The Falcón Basin in northwestern Venezuela and adjacent offshore basins developed within a zone of extensional tectonics during Oligocene and Miocene times. Extension resulted from right-lateral motion along offset, east-west-trending, transcurrent faults, including the Oca fault in western Venezuela, the Cuiza fault in northern Colombia, and the San Sebastián fault along the coastal areas of central Venezuela. On both local and regional scales, transcurrent and normal faults were active during the early evolution of the basins. These faults define rhomb-shaped pullapart basins in map plan. Extension occurred in a northeast direction causing normal faulting along north-west trends. Basin subsidence was accompanied by crustal thinning and injection of basaltic magmas. Evidence of Oligocene magmatic activity is found in the central part of the Falcón Basin where volcanic rocks and hypabyssal intrusions are exposed. These rocks are similar to other suites of continental igneous rocks typically associated with rifting environments. Basaltic rocks of both alkalic and subalkalic affinities are present. Xenoliths of the underlying crust and mantle are abundant. Felsic igneous rocks are relatively rare. Other structures within the Falcón and Bonaire basins formed during later stages of basin development. These include folds and reverse faults of northeast trend and conjugate sets of small transcurrent faults. These structures were amplified by greater compressional stresses during late Miocene and Pliocene time. A similar Tertiary tectonic regime is postulated for the larger area of the Bonaire Crustal Block, a block that includes the Falcón and Bonaire basins. This block is a broad region of extensional pullapart structures which developed during the Oligocene to Miocene, right-lateral motion between the Caribbean and South American crustal plates. Minimum extension within the northern part of the block, along the Venezuelan and Netherlands Antilles and the Paraguaná and Guajira peninsulas, is estimated at 35 to 45 km in an east-west direction. The extension within this nonrigid block should be considered when determining Tertiary movements between the Caribbean and South American plates.