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Cryptic tectonic domains of the Klamath Mountains, California and Oregon
Engineering Geology, 27 (1989) 433-448
433
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
CRYPTIC TECTONIC DOMAINS OF THE KLAMATH MOUNTAINS, CALIFORNIA AND OREGON
WILLIAM P. IRWIN
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 (U.S.A.) (Accepted for publication November 4, 1988)
ABSTRACT Irwin, W.P., 1989. Cryptic tectonic domains of the Klamath Mountains, California and Oregon. In: A.M. Johnson, C.W. Burnham, C.R. Allen and W. Muehlberger (Editors), Richard H. Jahns Memorial Volume. Eng. Geol., 27: 433-448. Numerous fragments of oceanic crust and island arcs make up the Klamath Mountains province. These fragments were joined together (amalgamated) in an oceanic setting during Paleozoic and Mesozoic collisional events and were accreted to North America as a composite unit during latest Jurassic or earliest Cretaceous time. The roughly arcuate and concentric distribution of the terranes of the Klamath Mountains does not now seem to be a result of simple oroclinal bending as earlier believed. Although commonly described as a west-facing arcuate structure, the province is cut diagonally by a vaguely defined NW-trending zone of discontinuity, or hinge line, that divides the province into NE and SW tectonic domains. The zone of discontinuity is marked by a number of lithic and structural anomalies, and particularly by the distribution of a remarkable series of belts of plutonic rocks. The terranes, regional structures, and plutonic belts of the NE domain trend NE and are generally wider and more coherent than the narrow NWtrending terranes and plutonic belts of the SW domain. Most plutonic belts of the NE domain do not have equivalents in the SW domain. Paleomagnetic evidence suggests that all the plutonic belts, except possibly the youngest (the earliest Cretaceous Shasta Bally belt), were emplaced before the Klamath Mountains terranes finally accreted to North America.
INTRODUCTION
T h e K l a m a t h M o u n t a i n s c o n s t i t u t e a w e s t - f a c i n g a r c u a t e p r o v i n c e of 30,645 k m : i n a r e a , a r e g i o n s o m e w h a t l a r g e r t h a n t h e S w i s s Alps. T h e p r o v i n c e is s u b d i v i d e d i n t o s e v e r a l t e r r a n e s , e a c h of w h i c h r e p r e s e n t s a n a l l o c h t h o n o u s f r a g m e n t of o c e a n i c c r u s t o r i s l a n d a r c ( F i g . l ) . T h e K l a m a t h M o u n t a i n s p r o v i n c e is o n l y a s m a l l p a r t o f t h e v a s t c o l l a g e of s u s p e c t t e r r a n e s a l o n g t h e w e s t e r n m a r g i n of N o r t h A m e r i c a f r o m M e x i c o to A l a s k a ( C o n e y et al., 1980). A l l t h e t e r r a n e s o f t h e K l a m a t h M o u n t a i n s a r e o c e a n i c i n o r i g i n ; t h e r e is no e v i d e n c e t h a t a n y of the t e r r a n e s were ever u n d e r l a i n by c o n t i n e n t a l basem e n t r o c k s . O p h i o l i t i c t e r r a n e s of t h e p r o v i n c e , c o n s i s t i n g p a r t l y of o c e a n i c crust and upper mantle, probably originated at various oceanic spreading centers d u r i n g Ordovician, P e r m i a n , Triassic, a n d J u r a s s i c times. The ophiolitic 0013-7952/89/$03.50
~ 1989 Elsevier Science Publishers B.V.
434
components of the terranes are sequentially younger oceanward (westward), suggesting th at the amalgamation of the terranes also was sequential. The terranes also include parts of volcanic island arcs t hat formed at various times during the Paleozoic and Mesozoic, and these also tend to be sequentially y o u n g er oceanward. The deformation of the rocks of the terranes differs widely. In some terranes the stratigraphy and s t r uc t ur e is coherent, but more commonly the terranes consist of broken formations and melange. Along much of the southeastern and eastern perimeter of the province the terranes are overlain unconformably by relatively mildly deformed sequences of Cretaceous and T ert i ary strata t hat were deposited after the terranes had accreted to the continent. EASTERN K L A M A T H TERRANE
The Eastern Klamath t er r ane *~ is the oldest in the province. It consists mainly of the Trinity ultramafic sheet (ophiolite) and the overlying Paleozoic and Mesozoic island-arc deposits (Fig.l). The Trinity sheet, perhaps the largest ultramafic body in North America, crops out over a large area of the central part of the terrane, and is exposed for a length of 160 km where its edge is upturned along the western border of the terrane. It consists mainly of tectonized and partly serpentinized harzburgite and dunite, which are intruded by pyroxenite, gabbro, and plagiogranite. Isotopic ages as old as 480 Ma measured on the gabbro (Lanphere et al., 1968; Mattinson and Hopson, 1972) indicate th at the Trinity ultramafic sheet is Early Ordovician or older. The large central outcrop of the Trinity sheet divides the Paleozoic and Mesozoic strata of the Eastern Klamath t errane into two areas of exposure: the Yreka-Callahan area to the NW, and the Redding section to the SE. Evidence from magnetic, gravity, and seismic refraction studies indicate t hat the Trinity sheet underlies the strata of both areas (Lafehr, 1966; Griscom, 1973; Fuis and Zucca, 1984; Blakeley et al., 1985). The strata of the Y r e k a - C a l l a h a n area are Ordovician to Devonian in age (Potter et al., 1977). They are subdivided into seven or more formations whose relations to one anot her and to the Trinity ultramafic sheet are structurally complex and controversial. The strata of the Redding section range in age from Early Devonian through Middle Jurassic and are subdivided into sixteen formations with a total exposed thickness of more t ha n 10 km (Sanborn, 1953; Kinkel et al., 1956; Albers and Robertson, 1961). They are structurally more coherent t han the strata of the Y r e k a - C a l l a h a n area and form a generally homoclinal sequence that dips SE away from the broad central outcrop of the Trinity sheet. The
*lThe terrane n o m e n c l a t u r e used in this report differs s o m e w h a t from the n o m e n c l a t u r e used by some other authors. For example, each of the three subdivisions of the E a s t e r n K l a m a t h t e r r a n e described herein is considered a separate terrane by Blake et al. (1982).
435 124 °
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EXPLANATION
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Western Jurassic terrarle Western Paleozoic and Triassic belt {composite) _~ Rattlesnake Creek terrane ~
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Fig.1. Terranes of the Klamath Mountains province and distribution of the principal plutons. Some plutons too small to be shown; black dots indicate localities of small isotopically-dated Ridge; plutons. Letter symbols refer to names of dated plutons: A = A s h l a n d ; A R = A m m o n B G = Basin Gulch; B M = Bear Mountain; B W = Bear Wallow; C = Caribou Mountain; C C = Castle Crags; C M = Cracker Meadow; C P = Craggy Peak; D P = Deadman Peak; E F = East Fork; E P = English Peak; F S = Forks of Salmon; G = Greyback; G C = Glen Creek; G H = Gold Hill; G P = Grants Lake; I M = I r o n s i d e Mountain; I R = I l l i n o i s River; J = J a c k s o n v i l l e ; Pass; H L = H o r s e s h o e Coon Mountain; M M = M u l e Mountain; N G = U n n a m e d gabbro; P C = Price Creek; LC=Lower P R = Pit River; R P = Russian Peak; S = Slinkard; S B = Shasta Bally; S G = Saddle Gulch; S M = Star Mountain; S P = Sugar Pine; V B = Vesa Bluff; W = Wildwood; W C = Wooley Creek; W P = Walker Point; W R = White Rock; Y B = Yellow Butte.
436 large number of andesitic volcanic formations distributed through the Redding section, and the presence of reefal limestones, suggest that the Redding section represents a long-standing volcanic island arc built on Ordovician oceanic crust. CENTRAL METAMORPHICTERRANE The Central Metamorphic terrane generally lies west of, and structurally below, the Trinity sheet. It consists of a lower unit of metamorphosed mafic volcanic rocks (Salmon Hornblende Schist) and an upper unit of metamorphosed clastic sedimentary and carbonate rocks (Abrams Mica Schist). Where most fully developed, being west of the southern half of the Eastern Klamath terrane, it forms a nearly detached, north-trending synformal thrust plate that is probably 3-5 km thick at the axis. It is tectonically thinned eastward, where it dips beneath the Trinity sheet. Relatively small klippen of Eastern Klamath terrane rest on the Central Metamorphic terrane at three widely-spaced localities (Fig.l). The normal relationship between the Central Metamorphic and Eastern Klamath terranes appears structurally transposed for 35 km along their boundary SW of Yreka, where the Central Metamorphic terrane is represented by a relatively narrow sliver t h a t crops out along the east side rather than west of the northern extension of the Trinity ultramafic sheet. The stratigraphic age of the protoliths of the schists is unclear. However, isotopic ages of 380-399 Ma measured on the schists indicate that the schists formed during a Devonian tectonic event (Lanphere et al., 1968; Hotz, 1977). The metamorphism resulted from overriding of the protoliths by the Trinity ultramafic sheet, and was contemporaneous with Devonian volcanic activity represented by the oldest formations of the Redding section. NORTH FORK TERRANE An extensive area of structurally complex rocks west of the Central Metamorphic terrane was called the Western Paleozoic and Triassic belt during early studies of the region (Irwin, 1960). The belt is now subdivided, from east to west, into the North Fork, Hayfork, and Rattlesnake Creek terranes (Irwin, 1972) in the southern and central parts of the province. The North Fork terrane occupies a zone 2 10 km wide for approximately 100 km along the west side of the Central Metamorphic terrane. The structurally lowest part of the North Fork terrane is a dismembered ophiolite consisting of serpentinized peridotite, gabbro, plagiogranite, diabase, and pillow basalt. The ophiolitic rocks crop out along the western side of the terrane, and are succeeded upward to the east by mafic volcanic rocks with interlayered argillite, radiolarian chert, and minor limestone pods and lenses. One horizon of highly disrupted argillite in the Salmon River region contains blocks of glaucophane-lawsonite blueschist (Ando et al., 1983). The North Fork ophiolite is probably late Paleozoic in age, based on the isotopic age of a small plagiogranite body (Ando et al., 1983) and on an uncertain relationship
437 with nearby red radiolarian chert of Permian age (Blome and Irwin, 1983). Chert and siliceous tuff interlayered with the mafic volcanic rocks contain Triassic and Lower Jurassic radiolarians. However, the few limestone bodies that have yielded useful fossils contain upper Paleozoic fusulinids and foraminifers (Irwin and Galanis, 1976), which indicate that those limestone pods are either olistoliths or tectonic blocks. HAYFORK TERRANE The Hayfork terrane consists of three formations and the Ironside Mountain batholith and related plutons. The principal formation, and structurally lowest, is the Hayfork Bally Meta-andesite which consists mainly of c r y s t a l lithic tuff and tuff breccia of andesitic to basaltic composition. Isotopic ages ranging from 156 Ma (M.A. Lanphere, oral commun., 1977) to 168-177 Ma (Fahan, 1982) have been measured on the meta-andesite. These volcanic strata are overlain by, and interfinger with, a chert-argillite formation that in turn is structurally overlain by melange. The melange is separated from the underlying strata by the Wilson Point thrust fault (Wright, 1982), which divides the Hayfork terrane into two subterranes. The Western Hayfork subterrane includes the Hayfork Bally Meta-andesite and the chert-argillite unit; the Eastern Hayfork subterrane is the melange. The melange includes serpentinite, argillite, chert, quartzose sandstone, mafic and locally silicic volcanic rocks, limestone pods, and a few amphibolite knockers. The serpentinite occurs most commonly as slivers along the Wilson Point thrust. The chert yields both Triassic and Jurassic radiolarians (Irwin et al., 1982). Some of the limestone contains Permian fossils of Tethyan faunal affinity (Irwin and Galanis, 1976; Nestell et al., 1981). RATTLESNAKE CREEK TERRANE The Rattlesnake Creek terrane is the tectonically most disorganized unit of the province, and bedrock relations are obscured over much of the area by landslides on a grand scale. The terrane is a melange consisting of highly dismembered ophiolite, mafic to silicic volcanic and subvolcanic rocks, plagiogranite, chert, limestone pods, and minor sandstone and conglomerate. The chert contains Triassic and Jurassic radiolarians. Sparsely distributed fossils in the limestone pods are variously Devonian(?), late Paleozoic, and late Triassic. Isotopic ages ranging from 193 to 207 Ma have been measured on the plagiogranitic rocks (Wright, 1981, 1982). WESTERN JURASSIC TERRANE The Western Jurassic terrane includes the Josephine ophiolite of Snoke (1977), the Galice and Rogue Formations, and the Chetco River complex of
438 Hotz (1971). The Josephine ophiolite is excellently exposed and most completely preserved in the Smith River region just south of the Oregon border. There it displays a virtually complete ophiolite sequence from lowermost harzburgite to uppermost sheeted dikes and pillow basalt (Harper, 1984). Zircon from plagiogranite yields an isotopic age of 157 Ma for the ophiolite (Saleeby et al., 1982). The ophiolite is conformably overlain by the Upper Jurassic (Oxfordian to Kimmeridgian) Galice Formation which consists of slaty argillite, graywacke, and minor conglomerate (Harper, 1984). Northward, in Oregon, the Galice generally overlies and contains interlayers of the volcanic Rogue Formation, and these two formations are separated from the ophiolite by faults and by a narrow fault slice of amphibolite. The Rogue is a dominantly volcaniclastic formation that probably formed as part of a calc alkaline volcanic arc, with the Chetco River complex representing the core of the arc (Garcia, 1982). The Chetco River complex is a dominantly mafic plutonic complex that also includes the calc alkaline Illinois River batholith whose isotopic age is about 153 Ma (Hotz, 1971; Garcia, 1982). PREAMALGAMATIONAND POSTAMALGAMATIONPLUTONIC BELTS Granitoid plutonic rocks of the Klamath Mountains range in age from Ordovician to Cretaceous. They are non-uniformily distributed, being relatively few in number in the Y r e k a - C a l l a h a n region, the Redding section, and the Western Jurassic terrane. Many plutons are elongate parallel to the trend of their host terrane, and in some instances, such as the Ironside Mountain batholith, the orientation probably is controlled by the orientation of the host arc. Some plutons are truncated by terrane boundaries. Others cut across terrane boundaries, intruding one or more adjacent terranes. These boundary relations are important constraints on the time of amalgamation of the terranes. Isotopic age data from various published sources (Fig.2) indicate that most of the plutons are distributed in belts according to age, and t h a t the belts in most instances follow the trends of the terranes (Fig.3). A few plutons are outside the named belts. The pattern of plutonic belts I am proposing is highly provisional, because many plutons are not dated and some of the isotopic ages measured by different methods are widely variant. Despite these uncertainties, the overall pattern of distribution of the plutonic belts provides valuable insight regarding the tectonic development of the province. The plutonic belts trend NE-SW in the NE part of the province, and trend NW-SE, nearly at right angles, in the SW part. The Paleozoic plutonic belts are all in the Eastern Klamath terrane and are NE-SW trending; remarkably, Jurassic plutons are unknown in the Eastern Klamath terrane except possibly the Castle Crags pluton. Oceanward, the NE-SW trending belts are of successively younger Jurassic and Cretaceous ages except for the westernmost belt, the Chetco, which is Late Jurassic. The NW-SE trending plutonic belts also are of Jurassic and Cretaceous ages, but
439
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GRANTS STAR WOOLEY . . . . . . . . . . PASS . . . . . . . MOUNTA~ . . . . . . . CREEK . . . . . . . BELT BELT BELT
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Fig.2. Comparison of ages of plutons of the Klamath Mountains province (modified from Irwin, 1985). The plutons are grouped according to terranes (top of figure) and plutonic belts (braces near bottom of figure). Sources for the isotopic age data are referred to by right-reading numbers in the diagram: 1 = Albers et al., 1981; 2 = J.P. Albers, oral commun., 1982; 3 = Allen et al., 1982; 4 = Ando et al., 1983; 5 = Curtis et al., 1958; 6=Davis, 1961; 7 = Evernden and Kistler, 1970; 8 = Fahan, 1982; 9=Harper and Saleeby, 1980; l O = H o l d a w a y , 1963; 1 1 = H o t z , 1971; 1 2 = I r w i n et al., 1974; 1 3 = Lanphere and Jones, 1978; 1 4 = Lanphere et al., 1968; 1 5 = M.A. Lanphere, written commun., 1977 and 1983; 1 6 = Mattinson and Hopson, 1972; 1 7 = Romey, 1962; 1 8 = Saleeby, 1984; 1 9 = Saleeby et al., 1982; 2 0 = S n o k e , 1977; 21=Snoke et al., 1981; 2 2 = W r i g h t , 1981 and 1982; 23=Young, 1978; 2 4 = Zeller, 1965. The time scale is from Harland et al., 1982. Numerical age is in millions of years.
the y o u n g e s t ( S h a s t a B a l l y belt) is f a r t h e s t f r o m the o c e a n a n d is p a r t l y s u p e r i m p o s e d on the oldest P a l e o z o i c belt (Alpine G a b b r o belt). S o m e g r a n i t o i d p l u t o n s a r e o r i g i n a l c o m p o n e n t s of t h e t e r r a n e s , h a v i n g f o r m e d at s p r e a d i n g c e n t e r s or in t h e r o o t zones of v o l c a n i c island arcs, a n d p r e d a t e t h e t i m e of collision w i t h o t h e r t e r r a n e s . T h e s e p r e a m a l g a m a t i o n p l u t o n s in s o m e i n s t a n c e s a r e s i m i l a r in a g e a n d c o m p o s i t i o n to the v o l c a n i c s t r a t a t h e y i n t r u d e , f o r m i n g p l u t o n i c - v o l c a n i c pairs. E x a m p l e s of p l u t o n i c v o l c a n i c p a i r s are: t h e M u l e M o u n t a i n s t o c k i n t r u d i n g t h e B a l a k l a l l a R h y o l i t e ( D e v o n i a n ) of t h e R e d d i n g section, t h e p l u t o n s of the M c C l o u d belt i n t r u d i n g the D e k k a s A n d e s i t e ( P e r m i a n ) o f t h e R e d d i n g section, a n d the I r o n s i d e M o u n t a i n b a t h o l i t h i n t r u d i n g t h e H a y f o r k B a l l y M e t a - a n d e s i t e ( J u r a s s i c ) of t h e H a y f o r k t e r r a n e . I n c o n t r a s t , t h e p o s t a m a l g a m a t i o n p l u t o n s a r e signific a n t l y y o u n g e r t h a n t h e i r h o s t r o c k s a n d a r e g e n e t i c a l l y u n r e l a t e d to them. T h e s e p l u t o n s a r e a s s i g n e d to belts on t h e basis of isotopic age, a n d a r e d e s i g n a t e d as p o s t a m a l g a m a t i o n if s o m e of t h e p l u t o n s of t h e belt a r e k n o w n to i n t r u d e m o r e t h a n one t e r r a n e or if, on t h e basis of o t h e r r e g i o n a l cons i d e r a t i o n s , t h e p l u t o n s a r e k n o w n to be y o u n g e r t h a n t h e t e r r a n e b o u n d a r y . T h e p o s t a m a l g a m a t i o n p l u t o n s i n t r u d e d t h e h o s t t e r r a n e s p r e s u m a b l y as a r e s u l t of c r u s t a l s u b d u c t i o n events.
440
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EXPLANATION
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Fig.3. Plutonic belts of the Klamath Mountains province. Names of plutonic belts are followedby approximate ages in millions of years. THE TECTONIC DOMAINS Discontinuities in the orientation and distribution of the plutonic belts, in addition to other features, indicate that the Klamath Mountains province can be subdivided into two tectonic domains, although the significance of these discontinuities is unknown. The change in trend of the plutonic belts is along a vaguely defined NW-trending zone of discontinuity that divides the province into NE and SW tectonic domains (Fig.3). The difference in orientation of the plutonic belts is accompanied by differences in the ages and numbers of belts. All the Paleozoic plutonic belts are in the NE domain. Jurassic and Cretaceous belts are present in both domains, but their precise ages differ from one domain to the other. The number of belts in the NE domain is nearly double the number in the SW domain. In the NE domain the Western Paleozoic and Triassic belt contains the greatest number of plutons, including virtually all the Jurassic postamalgamation plutons of the province (Fig.4). It is notable that this vast region appears devoid of preamalgamation plutons. In contrast, the plutons of the Western Paleozoic and Triassic belt of the SW domain are virtually all preamalgamation plutons.
441
Fig.4. Distribution of the isotopically dated postamalgamation plutons of the Klamath Mountains province. Small dated plutons are shown by solid circles with letter symbol indicating Jurassic (J) or Cretaceous (K) ages. Undated and preamalgamation plutons are not shown.
These differences in the plutonic belts from one domain to the ot her are particularly puzzling where the host terranes seem virtually continuous across the zone of discontinuity. The Ironside M o u n t a i n (,~ 170 Ma) and Wooley Creek ( ~ 163 Ma) plutonic belts are interesting examples because both intrude the meta-andesite of the H a yf or k t e r r a n e and virtually coincide at the zone of discontinuity. The Ironside M ount a i n batholith is preamalgamation, not only because of its genetic relation to the meta-andesite but because it also is t r u n c a t e d by the t e r r a n e boundaries. However, the Wooley Creek belt is postamalgamation, even though only a few million years younger t han the Ironside Mo u n ta i n belt, because the Wooley Creek, Slinkard, and Vesa Bluff plutons of the belt cross the t er r ane boundaries (Donato et al., 1982; Barnes, 1983; Mortimer, 1984). In addition, Cretaceous plutons of the Grants Pass belt intrude the boundary between the Western Jurassic terrane and the rocks of the Western Paleozoic and Triassic belt in the NE domain, but plutons of comparable age and intrusive relations are u n k n o w n in equivalent terranes of the SW domain (Fig.4). The terranes of the NE domain, including the Eastern Klamath terrane, face
442
NW, n e a r l y parallel to the t r e n d of the zone of d i s c o n t i n u i t y (Fig.5). T h e i r combined width of outcrop, m e a s u r e d in the d i r e c t i o n of facing, is double the combined w i d t h of e q u i v a l e n t t e r r a c e s of the SW domain. The r e g i o n a l s t r u c t u r e s in the NE d o m a i n t r e n d N E - S W and either t e r m i n a t e or c h a n g e at the zone of d i s c o n t i n u i t y . S t r u c t u r e s of the E a s t e r n K l a m a t h t e r r a n e include the S c o t t M o u n t a i n s antiform, w h i c h appears to be a b r o a d u p w a r p in the T r i n i t y u l t r a m a f i c sheet, and the SE-dipping h o m o c l i n a l Redding section. The W e s t e r n Paleozoic and Triassic belt of the N E d o m a i n r e p r e s e n t s a large, r e l a t i v e l y thin t h r u s t plate t h a t is r o o t e d to the SE and was u n d e r t h r u s t from the N W by the W e s t e r n J u r a s s i c terrane. The axis of a b r o a d d o w n w a r d flexure in the t h r u s t plate, the Gold Hill synform, trends N E - S W a p p r o x i m a t e l y in the position of the G r a y b a c k p l u t o n i c belt; and a n a r r o w c o r r i d o r of W e s t e r n J u r a s s i c t e r r a n e is exposed a l o n g the K l a m a t h River in the b r e a c h e d southe a s t e r n limb of the synform. The part of the W e s t e r n J u r a s s i c t e r r a n e in the
Fig.5. Major structural features of the Klamath Mountains province, and the approximate location of the boundary between the NE and SW domains. Plutons are not shown. Shown are axes of: the Gold Hill synform (GHS) in the overthrust plate of undivided rocks of the Western Paleozoic and Triassic belt; the Scott Mountains antiform (SMA) in the Trinity ultramafic sheet; the Douglas City synform (DCS) in the overthrust plate of Central Metamorphic terrane; and the Condrey Mountain dome (CD) of Mortimer and Coleman (1984). Symbols also indicate the homoclinal SE dip of Devonian to Jurassic strata of the Redding section.
443
NE domain includes imbricate thrust slices that trend lengthwise to the terrane and dip SE. Neither the precise location nor the nature of the boundary between the two domains is known. The location of the southern part of the boundary between the two domains is particularly equivocal. The Shasta Bally batholith trends NW, parallel to the trend of the plutonic belts of the SW domain and nearly at right angles to the principal regional structures of the Eastern Klamath terrane, and thus would seem to belong to the SW domain. On the other hand, Shasta Bally batholith is part of the belt of youngest plutons of the province, some of which intrude the main expanse of the Trinity ultramafic sheet. Farther northward the domain boundary may be represented by the NWtrending Browns Meadow fault of Davis (1968), which is near the abrupt change in the Central Metamorphic terrane from the broad synformal thrust plate (Douglas City synform) of the SW domain to the narrow northward continuation of the terrane in the NE domain (Fig.5). In the Western Paleozoic and Triassic belt, the small Forks of Salmon pluton is the northernmost pluton of the Ironside Mountain belt and narrowly constrains the boundary between that belt and the English Peak pluton of the Wooley Creek belt. The trend in the western boundary of the Western Jurassic terrane changes abruptly from NW to NE a few kilometers NE of Crescent City and may represent a point on the domain boundary. At that point, the upper units of the Josephine ophiolite sequence and the overlying Galice Formation of the NW-trending part of the terrane are abruptly truncated and then continue with considerable disruption along the NE trending part of the terrane. Although the Josephine ophiolite south of the latitude of Crescent City trends SE, suggestive of the SW domain, the greatest disruption of the ophiolite is approximately 55 km along its trace to the SE to where there is a sharp embayment in the boundary of the province. South of the embayment, the Josephine ophiolite is present only as occasional fragments and narrow fault slivers along the western boundary of the province. SEQUENCE OF TERRANE AMALGAMATION
Several kinds of evidence determine the times and sequence of amalgamation of the various terranes. These incude the age of the youngest strata of the subducting terrane, the age of the metamorphism that in some instances affects the rocks of the lower plate adjacent to the terrane suture as a result of underthrusting, the age of postamalgamation plutons, and the age of strata that overlap the terrane boundary. The time of suturing of the Eastern Klamath and Central Metamorphic terranes is established by the Early or Middle Devonian isotopic age (Lanphere et al., 1968; Hotz, 1977) of the metamorphism that was caused by the overriding of the protoliths of the schist along the Bully Choop thrust (Irwin, 1963) by the Trinity ultramafic sheet. The suture is intruded by Early Cretaceous plutons of the Shasta Bally belt, and, like sutures between other Klamath terranes, is overlain by Cretaceous strata of the Great Valley sequence at the southeast end of the province.
444
The suture between the Central Metamorphic and North Fork terranes is the Siskiyou thrust of Davis (1968). The youngest dated strata of the North Fork are radiolarian chert and tuff of Early Jurassic (Pleinsbachian) age (Blome and Irwin, 1983). The thrusting has metamorphosed some of the rocks along the fault, but isotopic ages of 133 and 158 Ma measured on these rocks (Lanphere et al., 1968) are equivocal. The Siskiyou thrust is intruded by the small East Fork pluton (biotite, 164 Ma; hornblende, 149 Ma; M.A. Lanphere, written commun., 1983) and Russian Peak pluton (144-147 Ma; Romey, 1962; Evernden and Kistler, 1970), and by the Early Cretaceous Deadman Peak pluton. Thus the age of the suture is not precisely known but is in the range of ]~arly Jurasmc (Pleinsbachian) to Late Jurassic. The next lower (westerly) terrane in the order of structural stacking is the Hayfork. A zone of metamorphism is unapparent along the boundaries between the Hayfork and either the structurally higher (North Fork) or the next lower (Rattlesnake Creek) terrane. The plutons of the Ironside Mountain belt ( ~ 170 Ma) of the SW domain, which form a plutonic--volcanic pair with the host rocks, do not cross the terrane boundary; but the plutons of the Wooley Creek belt ( ~ 163 Ma) of the NE domain are reported to cross not only the boundary of the structurally higher terrane (Mortimer, 1984) but also the boundary of the structurally lower terrane (Barnes, 1983). These relations suggest that the ages of both sutures are between 170 and 163 Ma, and are compatible with the Middle Jurassic isotopic age (168 177 Ma; Fahan, 1982) of the Hayfork Bally Meta-andesite. The Western Jurassic terrane has underthrust the Rattlesnake Creek terrane of the SW domain and also the rocks of the undivided Western Paleozoic and Triassic belt of the NE domain. In the NE domain, the Galice Formation of the Western Jurassic terrane contains Upper Jurassic (Oxfordian to middle Kimmeridgian) fossils (Imlay, 1959). Much of the Galice in the SW domain has been metamorphosed to semischist that has yielded Late Jurassic (late Kimmeridgian and early Tithonian) isotopic ages (148-153 Ma; Lanphere et al., 1978). The boundary between the Western Jurassic terrane and the rocks of the Western Paleozoic and Triassic belt is intruded by Early Cretaceous plutons of the Grants Pass belt ( ~ 140 Ma) in the NE domain, but postamalgamation plutons are not known in equivalent rocks of the SW domain. These relations indicate that the amalgamation of the Western Jurassic terrane probably was latest Jurassic (Tithonian) and certainly no later than earliest Cretaceous. Thus the amalgamation of the Klamath terranes was mainly a series of Jurassic tectonic events, except the early Paleozoic suturing of the Eastern Klamath and Central Metamorphic terranes, and was sequential oceanward. The sequence of amalgamation does not end at the western border of the province, but continues oceanward with the amalgamation of Early Cretaceous schist (120 Ma; Lanphere et al., 1978) and Jurassic and Cretaceous graywacke formations of the Coast Range province. The various times of amalgamation during the Jurassic are fairly well constrained in most instances to only a few million years.
445 ACCRETION TO NORTH AMERICA Comparable evidence for the time of accretion of the K l am at h terranes to N or th America is unavailable because the t e r r a n e boundaries are concealed along the eastern perimeter of the province by a broad mantle of Cretaceous and y o u n g er strata. However, some evidence for the time of accretion to North America is provided by paleomagnetic data. Results of paleomagnetic studies indicate t hat the plutons and other elements of the NE domain have rotated clockwise substantially relative to stable No r th America. However, comparison with the SW domain is now infeasible because paleomagnetic data are unavailable for plutons of the SW domain except for the Shasta Bally batholith. Five plutons in the Western Paleozoic and Triassic belt of the NE domain show clockwise rotations ranging from 53 ° to 113 ° except the Ashland pluton which may show small counterclockwise r o t a t i o n (Schultz, 1983). All are Jurassic except the Grants Pass pluton which is Early Cretaceous. These rotations compare favorably with rotations of about 60 ° for Lower to Middle Jurassic strata of the Eastern K l amath t e r r a n e (Mankinen et al., 1984). Sparse paleomagnetic data suggest t h a t the large angular difference in trend between the Cretaceous plutonic belts (Grants Pass and Shasta Bally) of the two domains is accompanied by a comparable difference in magnetic orientation. The Grants Pass pluton has rotated approximately 78° _ 33 ° clockwise relative to stable N or t h America (Schultz, 1983), whereas preliminary results from measurements on the Shasta Bally batholith indicate little or no rot at i on (E.A. Mankinen, pers. commun., 1985). Rotation of the Grants Pass pluton back to its apparent original orientation would align it more nearly parallel to the Shasta Bally batholith. Except for the plutons just mentioned, the only substantial paleomagnetic data available for the province pertain to the Redding section of the Eastern K l amath terrane. The original or i e nt a t i on of the pre-Permian rocks of the Redding section is unknown. However, the Permian and Triassic strata have
Pre latest Tdassic
Post-earliest Cretaceous
le
c':'::3:d assemblage
Fig.6. Schematic diagram showing the sequence of rotation of the volcanic island arc represented by the Redding section of the Eastern Klamath terrane and its accretion to North America. There is no paleomagnetic evidence for substantial latitudinal displacement of the terrane.
446 r o t a t e d c l o c k w i s e a b o u t 100 ° , a p p r o x i m a t e l y 40 ° of w h i c h o c c u r r e d b e f o r e t h e d e p o s i t i o n of t h e J u r a s s i c s t r a t a ( M a n k i n e n et al., 1984). T h e s e r o t a t i o n s a r e a s s u m e d to h a v e i n c l u d e d t h e p r e - P e r m i a n s t r a t a a n d t h e p r e a m a l g a m a t i o n plutons. The l o n g - s t a n d i n g v o l c a n i c arc r e p r e s e n t e d by the R e d d i n g section m u s t h a v e f a c e d S W d u r i n g P e r m i a n a n d T r i a s s i c t i m e , r o t a t e d to face W by E a r l y or M i d d l e J u r a s s i c , a n d c o n t i n u e d r o t a t i n g to a t t a i n e s s e n t i a l l y its p r e s e n t N W f a c i n g o r i e n t a t i o n b y E a r l y C r e t a c e o u s t i m e (Fig.6). T h e t i m e of a t t a i n i n g i t s p r e s e n t f a c i n g is i n d i c a t e d b y t h e v i r t u a l l a c k of r o t a t i o n of t h e S h a s t a B a l l y b a t h o l i t h a n d of t h e L o w e r C r e t a c e o u s ( H a u t e r i v i a n ) s t r a t a of t h e G r e a t V a l l e y o v e r l a p s e q u e n c e ( M a n k i n e n a n d I r w i n , 1982). T h e v i r t u a l c e s s a t i o n of r o t a t i o n , c l o s e l y f o l l o w e d b y d e p o s i t i o n of t h e o v e r l a p s e q u e n c e , p r o b a b l y r e p r e s e n t s t h e t i m e of a c c r e t i o n to N o r t h A m e r i c a . REFERENCES Albers, J.P. and Robertson, J.F., 1961. Geology and ore deposits of the East Shasta copper-zinc district, Shasta County, California. U.S. Geol. Surv. Prof. Pap., 338, 107 pp. Albers, J.P., Kistler, R.W. and Kwak, L., 1981. The Mule Mountain Stock, an early Middle Devonian pluton in northern California. ISOCRON/West, 31: 17. Allen, C.M., Barnes, C.G., Kays, M.A. and Saleeby, J.B., 1982. Comagmatic nature of the Wooley Creek batholith and the Slinkard pluton and age constraints on the tectonic and metamorphic events in the western Paleozoic and Triassic belt, Klamath Mountains, N. California. Geol. Soc. Am., Abstr. Progr., 14: 145. Ando, C.J., Irwin, W.P., Jones, D.L. and Saleeby, J.B., 1983. The ophiolitic North Fork terrane in the Salmon River region, central Klamath Mountains, California. Geol. Soc. Am. Bull., 94: 236-252. Barnes, C.G., 1983. Petrology and upward zonation of the Wooley Creek batholith, Klamath Mountains, California. J. Petrol., 24:495 537. Blake, M.C., Jr., Howell, D.G. and Jones, D.L., 1982. Preliminary tectonostratigraphic terrane map of California. U.S. Geol. Surv. Open File Rep., 82-593. Blakely, R.J., Jachens, R.C., Simpson, R.W. and Couch, R.W., 1985. Tectonic setting of the southern Cascade Range as interpreted from its magnetic and gravity fields. Geol. Soc. Am. Bull., 96: 43-48. Blome, C.D. and Irwin, W.P., 1983. Tectonic significance of Late Paleozoic to Jurassic radiolarians in the North Fork terrane, Klamath Mountains, California. In: C.H. Stevens (Editor), PreJurassic Rocks in Western North American Suspect Terranes. Society of Economic Paleontolo~ gists and Mineralogists, Pacific Section, pp.77- 89. Coney, P.J., Jones. D.L. and Monger, J.W.H., 1980. Cordilleran suspect terranes. Nature, 288: 329 333. Curtis, G.H., Evernden, J.F. and Lipson, J., 1958. Age determination of some granitic rocks in California by the potassium-argon method. Calif. Div. Min. Spec. Rep., 54, 16 pp. Davis, G.A., 1961. Metamorphic and Igneous Geology of Pre-Cretaceous Rocks, Coffee Creek Area, Northeastern Trinity Alps, Klamath Mountains, California. Ph.D. thesis, University of California, Berkeley, Calif., 199 pp. Davis, G.A., 1968. Westward thrust faulting in the south-central Klamath Mountains. Geol. Soc. Am. Bull., 79:911 934. Donato, M.M., Barnes, C.G., Coleman, R.G., Ernst, W.G. and Kays, M.A., 1982. Geologic map of the Marble Mountains Wilderness, Siskiyou County, California. U.S. Geol. Surv., Miscellaneous Field Studies Map Ml$:1452-A,scale li48,000. Evernden, J.F. and Kistler, R.W., 1970. Chronology of emplacement of Mesozoic batholithie complexes in California and western Nevada. U.S. Geol. Surv. Prof. Pap., 623, 42 pp.
447 Fahan, M.R., 1982. Geology and geochronology of a part of the Hayfork terrane, Klamath Mountains, northern California. M.S. thesis, University of California, Berkeley, Calif., 127 pp. Fuis, G.S. and Zucca, J.J., 1984. A geologic cross section of northeastern California from seismic refraction results. In: T.H. Nilsen (Editor), Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California. Society of Economic Paleontologists and Mineralogists, Pacific Section, 42: 203-209. Garcia, M.O., 1982. Petrology of the Rogue River island-arc complex, southwestern Oregon. Am. J. Sci., 282: 783-807. Griscom, A., 1973. Bouguer gravity map of California; Redding Sheet. California Division of Mines and Geology, scale 1:250,000. Harland, W.B., Cox, A.V., Llewellyn, P.G., Picton, C.A.G., Smith, A.G. and Walters, R., 1982. A Geologic Time Scale. Cambridge University Press, Cambridge, 131 pp. Harper, G.D., 1984. The Josephine ophiolite, northwestern California. Geol. Soc. Am. Bull., 95: 1009-1026. Harper, G.D. and Saleeby, J.B., 1980. Zircon ages of the Josephine ophiolite and the Lower Coon Mountain pluton. Geol. Soc. Am., Abstr. Progr., 12: 109-110. Holdaway, M.J., 1963. Petrology and structure of metamorphic and igneous rocks of parts of northern Coffee Creek and Cecilville quadrangles, Klamath Mountains, California. Ph.D. thesis, University of California, Berkeley, Calif., 180 pp. Hotz, P.E., 1971. Plutonic rocks of the Klamath Mountains, California and Oregon. U.S. Geol. Surw, Prof. Pap., 684-B, 20 pp. Hotz, P.E., 1977. Geology of the Yreka quadrangle, Siskiyou County, California. U.S. Geol. Surv. Bull., 1436, 72 pp. Imlay, R.W., 1959. Succession and speciation of the Pelecypod Aucella. U.S. Geol. Surv. Prof. Pap. 314-G, pp.155 169. Irwin, W.P., 1960. Geologic reconnaissance of the northern Coast Ranges and Klamath Mountains, California, with a summary of the mineral resources. Calif. Div. Min. Bull., 179, 80 pp., with map, scale 1,500,000. Irwin, W.P., 1963. Preliminary geologic map of the Weaverville quadrangle, California. U.S. Geol. Surv. Mineral Investigations Field Studies Map MF-275, scale 1:62,500. Irwin, W.P., 1972. Terranes of the western Paleozoic and Triassic belt in the southern Klamath Mountains, California. In: Geological Survey Research, 1972. U.S. Geol. Surv. Prof. Pap., 800-C: C103-Clll. Irwin, W.P., 1985. Age and tectonics of plutonic belts in accreted terranes of the Klamath Mountains, California and Oregon. In: D.G. Howell (Editor), Tectonostratigraphic Terranes of the Circumpacific Region. Circumpacific Council for Energy and Mineral Resources, Earth Science Series, No. 1, pp,187-199. Irwin, W.P. and Galanis, S.P., Jr., 1976. Map showing limestone and selected fossil localities in the Klamath Mountains, California and Oregon. U.S. Geol. Surv., Miscellaneous Field Studies Map MF-749, scale 1:500,000. Irwin, W.P., Wolfe, E.W., Blake, M.C., Jr. and Cunningham, C.G., Jr., 1974. Geologic map of the Pickett Peak quadrangle, Trinity County, California. U.S. Geol. Surv. Geologic Quadrangle Map GQ-1111, scale 1:62,500. Irwin, W.P., Jones, D.L. and Blome, C.D., 1982. Map showing sampled radiolarian localities in the Western Paleozoic and Triassic belt, Klamath Mountains, California. U.S. Geol. Surv., Miscellaneous Field Studies Map MF-1399, scale 1:250,000. Kinkel, A.R., Jr., Hall, W.E. and Albers, J.P., 1956. Geology and base-metal deposits of West Shasta copper-zinc district, Shasta County, California. U.S. Geol. Surv. Prof. Pap., 285, 156 pp. LaFehr, T.R., 1966. Gravity in the eastern Klamath Mountains, California. Geol. Soc. Am. Bull., 77:1177 1190. Lanphere, M.A. and Jones, D.L., 1978. Cretaceous time scale from North America. In: G.V. Cohee et al. (Editors), Contributions to the Geologic Time Scale. American Association of Petroleum Geologists, Studies in Geology, 6: 259-268. Lanphere, M.A., Irwin, W.P. and Hotz, P.E., 1968. Isotopic age of the Nevadan orogeny and other plutonic and metamorphic events in the Klamath Mountains, California. Geol. Soc. Am. Bull., 79:1027 1052.
448 Lanphere, M.A., Blake, M.C., Jr. and Irwin, W.P., 1978. Early Cretaceous metamorphic age of the South Fork Mountain Schist in the northern Coast Ranges of California. Am. J. Sci., 278: 798-815. Mankinen, E.A. and Irwin, W.P., 1982. Paleomagnetic study of some Cretaceous and Tertiary sedimentary rocks of the Klamath Mountains province, California. Geology, 10:82 87. Mankinen, E.A., Irwin, W.P. and Gromme, C.S., 1984. Implications of paleomagnetism for the tectonic history of the Eastern Klamath and related terranes in California and Oregon. In: T.H. Nilsen (Editor), Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California. Society of Economic Paleontologists and Mineralogists, Pacific Section, 42:221 229. Mattinson, J.M. and Hopson, C.A., 1972. Paleozoic ophiolitic complexes in Washington and northern California. Carnegie Institution, Annual Report of the Director, Geophysical Laboratory, 1971 1972, pp.578 583. Mortimer, N., 1984. Petrology and Structure of Permian to Jurassic Rocks near Yreka, Klamath Mountains, California. Ph.D. thesis, Stanford University, Stanford, 84 pp. Mortimer, N. and Coleman, R.G., 1984. A Neogene structural dome in the Klamath Mountains, California and Oregon. In: T.H. Nilsen (Editor), Geology of the Upper Cretaceous Hornbrook Formation, Oregon and California. Society of Economic Paleontologists and Mineralogists, Pacific Section, 42: 179-486. Nestell, M.K., Irwin, W.P. and Albers, J.P., 1981. Late Permian (Early Djulfian) Tethyan Foraminifera from the southern Klamath Mountains, California. Geol. Soc. Am., Abstr. Progr.. 12: 519. Potter, A.W., Hotz, P.E. and Rohr, D.M., 1977. Stratigraphy and inferred tectonic framework of lower Paleozoic rocks in the eastern Klamath Mountains, northern California. In: J.H. Stewart et al. (Editors), Paleozoic Paleogeography of the Western United States. Society of Economic Paleontologists and Mineralogists, Pacific Section, Pacific Coast Paleogeograpy Symposium l, pp.421-440. Romey, W.D., 1962. Geology of a Part of the Etna Quadrangle, Siskiyou County, California. Ph.D. thesis, University of California, Berkeley, Calif., 93 pp. Saleeby, J.B., 1984. Pb/U zircon ages from the Rogue River area, Western Jurassic belt, Klamath Mountains, Oregon. Geol. Soc. Am. Abstr. Progr., 16: 331. Saleeby, J.B., Harper, G.D., Snoke, A.W. and Sharp, W.D., 1982. Time relations and structuralstratigraphic patterns in ophiolite accretion, west-central Klamath Mountains, California. J. Geophys. Res., 87: 3831-3848. Sanborn, A.F., 1953. Geology and paleontology of the southwest quarter of the Big Bend quadrangle, Shasta County, California. Calif. Div. Min. Spec. Rept., 63, 26 pp. Schultz, K.L., 1983. Paleomagnetism in the Klamath Mountains, Southern Oregon and Northern California. M.S. thesis, Oregon State University, Corvallis, Ore., 153 pp. Snoke, A.W., 1977. A thrust plate of ophiolitic rocks in the Preston Peak area, Klamath Mountains, California. Geol. Soc. Am. Bull., 88: 1641-1659. Snoke, A.W., Quick, J.E. and Bowman, H.R., 1981. Bear Mountain igneous complex, Klamath Mountains, California: an ultrabasic to silicic calc-alkaline suite. J. Petrol., 22:501 552. Wright, J.E., 1981. Geology and Uranium-Lead Geochronology of the Western Paleozoic and Triassic Subprovince, Southwestern Klamath Mountains, California. Ph.D. thesis, University of California, Santa Barbara, ("alif., 300 pp. Wright, J.E., 1982. Permo-T, iassic accretionary subduction complex, southwestern Klamath Mountains, northern California. J. Geophys. Res., 87: 3805-3818. Young, J.C., 1978. Geology of the Willow Creek Quadrangle, Humboldt and Trinity Counties, California. California Division of Mines and Geology Map Sheet 31, scale 1:62,500. Zeller, E.J., 1965. Modern Methods for Measurement of Geologic Time. California Division of Mines and Geology Mineral Information Service, 18: 9-16.
433
Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands
CRYPTIC TECTONIC DOMAINS OF THE KLAMATH MOUNTAINS, CALIFORNIA AND OREGON
WILLIAM P. IRWIN
U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 (U.S.A.) (Accepted for publication November 4, 1988)
ABSTRACT Irwin, W.P., 1989. Cryptic tectonic domains of the Klamath Mountains, California and Oregon. In: A.M. Johnson, C.W. Burnham, C.R. Allen and W. Muehlberger (Editors), Richard H. Jahns Memorial Volume. Eng. Geol., 27: 433-448. Numerous fragments of oceanic crust and island arcs make up the Klamath Mountains province. These fragments were joined together (amalgamated) in an oceanic setting during Paleozoic and Mesozoic collisional events and were accreted to North America as a composite unit during latest Jurassic or earliest Cretaceous time. The roughly arcuate and concentric distribution of the terranes of the Klamath Mountains does not now seem to be a result of simple oroclinal bending as earlier believed. Although commonly described as a west-facing arcuate structure, the province is cut diagonally by a vaguely defined NW-trending zone of discontinuity, or hinge line, that divides the province into NE and SW tectonic domains. The zone of discontinuity is marked by a number of lithic and structural anomalies, and particularly by the distribution of a remarkable series of belts of plutonic rocks. The terranes, regional structures, and plutonic belts of the NE domain trend NE and are generally wider and more coherent than the narrow NWtrending terranes and plutonic belts of the SW domain. Most plutonic belts of the NE domain do not have equivalents in the SW domain. Paleomagnetic evidence suggests that all the plutonic belts, except possibly the youngest (the earliest Cretaceous Shasta Bally belt), were emplaced before the Klamath Mountains terranes finally accreted to North America.
INTRODUCTION
T h e K l a m a t h M o u n t a i n s c o n s t i t u t e a w e s t - f a c i n g a r c u a t e p r o v i n c e of 30,645 k m : i n a r e a , a r e g i o n s o m e w h a t l a r g e r t h a n t h e S w i s s Alps. T h e p r o v i n c e is s u b d i v i d e d i n t o s e v e r a l t e r r a n e s , e a c h of w h i c h r e p r e s e n t s a n a l l o c h t h o n o u s f r a g m e n t of o c e a n i c c r u s t o r i s l a n d a r c ( F i g . l ) . T h e K l a m a t h M o u n t a i n s p r o v i n c e is o n l y a s m a l l p a r t o f t h e v a s t c o l l a g e of s u s p e c t t e r r a n e s a l o n g t h e w e s t e r n m a r g i n of N o r t h A m e r i c a f r o m M e x i c o to A l a s k a ( C o n e y et al., 1980). A l l t h e t e r r a n e s o f t h e K l a m a t h M o u n t a i n s a r e o c e a n i c i n o r i g i n ; t h e r e is no e v i d e n c e t h a t a n y of the t e r r a n e s were ever u n d e r l a i n by c o n t i n e n t a l basem e n t r o c k s . O p h i o l i t i c t e r r a n e s of t h e p r o v i n c e , c o n s i s t i n g p a r t l y of o c e a n i c crust and upper mantle, probably originated at various oceanic spreading centers d u r i n g Ordovician, P e r m i a n , Triassic, a n d J u r a s s i c times. The ophiolitic 0013-7952/89/$03.50
~ 1989 Elsevier Science Publishers B.V.
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components of the terranes are sequentially younger oceanward (westward), suggesting th at the amalgamation of the terranes also was sequential. The terranes also include parts of volcanic island arcs t hat formed at various times during the Paleozoic and Mesozoic, and these also tend to be sequentially y o u n g er oceanward. The deformation of the rocks of the terranes differs widely. In some terranes the stratigraphy and s t r uc t ur e is coherent, but more commonly the terranes consist of broken formations and melange. Along much of the southeastern and eastern perimeter of the province the terranes are overlain unconformably by relatively mildly deformed sequences of Cretaceous and T ert i ary strata t hat were deposited after the terranes had accreted to the continent. EASTERN K L A M A T H TERRANE
The Eastern Klamath t er r ane *~ is the oldest in the province. It consists mainly of the Trinity ultramafic sheet (ophiolite) and the overlying Paleozoic and Mesozoic island-arc deposits (Fig.l). The Trinity sheet, perhaps the largest ultramafic body in North America, crops out over a large area of the central part of the terrane, and is exposed for a length of 160 km where its edge is upturned along the western border of the terrane. It consists mainly of tectonized and partly serpentinized harzburgite and dunite, which are intruded by pyroxenite, gabbro, and plagiogranite. Isotopic ages as old as 480 Ma measured on the gabbro (Lanphere et al., 1968; Mattinson and Hopson, 1972) indicate th at the Trinity ultramafic sheet is Early Ordovician or older. The large central outcrop of the Trinity sheet divides the Paleozoic and Mesozoic strata of the Eastern Klamath t errane into two areas of exposure: the Yreka-Callahan area to the NW, and the Redding section to the SE. Evidence from magnetic, gravity, and seismic refraction studies indicate t hat the Trinity sheet underlies the strata of both areas (Lafehr, 1966; Griscom, 1973; Fuis and Zucca, 1984; Blakeley et al., 1985). The strata of the Y r e k a - C a l l a h a n area are Ordovician to Devonian in age (Potter et al., 1977). They are subdivided into seven or more formations whose relations to one anot her and to the Trinity ultramafic sheet are structurally complex and controversial. The strata of the Redding section range in age from Early Devonian through Middle Jurassic and are subdivided into sixteen formations with a total exposed thickness of more t ha n 10 km (Sanborn, 1953; Kinkel et al., 1956; Albers and Robertson, 1961). They are structurally more coherent t han the strata of the Y r e k a - C a l l a h a n area and form a generally homoclinal sequence that dips SE away from the broad central outcrop of the Trinity sheet. The
*lThe terrane n o m e n c l a t u r e used in this report differs s o m e w h a t from the n o m e n c l a t u r e used by some other authors. For example, each of the three subdivisions of the E a s t e r n K l a m a t h t e r r a n e described herein is considered a separate terrane by Blake et al. (1982).
435 124 °
123°
I
I
122 ° I
EXPLANATION
~ ~
43°
Port Orford
Western Jurassic terrarle Western Paleozoic and Triassic belt {composite) _~ Rattlesnake Creek terrane ~
Hayfork terrane
~
North Fork terrane
~ ~
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Fig.1. Terranes of the Klamath Mountains province and distribution of the principal plutons. Some plutons too small to be shown; black dots indicate localities of small isotopically-dated Ridge; plutons. Letter symbols refer to names of dated plutons: A = A s h l a n d ; A R = A m m o n B G = Basin Gulch; B M = Bear Mountain; B W = Bear Wallow; C = Caribou Mountain; C C = Castle Crags; C M = Cracker Meadow; C P = Craggy Peak; D P = Deadman Peak; E F = East Fork; E P = English Peak; F S = Forks of Salmon; G = Greyback; G C = Glen Creek; G H = Gold Hill; G P = Grants Lake; I M = I r o n s i d e Mountain; I R = I l l i n o i s River; J = J a c k s o n v i l l e ; Pass; H L = H o r s e s h o e Coon Mountain; M M = M u l e Mountain; N G = U n n a m e d gabbro; P C = Price Creek; LC=Lower P R = Pit River; R P = Russian Peak; S = Slinkard; S B = Shasta Bally; S G = Saddle Gulch; S M = Star Mountain; S P = Sugar Pine; V B = Vesa Bluff; W = Wildwood; W C = Wooley Creek; W P = Walker Point; W R = White Rock; Y B = Yellow Butte.
436 large number of andesitic volcanic formations distributed through the Redding section, and the presence of reefal limestones, suggest that the Redding section represents a long-standing volcanic island arc built on Ordovician oceanic crust. CENTRAL METAMORPHICTERRANE The Central Metamorphic terrane generally lies west of, and structurally below, the Trinity sheet. It consists of a lower unit of metamorphosed mafic volcanic rocks (Salmon Hornblende Schist) and an upper unit of metamorphosed clastic sedimentary and carbonate rocks (Abrams Mica Schist). Where most fully developed, being west of the southern half of the Eastern Klamath terrane, it forms a nearly detached, north-trending synformal thrust plate that is probably 3-5 km thick at the axis. It is tectonically thinned eastward, where it dips beneath the Trinity sheet. Relatively small klippen of Eastern Klamath terrane rest on the Central Metamorphic terrane at three widely-spaced localities (Fig.l). The normal relationship between the Central Metamorphic and Eastern Klamath terranes appears structurally transposed for 35 km along their boundary SW of Yreka, where the Central Metamorphic terrane is represented by a relatively narrow sliver t h a t crops out along the east side rather than west of the northern extension of the Trinity ultramafic sheet. The stratigraphic age of the protoliths of the schists is unclear. However, isotopic ages of 380-399 Ma measured on the schists indicate that the schists formed during a Devonian tectonic event (Lanphere et al., 1968; Hotz, 1977). The metamorphism resulted from overriding of the protoliths by the Trinity ultramafic sheet, and was contemporaneous with Devonian volcanic activity represented by the oldest formations of the Redding section. NORTH FORK TERRANE An extensive area of structurally complex rocks west of the Central Metamorphic terrane was called the Western Paleozoic and Triassic belt during early studies of the region (Irwin, 1960). The belt is now subdivided, from east to west, into the North Fork, Hayfork, and Rattlesnake Creek terranes (Irwin, 1972) in the southern and central parts of the province. The North Fork terrane occupies a zone 2 10 km wide for approximately 100 km along the west side of the Central Metamorphic terrane. The structurally lowest part of the North Fork terrane is a dismembered ophiolite consisting of serpentinized peridotite, gabbro, plagiogranite, diabase, and pillow basalt. The ophiolitic rocks crop out along the western side of the terrane, and are succeeded upward to the east by mafic volcanic rocks with interlayered argillite, radiolarian chert, and minor limestone pods and lenses. One horizon of highly disrupted argillite in the Salmon River region contains blocks of glaucophane-lawsonite blueschist (Ando et al., 1983). The North Fork ophiolite is probably late Paleozoic in age, based on the isotopic age of a small plagiogranite body (Ando et al., 1983) and on an uncertain relationship
437 with nearby red radiolarian chert of Permian age (Blome and Irwin, 1983). Chert and siliceous tuff interlayered with the mafic volcanic rocks contain Triassic and Lower Jurassic radiolarians. However, the few limestone bodies that have yielded useful fossils contain upper Paleozoic fusulinids and foraminifers (Irwin and Galanis, 1976), which indicate that those limestone pods are either olistoliths or tectonic blocks. HAYFORK TERRANE The Hayfork terrane consists of three formations and the Ironside Mountain batholith and related plutons. The principal formation, and structurally lowest, is the Hayfork Bally Meta-andesite which consists mainly of c r y s t a l lithic tuff and tuff breccia of andesitic to basaltic composition. Isotopic ages ranging from 156 Ma (M.A. Lanphere, oral commun., 1977) to 168-177 Ma (Fahan, 1982) have been measured on the meta-andesite. These volcanic strata are overlain by, and interfinger with, a chert-argillite formation that in turn is structurally overlain by melange. The melange is separated from the underlying strata by the Wilson Point thrust fault (Wright, 1982), which divides the Hayfork terrane into two subterranes. The Western Hayfork subterrane includes the Hayfork Bally Meta-andesite and the chert-argillite unit; the Eastern Hayfork subterrane is the melange. The melange includes serpentinite, argillite, chert, quartzose sandstone, mafic and locally silicic volcanic rocks, limestone pods, and a few amphibolite knockers. The serpentinite occurs most commonly as slivers along the Wilson Point thrust. The chert yields both Triassic and Jurassic radiolarians (Irwin et al., 1982). Some of the limestone contains Permian fossils of Tethyan faunal affinity (Irwin and Galanis, 1976; Nestell et al., 1981). RATTLESNAKE CREEK TERRANE The Rattlesnake Creek terrane is the tectonically most disorganized unit of the province, and bedrock relations are obscured over much of the area by landslides on a grand scale. The terrane is a melange consisting of highly dismembered ophiolite, mafic to silicic volcanic and subvolcanic rocks, plagiogranite, chert, limestone pods, and minor sandstone and conglomerate. The chert contains Triassic and Jurassic radiolarians. Sparsely distributed fossils in the limestone pods are variously Devonian(?), late Paleozoic, and late Triassic. Isotopic ages ranging from 193 to 207 Ma have been measured on the plagiogranitic rocks (Wright, 1981, 1982). WESTERN JURASSIC TERRANE The Western Jurassic terrane includes the Josephine ophiolite of Snoke (1977), the Galice and Rogue Formations, and the Chetco River complex of
438 Hotz (1971). The Josephine ophiolite is excellently exposed and most completely preserved in the Smith River region just south of the Oregon border. There it displays a virtually complete ophiolite sequence from lowermost harzburgite to uppermost sheeted dikes and pillow basalt (Harper, 1984). Zircon from plagiogranite yields an isotopic age of 157 Ma for the ophiolite (Saleeby et al., 1982). The ophiolite is conformably overlain by the Upper Jurassic (Oxfordian to Kimmeridgian) Galice Formation which consists of slaty argillite, graywacke, and minor conglomerate (Harper, 1984). Northward, in Oregon, the Galice generally overlies and contains interlayers of the volcanic Rogue Formation, and these two formations are separated from the ophiolite by faults and by a narrow fault slice of amphibolite. The Rogue is a dominantly volcaniclastic formation that probably formed as part of a calc alkaline volcanic arc, with the Chetco River complex representing the core of the arc (Garcia, 1982). The Chetco River complex is a dominantly mafic plutonic complex that also includes the calc alkaline Illinois River batholith whose isotopic age is about 153 Ma (Hotz, 1971; Garcia, 1982). PREAMALGAMATIONAND POSTAMALGAMATIONPLUTONIC BELTS Granitoid plutonic rocks of the Klamath Mountains range in age from Ordovician to Cretaceous. They are non-uniformily distributed, being relatively few in number in the Y r e k a - C a l l a h a n region, the Redding section, and the Western Jurassic terrane. Many plutons are elongate parallel to the trend of their host terrane, and in some instances, such as the Ironside Mountain batholith, the orientation probably is controlled by the orientation of the host arc. Some plutons are truncated by terrane boundaries. Others cut across terrane boundaries, intruding one or more adjacent terranes. These boundary relations are important constraints on the time of amalgamation of the terranes. Isotopic age data from various published sources (Fig.2) indicate that most of the plutons are distributed in belts according to age, and t h a t the belts in most instances follow the trends of the terranes (Fig.3). A few plutons are outside the named belts. The pattern of plutonic belts I am proposing is highly provisional, because many plutons are not dated and some of the isotopic ages measured by different methods are widely variant. Despite these uncertainties, the overall pattern of distribution of the plutonic belts provides valuable insight regarding the tectonic development of the province. The plutonic belts trend NE-SW in the NE part of the province, and trend NW-SE, nearly at right angles, in the SW part. The Paleozoic plutonic belts are all in the Eastern Klamath terrane and are NE-SW trending; remarkably, Jurassic plutons are unknown in the Eastern Klamath terrane except possibly the Castle Crags pluton. Oceanward, the NE-SW trending belts are of successively younger Jurassic and Cretaceous ages except for the westernmost belt, the Chetco, which is Late Jurassic. The NW-SE trending plutonic belts also are of Jurassic and Cretaceous ages, but
439
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GREY. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . BACK BELT
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Fig.2. Comparison of ages of plutons of the Klamath Mountains province (modified from Irwin, 1985). The plutons are grouped according to terranes (top of figure) and plutonic belts (braces near bottom of figure). Sources for the isotopic age data are referred to by right-reading numbers in the diagram: 1 = Albers et al., 1981; 2 = J.P. Albers, oral commun., 1982; 3 = Allen et al., 1982; 4 = Ando et al., 1983; 5 = Curtis et al., 1958; 6=Davis, 1961; 7 = Evernden and Kistler, 1970; 8 = Fahan, 1982; 9=Harper and Saleeby, 1980; l O = H o l d a w a y , 1963; 1 1 = H o t z , 1971; 1 2 = I r w i n et al., 1974; 1 3 = Lanphere and Jones, 1978; 1 4 = Lanphere et al., 1968; 1 5 = M.A. Lanphere, written commun., 1977 and 1983; 1 6 = Mattinson and Hopson, 1972; 1 7 = Romey, 1962; 1 8 = Saleeby, 1984; 1 9 = Saleeby et al., 1982; 2 0 = S n o k e , 1977; 21=Snoke et al., 1981; 2 2 = W r i g h t , 1981 and 1982; 23=Young, 1978; 2 4 = Zeller, 1965. The time scale is from Harland et al., 1982. Numerical age is in millions of years.
the y o u n g e s t ( S h a s t a B a l l y belt) is f a r t h e s t f r o m the o c e a n a n d is p a r t l y s u p e r i m p o s e d on the oldest P a l e o z o i c belt (Alpine G a b b r o belt). S o m e g r a n i t o i d p l u t o n s a r e o r i g i n a l c o m p o n e n t s of t h e t e r r a n e s , h a v i n g f o r m e d at s p r e a d i n g c e n t e r s or in t h e r o o t zones of v o l c a n i c island arcs, a n d p r e d a t e t h e t i m e of collision w i t h o t h e r t e r r a n e s . T h e s e p r e a m a l g a m a t i o n p l u t o n s in s o m e i n s t a n c e s a r e s i m i l a r in a g e a n d c o m p o s i t i o n to the v o l c a n i c s t r a t a t h e y i n t r u d e , f o r m i n g p l u t o n i c - v o l c a n i c pairs. E x a m p l e s of p l u t o n i c v o l c a n i c p a i r s are: t h e M u l e M o u n t a i n s t o c k i n t r u d i n g t h e B a l a k l a l l a R h y o l i t e ( D e v o n i a n ) of t h e R e d d i n g section, t h e p l u t o n s of the M c C l o u d belt i n t r u d i n g the D e k k a s A n d e s i t e ( P e r m i a n ) o f t h e R e d d i n g section, a n d the I r o n s i d e M o u n t a i n b a t h o l i t h i n t r u d i n g t h e H a y f o r k B a l l y M e t a - a n d e s i t e ( J u r a s s i c ) of t h e H a y f o r k t e r r a n e . I n c o n t r a s t , t h e p o s t a m a l g a m a t i o n p l u t o n s a r e signific a n t l y y o u n g e r t h a n t h e i r h o s t r o c k s a n d a r e g e n e t i c a l l y u n r e l a t e d to them. T h e s e p l u t o n s a r e a s s i g n e d to belts on t h e basis of isotopic age, a n d a r e d e s i g n a t e d as p o s t a m a l g a m a t i o n if s o m e of t h e p l u t o n s of t h e belt a r e k n o w n to i n t r u d e m o r e t h a n one t e r r a n e or if, on t h e basis of o t h e r r e g i o n a l cons i d e r a t i o n s , t h e p l u t o n s a r e k n o w n to be y o u n g e r t h a n t h e t e r r a n e b o u n d a r y . T h e p o s t a m a l g a m a t i o n p l u t o n s i n t r u d e d t h e h o s t t e r r a n e s p r e s u m a b l y as a r e s u l t of c r u s t a l s u b d u c t i o n events.
440
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Fig.3. Plutonic belts of the Klamath Mountains province. Names of plutonic belts are followedby approximate ages in millions of years. THE TECTONIC DOMAINS Discontinuities in the orientation and distribution of the plutonic belts, in addition to other features, indicate that the Klamath Mountains province can be subdivided into two tectonic domains, although the significance of these discontinuities is unknown. The change in trend of the plutonic belts is along a vaguely defined NW-trending zone of discontinuity that divides the province into NE and SW tectonic domains (Fig.3). The difference in orientation of the plutonic belts is accompanied by differences in the ages and numbers of belts. All the Paleozoic plutonic belts are in the NE domain. Jurassic and Cretaceous belts are present in both domains, but their precise ages differ from one domain to the other. The number of belts in the NE domain is nearly double the number in the SW domain. In the NE domain the Western Paleozoic and Triassic belt contains the greatest number of plutons, including virtually all the Jurassic postamalgamation plutons of the province (Fig.4). It is notable that this vast region appears devoid of preamalgamation plutons. In contrast, the plutons of the Western Paleozoic and Triassic belt of the SW domain are virtually all preamalgamation plutons.
441
Fig.4. Distribution of the isotopically dated postamalgamation plutons of the Klamath Mountains province. Small dated plutons are shown by solid circles with letter symbol indicating Jurassic (J) or Cretaceous (K) ages. Undated and preamalgamation plutons are not shown.
These differences in the plutonic belts from one domain to the ot her are particularly puzzling where the host terranes seem virtually continuous across the zone of discontinuity. The Ironside M o u n t a i n (,~ 170 Ma) and Wooley Creek ( ~ 163 Ma) plutonic belts are interesting examples because both intrude the meta-andesite of the H a yf or k t e r r a n e and virtually coincide at the zone of discontinuity. The Ironside M ount a i n batholith is preamalgamation, not only because of its genetic relation to the meta-andesite but because it also is t r u n c a t e d by the t e r r a n e boundaries. However, the Wooley Creek belt is postamalgamation, even though only a few million years younger t han the Ironside Mo u n ta i n belt, because the Wooley Creek, Slinkard, and Vesa Bluff plutons of the belt cross the t er r ane boundaries (Donato et al., 1982; Barnes, 1983; Mortimer, 1984). In addition, Cretaceous plutons of the Grants Pass belt intrude the boundary between the Western Jurassic terrane and the rocks of the Western Paleozoic and Triassic belt in the NE domain, but plutons of comparable age and intrusive relations are u n k n o w n in equivalent terranes of the SW domain (Fig.4). The terranes of the NE domain, including the Eastern Klamath terrane, face
442
NW, n e a r l y parallel to the t r e n d of the zone of d i s c o n t i n u i t y (Fig.5). T h e i r combined width of outcrop, m e a s u r e d in the d i r e c t i o n of facing, is double the combined w i d t h of e q u i v a l e n t t e r r a c e s of the SW domain. The r e g i o n a l s t r u c t u r e s in the NE d o m a i n t r e n d N E - S W and either t e r m i n a t e or c h a n g e at the zone of d i s c o n t i n u i t y . S t r u c t u r e s of the E a s t e r n K l a m a t h t e r r a n e include the S c o t t M o u n t a i n s antiform, w h i c h appears to be a b r o a d u p w a r p in the T r i n i t y u l t r a m a f i c sheet, and the SE-dipping h o m o c l i n a l Redding section. The W e s t e r n Paleozoic and Triassic belt of the N E d o m a i n r e p r e s e n t s a large, r e l a t i v e l y thin t h r u s t plate t h a t is r o o t e d to the SE and was u n d e r t h r u s t from the N W by the W e s t e r n J u r a s s i c terrane. The axis of a b r o a d d o w n w a r d flexure in the t h r u s t plate, the Gold Hill synform, trends N E - S W a p p r o x i m a t e l y in the position of the G r a y b a c k p l u t o n i c belt; and a n a r r o w c o r r i d o r of W e s t e r n J u r a s s i c t e r r a n e is exposed a l o n g the K l a m a t h River in the b r e a c h e d southe a s t e r n limb of the synform. The part of the W e s t e r n J u r a s s i c t e r r a n e in the
Fig.5. Major structural features of the Klamath Mountains province, and the approximate location of the boundary between the NE and SW domains. Plutons are not shown. Shown are axes of: the Gold Hill synform (GHS) in the overthrust plate of undivided rocks of the Western Paleozoic and Triassic belt; the Scott Mountains antiform (SMA) in the Trinity ultramafic sheet; the Douglas City synform (DCS) in the overthrust plate of Central Metamorphic terrane; and the Condrey Mountain dome (CD) of Mortimer and Coleman (1984). Symbols also indicate the homoclinal SE dip of Devonian to Jurassic strata of the Redding section.
443
NE domain includes imbricate thrust slices that trend lengthwise to the terrane and dip SE. Neither the precise location nor the nature of the boundary between the two domains is known. The location of the southern part of the boundary between the two domains is particularly equivocal. The Shasta Bally batholith trends NW, parallel to the trend of the plutonic belts of the SW domain and nearly at right angles to the principal regional structures of the Eastern Klamath terrane, and thus would seem to belong to the SW domain. On the other hand, Shasta Bally batholith is part of the belt of youngest plutons of the province, some of which intrude the main expanse of the Trinity ultramafic sheet. Farther northward the domain boundary may be represented by the NWtrending Browns Meadow fault of Davis (1968), which is near the abrupt change in the Central Metamorphic terrane from the broad synformal thrust plate (Douglas City synform) of the SW domain to the narrow northward continuation of the terrane in the NE domain (Fig.5). In the Western Paleozoic and Triassic belt, the small Forks of Salmon pluton is the northernmost pluton of the Ironside Mountain belt and narrowly constrains the boundary between that belt and the English Peak pluton of the Wooley Creek belt. The trend in the western boundary of the Western Jurassic terrane changes abruptly from NW to NE a few kilometers NE of Crescent City and may represent a point on the domain boundary. At that point, the upper units of the Josephine ophiolite sequence and the overlying Galice Formation of the NW-trending part of the terrane are abruptly truncated and then continue with considerable disruption along the NE trending part of the terrane. Although the Josephine ophiolite south of the latitude of Crescent City trends SE, suggestive of the SW domain, the greatest disruption of the ophiolite is approximately 55 km along its trace to the SE to where there is a sharp embayment in the boundary of the province. South of the embayment, the Josephine ophiolite is present only as occasional fragments and narrow fault slivers along the western boundary of the province. SEQUENCE OF TERRANE AMALGAMATION
Several kinds of evidence determine the times and sequence of amalgamation of the various terranes. These incude the age of the youngest strata of the subducting terrane, the age of the metamorphism that in some instances affects the rocks of the lower plate adjacent to the terrane suture as a result of underthrusting, the age of postamalgamation plutons, and the age of strata that overlap the terrane boundary. The time of suturing of the Eastern Klamath and Central Metamorphic terranes is established by the Early or Middle Devonian isotopic age (Lanphere et al., 1968; Hotz, 1977) of the metamorphism that was caused by the overriding of the protoliths of the schist along the Bully Choop thrust (Irwin, 1963) by the Trinity ultramafic sheet. The suture is intruded by Early Cretaceous plutons of the Shasta Bally belt, and, like sutures between other Klamath terranes, is overlain by Cretaceous strata of the Great Valley sequence at the southeast end of the province.
444
The suture between the Central Metamorphic and North Fork terranes is the Siskiyou thrust of Davis (1968). The youngest dated strata of the North Fork are radiolarian chert and tuff of Early Jurassic (Pleinsbachian) age (Blome and Irwin, 1983). The thrusting has metamorphosed some of the rocks along the fault, but isotopic ages of 133 and 158 Ma measured on these rocks (Lanphere et al., 1968) are equivocal. The Siskiyou thrust is intruded by the small East Fork pluton (biotite, 164 Ma; hornblende, 149 Ma; M.A. Lanphere, written commun., 1983) and Russian Peak pluton (144-147 Ma; Romey, 1962; Evernden and Kistler, 1970), and by the Early Cretaceous Deadman Peak pluton. Thus the age of the suture is not precisely known but is in the range of ]~arly Jurasmc (Pleinsbachian) to Late Jurassic. The next lower (westerly) terrane in the order of structural stacking is the Hayfork. A zone of metamorphism is unapparent along the boundaries between the Hayfork and either the structurally higher (North Fork) or the next lower (Rattlesnake Creek) terrane. The plutons of the Ironside Mountain belt ( ~ 170 Ma) of the SW domain, which form a plutonic--volcanic pair with the host rocks, do not cross the terrane boundary; but the plutons of the Wooley Creek belt ( ~ 163 Ma) of the NE domain are reported to cross not only the boundary of the structurally higher terrane (Mortimer, 1984) but also the boundary of the structurally lower terrane (Barnes, 1983). These relations suggest that the ages of both sutures are between 170 and 163 Ma, and are compatible with the Middle Jurassic isotopic age (168 177 Ma; Fahan, 1982) of the Hayfork Bally Meta-andesite. The Western Jurassic terrane has underthrust the Rattlesnake Creek terrane of the SW domain and also the rocks of the undivided Western Paleozoic and Triassic belt of the NE domain. In the NE domain, the Galice Formation of the Western Jurassic terrane contains Upper Jurassic (Oxfordian to middle Kimmeridgian) fossils (Imlay, 1959). Much of the Galice in the SW domain has been metamorphosed to semischist that has yielded Late Jurassic (late Kimmeridgian and early Tithonian) isotopic ages (148-153 Ma; Lanphere et al., 1978). The boundary between the Western Jurassic terrane and the rocks of the Western Paleozoic and Triassic belt is intruded by Early Cretaceous plutons of the Grants Pass belt ( ~ 140 Ma) in the NE domain, but postamalgamation plutons are not known in equivalent rocks of the SW domain. These relations indicate that the amalgamation of the Western Jurassic terrane probably was latest Jurassic (Tithonian) and certainly no later than earliest Cretaceous. Thus the amalgamation of the Klamath terranes was mainly a series of Jurassic tectonic events, except the early Paleozoic suturing of the Eastern Klamath and Central Metamorphic terranes, and was sequential oceanward. The sequence of amalgamation does not end at the western border of the province, but continues oceanward with the amalgamation of Early Cretaceous schist (120 Ma; Lanphere et al., 1978) and Jurassic and Cretaceous graywacke formations of the Coast Range province. The various times of amalgamation during the Jurassic are fairly well constrained in most instances to only a few million years.
445 ACCRETION TO NORTH AMERICA Comparable evidence for the time of accretion of the K l am at h terranes to N or th America is unavailable because the t e r r a n e boundaries are concealed along the eastern perimeter of the province by a broad mantle of Cretaceous and y o u n g er strata. However, some evidence for the time of accretion to North America is provided by paleomagnetic data. Results of paleomagnetic studies indicate t hat the plutons and other elements of the NE domain have rotated clockwise substantially relative to stable No r th America. However, comparison with the SW domain is now infeasible because paleomagnetic data are unavailable for plutons of the SW domain except for the Shasta Bally batholith. Five plutons in the Western Paleozoic and Triassic belt of the NE domain show clockwise rotations ranging from 53 ° to 113 ° except the Ashland pluton which may show small counterclockwise r o t a t i o n (Schultz, 1983). All are Jurassic except the Grants Pass pluton which is Early Cretaceous. These rotations compare favorably with rotations of about 60 ° for Lower to Middle Jurassic strata of the Eastern K l amath t e r r a n e (Mankinen et al., 1984). Sparse paleomagnetic data suggest t h a t the large angular difference in trend between the Cretaceous plutonic belts (Grants Pass and Shasta Bally) of the two domains is accompanied by a comparable difference in magnetic orientation. The Grants Pass pluton has rotated approximately 78° _ 33 ° clockwise relative to stable N or t h America (Schultz, 1983), whereas preliminary results from measurements on the Shasta Bally batholith indicate little or no rot at i on (E.A. Mankinen, pers. commun., 1985). Rotation of the Grants Pass pluton back to its apparent original orientation would align it more nearly parallel to the Shasta Bally batholith. Except for the plutons just mentioned, the only substantial paleomagnetic data available for the province pertain to the Redding section of the Eastern K l amath terrane. The original or i e nt a t i on of the pre-Permian rocks of the Redding section is unknown. However, the Permian and Triassic strata have
Pre latest Tdassic
Post-earliest Cretaceous
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c':'::3:d assemblage
Fig.6. Schematic diagram showing the sequence of rotation of the volcanic island arc represented by the Redding section of the Eastern Klamath terrane and its accretion to North America. There is no paleomagnetic evidence for substantial latitudinal displacement of the terrane.
446 r o t a t e d c l o c k w i s e a b o u t 100 ° , a p p r o x i m a t e l y 40 ° of w h i c h o c c u r r e d b e f o r e t h e d e p o s i t i o n of t h e J u r a s s i c s t r a t a ( M a n k i n e n et al., 1984). T h e s e r o t a t i o n s a r e a s s u m e d to h a v e i n c l u d e d t h e p r e - P e r m i a n s t r a t a a n d t h e p r e a m a l g a m a t i o n plutons. The l o n g - s t a n d i n g v o l c a n i c arc r e p r e s e n t e d by the R e d d i n g section m u s t h a v e f a c e d S W d u r i n g P e r m i a n a n d T r i a s s i c t i m e , r o t a t e d to face W by E a r l y or M i d d l e J u r a s s i c , a n d c o n t i n u e d r o t a t i n g to a t t a i n e s s e n t i a l l y its p r e s e n t N W f a c i n g o r i e n t a t i o n b y E a r l y C r e t a c e o u s t i m e (Fig.6). T h e t i m e of a t t a i n i n g i t s p r e s e n t f a c i n g is i n d i c a t e d b y t h e v i r t u a l l a c k of r o t a t i o n of t h e S h a s t a B a l l y b a t h o l i t h a n d of t h e L o w e r C r e t a c e o u s ( H a u t e r i v i a n ) s t r a t a of t h e G r e a t V a l l e y o v e r l a p s e q u e n c e ( M a n k i n e n a n d I r w i n , 1982). T h e v i r t u a l c e s s a t i o n of r o t a t i o n , c l o s e l y f o l l o w e d b y d e p o s i t i o n of t h e o v e r l a p s e q u e n c e , p r o b a b l y r e p r e s e n t s t h e t i m e of a c c r e t i o n to N o r t h A m e r i c a . REFERENCES Albers, J.P. and Robertson, J.F., 1961. Geology and ore deposits of the East Shasta copper-zinc district, Shasta County, California. U.S. Geol. Surv. Prof. Pap., 338, 107 pp. Albers, J.P., Kistler, R.W. and Kwak, L., 1981. The Mule Mountain Stock, an early Middle Devonian pluton in northern California. ISOCRON/West, 31: 17. Allen, C.M., Barnes, C.G., Kays, M.A. and Saleeby, J.B., 1982. Comagmatic nature of the Wooley Creek batholith and the Slinkard pluton and age constraints on the tectonic and metamorphic events in the western Paleozoic and Triassic belt, Klamath Mountains, N. California. Geol. Soc. Am., Abstr. Progr., 14: 145. Ando, C.J., Irwin, W.P., Jones, D.L. and Saleeby, J.B., 1983. The ophiolitic North Fork terrane in the Salmon River region, central Klamath Mountains, California. Geol. Soc. Am. Bull., 94: 236-252. Barnes, C.G., 1983. Petrology and upward zonation of the Wooley Creek batholith, Klamath Mountains, California. J. Petrol., 24:495 537. Blake, M.C., Jr., Howell, D.G. and Jones, D.L., 1982. Preliminary tectonostratigraphic terrane map of California. U.S. Geol. Surv. Open File Rep., 82-593. Blakely, R.J., Jachens, R.C., Simpson, R.W. and Couch, R.W., 1985. Tectonic setting of the southern Cascade Range as interpreted from its magnetic and gravity fields. Geol. Soc. Am. Bull., 96: 43-48. Blome, C.D. and Irwin, W.P., 1983. Tectonic significance of Late Paleozoic to Jurassic radiolarians in the North Fork terrane, Klamath Mountains, California. In: C.H. Stevens (Editor), PreJurassic Rocks in Western North American Suspect Terranes. Society of Economic Paleontolo~ gists and Mineralogists, Pacific Section, pp.77- 89. Coney, P.J., Jones. D.L. and Monger, J.W.H., 1980. Cordilleran suspect terranes. Nature, 288: 329 333. Curtis, G.H., Evernden, J.F. and Lipson, J., 1958. Age determination of some granitic rocks in California by the potassium-argon method. Calif. Div. Min. Spec. Rep., 54, 16 pp. Davis, G.A., 1961. Metamorphic and Igneous Geology of Pre-Cretaceous Rocks, Coffee Creek Area, Northeastern Trinity Alps, Klamath Mountains, California. Ph.D. thesis, University of California, Berkeley, Calif., 199 pp. Davis, G.A., 1968. Westward thrust faulting in the south-central Klamath Mountains. Geol. Soc. Am. Bull., 79:911 934. Donato, M.M., Barnes, C.G., Coleman, R.G., Ernst, W.G. and Kays, M.A., 1982. Geologic map of the Marble Mountains Wilderness, Siskiyou County, California. U.S. Geol. Surv., Miscellaneous Field Studies Map Ml$:1452-A,scale li48,000. Evernden, J.F. and Kistler, R.W., 1970. Chronology of emplacement of Mesozoic batholithie complexes in California and western Nevada. U.S. Geol. Surv. Prof. Pap., 623, 42 pp.
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