Brittle fault arrays, regional structural patterns, and marine multichannel seismic reflection profiles across the Magallanes basin near 53 ¿S latitude show contrasting styles of superposed extensional, contractional, and transtensional deformation. Below the northeastern end of a NE-SW transect of the basin, seismic reflection profiles show deepening levels of pre-Late Jurassic basement and Upper Jurassic to Lower Cretaceous volcanic horizons toward the southwest. This trend is associated with apparent normal fault offsets that may be attributed to a widespread Late Jurassic-Early Cretaceous extensional event. Contrastingly, below the southwestern end of Magallanes basin subsurface depths of basement and volcanic horizons shallow to the southwest. Integration of seismic reflection data with published seismic refraction data,geologic mapping, and satellite gravity anomaly data suggest that this shallowing trend is controlled structurally by craton-vergent, basement-involved contractional deformation. This style of shortening contrasts with thin-skinned styles of thrusting in the northern and central parts of the basin on Tierra del Fuego. Structural relationships, stratigraphic data, and previously published metamorphic data indicate that basement-involved contraction occurred during latest Cretaceous to early Tertiary time. Overprinting all contractional structures, steep Tertiary strike-slip and oblique-slip faults in the central and southern parts of the Magallanes basin display oblique-normal (mostly down-to-the-southwest) offsets across WNW-ESE trending segments of the South American-Scotia transform plate boundary on the South American continent. Newly discovered features along the plate boundary segment include sediment-filled grabens and half grabens up to 7 km wide that occur between subparallel fault splays in the Magallanes fault zone. Kinematic analyses of fault-slip data indicate that sinistral transtensional motion occurred within this fault zone between the eastern arm of the Straits of Magellan and central Tierra del Fuego. Together these data indicate that (1) the southernmost Andes were under compression during rapid cooling and exhumation of unique high-pressure (~8 kbars) basement rocks now exposed in the Darwin Metamorphic Complex (DMC) near 55 ¿S latitude and (2) final exhumation of basement rocks in the DMC occurred during a shift away from compression toward transtension during late Tertiary crustal relaxation. We suggest that preexisting structural anisotropies formed during basement-involved contractional deformation and the initiation of seafloor spreading south of South America during the Oligocene controlled late Tertiary sinistral transtensional patterns in the Magallanes fault zone.¿ 1997 American Geophysical Union