ARE OLISTOSTROMES OF THE GREAT VALLEY GROUP, CALIFORNIA REMNANTS OF FOREARC MUD VOLCANO DEPOSITS OR SUBMARINE LANDSLIDE DEPOSITS FROM SEA-FLOOR HORSTS?

The Great Valley Group (GVG) of California is considered to be the classic example of strata deposited in a forearc basin. The basal strata of the GVG contains sedimentary mélanges of serpentinite-siliciclastic matrix which is thought of as analogs of modern forearc mud volcano deposits

Although there have been several studies published on the basal GVG olistostromes, important details regarding metamorphic blocks within GVG olistostromes have not been examined. Field relationships of these blocks play a huge role in determining whether or not these olistostromes are Franciscan derived windows or if the blocks were transported into the GVG via diapirs, submarine landslide, or tectonically driven processes.

Field relationships have shown intensity of foliation in the GVG serpentinite-siliciclastic matrix in many outcrops, which has yet to be documented in this section of GVG. At least two rutile-bearing amphibolite blocks have also been found. A blueschist overprinted amphibolite or gabbro block in which blue amphibole grew after an episode of tectonic brecciation was also found and verified with petrologic analysis. Field relationships at Lake Berryessa combined with drone imagery and petrologic analysis have shown a block of foliated metavolcanics with local development of blue amphibole within the serpentinite matrix olistostrome to be much larger than originally anticipated (upwards of 800-1000m). The metavolcanic block has sedimentary contacts with the matrix showing that it is a block in olistostrome and not a window into Franciscan. The emplacement of this metavolcanic block is difficult to explain by diapiric emplacement due to its size. This may suggest that the larger blocks of GVG basal olistostromes were sourced from exhumed rocks on the sea floor rather than as a result of forearc mud volcanism.

Further analysis of these large blocks via geochemistry and geochronology will help to provide constraints on how far down in the subduction channel these blocks travelled before being deposited in the GVG.