Constraints from magnetotelluric measurements on magmatic processes and upper mantle structure in the vicinity of Lassen volcanic center, northern California

摘要

A 250-km-long broadband and long-periodn(0.01–20,000 s periods) magnetotelluric (MT)nline along latitude 40½ N spans from the California-nNevada border at longitude 120 Wnacross the buried extension of the northernnSierra Nevada, the southern Cascades arc,nand the subducting Gorda plate to longituden123 W. The resulting resistivity cross sectionnreveals conductors at the locations ofndewatering of the subducting metasomatizedncrust at 100 km depth, partial melting of thenmantle wedge at 40–60 km depth, and meltingnin the crust shallower than 40 km. Thenconductor at 100 km is too conductive (8 S/m)nto result solely from magma; very conductivenfl uids must be present either separately ornincorporated into hydrous melts. A melt fractionnof 7% is estimated for the mantle wedgen(average conductivity is 0.06 S/m). Sites ofnPleistocene and Holocene volcanism at LassennPeak, Hat Creek, and Poison Lake arenclosely associated with crustal conductorsninferred to be shallow magma. The MT studynalso reveals presumably basaltic magma atn40–50 km depth in the upper mantle wherenthe eastern end of the profi le extends into thenBasin and Range province.