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QUATERNARY UPLIFT OF THE TORRES ISLANDS, NORTHERN NEW HEBRIDES FRONTAL ARC: COMPARISON WITH SANTO AND MALEKULA ISLANDS, CENTRAL NEW HEBRIDES FRONTAL ARC' F. W. TAYLOR, C. JOUANNIC, AND A. L. BLOOM Institute for Geophysics, University of Texas at Austin, Austin, Texas 78712 ORSTOM (Office de la Recherche Scientifique et Technique Outre-mer), c/o LGPN/LIPI, Bandung, Indonesia Department of Geological Sciences, Cornell University, Ithaca, New York 14853 ABSTRACT Coral reef terraces on the Torres Islands have recorded the Quaternary uplift of part of the northern New d Hebrides frontal arc. These isles lie about midway between the volcanic chain and trench axis, where the Indian plate underthrusts the arc from the west. 14C and 230Thp34Uages for Torres fossil corals indicate that for approximately the past 100,000 years the islands uplifted at a constant rate ranging geographically from ,I 0.7 to 0.9 "/yr. The occurrence of similar uplift rates and eastward-tilting directions and the absence of 1 '* major crosscutting structures or rupture zone boundaries suggest that the islands may be part of a single tectonic block or arc segment. The frontal arc morphology in the Torres area is typical of frontal arcs. Farther south, the central New Hebrides frontal arc has no physiographic trench at the plate boundary west Il ' of Santo and Malekula Islands, and the axes of the maximum uplift rate are at the seaward edge of the plate. The d'Entrecasteaux Ridge (DR), a massive E-W trending bathymetric feature on the Indian plate, is underthrusting Santo and Malekula. The eastern parts of Santo and Malekula are physiographically equiva- lent to the Torres Islands and seem to have been on a former axis of maximum uplift rate. However, in response to subduction of the DR, the uplift axes of Santo and Malekula may have shifted westward. In a few hundred thousand years the new uplift axes caused the western parts of these islands to achieve their present topographic dominance over the earlier uplift axes. Holocene uplift rates on southern Santo and northern Malekula are about twice the average rates of the previous 100,000 years. The accelerated Holocene uplift could be explained if a particularly prominent part of the DR has recently underthrust Santo and Malekula. INTRODUCTION from 10 to 1,000,000 years (Chappell 1974; Vertical deformation patterns at conver- Konishi et al. 1974; Taylor and Bloom 1977; gent plate boundaries allow us to infer impor- Plafker and Rubin 1978; Taylor et al. 1980). tant information about interactions between In the New Hebrides island arc we have used plates and the earthquake generation pro- instrumental methods to measure short term * cess. Over times of 100 years or less, precise vertical movements (Marthelot et al. 1980; leveling, tiltmeters, tide gauges, and gravity Bevis and Isacks 1981; Isacks et al. 1981). To I/ measurements are valuable for documenting interpret the significance of these results, we ' vertical deformation (e.g., Fitch and Scholz have used reef terraces to establish the long I( 1971; Yonekura 1975; Bevis and Isacks 1981; term rates and patterns of net vertical defor- Mogi 1981). However, over longer periods of mation on the islands of Santo, Malekula, time only geological methods can determine Efate and, now, the Torres Islands (Bloom et the net deformation pattern resulting from the al. 1978; Taylor et al. 1980, 1981; Jouannic et coseismic and aseismic vertical motions (see al. 1980, 1982; Gilpin 1982; Lecolle et al. in Sieh 1981 for review). On tropical coasts, prep.). emerged marine and coral reef terraces have At convergent margins, uplift rates provide proven to be valuable geological recorders of a measure of the intensity of interaction be- net vertical deformation on the time scale tween the subducting and overthrusting plates. In the classification of Uyeda and Kanamori (1979), when the subducting and Manuscript received July 26, 1984; revised overthrusting plates are strongly coupled the February 25, 1985: arc system is called the Chilean-type. In this case, the arc is in a state of compression, [JOURNAL OF GEOLOGY,1985, Vol. 93, p. 419-4381 O 1985 by The University of Chicago. All rights much of the interplate motion occurs as reserved. seismic slip, and uplift rates tend to be rapid 0022- 1376/85/9304-003$1.00 (Dewey 1980; Yonekura 1983). Where inter- 420 F. W. TAYLOR, C. JOUANNIC, AND A. L. BLOOM nism attribute a major role to subduction of sediment (Cloos 1982) or bathymetric fea- tures (McCann and Habermann in prep.). In these models, increased strength of interplate coupling is not a cause of uplift, but only a symptom of the nature of the material being subducted. The main purpose of this paper is to de- scribe the uplift of the Torres Islands sector of the New Hebrides frontal arc, where sub- ducting topography apparently has had little influence on upper plate tectonics (fig. 1). This research is the result of a rare opportu- nity to visit the remote Torres Islands and presents the only existing isotopic ages of fossil corals from the uplifted Torres Island reefs. In terms of arc geometry and seismicity, the Torres Islands frontal arc is reasonably typical. However, the fact that the islands are emerged makes them a somewhat unusual part of the frontal arc. The Torres Islands parallel the arc trend and lie along the Torres Ridge at the edge of the inner trench slope (fig. 1). The volcanic axis is about 80 km east FIG. 1.-Bathymetry (meters) of the Vanuatu of the Torres Islands, and the trench axis lies (New Hebrides) arc (after Kroenke et al. 1983). 50 to 60 km west of the isles. Asterisks indicate the locations of volcanoes. Note After describing the Torres Islands we will the trends of the d’Entrecasteaux Ridge and the compare their uplift history with that of Santo West Torres Plateau on the Indian plate which is thrusting eastward beneath the New Hebrides arc. and Malekula Islands (Neef and Veeh 1977; The New Hebrides trench does not exist west of Taylor et al. 1980; Jouannic et al. 1980, 1982; Santo and Malekula. The lines across the Torres Gilpin 1982). A comparison is valuable be- Islands and Santo indicate the locations of topo- cause Santo and Malekula are underthrust by graphic profiles figure 8. in the d’Entrecasteaux Ridge (DR), a major to- pographic feature on the Indian plate (fig. 1). The detailed morphology of this ridge and its plate coupling is weak and an arc is classified intersection with the New Hebrides arc are as Mariana-type (Uyeda and Kanamori 1979), described by Collot et al. (1985). Subduction virtually all underthrusting is aseismic. In this of the DR has profoundly influenced the arc classification, the New Hebrides arc proba- geomorphology, uplift history, and seismicity bly represents an intermediate case where in- of the Santo and Malekula area (e.g., terplate coupling is somewhat strong and fre- Ravenne et al. 1977; Pascal et al. 1978; Chung quent shallow-thrusting earthquakes indicate and Kanamori 1978a, 1978b, Taylor et al. that a significant proportion of the interplate 1980, 1981; Isacks et al. 1981; Daniel and motion is seismic (Pascal et al. 1978; Isacks et Katz 1981; Collot et al. 1985). Vertical tec- al. 1981). However, the New Hebrides has an tonism has greatly modified the geometry of actively opening back-arc basin (Chase 1971; this part of the arc. Due to so much uplift of Falvey 1975; Malahoff et al. 1982) as well as the extreme western edge of the frontal arc, Quaternary rifting in the immediate back-arc its morphology is very unusual. Large stress (Karig and Mammerickx 1972; Ravenne et al. drops accompany earthquakes here, but we 1977). These features suggest relatively weak do not know if increased coupling or other interplate coupling (Uyeda and Kanamori effects are the causes for the rapid uplift and 1979). other phenomena at Santo and Malekula. In Other explanations for vertical arc tecto- contrast with the Torres Islands, there is no QUATERNARY UPLIFT OF THE TORRES ISLANDS 42 1 geomorphic trench along the plate boundary altimeter traverses to measure altitudes. For west of Santo and Malekula Islands. In fact, altimetry measurements we used two Wal- the western parts of Santo and Malekula are lace-Tiernan FA-181 surveying altimeters where the inner trench slope would be if the with one altimeter based at sea level giving northern and southern segments of the trench altitude measurements that were reproduc- were continuous along a straight line (fig. 1). ible within k2 m when stations were reoc- By comparing the Torres Islands and the cupied. Before and after our work on the Tor- Santo-Malekula area, we identify some pecu- res Islands, the altimeters were checked liar effects of subduction of the DR on the relative to surveyed altitudes on other is- Santo-Malekula frontal arc. This is important lands, where we found them to be quite ac- because subduction of large aseismic ridges is curate. common (e.g., Kelleher and McCann 1976; The altimeters consistently indicated al- Vogt et al. 1976; Dupont 1979, 1982; McCann titudes significantly lower than those indi- and Habermann in prep.), but the extent of cated by the topographic map of the Torres their influence is poorly understood. For ex- Islands. For example, where the contour ample, the Louisville Ridge may have in- lines show ‘altitudes of 50 to 200 m above sea fluenced development of the Tonga frontal level (ASL), we measured altitudes between arc (Vogt et al. 1976, p. 23; Dupont 1982). 30 and 160 m ASL. The map indicates that no McCann and Habermann (in prep.) proposed inland surveyed altitudes were used to plot that nearly all frontal arc uplift not related to the topographic contours.

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