GEOPHYSICAL RESEARCH LETTERS, VOL. 15, NO. 5, PAGES455-458, MAY 1988 HAS THE WHARTON BASIN'S HEAT FLOW BEEN PERTURBED BY THE FORMATION OF A DIFFUSE PLATE BOUNDARY IN THE INDIAN OCEAN? Carol A. Stein, 1 Michael A. Hobart, 2Dallas H. Abbott, 2 1Department ofGeological Sciences, University ofIllinois at Chicago 2Lamont-Doherty Geological Observatory, Columbia University Abstract. The CentralIndian Ocean region has heatflow higherthanexpected for its lithospheric age.This heatflow anomaly is thought to be associated with deformation of sed- iment and crustand high seismicity. To betterconstrain the nature of this deformation, we examine the spatial variation of the heat flow. Previouswork suggested high heat flux also in the Wharton Basin to the east, which shows less seismicityand deformation. Using new values for lithos- phericage from reinterpretation of the magnetic anomalies, we have reexamined the heat flow and found it no higher than expected, in contrast to the CentralIndian Basin. This spatial distribution of heat flow highs andexpected values is consistent with thepattern of seismicity anddeformation and the predictions of the recent diffuse plate boundary model [Wienset al., 1985] for theregion. Introduction The Central Indian Basin, betweenthe Ninetyeastand Chagos-Laccadive Ridges, haslongbeen knownasa seismi- cally active region [Gutenberg and Richter, 1954; Sykes, 1970; Stein and Okal, 1978; Wiens, 1985; Bergman and Solomon, 1985] (Figure 1). In this area, sediments and acoustic basement are sometimes deformed into long- wavelength (100-300 km) "undulations", oftenwith closely- spaced (1-3 km) faults superimposed and up to 3 km of relief. This deformation is alsoassociated with large (30-80 mgal)free-air gravity anomalies [Weissel et al., 1980; Geller et al., 1983], and east-west trendinggeoid anomalies with amplitudes up to '2 m [McAdooand Sandwell, 1985]. In addition, the heatflow is higher than expected for the lithos- pheric age[Anderson et al., 1977; Weissel et al., 1980]. The seismicity, highheat flow and deformation arethought to be associated with a diffuse plate boundary separating distinct Indianand Australian plates [Wiens et al., 1985]. .. Whether this deformation extends eastward to the Whar- ton Basin is unclear. The WhartonBasinis lessseismically active than the Central Indian Basin (Figure 1), and the deformation and faulting are lessspectacular [Geller et al., 1983]. On the other hand, the SW-NE trending geoid anomalies are about the same magnitude aswestof theNine- tyeastRidge [Haxby, 1987]. Anderson et al. [1977] calcu- lated that the heat flow in the Wharton Basin is about 8 mWm -2 greater than predicted for its age. In this paper,we examinespatialvariation of the heat Copyright 1988 by theAmerican Geophysical Union. Paper number 7L7290. 0094-8276/88/007L-7290503.00 40'E I 60 'E I 80'E I 1 O0 'E I t':•' •"•'"' '" ;' --• "'•' ':'" ' ;•'•••:•(:'•:!•-• INDIAN OCEAN '•'• .... ... •..[.•: •::•7• RIDGE •:;• ' '-•'• ?•7•':: • ;•:• AFRICAN DOE -,,I II :- zJ•:•-•.;....... NTRAL 20'•"••?) ß •:• • • _ el/ RIDGE ß • " ••• •A R C T I L I A N Fig. 1. Plate geometryand "intraplate" seismicity (1912- 1983) in the Indian Ocean. The epicenters plottedindicate the proposed diffuse plate boundary, and remaining intra- plate deformation. The Wharton Basin has less seismicity than the Central Indian Basin and the Ninetyeast Ridge and Chagos-Laccadive Ridge. Solid triangleand ellipseindicate the location of the Euler pole and 95% confidence limits. Arrowsshow expected motion in the diffuse plateboundary. After Wiens et al. [ 1985]. flow within the Wharton and Central Indian Basins. For the Wharton Basin, we use new magnetic anomaly identifications [Liu et al., 1983], which imply generally younger lithospheric ages for the heat flow measurement sitesthan the earlier model of Sclaterand Fisher [1974]. We find that heat flow in the Wharton Basin, in contrast to the Central Indian Basin, is no higher than expected. This difference is in accord with the predictions of the diffuse plate boundary model, sincethat boundary appears not to extend east of the Ninetyeast Ridge. Data We excluded all measurements on hot spot tracks,on oceanic plateaus with possiblecontinentalfragments,in areasof insufficient sediment cover [Anderson and Hobart, 1976] and from stations with dubious experimental quality (see Figure 2). We have reanalyzed the Lamont-Doherty Geological Observatory heat flow data. Magneticanomaly 455