Arakapas fault belt, : A fossil transform fault

l' (f G^ASS^'^ [ department of Earth Sciences, The Open University, Milton Keynes, MK7 6 A A, England

ABSTRACT undeformed. Furthermore, the major structure and component rock units were identified in the 1950s by detailed mapping at the It is widely accepted that the massif of Cyprus is a frag- scale of 1:5,000 by the Cyprus Geological Survey. Also, in those ment of oceanic lithosphere formed at a constructive margin be- pre—plate-tectonic days, the geology and the known geophysics neath a small marginal sea some 85 m.y. ago. The Arakapas fault were combined (Gass and Masson-Smith, 1963) to suggest that the belt is an elongate east-west fracture zone where an intensely brec- massif was a fragment of Mesozoic oceanic crust and mantle ciated basement of ocean crust is overlain by a variety of mafic vol- formed between the then more widely spaced continents of Africa canic rocks and clastic sediments. While constructive margin proc- and Eurasia. Following the introduction of plate-tectonic theory, it esses were still active elsewhere on the massif, the Arakapas fault was proposed (Gass, 1968) that the Troodos massif had formed at belt existed as a trough with a rugged bathymetry formed of an east-west Tethyan constructive margin, and, to explain the numerous fracture zones between which were relatively unde- north-south orientation of the sheeted dike swarm, a 90° anti- formed blocks. The north-south-trending dikes of the main massif clockwise rotation was postulated. In 1971, Moore and Vine sub- swing progressively westward into an east-west alignment as the stantiated the constructive margin origin of the massif by field and fault is approached. This deviation could be due to horizontal drag petrochemical studies. Since that time, numerous geological and along the fault, but dike injection into a sygmoidal stress field prior geophysical investigations either positively support or do not con- to the development of the fault is considered. Onto the rugged tradict an origin of the Troodos massif at a constructive margin bathymetry, in which fault scarps and associated scree deposits are some 85 m.y. ago, although geochemical evidence (Pearce, 1973) still identifiable, mafic lavas were extruded, and a variety of sedi- suggests that this was in a small back-arc basin. In Figure 1, the ments, produced by the submarine erosion of bathymetric highs main units of the Troodos ophiolite are given in chronological within and on the flank of the trough, were deposited. Some of the order and briefly described. volcanic rocks within the fault belt are more primitive than those of Along the southern flank of the massif, a prominent linear east- the main massif, and this is interpreted as owing to a higher per- west depression separates the main massif from the centage of melt in the underlying mantle and easier egress for the Forest, a mountainous area of similar but more tectonized rocks, to basaltic magmas. The metamorphic imprint indicates that the the south (Fig. 1, inset). Although this valley has long been recog- thermal gradient in the fault zone was steeper than elsewhere on the nized as an area of intense faulting (Wilson, 1959; Bear, 1960; massif and that quantities of circulating sea water were greater. Bagnall, 1960, 1964; Pantazis, 1967; Lapierre and Rocci 1967; Later, once this part of the fault zone had moved well outside the Lundberg, 1969) it was only in 1971 that Moores and Vine constructive margin offset, serpentinite masses were emplaced and suggested that it might represent a fossil transform fault. For the normal and reverse faulting occurred. Structural, petrochemical, past three years, this fault zone, hereafter termed the Arakapas and sedimentary features combine to suggest that the Arakapas fault belt, has been the subject of detailed study by one of us fault belt is a fossil transform fault. (Simonian, 1975); the principle findings of this study and a com- parison with oceanic fracture zones are presented here. INTRODUCTION ARAKAPAS FAULT BELT An ophiolite, as defined by the Geological Society of America's 1972 Penrose Conference on ophiolites, is a distinctive assemblage The topographic depression that marks the site of the Arakapas of mafic and ultramafic rocks that range downward from mafic pil- fault belt extends east-west for some 35 km before disappearing, at low lavas through a sheeted dike complex, layered gabbros, and ul- both ends, beneath an undeformed cover of Tertiary sedimentary tramafic cumulates to an underlying tectonized peridotite. rocks (Fig. 2). The fault belt varies in width from 0.5 to 1.5 km and Although this widely accepted definitipn deliberately avoids any in elevation from 400 to 500 m, and it has the surface configuration genetic implications, the ophiolite assemblage and that deduced of either a single valley or a series of near-parallel troughs separated from geophysical evidence for the oceanic lithosphere as formed at by elongate ridges. The main massif to the north reaches heights of constructive margins have much in common — so much so that the 1,200 to 1,500 m and consists primarily of sheeted dike complex, details of ophiolite geology are commonly used to clothe the neces- whereas in the lower lying (600 to 900 m) Limassol Forest to the sarily meager skeleton of geophysical data on present-day con- south, serpentinites, together with gabbros and sheeted dike com- structive margin processes. plex, are the main rock types. Within the fault belt, the main rock The Troodos massif of Cyprus has played a key role in the outcrops are of pillow lavas that are intercalated with thick wedges ophiolite saga because, unlike so many others, it is complete and of clastic sedimentary rock and overlie an intensely fractured base-

Geological Society of America Bulletin, v. 89, p. 1220-1230, 7 figs., 1 table, August 1978, Doc. no. 80809.

1220

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ment of mechanically brecciated Sheeted Intrusive Complex. This plex of the Axis Sequence containing screens and invaded by minor threefold division into main massif, Arakapas fault belt, and intrusions of unlayered gabbro and plagiogranite. Some 10 km Limassol Forest is a realistic framework within which the disposi- north of the fault belt, the Axis Sequence dikes, as elsewhere on the tion, structure, and composition of the various rock types in and massif, are between 0.3 and 3.0 m wide and near vertical and strike adjacent to the fault belt can be described. north-south. Southward, approaching the fault belt, there is a pro- gressive change in strike from north-south to east-west. As the Main Massif near-vertical orientation is maintained throughout the trace of the dike trend is concave to the west (see Fig. 2). Throughout the zone The domal structure of the Troodos massif (Wilson, 1959) is re- of strike change, the width of the dikes does not vary in any sys- sponsible for the annual disposition of the main rock types, in tematic way, and the well-developed sheeted aspect of the complex which the deepest levels of tectonized peridotite and cumulate is maintained. Individual dikes show no more internal jointing than plutonic rocks crop out in the elevated center of the massif, and the elsewhere on the massif. If the change in strike has been tectonically upper unit, of pillow lavas, occupies the periphery. Between these impressed upon the dikes subsequent to their injection along a two units, most of the massif is formed of the Sheeted Intrusive north-south azimuth, the movement has been taken up along the Complex, which consists of 100% near-vertical metabasalt dikes dike contacts. As the fault belt is approached, brecciation zones with a dominant north-south strike and a unit thickness of about parallel to the dike trend develop and become progressively more 1.5 km. Gass and Smewing (1973) recognized that the two types of extensive until they merge into the fault belt itself; within 1 km of pillow lavas on Cyprus (Wilson, 1959) could be identified, by their the fault belt, jointing within individual dikes is intense. Screens metamorphic imprint, as having been erupted in different tectonic and irregular intrusions of high-level unlayered gabbro and regimes. The Lower Pillow Lavas were poured out at a constructive plagiogranite within the Axis Sequence are markedly more abun- axis, whereas the Upper Pillow Lavas represent off-axis activity. dant in the area adjacent to the fault belt than elsewhere on the The term "Axis Sequence" was coined to describe the Lower Pillow massif. Lavas and the genetically related underlying Sheeted Intrusive Complex. No marked time gap is envisaged between the Axis Se- Arakapas Fault Belt Basement quence and the Upper Pillow Lavas (Smewing and others, 1975; Robertson, 1975) except that the near-axis hydrothermal activity The present land form of the Arakapas fault belt varies along its that metamorphosed the Axis Sequence had ceased before the length from a single valley to a series of valleys separated by paral- Upper Pillow Lavas were erupted. lel ridges. In this it reflects, albeit in a subdued manner, not only the Adjacent to the Arakapas fault belt, the erosion levels are such form of the linear oceanic depression but also the configuration of that the main massif rock types exposed are the sheeted dike com- the underlying basement. As this basement was largely infilled by a

km Maestrichtian sediments rO

v Upper Pillow Lavas Umbers Zeolite p|Ä% facies -1 Pillow Lavas

Green- -2 schist Axis Sheeted Dike facies Sequence Complex

Figure 1. Main rock units of Troodos ophio- lite. Trondhjemites High-level intrusives High-level gabbros

•5 Layered gabbros o o o o Layered series o o o Layered ultramafic rocks

A A A A -6 AAA Tectonized harzburgite-depleted mantle

A A A A

AAA

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Fold axis in Yerassa fold belt

Pillow Lavas Sheeted Dike Complex Plutonic Complex (Mainly Gabbros)

Serpentinite

Figure 2. Main structural elements and rock types in and adjacent to Arakapas fault belt.

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mixture of volcanic and sedimentary material, it is both realistic are not easily discerned from the overlying submarine scree de- and convenient to divide the description of the fault belt into that of posits. However, the screes, which appear to have formed by grav- the basement, given in this section, and that of the infill, discussed ity collapse of the depression walls triggered by fault movement, in the next section. occasionally show vague bedding and also have a more consoli- The geologic limits of the fault belt are readily defined. To the dated groundmass. Furthermore, the scree deposits have a lenticu- north, the contact with the main massif is taken as the line or nar- lar outcrop pattern in contrast to the linear disposition of the fault row zone across which the uniform sheeted structure of the dike breccias. The widespread occurrence of undeformed metamorphic complex is replaced by dominantly brecciated rocks. To the south, mineralization demonstrates that all the rocks of the fault belt it is the intrusive, intensely sheared contact with the younger ser- basement underwent metamorphism after brecciation. pentinites of the Limassol Forest (see Fig. 2). Within the fault belt, the surface configuration of the basement varies markedly along its Infill length. A north-south section between Kato Dhrys and Vavla (Fig. 3) suggests an extremely rugged surface, with linear depressions of Erupted and deposited into the bathymetric depression, and rest- intensely brecciated basement separated by east-west-elongated ing unconformably on the brecciated Arakapas fault belt basement, masses of undeformed Axis Sequence (Figs. 2, 3). To the west, for is a thick sequence of pillowed flows and intercalated sedimentary the 6 km between Perapedhi and Ayios Mamas, the rocks, all invaded by sills, bosses, and irregular intrusions. palaeobathymetry is defined by an east-west south-facing scarp Elsewhere on the massif, Axis Sequence and Upper Pillow Lava aproned by extensive fossil scree deposits. rocks can be distinguished by geochemical and petrographic Altered metabasalts of the Axis Sequence dikes, together with characteristics, whereas in the fault belt this can only be done on rare microgabbro and plagiogranites and rarer pillow lavas, are the metamorphic criteria. However, in contrast to the main massif, a dominant rock type of the basement, and they are present as both strong unconformity exists between these two units. The sedimen- fault breccias and as undeformed blocks. The fault breccias form tary intercalations are here labeled Axis Sequence interlava and the depressions and the undeformed blocks the "highs" in the Upper Pillow Lava sedimentary rocks, and the infill is described configuration of the fault belt basement. The orientation of the under "igneous" and "sedimentary" headings. dikes with the undeformed blocks varies widely and is, seemingly, Igneous Infill. In both the Axis Sequence and the Upper Pillow randomly orientated (Fig. 2). Although the fault breccias are widely Lavas of the Arakapas fault belt, the main eruptive types are pillow distributed, it is evident that brecciation becomes more intense lavas in which many of the "pillows" are unidirectionally elon- toward fault zones, which are also commonly mineralized. There is gated. However, unlike the pillowed sequences elsewhere on no doubt that the brecciation of the basement is mechanical and Troodos, the pillowed units are interbedded with breccias and in- was produced by east-west transcurrent movements. The fault vaded by sills, bosses and irregular intrusions; a few massive flows breccias consist of highly angular fragments of metabasalt and mi- have been identified. Axis Sequence dikes, so abundant elsewhere, crogabbro, ranging in size from 0.5 to 0.01 m, set in a matrix of pul- are rare and, when present, are thin, crosscutting, and discon- verized rock flour. In both texture and structure the fault breccias tinuous.

Upper Pillow Lavas

Sediments

Lavas Axis Sequence 0 TfTvTrV • Sediments

Pillow Lavas

Scree deposits u' i ! m Brecciated Sheeted Complex V Sheeted Complex H o Approximate Horizontal and Vertical Scale 1.0 km^ I

Figure 3. Composite schematic north-south cross section across Arakapas fault belt, showing rugged brecciated basement and disposition of volcanic and sedimentary infill.

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Although pillow lavas can be divided into a variety of pétro- cpx, and cpx + plagioclase (pi). Accessory phenocrystal chromite graphie types on the basis of their megascopic characteristics, lack occurs in all olivine-bearing varieties. Olivine, which shows no

of exposure and structural complexity preclude a lava-type stratig- reaction relation, is Fo90_8o, opx is strongly magnesian (Ca4Mg85-

raphy; all evidence suggests that the various lava types are ran- FeH — probe data; C. Allen, 1974, personal commun.), and cpx

domly interbedded. The abundant presence within the breccias of has the composition Ca41Mg5iFe8 (probe data; C. Allen, 1974, per- fragments having the original chilled margin of pillows strongly sonal commun.). Chromite obviously crystallizes first and is fol- suggests that these horizons were produced by the gravity collapse lowed by ol, then either by opx or cpx, and finally by pi. The rare of newly erupted pillow lavas and, therefore, they could realisti- occurrence of the assemblage ol + cpx + pi indicates that only oc- cally be termed collapse breccias. Both the elongate form of the pil- casionally was the ternary point in the basalt system reached. It is lows and the interbedded collapse breccias indicate eruption onto proposed here that the olivine phyric varieties crystallized from the steep slopes of a rugged surface. The form and textures of some more primitive melt than did those containing cpx and pi, which of the massive flows are envisaged as lava lakes erupted and ponded could only have developed as a result of low-pressure fractionation in bathymétrie depressions. (O'Hara, 1968). About 30% of all lavas in the fault belt are primi- Elsewhere on Troodos, the higher levels of the Axis Sequence tive olivine-bearing types. contain abundant near-vertical dikes with a regular, well-defined When pillowed, the primitive lavas usually have glassy (quench) north-south strike. Dikes in the Arakapas fault belt are few, thin, and variolitic textures, whereas the evolved (cpx + pi) types are narrow, sinuous, and discontinuous. This implies that the east-west more crystalline, with hyalopilitic textures grading into intersertal tensional stress field that affected the main massif did not exist in interiors. In massive flows intersertal textures are ubiquitous. Most the fault belt area during the "infill" period. In two small areas speciments show alteration of the glassy mesostasis to a variety of where fault belt dikes occur, they show the same curved trend as clay minerals. Furthermore, the evolved more crystalline lavas, the Axis Sequence intrusives to the north of the fault belt, and this which unlike the glassy rocks contain modal plagioclase in the suggests the intermittent continuation of the torsional stress that groundmass, have been saussuritized, with the resultant develop- produced the brecciation in the underlying basement. The presence ment of secondary replacement minerals. of sills, bosses, and irregular intrusions also implies the absence of a Elsewhere on Troodos the Axis Sequence lavas either are aphyric continuous directional stress field. or carry microphenocrysts of cpx and pi; ol is extremely rare. Petrographically, the principle differences in the igneous rocks of Primitive ol-bearing lavas, rarely containing opx, occur only at the the Arakapas fault belt are in phenocryst assemblage and texture. top of the pillow lava sequence in the Upper Pillow Lavas (Gass, The following assemblages have been identified: olivine (ol), ol + 1958; Smewing and others, 1975), whereas in the Arakapas fault orthopyroxene (opx), ol + opx + clinopyroxene (cpx), ol + cpx, belt they occur throughout the sequence. It is obvious that there are

TABLE 1. MAJOR OXIDE, TRACE-ELEMENTS AND RARE-EARTH COMPOSITION OF SOME PRIMITIVE ARAKAPAS FAULT BELT BASALTS

Sample Axis Sequence Upper Pillow Lavas No.: 2416 34 1303 1074 8946 531 704 786 1081 734 1181 1142 1141

Si02 51.47 51.97 52.18 52.62 51.49 52.29 50.74 51.23 53.03 50.24 50.62 51.46 52.48 Ti02 0.20 0.19 0.20 0.16 0.18 0.19 0.28 0.26 0.33 0.29 0.23 0.23 0.22 A12OT 12.48 10.89 12.07 10.37 10.72 11.21 12.59 12.56 13.41 12.85 12.83 13.50 12.70 Fe20, 3.04 3.67 2.41 3.69 3.55 3.76 4.11 2.44 3.46 3.28 3.58 2.87 1.77 FeO 4.83 4.97 6.48 5.41 4.76 4.78 4.90 5.78 4.08 4.66 4.80 4.95 5.98 MnO 0.14 0.17 0.17 0.16 0.16 0.15 0.18 0.17 0.15 0.16 0.15 0.15 0.15 MgO 11.74 12.14 12.14 14.23 12.72 11.74 11.67 10.91 10.25 9.54 9.97 9.18 11.38 CaO 10.31 9.62 10.01 7.45 10.11 10.22 10.54 10.93 11.22 12.52 10.80 10.83 10.54 Na20 0.44 0.99 0.70 1.57 0.67 0.45 0.46 0.78 1.19 0.75 0.41 0.71 0.97 K2O 0.20 0.30 0.25 0.22 0.27 0.06 0.16 0.20 0.48 0.21 0.67 0.53 0.31 + H2O 5.34 4.22 3.49 4.25 4.60 5.63 5.14 5.39 3.13 5.15 5.29 4.44 4.02 P2O5 0.04 0.03 0.04 0.03 0.04 0.04 0.01 0.03 0.04 0.01 0.05 0.03 0.03 Total 100.23 99.69 100.14 100.16 99.32 100.52 100.78 100.68 100.77 99.72 99.40 98.88 100.55

Nb 33 5 4 3 2 2 2 1 2 2 2 2 0 Y 6 5 4 1 1 5 6 5 8 7 6 4 4 Sr 135 261 59 63 70 77 210 295 236 212 170 161 131 Rb 6 0 4 3 3 0 1 2 1 2 8 8 7 Zr 12 14 14 13 9 10 8 16 14 13 9 18 13 Cr 742 960 830 1294 956 872 663 807 599 601 1005 708 788 Ni 342 408 379 511 401 319 402 367 317 288 515 325 316 Co 78 100 95 86 85 83 76 84 72 81 88 81 95 Ce 1.23 1.32 1.19 Nd 0.60 1.31 0.12 Sm 0.25 0.72 0.57 Eu 0.10 0.25 0.29 Gd 0.77 0.98 Yb 0.10 0.22 0.25 Tm 0.17 0.24 0.26 Ys 0.93 1.12 1.37 Lu 0.17 0.20 0.26

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major differences in the magma-genetic and eruptive mechanisms increase in A1203 and a decrease in MgO is discernible. Na20 and between the two regions. For the main massif, it has been proposed CaO, however, have been greatly mobilized during metamorphism, (Smewing and others, 1975) that the Axis Sequence lavas were so that in extreme cases some of these evolved basalts, which are erupted at a slow-spreading ridge, where magma had abundant mostly olivine tholeiites, are nepheline normative. The wide com- time to fractionate and only evolved basalt melts were tapped. For position spectrum of the fault belt lavas is shown in Figure 4, a Ti the fault belt, it is suggested that magmas had easier egress to the versus Zr plot on which the primitive basaltic komatiites, being im- surface through the plentiful fracture zones, and so primitive melts poverished in both Ti and Zr, plot close to the origin. were more readily extruded. Chondrite-normalized rare-earth element (REE) abundances The geochemistry of these igneous rocks, as with other ophiolite show the fault belt basalts to have light-REE-depleted patterns. basalts, has been modified by the hydrothermal metamorphism The absence of a Eu anomaly is particularly relevant, as this implies they have undergone. Thermodynamic considerations (Pearce, that there was little plagioclase fractionation. Simonian (1975), 1973) suggest that those elements with valencies less than three, quoting the works of Schilling (1971, for example), used the term and others diadhochically compatible with them, are mobile during "profile parameter" (PP), the ratio of enrichment factor of Ce/Tm,

metamorphism. It is widely accepted that Ti02, Zr, Y, Nb (Cann, to subdivide Arakapas fault belt basalt REE patterns into three 1970), Cr (Pearce, 1973), Ni (Simonian, 1975), and the rare earths groups, labeled A, B, and C. Group A has a PP greater than 0.4 to (Frey and others, 1968) are immobile during hydrothermal 0.5; the PP for group B falls between 0.3 and 0.5 and that for group metamorphism involving the circulation of sea waters through C below 0.3. The grouping correlates directly with Ti abundances. -1 oceanic crust with a thermal gradient of about 150 °C km (Gass Ti02 in groups A and B are >0.4% by weight, whereas for group C and Smewing, 1973). they are lower. REE patterns of group A and B are typically those In Table 1 the chemical analyses of the most primitive Arakapas of oceanic tholeiites, but group C patterns are atypical (Fig. 5) in fault belt basalts are presented. The samples chosen were those that that they show a continuous increase in chondrite-normalized petrographically showed least signs of alteration. These basalts are abundances from Sm to Tm.

quartz normative and are characterized by (1) Si02, 50% to 53% Accepting Menzies and Allen's (1974) calculation that the un- by weight; (2) MgO >9% by weight; (3) very low concentrations depleted mantle beneath Troodos was a plagioclase lherzolite with of incompatible elements; (4) high Cr and Ni concentrations; a composition of 65% ol, 15% opx, 10% cpx, and 8% pi, Smew- (5) low alkali content; and (6) glassy (quench) textures. With the ing and others (1975) proposed that the incompatible-element var- exception of their slightly low CaO/Al.,0., values, these rocks may iation in the main massif was due primarily to the composition of be classified as basaltic komatiites according to the definition of successive magma batches becoming progressively depleted in in- Brooks and Hart (1974). It is evident from the high H20, high compatible elements. Low-percentage partial melts would be rich Fe20;!/Fe0, and fluctuating Na20 and Sr concentrations that in incompatible elements, whereas their concentration in higher chemical alteration did occur during the hydrothermal metamor- percentage melts would be lower because of the melt contribution phism. During such alteration, glass changes to clay minerals and of low incompatible-element mineral phases. Furthermore, as a re- CaO is lost, although A1203 remains stable (see, for example, sult of an incremental removal of melt during the partial melting Matthews, 1971; Pearce, 1973); this may account for the low process, the REE patterns of successive magma batches would be- Ca0/Al20:1 values. come increasingly more depleted (Smewing and Potts, 1976) — The evolved basalt varieties have been produced by fractionation that is, the PP value would decrease. in which the incompatible elements show a systematic increase, In the Arakapas fault belt basalts, the overall concentration of whereas Cr and Ni decrease in concentration. Despite the obvious incompatible elements is lower and the range more limited than for possibility of variations due to hydrothermal metamorphism, an the main massif (see Fig. 4). This implies that low-percentage par- tial melts were never available. It is evident, therefore, that the pet- rogenetic processes of the fault belt differed fundamentally from Wt%Ti(X those of the main massif. Furthermore, in the fault belt, a greater degree of partial melting was achieved, which, if Gale's (1973) and 2 0-

Rock Chondrite

10- 10-

Figure 5. Type C chondrite-normalized rare-earth element pat- tern. Analyses taken from Table 1. Typical type A pattern (dashed line) is shown as comparison. 50 100 2-

Zr ppm

Figure 4. Variation diagram of percentage by weight Ti02 versus parts per million Zr. Shaded areas are Axis Sequence rocks of Arakapas fault belt; dashed line encloses Axis Sequence rocks of main massif. Ce Nd Sm Eu Gd Tb

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Green's (1972) estimations are accepted, must have been between 30% and 40% to produce the basaltic komatiites. We suggest that Fine sand layer within laminated mud the fault belt basalts resulted from melts produced at different stages during incremental partial melting (25% to 40%) of plagio- clase lherzolite mantle (Menzies and Allen, 1974) at depths equiva-

Unit grading from coarse grit up through sands and lent to the 5- to 10-kb range. Type C REE patterns may have been silts to laminated muds inherited from melts resulting from the last stages of partial melt- ing. O o0 ° o I Sedimentary Infill. Unfossiliferous breccias, grits, sandstones, . O_ "o O O > ° o oo siltstones, and mudstones occurring in well-bedded, usually graded units unconformably overlie the brecciated basement of the Sharp contact Arakapas fault belt. The thickness of these sedimentary units is seemingly dependent on the configuration of the highly irregular o ^ ^ - °

Sequence of breccias, grits, sands, silts and suggest a rugged bathymetry in which positive features were sub- laminated muds disturbed by slumping jected to erosional processes in which collapse under gravity was the principal agent. Analytical studies by A.H.F. Robertson show

Sharp contact, shearing in underlying laminated muds that the argillitic fraction is enriched in Cr, Zr, V, and Zn and low in Cu, Ti, Mn, Ni, and Pb compared to oceanic clays. In composi- tion they are comparable with argillized ocean-floor tholeiites and - Laminated muds with thin silt layers in their high Cr and low Ti content they are geochemically akin to the fault belt lavas. It is proposed, therefore, that these fine-grained sedimentary strata originated as clay minerals produced by the submarine weathering of fault belt basaltic sea floor by processes Pillow lavas similar to those reported and discussed by Matthews (1971) and Bertine (1974). As no carbonates are present, these erosional and Figure 6. Composite stratigraphie section of Arakapas fault belt sedimentary strata. Although actual sections are depicted, these have been depositional processes may have taken place below the carbonate chosen to illustrate most fully the various sedimentary rock types and struc- compensation depth. tures exposed. The scenario for Arakapas fault belt sedimentary processes is a

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seismically active elongated bathymetric trough along which right- sedimentary strata have not been deformed by compressional lateral transverse movement is taking place. In such an area, solid forces. Enrichment in 8180 values (T. Heaton, 1976, personal rock (basement) will become fractured and fall under gravity, only commun.) suggests that the Limassol Forest serpentinite was pro- to be moved again by the next tremor, unless it reaches zero poten- duced by the reaction of ultramafic rocks with sea water. In a tial in a bathymetric depression. Obviously, each earth movement highly faulted zone, such as the Arakapas fault belt, sea water could will activate mass movement from various parts of the trough penetrate more deeply into the lithosphere and serpentinize sub- walls, not only triggering new rock falls but also reactivating the crustal peridotites. It is proposed that the density inversion so pro- products of previous ones. duced would result in the formation of serpentinite diapirs which, A continuum of subaqueous mass transport processes, ranging on moving upward through the brecciated zone, could entrain from slumping and sliding to mass and turbidity flows, is postu- blocks of cumulate plutonic rocks and Axis Sequence on the way. lated as the mechanisms by which the Arakapas fault belt sediments were transported and deposited. These various processes have been DISCUSSION fully described and documented by Dott (1963), Fisher (1971), Hampton (1972), and Carter (1975). Here, we discuss only the Once the Troodos massif had been widely accepted as a fragment means by which the various sedimentary units were formed, of oceanic lithosphere produced at a constructive margin, the sub- although it is emphasized that any one body of debris may pass, sequent suggestion that the Arakapas fault belt was a fossil trans- with increasing distance from the original source, progressively form fault (Moores and Vine, 1971) was almost inevitable. Indeed, through stages of slide to mass flow and then turbidity current be- the principal objective of the research project on which this paper is fore eventual deposition. The simplest process in this spectrum of based was to determine whether or not the fault belt was a trans- mechanisms is submarine rock fall (Dott, (1963), to which category form fault. We therefore now review the features of present-day the fault belt scree breccias belong. Unsorted, matrix-rich breccias oceanic fracture zones and compare them with those of the fault (Fig. 6) are interpreted as the products of high-density mass flows, belt. whereas the doubly graded course units are considered to represent a transition-stage mass flow and turbidity current. Evenly graded Comparison with Present-day Oceanic Fracture Zones units are identified as turbidity-current deposits. The interstrat- ification of graded units of varying grain size (see Fig. 6) is inter- Oceanic fracture zones are linear belts of rugged bathymetry, preted as turbidity deposits derived from different distances and characterized by elongate ridges and troughs bounded by fault sources. The common absence of the fine-grained top of a graded scarps (Menard and Chase, 1970), which have a vertical relief of as unit is due, as previously mentioned, to its removal by the erosional much as 4 km and are as wide as 30 km. Several hundreds of metres incoming of the next turbidity flow. of well-stratified sediments occur in the fault zone depressions, whereas scarps and bathymetric highs are sediment free. This im- Limassol Forest plies (van Andel and others, 1973; Feden and others, 1975) that downslope transportation of sediments is significant. It has been The southern boundary of the narrow Arakapas fault belt gra- noted that magnetic linear strip anomalies that are usually parallel ben, at its contact with the Limassol Forest (see Fig. 2), is a zone of to spreading axes do, in some cases, bend on approaching a trans- east-west reverse faults that formed after the main transcurrent form fault zone to become parallel or subparallel to it (van Andel movement. Although not studied in the same detail as the fault belt and others, 1973). The existence of strong magnetic anomalies (Bear, 1960; Morel, 1960; Pantazis, 1967), the Limassol Forest is along fault zones (Rea, 1972) has been explained as being due to known to be composed of an elevated, east-west-elongated core of the emplacement of magma or serpentinite and the tendency for serpentinite around which there is a discontinuous envelope of Axis lavas to accumulate in fault zone depressions (Vogt and others, Sequence and Plutonic Complex. Upper Pillow Lavas unconform- 1971). In the FAMOUS area (ATCYANA, 1975; Whitmarsh and ably overlie the serpeninites of the Limassol Forest, which, at their Laughton, 1976) a similar deviation is noted in fault scarps that southern margin, are in contact with strongly folded and faulted significantly veer progressively toward the related spreading axis as lower Miocene calcareous sedimentary rock of the Yerasa fold and the transform fault is approached. fault belt (Fig. 2). Within the serpentinites, and particularly along Both the topography of the Arakapas fault belt basement and the their northern faulted contact, are large entrained masses of layered occurrence within the depression of well-stratified sediments is in peridotite and gabbros and Axis Sequence dike complex. The lack keeping with oceanic fracture features. Furthermore, the turbidity- of any concordance in the layering from one block to another current transportation mechanism, so obviously involved in infill suggests that they are not connected at depth and were caught up sedimentation of the fault belt, is similar to that proposed for sedi- and transported by the invading serpentinite. As there is no nega- ments in the Vema fracture zone (van Andel, 1969). The swing in tive gravity anomaly over the area (Gass and Masson-Smith, 1963) the Axis Sequence dike trend north of the fault belt (Fig. 2) is it is suggested that the serpentinite is rootless and is possibly closely comparable to the deviation of magnetic anomalies and that diapiric in form. This contention is supported by internal structures of the fault scarps in the FAMOUS area of the Mid-Atlantic Ridge. that also show that later upward movement was along a series of Fracture zones seem to vary in structure, topography, and com- stacked, gently dipping, northerly inclinced thrust sheets (Lapierre position from one ocean to another. However, those studied in de- and Rocci, 1967; C. Xenophonotos, 1974, personal commun.) tail, such as the Vema, Romanche, and Chain fracture zones of the Outcrops of brecciated Axis Sequence within the Limassol Forest equatorial Mid-Atlantic Ridge (Bonatti and Honnorez, 1976; Prinz indicate that the serpentinite was emplaced into a wide fracture and others, 1976) have yielded a wide variety of rock types, includ- zone of which the Arakapas fault belt represents the northern mar- ing basalts, metabasalts, and basalt breccias, various types of gab- gin. The emplacement occurred after the transcurrent movement of bro and metagabbro, and ultramafic rocks, including serpentinite the fault belt but before the extrusion of the Upper Pillow Lavas, and peridotite. All these rock types occur in the Arakapas fault belt, and it continued, as the presence of deformed sediments in the but it is particularly significant that Bonatti and Honnorez (1976), Yerasa area indicates, into early Miocene time. Middle Miocene describing the Vema fracture zone, identified the north wall of the

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valley as being "normal" oceanic crust with basalts and gabbros, dike deviation took place. Two models come to mind, and these are whereas the southern wall is anomalous in that it is composed of illustrated in Figure 7. serpentinite. Bonatti and Honnorez (1976) contended that serpen- If sea-floor spreading occurred along two constructive margins tinites are a significant component of the ocean crust and were joined by a well-defined fault zone (Fig. 7, A), then the dike devia- probably emplaced as vertical intrusions in deep fracture zones tion could have been progressively impressed by continuing right- parallel to ridge axes. It is relevant to note that although the Limas- lateral movement along the fault zone (Fig. 7, B). Alternatively, if sol Forest serpentinite was emplaced into the fracture zone, it was two spreading axes were not linked by a transform, as is commonly intruded parallel to the fault belt and after the main transcurrent the case in the Afar depression (Barberi and others, 1975; Harrison movement was over. and others, 1975), then spreading in opposite directions in the em- Volcanic products have been dredged from the walls and axes of bryonic constructive margins would create a sigmoidal stress field several fracture zones. Those from the Oceanographer fracture across the offset zone (Fig. 7, C). Any dike injected into such a field zone have been described and discussed by Fox and others (1976). would have a sigmoidal form. The bend in the S would increase Menard and Atwater (1969) postulated that basaltic eruptions until fracturing took place, but by this time the dike deviation from "leaky transforms" would occur when change in spreading would have already occurred (Fig. 7, D). In choosing one of these direction caused extension across fracture zones. However, models, we are impressed by the lack of fracturing of individual although very primitive lavas have occasionally been recorded from dikes within the deviation zone and also by the zones' broad (about dredge hauls (Frey and others, 1974), basalts as primitive as those 10 to 15 km) radius of curvature. These factors lead us to favor the of the Arakapas fault belt have not been described. second "sigmoidal" stress field hypothesis, although obviously There can be little doubt that the fault belt was a zone of major further dike deviation could be caused by along-fault drag. transcurrent movement. So, as it compares closely in topography, Within the Arakapas fault belt itself there is a marked difference basement configuration, composition, and sedimentary infill and in between the structure of the basement and that of the infill. The its association with serpentinite intrusions with present-day oceanic basement is brecciated, whereas the infill, although faulted, is not. fracture zones, the proposal that it is a fossil transform fault is ac- To produce brecciation of the type displayed by the fault belt ceptable as a working hypothesis. Indeed, those who accept the basement, there must have been some element of compression Troodos massif as a fragment of oceanic lithosphere formed at a across the fault zone. That this was subsequently replaced by a ten- constructive margin will undoubtedly regard the Arakapas fault sional component is witnessed by the presence of infill lavas. The belt "fossil transform" as yet another line of evidence in support of change in stress field could have been caused by a slight change in their case. the spreading direction along the two adjacent constructive mar- gins (Menard and Atwater, 1969; Sleep and Biehler, 1970). Not Arakapas Fault Belt (Transform Fault) Mechanisms only could such a change produce a linear bathymetric depression, but also loss of load pressure would allow greater percentage melts Paleomagnetic evidence (Vine, and others, 1973) indicates that than elsewhere along the constructive margin. This and the easy the Troodos massif has been rotated anticlockwise through 90° egress for the magmas could combine to explain the primitive na- since its formation some 85 m.y. ago. However, in this discussion present-day coordinates and vectors will be used to avoid unneces- sary confusion. If the Troodos massif is oceanic lithosphere formed at a con- structive margin and if the Arakapas fault belt is a transform fault, then the fault belt structures and their relation to the main massif may be of use in interpreting the mechanisms. Undoubtedly, it is the sheeted nature of the dikes in the lower part of the Axis Sequence that is the most convincing evidence that the Troodos massif was produced at a constructive margin. Fur- thermore, as the dike trend is north-south, then that must also be the orientation of the spreading axis. Kidd and Cann (1974) sub- jected data on the one-sided chilling of Axis Sequence dikes to statistical treatment and concluded that the Troodos spreading axis lay to the present-day west of the massif and that the dikes were all injected along a narrow linear zone only a few metres wide. We ac- cept these conclusions because they are in keeping with other evi- dence on Troodos and also on structures at embryonic spreading axes such as those in the Afar depression (Barberi and others, 1975). Accepting that the Troodos spreading axis lay to the west and Figure 7. Postulated mechanism for dike variation in main massif north that it and the dominant dike trend are orientated north-south, of Arakapas fault belt. Diagrams A and B illustrate a case where a fault line then it is necessary to explain how and why the dike trend de- exists between two spreading axes. In this case, dike deviation is most likely viates markedly and progressively westward as the Arakapas fault to be due to drag produced by horizontal movement along fault. Diagrams belt is approached from the north (see Fig. 2). Although there can C and D indicate two spreading axes not originally connected by transform fault. Dike injection along these axes could result in sigmoidal stress field be little doubt that the main massif lies to the north of the fault belt (shaded area in C), deviation at ends of constructive margins before de- transform along which right-lateral movement was taking place, it velopment of transform, and, hence, wide radius curvature of internally un- is less easy to envisage at what stage and by what mechanism the deformed dikes.

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ture of the fault belt basaltic komatiites (see Table 1). Furthermore, gradients and greater volumes of sea water than elsewhere. (6) Sea the same processes would explain why the hydrothermal water penetrating deeply into the oceanic lithosphere along the metamorphism in the fault belt differs from that elsewhere on the fracture zone reacted with the underlying ultramafic rocks (Table massif in requiring a steeper thermal gradient and greater volumes 1), producing serpentinites (of the Limassol Forest), which rose of sea water involved in the basalt-water hydrothermal interaction. diapirically, carrying with them detached blocks of cumulates and The abundance of clastic sedimentary rocks in the Arakapas fault Axis Sequence. (7) Volcanic and sedimentary processes similar to belt and their virtual absence elsewhere on the massif are explained those of the Axis Sequence continued after the emplacement of the by the presence, along a seismically active zone, of a linear serpentinite, but, as they lack the characteristic Axis Sequence as bathymetric depression. In this context it is relevant to note that metamorphism, they are designated as belonging to the Upper Pil- although the main metamorphic episode affected the Axis Sequence low Lavas. (8) Continued upward movement of the serpentinite rocks, it did not (by definition — Gass and Smewing, 1973) affect produced east-west normal and reversed faulting until early the Upper Pillow Lavas and their sedimentary strata. This supports Miocene time. the suggestion (Smewing and others, 1975) that metamorphism took place close to the spreading axis but that magmatism and ACKNOWLEDGMENTS sediment-producing seismic activity continued thereafter. Various lines of field evidence indicate that the serpentinites of We thank J. R. Cann, A. D. Lewis, J. A. Pearce, J. P. Potts, J. D. the Limassol Forest, here considered to be part of the transform Smewing, and R.A.M. Wilson for their constructive criticisms of fault, must have been originally emplaced in the geologically brief the first draft of this manuscript and I. L. Gibson and his colleagues period between the metamorphism of the Axis Sequence and the at Bedford College, London, for analytical services. eruption of the Upper Pillow Lavas. To explain this, we suggest that in this highly brecciated zone the sea water responsible for the hydrothermal metamorphism of the Axis Sequence penetrated REFERENCES CITED more deeply into the oceanic lithosphere than elsewhere and reacted with the ultramafic rocks of the cumulate sequence and the ARCYANA, 1975, Transform fault and rift valley from bathyscaph and depleted mantle (see Fig. 1). 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MANUSCRIPT RECEIVED BY THE SOCIETY OCTOBER 22, 1976 Pearce, J. A., 1973, Some relationships between the geochemistry and REVISED MANUSCRIPT RECEIVED JULY 14, 1977 tectonic setting of basic volcanic rocks [Ph.D. thesis]: Norwich, Great MANUSCRIPT ACCEPTED AUGUST 8, 1977

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