THE SOUTHEAST INDIAN RIDGE BETWEEN 88-DEGREES-E AND 118-DEGREES-E - GRAVITY-ANOMALIES AND CRUSTAL ACCRETION AT INTERMEDIATE SPREADING RATES

Although slow spreading ridges characterized by a deep axial valley an d fast spreading ridges characterized by an axial bathymetric high hav e been extensively studied, the transition between these two modes of axial morphology is not well understood. We conducted a geophysical-su rvey of the intermediate spreading rate Southeast Indian Ridge between 88 degrees E and 118 degrees E, a 2300-km-long section of the ridge l ocated between the Amsterdam hot spot and the Australian-Antarctic Dis cordance where satellite gravity data suggest that the Southeast India n Ridge (SEIR) undergoes a change from an axial high in the west to an axial valley in the east. A basic change in axial morphology is found near 103 degrees 30'E in the shipboard data; the axis to the west is marked by an axial high, while a valley is found to the east. Although a well-developed axial high, characteristic of the East Pacific Rise (EPR), is occasionally present, the more common observation is a rifte d high that is lower and pervasively faulted, sometimes with significa nt (> 50 m throw) faults within a kilometer of the axis. A shallow axi al valley (< 700 m deep) is observed from 104 degrees E to 114 degrees E with a sudden change to a deep (>1200 m deep) valley across a trans form at 114 degrees E. The changes in axial morphology along the SEIR are accompanied by a 500 m increase in near-axis ridge flank depth fro m 2800 m near 88 degrees E to 3300 m near 114 degrees E and by a 50 mG al increase in the regional level of mantle Bouguer gravity anomalies over the same distance, The regional changes in depth and mantle Bougu er anomaly (MBA) gravity can be both explained by a 1.7-2.4 km change in crustal thickness or by a mantle temperature change of 50 degrees C -90 degrees C. In reality, melt supply (crustal thickness) and mantle temperature are linked, so that changes in both may occur simultaneous ly and these estimates serve as upper bounds. The along-axis MBA gradi ent is not uniform. Pronounced steps in the regional level of the MBA gravity occur at 103 degrees 30'E-104 degrees E and at 114 degrees E-1 16 degrees E and correspond to the changes in the nature of the axial morphology and in the amplitude of abyssal hill morphology suggesting that the different forms of morphology do not grade into each other bu t rather represent distinctly different forms of axial (s)tructure and tectonics with a sharp transition between them. The change from an ax ial high to an axial valley requires a threshold effect in which the s trength of the lithosphere changes quickly. The presence or absence of a quasi-steady state magma chamber may provide such a mechanism. The different forms of axial morphology are also associated with different intrasegment MBA gravity patterns. Segments with an axial high have a n MBA low located at a depth minimum near the center of the segment, A t EPR-like segments, the MBA low is about 10 mGal with along-axis grad ients of 0.15-0.25 mGal/km, similar to those observed at the EPR, Rift ed highs have a shallower low and lower gradients suggesting an attenu ated composite magma chamber and a reduced and perhaps episodic melt s upply. Segments with a shallow axial valley have very flat along-axis MBA profiles with little correspondence between axial depth and axial MBA gravity.