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. 399 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/3/399/3381730/i0016-7606-105-3-399.pdf by guest on 03 October 2021 SUHR AND CA WOOD 15 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 detach- ment. 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 Downloaded from http://pubs.geoscienceworld.org/gsa/gsabulletin/article-pdf/105/3/399/3381730/i0016-7606-105-3-399.pdf by guest on 03 October 2021 OPHIOLITE OBDUCTION, BAY OF ISLANDS, NEWFOUNDLAND the allochthon onto the North American mar- gin. This later emplacement phase of the ob- duction 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 de- veloped within the northern and central Blow Me Down Mountain where a gradual over- print 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 map- ping 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 in- creases gradually toward the base of the pe- ridotites and culminates in ultramafic ul- tramylonites, 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 fab- ric 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