Compositionally zoned ash flow sheets provide convincing evidence for chemically zoned magma bodies. Most workers have assumed that the high-silica portions of these magma bodies evolved largely by differentiation processes that occurred within the magma chamber. However, chemical heterogeneities within some ash flow sheets are not consistent with these differentiation processes. The chemical variation of pumice fragments in the large volume (>1200 km3), Rainier Mesa ash flow sheet ranges from 55 to 76.3% silica. These pumice fragments occur in three distinct chemical groups. A low- and high-silica group is separated by a compositional gap of about 72% silica, and within the high-silica group there are two distinct populations based on trace element variations. There is little overlap between populations. These three magma types have been resident in same magma chamber at the same time and cannot be produced by any differentiation process of a single magma body. They must reflect discrete magma batches generated in the source area. Furthermore, the lower silica portion (<72% SiO2) of the Rainier Mesa ash flow sheet is chemically distinct from the lower silica portion of the overlying Ammonia Tanks ash flow sheet, even though they erupted within 200,000 years of each other. These ash flow sheets from the SW Nevada volcanic field are associated in time and place with Basin and Range extension, and all models for extension involve detachment surfaces that extend to great depth. A model for the relationship of these compositional heterogeneities and the regional extension involves (1) the generation of magma batches by either continuous melting of the source at different temperatures, or by melting of different sources, (2) the use of faults (shears) as conduits for transport of magma, and (3) the use of a dilatant releasing step on a detachment as storage chamber for the magma. We first address the evidence for separately generated magma batches and then propose a model for their coalescence in a single chamber prior to eruption. ¿ American Geophysical Union 1995