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Deformation and Metamorphism of the Franciscan Subduction Zone Complex Northwest of Pacheco Pass, California

DARREL S. COWAN* Shell Oil Company, P.O. Box 527, Houston, Texas 77001

ABSTRACT juxtapose rock units that bear no apparent tion and metamorphism, along a major stratigraphie, deformational, or metamor- fault system of regional extent that is an The Franciscan Complex northwest of phic relation to one another. The structural important crustal expression of lithospheric Pacheco Pass, California, includes three units were separately deformed ' and consumption at depth. fault-bounded units, each characterized by metamorphosed under a variety of condi- Miyashiro (1961) suggested that the a different deformational style and suite of tions prior to their tectonic juxtaposition Franciscan and grossly parallel Sierra metamorphic mineral assemblages. Struc- during late Mesozoic continental margin Nevada batholith and related high- turally highest is jadeitic pyroxene-bearing subduction. Field and pétrographie evi- temperature, low-pressure metamorphic metagraywacke semischist. The areally ex- dence permit, but do not prove, the rocks are part of a series of circum-Pacific tensive Garzas tectonic mélange separates hypothesis that both the semischist and paired metamorphic belts. As such, they are the semischist from the structurally lowest exotic, high-grade mélange inclusions once in essence late Mesozoic analogs of rock as- pumpellyite-bearing Orestimba meta- were more deeply buried and have been semblages forming in active subduction graywacke. The Garzas mélange is emplaced upward into their present zones and associated magmatic arcs, re- representative of Franciscan mélanges in anomalously shallow structural positions. spectively (Takeuchi and Uyeda, 1965). general. These mappable bodies have an in- Key words: structural geology, Franciscan, When viewed in the context of plate tec- ternal fabric dominated by penetrative, blueschist metamorphism, tectonic tonic theory and paired metamorphic belts, mesoscopic shear fractures and contain tec- mélange, subduction, California. the Franciscan is hardly a unique assem- tonic inclusions of all sizes immersed in a blage, nor is it a particularly anomalous pervasively sheared, generally fine-grained INTRODUCTION one. It offers an excellent opportunity to matrix. The shear fractures record brittle The Franciscan Complex, a heterogene- learn about the metamorphic and deforma- deformation of consolidated rock bodies. ous assemblage of metagraywacke, chert, tional history of subduction zone assem- Many mélanges contain exotic inclusions, mafic volcanic rocks, and minor isolated blages and to further our understanding of clearly not derived from adjacent units, and blocks and slabs of high-grade blueschist- how plate interactions are expressed in the metamorphic mineral assemblages and tex- and amphibolite-facies metamorphic rocks, geologic record. tures indicate that they were metamor- is widely exposed in the California Coast The purpose of this paper is to sum- phosed before being tectonically mixed Ranges and parts of southwestern Oregon marize structural and petrologic data from with more voluminous, generally lower and Baja California. The complex is cur- a representative Franciscan terrane in the grade, metagraywacke inclusions. rently interpreted as having accumulated in Diablo Range northwest of Pacheco Pass, The structural units are grossly sheetlike a late Mesozoic-early Tertiary subduction California, and to emphasize certain as- in external form and are separated from zone at the western margin of the North pects of the tectonic framework of the study one another by gently to steeply dipping American plate (for example, Hamilton, area that may characterize the Franciscan, major faults. Unlike low-angle thrusts in 1969; Bailey and Blake, 1969; Ernst, 1970; and perhaps some other subduction zone imbricated Cordilleran terranes, the faults and many others). Franciscan "trench" complexes, in general. A more detailed pre- do not systematically repeat or offset a materials were carried against and beneath sentation of lithologic and petrographic normal stratigraphie sequence but rather the coeval Great Valley sequence, a thick data is in Cowan (1972), and the reader is succession of clastic rocks of Tithonian to referred to Bailey and others (1964) for a Maestrichtian age that contrasts strikingly * Present address: Department of Geological Sciences, comprehensive summary of Franciscan University of Washington, Seattle, Washington 93195. with the Franciscan in its lesser deforma- geology.

Geological Society of America Bulletin, v. 85, p. 1623-1634, 5 figs., October 1974

1623

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FRANCISCAN COMPLEX Northwest of the Garzas Creek Narrows, both s surfaces locally have been folded on NORTHWEST OF PACK E CO PASS the Tesla-Ortigalita fault is sharp and ;i small scale. Interbedded graywacke and nearly vertical (Fig. 2). Southeast of the phyllite display groups of kinklike folds The core of the northern part of the Di- narrows, an unconsolidated, locally pow- with sharp hinges and amplitudes of a few ablo Range (Fig. 1) consists of the Francis- dery, bluish-gray matrix of thoroughly centimeters; whereas individual graywacke can Complex, flanked by Upper Jurassic to comminuted rock debris containing iso- beds commonly contain isolated shear Upper Cretaceous sedimentary and vol- lated blocks of graywacke, semischist, bands, transverse to the foliation, that canic rocks of the Great Valley sequence greenstone, thinly b:dded red and green define the axial planes of small asymmetric that are lithologically similar to Great Val- chert, and glaucophane-lawsonite schist angular folds with amplitudes that range ley strata elsewhere in the Coast Ranges occupies the fault zone and expands to a from a few millimeters to several centime- (Dickinson and Rich, 1972). Vickery width of 400 m at Richard Creek. The con- ters. Apparently, no secondary axial plane (1924), Huey (1948), Leith (1949), Briggs tact of the shear zone debris with fractured foliation was developed in either case. The (1953), Schilling (1962), Maddock (1964), Great Valley strata is approximately verti- rather limited, nonrepresentative exposures and Raymond (1973a) mapped parts of the cal or dips steeply southwest. The sand- of this unit prevented a systematic analysis northern Diablo Range and together estab- stone, conglomerate, and shale of the Great of the folds, but their existence does not de- lished that the entire boundary of the Fran- Valley sequence were not examined in de- tract from the remarkably consistent at- ciscan mass is a series of steeply dipping tail during this study, but the age of the titude of bedding and foliation, which faults now collectively termed the "Tesla- strata immediately adjacent to the Francis- grossly parallel the lower tectonic contact Ortigalita fault" (Page, 1966; Raymond, can is probably Turonian. Both Maddock cf the unit and seem to preclude the pres- 1973b). (1964) and Schilling (1962) found fossils of ence of any large folds within it. The area mapped during this study is ap- this age in stratigraphically equivalent Microscopically, the foliation that proximately 20 km northwest of Pacheco rocks to the northwest and southeast. characterizes the entire unit is defined by Pass (Fig. 1). The northern part of the area the planar preferred orientation of the slightly overlaps the southern edge of the METAGRAYWACKE SEMISCHIST boundaries of mineral grains and grain Mount Boardman quad mapped by Mad- The structurally highest unit within the aggregates (Fig. 3A). It is clear that the dock (1964). Raymond (1973a, 1973b) Franciscan locally is a semischistose meta- rocks are tectonites with planar fabrics ac- mapped the Mount Oso area to the north, graywacke that ranges in thickness from ap- quired during recrystallization and flow and Cotton (1972) differentiated Francis- proximately 200 m to 600 m. This unit oc- under stress. There is very little evidence for can bedrock units in the western half of the curs immediately adjacent to the sharp simple cataclasis or mechanical granulation range north of California F.oute 152. The trace of the Tesla-Ortigalita fault, or ac- unaccompanied by concurrent recrystalliza- Franciscan Complex at Pacheco Pass has companying shear zone debris, and is sepa- tion. Metamorphic white mica, chlorite, been exhaustively studied by McKee rated from structurally underlying tectonic and lawsonite have grown parallel and (1962a, 1962b), Ernst and Seki (1967), and mélange by a fault that dips moderately to subparallel to the foliation. Recrystallized Ernst and others (1970). Ernst (1971b) steeply eastward. No fossils were found in volcanic and metamorphic lithic fragments later made a petrographic reconnaissance the unit, but whole-rcck K-Ar dates (Table and chert grains in metagraywacke and of Franciscan metagraywacke from the core 1) indicate it is at least: as old as Albian age. conglomerate are markedly flattened and of the northern Diablo Range and deter- The dates also show the unit was attenuated in this plane. Deformed clastic mined the approximate areal distribution of metamorphosed before the deposition of quartz grains, the most abundant detrital metaclastic rocks containing jadeitic pyrox- presently adjacent Great Valley strata. ene. Investigations to date indicate the Bedding in the semischist is generally well Franciscan in the Diablo Range is represen- preserved and becomes only locally dis- tative, both petrologically and structurally, rupted near the Tesla-Ortigalita fault. of the complex in general. Medium- to coarse-grained graywacke pre- The Franciscan in the study area includes dominates in beds that average several cen- three major fault-bounded structural units, timeters, but range up to 2 m, in thickness. each characterized by a distinctive deforma- A single, discontinuous layer of thin- tional style and suite of metamorphic min- bedded, white to pale-green chert several eral assemblages. In descending structural meters thick crops out in Richard Creek. order, the three semitabular units include Transverse, concordant, and ptygmatic (1) metagraywacke semischist, (2) "Gar- veins of quartz and albite are abundant in zas" tectonic mélange, and (3) gently folded metagraywacke. "Orestimba" metagraywacke. The defor- The most striking c.nd distinctive struc- mational styles represented here are not tural feature of the metagraywacke is a foli- unique to the area but probably characterize ation, with an accompanying incipient the complex throughout the Coast Ranges. cleavage, that is exclusively parallel to bed- The Tesla-Ortigalita fault that separates the ding. Although the foliation ranges from Franciscan from Great Valley rocks today is moderate to strong, no systematic zonal probably a steeply dipping to vertical variations in textural reconstitution related modification of the regional, presumably to the faults bounding the unit were ob- low-angle, "Coast Range thrust" along served. Mesoscopical y, original detrital Y//\ GREAT VALLEY SEQUENCE which little-deformed strata of the Great grains and larger clasts in conglomerates, | | FRANCISCAN COMPLEX Valley sequence were originally juxtaposed especially lithic fragments, are noticeably over the complexly deformed Franciscan flattened parallel to the foliation. Although TFFFFI] ULTRAMAFIC ROCKS (Bailey and others, 1970; Raymond, the clastic nature o : the rocks is still obvi- 1973b). Tertiary and Quaternary uplift of ous, grain boundaries are diffuse, and the F MAJOR FAULTS the Franciscan core of the Diablo Range graywackes have a semischistose fabric. was accommodated at least in part by dis- Figure 1. Generalized geologic map of northern Di- Even though bedding and foliation strike ablo Range, California. Simplified from San Jose Sheet, placements along this fault. northwest and dip consistently northeast, Geologic Map of California (Rogers, 1966).

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constituent in the rocks, are the most im- and dimensionally variable blocks of resis- p. 2401; Photo 65 in Bailey and others, portant elements of the tectonite fab::ic. tant rock types enclosed in a pervasively 1964). The strongly sheared fine-grained They display strong undulatory extinction sheared, fine-grained matrix. The blocks matrix in the mélange is semicontinuously and deformation lamellae, and the ends of include both a wide variety of metamorphi- exposed only along major intermittent- tabular grains have polygonized into cally exotic high-grade amphibolite- and stream canyons. fine-grained granoblastic aggregates that in blueschist-facies rocks, as well as lower Because the original strafai continuity of part define the foliation (Fig. 3A). The grade greenstone and chert that typify an metagraywacke in the tectonic mélange has aggregates, which often contain oriented ocean-floor assemblage. Metagraywacke been destroyed, bedding is rarely observed. plates of white mica and chlorite, are well blocks are most abundant by far. Although Bedding is preserved in the interiors of developed in most metagraywacke of the the rock types and degree of shearing vary some large inclusions of metagraywacke, unit, and quartz in even the least recrystal- within the unit, the essential components of but these attitudes are not reliable guides to lized rocks is incipiently polygonized. its chaotic structural geometry are penetra- structural relations, because the inclusions The metaclastic rocks in the unit contain tive mesoscopic shear surfaces that have a are but fragments of larger blocks or layers the metamorphic mineral assemblages tectonic origin (Fig. 4). No further genetic and are not necessarily related to surround- quartz + albite + lawsonite ± jadeitic interpretations are implied for the moment ing inclusions. The most prominent pyroxene ± sodic amphibole ± aragonite + in the term tectoniç mélange. A more com- measurable feature in the mélange is the white mica + chlorite + stilpnomelane + plete discussion of mélanges is deferred to a marked foliation defined by abundant sub- sphene. Albite and lawsonite are present in later section. parallel surfaces of shear and dislocation in each sample studied petrographically, and the matrix. This foliation replaces bedding incipient to well-developed jadeitic pyrox- Mapping Procedure as the most important mesoscopic fabric ene occurs in most, but not all meta- Because of the extreme range in size, element. Systematic measurement of folia- graywacke samples. The pyroxene always shape, and lithology of the resistant frag- tion attitudes throughout the entire coexists with albite; samples with the high- ments and blocks encountered in the Gar- mélange in the area was precluded by the est modal percentages of lawsonite and zas mélange, it is important to carefully limited, nonrepresentative exposures. pyroxene contain some albite grains that select only the most meaningful data for in- Often, the attitudes vary within a single completely lack these metamorphic miner- clusion in the geologic map. During this in- outcrop, and the shear foliation wraps als. Pyroxene prisms have grown within vestigation, the mélange was treated as a around larger inclusions. Attitudes plotted plagioclase grains and apparently do not single structural unit, and with the excep- on the geologic map (Fig. 2) are visual aver- have any preferred orientation parallel to tion of two large, stratigraphically coherent ages. Strikes vary from northeast to north- the foliation, nor have they been granulated blocks of metagraywacke with dimensions west, but the foliations measured in Garzas by postmetamorphic cataclasis. The dis- of several kilometers, tectonic inclusions Creek dip consistently eastward, grossly tribution of the assemblages is shown in were not differentiated from the parallel to the eastward-dipping fault con- Figure 2. Bloxam (1960), McKee (1962a), fine-grained sheared matrix that encloses tacts of the mélange with adjacent units. In Suppe (1969), and Ernst (1965; 1971a, them. On this scale, it was impossible to ac- the field, I placed these contacts at the first 1971b; Ernst and others, 1970) have de- curately map the boundaries of all but the appearance of tectonic inclusions (such as scribed in detail mineralogically similar largest inclusions. Instead, 1 made several greenstone, chert, and high-grade schist) jadeitic pyroxene-bearing metagraywacke reconnaissance traverses through the that were clearly not derived from adjacent from elsewhere in the Franciscan. mélange in an effort to establish gross pat- metagraywacke terranes. The characteristic terns in the distribution of rock types. Some sheared mélange matrix is only rarely ex- GARZAS TECTONIC MÉLANGE of the more resistant inclusions, including posed, and stratal continuity in adjacent Structurally beneath the metagraywacke foliated glaucophane-lawsonite schist and metagraywacke becomes locally disrupted semischist is an areally extensive unit in- albite-chlorite greenstone, form the familiar by similar shearing within a few meters of formally termed the "Garzas tectonic craggy outcrops, averaging several meters the contact with mélange. mélange." The unit displays a distinctive in diameter, that protrude from grassy deformational style characterized by small slopes and are commonly referred to as Matrix and Inclusions angular phacoids and larger geometrically "knockers" (Coleman and Lanphere, 1971, The matrix of the Garzas mélange is predominantly sheared siltstone and mudstone TABLE 1. POTASSIUM-ARGON DATES and is similar to the pelitic matrix of other

Sample no. Date Type date mélanges described by Reed (1957, p. (m.y.) 31-32), Dickinson (1966, p. 462), and Hsu (1968). Weathering commonly emphasizes Metagraywacke Semischist discrete shear planes. The matrix is cohe- 185 110 ± 2 Whole rock sive and contrasts significantly with the 2028 111 ± 2 do. 203A 76 ± 2 do. powdery, completely milled gougelike debris that occupies the Tesla-Ortigalita shear Inclusions from Garzas Tectonic Mélange zone and other high-angle fault zones in the 55 106 ± 2 Whole rock Franciscan. On close inspection, small 291 116 ± 2 do. 64 (coarse) 130 ± 2.6 White mica concentrate; M>%: 5.46, 5.42; 29.15 x elongate lenses of fine-grained graywacke 10'6cm3 radiogenic '"Ar; 16% air correction are interleaved with the sheared pelitic ma- 64 (60-150 mesh) 131 ± 2.6 White mica concentrate; K20%: 6.84, 6.88; 37.22 x trix, and a small part of the dominantly ar- 10"6cm3 radiogenic ''"Ar; 10% air correction Orestimba Metagraywacke gillaceous matrix may have been derived 7 93 ± 2 Whole rock from cataclasis and comminution of larger 8 90 ± 2 do. graywacke inclusions. 79 90 i 3 do. The tectonic inclusions in the mélange in- 249 88 ± 2 do. clude metagraywacke with well-preserved

Note: Dates determined by R. L. Armstrong. See Huppe and Armstrong (1972) for brief pétrographie descriptions clastic textures and a great variety of and complete analytical data for all samples except number 64. Locations of dated samples indicated on Figure 2 metaigneous and schistose metaclastic of this report. rocks and chert. Their heterogeneous dis-

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ALL ASSEMBLAGES INCLUDE: QUARTZ + WHITE MICA + CHLORITE ±

(of,- SPHENE ± STILPNOMELANE ± CALCIUM CARBONATE V w V ZJ , INCLUSIONS IN TECTONIC MÉLANGE (SEE TABLE 2 FOR METAMORPHIC MINERAL ASSEMBLAGES) METAGRAYWACKES WITH ISOTROPIC DETRITAL TEXTURES ARE NOT INCLUDED. 315• V zf ^ #224 / V _ ^ * »223 A A METACLASTIC ROCKS V Wì i »CA'' • METACHERTS

% y a METAVOLCANIC AND OTHER METAIGNEOUS ROCKS • f — Aio • ______7 wrnËmmim'¿a I w^mmm M§ • PROTOLITH UNKNOWN m e 6; - I g^ m w-»» •• i à V ' 'M , r, #r ¡ • ' ' ' .. •• .'/ ; ; / C UNMAPPED ---/ / 247 ANALYZED AND/OR DATED SAMPLES; SEE TABLES 1 AND 3 ^ Fr -/' ¿ fa 'Mir- GEOLOGIC MAP AND CROSS SECTION OF FRANCISCAN COMPLEX iiiii' NORTHWEST OF PACHECO PASS, CALIFORNIA GEOLOGY BY D.S. COWAN \ 1969-1971 •

CROSS SECTION (ALL STRUCTURES DIAGRAMMATIC) FIGURE 2

Figure 2. Geologic map and cross section of Franciscan Complex northwest of Pacheco Pass, California. COWAN, FIGURE 2 Geological Society of America Bulletin, v. 85, no. 10

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tribution is immediately apparent in Figure variety of metamorphic mineral assem- inclusions. Metamorphic calcium carbon- 2, which displays only the locations of blages with markedly heterogeneous dis- ate is present in several samples; aragonite samples collected on traverses. Hsii (1968, tribution (Fig. 2): was positively identified only in assem- p. 1065), followed by Berkland and others blages containing lawsonite ± pumpellyite (1972), classified inclusions in mélanges as albite + quartz + and was often partly recrystallized to cal- native and exotic-, his native inclusions are albite + pumpellyite chlorite + cite. Because of the rapid postmetamorphic fragments of rocks originally interbedded albite + pumpellyite + hwsonite white mica ± i nversion of aragonite to calcite (Brown and with fine-grained pelitic material that now albite + lawsonite stilpnomelane others, 1962), it is probably impossible to forms mélange matrix, and exotic inclu- determine petrographically whether calcite sions are foreign to this now-disrupted ter- Sodic amphibole was not identified in any coexisted stably with any or all of these as- rane. In practice, however, it is often difficult to decide which rock types now present in a tectonic mélange were originally interbedded in a stratigraphically coherent rock-stratigraphic precursor; chert and greenstone are particularly troublesome examples. In this paper, I will restrict the term "exotic" to blocks of any protolith that were metamorphosed prior to their associa- tion with the sandstones and mudstones that form the bulk of the mélange.

Metagraywacke Inclusions The most abundant inclusions in the Garzas mélange are metagraywacke. Al- though recrystallized, they have well- preserved, essentially isotropic, clastic fabrics. The metagraywackes are quite variable petrologically; detrital modes of several samples, selected to illustrate the variabil- ity, are shown in Figure 5. No two samples are from, a single continuous inclusion. Only one inclusion was found that con- tained interbedded conglomerate. The inclusions range in size from a few millime- A. ters to approximately 1 km in their longest 0.1 MM dimension; the largest inclusion, the Mus- tang Peak metagraywacke slab, is at least 4 km long and is discussed separately below. Fragments averaging a few centimeters or less in diameter are irregularly rounded to elongate and are scattered through a strongly sheared fine-grained matrix. Inclu- sions several meters or less in length are most commonly elongate angular phacoids with their longest dimensions approximately parallel to the shear-plane foliation; this particular geometry is most characteristic of tectonic mélanges (Fig. 4; figures in Hsii, 1968, 1969; Dickinson, 1971b). Most of the largest inclusions have indeterminate shapes but are probably elongate with long axes subparallel to the regional strike of the shear-plane foliation. These large inclusions, which are clearly surrounded by sheared mélange, have sheared margins and are fractured throughout, but bedding is often preserved intact in their interiors. Ap- parently, inclusions of interbedded graywacke and siltstone must be attenuated to a certain critical size by fracturing and B. 0.5 MM fragmentation before stratal continuity be- Figure 3. A. Deformed clastic quartz grains in metagraywacke semischist. Crossed nicols. Large relict grains are comes completely disrupted by penetrative undulose and have grossly lensoid shapes. Ends of grains have polygonized into fine-grained granoblastic aggregates mesoscopic shearing. No fossils have been and contain plates of newly grown white mica and chlorit;. Tabular aggregates, relict grains, and phyllosilicates found in the inclusions. define metamorphic foliation. B. Undeformed isotropic clastic fabric in Orestimba metagraywacke. Crossed nicols. Nearly all grains visible are quartz and detrital plagioclase, now altered to albite. Sample contains abundant The metagraywacke inclusions contain a metamorphic pumpellyite, white mica, and chlorite replacing framework grains.

semblages. Veins of quartz and carbonate graywacke inclusions in the mélange (Fig. sporadically on traverses through the in- are abundant. 5); they contain more volcanic lithic frag- terior of the inclusion. These zones may ments and detrital biotite grains, partly to represent the incipient fragmentation of the Mustang Peak and Zimba completely replaced by chlorite stilp- large inclusion into smaller tectonic blocks. Metagraywacke Slabs nomelane, than Orestimba meta- Contacts of the Mustang Peak meta- The Mustang Peak and Zimba metagraywackes. Both the Mustang Peak and graywacke with surrounding mélange have graywacke slabs, informally named alter Zimba inclusions contain metamorphic been, in most places, only approximately the topographic highs they respectively up- mineral assemblages that characterize other located. The eastern tectonic contact dips hold, are the largest inclusions in the tec- mélange metagraywacke units (Fig. 2); two gently eastward but has been modified by a tonic mélange (Fig. 2). Together they illus- samples from the Zimba inclusion contain nearly vertical fault south of Mustang Can- trate, on a large scale, the geometric rela- minor glaucophane in addition to pumpel- yon. The western contact dips steeply to the tions of metagraywacke inclusions to sur- lyite and lawsonite. southeast. The Mustang Peak inclusion rounding pervasively sheared mélange ma- Bedding is gently folded and generally in- thins progressively to the northeast and can trix and associated smaller inclusions. tact in the interior of the Mustang Peak in- best be visualized as an eastward-dipping, The Mustang Peak inclusion is at least 4 clusion, but within a few meters of its mar- largely coherent, slab of metagraywacke km long and consists entirely of well- gins, stratal continuity becomes progres- embedded in sheared mélange. bedded fine- to medium-grained meta- sively disrupted by penetrative shearing The Zimba metagraywacke is separated graywacke and siltstone. Some sedimentary geometrically identical to that displayed by from underlying mélange by a tectonic con- structures, including small-scale cross- the surrounding mélange. The disruption is tact that dips gently northeastward. The laminations, sole markings, and convolute best exposed on the ridge that culminates in metagraywacke has a weak bedding-plane bedding, are present. The metagraywacke Mustang Peak, and other unmappable foliation produced by compaction of soft units have detrital modes distinct from both zones of sheared metagraywacke and silt- lithic fragments and rotation of tabular de- the Orestimba unit and most other meta- stone less than 10 m wide were encountered trital grains of quartz and plagioclase. Bed-

Figure 4. A. Tectonic mélange consisting predominantly of metagraywacke with small amounts of sheared siltstone. Bedding and stratal continuity have been completely disrupted by penetrative brittle-shear fractures that define foliation oriented approximately parallel to hammer handle. B. Tectonic mélange exposed along Garzas Creek. Lens- shaped inclusions of relatively resistant metagraywacke embedded in cohesive, strongly sheared fine-grained matrix.

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ding becomes disrupted by penetrative sphene; quartz and phengite are sometimes lar metachert has been described by McKee shearing within 1 to 2 m of the contact with present. These are the familiar "blue- (1962a), Coleman and Lee (1963), and mélange; smaller inclusions similar to the schists" that occur as small tectonic blocks Ernst (1965; Ernst and others, 1970). Zimba metagraywacke— and probably de- and larger structural units throughout the Only two small bodies of sheared serpen- rived from it — are locally abundant below Franciscan. Switzer (1951), Brothers tinite, neither of which exceeds 100 m in its this contact. In contrast to the Mustang (1954), Borg (1956), McKee (1962a), Cole- longest dimension, were located in the field, Peak inclusion, the Zimba metagraywacke man and Lee (1963), Bailey and others and each lies on the projected extension of a is essentially a structurally and topographi- (1964), Ernst (1965; Ernst and others, high-angle fault that terminates the meta- cally high klippe. There is no field evidence 1970), and Suppe (1969) have described the graywacke tectonite unit east of Days Pass that it was originally overlain by tectonic petrology and mineralogy of these and (Fig. 2). mélange, and the present structural position higher grade mafic blueschist rocks in de- suggests that the Zimba unit: may once have tail. Even though igneous textures were Discussion of Exotic Blocks been a part of the metagraywacke semi- generally obliterated by recrystallization, Even though rocks of similar petrology schist, even though their mineral assem- the composition of a tectonic block from and mineralogy to the inclusions described blages are different and they are now sepa- locality 64 in the area (Table 3, see footnote here are discussed in detail in the literature rated by tectonic mélange. The strong fab- 1) is similar to mafic blueschist from other cited above, their structural relations to ric of the semischist precluces point count- areas with predominantly spilitic composi- adjacent Franciscan rocks in other areas are ing for comparison with Zimba detrital tions. Some samples have relict fragmentai, locally ambiguous. Some are clearly tec- modes. agglomeratic, and diabasic textures. Two tonic inclusions in serpentinite (Brothers, samples contain pale-green omphacite, 1954), whereas others occur as isolated tec- Other Inclusions chlorite, and incipient glaucophane; similar tonic blocks in linear "shear zones" of un- In addition to the metagraywacke blocks omphacitic metabasalt occurs at Pacheco certain origin and structural geometry and slabs, the mélange contains inclusions Pass (Ernst and others, 1970). (McKee, 1962a; Borg, 1956). Some larger of metavolcanic rocks, semischistose and The highest grade inclusions in the masses are essentially mappable, discrete schistose metaclastic rocks, and metacherts. mélange are generally coarse-grained schist structural units (Coleman and Lee, 1963; A variety of protoliths and metamorphic that contains the characteristic assemblage Suppe, 1969). It is important, therefore, to mineral assemblages are represented. They sodic amphibole + garnet + phengite and, emphasize again the tectonic environment range in size from approximately 10 cm to less commonly, sodic amphibole + epidote of these inclusions in the study area. perhaps 50 m in their longest dimension; and omphacite 4- garnet + glaucophane. Exposures along Garzas Creek show smaller inclusions are often demonstrably Lawsonite is also present in some assem- conclusively that the inclusions, like the elongate with long axes oriented subparal- blages, and chlorite commonly replaces metagraywacke described above, are com- lel to the shear foliation in the surrounding garnet. These minerals and their textures pletely enclosed in a cohesive, pervasively fine-grained matrix. The larger inclusions indicate retrogression, and all originally sheared fine-grained matrix. The matrix, project above grassy hills des as rugged eclogitic samples containing both ompha- and associated metagraywacke, displays the outcrops that are irregularly rounded in cite and garnet also contain glaucophane generally well-developed shear-plane folia- plan; their actual dimensions can only be and lawsonite. One coarse-grained am- estimated. phibolite contains green hornblende + epi- A detailed pétrographie description of dote + quartz; the hornblende is fringed the inclusions sampled in the field is beyond with blue amphibole. These assemblages the scope of this report; their distribution is also characterize the highest grade indicated in Figure 2, and Table 21 lists blueschist- and amphibolite-facies their mineral assemblages and protolith metamorphic rocks found elsewhere in the where determinable. Franciscan; they occur in other parts of the Metavolcanic rocks are most abundant. Franciscan only as isolated, clearly tectonic Similar inclusions are widespread in the blocks often associated with serpentinite Franciscan, and excellent descriptions of and lower grade metabasalt, meta- their petrology and distribution are in graywacke, and chert in tectonic mélanges Coleman and Lanphere (1971), Coleman (Coleman and Lee, 1963; Coleman and and Lee (1963), Ernst (1965), and Ernst Lanphere, 1971). and others (1970). In the study area, Semischistose and schistose metaclastic metavolcanic rocks with the lowest grade inclusions in the mélange contain quartz + mineral assemblages typically contain albite lawsonite + white mica ± albite ± chlorite + chlorite -f- pumpellyite + sphene. Relict ± glaucophane. Chert and metachert inclu- pilotaxitic and porphyritic textures are sions are uncommon. Some are thin- • ORESTIMBA METAGRAYWACKE commonly preserved in these rocks, but un- bedded, white, light green, tan, or red in equivocal pillow structures were observed color, and they contain ovoid recrystallized A MELANGE METAGRAYWACKE INCLUSIONS at only one locality. relict radiolarian tests. These inclusions are Some of the inclusions in the area are identical to the essentially unmetamor- • MUSTANG PEAK METAGRAYWACKE phosed well-bedded chert that is wide- fine-grained foliated schists that contain • ZIMBA METAGRAYWACKE sodic amphibole, lawsonite, chlorite, and spread in the Franciscan Complex (Bailey and others, 1964, p. 55-68). In addition, a Figure 5. Volumetric Q-F-L percentages in meta- few strongly foliatec, lineated, and com- graywacke from Orestimba unit, tectonic mélange, and 'See NAPS document 02420 for four pages of sup- 2imba slab. See Dickinson (1970a) for general discus- plementary material. Order from ASIS/NAPS do pletely recrystallized metachert samples sion of point-counting procedures and philosophy and Microfiche Publications, 305 E. 46th Street, New York, contain small porphyroblasts of garnet, interpretations of detrital sandstone modes. Q = total New York 10017. Remit in advance for each NAPS ac- lepidoblastic sodic amphibole, and locally quartzose grains; F = feldspar (plagioclase) grains; L — cession number $1.50 for microfiche or $7.25 for total unstable lithic grains of volcanic, metamorphic, photocopies. Make checks payable to Microfiche Publi- pale-green phengite plates in a mosaic of and sedimentary origins. 400 framework points cations. medium-grained granoblastic quartz. Simi- (Q+F+L) counted in each sample.

tion that characterizes tectonic mélanges. moderately north, but constituent chert in albite that locally are intergrown with Because the sheared matrix weathers easily, beds are folded, fractured, and brecciated. pale, yellow-green chlorite or with golden it is only rarely observed as polished rem- Thin, discontinuous pods of fine-grained, brown stilpnomelane. Glaucophane locally nants plastered around the margins of in- pumpellyite-bearing green diabase occur coexists with chlorite. clusions exposed on hillsides. It is apparent along its upper surface. from Figure 2 and Table 2 (see footnote 1) The metagraywacke in the unit is dis- METAMORPHISM that inclusions of markedly different tinctly quartzofeldspathic and contains a metamorphic grades and lithologies have very small amount of lithic detritus (Fig. 6, Metamorphic Mineral Assemblages been intimately mixed together. Many in- see footnote 1). It contrasts significantly in Franciscan metaclastic rocks in the study clusions have well-developed, and often this respect with metagraywacke inclusions area contain the following metamorphic complex, internal metamorphic structures; in the tectonic mélange and with the meta- mineral assemblages: (1) albite, (2) albite these foliations and lineations record an graywacke semischist, although determina- + pumpellyite, (3) albite + pumpellyite + earlier metamorphic history, and, as tion of detrital modes is difficult because of lawsonite, (4) albite + lawsonite, and (5) pointed out by Coleman and Lee (1963), metamorphic recrystallization. K-feldspar albite + lawsonite + jadeitic pyroxene. All . they have no systematic structural or genet- is completely absent from these rocks, even assemblages contain quartz + chlorite + ic relation to the deformational geometry though the high quartz-feldspar content white phengitic mica. Assemblages (1), (2), of the surrounding mélange or adjacent: in- and a heavy mineral suite that includes al- and (3) may contain calcite; (3) through (5) clusions. Finally, it is clear from field evi- lanite and zircon signal a plutonic prove- may contain aragonite. Sodic amphibole dence in the area that exotic inclusions, nance. may occur locally in all but (1). Experimen- which are in metamorphic disequilibrium In marked contrast to the structural style tal studies strongly suggest that assem- with surrounding metagraywacke, are not of the Garzas mélange, bedding in the Ores- blages (1) through (5) form in response to restricted to narrow, high-angle fault zones. timba unit is intact and gently folded. increasing pressures at nearly constant, re- Stratal continuity becomes disrupted only latively low, temperatures (see Ernst 1971a, ORESTIMBA METAGRAYWACKE locally within a few meters of the contact for a comprehensive review). Metastable The structurally lowest unit in the Fran- with structurally overlying mélange. The recrystallization and metasomatic changes ciscan Complex consists predominantly of discontinuous and massively bedded nature in bulk composition are of only minor and massive to thin-bedded metagraywacke of most outcrops makes accurate determi- local importance in Franciscan metamor- with minor amounts of interbedded silt- nations of attitudes impossible; they com- phism. stone. This unit is informally termed "Ores- monly vary in dip and strike over distances Each structural unit contains a charac- timba metagraywacke"; excellent expo- of as little as 30 m. The impression is one of teristic suite of metamorphic mineral as- sures occur along Orestimba Creek and its gently undulating fold surfaces with am- semblages. The metagraywacke semischist major intermittent tributaries. The Ores- plitudes of this order and larger. Thin- is dominated by assemblage (5), which ap- timba metagraywacke lies structurally be- bedded metagraywacke and phyllite locally parently is stable at pressures of 7 to 9 kb at neath the Garzas tectonic mélange and is display cylindrical parallel folds with wave temperatures of 200° to 300°C. Individual separated from it by a fault that dips gently lengths and amplitudes that average 0.5 m. metagraywacke inclusions from the Garzas to moderately eastward. Bedding is folded Even smaller folds occur on the limbs of mélange contain (1), (2), (3), or (4). These but not dislocated, and metamorphic folia- these larger mesoscopic features. Appar- assemblages, lacking jadeitic pyroxene, are tion is generally absent. Although I found ently, fold wave lengths are a function of probably stable at correspondingly lower no fossils in the study area, whole-rock bed thicknesses in the unit. Microscopi- pressures of 3 to 7 kb and temperatures of K-Ar dates on four samples indicate it must cally, most sandstone in the unit has gener- 150° to 250°C. Pumpellyite-bearing as- be at least as old as Cenomanian (Table 1). ally well-preserved isotropic clastic fabrics semblages (2) and (3) predominate in the Cotton (1972) found two fossils of proba- (Fig. 3B), in contrast to the metagraywacke Orestimba metagraywacke, but jadeitic bly Alban or younger age in a lithologically semischist. A planar metamorphic fabric pyroxene is developed locally at its north- similar unit 10 km from the southwestern- defined by the preferred orientation of lens- eastern margin (Fig. 2). I found no evidence most exposures of the Orestimba meta- oid quartz grains and newly formed phyl- that major structures within the latter unit graywacke. The units, which may be equiv- losilicates is incipiently developed in Ores- control the distribution of each assemblage, alent, are presently separated by faults and timba graywacke only in the northeastern and chemical analyses (Table 3, Fig. 6, see mélange. part of the unit. footnote 1) indicate that metamorphism Massively bedded, monotonously The Orestimba metagraywacke contains was isochemical. The assemblages might medium-grained metagraywacke predomi- the following metamorphic mineral as- have formed contemporaneously in re- nates but is interbedded with a thin-bedded, semblages arranged in a crudely zonal pat- sponse to a simple lithostatic pressure gra- fine-grained metagraywacke and phyllitic tern from southwest to northeast (Fig. 2): dient; the close spatial proximity of siltstone facies east of Orestimba Creek. Al- albite + pumpellyite, albite + pumpellyite pumpellyite- and pyroxene-bearing assem- though faint laminations parallel to bed- + lawsonite ± glaucophone, albite + law- blages would then suggest that pumpellyite ding and size grading are extremely rare in sonite, albite + lawsonite + jadeitic pyrox- can be stable at pressures somewhat higher the former facies, sedimentary structures, ene ± glaucophane. All assemblages con- than previously assumed. Conversely, including wavy current laminations, tain quartz + chlorite + white mica ± cal- pumpellyite-bearing assemblages might fine-scale cross-laminations, and graded cium carbonate + sphene ± stilpnomelane. have been converted to higher pressure as- bedding, are abundant in the latter. Con- Aragonite was positively identified only in semblages during a later, separate glomerate and pebbly mudstone apparently samples containing lawsonite. Two sam- metamorphic event. The metastable coexis- are absent from the entire unit. ples, shown on Figure 2, contain pumpel- tence of pumpellyite with jadeitic pyroxene A few thin discontinuous lenses of thin- lyite, lawsonite, glaucophane, and very rare might then be due to sluggish reaction rates. bedded, red-brown radiolarian chert are in- incipient needles of jadeitic pyroxene in al- Tectonic inclusions of metaigneous rocks terbedded with graywacke southwest of bite. Pumpellyite is well developed in the in the Garzas mélange include low-grade Gill Ranch. The largest of these is 175 m central and southwestern parts of the unit. greenstone with relict igneous textures thick and holds up the Rooster Comb (Fig. To the northeast, pyroxene always occurs and containing albite + chlorite + pumpel- 2). This lens, and bedding within it, dip sparingly as radiating sprays of fine prisms lyite; foliated and lineated blueschists con-

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taining sodic amphibole + lawsonite; and Orestimba metagraywacke are clustered metamorphic relation to one another. high-grade, completely recrystallized rocks about a mean of 90 ± 2 m.y. These dates However, a comparison of metamorphic containing sodic amphibo.e, garnet, epi- overlap with dates from the metagraywacke mineral assemblages across a given fault dote, hornblende, and omphacite in various semischist, but, as a group, they are may help to decipher the final relative dis- combinations. These assemblages record significantly younger than the two oldest placements of adjacent structural units. progressively higher temperatures of dates from the semischist. This age differ- metamorphism (Ernst and others, 1970; ence permits, but does not prove, the Faulting and Metamorphism Taylor and Coleman, 1968), possibly as hypothesis that the semischist was initially In the study area, the jadeitic pyroxene- high as 400° to 535°C in the latter case. On recrystallized earl er than the Orestimba bearing metagraywacke semischist struc- the basis of metamorphic mineral assem- metagraywacke, even though rocks in at turally overlies the Garzas mélange and blages alone, it is probable that all least the former unit remained open systems Orestimba metagraywacke, both of which metaigneous inclusions but those contain- until Late Cretaceous time. consist predominantly of pumpellyite- and ing albite + chlorite + pumpellyite are exot- Dates from exotic inclusions in the lawsonite-bearing metaclastic rocks (Fig. ic and did not recrystallize in metamorphic mélange range from 131 to 106 m.y. 2). This structural arrangement, noted equilibrium with associated albite-bearing Coarse and fine fractions of white mica elsewhere in the Franciscan (Blake, 1965; and pumpellyite + lawsonite-bearing meta- separated from a foliated glaucophane- Suppe, 1973), is the reverse of what would graywacke. However, mineral foliations lawsonite inclusion in Garzas Creek (local- be expected had the rocks recrystallized in and lineations that characterize many of the ity 64) yielded dates of 130 m.y. and 131 response to simple burial, because the high- exotic inclusions contrast strikingly with m.y., respectively. This striking concor- est pressure assemblages occur in rocks the essentially unmodified relict clastic tex- dance suggests that argon loss ceased rather above and at shallower structural depths tures of the graywacke. This fundamental abruptly in the rock and was not a function tian rocks containing presumably lower textural contrast proves that the exotic in- of grain size. Although the two whole-rock pressure assemblages. clusions could not have been in their pres- dates obtained from mélange inclusions are At least two hypotheses could explain the ent structural positions during at least part, comparable to dates from the semischist, observed anomaly. The semischist either if not all, of their metamorphic history. white mica from sample 64 is significantly was once more deeply buried and has been Not a single grain of X-feldspar was older than all other samples dated. Similar c isplaced several kilometers upward along found in graywacke or other rocks in the high-grade inclusions from mélanges else- major faults since its metamorphism, or study area. It is difficult to imagine a source where in the Franciscan generally yield was metamorphosed in its present struc- terrane that could have provided hundreds older dates for white mica than adjacent tural position at depths less than those re- of cubic kilometers of sediment completely lower grade units (Coleman and Lanphere, quired for production of observed high-

devoid of K-feldspar. K20 contents of 1971; Suppe and Armstrong, 1972). pressure assemblages. The second quartzofeldspathic Orestimba metagray- hypothesis would in turn require that wacke (Table 3, see footnote 1) are about MAJOR FAULTS lithostatic pressures be supplemented by the same as "Coastal Belt" Franciscan The Franciscan in the study area can best tectonic overpressures, perhaps generated graywacke samples that contain abundant be visualized as a seres of discrete, grossly beneath major low-angle faults (Blake and modal K-feldspar (Bailey and others, 1964, sheetlike structural units separated from others, 1967, 1969). Theoretically, this p. 33). Although original K-feldspar con- one another by gently to steeply dipping process would produce an inverted se- tent of Franciscan graywacke may in part major faults. The preceding descriptions of cuence of mineral assemblages within a be a function of its stratigraphic age (Bailey each unit clearly show that rocks with dif- structural unit. However, the upper fault and others, 1964, p. 140-141), a more ferent deformational and metamorphic his- boundary of the semischist, to which reasonable explanation for the total ab- tories and different ages have been jux- assemblages within the unit could be re- sence of this phase in rocks from the study taposed. The faults separating the Garzas ferred, is not preserved. Although the Great area and at least some other Franciscan tectonic mélange from the metagraywacke Valley sequence may have originally over- metagraywacke is that K-feldspar is unsta- semischist and the Orestimba meta- lain the semischist along the low-angle ble in rocks of a wide range of bulk com- graywacke are each required by three "Coast Range thrust" (Bailey and others, positions under the high-pressure- mutually independent criteria: a striking 1970; Raymond, 1973b), they are at pres- low-temperature metamorphic conditions contrast in deformational styles, different ent juxtaposed along the later, high-angle accompanying recrystallization to blueschist metamorphic mineral assemblages in Tesla-Ortigalita fault. facies and prehnite-pumpellyite facies metaclastic rocks, e.nd juxtaposition of The most important evidence in support mineral assemblages. graywackes of different detrital modes. of the first hypothesis is the major faults Suppe (1969, 1973) mapped similar faults themselves. In view of the striking contrast Metamorphic Ages in the Franciscan in the northern Coast in mineral assemblages and metamorphic Ten K-Ar isotopic dates from the area are Ranges. These enigmatic structures, also ages across the faults throughout the study presented in Table 1. With the exception of noted by Brown (1964a, 1964b) and Ghent area, it seems unlikely that the tectonic locality 64, all dates were obtained from (1965), probably characterize the entire i:nits could have been metamorphosed whole-rock samples. Suppe and Armstrong complex. while they occupied their present relative (1972) suggested that whole-rock dates ap- Unfortunately, the ages and displacement structural positions. Postmetamorphic dis- parently reflect the time span during which histories of these faults may be as obscure placements along the fault separating the metamorphic minerals, over an entire re- as their existence is obvious. Unlike low- semischist from underlying mélange could gion consisting of several structural units, angle Cordilleran thrusts and conventional have accommodated the emplacement of became closed systems with respect to high-angle normal, reverse, and strike-slip the semischist into its present anomalously argon diffusion. White mica is more faults, they do not systematically repeat or shallow level. Suppe (1970) first proposed argon-retentive than whole rocks. Although offset a dated stratigraphic succession but such a hypothesis to explain the juxtaposi- whole-rock ages from the region range from rather juxtapose rock units that bear no tion of low- and high-grade Franciscan 116 to 76 m.y., the four dates from the apparent stratigraphic, deformational, or rocks.

Ages of Faulting to describe all manner of chaotic rocks and spectrum of specialized terms. For example, even the Franciscan Complex as a whole. some mélanges consist predominantly of The absolute age of the final displace- Clearly, Hsii (1966; 1968, p. 1065) recog- graywacke and may contain minor inclu- ments along major faults in the study area is nized the unique fabric of the chaotic ter- sions of chert and greenstone. Other impossible to specify because the strati- ranes when he carefully defined Franciscan mélanges, such as the Garzas mélange, con- graphic age of displaced units is unknown. mélanges as "mappable bodies of deformed tain exotic blocks that have clearly had dif- Substantial displacements probably did not rocks characterized by the inclusion of tec- ferent metamorphic histories than the more occur after argon diffusion from metamor- hnically mixed fragments or blocks. . . in a voluminous graywacke inclusions with phic minerals ceased approximately 76 m.y. pervasively sheared, fine-grained, and which they are now intimately associated. ago. Similar dates are the youngest reported commonly pelitic matrix." Greenly (1919, Hsii (1968, p. 1065-1066) termed the from the Diablo Range as a whole (Suppe p. 193-195, Pl. VIII) had first used the term former "broken formations" and the latter and Armstrong, 1972). "autoclastic mélange" to describe structur- true "mélanges" and characterized "broken Several high-angle faults have a weak ally analogous rocks in Anglesey consisting formations" as disrupted rock-stratigraphic erosional topographic expression and are of lenticular fragments of all sizes embed- units. Berkland and others (1972) accepted visible on air photos (Fig. 2). These faults ded in a foliated matrix. Both Hsii and Hsii's definition of broken formation. I are clearly younger than the major, gener- Greenly believed that tectonic processes propose that this term be abandoned be- ally lower angle, faults they locally offset. were responsible for the distinctive fabric of cause the style of deformation in both Faults in the mélange have pulverized mélanges, but Hsii restricted the term to broken formations and exotic-bearing mélange matrix to a bluish-gray gouge con- pervasively sheared bodies containing cer- mélanges is identical. Moreover, it is very taining small inclusions of vein quartz and tain types of inclusions. In effect, I have difficult in practice to establish whether cer- other rock types. The gouge resembles the broadened Hsii's definition of mélanges to tain inclusions in a mélange are fragments finely comminuted debris in the Tesla- include all similarly deformed rock bodies, of rocks formerly interbedded in a discrete Ortigalita fault zone. Raymond (1973a) regardless of the types of inclusions or the rock-stratigraphic unit. If necessary, mapped similar faults within the Franciscan earlier history of the affected rocks. I have modifiers such as "graywacke-chert- in the Mount Oso area. I interpret these also added the modifier "tectonic" to em- greenstone" or "exotic" can precede "tec- faults as broadly contemporaneous with the phasize that, by definition, mélanges record tonic mélange" to indicate rock types pres- Tesla-Ortigalita fault system. Most of them a deformation. No other genetic implica- ent as inclusions. tions hinder use of the term tectonic probably formed during late Cenozoic Some needless confusion has also arisen mélange to describe rocks displaying a par- diapiric uplift of the Franciscan core of the regarding the distinction between mélanges ticular, easily recognizable, deformational Diablo Range. and olistostromes. Olistostromes are style. NATURE AND ORIGIN OF sedimentary accumulations of generally The internal features of tectonic rounded, resistant inclusions embedded in TECTONIC MÉLANGES mélanges are characteristic and provide the massive pelitic, sandy, or marly matrices Definitions most important criteria for recognition and (Abbate and others, 1970). They are prob- The structural units juxtaposed along differentiation of chaotic terranes in the ably deposited from submarine slides and major faults in the study area display con- field. Pervasive, mesoscopic shear fractures debris flows. Because the deformation that trasting deformational styles. Conventional are the essential elements of their internal produces tectonic mélanges can in theory foliated metamorphic tectonites and simply fabric. The shear surfaces occur as sub- affect rock bodies of any kind, olisto- folded sequences of metagraywacke with parallel anastomosing fractures in brittle stromes, as well as nonchaotic bedded se- relict clastic textures and intact bedding are graywacke and more closely spaced frac- quences of sandstone and mudstone, can be common throughout the Franciscan and are tures in less competent argillaceous rock deformed after deposition by surfaces of represented by the metagraywacke semi- (Fig. 4; Hsii, 1968, Pl. 1; Hsii, 1969, Photos brittle shear and thus become tectonic schist and Orestimba metagraywacke, re- 2 and 8; Dickinson, 1971b, Fig. 4). The lat- mélanges. As discussed below, it may in spectively. However, it is obvious from field ter commonly have a scaly appearance, and practice be difficult to establish whether a evidence alone that vast chaotic terranes individual lenticular chips are polished and given mélange was originally an olisto- cannot be described by conventional labels slickensided. The fracturing resulted in dis- strome or a bedded sequence. such as semischist and bedded sequence. ruption of original sfratai continuity in These mappable bodies, which display a bedded sequences of sandstone and mud- Origin of Tectonic Mélanges third fundamental style of deformation, stone. In incipiently deformed sequences, Unfortunately, our knowledge of tectonic have a characteristic internal fabric domi- individual sandstone beds are simply dis- mélanges is too scanty to allow a com- nated by penetrative mesoscopic shear frac- placed along fractures that initially form at prehensive and convincing hypothesis for tures and contain tectonic inclusions of all low angles to bedding; disruption progress- their origin. Instead, I will focus on three sizes immersed in a pervasively sheared, es until only isolated, phacoidal fragments important questions (and discuss some pos- generally fine-grained matrix. The essence of beds, completely surrounded by a sible solutions): (1) What was the nature of of the shear-fracture fabric is that it records sheared argillite matrix, remain. Because a given tectonic mélange prior to its defor- a deformation. Tectonic inclusions com- the shearing is progressive in nature, there mation? (2) How are exotic inclusions in- prise a range of rock types, and the defor- is no established minimum degree of de- troduced into mélanges? and (3) What do mational style in question is not limited to formation that distinguishes a tectonic observed structures imply about the defor- terranes containing demonstrably exotic mélange from an undeformed precursor. mational environment of mélanges? This blocks. The division is rather arbitrary and depends entire discussion of mélanges draws heavily Given that such deformed rock bodies on the judgment of the observer. on my experience in the study area and my exist, we must decide what to call them. I By accepting an expanded, more general reconnaissance observations elsewhere in have chosen the term "tectonic mélange," definition of tectonic mélange, we can em- the Franciscan and on Hsii's (1968, 1969) even though "mélange" itself is in danger of phasize the structural kinship of similarly description of mélanges in the Morro Bay becoming a useless, wastebasket term used deformed bodies and thus avoid a whole area, California.

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In some areas, it is obvious that a tectonic tures, and, in some cases, ages together have been tectonically transported upward mélange was derived from either a bedded prove that the inclusions were metamor- from their sites of metamorphism to their sequence or an olistostrome. The Mustang phosed before they were mixed with pressnt structural positions among less Peak slab, for example, has an interior of associated mélange metagraywacke. The dense, lower grade metagraywacke. Some folded, but unsheared, interbedded Garzas mélange contains a representative blocks in other Franciscan mélanges have graywacke and mudstone; bedding be- suite of these inclusions. Coarse-grained actinolite ± talc rinds that indicate they comes progressively disrupted by penetra- blueschist and amphibolite with the highest were once immersed in serpentinite (Cole- tive shearing near its margins. This mélange grade mineral assemblages are rare. Cole- mar. and Lanphere, 1971). It is conceiv- is structurally indistinguishable from the man and Lanphere (1971) measured K-Ar able that exotic inclusions could have been Garzas mélange enclosing the slab, and the ages of 155 ± 8 m.y. on actinolite and 149 carried upward some distance in pods of two can be separated only on the basis of ± 5 m.y. on hornblende from a coarse- serpentinite that were rising through rela- exotic inclusions in the Garzas. In contrast, grained garnet amphibolite of this general tively more dense metasediments, but most of the "Williams chaos facies," an type collected from the Garzas mélange just mélange inclusions in the study area are en- olistostrome in the northern Coast Ranges southeast of the mapped area. Mineralogi- closed in strongly sheared metagraywacke (Suppe, 1969), is a penetratively sheared cally similar high-grade blocks elsewhere in and argillite, and serpentinite hosts, if ever mélange, but soft-sediment deformational the Franciscan consistently give K-Ar min- present, have become comminuted or have features and primary chaotic structures in eral ages of about 150 m.y. More common migrated away and disappeared. A more some parts are unaffected by shearing. are foliated glaucophane—lawsonite-white appealing suggestion is that the blocks have However, it may be difficult or impossible mica metavolcanic rocks; white mica from been emplaced upward along systems of in- to determine the parent of a mélange that a typical example on Garzas Creek (locality numerable shear planes in tectonic cannot be traced into an undeformed pre- 64) yielded K-Ar ages of 130 and 131 ± 2.6 mélanges that intersect high-grade parent cursor. Shearing may have completely ob- m.y. (Table 1). Blueschist samples with terranes at depth (Suppe, 1972). This pro- literated diagnostic sedimentary features. similar mineral assemblages from other cess is kinematically analogous to the up- The Garzas mélange typifies the dilemma. Franciscan terranes have yielded nearly ward emplacement of much larger, fault- Abundant inclusions of Mustang Peak and identical K-Ar dates (Lee and others, 1964; bounded slabs of high-grade rocks such as less common Zimba-type metagraywacke Suppe and Armstrong, 1972). For the Fran- the metagraywacke semischist and the are concentrated near each slab, suggesting ciscan in general, the demonstrably exotic Taliaferro metamorphic complex in the that at least part of the mélange formed as a inclusions are older and of distinctly higher northern Coast Ranges (Suppe, 1969, result of systematic fragmentation and dis- metamorphic grade than associated meta- 1970). ruption of bedded sandstone mudstone se- graywacke. Although some rare metachert Finally, it is interesting to speculate on quences. However, the mélange apparently is included in the exotic group, most inclu- the nature of the deformational environ- contains few inclusions derived from the sions have "basaltic" compositions and are ment that produced Franciscan tectonic adjacent metagraywacke semischist and inferred to have had igneous protoliths. mélanges. Several observations are perti- Orestimba unit. In fact, the range in If all inclusions of this type were simply nent. Clearly, the internal surfaces of shear graywacke modes (Fig. 5) and the variety of large blocks in olistostromes that developed and dislocation in mélanges represent the nongraywacke inclusions (Table 2, see into tectonic mélanges after deposition and postdepositional deformation of consoli- footnote 1) suggest that parts of several consolidation, their origin and emplace- dated, lithified rock bodies. Sandstone beds, formerly undeformed, but presently unex- ment would be rather straightforward. and, less obviously, associated mudstone, posed rock-stratigraphic units may have Rocks that once were deeply buried and record large-scale brittle deformation. Brit- been incorporated into the mélange. Al- subjected to moderate temperatures and tle behavior is not included in the catalog of though I did not observe any soft-sediment high pressures were elevated tectonically structures usually ascribed to submarine deformational structures in the matrix or above the ocean floor. The uplifted terranes slumping of "soft," unlithified sediments. inclusions of the mélange, it is possible that provided detritus to submarine chaotic Sandstone beds were initially fragmented parts of the deformed body were initially debris-flow deposits interbedded with by shear fractures that formed subparallel deposited as olistostromes. Thus, even stratified sandstone and mudstone. The to bedding (Fig. 4); this compressional though the geometry and relative structural "olistostrome" hypothesis is unsatisfactory strain contrasts with extensional strain re- positions of inclusions are at present defor several reasons. In the Garzas mélange, corded by fractures perpendicular to bed- monstrably or inferentially relatable to sur- for example, there is no positive evidence ding. On a larger scale, mélanges typically, faces of shear, the distinctive inclusion-in- that any part of the deformed body was ini- but not necessarily, separate discrete tec- matrix fabric, and the exotic inclusions tially an olistostrome. Significantly, the tonic units, such as the semischist and Ores- themselves, may have been inherited from highest grade inclusions seemingly always timba unit, with ages and metamorphic chaotic submarine slumps. It seems that "occupy disturbed zones (mélange or shear nineral assemblages that preclude a normal similar reasoning must be applied to similar dislocation)" in the Franciscan as a whole stratigraphie, depositional relation. mélanges if positive evidence of a parent is (Coleman and Lanphere, 1971, p. 2398). lacking. In view of these observations, I believe Unless we accept the premise that all tec- tectonic mélanges are structurally equiva- The origin of exotic inclusions and the tonic mélanges containing exotic inclusions lent to faults. Mélanges can best be charac- processes responsible for their emplacement with no presently exposed source were orig- terized as sheetlike zones of distributed in Franciscan mélanges are outstanding inally olistostromes, we must conclude, as shear formed during brittle deformation of problems. Most of the controversy centers did Coleman and Lanphere, that most exot- consolidated rock under a considerable, but on the familiar isolated blocks of foliated ic inclusions are fragments of relatively old as yet unspecified, overburden. Displace- high-grade "blueschists" that are wide- metamorphic terranes that have been tec- ments have occurred along innumerable spread, but volumetrically insignificant, in tonically mixed with younger Franciscan ¡subparallel shear fractures, and the locus of the Franciscan (Bailey and others, 1964, p. clastic rocks. tectonic dislocation has in effect expanded 89—111). These blocks were clearly not de- Although the exact nature of the mixing from a plane (=fault) to a zone several me- rived from units adjacent to the enclosing process is unknown, the mineralogy of ters to kilometers in width (=tectonic mélange and have no exposed source. exotic inclusions requires that relatively mélange). Remnants of rock-stratigraphic Metamorphic mineral assemblages, tex- dense blocks from deeply buried terranes units that became progressively attenuated

as their margins contributed inclusions to a cession. In the Franciscan, adjacent struc- Bailey, E. H., Irwin, W. P., and Jones, D. L., mélange are sometimes preserved within or tural units need never have been parts of an 1964, Franciscan and related rocks and adjacent to it; the Mustang Peak and orderly stratigraphic sequence. Instead, the their significance in the geology of western Zimba slabs are examples. However, a units include rocks of different ages derived California: California Div. Mines and mélange need not be directly related to adj- from different sources; terrigenous sedi- Geology Bull. 183, 177 p. Bailey, E. H., Blake, M. C., Jr., and Jones, D. L., acent units. The Orestimba metagraywacke ments have been mixed with oceanic basalts 1970, On-land Mesozoic oceanic crust in and the semischist apparently contributed and pelagic cherts on both a large and small California Coast Ranges: U.S. Geol. Survey little, if any, material to the Garzas scale. Rocks were deformed and Prof. Paper 700-C, p. C70-C81. mélange; they were juxtaposed with it by metamorphosed under a variety of condi- Beck, R. H., and Lehner, P., 1974, Oceans, new displacements on the major faults that pres- tions and then juxtaposed along major frontier in exploration: Am. Assoc. Pe- ently separate the units from the mélange. faults. There is as yet no satisfactory troleum Geologists Bull., v. 58, p. 376-395. dynamic interpretation to account for the Berkland, J. O., Raymond, L. A., Kramer, J. C., SUMMARY broadly distributed shear recorded in Moores, E. M., and O'Day, M., 1972, The Franciscan Complex is now inter- mélanges or the apparent upward dis- What is Franciscan?: Am. Assoc. Petroleum Geologists Bull., v. 56, p. 2295-2302. preted as having accumulated in a late placement of rocks that were once more Blake, M. C., Jr., 1965, Structure and petrology deeply buried. Perhaps, as Ernst (1971c) Mesozoic—early Tertiary subduction zone of low-grade metamorphic rocks, at the North American continental margin. proposed, such deformation occurred dur- blueschist-facies, Yolla Bolly area, northern This interpretation, with minor ing buoyant upward rise of subducted ma- California [Ph.D. thesis]: Stanford, Calif., modifications, has been reiterated so often terials in response to an over-all isostatic Stanford Univ., 91 p. it has become hoary with age (for example, necessity as new materials were added in- Blake, M. C., Jr., Irwin, W. P., and Coleman, R. Hamilton, 1969; Bailey and Blake, 1969; crementally, or "underplated," to the west- G., 1967, Upside-down metamorphic zona- Ernst, 1970, 1971a, 1971b; Ernst and ern margin of the subduction zone. tion, blueschist facies, along a regional thrust in California and Oregon: U.S. Geol. others, 1970; Page, 1970; Dickinson, In the broadest sense, Cordilleran struc- Survey Prof. Paper 575-C, p. C1-C9. 1970b, 1971a), but it offers an elegant and tures record crustal shortening accom- 1969, Blueschist-facies metamorphism re- readily acceptable explanation for both the plished by imbrication and repetition along lated to regional thrust faulting: Tec- metamorphic and tectonic features of the low-angle faults. Tectonic mélanges and tonophysics, v. 8, p. 237-246. complex and its relation to adjacent coeval major faults that have juxtaposed unrelated Bloxam, T. W., 1960, Jadeite-rocks and rocks. Ideally, we could directly compare rocks record continental margin accretion glaucophane-schists from Angel Island, San the Franciscan with its most likely modern accompanying subduction. These features Francisco Bay, California: Am. Jour. Sci., v. analogs, the structurally complex inner may prove as useful as blueschist-facies 258, p. 555-573. walls of oceanic trenches, but available minerals in identifying ancient subduction Borg, I. Y., 1956, Glaucophane schists and eclo- marine geophysical data offer only tantaliz- zone assemblages in the geologic record. gites near Healdsburg, California: Geol. ing hints of major arcward-dipping faults Soc. America Bull., v. 67, p. 1563-1584. within highly deformed, but otherwise ir- ACKNOWLEDGMENTS Briggs, L. I., Jr., 1953, Geology of the Ortigalita Peak quadrangle, California: California resolvable, acoustic basement (for example, I am indebted to W. R. Dickinson for his Div. Mines Bull. 167, 61 p. Beck and Lehner, 1974). stimulating advice, criticism, and Brothers, R. N., 1954, Glaucophane schists from Although field work in other ancient unflagging interest in this research. I also the North Berkeley Hills, California: Am. subduction zone assemblages will doubtless thank M. C. Blake, Jr., R. G. Coleman, K. Jour. Sci., v. 252, p. 614-626. reveal additional facets of the deformation Crawford, W. G. Ernst, D. L. Jones, C. F. Brown, R. D., Jr., 1964a, Geologic map of the accompanying subduction, several funda- Mansfield, B. M. Page, L. A. Raymond, R. Stonyford quadrangle, Glenn, Colusa, and mental features of the study area may A. Schweickert, and John Suppe, each of Lake Counties, California: U.S. Geol. Sur- vey Mineral Inv. Map MF-279. characterize both the Franciscan elsewhere whom freely discussed with me many of the 1964b, Thrust-fault relations in the north- and, to the extent that the Franciscan is a ideas and observations presented here. ern Coast Ranges, California: U.S. Geol. representative example, subduction zone Blake, Ernst, and Suppe critically read ear- Survey Prof. Paper 475-D, p. D7-D13. assemblages in general: (1) the complex lier versions of the manuscript. R. L. Arm- Brown, W. H., Fyfe, W. S., and Turner, F. J., consists of discrete structural units, grossly strong dated a suite of samples from the 1962, Aragonite in California glaucophane sheetlike in geometry and separated from area, and the University of Mexico pro- schists, and the kinetics of the aragonite- one another by gently to steeply dipping vided chemical analyses under a coopera- calcite transformation: Jour. Petrology, v. major faults; (2) the structural units contain tive agreement with the Department of 3, p. 566-582. rocks of different ages, bear no apparent Geology at Stanford University. Coleman, R. G., and Lanphere, M. A., 1971, Distribution and age of high-grade blue- stratigraphic relation to one another, and I am grateful to Mrs. Ernest Gill and schists, associated eclogites, and amphibo- have each experienced a different deforma- Kaiser-Aetna for permission to enter the lites from Oregon and California: Geol. tional and metamorphic history; (3) tec- Gill Ranch. Soc. America Bull., v. 82, p. 2397-2412. tonic mélanges are major zones of distrib- This research was supported by a Na- Coleman, R. G., and Lee, D. E., 1963, uted shear that record large-scale tectonic tional Science Foundation Graduate Fel- Glaucophane-bearing metamorphic rock transport and deformation of consolidated lowship and research grants from the types of the Cazadero area, California: rock bodies; and (4) rocks that were once Geological Society of America and the Shell Jour. Petrology, v. 4, p. 260-301. more deeply buried may have been dis- Companies Foundation. Cotton, W. R., 1972, Preliminary geologic map placed upward from structural positions of the Franciscan rocks in the central part of occupied during metamorphism. the Diablo Range, Santa Clara and REFERENCES CITED Alameda Counties, California: U.S. Geol. The essence of subduction zone deforma- Abbate, E., Bortolotti, V., and Passerini, P., Survey Misc. 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