Size and Depth of Ancient Magma Reservoirs Under Atolls and Islands of French Polynesia Using Gravity Data
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 105, NO. B4, PAGES 8173{8191, APRIL 10, 2000 Size and depth of ancient magma reservoirs under atolls and islands of FrenchPolynesia using gravity data Val erie Clouard and Alain Bonneville Lab oratoire de G eosciences Marines et T el ed etection, Universit edelaPolyn esie francaise, Pap eete, Tahiti, FrenchPolynesia 1 Hans G. Barsczus, GBE - UMR 5569 - CNRS and Universit edeMontp ellier I I, Montp ellier, France Abstract. New insights into the structural and tectonic evolution of islands and atolls in FrenchPolynesia are derived from the analysis of gravity data. Free-air anomaly maps were constructed using gravity data from land surveys of 25 islands combined with free-air anomaly data derived from satellite altimetry and shipb orne gravimeters. Residual isostatic anomalies were calculated using a three-dimensional 3-D, four-layer crustal mo del taking into account the bathymetry of the sea o or, the top ography of the islands, and the de ection of the lithosphere under the load of the volcano es. Twenty of the 25 islands yield a p ositive residual anomaly, ranging b etween 9 and 60 mGal. Negative residual anomalies over four islands probably corresp ond either to limestone dep osits or to fragmented material from aerial volcanism. The o ccurrence of a p ositive anomaly provides some evidence that there is a solidi ed magma chamb er at depth b eneath each of several islands in FrenchPolynesia. A linear relation b etween the amplitude of the p ositive gravity anomaly and island volume is also observed. Geological features which generate these p ositive anomalies are describ ed by simple geometric mo dels 3-D ellipsoidal dense b o dies which lead to a rough estimate of the size and depth of the magma chamb ers. We then prop ose a simple linear relation between the volume of such magma chamb ers and the volume of islands that can b e used for other extinct intraplate basaltic volcano es. The diameter of the inferred magma chamb er and its pro jection on the surface of the islands imply that calderas corresp ond mostly to collapses of the anks of the edi ces rather than to caldera vertical subsidence. 1 1. Intro duction yr [Herron, 1972; Minster and Jordan, 1978]. The Austral, So ciety, Pitcairn-Gambier, and Marquesas is- FrenchPolynesia Figure 1 is lo cated to the south land chains are comp osed mainly of islands, while the of the central part of the Paci c Plate, b etween latit- Tuamotu archip elago is comp osed entirely of atolls. All udes 5 S and 30 S and longitudes 130 W and 160 W. together, FrenchPolynesia consists of 34 islands and 84 The Austral, So ciety, Pitcairn-Gambier, and Tuamotu atolls. island chains trend N120 , the same direction of the The islands rise 4500 m ab ove the sea o or, and their presentPaci c Plate motion. The Marquesas Islands maximum elevation ab ove sea level, at Tahiti, is 2200 have a N140 strike. The region is crossed by the Aus- m. To a rst approximation, island ages and morpho- tral and Marquesas Fracture Zones FZ with a N70 logies are consistent with a hotsp ot theory: the age of strike. These FZs are the main tectonic features of the each island within a chain increases from the southeast sea o or and are related to the Farallon Ridge, which to the northwest, based on radiometric dating of volcan- b ecame extinct 9 Myr ago [Mammerickx et al., 1980]. ism from the present to ab out 16 Ma Table 1. The age The average velo cityofPaci c Plate motion is 11 cm of the o ceanic crust ranges b etween 45 and 85 Ma [Her- ron, 1972; Mayes et al., 1990; Munschy et al., 1996]. 1 Also at Centre IRD, Montp ellier, France The Tuamotu Islands are separated from the other is- land chains by basins exceeding 5000 m in depth. The Copyright 2000 by the American Geophysical Union. origin of the Tuamotu Islands, which rise ab ove a broad plateau, is uncertain but has b een attributed to an ab or- Pap er numb er 1999JB900393 ted ridge [Pautot, 1975], a hotsp ot [Okal and Cazenave, 0148-0227/00/1999JB900393$09.00 8173 8174 CLOUARD ET AL.: GRAVITY SIGNATURE OF MAGMA RESERVOIRS Marquesas Nuku Hiva Ua Pou Hiva Oa 10˚S Marquesas FZ Manihi Mataiva Rangiroa Tuamotu Tupai Makatea Maupiti Bora Bora Tahaa Tetiaroa Manuae Huahine Moorea Maiao Tahiti Society 20˚S Pitcairn-Gambier Maria Moruroa Rimatara Rurutu Gambier Tubuai Austral FZ Austral Raivavae Rapa 500 km 30˚S 150˚W 140˚W -6000 -5000 -4000 -3000 -2000 -1000 0 meters Figure 1. Bathymetric map [Sichoix and Bonnevil le, 1996] showing the volcanic chains of French Polynesia, with archip elago names underlined. The names of the islands discussed in text are shown. FZ stands for fracture zone. 1985], or an emerged submarine chain which originated 2. Data Presentation and Pro cessing close to the East Paci c Rise [Ito et al., 1995]. 2.1. Gravity Data The age, p etrology, and geo chemistry of the French Polynesian islands are relatively well known see sum- From 1978 to 1981, 800 measurements were made by maries by Brousse et al. [1990], Diraison [1991], and Barsczus [1979, 1980, 1981] using a Worden 600 gravi- Leroy [1994], but their subsurface structure, apart from meter at Manuae, Maupiti, Tupai, Bora-Bora, Tahaa, Moruroa and Fangataufa atolls [Guil le et al., 1992, Huahine, Maiao, Tetiaroa, Mo orea, and Tahiti in the 1993], remains p o orly known. In this pap er, we address So ciety Islands; Nuku Hiva, Hiva Oa, and Ua Pou in the lateral density distribution using existing gravity the Marquesas Islands; Maria, Rimatara, Rurutu, Tu- data, which leads to a three-dimensional mo del of the is- buai, Raivavae, and Rapa in the Austral Islands; Mor- lands. Shipb oard and satellite-derived gravity data are uroa and Gambier Islands in Gambier-Pitcairn align- merged with previously unpublished land data. After ment; and Mataiva, Manihi, Makatea, and Rangiroa in adjusting these di erent data sets to a common ref- the Tuamotu Islands. These data were merged with 137 erence system, a free-air anomaly FAA map is ob- measurements made by Macheski [1965] at Tahiti and tained. We then subtract a theoretical gravity eld Mo orea using a Worden 300 gravimeter and with 320 computed using a three-dimensional, four-layer crustal measurements made by Leroy [1994] at Tahiti using a mo del based on a digital terrain mo del for bathy- Lacoste and Romb erg mo del G gravimeter. All data are metry and elevation. Residual isostatic anomaly maps tied to PPT J7912, a reference station of the Interna- [Blakely, 1995] are then pro duced for 17 islands and 8 tional Gravity Standardization Net established in 1971 atolls to obtain insights into the makeup of shield vol- IGSN71 and lo cated at Pamatai in Tahiti. The PPT cano es. J7912 station where PPT stands for Pap eete and J7912 CLOUARD ET AL.: GRAVITY SIGNATURE OF MAGMA RESERVOIRS 8175 Table 1. Radiometric Ages of Aerial Volcanism in FrenchPolynesian Studied Islands Island Longitude W Latitude S Age, Ma Reference Austral 0 0 Raivavae 147 39 23 52 5.6-7.6 Duncan and McDougal l [1976] 0 0 Rapa 144 20 27 36 5.9-5.2 Krummenacher and Noetzlin [1966] 0 0 Rimatara 152 48 22 39 4.8-28.6 Turner and Jarrard [1982] 0 0 Rurutu 151 21 22 29 0.6-12.3 Dalrymple et al. [1975], Duncan and McDougal l [1976], and Turner and Jarrard [1982] 0 0 Tubuai 149 30 23 23 8.5-10.6 Duncan and McDougal l [1976] Marquesas 0 0 HivaOa 139 03 09 45 1.6-2.5 Duncan and McDougal l [1974] 0 0 Nuku Hiva 140 12 08 54 3.0-4.3 Duncan and McDougal l [1974] So ciety 0 0 Bora Bora 151 44 16 30 3.1-3.4 Duncan and McDougal l [1976] 0 0 Huahine 151 00 16 45 2.0-2.6 Duncan and McDougal l [1976] 0 0 Maupiti 152 16 16 27 4.0-4.5 Duncan and McDougal l [1976] 0 0 Mo orea 149 50 17 33 1.5-1.7 Duncan and McDougal l [1976] 0 0 Tahaa 151 30 16 39 2.6-3.2 Duncan and McDougal l [1976] 0 0 Tahiti-iti 149 30 17 36 0.4-0.5 Leroy [1994] 0 0 Tahiti-nui 149 15 17 48 0.19-1.37 Leroy [1994] Pitcairn-Gambier 0 0 Gambier Manureva 134 57 23 09 6.2 Guil lou et al. [1994] 0 0 Moruroa 138 54 21 51 11.0 Gil lot et al. [1992] means that it was the twelth station made in 1979 of the Austral Islands A. Bonnevil le, personal commu- was established by a Japanese team during a circum- nication, 1999. The entire o shore data set is referred Paci c gravimetric connections campaign [Nakagawa et to here as the "marine data set." The error on FAA al., 1983]. From this total "land data set", a FAA was satellite-derived data was estimated to b e 5 mGal by then calculated using the GRS80 ellipsoid mo del. The Sandwel l [1992]. The shipb oard data error is 1 to 2 exp erimental error e related to this land data set can b e mGal. Consequently, a maximum error of 5 mGal for expressed as the sum of the measurement error e es- the whole marine data set is assumed.