Fig. 10

Field photographs of OPS in the Franciscan Complex. In these photos, all contacts (dashed lines) of apparent “inverted” OPS stratigraphy (such as basalt over chert or sandstone, or chert over sandstone) and “omitted” OPS (clastics over basalt when pelagic rocks are present in the full sequence) are faulted. Some of the contacts in “normal” OPS stratigraphic order are depositional whereas others are faults. Dashed lines drawn within the same lithology represent faults. a Marin Headlands, prehnite-pumpellyite facies OPS imbricates at the western side of Black Sand Beach, Bonita Cove (Fig. 6), where basalt and chert (itself internally imbricated) is thrust over siliciclastic rocks. Whereas the siliciclastic section is primarily bedded with local broken formation, sedimentary mélange/conglomerate horizons are present. “b” and “c” show outcrops within the Rodeo Cove shear zone of the Marin Headlands (Fig. 6), a locality described in detail in Meneghini and Moore (2007). At the scale of Fig. 6, the Rodeo Cove shear zone is one of the faults repeating the OPS section and this imbrication is seen on a smaller scale within the shear zone itself as shown in the outcrop photos. The imbrication locally leads to block-in-matrix relationships such as the chert-in-sandstone seen in the “ch/ss” parts of “b”, the basalt in basalt matrix part (labeled as “bs”), and the basalt and chert in shale matrix (“ch/sh/bs”) part of “c”. “d” and “e” show close ups of sedimentary block-matrix horizons in the outcrops of (“a”). d Pebbly mud-rich sandstone/conglomerate. This conglomerate grades into cleaner sandstone with blocks shown in (“e”), and this is interbedded with deformed shale with blocks of other lithologies. These sorts of textures and relationships are common in siliciclastic matrix mélange as seen in “k-n” as well as Figs. 16, 17, 18, and 20 of Wakabayashi 2015). f Siliciclastic-chert-basalt OPS from blueschist facies rocks, Sunol Regional Wilderness. The preserved parts of chert and siliciclastic sections in the imbricates of mainly metavolcanic exposures are very thin in this area (commonly <10 m and too small to show on the Fig. 7 map). g OPS stack of sandstone-limestone-chert-basalt in blueschist facies rocks along the Skaggs Road, Sonoma County (location on Fig. 2). This type of OPS stack is very rare in the Franciscan. h Block-in-sandstone matrix, Sunol Regional Wilderness showing OPS of sandstone overlying chert overlying limestone that overlies basalt. Coherent imbricates of this type of OPS have not been found to date in the Franciscan. i Skaggs Spring schist (completely recrystallized quartz-rich glaucophane-lawsonite-phengite ± jadeite schist) imbricated with intact serpentinized peridotite. Close up in “j” shows silica carbonate rock (hydrothermally replaced serpentinite, also called listvenite) blocks along one of the contacts that places Skaggs Springs Schist on serpentinite. This may represent a depositional contact. Note that there is a lot of surficial material draped over the outcrop in this photo. k Photo showing interbedded sandstone and pebbly sandstone and shale matrix sedimentary breccia/conglomerate, blueschist facies rocks along Panoche Road (location on Fig. 2). This shows the interbedding of ordinary turbidites and sedimentary mélange matrix. These horizons locally have blocks of tens of meters in size (compare with different photos in Fig. 18 of Wakabayashi 2015). l Interbedding of sedimentary breccia/conglomerate and sandstones and gradational depositional contact between sedimentary breccia/conglomerate (sedimentary mélange matrix), blueschist facies rocks, Sunol Regional Wilderness (location on Fig. 7). The elevation difference between the bottom of the photo and top is about 15 m. m, n Sunol Regional Wilderness exposures showing local deformational overprint on conglomerate/sedimentary breccia that makes up the siliciclastic mélange matrix in this area. In “m,” note the pencil on the center of the upper photo border for scale. In addition to the locally developed deformational fabric, note the late brittle fault in (“m”). The most deformed zones in these outcrops are typical of what has been interpreted as “tectonic” mélange matrix in the Franciscan Complex. o–r Sedimentary serpentinite mélange, Ring Mountain (locations on Fig. 8) o metamafic block in sedimentary serpentinite matrix. The block is an omphacite-chlorite schist on the left side of the photo (chlorite replaces most of the omphacite and there is late growth of lawsonite), and this grades to or is bordered by nearly monomineralic chlorite schist on the right. The matrix is serpentinite conglomerate and sandstone with many exotic clasts ranging from <0.1 mm to cm scale. p Close up of part of “o” showing texture of serpentinite conglomerate/pebbly sandstone. q Close of a separate outcrop of sandy/pebbly serpentinite matrix with many rounded grains, including a coarse blueschist clast. r Mafic block in serpentinite matrix, showing internal faulting and folding. The block is garnet amphibolite (grt-am) nearly completely overprinted by blueschist facies assemblages (bsch) in most areas. The block is internally imbricated and tightly folded resulting in repetitions of mafic (bsch/grt-am) and metaultramafic (UM) schist. The latter is composed of tremolite/actinolite + phengite + chlorite. Progressive deformation has created local block-in-matrix textures in which mafic blocks (bsch) are enclosed in metaultramafic schist (UM). Most of these block-in-matrix features are too small to be seen in this photo. Similar features are present in other blocks in this mélange. They apparently predate the incorporation into the clastic serpentinite matrix which is not part of the imbricates. The features preserved in these blocks may preserve an ocean plate stratigraphic relationship similar to that shown at map scale in Fig. 9 (Willow Spring Slab). s Pebbly serpentinite sandstone matrix from Sunol Regional Wilderness (location on Fig. 7) showing well-preserved sedimentary texture. This type of material lacks erosional resistance and tends to be poorly exposed, except on the steepest slopes or artificial excavations. t, u Examples of imbrication of siliciclastic (sandstone and shale) (locations on Fig. 2). Without good exposures, these structures are much more difficult to identify in these rocks in contrast to rock assemblages with volcanic and/or pelagic sedimentary rocks. “t” shows exposures of the prehnite-pumpellyite facies Novato Quarry terrane along Nave Drive in Marin County and “u” shows lawsonite-albite facies rocks that may be correlative to the Burnt Hills terrane along Panoche Road of the southern Diablo Range. For a higher resolution image, please see Additional file 1