Structural context and variation of ocean plate stratigraphy, Franciscan Complex, California: insight into mélange origins and subduction-accretion processes

Abstract

The transfer (accretion) of materials from a subducting oceanic plate to a subduction-accretionary complex has produced rock assemblages recording the history of the subducted oceanic plate from formation to arrival at the trench. These rock assemblages, comprising oceanic igneous rocks progressively overlain by pelagic sedimentary rocks (chert and/or limestone) and trench-fill clastic sedimentary rocks (mostly sandstone, shale/mudstone), have been called ocean plate stratigraphy (OPS). During accretion of OPS, megathrust slip is accommodated by imbricate faults and penetrative strain, shortening the unit and leading to tectonic repetition of the OPS sequence, whereas OPS accreted at different times are separated by non-accretionary megathrust horizons. The Franciscan subduction complex of California accreted episodically over a period of over 150 million years and incorporated OPS units with a variety of characteristics separated by non-accretionary megathrust horizons. Most Franciscan OPS comprises MORB (mid-ocean-ridge basalt) progressively overlain by chert and trench-fill clastic sedimentary rocks that are composed of variable proportions of turbidites and siliciclastic and serpentinite-matrix olistostromes (sedimentary mélanges). Volumetrically, the trench-fill component predominates in most Franciscan OPS, but some units have a significant component of igneous and pelagic rocks. Ocean island basalt (OIB) overlain by limestone is less common than MORB-chert assemblages, as are abyssal serpentinized peridotite slabs. The earliest accreted OPS comprises metabasite of supra-subduction zone affinity imbricated with smaller amounts of metaultramafic rocks and metachert, but lacking a clastic component. Most deformation of Franciscan OPS is localized along discrete faults rather than being distributed in the form of penetrative strain. This deformation locally results in block-in-matrix tectonic mélanges, in contrast to the sedimentary mélanges making up part of the clastic OPS component. Such tectonic mélanges may include blocks and matrix derived from the olistostromes. Franciscan subduction and OPS accretion initiated in island arc crust at about 165-170 Ma, after which MORB and OIB were subducted and accreted following a long (tens of mega-ampere) gap with little or no accretion. Following subduction initiation, a ridge crest approached the trench but probably went dormant prior to its subduction (120-125 Ma), after which the subducted oceanic crust became progressively older until about 95 Ma. From 95 Ma, the age of subducted oceanic crust decreased progressively until arrival of the Pacific-Farallon spreading center led to termination of subduction and conversion to a transform plate boundary.

A 1.9‐Ga Mélange Along the Northern Margin of the North China Craton: Implications for the Assembly of Columbia Supercontinent

“…In this paper, we present the results of new detailed field mapping, geochronology data, and geochemistry analysis of the mélange complex (Figure ). The mélange rocks here are generally dominated by chaotic mixtures of metapelites, metapsammites, mica schist, paragneisses, and amphibolite as metapelite matrices, and exotic mafic, metagabbroic rocks, and metabasalts as blocks (Silver & Beutner, ; Wakabayashi, , ). Our field observations indicate that the Bayan Obo mélange contains a structurally complex mixture of metapelites (Figures e, f, and j), metapsammites (Figure j), mica‐quartz schist matrix (Figures k–m) mixed with exotic blocks of ultramafic (Figures a and g–k) and metagabbroic rocks (Figures a and f), metabasalts (Figures e–g and i) that locally include possible relict pillow structures (Figure e), plagiogranite (Figures f and i), amphibolite (Figures c–f and i), metacarbonate rocks (Figures l and m), alkaline granulite (Figure m) and tonalite‐trondhjemite (TT) gneisses (Figures b–e, i, and k).…”

“…In general, a subduction system is required to consume an intervening oceanic plate separating two cratons/blocks prior to the collisional orogeny that ultimately joins the two continents. The mélange associated with oceanic subduction, accretion, and eventual continental collision is thus significant in documenting the tectonic process of orogen (e.g., Festa et al, ; Ghatak et al, ; Greenly, ; Wakabayashi, , ; Wakabayashi & Medley, ; Wang et al, ). In this contribution, we report for the first time a new Paleoproterozoic mélange located in the Bayan Obo region on the northernmost margin of the North China craton (present‐day coordinates).…”

Progressive changes in lithological association of the Sanbagawa metamorphic complex, Southwest Japan: Relict clinopyroxene and detrital zircon perspectives

“…Mid‐ocean ridge basalt (MORB) and oceanic island basalt (OIB) are the main protoliths of metabasite in accretionary complexes and high‐pressure metamorphic complexes (e.g. Isozaki, Maruyama, & Fukuoka, ; Safonova et al, ; Wakabayashi, ). The MORB oceanic crust generally lies beneath a thick sedimentary cover at the trench.…”

Carboniferous–Early Permian sedimentary rocks from the north‐eastern Erenhot, North China: Implications on the tectono‐sedimentary evolution of the south‐eastern Central Asian Orogenic Belt

“…Accretionary complex related sedimentation is probably responsible for the Halatumiao Formation as well since high aquatic productivity is common in olistostromes deposition under an accretionary wedge environment. However, the accretionary complex is dominated by turbidite and clastic rocks with a percentage of ~90% (Wakabayashi, , ). This differs from the Halatumiao Formation which is mainly composed of fine‐grained carbonaceous and sandy slate (Figure 11a), making it difficult to stand for an accretionary wedge or fore‐arc setting.…”