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Origin of Blueschist-Bearing Chaotic Rocks in the Franciscan Complex, San Simeon, California

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Origin of Blueschist-Bearing Chaotic Rocks in the Franciscan Complex, San Simeon, California Origin of blueschist-bearing chaotic rocks in the Franciscan Complex, San Simeon, California DARREL S. COWAN Department of Geological Sciences, University of Washington, Seattle, Washington 98195 ABSTRACT INTRODUCTION posed on the coast south of San Simeon, California, are described here in detail, and Chaotic rocks exposed in sea cliffs south The Franciscan Complex of the Califor- the relative roles that sedimentary and of San Simeon, California, consist of sub- nia Coast Ranges is now widely regarded as tectonic processes might have played in the rounded to lens-shaped fragments of having formed during late Mesozoic sub- development of their distinctive mesoscopic graywacke, greenstone, and less abundant duction at a convergent plate boundary. fabric are discussed. I chose San Simeon for blueschist and chert dispersed in a matrix of This interpretation is supported by the my study in part because the sea cliffs af- argillite. This nonbedded mélange has been compelling spatial and temporal relation- ford exposures far superior to those inland. deformed twice. An earlier deformation, ship of the Franciscan to the coeval Great Also, the Franciscan rocks in the vicinity of Di, produced a strong northwest-striking, Valley sequence and Sierra Nevada batho- San Simeon are significant in a historical northeast-dipping foliation defined by both lith and by the striking geometrical corre- sense, for they were the first in the complex tectonically flattened inclusions and a paral- spondence of these Mesozoic petrotectonic to be explicitly characterized as chaotic, lel penetrative cleavage in argillite. Most assemblages to the inner walls of oceanic tectonic mixtures or mélanges by K. J. Hsii inclusions, even of blueschist, are imperfect trenches, fore-arc sedimentary basins, and in a series of papers that have had a pro- oblate ellipsoids or display pinch-and-swell magmatic arcs that together characterize found influence on our perception of Fran- structure and, locally, extreme necking and modern convergent margins. Careful study ciscan geology. boudinage. Overall ductile behavior during of ancient rocks in the Franciscan Complex Dj was succeeded by the development of will yield some general insights into sub- FRANCISCAN COMPLEX subparallel shear fractures that record a duction processes which, by their very na- AT SAN SIMEON brittle deformation, D2. Displacements on ture, are difficult or impossible to examine these fractures were generally small or neg- directly. The Franciscan basement west of the ligible. One of the outstanding unsolved prob- Sur-Nacimiento fault zone (Fig. 1) is over- It is clear that neither Dj nor D2 was re- lems of Franciscan geology is the origin of lain structurally by remnants of the sponsible for the original lithologic heter- the nonbedded, chaotic "mélanges" that lo- Mesozoic Great Valley sequence (Hsii, ogeneity and chaotic fabric of the mélange. cally contain isolated blocks of blueschist. It 1969; Page, 1972) and depositionally by Mixing of foliated, glaucophane-lawsonite is ironic that the origin of these distinctive scattered sections of predominantly blueschist with lower-grade graywacke and rocks is still hotly debated, for they are Neogene sedimentary rocks (Jennings, greenstone to yield a nonbedded diamictite widespread in the Franciscan and have be- 1958). Hsü (1967, 1969) reinterpreted the composed of variously sized clasts in a come not only synonymous with the Com- geologic mapping compiled by Jennings mudstone matrix must have occurred prior plex itself, but also, to some extent, prima (1958) at a scale of 1:250,000 and first rec- to D^ probably by sedimentary processes facie evidence for subduction elsewhere. ognized that the Franciscan in the Morro involving submarine sliding and downslope Uncertainty usually centers about whether Bay—San Simeon area consists of relatively transport of debris flows. There is no evi- mélanges have acquired their chaotic aspect coherent and lithologically homogeneous dence that the mélange was bedded or that because of sedimentation from submarine units that are seemingly surrounded by the blueschist inclusions were tectonically debris flows as olistostromes, postdeposi- nonbedded, lithologically heterogeneous introduced among lower-grade rocks, prior tional tectonic deformation, or both. The rocks that he termed melange. This funda- to D(. The sequence of sedimentary and conspicuous association of blocks of mental distinction has been a valuable tectonic events suggests that as the subduc- blueschist with chaotic rocks strongly mapping criterion elsewhere in the Francis- tion ultimately responsible for the Francis- suggests that their histories are somehow re- can (for example, Cowan, 1974), and it was can Complex as a whole proceeded, lated, but the mechanism responsible for utilized by Hall (1974, 1976) on his more blueschists in elevated parts of previously mixing metamorphically exotic, high-grade detailed maps of the Cambria and San Sim- accreted material were eroded, mixed with blocks with more voluminous, lower-grade eon areas at a scale of 1:24,000. Hsii (1968, lower-grade rock, and deposited as olisto- graywacke, chert, and greenstone is still 1969) developed an elaborate nomenclature stromes that were subsequently accreted controversial. that he felt was generally applicable to and deformed. The blueschist-bearing chaotic rocks ex- chaotic terranes in the Franciscan, and he Geological Society of America Bulletin, v. 89, p. 1415- 423, 8 figs., September 1978, Doc. no. 80913. 1415 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/9/1415/3418822/i0016-7606-89-9-1415.pdf by guest on 30 September 2021 1416 D. S. COWAN interpreted the mélange at San Simeon as tail are exposed in sea cliffs between the vil- 1969), that Hsii named both "Cambria having formed by tectonic mixing and lage of San Simeon and the mouth of Santa Pines Slab" and "broken formation A" and fragmentation on both small and large Rosa Creek at Cambria, 12 km to the that Hall (1974, 1976) mapped as "un- scales. I have deferred further discussion of southeast (Fig. 1). Chaotic rocks, included named sedimentary rocks," extends from the nomenclature and interpretation of by Hsii (1969) in "mélange unit #3," pre- San Simeon Creek to Villa Creek, 20 km to melanges to a later section in order to im- dominate northwest of San Simeon Creek. the southeast. This fault-bounded unit part as little genetic prejudice as possible to A well-bedded, gently folded unit of (Hall, 1974, 1976) could be either a the observations that follow. feldspathic graywacke and mudstone, at tectonic outlier of Great Valley sequence or The Franciscan rocks that I studied in de- least in part of Late Cretaceous age (Hsii, a mappable coherent unit within the Fran- ciscan. I also briefly examined rocks crop- ping out on the coast between Villa Creek and Cayucos which are indistinguishable from those exposed northwest of San Sim- eon Creek. My own reconnaissance and the work of Hsu (1969) and Hall (1974, 1976) suggest that the coastal exposures described here are representative of the chaotic Fran- ciscan terrane depicted in Figure 1. Mélange Most of the exposures in the study area consist of subrounded to lenticular inclu- sions of a variety of rock types that are ap- parently randomly dispersed in a matrix of black argillite (Figs. 2, 3, 4, 6). I will use the term "mélange" as a purely descriptive, nongenetic term for such chaotic rocks that, on a mesoscopic scale, consist predomi- nantly of argillite. The most important characteristics of the mélange are its Figure 1. Generalized geologic and location map of southern Coast Ranges in vicinity of San Sim- lithologic heterogeneity and the lack of any eon (modified from Jennings, 1958; Page, 1972; Hall, 1974, 1976); gr = area northeast of Sur- obvious sorting or orderly internal organi- Nacimiento fault zone (SNFZ) underlain by granitic and metamorphic basement of Salinian block; F— zation such as layering or bedding. Hsii basement of Franciscan Complex; C = bedded sandstone of "Cambria Pines slab" of Hsii (1969). (1968, 1969) noted that inclusions vary tremendously in size. In the sea cliffs I studied, the sizes of inclusions that demon- strably are completely surrounded by argil- lite range from 1 or 2 mm to perhaps 10 m. Larger bodies, which may or may not have exposed contacts with mélange, locally ex- ceed the lengths of individual outcrops. They would be mappable as discrete inclu- sions in a matrix of undifferentiated argil- lite and smaller inclusions only at map scales smaller than those appropriate for the analysis of mesoscopic features em- phasized in this study (for example, Hall, 1974, 1976). Most inclusions are monolithologic and nonbedded, but bed- ding or remnants thereof are preserved in some inclusions of graywacke and mudstone larger than 1 m in diameter (Fig. 8). Inclusions The inclusions in mélange consist of grajwacke, greenstone, chert, and a variety Figure 2. Typical foliated mélange northwest of Pico Creek. Most larger inclusions are lenticular and are statistically oriented parallel to cleavage in matrix of argillite. S, is horizontal in photo. Large of completely recrystallized, foliated inclusion of graywacke in center is necked. In detail, foliation in argillite flows into these concave re- metamorphic rocks. Although relative pro- entrants and around ends of inclusion. portions are difficult to estimate, probably Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/9/1415/3418822/i0016-7606-89-9-1415.pdf
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