New interpretation of the Franciscan mélange at San Simeon coast, California: tectonic intrusion into an accretionary prism

Abstract

Many concepts and interpretations on the formation of the Franciscan mélange have been proposed on the basis of exposures at San Simeon, California. In this paper, we show the distribution of chaotic rocks, their internal structures and textures, and the interrelationship between the chaotic rocks and the surrounding sandstones (turbidites). Mélange components, particularly blueschists, oceanic rocks, including greenstone, pillow lava, bedded chert, limestone, sandstone, and conglomerate, have all been brecciated by retrograde deformation. The Cambria Slab, long interpreted as a trench slope basin, is also strongly deformed by fluidization, brecciation, isoclinal folding, and thrusting, leading us to a new interpretation that turbiditic rocks (including the Cambria Slab) represent trench deposits rather than slope basin sediments. These rocks form an accretionary prism above mélanges that were diapirically emplaced into these rocks first along sinistralthrust faults, and then along dextral-normal faults. Riedel shear systems are observed in several orders of scale in both stages. Although the exhumation of the blueschist blocks is still controversial, the common extensional fractures and brecciation in most of the blocks in the mélanges and further mixture of various lithologies into one block with mélange muddy matrix indicate that once deeply buried blocks were exhumed from considerable depths to the accretionary prism body, before being diapirically intruded with their host mélange along thrust and normal faults, during which retrograde deformation occurred together with retrograde metamorphism. Recent similar examples of high-pressure rock exhumation have been documented along the Sofugan Tectonic Line in the Izu forearc areas, in the Mineoka belt in the Boso Peninsula, and as part of accretionary prism development in the Nankai and Sagami troughs of Japan. These modern analogues provide actively forming examples of the lithological and deformational features that characterize the Franciscan mélange processes.

Assembling the world’s type shallow subduction complex: Detrital zircon geochronologic constraints on the origin of the Nacimiento block, central California Coast Ranges

“…Type E spectra in the schist of Sierra de Salinas are explained by longdistance turbidite flow of sediment from central California down the trench axis (Dumitru et al, 2016). Furthermore, recent structural work by Ogawa et al (2014) suggests that Naci miento slabs may in fact represent trench axis deposits, not trenchslope de posits as traditionally interpreted, and so could receive detritus from Idaho in the same fashion as that invoked for the schist of Sierra de Salinas.…”

“…We suggest that this apparent linear outcrop pattern, parallel to the strike of the Nacimiento belt, was due to wedge flexure adjacent to thrust faults (e.g., Smith et al, 1979). We propose that the proto-San Gregorio-Hosgri fault represents a Late Cretaceous intra wedge thrust fault, adjacent to which a trenchslope (e.g., Smith et al, 1979) or trenchaxis (Ogawa et al, 2014) basin system developed, and that the fault reactivated in Neogene time as a strand of the San Andreas wrench system.…”

“…The Nacimiento Franciscan is a tectonic assem blage of sandstone, siltstone, shale, chert, conglomerate, serpentinite, and basaltic rocks variably metamorphosed along a HP/LT trajectory (Hsü, 1969;Gilbert, 1973;Cowan and Page, 1975;Ernst, 1980;Underwood et al, 1995;Underwood and Laughland, 2001). Three distinct types of units comprise the Nacimiento Franciscan: (1) broken formation consisting of internally sheared yet coherent wellbedded sandstone and turbidite sequences that lack exotic blocks; (2) a chaotically sheared (mélange) unit consisting mainly of clastic and greenstone blocks within a foliated siliciclastic matrix and containing <1% exotic blocks of chert, blueschist, and serpentinite; and (3) coherent unmeta morphosed submarine fan systems interpreted as trenchslope or trenchaxis basin assemblages (slabs) lying in depositional contact above, and contain ing detritus from, the subduction complex (Hsü, 1968;Gilbert, 1973;Smith et al, 1979;Becker and Cloos, 1985;Underwood and Howell, 1987;Hall, 1991;Under wood and Laughland, 2001;Dickinson et al, 2005;Jacobson et al, 2011;Ogawa et al, 2014).…”

“…2), has been debated since the pio neering work of Hsü (1968). Models for the incorporation of lithologically di verse fragments, including "exotic" blocks such as blueschist, in serpentinite or finegrained siliciclastic material within the Nacimiento Franciscan fall generally into three categories: (1) tectonic-previously accreted blocks were plucked from the hanging wall of the subduction channel and exhumed by return flow (e.g., Cloos, 1982); (2) sedimentary-previously exhumed blocks and siliciclastic deposits slid down the trench slope and were (re)subducted (e.g., Cowan, 1978;Platt, 2015); and (3) diapiric-diapirs entrained blocks of subduction assemblages as they rose (e.g., Becker and Cloos, 1985;Ogawa et al, 2014). The latter model proposes that earlier formed tectonic mélange was reworked by diapirism; hence, we consider the diapiric model as a varia tion on the tectonic model.…”

Sandstone-matrix mélanges, architectural subdivision, and geologic history of accretionary complexes: A sedimentological and structural perspective from the Franciscan Complex of Sonoma and Marin counties, California, USA

“…In contrast, thinner, especially mud-richer sections may convert to shale-matrix, fault-zone mélange, some of which may form and accrete incrementally (e.g., Rowe et al, 2013). The Subduction Channel model (Cloos, 1982(Cloos, , 1984Shreve, 1988a, 1988b) is used widely to explain formation and emplacement of laterally extensive and generally thick shale-matrix or serpentinite-matrix mélange units within accretionary complexes (e.g., Hansen, 1992;Vannucchi et al, 2008;Blanco-Quintero et al, 2011;Ukar, 2012;Ogawa et al, 2014;Ernst, 2015). In the Subduction Channel model, continuous ductile flow and pervasive deformation of the shale or serpentinite matrix are essential for offscraping, fragmentation, and mixing of high-and low-grade blocks and other rocks within mélange packages (Cloos, 1982;Cloos and Shreve, 1988a;Ukar, 2012).…”