Distribution and character of upper mesozoic subduction complexes along the west coast of North America

Tectonophysics, 47 (1978) 207-222 0 Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands

207

DISTRIBUTION AND CHARACTER OF UPPER MESOZOIC SUBDUCTION COMPLEXES ALONG THE WEST COAST OF NORTH AMERICA

D.L. JONES,

M.C. BLAKE,

Jr., E.H. BAILEY

and R.J. MCLAUGHLIN

U.S. Geological Survey, Menlo Park, Calif. (U.S.A.) (Submitted

August 17, 1976; accepted for publication

September

6, 1977)

ABSTRACT Jones, D.L., Blake, M.C., Jr., Bailey, E.H. and McLaughlin, R.J., 1978. Distribution and character of upper Mesozoic subduction complexes along the west coast of North America. In: K.L. Burns and R.W.R. Rutland (Editors), Structural Characteristics of Tectonic Zones. Tectonophysics, 47 : 207-222. Structurally complex sequences of sedimentary, volcanic, and intrusive igneous rocks characterize a nearly continuous narrow band along the Pacific coast of North America from Baja California, Mexico to southern Alaska. They occur in two modes: (1) as complexly folded but coherent sequences of graywacke and argillite that locally exhibit blueschist-grade metamorphism, and (2) as melanges containing large blocks of graywacke, chert, volcanic and plutonic rocks, high-grade schist, and limestone in a highly sheared pelitic, cherty, or sandstone matrix. Fossils from the coherent graywacke sequences range in age from late Jurassic to Eocene; fossils from limestone blocks in the melanges range in age from mid-Paleozoic to middle Cretaceous. Fossils from the matrix surrounding the blocks, however, are of Jurassic, Cretaceous, and rarely, Tertiary age, indicating that fossils from the blocks cannot be used to date the time of formation of the melanges Both the deformation of the graywacke, with accompanying blueschist metamorphism, as well as the formation of the melanges, are believed to be the result of late Mesozoic and early Tertiary subduction. The origin of the melanges, particularly the emplacement of exotic tectonic blocks, is not understood.

INTRODUCTION

Structurally complex assemblages of elastic sedimentary rocks, mainly graywacke and argillite, together with various amounts of volcanic rocks, chert, ultramafic and mafic intrusive rocks, and minor limestone, are abundant along the Pacific margin of North America (Fig. 1). These rocks occur in two modes: (1) melanges, wherein rocks of widely differing ages and compositions are chaotically mixed, and (2) coherent units of dominantly flyschlike graywacke and argillite that may be complexly deformed and metamorphosed but lack the chaotic nature of the melanges. These two types of deposits may occur together, usually as structurally interleaved

CANADA

Fig. 1. Distribution of upper Mesozoic and lower Tertiary subduction complexes along the Pacific Coast of North America.

units, or they may constitute separate parallel belts. Dating these rocks is usually difficult. The melanges commonly contain blocks of fossiliferous rocks, but these blocks are generally older than the argillaceous matrix. This difference in age has led to much confusion, as there has been a tendency to ascribe an age to the entire melange unit based on that of the blocks. The coherent graywacke and argillite units are generally poorly fossiliferous, and only persistent searching yields fossils sufficiently well preserved to establish the age. Despite these problems, we now have enough data to show that very large areas of western North America are characterized by these types of deposits, which we believe were formed by accretion due to subduction of an oceanic plate during late Mesozoic and early Tertiary time. Space does not permit detailed discussion of the entire belt of subduction complexes which stretches along the west coast of North America for over 4,500 km (Fig. 1). Instead we will very briefly describe the major deposits and then concentrate on the Franciscan rocks of California as being representative of the kinds of rocks and structures that characterize the entire belt.

209 KODIAK

ISLAND

AND KENAI

PENINSULA

A thick sequence of deformed argillite and graywacke forms much of Kodiak Island and the Shumagin and Sanak Islands to the southwest. Similar rocks continue to the northeast and underlie the main part of the Kenai Peninsula. These rocks have long been recognized as having been deposited in deep water near the continental margin (Burk, 1965; Plafker and MacNeil, 1966; G.W. Moore, 1969; J.C. Moore, 1972, 1973; Jones and Clark, 1973), and having relatively undeformed shallow-water equivalents to the north. Fossils from a number of localities are all of Late Cretaceous age (Campanian to Maastrichtian; see Jones and Clark, 1973), indicating that subduction and accretion occurred during latest Mesozoic or early Cenozoic time. Structurally overlying these folded beds of graywacke and argillite are melange units (Uyak Formation of Kodiak Island and the McHugh Complex near Anchorage) containing blocks of mafic and ultramafic rock together with blocks of Permian Tethyan fusulinid-bearing limestone, chert, and graywacke in a highly sheared matrix of fine-grained sedimentary rocks (see Clark, 1973; Connelly et al., 1976). Moore and Connelly (1976) and Connelly et al. (1976) argue that these rocks represent an early Mesozoic subduction complex, but the presence of rocks containing Cretaceous foraminifers and radiolarians (E.A. Pessagno, Jr., oral commun., 1976) indicates that at least part of the melange was formed no earlier than the late Mesozoic. CHUGACHAREA

A similar thick sequence of deformed graywacke and argillite with associated melanges characterizes the Chugach Mountains of southern Alaska. They have been studied in detail by Plafker in the Saint Elias Mountains, where fossils of Berriasian, Valanginian, and Campanian ages have been found (Jones, 1973; Plafker et al., 1976). Fossils as old as Early Jurassic occur in the melanges, and limestone blocks similar to the Upper Triassic Chitistone Limestone are present locally. Radiolarian chert associated with pillow basalt; diabase, and gabbro is abundant, and at one locality Valanginian radiolarians were recovered. Less-deformed, coeval, shallow-water deposits occur to the north where they constitute part of the upper plate of the Border Ranges thrust (MacKevett and Plafker, 1974). KELP BAY

Both the melanges and well-bedded graywacke and argillite sequences of the Saint Elias Mountains can be traced to the southeast (Plafker et al., 1976), where they constitute part of the Kelp Bay Group (Berg and Hinckley, 1963; Loney et al., 1975). These rocks comprise a highly deformed complex of graywacke, pelite, tuffaceous pelite, massive and pillowed green-

210

stone, radiol~ian-bearing ribbon chert, conglomerate, blocks of limestone, gabbro, and thin lenses of highly sheared serpentinite. This complex has been assigned a Triassic and(or) Jurassic age (Loney et al., 1975), but the presence of Valanginian radiolarian chert and Hauterivian to Barremian Intoceramus and belemnites in siltstone associated with volcanic rocks (Plafker et al., 1976) indicate that much younger rocks are present. Deformation and production of the melanges probably occurred in latest Cretaceous to early Tertiary time. These deformed rocks are structurally overlain by a complex sequence including volcanic and shallow-water sedimentary rocks (Goon Dip Greenstone and Whitestripe Marble) that are lithologically equivalent to the Middle and (or) Upper Triassic Nikolai Greenstone and the Upper Triassic Chitistone Limestone of the Wrangell Mountains, Alaska (Plafker et al., 1976). Middle Jurassic (180 m.y,--164 m.y.) granitic rocks intrude the Goon Dip and Whitestripe (Loney et al., 1975), but nowhere do they cut the adjacent or structurally underlying deep-water subduction complexes. PACIFIC

RIM SEQUENCE

A highly deformed sequence of graywacke, argillite, tuffaceous argillite, chert, and greenstone that occurs along the west coast of Vancouver Island is named the Pacific Rim sequence (Muller, 1973; Muller et al., 1974). Page (1974) clearly pointed out the melange-like nature of this deposit, which is ch~acte~zed by chaotic structural style, intense deformation, and juxtaposition of rocks formed in several different environments and subjected to different degrees of metamorphism. The Lower Cretaceous (Valanginian) fossil Buchia pacifica is locally abundant (Muller, 1973) in siltstone, and Upper Jurassic (zone 2A) radiolarians are known from red ribbon chert (Jan Muller and E.A. Pessagno, Jr., oral commun., 19’76). Because of similarities in lithologic association, general age, and style of deformation, these rocks confidently can be correlated with the part of the Kelp Bay Group of Baranof and Chichagof Islands, Alaska. SAN JUAN

ISLANDS

Complex melange deposits characterize much of the San Juan Islands (Whetten, 1975, p. 387; Whetten et al., 197’7), but they are still not well understood. In general, much of the area is characterized by a chaotic mixture of argillite, tuff, graywacke, chert, pillow lava, mafic and uitramafic rocks, and scattered blocks of limestone containing Devonian, Pennsylvanian, and Permian fossils. Low-grade blueschist-facies (lawsonite and aragonite) metamorphism has affected much of the terrane (Glassley et al., 1976). Mixing of rocks of several ages is well documented by the juxtaposition of pillow basalts with interbedded limestone containing Permian Tethyan fusulinids (Danner, 1965) with red ribbon chert containing upper Jurassic radiolarians

211

1976). Elsewhere, red argillaceous tuff (E. A. Pessagno, Jr., oral commun., interbedded with pillow basalt has yielded Cretaceous planktonic foraminifers (Pessagno, oral commun., 1976). From these data it is apparent that the islands have undergone a very late Mesozoic or early Tertiary period of intense deformation and high-pressure metamorphism characteristic of subduction complexes. It is not clear, however, whether the entire mixed terrane actually constitutes one melange, or whether older and younger melanges have been juxtaposed and sheared together during the last deformation. The structural position of this melange terrane, lying inboard, or east, of Vancouver Island, is also anomalous, and it suggests that Vancouver Island may have been displaced from a more southerly position in a manner similar to that suggested by Jones et al. (1976a) for comparable rocks in southern Alaska. CENTRAL

OREGON

The presence of an extensive melange in central Oregon has been suggested by Thayer and Brown (1976) but they did not specifically link it to subduction. The melange comprises blocks of argillite: chert, limestone, pillow basalt, amphibolite, keratophyre, and andesitic sandstone in a highly sheared argillite or locally a serpentinite matrix. Large blocks of ultramafic to mafic rocks, such as the Canyon Mountain Complex, may also be incorporated in the melange. These rocks are thought by Thayer and Brown (1973, 1976) to be of mainly Permian age, and this view is supported by the presence of Permian Tethyan fusulinids in some limestone blocks (Bostwick and Nestell, 1967). However, the discovery of Mesozoic radiolarians (Jones et al., 1976b) in a block of chert in the melange shows that much younger rocks are present, and that the melange itself could not have been formed during late Paleozoic to early Mesozoic time, as suggested by Thayer and Brown (1976). In the Mitchell area of central Oregon, a similar melange containing Permian fusulinid-bearing limestone, marble, chert, quartzite, mafic metavolcanic rocks, and serpentinite (Swanson, 1969), is unconformably overlain by Lower Cretaceous (Albian) sedimentary rocks (Wilkinson and Oles, 1968). Swanson reports that the melange contains the blueschist-facies metamorphic minerals lawsonite and crossite, but whether these occur in a regionally metamorphosed terrane or in tectonic blocks is not clear. In any case, the overlying Lower Cretaceous sedimentary rocks place an upper age limit on the time of melange formation in central Oregon, which must be preAlbian. This melange is thus somewhat older than the similar west coast subduction complexes discussed herein. BAJA CALIFORNIA

the

The presence of metamorphosed (blueschist) subduction complexes Vizcaino Peninsula and Cedros Island, Baja California, Mexico,

on has

212

recently been discussed by Jones et al. (1976c). They are best developed on the southern half of Cedros Island, where they form a number of thrust slices or sheets, marking the position of former subduction zones. Included are units of strongly foliated meta~aywacke, metabasalt, and metachert, serpentinite melanges, and metagraywacke melanges in which occur blocks of chert, greenstone, blueschist, quartzite, and limestone. One chert block yielded Upper Jurassic radiolarians (E.A. Pessagno, Jr., written commun., 1976), and a huge limestone block yielded crinoidal remains and fusulinids of possible Early Permian age (R.C. Douglass, written commun., 1976). California Coast Ranges The late Mesozoic to early Tertiary subduction complex of the California Coast Ranges consists of the mixture of sedimentary, igneous, and metamorphic rocks known as the Franciscan assemblage or complex. It is the most thoroughly studied of a series of similar packages of rock that border the west side of the North American continent. The upper limit of the Franciscan rocks is by definition the Coast Range thrust (Bailey et al., 1970), which in the northern Coast Ranges extends as a regionally flat but locally folded, nearly continuous structure from the San Andreas fault on the west across the entire Coast Range to its eastern margin where it plunges steeply to the east under the Great Valley sequence (Figs. 2, 3). Above the Coast Range thrust is in most places an ophiolite, interpreted as oceanic crust, on which the sedimentary rocks of the Great Valley sequence were deposited. These sedimentary rocks range in age from Late Jurassic to early Tertiary, and they contain many of the same fossils found in the coeval Franciscan rocks now lying structurally below them. An island arc has been postulated to separate the Franciscan and the Great Valley sequence (Blake and Jones, 1974, p. 354) but direct evidence to support this suggestion is lacking. Both the Franciscan and the Great Valley rocks formed on oceanic crust off the continent, so that subduction took place beneath an oceanic plate, not a crystalline continental block. We will first describe the rocks of the Great Valley sequence, and then turn to the underlying Franciscan subduction complexes. Great Valley sequence The Great Valley sequence consists of sandstone and mudstone, with minor conglomerate, piled up in a virtually conformable stratified sequence at least 16,000 m thick. Lying on the basal ophiolite in eastern parts of the Coast Ranges are mudstones of late Kimmeridgian age (Jones, 1975), and in the west the basal beds appear to be a bit younger. The age of the youngest beds lying on the basal ophiolite has not been defined in all areas, but locally it is as young as Eocene. The rocks are flysch-like with a preponderance of mudstone and thin sandstone in the older part of the section, and more and

liS0

1

iP

OREOON ----__----__-__ XLIFORNIA

1 :PLANATION

Franciscan auemblage Great V&y sequence. 8 mineral zones in met& mtially unmetamorphoeed morphmed gnywacke: m 1. Laumwtite 2, Pumpellyite Pre-Fran&can rocks

0 t 0

100

KILOMETERS f 100

MILES

Fig. 2. Map of California showing distribution of the Franciscan assemblage and Great Valley sequence. Position of cross-section (Fig. 3) shown by A-A’.

214 I

A

ic

4

A’

SL UJ-UK FRANCISCAN tCkm ~- ---IO0 KM

Fig. 3. Cross-section across northern Coast Ranges. See Fig. 2 for location (Section based on unpublished data of R.J. Mc~ughlin and E.H. Bailey.) 0 = ophiolite at base of Great Valley sequence; C = Cenozoic volcanic and sedimentary rocks.

thicker sandstone in the upper part. The basal sandstones contain mafic volcanic detritus and no potassium feldspar, whereas the youngest beds have less matrix and contain as much as 30% potassium-feldsp~. Individual beds show remarkable continuity, being traceable where best exposed along the west edge of the Great Valley for tens of kilometers. The lowest part of the pile has undergone only low-grade load metamorphism with the generation of laumontite, prehnite, or pumpellyite (Bailey and Jones, 1973). The lack of structural distortion or regional metamorphism in these rocks deposited over the active subduction zone is remarkable. The ophiolite lying beneath the Great Valley sequence is generally dismembered, but locally, as at Point Sal near Santa Barbara (Hopson et al., 1975), the entire ophiolite sequence from basal harzburgite through gabbro, basaltic sheeted zone, and overlying pillow lavas can be observed. These rocks are regarded as oceanic crust and mantle, although the sequence is thin as compared to modern oceanic sections. The oceanic crust above the harzburgite is generally less than 1 km thick, leading some to suggest it originated in a back-arc basin or marginal sea rather than at an accreting plate margin (Blake and Jones, 1974). It has been radiometrically dated at several places and shows an age of 150-160 m.y. (Lanphere, 1971; Hopson et al., 1975). Most of the basal, or mantle, part of the ophiolite is completely serpentinized h~zbu~te that now directly overlies the Coast Range thrust and subjacent subducted Franciscan rocks. We know of no way to determine how much of the original thickness of ophiolite has been removed by shearing along the Coast Range thrust, but the total thickness of ophiolite now exposed is nowhere more than 4 km and is generally not more than 2 km. Locally, however, the ophiolite belt is up to 8 km thick owing to tectonic repetition. Franciscan

assemblage

The assemblage of Franciscan rocks lying structurally beneath the Coast Range thrust includes, in decreasing order of abundance, graywacke, shale, mafic volcanic rocks, serpentinite, chert, and limstone, all at least somewhat

215

metamorphosed and with a minor part completely converted to schist, jadeitized metagraywacke, and even eclogite (Bailey et al., 1964). Megafossils are rare, but in recent years many dates have been obtained from radio&an chert (Pessagno, 1973) or palynomorph-bearing limy rocks (Evitt and Pierce, 1975). The fossils indicate a depositional span from Late Jurassic (Tithonian) to Eocene time. Radiometric dates on metamorphic rocks give the same span (Suppe, 1969), except some included blocks of high-grade blueschists yielded ages of 150 m.y. (Coleman and Lanphere, 1971) - a bit older than any of the fossils-bearing sedimentary rocks - and none has yielded Tertiary dates. The structure of the Franciscan rocks is complex and still poorly understood. In outcrop, the most consistent feature is their pervasively sheared appearance. Although gradations exist, the rocks can usually be subdivided in the field into three major groups: (1) metagraywacke units characterized by an incipient to pronounced penetrative schistosity that can be further subdivided into several “texture zones” (Blake et al., 1967); (2) “broken formations” defined as relatively well-bedded graywacke-shale units disrupted by nonpenetrative brittle-shear fractures but not containing exotic inclusions; and (3) melange consisting of fragments and blocks of graywacke, chert, and greenstone, plus exotic rocks such as glaucophane schist, eclogite, and rarely limestone, in a sheared matrix of argillite or less commonly serpentinite (Hsii, 1971). Another feature common to all of the Franciscan rocks is the presence of metamorphic mineral assemblages, which, from experimental and theoretical (thermodynamic) data, indicate a high ratio of pressure to temperature (Ernst, 1971). The most typical of these assemblages contain the diagnostic blueschist minerals lawsonite and glaucophane; however, zeolite (laumontite), prehnite-pumpellyite, glaucophanitic greenschist, and even amphibolite facies are locally present. The distribution of these facies suggests that the observed mineral assemblages formed along the sole of major thrust faults in inverted sequences, probably in response to higher thermal gradients within the hanging wall of the upper plate (Blake et al., 1969; Platt, 1975). Subsequent deformation has largely obscured these original relations, however, and in many places units carrying very different metamorphic mineral assemblages are tectonically juxtaposed. The coherent units of graywacke form large slabs and nappes, surrounded by or engulfed in the melange units (Blake and Jones, 1974). Structurally lower slabs tend to be younger than overlying slabs, and those to the west are younger than those to the east. For example, rocks of Tertiary (Eocene) age are found only in western exposures, in a region called the coastal belt (Evitt and Pierce, 1975). In order to illustrate the structural style and complexity of subduction complexes, we will describe in some detail a structural cross section across the Coast Ranges north of San Francisco (Figs. 3, 4). Because of the great heterogeneity of the entire belt, stretching from Mexico to Alaska, no other

216

0

5KM

EXPLANATION GREAT

FRANCISCAN

r-l 2

VALLEY

SEQUENCE

ROCKS

Radiolarian

chert

Fig. 4. Cross-section showing relation of Great Valley sequence to Franciscan rocks, and positions of Coast Range thrust and Coastal belt thrust. (Based on unpublished data of R.J. McLaughlin and E.H. Bailey.)

place will exhibit exactly the same units in similar sequence, but the style of imbricate slabs with inte~ening melange probably does prevail for the entire belt.

217

In detail (Fig. 4), a thick unit of cohesive graywacke of prehnitepumpellyite metamorphic grade immediately underlies the Coast Range thrust. Elsewhere along this contact, the rocks tend to be more metamorphosed, and include lawsonite-bearing quartz mica schist of textural zone III (Blake et al., 1967). A unit of well-crystallized foliate blueschist metagraywacke underlies the prehnite-pumpellyite graywacke. It has an outcrop width of as much as 2 km and has been traced over a distance of 45 km. Below is prehnitepumpellyite-bearing melange. Because of the contrast in metamorphic grade with the rocks above and below, we infer that the blueschist unit reached its present position by sliding or thrusting. Elsewhere in the northern Coast Ranges are other sheets of similarly metamorphosed blueschists which because of their large size are best described as nappes, rather than simply as tectonic blocks. Beneath the units of cohesive metagraywacke and blueschist is a melange underlain by cohesive graywacke and shale. In the area crossed by the section shown in Fig. 4, at least three cohesive units and two melange zones are recognized, and there seem to be others elsewhere in the Coast Ranges (Maxwell, 1973). All of these extend for tens of kilometers and are mappable units, but their distinctions are subtle and correlation from one area to another is difficult. The more cohesive graywacke-shale units contain beds characterized by their diversity of thickness, with graywacke beds ranging in thickness from a few centimeters to a few meters. In general, individual beds cannot be traced, partly because of poor exposure, but also because all the cohesive units are cut by innumerable faults that cross the bedding at low angles, shearing the sedimentary layers. The assemblage of rocks as a whole has continuity but individual layers do not. Where the shearing is most intense the rocks become a “broken formation,” with all gradations from disrupted beds to boudins in a shale matrix. This style of deformation appears to be the result of pulling apart or massive extension nearly parallel to bedding. Some of the cohesive graywacke units include large interbedded lenses of radiolarian chert. Melange units, some more than 1 km thick, separate the more cohesive graywacke units. The melanges have a matrix of siltstone and disaggregated graywacke in which are suspended lenses and blocks of more cohesive graywacke, mafic volcanic rocks, chert, local serpentinite, and metamorphic rocks including high-grade blueschists, amphibolite, and eclogite “knockers”. The metamorphic mineral of the matrix generally is pumpellyite, but lawsonite is locally present. The blocks range in size from one meter up to many kilometers, but most fall in the range from ten meters to a few hundred meters. The mixture of hard lumps in a softer matrix results in extensive landsliding and the development of a characteristic knobby topography. Mapping the limits of melange zones can be difficult because melange composed mostly of sheared matrix does not differ greatly from the “broken formation” parts of the cohesive graywacke units. At the other extreme some of

218

the volcanic “blocks” are 10 km or more in extent, making it difficult to identify these massive units as simply blocks in a sheared matrix. In addition, where younger major faults, such as the San Andreas, locally cut the Franciscan terrane, the breccia zones generated along them closely resemble the older Franciscan melanges that may have an entirely different origin. The westernmost Franciscan unit - the coastal belt - consists of arkosic sandstone and lesser amounts qf shale and very little volcanic rock and chert that crop out in a belt extending inland about 50 km from the coast. The coastal belt sandstone contains up to 30% potassium feldspar, in contrast to the other Franciscan units, which contain almost none. In most places these rocks are thin-bedded, but locally they are massive with no apparent bedding throughout several tens of meters. The rocks locally are isoclinally folded, sheared, and fractured (Beutner, 1975), but less so than the Franciscan rocks to the east. Laumontite is widespread and prehnite and pumpellyite occur locally. Fossils indicate an age span from Late Cretaceous (Cenomanian) to Eocene (Evitt and Pierce, 1974). Most of these young rocks dip eastward beneath older Franciscan rocks. The contact between the two is a fault named the Coastal Belt thrust by McLaughlin and Bailey (see Fig. 4). Recent studies suggest that nearly all of the high-grade glaucophane schist and eclogite knockers are confined to the melange zone overlying the Coastal Belt thrust (Fig. 4), and have no relation to the Coast Range thrust. CONCLUSION Based on this brief review of the late Mesozoic and early Tertiary subduction complexes, we can draw several conclusions regarding the structural evolution of western North America. (I) The process of accretion due to subduction has added a very large area of new land to the North American continent. A conservative estimate indicates that nearly 200,000 km* are now presently underlain by late Mesozoic to early Tertiary subduction complexes. Obviously, this process is an important means by which continents grow. (2) The presence in some of the melanges (Kodiak Island and Kenai Peninsula, San Juan Islands, central Oregon, Baja California?} or large blocks of limestone containing Tethyan, or tropical, fusulinid faunas which are very different from nearby North American coeval faunas (Monger and Ross, 1971) indicates that an enormous amount of northward plate movement has occurred to bring about the displacement of these blocks. The fact that these Tethyan fusulinid-Bering blocks occur in young melanges suggests that similar displaced terranes with Tethyan fusulinids, such as the Cache Creek belt of British Columbia (Monger and Ross, 1971; Monger et al., 1972; Monger, 1975) may have accreted in late, rather than early, Mesozoic time. (3) These subduction complexes mark the continental margin at their time of formation. As such, they can be used as “mega-key beds” to delimit and analyze younger tectonic movements that have affected and changed the

219

continental margin (Jones et al., 1976). With regard to the deposition and subsequent deformation of the Franciscan rocks, several aspects should be emphasized: (1) The Franciscan assemblage contains some pelagic deposits, but they are minor compared to the vast quantity of qua&o-feldspathic graywacke and shale that was derived from a continental source. Hence, it is incorrect to refer to the Franciscan as representing scraped-off oceanic deposits. The source of the qua&o-feldspathic detritus and its paleogeographic setting are still conjectural. (2) The style of deformation clearly indicates nearly horizontal transport of the Franciscan rocks beneath an upper plate that includes at its base a slab of ophiolite. The plates were probably completely decoupled, because sedimentary rocks of the Great Valley sequence are relatively undeformed. This decoupling probably occurred within a zone of intense imbrication located in the lower part of the ophiolite, which is extensively sheared, disrupted, and locally reduced to tectonic melange. (3) Most workers now agree that the underthrusting of Franciscan rocks was the result of subduction, and that during this process the melanges and blueschists were formed. However, several problems remain unresolved: (a) The vast differences in structural style and metamorphic mineral assemblages between the melanges and blueschists implies that either the tectonic burial differed greatly, subduction rates varied throughout time, or some other as yet unknown factors were involved. (b) The pervasive mixing of high- and low-grade metamorphic rock implies extensive postmetamorphic tectonic transportation, but the source, timing, and direction of movement of the metamorphic units are not known. (c) The origin of the melanges remains enigmatic. Gravity sliding followed by subduction could explain the mixing, but this would not account for the origin and emplacement of the high metamorphic-grade knockers, many of which show evidence of having been transported within serpentinite. The relation between the knockers and the coastal belt thrust suggests that the knockers may have been introduced into the Franciscan assemblage from a western source, but that source has not been identified. REFERENCES Bailey, E.H. and Jones, D.L., 1973. Metamorphic facies indicated by vein minerals in basal beds of the Great Valley sequence, northern California. U.S. Geol. Surv. J. Res., l(4): 383-385. Bailey, E.H., Irwin, W.P. and Jones, D.L., 1964. Franciscan and related rocks and their significance in the geology of western California. Calif. Div. Mines Geol. Bull., 183; 177 pp. Bailey, E.H., Blake, M.C., Jr. and Jones, D.L., 1970. On-land Mesozoic oceanic crust in California Coast Ranges. In: Geological Survey Research, 1970. U.S. Geol. Surv. Prof. Pap., 700-C: C70-81. Berg, H.C. and Hinckley, D.W., 1963. Reconnaissance geology of northern Baranof Island, Alaska. U.S. Geol. Surv. Bull., 1141-O: 01-024.

220 Beutner, E.C., 1975. Folds in coastal Franciscan rocks, Cape Mendocino area, California. Geol. Sot. Am. Abstr. Programs, 7 (3): 298-299. Blake, M.C., Jr. and Jones, D.L., 1974. Origin of Franciscan melanges in northern California: Sot. Econ. Paleontol. Mineral., Spec. Publ., 19: 255-263. Blake, M.C., Jr., Irwin, W.P. and Coleman, R.G., 1967. Upside-down metamorphic zonation, blueschist facies, along a regional thrust in California and Oregon. U.S. Geol. Sui-v. Prof. Pap., 575-C: Cl-C9. Blake, M.C. Jr., Irwin, W.P. and Coleman, R.G., 1969. Blueschist-facies metamorphism related to regional thrust faulting: Tectonophysics, 8: 237-246. Bostwick, D.A. and Nestell, M.K., 1967, Permian tethyan fusulinid faunas of the northwestern United States. Syst. Assoc. Publ., 7: 93-102. Burk, C.A., 1965. Geology of the Alaska Peninsula-Island Arc and continental margin, (1). Geol. Sot. Am. Mem. 99: 250 pp. Clark, S.H.B., 1973. The McHugh Complex of south-central Alaska. U.S. Geol. Surv. Bull., 1372-D: Dl-DlO. Coleman, R.G. and Lanphere, M.A., 1971. Distribution and age of high-grade blueschists, associated eclogites, and amphibolites from Oregon and California. Geol. Sot. Am. Bull., 82: 2397-2412. Connelly, W., Hill, M., Beyer, B. and Moore, J.C., 1976. The Uyak Formation, Kodiak Islands, Alaska: an early Mesozoic subduction zone complex. Geol. Sot. Am. Abstr. Programs, 8 (3): 364. Danner, W.R., 1965. Guidebook for field trips on San Juan Island, Washington. B. C. Univ. Dep. Geol., Rep., 1 (2nd ed.), 34 pp. Ernst, W.G., 1971. Do mineral parageneses reflect unusually high-pressure conditions of Franciscan metamorphism? Am. J. Sci., 270: X1-108. Evitt, W.R. and Pierce, S.T., 1975. Early Tertiary ages from the coastal belt of the Franciscan Complex, northern California. Geology, Aug. 1975: 433-436. Glassley, W.E., Whetten, J.T., Cowan, D.S. and Vance, J.A., 1976. Significance of coexisting lawsonite, prehnite, and aragonite in the San Juan Islands, Washington. Geology, 4 (5): 301-302. Hopson, CA., Frano, C.J., Pessagno, E.A., Jr. and Mattinson, S.M., 1975, Preliminary report and geologic guide to the Jurassic ophiolite near Point Sal, southern California coast. Guidebook Field Trip, 5, Geol. Am., Cordilleran Sect., 71st Annu. Meet., Los Angeles, 22 pp. Hsii, K.J., 1971. Franciscan melanges as a model for eugeosynclinal sedimentation and underthrusting tectonics. J. Geophys. Res., 76: 1162-1170. Jones, D.L., 1973. Structural elements and biostratigraphic framework of Lower Cretaceous rocks in southern Alaska. In: The Boreal Lower Cretaceous. See1 House Press, Liverpool, pp. l-18. Jones, D.L., 1975. Discovery of Buehiu rugosa of Kimmeridgian age from the base of the Great Valley sequence. Geol. Sot. Am. Abstr. Programs, 7 (3): 330. Jones, D.L. and Clark, S.N.B., 1973. Upper Cretaceous (Maestrichtian) fossils from the Kenai-Chugach Mountains, Kodiak and Shumagin Islands, southern Alaska. U.S. Geol. Surv. J. Res., 1 (2): 125-136. Jones, D.L., Pessagno, E.A., Jr. and Csejtey, B. Jr., 1976a, Significance of the Upper Chulnita Ophiohte for the Late Mesozoic evolution of southern Alaska. Geol. Sot. Am, Abstr. Programs, 8 (3): 385-386. Jones, D.J.,., Pessagno, E.A., Jr., Force, E.R. and Irwin, W.P., 1976b. Jurassic radiolarian chert from near John Day, Oregon. Geol. Sot. Am. Abstr. Programs., 8 (3): 386. Jones, D.L., Blake, M.C., Jr., and Rangin, C., 1976c. The four Jurassic belts of northern California and their significance to the geology of the southern California borderland: Am. Assoc. Pet. Geol. Pac. Sect., Misc. Publ. 24: 343-362. Lanphere, MA., 1971. Age of the Mesozoic oceanic crust in the California Coast Ranges. Geol. Sot. Am. Bull.. 82: 3209-3212.

221 Loney, R.A., Brew, D.A., Muffler, L.J. and Pomeroy, J.S., 1975. Reconnaissance geology of Chicagof, Baranof, and Kruzot Islands, southeastern Alaska. U.S. Geol. Surv. Prof. Pap., 792: 105 pp. MacKevett, E.M., Jr. and Plafker, Cr., 1974. The Border Ranges fault in south-central Alaska. U.S. Geol. Surv. J. Res., 2 (3): 323-329. Maxwell, J.C., 1973. Anatomy of an orogen. Geol. Sot. Am. Bull., 85: 1195-1204. Monger, J.W.H., 1975. Correlation of eugeosynclinal tectono-stratigraphic belts in the North America Cordillera. Geol. Sci. Can., 2 (1): 4-10. Monger, J.W.H. and Ross, CA., 1971. Distribution of fusulinaceans in the western Canadian cordillera. Can. J. Earth Sci., 8: 259-278. Monger, J.W.H., Souther, J.G. and Gabrielse, H., 1972. Evolution of the Canadian cordillera. Am. J. Sci., 272: 577-602. Moore, J.C., 1972. Uplifted trench sediments: southwestern Alaska-Bering Shelf edge, Science, 175: 1103-1105, Moore, J.C., 1973. Cretaceous continental margin sedimentation, southwest Alaska. Geol. Sot. Am. Bull., 84: 595-614. Moore, J.C. and Connelly, W., 1976. Subduction, arc volcanism, and fore-arc sedimentation during the early Mesozoic, southwest Alaska. Geol. Sot. Am., Abstr. Programs, 8 (3): 397-398. Moore, G.W., 1969. New formations on Kodiak and adjacent islands, Alaska. In: G.V. Cohee, R.G. Bates and W.B. Wright (Editors), Changes in Stratigraphic Nomenclature by the U.S. Geological Survey, 1967. U.S. Geol. Surv. Bull., 1274-A: A27-A35. Muller, J.E., 1973. Lower Cretaceous flysch sequence of the west coast of Vancouver Island. Geol. Sot. Am., Abstr. Programs, 5 (1): 84. Muiler, J.E., Northcote, K.E. and Carlisle, D., 1974. Geology and mineral deposits of Alert-Cape Scot Map Area, Vancouver Island, British Columbia. Geol. Surv. Can. Pap., 74-8: 77 pp. Page, R.J., 1974. A tectonic melange on the west coast of Vancouver Island, British Columbia. Geol. Sot. Am. Abstr. Programs, 6 (3): 233. Pessagno, E.A., Jr., 1973. Age and significance of radiolarian cherts in the California Coast Ranges. Geology, Dec. 1973: 153-156. Plafker, G. and MacNeil, F.S., 1966. Stratigraphic significance of Tertiary fossils from the Orca Group in the Prince William Sound region, Alaska. U.S. Geol. Surv. Prof. Pap., 560-B: B62-B68. Plafker, G., Jones, D.L., Hudson, T. and Berg, H.C., 1976. The Border Ranges fault system in the Saint Elias Mountains and the Alexander Archipelago. in: E.E. Cobb (Editor), The United States Geological Survey in Alaska: Accomplishments during 1975. U.S. Geol. Surv. Circ., 733: 14-16. Platt, J.P., 1975. Metamorphic and deformational processes in the Franciscan complex, California. Some insights from the Catalina Schist terrane. Geot. Sot. Am. Bull., 86: 1337-1347. Suppe, J., 1969. Times of metamorphism in the Franciscan terrain of the northern Coast Ranges, California. Geol. Sot. Am. Bull., 80: 135-142. Swanson, D.A., 1969. Lawsonite blueschist from north-central Oregon: U.S. Geol. Surv. Prof. Pap., 650-B: B8-Bll. Thayer, T.P. and Brown, C.E., 1973. Blue Mountain region of northeastern Oregon and western Idaho interpreted as a pre-Cretaceous island-arc system: Geol. Sot. Am. Abstr. Programs, 5 (1): 115-116. Thayer, T.P. and Brown, C.E., 1976. The John Day area: an exemplar of pre-Tertiary geology in the Blue Mountain region, Oregon-Idaho. Geol. Sot. Am. Abstr. Programs, 8 (3): 415-416. Whetten, J.T., 1975. Tertiary f?) melange in the southeastern San Juan Islands, Washington. Geol. Sot. Am. Abstr. Programs, 7 (3): 387-388.

222 Whetten, J.T., Zartman, R.E., Cowan, D.S., Glassley, W.E., Jones, D.L. and Pessagno, E.A., Jr., 1977. New dates and their significance from the San Juan Islands, Washing ton. Geol. Sot. Am. Abstr. Programs (in press). Wilkinson, W.D. and Oles, K.F., 1968. Stratigraphy and paleoenvironments of Cretaceous rocks, Mitchell quadrangle, Oregon. Am. Assoc. Pet. Geol. Bull., 52 (1): 129-161.