Bransfield Strait, a backarc basin off the northwestern Antarctic Peninsula, is a modern analog for Cretaceous basins inverted in the compressional tectonic regime that initiated the Andean Cordillera. Eight new refraction ocean-bottom seismograph profiles in the strait demonstrate that crustal thickness in the deep central basin increases from northeast to southwest, from ∼10 km to ∼14–16 km. This confirms multichannel seismic interpretation of upper crustal structures suggesting that the Bransfield basin is opening by northeast to southwest rift propagation within arc crust of the Antarctic Peninsula, a process also recorded in the obducted Cretaceous Rocas Verdes basin of the southernmost Andes. Thinning is most prominent along the axis of the strait, where the crust is ∼9–11 km thick. In contrast, thicknesses beneath the Antarctic Peninsula margin and the inactive South Shetland Islands pedestal are ∼18 km and ∼24 km, respectively. Seismic velocities and thicknesses suggest that new oceanic crust is not yet being generated. Extension is focused along the northwest margin, imparting the physiographic asymmetry to the strait. Comparing the Bransfield basin with the inverted Rocas Verdes basin and intraoceanic counterparts in the western Pacific suggests that rift propagation and trench-side focusing of extension may be fundamental features of young backarc basins. Resultant asymmetry may facilitate observed obduction of backarc basin floor and arc rocks onto continental margins during compressional orogenesis.