Kings River Ophiolite, Southwest Sierra Nevada Foothills, California

Kings River ophiolite, southwest Sierra Nevada foothills, California

JASON SALEEBY Department of Geology and Geophysics, University of California, Berkeley, California 94720

ABSTRACT INTRODUCTION

In the lower Kings River area, rocks older than the Sierra Nevada Distinctive rock sequences consisting (from bottom to top) of batholith include a disrupted and metamorphosed ophiolite. The peridotite, gabbro, mafic dikes, and pillow basalt are referred to as Kings River ophiolite consists of tectonically emplaced slabs as ophiolites. Ophiolites are believed to represent fragments of oceanic much as 20 km long that are separated by serpentinite-matrix crust and upper mantle that were detached and emplaced into melange zones and by crosscutting plutons of the batholith. Within orogenic zones along once-active plate junctures (Dietz, 1963; the slabs, various segments of the original ophiolite section are Thayer, 1969; Coleman, 1971a; Dewey and Bird, 1971; Church, preserved. From the base upward, the reconstructed section con- 1972; Brookfield, 1977). The widespread occurrence of ophiolite sists of (1) a harzburgite zone (more than 4 km thick), (2) a transi- remnants now documented in the Sierra Nevada of California tion zone between ultramafic and mafic tectonites and cumulates (Morgan, 1973; Menzies and others, 1975; Moores and Menzies, (2.5 km thick), (3) a gabbro zone with cumulates (2 km thick), (4) a 1975; Ehrenberg, 1975; Saleeby, 1975a, 1975b, 1975c, 1977a, mafic-dike zone (0.7 km thick), and (5) a pillow-basalt zone (1.8 1977b; Behrman, 1978) is important because they record the role km thick). The pillow basalt is overlain by at least 20 m of metal- of plate-margin processes and the involvement of oceanic liferous radiolarian chert. After tectonic mixing and emplacement lithosphere in the pre-batholith tectonic evolution of the Sierran into the Sierran terrane, the ophiolite was metamorphosed to the terrane. hornblende-hornfels facies by the batholith. Ophiolite remnants exposed almost continuously as a 125-km- The Kings River ophiolite is interpreted as a disrupted fragment long northwest-trending belt in the southwestern Sierra Nevada of oceanic crust and upper mantle. Isotopic ages along with struc- foothills are referred to informally as the Kings-Kaweah ophiolite tural and petrographic data indicate that the igneous part of the belt (Fig. 1, inset map). The Kings-Kaweah ophiolite belt forms section originated in latest Paleozoic or possibly earliest Mesozoic part of the western wall of the Sierra Nevada batholith in the re- time. Intense deformation of the ophiolite began at its point of ori- gion. The Kings River ophiolite is located at the northern end of the gin. Deep levels of the ophiolite were penetratively mylonitized, in- belt; it is exposed for 33 km along strike and covers an area of 340 termediate levels were deformed by ductile faulting, and upper km2. The purpose of this paper is to (1) describe the general setting levels were deformed by brittle shear. As deformation and disrup- of the Kings River ophiolite and its relation with the rest of the tion progressed, serpentinization of the ophiolite's lower levels also Kings-Kaweah ophiolite belt, (2) describe in detail the petrology progressed. Serpentinization and differential tectonic movements and structure of the Kings River ophiolite, and (3) discuss the were concentrated along zones that became serpentinite-matrix emplacement and related deformation of the ophiolite. A more in- melange. The inclusion of only ophiolite-assemblage rocks in the depth discussion of the emplacement and regional tectonic sig- melange zones indicates that the melange mixing was oceanic. nificance of the entire ophiolite belt is presented elsewhere (Saleeby, The ophiolite originated and began its deformational history at a 1977b, and ms. in prep.). mid-ocean spreading center where that center was cut by a trans- Prior to my work, the northern part of the ophiolite had been verse fracture zone. The progression from brittle to ductile be- mapped in reconnaissance and studied petrographically by Mac- havior with stratigraphic depth during initial deformation is at- donald (1941). The southern part had been mapped in reconnais- tributed to a steep thermal gradient, typical of an ocean ridge. sance (Mathews and Burnett, 1966). Neither of these studies rec- Progressive deformation and disruption and, ultimately, ophiolite ognized the existence of an ophiolite. Mafic and ultramafic rocks of emplacement occurred along a wrench zone that cut obliquely into the region have been generally interpreted as magmatically western North America and truncated earlier-formed tectonic ele- emplaced "forerunners" of the Sierra Nevada batholith (Mayo, ments. The wrench zone is believed to have been an extension of 1941; Bateman and others, 1963). It will be shown that rocks of the mid-ocean fracture zone that widened and became more com- the Kings River ophiolite are not magmatic forerunners of the plex with time. During the later stages of wrench movement, a batholith and that the ophiolite is of a petrogenetic domain re- component of eastward underthrusting commenced. Disrupted moved from that of the batholith in terms of both time and space. ocean floor of the wrench zone was left as an accretionary hanging Magmatically emplaced mafic rocks do occur in the vicinity of the wall of a newly formed subduction zone. A Jurassic volcanic arc Kings River ophiolite (Saleeby, 1975c, 1976a, 1976b; Saleeby and was built across the already weakened oceanic basement as it un- Sharp, 1977), but these rocks along with other batholithic rocks derwent transverse shortening and continued wrench movements in will not be discussed in depth here. response to oblique subduction. Final truncation of North Ameri- can tectonic elements and emplacement of the ophiolite probably GEOLOGIC SETTING overlapped in time with arc activity. Similar deformation and truncation zones are a common feature in modern subduction-arc The Kings-Kaweah ophiolite belt is exposed along the western complexes of the circum-Pacific. edge of the Sierran foothills between lat 36°00'N and 37°00' (Fig.

Geological Society of America Bulletin, v. 89, p. 617-636, 7 figs., April 1978, Doc. no. 80413.

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Figure 1. Geologic location map showing entire Kings-Kaweah ophiolite belt and area covered in adjoining detailed geologic map of the

1, inset map). The southern half of the belt consists of a tectonic since some of them contain mappable internal stratigraphic units. melange in which the various rock types of the ophiolite assem- Collectively, these slabs are the Kings River ophiolite. blage are mixed within a schistose serpentinite matrix. The A complete ophiolite succession is preserved within the tectonic melange is referred to informally as the Kaweah serpentinite slabs of the Kings River area. The slabs are separated by melange melange and is discussed in detail elsewhere (Saleeby, 1975a, zones containing ophiolitic material and by crosscutting plutons of 1975b, 1975c, 1977a, 1977b). North of the Kaweah River area the the Sierra Nevada batholith (Fig. 1). The slabs are elongated in a tectonic blocks increase in size (Fig. 1, inset map). The large blocks northwest direction and range from 5 to 20 km in length. The slabs of the Kings River area are referred to as slabs (after Hsu, 1968), form the topographic highs in the area and are named informally

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Kings River ophiolite. Geology by Saleeby supplemented by Macdonald (1941), Matthews and Burnett (1966), and W. D. Sharp (unpub. data).

after the highest peaks that they underlie. The Bald Mountain and The melange zone that bounds the east side of the Hog Mountain Hughes Mountain slabs consist mainly of metamorphosed pillow slab separates the ophiolite from metasedimentary rocks of the basalt, breccia, and mafic dikes. The Tivy Mountain and Hog Calaveras Complex (Saleeby and Goodin, 1977). The melange zone Mountain slabs consist mainly of metamorphosed gabbro and that bounds the western margin of the Tivy Mountain slab is simi- peridotite. lar to the Kaweah melange and forms the western known border of Contacts between ophiolitic slabs and adjacent melange zones the ophiolite. are vertical and gradational over short distances. Exotic blocks of Plutons ranging in composition from olivine-hornblende mainly metabasalt and metachert characterize the melange zones. melagabbro to biotite quartz diorite intruded and metamorphosed

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GENERALIZED RECONSTRUCTED OPHIOLITE SECTION OCEANIC uj co BASEMENT PROTOLITHS METAMORPHIC DERIVATIVES 20m km o —I METALLIFEROUS AND PURE METAQUARTZITE LAYER OQ RAD IOLARIAN CHERT 2 PILLOW BASALT ZONE PILLOW BASALT WITH LOCAL INTER- MAFIC HORNBLENDE BASALT, 7.0km PILLOW CHERT, PILLOW BRECCIA, HORNFELS AND SCHISTOSE DIABASE MASSIVE BASALT AND MAFIC DIKE ROCK AMPHI BOLITE

Figure 2. Reconstructed co- MAFIC DIKE ZONE lumnar section of the Kings LAYER BASALT-DIABASE DIKES WITH LOCAL MAFIC HORNBLENDE River ophiolite. Brief lithologie 0.7km DIKES AND SCREENS OF CLINOPYROXENE HORNFELS AND SCHISTOSE 3A GABBRO AMPHI BOLITE descriptions given in terms of

DIABASE, both protoliths and metamor- GABBRO, phic derivatives. Lines to left of LOW GRADE GABBRO ZONE METAGABBRO CLINOPYROXENE (1H0RNBLENDE OR MAFIC HORNBLENDE columnar section represent OLIVINE) GABBRO CUMULATES AND HORNFELS, SCHISTOSE section intervals taken from DIKES WITH LOWER LEVELS CONTAINING AMPHI BOLITE AND TREMOLITE± 2km CLINOPYROXENITE AND WEHRLITE Mg-CHLOR ITE±TALC HORNFELS ophiolitic slabs and blocks used CUMULATES AND SCHIST to reconstruct the sequence and LAYER thickness. Also shown for com- JB parison is a generalized oceanic basement section after Ludwig

GABBRO, TRANSITION ZONE and others (1970), Sutton and PARTIALLY INTERLAYERED TECTONITIC OUNITE, TALC-ANT I GORITE HORNFELS others (1971), Christensen and SERPENT INIZED HARZBURGITE AND WEHRLITE WITH AND SCHIST, LOCAL TALC- PER I DOT I TE, INCLUSIONS AND DIKES OF CLINO" MAGNESITE HORNFELS, MAFIC Salisbury (1975), and Clague GABBRO- PYROXENITE, GABBRO, DlORITE AND AND FELSIC HORNBLENDE and Straley (1977). PERIDOT I TE PLAGIOGRANITE, AND VEINS OF TALC- HORNFELS, AND AMPHIBOLITE MIXTURE •2.5km Cr-CHLORITE-SERPENT I NE ROCK GNEISS

MOHO

MANTLE HARZBURGITE ZONE TECTONITIC HARZBURGITE AND DUNITE TALC-ANT I GOR ITEiTREMOLI TE WITH LOCAL INCLUSIONS OF CHROMITE- HORNFELS AND SCHIST, »4km OLIVINE CUMULATE, WEHRLITE AND LOCAL TALC-MAGNESITE GABBRO HORNFELS, AND AMPHI BOL ITE GNEISS

the disrupted ophiolite between 169 m.y. and 98 m.y. ago (Saleeby, the suture is interpreted to represent a fossil continental-oceanic 1975c, 1976a, 1976b). Contact metamorphism is nearly penetra- contact (Kistler and Peterman, 1973; Saleeby, 1975c, 1976b; Chen, tive and in the hornblende-hornfels facies as defined by Turner 1977). (5) The Calaveras Complex and its internal structures are (1968). Crosscutting plutons of Jurassic age are schistose generally truncated by the suture, and similar rocks are apparently missing parallel to structures within the ophiolite (Saleeby, 1975c, 1976a; west of the suture (Schweickert, 1976a; Schweickert and others, Saleeby and Sharp, 1977). However, the intensity of deformation 1977; Saleeby and Goodin, 1977). features is far greater in the ophiolite, and furthermore, the Jurassic The Sierran foothill suture is thus an important regional tectonic plutons are structurally intact. Therefore, the Jurassic plutons were feature. My work has thus far shown that its active history is long emplaced late in the deformational history of the ophiolite after and complex and that a meaningful understanding of it will only significant tectonic mixing ceased. Crosscutting plutons of Creta- come through the integration of a vast amount of detailed struc- ceous age lack deformational features and thus postdate deforma- tural, petrologic, and geochronologic data. The Kings River tion of the ophiolite. ophiolite represents the only complete ophiolite known to exist The Kings-Kaweah ophiolite belt represents a tectonic suture in along the suture. Thus, the structural and petrologic details of the the Earth's crust (Saleeby, 1975c, 1977a, 1977b). The suture ap- ophiolite are of regional significance. parently extends at least 200 km to the northwest as the Sierran foothills fault system (Clark, 1960; Schweickert and others, 1977). THE OPHIOLITE PRIOR TO DISRUPTION The importance of the Sierran foothill suture is exemplified by the following: (1) All known Sierra Nevada ophiolite fragments occur A continuous, intact ophiolite section does not exist in the study along it (2) The high degree of deformation and tectonic mixing area, although one is inferred to have existed prior to tectonic dis- displayed along it (Clark, 1960,1964; Morgan, 1973; Duffield and ruption. A complete ophiolite lithologic assemblage is present, and Sharp, 1975; Ehrenberg, 1975; Saleeby, 1975a, 1975b, 1977a; the inferred originally intact section can be reconstructed from field P. G. Behrman, 1977, unpub. data) indicates that it was a zone of and petrographic data. The reconstructed columnar section of the large translative movement. (3) There are distinct changes in the ophiolite is shown in Figure 2. The lithologic descriptions to the gross structure of the crust and upper mantle that coincide with the right of the columnar section are in terms of both the protoliths and suture, and these changes can be best explained by its representing their metamorphic derivatives. The protoliths are readily deduced a fossil contact between pre-Cretaceous oceanic and continental since relict primary minerals, textures, and structures are common lithosphere (Cady, 1975). (4) Jurassic and younger plutonic rocks (Figs. 3, 4). The thicknesses shown in Figure 2 must be considered emplaced magmatically into and to the east of the suture have sys- approximate because of the disrupted state of the ophiolite. The tematic petrochemical variations that can also be best explained if thicknesses are taken from the Tivy Mountain and Bald Mountain

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slabs, and most of the discussion of the ophiolite section is based on mafic rocks of the transition and harzburgite zones. The first these two slabs. They span the entire ophiolite section and are spa- episode occurred prior to or during mylonitization of the lower tially related in such a way that a nearly continuous section could part of the section. Talc and Cr-chlorite also formed during early- be traversed if the intervening crosscutting pluton were removed stage serpentine alteration. The alteration zones are concentrated and the slabs fitted back together. The ophiolite zones of the Tivy around meta-igneous bodies containing diorite, plagiogranite, and Mountain and Bald Mountain slabs are also shown in Figure 1. gabbro with relict primary hornblende. The meta-igneous bodies along with the alteration zones were highly deformed during Description of the Ophiolite Section mylonitization of the ophiolite's lower levels. Deformational features of these rocks are discussed in the section on disruption of the Harzburgite Zone. The restored Kings River ophiolite section is ophiolite. The second episode occurred during disruption of the similar to ophiolite sections described from numerous other ophiolite and formation of the serpentinite melange zones. These localities. As with many of these localities (Dewey and Bird, 1971; serpentinites are highly schistose and are also discussed in the sec- Church, 1972), the base of the Kings River section is composed of a tion on ophiolite disruption. The third episode of serpentine great thickness of interlayered dunite-harzburgite tectonite (Figs. growth was during contact metamorphism of the ophiolite. During 3a, 3b). Relict igneous textures have not been recognized in the this episode, granoblastic clusters of antigorite and talc were dunite-harzburgite tectonite. Sparse pods of chromite-olivine superimposed over many of the earlier features (Fig. 3b). Antigorite cumulates (Fig. 3c) and cumulate dunite and wehrlite have been now predominates over chrysotile and lizardite throughout the found, but their original relationships with the dunite-harzburgite ophiolite. This is believed to be primarily a result of contact tectonite have been obscured by deformation and metamorphism. metamorphism with antigorite being the stable serpentine mineral Scattered grains of chrome spinel also occur in the dunite- under hornblende-hornfels facies conditions (Turner, 1968; Cole- harzburgite tectonite. Small highly deformed mafic inclusions simi- man, 1976b). lar to those present in the overlying transition zone also occur Gabbro Zone. Above the transition zone is the 2-km-thick gab- sporadically in the basal harzburgite zone. Most of these inclusions bro zone that consists mainly of clinopyroxene gabbro and are amphibolite-facies metamorphic tectonites that lack relict pri- hornblende-clinopyroxene gabbro. Patches of plagioclase mary features. Some contain relict primary minerals and textures clinopyroxenite and olivine clinopyroxenite also occur near the that indicate derivation from gabbro and lesser clinopyroxenite. base of the gabbro zone. Cumulate structures and textures are Preliminary bulk chemical work shows that the amphibolite inclu- common in the gabbro zone. The upper 1.5 km of this zone is com- sions are similar to clinopyroxene gabbro of the transition and posed of uniform plagioclase-clinopyroxene adcumulates and rare gabbor zones. mesocumulates. Brown hornblende occurs as a postcumulus phase Transition Zone. The transition zone is 2.5 km thick and in the mesocumulates and as an important primary phase in lithologically heterogeneous; it consists of rocks similar to the basal hornblende-clinopyroxene gabbro dikes that cut the cumulate harzburgite zone and overlying gabbro zone. The bottom of the rocks. Diabase dikes similar to those of the overlying mafic-dike transition zone is defined by the near absence of mafic rock; the top zone locally cut through the gabbro zone. Diorite and plagiogranite is defined by the predominance of mafic rock. Extreme deformation are absent in the gabbro zone. has obscured original relationships within this zone, but it appears Penetrative mylonitization typical of the underlying transition to be characterized by mafic dikes cutting mainly harzburgite with and harzburgite zones dies out in uppermost levels of the transition layers of dunite and wehrlite. The dikes consist mainly of clinopy- zone and in the lowermost levels of the gabbro zone. Higher in the roxene and hornblende-clinopyroxene gabbro with lesser amounts section, mylonitization is concentrated along narrow belts. These of clinopyroxenite. Most of the dikes have been deformed into mylonite belts bound nondeformed domains in which relict pri- linear and tabular inclusions (Fig. 1). Toward the top of the transi- mary features are well preserved. Consistent with the trend of de- tion zone, wehrlitic layers with relict cumulate textures become an creasing intensity of mylonitization, mylonites are apparently lack- important component in the ultramafic host, and some of the mafic ing in the overlying mafic-dike and pillow-basalt zones. bodies appear as sill-like layers that also have relict cumulate tex- Mafic-Dike Zone. The mafic-dike zone consists of a 0.7-km- tures and structures (Fig. 4a). thick section of mainly basalt and diabase dikes (Figs. 3e, 4b). The The transition zone is also characterized by trace amounts of dikes extend up into the pillow-basalt zone, but the top of the diorite and plagiogranite. These rocks occur as pods as much as 3 mafic-dike zone is defined as the base of the lowermost pillows. m in diameter in ultramafic rock and as dikes as much as 30 cm in Sheeting structure similar to that found in the Troodos ophiolite thickness in mafic rock. They are devoid of K-bearing minerals; (Moores and Vine, 1971) is a common feature, but it has not been preliminary chemical work shows that they contain less than 0.2% found to occur in more than six successive dikes and commonly in

K20. A complete gradation exists between hornblende- no more than three dikes. The sheeted dikes are commonly cut at clinopyroxene gabbro and diorite and between diorite and plagio- various angles by thicker nonsheeted dikes. Thickness of sheeted granite. This is manifested by (1) disappearance of clinopyroxene, dikes ranges from 3 to 30 cm. Nonsheeted dikes are as much as 6 m (2) decrease and then disappearance of hornblende, (3) change in thick. Dike breccias similar to those found in the Bay of Islands relict primary hornblende from brown to green, (4) plagioclase ophiolite (Williams and Malpas, 1972) occur locally. Contradic- composition becoming progressively more sodic, and (5) appear- tory intrusive relationships occur between the basalt and diabase ance of quartz. Diorite and plagiogranite are volumetrically insig- dikes and the subordinate gabbro masses. The gabbro is similar to nificant relative to other rock types of the transition zone. The facts hornblende-clinopyroxene gabbro and clinopyroxene gabbro of the that these trace rock types are gradational with gabbro and that underlying gabbro zone. In the mafic-dike zone the gabbro occurs they both cut and are cut by gabbro indicate that they are an inte- both as dikes cutting basalt and diabase and as small screens cut by gral part of the ophiolite. basalt and diabase. Most phenocrysts in the basalt and diabase of Three episodes of serpentine growth are recorded in the ultra- the mafic-dike zone are Ca-plagioclase, but clinopyroxene and

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/4/617/3444453/i0016-7606-89-4-617.pdf by guest on 26 September 2021 Figure 3. Photomicrographs showing deformation and relict primary textures (magnification about 8 x, except Fig. 3f, which is 24 x ). OL = olivine. OPX = orthopyroxene. ANT = antigorite. CHR = chromite. CPX = clinopyroxene. HB = hornblende. PL = plagioclase. (a) Harzbuigite protomylonite. (b) Harzburgite mylonite showing strong hornfelsic overprinting by antigorite. (c) Olivine-chromite cumulate showing faint cumulate layering, (d) blas- tomylonitic or hornfelsic mylonitic clinopyroxene gabbro. (e) Relict ophitic texture in hornfelsic diabase, (f) Relict radiolarian tests in completely recrystallized chert (note abundance of fine disseminated opaque minerals, faint flattening of tests, and abundance of quartz-filled brittle fractures). Mylonitic banding in d, a, and b is S,.

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possible relict olivine phenocrysts also occur at several localities. oceanic seismic velocity sections (Ludwig and others, 1970; Sutton Diorite and plagiogranite are apparently lacking in the mafic-dike and others, 1971; Christensen and Salisbury, 1975; Clague and zone. Straley, 1977) (Fig. 2). (3) The physical features of the volcanic Pillow-Basalt Zone. The mafic-dike zone grades upward into a rocks, particularly the nonvesicular pillow rinds, indicate eruption 1.8-km-thick pile of pillowed and locally massive basalt and basalt in deep water. (4) Possible interpillow pelagic sediments and pillow breccia. Metamorphic recrystallizarion of the pillow basalt radiolarian chert deposited on the upper surface of the ophiolite is complete. Pillows range from 5 cm to 2m in diameter (Fig. 4c). indicate that the igneous part of the section evolved and remained Vesicles in pillow rims appear to be lacking. This is not a result of for some time under deep water and beyond the reach of terrige- metamorphism since vesicles and amygdules in pillow interiors and nous sedimentation. In this section the petrogenesis of the Kings rarely in massive basalts have survived metamorphism. Thus, a River ophiolite is discussed in the light of what is known about deep-water origin for the pillow basalt is indicated (Moore and ocean-floor genesis. Schilling, 1973). Breccias of this zone consist mainly of pillows and Harzburgite Zone. The basal harzburgite zone is interpreted as pillow fragments. However, nonsorted breccias containing angular oceanic upper mantle. Three hypotheses on the relationships be- clasts of massive basalt and rare diabase also occur locally. Contact tween the harzburgite and the rest of the ophiolite section are con- relations between these breccias and adjacent pillow basalt could sidered: (1) the harzburgite is petrogenetically linked to the rest of not be established owing to insufficient exposures. Interpillow the section and forms the lowermost cumulates of the section. (2) siliceous sedimentary deposits occur locally; these are probably the harzburgite is depleted refractory material from which the rest biogenic in origin, but any diagnostic features that may have of the section fractionated. (3) The harzburgite is not petrogeneti- existed have been destroyed by metamorphism. The pillow basalts cally linked to the rest of the section, but is older mantle substrate are generally aphyric; however, small granoblastic clusters of upon which the rest of the section was built. Unfortunately, intense metamorphic plagioclase or hornblende occur commonly in pillow deformation and contact metamorphism have obliterated diagnos- interiors. These may be relict plagioclase or clinopyroxene micro- tic features that would allow a definite choice between these three phenocrysts. Relict plagioclase and clinopyroxene phenocrysts hypotheses. Cumulate processes have operated in the harzburgite occur in some pillows and are common in dike rocks of this zone. zone, but in each case the mineral assemblage is olivine ± chromite Preliminary bulk chemical data show that the metamorphosed pil- ± clinopyroxene. The cumulates lack orthopyroxene. Furthermore, low basalts and metamorphosed basaltic dike rocks are similar to orthopyroxene is lacking as a definite igneous phase throughout the abyssal tholeiites (Kay and others, 1970; Shido and Miyashiro, entire ophiolite section. This suggests that the harzburgite did not

1971) with 47% to 50% Si02 and less than 0.2% KzO. Felsic rocks originate as the lowermost cumulate of the ophiolite. The cumulate appear to be lacking in the pillow-basalt zone. pods that do occur within the harzburgite zones are thus inter- Radiolarian Chert. The top of the ophiolite section is capped by preted as isolated masses of early-formed crystals that settled out radiolarian chert. The largest mass of radiolarian chert found in and were left behind by magma bodies that rose through the depositional contact with pillow basalt is less than 3 m in thickness. harzburgite and formed the upper part of the ophiolite section. Well-bedded chert blocks that range up to 20 m in thickness are, Similar pods of ultramafic cumulates exist within harzburgite however, common in melange zones that bound the ophiolitic tectonite in the Vourinos ophiolite of Greece (Harkins, 1976) and slabs. The chert melange blocks are interpreted as having rested on in the Semail ophiolite of Oman (Allemann and Peters, 1972) and top of the ophiolite prior to disruption. This is supported by the are common in Alpine-type peridotite massifs throughout the fact that the chert blocks are similar to both the small chert masses world (Thayer, 1970). It is tempting to speculate that the magmas situated within the ophiolitic slabs and other numerous chert which gave rise to the cumulate pods and upper zones of the melange blocks that occur along the entire length of the Kings- ophiolite were derived from nondepleted oceanic mantle [as dis- Kaweah ophiolite belt (Saleeby, 1975a, 1975b, 1975c, 1977b). cussed by Green and Ringwood (1967), Kay and others (1970) and Thus a minimum stratigraphic thickness of 20 m is shown on the Green (1970, 1971)] and that the harzburgite zone represents the columnar section in Figure 2. The cherts are rhythmically bedded depleted refractory material from which these magmas fraction- with gray or light purple, nearly pure silica layers ranging from 1 to ated. This speculation is consistent with a model for ophiolite 5 cm in thickness and black to dark purple impure layers ranging genesis in which the ophiolite-forming fractionation event occurs at from 1 mm to 2 cm in thickness. The impure layers are enriched in an oceanic spreading center. This view of ophiolite genesis is metalliferous minerals and are notably lacking in pelitic minerals. adopted here, but it must be emphasized that evidence indicating Metamorphic recrystallization has obliterated or highly modified that the igneous part of the Kings River section is related spe- radiolarian tests (Fig. 3f); thus, the age of the chert is unknown. cifically to the harzburgite of the harzburgite zone is lacking. Thus, Several chert blocks contain relict soft-sediment deformation struc- of the three hypothetical relationships between the harzburgite and tures. These consist of chert-cemented, chert-clast breccias and as- higher zones of the ophiolite listed above, hypothesis two is favored sociated chaotic folds. but three cannot be dismissed. Transition Zone. The Kings River transition zone is the interval Petrogenesis of the Ophiolite Section of the ophiolite section in which two important changes occur: (1) Mafic rocks become progressively more common up-section rela- The Kings river ophiolite is interpreted as a disrupted fragment tive to ultramafic rocks, and (2) rocks of clearly magmatic origin of oceanic crust and upper mantle. This is supported by several (dunite-wehrlite-gabbro) become progressively more common up- lines of evidence. (1) The protoliths of the ophiolitic rocks are simi- section relative to harzburgite. Other important features of the lar to rocks dredged from oceanic ridges and fracture zones (Au- transition zone are the occurrence of diorite and plagiogranite mento, 1969; Engel and Fisher, 1969; Melson and Thompson, dikes, and the presence of early-stage serpentinite in conjunction 1970; Aumento and others, 1971; Bonatti and others, 1971; Cann, with hornblende-bearing gabbro, diorite, and plagiogranite. The 1971; Melson and Thompson, 1971; Engel and Fisher, 1975). (2) transition zone is interpreted as (1) the frozen feeder system for the The stratigraphic succession of protoliths in the ophiolite section overlying igneous part of the section and (2) the lower levels of a would yield a seismic velocity section comparable to modern-day stratiform plutonic complex of which the upper levels are repre-

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Figure 4. Relict primary features in hornblende hornfels. (a) Sequence of graded layers ranging from plagioclase clinopyroxenite (PL CPXITE) to clinopyroxene gabbro (CPX GAB), (b) Diabase and basalt dikes; large dike running across field is chilled against smaller dikes and is cut longitudinally by two small dikes, (c) Relict pillows and interpillow sediment (note that chilled rinds are still visible).

sented by the gabbro zone. Similar zones exist in the Vourinos ophiolite of northern Greece (Moores, 1969) and in the Semail ophiolite of Oman (Allemann and Peters, 1972). Some masses of magma that came to rest in the transition zone differentiated and gave rise to small dikes of diorite and plagiogranite. It is significant that these are the only highly differentiated rocks found throughout the entire ophiolite section. Thus, stagnation of magma bodies was apparently restricted to the transition zone. Highly differentiated rocks in the lower parts of other ophiolites have been discussed by Thayer (1974) and Coleman and Peterman (1975). The early-stage serpentine alteration zones are interpreted as being genetically related to the differentiated rocks of the transition zone. As stated above, the alteration zones are spatially related to primary Gabbro and Mafic-Dike Zones. The gabbro zone and the hornblende-bearing gabbro, diorite, and plagiogranite. The altera- mafic-dike zone of the Kings River ophiolite are interpreted as tion zones also contain talc and Cr-chlorite, which are commonly material of oceanic layer 3A (Fig. 2). The break in the ophiolite formed by hydrothermal alteration. The alteration zones are section occurs at the contact between these two zones; therefore, thought to be hydrothermal aureoles and veins developed around some interval of the section including upper gabbro and (or) lower differentiated magma bodies that, in advanced stages of crystalli- mafic dikes may be missing. Formation of both zones was closely zation, were concentrating and expelling magmatic water into ul- linked in time since screens and dikes of gabbro occur locally in the tramafic wall rocks. dike zone and diabase dikes occur in the gabbro zone. Unfortu- Whether the transition zone represents uppermost oceanic man- nately, the details of the contact relationships between these zones tle or lowermost oceanic crust (layer 3B) is an important question. cannot be established because of the break in the section. If it represents uppermost mantle, then the Kings River ophiolite The gabbro zone and the upper part of the transition zone to- section is like many other ophiolite sections in that its crustal part is gether represent a stratiform plutonic complex that fed the mafic- 2 to 4 km thinner than normal oceanic crust (Bailey and others, dike and pillow-basalt zones from below. This plutonic complex 1970; Dewey and Bird, 1971). If the transition zone is considered consists (from the base up) of mainly wehrlite, clinopyroxenite, and lowermost crust, then the crustal thickness of the Kings River sec- clinopyroxene gabbro. Lesser dunite and olivine- and/or tion is comparable to that of oceanic crust (Fig. 2). It is suggested plagioclase-bearing clinopyroxenite occur toward the base of the that the mixture of partly serpentinized ultramafic rock, mafic rock, complex. If the pods of chromite-olivine cumulate present lower in and leucocratic rock of the transition zone would yield seismic ve- the section are considered as having segregated from the magma locities intermediate between oceanic upper-mantle and lower-crust bodies that fed the plutonic complex, then coupled with the se- velocities. If so, then the transition zone may correspond to the quence of rocks listed above, the following crystallization sequence basal layer of oceanic crust, as in seismic sections reported by Sut- is implied: olivine —» chromite (or chromite —» olivine) —» clinopy- ton and others (1971) and Clague and Straley (1977). The transi- roxene —» plagioclase —> hornblende. Similar crystallization se- tion zone is correlated with layer 3B in Figure 2; this is discussed quences lacking orthopyroxene have been reported from the further in Saleeby (1976c). Troodos ophiolite, Cyprus (Moores and Vine, 1971), Bay of Island

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ophiolite, Newfoundland (Irvine and Findlay, 1972), and Point Sal terpreted as hydrothermal in origin as shown for similar modern ophiolite, California (Hopson and others, 1975). deep-sea deposits (Bostrom and Peterson, 1969; Bostrum and Additional important features of the Kings River plutonic com- others, 1976; Heath and Dymond, 1977). The soft-sediment de- plex are (1) the thick sequence of uniform adcumulate gabbro that formation exhibited by the chert resulted from either slope insta- indicates a slow crystal accumulation rate (Jackson, 1961) and (2) bility or oceanic tectonism. the lack of highly differentiated rocks throughout the gabbro, mafic-dike, and pillow-basalt zones. The lack of highly differ- Age of the Ophiolite Section entiated rocks in these zones is contrary to what is found in the California Coast Range ophiolite (Bailey and Blake, 1974; C. A. Zircon populations from diorite and plagiogranite of the transi- Hopson, J. M. Mattinson, and J. Saleeby, 1974, field reconnais- tion zone yield a complex spread of U-Pb ages. Minimum ages sance), the Vourinos ophiolite (Moores, 1969), and the Newfound- range between 205 m.y. and 270 m.y. Upper intercept ages fall land ophiolites (Dewey and Bird, 1971) where highly differentiated around 300 m.y. Similar patterns exist in zircon populations from dikes and lava flows are common. This suggests that the Kings diorite and plagiogranite from three widely spaced tectonic blocks River plutonic complex underwent only limited magmatic differ- within the Kaweah serpentinite melange. entiation relative to plutonic complexes of many other ophiolites. Zircon discordance is attributed to thermal metamorphism re- The lack of differentiation cannot be attributed to the Kings River lated to Cretaceous emplacement of adjacent plutons. The interpre- ophiolite having a short residence time in the area of ophiolite tation of young discordant zircon ages is difficult and will be genesis since adcumulate textures predominante in the plutonic treated in a future publication along with presentation of the ana- complex. Differentiation may have been hindered because of con- lytical data. The tentative interpretation is that initial crystalliza- tinuous tapping of the upper plutonic complex to form the mafic- tion of the zircon could have occurred as early as 300 m.y. ago. dike and pillow-basalt zones. This is consistent with the phenocryst However, there is a strong possibility that initial crystallization oc- assemblage of the upper zones being the same as the cumulate curred as late as 250 m.y. ago. Since the diorite and plagiogranite phases throughout the upper 1.5 km of the plutonic complex. The dikes are an integral part of the transition zone and since the U-Pb complicated structural relations between sheeted and nonsheeted systematics are consistent along the 125-km length of the Kings- dikes of the dike zone indicate that tapping of magma from the un- Kaweah ophiolite belt, the zircon ages are considered ophiolite- derlying gabbro zone did not follow a simple axial-zone rift as en- genesis ages. Thus, the age of ophiolite formation is considered visioned for the Troodos ophiolite, Cyprus (Moores and Vine, latest Paleozoic or possibly earliest mesozoic. 1971). Perhaps this complicated dike pattern is related to the K-Ar systems of mafic rocks from the ophiolite's lower zones and presumed continuous tapping of magma from the underlying gab- from crosscutting Jurassic plutons have been strongly affected by bro zone. Cretaceous thermal metamorphism. In many instances the K-Ar Pillow-Basalt Zone. The pillow-basalt zone is interpreted as ages have been completely reset to Cretaceous age. Minimum ages layer 2 of oceanic crust. This zone is somewhat thicker than the on metagabbro and amphibolite tectonite from the transition and pillow-basalt zones of many other ophiolites (Bailey and others, harzburgite zones range back to 190 m.y. As discussed below, pre- 1970; Dewey and Bird, 1971). Perhaps the greater thickness of this batholithic amphibolite-facies metamorphism of these lower zones zone also reflects continuous tapping of magma from the underly- is believed to have been related to ophiolite genesis. Further treat- ing gabbro zone, as discussed above. Mafic dikes that extend up ment of the K-Ar data and the metamorphic history of the entire into the pillow-basalt zone are interpreted as feeders for the pillow Kings-Kaweah ophiolite belt is given by Saleeby (1977b) as well as basalt. Most of the breccias of the pillow-basalt zone are composed further treatment of the zircon data. of pillow fragments. The rare breccias that contain angular clasts of massive basalt and diabase in addition to pillow fragments have Summary of Ophiolite Formation features that suggest fault-zone brecciation in some instances and talus-slope accumulations in other instances. Local massive basalts The Kings River ophiolite formed during latest Paleozoic or pos- may be thick flows or sills, although it is possible that some of them sibly earliest Mesozoic time at an oceanic spreading center located could have at one time contained pillows, but such structures have beyond the reach of terrigenous sedimentation. The crustal part of since been destroyed by deformation and contact metamorphism. the ophiolite grew by accretion of basaltic magma derived from Lack of relict primary minerals and textures throughout the pillow nondepleted oceanic mantle. A 1.8-km-thick sequence of pillow basalt is attributed to complete metamorphic recrystallization that lava and pillow breccia accumulated on the ocean floor while a was possible because of the original fine grain size of the lava flows. 5.2-km-thick subvolcanic intrusive complex grew beneath. De- This is in contrast to rocks lower in the section where relict primary pleted harzburgite was left beneath the intrusive complex as minerals are coarse and commonly have been recrystallized only oceanic upper mantle. The young ocean floor stayed beneath the

along grain margins (Figs. 3a, 3d, 3e). CaC03 compensation depth and accumulated a modest thickness Radiolarian Chert. The radiolarian chert represents pelagic of siliceous biogenic sediment prior to tectonic disruption and mix- material deposited on top of the ophiolite section during the after ing. the formation of the pillow-basalt zone. The chert is thus interpreted as layer one of oceanic crust. The lack of calcareous deposits STRUCTURE OF THE DISRUPTED OPHIOLITE

suggests that the ophiolite section stayed beneath the CaCo3 compensation depth. Lack of terrigenous deposits suggests that the As the Kings River ophiolite was transported from the oceanic ophiolite originated at a significant distance from any land mass or, spreading center and emplaced onto the continental margin, it was alternatively, that there was a sediment trap or barrier between the highly deformed and disrupted into large slabs. The slabs were site of ophiolite origin and any adjacent land mass. The metallifer- separated from one another by narrow serpentinite-matrix melange ous deposits, which are the Al-poor ferromanganoan type, are in- zones. The melange zones record the tectonic mixing that occurred

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between the slabs as the ocean floor was disrupted and differen- breccias, and dike rocks are mainly hornfels now. Schistose zones tially transported. The early stages of ocean-floor disruption that do occur in the metabasalt, but these are concentrated near con- led to melange formation are recorded in the structures that occur tacts with Jurassic crosscutting plutons that share a parallel schis- within the slabs. In this section, structural data on the slabs and tosity (Saleeby, 1975c; Saleeby and Sharp, 1977). Therefore, it ap- melange zones are presented. The simplest structural relations are pears that the Bald Mountain slab was in an only moderately discussed first with the more complicated relations following. The faulted state prior to pluton emplacement and contact metamor- structure of the ophiolite will then be interpreted in the light of phism. The Bald Mountain slab along with cross-cutting plutons of what is known about modern-day ocean-floor and continental- the region and the rest of the ophiolite contain localized fracture margin tectonics. systems that developed during the late stages of contact metamorphism. These fracture systems are unrelated to ophiolite Bald Mountain and Hughes Mountain Slabs genesis or emplacement and will therefore not be discussed any further. Figure 1 shows the large metabasalt slabs surrounded on all sides The Hughes Mountain segment of the reconstructed slab is struc- by crosscutting plutons of the batholith. The larger xenoliths of turally more complex than the Bald Mountain segment. Its south- metabasalt derived from these labs are also shown. The Bald ern part is mainly hornfels with abundant pillows, breccia, and Mountain slab is 8 km long; the Hughes Mountain slab is 5 km local dikes. The hornfells also contains relict shear zones that in long. The two slabs are interpreted as having once formed one 13- some cases have a schistosity. Northwest along the slab's long axis, km-long slab that was rifted apart along a pre-existing zone of the relict shear zones become more intense and nearly penetrative. weakness during pluton emplacement (Fig. 7). The shear surfaces strike northwest and rarely dip less than 80°. The Bald Mountain segment of the reconstructed slab is locally Deformed pillows, breccia, and dikes are common, but in many deformed internally. Local brecciated rocks that appear to be relict cases these features are highly distorted or obliterated by the shear fault breccias have been strongly overprinted by contact fabric. At the north end of the slab, the shear fabric is enhanced by metamorphism. These breccias along with pillow basalt, pillow a schistosity formed by oriented hornblende crystals. The shear

Figure 5. Photographs of deformation features from Tivy Mountain slab, (a) Margin of strongly lineated mafic inclusion (note large mullion structure and dark serpentinite rods in adjacent ultramafic host), (b) Distorted relict igneous texture in gabbro mylonite from mafic inclusion; erosion has cut through coarse-grained layer and exposed finer-grained layers at bottom of photo, near finger, and in upper left; plane of photo is plane of S,. (c) Open F, folds in S, and L: of harzburgite; folded S, in this case is exemplified by partings that run along transposed and rodded serpentinite zones, (d) Complex relations between dikes and mylonite structures of transition zone; remnants of diorite (DRT) dikes cutting obliquely across S, in mylonitic gabbro are in turn truncated by S, (also note patches of mylonitic plagiogranite (PG) that have complex relations with S,.

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fabric and the overprinted schistosity together produce a banded striking Sj commonly grades into S2, whereas Si in other orienta- amphibolite. The banding is locally folded on a small scale about tions is commonly cut by S2. The dips of both surfaces are usually steeply plunging axes. The Schistosity is believed to have formed nearly vertical. S, and S2 as well as steep-plunging folds occur in parallel to the earlier shear fabric during emplacement of adjacent ultramafic rocks throughout the ophiolite and are discussed further Jurassic plutons (Saleeby, 1975c; Saleeby and Sharp, 1977). The in- in the section on the Tivy Mountain Slab. Stereo plots of Si and S2 tensity of the schistosity increases near pluton contacts, and the are also presented with data on the Tivy Mountain slab in Figure 6. plutons contain a parallel schistosity. Along the southern end of the Mafic inclusions of the Hog Mountain slab are also highly de- Hughes Mountain slab the marginal schistosity diverges from a formed. The smaller bodies, which range down to 2m in diameter, northwest-strike and turns nearly due west parallel to the intrusive are invariably strongly foliated and/or lineated amphibolite. The contact with strongly schistose Jurassic diorite and gabbro (Fig. 1). larger bodies such as the one at the northwest end of the slab are The Jurassic rocks were apparently intruded into a pre-existing foliated amphibolite with large domains in which vestiges of the shear zone along which the Hughes and Bald Mountain segments gabbro protolith remain. The foliation in the amphibolite bodies is

were originally joined (Fig. 7). always parallel to St of the ultramafic host rock and is equated with A progression in intensity of deformation is observed from S:. Si'in the amphibolites formed mainly by syntectonic recrystalli- southeast to northwest along the Bald Mountain and Hughes zation. The structure of the amphibolite bodies is also discussed Mountain slabs. This is manifested by localized shear zones increas- further in the section of the Tivy Mountain slab. ing in abundance and intensity of shear fabric followed by coales- cence of shear zones into a penetrative shear fabric. This pre- Tivy Mountain Slab batholith structural feature of the basalt influenced its behavior during pluton emplacement and contact metamorphism. The The Tivy Mountain slab is surrounded by melange zones and highly deformed zones commonly formed schists, whereas the non- crosscutting plutons of the batholith (Fig. 1). It is 20 km long and is deformed zones formed hornfels. The relict shear zones of the also elongated in a northwest direction. It is composed of dunite,

metabasalt are probably related to Si and S2 of the melange zones harzburgite, wehrlite, clinopyroxenite, gabbro, and amphibolite and neighboring slabs. These structures are discussed further be- derived from gabbro. It comprises the lower three zones of the re- low. The schistosity shared by the metabasalt and Jurassic plutons constructed ophiolite section (Fig. 2).

is referred to as S3. A stereo plot of S3 is shown with other structural The structure of the Tivy Mountain slab is characterized by the data in Figure 6. presence of mylonites that are penetrative in the harzburgite zone and most of the transition zone but localized in the gabbro zone. In Hog Mountain Slab the harzburgite zone, only small pockets of chromite-olivine cumulate have survived mylonitic deformation. Cumulate dunite, The main part of the Hog Mountain slab is shown in Figure 1. It wehrlite, and gabbro have locally survived in the transition zone. is composed of harzburgite, wehrlite, and lesser gabbro and am- Ductile faults, which bound moderately deformed domains with phibolite derived from gabbro. It is 17 km long, elongate in a abundant primary features, extend into the lower 0.5 km of the northwest direction, and bounded along its margins by melange gabbro zone. The upper 1.5 km of the gabbro zone is nondeformed zones. except for local ductile faults. Contact metamorphism has im- The structure of the Hog Mountain slab is characterized by in- printed hornfels textures over deformation and primary features of tense deformation zones that bound less-deformed domains. Relict all three zones. An exception to this pattern exists at the southeast textures are well-preserved within the less-deformed domains. end of the slab. Here a 6-km-long strip of metagabbro is strung out However, intense contact metamorphism has in many cases com- between crosscutting plutons of the batholith. The strip of pletely reconstituted large parts of these domains into talc- metagabbro was converted into a schistose amphibolite during con- antigorite hornfels. The less-deformed domains grade into defor- tact metamorphism. The schistosity is interpreted as having had a mation zones in two manners. (1) Mineral grains and aggregates developmental history similar to the schistosity (S3) that pervades are progressively uniformly distorted into a planar fabric until the the north end of the Hughes Mountain slab. rock is strongly foliated. This is visible in hand sample by the dis- Primary features of the Tivy Mountain slab have been deformed tortion of relict pyroxene grains. In thin section, olivine grains can into planar and linear elements by mylonitization. The planar ele- also be seen distorted with strong intragranular flow features. In ments are collectively designated S, as mentioned above. The linear the more strongly deformed rocks, cataclasis has worked in con- elements are collectively designated Lj. S, is mesoscopically defined junction with intragranular flow. (2) The peridotite is disrupted by by (1) planar distortion of relic orthopyroxene porphyroclasts in a fracture cleavage or blocky fracture system that grades into a harzburgite (Figs. 3a, 3b), (2) transposition of early-stage serpenti- penetrative schistosity. In this case schistose serpentinite develops nite zones into planar domains, (3) parting surfaces that coincide along the fracture surfaces with the intensity of deformation in- with highly transposed serpentinite zones, (4) long and inter- creasing as the fracture or schistose zones widen and ultimately en- mediate axes of mafic inclusions, and (5) planar distortion of relict gulf the entire rock. Thus, two ypes of deformation zones exist, igneous textures into gabbro and clinopyroxenite. Various combi- each of which is characterized by a distinct fabric: (1) strongly nations of these elements occur together and are invariably copla- foliated harzburgite or wehrlite in which relict pyroxene and nar. A stereo plot of Si from the Tivy Mountain slab is shown in olivine grains have been flattened into the foliation plane and (2) Figure 6. The plot shows a faint nearly horizontal girdle with a schistose serpentinite in which no vestiges of original mineral grains broad northwest-striking maxima. L] is in the plane of Si and is remain. Two important relationships exist between these two defined mainly by long axes of various dimensional markers. The types: (1) the peridotite foliation (Si) is cut and disrupted at various intermediate axes of the markers are also within Si, and thus the

angles by the serpentinite schistosity (S2), and (2) the peridotite short axes are normal to Si. L, is mesoscopically defined by (1) foliation becomes progressively schistose as relict primary minerals elongate distortion of relict orthopyroxene porphyroclasts in disappear and the rock grades into schistose serpentinite. Strikes of harzburgite, (2) rodding in transposed early-stage serpentinite S, are variable because of folding and rotation about steep axes. zones (Fig. 5a), (3) long axes of mafic inclusions, (4) long axes of

Strikes of S2 are northwest (Fig. 1). It follows that northwest- distorted relic clinopyroxene grains within gabbro and clinopyrox-

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enite (Fig. 5b), and (5) mullion along margins of mafic inclusions Numerous observations show that this variability is partly due to a (Fig. 5a). Various combinations of collinear L, elements occur to- significant range in the pre-F, orientations of surfaces that best gether. A stereo plot of L, from the Tivy Mountain slab is shown in display folds. For example, complex networks of early-stage ser- Figure 6. The faint steep southeast-plunging maxima is consistent pentinite zones commonly have irregular segments that are trans- with the mesoscopic observation of lying within S,. posed into Sj to differing degrees. Thus, a significant range in axial The structural elements outlined above evolved mainly from orientations resulted when the networks were folded. Variability in ductile and cataclastic flow of gabbro and locally serpentinitic ul- F, orientations is also a result of superimposed folds. Consistent tramafic rock. However, mafic inclusions of the harzburgite zone patterns in overprinting have not been observed, however. In addi- deformed mainly by syntectonic recrystallization during ductile and tion, open and isoclinal folds share similar maxima and ranges in cataclastic flow of the ultramafic host. In similar inclusions of the orientations of axes and axial surfaces. Open and isoclinal folds transition zone and in ductile faults of the gabbro zone, ductile and commonly occur together with similar orientations. An important cataclastic flow dominated over recrystallization. In these cases the feature of F, is the common clustering of such folds in domains true extent to which recrystallization was involved on a fine scale that are truncated or bounded by domains in which S! is not folded cannot be resolved because of hornfelsic overprinting during con- (Fig. 6). Thus, developmental stages of Sj and F] are at least in part tact metamorphism (Fig. 3d). contemporaneous. Another important feature of F, is the predomi- The structural elements outlined above give an indication of nance of steep plunges. The stereo plot of F, axes (Fig. 6) may in tectonic flow patterns in the harzburgite and transition zones. In fact be biased against the steep-plunging folds since moderate- to

these zones, Lj is not always present with St. In these cases dimen- shallow-plunging folds of mesoscopic scale are in this area exposed sional markers that define L, have subequal large and intermediate preferentially over steep-plunging folds. Steep-plunging F,-folds in axes. It thus appears that a continuum exits between flattening and many instances have asymmetries which suggest a dextral sense of constrictional flow features. This is exemplified by the distorted motion. Asymmetries suggestive of sinistral motion also occur igneous textures of gabbro from the lower two zones of the ophio- rarely. Most F, folds are so complex that a simple sense of asym- lite. Prior to deformation, clinopyroxene grains were roughly metry cannot be deduced. The girdle present in the S[ stereo plots equidimensional as shown by relict primary textures that exist lo- for both the Tivy Mountain and Hog Mountain slabs (Fig. 6) is at- cally in the transition zone and throughout the gabbro zone. Thus tributed to F, rotations. The spread in L, orientations may also be the relative dimensions of the deformed clinopyroxene grains (Fig. related to F, rotations. In a number of instances, L, can be seen to 5b) can be used as approximate indicators of finite strain. Figure 6 be slightly to highly folded by F, (Fig. 5c). In these instances, L^ is also shows a deformation plot (Flinn, 1956, 1962; Wood, 1973, certainly not an axial lineation of Fj. However, in some instances, 1974) for distorted relict clinopyroxene grains measured from L, does appear as an axial lineation. In many instances, Lt does not meta-gabbro inclusions in the upper harzburgite and transition occur with mesoscopic F[ and thus their mutual relaionships cannot zones. The plot shows between 300% and 650% elongation paral- be established.

lel to Lj. The data points fall mainly in the L-tectonite field, but lap The Tivy Mountain slab is locally cut by S2 as defined earlier in across the plane strain line into the S-tectonite field. In some in- the section on the Hog Mountain slab. Figure 6 shows a stereo plot

stances inclusions that plot toward the S-field can be seen to be for S2 from the entire ophiolite. There is a vertical-dipping, more tabular shaped than the elongate inclusions that plot well northwest-striking maxima that is close to the S, and F, axial-plane

within the L-field. In addition, L-elements of the ultramafic host are maxima. S; in maxima orientations grades into S2 by progressive commonly lacking in areas containing the more tabular inclusions. enrichment in schistose serpentinite. This relationship exists along Within metagabbro inclusions, the approximate finite strain de- the western margin of the slab (Fig. 1) and is shown schematically

duced from the deformation plot represents minimum strain for the in Figure 6. In contrast, Sj in other orientations is cut by S2 where S2 mylonites. This is from the following two situations: (1) metagab- is developed. This is exhibited along the northeastern margin of the

bro inclusions (unless highly recrystallized) always have definite re- slab (Fig. 1). It is tempting to equate S2 with the axial surface of

lict igneous textures, whereas the ultramafic host rocks are com- Fj. However, S2 has not been observed as an axial surface in

monly mylonite and ultramylonite in which original grains have mesoscopic folds. S2 in the Tivy Mountain slab is developed been obliterated or totally transposed into L, and Sj. (2) Metagab- primarily adjacent to melange zones. bro inclusions commonly lie in parallel trains that appear to be large boudins (Figs. 1, 6). This is consistent with the presence of Melange Zones healed extension fractures that occur within the inclusions oriented at high angles to Lj and S,. As the inclusions were deformed inter- Melange zones bound the Hog Mountain slab and much of the nally, they were also pulled apart along such extension fractures Tivy Mountain slab (Fig. 1). The melange zones probably separated while the ultramafic host moved in around them by more uniform all of the slabs prior to emplacement of the crosscutting plutons. flow. Thus, the total strain for the mafic-ultramafic mixture of these The melange matrix is schistose serpentinite. The matrix schistosity

zones is somewhat greater than that recorded within the inclusions. is continuous with S2 of the adjacent slabs and is thus considered S2. Similar types of strain patterns occur in the upper transition and A similar schistosity pervades the matrix of the Kaweah serpenti- gabbro zones, but in these zones, strain is concentrated along do- nite melange to the south. Near contacts with crosscutting plutons, mains or belts that bound less-deformed domains. the matrix of the Kings River melange is recrystallized to talc- The structure of the Tivy Mountain slab is also characterized by antigorite schist and hornfels. Along the southwest margin of the

folds (F,) in Sj and Lt. F, is best defined where the ultramafic rocks Tivy Mountain slab, talc-magnesite schists and hornfels are also contain abundant transposed early-stage serpentinite zones and re- abundant. The schistosity developed during contact metamorphism

lated foliation-plane partings. Geometry of Ft is highly variable. A followed S2 and in some cases probably represents S3. This is consis- continuum exists between extremely open and isoclinal varieties. tent with the fact that ultramafic hornfels that lack schistosity are Orientations of axes and axial surfaces are also variable (Fig. 6). restricted to contacts with Cretaceous plutons that lack S3.

Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/89/4/617/3444453/i0016-7606-89-4-617.pdf by guest on 26 September 2021 L S] POLES 1 F, AXES F. AXIAL SURFACE POLES TIVY MTN SLAB TIVY MTN SLAB TIVY MTN SLAB TIVY MTN SLAB ^53 PTS 207 PTS 106 PTS 101 PTS 1,3,^,5,6% 2 ,6,10,15,20% 1,3,6,9,12% 1,2,5,9,15% ~200m

LOG Z/Y -1

CLINOPYROXENE DEFORMATION PLOT

S, POLES L| S2 POLES S, POLES HOG MOUNTAIN SLAB HOG MTN SLAB TOTAL OPHIOLITE JURASSIC PLUTONS AND AUREOLES 198 PTS hS PTS 289 PTS 108 PTS 1,3,6,9,12,15% 2,'5,10% 1,2,6,12,18% 1,3,6,9,13%

Figure 6. Schematic block diagram showing mesoscopic relations of structures within the Tivy Mountain and Hog Mountain slabs. Diagram is based on a plane table—alidade map (Saleeby and G. E. Melosh, unpub. data) made in the Tivy Mountain slab along the Kings River. Also shown are equal-area stereo plots of S« Li, and F] from the Tivy Mountain slab; S, and Lj from the Hog Mountain slab; S2 from the entire ophiolite; and S3 from Jurassic crosscutting plutons and their schistose aureoles. Next to each stereo plot, the number of data points are given along with the percentage of total data points per 1% area represented by the contours. A deformation plot on clinopyroxene dimensional markers from gabbro mylonite of 24 mafic inclusions is also shown. Between 12 and 20 grains were measured and then averaged for each inclusion. (Vertical dimension exaggerated about 2 x.)

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The matrix of the melange zones was apparently derived from Similar data for the Hughes and Bald Mountain slabs are lacking.

adjacent peridotite slabs. The melange zones contain blocks of Small-scale folds occur locally in S2 of the melange matrix. All folds

metagabbro and peridotite derived from the adjacent slabs in addi- in S2 have steep-plunging axes. The majority of these folds have tion to exotic blocks of metabasalt and metachert. Macdonald asymmetries which suggest a dextral sense of motion. (1941) originally mapped the melange zones as metabasalt because the mafic melange blocks are much more readily exposed than the Chronology of Ophiolite Disruption serpentinite matrix. The melange zone that bounds the southwest margin of the Hog Mesosocopic relations of the structures within the Tivy Moun- Mountain slab is of particular interest. Near the southeast end of tain slab and its bounding melange zones are summarized in Figure

Hog Mountain, the melange blocks consist of metabasalt and 6. Also shown are stereo plots for Si, Lu and F! of the Tivy Moun-

metachert. This segment of the melange zone is believed to have tain slab, Si and L! of the Hog Mountain slab, S2 and S3 of the en- once separated the Hog Mountain slab from the Hughes Mountain tire ophiolite, and a deformation plot for metagabbro from the metabasalt slab prior to emplacement of the intervening plutons lower zones of the Tivy Mountain slab. The field relations suggest

(Fig. 7). The predominance of metabasalt and metachert melange that S i and Li were followed by Fi, which was followed by S2. S2

blocks is believed to have resulted from simple two-way mixing be- and S3 have similar orientations but are distinguished from one

tween the two slabs. Along the northern segment of this melange another by the following critical relationship: S2 represents a sur- zone, metabasalt and metachert blocks are rare. This segment sepa- face along which tectonic mixing of different stratigraphic elements rates the Hog Mountain slab from the Tivy Mountain slab that of the ophiolite occurred. This surface is truncated by Jurassic plu-

lacks these rock types. A large mafic-ultramafic block occurs within tons that also contain S3. Furthermore, the Jurassic plutons are not this segment of the melange zone (Fig. 1). The block is mylonitic tectonically mixed into the melange zones. Thus, the gross sequence

peridotite and gabbro with the gabbro occurring as large boudins. of deformation is that described above with S3 following S2. An

The peridotite has been partially digested into the melange as additional subtlety in the sequence exists in the metabasalt slabs. S3 shown by the abundance of S2. developed parallel to a pre-existing shear fabric as described above. The melange zone that bounds the northeastern margin of the The shear fabric appears to be the structure along which tectonic Hog Mountain slab is also of interest. It separates the ophiolite blocks were dislodged from the parent slabs and mixed into the from the Calaveras Complex (Fig. 1). The melange blocks consist melange zones. Thus, the shear fabric is apparently related to Si mainly of various mafic rocks and metachert and lack the distinc- and I or S2. tive quartz-mica schist of the Calaveras Complex. These relation- Detailed field observations suggest that the sequence of struc- ships indicate (1) that tectonic juxtaposition of the ophiolite against tures presented above did not develop in distinct unrelated pulses. the Calaveras Complex occurred late in the ophiolite's disruption This is evident by several relationships: (1) Fj folds in Si are trun- history following melange mixing and (2) that rocks of Calaveras cated by surfaces that are indistinguishable from S^ (2) Folds with

affinity were not situated depositionally above the ophiolite prior similar orientation as Fi maxima occur locally in S2 and rarely in S3

to mixing. Thus, melange formation was apparently oceanic. Fur- (Fig. 6). (3) S2 in some instances grades from S, and in other in- thermore, there are several distinctive blocks of sedimentary brec- stances crosses S^ These relationships coupled with the fact that Si,

cia and conglomerate that occur primarily within this melange S2, S3, and F, axial surfaces all have similar maxima orientations zone. The sedimentary rocks contain clasts and fine detritus of suggests that deformation was progressive along fairly uniform chert, basalt, gabbro, and amphibolite. In one instance, the matrix trends and that the formation of each structure overlapped in time is fine-grained, now schistose, ultramafic material; in another in- and space with succeeding structures. Furthermore, folding and ro- stance the matrix is calcite-dolomite. The ophiolite detrital rocks tation of existing surfaces apparently accompanied both further are important from three standpoints: (1) They further demon- formation of the same surface and later formation of succeeding strate the absence of continental-derived material resting above the surfaces. ophiolite during or prior to tectonic mixing. (2) Gabbro and am- The absolute timing of deformation and disruption bears phibolite detritus indicate that the deeper levels of the ophiolite strongly on the interpretation of ophiolite emplacement and re- were uplifted and eroded prior to melange development. (3) If sur- gional tectonics. Detailed structural relationships and geo- face accumulations of sediment have migrated down into the chronologic data from the ophiolite's transition zone yield impor- melanges, then certainly ocean water also migrated to at least that tant information on the onset of SpL, development. Numerous depth, which has important implications concerning serpentiniza- dikes (mainly of gabbro, but also of diorite and plagiogranite) have tion and melange formation. complex crosscutting relationships in this zone. Some dikes

The melange blocks are composed entirely of ophiolite- definitely cut across SrL, along part of their length but are in turn assemblage rocks. Peridotite blocks are usually strongly serpen- transposed into Si-Li and mylonitized along their remaining length. tinized and rarely retain primary features. Metagabbro blocks re- Other dikes cut Si-Li but are simply truncated by the same struc- tain both igneous and tectonite features. Metabasalt blocks display tures (Fig. 5d). Analogous relationships exist with zones of early- rare primary features and common relict shear fabrics and as- stage serpentine alternation. Different generations of these zones sociated schistosity. Metachert blocks almost always have bedding are transposed into Si and Li to different degrees. Some veins cut Si preserved. The chert blocks appear to have been deformed primar- but are in turn cut by S^ As discussed above, the early-stage altera- ily by brittle fractures oriented at large angles to bedding. tion is thought to represent hydrothermal metamorphism related to The Hog Mountain and Tivy Mountain slabs do not appear to transition-zone igneous activity. The relationships discussed above have undergone significant differential rotations relative to one for transition-zone dike rocks and early-stage serpentinites indicate another during melange formation. This is evident from the fact that Si-Li formation began during the igneous history of the that Si, Li, and F! orientations within the two slabs are similar. ophiolite. These relationships also indicate that mylonitization pro-

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ceeded in steps and that translative displacements were an integral are difficult to envision as forming at a normal oceanic spreading part of that deformation. center. Tectonites similar to those of the Kings River ophiolite have Further time constraints can be imposed on ophiolite deforma- not been recovered from modern-day oceanic spreading centers.

tion and disruption. As discussed above S2 preceded the Middle to These types of rocks have been recovered from transverse fracture

Late Jurassic emplacement of plutons that contain S3. Thus, S2 de- zones, however (Aumento and others, 1971; Bonatti and others,

velopment ceased by Middle Jurassic time, and S3 development ex- 1971; Melson and others, 1972; Thompson and Melson, 1972; tended through Late Jurassic time. Plutons of the region that were Bonatti and Honnorez, 1976; Fox and others, 1976; Schreiber and emplaced in Early Cretaceous time lack tectonic structures; their Fox, 1977). emplacement thus postdates the deformation history of the ophio- It is proposed that the Kings River ophiolite originated at an lite. Another important time constraint concerns the age of the oceanic spreading center where it was cut by a transverse fracture Calaveras Complex. Based on regional structural and stratigraphie zone. Furthermore, progressive disruption and emplacement of the studies and on published and new paléontologie data (Christensen, ophiolite is believed to have been primarily a result of wrench tec- 1963; Jones and Moore, 1973; Saleeby and S. E. Goodin, unpub. tonics related to the fracture zone. The proposed fracture or data), Saleeby and Goodin (1977) considered the Calaveras wrench zone thus extended from at least the oceanic spreading cen- quartz-mica schist unit adjacent to the ophiolite to be of Late ter to the Sierran foothill suture that defined a segment of the an- Triassic to Early Jurassic age. Therefore, final tectonic juxtaposi- cient continental margin. The San Andreas—Queen Charlotte fault tion of the ophiolite against the Calaveras Complex occurred dur- system (Wilson, 1965), the Macquarie Ridge-Alpine fault system ing Jurassic time. (Griffiths, 1971; Griffiths and Varne, 1972) and the Spitsbergen The relationships discussed above indicate that deformation and fracture zone (Lowell, 1972) are outstanding modern examples of disruption of the ophiolite began in latest Paleozoic to possibly large wrench zones which involve both oceanic and continental earliest Mesozoic time and progressed through Jurassic time. domains. Wrench-zone tectonics has recently been called upon as Furthermore, the deformation trends remained fairly constant over an important ophiolite emplacement mechanism by other workers this time interval. A similar deformation history is emerging from (Dewey and Karson, 1976; Brookfield, 1977). Hamilton and Myers detailed structural, stratigraphie, and geochronologic studies of the (1966), Burchfiel and Davis (1972), and Schweickert (1976a) have Kaweah serpentinite melange (Saleeby, 1977b). postulated an early Mesozoic tectonic truncation event along the southwest margin of North America. The foothill suture has been Tectonic Setting of Ophiolite Disruption

A palinspastic reconstruction of the ophiolite prior to emplacement of Jurassic plutons and juxtaposition against the Calaveras Complex is shown in Figure 7. The Bald Mountain and Hughes Mountain basalt slabs are joined together and inserted into the gap between the Tivy Mountain slab and the southeast end of the Hog Mountain slab. The reconstruction shows the three interdigitated slabs separated from one another by narrow steep-dipping melange zones. Melange zones also bound the outer margins of the cluster of slabs. Each slab contains a different interval of the ophiolite and a different expression of the sequence of structures discussed above. Evidence indicating that dips of the ophiolite have been altered significantly by emplacement of the batholith or Tertiary uplift is lacking. Thus, the structural configuration shown in Figure 7 is taken as the Early Jurassic state of the ophiolite. The structural configuration shown in Figure 7 differs significantly from that of other large ophiolites such as Papua (Davies, 1971), Semail (Alleman and Peters, 1972), Troodos (Gass, 1968; Moores and Vine, 1971), and Vourinos (Moores, 1969). These ophiolites occur as moderately dipping sheets rather than a cluster of slabs bounded by steeply dipping melange zones. The structure of the Kings River ophiolite and its disruption history indicate that it was not emplaced during an obduction or overthrust event as envisioned for most large ophiolite sheets (Coleman, 1971a; Dewey and Bird, 1971; Church, 1972). Evidence indicating disruption and emplacement within a continental-margin subduction zone (such as high-pressure metamorphism or tectonic mixing with continental-margin sedimentary rocks) is also lacking. An alterna- Figure 7. Interpretive palinspastic reconstruction showing the structural tive emplacement mechanism that accounts for the structure and configuration of the ophiolite prior to emplacement of Jurassic plutons and disruption history of the Kings River ophiolite is presented below. tectonic juxtaposition against the Calaveras Complex. The tectonic slabs are shown interdigitated with one another and separated by serpentinite- Intense deformation of the Kings River ophiolite began at its matrix melange zones. Primary and deformation features of slab interiors point of origin. Thick mylonites with high amounts of constric- are schematically shown. Motifs used in slab interiors are same as in col- tional and flattening strain and abundant translational structures umnar section of Figure 2.

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cited as the logical zone for this truncation. This view is adopted slumped masses of chert were further lithified and then tectonically here with wrench tectonics and ophiolite emplacement being an in- broken and mixed into oceanic melange zones. tegral part of the continental-margin truncation. However, the The complex disruption patterns exhibited along modern trans- difficulties that have arisen in establishing the precise age of the verse fracture zones are evident in the structural patterns of the truncation are believed to be a result of the truncation being pro- ophiolite's tectonites. The complex crosscutting relationships be- gressive over an extended time period rather than a discrete event. tween successive generations of dikes, early-stage serpentine Comprehensive treatment of the views set forth above requires alteration zones, and mylonite structures of the transition zone in- presentation of a large amount of detailed structural, stratigraphic, dicate that mylonitization progressed in incremental steps. Fur- and geochronologic data on the Kaweah serpentinite melange and thermore, some domains were either undergoing rotations in time the Calaveras Complex, and additional reconnaissance field and relative to the trend of mylonitization, or alternatively mylonitiza- geochronologic data on other segments of the suture. This is tion trends varied in time and crosscut one another. This is also indi- beyond the scope of this paper. The remainder of this section will cated throughout the lower levels of the ophiolite by the presence of deal primarily with the structure of the Kings River ophiolite and folded and rotated mylonite domains that aré in turn bounded by how it may relate to the wrench-zone emplacement mechanism. later-generation mylonites (Fig. 6). In this context, F, folding is at- Further substantiation of the wrench-zone hypothesis will be pre- tributed to nonhomogeneous flow, drag, and complex buckling of sented elsewhere (Saleeby, 1977b). developing and previously developed mylonite domains during The mechanical behavior of the ophiolite during fracture-zone progressive mylonitization. Similar complex patterns in disruption, disruption is an important consideration. A variation in behavior as folding, and progressive mylonitization are typical of large wrench a function of stratigraphic depth is apparent. The harzburgite and zones that extend into continental areas (Moody and Hill, 1956; lower transition zones responded mainly by ductile and cataclastic Lillie and Gunn, 1964; Reed, 1964; Dickinson, 1966; Higgins, flow with the exception of the lowermost mafic bodies that were 1971; Harding, 1973, 1974; Wilcox and others, 1973; Sylvester syntectonically recrystallized in the amphibolite fades. In the upper and Smith, 1976). The steep-plunging elongation lineation (Lj) is transition and lower gabbro zones, ductile and cataclastic flow pre- believed to be primarily a result of vertical flow of hot upper- dominated, but were not as uniform as lower in the section. The mantle and lower-crustal material into the fracture zone in the main part of the gabbro zone responded by localized flow and slip vicinity of its intersection with the spreading axis. Similar lineations along ductile faults. The mafic-dike and pillow-basalt zones re- are common in peridotite and included metagabbro along the sponded by the development of brittle shear domains. Chert re- Kaweah serpentinite melange (Saleeby, 1977b). The peridotite sponded to ocean-floor disruption by localized ductile flow and melange blocks were apparently protrusive in origin. The blocky brecciation prior to lithification and by brittle fracture into tabular fracture systems associated with S2 development both in the Kings blocks following lithification. The variation from brittle to pro- River peridotite slabs and in Kaweah peridotite melange blocks gressively more ductile behavior with increasing depth is believed suggest expansion during serpentinization (Coleman and Keith, to be related to a steep thermal gradient in the ocean ridge. As 1971). The expansion is believed to have accelerated upward flow mentioned above, the harzburgite zone was under amphibolite- of the peridotite. Upward flow and protrusion of ultramafic rocks facies conditions, which ranges from about 450 to 650 °C (Turner, and its included mafic tectonites are known to be important pro- 1968). This temperature range at a stratigraphic depth of 7 to 11 ceses along modern oceanic fracture zones (Melson and others, km (Fig. 2) corresponds well with calculated ocean-ridge geotherms 1972; Thompson and Melson, 1972; Bonatti and Honnorez, 1976; (Sclater and Francheteau, 1970). Fox and others, 1976). Fracture zones are known to have complex tectonic disruption Serpentinization is known to be an important process along patterns that involve progressively more ocean floor with time modern transverse fracture zones (Bonatti and others, 1971; Mel- (Cann and Vine, 1966; Menard and Atwater, 1968; Van Andel and son and Thompson, 1971; Bonatti and Honnorez, 1976). others, 1969, 1973; Detrick and others, 1973; ARCYANA, 1975; Modern-day fracture-zone setpentinization is intimately related to Fox and others, 1976; Macdonald and Luyendyk, 1977; Delong protrusion tectonics. Partial serpentinization of the ophiolite's and other, 1977; Ramberg and others, 1977). This pattern is lower zones appears to have been progressive. Earliest-formed ser-

suggested by the structure and petrology of sedimentary blocks that pentinites are transposed into St and L, to varying degrees. As occur in melange zones. A discussion above, the deeper levels of the stated above, these serpentinites appear to have formed by hydro- ophiolite were exposed and locally eroded prior to melange mixing thermal metamorphism related to igneous activity in the transition

as shown by the presence of gabbro and amphibolite detrital zone. Development of serpentinite with the S2 fabric may have over- melange blocks. Deep-level exposures of ocean floor are only lapped in time with development of the early-stage serpentinites. known to occur along fault scarps of transverse fracture zones The incomplete transposition of the latest-formed hydrothermal

(Bonatti and others, 1971; Melson and Thompson, 1971; Melson serpentinites into Si and the local intergradation of S, and S2

and others, 1972; Thompson and Melson, 1972; Fox and others, suggest such a temporal relationship. Development of S2 is believed 1976; Bonatti and Honnorez, 1976). The fact that these sedimen- to be primarily related to the progressive introduction of ocean tary accumulations were reincorporated into oceanic melange indi- water into the ophiolite's deeper levels as disruption proceeded. cates that the zone of ocean-floor faulting and exposure became Disruption and water migration were probably a result of both more chaotic with time. Similar patterns exist with significant ac- wrench and protrusion tectonics. The presumed acceleration of cumulations of ophicalcite that occur in the Kaweah serpentinite protrusion activity by progressive serpentinization is believed to melange. Similar ophicalcites have so far been recognized in two have resulted in a runaway effect with deformation and tectonic melange blocks of the Kings River area. Ophicalcites are an impor- mixing ultimately being concentrated along serpentinite melange tant feature in modern transverse fracture zones of the Atlantic zones. (Bonatti and others, 1974). Soft-sediment deformation in chert As stated above the lack of terrigenous material mixed into the blocks is also attributed to fracture-zone faulting and tilting. As the melange zones indicates that the ophiolite was still in transport zone of ocean-floor disruption grew in intensity and width, the along an oceanic segment of the wrench zone during melange mix-

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ing. Time constraints on this stage of the ophiolite's disruption his- Tobisch, oral commun., 1977). The precise location of the subduc- tory are poorly defined. Detailed structural and stratigraphic tion zone is not clear because of overprinting by several later pro- studies of the Kaweah serpentinite melange (Saleeby, 1977b) indi- cesses: (1) Late Jurassic to Holocene sedimentation, (2) Cretaceous cate that a significant amount of pelagic and later hemipelagic to early Tertiary subduction involving the Franciscan Complex, sedimentation occurred along the ophiolite belt during melange de- and (3) middle Tertiary to Holocene San Andreas wrench tectonics. velopment. Structural relations between the ophiolite and Jurassic Recent tectonic syntheses of the Sierran foothill metamorphic belt plutons and between the Kaweah serpentinite melange and proba- cite the foothill suture as a Jurassic subduction zone along which ble Jurassic volcanic and epiclastic rocks indicate that melange mix- the Jurassic arc terrane collided with North America (Moores, ing ceased by middle Jurassic time. Thus melange development 1970, 1971; Schweickert and Cowan, 1975). This view is not could have extended from Permian through Early Jurassic time. adopted here. The foothill metamorphic belt is here interpreted as

Development of S3 and tectonic juxtaposition of the ophiolite an intra-arc transpression zone that formed along an earlier trans- against the Calaveras Complex represent the final stages of ophio- form juncture. lite deformation and emplacement. These are the only structural CONCLUSIONS features of the ophiolite that may be related to the Late Jurassic Nevadan orogeny as redefined by Bateman and Clark (1974). Thus, The Kings River ophiolite represents a disrupted fragment of most of the ophiolite deformation is pre-Nevadan. oceanic crust and upper mantle that was tectonically emplaced into Jurassic rocks of the Sierran foothill metamorphic belt have been the Sierran terrane prior to emplacement of the batholith. The interpreted as the remnants of a volcanic arc terrane (Moores, ophiolite was subsequently metamorphosed in the hornblende- 1970, 1971; Schweickert and Cowan, 1975). This view is sup- hornfels facies by the batholith. Generation of the ophiolite took ported by detailed studies on probable Jurassic rocks that rest dep- place at a late Paleozoic to possibly early Mesozoic oceanic spread- ositionally above the Kaweah serpentinite melange (Saleeby, ing center where the center was cut by a transverse fracture zone. 1977b; Saleeby and Sharp, 1977). It must be emphasized, however, Intense deformation related to fracture-zone tectonics began at the that continent-derived epiclastic rocks are a significant component ophiolite's point of origin. However, the remnants of standard of the Jurassic arc assemblage along the entire length of the foothill ophiolite and modern ocean-floor stratigraphy are well-preserved metamorphic belt (Clark, 1964, 1976; Behrman and Parkison,, within slabs that range up to 20 km in length. 1977; Saleeby, 1977b; Saleeby and Sharp, 1977). The Kings- Disruption of the ophiolite commenced with mylonitization of its Kaweah ophiolite belt represents oceanic basement upon which the lower levels and brittle shear of upper levels. This variation in de- Jurassic arc terrane was built. Reconnaissance work on other formation behavior is attributed to a steep ocean-ridge thermal ophiolite fragments of the foothill suture supports this view. Juras- gradient favoring more ductile behavior with depth. Progressive sic plutonic rocks that intrude the Kings River ophiolite probably disruption along the fracture zone facilitated migration of water

represent local roots of the arc terrane. Development of S3 in the into ever deeper levels; this enhanced serpentinization. The zone of

Jurassic plutons and the ophiolite, steep plunging folds in S3, and ocean-floor disruption widened with time. Ultimately, motion was the Jurassic juxtaposition of the ophiolite against the Calaveras concentrated along highly serpentinized zones that served as pas- Complex suggest that both wrench and compression tectonics op- sageways for tectonic mixing. Peridotite and gabbro blocks from erated along the suture during arc activity. Longitudinal wrench lower levels were mixed with basalt and small chert blocks from zones are common along modern arc terranes of the circum-Pacific upper levels to form serpentinite melange. Prolonged disruption (Allen, 1962, 1965; Allen and others, 1970; Wilson, 1965; Fitch, and mixing of the ophiolite resulted from large translational 1972; Karig, 1974; Karig and others, 1975, 1977; Brookfield, movements along a wrench zone that started at the mid-ocean 1977; J. R. Curray, D. G. Moore, L. A. Lawver, F. J. Emmel, R. W. transverse fracture. Upward protrusion of ultramafic rock also ac- Raitt, and M. Henry, unpub. ms.). celerated ocean-floor disruption. Foliation and schistosity surfaces Longitudinal wrench zones within present-day subduction-arc progressively developed along the wrench zone and were in many complexes are particularly prevalent along zones of oblique plate instances folded about steep-plunging axes. Disruption and convergence. It is suggested that the long-active oceanic wrench tectonic mixing of the ophiolite was oceanic and ceased by Middle zone evolved into an oblique convergence juncture and that the Jurassic time. zone of disrupted ocean floor was left as the hanging wall of an During Middle and Late Jurassic time the Kings River ophiolite east-dipping subduction zone. If the subduction complex terminol- formed part of the disrupted oceanic basement beneath a volcanic ogy of Karig and Sharman (1975) is applied, the stranded oceanic arc terrane. Tectonic movement continued along the pre-existing basement material would represent the most primitive form of an wrench zone, however. This movement resulted in deformation of accretionary wedge. The foothill suture kept the accretionary arc plutons that had intruded the ophiolite, further deformation of wedge mechanically uncoupled from North America throughout the ophiolite, and final juxtaposition of the ophiolite against the the remainder of Jurassic time by wrench tectonics with a com- Calaveras Complex of the North American continent. Thus, pressive component. Harland (1971) has proposed the term ophiolite emplacement in this instance is primarily a result of "transpression" for tectonic movements involving both transcur- wrench tectonics. Middle to Late Jurassic subduction occurred to rent (wrench) and compressive components. The application of this the west during the waning stages of Sierran foothill wrench concept to the tectonic development of the Sierran terrane is out- movements. The already weakened oceanic basement and overlying lined in Saleeby (1977b) and will be discussed further in a future Jurassic arc rocks underwent compressive deformation in response publication. to the adjacent subduction tectonics. Thus, transverse shortening The precise time at which oblique subduction began is not clear. worked in conjunction with longitudinal wrench movements in the Regional age data on plutonic and volcanic rocks of the Sierran ter- Sierran foothills during Middle and Late Jurassic time. This stage rane suggest that east-dipping subduction began in Triassic time of the ophiolite's long deformational history may be regarded as (Evernden and Kistler, 1970; Crowder and others, 1973; Schweic- the Nevadan orogeny. Similar longitudinal transpression zones are kert, 1976b; Morgan and Stern, 1977; P. C. Bateman and O. T. common in modern arc terranes of the circum-Pacific.

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