In 1999 a marine geophysical survey collected bathymetric imagery, seismic reflection and refraction data to investigate the crustal structure and evolution of the Sunda Strait, which is linked to the obliquely convergent geodynamic setting of the Sumatran trench. The transtensional character of the study area is manifested in faulted blocks of arc basement and active normal faults detected on both sides of a large graben at the western entrance to the Sunda Strait. The combined interpretation of reflection and wide-angle data reveals over 6 km of sediment graben fill associated with substantial crustal thinning. The southern part of the study region is only 50 km from the trench and the Moho of the downgoing plate is found at a depth of 28 km. ¿ 2000 American Geophysical Union