Structural History of Ophiolite Obduction, Bay of Islands, Newfoundland

Structural history of ophiolite obduction, Bay of Islands, Newfoundland

GÜNTER SUHR* 1 Centre for Earth Resources Research, Department of Earth Sciences, Memorial University, St. John's, PETER A. CAWOOD J Newfoundland, Canada, A1B 3X5

ABSTRACT INTRODUCTION tachment-related rock units in the Bay of Islands ophiolite complex (BOIC), west Obduction-related structures in the ophi- The mantle section of ophiolites preserves Newfoundland. Along-strike variation in ob- olitic Bay of Islands Complex, west Newfound- the record of two different tectonic events duction-related structures allows definition of land, are classified into shearing dominated (Nicolas and others, 1980). The entire mantle two different obduction styles within the and accretion dominated. Mapping of these section was affected by mantle flow occurring ophiolite: shearing dominated and accretion structures is combined with other geological during the extensional phase of an oceanic dominated. We correlate the different obduc- constraints and provides the basis for a recon- spreading center. These spreading-related tion styles with a new palinspastic restoration struction of the obduction history of the ophi- mantle structures are overprinted along the of the BOIC (Suhr, 1992) and discuss Theo- olite. The earliest detachment structures are base of the ophiolite by strain related to the logical controls which may have determined shearing dominated and are preserved in the obduction of the ophiolite. Spreading-related the nature of obduction structures developed basal peridotites. They indicate north-directed structures are of a low-stress, high-tempera- in the BOIC. thrusting. Detachment was facilitated by the ture nature, strain is distributed homogene- shallow position of the lithosphere-astheno- ously, and mineral shape fabrics are generally REGIONAL SETTING sphere boundary below the recently extinct Bay inconspicuous. Strong olivine lattice fabrics, of Islands spreading ridge. Detachment in- however, indicate large accumulated strain The BOIC represents the highest structural volved lithosphere of the southern side and im- (Ceuleneer and others, 1988). The basal parts slice within the Humber Arm allochthon of mediate northern side of the ridge. During the of the peridotites are characterized by a high- western Newfoundland (Fig. 1). The ophi- early stages of detachment, the Coastal Com- stress, lower temperature microstructure, a olitic St. Anthony Complex in the Hare Bay plex, an arc-related unit located to the west of strong mineral shape fabric, and a heteroge- allochthon (Williams, 1975) in northwest the Bay of Islands Complex, acted largely as a neous structural make-up involving strain lo- Newfoundland occupies a similar structural lateral ramp to obduction. Early accretion oc- calization. The basal peridotites contain the position. Formation of these ophiolite com- curred in two tectonic situations: (1) at the stage earliest information about the process by plexes and their emplacement onto the an- when the basal décollement plane intersected which dense oceanic lithosphere was de- cient continental margin of eastern North the boundary to the structurally and litholog- tached from its original oceanic substrate and America took place during the Early to Mid- ically diverse Coastal Complex; this resulted in was emplaced onto buoyant continental dle Ordovician Taconian Orogeny (Church accretion of blocks up to several hundred crust. The abundance of mantle peridotites in and Stevens, 1971; Williams and Stevens, meters thick with a lithological make-up not ophiolites constrains the level of décollement 1974; Cawood and others, 1988). The BOIC is known from the Bay of Islands Complex; and to within the upper mantle. exposed in four massifs: Table Mountain, (2) at the stage when the basal thrust plane Accreted to the base of the mantle section North Arm Mountain, Blow Me Down intersected the oceanic crust. Deformation in many ophiolites is the metamorphic sole Mountain, and Lewis Hills. To the west of the switched into the mafic and hydrated footwall (Fig. 1). The sole represents a sequence of BOIC is the Little Port Complex, an assem- lithologies and led to intermittent accretion of oceanic crustal lithologies which shows a blage of volcanic and plutonic rocks showing mafic oceanic crustal rocks now preserved in complex inverted metamorphic zonation a variety of metamorphic grades and struc- the metamorphic sole. Only the Lewis Hills from amphibolite- and granulite-facies rocks tural styles (Comeau, 1972; Williams, 1973; massif, located close to the Coastal Complex at the top to subgreenschist facies at the base Karson and Dewey, 1978;Karson, 1984). The during detachment, preserves evidence for (Williams and Smyth, 1973; Malpas, 1979a; Little Port Complex is restricted to outcrops both types of early accretion. At a later stage of Jamieson, 1980, 1981, 1986; Spray and Wil- west of Table Mountain, North Arm Moun- obduction, localized removal of the basal se- liams, 1980; Spray, 1984, 1988; Ghent and tain, and Blow Me Down Mountain (Wil- quence of the ophiolite was associated with ac- Stout, 1981; Searle and Malpas, 1982; liams, 1973). Karson and Dewey (1978) and cretion of a diverse assemblage of harzburgites, McCaig, 1983). The metamorphic sole con- Karson (1984) extended the definition of the amphibolites, and greenschists. tains information about that stage of the ob- Little Port Complex to include the western duction history when the ophiolite was thrust Lewis Hills and proposed the name "Coastal over oceanic crust. Complex" (CC) for this new assemblage. Ac- In this paper, we present new data from the cording to their interpretation, the original *Present address: Department of Mineralogy, oceanic contact between the BOIC and the University of Köln, Zülpicher Str. 49,5000 Köln 1, basal peridotites, the metamorphic sole, and Germany. related lithologies, based on mapping of de- CC is preserved in the Lewis Hills.

Geological Society of America Bulletin, v. 105, p. 399-410, 7 figs., March 1993.

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KILOMETRES

Lookout Hills

Table v Mountain

^ Fig. 5

North Arm Figure 1. Geologic map showing the distribution of the Bay of Islands Mountain Complex, the Coastal Complex, and rock units accreted during detachment. Dashed boxes indicate the location of maps shown in Figures 2,4, and 5.

Blow Me Down Mountain

Fig. 2 Lewis Hills

Fig. 4 58°00'W

Bay of Islands Coastal rocks accreted Complex Complex during detachment

Based largely on structural evidence, the recent geochemical data rule out such a model 1973; Malpas and others, 1973; Malpas, CC was interpreted as a fracture zone assem- (Jenner and others, 1992; Elthon, 1992). In a 1979b; Searle and Stevens, 1984; Jenner and blage adjacent to which the BOIC formed different interpretation, based mainly on others, 1992). along an oceanic spreading center (Karson lithological, geochemical, and age con- Our work is limited to the BOIC where a and Dewey, 1978; Karson, 1984). Geochem- straints, the Little Port Complex (that is, the metamorphic sole forms a continuous sheet ical evidence was later used to suggest a clas- CC excluding the western Lewis Hills) has on the eastern flank of all four massifs (Fig. 1; sic mid-ocean-ridge environment for this con- been assigned to formation of an island arc, Williams, 1973; Williams and Smyth, 1973; figuration (Casey and others, 1985), but distinct from the BOIC (Williams and Payne, Malpas, 1979a; McCaig, 1983). Late folding

400 Geological Society of America Bulletin, March 1993

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the allochthon onto the North American margin. This later emplacement phase of the obduction process is marked by the widespread development of mélange (Stevens, 1970; Wil- liams and Stevens, 1974; Williams, 1975).

Shearing-Dominated Structures

Shearing-dominated structures are best developed within the northern and central Blow Me Down Mountain where a gradual overprint of the spreading-related structures by detachment structures occurs 300-500 m above the base of the mantle sequence (Fig. 2). This estimate contrasts with the mapping of Savie (1988), who considers only the basal 50-170 m to be affected by detachment- related strain. Foliations in the detachment domain dip to the northwest, parallel to those in the spreading domain. Mineral-stretching lineations change, however, from northwest plunging in the spreading-related structures (mean 34° to 324°) to gently north plunging in the detachment structures (Fig. 3a; compare with Girardeau and Nicolas, 1981). Judging from a progressively stronger mineral shape fabric, the detachment-related strain increases gradually toward the base of the peridotites and culminates in ultramafic ultramylonites, as much as 20 m thick, located immediately above the metamorphic sole. The ultramylonites are, however, not later- ally continuous. Shear-sense determinations of the peridotites were performed using a well-established microstructure method based on the obliquity between the lattice fabric of olivine grains and the shape fabric de- Figure 2. Simplified map of detachment-related structures in the eastern part of Blow Me Down fined by spinel (Nicolas and others, 1972; Mountain. Ceuleneer and others, 1988). The obliquity of the lattice fabric can be inferred from the ori- has affected the massifs, and the associated FIELD RELATIONSHIPS entation of tilt walls seen in thin sections cut fold axes trend north-northeast-south-south- parallel to the stretching lineation and per- pendicular to the foliation. This method indi- west and are subhorizontal (Williams, 1973; Field and pétrographie work indicates that cates northward movement of the hanging Karson, 1979; Williams and Cawood, 1988). the detachment structures of the BOIC can be wall (15 samples northward, 2 southward), in Due to this folding, structural data presented divided into three categories: (1) a detach- agreement with earlier results (6 northward, in the paper do not correspond to original ment stage marked by shearing-dominated 0 southward) of Girardeau and Nicolas attitudes in the oceanic environment. By us- structures; (2) an early, ductile accretion (1981). As this movement brought deep-level ing the regional attitude of the contact be- stage marked by the incorporation of mafic rocks (peridotites) above higher-level rocks tween gabbros and peridotites as a paleohor- and ultramafic material to the base of the (oceanic crustal lithologies exposed in the izontal, however, the original attitudes of the ophiolite—a phase predating the sole can be metamorphic sole), the movement is inter- structures can be restored. As the gabbro- distinguished from the formation of the sole preted as a reverse motion. peridotite boundary is regionally parallel to itself; and (3) a later accretion stage during the detachment-related structures, the de- which a variety of mafic and ultramafic lithol- In the Knight's Brook area, at a short dis- tachment plane is inferred to have been sub- ogies were attached to the ophiolite after or tance above the metamorphic sole, there is a horizontal. The trend of late fold axes is ap- during a part of the metamorphic sole and lensoid body of peridotites which shows proximately parallel to the orientation of overlying units had been removed. These structures and microstructures typical of detachment-related mineral stretching linea- three types of detachment-related structures spreading-related deformation (Fig. 2). With- tions. Hence, unfolding has only a small to largely predate the incorporation of the ophi- in this lower strain lens, the mineral foliation negligible effect on the trend of the stretching olite within the thrust stack of the Humber and lineation are poorly developed, and the lineations. Arm allochthon and the final emplacement of compositional banding is locally folded. Lo-

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Figure 3. Stereographic projections of mineral stretching lineations in Blow Me Down Mountain (BMD), Table Mountain (TM), and the Lewis Hills (LH). Lower-hemisphere projection, equal area. Contours are at 1%, 2%, 4%, 8%, and 12% of 1% area, (a-c) Detachment-related, basal peridotites in BMD, LH, and TM, respectively; (d) upper detachment unit in TM; (e) east-west-striking ductile shear zone in LH; (f) early-accretion assemblage in LH; (g-i) amphibolite unit of metamorphic sole in BMD, LH, and TM, respectively; (j) greenschist unit of metamorphic sole in LH; (k) late-accretion assemblage in LH.

cally, the stretching lineation plunges moder- late folding of the entire massif associated ture cut by thin bands with very fine grained, ately to the west-northwest, identical to the with final emplacement. Primary flow com- recrystallized olivine, indicating high-stress spreading-related deformation preserved far- plexities must have occurred during the early deformation. Attitudes of mineral foliations ther up-section. detachment (folding, minor duplex forma- and lineations in the basal peridotites are sim- In the southern Blow Me Down massif, tion?) and are evidence for deviations from ilar to the Blow Me Down Mountain section: more complex detachment structures occur. shearing-dominated structures. foliations dip to the northwest, and stretching The lower portion of the detachment-related In the eastern Lewis Hills massif, shearing- lineations plunge to the north-northwest foliation of the peridotites is folded together dominated structures occur in a zone —700 m (Fig. 3b). Toward the lower part of the pe- with the metamorphic sole. Cross-shearing thick within the peridotites (Fig. 4). The ridotites, folding and cross-shearing of the is abundant in the peridotites. Where the boundary with the spreading-related struc- high-strain fabric is more common. Move- metamorphic sole is folded along with the pe- tures was difficult to locate, as it is very broad ment senses in the basal peridotites indicate a ridotites, it is considerably thickened and pre- and diffuse. A local overprint of higher-stress, northward movement of the hanging wall (7 serves relic gabbroic textures. The higher- lower-temperature microstructures is com- northward, 1 southward). Locally, meso- level detachment structures in the peridotites mon even above the suggested boundary be- scopic observations indicating southward-di- and the upper limit of detachment-related tween spreading and detachment strain. For rected shearing are present. strain, however, are not folded. Overall, the example, samples from higher in the section Within the peridotite section of the Table structural pattern in the souther.. Blow Me along the dunite-harzburgite boundary pre- Mountain massif, the most complete se- Down massif is inconsistent with an origin by serve a coarse, low-stress olivine microstruc- quence is exposed in the southeastern part

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ophiolite. Contacts between the ophiolite and accreted material are marked by ductile shear zones or, in the Lewis Hills, may be obscured by late ultramafic intrusions and minor late faults. Early accretion structures are most exten- sively developed in the Lewis Hills (Fig. 4). In the eastern Lewis Hills, below the highly deformed harzburgite described in the previous section, harzburgite which locally preserves a high-temperature fabric is found. In addition, it contains abundant gabbro dikes and signif- icantly more dunite pods than are present in the overlying harzburgites. Where a course (low-stress) olivine microstructure is present, large, delicate, spinel-clinopyroxene sym- plectites, up to 5 mm in diameter, are preserved. All features point to a type of harzburgite distinct from that in the overlying sequence. Neither the characteristic sym- plectites nor the deformed, crosscutting gabbroic dikes have previously been reported from anywhere in the mantle sequence of the BOIC. In addition, the abundance of dunite pods is atypical for the basal peridotites of the BOIC. The distinct harzburgite unit was het- erogeneously overprinted by high-strain fabrics and grades over a short distance into ul- Figure 4. Simplified map of detachment-related structures in the eastern and southeastern tramafic ultramylonites near the contact with Lewis Hills. amphibolites of the metamorphic sole. Around Hines Pond, a remarkable lithological sequence is exposed (Fig. 4). Mantle (Fig. 5). There, the basal high-strain fabric is which are assigned to the spreading phase of harzburgite to the south and west of Hines developed in a 600-m-thick section and is less the ophiolite (Suhr, 1992). Above this spread- Pond is intruded by gabbroic to anorthositic, homogeneously developed than in the central ing-related unit are harzburgites which show anastomosing dikes (Fig. 6a). To the south- and northern Blow Me Down Mountain mas- a variable overprint by mylonitic shear zones east of Hines Pond, a well-banded sequence sif. It appears that an earlier, detachment- with microstructures similar to the basal pe- of harzburgite- (plagioclase-) lherzolite and related mineral fabric has been folded/reori- ridotites. Mineral shape fabrics in the shear gabbro occurs (Fig. 6b). It was strongly af- ented by mylonitic shear zones that have zones and host rocks are subparallel. Folia- fected by high-stress, lower-temperature orientations parallel to the metamorphic sole. tions dip gently to the north-northwest, and strain. This upper-mantle sequence is inter- Due to the overprinting, the mineral-stretch- mineral lineations trend northeast-southwest leaved with amphibolites (as much as 200 m ing lineations have more variable orientations (Fig. 3d). The upper boundary of the unit wide) which are best exposed in two promi- (Fig. 3c) than are in the Blow Me Down mas- climbs up-section toward the south with re- nent, curved ridges. No garnet-bearing am- sif. In plan view, most shear zones show spect to the gabbro-peridotite boundary. The phibolite was found. Some of the contacts a sinistral (upper block to south) component abundance of mylonitic shear zones de- between amphibolite and harzburgite are sep- of movement. Microstructure-based shear- creases toward the south. Field- and micro- arated by an amphibole-olivine ± garnet ± sense determinations (14 measurements) structure-based shear-sense determinations spinel augen schist, probably representing yielded no consistent movement sense. Data (ten samples investigated) indicate a consist- deformed and hydrated poikilitic wehrlite. from Girardeau and Nicolas (1981) suggest ent southwest movement of the upper block. To the west of this complex sequence lies exclusively northward movement of the Asymmetric folds suggest a reverse-fault the morphological ridge of Cloud Mountain. hanging wall in the basal peridotites from Ta- movement. We interpret these shear zones to It exposes a mafic-ultramafic "megalens." ble Mountain. Ultramafic ultramylonites are reflect detachment-related strain at relatively Megalenses represent concentrations of located immediately above the metamorphic high levels within the mantle section. semi-regularly arranged wehrlite, plagioclase sole. They show synkinematic amphibole and wehrlite, troctolite, and gabbro within dunitic are locally metasomatized (Church, 1972; Early Accretion host rocks; these megalenses are widespread Malpas, 1979a; McCaig, 1981). and typical for the eastern and northern Lewis Overlying the basal, detachment-related Mantle harzburgites, amphibolites, and Hills (Karson, 1975, 1977, 1979). Therefore, peridotites of the Table Mountain massif, various lithologies preserved in the metamor- the lithologies of Cloud Mountain are consid- there is a section, as much as 700 m thick, of phic sole represent material which was ac- ered to be an integral part of the Lewis Hills lower-stress, higher-temperature peridotites creted at an early stage to the base of the ophiolite, whereas the harzburgite-amphibo-

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The shear zone branches farther westward and appears to correlate with the "late, anastomosing shear zones" recognized by Karson (1977, 1979). Within the Hines Pond region, lithological and structural contacts are generally steeply dipping, typically to the northwest. This in- cludes (1) the east-west-striking shear zone, (2) contacts between amphibolites and harzburgites in the accreted blocks, (3) the contact between the accreted blocks and the foliation and stretching »/î lineation with dip metamorphic sole in the south and (4) between the accreted blocks and the "proper" ophiolitic sequence to the north and west, and ultramafic ultramylonite (5) the (slightly faulted) contact between the metamorphic sole and harzburgite of the major structural boundary BOIC to the east of Hines Pond. A late, ductile shearing is present on most of these contacts; it led to steepening of the contacts and late fault plunges of mineral lineations (compare Figs.

intermediate structural slices 3b, 3e, 3f, and 3h). o o of Humber Arm allochthon The original attitude of the lithologies and OO (Blow Me Down Bk. and Skinners Cove Fms.) contacts in the Hines Pond region is not well constrained because of the unknown style „—_ late accretion asemblage (harzburgite, amphlbolite, and intensity of late folding along the southern LZXJ greenschist) edge of the Lewis Hills. If the contacts were originally subhorizontal, most of the late metamorphic sole movements observed on them would indicate (amphibolite, greenschist, diabase, sedimentary rocks) a northward movement of the hanging wall. If, however, they were originally steep, they ,——upper mantle harzburgite would indicate a normal sense of movement | i with high stress deformation with north down. t—1 (detachment related) Underlying the ductilely accreted blocks of upper mantle harzburgite mafic and ultramafic material, the metamor- with low stress deformation phic sole sensu stricto can be found. It is (spreading related) recognized as a thin, continuous belt along the eastern margin of all four massifs of the BOIC. Within the sole, the apparent meta- Figure 5. Simplified map of detachment-related structures in the eastern Table Mountain. morphic gradient is extremely steep, reverse, and initially discontinuous but readjusted dur- lite sequence around Hines Pond is consid- indicates that the wehrlite is part of a late ing partial re-equilibration after juxtaposition ered exotic and accreted. The contact be- intrusive suite which occupies the same tec- (Spray, 1984; Jamieson, 1986). The original, tween both sequences must later have been tonic position along strike to the southwest. high-temperature contact between the duc- infiltrated by ultramafic intrusions, as wehr- To the north of Hines Pond, dunites, wehr- tilely accreted blocks and the metamorphic lite and dunite, partly ultramylonitic, partly lites, troctolites, and gabbros are caught in a sole is shown by (1) the structural concor- undeformed, occur commonly along the con- shear zone that is several meters to tens of dance between the ultramafics and the up- tact between the two (Fig. 6c). As a result, meters wide and that strikes east-west; it has permost sole, (2) the presence of (metaso- highly deformed amphibolite may be juxta- an exceptionally strong L > S fabric (Fig. 4). matic) ultramafic ultramylonites immediately posed against poikilitic wehrlite. Only within Stretching lineations in the shear zone show above the sole in all three investigated mas- the immediate contact area has the fabric of gentle to steep plunges ranging from north- sifs, (3) the interbanding of the ultramylonites the amphibolites been erased. Our mapping west, north, northeast, to east (Figs. 3e, 6d). (several millimeters to decimeters thick) and •

Figure 6. Field relationships; pen for scale is 15 cm long, (a) Deformed gabbroic dikes arranged as a network in mylonitic harzburgite; west of Hines Pond, early-accretion assemblage; (b) highly sheared harzburgite (low relief) with repetitive banding of anorthosite/gabbro (high relief); south of Hines Pond; early-accretion assemblage; fieldboo k for scale; (c) weakly deformed wehrlite with subhedral to euhedral, oikycrystic clinopyroxene; east of Cloud Mountain; (d) highly deformed plagioclase wehrlite from east-west-striking, ductile shear zone north of Hines Pond. Apparently different strength of fabric is caused by variable orientation of the extraordinarily strong L > S fabric; (e) well-banded, coarse amphibolites from metamorphic sole in the southeastern Blow Me Down Mountain.

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amphibolites restricted to the uppermost low the ultramafics (Fig. 6e). They are cut by whereas in 1984, he regarded them as "small meters of the sole (eastern Lewis Hills and deformed, pegmatitic dikes. Gabbroic relics ophiolitic slices underlying the main alloch- Table Mountain), and (4) the inclusion of met- have also been discovered in the sole of Table thon." The late-accretion assemblage con- amorphic ultramafics (chlorite-amphibole- Mountain (McCaig, 1983) and to the south of sists of slivers of fine-grained, mylonitic olivine schists) as lenses in the uppermost Hines Pond. On the basis of geochemical ar- greenschists with rare, crosscutting diabase amphibolites (southeast of Hines Pond). The guments, Savic (1988) suggested, however, dikes, amphibolites, harzburgite, and meta- total thickness of the sole in the BOIC is that the majority of the uppermost amphibo- morphic sole. Relic textures of amphibolite in less than 300 m (Williams and Smyth, 1973; lites in Blow Me Down Mountain were de- greenschists demonstrate that some of the Malpas, 1979a). In the upper part of the rived from basaltic protoliths. greenschists derive by mylonitization of the sole, ductile accretion occurred. In the lower Foliations within the metamorphic sole are amphibolites during greenschist-facies condi- part, deformation structures during accretion moderately northwest dipping. The only ex- tions. Less commonly, the presence of mica- switched progressively from homogeneous ception occurs in a small, fault-bound sliver in schists points to sedimentary protoliths for (typical for the upper part of the sole) to het- the northeast of Table Mountain (Fig. 5). The the greenschists. No garnet-bearing amphib- erogeneous. Only the upper part of the sole is stretching lineation is only poorly developed, olite was observed. Stretching lineations well exposed along strike. especially in the greenschist part of the se- within all lithologies plunge gently to the north A detailed study of the sole in the BOIC quence. In the amphibolites, the orientation is (Fig. 3k). More recent mapping shows that was recently performed by Savie (1988) in the gently north plunging (Figs. 3g, 3h) or north- the late accretion unit in the Lewis Hills wid- Blow Me Down Mountain massif. Savie east plunging (in Table Mountain; Fig. 3i), but ens considerably to the southwest and may (1988) established a composite profile which it appears to change to westerly plunges in the actually represent a complex thrust belt. Con- is similar to the Pond Point section from greenschists (Fig. 3j). A similar change in ori- tacts between the different units are poorly southern North Arm Mountain, described by entation within the sole is observed within the exposed. In one stream bed, serpentinite was Williams and Smyth (1973) and Malpas St. Anthony Complex (Jamieson, 1986). Ver- recognized between a greenschist and am- (1979a). gence directions of asymmetric folds in the phibolite unit. The contact between the In the composite profile of Savie (1988), amphibolite unit of the Blow Me Down massif harzburgitic lens and the amphibolite to the thin granulite-grade rocks are developed lo- indicate northward movement of the hanging north of it (Fig. 4) is marked by the same cally at the top of the sole. They grade into wall (Savic, 1988), consistent with our meas- amphibole-olivine augen schist as is in the amphibolites which contain relic clinopy- urements from the ultramafics. early-accretion assemblage. roxenes and locally garnet in their upper part. A common feature of the late-accretion as- Below, epidote amphibolites are developed. Late Accretion semblage, in both the Lewis Hills and Table Below a structural break, greenschist-facies Mountain, is that the blocks were not just rocks are exposed. They are divided into an The characteristic feature of late-accretion simply accreted to the base of the sole, but the upper biotite subfacies and a lower chlorite structures is that prior to, or during, their ac- sole has also been truncated and removed, so subfacies. Both mafic volcanics and pelitic cretion, the metamorphic sole has been re- that the blocks are juxtaposed against the rocks can be recognized as protoliths for the moved. Therefore, the late-accretion litholo- basal peridotite section (in Table Mountain) greenschist unit, which is interpreted by gies postdate formation of the sole. or against the early-accretion assemblage (in Savie (1988) as metamorphosed mélange. In Evidence of brittle accretion is best devel- the Lewis Hills). The accreted blocks repre- the lower part of the greenschist unit, original oped in the northeastern Table Mountain sent a wide range of structural levels, and structures of the volcanic rocks are pre- massif (Fig. 5) where a series of fault-bounded most lithologies show intense ductile defor- served. Biotite-rich mylonites are abundant in blocks of metamorphic rocks and peridotite mation before accretion to the base of the the lower part of the amphibolite unit. were first described by Quinn (1985). From ophiolite. It is not clear whether the late-ac- Although the lithologic or metamorphic zo- west to east, four lithological units are ob- cretion assemblage was juxtaposed to the nation within the sole shows a broad-scale, served: multiply deformed, fine-grained, gar- ophiolite one sliver at a time or whether it was along strike similarity, a detailed fine-scale netiferous amphibolites and felsic gneiss; accreted as a pre-assembled unit. continuity does not exist (see also Savie, moderately strained, altered harzburgites; Below both the sole and the late-accretion 1988). For example, amphibolite-grade mafic multiply deformed plagioclase amphibole ± assemblage in the southern Lewis Hills, gab- rocks are positioned in the highest structural epidote ± garnet ± quartz schists with prom- broic to micro-gabbroic rocks have been level of the sole. The thickness of the am- inent lenses and bands of epidote ± sphene ± recognized. They vary from undeformed phibolites, however, varies from less than 10 garnet; and moderately strained harzburgites. and unmetamorphosed to highly deformed, m to more than 100 m. Where the greenschists Farther east, gray slates and arkoses are ex- greenschist-facies rocks and are assigned to reach high up in the sole, micaceous schists posed. They are probably part of the under- the metamorphosed mélange typically may occur at a short distance below the am- lying structural slice of the allochthon. Con- present at the base of the sole (compare with phibolites, as is the case in the southern tacts between each of the four units are not Savic, 1988). branch of the Knight's Brook profile and far- exposed. The westernmost amphibolite con- ther south, where abundant quartz-biotite- tains spessartine sillimanite-bearing coticule muscovite-plagioclase-garnet schists are ex- lenses. BOUNDARY CONDITIONS FOR posed. Where the amphibolites are very A late-accretion assemblage is also recog- OBDUCTION HISTORY thick, as they are in the southern Blow Me nized in the southern Lewis Hills. Initially, Down massif, layered, coarse-grained, lo- Karson (1979) correlated these lithologies The main results from our study of the de- cally garnetiferous amphibolites with relict ig- with rocks exposed in the western Lewis Hills tachment-related lithologies of the BOIC are neous, gabbroic textures are developed be- (then termed Mount Barren Assemblage), as follows.

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As was pointed out by Nicolas (1989, p. 302), subduction-zone environments are, however, too cold to allow formation of the high-grade features observed in the basal peridotites and the sole. In addition, deformation of the basal peridotites occurred under dry conditions with the exception of the very basal ultramylonites. This feature is inconsistent with con- tinued thrusting of peridotites over hydrated oceanic basalts in a subduction zone, in par- ticular if the subducted material is derived from a fracture zone (Casey and Dewey, 1984). Finally, the study of Suhr (1992) considers the mantle section of Table Mountain as continuous, without any major structural breaks. Such breaks would be required if Figure 7. Model for detachment of the Bay of Islands ophiolite. BMD, Blow Me Down mantle wedges were expelled, as suggested in Mountain; NAM, North Arm Mountain; TM, Table Mountain; LH, Lewis Hills; double dashed the model of Casey and Dewey (1984). We line, position of former ridge. Lithosphere-asthenosphere boundary (taken as 1000 °C) has been consider a subduction zone as a very unlikely adjusted during static cooling after cessation of spreading. Paths 1 and 2 refer to different detachment horizon for the BOIC. detachment structures preserved in the four massifs. In the LH, northward-directed thrusting led Any model to explain the obduction struc- to early accretion of material derived from the Coastal Complex, a feature not observed in the tures must be based on a palinspastic recon- three other massifs. Syn-detachment ultramafie intrusions, present in the LH, are not shown. struction of the Bay of Islands ophiolite. Our Back-thrusting is observed in TM. Note that the Coastal Complex acted mainly as a lateral ramp interpretation uses a recent model developed during detachment. from a mantle tectonic study of the BOIC (Fig. 7; Suhr, 1992). It contrasts to previous (1) Detachment structures of the BOIC ites and amphibolites) is a typical feature of models by Karson and Dewey (1978), Casey can be classified into three groups: structures the CC. and others (1983), and Karson and Elthon dominated by shearing, followed by early, Further constraints are provided by peak (1987) by suggesting that (1) the Lewis Hills ductile accretion, and late accretion. Late ac- temperatures from the metamorphic sole and massif was not aligned together with the three cretion has been accompanied by apparent temperatures of deformation estimated for other massifs of the BOIC but was spatially removal of part of the ophiolite sequence. the basal peridotites. For the latter ones, a separated; the current proximity of the Lewis (2) Early-accretion structures are most temperature of 900-1000 °C has been esti- Hills to the other massifs is a late feature pro- abundant in the Lewis Hills. Material ac- mated (Suhr, 1991). For the metamorphic duced during the later emplacement history; creted at this stage is exotic to the BOIC. sole, peak temperatures/pressures of 750- (2) both the Lewis Hills and the Blow Me (3) Textural evidence shows that the first 850 "C/7-1 1 kbar were calculated by McCaig Down massif were located close to the ridge accreted rocks within the metamorphic sole (1983) for an occurrence in Table Mountain, axis at the time of detachment; and (3) the were at least in part derived from gabbroic and 650-850 °C/6-9 kbarfor an occurrence in plate movement was in the process of chang- protoliths. Following geochemical arguments the Blow Me Down massif (Savic, 1988). In ing from an extensional plate boundary gen- by Savic (1988), early accretion in the sole order to explain the high temperatures, a erating the oceanic lithosphere of the BOIC must also have involved basaltic rocks. source of heat is required. Residual heat from to a contractional setting. Consistent with (4) The main detachment movement was a still young and hot ophiolite has been in- earlier models (Karson and Dewey, 1978; directed north to north-northwest and not voked by numerous authors (Malpas, 1979a; Casey and others, 1983), the three northern west to northwest as implied in previous mod- Jamieson, 1980, 1981, 1986; Spray, 1984; massifs are located to the north of a west- els (Williams and Stevens, 1974; Karson and Savic, 1988). Although modeling studies have northwest-east-southeast-trending spread- Dewey, 1978; Searle and Stevens, 1984). In suggested that the high-grade nature of the ing-center axis and to the east of the boundary Table Mountain, and locally in the other sole could also result from frictional heating between the CC and the BOIC. massifs, additional south to southwestward alone (Pavlis, 1986), more recent work of Evidence presented previously suggests movement is indicated. Hacker (1991) indicated that residual heat is that in addition to a structural contrast (Kar- A fundamental concern in developing a critical to attain the high temperatures. This son and Dewey, 1978; Karson, 1984), there is model for the obduction history of the Lewis would imply that the ophiolite was still very a significant age and geochemical difference Hills is determining the source of the exotic, young at the time of detachment. between the CC and BOIC (Jacobsen and early-accretion assemblage. The following The high pressures indicated for the upper- Wasserburg, 1979; Malpas, 1979b; Dunning two features suggest to us a derivation from most sole rocks are inconsistent with the cur- and Krogh, 1985; Jenner and others, 1992). the CC. (1) Mantle harzburgites containing rently observed thickness of the oceanic lith- By combining the geochemical and structural crosscutting gabbroic dikes have been found osphere in the BOIC (about 12 km plus evidence, we suggest that the boundary be- in the eastern CC of the Lewis Hills and the overlying water column). This has led Casey tween the CC and BOIC represents the oce- Lookout Hills massif of the CC (Fig. 1). (2) and Dewey (1984) to suggest a model involv- anic contact between an older arc terrane and Juxtaposition of highly deformed lithologies ing obduction of the BOIC in the hanging wall a younger oceanic basin, i.e., the typical re- from contrasting structural levels (harzburg- of a progressively flattening subduction zone. lation for a boundary between an arc and a

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back-arc basin (Cawood and Suhr, 1992). sation of spreading, lithospheric thickness more frequent; this explains the more com- Based on evidence from the Lewis Hills and increases rapidly. For example, after 1 m.y. mon occurrence of volcanic protoliths within the internal structure of the CC, the contact of static cooling, the lithosphere-astheno- the lower part of the sole (compare with between both tectonic units trends between sphere boundary beneath a ridge is located at Jamieson, 1980; Spray, 1984). N10°W and N20°E. This trend implies that a depth of 12 km (Boudier and others, 1988). The obduction path of the Lewis Hills mas- during northward movement of the BOIC, The increased rigidity involved with litho- sif was different from that of the other massifs the CC acted mainly as a lateral ramp to ob- spheric thickening during static cooling could due to the proximity of the eastern Lewis duction. A similar movement was inferred by explain why, during northward obduction of Hills to the colder CC (Fig. 7). This difference Casey and Dewey (1984). the BOIC, part of the northern side of the is manifested in ductile accretion of exotic The thermal regime at the time of detach- ridge was undercut by the detachment hori- lithologies (probably derived from the CC) to ment is that of a recently extinct spreading- zon and obducted (Fig. 7). Otherwise, for a the base of the Lewis Hills. Structural dis- center environment. Models of spreading very young spreading center it is intuitive to ruption and complex intrusive relationships centers require that the isotherms dip gently have the basal thrust emerge at the ridge it- are characteristic for the CC and its boundary away from the ridge axis, the exact angle de- self, as was postulated for the Oman ophiolite with the BOIC and resulted in thermal and pending on the spreading rate (Parker and (Boudier and others, 1982, 1988). lithological heterogeneities (Comeau, 1972; Oldenburg, 1973; Sleep, 1975; Kusznir and In the case of the three northern massifs of Karson and Dewey, 1978; Karson, 1984). As Bott, 1976). During static cooling between the the BOIC, northward shearing was initially a result, the shape of the basal décollement time of formation of the oceanic lithosphère not influenced by the CC due to the large plane assumed an irregular orientation, in and initiation of detachment, the isotherms distance from the CC-BOIC boundary contrast to the relatively smooth profile re- will adjust to the lower temperatures, and (Fig. 7). The temperature distribution across corded in the three northern massifs of the their orientation will be shallower than at the the décollement plane was inverted. Heat was BOIC. The warped shape of the plane along time of formation (Boudier and others, 1988). added to the footwall from the still hot perid- which early accretion occurred within the LH Near the CC, the isotherms of the BOIC will otite slab and perhaps also due to frictional is partly attributed to this irregular move- be depressed due to the difference in age and energy produced along the thrust plane. In the ment. Ramping and subsequent footwall associated thermal structure between both absence of any significant lithological heter- propagation of the detachment plane were complexes (Forsyth and Wilson, 1984; Phipps ogeneities within the olivine-dominated up- common in the heterogeneous lithosphère of Morgan and Forsyth, 1989). Some of this per-mantle lithologies, higher temperatures the CC and caused the ductile accretion of thermal contrast might have been leveled out in the hanging wall tended to partition the blocks of ultramafic and mafic material. Fol- due to ultramafic intrusions along the bound- deformation up-section into Theologically lowing accretion of the blocks, the boundary ary (Karson and Dewey, 1978; Karson and weaker (as hotter) peridotites. Accretion of was infiltrated by ultramafic intrusions. The others, 1983; Dunsworth and others, 1986) material to the base of the ophiolite is thus intrusion might have emerged from the CC- and frictional heating along transform faults unlikely. The shearing-dominated deforma- BOIC boundary, where they are a common (Chen, 1988), or, more generally, along crust- tion is characteristic for this stage of detach- feature (Karson and Dewey, 1978). al-scale, strike-slip faults. ment. The south- to southwestward-directed Accretion of the mafic rocks in the Lewis movement recorded in Table Mountain and Hills occurred in a fashion similar to that of OBDUCTION HISTORY locally in the other massifs is interpreted as the three other massifs, that is, as thin slivers minor back-thrusting. of sheared amphibolites and greenschist pre- A Theologically prominent detachment ho- When the décollement plane had reached served in the sole. Our model suggests that, at rizon is required in order to explain the ob- the base of the crust, deformation switched least within the eastern Lewis Hills massif, duction of a tectonically undisturbed piece of rapidly from within the ultramafics into the the sole is derived from protoliths of the CC. oceanic lithosphère which, in the case of the mafic lithologies of the footwall, as mafic Late accretion must have involved out-of- BOIC, was at least 75 km long and may have rocks are Theologically weaker than olivine- sequence thrusting in order to explain the re- originally been much larger. For the model of dominated Theologies during ductile deforma- moval or shifting of the sole in areas where a dying spreading center, the lithosphere/as- tion (Pavlis, 1986; Savic, 1988; Hacker, 1990, late accretion occurred. In the Lewis Hills, thenosphere boundary constitutes one of the 1991). As a result of the switching into the structural reworking of the early-accretion as- most pronounced Theological boundaries and footwall, a low-temperature overprint of the semblage within the late-accretion assem- an ideal basal décollement. Our model there- peridotites did not take place except for the blage is indicated by (1) the characteristic oc- fore uses this boundary (taken as 1000 °C, formation of the ultramylonite and rare am- currence of amphibole-olivine augen schists Boudier and others, 1988) within the con- phibole-bearing shear zones. A further result between harzburgite and amphibolite and (2) straints provided in the previous section. The was that the upper-mantle sequence remained the presence of slivers of metamorphic sole in other potentially significant boundaries internally coherent during subsequent defor- both the early and late accretion assemblage. within the proposed paleogeographic frame- mation. Thrusting over crustal rocks involved Possibly, this stage was associated with a work are represented by the CC-BOIC intermittent accretion of material derived more westerly directed movement than was boundary and the former plate boundary of from the footwall, as evidenced by the incor- the case in the early phase of detachment as the ridge itself. The effects of the CC-BOIC poration of amphibolites with gabbroic pro- shown by stretching lineations in the green- boundary should be seen in the Lewis Hills toliths derived from the lower oceanic crust. schist part of the metamorphic sole. The in- massif (see below). The role of the extinct As the thrust climbed farther into progres- corporation of continental-margin clastic sed- ridge is difficult to evaluate. During spread- sively more hydrated, and hence weaker, ma- imentary lithologies in the greenschist and ing, the thin lithosphère at the ridge consti- fic oceanic crust, thrust propagation to the subgreenschist portions of the metamorphic tutes a major zone of weakness, but after ces- foreland (and therefore accretion) became sole also suggests that this phase of accretion

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was associated with emplacement of the de- blage of mylonitic greenschist, amphibolites, ature structure of a ridge-transform-ridge system: Earth and Planetary Science Letters, v. 70, p. 355-362. tached ophiolite onto the North American and harzburgites was accreted. Ghent, E. D.,and Stout, M. Z., 1981, Metamorphism at the base of the Samail Ophiolite, southeastern Oman Mountains: Journal margin. of Geophysical Research, v. 86, p. 2557-2571. Girardeau, J., and Nicolas, A., 1981, The structures of two ophiolite Two remarkable features of the ophiolite ACKNOWLEDGMENTS massifs, Bay of Islands, Newfoundland: A model for the oce- are (1) that detachment-related deformation anic crust and upper mantle: Tectonophysics, v. 77, p. 1-34. Hacker, B. R., 1990, Simulation of the metamorphic and deforma- in the three northern massifs was limited to Our work was supported by research tional history of the metamorphic sole of the Oman ophiolite: the basal 500 m of the 12-km-thick litho- Journal of Geophysical Research, v. 95, p. 4895-4907. grants from the Natural Sciences and Engi- Hacker, B. R., 1991, The role of deformation in the formation of sphère, and (2) there is a lack of internal im- neering Research Council of Canada, Energy metamorphic gradients: Ridge subduction beneath the Oman ophiolite: Tectonics, v. 10, p. 455-473. brication of the ophiolite. These circum- Mines and Resources ; and Parks Canada. We Jacobsen, S. B., and Wasserburg, G. J., 1979, Nd and Sr isotopic study of the Bay of Islands ophiolite complex and the evo- stances were favored by three factors: (1) thank Tony Berger for accommodation and lution of the source of mid-ocean ridge basalts: Journal of movement of material of very high rigidity, enjoyable discussions on the geology of west Geophysical Research, v. 84, p. 7429-7445. Jamieson, R. A., 1980, Formation of metamorphic aureoles beneath that is, oceanic lithosphere (Brace and Kohl- Newfoundland. Many thanks also to ophiolites—Evidence from the St. Anthony complex, New- stedt, 1980; Kusznir and Park, 1987), over the foundland: Geology, v. 8, p. 150-154. F. Boudier, G. Dunning, J. van Gool, R. A. Jamieson, R. A., 1981, Metamorphism during ophiolite emplace- weaker asthenospheric mantle; (2) at a later ment—The petrology of the St. Anthony Complex: Journal of Jamieson, and A. Nicolas, who critically read Petrology, v. 22, p. 397-449. stage, movement over Theologically weaker early versions of this manuscript. T. Rivers Jamieson, R. A., 1986, P-T path from high temperature shear zones beneath ophiolites: Journal of Metamorphic Geology, v. 4, crustal lithologies (Pavlis, 1986; Hacker, helped in identification of the coticule sample p. 3-22. 1990,1991); and (3) lack of preservation of the Jenner, G. A., Dunning, G. R., Malpas, J., Brown, M., and Brace, from Table Mountain. Constructive reviews T., 1991, Bay of Islands and Little Port complexes, revisited: leading edge of the ophiolite. The last point by Jack Casey and R. A. Jamieson are grate- Age, geochemical and isotopic evidence confirm supra-sub- leaves open the possibility that imbrication duction zone origin: Canadian Journal of Earth Sciences, fully acknowledged. They considerably im- v. 28, p. 1635-1652. has occurred in oceanic lithosphere which Karson, J. A., 1975, Structural studies in the mafic and ultramafic proved the manuscript. rocks of the Lewis Hills, western Newfoundland [M.Sc. the- was once present to the north of Table Moun- sis]: Albany, New York, State University of New York at tain. It is interesting in this respect that back- Albany, 125 p. Karson, J. A., 1977, The geology of the northern Lewis Hills, west- thrusting is observed in Table Mountain, the REFERENCES CITED ern Newfoundland [Ph.D. dissert.]: Albany, New York, State University of New York at Albany, 474 p. massif closest to the proposed leading edge. Boudier, F., Nicolas, A., and Bouchez, J. L., 1982, Kinematics of Karson, J. A., 1979, Geological map and descriptive notes of Lewis oceanic thrusting and subduction from basal sections of ophi- Hills massif, western Newfoundland: Department of Energy, Although numerous obduction-related fea- olites: Nature, v. 296, p. 825-828. Mines and Resources: Geological Survey of Canada, open file Boudier, F., Ceuleneer, G.. and Nicolas, A., 1988, Shear zones, 628. tures of the BOIC are addressed in our model, thrusts and related magmatism in the Oman ophiolite: Initi- Karson, J. A., 1984, Variations in structure and petrology in the other features remain problematical. Among ation of thrusting on an oceanic ridge: Tectonophysics, v. 151, Coastal Complex, Newfoundland: Anatomy of an oceanic p. 275-296. fracture zone, in Lippard, S. J., Shelton, A. W., and Gass, these are the enigma of high pressures re- Brace, W. F., and Kohlstedt, D. L., 1980, Limits on lithospheric I, G., eds., Ophiolites and oceanic lithosphere: Geological stress imposed by laboratory experiments: Journal of Geo- Society of London Special Publication 13, p. 131-144. corded in the sole and the apparent abun- physical Research, v. 85, p. 6248-6252. Karson, J., and Dewey, J. 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flow and temperature perturbations due to a transform offset: Spray, J. G., 1984, Upper mantle decoupling and ophiolite displace- Williams, H., and Payne, J. G., 1975, The Twillingate granite and Effects on oceanic crustal and upper mantle structure: Journal ment, in Gass, I. G., Lippard, S. J., and Shelton, A. W., eds., nearby volcanic groups: An island arc complex in northeast of Geophysical Research, v. 93, p. 2955-2966. Ophiolites and oceanic lithosphere: Geological Society of Newfoundland: Canadian Journal of Earth Sciences, v. 12, Quinn, L.,1985, The Humber Arm allochthon at South Arm, Bonne London Special Publication 13, p. 255-268. p. 982-995. Bay, with extension in the Lomond area, west Newfoundland Spray, J. G., 1988, Thrusi-related metamorphism beneath the Sh- Williams, H., and Smyth, W. R., 1973, Metamorphic aureoles be- IM.Sc. thesis]: St. John's, Memorial University of Newfound- etland Islands oceanic fragment, northeast Scotland: Cana- neath ophiolite suites and alpine peridotites: Tectonic impli- land, 188 p. dian Journal of Earth Sciences, v. 25, p. 1760-1776. cations with west Newfoundland examples: American Jour- Savie, G., 1988, Structural and metamorphic geology of the subo- Spray, J. G., and Williams, G. D., 1980, The sub-ophiolite meta- nal of Science, v. 273, p. 594-621. phiolitic dynamothermal metamorphic sole and peridotite tec- morphic rocks of the Ballantrae igneous complex, SW Scot- Williams, H., and Stevens, R. K.., 1974, The ancient continental tonites, Blow Me Down massif, Newfoundland-Canada: Tec- land: Geological Society of London Journal, v. 137, margin of eastern North America, in Burk, C. A., and Drake, tonic implications for subduction and obduction [Ph.D. p. 359-368. C. L., eds., The geology of continental margins: New York, dissert.]: Houston, Texas, University of Houston, 361 p. Suhr, G., 1991, Structural and magmatic history of upper mantle Springer-Verlag, p. 781-796. Searle, M. P., and Malpas J., 1982, Petrochemistry and origin of peridotites in the Bay of Islands Complex, Newfoundland sub-ophiolitic metamorphic and related rocks in the Oman [Ph.D. thesis]: St. John's, Newfoundland, Memorial Univer- Mountains: Geological Society of London Journal, v. 139, sity, 426 p. p. 235-248. Williams, H., 1973, Bay of Islands map area, Newfoundland, (12G), Searle, M. P., and Stevens, R. K., 1984, Obduction processes in report and map 1355A: Geological Survey of Canada, Paper ancient, modern and future ophiolites, in Gass, I. G., Lippard, 72-34,7 p. S. J., and Shelton, A. W., eds., Ophiolites and oceanic lith- Williams, H., 1975, Structural succession, nomenclature, and inter- osphere: Geological Society of London Special Publication pretation of transported rocks in western Newfoundland: Ca- 13, p. 303-315. nadian Journal of Earth Sciences, v. 12, p. 1874-1894. Sleep, N. H., 1975, Formation of oceanic crust: Some thermal con- Williams, H., and Cawood, P. A., 1989, Geology, Humber Arm MANUSCRIPT RECEIVED BY THE SOCIETY MAY 14, 1991 straints: Journal of Geophysical Research, v. 80, Allochthon, Newfoundland: Geological Survey of Canada, REVISED MANUSCRIPT RECEIVED JUNE 12,1992 p. 4037-4042. map 1678A, scale 1:250,000. MANUSCRIPT ACCEPTED JULY 15,1992

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