Seafloor Morphology and Tectonics of the Christmas Island Area, Indian Ocean

Christmas Island lies about 1600 km north-north west of Australia's Northwest Cape and approximately 350 km south of Java in the northern part of the Wharton basin (IndianOcean). Recently Australia declared a 200 mile Fisheries Zone around the island andAGSO was asked to assess seabed morphology, sediment thickness and offshore mineralresources in this area. In February 1992 RAT "Rig Seismic" carried out a detailed survey ofthe region, providing relevant data for the required assessment. Eight seismic profiles wereacquired on this cruise, totalling about 2000 km, and almost twice as much bathymetricdata was recorded. In conjunction with seismic and bathymetic data collected by otherinstitutions, our data provides a good coverage of the area, which enabled us to compile anew bathymetric map and to produce the first sediment thickness map.

Among the published bathymetric maps only three cover the Christmas Island area: 1)published by Udintsev (Geophysical Atlas of the Indian Ocean, 1975; 1:5,000 000), 2) byMammerickx et al. (1976, 1:5,000 000) and 3) 1:10,000 000 General BathymetricCompilation (GEBCO) map, published by the International Oceanographic Service (1982).All published bathymetric maps were compiled in the end of the 1970s and the beginningof the 1980s, and all of them were based on processing analog records of water depths andwere drafted manually. Moreover, most of the data for map compilation were collectedusing a sextant, and only a very limited using satellite navigation.

The amount and quality of data collected by the end of the 1970s allowed the production ofthe fairly accurate 1:5,000 000 and 1:10,000 000 maps of the Indian Ocean listed above,however a lot of smaller features, such as individual seamounts, are missing on those maps.Insufficient data coverage led to broad extrapolation of bathymetric trends, sometimesderived purely from magnetic lineation pattern (Fig.1). To the east of Christmas Island thelack of information is particularly evident: all the maps differ in their interpretation of this area.

New high quality data collected by "Rig Seismic", and digital water depths obtained fromthe USA National Geophysical Data Bank (NGDC), were used for compilation of a newrevised version of the bathymetric map on the Christmas Island area in a 1:1,000 000 scale.The new map (to be published in AGSO's Offshore Resources Map Series) contains a lotmore detail on the complex bathymetry of the area, and gives a more realistic picture ofseamount distribution and the structure of the Java Trench and Java's outer-arc ridge. Theamount of added information can be clearly deduced from comparison of Fig.1 and Fig.2.The time scale used in this report is that of McDougall (1974) and Fanoon et al. (1993).

圣诞岛（Christmas Island）位于澳大利亚西北岬以北西北约1600公里处，地处印度洋沃顿盆地北部，距爪哇岛以南约350公里。近期澳大利亚宣布在该岛周边划定200海里渔业专属经济区，澳大利亚地质调查局（AGSO）受委托评估该区域的海底地貌、沉积物厚度与近海矿产资源。1992年2月，代号RAT的"Rig Seismic"号勘探船开展了该区域的详细勘测，为评估工作提供了核心数据支撑。本次航次共采集8条地震剖面，总长度约2000公里，同期记录的水深测量数据量约为地震剖面的两倍。结合其他机构获取的地震与水深数据，本次采集的数据实现了该区域的良好覆盖，借此我们得以编制全新的水深地形图，并绘制出首张区域沉积物厚度分布图。

已公开的水深地形图中，仅有3份覆盖圣诞岛海域：1）乌丁采夫（Udintsev）编制的《印度洋地球物理图集》（1975年，比例尺1:500万）；2）马默里克斯等（Mammerickx et al.，1976年，比例尺1:500万）；3）国际海洋服务局1982年发布的1:1000万通用海底地形汇编图（GEBCO）。上述所有公开水深地形图均编制于20世纪70年代末至80年代初，全部基于模拟水深记录的处理结果，并通过手工编绘完成。此外，大部分编图数据采用六分仪采集，仅极少数使用卫星导航技术。

20世纪70年代末采集的数据规模与质量已可支撑编制前述精度较高的1:500万与1:1000万印度洋海域地形图，但此类地图遗漏了大量小型地貌特征，例如单个海山。数据覆盖不足导致水深趋势被大范围外推，有时仅基于磁异常线理模式进行推断（如图1）。在圣诞岛以东海域，信息匮乏的问题尤为突出：所有现有地图对该区域的解译结果均存在显著差异。

本次"Rig Seismic"号采集的新型高质量数据，以及从美国国家地球物理数据中心（NGDC）获取的数字化水深数据，被用于编制圣诞岛海域1:100万比例尺的修订版全新水深地形图。该新图将收录于AGSO的《近海资源地图集》系列，其详细展现了该区域复杂的海底地形，更真实地反映了海山分布以及爪哇海沟与爪哇外弧脊的构造形态。新增的信息量可通过图1与图2的对比清晰体现。本报告采用的时间标尺为麦克道格尔（McDougall，1974）与法努恩等（Fanoon et al.，1993）提出的方案。