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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 perido tites 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-amphi- bole–bearing mylonitic perid otites, ~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 harz burgitic 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. This mantle 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 © 2017 Geological Society of America 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 136 by guest on 27 September 2021 Research Paper 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).
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