A Thrust Plate of Ophiolitic Rocks in the Preston Peak Area, Klamath Mountains, California

A thrust plate of ophiolitic rocks in the Preston Peak area, Klamath Mountains, California

ARTHUR W. SNOKE Department of Geology, University of South Carolina, Columbia, South Carolina 29208

ABSTRACT ultramafic-mafic complexes and their significance in the tectonic evolution of the Klamath Mountains has been emphasized by Irwin In the Preston Peak area, Klamath Mountains, California, a re- and Lipman (1962), Irwin (1964), Lipman (1964), and Davis gional thrust fault separates metasedimentary rocks of the Late (1968). However, the ophiolitic character and local preservation of Jurassic Galice Formation from an overlying plate of older ophio- intact pseudostratigraphic sequences has only recently been investi- litic rocks. The ophiolite consists of a basal sheet of ultramafic gated (Snoke, 1971; Irwin, 1972; Lindsley-Griffin, 1973). tectonite overlain and intruded by a heterogeneous mafic complex In the Preston Peak area (Fig. 1), glaciated exposures and deeply that in turn is overlain by metabasaltic and metasedimentary rocks. dissected canyons permit an exceptional opportunity to examine Field relations indicate that the ophiolite is polygenetic, with a the ophiolite sequence that forms the basal part of the western major temporal hiatus separating the tectonitic ultramafic rocks Paleozoic and Triassic belt of Irwin (1960). The ophiolite is divisi- and the associated mafic rocks. Mineral assemblages and textures ble into three distinct units: (1) a basal sheet of alpine-type perido- in the ultramafic rocks suggest high-temperature recrystallization tite and sparse dunite, locally with a tectonite fabric; (2) a mafic and penetrative deformation. In contrast, diabase and diabase complex, consisting predominantly of diabase, basalt, and diabase breccia, the most abundant constituents of the mafic complex, are breccia all metamorphosed to nonschistose rocks of the greenschist nonschistose rocks metamorphosed to lower greenschist facies. facies; and (3) an originally overlying but now fault-bounded se- Contacts between ultramafic rocks and rocks of the mafic complex quence of interlayed basaltic and sedimentary rocks regionally are fault contacts, intrusive contacts, or both. Mafic rocks occur in metamorphosed to the greenschist facies. Other eleméhts of the the ultramafic rocks as diabase dikes with chilled margins and as ophiolite include scarce masses of jackstraw-textured talc-olivine tectonic inclusions. Piecemeal growth of the ophiolite is also indi- rock in tectonitic peridotite, tectonic inclusions of amphibolite in cated by minor features: scarce jackstraw-textured talc-olivine serpentinite mélange, gabbro and olivine clinopyroxenite fragments rocks in tectonitic peridotite, cognate xenoliths of gabbro and in the diabasic and basaltic rocks of the mafic complex, and dikes olivine clinopyroxenite in diabase, and scattered dikes of inter- of intermediate composition intrusive into the mafic complex. A mediate composition in both ultramafic and mafic rocks. schematic summary of the lithology of the ophiolite is given in Fig- Field aspects of the ophiolite appear more compatible with a ure 2; the distribution of rock units is shown in Figure 3. primitive island-arc setting than with a spreading oceanic ridge or Although the ophiolite appears to be a distinct petrotectonic as- marginal-basin model. The temporal relations between the ultra- semblage, contact relations between the component parts suggest a mafic and mafic rocks, the presence of pyroclastic breccias, and the polygenetic history, with a major hiatus between the tectonitic ul- character of associated epiclastic rocks support this hypothesis. On tramafic rocks and the overlying mafic segments. This reasoning is the basis of this interpretation, the tectonic history of this segment supported by a sharp metamorphic contrast between the tectonitic of the Klamath Mountains during late Paleozoic to Jurassic time alpine-type peridotite and the nonschistose, lower greenschist- was dominated by island-arc genesis and westward extensional rift- facies, metadiabasic and metabasaltic rocks. A distinct time break ing. The ultimate collapse of this system occurred during the Late in the development of the ophiolite is also indicated by the occur- Jurassic (Nevadan orogeny) when the Galice Formation (Jurassic rence of jackstraw-textured talc-olivine rock in the tectonitic island arc and associated sedimentary basin) was thrust beneath the peridotite, with mafic dikes in both ultramafic rock types. Preston Peak ophiolite, a Permian-Triassic remnant arc. ALPINE-TYPE ULTRAMAFIC ROCKS INTRODUCTION A folded and faulted ultramafic sheet is separated from the un- An arcuate belt—like arrangement of lithologically distinct derlying metasedimentary rocks of the Galice Formation by a eugeosynclinal rocks is the most obvious structural feature of the thrust fault and is in turn overlain by rocks of the mafic complex or Klamath Mountains province of northwestern California and western Paleozoic and Triassic belt. The ultramafic sheet consists of southwestern Oregon (Irwin, 1960). The belts are bounded by highly serpentinized peridotite, a little dunite, and very minor east-dipping faults that are interpreted to be thrust faults of re- pyroxenite. Similar but larger ultramafic sheets lie southeast (the gional extent (Irwin, 1964; Davis, 1968). The origin of this ar- Trinity sheet) and west (the Josephine sheet) of the Preston Peak rangement is not fully understood, but the concepts of plate tecton- area (Fig. 1). These ultramafic sheets lie directly on thrust faults and ics and sea-floor spreading suggest a gradual accretionary process presumably are tectonically emplaced (Irwin and Lipman, 1962; involving the subduction of eugeosynclinal sedimentary and vol- Irwin, 1964). canic rocks beneath an overriding continental margin (Hamilton, 1969; Irwin, 1973). Field Appearance Sheetlike and linear bodies of alpine-type ultramafic and associated mafic rocks are concentrated along many of the bound- About half of the ultramafic rocks are massive and uncrushed, aries between the lithic belts. The deep-seated origin of the and half are highly sheared serpentinites (slickentite). The massive

Geological Society of America Bulletin, v. 88, p. 1641-1659, 14 figs., 1 table, November 1977, Doc. no. 71110.

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ultramafic rocks typically weather to a brown or reddish-orange especially prevalent near the complexly faulted thrust zone between crust whose surface is studded with aggregates of relict olivine and the Galice Formation and the upper ophiolite plate; however, slick - pyroxene. Other unsheared ultramafic rocks may be totally serpen- entite also crops out in scattered interior areas of the ultramafic tinized but retain a vestige of their original coarse texture on sheet. weathered surfaces. The sheared serpentinites are dark olive green Locally in the massive ultramafic rock a mineral foliation is to bluish green and usually have a crude foliation. The unsheared defined by a planar arrangement of ellipsoidal aggregates of olivine ultramafic rocks form blocky, resistant outcrops, whereas the slick- and pyroxene. Scarce segregation banding is seen in the Middle entite forms smooth broad ridges that are subject to landsliding and Fork of the Smith River, where pyroxene-rich layers (about 5 mm other downslope movements. Highly sheared serpentinites are thick) alternate with olivine-rich layers (now almost completely

METASEDIMENTARY AND METAVOLCANIC ROCKS

'mm Schists of Western Jurassic belt Condrey Mountain

Western Paleozoic and Triassic belt

Pm - Permian where differentiated Tr - Triassic

Central metamorphic belt

Eastern Klamath belt

PLUTONIC ROCKS v:

Granitic rocks

Ultramafic and associated mafic rocks

Modified from Hotz (1971) and Irwin (1972)

Figure 1. Generalized geologic map of Klamath Mountains province, California and Oregon.

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serpentinized). These structures are found in other California ul- ultramafic rocks from the Galice Formation. The contact between tramafic rocks (Burro Mountain, southern Coast Ranges — Burch, the Galice and the mafic rocks is a segment of the Preston Peak 1968; Loney and others, 1971; and Trinity ultramafic sheet — Lip- fault. The contact between the mafic rocks and the overlying ultra- man, 1964; Goullaud, 1975) and are interpreted to be the product mafic rocks is the Wounded Knee fault, an east-dipping, high-angle of penetrative flowage and «crystallization. reverse fault. Lying above the ultramafic rocks are either a heterogeneous as- Contact Relations with Adjacent Rock Units semblage of fine- to medium-grained metabasaltic rocks called the mafic complex or metavolcanic and metasedimentary rocks corre- The contact between the ultramafic rocks and the underlying lated with the western Paleozoic and Triassic belt of Irwin (1960). metasedimentary (rarely metavolcanic) rocks of the Galice Forma- Commonly the contacts between these rocks and the ultramafic tion is the east-dipping Preston Peak fault (Fig. 3). The ultramafic rocks are faults, but in several places the mafic rocks are intrusive rocks along the contact are highly sheared, and in many places the into the ultramafic rocks. shear planes dip less than 45° to the east. Rodingite dikes and segregations are widespread in the ultramafic rock near the fault, as Petrography are highly altered mafic dikes and small intrusions. In the west-central part of the area, a sliver of mafic rocks The primary minerals of the ultramafic rocks are magnesian (greenstone, metadiabase, and highly altered gabbro) separates the olivine, enstatite, diopside, and red-brown to yellow-brown chro-

METAVOLCANIC AND META- ...... SEDIMENTARY ROCKS OF THE oooo COÙO O^gog WESTERN PALEOZOIC AND TRIASSIC BELT

SILICEOUS ARGILLITE

DIABASIC BRECCIA

MAFIC

COMPLEX DIABASE-multiple intrusions Figure 2. Diagrammatic columnar section of Preston Peak ophiolite. GABBRO AND PYROXENITE XENOLITHS INTERMEDIATE DIKES

ALPINE-TYPE ULTRAMAFIC ROCKS (urn), locally with a tectonite fabric

JACKSTRAW-TEXTURED TALC-OLIVINE ROCK

AMPHIBOLITE TECTONIC INCLUSIONS IN SHEARED SERPENTINITE

METASEDIMENTARY ROCKS OF THE GALICE FORMATION

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•Vj-Yj-^'j-y 123°45' Jl. Arthur W. Snoke 1969-71,74 i A. 2. Modified from Cater and izj—«J Wells 1953, Plate 1 Base from parts of Preston Peak (1956) and Gasquet |1951j 15 minute

quadrangles— U.S. Geological Survey

Bend in'section

Figure 3. Geologic map and cross sections of Preston Peak area, Klamath Mountains, California. (See explanation on following page.)

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mian spinel. Feldspar was not seen in the alpine ultramafic rocks. less peridotite has an equigranular aspect which suggests that Alteration products include serpentine-group minerals, tremolite, widespread annealing recrystallization and grain growth have talc, chlorite, magnetite, carbonate, and brucite. obliterated considerable small-scale evidence of the deep-seated Mesoscopic foliation defined by tabular aggregates of primary deformation history. However, in other parts of the ultramafic mineral grains is locally well developed in parts of the ultramafic sheet pronounced mylonitic textures are well preserved. The sheet, but the peridotite commonly is structureless. This structure- peridotite on the east side of Haystack Mountain is an excellent

INTRUSIVE PLUTONIC ROCKS SURFICIAL DEPOSITS

r Predominantly hornblende and hornblende-pyroxene u gabbro with subordinate mica-pyroxene diorite and < syenodiorite. Small intrusive masses of biotite and hornblende - biotite quartz diorite, biotite grano- I QI I Landslide diorite and quartz monzonife < D I Qg I Glacial deposits

Predominantly olivine clinopyroxenite, hornblende- O olivine clinopyroxenite, and wehrlite with subordinate

dunite; locally includes olivine gabbro, two-pyroxene V gabbro, and hornblende-plagioclase pegmatite

METASEDIMENTARY ROCKS

u LU l/> I<- t/<> Galice Formation SYMBOLS -> 0Í D Contact,dashed where approximately located , dotted where inferred or concealed PRESTON PEAK OPHIOLITE

">"iiimiiim«iiiii!fiiijimiimi Gradational contact Upper metavolcanic unit Includes metasedimentary rocks -stippled pattern Metavolcanic and P-Trvs Conglomerate - grit unit metasedimentary Fault,dashed where approximately m rocks (undivided)-- located, dotted where inferred or Argillite unit northeast of the concealed S.Siskiyou fault Imv Lower metavolcanic unit — m- marble

u u Thrust fault, dashed where approximately located, sawteeth t/1 V) < < Rocks of the inner zone of the on upper plate DD contact aureole of the Bear Z oc Mountain pluton Contact metamorphosed equivalent Axis of overturned anticlinal of umv and P-Trvs ±h fold in Galice Formation Predominantly hornblende schist, amphibolite,and basic gneiss u z Ö < Strike and dip of bedding, M « Mostly metadiabose with scattered overturned ( right) O • a i i metagabbro and metapyroxenite xenoliths as well as dikes and small intrusions of fine-grained 2 metabasalt Strike and dip of fofiation b, metadiabosic breccia where in metamorphic and alpine- differentiated type ultramafic rocks

Amphibolite and amphibolite gneiss

Drainage Alpi ne-type ultramafic rocks sp, serpentinite

Figure 3. (Continued). Explanation of geologic map and cross sections.

blastomylonite characterized by fluxion structure with multigrain are mineralogically similar to the metadiabases of the ophiolite and porphyroclastic augen of olivine, orthopyroxene, and clinopyrox- are thought to have fed the mafic constructional pile. Therefore, the ene (Fig. 4). The mineral grains that compose the multigrain augen development of the jackstraw talc-olivine rocks is considered to invariably exhibit pronounced strain effects such as undulatory ex- postdate the cessation of penetrative deformation imprinted on the tinction, kink bands, and deformed exsolution lamellae. These re- tectonitic peridotite and to predate the beginning of basaltic mag- sistant aggregates are set in a fine-grained, xenoblastic granular matism that built the mafic segments of the ophiolite. These age re- matrix consisting mainly of olivine. Similar textures are described lations emphasize the polygenetic history of the ophiolite. from many alpine-type ultramafic masses (Raleigh, 1965; Ave Lal- lemant, 1967; Ragan, 1967; Medaris, 1972). Summary Nearly all the alpine ultramafic rocks in the Preston Peak area are intensely serpentinized. Sheared serpentinites rarely contain any The ultramafic rocks of the Preston Peak sheet are typical of the primary grains, but the massive ultramafic rocks contain many, alpine type in that they exhibit the following distinctive charac- although the amount is quite variable. Many of the massive ultra- teristics: (1) evidence of solid-state deformation and recrystalliza- mafic rocks show a relict coarse texture on weathered surfaces but tion, (2) proximity to a major fault zone, (3) the predominance of contain more than 60% serpentine-group minerals. This high con- olivine over pyroxene and the lack of plagioclase, (4) lack of cumu- tent of serpentine minerals and associated alteration makes any late texture, (5) podiform chromite deposits, and (6) no evidence of volume percentage estimation of the primary constituents uncer- contact metamorphism near country-rock contacts. tain. A composite and typically complex tectonic history is well Olivine (Fo8g-Fo92, estimated from near-centered optic axis documented for many alpine-type ultramafic bodies (Thayer, 1960; figures on a flat stage) is the most abundant primary mineral in the Coleman, 1971; Loney and others, 1971). In particular, evidence serpentinized peridotite, originally probably forming between 75% for deep-seated deformation prior to emplacement into the crust and 90% of the rock. Pyroxene was the other major constituent of has been confirmed by the examination of coexisting primary min- the rock, but estimation of the original orthopyroxene :clinopyrox- eral phases (Loney and others, 1971; Medaris, 1972; Himmelberg ene ratio is made difficult by secondary alteration. In other alpine- and Loney, 1973), as well as textural and fabric studies (Avé type peridotites from the California-Oregon ultramafic belts, this Lallemant, 1967; Christensen, 1971; Loney and others, 1971). ratio is exceedingly high, and in some cases clinopyroxene occurs in Detailed studies of the Preston Peak alpine-type ultramafic rocks traces or is missing (Lipman, 1964; Himmelberg and Coleman, have not been attempted and, because of the extensive serpentini- 1968; Loney and others, 1971; Himmelberg and Loney, 1973). zation, probably will not be. The metamorphic structures and tex- Most of the alpine-type peridotite in the Preston Peak area is ser- tures locally well preserved in the ultramafic sheet suggest that pentinized harzburgite, but some of the peridotite contains these rocks have undergone a deep-seated deformation similar in sufficient clinopyroxene to be lherzolite according to Jackson's style and intensity to other Klamath Mountains alpine-type (1968) classification. Dunite is locally abundant, and it is par- peridotites (Trinity ultramafic sheet — Lipman, 1964; Goullaud, ticularly widespread near the Cyclone Gap mine, an abandoned 1975; Josephine ultramafic sheet — Himmelberg and Loney, chromite deposit. 1973). These ultramafic sheets (Trinity, Preston Peak, Josephine) represent the basal part of ophiolitic assemblages that partially Jackstraw-textured Talc-Olivine Rock form the hanging-wall plate of at least three of the major regional thrust sheets outlined by Irwin (1964). Several small masses of jackstraw-textured talc-olivine rock occur in tectonitic serpentinized peridotite south of Rattlesnake MAFIC COMPLEX Meadow, near Preston Peak (Fig. 3). The rock is characterized by elongate olivine crystals commonly arranged in a crisscross fashion. A heterogeneous assemblage of metamorphosed mafic intrusive Detailed field, petrographic, and chemical studies of these unusual and extrusive rocks forms a series of rugged glacially sculptured rocks (Snoke and Calk, 1978) indicate that they formed from ser- peaks in the eastern and northern parts of the Preston Peak area. pentinized peridotite by a replacement phenomenon involving a Sanger Peak, Youngs Peak, Rocky Knob, El Capitan, Copper volatile-rich phase. Mafic dikes intrude the jackstraw talc-olivine Mountain, Preston Peak, and Twin Peaks are resistant fine- to rocks as well as the surrounding country-rock peridotite. The dikes medium-grained mafic rocks (Fig. 3). Diabase and diabasic breccia constitute more than 95% of the exposures. Scattered throughout the diabasic rocks are medium-grained gabbroic inclusions and associated clinopyroxenite, irregular aphanitic greenstone masses, and fine-grained metabasaltic dikes. Amphibolite occurs as tectonic inclusions in serpentinite mélange near the base of the ultramafic sheet. Scarce dikes of intermediate composition are found at several localities. During a reconnaissance of the western Siskiyou Mountains, including the Preston Peak area, Maxson (1931, 1933) recognized this assemblage of mafic rocks as a distinct mappable unit and called it the Preston hornblende diorite. My study shows that the name is inappropriate for the following reasons: (1) the rocks vary appreciably in texture, in composition, and in degree of metamorphism; (2) the predominant rock type is metadiabase; and (3) actinolite is the typical amphibole rather than hornblende (ex- ceptions being the amphibolites and contact-metamorphosed varieties). To rectify this misnomer, the rocks are here called the mafic complex and are considered part of the Preston Peak ophiolite. Broad terranes of aphanitic metavolcanic rock (see Fig. 3, Broken Figure 4. Mylonitic mineral foliation in seipentinized peridotite. Note Rib Mountain area) are excluded from the mafic complex and are ellipsoidal aggregates of pyroxene porphyroclasts. Haystack Mountain. mapped as a part of the western Paleozoic and Triassic assemblage.

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However, these rocks are mineralogically identical to the rocks of monly encountered during a typical traverse across groups of iso- the mafic complex and are interpreted to represent the extrusive lated exposures. Gabbroic and, less commonly, pyroxenite inclu- segment of an ophiolite sequence. The diabase breccias in the mafic sions are locally abundant. Typically, these rocks occur as magma- complex are also probably extrusive (that is, pyroclastic), but the tic xenoliths in diabase (Fig. 5), but some occurrences are ex- predominance of phaneritic texture within the massive diabase ceptionally complex and may indicate a more complicated igneous suggests that much of the mafic complex is of hypabyssal or sub- history (Fig. 6). Aphanitic greenstone dikes and irregular masses volcanic origin. are also quite common. Exposures of diabasic breccia lie structurally or topographically above the massive diabase; the transition General Features between the two kinds of rock involves a thin zone of interlayered diabase and breccia. Dike injections do not occur in the diabasic The rocks of the mafic complex are massive and nonschistose breccia above the transition zone. (except for the amphibolite tectonic inclusions); a well-developed east-west to northwest-southeast jointing is usually the only Contact Relations measurable structure. Multiple intrusion is evident in many areas within the mafic complex. Many diabase dikes with chilled margins The contacts between the alpine-type ultramafic rocks and the intrude texturally identical diabase. A less convincing but com- rocks of the mafic complex are of three types: (1) fault — sheared plementary observation is the variation in diabasic texture corn- serpentinite in contact with diabase, no diabase dikes or intrusions, and few if any diabase tectonic inclusions in the serpentinite; (2) intrusive — diabase dikes and small intrusions injected into serpentinized ultramafic rocks; in some extreme cases only lensoidal serpentinite screens remain in a complexly diked "edge zone" (adapted from the term "edge fades" used by Reinhardt [1969] to describe a similar phenomenon in the Semail ophiolite of Oman); and (3) fault with many diabase tectonic inclusions — a hybrid between the previously described types; the diabase inclusions are interpreted to be tectonically fragmented dikes resulting from late ^-s-y.-'.VsV movements within plastic serpentinite. Metadiabase Faults between rocks of the mafic complex and serpentinized alpine peridotites are common throughout the upper-plate terrane and appear to be high angle. Well-defined steeply dipping reverse faults occur in the western part of the area, exemplified by the Twin 4 + + Peaks and Wounded Knee faults (Fig. 3). + + +V W + + + 1 Intrusive contacts, where well preserved, provide spectacular and irrefutable evidence that the rocks of the mafic complex postdate the alpine-type ultramafic rocks. The diabase intrusions occur as steeply dipping dike swarms or small individual masses. In a saddle just north of Rocky Knob, a well-developed edge zone occurs. The + 4- -i . 4 4. 4. . 4- . 4- -L 4-i edge zone consists of a dike complex in which the country rock is Figure 5. Pyroxenite xenolith within laige gabbro inclusion that has preserved only as irregular screens. The zone is roughly 120 to been intruded by diabase dike (right of center). Diabase dike has narrow 150 m wide. chilled margin about 5 cm thick. Sketched from a photograph. The faulted intrusive contact often is the most difficult to inter-

Figure 6. Irregular pyroxenite layer intimately associated with gabbro. Knife rests on fine- grained greenstone mass; texturally distinct diabases occur on both sides of layers.

pret. The occurrence of diabase masses in serpentinite and of ser- and vesicular greenstone are common, whereas medium-grained pentinite smears in diabase is typical. Having seen well-preserved gabbro is very rare. intrusive contacts, one can interpret the hybrid contacts as intrusive The breccias are poorly sorted, and all the fragments are angular. types that have been modified by faulting. The diabase inclusions No evidence was found of aqueous transport. Clast size ranges were probably once continuous dikes or small intrusive masses. from probably dust particles to more than 30 cm. The large clasts The serpentinite smears are thought to be tectonic injections along are uncommon, and the average size ranges from 2.5 to 8 cm. small fault planes or in some few cases partly remobilized serpen- Metamorphic recrystallization has obliterated matrix textures, but tinite screens. In Figure 3 such contacts have been plotted as faults, field observations of weathered surfaces suggest that original ma- but the presence of diabase tectonic inclusions distinguishes them trix material was minor and that the rock consists essentially of from simple fault contacts (type 1). fragments. Contacts between rocks of the mafic complex and the metavol- The interpretation of volcanic breccias is a difficult task, judging canic and metasedimentary rocks of the western Paleozoic and from the many possible origins outlined by Fisher (1960). The Triassic belt are scarce within the Preston Peak area. Those exposed compositional similarity yet textural variation of the clasts, coupled are either faults (for example, the fault west of Twin Peaks) or dep- with the wide range in clast size and poor sorting, suggests a pyro- ositional (for example, the siliceous argillite patch northeast of clastic origin for these rocks. They may thus record the occurrences Copper Mountain). of vents, but this interpretation cannot be tested until the geometry of the breccias is thoroughly mapped. Lithology Gabbro and Pyroxenite. Irregular masses and angular inclusions of gabbro and less commonly associated olivine clinopyrox- Diabase. Fine- to medium-grained, grayish-green diabase is the enite occur in and are intruded by diabasic rocks. The gabbros and most abundant rock type in the mafic complex. The rocks are mas- pyroxenites are scattered throughout the mafic complex but appear sive, exceedingly tough, and often intricately veined with clinozois- to be somewhat preferentially concentrated near basal ultra- ite and carbonate. Typically the diabases are characterized by uni- mafic-mafic rock contacts. form grain size; however, porphyritic varieties have been found. The irregular distribution of gabbro and pyroxenite makes a vol- Altered plagioclase-actinolitic amphibole-epidote-chlorite-leucox- umetric estimate difficult, but certainly their cumulative total is ene is the characteristic mineral assemblage of the diabases unaf- fected by contact metamorphism. Relict igneous texture is well preserved, and there is no indication of metamorphic textural equilibrium in these rocks. The occurrence of relict pyroxene and calcic plagioclase, coupled with the presence of zeolites in several specimens, indicates the lack of chemical equilibrium attained in these apparent greenschist-facies rocks. Table 1 lists some representative modes of the diabasic rocks. Specimens 69-62, 69-63, and 70-376 are definitely beyond the effects of thermal metamorphism, while the other specimens show incipient to moderate contact-metamorphic alteration.1 Diabasic Breccia. Diabasic breccias (Fig. 7) are locally abundant in the mafic complex and may serve as a very useful map unit in future, more detailed studies. Breccia fragments are entirely metabasitic in composition and range in texture from aphanitic greenstone to medium-grained gabbro. Diabase and greenstone clasts are always predominant, but microgabbro and porphyritic

1 A more detailed description of the diabasic rocks, GSA supplementary material 77-9, may be ordered from Documents Secretary, Geological So- Figure 7. Diabase breccia consisting of angular clasts of mafic igneous ciety of America, 3300 Penrose Place, Boulder, Colorado 80301. rocks. Diabase and greenstone fragments predominate.

TABLE 1. MODAL ANALYSES OF DIABASES FROM MAFIC COMPLEX, PRESTON PEAK

Sample no.: 69-62 69-63 69-65 70-3073' 70-317* 70-320* 70-321"" 70-323* 70-376

Plagioclase 38 40 38.5 51.5 23 35.5 30 28 41.5 Amphibole 41 38.5 41.5 42.5 69 46 56 44.5 20.5 Epidote 10.5 9 2 2 6 9.5 15 14.5 Chlorite 5 5 11.51 tr. 3.5 3 1 6.5f 8 Pyroxene 0.5 0.5 13 Leucoxene—sphene- opaque oxides 3 3.5 6.5 4.5 4.5 3.5 3 4.5 2.5 Quartz 3 tr. Zeolite 1.5 tr. Stilpnomelane tr. tr. tr. Biotite tr. 6 tr. Calcite tr. 1.5 Note: Thin sections selected for modal analysis avoided metasomatic clinozoisite-carbonate veinlets. Approximately 800 points covering the entire area of a standard pétrographie thin section were counted per section. Values in volume percent. Tr. = trace. " Contact metamorphosed. * Includes incipient contact-metamorphic(?) biotite.

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small, probably not exceeding 2% to 3%. This minor amount of dote, sphene, and opaque minerals. Common alteration products gabbroic rock is in distinct contrast to some of the Mediterranean are chlorite, actinolite, and clinozoisite. (Wilson, 1959) and Pacific (Davies, 1971) ophiolites, as well as to Hornblende varies from 50% to 80%, being most abundant in the Canyon Mountain Complex (Thayer and Himmelberg, 1968). the hornblende schists and least in the amphibolitic gneisses. All of these rocks are highly altered by low-grade metamor- Plagioclase grains are equant to elongate and untwinned. Refrac- phism, calcium metasomatism, and local brecciation and shearing. tive index estimations are near or slightly higher than balsam, The rocks are medium to coarse grained, and the gabbros are suggesting that the An content is approximately intermediate sometimes characterized by a distinct foliation (flow structure?). oligoclase. In several specimens, epidote is a prograde metamorphic Gabbro-pyroxenite relationships are poorly understood, but the phase, while colorless clinozoisite is a common retrograde mineral occurrence of pyroxenite inclusions in gabbro (Fig. 5) and gabbro in all the rocks. In a rather unusual amphibolite, bright-yellow epi- in pyroxenite (Fig. 6) suggests that they are contemporaneous. dote, green diopside, and plagioclase form coarse discontinuous The gabbros originally consisted of calcic plagioclase, diopsidic segregations parallel to the foliation of the rock. In summary, the pyroxene, and minor opaque minerals. Their present mineralogy is following parageneses have been found in the amphibolites: complex and characteristic of low-grade greenschist-facies hornblende-oligoclase-epidote-sphene, hornblende-oligoclase- metamorphism. Plagioclase is typically heavily saussuritized, but sphene, and hornblende-oligoclase-epidote-diopside-sphene. These original features, such as hypidiomorphic-granular texture and assemblages are indicative of low to intermediate amphibolite- twin lamellae, may be well preserved. Pyroxenes are pseudomor- facies metamorphism (Turner, 1968, p. 306—309). phically altered to fibrous, pale-green actinolitic amphibole and The typical contact relation of amphibolite adjacent to sheared irregular chlorite patches. serpentinite, coupled with the shape and distribution of the am- The original pyroxenite minerals consisted essentially of olivine phibolite bodies, indicates a history of tectonic fragmentation and and clinopyroxene. The olivines are totally serpentinized, whereas transport. These late tectonic events have obscured or eliminated pyroxene grains are partially pseudomorphosed by amphibole but much evidence critical in the determination of the site of origin. The may be locally well preserved. Thin exsolution lamellae parallel to available data are inconclusive but are compatible with the sugges- (100) are common in the clinopyroxenes. No orthopyroxene has tion that some amphibolites within ophiolite are part of the base- been found in any of the examined gabbro or pyroxenite speci- ment complex upon which younger igneous rocks have accumu- mens. lated (Malpas and others, 1973; Miyashiro, 1975). The meta- Amphibolite. Several masses of amphibolite and amphibolitic diabase and metagabbro dikes that intrude the amphibolites sup- gneiss are located south and southwest of Haystack Mountain and port the interpretation. Additional evidence is preserved on the are moderately well exposed along Knopki Creek Road (Figs. 3, 8). west side of Haystack Mountain, where foliation in amphibolite A small and much-altered amphibolite mass also occurs just north and overlying peridotite tectonite is parallel. There is little indica- of the Middle Fork of the Smith River near the Preston Peak thrust tion of postmetamorphic shearing along the amphibolite-peridotite fault. All of the bodies are within serpentinite and occasionally are contact, suggesting that this exposure is part of a large composite partitioned by narrow serpentinite smears. The rocks are also in- block of tectonitic rocks in serpentinite mélange. truded by fine- to medium-grained mafic dikes; these mafic rocks Dikes of Intermediate Composition. Intermediate dikes intrude appear texturally and mineralogically similar to some of the metadiabase and serpentinized peridotite in several localities nonschistose diabases and gabbros in the mafic complex. throughout the Preston Peak area. Field relations and petrography The amphibolites are fine- to medium-grained, dark-green, dis- suggest that these rocks are part of the ophiolite, but the abundance tinctly foliated rocks. Some of the finer-grained varieties are highly of Late Jurassic magmatic activity in the area (Snoke, 1974) makes schistose and well lineated. Some coarser types are banded and this interpretation uncertain. The dikes appear to be concentrated gneissose. Typically, these metamorphic rocks are highly altered, in distinct areas and are classified into three groups: (1) fine- and many are permeated by thin white calc-silicate veinlets. grained, nonporphyritic biotite quartz diorite, south of Preston The typical hornblende schist and amphibolite consist essentially Peak (approximately at 2,185 m elevation); (2) porphyritic of hornblende and plagioclase, with subordinate amounts of epi- hornblende microdiorite, ridge southwest of Cracker Meadows;

HaystOik Mountain met adi ob ate md dike" tworm Figure 8. Area south of Haystack Mountain i er pen tini and Sanger Peak, looking north from Broken Rib peridotite Mountain. Preston Peak fault separates overlying serpentinite mélange from Late Jurassic Galice metavolcanic and metasedimentary rocks. Tectonic inclusions in mélange include amphibo- om phi boti te lite, metadiabase (md), and Galice metavolcanic and metasedimentary rocks (Jg).

iiirfn rlrrinir ffuln^f the Broken RibJ^fugiain are*

and (3) hornblende andesite porphyry and rodingitized equivalents, sequence are intruded and metamorphosed by the Bear Mountain in the serpentinite terrane south and southeast of the ridge connect- and Blue Ridge plutons. As contacts with the intrusions are ap- ing Sanger Peak and Haystack Mountain. proached, textural and mineralogical changes are readily apparent in the metavolcanic and metasedimentary rocks. The thickness of METAVOLCANIC AND METASEDIMENTARY the total sequence is estimated to be at least 2,000 to 2,500 m; ROCKS OF THE WESTERN PALEOZOIC however, in view of possible tectonic repetition and the deforma- AND TRIASSIC BELT tion associated with the emplacement of the Bear Mountan pluton, this estimate is highly speculative. Maxson (1931, 1933) mapped and described a heterogeneous Lower Metavolcanic Rocks. The lowest unit is a poorly ex- assemblage of argillite, mafic volcanic rock, chert, and limestone in posed heterogeneous assemblage consisting predominantly of mas- the northeastern corner of the Preston Peak quadrangle. He named sive metabasaltic flows, sills, and dikes. Poorly preserved pillow this sequence the Grayback Formation and considered the rocks to structure is seen at a few localities. Minor mafic tuff and tuff brec- be of Devonian age. This age assignment was based on a determi- cia metamorphosed to chlorite-epidote-calcite (albite + quartz) nation made by Kindle from a small collection of invertebrate fos- semischists are present. Thin-bedded gray to black chert and sili- sils secured by Diller (1909) "10 miles south of Waldo along the ceous argillite, as well as fine-grained phyllitic rocks, are locally Happy Camp trail." abundant. Small discontinuous lenses of gray, cream, and white Along the eastern margin of the Gasquet quadrangle, Cater and marble occur near the top of the unit. Wells (1953) mapped the edge of a rather extensive sequence of Siliceous Argillite. Lying depositionally above the lower metavolcanic and subordinate metasedimentary rocks as a metavolcanic unit is a sequence of black fine-grained detrital rocks volcanic-rich segment of the Galice Formation. However, these characterized by a platy cleavage. Original stratification is rarely rocks are underlain and locally tectonically injected by serpentinite apparent but when found is usually parallel or nearly parallel to the of the Preston Peak ultramafic sheet. This structural position, cleavage. East-west cross jointing is common. All the rocks are coupled with the occurrence of thin-bedded chert and occasional highly indurated. Argillaceous varieties are nearly indistinguishable marble (rocks exceptionally rare in the Galice Formation), indicate from rocks of similar grain size in the Galice Formation. Siliceous that the rocks are part of the upper-plate western Paleozoic and layers are more massive and often break with a conchoidal fracture. Triassic belt and are correlative with the Grayback Formation, as At one locality a small, isolated pod of black carbonaceous marble originally suggested by Maxson (1933). is present in the unit. Clastic texture defined by silty lenses and No fossils have been found in the rocks of the western Paleozoic layers is visible in thin section. Relict multigrain ovoids considered and Triassic belt in the Preston Peak area, but the revised (Wells to be recrystallized radiolaria are plentiful in some siliceous layers. and others, 1949) and recent fossil determinations (Irwin, 1972; The siliceous argillite consists essentially of quartz, phyllosilicates, Irwin and Galanis, 1976) elsewhere along this belt suggest that a and opaque oxides, with occasional carbonate, albite, pyrite, and a Permian-Triassic age is more likely than the Devonian determina- variety of accessories (sphene, zircon, apatite). Along the Bear tion reported by Diller (1909). Basin Road, biotite in these rocks as well as in the overlying In the Preston Peak area, rocks of the western Paleozoic and conglomerate-grit unit probably is a contact-metamorphic rather Triassic belt occur primarily as a complexly fault-bounded se- than a regional-metamorphic effect. quence in the west-central and southwestern parts of the map area Conglomerate-Grit. Conformably overlying the argillites is (Fig. 3.) The rocks are intruded and extensively metamorphosed by another clastic unit consisting predominantly of polymictic pebble the Bear Mountain pluton in the south-central part of the area and conglomerate and grit but also including wacke, siltstone, and ar- by the Blue Ridge intrusion in the southwest. At a sufficient dis- gillite. The rocks weather to dull-brown massive exposures and tance from the intrusions, where the rocks have undergone only often are vuggy where carbonate clasts have preferentially weath- lowest greenschist-facies metamorphism, relict textures and struc- ered out. Clast size within this unit is variable both across and tures are locally well preserved. Metabasalt, apparently lava flows along strike. Along Bear Basin Road, grit (Fig. 9) is predominant in and shallow intrusions, is the most common rock. Fine-grained the lower part of the section, while coarse, muddy conglomerates black argillite forms interlayers ranging from 1 m or so to mappa- (Fig. 10) are especially prominent in the middle of the unit. Near ble sequences hundreds of metres thick. Other metasedimentary the top of the unit, siltstone and argillite are locally abundant. rocks include siltstone, sandstone, and pebble conglomerate as well The rocks are poorly to moderately sorted, and grains and rock as chert and limestone. clasts are angular to subangular. Graded bedding is seen in some Several isolated exposures of rock correlated with the western wacke interbeds. Clast types are diverse and include siliceous argil- Paleozoic and Triassic belt lie east of the main fault-bounded band lite, impure chert (sometimes with relict radiolaria), sugary to (Fig. 3). Two of these localities (northeast of Copper Mountain and tectonitic quartzite, monocrystalline quartz, granitoid fragments north-northeast of El Capitan) are especially critical, for field rela- (medium-grained biotite quartz diorite; also some fine-grained and tions indicate that the rocks lie depositionally on rocks of the mafic porphyritic types which may be hypabyssal equivalents), calcite complex. marble, mafic volcanic rock (exhibiting porphyritic, intergranular, and spherulitic textures; pyroclastic types — crystal tuffs — are Bear Basin Road Sequence also present), felsite, siltstone and sandstone, and low-grade siliceous schists. Chert'y and siliceous fragments are by far the most Along the Bear Basin Road (Forest Service Road 17N04) east of abundant lithic type. Volcanic phenoclasts are widespread but are the intersection with Forest Service Road 17N05 is a moderately never in excess of metasedimentary fragments. well-exposed sequence of metavolcanic and metasedimentary The original matrix material was argillaceous but has been to- rocks. Four broad units are designated: (1) lower metavolcanic tally recrystallized to a fine-grained aggregate of biotite, quartz, rocks, (2) siliceous argillite, (3) conglomerate-grit, and (4) upper and opaque minerals. The quantity of matrix material is variable; metavolcanic rocks. The sequence forms a right-side-up homocline some specimens with unusually high amounts of interclastic mud dipping 40° to 60° to the southeast. The lower contact is may be more properly called argillaceous conglomerates or presumably faulted against sheared serpentinite. Lenses and smears diamictites. of serpentinite occur within the lower metavolcanic unit, suggesting Cater and Wells (1953) mapped this unit as a volcanic agglom- additional structural complications. The uppermost rocks in the erate layer within the Galice Formation. As previously implied, the

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Figure 10. Muddy conglomerate facies in conglomerate-grit unit, Bear Basin Road sequence. Pencil point rests on granitoid clast; light-colored Figure 9. Photomicrograph of grit from conglomerate-grit unit, Bear clast directly above is marble. Basin Road sequence. Clast types include variety of sedimentary and metasedimentary fragments; igneous detritus is subordinate. Q = mono- rocks in the map area. Two occurrences (northeast of Copper crystalline or multicrystalline quartz, C, = impure chert, C = granoblastic 2 Mountain and north-northeast of El Capitan) appear to lie deposi- recrystallized chert or fine-grained quartzite, Cr = impure chert with radiolarian ovoids, S = siliceous schist, P = microporphyritic igneous frag- tionally on the rocks of the mafic complex; at several other ments containing plagioclase phenocrysts, and F = felsite. Plane polarized localities near Raspberry Lake, complex faulting may explain the light. present position of these rocks within serpentinized peridotite (Fig. 3). Northeast of Copper Mountain. Approximately 0.5 km north- lithologic character and structural position of the Bear Basin se- east of Copper Mountain, a small patch of siliceous argillite lies quence preclude a correlation with the Galice Formation. Further- depositionally on diabasic breccia of the mafic complex. In thin more, the term "volcanic agglomerate" is inappropriate in light of section, the metasedimentary rock is an exceedingly fine-grained the variety and types of clasts, subordinate nature of the volcanic aggregate of quartz and phyllosilicate minerals. The phyllosilicate fragments, and presence of an argillaceous matrix. Lithology paragenesis is complex, including pale-green muscovite, yellowish- coupled with texture and bedding characteristics suggest that the brown biotite, and minor amounts of chlorite. Thin silty layers of conglomerate-grit unit may be a subaqueous debris flow deposit. angular grains of quartz and feldspar with miscellaneous heavy Upper Metavolcanic Rocks. Lying above the conglomerate-grit minerals define bedding. unit is a sequence of metabasaltic rocks and subordinate siltstone, North-Northeast of El Capitan. Two small patches of argillite, and chert. The metavolcanic rocks consist predominantly metasedimentary rock overlie the mafic complex and in part are in- of massive flows and subordinate dikes and sills. Some of the terlayered with diabasic breccia on a north-trending ridge approx- metabasalt is vesicular, while amygdules are common in many of imately 1.8 km north-northeast of El Capitan (Fig. 3). Siliceous ar- the specimens collected. Metasedimentary rocks, typically impure gillite is the most common metasedimentary rock, but volcanic grit chert, form scattered intercalations within the metavolcanic rocks, forms distinct interlayers in the northernmost exposure. These but most are concentrated in a band (crudely defined in Fig. 3). The patches locally appear to have faulted contacts with the rocks of the rocks are similar to those of the lower metavolcanic unit, except surrounding mafic complex, as indicated by occasional sheared that marble is not found in the upper unit. All the rocks mapped serpentinite smears. However, the shape and structural position of within the upper metavolcanic unit are somewhat affected by con- the patches, the interlayered aspect, and the presence of diabasic tact metamorphism. Rocks near intrusive contacts have been breccia suggest that these rocks are depositional remnants similar altered to schists, amphibolites, hornfelses, mafic gneisses, and to the occurrence northeast of Copper Mountain. migmatitic rocks. Summary Northeast of the South Siskiyou Fork Fault The following general observations apply to the rocks of the Most of the metavolcanic and metasedimentary rocks northeast western Paleozoic and Triassic belt exposed in the Preston Peak of the South Siskiyou Fork occur in a 1 to 3.5-km-wide fault- area: (1) mafic metavolcanic rocks predominate over metasedimen- bounded band that extends approximately northeast from the tary rocks; (2) fragmental metavolcanic rocks are present but are South Siskiyou Fork to just beyond Broken Rib Mountain, where it distinctly subordinate to massive, often nonvesicular, greenstone; is truncated by the northwest-trending Middle Fork fault. Mafic and (3) typical metasedimentary rocks are siliceous argillite and volcanic rocks with subordinate amounts of black fine-grained de- impure chert, while the coarse clastic rocks (grits and conglomer- trital rocks and impure chert constitute more than 98% of the ex- ates) apparently record the rapid influx of immature detritus from pose rocks northeast of the fault. Interbedded limestone, wacke, local(?) source areas. and conglomerate are present at scattered localities. GALICE FORMATION Isolated Exposures A sequence of interbedded shale, siltstone, and sandstone with Several small, isolated exposures of metasedimentary and minor pebble conglomerate lying beneath the Preston Peak thrust metavolcanic rocks occur east of the main belt of Paleozoic-Triassic fault is the youngest but structurally lowest pre-thrust unit in the

Figure 11. Fold types in Galice metasedimentary rocks. Siskiyou Fork of Smith River. A, Slightly overturned metamorphic fold in semischistose meta- graywacke and slate. Note well-developed axial-plane cleavage as well as thickened crest and thinned limbs. B, Postmetamorphic fold with near-vertical axial plane in laminite sequence. Preston Peak area. The rocks have been strongly folded and meta- immediately adjacent to the Preston Peak thrust fault (Fig. 8). The morphosed to semischist and slate. Postmetamorphic folding and metavolcanic rocks appear depositionally conformable on typical kinking have deformed the schistosity. These metasedimentary Galice fine-grained lineated semischist. Several slivers of these so- rocks are correlated with the Galice Formation of southwestern called Galice rocks occur as tectonic inclusions within serpentinite Oregon and are part of the western Jurassic belt (Irwin, 1960). mélange of the upper plate (Fig. 8). Abundant metavolcanic rocks, predominantly meta-andesitic flows and flow breccias, form the lower part of the Jurassic section west Deformation Features and Metamorphic Structures of the Preston Peak area (Cater and Wells, 1953). Metabasalt, keratophyre, and quartz keratophyre are subordinate. Galice beds typically strike 10° to 15° east or west of north and The Galice Formation was originally described and named by dip to the east from as little as 20° to 25° to vertical. In some places Diller (1907) for exposures of dark slaty shales and subordinate the beds are overturned. Two types of folds, metamorphic and sandstone and conglomerate along Galice Creek, Josephine postmetamorphic, are apparent (Fig. 11). County, southwestern Oregon. Hershey (1911) mapped a long belt The wavelength of the metamorphic folds varies from a few cen- of "Galice slate" west of the Preston Peak fault. Likewise, Maxson timetres to several kilometres. Axial planes dip steeply to the east, (1933) and Cater and Wells (1953) considered the metasedimen- and mesoscopic folds are characterized by well-developed axial- tary rocks between the Preston Peak fault and the Josephine ultra- plane cleavage and thickened crests and thinned limbs. Fold axes in mafic sheet to be an extension of the Galice Formation. Irwin the mesoscopic metamorphic folds generally plunge less than 30° to (1960) during reconnaissance mapping of the California Klamaths either the north or south. The axis of a large asymmetrical has found that lithologically similar rocks extend nearly the length metamorphic fold has been located in the Siskiyou Fork of the of the province. Smith River (Fig. 3). The western limb of the fold is overturned (on Fossil collections from the Galice are small and were collected the basis of graded bedding), and the axial surface strikes approxi- mainly by Diller (1907). The Late Jurassic pelecypod Buchia con- mately north-south and dips 50° to 60° to the east. The wavelength centrica (Sowerby), which ranges from late Oxfordian to middle of this anticlinal fold is approximately 3 to 3.5 km. Kimmeridgian, defines the unit's age as far as it is known (Imlay, A variety of fold styles, including chevron, open, and megakink, 1959). The only fossils found in the Preston Peak area are uniden- as well as kink bands, are postmetamorphic. The essential charac- tified plants in a loose creek boulder from the Siskiyou Fork of the teristics of all these folds is that they deform the pre-existing Smith River (collected by R. Evarts, July 1971). metamorphic cleavage or schistosity. The variations in fold form are controlled by layer thicknesses. Large and small chevron folds Lithology occur in thin-bedded (usually less than 2 cm) rhythmic sequences. Kink bands occur almost exclusively within semischistose wacke The typical Galice graywacke is a steel-gray semischistose rock. beds, where micaceous folia measure on the order of tenths of mil- A weak to moderate lineation parallel to fold axes is seen on the limetres. Open folds occur in interbedded shale, siltstone, or foliation (cleavage) surfaces of some specimens. According to the sandstone sequences where layer thickness varies from 5 to 15 cm. terminology developed by Hutton and Turner (1936), Turner The most pronounced metamorphic structure is a pervasive (1938), and Hutton (1940) for New Zealand metagraywackes, the axial-plane cleavage or schistosity that commonly appears nearly Galice in the Preston Peak area is typical of the Chlorite 2 subzone. parallel to original bedding. However, the almost ubiquitous oc- Clastic texture has been partly obliterated by the development of an currence of transposed bedding in the thin-bedded rocks suggests irregular schistosity, and recrystallization is advanced but not that schistosity typically is somewhat oblique to bedding. Silty complete. Galice pelites are black to dark gray, highly fissile silty to layers in such sequences pinch and swell and commonly lens out argillaceous slates. Pebble conglomerate typically occurs as small along strike because of tectonic transposition. Transposed bedding discontinuous lenses or as thin basal layers within turbidite strata, is spectacularly developed at many places in shale-graywacke se- but occasionally thick (approximately 1.5 m in one case) quences where originally thin shaly layers are transposed into conglomerate-rich layers are found. Pebbly mudstone occurs as parallel lenses within massive graywacke (Fig. 12). Fracture cleav- scarce layers in the normal shale-siltstone-sandstone sequences. age is developed in some massive graywacke beds and is typically a An unusual occurrence of metavolcanic rocks intruded by mafic local feature. and quartz keratophyre dikes occurs along the Knopki Creek Road Small-scale faulting is common within the Galice Formation, but

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Figure 12. Transposition of bedding in thin shaly layers interbedded Figure 13. Minor thrust located 30 m west of Preston Peak thrust fault. with massive graywacke. Chlorite-carbonate-quartz dikes are truncated, and one dike has been partially smeared out along fault plane. no major fault or shear zone is known to exist within the Galice rocks exposed in the Preston Peak area. An interesting minor thrust fault is exposed in the Siskiyou Fork of the Smith River approxi- The heavy minerals zircon, tourmaline, pyrite (some also au- mately 30 m west of the Preston Peak fault. It dips to the east and togenic), and sphene were commonly seen in thin section. Some cuts several chlorite-carbonate-quartz dikes, one of which was apatite, hornblende, and biotite were seen; chromian spinel, smeared out along the fault plane (Fig. 13). The proximity of the staurolite, and garnet were very scarce. fault to the Preston Peak thrust zone suggests that this fault may be These data suggest that the source terrane consisted predomin- a manifestation of the underthrusting of the Galice rocks beneath antly of siliceous sedimentary rocks such as impure chert and argil- the western Paleozoic and Triassic plate. lite, with unknown but less abundant amounts of volcanic rocks. Despite minor faulting and shearing, the Galice Formation is a Chromian spinel implies the presence of ultramafic rocks. The zir- structurally coherent unit, lacking the mélange structures charac- con, tourmaline, sphene, apatite, biotite, and hornblende suggest teristic of subduction turbidite assemblages (for example, the granitic sources. Staurolite and garnet indicate metamorphic rocks. Franciscan Complex of the California Coast Ranges; Hsii, 1971). These qualifications are met by the rocks within the western Paleozoic and Triassic belt immediately east of the Galice Forma- Graywacke Petrography tion (Irwin, 1966, p. 21). It is suggested that these rocks, as well as metasedimentary, metavolcanic, and intrusive rocks of the Central The sandstones are lithic wackes and subfeldspathic lithic Metamorphic and Eastern Klamath belts supplied most of the de- wackes. Original matrix material, perhaps 5% to 15% of the rocks, tritus for Galice sedimentation. has been totally recrystallized to dark streaks of fine phyllosilicates, quartz, and disseminate opaque oxides. STRUCTURAL FEATURES The average wacke consists predominantly of siliceous lithic fragments, monocrystalline quartz, and plagioclase in a schistose Preston Peak Thrust matrix of fine-grained colorless mica and opaque minerals. Suban- gular plagioclase and quartz grains are common, but often quartz The Preston Peak fault is an eastward-dipping thrust fault that forms flattened lenses parallel to schistosity. Similarly, nearly all the separates the Late Jurassic Galice metasedimentary rocks from a siliceous lithic grains are flattened and form elongate lenses due to hanging-wall plate of ophiolitic rocks. Hershey (1911) was the first the pervasive foliation present in these subzone 2 rocks. No mineral to recognize and map this fault, showing it as the boundary be- phases diagnostic of the blueschist facies (lawsonite and tween "Galice slates, etc." on the west and "Devonian(P) slate," glaucophane) occur in these metagraywackes. Furthermore, whole- serpentinite, and metagabbro on the east. On his Del Norte County rock x-ray diffraction patterns show only quartz, Na-plagioclase, map, he called it the Orleans fault, considering it to be correlative muscovite, and chlorite. These data indicate lowest greenschist- with a fault just east of the village of Orleans far to the south on the facies metamorphism (Turner, 1968). The siliceous lithic fragments Klamath River. In my study area, I have abandoned the name Or- vary from chert to siliceous argillite. All of these fragments have leans fault in favor of applying a local name, the Preston Peak been partially reconstituted, so that tiny sericite flakes, indicative of thrust, because the fault near Orleans has not been conclusively original clay content, typically lie in a micromosaic of quartz. shown to be the same as that in the Preston Peak area. That the Fragments with sericite are far more abundant than pure chert faults of the two areas may not be correlative can be inferred from grains. No relict radiolaria were seen. recent mapping by Irwin (1972) that suggests that the hanging- Less abundant than the siliceous sedimentary grains are mafic wall plate east of Orleans is lithologically distinct (Hayfork terrane) volcanic fragments including lathwork, felsite, and porphyritic from the ophiolitic rocks of the Preston Peak area. types. Still less abundant fragments of phyllite, argillite, and shale Maxson (1931) considered the Preston Peak fault to be a steeply are often smeared out parallel to the schistosity. dipping reverse fault with a throw of thousands of metres. How- Metamorphic muscovite is characteristically fine-grained, but ever, not until Irwin (1964) suggested that the major faults bound- larger plates (0.15 to 0.25 mm), perhaps detrital, occur both paral- ing the lithic belts of the Klamath Mountains province are regional lel and perpendicular to the foliation. Carbonate replacement of thrust faults was the magnitude of the Preston Peak fault realized. plagioclase is common. No potassium feldspar has been identified Hamilton (1969) subsequently suggested that the regional thrust in thin section or on stained slabs. faults of the Klamath Mountains "include fossil Benioff zones" and

thereby define zones of progressive underthrusting of oceanic rocks extension of the Broken Rib fault occurs at the western edge of the against an accreting continental margin. map area, where sheared serpentinite is in contact with the lower Post-thrusting folding and high-angle faulting have complicated metavolcanic unit of the Bear Basin Road sequence. field relations, so that unequivocal evidence of thrusting (that is, Twin Peaks Fault. Near Twin Peaks, ultramafic rocks, locally low-angle faulting) is not obtained at outcrop scale. However, the sheared, structurally overlie diabases of the mafic complex along a following diverse evidence greatly favors the thrust-fault hypothe- steep eastward-dipping contact. This boundary is interpreted to be sis: (1) the regional extent and magnitude of stratigraphic displace- a high-angle reverse fault and can be followed along a linear ment, (2) klippenlike and fensterlike features, (3) the sheetlike na- north-northeast trend until it is covered by glacial deposits in the ture of the overlying ultramafic rocks, (4) the occurrence of tran- Middle Fork of the Smith River. North of the river, serpentinite secting tear faults, (5) low-dipping shears in the upper-plate ser- mélange is well developed, and a single well-defined fault contact is pentinites, and (6) minor thrusts in the footwall (Fig. 13). not recognized. These data indicate that either the Twin Peaks fault The age of the Preston Peak thrust fault can be bracketed by a is truncated by the northwest-trending Middle Fork fault or it combination of stratigraphic, structural and geochronological data. merges into the mélange shear zone. South of Twin Peaks, the fault A post—middle Kimmeridgian age seems certain in that the age of trends directly toward the center of the Bear Mountain pluton, but the underlying Galice Formation appears to be firmly established as there is no evidence of extensive faulting within the plutonic rocks. late Oxfordian to middle Kimmeridgian. A minimum age is less This, coupled with the eastward dip and elongate nature of the in- definitive and is based on the radiometric age of apparent post- trusion, suggests that the Bear Mountain pluton intruded into the thrusting plutons. Both the Grants Pass pluton and the northern ex- Twin Peaks fault zone. tension of the White Rock pluton (Fig. 1) appear to intrude the northern continuation of the Preston Peak thrust zone (Wells and Northwest-Trending Tear Faults others, 1940; Diller and Kay, 1924; Hotz, 1971a). Wells and others (1940) delineated a contact aureole in the Applegate Forma- In the northern and west-central parts of the map area, the tion (Late Triassic?) around the east side of the Grants Pass pluton; Preston Peak thrust fault is transected by two major northwest- I have seen contact-metamorphosed lower-plate Galice graywacke trending high-angle faults, the Middle Fork and South Siskiyou near the southwestern border of the pluton. Hotz (1971b) reported Fork faults, respectively. A small northwest-trending fault parallels that hornblende from the Grants Pass pluton yields an age of 136 the Middle Fork fault (Fig. 3). The Preston Peak fault zone is offset m.y., and biotite from the White Rock body gives an age of 138 2.6 km by the South Siskiyou Fork and approximately 1.6 km by m.y. These ages are concordant with dates from the Bear Mountain the Middle Fork fault. The magnitude of these offsets precludes a pluton (126 to 143 ± 4 m.y.; J. C. Von Essen, 1970, written com- simple dip-slip motion, and almost certainly strike-slip or mun.), which I consider to be post-thrusting, indicated by its oblique-slip movement has played a major role in the development emplacement into a fault zone developed concurrent with or sub- of these faults. Extension of either of the faults into the Galice For- sequent to the underthrusting of the Galice Formation. mation has not been found, and it seems likely that these features The available data indicate that the Preston Peak thrust fault is are tear faults in the upper-plate rocks. post—middle Kimmeridgian but pre—Bear Mountain, Grants Pass, and White Rock plutonism (126 to 143 m.y.) and that the tectonic Tectonic Inclusions of Western Paleozoic and Triassic Rocks emplacement of the upper plate of ophiolitic rocks is unquestion- ably a Late Jurassic event (that is, Nevadan orogeny). North of Rattlesnake Meadow and west of Raspberry Lake, two masses of western Paleozoic-Triassic rocks occur within alpine-type East-dipping Upper-Plate Reserves Faults peridotite (Fig. 3). A third smaller mass is exposed on the east wall of the Raspberry Lake cirque. These occurrences are interpreted to East of the Preston Peak thrust fault, a series of eastward-dipping be tectonic inclusions in serpentinized peridotite and appear to reverse faults suggest imbrication of the upper-plate rocks concur- define a thrust fault within the ultramafic sheet. The extent of this rent with or partly subsequent to major underthrusting. From west fault is uncertain, but the available data suggest that it is a minor to east, the major faults Wounded Knee, Broken Rib, and Twin manifestation of the east-to-west tectonic transport evident Peaks are shown in Figure 3. throughout the upper plate of ophiolitic rocks. Wounded Knee Fault. In the west-central part of the map area, a slice of mafic rocks directly overlies the Galice metasedimentary OPHIOLITE GENESIS rocks. The slice is bounded on the west by the Preston Peak thrust fault and on the east by the Wounded Knee reverse(?) fault. Exami- The rocks above the Preston Peak thrust fault are apparently an nation of the Wounded Knee fault is hindered by both poor access ophiolite suite, originally a pseudo-stratigraphic sequence includ- and poor exposures. Available evidence suggests that the fault is ing a basal ultramafic sheet, an overlying mafic complex of intru- truncated on the north by the northwest-trending Middle Fork sive and extrusive rocks, and an uppermost assemblage of inter- fault, while at its southern end it appears to merge with the Preston bedded metavolcanic and metasedimentary rocks (part of the west- Peak thrust zone. ern Paleozoic and Triassic belt of Irwin, 1960). Remnants of this Within the bedrock slice between the Preston Peak and Wounded sequence are locally preserved, but complex faulting and folding Knee faults, the juxtaposition of a variety of rocks and the presence have greatly complicated and modified its original order. (Fig. 2 of linear smears of serpentinite suggest internal faulting. The com- summarizes the proposed stratigraphy of the ophiolite). plexity and extension of this faulting are unknown, but in view of An important discovery during the study of the Preston Peak the proximity of the Preston Peak thrust fault such tectonic ophiolite is that the ultramafic rocks crystallized distinctly before shuffling seems to be a reasonable and expected response of the the mafic rocks. This is indicated by the sharp metamorphic con- upper-plate rocks. trast between the tectonitic alpine-type peridotites and the Broken Rib Fault. The Broken Rib fault is a north-northeast- nonschistose lower greenschist-facies metadiabases and metagab- trending boundary between metavolcanic and metasedimentary bros. The preservation of intrusive edge zones, as well as isolated rocks of the western Paleozoic and Triassic belt and a footwall slice dike injections with chilled margins, indicates that the ultramafic of sheared serpentinite. The fault is truncated at its northern and rocks have acted as country rock rather than source rock for the southern ends by northwest-trending high-angle faults (Middle basaltic magmas. This observation is not a new concept in ophiolite Fork and South Siskiyou Fork faults, respectively). A possible offset geology, for Reinhardt (1969) clearly described a similar hiatus be-

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tween the basal peridotite and the overlying mafic rocks of the 1972). Therefore, ophiolite formed at a spreading center should Semail ophiolite in Oman. Furthermore, 87Sr/86Sr ratios from ul- include cumulates as an integral part of the sequence. Curiously, tramafic complexes of other Cordilleran ophiolites (Lanphere, possible cumulates are very scarce in the Preston Peak ophiolite, 1973) are higher than rocks from the overlying mafic construc- suggesting either that spreading rates were so rapid as to preclude tional pile, suggesting contrasting crystallization histories for these the development of extensive magma chambers or that another site two rock units. of origin is indicated. With the polygenetic history of the Preston Peak ophiolite as a 6. Sediments found on active oceanic ridges are typically fine- critical restraint, the current hypotheses for the origin of ophiolite grained pelagic accumulations that are often clay-rich and siliceous. can be considered: (1) They formed at an oceanic spreading center Analogous metasedimentary rocks (impure chert and siliceous ar- (Coleman, 1971; Dewey and Bird, 1971; Moores and Vine, 1971); gillite) are common in the upper part of the Preston Peak ophiolite. (2) they formed in a marginal basin behind an active island arc However, coarse clastic rocks (grit and conglomerate) are locally (Dewey and Bird, 1971; Blake and Jones, 1974); and (3) they rep- abundant and suggest rapid transport of immature detritus from resent the primitive stage (the basal part) of an island-arc complex nearby source areas. This type of sedimentation seems incompati- (Ewart and Bryan, 1972; Miyashiro, 1973, 1975). Most models ble with a mid-oceanic ridge environment. that interpret ophiolite as developing at a spreading oceanic ridge Karig (1970, 1971) has argued that extensional rifting within begin with an upwelling of primitive upper mantle (lherzolite or island-arc systems leads to the creation of interarc basins and new garnet peridotite) undergoing adiabatic decompression and partial oceanic crust behind migrating arcs. Dewey and Bird (1971) have melting. At 25 to 30 km, primitive upper-mantle rock can yield ap- suggested that this mechanism may be important in the generation proximately 30% tholeiitic partial melt (Kay and others, 1970). of some ophiolites, and they listed criteria useful in the identifica- With continued diapiric uprise, this tholeiitic magma is segregated tion of such assemblages. Among the diagnostic features of a from a refractory ultramafic residuum and ascends rapidly to form marginal-basin ophiolite is the presence of an overlying volcanic- a volcano-plutonic complex overlying a rind of depleted peridotite lastic sequence built from erosion of the adjacent areas. Such thick (Kay and others, 1970; Dewey and Bird, 1971). In this model, the volcanogenic deposits are lacking in the Preston Peak area, tectonite fabric that is invariably characteristic of the basal ultra- although the coarse clastic rocks (grits and conglomerates) with mafic segment of ophiolite is developed either during the upward volcanic detritus indicate local igneous source areas. An intra-arc rise of the upper-mantle rocks (Moores and Vine, 1971) or during basin would contain similar deposits (Dickinson, 1971). the subsequent movement away from the axis of the ridge (Cole- The marginal-basin hypothesis, like the spreading oceanic-ridge man, 1971). model, begins with an uprising mantle diapir (Karig, 1971). Crit- If this model is applied to the Preston Peak ophiolite, the follow- icisms 1 and 2 previously emphasized in the evaluation of the ing problems are evident. spreading oceanic-ridge model appear applicable to the marginal- 1. Contacts between mafic rocks and the tectonitic ultramafic basin hypotheses. Furthermore, the basal ultramafic rocks lack any rocks are sharp, either intrusive or faulted. Basaltic rocks occur in vestige of the magmatic-type layering reported in hot mantle the peridotite either as discrete dikes with chilled margins or as diapirs (Dickey, 1970), and contact-metamorphic effects are absent tectonic inclusions. These relations suggest that the magmas that in the surrounding country rocks. A possible exception is the am- built the mafic complex were not extracted from the immediately phibolite blocks in serpentinite mélange, which could be inter- underlying ultramafic sheet. The refactory character of the alpine- preted as remnants of a contact aureole fragmented during later type peridotites must reflect a pre-existing condition developed dur- tectonic movements (Williams and Smyth, 1973). ing an earlier episode of mafic magma genesis. In contrast to the previous hypotheses, which are based on a 2. The tectonite fabric (that is, metamorphic recrystallization) of spreading mechanism in either an oceanic ridge or marginal basin, the basal peridotite predates most of the mafic magmatism of the the island-arc model involves the development of part of the Preston Peak ophiolite. An exception may be the amphibolites in ophiolite above a descending lithosphere plate. It is a possible in- serpentinite melange south-southwest of Haystack Mountain. terpretation for the Preston Peak ophiolite, for it is readily compat- Parallel fabrics of the tectonitic lherzolite and adjacent amphibolite ible with the apparent differing crystallization histories of the ul- on Haystack Mountain suggest similar deformation histories. The tramafic and mafic rocks. According to this model, the basal sheet amphibolities, therefore, are synkinematic metamorphic rocks that of tectonitic ultramafic rock represents upper-mantle basement developed early in the history of the ophiolite. upon which the intrusive-extrusive mafic complex was built. The 3. Recent investigation of Mid-Atlantic Ridge volcanism rare amphibolite masses in serpentinite mélange may be remnants (Moore and others, 1974) indicates a preponderance of pillow lava of mafic ocean crust developed during the deformation and deple- erupted quietly from fissure vents. Pillow lavas are not abundant in tion of these mantle rocks. Amphibolite, typically closely associated the Preston Peak ophiolite; in fact, pyroclastic mafic breccias are with tectonitic peridotite or its serpentinized equivalent, is an inte- common and probably are evidence of local explosive volcanic cen- gral part of other ophiolites in the Klamath Mountains (for exam- ters. ple, the Josephine ophiolite; Wells and others, 1949). Furthermore, 4. Dredged abyssal tholeiites often contain modal olivine as amphibolite is a common constituent of the basement complex of phenocrysts and (or) in thegroundmass (Miyashiro and others, some modern island-arc terranes (see, for example, Tobisch, 1968; 1969; Shido and others, 1971). Olivine pseudomorphs even occur Shiraki, 1971). Major- and trace-element chemical data from the in metamorphosed abyssal basalts (Melson and van Andel, 1966, Preston Peak ophiolite (Snoke and others, 1977) tend to support Fig. 8). Careful petrographic examination has failed to find primary the primitive island-arc model. In particular, metadiabase and or pseudomorphosed olivine in any mafic rocks of the ophiolite. metadiabasic breccia of the mafic complex are characterized by low Scarce clinopyroxenite xenoliths are the only rocks in the construc- Ti02, Cr, and Ni contents and appear more similar to island-arc tional sequence to contain olivine or recognizable pseudomorphs. tholeiite than to abyssal tholeiite. An analyzed quartz diorite, a rep- 5. Coarse-grained mafic to ultramafic rocks with cumulate resentative of the intermediate dike rocks, is analogous to Na-rich textures have been dredged from oceanic ridges and associated silicic rocks characteristic of the island-arc tholeiitic series as fracture zones (see, for example, Melson and Thompson, 1970; defined by Jakes" and Gill (1970). Bonatti and others, 1971). Likewise, many models for the develop- In summary, although the available field and chemical data are ment of oceanic crust assume that magma chambers often form not sufficient to unequivocally discriminate between the current at the spreading center and subsequently fractionate during cool- hypotheses for the origin of ophiolite, a primitive island-arc built ing (Thayer, 1969; Cann, 1970; Coleman, 1971; Dickinson, on oceanic lithosphere is perhaps the most viable and flexible

migration of volcanic arc Preston Peak remnant arc Callovian-Oxfordian

'.VffllNVH proto-Klamath Mountains Figure 14. Schematic n i i a llll/ll' \ simatic crust summary of emplacement of Preston Peak ophiolite. A, Westward migration of Jurassic volcanic arc rifted spreading center from foundering Permian- Triassic arc-trench complex; deposition of Galice Northern plutonic area 145-155 m.y. sediments in small margi- lanphere & others, 1968 nal ocean basin between Oxfordian- volcanic arc and an inac- tive remnant arc. B, Flat- Middle Kimmeridgian tening of subduction zone and consequent eastward shift of magmatic locus; Voluminous sedimentation in Galice basin now located in front of magmatic arc (arc-trench gap). C, Collapse of Galice basin as active subduction U-Western Jurassic -»^-«-Western Paleozoic ? & Triassic belt shifts farther west; Galice I belt rocks are underthrust Preston Peakophiolite, beneath older Permian Triassic remnant arc C. Late Kimmeridgian (~145m.y.) (Preston Peak ophiolite).

model in accord with the data. Nevertheless, regardless of which plutonic belt; Lanphere and others, 1968). The Galice basin con- model is favored, a polygenetic history for the Preston Peak tinued to accumulate abundant detritus; but it was now positioned ophiolite is clearly indicated, and this aspect must be incorporated in front of an active magmatic arc rather than behind it (Fig. 14 B). into any evolutionary scheme. The final phase (Fig. 14 C) of the tectonic sequence was the collapse of the Galice basin as subduction shifted farther to the west during EMPLACEMENT OF OPHIOLITE Late Jurassic time (Blake and Jones, 1974). It was during this episode that the Galice Formation was thrust beneath the older Although the origin of the ophiolite is problematical, the fault Preston Peak ophiolite (Permian-Triassic remnant arc?). contact at its base indicates a final tectonic emplacement. As previously indicated, substantial evidence suggests that the Preston Peak ACKNOWLEDGMENTS fault is essentially a Late Jurassic (Nevadan) event. A numerical age is uncertain; but the available radiometric dating of post-thrusting This investigation was initially undertaken as partial fulfillment plutons (Lanphere and others, 1968; Hotz, 1971a) suggests that of the requirements for the Ph.D. degree at Stanford University. 150 ± 5 m.y. is a reasonable approximation. The timing of the Early versions of the manuscript were reviewed by Robert R. latest slippage along this fault is unknown, but the widespread late Compton, William R. Dickinson, W. P. Irwin, and Benjamin M. Mesozoic and Tertiary tectonic activity (Coast Range orogeny) Page. Generous financial support for the research was provided by west of the Preston Peak area probably caused some tectonic over- two Penrose research grants from the Geological Society of printing. America, two grants from the Shell Research Fund at Stanford Uni- Prior to the final tectonic emplacement of the Preston Peak versity, and a grant-in-aid-of-research from the Society of Sigma ophiolite, Jurassic sedimentary and volcanic rocks of the Galice Xi. Bruce Dickson, Stuart Gill, and Joe Harrigan served ably as Formation were deposited along the western margin of the proto- field assistants during various periods of the research. Klamath Mountains. Similar rocks apparently extended southward and now form the Logtown Ridge—Mariposa sequence of the western Sierra Nevada foothills. If the Preston Peak ophiolite is a rem- REFERENCES CITED nant of a primitive island arc, the lower metavolcanic-rich section of the Galice Formation may represent a Jurassic arc rifted from a Avé Lallemant, H. G., 1967, Structural and petrofabric analysis of an foundering Permian-Triassic arc-trench complex (Figure 14 A). In- "alpine-type" peridotite: The lherzolite of the French Pyrenees: Leidse tercalated sediments as well as some sedimentary rocks im- Geol. Meded., v. 42, p. 1-57. mediately above the volcanic-rich section of the Galice Formation Blake, M. C., Jr., and Jones, D. L., 1974, Origin of Franciscan mélanges in probably represent either intra-arc or marginal-basin deposits. In northern California; in Dott, R. H., Jr., and Shaver, R. H., eds., Mod- ern and ancient géosynclinal sedimentation: Soc. Econ. Paleontologists that most of the Galice sediments are younger than the metavol- and Mineralogists Pub. 19, p. 345-357. canic rocks, sedimentation apparently increased as arc magmatism Bonatti, E., Honnorez, J., and Ferrara, G., 1971, Peridotite-gabbro-basalt subsided. At this time, westward migration of the volcanic arc complex from the equatorial Mid-Atlantic Ridge: Royal Soc. London slowed, and the subduction zone rotated to a shallower inclination, Philos. Trans., ser. v. 268, p. 385-402. causing the locus of magmatism to shift eastward (northern Burch, S. H., 1968, Tectonic emplacement of the Burro Mountain ultra-

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