Inferences on crustal velocities and densities from P wave delays and gravity anomalies

A correlation is demonstrated between P wave delays and Bouguer gravity anomalies. For California data it is given approximately by 〈Δg〉 = −355PD where 〈Δg〉 is the Bouguer (slab equivalent) gravity change in milligals and PD is the delay in seconds. The increases in P wave delays and negative gravity anomalies associated with topographic highs are manifestations of isostatic adjustment. However, composition changes of the type implied by empirical velocity-density relationships cannot explain the gravity changes associated with P wave delays in California batholiths. One way to reconcile the discrepancy is to postulate velocity reversals in the batholith. A thermal argument shows that anomalously high temperatures are likely to occur under the Sierra Nevada because of the unusually large thickness of batholithic rock. Velocity reversals under such circumstances are plausible.