Analysis of broadband seismograms from the 15 July 2003 large earthquake (M 7.6) in the central Indian Ocean reveals an unusual source process. The source duration of longer than a minute is more than twice as long as expected from earthquake scaling relations, yet ~80% of the moment release occurred in two energetic asperities near the end of the rupture. These two asperities were located in lithosphere with an age of 7 Ma or greater. A previous study has suggested that strike-slip earthquakes in oceanic lithosphere having much longer than expected source durations also have a slow, dissipative rupture process characterized by low radiated seismic energy (and therefore low apparent stress). We find no evidence for a slow rupture process to the 2003 earthquake. Instead, the long duration appears to be due only to nucleation close to the actively spreading Carlsberg Ridge, in lithosphere younger than 7 Ma. Younger oceanic lithosphere may be able to generate small to moderate earthquakes but be unable to sustain slip in a large event due to steady release of strain in aseismic creep events. Large strike-slip earthquakes within oceanic lithosphere may occur only in the central portions of long transform faults or in intraplate regions, rupturing energetic asperities like those that failed in the mid-Indian Ocean earthquake and leading to the observation that oceanic strike-slip earthquakes have the largest apparent stresses among the global population of shallow earthquakes.