Albania): Oceanic Detachment Shear Zone

Research Paper

GEOSPHERE Mylonites in ophiolite of Mirdita (Albania): Oceanic detachment shear zone

1 2 1 3 2 GEOSPHERE; v. 13, no. 1 A. Nicolas , A. Meshi , F. Boudier , D. Jousselin , and B. Muceku 1Geosciences, Université de Montpellier II, 34095 Montpellier, France 2Faculty of Geology, Polytechnic University of Tirana, Rruga Elbasanit, 1010, Albania doi:10.1130/GES01383.1 3Université de Lorraine, Ecole Nationale Supérieure Géologie Centre de Recherches Pétrographiques et Géochimiques (ENSG-CRPG), 54501 Vandoeuvre-les-Nancy, France

11 figures; 1 table ABSTRACT west (Fig. 1A). Based on several field campaigns in the northern part of Mirdita CORRESPONDENCE: Francoise.Boudier@gm during the 1990s, it seems to be one of the few and, so far, the best ophio- .univ-montp2.fr The northern Mirdita ophiolite massifs in Albanian Dinarides formed at a lite where an oceanic core complex (OCC) has been identified, located within slow-spreading ridge, active during the Jurassic (160–165 Ma). They share a the western Mirdita massifs, Puka, and Krabbi (Fig. 1A insert) (Nicolas et al., CITATION: Nicolas, A., Meshi, A., Boudier, F., Jous- selin, D., and Muceku, B., 2017, Mylonites in ophiolite common horizontal Jurassic–Lower Cretaceous sedimentary cover showing 1999; Meshi et al., 2009). Other OCC candidates include Chenaillet (Manatschal of Mirdita (Albania): Oceanic detachment shear zone: that they were not deeply and intrinsically affected by later Alpine thrusting. et al., 2011) and Thetford Mine in eastern Canada, which has been compared to Geosphere, v. 13, no. 1, p. 136–154, doi:10.1130 The western massifs of Mirdita, first oceanic core complex (OCC) and detach- Mirdita (Tremblay et al., 2009). We suspect that the Othrys ophiolite in Greece /GES01383.1. ment shear zone described in ophiolites, compare with OCCs in slow-spread- could be another excellent OCC candidate, as part of the Othrys band of the ing ridges and provide continuous exposure of the deep internal structure of same Dinaric belt (Dijkstra et al., 2001). The evidence for OCC interpretation Received 22 June 2016 Revision received 19 October 2016 this system, revealing its kinematics, thanks to detailed structural mapping in these massifs is based on similarities with the OCCs properties discovered Accepted 7 December 2016 in peridotites and gabbros. The Mirdita detachments root in the Moho tran- in the Atlantic Ocean (Karson and Dick, 1983; Karson et al., 1987; Mevel et al., Published online 10 January 2017 sition zone (MTZ), a weak zone at the top of asthenospheric mantle, where 1991; Tucholke, 1998) and since, described in other slow spreading ridges. basaltic melts impregnate dunites. The OCC domes are plagioclase-amphibole–bearing mylonitic peridotites, ~400 m thick, grading downward within 200 m to harzburgitic mantle. The mylonitic detachments crossed Moho be- Western Mirdita Ophiolite, Largely Preserving Its Oceanic Setting neath a NNE-SSW–trending ridge. On the western side of OCC domes, the hanging wall of the ridge, crustal gabbros, and basalts are still preserved, An unexpected feature increasing our interest for the western Mirdita mas- despite being deeply affected by hydrothermal alteration. From there, the par- sifs is that the Upper Jurassic and Lower Cretaceous marine sediments were tially molten MTZ was detached as a shear zone, mixing with lower gabbros. transgressive over the ophiolite seafloor (Figs.1A and 1B). The Upper Jurassic The OCC emerged, migrating upsection and eastward over 5 km. Finally, the sedimentary cover starts with shallow-water radiolarian flysch and ophiolitic OCC front is observed in hornblende-rich syntectonic mylonites derived from conglomerates grading to the limestone cliffs of Lower Cretaceous (Gawlick upper gabbros and from the overlying former lid. Serpentinization is static et al., 2008). These formations remained nearly horizontal in the present situa- within these mylonites. A low-temperature detachment fault is expressed tion with an average 170E15° strike and dip in map (Fig. 1B). This discordance as a sheared antigoritic mélange at the margin of the mylonitic shear zone. is mainly observed in the central zone and the eastern massifs, and only locally Asthenospheric flow in the harzburgitic mantle beneath the ridge of origin in small outcrops of western massifs, except south of Puka massif where the has been preserved below the OCC rooting. The dominant asthenospheric Upper Jurassic cover extends several kilometers (Anonymous, 1983). For these flow direction trends parallel to the ridge axis. Thismantle flow rotates over reasons, the present-day relief in the northern Mirdita ophiolite remains close 200 m into the low-temperature mylonitic detachments, where OCC motion to the original marine topography (Nicolas et al., 1999; Tremblay et al., 2009), turns transversal to the ridge. Crystal preferred orientation measurements on despite local Dinaric flat-lying thrusts (Gawlick et al., 2008) that do not affect six samples point to brown hornblende crystal growth during mylonitic flow the horizontal attitudes of the marine cover of the Mirdita ophiolite. Besides the and illustrate the change of olivine intra-crystalline slip system in mylonites intense tectonic activity in the surrounding alpine units, this suggests that the compared to porphyroclastic harzburgite. Mirdita ophiolite was the uppermost nappe (Schmid et al., 2008; Roure et al., 2010), ruling out any major rotation during alpine events, a situation suggest- INTRODUCTION ing that the preserved structures have the same orientations that developed at a spreading center. For permission to copy, contact Copyright Mirdita ophiolite is a NE-SW–trending structural belt in Albania, located This new paper on the Mirdita ophiolite focuses on the OCCs. It contributes Permissions, GSA, or [email protected]. between the alpine Pelagonian in the east and external Dinarides units in the only indirectly to the regional geology that is complex and has been prepared

19°45′E 20°00′E 20°15′E o o o o o o o o oo Cretaceous V o A Quaternary o o o o o o o o limestones o o o o o o o o o o o o o o o o o o o o o o o + + + o o o o o o Gabbro + + hz + o o o o o o o o o o + Plagiogranite + o o o o o o o 42°10′N + o o o o o o o + o o o o o o o o + + o o o o o o o o o o Amphibolite KRABBI o o o o o o o Volcanics + o o o o o lh + + o o o o o o o o + + o o o o + o o o o o o 90 Diabase dike + + o o o o o Peridotite + o o o o 70-85 + + + o o o dip + oo o o o hz lh Lherzolitic + + + + o o o o mylonite 5-30 o o o o o o o o o o o o o o o hz Harzburgite o o o o o o o o o o o o o o + hz o o o + o o hz + o + KUKES + + hz + GOMSIQE + 42°00′N o lh o o + o o lh + o o + + o o o o o o o o o o o PUKA o o o oo o o lh o o o o o o o oo o o N o o o o o o o o o o

o o o o o o o V o o o oo o o o o o o o o o o o o o o o C E N T R A L + o o o o o o o + o o o o o o o V o o o o o A lh o o o o o o + o o o o o o D I N A R I D E S R + o o o o o o + o o o o o o o D o o o o o o A N o o o o o o o o R + o o o o o + o o o Z o hz P o m o o o o O + + o o o o E L o 42° i

o o r N + + o o o o o o d o o o K R i E + o o o o o t A o o oo o o a o A d r i a t i c G o o o o o o o o o A S o + o o s e a O o o o o o p 41°50′N o o o o h N + o T i 5 km o o o o hz o o A Z O l I o i + + o o o LURA t A I t a l y e + o o o o N Z O N ALBANIE

N Figure 1 (on this and following page). (A) Simplified map of Mirdita northern massifs and its location in the Dinaric ophiolitic belt (ophiolites E shown in dark gray). Detailed study concerns the western massifs, Gomsiqë, Puka, and Krabbi (see Fig. 2). The limit between western mas- E sifs (Gomsiqë, Puka, and Krabbi), which have a tholeiitic trend, and the eastern massifs (Kukes and Lura), which have a mixed calc-alkalic volcanics trend and plagiogranite intrusions, follows the NE-SW central Mirdita synform (dotted line). 100 km 20°E

by the excellent mapping at the 1/200,000 scale by the Albanese geologists a mechanically weak formation initiating detachments has been located in (Anonymous, 1983). As a secondary result only, we will contribute by precisely serpentinites at shallow and accessible depth (MacLeod et al., 2002; Escartín locating the ridge from which the OCC emerged in Mirdita, so far a subject of et al., 2003). A deeper origin has been claimed in oceanic OCCs where major vivid discussion (Maffione et al., 2013; Tremblay et al., 2015). detachment shear zones include a high-temperature mylonitic component. A mechanically weak melt-rich zone thus appears as a potential decoupling site Oceanic Core Complex Detachment Zone (Schroeder and John, 2004; Karson et al., 2006; Hansen et al., 2013). Following our 1999 paper, the present contribution relies on (1) a detailed Extensional core complexes in the present setting are only known through survey of contact relationship between ophiolitic formations relating to OCC indirect geophysical tools (DeMartin et al., 2007), by seafloor drilling (Ildefonse dynamics; (2) an improved cover in foliation and lineation measurements et al., 2007a; Blackman et al., 2011), or by submersible means (Karson et al., within the mylonitic and porphyroclastic peridotites leading to detachment 2006). Typically, in the literature on OCCs from the Mid-Atlantic Ridge (MAR), kinematics; and (3) detailed petrostructural studies linked to mylonites devel-

B N maxmax plane layering : plane : 170 E 15170 E 15

N = 24N = 24

Cretetacaceoeousus lilimemessttoonesnes CrCretetacaceoeousus lilimemeststonesones KrKryayaitit volcanics GuGuriritt tete KuKuaa central Mirdita synform

Puka dome

Figure 1 (continued). (B) View from Krabbi dome, looking east on the Upper Jurassic–Lower Cretaceous marine limestone cliffs overlying Mirdita ophiolite. White and sharp limestone ridges, nearly horizontal, contrast with the brownish and smooth ophiolites. The background limestone ridges lay on top of the eastern Kukes-Lura massifs. Insert: polar projection (lower hemisphere) of the layering measured in this sedimentary sequence. Measurements are obtained from the 1/200,000 geologic map of Albania (Anonymous, 1983). Average plane orientation is 170E15 for 24 measurements.

opment after melt impregnated mantle rocks. The aim of this new contribution The eastern massifs (Kukes, Lura, and Tropoje, Fig. 1A) represent typical is to understand better the rooting and evolution of OCC detachment, integrat- harzburgitic ophiolites, rich in chromite deposits, showing the internal crustal ing the many recent developments concerning modern OCCs. organization of a classical ophiolite sequence (Pamic, 1983; Hoxha and Boullier, 1995; Nicolas et al., 1999). This eastern domain is largely covered by cuestas of Early Cretaceous limestones (Fig. 1B). The western massifs in northern Mirdita THE WESTERN MASSIFS IN NORTHERN MIRDITA OPHIOLITE (Gomsiqë, Puka, and Krabbi) are mantle domes (Fig. 2), variously capped by a horizon of mylonitic plagioclase-amphibole peridotites (Figs. 3A and 3B). The General Description nature of the mantle is clearly harzburgitic and not lherzolitic. Gomsiqë massif is thrust westward on the Krasta Formation of external Albanides and is sep- The Mirdita ophiolites are divided in the western and eastern domains on arated eastward from the Puka massif by the Krë deep volcano-sedimentary each side of the central Mirdita synform trending N-S. It is a saddle, up to shear zone (Figs. 2 and 3A). This shear zone contains an ophiolitic mélange 10 km wide, and extends over some 40 km to the south (Fig. 1A). This central consisting of a serpentinite matrix (Figs. 3C and 4A) and including blocks up to domain, lying between the eastern and western peridotite massifs, represents hundreds of meters of gabbros and basalts, nearly undeformed (Fig. 4B), such oceanic crust intruded by copious plagiogranites. as in a typical root zone of diabase dikes complex. It is along the western foot-

o o o o o Mylonitic o o o o ap amphibole peridotite o o o Iballe o pp pp Mylonitic o hz 12A30• plagioclase peridotite o 12A33 42°10'N 1000 N " " • pp hz Harzburgite " " " pp " " o KRABBI S Ophiolite o pp breccia o o " " " o " " """ " "" o " "" " " " " + o " " "" " " " 96A47 + + o " " " " "" "" 12A41 o o " " • • o o " pp ap 5 km o o " 1200 + Figure 2. Geologic map of northwestern o o o + + o o + Mirdita massifs, modified after Anony- o o o o o o Dedaj mous (1983), Nicolas et al. (1999), and o o o o Meschi et al. (2009). Numbered locations o o o

o o refer to selected thin sections represented o v o o o o in Figure 7, including electron backscatter o hz o o o diffraction (EBSD) samples (bold label,

o o Fushë-Arres o " v Table 1 and Fig. 9). The dotted line marks o o o " " 12A16• o o " ap + the Fushë-Arres synform, exposing a thin o o hz GOMSIQE o " Puke + o oceanic crust composed of basalts cut by o o o o o 12A13 ap + diabase dikes, overlying an extremely thin o • + Road o o " " " gabbro section cropping only in the ba- S " "" "" " pp pp + + o o o " " " + + salt windows. The margins of the synform " " • 98P6 + " " " 96A54 + + ++ expose in direct contact basalts and my o hz Krë "" "" • • + + + + Gomsiqe o 12A48 lonitic amphibole peridotite. See Figure 3 + + Plagiogranite 96A58•S + + for geologic cross sections. Abbreviations: o hz + + 42°00'N ap—amphibole peridotite; hz—harzburgite; 400 Basalt o + pp—plagioclase peridotite. PUKA + o hz + + Gabbro pp + + + + + o + " " " Troctolite o " " o + o 1200 o + Amphibolite o 1000 o o o o o hz o o o Peridotite o o • o o o pp 98P21 o o o Main faults o o o o ap o o o o Volcano- o o o o o sedimentary ysch

19°40'E 19°50'E 20°00'E

hills of Krabbi and Puka massifs that an undeformed crustal unit is exposed ered with anorthositic and wehrlitic bands. North of Dedaj (Fig. 2), wehrlites, extending along the Dedaj Valley (Fig. 2). These lowlands are occupied by issued from the MTZ, and intruding the lower gabbros, develop spectacular highly altered gabbros (Figs. 5A and 5B), locally covered by undeformed pillow magmatic folds with mutual recrystallizations (Figs. 4D and 4E). basalts (Fig. 4C). Kaolin responsible for the whitish alteration of these gabbros The western slopes of Puka and Krabbi domes are mostly composed of has been mined in a few places. North in this valley, these gabbros are poorly mylonitic peridotites that stand over porphyroclastic harzburgites (Fig. 6A), layered with a weak magmatic foliation. Locally they are olivine-rich, interlay- grading to high-temperature harzburgites at the base of the domes (Fig. 6B).

100 µm C 50 µm F W Krë shear zone 2 km E G O M S I Q E P U K A C E N T R A L M I R D I T A

1 ======oo o oooo == o o o o + o ==o== o o o + =o o o + o = o o o o ======o o o o + o o o = oo o + === o o ==== o o o = oo + o o o ==== o ======o = ==== o ==== = o == o o o o o o + ======o + o o o = o o == o + + o o o o o oo + = o 0 = o o o + == o o o + + o o o o o o+ = + + oo

======o=== o o Serpentinite and o o o Jurassic-Cretaceous MTZ mylonite o oo Lid A 5 km ophiolite blocks o sediments Porphyroclastic Gabbro + + ++ Plagiogranite Fault harzburgite + +

P U K A K R A B B I S 1630 1476 1346 Fushë-Arres N 2 km synform Kushnen

1 o oo o ooooooo o o oo oo o o o oo o o o o o o oo o o o oo oooo 0 o o o + oooo ====o o + o o+

B 5 km

D 3 mm 2 mm E

Figure 3. Cross sections in the northwestern Mirdita ophiolite (see Fig. 2 for location). (A) West-east cross section oriented perpendicular to the paleospreading axis. It shows Gomsiqë mantle body thrust westward upon a Krasta Formation cushion, and eastward in faulted contact with the Krë shear band, serpentinites (C) with knockers of the ophiolite. Eastward, the Puka oceanic core complex (OCC) dome exposes mantle harzburgite overlain by the mylonitic shear zone beneath the lower gabbros of the oceanic ridge. Like the underlying high-temperature mantle harzburgite, mylonites are affected by static development of lizardite veining (F). Beyond, the eastern limit of Puka dome is in faulted contact with the Central Mirdita crustal section. (B) North-south section nearly parallel to sheeted dikes and to the presumed ridge axis. Crossing Krabbi and Puka summits, this section illustrates the thickness of the mylonitic cover: 700 m elevation between Krabbi summit and the Fushë-Arres crustal synform that trends perpendicular to the paleoridge axis. Lower mylonites are harzburgitic (D); mylonites at contact with basalt of the lid are amphibolites locally (E). MTZ—Moho transition zone.

GABBRO

SHEARED SERPENTIN

IT E

RO GABB TE WEHRLI E A

Figure 4. Lithotypes that crop out in the northwest massifs. (A) Serpentinite (antigorite schist) including xenoliths of oceanic crust, at all scales (Krë Valley). (B) Vari-textured upper gabbros, recrystallized with black hornblende and intruded GABBRO by a diabase dike, as xenolith in sheared serpentinite (Krë Valley). (C) Undeformed pillow lavas, few meters above weathered gabbbro (Dedaj Valley). (D) and (E) Mag- DUNITE-GABBRO GNEISSIC matic mixing between olivine gabbro and MYLONITE GABBRO F wehrlitic intrusion (north of Dedaj Valley), indicative of mixing at solidus temperature of gabbro-wehrlite 1200–1100 °C. DIABASE This Dedaj Valley exposure, representa- B DIABASE tive of lower crust is devoid of any solid- state deformation. (F) Gneissic gabbro amphibolitized, collected nearby inter layered gabbro-dunite mylonite (eastern margin of Krabbi Dome). (G) Gneissic gabbro exposed in the southeastern border of Krabbi Dome, near Fushë-Arres. Notice the small, undeformed diabase dikelets perpendicular to the gabbro foliation (scale bar = 10 cm).

G D

CARBONACEOUS LIMESTONE

O R G A B B R O B B A G B A S A L T

H A R Z B U R G I T E

Puke G A B B R O

B D e d a j R i v e r

Krabbi Summit

KRASTA-SUKKALI FORMATION

GABBRO-WEHRLITE L A H E R Z O L I T I C M Y L O N I T E S Dedaj B B R O G A F L A S E R O L I V Puke I N E - H O O N I T E S R N B L E N D E M Y L

B Figure 5. Large-scale field structures. (A) Gabbros highly hydrothermalized (whitish kaolinite), forming the Dedaj Valley A S A L T lowlands. (B) Flat layering in weathered gabbro (yellowish) overlying mantle peridotite, looking southwest in the Dedaj F U S H E-A R M Valley. (C) View on the Krabbi dome culminating at 1630 m with the Dinaric Krasta-Sukkali Formations in the back- R E S C R U S T A L S Y N F O R ground. Looking NW, the view is oblique to both the ridge axis and the detachment thrust. The steep cliff beneath the summit is in mylonitic formation “lherzolitic mylonites,” making the estimated detachment 400 m thick, grading down in hornblende-bearing peridotite, overlain by crustal formation of Fushë-Arres synform. Crustal section exposed between Gomsiqë and Krabbi massifs “gabbro-wehrlite” preserves magmatic structures.

O L I V I N E - H O R N B L E N D E M Y L O N I T E S C

The Puka and Krabbi domes are wrapped by mylonitic covers, both ~400 ± ern Puka margins, along volcanic exposures of the Fushë-Arres synform, are 100 m thick (Figs. 3A, 3B, and 5C). The mylonites largely extend at the south- represented by peridotite-rich brown hornblendic amphibole (Fig. 3E). Ultra ern and northern margins of Krabbi and Puka domes, respectively, along the mylonitic bands, dominantly represented by hornblende-bearing peridotite, W-E–trending crustal synform of Fushë-Arres (Fig. 2). Mylonites are well ex- are either interlayered with or locally crosscutting the mylonite foliation (Figs. posed again on top of Krabbi dome (Fig. 5C) and along its northeastern limit 6F and 6G). (Fig. 2). Mylonites have either a dominant harzburgitic composition (Fig. 3D), The base of the crustal section at the eastern contact of Krabbi and interlayered with mylonitic gabbroic layers at various scales, from tens of cen- Puka massifs is represented by gabbros stretched in low-grade amphibolite timeters (Fig. 6C) to millimeters, intermixing olivine and plagioclase (Figs. 6D facies (Figs. 4F and 4G). These gabbros belong to the north Mirdita central and 6E). Parts of the mylonitic cover, in particular at southern Krabbi and north- crustal domain.

opx ol

pl + cpx ol A D

Figure 6. Field structures in peridotites. (A) Mid-temperature porphyroclastic texture in harzburgite, with a strong foliation and lineation (see Fig. 7B). (B) High-tem- ol perature protogranular clinopyroxene-rich harzburgite. This peridotite is coarse opx + cpx du grained (Fig. 7A) with some evidence of plastic strain (core of Puka massif). Proto gb granular texture is defined by Mercier and Nicolas (1975) in mantle xenoliths as be- E ing in equilibrium with asthenospheric BA mantle at ~1200 °C. (C) Peridotite mylonite with gabbro layering (marker 3 cm) (near Iballe, Krabbi massif). (D) Transition to mylonitic texture in plagioclase-rich perido

e tites, ascribed to Moho transition zone (MTZ) (marker 10 cm), west Krabbi dome.

ylonit (E) Tight isoclinal folding in plagioclase-

m rich peridotite mylonite (marker 10 cm)

a- mylonite (NW margin of Puka dome). Compare ultr with thin sections of Figures 7C and 7D. (F) Isoclinal fold marked by gabbro layers in mylonitic peridotite. The folds are trun- cated by an ultramylonite band (top of e F northern Krabbi). (G) Peridotite mylonite, pl-peridotit irregularly cut by an ultramylonite con- torted “dikelet,” exposed on Krabbi sum- o mit where mylonites are steeply dipping gabbr (marker 10 cm). Abbreviations: cpx—clino pyroxene; du—dunite; gb—gabbro; ol— olivine; opx—orthopyroxene; pl—perido dite—plagioclase-bearing peridotite.

lonite y

a-m

mylonite ultr

CC G

Deformation in Mantle Rocks: Temperature and Strain Estimates mantle domes, high-T harzburgites are present in the lower zones northeast of Krabbi and in the east-central part of Puka. In harzburgites from Krabbi, as well Photographs of representative thin sections in the mantle rocks from the as in Puka and Gomsiqë, the high-T foliation planes (Fig. 8A) are steeply dip- studied massifs illustrate an evolution of harzburgite from coarse-grained to ping and parallel to the NE-SW–trending paleoridge, marked by sheeted dike porphyroclastic texture (Figs. 7A–7C) observed in the mantle section of the orientation (see also Fig. 1A). High-T lineations consistently trend NE-SW with studied massifs (see also Figs. 3A and 3B). This progressive evolution oper- dominant shear sense top to SW (Fig. 8B). Transition to the low-T porphyro ates at decreasing temperature and increasing strain (Nicolas, 1989). (1) The clastic conditions is marked by rotation of foliation into a NW-SE trend, with protogranular or coarse-grained textures of Mercier and Nicolas (1975) mildly lineation gently plunging northwest, in both Krabbi and Puka massifs. deformed at high-temperature ([high-T] ~1200 °C) in asthenospheric condi- The mylonitic zone in the northeast part of Krabbi massif is in continuity tions (Fig. 7A); (2) medium-temperature ([mid-T] ~1000–1100 °C) porphyroclas- with low-T mantle flow, marking a shear zone trending NW-SE with a domi- tic conditions, marked by grain-size reduction and irregular grain boundaries nant dextral shear sense. In the upper Krabbi dome, mylonitic foliations are flat (Fig. 7B), and finally, (3) low-temperature ([low-T] <900 °C) porphyroclastic or gently dipping west with lineation downdip. conditions, characterized by a flowing olivine matrix including orthopyroxene Mylonites crop out in the northern end of the Puka dome. Mylonitic folia- porphyroclasts (Fig. 7C). In these deformations, the flow proceeds dominantly tions are flat at the northern contact with the basaltic lid of Fushë-Arres syn- by dislocation creep (Nicolas and Poirier, 1976). Shear senses are deduced form; they steepen when relayed in porphyroclastic low-T conditions at the from the obliquity of olivine (or orthopyroxene) crystallographic fabric (lattice western contact with gabbros of the Dedaj-Krë zone. Mylonitic lineations trend slip plane and slip line), relative to the strain ellipsoid marked by foliation and E-W, with a dominant westward shear sense marked at the western margin. It lineation, respectively (Nicolas, 1989). In most cases, these relationships are should be recalled that mylonites in the domes vary on a local scale, and shear- observed directly through the optical microscope, in oriented thin sections, cut sense determinations lose confidence. properly perpendicular to foliation and parallel to lineation. Altogether in both Krabbi and Puka domes, high-T foliations beneath the Mylonites and then ultramylonites are represented here by their typical OCC envelope are trending NE-SW, parallel to the sheeted dikes marking the various lithologies, plagioclase-bearing peridotite, hornblende-bearing perido Mirdita paleoridge NE-SW orientation (see also Fig. 1). High-T lineations gently tite or amphibolite (Figs. 7D and 7E), all marking a sharp transition to the por- plunging SW indicate that asthenospheric mantle flow is ridge parallel, and phyroclastic harzburgite, due to their mineral phases layering. Shear-sense de- flow direction is moderately south plunging. The structural maps of the mantle termination loses performance in mylonites when grain size decreases below section confirm the structural organization shown in cross sections (Fig. 3). 200 µm, and grain boundary sliding gets prominent (see section below on Puka and Krabbi domes are mostly composed of a mylonitic wrapper ~400 ± EBSD). Finally, kinematic analysis gets limited to flow plane and fold axes in 100 m thick, overlying porphyroclastic peridotites grading to high-T harzburg- ultramylonites, when the fine-grained matrix <50 µm behaves as a high-vis- ites at the base of the domes. The mylonites grade upsection from harzburgitic cosity medium (Fig. 7F). composition to plagioclase-hornblende–bearing peridotites, with increasing The last episode recorded in mantle rocks is a static lizardite network de- hornblende content at the contact with the basaltic lid. veloped in olivine at low temperatures (below 400 °C) within high-T and low-T porphyroclastic peridotites, and even in the olivine-rich bands from mylonites (Figs. 3F and 7D). In this respect, ultramylonites are commonly absolutely fresh Electron Backscatter Diffraction Measurements and Mylonites being largely impermeable to fluid circulation below 400 °C. Alternatively, sheared antigorite forms the matrix of the extended shear zone of Krë, be- Electron backscatter diffraction (EBSD) is a powerful tool to study the fab- tween Gomsiqë and Puka massifs (Figs. 3A, 3C, 4A, and 7G). rics and the mineralogy in fine-grained mylonites. The EBSD measurements were carried out using the CamScan X500FE Crystal Probe equipped with a HKL Nordlys camera and HKL Channel 5 suite of programs installed at Geo- Structural and Kinematic Analysis in Mirdita Western Massifs sciences Montpellier. Texture analysis was carried out using the MTEX, the open-source MATLAB toolbox for texture analysis (Hielscher and Schaeben, With respect to our earlier structural and kinematic map covering the entire 2008). Due to the reduced grain size (500 µm down to <50 µm), measurements North Mirdita ophiolite (Nicolas et al., 1999), we have focused here on the two covered only a partial surface of the thin section, 7–25 mm2, at a step size of OCCs of Krabbi and Puka domes, integrating recent study (Meshi et al., 2010). 5 µm. The indexation rate is between 70% and 75%. We have complemented the coverage of Krabbi and Puka domes, with addi- Six samples were collected for EBSD investigations (Table 1). The results tion of a few measurements in the western ultramafic Gomsiqë massif (Figs. of the three selected samples, representative of low-T porphyroclastic (sample 8A and 8B). These data tend to confirm a dominant exposure of high-T harz- 12A33), mylonitic (sample 12A16), and ultramylonitic (sample 12A13) textures, burgite in this latter massif, with respect to mylonites. In the Puka and Krabbi will be described and discussed (see location in Fig. 2 and texture in Fig. 7).

A 2mm E 2mm high-T Porphyroclastic Harzburgite Mylonitic Amphibolite Figure 7. Textural evolution in thin section. Plain-light and same magnification to compare the contrasts in grain size (see sample locations in Fig. 2). The thin sections are opx cut in the XZ plane of finite strain (perpen- opx dicular to foliation and parallel to mineral lineation). (A) High-temperature porphyroclastic harzburgite (98P21), protogranular texture, polygonal olivine grains 2–5 mm, with sub-boundaries; note the undeformed network of lizardite veining. (B) Mid-temperature porphyroclastic harzburgite (98P6), foliated, grain size 2–3 mm. (C) Low-tem- 2mm 2mm perature porphyroclastic harzburgite B F (12A33), olivine matrix, grain-size 200–500 µm, orthopyrox ene (opx) porphyroclasts mid-T Porphyroclastic Harzburgite Ultramylonitic Harzburgite (12A13) gliding on (100) slip plane, or ribbon shaped. (D) Plagioclase-bearing peridotite (12A16), olivine layers and/or plagioclase-rich layers, grain-size ~100 µm, lizardite (liz) ribbons. (E) Mylonitic amphibolite (96AA47), brown hornblende (hb), grain size ~1 mm, recrys- opx cpx tallized plagioclase (pl) ~100 µm. (F) Ultra mylonitic harzburgite (12A13), viscous matrix of olivine, grain size <50µm, body rotation of orthopyroxene porphyroclasts. (G) Sheared serpentinite (96AA58), lamellar antigorite including lenticular clinopyrox- opx ene (cpx) xenocrysts 1–5 mm. (H) Olivine 2mm 2mm gabbro (96AA54), magmatic texture, con- C G cave olivine grains (ol) 2–3 mm. See sample low-T Porphyroclastic Harzburgite (12A33) Sheared Serpentine locations on map (Fig. 2). Electron backscatter diffraction (EBSD) samples are labeled on photographs.

ol pl

liz ol 2mm 2mm D H Mylonitic Plagioclase Peridotite (12A16) Magmatic Olivine Gabbro

FOLIATION AND DIKES o o o o o A o o o o Iballe DIP: 0–30° 35–60° 65–85° o o o Porphyroclastic HT o pp o hz in peridotite/ N 42°10’N magmatic in gabro o 1000101 " " pp Porphyroclastic LT " " " pp " " Mylonitic o KRABBI o ppp o o " " DIKES o " " """ " " o """" " " " " + o " " "" " " " " + + o " " " " "" "" o o " " o o " pp ap o o " 12002 + o o o + + o o + 5 km o o Dedaj o o o o o o o o Figure 8 (on this and following page). o o o Petrostructural maps of the massifs of o o o o o northwest Mirdita ophiolite. (A) Foliations. o o o hz This map provides information on orienta- o o o o o Fushë-Arres tions but also on the type of increasing o o " o o " " strain and whether it is plastic in perido o o " app + o o hz GOMSIQE o " PPuukek + tites or magmatic in crustal gabbros. Trac- o o o Road ing mantle flow trajectories (purple lines) o o o ap + o + + + for low-temperature (LT) porphyroclastic o o " " " + + S " "" "" " pp pp + + peridotites and for mylonites provides a o o o " " " + + +Plagiogranit+ e " " + "" """" + visualization of the sheared domain wrap- o Gomsiqe hz Krëo " + + Basalt ping both mantle domes. + + S + + o Gabbro hz + + 42°00’N 400 o + PUKA + Troctolite o " " " hz + + " " + + pp Amphibolite + + + o + o o + Peridotite 1200 o o + mylonitic o 1000 amphibole o o o ap o o hz peridotite o o pp o mylonitic o o o o Main faults plagioclase o o pp o o peridotite o o o o ap o o o o o hz harzburgite o o o Volcano- Ophiolite o sedimentary ysch S breccia

19°40’E 19°50’E 20°00’E

Figure 9 illustrates the phase distribution map and pole figures of crystal pre- Sample 12A33, a clinopyroxene-plagioclase–bearing harzburgite from ferred orientation (CPO). The phase map provides the grain size and the modal north Krabbi is representative of low-T porphyroclastic texture (Fig. 7C) com- composition based on the total indexed phases (Table 1). Based on the obser- posed of a recrystallized matrix (grain-size 200–500 µm), including all phases vation on the large set of thin sections, the non-indexed phase in the six stud- (Table 1), 49% olivine, 6% clinopyroxene, 5% plagioclase, 2% hornblende, 13% ied samples is dominantly serpentine and iron hydroxide. Thus this parameter orthopyroxene partly included in the matrix, and also present as residual por- in Table 1 is a measure of the rate of greenschist alteration, ~50% in coarse- phyroclasts. The 27% non-indexed phase is serpentine (lizardite). The olivine grained mantle harzburgite, decreasing to 30% in mylonites. (forsterite) CPO is consistent with the [100](010) high-T olivine slip system,

o o o o o LINEATION o o B o o Iballe o PLUNGE 0–30° 35–60° 65–85° o o o pp Porphyroclastic N o hz HT & LT o 42°10’N 1000 Mylonitic " " pp " " " " " pp 5 km o KRABBI o pp SHEAR SENSE o o " " " " " (motion of the upper block) o " " " " " "" o " """"" " " " " + o " " " " " + + o o " " " "" "" Porphyroclastic HT o " " o o " ap o o " pp + o o o 1200 + + Porphyroclastic LT o o + o o o o o o Dedaj o o o o Mylonitic o o o o o o Figure 8 (continued). (B) Lineations and o o o o o associated shear senses in high-tempera- o hz Fushë-Arres o o o ture (HT) asthenospheric mantle flow and o o " o o o " " mid-temperature and increasing strain in o o " ap + o o hz GOMSIQE o " Puke + the detachment shear zone. Shear senses o o o + Road are represented with the movement of o o o pp ap o + + + the upper block on shear plane. Abbrevia- o o " " " + + S " "" "" " pp + + + + tions: ap—amphibole peridotite; hz—harz- o o o " " " + + + + " " + +Plagiogranit+ e burgite; pp—plagioclase peridotite. "" """" + o Gomsiqe hz Krëo " + + Basalt + + S + + o Gabbro hz + + 42°00’N 400 o PUKA + + Troctolite o " " " hz + + " " + + pp Amphibolite + + + o + o o + Peridotite 1200 o o + mylonitic o 1000 amphibole o o o ap o o peridotite o o pp hz o mylonitic o o o o o Main faults plagioclase o o pp o peridotite o o o o o o o o ap o hz harzburgite o o o Volcano- o Ophiolite sedimentary ysch breccia

19°40’E 19°50’E 20°00’E

consistent with its porphyroclastic texture. The mild CPO of enstatite restricts blende, departs from composition of plagioclase-amphibole–bearing perido the importance of intracrystalline glide on the expected slip plane (100), antici- tite, falling in the troctolitic field (with pargasite developed from diopside). The pated from the orientation and habitus of some porphyroclasts (Fig. 7C). 31% non-indexed phases correspond partly to iron hydroxide and largely to The mylonite sample 12A16 is a plagioclase-amphibole–bearing perido the static development of serpentine lizardite in the olivine-rich bands (Figs. 3F tite from north Puka, composed of a recrystallized matrix (grain-size ~100 µm) and 7D). Only pargasitic amphibole exhibits a strong CPO of its three indexes strongly banded at the millimeter scale (Fig. 7D). Its composition (Table 1), 29% consistent with oriented growth. Enstatite has a weak CPO similar to that ob- olivine, 10% orthopyroxene, 1% clinopyroxene, 8% plagioclase, and 20% horn- served in porphyroclastic harzburgite 12A33, (001) close to foliation.

TABLE 1. MODAL COMPOSITION OF PERIDOTITES (FROM EBSD DATA) Sample Type OlivineOrthopyroxene Clinopyroxene Plagioclase AmphiboleNon-indexed* 12A48 High-temperature porphyroclastic harzburgite 30.4 10.8 5.13 4.11 0.78 48.8 E-Puka 12A30B Mylonite43.9 11 8.71 4.21.3730.8 N-Krabbi 12A33 Medium-temperature porphyroclastic harzburgite 49 13.3 5.76 4.54 1.77 26.8 N-Krabbi 12A13 Ultramylonite54.4 10 1.33 2.05 8.823.4 N-Puka 12A41 Mylonite47. 7 3.11.070.1414.633.4 C-Krabbi 12A16 Mylonite29.3 9.51.2 8.420.930.7 N-Puka *Non-indexed phases = oxides and serpentine. Note: Modal composition obtained from electron backscatter diffraction (EBSD) measurements on six samples studied. The modal composition is computed excluding non-indexed phases (dominantly serpentine, oxides, and low-grade alteration microphases). Samples are classified on the basis of increasing pargasite content.

The ultramylonite 12A13 is a peridotite from north Puka, composed ponents located at the western margin of Krabbi massif, along the Dedaj of a fluidal matrix (grain size <50 µm) (Fig. 7F), including rounded ortho valley. This valley offers the best exposures of lower gabbros including pyroxene clasts enclosed by an asymmetrical pressure shadow that points wehrlites, highly deformed in magmatic conditions (Figs. 4D and 4E), crop- to rotation in the flowing matrix. Its composition (Table 1), 54% olivine, 10% ping near mylonitic peridotites of the western slope of Krabbi massif. East- orthopyroxene, 1% clinopyroxene, 2% plagioclase, and 9% hornblende, is ward (Figs. 2 and 8), a second type of contact between the OCC detachment closer to lherzolite, assuming that pargasitic amphibole develops at the ex- and the overlying basaltic lid (basalts and diabase dikes) is located along pense of clinopyroxene. The relatively low 23% non-indexed phases suggest the Fushë-Arres synform between Krabbi and Puka domes. Here, mylonitic a limited fraction of seawater penetrating the ultramylonite. Strong olivine amphibole-rich peridotites and amphibolites issued from this lid crop out CPO evolves when compared to sample 12A33. Obliquity of crystallographic at both contacts with Krabbi and Puka domes. Continuing ridge magmatic axes on strain ellipsoid disappears, and olivine slip system [001](010) is now activity during slip on the shear zone is attested to by a number of new dia- prominent. Pargasitic amphibole presents an expected CPO [001] axis par- base dikes cutting the mylonitic detachment zone. Finally, mylonites devel- allel to lineation, indicative of crystal growth in mantle flow. Enstatite CPO oped from plagioclase-rich peridotites are exposed on the eastern margin is random. of Krabbi dome, in contact with sheared crustal gabbros from the Mirdita central crustal section. The thickness of the transition from high-T harzburgite to mylonitic detach- DISCUSSION ments is ~400 m, measured in both OCCs (Figs. 3 and 5C). The intensely deformed peridotites at the base of the detachment are typically rich in olivine, Critical Interfaces Related to Detachment with a few orthopyroxene large clasts, derived from harzburgitic mantle (Fig. 7C). The overlying mylonitic peridotites are finely layered with gabbroic A dunitic Moho transition zone (MTZ) on top of the uppermost harzburg bands richer in minute grains of plagioclase and clinopyroxene (Figs. 6C–6E). itic mantle of ophiolites is observed beneath Moho and the crustal lower These composite mylonites located between high-T porphyroclastic mantle gabbros, either in a slow-spreading (Karson et al., 1984; Nicolas, 1989) or in a harzburgite and crustal gabbro are inferred to be issued from a Moho tran- fast-spreading system like Oman, where the MTZ is marked by an interlayering sition zone (MTZ) composed of mantle peridotite irregularly impregnated by of dunite with gabbro sills (Boudier and Nicolas, 1995). gabbroic melt (Fig. 10A). Thus in Mirdita, a former MTZ is identified as being The detailed mapping of mylonites distribution in the studied domain the main component of the mylonitic domes, mapped so far as plagioclase of Mirdita ophiolite (Figs. 2, 3, and 8) has shown their dominant location lherzolites (Anonymous, 1983). These facies represented the main signature of at the interface of the mantle domes and the continuous crustal exposure. Mirdita western massifs contrasting with the harzburgitic mantle of the eastern Mylonites mark the transition from high-T harzburgite to lower-crust com- massifs (Anonymous, 1983; Pamic, 1983).

GEOSPHERE | Volume 13 | Number 1 Nicolas et al. | Mirdita mylonites Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/1/136/1000829/136.pdf 148 by guest on 27 September 2021 on 27 September 2021 by guest Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/13/1/136/1000829/136.pdf Research Paper B A 12A13 from north Puka, ultramylonite peridotite, grain size <50 µm. porphyroclastic plagioclase-bearing lherzolite; grainµm. size (B) Sample 12A16, 200–500 amphibole-rich mylonite from northwest Puka, grain size ~100 µm. (C) Sample (same orientation as in the phase map). Lower hemisphere stereoplots; contours at 0.5 multiple of a uniform distribution (m.u.d.). (A) Sample 12A33 from north Krabbi, crystal preferred orientation (CPO) of forsterite (Fo), enstatite (En), and pargasitic amphibole (Pa) (modes bracketed; see Table 1). White line is the trace of foliation green—diopside (Di); yellow—bytownite (By); gray—pargasite (Pg); black—non-indexed phases (see also Table 1). In the right column, corresponding pole figures show finite strain (perpendicular to foliation and parallel to mineral lineation). Maps in the left column show distribution phases. Red—forsterite (Fo); blue—enstatite (En); Figure 9. Electron backscatter diffraction (EBSD) results from Mirdita selected samples (see location on Fig. 2) (Table 1). The thin sections are cut in the XZ plane of C 1000 µm 1000 µm 500 µm 12 A 13 12 A 16 12 A 33 (29.3) (20.9) (13.3) (49.0) En (9.5 ) Pg En Fo Fo (5.8 ) (54.4) Di (8.8 ) (10.0) Pg Fo En <100> <100 > <100 > <100 > <100> <100 > <100 > <100 > <100> <010> <010> <010> <010> <010> <010> <010> <010> <010> <001> <001> <001> <001> <001> <001> <001> <001> <001> 10 1 8 6 2 1 8 6 2 1 2 4 4 3 2 4 2 2 3 1 3 2 1 2 1 2 4 3 1 3

GEOSPHERE | Volume 13 | Number 1 Nicolas et al. | Mirdita mylonites 149 Research Paper

Time sequences

Figure 10. Time sequences represent the time evolution of the oceanic core complex (OCC) detachment as recorded in the fos- Plagioclase-bearing silized OCC of Mirdita. (A) Step 1 is initia- mylonites tion of the detachment at the Mohorovičić Amphibole-rich discontinuity (the Moho), in a zone located mylonites at the ridge margin of a slow-spreading system, still composed of mantle impreg- Hydrothermal circulation nated by melt. This Moho transition zone Nascent crust O 500m high% H2 (MTZ) would evolve, during cooling, and O 0.6% H2 O melt extraction toward the gabbro crustal 0.2% H2 O section, in a thick dunitic MTZ, as observed 2km 0.02% H2 in ophiolites representative of low- to LT porphyroclastic mid-spreading rate. The OCC detachment N harzburgite is an alternative evolution. (B) Step 2 ac- Gabbro TIO counts for the hydrothermal path as re- HT porphyroclastic corded in studied mylonites, enriched in harzburgite hornblendic amphibole upsection. Quan- EXHUMA tification of progressive hydration during e upward progression along the detachment i t D u n is based on electron backscatter diffraction C (EBSD) measurements (Table 1) of amphibole content (average 2% water in hornblende). (C) Step 3 is the final situation as Detachment shear zone B represented in the fossil OCC from Mirdita Harzburgite A s t h e n o s p h e r e ophiolite. HT—high-temperature; LT—low temperature.

The Moho Transition Zone as a Detachment Surface abut on flat, layered gabbros through a pyroxene-rich banded unit a few hundred meters thick. The Moho transition zone (MTZ) plays a major role in oceanic accretion. Its Due to (1) the systematic location of the mylonite at contact between man- composition, shape, and orientation are highly dependent on the spreading tle harzburgite and crustal formations and (2) the textural transition from por- rate (see below). Due to its partially molten nature, MTZ rheology is weaker phyroclastic to mylonitic harzburgite and to plagioclase and/or hornblende than its mantle environment, and the OCC detachment tends to remain chan- mylonitic peridotite, we concluded that the shear detachment is initiated at neled within it. a Moho transition zone. Merging from asthenospheric harzburgites, sheared The MTZ concept has been developed in the fast-spreading Oman deformation localized in the melt-rich MTZ, accounts for plagioclase-bearing ophiolites (Boudier and Nicolas, 1995), and it is now described in the peridotite mylonites and gabbroic layers. Upward migration of the detachment East Pacific Rise (EPR) (Crawford and Webb, 2002; Carbotte et al., 2013). along crustal gabbros and basaltic dikes accounts for amphibole-rich mylonitic The MTZ is locally much thicker in slow-spreading ophiolites than in peridotite (Fig. 10A). fast-spreading ophiolites, despite a much larger heat supply within it. In This conclusion concerning rooting of the detachment at depth, down to ophiolites representative of a slow to moderate spreading rate, the MTZ asthenospheric mantle, was also reached at Trans-Atlantic Geotraverse (TAG) thickness ranges from 1 to 3 km (see compilation in Nicolas and Boudier, (Mid-Atlantic Ridge 26°N) by deMartin et al. (2007), who reported results of a 2003). In Kukes massif from eastern Mirdita (Fig. 1), MTZ documented as a seismic study of a steep and active fault zone. Temperatures in detachment dunitic domain, varies from 1.5 to 2.5 km (Hohxa and Boullier, 1995); in Bul- have been evaluated between 700 and 900 °C for an estimated depth ~7 km qiza massif (Fig. 1), MTZ thickness marked by dunite is over 2 km (Meshi (Schroeder and John, 2004; Hansen et al., 2013) in mylonites from the Mid- et al., 2009). In both cases, foliations in peridotite are steeply dipping and Atlantic Ridge, Atlantis, and in Kane OCCs, respectively.

Seawater Penetration at the Ridge have merged. Was it located to the west between Gomsiqë and these OCCs or to the east in the central Mirdita synform, between these western massifs Phase distribution maps in our EBSD study provide accurate modal com- and the Kukes ophiolite (Fig. 1)? In our previous paper, we had no argument positions. The seawater fraction in the detachments up to Moho is tentatively supporting either side for the ridge of origin. Here, we have located the hang- evaluated based on amphibole fractions in the mylonites, assuming an ~0.2% ing wall on the eastern side of the Krë shear zone, between Gomsiqë and Puka water content in amphiboles. The six representative mylonite samples analyzed massifs (Figs. 2 and 3). This is supported by observations of the absence of (Table 1) are classified according to their amphibole content. Our field observa- plastic strain and a common magmatic fabric in these lower gabbros asso- tions show that the harzburgitic mylonites are in contact with the low-T porphyro ciated with wehrlitic intrusions from Dedaj Valley (Fig. 4D and 4E). This do- clastic harzburgites, at the base of the detachment, although hornblende-rich main of crustal exposure devoid of solid-state deformation, a remnant of ridge mylonites are located at the upper contact with the basaltic lid (Figs. 3 and 8). accretion, would represent the hanging wall of detachment, west of Krabbi This distribution suggests that a decreasing fraction of seawater has penetrated dome. The next exposure of the hanging wall is represented by the lid forma- along the ridge plane attaining on the order of 0.2% at Moho and rapidly de- tions, lava flows, and dikes of the Fushë-Arres synform. Here, the flat setting of creasing below within the MTZ, to virtually disappear within the underlying the diabase dikes (Fig. 8A) may account for the hanging-wall tilting. mantle harzburgites (Fig. 10B). This compares with the deep-seawater crustal The detachments, progressing upsection within the crust and cooling, penetration, 6–7 km below seafloor as estimated at the Mid-Atlantic Ridge OCCs reached upper gabbros that were plastically deformed, together with the MTZ, (Schroeder and John, 2004; deMartin et al., 2007; Hansen et al., 2013). thinned and transformed into plagioclase-bearing peridotite mylonites (Fig. The lower-grade hydrous alteration, low-amphibolite, greenschist meta- 10A). At the level of the lid, part of the sheeted dikes was transformed to my- morphism in the detachment mylonites is not significant. This is inferred from lonitic amphibolites (Figs. 2 and 7E). Fresh diabase dikes that cut the mylo- the EBSD maps and modal composition (Table 1) stating that the non-indexed nitic detachment point to continuing magmatic activity during the exhumation phases point to similar value in porphyroclastic harzburgite and in mylonites. (Fig. 4G). The major shear zone of Krë containing large undeformed crustal The porphyroclastic harzburgite is devoid of greenschist phases except ser- knockers in a matrix of antigorite schists (Figs. 3A, 3C, 4A, and 4B) may derive pentine lizardite observed under the optical microscope. Similar observation from the last stage of detachments. The deep and extensive kaolinization of is valid concerning the mylonites studied. The hydrous phase observed is plagioclase in gabbros from the lowlands of Dedaj Valley is specific to this lizardite largely developed at the expense of olivine-rich bands (Fig. 3F) and zone. It requires massive hot-water circulation within the hanging wall. consistently contributes to the 30% non-indexed phases, in addition to iron This query about the site of the paleo-ridge is important for the regional geol hydroxides. This leaves little space for the development of hydrous phases, ogy and for the modeling of west Mirdita OCCs. Oceanic core complex detach- actinolite, talc, and chlorite, during the low-T stage of the exhumation process. ments in the Atlantic ridges could have emerged on either side of the ridge as The development of static lizardite suggests that the tectonic activity along the documented at Kane OCC from the Mid-Atlantic Ridge (MAR) (Dick et al., 2008). shear detachment had ceased below 400 °C, the upper limit for alteration in liz- The question has been addressed in Mirdita by Maffione et al. (2013), based on ardite. The detachment was possibly relayed along antigorite-schist–sheared comparison of magnetization measured in the OCC footwall and hanging wall, mélange exposed in particular in the Krë shear zone (Figs. 3C and 4A) that a method experimented in Mid-Atlantic Ridge OCCs (Morris et al., 2009). Com- could represent the latest and separate stage of the exhumation process. parison of magnetization in gabbro and peridotite on the Puka eastern slope in Finally, the deep, whitish alteration affecting gabbros and basalts west of Mirdita, lead Maffione et al. (2013) to assume a 46° counterclockwise rotation of Puka and Krabbi domes (Fig. 5B) has been ascribed to a Pliocene–Quaternary the footwall around a subhorizontal axis trending N-S, thus localizing the ridge episode in the Albania 1/200,000° geologic map (Anonymous, 1983). However, axis east of Puka massif. Alternative contribution to this question is provided weathering of their paleosoils could be older. This extensive alteration product by fission-track thermochronology (Muceku et al., 2008). In the western massifs is kaolinite, mined in a few places, and this necessitates temperatures above concerned here, these authors identify the Krë shear zone between Gomsiqë 80 °C (Ross and Kerr, 1931). It points to equilibrium temperature too high for and Puka massifs as part of a series of pre-Cretaceous thrust. This early group of groundwater. We assume that the detachment fault could have introduced thrust affects the ophiolite and relates to a major discontinuity during obduction seawater resulting from hot hydrothermal fluids percolating from the uprising or accretion, which is thus consistent with possible rooting of OCCs detachment OCCs (Fig. 10). along this line, as inferred from our detailed structural mapping.

Ridge Location in Western Mirdita Model for the Detachment

In Mirdita, the sheeted dikes are steep and oriented N-S to NNE-SSW (Nico- Recent papers (Nicolas et al., 1999; Tremblay et al., 2009) fully confirm the las et al., 1999), suggesting that the ridge was oriented accordingly. Here, we Upper Jurassic nearly intact seafloor topography above the internal structure discuss the location of the ridge axis from which the Puka-Krabbi detachments of the northern Mirdita ophiolite, beneath nearly horizontal, transgressive

marine sediments. This fortunate situation of Mirdita OCCs in terms of tec- related flow. Nevertheless, Figure 11 illustrates the complexity introduced by tonics simplifies the interpretation because it excludes late large rotations. the detachment of the Mirdita domes, from their origin in the melt-rich, ~2-km- Thanks to exposure of rocks with preserved asthenospheric fabric, and their thick MTZ and their intrusion along the slope of the lithosphere-asthenosphere relationship to shallower units, these entries improve the knowledge of deep limit below the ridge axis. The shear detachment has been initiated into mantle kinematics of OCC. The frame of the model for Mirdita ophiolite of Figure 11 peridotites, with prints of the high-T asthenospheric mantle flow, attached to is inspired from geophysical models providing information on the deep struc- the ridge of origin (Fig. 10A) and reflecting its kinematics (Fig. 8B). Progressing tures of OCC detachments along the MAR ridge axis. These models are based upward, the detachment tends to orient into a transverse northwest trend, as on seismic data across the TAG at 26°N (deMartin et al., 2007), on gravimetric recorded in the overlying low-T porphyroclastic harzburgites and in mylonites data at the 30°N Atlantis detachment (Nooner et al., 2003; Henig et al., 2012), near the hanging wall of detachments (Fig. 8B). On top of the mantle dome, and on detachment geology (Schroeder and John, 2004; Karson et al., 2006; mylonites trend into a flat settling and reflect some disorder of the flow direc- Ildefonse et al., 2007b; Dick et al., 2008, Hansen et al., 2013). tions (Figs. 8A and 8B). The shear detachment stretched to 400-m-thick mylo- The first evidence from the detailed mapping in western Mirdita (Figs. 8A nites points to an estimate close to the 450 m inferred in Kane OCC (Hansen and 8B) is the poor structural organization compared to other ophiolite bod- et al., 2013). The clockwise rotation of the OCC footwall is consistent with the ies (Nicolas, 1989). In the field, the spectacular mylonites result in fairly scat- dominant shear sense deduced from the kinematic mapping (Fig. 8B). Kine- tered local orientations. At the scale of the map, the disorder also has another matic indicators gathered in the high-T asthenospheric harzburgites, having source that is the complexity of the kinematic information carried: from high-T, preserved the print of mantle flow parallel to the ridge of origin, suggest that ridge-related longitudinal mantle flow to low-T transversal, detachment- the ridge was active at the same time as the detachments were being initiated.

10 km 5 0 5 10

Corrugated NW detachment surface Break-away SE D e t a c h m e n t Krabbi Dome f a u l t Neovolcanic zone Debris ow Dedaj Valley ... o ...... 3 1 ... o km ...... o .. o ...... o o.. 5 0 oo Lid Lithospheric mantle 4 Lithospheric mantle Crust 2 Gb

Wh 4 2 Du o s p h e h e n r e ( 1 2 0 0 ° C A s t ) 6 Hz

Figure 11. Model of emplacement of core complex in Mirdita western massifs. The model integrates the best documented structures in Krabbi massif: (1) The frozen spreading axis, represented by oceanic crust of Dedaj valley, devoid of sheared deformation, is the hanging wall. Crust and mantle section represented in this domain is based on structural mapping in Kukes and Bulqiza eastern massifs (Hoxha and Boullier, 1995; Meshi et al., 2010). (2) The detachment roots in the soft zone at Moho level, progress upward, integrating first gabbros, then basaltic lid at contact. (3) Foliation and shear sense along the detachment recorded in the lithospheric mantle flow and in mylonites refer to structural map of Figure 8. The flow line is consistently trending NW-SE; the shear sense in the model is optimized from poor resolution of shear-sense measurements (Fig. 8B). (4) Flow direction and shear sense in asthenospheric mantle rely on the most consistent kinematic results (Fig. 8), indicative of asthenospheric mantle flow parallel to the ridge. The dynamics of the system result in clockwise rotation and upward movement of the footwall. (5) The break-away is assigned to the eastern contact of mantle domes with gneissic gabbros of the central Mirdita crustal domain (Figs. 2, 4F, and 4G). Abbreviations: Du—dunite; Gb—gabbro; Hz—harzburgite; Wh—wehrlite.

Mid-Atlantic Ridge OCCs Detachment Surface The detachment shear zone is composed of mylonitic amphibole-plagio clase peridotites, underlain by porphyroclastic harzburgite, and capping Initiating a detachment surface beneath the rift valley necessitates the pres- asthenospheric harzburgitic mantle rocks. The detachment is localized at ence of a mechanically weak horizon. Good candidates are either serpentinites Moho level and initiated in a melt-impregnated mantle. This option departs forming at T<500 °C from shallow mantle or at greater depth of the domain of from some OCC models that invoke an intrusive magma chamber at the deep melt impregnation. The major shear zones in oceanic core complexes include root of the detachment shear zone. Both models could relate to slightly dif- both a high-T mylonitic component, typically capped by a lower-T semi-brittle ferent spreading rates. The shear-detachment mylonites are devoid of green- shear zone (detachment fault). schist-facies hydrous phases, and serpentinization is static. A low-T antigorite In particular, outcrops at the MAR (15°45′N) and at the central part of the At- schist mélange at the western margin of the shear detachment may represent lantis massif where the sampling is superficial yield essentially low-tempera- a separate last phase of the exhumation. ture fault schists and sheared serpentinites. Based on such evidence, MacLeod Root of the detachment is localized west of the domes along a N-S–trend- et al. (2002) and Escartín et al. (2003) tend to extend their results to other core ing paleoridge. Tracing progressive textural and kinematic evolution, a rapid complexes on shallow detachments within serpentinites. In Mirdita, sheared transition from ridge-parallel asthenospheric accretion to ridge-transverse serpentinites have only a local contribution along the frozen limit of the de- OCC shear detachment is inferred. Detachment progressed eastward, includ- tachment (Krë shear zone). In serpentinized peridotites and MTZ mylonites, all ing lid formations in the shear domain (margins of the Fushë-Arres synform). thin sections show that lizardite postdates any deformation. Lattice-preferred orientation of mineral phases in mylonites reveals dominant Oceanic sampling has also produced a significant number of mylonites on activation of the low-T [001](010) slip system in olivine; strong orientation of the southern slope of Atlantis Massif, 30°N MAR (Schroeder and John, 2004; [001] hornblende along the lineation is indicative of amphibole growing in the Karson et al., 2006), as well as on domes of Kane megamullion (Dick et al., deformation path. 2008; Hansen et al., 2013). Studies of the high-T sheared samples offer many

similarities with the detachment facies described in Mirdita, in particular the ACKNOWLEDGMENTS strain evolution recorded on the 400 m reconstructed petrostructural log at We wish to recognize Albanese field geologists who worked with exceptional field expertise and Kane (Hansen et al., 2013). accuracy in the Mirdita ophiolite in producing the 1/200,000 map published in 1983 under the Based on this transition in Mirdita, and on mixed layers of gabbro-perido People’s Socialist Republic of Albania. Christophe Nevado and Doriane Delmas provided excellent tite, we have ascribed the OCC detachment to the MTZ as described in polished thin sections, and Fabrice Barou’s expertise facilitated the EBSD acquisition. The con- structive and detailed remarks of both reviewers, Jeffrey Karson and Chiara Frassi, contributed to ophiolites. This option departs from some OCC models that assign the the improvement of the manuscript. weak zone initiating the detachment to gabbro intrusion in mantle rocks for the Atlantis Massif (e.g., Ildefonse et al., 2007b; Blackman et al., 2011) REFERENCES CITED or for Kane OCC (Dick et al., 2008), according to Cannat’s (1993) model of a Anonymous, 1983, Harta Gjeologjike e RPS të Shoipërisë, 1/200 000°: Tirana, Albania, Ministry slow-spreading ridge. Altogether, we notice that some MTZ characteristics of Industry and Minerals. are identified as mantle components in the gabbro section of the Atlantis Blackman, D.K., et al., 2011, Drilling constraints on lithospheric accretion and evolution at Atlan- Massif (Suhr et al., 2008; Drouin et al., 2010), providing some credit to the tis Massif, Mid-Atlantic Ridge 30°N: Journal of Geophysical Research, v. 116, B07103, doi:10 .1029/2010JB007931. existence of a MTZ at the root of the oceanic core complex. The MTZ option Boudier, F., and Nicolas, A., 1995, Nature of the Moho transition zone in the Oman ophiolite: in Mirdita is also supported by the continuous exposure of the oceanic crust Journal of Petrology, v. 36, p. 777–796, doi:10.1093/petrology/36.3.777. overlying the mantle domes in the mapped area. This discussion may relate Cannat, M., 1993, Emplacement of mantle rocks in the seafloor at mid-ocean ridges: Journal of Geophysical Research, v. 98, p. 4163–4172, doi:10.1029/92JB02221. to the complex structure of oceanic lithosphere and its strong dependence Carbotte, S.M., Marjanivic, M., Carton, H., Mutter, J.C., Canales, J.P., Nedimovic, S.M.R., Han, S., on spreading rate. and Perfit, M.R., 2013, Fine-scale segmentation of the crustal magma reservoir beneath the East Pacific Rise: Nature Geoscience, v. 6, p. 866–870, doi:10.1038/ngeo1933. Crawford, W.C., and Webb, S.C., 2002, Variations in the distribution of magma in the lower crust and at the Moho beneath the East Pacific Rise at 9°–10°N: Earth and Planetary Science Let- CONCLUSIONS ters, v. 203, p. 117–130, doi:10.1016/S0012-821X(02)00831-2. deMartin, B.J., Sohn, R.A., Canales, J.P., and Humphris, S.E., 2007, Kinematics and geometry Oceanic core complexes (OCCs) exposed in the northern Mirdita ophio- of active detachment faulting beneath the Trans-Atlantic Geotraverse (TAG), hydrothermal field on the Mid-Atlantic Ridge: Geology, v. 35, no. 8, p. 711–714, doi:10.1130/G23718A.1. lite complement fragmentary information available for modern OCCs. The Dick, H.J.B., Tivey, M.A., and Tucholke, B.E., 2008, Plutonic foundation of a slow-spreading ridge dome-shaped structure of both massifs in Mirdita comprising a mantle core segment: Oceanic core complex at Kane Megamullion, 23°30′N, 45°20′W: Geochemistry, wrapped by the detachment zone provides a 3D image of the OCC. Thermal Geophysics, Geosystems, v. 9, no. 5, Q05014, doi:10.1029/2007GC001645. Dijkstra, A.H., Drury, M.R., and Vissers, R.L., 2001, Structural petrology of plagioclase-peridotites evolution and kinematics of the detachment are deduced from detailed petro- in the West Othrys Mountains (Greece): Melt impregnation in mantle lithosphere: Journal of structural mapping. Petrology, v. 42, p. 5–24, doi:10.1093/petrology/42.1.5.

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