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.

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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 inclusions of a variety of rock types that are apparently 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 predominantly 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 argillite 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 inclusions in a matrix of undifferentiated argillite and smaller inclusions only at map scales smaller than those appropriate for the analysis of mesoscopic features emphasized in this study (for example, Hall, 1974, 1976). Most inclusions are monolithologic and nonbedded, but bedding 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

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at least 75% of the inclusions are cludes light- to dark-green, fine-grained, murky matrix of chlorite, very fine-grained graywacke and most of the remainder are nonfoliated metavolcanic rocks. Textural sphene, and, locally, epidote. Most samples greenstone. Hsu (1969) also found serpen- and mineralogical criteria for a volcanic contain strongly pleochroic pumpellyite tinite and diabase, gabbro, and associated origin are usually evident only in thin sec- that is locally restricted to tiny veinlets. ultramafic rocks in his "mélange unit #3," tion; I observed well-formed pillows in only Veins of carbonate are abundant. Inclusions but I encountered only one exposure of two outcrops. Some samples have relict of chert are chiefly pale green to light gray, sheared serpentinite, 500 m east of the San diabasic textures; most consist of microlites although typical red-brown varieties also Simeon pier, in the study area. Many pale- and occasional phenocrysts of albite set in a occur. Some chert is interbedded with green wisps and lenses in mélange superfi- cially resemble serpentinite but are actually greenstone consisting predominantly of chlorite and albite. Graywacke is typically fine grained and poorly sorted and has a well-preserved clastic texture. Most inclusions contain no trace of bedding. In some, the sandstone is finely laminated. Other inclusions consist of alternating thin, medium, or thick beds of sandstone and mudstone. Graded beds, which are uncommon, are upright. Thirty- three samples of both bedded and nonbedded sandstone from the study area and from east of Villa Creek were studied petro- graphically. There is a significant range in both detrital and metamorphic mineralogy. About 8% to 50% of the grains are fragments of volcanic rock; the remainder are subangular quartz, feldspar, and chert. Ten samples that were collected between Pico Creek and San Simeon Creek, and south of Villa Creek, contain as much as 7% detrital K-feldspar. In contrast, sandstone in the Cambria slab contains 10% to 20% Figure 3. Relatively little-deformed mélange west of Cayucos, illustrating subrounded shape of K-feldspar (Hsu, 1969; my data). In thin some inclusions and intimate mixing of diverse rocks. A is graywacke with well-preserved clastic fab- section, very fine-grained metamorphic ric; B is nonfoliated albite-chlorite-sphene greenstone. Matrix of argillite contains smaller clasts and is weakly foliated. phyllosilicates are generally widespread. Eight samples contain rare, apparently metamorphic, pleochroic pumpellyite in quartz, albite, and matrix; one of these also contains prehnite. In some other samples, relations are equivocal, and pumpellyite may be detrital. Two thin sections each contain a single grain of detrital blue amphibole. Veins of calcite are abundant, and some inclusions are cut by veins of laumon- tite. Granules and pebbles in graywacke and in inclusions of conglomerate consist principally of sandstone, varicolored chert, and a variety of aphanitic and porphyritic volcanic rocks. Conglomerate exposed in the sea cliff adjacent to the pier at San Simeon contains rare clasts of Franciscan-like red, veined radiolarian chert, pumpellyite- bearing greenstone, and serpentinite, and B. M. Page found two clasts of blueschist in a small body of conglomerate near Cayucos (Cowan and Page, 1975). Hsii (1969) re- ported that Franciscan clasts occur in other Figure 4. Elongate, necked inclusion of glaucophane + lawsonite + quartz + white mica blueschist conglomerates in the area that I did not enclosed in strongly foliated argillite. Inclusion is oriented parallel to and has been extended within S,. Recrystallization foliation in blueschist was considerably disrupted by mesoscopic and microscopic study in detail. catadasis. Surrounding argillite contains inclusions of nonfoliated graywacke (± pumpellyite), Greenstone is a general category that in- greenstone, and chert.

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siliceous argillite. Radiolaria from a bedded slab of red-brown chert overlain by pillow lava 1.7 km southeast of Villa Creek (Hall, 1974) are of Late Jurassic age (E. A. Pes- sagno, Jr., 1977, personal commun.). Less than 1% of the fragments in mélange are foliated metamorphic rocks. Nearly all of these are fine- to medium- grained mafic blueschist containing glaucophane + lawsonite + quartz + white Figure 5. Sketches of inclusion of non- mica + sphene ± chlorite ± epidote. Min- foliated albite + chlorite greenstone exhumed from matrix of argillite. Inclu- eralogically identical blueschist is wide- sion has been deformed into an imperfect spread in the Franciscan Complex (for oblate ellipsoid by tectonic flattening. A. example, Coleman and Lee, 1963; Ernst, Top view, illustrating roughly circular 1965) and probably represents metabasalt shape in section normal to direction of recrystallized during relatively high- maximum finite shortening. B. Side view, normal to X-Y plane of maximum finite pressure-low-temperature metamorphism. elongation oriented parallel to S,. Draw- One blueschist block contains layers of ings by Paul Boving. foliated metachert consisting of garnet, sodic amphibole, and white mica set in a matrix of granoblastic quartz. The smallest recognizable blueschist I found was 5 cm in diameter, and the largest was a lens 5 m long. Other metamorphic rock types in- clude a fine-grained, crenulated actinolite schist and a semischistose metagraywacke containing abundant metamorphic chlorite and white mica. The mélange at San Simeon apparently does not contain coarsely crys- talline inclusions or knockers of "Type IV" garnet- or glaucophane-bearing blueschist and eclogite (Coleman and Lee, 1963), which occur locally in the Franciscan east of the San Andreas fault. It is important to emphasize that each fragment of blueschist is completely surrounded by mélange containing a normal complement of nonfoliated and markedly lower-grade inclusions in pelitic matrix.

DEFORMATION Figure 6. Isolated lenticular inclusion of graywacke flattened during D, and subsequently offset Most of the Franciscan rocks at San Si- along shear fracture (arrow) during D2. meon have a pronounced mesoscopic fabric. Many of the inclusions in mélange are few subrounded to irregularly angular in- shape of flattened, oblate ellipsoids. Thus, vaguely to markedly elongate in cross sec- clusions, generally less than 3 cm in diame- they appear lenticular in any outcrop or tion and are oriented parallel to one ter, are typically interspersed among larger section oriented approximately normally to another. The matrix of argillite is typically and smaller phacoids (Fig. 3). In cross sec- the plane containing their intermediate scaly and slickensided. Both inclusions and tion, many individual inclusions could best axes. I exhumed several isolated inclusions matrix locally have been offset along dis- be characterized as irregularly elongate of graywacke and greenstone about 10 to crete fractures. I infer that these rocks have rather than ideally lenticular, and some, 20 cm in diameter to determine their experienced two deformations that have even of blueschist, display pinch-and-swell geometry. They were roughtly circular in substantially, but not completely, modified structure and, more rarely, extreme necking plan, but their surfaces were gently scal- the record of an even earlier history (dis- and even boudinage (Figs. 2, 4). It is my im- loped and were plastered with a thin, discussed more fully below). pression that, on the average, inclusions of continuous rind of polished argillite. They greenstone are more strongly elongate than tapered to sharp or slightly rounded distal S, Foliation inclusions of graywacke. However, inclu- edges at their equators (Fig. 5). sions of every rock type are represented The strong preferred orientation of the The inclusions in any given outcrop of within the spectrum of shapes. In three di- lenticular inclusions defines a foliation that mélange are variously shaped, but distinctly mensions, the inclusions are not linear; is statistically parallel to a foliation in the lenticular phacoids predominate (Fig. 2). A rather, they approximate the geometric accompanying argillite defined by penetra-

+ POLES TO S0 IN BEDDED INCLUSIONS

X POLES TO S0 IN CAMBRIA PINES SLAB

• D2 LINEAR STRUCTURES Figure 7. Lower-hemisphere equal-area stereographic projections of structural data. A. Ninety poles to S„ measured from San Simeon to San Simeon Creek and from Villa Creek to Cayucos. Contoured using method of Kamb (1959) in intervals of 4 a", counting area = 0.91 ; E (number of points expected to fall within counting area for random distribution) = 8.18; concentrations of points >3 a are interpreted as nonrandom. B. Poles to S0 in bedded inclusions northwest of San Simeon Creek and in "Cambria Pines" sandstone between San Simeon and Santa Rosa Creeks; 16 a point maximum of S, from diagram A shown for reference. Linear structures formed by intersecting D2 shear fractures were measured between San Simeon and San Simeon Creek. Solid line is trace of S, normal to point maximum. C. Traces of Cenozoic high-angle faults, San Simeon to San Simeon Creek.

tive, subparallel, anastomosing surfaces of discontinuity that impart a distinctive scaly appearance to weathered outcrops (Figs. 4, 6). Fresh samples and exposures are more cohesive but still tend to cleave readily along the foliation. These parallel foliations together define Sj. Domains larger than a single inclusion are statistically homogene-

ous with respect to St, and the strong preferred orientation of S, throughout the study area is obvious from Figure 7. In smaller domains, an important characteristic of Sj is that it tends to conform closely to the outer surfaces of adjacent inclusions (Fig. 2). It departs from its statistical preferred orientation as it diverges at the distal edges of phacoids and locally flows into concave re-entrants. Even where individual s surfaces are not visible in argillite, small lenticular inclusions have their long dimen- sions faithfully oriented parallel to surfaces of nearby larger inclusions. On the basis of these relations in individual outcrops, it is probable that even the largest inclusions at Figure 8. Thin-bedded sandstone and mudstone in bedded inclusion. Sandstone displays pinch- San Simeon are lenticular and are oriented and-swell structure, necking, and boudinage. Extension is parallel to both bedding and foliation in parallel to Si on a macroscopic scale. mudstone. Si is horizontal in photo. Extended bed at top of photo has been offset by opposing shear fractures recording D2. Faint laminations in inclusions of massive graywacke and bedding (S„) in inclusions of nounced pinch-and-swell structure and, stone. In each case, sandstone beds have

interbedded graywacke and mudstone are rarely, boudinage (Fig. 8); some also dis- been extended parallel to S0 and, apparent- apparently grossly parallel to Sj (Fig. 8). play calcite-filled extension fractures ly, to S,. The only structures generally visible within oriented normal to bedding. Boudins are I refer to the deformation responsible for massive beds are randomly oriented veins of generally lenticular and have not rotated foliation S, as D„ since this earlier fabric as calcite and quartz. However, sandstone appreciably. The development of these affected by less well-developed structures beds in thin-bedded sequences comprising structures was clearly favored in thin- to that record a subsequent deformation, D2. at least 20% mudstone typically show pro- medium-bedded inclusions rich in mud- Kinematic Interpretation of S,. The

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fabric of the mélange reflects both tectonic has occurred in all directions in bi, since distributed intergranular movements, and, flattening during deformation and the some inclusions display pinch-and-swell to a lesser extent, by localized shear and ex- influence of the pronounced contrast in structure in all exposures oriented approx- tension fracturing, perhaps suggesting that ductility between the inclusions and the imately normal to the foliation. Ramsay the sandstones contained some interstitial more ductile, relatively homogeneous argil- (1967) and Sanderson (1974) have noted pore fluid and that total fluid pressure was lite in which they are immersed. Ramsay that such relations, and "double boudin- high and effettive confining pressure was (1967), Gay and Jaeger (1975a, 1975b), age" in general, imply flattening-type de- correspondingly low, during D, (Handin and others have discussed the deforma- formation with k < 1; all directions in the and others, 1963). This inferred environ- tional behavior of mechanically analogous plane of flattening will have undergone ment of high fluid pressure is compatible materials consisting of relatively competent finite elongation. Finally, extension frac- with the observations that veins of calcite or rigid particles in a ductile matrix. Be- tures, pinch-and-swell structure (Fig. 8), are abundant and that nonporous rocks cause of the contrasts in ductility between and boudinage occur in thin sandstone beds have selectively experienced widespread the inclusions and matrix and among the in mudstone-rich bedded inclusions. These cataclasis. Macroscopically ductile be- inclusions themselves, and since the shapes beds have been extended parallel to layer- havior of argillite is best illustrated where it and orientations of the inclusions prior to ing, and the bedded inclusions have experi- has flowed around the edges and into the deformation are unknown, it is impractical enced the same deformation as the sur- necked regions of flattened and extended if not impossible to use the deformed inclu- rounding mélange. inclusions. This behavior resembles the pat- sions to determine exactly the finite tectonic The inclusions in mélange were deformed terns of flow of matrix in natural and ex- strain of the mélange as a whole. However, in response to large-scale plastic flow in the perimental examples of pinch-and-swell the shapes of individual inclusions can be relatively more ductile matrix of argillite. structure and boudinage described by used to infer qualitatively the kind of total The foliation in argillite is statistically Ramberg (1955) and Ramsay (1967). finite strain that each has experienced dur- parallel to the plane of elongation of flat- ing Dj. Many of the inclusions are imperfect tened inclusions and, hence, formed normal D2 Deformation oblate ellipsoids (Fig. 5). In terms of the pa- to the direction of maximum finite shorten- rameter k of Flinn (1962), where k = (a ing. Although Si is not a true slaty cleavage, Overall ductile behavior during D, was — 1 )l(b — 1), and a is the ratio of the major defined by the microscopic preferred orien- succeeded by the development of shear frac- and intermediate axes, and b is the ratio of tation of fine-grained metamorphically re- tures during a subsequent brittle deforma- the intermediate and minor axes of a strain crystallized phyllosilicates, it may have a tion, D2. The D2 fractures are developed ellipsoid, these inclusions have 1 > k > 0, similar kinematic significance. only locally, and they clearly cut across S,. and the deformation could be characterized Many examples of pinch-and-swell struc- They are most easily recognized where they as flattening. The strong preferred orienta- ture and boudinage discussed in the litera- transect flattened inclusions and beds of tion of these ellipsoids and the apparent ture, such as those illustrated by Ramberg sandstone (Figs. 6, 8), and are indistinct in absence of any discernible lineation in Si (1955) and Ramsay (1967), occur in argillite except where they have been sub- suggests that they have been shortened nor- medium- to high-grade metamorphic rocks sequently filled with calcite. These fractures mal to the foliation and extended parallel that had a high mean ductility during de- are not as significant as the structures to it. "Perfect" oblate ellipoids have ex- formation and recrystallization due to high formed during D,, because they are not perienced axially symmetrical deformation. temperatures and confining pressures. In penetrative on a mesoscopic scale and total The range in shapes and axial ratios of contrast, D, took place at relatively low displacements along them are compara- inclusions reflects in part their different temperatures and effective confining pres- tively minor. Offsets generally range from a compositions and ductilities. sures, and mechanical processes of flow, few millimetres to several centimetres. The Numerous irregularly shaped inclusions rather than synkinematic recrystallization, fractures occur sporadically in both displaying pinch-and-swell structure and, predominated in the widespread matrix of mélange and associated bedded inclusions. more rarely, boudinage support the in- mudstone. The deformation of the mélange A kinematic analysis of the shear frac- terpretation that tectonic flattening has oc- is an excellent example of macroscopically tures is hampered by the fact that their curred during Dj. Experimental and ductile behavior. At the scale of outcrops, geometry was strongly influenced by the theoretical studies of these phenomena by the rocks were deformed permanently and pronounced mechanical anisotropy im- Ramberg (1955), Ramsay (1967), Sander- plastically without any wholesale rupture parted by Si. In mélange, they are generally son (1974), Gay and Jaeger (1975b), and or loss of cohesion. In hand specimens and subparallel to the foliation in argillite, and others have shown that they also result thin sections, however, it is clear that most they tend to be localized at the margins of from the extension of relatively competent inclusions accommodated their changes in lenticular and lozenge-shaped inclusions. In materials surrounded by a more ductile shape by cataclastic flow on mesoscopic general, more than one set of fractures matrix. The development of pinch-and- and microscopic scales. In particular, in- occur in an outcrop, but their poor de- swell structures, rather than boudinage, is clusions of greenstone, chert, and foliated velopment and variable orientation pre- apparently favored if the contrast in blueschist are partly or wholly brecciated, clude any detailed, comprehensive analysis ductility between the inclusions or layers and this deformation may have obliterated of their geometry. However, it was my im- and matrix is relatively small. At San Sim- pre-existing features such as pillows in pression that in many outcrops the fractures eon, three features of the inclusions display- metabasalt. In contrast, most inclusions of occur in roughly conjugate sets with acute ing these structures are especially sig- graywacke generally show little evidence of angles of intersection of 60° or less that con- nificant. First, the inclusions have been ex- cataclasis. In some thin sections, mechani- tain Si. The lines formed by the intersec-

tended parallel to Sl5 as defined by the pre- cal granulation is restricted to one or two tions of several pairs of well-developed frac- ferred orientation of flattened ellipsoids and narrow zones averaging less than 1 mm in tures lie along a great circle that is nearly, the foliation in argillite, and shortened width. Macroscopic changes in shape ap- but not exactly, coincident with the great normal to it. Second, extension apparently parently were accommodated principally by circle representing the trace of the plane

normal to the statistical maximum of poles straint on models for the origin of the at San Simeon, or, to my knowledge, any- to S, (Fig. 7). Displacements along most mélange. Both the recrystallization fabrics where west of the Sur-Nacimiento fault fractures were geometrically similar to and metamorphic mineral assemblages of zone, even though small inclusions, analo- those illustrated in Figures 6 and 8 con- these foliated inclusions prove that they gous to those at San Simeon, are wide- sequently resulted in additional net exten- were metamorphosed in an environment far spread (Hall, 1974, 1976). Since the sion subparallel to S0 and S,. removed from their present one and were blueschists were metamorphosed under

Although D2 has only slightly modified mixed with lower-grade nonfoliated gray- substantially higher pressures and tempera- the strong fabric developed during Dl5 one wacke, greenstone, and chert and a pelitic tures than the associated lower-grade in- can visualize from Figure 8, for example, matrix sometime after metamorphism. A clusions, one is left with hypothesizing that that an increase in both the intensity of frac- similar conclusion applies to blueschist- the blueschists were tectonically eroded turing and the amount of offset along frac- bearing chaotic terranes elsewhere in the from a completely cryptic, high-grade fer- tures in bedded rocks could promote the Franciscan Complex (Hsii, 1968; Cowan, rane at depth, but the kinematic difficulties progressive disruption of strafai continuity 1974). At San Simeon, nearly every inclu- of selectively transporting small fragments and result in isolated lozenge-shaped inclusion of blueschist has been flattened normal of blueschist individually upward for sev- sions, or phacoids, of graywacke in a mat- to S, and extended within it (Fig. 4), and, in eral kilometres, from their sites of rix of originally interbedded mudstone. this regard, blueschist is entirely analogous metamorphism through and into a sea of Such a process, described by Rast (1956), to more voluminous lower-grade inclu- lower-grade rocks, seem almost overwhelm- did not contribute significantly to the total sions. S! records an episode of tectonic flat- ing. It is also doubtful that fragmentation fabric of the rocks at San Simeon. tening, D,, which was not in itself respon- and mixing resulted from the isoclinal fold- sible for the dispersal of the inclusions ing and complete transposition of an inter- Cenozoic Faults throughout the matrix. Thus, the inclusions bedded sequence of mudstone, sandstone, of blueschist must have been emplaced into greenstone, and chert. I found no floating Hall (1974, 1975, 1976) mapped and de- approximately their present positions rela- hinges or other remnants of folds that scribed several Cenozoic and Quaternary tive to other, nearby inclusions, prior to Dj. would likely have formed during such a de- high-angle faults in the vicinity of San Sim- In fact, if the effects of Dj (and D2) are men- formational event, which, in any case, could eon, Cambria, and Cayucos. The chaotic tally subtracted, strongly flattened mélange not in itself have accounted for the rocks in the study area lie east of the right- would originally have resembled some of emplacement of metamorphically exotic slip San Simeon fault zone, but I encoun- the least deformed mélange, exposed very blueschist. tered several high-angle faults with un- locally at San Simeon, in which inclusions In contrast to these enigmatic tectonic known, but probably very minor, displace- of graywacke, greenstone, and blueschist processes, sedimentary processes, including ments exposed in the sea cliffs. Most are are decidedly nonlenticular in shape but still submarine sliding and downslope transport sharp and well defined and contain little or chaotically disposed in a matrix of argillite of turbid, mud-rich debris flows, can readily no gouge. They are vertical or dip steeply to (Fig. 3). Therefore, I conclude that Dl5 the accomplish small- and large-scale mixing the southwest, in contrast to structures earliest in situ deformation for which I and result in internally chaotic, nonbedded found any evidence in the fabric of the formed during D, and D2 (Fig. 7). The con- deposits of diamictite. I favor the hypoth- tact of the Cambria slab and mélange ex- rocks at San Simeon, was responsible for esis, permitted but not demanded by the posed north of the mouth of San Simeon neither the original heterogenity nor the fabric data discussed here, that the mélange Creek is probably the Cambria fault (Hall, chaotic fabric of the mélange, and that the at San Simeon is essentially a pebble- and mixing of blueschist with lower-grade in- 1974,1975,1976). Attitudes of both St and boulder-bearing mudstone, or diamictite, bedding are erratic in highly sheared rock clusions to yield a nonbedded assemblage of that was deposited as one or more olistos- within 20 to 30 m of the fault, but little variously sized clasts in a matrix of tromes; the widely dispersed inclusions of mixing of the two lithologic units has oc- mudstone must have occurred prior to Dt. blueschist are fragments of a metamorphic curred within the fault zone. S, has also The various models that have been pro- terrane which were mixed, transported, and been disrupted by intense shearing approx- posed to account for the origin of chaotic deposited with clasts of sandstone, green- imately 600 m southeast of Little Pico blueschist-bearing terranes in the Francis- stone, and chert that were derived, at least Creek and 600 m northwest of Pico Creek, can essentially involve either tectonic or in part, from a Franciscan source. The im- but I was unable to ascertain whether sedimentary processes (Hsii, 1968, 1973, plications of these events are discussed be- Cenozoic faulting or a local intensification 1974; Cowan, 1974; Blake and Jones, low. The predeformational fabric of the of D2 was responsible for this deformation. 1974; Maxwell, 1974). The tectonic models mélange, and, in particular, the shapes, ar- postulate that both the lithologic heter- rangement, and dispersal of the clasts them- DISCUSSION ogeneity and chaotic fabric of mélanges re- selves, correspond well to the criteria for sult from wholesale fragmentation of pre- olistostromes categorized and illustrated by Origin of the Mélange existing terranes, which are represented as Hsii (1968, 1974) and Abbate and others inclusions in a thoroughly milled matrix. A (1970). Two fundamental questions regarding mélange of this type in effect would have Other sedimentary attributes of the the genesis of chaotic rocks at San Simeon, formed in a major fault zone. Although this diamictite, such as the geometry and thick- and similar terranes elsewhere, are (1) how process could arguably have produced the ness of individual stratigraphic units, are did the lithologic heterogeneity of the pre-Dj chaotic fabric of the mélange at San very difficult to identify through the screen mélange originate, and (2) what was re- Simeon, there is no compelling independent of subsequent deformation. Although it sponsible for the nonbedded, chaotic fab- evidence that the metamorphically exotic cannot be unequivocally demonstrated that ric? The inclusions of blueschist, which are blueschists were mixed with lower grade the diamictite is in depositional contact as significant tectonically as they are insig- rocks by such a mechanism. There are no with bedded rocks or that it forms strati- nificant volumetrically, are a useful con- suitable high-grade source terranes exposed graphic units constituting part of a con-

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formable sedimentary succession, some or were strongly deformed after deposition as chaotic rocks at San Simeon is appropriate, all of the bedded inclusions of sandstone they also became incorporated into the in view of their historical significance. Hsu and mudstone, which range in thickness younger parts of the Franciscan subduction (1963 p. 1065), very explicitly defined from about 1 m to at least 200 m, may be complex. In this way, previously accreted mélanges as "mappable bodies of deformed remnants of stratified rocks that were origi- Franciscan material, including older, high- rocks characterized by the inclusion of nally interbedded with olistostromes. The grade blueschist, was recycled as subduc- tectonically mixed fragments or blocks . . . attitude of bedding is fairly constant tion continued. Cowan and Page (1975) in- in a pervasively sheared, fine-grained, and throughout the study area (Fig. 7), and the ferred a similar sequence of events to ex- commonly pelitic matrix." It is clear that contacts of at least two inclusions grossly plain the Las Tablas conglomerate-breccia Hsii ¡1968,1969) originally considered that parallel both internal bedding and S, in ad- exposed 20 km east of San Simeon. The Las tectonic mixing and fragmentation were re- jacent mélange. It is tempting to label such Tablas unit also contains reworked Francis- sponsible for both the chaotic fabric of the sharp contacts as depositional, even though can debris and is unequivocally of sedimen- diamictite at San Simeon and the emplace- there may have been movement along them tary origin. The types of clasts in the ment of exotic inclusions of blueschist into

during Dt. On the other hand, the bedded conglomerate-breccia and diamictite are a lower-grade host. It is rather ironic that inclusions could be olistoliths. In any case similar, but the Las Tablas unit has a matrix the "type" Franciscan mélange is actually they show no internal evidence of having of sandstone, and the clasts were con- not a mélange at all, according to Hsu's experienced deformations other than D[ sequently not strongly flattened as they definition, if the sequence of sedimentary

and D2. were at San Simeon. Cowan and Page and deformational events outlined in this Although the exact depositional envi- (1975) suggested that the source of the paper is correa. The mélanges at San Sim- ronment of the olistostromes at San Simeon Franciscan detritus at Las Tablas was eon are best described as deformed olisto- cannot be specified at present, they are lithologically heterogeneous and possibly stromes. Chaotic terranes elsewhere in the clearly part of the Franciscan Complex, chaotic. I now slightly modify this hypoth- Franciscan Complex have probably origi- which accumulated in and near an oceanic esis and suggest that the clasts of blueschist nated by a variety of processes, including trench in an active subduction zone. Study at San Simeon and Las Tablas could have the deformation of olistostromes containing of modern trenches with reflection seismol- been derived from sheetlike units of high- dispersed and intimately mixed recycled ogy has revealed that sediments are depos- grade metamorphic rocks, similar to those material, as hypothesized in this paper. ited not only in the trench itself but also in mapped by Suppe (1973), myself (Cowan, Some might well be characterized as numerous trench-slope basins perched on 1974), and Piatt (1975), that were mélanges sensu Hsii, or tectonic mélanges, the topographically complex inner wall of tectonically elevated during subduction in if their composition and fabric can be the trench. These sediments subsequently the arcward part of the evolving Franciscan shown to have originated largely by become deformed and accreted to the inner accretionary wedge (Silling and Cowan, tectonic mixing. wall as subduction proceeds (Karig and 1977). Sharman, 1975; Moore and Karig, 1976). SUMMARY Seismic data have neither proven nor pre- Mélanges in the Franciscan Complex cluded the existence of chaotic olisto- The chaotic rocks at San Simeon owe stromes in trenches and trench-slope basins, Hsu (1967, 1968, 1969) referred to the their lithologie heterogeneity and distinctive but the general tectonic setting of these en- blueschist-bearing chaotic rocks in the mesoscopic fabric to a combination of vironments is ideal for the generation of vicinity of San Simeon as "mélanges." His sedimentary and tectonic processes accom- submarine debris flows in view of their ir- outstanding contribution to Franciscan panying subduction. Diverse rocks, includ- regular topography and frequent earth- geology was the recognition that some ing high-grade foliated blueschist and quakes. Suppe (1973), Maxwell (1974), Franciscan terranes are characterized by es- lower-grade nonfoliated graywacke, Kleist (1974), Blake and Jones (1974), sentially chaotic fabrics and by the partial greenstone, and chert, were mixed together Gucwa (1975), and Crawford (1975) have to complete disruption of sfratai continuity. in submarine debris flows and deposited in described olistostromes and pebbly and The concept of mélange has subsequently or near a trench as olistostromes with cha- bouldery mudstones elsewhere in the Fran- been applied to other parts of the Francis- otic primary fabrics and mud-rich matrices.

ciscan, but it is not yet known how abun- can. With the realization that the Francis- A subsequent deformation, D1; produced a dant they are in the complex as a whole. can Complex formed in an ancient subduc- strong foliation defined by both tectonically tion zone, mélanges acquired a broader flattened inclusions and penetrative cleav- Recycling during Subduction significance, and many geologists have age in the matrix of argillite. The tectonic come to regard them as characteristic and fabric, which developed in the olistostromes The sequence of sedimentary and tectonic even diagnostic features of subduction-zone as they were subducted, reflects the super- events recorded at San Simeon is complex complexes. It should be emphasized, how- position of a specific type of finite strain on but specific. Many of the clasts in the ever, that enough uncertainty has arisen mechanically heterogeneous argillite-rich diamictite, and especially those of blue- concerning the definition and properties of rocks that comprise relatively rigid inclu- schist, red radiolarian chert, and mélanges to warrant some caution in the sions enclosed in a more ductile matrix. pumpellyite-bearing graywacke and application of the concept unless the history In a broader sense, the sedimentary and greenstone, were derived from Franciscan and tectonic setting of the rocks in question tectonic events hypothesized for the rocks that were exposed to submarine or are known in some detail. mélange at San Simeon reveal something possibly subaerial erosion, very likely on This paper is not the proper forum for a about the complex process of subduction the inner wall of a trench. The olistostromes comprehensive review of mélanges, but a that ultimately was responsible for the ac- containing reworked Franciscan detritus brief discussion of the nomenclature of the cumulation of the Franciscan as a whole.

The geologic evidence at San Simeon and Franciscan metamorphic rocks, Panoche and ancient geosynclinal sedimentation: Las Tablas supports the inference that the Pass, California: Geol. Soc. America Bull., Soc. Econ. Paleontologists and Miner- inner walls of trenches are dynamic systems v. 76, p. 879-914. alogists Spec. Pub. 19, p. 321-333. Flinn, D., 1962, On folding during three- Jennings, C. W., compiler, 1958, Geologic map that experience significant internal defor- dimensional progressive deformation: Geol. of California, San Luis Obispo sheet: mation even as new oceanic and terrigenous Soc. London Quart. Jour., v. 118, p. 385- California Div. Mines and Geology, scale materials are accreted and deformed. 433. 1:250,000. Gay, N. C., and Jaeger, J. C., 1975a, Cataclastic Kamb, W. B., 1959, Ice petrofabric observations deformation of geological materials in ma- from Blue Glacier, Washington, in relation ACKNOWLEDGMENTS trices of differing composition: I. Pebbles to theory and experiment: Jour. Geophys. and conglomerates: Tectonophysics, v. 27, Research, v. 64, p. 1891-1909. Many of the observations and interpre- p. 303-322. Karig, D. E., and Sharman, G. F., 1975, Subduc- tations in this paper were first developed on 1975b, Cataclastic deformation of geologi- tion and accretion in trenches: Geol. Soc. a field trip to San Simeon in March 1975 cal materials in matrices of differing com- America Bull., v. 86, p. 377-389. position: II. Boudinage: Tectonophysics, Kleist, J. R., 1974, Deformation by soft-sediment with B.W.D. Yardley and several students v. 27, p. 323-331. extension in the coastal belt, Franciscan from the University of Washington. I ap- Gucwa, P. R., 1975, Middle to Late Cretaceous Complex: Geology, v. 2, p. 501-504. preciate many hours of discussion on the sedimentary mélange, Franciscan Complex, Maxwell, J. C., 1974, Anatomy of an orogen: general problem of the origin of chaotic northern California: Geology, v. 3, Geol. Soc. America Bull., v. 85, p. 1195- rocks with G. A. Davis, B. M. Page, R. A. p. 105-108. 1204. Hall, C. A., 1974, Geologic map of the Cambria Moore, G. F., and Karig, D. E., 1976, Develop- Schweickert, B.W.D. Yardley, and several region, San Luis Obispo County, Califor- ment of sedimentary basins on the lower others. I especially thank Paul Boving for nia: U.S. Geol. Survey Misc. Field Studies trench slope: Geology, v. 4, p. 693-697. his contribution, and I am indebted to Page Map MF-599. Page, B. M., 1972, Oceanic crust and mantle and Yardley for critical reviews of the man- 1975, San Simeon-Hosgri fault system, fragment in subduction complex near San uscript. coastal California: Economic and environ- Luis Obispo, California: Geol. Soc. mental implications: Science, v. 190, America Bull., v. 83, p. 957-972. p. 1291-1294. Piatt, J. 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