Metamorphic and Deformational Processes in the Franciscan Complex, California: Some Insights from the Catalina Schist Terrane

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Metamorphic and deformational processes in the Franciscan Complex, California: Some insights from the Catalina Schist terrane

J. P. PLATT Department of Geology and Mineralogy, University of Adelaide, Adelaide, South Australia 5001

ABSTRACT INTRODUCTION Maxwell, 1974; Blake and Jones, 1974). The paleogeography suggested by Dickin- On Santa Catalina Island, blueschist is The Franciscan Complex (Berkland and son (1971) is adopted here, however. The structurally overlain by glaucophanic others, 1972) of western California consists purpose of this paper is to describe the greenschist, which is overlain in turn by a largely of graywacke, chert, and basaltic Franciscan schist on Santa Catalina Island, unit of amphibolite and ultramafic rock. volcanic rocks deposited in Late Jurassic to present some tectonic hypotheses, and These three units are juxtaposed along sub- Eocene time on simatic basement (Bailey show how they might help explain some horizontal postmetamorphic thrusts; tec- and others, 1964). The present disruptive problems posed by the Franciscan Com- tonic blocks of amphibolite are distributed deformational style and high-pressure, plex. along the thrust between the greenschist low-temperature metamorphism are the re- and the blueschist. Physical conditions of sults of eastward lithospheric underthrust- CATALINA SCHIST TERRANE metamorphism are estimated to be approx- ing beneath the western margin of the imately 300°C and 9 kb for blueschist, North American plate during late Mesozoic General Description 450°C and 8 kb for greenschist, and 600°C and early Cenozoic time (Hamilton, 1969; and 10 kb for amphibolite. I suggest that Bailey and Blake, 1969; Ernst, 1970). The In California south of the Transverse metamorphism occurred in a newly started Franciscan Complex lies beneath a major Ranges, the Franciscan Complex crops out subduction zone, where an inverted thermal tectonic discontinuity, the Coast Range as two small areas of schist, one in the Palos gradient developed below the hot thrust (Bailey and others, 1970). Above this Verdes Hills, and the other on Santa hanging-wall peridotite. Postmetamorphic thrust lie Paleozoic and Mesozoic granitic Catalina Island (Fig. 1). The distribution of eastward underthrusting along surfaces of and metamorphic rocks in the Klamath clasts of similar schist in the Miocene San varying dip can explain the present struc- Mountains and the Great Valley sequence Onofre Breccia indicates that the schist ter- tural relationships. with its ophiolitic basement in the Coast rane may underlie much of the offshore Tectonic blocks of glaucophane-epidote Ranges (Fig. 1). The Great Valley sequence area of southern California (Woodford, schist, amphibolite, and eclogite elsewhere consists of Late Jurassic and Cretaceous 1925; Vedder and others, 1974). The fol- in the Franciscan Complex may be dis- graywacke turbidites; it lies unconformably lowing description of the metamorphic rupted remnants of similar metamorphic on older continental rocks in the east and rocks on Santa Catalina Island is sum- zones. The inverted thermal gradient will directly on ophiolite, interpreted as an marized from Piatt (1975). only exist in the early stages of subduction, oceanic crustal remnant (Bailey and others, The Catalina Schist consists of three tec- which explains why the blocks are the old- 1970), in the west. tonic units, distinguishable by their est rocks in the Franciscan Complex. Prior to major right slip on the San An- metamorphic mineral assemblages. The The gross decrease in age and metamor- dreas fault (Hill and Dibblee, 1953; units are separated by regionally subhori- phic grade westward across the Franciscan Crowell, 1962), which doubled up the zontal folded thrusts (Figs. 2, 3). Mineral results from successive underthrusting and north-northwest—trending Mesozoic belts assemblages in the three units are sum- accretion of progressively younger slices of (Fig. 1), California probably was an active marized in Figure 4. supercrustal material, concurrent with up- continental margin resembling the west The Catalina Blueschist Unit is struc- lift and erosion. Pressure-temperature (P-T) coast of South American (Hamilton, 1969). turally lowest and the most extensive in conditions of metamorphism in each east- According to this model, the site of the outcrop. It consists of metagraywacke, dipping tectonic slice will increase down- Franciscan Complex corresponds to the metachert, mafic metavolcanic rocks, and dip. At any given time, older, more easterly oceanic trench and inner trench wall, the ultramafic rock, uniformly metamorphosed slices will have been uplifted further, hence Great Valley sequence to the arc-trench under blueschist-facies conditions (Fig. 4). metamorphic grade in the exposed edges gap, and the Sierra Nevada—Salinia— A large proportion of the volcanic rocks will increase eastward and structurally up- Peninsular Range batholith belt to the con- were deposited as well-bedded basaltic sand ward. tinental volcanic-plutonic arc (Dickinson, and conglomerate, which have been recrys- If erosion is faster than accretion for a 1971). tallized to distinctive glaucophane- time, younger slices will be metamorphosed The environment of deposition and the lawsonite schist and phyllite. Massive at lower pressures than were the older sources of Franciscan sediment continue to omphacite-lawsonite "greenstone" was de- higher ones. Simple reverse faulting can be debated (for example, Bailey and Blake, rived from diabase, flow breccia, and pillow then produce the observed interleaving of 1969, p. 229; Scholl and Marlow, 1972, lava. Much of the metagraywacke lacks rocks of different metamorphic grade. Key 1974; Matthews and Wachs, 1973), and glaucophane, but the presence of lawsonite words: metamorphic petrology, tectonics, several alternative models for Franciscan and jadeitic pyroxene indicate that it was Franciscan, subduction, blueschist, eclogite. tectonics have been advanced (Hsu, 1971; metamorphosed under the same conditions

Geological Society of America Bulletin, v. 86, p. 1337-1347, 9 figs., October 1975, Doc. no. 51002.

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as the metavolcanic rocks. Synmetamorphic deformation produced isoclinal folds and a schistosity, and there were several subsequent folding events, but primary textures are sufficiently well preserved locally to allow determination of original rock types. The Catalina Greenschist Unit structurally overlies the Blueschist Unit in the central part of Santa Catalina Island (Figs. 2, 3). It is derived from a rock assemblage similar to that of the Blueschist Unit, although the proportion of mafic volcanic rocks is higher and the metasediments are predominantly pelitic. Intense deformation and thorough recrystallization have largely destroyed primary textures and have produced a pervasive schistosity. The schist and phyllite of the Greenschist Unit are readily distinguished from those of the Blueschist Unit by the conspicuous por- phyroblasts of clinozoisite and epidote in mafic schist and of albite, almandine garnet, and, locally, biotite in the metasediments. The critical blueschist-facies minerals, lawsonite, aragonite, and jadeitic pyroxene are absent, but glaucophane and crossite are locally abundant in mafic schist (Fig. 4). Crossite and biotite occur locally in equilibrium association in metachert (Piatt, 1975). Piemontite was reported from Greenschist Unit metachert by Bailey (1940). An episode of retrograde metamorphism, accompanied by a second deformation, caused chloritization of biotite and garnet, renewed growth of albite and other lower greenschist-facies minerals, and local development of pumpellyite. The structure has been complicated further by two postmetamorphic periods of folding. Greenschist-Blueschist Thrust. The contact between the Blueschist and Greenschist Units is regionally subhorizontal (Fig. 3), with the Greenschist Unit on top. Experi- mentally determined phase relations (Fig. 5) suggest that the two units were metamorphosed under distinctly different conditions, and neither unit has been affected by the conditions attending the metamorphism of the other. The contact is therefore a postmetamorphic fault, which I name the Greenschist-Blueschist thrust. Based on the Figure 1. Generalized geologic map of California, showing the distribution of the principal late outcrop pattern of the Greenschist and Mesozoic tectonic elements. Modified from U.S. Geological Survey (1966) and Bailey and others Blueschist Units, displacement must exceed (1970, Fig. 5). CRT: Coast Range thrust. NF: Sur-Nacimiento fault zone. 9 km. The thrust is a zone locally up to 200 m The Catalina Amphibolite Unit overlies originally have been a body of differen- thick, filled with tectonic blocks of both the Blueschist and Greenschist Units in tiated gabbro. The semipelitic schist con- amphibolite-facies rocks and serpentinite in the center of the island (Figs. 2, 3). It contains quartz -I- plagioclase + garnet + bio- a matrix of talc-chlorite-actinolite schist. sists largely of green hornblende—zoisite tite -I- muscovite + kyanite + zoisite; no Many of these blocks are massive schist, with minor brown hornblende- primary textures remain, but the aluminous garnet—brown hornblende rocks, identical garnet schist, semipelitic schist, and garnet composition indicated by the mineralogy to garnet amphibolite inclusions in the Am- quartzite; these are overlain by a body of suggests that it is a metasediment. The gar- phibolite Unit ultramafic body (see below). massive serpentinite. The green hornblende net quartzite is probably metachert. Hence, Other types of block include eclogite, green schist appears to have been derived from an the original rock assemblage included com- hornblende—zoisite schist, and garnet, igneous protolith, but it is richer in calcium ponents similar to those of the lower grade quartzite; the last two rock types also occur and magnesium and poorer in iron and schist units, although the proportion of as coherent bodies of schist in the Am- titanium than basalt (Ernst and others, mafic igneous rock was much higher. phibolite Unit (Figs. 2, 3). 1970, Table 9, sample CAT-11). It may The amphibolite-facies schist has under-

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gone intense deformation and thorough re- A A' crystallization; all rock types now have West East coarse-grained metamorphic textures. Min- FAULT I NORTH SIDE eral assemblages are typical of the am- FAULT phibolite facies; kyanite and diopside appear in rocks of suitable composition, and blue amphibole and primary chlorite are •I \ CBS -

absent. Kyanite and corundum were re- TECTONIC BLOCK ' CBS \ ported from this unit by Bailey (1940). GREENSCHIST-BLUESCHIST THRUST Retrograde metamorphism caused saus- 0 1 2 suritization of the originally intermediate km metamorphic plagioclase, chloritization of garnet and biotite, and sericitization of Serpentinite Amphibolite Unit Greenschist Unit kyanite.

The amphibolite-facies schist is overlain Garnet amphibolite blocks CBS Blueschist Unit by massive serpentinite derived from dunite and harzburgite (Figs. 2, 3). Some of the Figure 3. Schematic section from Figure 2 across central Santa Catalina Island, showing the relationships of the main metamorphic units. contacts of this ultramafic body are late- stage faults and landslide surfaces, but the Greenschist and Blueschist Units have been constrained between the stability field of following evidence indicates that it was brought up again on the north side of the the zeolite facies and the upper temperature present in the Amphibolite Unit during island along high-angle faults (Figs. 2, 3) limit of lawsonite. Oxygen-isotope work on amphibolite-facies metamorphism: that probably postdate the Ollas thrust. Franciscan blueschist containing similar 1. Locally, it has a sharp, apparently un- mineral assemblages suggests metamorphic faulted lower contact with the underlying Physical Conditions of Metamorphism temperatures in the range 270° to 315°C amphibolite-facies schist. (Taylor and Coleman, 1968). Likely condi- 2. Blocks of garnet amphibolite (with Available experimental evidence provides tions for the Blueschist Unit are thus about brown hornblende and zoisite) and eclogitic some P-T constraints on the conditions of 300°C at 9 kb (Fig. 5). rock are abundant within the ultramafic metamorphism of the three schist units. The The presence of biotite and almandine body (Figs. 2, 3). Mineral assemblages and pressure range for the Blueschist Unit is garnet in the Greenschist Unit suggests a mineral chemistry suggest that these blocks constrained by the presence of lawsonite metamorphic temperature in the range 400° formed under similar physical conditions to and aragonite and more precisely by the as- to 450°C, corresponding to the almandine the rest of the Amphibolite Unit (Piatt, sociation of jadeitic pyroxene with both isograd of Barrovian regional metamor- 1975). Several of these blocks are sur- quartz and albite. The field shown in Figure phism (Turner, 1968, Fig. 8.5). Higher rounded by massive serpentinite, and there 5 is centered on the lower pressure-stability temperatures are unlikely, in view of the is no evidence that they were emplaced in limit of a natural pyroxene from a Francis- presence of chlorite and blue amphibole the ultramafic body by postmetamorphic can metagraywacke: Jd82Ac14Di4 (Newton and the absence of amphibolite-facies min- faulting. and Smith, 1967). The temperature range is erals. The presence of glaucophane in mafic 3. Structural analysis demonstrates that METAVOICANIC BLUESCHIST GREENSCHIST AMPHIBOLITE the ultramafic body is involved in a large- ROCKS scale synmetamorphic fold with the ALBITE CA-PLAGI0CLASE amphibolite-facies schist (Piatt, 1975, Fig. LAWSONITE 30). This is a second-generation fold that EPIDOTE GROUP -(2 Figure 4. Comparative deforms the schistosity, but it is associated DIOPSIDE mineral assemblages of tec- with a pervasive mineral lineation parallel 0MPHACITE tonic units on Santa Cata- BLUE AMPHIBOLE lina Island. Retrograde min- to major and minor F2 fold axes. The min- CA-AHPHIB0LE (G) (7) erals and some accessory eral assemblages shown for the Amphibo- GARNET ... minerals are omitted. Heavy lite Unit in Figure 4 were stable during this CHLORITE lines: more than 5 percent. deformation. MUSCOVITE STILPNOMELANE Light lines: less than 5 per- Serpentinization must have postdated the SPHENE cent. Dashed lines: present amphibolite-facies metamorphism, because RUTILE in some samples only. Over- there is no evidence of penetrative deforma- QUARTZ lapped lines show approxi- tion subsequent to serpentinization, and the metatuff mate mineral compatibili- METASEDIMENTARY ties. Notes: 1. saussuritized. serpentine minerals present (lizardite and ROCKS chrysotile) are unstable under amphibolite- QUARTZ 2. pistacite. 3. clinozoisite facies conditions (Coleman, 1971, p. 907). ALBITE and iron-poor epidote. 4. CA-PLAGIOCLASE mainly a-zoisite. 5. actino- Olivine pseudomorphs and relict or- LAWSONITE lite. 6. green hornblende. 7. thopyroxene grains define a planar fabric EPIDOTE GROUP brown hornblende. Blue- relict from the original peridotite; this fab- JADEITE schist Unit: gst = ompha- ric may have been of deformational origin. BLUE AMPHIBOLE CA-AMPHIB0LE citic greenstone. Meta- Ollas Thrust. The Amphibolite Unit has GARNET tuff = mafic metavolcan- been thrust over both the Blueschist and CHLORITE iclastic rocks. Greenschist Greenschist Units along the Ollas thrust STILPNOMELANE Unit: qs = quartz schist (Figs. 2, 3) This is a north to northeasterly (metachert). ps = pelitic BIOTITE schist. dipping postmetamorphic fault, which KYANITE crosscuts the Greenschist-Blueschist thrust. SPHENE Like the latter, it has been disturbed by sub- RUTILE sequent folding and minor faulting. The t- ps

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rium albite jadeite + quartz provides an ul- described above. The metamorphic condi- timate upper pressure limit (Fig. 5, curve 2). tions of the Amphibolite Unit are thus con- The heavy saussuritization of the metamor- strained between 580°C at 8.5 kb and phic plagioclase indicates that it was origi- 620°C at 12.5 kb. nally significantly more calcic than albite, Retrograde metamorphism of both the and so the pressure probably did not exceed Greenschist and Amphibolite Units took that of curve 4 (Fig. 5) by a large amount. place at temperatures below the biotite Also, the low jadeite content of the isograd (less than 400°C) and at relatively clinopyroxene (less than 5 mol percent low pressures (below curve 5 in Fig. 5), be- where associated with quartz, plagioclase, cause lawsonite did not crystallize. and hornblende and less than 20 mol percent in eclogite), suggests that the pressure Tectonic History of the was substantially below that of curve 2. An Catalina Schist Terrane upper pressure constraint halfway between curves 4 and 2 seems reasonable, therefore. The evidence critical to the tectonic For an amphibolite-facies temperature model is summarized as follows: (1) The range of 500° to 700°C, P is constrained three tectonic units (Blueschist, Green- between 7 and 14 kb. schist, and Amphibolite) were derived from Recent work by Raheim and Green assemblages of clastic sedimentary rocks, (1975) allows the distribution coefficient chert, mafic igneous rocks, and ultramafic Figure 5. Pressure-temperature curves for ex- material. The proportions are different in ( r.a-cpx _ (Fe/Mg) \ perimentally determined or computed phase K Ga each unit, but the rock types involved are V D (Fe/Mg) / equilibria pertinent to the metamorphism of the Cpx similar. (2) The units were metamorphosed Catalina Schist. 1 and 4, Newton and Kennedy for the partition of ferrous iron and mag- at high pressures and at temperatures rang- (1963). 2, Newton and Smith (1967). 3, Johan- nesium between garnet and clinopyroxene ing from 300° to 600°C (Fig. 5). (3) The nes and Puhan (1971). 5, Liou, (1971). 6, Hol- to be used as a geothermometer. A value of ultramafic body in the Amphibolite Unit daway (1971). Mineral abbreviations: ab = al- K was determined on nine garnet-clino- was present, probably in the form of bite, jd = jadeite, ky = kyanite, law = law- d pyroxene pairs from a garnet—brown peridotite, during the amphibolite-facies sonite, qtz = quartz, sill = sillimanite. metamorphism. (4) Retrograde metamor- CBS, CGS, CA: P-T constraints on the condi- hornblende—clinopyroxene inclusion in the tions of metamorphism of the Catalina Blue- Amphibolite Unit ultramafic body. phism of the Amphibolite and Greenschist schist, Greenschist, and Amphibolite Units, re- Analyses were performed by electron Units occurred at low temperatures and rel- spectively. X = preferred values. microprobe on garnet rims and adjacent atively low pressures. (5) The three units clinopyroxenes; analytical procedures and were juxtaposed by postmetamorphic schist suggests pressure in excess of about 7 results are given by Piatt (1975). The mean thrusting. kb at 450°C (Ernst, 1963; Maresch, 1973), value of KD was 9.24 ± 0.04. The low vari- and at that temperature, the presence of ation among the nine values suggests that Discussion clinozoisite in place of lawsonite provides the minerals analyzed had reached chemical an upper pressure limit (assuming PL = equilibrium. The similar rock assemblage, high- PH20) of about 10 kb (Fig. 5, curve 1). If P is assumed to lie in the range 7 to 14 pressure metamorphism, complicated Thus, the Greenschist Unit was probably kb, the equation deformational history, and present intimate metamorphosed at ~450°C and 7 to 10 kb. _3686 + 28.35 xP(kb) association of the three Catalina Schist The mineralogy of the Amphibolite Unit ToK units suggest that they represent different nl K + 2.33 does not closely constrain the conditions of d parts of a zoned metamorphic complex. metamorphism. The equilibrium anorthite (Raheim and Green, 1975) gives an equil- K-Ar ages of about 110 m.y. on amphibo- + vapor zoisite + kyanite + quartz pro- ibration temperature in the range 580° to lite and blueschist from Santa Catalina Is- vides a lower pressure constraint (Fig. 5, 620°C. This is probably close to the peak land (Suppe and Armstrong, 1972, Fig. 3) curve 4), since the assemblage zoisite + metamorphic temperature for the Am- are consistent with this. kyanite + quartz appears to have been sta- phibolite Unit as a whole. If this is true, the relative P-T conditions ble at the peak of metamorphism in Using this more accurate temperature de- of metamorphism of the units are of interest metasediments and in veins. Plagioclase, termination, the pressure can be con- (Fig. 5). It is generally accepted that the however, was also stable, hence the equilib- strained more closely using the reasoning blueschist-facies metamorphism of the Franciscan Complex occurred under a low regional geothermal gradient. For example, the conditions of metamorphism of the Figure 6. Idealized section through a recently ini- Catalina Blueschist Unit require a P-T gra- tiated subduction zone, dient of about 33°C/kb. It is clear, however, showing approximate dis- that the Greenschist and Amphibolite Units tribution of isotherms. Ini- were not metamorphosed along this gra- tial geothermal gradients dient, because the required temperatures within the overthrust and (450° and 600°C) would only be reached at underthrust lithospheric pressures of 13.5 and 18 kb, respectively. plates were calculated, as- The higher temperatures must rather have suming heat-flow values of resulted either from lateral changes in the 1.6 and 1.3 ¿ical/cm2 sec, re- regional geothermal gradient or from the spectively. Heat derived o io a? M 4p so so TO BO 90^ from radioactivity or fric- ' ' ' ' ' ' ' ' ' ' effects of a local heat source. tion is neglected. CBS, CGS, CA: regions where the physical conditions correspond to the estimated Possible sources of lateral or localized metamorphic conditions of the Catalina Schist tectonic units. thermal gradients in a subduction zone are

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(1) the oceanic lithosphere below the sub- Metamorphism of the Amphibolite Unit in others, 1967, Fig. 1) and to the Diablo ducted supercrustal prism, (2) the Benioff this environment is thus reasonable. Range (Ernst, 1965, 1971a). slip surface itself, and (3) the upper mantle After this prograde metamorphic The regional structure of the Franciscan in the hanging wall above the subducted episode, the zoned metamorphic complex Complex is still poorly understood, owing prism. cooled and was uplifted. The greenschist to the scarcity of fossils or distinctive If the subducted oceanic lithosphere has a and amphibolite zones underwent limited marker horizons and the complex and ir- normal thermal gradient, it will not be a low-temperature, relatively low-pressure regular structural style. Recent detailed source of significant heat to material lying retrograde metamorphism. Serpentiniza- work in the northern California Coast on its upper surface. In the plate-tectonic tion of the hanging-wall peridotite occurred Ranges (Suppe, 1973; Blake and Jones, model adopted here, the downflow of the before break-up of the zoned metamorphic 1974; Maxwell, 1974), the San Francisco subducted slab is considered responsible for complex into the present tectonic units, and peninsula (Hsii and Ohrbom, 1969), and the low regional geothermal gradient, and it may have accompanied the retrograde the Diablo Range (Cowan, 1974) suggests thus it cannot also function as a heat metamorphism. Subduction probably con- that much of the Franciscan is composed of source. tinued beneath the complex during these internally coherent slabs of widely varying Frictional heating along the Benioff slip events, since the preservation of the original rock type, age, and metamorphic grade, surface was shown by Oxburgh and Tur- mineral assemblage in the Blueschist Unit separated either by faults or by mappable cotte (1970, p. 1669) to be of great poten- implies that the regional geothermal gra- bodies of tectonic mélange (Cowan, 1974). tial importance, but at crustal levels the un- dient was kept low. The forces imposed by Most of the mappable slabs consist of gray- derthrusting movement appears to be dis- this continuing subduction may have wacke and some of basaltic rocks, chert, tributed throughout the subducted super- caused the postmetamorphic folding and limestone, or serpentinite. The fault- crustal prism sandwiched between the two break-up of the zoned metamorphic com- bounded slabs and the mélange bodies, lithospheric plates, and significant local plex. both of which may have outcrop widths of heating is unlikely. The ultramafic body in the Amphibolite several kilometres, appear to have regional- At the start of subduction, the super- Unit is explained by the model as a frag- ly gentle dips, usually to the east, but at- crustal prism is thrust beneath the crust ment of the original hanging-wall upper titudes and contacts have been considerably and upper mantle of the hanging-wall mantle. The apparently unfaulted contact complicated by later folding and faulting. lithospheric plate, and the isotherms in the between the ultramafic body and the Outcrop-sized blocks in mélanges include latter will be transected by the thrust sur- amphibolite-facies schist was presumably a graywacke, chert, and basaltic volcanic face (Fig. 6). At the depths responsible for movement surface at some time during rocks. Like the coherent Franciscan units blueschist-facies metamorphism, the initial subduction, but it was welded during the from which they are probably derived, the temperature in the hanging-wall upper subsequent amphibolite-facies metamorph- blocks contain zeolite-, prehnite- mantle would be 600° to 700°C, sufficient ism and thus differs from the post- pumpellyite—, or blueschist-facies mineral to create an inverted thermal gradient in the metamorphic Ollas and Greenschist- assemblages; considerable variations in cold subducted prism. Blueschist thrusts. metamorphic grade may be found even Tectonic Model. I suggest, therefore, Figure 7 shows a possible sequence of within one mélange body (Cowan, 1974). that the Catalina metamorphic complex events which explains the present distribu- Also present, although relatively rare, are was affected by an inverted temperature tion of the schist units in terms of eastward blocks of amphibolite, eclogite, and gradient, decreasing downward and west- underthrusting. The main problem is to ex- glaucophane-epidote schist — metamorphic ward from the hanging wall of the subduc- plain the presence of the garnet amphibolite grades higher than those found in most tion zone (Fig. 6). The overall geothermal tectonic blocks found between the Blue- coherent Franciscan units. gradient was kept low by continued sub- schist and Greenschist Units. As mentioned The tectonic model proposed for the duction of cold material. The underthrust- above, these are identical to inclusions Catalina Schist terrane may shed some light ing motion that accompanied metamor- found within the ultramafic body. To ex- on the origin and emplacement of the phism was distributed throughout the com- plain this, I postulate an early post- high-grade tectonic blocks in the Franciscan plex (and probably for some distance up metamorphic thrust (step 2 in Fig. 7), not Complex, the overall variation of age and into the hanging-wall peridotite) and pro- now preserved, that placed amphibolite and metamorphic grade across the Franciscan duced the synmetamorphic deformation. ultramafic rock over the lower grade schist. terrane, and the interleaving of fault- The complex was then welded to the hang- Once the ultramafic rock had been serpen- bounded slabs of different ages and ing wall, downward movement ceased, and tinized, pieces of serpentinite containing metamorphic grades. subduction continued beneath it. amphibolite blocks could be torn off during The downbowing of the isotherms shown step 3 and emplaced along the Green- High-Grade Tectonic Blocks in Figure 6 is schematic and is based on schist-Blueschist thrust. Oxburgh and Turcotte (1970, Fig. 14). Fig- One of the most intriguing problems of ure 6 is also similar in many respects to the Franciscan Complex is the origin and FRANCISCAN COMPLEX figures of Ernst (1970, Fig. 3; 1973, Fig. 5). emplacement of outcrop-sized tectonic The variation introduced here is that the blocks of higher metamorphic grade than hanging wall of the subduction zone is ini- Unlike the Catalina Schist terrane, much most of the coherent Franciscan units. The tially hot enough to create a limited thermal of the Franciscan Complex is only incip- occurrence, field relationships, and petrol- aureole in the subducted supercrustal iently metamorphosed, carrying zeolite- ogy of these high-grade blocks have been prism. Assuming an oceanic heat flow of and prehnite-pumpellyite—facies minerals reviewed by Bailey and others (1964, p. 1.6 jtical/cm2 sec behind the subduction (Bailey and others, 1970, Fig. 5). Significant 96-105) and Coleman and Lanphere zone (Oxburgh and Turcotte, 1970, p. areas of more thoroughly recrystallized (1971). More detailed descriptions of indi- 1669), the P-T conditions at a depth of 31 blueschist-facies rocks are restricted to the vidual occurrences are given by, among km within a standard section of oceanic more easterly parts of the Franciscan ter- others, Brothers (1954), Borg (1956), lithosphere would be about 710°C at 10 kb. rane in northern California (Blake and Bloxam (1959), Coleman and Lee (1963),

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Coleman and others (1965), Ernst (1965) fossil ages from the Franciscan Complex, and a glaucophane-lawsonite schist block Ernst and others (1970), Dudley (1972), which range from Late Jurassic to Eocene gives an age of 130 m.y. (Cowan, 1974). Hermes (1973), Cowan (1974), and Piatt (Blake and Jones, 1974, Table 1). Hence, Physical Conditions of Metamorphism. (1975). the high-grade tectonic blocks are among Tectonic blocks of normal blueschist and Most blocks have a basaltic composition the oldest rocks in the Franciscan Complex lower grade facies were metamorphosed in and have been metamorphosed to form and were probably all formed about 150 the P-T field indicated on Figure 8 for "in- blueschist (with glaucophane, lawsonite, m.y. ago. The only apparent exception to place Franciscan." aragonite, and omphacite), glaucophane- this age picture is the Catalina Amphibolite Coarse-grained glaucophane-epidote epidote schist (commonly with garnet, om- Unit, which yields K-Ar ages of about 110 schist (often referred to as "high-grade phacite, calcite, and rutile), amphibolite m.y. (Suppe and Armstrong, 1972, Table 1; blueschist," for example, Coleman and (with or without garnet, plagioclase, and Forman, 1970). Lanphere, 1971, p. 2403) generally lack the clinopyroxene), or eclogite (which may Tectonic blocks containing normal blueschist-facies index minerals lawsonite contain epidote and glaucophane or horn- blueschist-facies mineral assemblages have and aragonite and probably formed outside blende). They are found in fault zones, in not been dated systematically. They are their stability fields (Fig. 8). Pressures in ex- mélange, or in association with serpentin- probably derived from the presently excess of about 7 kb are indicated by the pres- ite, and they lie in a sheared shale or serpen- posed coherent Franciscan terranes and ence of glaucophane (Ernst, 1963; tine matrix. The margins of the blocks are thus have metamorphic ages spread Maresch, 1973) and omphacite (Essene and commonly retrograded, and they may have throughout late Mesozoic time. The Fyfe, 1967). Oxygen isotope geother- selvages of course-grained talc, actinolite, lawsonite-bearing metagraywacke blocks mometry indicates metamorphic tempera- and chlorite. from a tectonic mélange in the Diablo tures in the range 410° to 535°C (Taylor K-Ar dates reported by Coleman and Range give K-Ar ages of 106 and 116 m.y., and Coleman, 1968). P-T constraints on Lanphere (1971, Fig. 7) on mica and calcic amphibole from glaucophane-epidote schist, amphibolite, and eclogite all lie in Southwest Northeast the range 140 to 150 m.y. These contrast with K-Ar dates obtained from lawsonite- bearing Franciscan rocks (for example, Suppe and Armstrong, 1972, Table 1), which range from 70 to 151 m.y., and with .F.vi.'tó)"'/,-;;-^ slip surface ¡« step 2

'-'THOsp 'Wffi/C Figure 7. Hypothetical sequence of thrusting events to explain the dis- slab tribution of metamorphic units and tectonic blocks on Santa Catalina Island. Step 1. Situation during main metamorphic event, with upper- mantle peridotite in the hanging wall of the subduction zone and a metamorphic "aureole" decreasing in temperature downward into the subducted sedimentary prism. Active slip surface (a) shown as a heavy Source of Tectonic'':\V.:\\vV:;X:v;:;-. :.. ^ s.s. in line; Slip surface during the next phase of movement shown as a dashed blocks in step 3 "•••'••.:•^ICvVA^vV:::-.-. ^ "-'.j;step 3 line. Step 2. After uplift, cooling, and serpentinization of the hanging- wall peridotite, the low-grade schist units and thrust along slip surface (b) beneath the serpentinite and amphibolite facies rocks. Step 3. Green- schist-Blueschist thrust (Gs-Bs Thrust on map). Tectonic blocks of serpentinite and amphibolite are carried along it. Step 4. Ollas thrust. Step 5. North Side and Airport faults. Present topographic surface shown s.s. in for reference. Notes: Steps 1 to 4 all involve underthrusting with the ^step 4 same sense of motion, yet they sandwich high-grade blocks between lower grade units. Steps 2 to 4 probably occurred as a continuous event, Tectonic blocks"-; Gs-Bs Thrust and the tectonic units can be visualized as gigantic tectonic blocks themselves, rolling along in a megashear zone. If buoyant uprise and underthrusting are concurrent, the underthrust units will not be continuously transported to greater depths. Earlier thrusts were deformed during the later movements, the resulting folds and minor faults are omitted for clarity. ^SiSf8ib /.¿¿'^Thrus0Mast

Gs-Bs ' s.s. in'VJ^i??'Thrust step 5 s Airport Gs-Bs Thrust Faulty North Side Fault

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the conditions of metamorphism of these rological data, but it is now generally ac- XJD blocks are shown in Figure 8. Considerable cepted that the ultramafic bodies were em- CL overlaps into the stability fields of lawsonite placed at low temperatures. ,1 and aragonite are shown to allow for the Ernst (1973) considered the coherent amphibolite / stabilization of epidote into the lawsonite Franciscan terranes and the tectonic blocks •15 blocks field by Fe3+ and for reversion of aragonite to have been metamorphosed along a con- yff^ 3 to calcite during unloading. glaucophane-epidotey^-lj / \ sistent P-T path, corresponding to different schist blocks\/ / /;' / j Amphibolite-fades blocks are relatively depths of burial in an environment of low / rare in the Coast Range Franciscan, and lit- geothermal gradient. The P-T gradient of tin i tle data are available on which to base an about 33°C/kb required for the metamorph- •10 / / \ X / estimate of the physical conditions of their ism of jadeitic metagraywacke, however, yi / I 1 / is lies at pressures higher than the stability I formation. The presence of almandine- / // pyrope-grossularite garnet and diopsidic limit of albite (Fig. 8, curve 2). Ernst's ' / / J S clinopyroxene and the lack of chlorite or model does not seem generally applicable, / / / / blue amphibole as prograde phases in most therefore, since many amphibolite blocks 5 / / / / blocks suggest normal amphibolite-facies carry prograde plagioclase. temperatures, above 500°C at least. The Coleman and Lanphere (1971) attempted temperature could not have exceeded to bypass the problem by suggesting that 700°C, because sheet silicates and epidote the blocks represent disrupted parts of a 100 200 300 400 500 600 T°C were stable during metamorphism. The as- "cryptic metamorphic terrain," formed be- Figure 8. Pressure-temperature constraints on sociation of aluminous hornblende, clino- fore the rest of the Franciscan, presumably pyroxene, and garnet with sodic plagioclase the conditions of metamorphism of Franciscan under different P-T conditions. Lawsonite- metamorphic rocks. Phase equilibrium curves 1 and epidote is characteristic of high-pres- and jadeite-bearing rocks, however, were to 5 are as in Figure 5. sure metamorphic terranes such as the San- being formed at the same time. Examples bagawa belt in Japan; this association is ab- include the South Fork Mountain Schist facies units but that the pressures were not sent from low-pressure facies terranes such (143 m.y.); lawsonite-bearing metabasaltof greater by a corresponding amount. Thus, as the Abukuma Plateau. A lower pressure the Taliaferro Metamorphic Complex, as argued for the Catalina Schist terrane, a limit of 6 kb at 500°C for this association which contains jadeitic metagraywacke heat source at depth seems needed. I pro- (corresponding to the Barrovian-facies (151 m.y., both of these from Suppe and pose, therefore, that amphibolite, eclogite, series, Turner, 1968, Fig. 8.4) is reasonable. Armstrong, 1972); and jadeitic meta- and glaucophane-epidote schist were The presence of plagioclase indicates that graywacke from Angel Island (145 m.y., formed in response to an inverted thermal pressures were below the breakdown curve Peterman and others, 1967). Thus, the P-T gradient developed below the hot hanging for albite, as indicated in Figure 8. Most paradox remains. wall of the subduction zone. Once these amphibolite blocks have been partly retro- Hermes (1973, p. 30) suggested that metamorphic rocks were welded to the graded to greenschist- or blueschist-facies amphibolite at Panoche Pass developed upper plate, they would cause some mineral assemblages (Coleman and Lan- within the oceanic crust before subduction. buoyant uplift, while material continued to phere, 1971, p. 2404; Ernst and others, But the maximum probable pressure within be thrust beneath them. This younger ma- 1970, p. 29, 61-62; Hermes, 1973, p. 3). the oceanic crust (about 2.5 kb) is inade- terial would be separated from the hanging Eclogite is stable over a wide range of quate to explain the observed mineral as- wall by the previously accreted schist and temperatures, but it probably only forms semblages. would not be significantly heated. The heat under high-pressure conditions. Distribu- Many of the blocks have a basaltic com- supply in the overhanging wedge of lithos- tion coefficients for the partition of Fe2+ position, which suggests that they may have phere would also be fairly limited, and so and Mg between coexisting garnet and been derived from the deeper and hotter for both these reasons, high-temperature clinopyroxene analyzed by Coleman and parts of the subducted oceanic crust below metamorphic rocks would only form in the others (1965) range from 20.4 to 29.9; the Franciscan supercrustal prism (see also early stages of subduction, and they would these values indicate equilibration tempera- Ernst and others, 1970, p. 244; Dudley, therefore be the oldest rocks in the subduc- tures in the range 420° to 470°C, using the 1972, p. 3499). There are several problems tion complex. equation of Raheim and Green (1975) and with such a model: (1) The initial thermal Emplacement. Because the high-grade assuming a pressure of 10 kb. Much Fran- gradient in the subducted plate would have metamorphic rocks were at the top of the ciscan eclogite occurs as interlayers or pods to be abnormally high to produce the re- structural pile, any reverse fault of moder- within blocks of amphibolite or glauco- quired temperatures of 500° to 600°C ate displacement that dipped eastward phane-epidote schist, without evidence of within the 6-km-thick crust. The resulting more steeply than the main tectonic units disequilibrium. It is probable, therefore, heat flow would probably raise the ambient would emplace high-grade rocks into the that the eclogite crystallized under the same temperature in the subduction zone above younger, lower grade rocks beneath them. physical conditions as those rocks and that the limits of blueschist metamorphism. (2) Such faulting is a logical response to the the alternative mineral assemblage is due to The metamorphic rocks would somehow forces exerted by plate convergence, so differences in bulk chemistry (Piatt, 1975, have to be emplaced upward through the blocks could have been emplaced into suc- Fig. 31). overlying kilometres of gabbro and basalt cessively younger material as long as sub- Origin. The P-T constraints shown in into the Franciscan supercrustal prism. (3) duction lasted. Also, should the rate of up- Figure 8, although broad, help to evaluate The model cannot explain high- lift have exceeded for a time the rate of the various explanations of the metamorph- temperature metamorphism of sedimentary accretion, blocks could have been emplaced ism of these blocks. Essene and others rocks, as on Santa Catalina Island. (4) The downward into material metamorphosed at (1965, p. 702-703) proposed that hot restricted age range of the blocks remains lower pressures as well as lower tempera- ultramafic intrusions into the Franciscan unexplained. tures (see section on interleaved rocks of Complex developed thermal and It appears from Figure 8 that the tectonic metamorphic grade). metasomatic aureoles of amphibolite and blocks were metamorphosed at higher Both the metamorphism of the high- eclogite. This model allows for the pet- temperatures than the coherent blueschist- grade rocks and their emplacement into a

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lower grade environment are thus natural grade near the coast, through prehnite- Armstrong, 1972), covering the entire and predictable results of the subduction pumpellyite facies, to lawsonite-bearing range of fossil ages. Suppe and Armstrong process. blueschist in the east (Bailey and others, (1972) have shown that some of the older It should be noted that several workers 1970). Recent work has shown that the units such as the South Fork Mountain (Hsü, 1971, p. 1167; Lockwood, 1972, p. Franciscan Complex in this area is made up Schist were metamorphosed before most of 283; Maxwell, 1974, p. 1201) have sug- of tectonic slabs larger than outcrop size the lower grade Franciscan rocks were de- gested that the high-grade blocks were em- separated by faults and mélange units posited as sediments. placed in unmetamorphosed rocks by (Maxwell, 1974; Blake and Jones, 1974); Blake and others (1967), noting that the sedimentary processes, the mélanges being rocks of high and low metamorphic grade more easterly, higher grade rocks tend to be in fact deformed olistostromes. This are interleaved (Suppe, 1973); and, as dis- structurally higher, suggested thai they mechanism is compatible with the model cussed below, some of the blueschist-facies were metamorphosed in a zone of tectonic proposed here; the model requires erosion units were metamorphosed before the or fluid overpressure beneath the Coast of the Franciscan terrane, including the youngest ones were deposited as sediments. Range thrust, with the grade of meta- high-grade rocks at the top of the pile, con- Thus the zonation is only apparent on the morphism increasing upward and toward current with underthrusting. The only ob- scale of the Franciscan Complex as a whole, the thrust. Ernst (1971b), on the other jection to the sedimentary emplacement and many metamorphic facies boundaries hand, suggested that the metamorphic zo- mechanism is that no high-grade blocks are tectonic. nation reflects the dip of the paleosubduc- have been described from an undeformed The meager fossil evidence (Bailey and tion zone, the higher grade rocks having Franciscan olistostrome. others, 1964, p. 115-123; Blake and Jones, been buried more deeply. Neither of these 1974, Table 1 and Fig. 1) suggests that suggestions accounts for the pattern of ages. Age and Metamorphic Grade there is an age zonation that roughly cor- Both models imply that the zonation was in the Franciscan Complex responds to the change in metamorphic formed during a single metamorphic event, grade. Fossils from the zeolite-facies coastal which is clearly disallowed by the age data. Several workers have shown that there is belt rocks are mainly of Cenomanian to a rough zonation in both age and Eocene age; fossils from the pumpellyite- Tectonic Model for the metamorphic grade across the Franciscan bearing belt inland range in age from Franciscan Complex Complex as a whole. This zonation is best Tithonian to Cenomanian; and the eastern, seen in the northern California Coast predominantly blueschist-facies belt yields The relationships described above can be Ranges, where it has not been grossly dis- only Tithonian and Lower Cretaceous fos- explained by postulating that the Francis- rupted by Tertiary strike-slip faulting. In sils. can Complex was formed by the successive this area metamorphic grade can be shown K-Ar ages on coherent Franciscan units eastward underthrusting of progressively to increase from predominantly zeolite range from 76 to 151 m.y. (Suppe and younger sheets of sediment throughout late Mesozoic time (Fig. 9). Each thrust sheet was metamorphosed, with grade increasing to the east with depth, and welded to the upper plate. This accretion was compensated for by progressive uplift of the Fran- ciscan Complex as a whole and erosion of material at the surface. This process would create a pile of east-dipping thrust sheets, successively younger downward and westward, with metamorphic grade in the exposed edges of the thrust sheets decreasing to the west (Fig. 9). Figure 9. Hypothetical The concept of successive eastward un- sequence of events to ex- derthrusting of tectonic mélanges was sug- plain the zonation of age gested by Hsii (1971) and forms the basis of and metamorphic grade models by Maxwell (1974) and Blake and across the Franciscan Com- Jones (1974). Evidence for an imbricate plex. Progressively younger structure of this type in present-day inner slices are labeled 1 to 5. trench walls was discussed by Karig (: 974). Stipple indicates blueschist metamorphism, at depths of These models do not discuss the metamor- over 20 km. phic implications, however. The important point introduced here is that if underthrusting and accretion occur concurrently with uplift and erosion, the observed zonation of age and metamorphic grade will form as a direct consequence. A similar model has been presented by Ernst (1975). It should be pointed out that the westward migration of the subduction zone proposed by Hsii (1971) is not a necessary consequence of the model as presented here. If accretion is compensated for by erosion, there is a crude mass balance, and the subduction complex will remain the same width (Fig. 9).

250 km 1 The model illustrated in Figure 9 is

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simplified for clarity. Underthrusting must or jadeite-bearing rocks are interleaved greenschist or glaucophane-epidote schist, have been accompanied by intense defor- with relatively low-pressure units. The for- and blueschist. These complexes are likely mation, and underthrusting and accretion mation of these structures presents a prob- to be extensively disrupted by later tec- may have been continuous or irregular lem similar to that of the emplacement of tonism caused by continued underthrusting rather than episodic. In particular, compli- the high-grade tectonic blocks — but on a and uplift. The Catalina Schist terrane and cations were caused by the interleaving of larger scale. the high-grade tectonic blocks in the Fran- rocks of different age and metamorphic The configuration of the model in Figure ciscan Complex are interpreted as the dis- grade. The model also presents a problem 9 depends on the balance between rate of rupted remnants of such zoned complexes. because it requires the removal of a vast accretion at the bottom of the pile and rate Other occurrences of amphibolite and eclo- overburden, much of it ultramafic, from the of erosion at the top. If the rate of uplift and gite in association with alpine ultramafic Franciscan during late Mesozoic time. This erosion exceeds the rate of accretion for a rocks and glaucophane schist may have implies that the continental margin and the while (perhaps as a result of a slowing similar origins. Great Valley sequence were separated from down of the rate of subduction) the subduc- The gross zonation of age and metamor- the Franciscan trench by an offshore arc, tion complex will lose mass, it will become phic grade in the Franciscan Complex re- from which this material was being re- thinner, and its outcrop will become nar- sults from successive underthrusting and moved (see also Bailey and Blake, 1969, p. rower. In Figure 9, drawn for a balance be- accretion of east-dipping slices of super- 227-228). Some present-day trenches have tween accretion and erosion, the blueschist crustal rock concurrent with uplift and ero- a topographic ridge in the inner trench wall isograds in each successive slice lie one sion. (Karig, 1974, p. 53-54). These ridges are above the other, but if the subduction com- Interleaving of rocks of different ages and being uplifted, and some rise above sea plex becomes thinner, the isograds in metamorphic grades results from changes level. The postulated Franciscan uplift may younger lower slices will be displaced in the balance between accretion and ero- have been analogous. There is little direct landward relative to the older higher slices. sion, coupled with reverse faulting. evidence for the existence of this uplift, or Thus a seemingly anomalous situation for sediments derived from it. Blake and arises, with high-pressure rocks directly ACKNOWLEDGMENTS Jones (1974, p. 347) suggested that vol- overlying lower pressure ones. Any reverse canogenic sandstone near the base of the fault that dips eastward more steeply than Work on Santa Catalina Island was sup- Great Valley sequence and the extensive the subducted slices will then interleave ported by the University of California at Lower Cretaceous sedimentary serpentine high- and low-pressure rocks. Santa Barbara and by a Geological Society beds may have been derived from the un- Changes in the rate of subduction and of America Penrose Fund grant. Permission derlying ophiolitic rocks. Many of the small late reverse faulting along surfaces of vary- to work on the island and considerable bodies of serpentinite scattered throughout ing dip are both sufficiently likely that rocks logistic support were provided by the Santa the Franciscan may also be of detrital or of different age and metamorphic grade Catalina Island Company. I am indebted to gravity-slide origin (Lockwood, 1972, p. should be interleaved commonly in the C. A. Hopson, E. H. Bailey, W. G. Ernst, 276) and may be derived from the pos- Franciscan Complex. J. C. Crowell, A. O. Woodford, J. Suppe, tulated uplift. Bailey and others (1964, p. and C. J. Stuart for discussion, correspon- 147) indicated that some of the younger OTHER HIGH-PRESSURE dence, long-term interest and encourage- "coastal belt" graywacke is derived from an ment, and reviews of the manuscript. older Franciscan terrane. METAMORPHIC BELTS Many problems still remain. The 30-km Many of the high-pressure metamorphic thick wedge of oceanic crust and upper belts distributed around the Pacific margin REFERENCES CITED mantle illustrated in Figure 9 is no longer and in the European Alpine system show seen. The present-day Coast Range ophiol- metamorphic zonations from zeolite Bailey, E. H., 1940, Piedmontite and kyanite ite (Bailey and Blake, 1974) is presumably a through blueschist to epidote-amphibolite from the Franciscan of Santa Catalina Is- remnant of this, modified by later tec- facies. This has been documented by Ernst land [abs. ]: Geol. Soc. America Bull., v. 51, tonism. The high-pressure Franciscan rocks (1975). In several cases the rocks of highest p. 1955. presently lying below the Coast Range metamorphic grade are also structurally Bailey, E. H., and Blake, M. C., Jr., 1969, Tec- tonic development of western California in thrust cannot have been metamorphosed in highest and are closely associated with the late Mesozoic; article 2, metamorphism their present tectonic position; the presently ultramafic rocks. Epidote-amphibolite and and its relationship with regional tectonics: overlying Coast Range ophiolite would eclogite are found in the Sanbagawa terrane Geotectonics, no. 4, p. 225-230. provide an altogether inadequate overbur- in Shikoku, Japan, for example, in close as- 1974, Major chemical characteristics of den. Possibly, the high-pressure rocks sociation with ultramafic masses such as the Mesozoic Coast Range ophiolite in Califor- moved back up along the thrust, reversing Higasi-Akaisi body (Ernst and others, nia: U.S. Geol. Survey Jour. Research, v. 2, the movement direction during subduction, 1970, p. 109-122; Mori and Banno, 1973). p. 637-656. as suggested by Ernst (1971b, p. 48). Alter- This association may result from inverted Bailey, E. H., Irwin, W. P., and Jones, D. L., natively, much of the ultramafic wedge may thermal gradients developed beneath the 1964, Franciscan and related rocks, and have been cut out by later underthrusting; hanging walls of subduction zones. their significance in the geology of western California: California Div. Mines and this would imply that the present faulted Geology Bull. 183, 177 p. base of the Coast Range ophiolite is a rela- CONCLUSIONS Bailey, E. H., Blake, M. C., and Jones, D. L., tively young feature. 1970, On-land Mesozoic oceanic crust in Inverted temperature zonations develop California Coast Ranges: U.S. Geol. Survey Interleaved Rocks of Different Grade during high-pressure metamorphism in Prof. Paper 700-C, p. 70-81. newly initiated subduction complexes, Berkland, J. O., Raymond, L. A., Cramer, J. C., Moores, E. M., and O'Day, M., 1972, One of the most important findings dur- owing to the residual heat in the hanging- What is Franciscan?: Am. Assoc. Petroleum ing recent work in parts of the northern wall lithospheric wedge. From top to bot- Geologists Bull., v. 56, p. 2295-2302. California Coast Ranges (Suppe, 1973) and tom, such zoned complexes consist of Blake, M. C., and Jones, D. L., 1974, Origin of in the Diablo Range (Cowan, 1974) is that ultramafic rocks from the hanging wall, Franciscan melanges in northern California, sheets larger than outcrop size of lawsonite- amphibolite or eclogite, glaucophanic in Dott, R. H., Jr., and Shaver, R. H., eds.,

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Modern and ancient géosynclinal sedimen- 1975, Systematics of large-scale tectonics Japan — Subsolidus relation of anhydrous tation: Soc. Econ. Paleontologists and and age progressions in Alpine and phases: Contr. Mineralogy and Petrology, Mineralogists Spec. Pub. 19, p. 345-357. circumpacific blueschist belts: Tec- v. 41, p. 301-323. Blake, M. C., Irwin, W. P., and Coleman, R. G., tonophysics, v. 26, p. 229-246. Newton, R. C., and Kennedy, G. C., 1963, Some 1967, Upside-down metamorphic zonation, Ernst, W. G., Seki, Y., Onuki, H., and Gilbert, equilibrium reactions in the join blueschist facies, along a regional thrust in M. C., 1970, Comparative study of low- CaAl2Si208-H20: Jour. Geophys. Research, California and Oregon: U.S. Geol. Survey grade metamorphism in the California v. 68, p. 2967-2984. Prof. Paper 575-C, p. C1-C9. Coast Ranges and the outer metamorphic Newton, R. C., and Smith, J. V., 1967, Investiga- Bloxam, T. W., 1959, Glaucophane schists and belt of Japan: Geol. Soc. America Mem. tions concerning the breakdown of albite at associated rocks near Valley Ford, Califor- 124, 276 p. depth in the earth: Jour. Geology, v. 75, p. nia: Am. Jour. Sci., v. 257, p. 95-112. Essene, E. J., and Fyfe, W. S., 1967, Omphacite 268-286. Borg, I. Y., 1956, Glaucophane schists and eclo- in Californian metamorphic rocks: Contr. Oxburgh, E. R., and Turcotte, D. L., 1970, gites near Healdsburg, California: Geol. Mineralogy and Petrology, v. 15, p. 1-23. Thermal structure of island arcs: Geol. Soc. Soc. America Bull., v. 67, p. 1563-1584. Essene, E. J., Fyfe, W. S., and Turner, F. J., 1965, America Bull., v. 81, p. 1665-1688. Brothers, R. N., 1954, Glaucophane schists from Petrogenesis of Franciscan glaucophane Peterman, Z. E., Hedge, C. E., Coleman, R. G., the North Berkeley Hills, California: Am. schists and associated metamorphic rocks, and Snavely, P. D., 1967, Sr87/Sr88 ratios in Jour. Sci., v. 252, p. 614-626. California: Contr. 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