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Mono Basin is a post-Miocene, northeast-trending structural depression lying immediately east of the central Sierra Nevada. A negative gravity anomaly of about 50 milligals centers under Mono Lake in the western portion of the basin, which is surrounded by varied and voluminous volcanic rocks of Pliocene, Pleistocene, and Holocene age, and has been regarded by others as a volcano-tectonic depression. K-Ar ages and the distribution and relations of the young volcanic rocks show that they represent several separate volcanic episodes and that volcanism was synchronous around the basin only during the period between about 2 and 4 m.y. ago. Structural development of the present depression has occurred largely during the last 3 m.y. and is still in progress.

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The geology of the margins of the Mono basin, California, suggests that the basin is a relatively shallow warp. When interpreted in the light of detailed study of the basin fill and of the geologic history of the region, the gravity and seismic data are consistent with the basin's being filled to a depth of 1 to 1.5 km, largely with sedimentary deposits. The geophysical data have been interpreted by others to indicate that the basin subsided deeply along nearly vertical faults and is filled to a depth of approximately 5 km with Cenozoic deposits, including a large volume of volcanic material. The basin appears to be the product of a regional pattern of warping and faulting and is not to be regarded as a volcano‐tectonic depression, as has been suggested.