Geophysical Evidence for Mid-crustal Magma Reservoirs in the Lassen Volcanic Region, California

Regional-scale complete Bouguer gravity anomalies underlying the Lassen and Shasta -Medicine Lake regions in northern California and southern Oregon are associated with subduction of the Gorda plate beneath North America. These generally negative anomalies reflect where underplating has deepened to form the mantle wedge, and where subduction has given rise to a series of Quaternary volcanoes comprising the southernmost end of the Cascade range. Multiple conductive bodies were identified by Park and Ostos (2013) in their magnetotelluric (MT) study of the broader Lassen volcanic region. Their broadband and long period measurements were conducted along a 250 km profile spanning from the California-Nevada border, to just west of the Great Valley in California. Utilizing their MT conductor geometries as a starting point, a forward gravity model was generated along the same profile, and agrees well with what they interpret to be the locations and depths of mid-crustal magma bodies in the Lassen and surrounding regions. The excess mass and volume of modeled anomaly (a) - most closely attributed to underlying Lassen Peak - were estimated at -2 x 1014 kg and 7 x 1011 m3, respectively