Paper No. 4
Presentation Time: 9:00 AM
THE CRETACEOUS SALINIAN CONTINENTAL ARC: OVERVIEW AND NEW ADVANCES
We review the petrologic and tectonic development of the allochthonous Cretaceous Salinian continental arc, currently located in central coastal California, with special attention to new and recent advances: 1) U-Pb detrital zircon ages indicate a late Paleozoic maximum depositional age for the sedimentary framework rocks of the Salinian arc and strengthen pre- and post-arc connections with the Mojave region. 2) During peak metamorphism beginning ~93 Ma, the metasedimentary framework of the Coast Ridge belt was partially melted and heavily deformed, as it was intruded by and mixed with basaltic to tonalitic melts at ~8 kbar. By 81 Ma, this section of the arc had developed a mafic-intermediate average composition (SiO 2 ~54%), and migmatitic compositional banding interpreted by early workers as metasedimentary in origin. 3) In the Santa Lucia Mountains, the more felsic Salinian central block is dominated by mid to late Cretaceous granodiorite and quartz diorite plutons. GASP barometry and garnet-biotite thermometry indicate equilibration here at pressures of up to 10 kbar, twice as deep as previously recognized. 4) Differences between the Coast Ridge belt and Central block develop new significance when viewed as coeval middle- to lower-crustal lateral variations in the Salinian arc. One remarkable difference is the presence of scores of small ultrabasic bodies peppered throughout the Central block but never found in the Coast Ridge belt. The ultrabasic bodies intruded as liquids and contributed to migmatization and melt production in the lower arc. 5) Arc collapse beginning at ~76 Ma was accompanied by flattened subduction and underthrusting of recently deposited (< 81 Ma*) greywacke, now the Schist of Sierra de Salinas. Dehydration (melting?) in the shear zone between the schist and plutonic rocks, and growth of garnet in mylonitized diorite constrain deformation to > 6 kbar between 81 and 76 Ma.
*Barth et al, 2003, Geology v.31; no. 6; p. 517-520.