Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous , Franciscan Complex, Author(s): John Wakabayashi Source: The Journal of , Vol. 100, No. 1 (Jan., 1992), pp. 19-40 Published by: The University of Chicago Press Stable URL: http://www.jstor.org/stable/30082316 Accessed: 10/04/2010 17:15

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http://www.jstor.org Nappes, Tectonics of Oblique Plate Convergence, and Metamorphic Evolution Related to 140 Million Years of Continuous Subduction, Franciscan Complex, California'

John Wakabayashi Earth Sciences Associates Inc., 701 Welch Rd., Palo Alto, CA 94304

ABSTRACT This paper presents a new synthesis of Franciscan Complex tectonics, with the emphasis on the pre-San Andreas fault history of these rocks. Field relations suggest that the Franciscan is characterized by nappe structures that formed during sequential accretion at the trench. The presence of these structures along with other field relations, including the lack of evidence for large offset of conglomerate suites, indicates that strike-slip fault systems of large displacement (>500 km) did not cut the FranciscanComplex during subduction. Regional geology and comparisons to modern arc-trench systems suggest that strike-slip faulting associated with oblique subduction took place inboard (east) of the Franciscan in the vicinity of the magmatic arc. The Franciscan varies along strike, because individual accreted elements (packets of trench sediment, , etc.) did not extend the full length of the trench. Different depths of underplating, distribution of post-metamorphic faulting, and level of produced the present-day surface distribution of high P/T . FranciscanComplex tectonic history can be summarized as follows: (1) East-dippingFranciscan subduction initiated beneath, and shortly after formation of, the . (2) High-temperatureprecursors to Franciscanhigh-grade tectonic blocks formed as a dynamothermal aureole during subduction initiation beneath the hot hanging wall and were underplatedto the upperplate. (3) Subduction continued, the high-grade metamorphic rocks were overprinted with assemblages of increasing P/T ratio as the hanging wall cooled, and the aureole was dismembered into blocks. (4) As subduction progressed,more material was underplated and metamorphosed as coherent . Peak metamorphic temperatures of successively subducted units de- creasedwith time as hanging wall heat continued to dissipate. (5) Trench sediments with parts of seamounts, oceanic rises, and other small masses on the downgoing plate were underplatedor offscrapedduring approximately 140 m.y. of continuous subduction, forming stacks of nappes. (6) The Mendocino Triple Junction migrated northward, and the subduction zone was replaced by a transform plate boundary associated with the San Andreas fault. Deformation and faulting related to the Neogene transform tectonic regime obscured many subduction-related structures.

Introduction

The Franciscan Complex of the California Coast sent fragments of seamounts, other oceanic rises, Ranges is an assemblage of variably deformed and and the pelagic cover and upper part of the sub- metamorphosed units that formed as a sub- ducted (e.g., Hamilton 1969; Mac- duction complex associated with east-dipping sub- Pherson 1983; Tarduno et al. 1985), possibly with duction at the western mar- a component of olistostrome blocks from the upper gin from the late through the Tertiary, a plate (MacPherson et al. 1990). period of over 140 m.y. (e.g., Hamilton 1969; Page The Franciscan is well known for high P/T 1981). Franciscan lithologies are predominantly de- metamorphism (e.g., Ernst et al. 1970; Ernst 1970, trital sedimentary rocks with subordinate basaltic 1971). The Coast Range ophiolite structurally over- volcanic rocks and , and minor . lies the Franciscan and is depositionally overlain The detrital rocks may represent offscraped and by well-bedded and of the Great underplated trench-fill sediments (e.g., Dickinson Valley sequence coeval with the Franciscan (e.g., 1970). The pelagic and volcanic rocks may repre- Dickinson 1970). The Coast Range ophiolite and lack high P/T metamor- 1 Manuscript received May 1, 1991; accepted August 18, phism. From west to east, the three sub-parallel 1991. geologic provinces of the Franciscan, Great Valley

[Journalof Geology, 1992, volume 100, p. 19-40] © 1992 by The University of Chicago.All rights reserved.0022/1376/92/10001-003$1.00

19 20 JOHN WAKABAYASHI

PickettPk. Yolla Bolly terraneor similar CentralBelt CoastalBelt GVSGreat Valley Sequence Coast Range ophiolite MF Maacamafault CF Calaveras fault HF Haywardfault H-RCF Healdsburg-RodgersCreek fault MFZ MendocinoFracture Zone

Figure 1. Generalized map of Franciscan Complex exposures in California. Adapted from Jennings (1977), Blake et al. (1984), and Jayko et al. (1986) and Wakabayashiunpub. data.

sequence, and Sierra Nevada batholith (see Figure strike-slip faulting took place within the Francis- 1) may represent respectively the subduction com- can (e.g., McLaughlin et al. 1988, in favor; Seiders plex, forearc basin deposits, and arc of an ancient 1991 against). (2) The structural relationship of arc-trench system (e.g., Dickinson 1970; Ingersoll many Franciscan tectonostratigraphic to 1978). Few dispute this general model, but major one another is poorly understood. (3) Opinions dif- questions and controversy regarding Franciscan fer on whether coarse-grainedblueschist, , history remain, for example: (1) Disagreement ex- and tectonic blocks (called "high- ists over whether or not large-scale (>500 km) grade blocks" or "knockers" in Franciscan litera- Journalof Geology TECTONICS OF OBLIQUE PLATE CONVERGENCE 21 ture) are vestiges of a pre-Franciscanmetamorphic Range; and (4) the Nacimiento block (Sur terrane event (e.g., Coleman and Lanphere 1971; Blake of McWilliams and Howell 1982). et al. 1988) or instead formed during the initial 1) Northern Coast Ranges. In this region, the stages of Franciscan subduction (e.g., Platt 1975; Franciscan has traditionally been subdivided into Suppe and Foland 1978; Cloos 1985; Wakabayashi three NW-SE trending lithologic belts: the Eastern, 1990). Central, and Coastal belts (Berklandet al. 1972). In this paper the above problems and other as- Easternmost and structurally highest, the blue- pects of Franciscan geology will be evaluated in- grade Eastern Belt consists of thrust sheets cluding: (1) the occurrence and significance of of metaclastic and metavolcanic rocks with fine- low-angle faults that formed during subduction grained metamorphic minerals generally <0.2 mm and accretion, particularly those separating Fran- in size (Suppe 1973; Worrall 1981; Blake et al. ciscan tectonostratigraphic terranes; (2) obliquity 1988). The belt has been divided into the structur- of plate convergence and its consequences in Fran- ally highest and highest metamorphic grade Pick- ciscan tectonics; (3) metamorphism and its rela- ett Peak terrane, schistose rocks commonly with tionship to tectonic evolution; and (4) aspects of complete textural reconstitution, and the structur- Franciscan accretion that may not fit a "classical" ally lower Yolla Bolly terrane, unfoliated to foli- (e.g., Karigand Sharman 1975) model of accretion, ated rocks with variable preservation of protolith such as tectonic wedging (e.g., Wentworth et al. textures (Blake et al. 1982). The Yolla Bolly terrane 1984) and distribution of high-gradeblocks (a prob- also includes a subunit of matrix melange lem that bears on the mechanisms of Franciscan (Worrall 1981; Jayko and Blake 1989). Yolla Bolly melange formation). terrane metaclastic rocks have yielded Tithonian to Cenomanian (Blake et al. 1988). Eastern Belt and similar rocks are referredto as "coherent " in this paper. Among coherent (non- The Franciscan Complex: Regional Overview melange) Franciscan rock units, only the coherent The present outcrop pattern and structure of Fran- blueschists exhibit penetrative fabrics, with the ciscan rocks has been heavily influenced by Neo- exception of local weakly-developed in gene faulting and deformation that initiated (and some standstones. continues to the present) after east-dippingFrancis- The Central Belt lies west of and structurally can subduction ceased and was replaced by a dex- below the Eastern Belt and contains a large propor- tral plate boundary represented by tion of shale matrix melange that includes blocks the San Andreas and related strike-slip faults (fig- of varying lithologies and metamorphic grades ure 1). Considerable folding and dip-slip faulting (Hsfi 1968; Gucwa 1974; Cloos 1982; Blake et al. has accompanied the strike-slip faulting (e.g., Page 1988). The boundary between the Eastern and Cen- 1981). In this paper, an attempt is made to "see tral belts is somewhat ill-defined, and Seiders through" this later deformational overprint and (1991) has mapped conglomerate units that strike evaluate the subduction history of the Franciscan across the previously defined boundarywithout ap- Complex by focusing on features that have largely parent offset. The Central Belt includes units of escaped disruption by the later event. largely coherent internal structure ranging up to Within the Franciscan Complex are numerous tens of kilometers in size. Detrital rocks of these fault-bounded units, each with distinctive lithol- coherent units have yielded fossils ranging from ogy, age, metamorphism, and structure, that have Tithonian to Campanian in age (Blake et al. 1984, been termed "tectonostratigraphic terranes" (e.g., 1988). Franciscan melange matrix displays a strong Blake et al. 1982). Names of units discussed in this shear fabric, and most Franciscan melanges, in- paper follow Franciscan terrane nomenclature of cluding those in the Eastern and Coastal Belts, ap- Blake et al. (1982) except for composite or pre- pear to have functioned as shear zones during part viously unnamed terranes (for brief descriptions of of their history (e.g., Cloos 1982). Some Franciscan various Franciscan terranes discussed in this paper melanges may have originated as olistostromes see table 1, in the Depository File, available free of that were subsequently subducted (e.g., Aalto and charge upon request from The Journal of Geology). Murphy 1984; Cowan 1985; MacPherson et al. The Franciscan can be divided into four regions 1990). Few studies have been conducted on Fran- (fig. 1) that differ in structural and lithologic char- ciscan melange matrix mineralogy, but acter: (1) the northern Coast Ranges; (2) the re- identified in some samples suggests at least local gion between the San Andreas and Hayward/ high P/T metamorphism of the matrix (Cloos Healdsburg-Rodgers Creek faults; (3) the Diablo 1983), and paleotemperature studies are consistent 22 JOHN WAKABAYASHI

with deep burial (>10 km) of some of the melanges (e.g., Coleman and Lee 1963; Maruyama and Liou (Underwood et al. 1988). Metamorphism of me- 1988), will also be referred to as the Ward Creek lange matrix and included blocks of various meta- coherent blueschists. morphic grades may suggest circulation of melange 3) Diablo Range. In the Diablo Range, slabs of to variable depths and return flow (e.g., Cloos 1982, metagreywacke, mostly of blueschist grade, are 1984). Most coherent units within the Central Belt separated by melange zones that commonly con- are of prehnite-pumpellyite grade (Blake et al. tain high-grade blocks (Raymond 1973; Cotton 1984, 1988). 1972; Cowan 1974; Crawford 1975; Page 1981; The Coastal Belt (including the Yager terrane of Blake et al. 1984). Some of the coherent slabs (the Underwood and Bachman 1985), the westernmost Eylar Mountain terrane)have been correlatedwith of these three Franciscan belts, structurally under- the Yolla Bolly terrane of the Eastern Belt of the lies the Central Belt (Bachman 1978; Blake et al. northern Coast Ranges (Blake et al. 1982), although 1988). The Coastal Belt is composed of variably these slabs exhibit more widespread crystallization deformed and shale, including melange of jadeitic pyroxene. South of the Diablo Range and units, with minor , limestone, and chert and east of the San Andreas fault, the Franciscancrops is of mostly zeolite grade (Bachman 1978; Blake out in limited exposures consisting mainly of me- et al. 1988). Excluding pelagic rocks, the Coastal lange that locally contains high-grade blocks and Belt contains fossils ranging in age from Paleocene . The Diablo Range domain is bounded to (Blake et al. 1988). on the west by the San Andreas and Hayward 2) Block. The region bounded faults. West of the San Andreas fault is the Salinian roughly by the San Andreas, Hayward, and Healds- block, a 700 km x 70 km allochthonous slice of burg-Rodgers Creek faults, herein called the San with granitic and high T/P meta- Francisco block. In this region, the Franciscan morphic basement. comprises a stack of nappes composed of coherent 4) Nacimiento Block or Sur terrane. West of the terranes of varying lithology and metamorphic Salinian block across the Sur-Nacimiento fault grade, separatedby low-angle melange zones (Blake zone is a belt of Franciscan exposures composed et al. 1984; Wakabayashi and Moores 1988b; Wa- primarily of melange (e.g., Hsfi 1968; Page 1981; kabayashi 1989). Melange is subordinate to coher- the Sur terrane of McWilliams and Howell 1982). ent units in much of this area. With the exception These rocks may have originated farther south of locally severe deformation near the San Andreas along the western North American plate margin fault (Page 1981), this domain may be the least af- than the rest of the Franciscan based primarily on fected by Neogene deformation of any region in the paleomagnetic data (e.g., Page 1982; McWilliams Coast Ranges (Herd 1978; Fox 1983), and as such and Howell 1982; Vedder et al. 1983). However, provides a good field area to examine Franciscan if Neogene strike slip displacement is undone (a field relations that may have resulted from subduc- palinspastic reconstruction that undoes Neogene tion. The nappes of this region have been gently strike-slip faulting is presented in Depository File, folded about NW-SE subhorizontal axes. Erosion figure C), similar conglomerate compositions for has produced the present-day map pattern of belt- Franciscan east of the San Andreas fault (the San like rock units with their long axes oriented paral- Francisco block) and Sur terrane rocks suggest a lel to those axes. One of the more notable rock linking of the two terranes (Seiders 1988; Seiders units is a coherent blueschist that resembles the and Blome 1988). In addition, the apparentlatitudi- structurally high, schistose, Pickett Peak terrane nal shift suggested by the paleomagnetic data has of the northern Coast Ranges, but is coarser- been questioned (Tarduno 1990; Butler et al. 1991). grained with metamorphic minerals up to 1 mm in size (Wakabayashiunpub. data). The two largest Low-Angle Faults Related to exposures of this unit are a 17 x 4 km exposure Franciscan Subduction (metabasalt and metagreywacke) near Cazadero (Coleman and Lee 1963; Erickson 1991) and a 70 The central San Francisco Bay region (BayArea) of x 3 km belt (mostly metagreywacke) that extends the San Francisco block preserves some of the best porthwest from near Healdsburg (Bailey et al. examples of low-angle tectonic contacts formed 1964; Blake et al. 1971; Blake et al. 1984) (figures during Franciscan subduction and accretion. Fran- 1 and 4). These rocks will be informally referredto ciscan rock units in this region comprise a stack of as the Skaggs Springs . The specific expo- nappes (Blake et al. 1984; Wakabayashiand Moores sure of these schists near Cazadero, on which a 1988b) (figures 2 and 3) composed of seven coher- number of petrologic studies have been conducted ent tectonostratigraphic terranes separated by low- Figure 2. Franciscan Complex of the central San Francisco Bay area. Legend for figures 2 and 3 is on figure 3. All contacts shown are tectonic except the two east-west trending. Eocene contacts in the southwest part of the map. Map based on Wakabayashi (1989), Wakabayashi and Moores (1988a), Wahrhaftig (1984a, 1984b), Wahrhaftig and Wakabayashi (1989), Shervais (1989), Blake et al. (1984), Brabb and Pampeyan (1983), Blake et al. (1974), Schlocker (1974) and Bonilla (1971). 24 JOHN WAKABAYArnti

Franciscan Units: Non-Franciscan rock units: KJTM: TiburonMelange: shale and serpentiniteKSB: San BrunoMtn. nappe(San BrunoMtn. and JO: CoastRange ophiolite and correlative rocks matrixmelange with numerous'high-grade' blocks. Novato Quarry terranes): K-feldspar-bearing KG: plutonicrocks of the Salinianblock KJAI: Angel Island nappe: metavolanic and quartzofeldspathicsandstone and shale; prehniteTK: UpperCretaceous and (?) Paleocene(?) sandstone metaclastic rocks; blueschist grade; incipientlypumpellyite grade; Campanian age for NovatoQuarry and shale of the Salinianblock; locally melange. foliatedto schistose. terrane,no fossils fromSan Bruno Mtn. terrane TP: Paleocenesandstone and shale of the Salinian KA: Alcatrazterrane: quartzofeldspathic sandstone KJP: Permanenteterrane: basalt, sandstone,shale, block. and shale; prehnitepumpellyite grade; Valanginian limestone and chert; prehnite pumpellyite to E: Eocene and younger clastic rocks of the San age blueschistgrade Franciscopeninsula. KJPB: Rocks of Point Bonita: similar to MarinHunters Point Shear Zone, City College Headlandsterrane but basaltsdiffer chemicallyand Fault Zone, Hillside Fault Zone: major terrane-sense of shearin ductilefault zone containinterpillow limestone; prehnite pumpellyite bounding melanges; shale and/orserpentinite matrix grade with relativelyrare 'high-grade'blocks; shown in low-anglefault; barbs on upperplate KJMH: Marin Headlandsterrane: basalt, chert, stipple sandstoneand shale; prehnite pumpellyitegrade; KJU: undifferentiatedFranciscan detritalrocks of Albianto Cenomanianage. regionalfold axis

boundaryof majorareas of exposure

Figure 3. Schematic cross section of San Francisco Bay area Franciscan rocks. All contacts shown are tectonic. (Scale is the same as in figure2 with no verticalexaggeration.)

angle melange zones. Regional dips of the nappes Richmond is thus the same as that SW of the Ti- are defined by exposures of melange matrix folia- buron Peninsula. East of the Point Richmond expo- tion and the bedding in the coherent nappes, that sures, separated by an alluvium-covered area, co- generally dips parallel to the foliation in the herent strata and melange matrix foliation dip bounding melanges. The nappe stack is about 10 uniformly to the NE, repeating the same structural km thick, with dimensions of larger nappes of stacking order, although many of the units are about 30 km across by 70 km along strike. The thinner than their correlatives SW of the major nappe stack is folded into a broad NW-trending synclinal axis. synform centered on Tiburon Peninsula and a par- Among the features of the nappe stack are: allel antiform or faulted antiform centered on the (1) Over 99% of the high-grade blocks in this sec- eastern San Francisco Bay. Coherent strata and me- tion are present in the Tiburon melange that occu- lange matrix foliation SW of the axis of Tiburon pies the highest Franciscan structural horizon Peninsula, including the rocks in San Francisco, (Wakabayashi 1989). Most of the other high-grade dip NE, whereas those NE of the axis dip to the blocks reside in melanges separating coherent SW. NE of the regional synformal axis is Red Rock, nappes. (2) Coherent blueschists of the Angel Is- an island composed of chert and basalt of the land nappe, possibly correlative to the Pickett Peak terrane. The strata at Red Rock or Yolla Bolly terranes of the northern Coast dip to the SW (M. C. Blake, Jr. oral comm. 1986), Ranges, make up the structurally highest Francis- placing them structurally below the exposures on can coherent unit in the nappe stack. (3) Coherent the NE side of Tiburon Peninsula. The sandstones nappes appearto be internally imbricated (Blake et at Point Richmond, correlated in this study to al. 1984; Wahrhaftig 1984a; Larue et al. 1989). rocks at San Bruno Mountain, dip to the SW, plac- (4) The largest melange zones (200-1500 m thick) ing them structurally below the Marin Headlands are those separating the coherent nappes. Minor terrane rocks at Red Rock. The structural stacking melanges (up to 300 m thick) are present as part of order from the NE side Tiburon Peninsula to Point the internal imbrication of coherent nappes, partic- Journalof Geology TECTONICS OF OBLIQUE PLATE CONVERGENCE 25 ularly in the Permanente and Marin Headlands ter- tural level of accretion; (2) the Angel Island nappe: ranes. (5) The Permanente terrane,separated by the 130-150 Ma based on tentative correlations to San Andreas fault from the rest of the nappes, is similar rocks at Ward Creek and in the Pickett correlated to a structural level somewhere below Peak terrane of the northern Coast Ranges that the Marin Headlands terrane,and probablythe San have yielded metamorphic ages in this age range; Bruno Mountain nappe, based on field relations (3) the Alcatraz terrane: 135 Ma based on Valangin- south of the line of section where the Permanente ian fossils found in the unit (Blake et al. 1984); terrane crops out on the east side of the San An- (4) the Marin Headlands terrane: 95 Ma based on dreas fault. Cenomanian fossils found in detrial rocks of the Accretionary ages of nappes can be estimated unit (Blake et al. 1984); (5) the San Bruno nappe: from several types of data: (1) The age of overlap 75-85 Ma based on Campanian fossils in the No- assemblages combined with the ages of the youn- vato Quarry terrane (Blake et al. 1982), that is a gest strata in the overlapped terranes bracket the part of this structural horizon in the eastern San age of emplacement. (2) Metamorphic ages approxi- Francisco Bay area; (6) the Permanente terrane: 65 mate the time of subduction and incorporation Ma estimated by Tarduno et al. (1985) as described into the accretionary complex for metamorphosed above; and (7) the Salinian block: 50-60 Ma based terranes. (3) The age of the youngest clastic on the Paleocene fossil age of rocks involved in strata in a terrane may approximatewhen a terrane mylonitization at the Salinian-Franciscancontact approached the trench shortly before accretion. and Eocene fossil age of underformedoverlap strata Controversy exists over whether or not some Fran- (Wakabayashiand Moores 1988a). The progressive ciscan clastic rocks were translated a large distance decrease in apparent emplacement ages toward parallel to the plate margin after deposition and lower structural levels in the Bay area suggests se- before incorporation into the Franciscan.Blake and quential off-scraping or underplating in an accre- Jones (1981) and Jayko and Blake (1984) proposed tionary complex as envisioned by Karig and Shar- an exotic origin for Franciscan sandstones that dif- man (1975) (Wakabayashi and Moores 1988b). If fered in framework modes from coeval Great Val- the thrust sheets formed during the Cenozoic (e.g., ley sequence sandstones of similar age. Dickinson Suppe 1978), such a sequence of structurally down- et al. (1982) suggested that differences in frame- ward younging of apparent emplacement ages work modes of coeval Great Valley and Franciscan would be highly fortuitous. sandstones may be due to complexities within the The Cazadero region, also within the San Fran- same arc-trench depositional system, and that all cisco block, may display more complex low-angle Franciscansandstones were deposited near the site tectonic contacts (Gealey 1951; Bailey et al. 1964; of accretion. Seiders (1988, 1991) and Seiders and Blake et al. 1971; Wakabayashiunpub. data) (figure Blome (1988) noted the similarity of coeval Great 4). The regional low-angle nature of the contacts Valley sequence and Franciscanconglomerates and can be inferred on the basis of the sinuous trace of proposed that all Franciscan clastic rocks were de- the contacts and repeated units in map view (figure posited near the site of accretion. In the Bay area, 4). Based on crosscutting relationships, different only the Alcatraz terrane sandstones differ greatly generations of low-angle faults are present, some in framework modes from coeval Great Valley se- of which may post-date the accretion of the units. quence sandstones (Jayko and Blake 1984). Thus, The younger faults may be similar to Cenozoic whether one accepts an exotic or local origin for thrusts identified by Suppe (1978), and Phipps the Alcatraz terrane sandstones, the fossil ages (1983, 1984a, 1984b) elsewhere in the northern from the other sandstone units (Marin Headlands, Coast Ranges. If the younger faults are removed, Novato Quarry-SanBruno Mountain) may closely the older low-angle fault contacts of the Cazadero approximate the timing of accretion for these region appear to form an accretionary age nappe rocks. (4) The fossil age of in the Perma- stack similar to that of the Bay area, 5 km thick nente terrane, combined with the calculated paleo- (excluding the thickness of the Coastal Belt), with latitude of deposition and plate motion models, largernappe sheet dimensions of 45 km across and was used by Tarduno et al. (1985) to estimate the 70 km along strike. Similar to the San Francisco accretion age of this unit. Estimated accretionary Bay area, probable nappe emplacement ages de- ages for the nappe units in the Bay area are listed crease structurally downward, with the following in descending structural order as follows: (1) the sequence: (1) Skaggs Springs schist: 145 Ma based Tiburon melange: 140-160 Ma based on the meta- on metamorphic age; (2) greenstone with incipient morphic age of the included blocks, interpreted in blueschist metamorphism of uncertain age; (3) me- this structural horizon to be at their original struc- lange of uncertain age; (4) the Rio Nido terrane: 90 blue Ward stipple Valley the blueschist tectonic. of Great dense generations in are fine-grained incipient and/or shown schist;metabasalts units: different fabric; contacts ophiolite melange;lacks schist.grade rock Springs All GEOLOGY Range several matrix Skaggsblueschist Coast region.Note shalegreenstone; area. serpentinite AREA KJfu:KJfg:metamorphism KJms: amphibole-bearingKJmv: Creeksp: Non-Franciscan KJogv:Sequence. data).

Healdsburg and (unpub. lower, the sandstone and in sandstone lower grade anddeformed rocks deformed Wakabayashi grade variably and pumpellyite related units:variably terrane:stipple anddata), rockBelt: pumpellyite prehnite Nido light HEALDSBURG-CAZADEROin CoastalprehniteRio (unpub. shale; Complex FranciscanTfcb:shale;Kfr: and shown Erikson

Franciscan 1984), of

(1971,section. sectional. et cross cross the Blake and on from Map faults 4. derived thrust FigureMapof Journalof Geology TECTONICS OF OBLIQUE PLATE CONVERGENCE 27

Ma based on Turonian fossils; (5) the Coastal Belt The above examples illustrate that nappe struc- (structurallyhighest part):<60 Ma based on fossils tures that formed during subduction may be pres- in detrital rocks. ent throughout the Franciscan (interpreted nappe In Franciscan exposures in the northern Coast stacks, speculative correlations and accretionary Ranges composed mainly of melange, Kleist (1974), age constraints are illustrated in Repository File, Gucwa (1975), and Jordan(1978) differentiated dis- figures A and B). These relationships suggest that tinct structural horizons. Scarce age data from Franciscanterranes represent components of an ac- these units suggest that they may represent a cretionary wedge formed by sequential offscraping downward-younging sequence (Maxwell 1974), and underplating, rather than a series of unrelated with the following age constraints: (1) Bald Peak forearc slivers juxtaposed by strike-slip faulting. schist: 110-146 + Ma based on metamorphic age (Suppe and Armstrong 1972; Lanphereet al. 1978; Oblique Plate Convergence and Consequences McDowell et al. 1984) (probablycloser to the older for Franciscan Tectonics part of the age range, see later discussion of meta- morphism); (2) Poison Rocks melange: 130-150 Plate convergence during much of the Franciscan Ma based on fossils from the shale matrix (Jordan accretion may have been highly oblique (Engebret- 1978); (3) Elk Creek melange: 90-110 Ma based on son et al. 1985). Oblique plate convergence is con- in limestone concretions within shale sistent with the southerly latitude of formation of of the melange matrix (Gucwa 1975); (4) Tin Creek some Franciscan oceanic rock units (e.g., Alvarez melange: no age constraint; (5) melange: et al. 1980; Tarduno et al. 1985). At the Franciscan 90-110 Ma, based on microfossils in limestone trench (western North American plate margin), concretions within shale of the melange matrix convergence is calculated to have been oblique- (Gucwa 1975); (6) Laytonville melange: probably sinistral prior to 118 Ma, nearly orthogonal from 80 Ma, based on the Albian-Cenomanian age of 118 to 100 Ma and from 56 Ma to the cessation of the youngest beds in limestone blocks and the sug- subduction, and oblique dextral from 100 to 56 Ma gestion that these blocks probably traveled some (Engebretsonet al. 1985). Comparatively large un- distance after pelagic deposition to the site of ac- certainties in relative plate motions exist from cretion (e.g., Alvarez et al. 1980); and (7) Coastal 118 to 83 Ma, the period spanned by the Creta- Belt: <60 Ma based on the fossil age of detrital ceous normal superchron (Engebretsonet al. 1985). sedimentary rocks (Blake et al. 1988). Regional- Oblique plate convergence is generally thought scale thrust faults that formed during subduction to be resolved into orthogonal subduction and have been identified in the Eastern Belt of the trench-parallel strike-slip (e.g., Fitch 1972; Beck northern Coast Ranges (Worrall 1981; Jayko and 1983). Recent research suggests resolution of Blake 1989), and analogous structures may exist in oblique convergence into oblique thrusting and the Diablo Range (Cowan 1974). In the Coastal trench-parallel strike-slip with about 60% of the Belt, the geometry of fault contacts are consistent trench-parallel component partitioned into strike- with sequential offscraping and underplating of slip faulting (Ekstrom and Engdahl 1989). trench sediments at a subduction zone (Bachman Possible oblique subduction fabrics exist in the 1978), with the exception of possible trench slope Skaggs Springs schist of the San Francisco block. basin deposits (the Yager terrane of Underwood The 70 x 3 km blueschist belt (figure 4) has a and Bachman 1985). Seiders (1988, 1991) identified pervasive stretching lineation oriented sub-paral- the same distinctive conglomerate suites in both lel to the length of the belt with a gentle E-SE the Franciscan and the coeval Great Valley se- plunge (Wakabayashiunpub. data). Glaucophane is quence, with Franciscan conglomerates stacked aligned with long axes parallel to this lineation, upside-down compared to their Great Valley se- and lawsonite overgrows a later stage of folding quence counterparts, which were deposited as a that locally folds the lineation. Thus both the lin- normal stratigraphic sequence. Such a relationship eation and a subsequent deformational episode is suggestive of sequential accretion of the Francis- formed in blueschist facies conditions. The linea- can units (Seiders 1988, 1991). Seiders (1991) uti- tion trend is independent of the steepness of folia- lized these Franciscan conglomerate horizons as tion dip, indicating that the lineation has not been stratigraphic or structural markers and demon- rotated to near parallelism with the belt axis by strated that this accretionarystructure type is pres- later folding. Shear sense indicators and the linea- ent over much of the northern Coast Ranges (Seid- tion trend and plunge suggest W-NW-directed ers 1991, figure 2). oblique thrusting or sinistral strike-slip (Wakabay- 28 JOHN WAKABAYASHI ashi unpub. data). Based on the above it is con- The Franciscan Central Belt has been suggested as cluded that the fabric in the Skaggs Springs schist zone of large magnitude strike-slip during the pe- may have resulted from sinistral-oblique subduc- riod of oblique convergence (e.g., McLaughlin and tion. The 143 Ma metamorphic age of this unit Ohlin 1984; McLaughlin et al. 1988; Blake et al. (Wakabayashiand Deino 1989) is within the calcu- 1988). Several lines of field evidence argue against lated period of sinistral-oblique convergence at the large magnitude (>500 km or so) strike-slip within Franciscan trench (Engebretson et al. 1985). Sinis- the Franciscan: (1) The preservation of apparent tral-oblique convergence during this period is also accretionary sequences involving most or all of the in agreement with the sense of shearing of similar Franciscan is inconsistent with dissection of the age noted in the Galice Formation and the basal by strike-slip faults of large thrust of the Josephine ophiolite, units situated in- displacement. (2) Conglomerate suits show little board (east) of the Franciscan in southwestern Ore- evidence of large scale strike-slip displacement gon (Harperet al. 1990). Other oblique subduction within the Franciscan (Seiders 1991). (3) Vitrinite fabrics (including dextral fabrics)have not yet been reflectance data (Underwood et al. 1988; Blake recognized in the Franciscan, possibly in part be- et al. 1988) indicate a west-to-east gradient of in- cause of the extremely fine-grained nature of meta- creasing metamorphic temperature across Central morphic minerals in most Franciscan coherent and Eastern Belts of the northern Coast Ranges blueschists (much finer than the Skaggs Springs that suggests no major (hundreds to thousands of schist). km) post-metamorphic discontinuities. Distrib- Depending on how much of the trench-parallel uted strike-slip faulting with <100 km of aggregate plate motion was partitioned into oblique subduc- displacement, and less than -20 km of displace- tion/thrusting, a large magnitude of strike-slip dis- ment per fault, during subduction can probably be placement (hundredsof km) may have been accom- accommodated by the above relationships. Exam- modated inboard of the Franciscan trench. The ples of such faulting may include Paleocene- magnitude of tangential plate movement at the Eocene faulting suggested by Blake et al. (1985) and Franciscantrench need not be as great as the latitu- McLaughlin et al. (1988). dinal shift suggested by paleomagnetic data from If strike-slip faulting of large displacement took some of the Franciscan oceanic terranes. This is place during subduction, the faults must have been because of the differences in location of the terrane located inboard (east) of the Franciscan,possibly in during transit on the oceanic plate and the location the vicinity of the coeval magmatic arc, analogous of the Franciscan trench relative to the pole(s) of to modern examples of obliquely convergent arc- rotation between North America and the systems such as the Indonesian region plate(s), and the possibility of additional plate (e.g., Fitch 1972) and the Aleutians (e.g., Ekstrom boundaries. In addition, evidence for compaction and Engdahl 1989). Although few studies of syn- flattening of magnetic inclinations in deep sea sed- subduction strike-slip faulting east of the Francis- iments suggests that the paleomagnetic data from can Complex have been conducted, several exam- the oceanic rocks may overestimate the magnitude ples of such faults and related deformation may of latitudinal transport (Tarduno 1990). Plate mo- have been identified: (1) evidence for 400-500 km tion models of Engebretson et al. (1985) suggest of dextral slip that took place between approxi- about 2400 km of sinistral trench-parallel relative mately 150 Ma and 90 Ma along the east margin of plate motion at the Franciscan trench during the the Sierra Nevada (Lahren and Schweickert 1989; period from 163 to 118 Ma, and about 2400 km of Lahren et al. 1990; interpreted age of movement dextral relative motion from 100 to 56 Ma. Rela- slightly modified by evaluation of Sierra plutonic tively small magnitudes of trench-parallel motion ages); (2) the 85-100 Ma proto-KernCanyon fault are calculated for the period between 118 and 100 within the Sierra Nevada batholith, with an esti- Ma and the period between 56 Ma and the cessa- mated 40 or more km of dextral slip (Busby-Spera tion of subduction (Engebretsonet al. 1985). Note, and Saleeby 1990); (3) regional ductile deformation as mentioned previously, the large uncertainties of 151-123 Ma age with sinistral-oblique sense of for the period from 118 to 83 Ma. If the 60% parti- shear in the western Sierra Nevada (Patersonet al. tioning estimate of Ekstrom and Engdahl (1989) is 1987; Tobisch et al. 1989); (4) accommodation of a adopted for a first approximation, then the magni- large amount of arc-parallel displacement (hun- tude of trench-parellel motion accommodated by dreds of kilometers) within the Sierra Nevada strike-slip faulting may have been about 1400 km batholith by emplacement of the plutons in a of sinistral faulting from 163 to 118 Ma, with a transtensional environment (Saleeby 1991), and like amount of dextral faulting from 100 to 56 Ma. (5) the Pine Nut fault system of western Nevada, Journalof Geology TECTONICS OF OBLIQUE PLATE CONVERGENCE 29 with an undetermined amount of left slip from along strike variation of the Franciscan can be 155 Ma to the Late and right slip of partly explained by the discontinuous nature of ac- uncertain magnitude thereafter (Ave Lallemant cretion along the trench, because the various ac- and Oldow 1988). creted elements, such as seamounts, oceanic rises, Although plate convergence was at times highly and packets of trench sediment, did not extend the oblique, Franciscan subduction was probably con- length of the trench. Some variation, such as the tinuous during the 140 m.y. of the assembly of the discontinuous nature of the Pickett Peak terrane complex (although accretion need not have been), schists and equivalents, may have also resulted because cessation of subduction (with or without from differences in location, orientation, and dis- conversion to a transform fault boundary) would placements of post-accretionary thrust and normal have resulted in thermal overprinting of high P/T faults. rocks (Cloos and Dumitru 1987) and raised geo- The Coastal Belt is not present in the Bay area thermal gradients in the Great Valley sequence or southern Coast Ranges. This may be because (Dumitru 1988), which are not recognized (Dumi- Salinian block emplacement (50-60 Ma, Wakabay- tru 1988; Ernst 1988). Continuous subduction is ashi and Moores 1988a) was taking place while consistent with the continuity of U/Pb zircon ages early Coastal Belt accretion (40-60 Ma, Blake for plutons of the coeval Sierra Nevada arc and arc et al. 1988) was occurring to the north or because volcanism east of the Sierra Nevada (combined the older Coastal Belt rocks are offshore. Equiva- data of: Stern et al. 1981; Chen and Moore 1982; lents of younger Coastal Belt rocks are probably Tobisch et al. 1986; Hanson et al. 1987; Sharp present offshore at the latitude of the Bay area and 1989; Saleeby et al. 1989a, 1989b). southern Coast Ranges (Page 1981).

Franciscan Accretionary Details: Small Total Metamorphism and Its Importance in Volume and Along Strike Variation Franciscan Tectonic History The Franciscan accretionary wedge, particularly In addition to regional structural relations, meta- the Eastern and Central belts, comprises a small morphism of Franciscan rocks is helpful in recon- volume of material compared to some modern ac- structing tectonic history. A much-debated prob- cretionary prisms that formed over a shorter time lem of Franciscan tectonics is the apparent gap in period (e.g., Indonesia, Hamilton 1979; Aleutians, metamorphic age and difference in metamorphic Moore et al. 1991; parts of Cascadia, Davis and grade between the high-grade blocks and the oldest Hyndman 1989). Based on the lack of intra- coherent blueschists. These differences may sug- Franciscan strike-slip faulting noted above, strike- gest a pre-Franciscan origin for the high-grade slip truncation probably did not cause the reduced blocks (e.g., Coleman and Lanphere 1971; Blake accretionary prism volume. A process for reducing et al. 1984). Alternatively, high-grade blocks and accreted volume more consistent with Franciscan coherent blueschists may be part of the same sub- field relations is sediment subduction or subduc- duction event (e.g., Suppe and Foland 1978; Cloos tion erosion (e.g., Scholl et al. 1980; von Huene 1985; Wakabayashi 1990). This problem is ad- 1986), whereby sediment is subducted without ac- dressed using petrologic, geochronologic, and field cretion and/or previously-accreted hanging wall data from high-grade blocks and coherent blue- material is removed by abrasion by the downgoing schists. plate. In this respect, the Franciscan accretionary Many "high-grade" blueschist and eclogite blocks complex may be intermediate in character be- possess relict amphibolite mineralogy (Blake et al. tween complexes with voluminous sediment ac- 1984; Moore and Blake 1989). Many blocks are en- cretion noted above, and the Peru and Marianas closed by, or diaplay remnants of, an actinolite- margins, which have a history of prolonged sub- rich rind that may have formed by metasomatic duction with little or no accretion (e.g., von Huene reaction between the blocks and surrounding ser- and Lallemand 1990; Hussong and Uyeda 1981). pentinite (Coleman and Lanphere 1971; Moore Even if Neogene strike-slip faulting is accounted 1984). The rinds overprint some of the blueschist for, considerable along strike variable exists in the growth stages in the high-grade blocks (Moore Franciscan. Among the most interesting variations 1984). Textural, petrologic, and geochronologic is the large proportion of melange in the northern data indicate that high-grade blocks were initially Coast Ranges compared to mostly coherent units metamorphosed under amphibolite facies condi- in the Bay area, and the lack of an equivalent to tions, then cooled through conditions of increasing the Pickett Peak terrane in the Diablo Range. The P/T ratio to the blueschist facies, a "counterclock- 30 JOHN WAKABAYASHI

wise" P-T-t path (Wakabayashi 1990) (figure 5). The P-T conditions of late-stage blueschist over- prints in the high-grade blocks overlap the P-T con- ditions of coherent blueschist metamorphism. All Franciscan metamorphic rocks were uplifted under conditions of low geothermal gradient, with com- mon preservation of aragonite (e.g., Ernst 1988). The age of initial amphibolite-grade high-grade block metamorphism is probably about 158-163 Ma, based on Ar/Ar plateau ages from hornblendes (Ross and Sharp 1986, 1988). Blueschist overprint ages are 158-162 Ma (blue amphibole Ar/Ar pla- teau ages, Ross and Sharp 1986), 162 Ma (U/Pb isochron, Mattinson 1986) and 139-150 Ma (white mica K/Ar and Ar/Ar, Coleman and Lanphere 1971; Suppe and Armstrong 1972; McDowell et al. 1984; Wakabayashi and Deino 1989). Actinolite- bearing rinds that commonly enclose the blocks have yielded 157 and 159 Ma K-Ar dates (Coleman T°C and Lanphere 1971, recalculated using decay con- stants of Steiger and Jiger 1977). The blue amphi- bole, actinolite rind, and U/Pb ages, combined with a similar age range for hornblendes, suggest that the high-grade blocks may have evolved from to blueschists within 5 m.y. or less.

Figure 5. A: Evolution of Franciscanmetamorphism: P-T paths shown are for material subducted at about: (1) 160 Ma (two representative P-T paths for high-grade blocks shown), (2) 145 Ma (Ward Creek coherent blueschist), (3) 90 Ma (Diablo Range)and 65 Ma (Perma- nente terrane).Note the drop in peak metamorphic tem- peratures with time with most of the decrease within the first 15 m.y. A steady-state subduction geotherm may have developed at or before 100 Ma. The cooling of the subduction complex is attributed to the dissipation of heat from the hanging wall of the subduction zone. gl: stability field of glaucophane from Maresch (1977); ab = jd + q: albite = jadeite + quartz from New- ton and Smith (1967). Sources: (1) Wakabayashi(1990), (2) Maruyama and Liou (1988), (3) Maruyama et al. (1985), Larue et al. (1989). B: Cartoons illustrating Fran- ciscan metamorphic evolution. The cartoons depict the relative structural position of units subducted, un- derplated and metamorphosed at 160, 145, 90, and 65 Ma. Inset at top of figure is a schematic enlargement of the dynamothermal aureole formed during subduction initiation. The thickness of the aureole is greatly exag- gerated. This inset illustrates the origin of the wide vari- ation in peak metamorphic temperatures and pressures in Franciscan high-grade blocks (Wakabayashi 1990). Temperaturevariation.is influenced by rather small dif- ferences in distance from the hanging wall and pressure is influenced by depth of underplatingin the subduction zone. Points (a)and (b)in the inset correspondto the two representative P-T paths for high-gradeblocks shown in "A." Journalof Geology TECTONICS OF OBLIQUE PLATE CONVERGENCE 31

The younger white mica ages may reflect the lower to post-metamorphic argon loss (during cooling blocking temperature of white mica relative to am- or subsequent thermal disturbance) (e.g., Dodson phibole (e.g., compare Harrison 1981, for amphi- 1973), even the oldest Pickett Peak terrane ages bole, and Wijbransand McDougall 1988, for white probably represent only minimum ages of meta- mica) and suggest a variable cooling history for the morphism. Although the white micas of the coher- blocks. Preservation of metamorphic assemblages ent blueschists are much finer-grainedthan those intermediate in textural succession and P-T condi- of the high-grade blocks and are thus potentially tions between the early amphibolite and late less retentive of argon (e.g., Dodson 1973), the old- blueschist assemblages, and geochronologic data, est coherent blueschist ages (135-146 Ma) still indicate that high P-T overprints over the early am- overlap the 139-159 Ma age range of high-grade phibolite assemblages resulted from continuous block K/Ar and Ar/Ar white mica ages, suggesting evolution in P-T conditions, rather than an over- no gap in metamorphic age. printing of an unrelated, earlier metamorphic Yolla Bolly terrane and Diablo Range rocks have event (Wakabayashi 1990). yielded whole rock K/Ar dates of about 90-115 Franciscan coherent metamorphic rocks range Ma (Suppe and Armstrong 1972). Some Yolla Bolly in grade from blueschist- transition in terrane ages may be either too young (Ar loss from the structurally highest Pickett Peak terrane and fine-grained white mica) or too old (premetamor- WardCreek coherent blueschists (Brownand Ghent phic Ar from detrital white mica [Blake et al. 1983; Maruyama and Liou 1988) to lawsonite 1988]). Blueschist metamorphism in part of the blueschist in Yolla Bolly terrane (Blake et al. 1988) Yolla Bolly terrane occurred at least as recently as and Diablo Range (Ernst et al. 1970; Maruyama about 95 Ma, because blueschist grade metagrey- et al. 1985) metagreywackes. The prograde P-T wacke in the Hull Mountain area of the northern path for the Pickett Peak terrane and Ward Creek Coast Ranges has yielded Cenomanian fossils coherent blueschist metamorphism was slightly (Blakeet al. 1988). Mattinson and Echeverria(1980) clockwise (Brown and Ghent 1983; Maruyamaand obtained a 92 Ma U-Pb isochron metamorphic age Liou 1988) (see figure 5). The fine-grained coherent from a Diablo Range blueschist grade metagabbro. blueschists ( 0.05 mm white mica is the principal A <(74-85) Ma metamorphic age is suggested for K-bearingphase) of the Pickett Peak terrane have part of the blueschist facies Burnt Hills terrane of not allowed the extraction of mineral separates in the Diablo Range that has yielded Campanian fos- most cases. Consequently, nearly all coherent sils (M. C. Blake, Jr., oral comm. 1989). Locally blueschist ages have been whole rock K/Ar dates. preserved blueschist facies metamorphism in the The South Fork Mountain Schist of the Pickett Permanente terrane of the Bay area (Larue et al. Peak terrane has yielded whole rock K/Ar and 1989), combined with the accretion age estimate whole rock total fusion Ar/Ar dates of about 110- of Tarduno et al. (1985), suggest a blueschist meta- 146 Ma (Suppe and Armstrong 1972; Lanphere morphic age of about 65 Ma. et al. 1978; McDowell et al. 1984). An Ar/Ar white Based on the above data, the following age mica plateau age of 143 Ma has been obtained from ranges are suggested for Franciscan metamor- the somewhat coarser-grained (white mica 0.4 phism: (1) high-grade block amphibolite metamor- mm in size) Ward Creek coherent blueschists (Wa- phism: 163-158 Ma; (2) rapid cooling to blueschist kabayashi and Deino 1989). conditions within 5 m.y. after amphibolite meta- Lanphere et al. (1978) suggested that Pickett morphism and variable further cooling to below Peak terrane ages >125 Ma were caused by extra- the blocking temperature for Ar in white mica neous argon from detrital white mica and con- from 159 to 139 Ma; and (3) coherent blueschist cluded that 115-125 Ma representedthe true meta- metamorphism: 146 + to 80 Ma or somewhat morphic age of the rocks. McDowell et al. (1984) younger. Geochronologic and petrologic data sug- noted, however, that the >125 Ma ages are from gest continuity in age and metamorphic conditions the most texturally reconstituted samples, that between high-grade blocks and coherent blue- lack detrital white mica, and are therefore valid schists. It is thus likely that both high-gradeblocks dates. The Ward Creek white mica Ar/Ar step and coherent blueschists formed during Franciscan heating date supports this conclusion. Given the subduction. A model for the metamorphic evolu- temperature of metamorphism (up to 330-345°C, tion of the high-grade blocks is presented (Waka- Blake et al. 1988), that is close to or exceeds the bayashi 1990), followed by a discussion of coherent blocking temperature for Ar in white mica (e.g., blueschist evolution: Wijbrans and McDougall 1988), and susceptibility High-gradeblock precursorsformed as amphibo- of the extremely fine-grainedmicas of these schists lites under the hot hanging wall of the iust- 32 JOHN WAKABAYASHI initiated Franciscan subduction zone and were sub- simply an artifact of different levels of exposure of sequently underplated to the upper plate (Platt uplifted Franciscan units. 1975; Suppe and Foland 1978; Cloos 1985). With continued subduction, the hanging wall cooled, "Unconventional" Aspects of Franciscan and the amphibolites were overprinted with as- Accretion: Distribution of High-Grade semblages of higher P/T ratio. The upper mantle Blocks and "Tectonic Wedging" wedge present above the zone of amphibolite for- mation in the subduction zone may have been the Two aspects of Franciscan tectonics that may not source for the serpentinite often found in close as- fit a "conventional" model of accretion (such as sociation with the blocks. The metamorphic ages Karig and Sharman 1975) are: (A) the distribution of the block rinds are within the same range as of high-gradeblocks and (B)"tectonic wedging" ex- amphibolite metamorphism in the blocks, sug- pressed as east-vergent thrusting of the Franciscan gesting that the blocks were in contact with ul- Complex over the Great Valley sequence or Sierran tramafic rocks during or shortly after metamor- basement (e.g., Wentworth et al. 1984). phism. A) High-grade blocks are found in melange Thermal modeling by Hacker (1990) and Cloos zones other than the highest structural horizon in (1985) suggest that rocks underplated immediately the Franciscan. Emplacement of these blocks at after, and just beneath, the high-temperaturehori- structurally lower levels requires mechanisms of zon of the high-grade blocks may have experienced tectonic and/or sedimentary recycling (Cloos a slightly clockwise progradeP-T path with lower 1986; Wakabayashiand Moores 1988b). Some field peak temperatures than the high-grade blocks. relations relating to high-grade block distribution These thermal models are consistent with the P-T are: (a) Largeconcentrations of blocks occur in the paths determined for the Ward Creek coherent structurally highest Franciscan horizon, just be- blueschists (figure 5) and Pickett Peak terrane neath Coast Range ophiolite or Great Valley se- (Maruyama and Liou 1988; Brown and Ghent quence outliers. As interpreted in this paper, this 1983). Based on petrologic and geochronologic horizon represents the original structural level of data, Franciscan coherent blueschists may repre- high-grade block accretion. (b) Blocks are common sent a unit subducted after and underplated struc- in shear zones and melange zones cutting or turally below the accretionary horizon of the high- bounding coherent blueschist nappes, particularly gradeblocks. The structurally highest Pickett Peak the Skaggs Springs schist. These shear zones are terrane (and the correlative Ward Creek/Skaggs generally narrow (<50 m) and commonly contain Springs blueschists) was accreted and metamor- serpentinite or metamorphosed ultramafic rocks. phosed first, followed by the Yolla Bolly terrane/ (c) Blocks are common in melanges at or near the Diablo Range. The trend of decreasing peak meta- contact between the Central Belt and Coastal Belt. morphic temperatures and decreasing apparent (d) High-grade blocks rare in melanges within the geothermal gradient with decreasing age (figure 5) Coastal Belt. may be attributed to the continued dissipation of Mechanisms proposed for high-grade block dis- heat from the hot hanging wall of the subduction tribution include: (1) serpentinite diapirism with zone. Most of the temperature decrease occurred included blocks (e.g., Coleman 1961; Ernst 1965; within the first 15 m.y. or less after subduction Carlson 1981; Moore 1984); (2) uplift, erosion, in- initiation, as predicted by thermal models (Cloos clusion in olistostromes, and resubduction (e.g., 1985; Peacock 1988). Maxwell 1974; Page 1978); (3) uplift, strike-slip Accretionary ages for some sub-blueschist grade truncation, resedimentation, resubduction (Karig units overlap ages of blueschist metamorphism 1980); (4) entrainment in strike-slip faults (Karig (see also Repository figure B). For example, while 1980; Blake et al. 1988); and (5) melange return the prehnite-pumpellyite grade Marin Headlands flow in which blocks are plucked from the struc- terrane accreted in the Bay area in the Ceno- turally high source horizon and emplaced in struc- manian, blueschist metamorphism took place in turally lower melanges (e.g., Cloos 1982). the Yolla Bolly terrane of northern Coast Ranges Some strengths and/or weaknesses of these (Blake et al. 1988) and in the Diablo Range (Mat- mechanisms are summarized, numbered to corre- tinson and Echeverria 1980). Presence or lack of spond to the mechanism listed above: (1) Ser- blueschist metamorphism at the same time in dif- pentinite diapirism explains the common associa- ferent Franciscan regions may be a function of dif- tion of the blocks with serpentinite, the common ferent depths of underplating or offscrapingof cor- occurrence of blocks in narrow serpentinite-filled relative units along the Franciscan trench or shear zones that cut the foliation of the higher- Journal of Geology TECTONICS OF OBLIQUE PLATE CONVERGENCE 33 grade coherent blueschists such as the Pickett dressed by any of the mechanisms are the scarcity Peak terrane and correlatives, especially the Skaggs of blocks in Coastal Belt melanges and the high Springs schist (Wakabayashi unpub. data) and concentration of blocks in the vicinity of the Angel Island nappe (Wahrhaftig 1984b), and is con- Coastal Belt/Central Belt contact. A speculative sistent with the formation of the Mg-rich reaction explanation for this distribution pattern follows. rinds on the blocks. Association with serpentinite The trench-parallel component of plate motion de- early in the history of the high-grade blocks is sug- creased sharply while the normal component in- gested by reaction rind ages that are close to the creased somewhat from 75-56 Ma (Engebretson age of high-grade metamorphism in the blocks et al. 1985), coincident with the change from Cen- (Coleman and Lanphere 1971, see discussion in tral Belt to Coastal Belt accretion. Although return previous section), and possibly, the presence of ser- flow in obliquely subducting melanges has not pentinite assemblages including antigorite and been modeled, travel paths to and from the block tremolite (nearly all of the serpentinite in the Fran- source horizon should be longer than for trench- ciscan and Coast Range ophiolite is composed of normal subduction. The change from highly lower-grade chrysotile + lizardite assemblages) as- oblique convergence to rapid normal convergence sociated with the blocks in the structurally highest may have drastically shortened the return paths melange in the San Franciso Bay area and in shear from the source horizon of blocks in melanges, re- zones cutting the Skaggs Springs schist (Wakabay- sulting in a relatively high concentration of blocks ashi unpub. data) and Angel Island nappe (Wahrhaf- at the Central Belt/Coastal Belt boundarymelange. tig 1984b). Serpentinite diapirism cannot account The change from Central Belt to Coastal Belt ac- for common occurrences where blocks appear asso- cretion also coincides with a relative increase in ciated only with shale matrix in melanges (e.g., the volume of sediment accretion. (The Coastal Cloos 1982). (2) Redeposition of high-grade blocks Belt, including offshore exposures, represents 50 suggests prior uplift and erosion of high P-T rocks. m.y. of accretion but is comparable in volume to Blueschist detritus is rare in Eastern and Central the adjacent Central and Eastern belts that ac- belt sandstones and conglomerates (e.g., Cowan creted over a period of 100-110 m.y.) The increase and Page 1975). However, high-grade blocks repre- in volume of accreted sediment may have resulted sent a small enough volume of material that they from the overtopping of the forearc high, that had, may not have contributed a noticeable amount of up to this time, ponded much of the clastic sedi- blueschist detritus to Franciscan clastic rocks. Fur- ment east of it in the Great Valley forearc basin thermore, high-grade blocks are present as blocks (Ingersoll 1978). Increase in trench sediment vol- in basal Great Valley sequence olistostromes in the ume may have caused underplating beneath the northern Coast Ranges (Phipps 1984b, Carlson high-gradeblock horizon (comparedto periodic ac- 1981). (3,4) The lack of evidence for strike-slip cretion and tectonic erosion), sealing off the source faulting during subduction within the Franciscan horizon from further "plucking"by a flow melange argues against these mechanisms. (5) The melange and preventing emplacement of high-grade blocks return flow mechanism (e.g., Cloos 1984), includ- into Coastal Belt melanges. ing "intrusion" of melange between coherent units B) Tectonic wedging (e.g., Wentworth et al. (suggested by Cloos 1984; field evidence presented 1984), characterized by the east-directed thrusting in Becker and Cloos 1985; Cloos 1991), appears to of Franciscan over presumed Sierran basement, is explain high-grade block distribution fairly well. presently active along the eastern margin of the In conclusion, field relations suggest that high- Coast Ranges (e.g., Wentworth et al. 1984; Nam- grade block distribution may have resulted from son and Davis 1988; Unruh and Moores 1990). Re- a combination of processes (1), (2), and (5), in the lated east-vergent thrusting in the Great Valley se- following sequence: (a) early association of blocks quence may have initiated in the with serpentinite, (b) possible rapid upward move- based on structural evidence (Unruh and Moores ment of some blocks by entrainment in serpen- 1990, 1992). This may have preceded or coincided tinite diapirs and return flow of melange, (c) sur- with the initiation of east-directed thrusting of face exposure and deposition of early uplifted Franciscan over basement, constrained to be blocks in olistostromes in both the Great Valley younger than the youngest blueschists in the upper forearc basin and the Franciscan trench with resub- plate (-75-90 Ma). Initiation of this thrusting may duction of trench deposits, (d) continued emplace- have resulted from one or a combination of the ment and redistribution of blocks by melange flow following: (1) shallowing of the dip of subduction and intrusion. from -80 Ma (e.g., Dickinson and Snyder 1978); Aspects of block distribution not directly ad- (2) an increase in convergence rate and/or greater 34 JOHN WAKABAYASHI

frontal accretion during the latest Cretaceous to early Tertiary (Engebretson et al. 1985; Ingersoll 1978), corresponding to the initiation of Coastal Belt accretion, that may have promoted increased thrust faulting to maintain a stable wedge geome- try (e.g., Platt 1986); and (3) collision of buoyant material with the subduction zone (that may have caused the subduction shallowing in part). Such collisions during this time period may have in- volved aseismic ridges or chains, such as the Permanente terrane at --65 Ma (Henderson et al. 1984; Tarduno et al. 1985), or the continental Salinian block at 50-60 Ma (Wakabayashi and Moores 1988a). Accretion of the oceanic Nicasio Reservoir, Geysers, and Marin Headlands terranes at -95 Ma may have occurred too early for the initiation of east-directed thrusting but may have played a role in the shallowing of subduction dip. Clearly, much further study needs to be conducted to determine the timing and cause of the initiation Figure 6. Cross sectional cartoons illustrating the tec- of "tectonic wedging." tonic evolution of the Franciscan Complex. 169 to 163 Ma: The Coast Range ophiolite forms by back arc spread- ing over a west-dipping subduction zone (Moores 1970). Franciscan Tectonic History: A Brief Summary The location of the west-dipping subduction zone is somewhat speculative; one option is shown. Humboldt Combining the available structural, metamorphic, ophiolite is from Dilek et al. (1989). 163 to 158 Ma: Con- and age data, the following Franciscantectonic his- tinental margin chokes the west-dipping subduction tory is presented (see figure 6): Between about 163 zone and east dipping Franciscan subduction is initiated and 169 Ma the Coast Range ophiolite formed to the west (Moores 1970; Schweickert and Cowan (Hopson et al. 1981; Mattinson data cited in 1975). High-gradeblock precursorsform as amphibolite McLaughlin et al. 1988) in a back-arc basin over a aureole under the hot hanging wall of the new subduc- west-dipping subduction zone (Moores 1970; tion zone. 158 to <20 Ma: A series of seamounts, oceanic Schweickert and Cowan 1975) (figure6). The west- rises and packets of terrigenous sediment are offscraped ern North American choked or underplatedfrom the downgoing oceanic plate. These accreted elements form many this subduction zone, and east-dipping subduction the Franciscan tectono- stratigraphic terranes. Trench-parallel plate motion is initiated to the west beneath young the Coast accommodated by oblique thrusting coupled with Range ophiolite from 163-158 Ma, trapping the strike-slip faulting in the vicinity of the arc. The strike- ophiolite in a forearc setting relative to the new slip faults are schematically shown as dextral, depicting subduction zone. High-grade block precursors the sense of motion during the period <100 Ma. formed as amphibolites during the inception of subduction and were overprinted with higher P-T assemblages with cooling during continued sub- scraped or underplated, some experienced high P-T duction. The structurally highest coherent blue- metamorphism. The depth of accretion coupled schists were underplated next at about 145 + Ma, with subsequent differential uplift and exposure followed by a series of accreted slices composed controlled the distribution of exposed high P-T mostly of trench sediments with some materials metamorphism in Franciscan rocks. Contempora- rafted to the subduction zone on the down-going neous with Franciscan subduction was arc volca- oceanic plate. Franciscan melanges formed as shear nism in the Sierra Nevada arc and forearc basin zones, both as part of the internal imbrication of sedimentation in the Great Valley sequence to the coherent nappes and separating coherent nappes; east of the Franciscan. Oblique plate convergence and some of the melanges may represent material may have resulted in latitudinal displacement of originally deposited as olistostromes in the trench some accreted oceanic terranes, but not of trench- prior to being subducted. Accreted units did not deposited detrital sedimentary rocks. Convergence extend the full length of the trench, resulting in was sinistral-oblique prior to 100 Ma and dextral- along-strike variation of the Franciscan.Depending oblique thereafter (Engebretson et al. 1985). Part on the depth at which the materials were off- of the trench-parallel component of relative plate Journalof Geology TECTONICS OF OBLIQUE PLATE CONVERGENCE 35 motion was partitioned into oblique thrusting Engebretson 1984). Faulting and deformation asso- with the remainder accommodated by strike-slip ciated with this transform regime disrupted the faults inboard of the Franciscan in the region of the original structural relations in the Franciscan, se- magmatic arc. The magnitude of associated strike- verely overprinting the older structures. slip faulting may have been about 1400 km sinis- tral strike-slip from 163-100 Ma, with the same ACKNOWLEDGMENTS amount of dextral strike-slip from 100 Ma to the termination of subduction, based on the plate mo- An early version of this paper appearedin an un- tion calculations (Engebretson et al. 1985) and 60% published field trip guide, "Field Trip to the Napa- partitioning of trench-parallel plate motion into Sonoma Wine Region," distributed for a field trip strike-slip faulting (Ekstrom and Engdahl 1989). for the 1990 AAPG meeting. The author's knowl- East-directed thrusting of the Franciscan over the edge of the Franciscan has benefitted from discus- forearc basin and basement may have initiated sions and exchange of data from many individuals, some time after about 75 Ma with activity contin- particularly M. C. Blake, Jr., C. Wahrhaftig, V. M. uing to the present. From about 15 Ma, depending Seiders, E. M. Moores, S. P. Phipps, W. G. Ernst, on latitude, the Mendocino triple junction mi- and M. Cloos. I thank The Journal of Geology grated northward, and subduction was replaced by reviewers for thorough, constructive reviews and a strike-slip dominated tectonic regime (Page and R. C. Newton for helpful editorial comments.

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