JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 98, NO. B6, PAGES 9605-9618, JUNE 10, 1993 Multibeam Bathymetric, Gravity and Magnetic Studies Over 79øE Fracture Zone, Central Indian Basin K. A. KAMESH RAJU,T. RAMPRASAD, V. N. KODAGALI AND R. R. NAIR National Institute of Oceanography, DonaPaula, Goa, India A regional scale bathymetric map has been constructed for the 79øE fracture zone (FZ) inthe Central Indian Basin between 10ø15'S and 14ø45'S latitudes and 78ø55'E and 79ø20'E longitudes using the high-resolution capabilities of the Hydrosweep multibeam swath bathymetric survey system. The bathymetric map clearly reveals the 79øE FZ, depicting a ridge and trough topography with anelevation difference of over 300m. Prominent E-W bathymetric lineations and several new features such as seamounts are conspicuous onthe map. TheE-W bathymetdc lineations bend atthe fracture zone toward south resulting in oblique extensional features. These features are more prominent in the middle and northern portion of thestudy area and are less significant in thesouthern portion. The prominence of these features in thenonh and theirsubtle presence in the south areprobably related to the strong and weakshear coupling atthe RTI, respectively, during the various stages of FZ evolution. Gravity and magnetic data were analyzed together with the bathymetric data. The observed free-air gravity across the 79øE FZ is modeled in terms of the lithospheric thickness, taking intoconsideration thethermal evolution of the oceanic lithosphere. A lithosphere thickness of about 100 km is inferred from these studies. INTRODUCTION The Central Indian Basin located between Central Indian Ridge (CIR) on thewest, theNintyeast Ridge on theeast, andthe South- East IndianRidge(SEIR) on thesouth hasa complex evolutionary historycontrolled by the varied spreading rates alongCIR and SEIR and the presence of the Indian OceanTriple Junction. A partof theCentral Indian Basin (CIB) has been thefocus of study for quite some time, following the discovery of polymetallic nodule rich areas. As a part of the Polymetallic Nodules Project the area between 10øS and 16øS latitudes and 71øE and 80øE longitudes wasextensively covered with conventional echo sound- ing (using precision depth recorders), magnetics, and in part by gravity surveys. These studies [Kamesh Raju and Ramprasad, 1989; Kamesh Raju, 1990] together with the earlierstudies by McKenzie and Sclater [1971] and Schlich [1982] have revealed magnetic lineations from A22 to A26 and fracturezones (FZs) at flanking the transform fault slopes downtoward the transform axis. The morphotectonic signature is preserved along theaseismic extensions of the transforms (namely, FZs) and can be traced thousands of kilometers across ridgeflanks. This feature suggests thatthe generation andevolution of oceanic lithosphere proximal to transform boundaries are different from those of normal oceanic lithosphere [Gallo et al., 1986]. The diminished bathymetric fea- tures as a resultof sediment infill in the trough across Clipperton FZ resulted in small-amplitude geoid anomalies [Wessel and Haxby, 1990]. This suggests that while the subsequent cooling produces relative vertical subsidence of thecrust with age, if there is sufficient sedimentation the FZ might as well be buried.The vertical motionresulting in ridge and trough topography is due to thermomechanical interactions in the lithosphere [Sandwell and Schubert, 1982], which are drivenby the age contrast across the FZ. To explain bothtopographic andgravitational signatures of the 73øE, 75ø45'E, 79øE, and 83øE within the CIB. The kinematic FZs, a model was proposed by Sandwell [1984] based on the studies over SEIR by Royer and Schlich [ 1988], the synthesis of thermomechanical concept, considering the cooling of lithosphere the magnetic anomalies byPatriat and Segoufin [1988], the by conduction [Turcotte and Oxburgh, 1967]. The model predicts studies by Munschy and Schlich [1989], and the tectonic fabrican error function geotherm for ages less than 70 Ma[Parsons and map presented by Royer et al. [1989] have also contributed toward Sclater, 1977] and differential subsidence oflithosphere accom- understanding ofthe evolution ofthe CIB. panied bythermal contraction. Because the asthenosphere has The detailed bathymetric maps presented earlierat 100-m con- tour intervalbased on the echo sounding data (spaced about56 km apart) had shown subtle E-W lineations andindistinct expres- sion of the79øE FZ [Kamesh Raju andRamprasad, 1989]. A recentmultibeam sonarsurvey carriedout duringOctober1990 using Hydrosweep system onboard ORV Sagar Kanya (cruise SK- 58), revealed many new bathymetric features. In this paperwe present the regional bathymetric map and threedetailed mapsof selected features in the CIB between 10ø15'S and 14ø45'S latitudes and78ø55'E and 79ø20'E longitudes together with gravityandmagnetics. The moststriking linearbathymetric fea- ture is the 79øE FZ. FZs are linear scars in the seafloor produced by transform faulting [Wilson, 1965]. Topography along theiractive segments consists of long ridges, troughs, and scarps which separate regions of different depths[Menard and Atwater, 1969]. The seafloor Copyright 1993 by the AmericanGeophysical Union. Paper number 92JB01266. 0148-0227/93/92JB-01266505.00 fluid like properties over million year time scales, the thermally induced density increments are compensated by subsidence proportional to square root of the age [Parker and Oldenberg, 1973]. The mechanical modelpredicts that as the upperportion of the lithosphere cools,an elastic layer develops and grows in thickness. Flexure studies of subducting oceanic lithosphere and lithosphere loaded by Hawaian-Emperor chainhave shown that the effective elastic thickness of the lithosphere is approximately proportional to square root of the age [Watts,1978; Caldwell and Turcotte, 1979; Watts et al., 1980; Bodine et al., 1981]. Wessel and Haxby [1990] presented a combined model, indicating the flexuraleffects from both thermal bending stresses [Parmentier and Haxby, 1986] and differential subsidence of the lithosphere [Sandwell and Schubert, 1982] for explaining both geoid anomaliesand topographyacrossthe FZs. The model accounts for the slip alongFZ for betterexplanation of geoidanomaly. As thedensity structure withinthelithosphere is moresensitive to the temperature variations, we have usedthe model proposed by Sandwell [1984] andutilized the density structure thus derived to modelthe free-air gravityanomalies across the FZ. 9605