moderately enriched incompatible elements pat- tivity later produced an incremental caldera growth. tern with a clear negative Nb anomaly compared The ultimate episode of the caldera evolution was to elements of similar incompatibility, e.g., K and probably a paroxysmal tuff eruption that produced La. Its most peculiar chemical feature lies in its the collapse of the block delimited by the boundary strongly fractionated rare earth element spectrum, faults. The normal and strike-slip faults of the pull- characterised by extremely low Y and heavy rare apart were converted to caldera border fractures. The earth element abundance (e.g., Dy, Er and Yb con- extinction of the Ranau step-over was marked by the centrations lower than 1 ppm, i.e., less than twice

Fig. 4. Structural map from Lake Ranau to the Semangka Bay deduced from structural interpretations of SPOT (K=J: 278=360) and Landsat (133=64 and 133=63) images. Inset at top right shows the location of the study zone. Inset at bottom left is an interpretation sketch of the detailed views (Fig. 5) of combined SPOT and Landsat images showing, from north to south, offsets of the caldera rim (1), of an inactive fault (2), of capture streams (A), and of the Way Robok River (B); arrows point approximately to the horizontal offsets. Legend: 1 D major active faults of the GSF; 2 D inactive or=and minor faults; 3 D active faults with a dominant normal component (hachured on the downthrown block); 4 D caldera rims of Ranau and boundary of a Suwoh caldera cluster; 5 D Quaternary alluvial and volcano-sedimentary deposits. 354 O. Bellier et al. / Tectonophysics 312 (1999) 347±359

Table 1 4.2. Isotopic ages ICP±AES chemical analyses of a Ranau Tuffs rhyolitic pumice (sample R1) and of a Toba caldera pumice, checked using AC-E, 40K±40Ar datings have been performed on whole BE-N, JB-2 and PM-S standards a rock, and carefully separated groundmass and R1 HT 16 feldspars. Sample was crushed and sieved to 315± µ SiO2 (wt%) 72.50 70.50 160 m grain size for whole-rock dating and TiO2 0.25 0.23 then cleaned with distilled water. Groundmass and Al2O3 14.60 15.12 feldspar separations were achieved on a smaller size Fe O* 2.20 2.02 2 3 fraction (160±100 µm) using a magnetic separator. MnO 0.06 0.05 MgO 0.53 0.47 These fractions were used (1) for argon extraction CaO 2.88 1.93 from0.8to1gofsampleunder high vacuum by Na2O 3.69 3.12 HF (High Frequency generator) heating, and (2) K2O 1.98 3.95 after reduction to a powder for potassium analy- P2O5 0.02 0.02 sis by atomic absorption spectrometry. The isotopic LOI 1.60 2.34 TOTAL 100.31 99.75 composition of argon and radiogenic argon content were measured by mass spectrometry after isotopic Cr (ppm) 5 4 dilution with 38Ar prepared along the technique de- Ni 3 1 Co 3 3 scribed by Bellon et al. (1981). Atmospheric argon Sc 3 4.2 composition was checked after each measurement. V3419 Results are listed in Table 2 together with the Rb 110 168 most characteristic parameters of the measurements. Ba 326 620 One may notice the low 36Ar content of feldspars (6 Nb 2.2 14.4 10 3 La 15.2 33.5 to 9 ð 10 cm ) with respect to that in the pumice 9 3 Ce 26.6 67 groundmass (6 to 8 ð 10 cm ) and the inter- Sr 254 132 mediate resulting content for the whole rock which Nd 8.5 23 re¯ects the mass balance of its different components, Zr 35 52 i.e., the feldspars (ca. 80%) and the mesostasis (ca. Eu 0.63 0.73 Dy 0.9 3.8 20%). Because of these different atmospheric con- Y 7 25.5 tamination imprints, we will consider that resulting Er 0.7 2.5 ages with the lowest 36Ar contents (i.e., the feldspar Yb 0.81 2 age) indicate most probably the age of this tuff. a Relative standard deviations are ca. 2% for major elements and The isotopic age of the sample material is brack- 5% for trace elements (Cotten et al., 1995). eted between 0.41 and 1.0 Ma, these large uncertainty boundaries taking account of the estimated analytical error. In fact, the mean ages are 0:66 š 0:18 the primitive mantle values). Such a feature is com- Ma for groundmass and 0:73 š 0:13 Ma for whole monly attributed to the presence of residual garnet in rock, respectively, while isotopic ages for feldspars the magma source, whatever its continental (garnet- range between 0.45 and 0.64 Ma. Thus, the mean bearing metasediments) or oceanic (garnet-bearing age for feldspars is 0:55 š 0:15 Ma and corresponds amphibolite or eclogite) origin. The lack of isotopic to the Ranau Tuff age. These results are in agree- Sr, Nd and Pb data for these pumices precludes fur- ment with the 40K±40Ar age of 0:53 š 0:03 Ma on ther discussion on their source. However, this sample biotites from a Ranau Tuff sample collected east of clearly differs from the well-known Toba caldera Liwa (Hari Utoyo et al., 1995), at about 20 km south pumices (Chesner and Rose, 1991), thought to de- of our sampling, con®rming that the studied Ranau rive from melting of continental crust, by its lesser Tuffs correspond to the paroxysmal eruption of the contents in all the incompatible elements. Ranau caldera. In addition, these tuffs outpoured at the same time as the middle Toba Tuffs were erupted: at O. Bellier et al. / Tectonophysics 312 (1999) 347±359 355

Table 2 Whole-rock and separate phases 40K±40Ar isotopic ages for rhyolitic pumice R1

40 40 36 Sample Mean age Lab. ref. Age š error K2O ArR ArR Ar exp (Ma) (Ma) (wt%) (%) (ð 108 cm3 g1)(ð 10-10 cm3) Feldspars 0:55 š 0:15 B4096 0:64 š 0:04 1.18 7 2.41 6.5 B4730 0:56 š 0:04 10.4 2.13 8.1 B4748 0:54 š 0:04 13.4 2.04 7.1 B4762 0:45 š 0:04 6.1 1.71 7.2 Vitreous groundmass B4729 0:72 š 0:18 4.55 3.3 10.55 84 0:66 š 0:18 B4763 0:69 š 0:17 3.4 10.17 80 B4483 0:59 š 0:14 3.6 8.62 59 Whole-rock 0:73 š 0:13 B4073 0:90 š 0:10 1.98 8.2 5.77 22 B4736 0:61 š 0:13 3.5 3.87 36

40 40 40 The most characteristic parameters of the measurements: ArR (subscript `R' means radiogenic Argon), the percentage of ArR versus 40 40 36 the sum of ArR and atmospheric Ar and the amount of Ar isotope of only atmospheric origin for each experiment but uncorrected from the low blank value (<2 ð 1010 cm3 for 36Ar). Age calculations are carried out using the constants recommended by Steiger and JaÈger (1977) with the 1¦ error calculated according to Mahood and Drake (1982). Notes: Weight fused: 0.6 to 0.8 g for feldspars, 0.7 to 0.8 g for mesostasis, 1 g for whole-rock, including scarce and small biotite crystals.

0:501 š 0:005 Ma (Chesner and Rose, 1991) from immediately after the tuff emplacement (e.g., Bel- the Toba caldera at 2ë100N, i.e., 1000 km further lier and SeÂbrier, 1995). The paroxysmal volcanic north from the Ranau caldera. event that formed the major Ranau caldera collapse produced the widespread Ranau Tuffs sampled in the Way Robok River by 0:55 š 0:15 Ma. The es- 5. The southern Great Sumatran Fault slip rate timated rate of dextral displacement deduced from the Way Robok stream offset of about 2750 š 200 Detailed mapping of the Quaternary surface fault m (B in Figs. 4 and 5) is 3.6 to 7.4 mm=yr, i.e., traces and of the offsets on SPOT and Landsat im- 5:5 š 1:9mm=yr. Just northwest of the Way Robok ages permitted calculation of strike-slip rates along River, a drainage pattern of three streams incising the southern GSF. Note that SPOT images generally the Ranau Tuffs is displaced with apparent dextral overestimate geomorphic features with uncertainties offsets ranging between 150 š 30 m and 500 š 30 m. ranging between š30 m and š100 m due to observa- Different amounts of incision characterise the pre- tion mis®t. On the studied Landsat image, uncertain- sent-day analysed up-stream and down-stream chan- ties are of the order of 200 m. The southernmost part nels, which re¯ect stream capture (e.g., Gaudemer of the present-day GSF, south to the Ranau Lake, is et al., 1989). Taking into account this capture effect, formed by two major NW-trending fault segments, the corresponding up-stream=down-stream more in- the Ranau±Suwoh and the Semangka fault segments cised channels show a `true' cumulative river offset that are linked by a releasing step-over fault zone of about 2550 š 100 m (A in Figs. 4 and 5). This (Fig. 4). The Ranau±Suwoh fault segment is formed offset affecting the 0:55 š 0:15 Ma tuffs yields a slip by one right-lateral strike-slip fault that cross-cuts rate of 5:3 š 1:6mm=yr. the Ranau Lake and displaces its southeastern bank. Other geomorphic feature offsets of about 2500 m At about 5ëS, the strike-slip motion along the have been observed on SPOT images. They are the GSF produced dextral offsets of streams that incise Ranau caldera rim offset by about 2550 š 50 m (1 the Ranau Tuffs, ranging between about 500 š 30 in Figs. 4 and 5) and a NNE-trending inactive fault m on SPOT image, and 2750 š 200 m (Figs. 4 and offset of 2450 š 50 m (2 in Figs. 4 and 5). They 5) on Landsat image. Due to the high erosion rate yield slip rates of 5:0 š 1:5and4:8 š 1:2mm=yr, in a tuff lithology in a tropical climate, we can respectively. All the determined slip rates converge consider that the stream network has been installed to the same order of values in fair agreement with 356 O. Bellier et al. / Tectonophysics 312 (1999) 347±359 ), also observed on SPOT image. A 16th 1989) interpreted in Fig. 4, inset a. Inset right is a detailed Landsat image view of offsets of the Way = 360 (7 = J: 278 = ) and of the capture drainage pattern constituted by three streams ( B Robok River ( Fig. 5. Detailed view of the SPOT image (K O. Bellier et al. / Tectonophysics 312 (1999) 347±359 357 both the slightly constrained 6 š 4mm=yr slip rate MF. This is probably responsible for the Pleistocene previously proposed by Bellier et al. (1991) and high subsidence rate of about 1 mm=yr reported for the less than about 10 mm=yr estimated for the the Sumatran fore-arc basins (Zen, 1993). Moreover, Semangka fault, i.e., the southernmost segment of Quaternary reverse faulting in the onshore fore-arc the GSF (Bellier et al., 1997). of central Sumatra (0.5ë±1ëS) provides evidence that part of the fore-arc sliver is submitted to a compression characterised by an about N40ëE-trending short- 6. GSF slip rate and slip partitioning across the ening (Detourbet, 1995; Bellier and SeÂbrier, 1995) Sumatran subduction zone which is similar to the compression trend deduced from GPS measurements (Prawirodirdjo et al., 1997) Offsets of geomorphic features documented by and from bore-hole breakout measurements in the satellite images con®rm the GSF dextral strike-slip back-arc (Mount and Suppe, 1992) (Fig. 3). These displacement and support a slow slip rate for south- observations suggest that if present-day strike-slip ernmost segments of the GSF. All offsets affecting exists along the MF, it is with a small compo- the studied Ranau Tuffs range between 2400 m and nent. In addition, the fore-arc sliver being under a 2950 m. They yield slip rates ranging between 3.6 thrust faulting regime, this is not consistent with the and 7.4 mm=yr, corresponding to an average slip rate hypothesis by McCaffrey (1991) of fore-arc sliver of 5:5 š 1:9mm=yr. stretching. Previous studies determined GSF slip rates of 23š2mm=yr, at 2ë100N, 17š6mm=yr, at 0.5ë±1ëN, 12 mm=yr, at 0.5ëS and 11 š 5mm=yr, at 3ëS to 4ëS. 7. Conclusion The 5:5š1:9mm=yr slip rate calculated in this study at 5ëS is more accurate and close to that calculated Our data concern quartz-free rhyolitic pumices 250 km further north, at about 3±4ëS. These results from the Ranau Tuffs corresponding to island-arc indicate the northward increase in dextral slip rate, medium-K calc-alkaline rhyolite volcanism. Their with slip rates between 1ëN and 2ëN being 10±20 40K±40Ar age ranges between 0.7 and 0.4 Ma and mm=yr faster than rates determined between 3ëS and corresponds to the ultimate episode of the caldera 5ëS. The discrete variation of the GSF strike is ef- evolution related with the paroxysmal tuff eruption fective between 5ëS and 0ë. Moreover, the slip rate that produced the major collapse. This mean age increase is discontinuous and corresponds to changes of 0:55 š 0:15 Ma places the major Ranau caldera in the GSF orientation, i.e., at 3.5ëS and at 1ëS. The collapse in a period of paroxysmal calderic activity largest increase in slip rate should occur between along the Sumatran Arc. 0.5ëS and 2ëN, where a crustal-scale relay fault zone Offset measurements performed on SPOT and is observed (Fig. 1). This increase in GSF slip rate Landsat images allow us to estimate the long-term is approximately parallel to an increase in conver- dextral slip rate of the southernmost GSF at 5:5 š gence obliquity between 7ëS and 2ëN. Between 2ëN 1:9mm=yr. This geologically determined slip-rate and 4ëS several transpressional belts are observed con®rms that the GSF slip rate of 25 š 7mm=yr at splaying southeastward from the GSF with an acute 3.5ëS given by geodetic GEODYSSEA network is angle of ca. 20ë (Figs. 1 and 3). South of 2ëN where probably an overestimate as already suggested by the most of these transpressional structures exist, the clockwise rotation of the Sunda block. GSF slip rate variation is mostly accommodated by Combined reappraisal observations from Bellier these back-arc deformations. Indeed, available off- and SeÂbrier (1995) and this study imply for central shore geophysical data on the fore-arc indicate a and north Sumatra (i) that the fore-arc block located dominant thrust component of the MF faulting, with to the west of the GSF is, in a ®rst approximation, a western upthrust block (Karig et al., 1980) (Fig. 2), a rigid block, (ii) that most of the fore-arc sliver is the major deformations that affect the fore-arc sliver under compression, and (iii) that combination of dif- between the GSF and MF corresponding to ¯exural ferent modes of deformation throughout the fore-arc offshore basins that are bounded to the west by the platelet to back-arc domain contributes to accom- 358 O. Bellier et al. / Tectonophysics 312 (1999) 347±359 modate the oblique convergence. Thus, the central± Acknowledgements north Sumatra overriding plate is deformed on a 500 km wide zone to accommodate the high con- This study was realised within the co-operative vergence obliquity (higher than 20ë±25ë). Overriding agreement between the DeÂleÂgation aux Risques Ma- plate shearing distributed in a wide zone is unusual jeurs (DPPR.SDPRM-RN, French Ministry of En- in an oblique convergence domain, this particularity vironment) and the Directorate General of Geology of Sumatra can be due to speci®c conditions related and Mineral Resources (P3G of the Indonesian Min- to the arc domain, i.e., unusual thermal structure istry of Mines and Energy). We thank both P3G and inducing a speci®c rheology. Puslitbang Geoteknologi-LIPI for ®eld work assis- Platt (1993) considered a plastic rheology model tance. Funding was provided by DPPR.SDPRM-RN for the leading edge that yields a critical obliquity within the above-mentioned co-operative agreement of about 20ë. The leading edge accommodates con- while SPOT images were provided by the PNTS vergence obliquity by oblique thrusting along the and ISIS programs (CNES; INSU-CNRS). We thank subduction zone where the obliquity angle is smaller J.-C. Philippet and J. Cotten for their assistance dur- than 20ë and by slip-partitioning for larger than 20ë ing the analytical phase of this study and we are obliquity. The southern GSF horizontally displaced grateful to G. Roche and L. Daumas for drawing recent geomorphic features (Fig. 4) and has been the ®gures. Special thanks are due to N. Chamot- reactivated by earthquakes with consistent lateral Rooke for helpful discussions and to M. Diament, displacement (the Mw D 6:8 Liwa 1994 earthquake B.J. Pillans and J.P. Burg for comments, reviews and is characterised by a co-seismic deformation related criticisms which considerably improved this paper. with a GSF right-lateral displacement of about 70 cm, Duquesnoy et al., 1996), showing that even for a low obliquity, slip-partitioning exists. References Taking into account the oblique slip of the subduction and the uncertainty on the plate convergence Baroux, E., Avouac, J.-P., Bellier, O., SeÂbrier, M., 1998. Kine- vector, the total amount of expected shear (i.e., defor- matics of active deformations along the Sunda Arc (Indone- sia): implication for regional tectonics. Terra Nova 10, 139± mation parallel to the trench) that has to be accom- 144. modated by the overriding plate is 17 š 10 mm=yr, at Beaudouin, T., Bellier, O., SeÂbrier, M., 1995. Segmentation et this locality, at ¾5ëS. Indeed, considering the lower aleÂa sismique sur la Grande Faille de Sumatra (IndoneÂsie). C. bound of 7 mm=yr expected shear and the GSF R. Acad. Sci. 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