Miocene ductile thinning below the Folegandros Detachment System, Cyclades, Greece

Folegandros island lies between the established SW‐directed West Cycladic and S‐directed Santorini detachment systems. Preserving shear‐sense indicators with opposing kinematics, the geodynamics and timing of kinematics are debated. Using white mica 40Ar/39Ar geochronology complemented by structural data, we report a new Miocene top‐to‐S detachment system, the Folegandros Detachment System (FDS), that is coupled with pure‐shear flattening and juxtaposes the Cycladic Blueschist Unit below Early Cretaceous to early Eocene low‐grade marble and quartzitic‐phyllitic sequences, topped by a metaflysch. Chronostratigraphic correlation and detrital zircon geochronology reveal the low‐grade rocks from the hanging wall of the FDS likely belong to the Pelagonian Zone. Middle Miocene zircon and apatite (U‐Th)/He cooling ages correspond to the latest stages of exhumation. Observations of opposing kinematics on Sikinos, Ios and now Folegandros reveal that ductile thinning played a more significant role in accommodating Miocene exhumation of high‐pressure rocks in the southern Cyclades than previously postulated.

the exhumation of metamorphic core complexes (Figure 1; Avigad et al., 1989;Gautier et al., 1993;Grasemann et al., 2012). Folegandros island, of the southern Cyclades, is critically located in the relay between the SW-directed West Cycladic (Grasemann et al., 2012) and S-directed Santorini  detachment systems (WCDS and SDS, respectively). In addition, it is proximal to the projected surface trace of the Trans-Cycladic Thrust (TCT, Grasemann et al., 2018) and aligns with an extensional hinge zone controlling incipient bivergent extension in the southern Cyclades (Ring & Glodny, 2021). Notably, the implications of the regional geodynamics on the exhumation of the highpressure/low-temperature (HP/LT) rocks exposed in the southern Cyclades are highly debated (Augier et al., 2015;Huet et al., 2009;Poulaki et al., 2021;Ring & Glodny, 2021), and unfortunately, the detailed bedrock mapping of Folegandros by Sowa (1985) has been overly simplified. Most publications show the island as a single exposure of the Cycladic Blueschist Unit (CBU) over the last four decades (e.g., Augier et al., 2015;Avdis & Photiades, 1999). Our new geochronological and structural data support Sowa's (1985) original observations of high-pressure metasedimentary and metavolcanic units structurally beneath low-grade marble and quartzitic-phyllitic sequences topped by a metaflysch. Moreover, our results indicate that exhumation was likely accommodated by Miocene pure-shear flattening, as suggested to occur on nearby Ios and Sikinos (Ring & Glodny, 2021), in addition to top-to-S low-angle detachment faulting along the Folegandros Detachment System (FDS).

| Geology of the Cycladic Islands
Located in the Aegean Sea, the Cyclades formed in the back-arc position of the Hellenic trench and belong to the Attic-Cycladic Crystalline Complex (ACCC; Gautier et al., 1999). Late Mesozoic-Cenozoic subduction of the African plate beneath the Eurasian plate, followed by Miocene extension related to slab rollback of the African plate and gravitational collapse, led to the formation of the ACCC (Jolivet & Brun, 2010). Bivergent crustal-scale detachment systems resulted in the exhumation of the lower plate, greenschistfacies retrogressed HP/LT metamorphic rocks, and an upper plate of high-and low-grade units that are sparsely preserved as structurally higher fragments in the Cyclades (Grasemann et al., 2012;Lister et al., 1984). On many islands including Thera and Serifos, this relationship is expressed by a CBU footwall juxtaposed by low-angle normal faults to an Upper Unit (s.l.) that is commonly untouched by Eocene HP metamorphism and correlated with the Pelagonian Zone of the Internal Hellenides (Reinecke et al., 1982;Soukis & Stockli, 2013;Zeffren et al., 2005). Use of the term Upper Unit (s.l.) herein represents the Pelagonian Zone (s.l.) above the CBU.

| Geology of Folegandros Island
Folegandros (Φολέγανδρος) is a small, 'peanut'-shaped and NW-SEoriented island with narrow ridges running parallel to its orientation. It consists of a northern structural dome, which exposes the structurally lower CBU and an overlying NE-dipping homocline of Early Cretaceous to Eocene units in the south (Figure 2; Sowa, 1985;Augier et al., 2015).
At the base, the CBU consists of metabasites intercalated with metasediments including micaschists, glaucophane-bearing greenschists and thin-medium bedded coarse-grained marbles (Augier et al., 2015;Photiades & Keay, 2003;Sowa, 1985). Metabasite lenses locally preserve relict HP/LT assemblages, including glaucophane and lawsonite pseudomorphs (Figure 3a), defining subduction-related blueschist facies metamorphism that is consistent with published Raman spectroscopy of carbonaceous materials (RSCM) peak metamorphic temperatures of 395-415°C (Augier et al., 2015). A N-S trending dominant stretching lineation, defined most commonly by syn-kinematic chlorite and white mica, suggests stretching was recorded within greenschist-facies conditions (Augier et al., 2015). White mica in these rocks are kinked and oblique to quartz microlithons ( Figure 4a). Polycrystalline quartz display bulging-to-subgrain rotation recrystallization (Figure 4b) and sweeping undulose extinction indicative of dynamic recrystallization and temperature conditions between 350 and 450°C (Passchier & Trouw, 2005). Although there is pervasive top-to-N shearing, as mapped by Augier et al. (2015), we also mapped nu-  Structurally above the CBU, the NE-dipping homocline consists of lower grade rocks (Sowa, 1985) with a markedly different thermal history than the CBU, which can be grouped into two distinct units. The first comprises most of the southern half of the island and consists of alternating low-grade, deformed to little-deformed marble (Figure 3b, c) and subordinate quartzitic-phyllitic sequences including calc-micaschists (locally called the Eleftherios Unit; Sowa, 1985). The quarzitic-phyllite unit displays disjunctive cleavage ( Figure 4d) and bulging recrystallization of quartz (Figure 4e), indicative of low-temperature deformation (Passchier & Trouw, 2005).

Significance Statement
Crustal-scale fault geometry and kinematics are integral in understanding regional stress fields and evaluating how the lithosphere accommodates extension. Our paper reports a new, Miocene detachment in the southern Cyclades that explains the strain partitioning between detachments in a soft-linked relay system. Moreover, we provide evidence the detachment was coupled with ductile thinning, forming a stretching detachment system, to accommodate Miocene extension of the Aegean. Finally, we establish a previously unacknowledged exposure of the Pelagonian Zone on the island of Folegandros, which reveals the hanging wall of the entire soft-linked system consists of Pelagonian Zone. This contribution has significant implications for the interpretation of the Miocene tectonics in the southern Cyclades and the architecture of rifted margins.
At the top of the Eleftherios Unit is a lithotectonic stratigraphy of meter-scale ophiolite/ serpentinite phacoid bodies in calcschist and micaschist overlying a marble and quartzitic-phyllitic sequence (Figure 3d;Sowa, 1985;Photiades & Keay, 2003). Along the southeast coast near Livadi, the second and structurally highest unit is a metaflysch with mid-Cretaceous marble olistoliths (locally called the Vighlitsa Unit; Sowa, 1985). The relatively lower metamorphic grade of both the Eleftherios and Vighlitsa units are consistent with published RSCM temperatures of 321-347°C (Augier et al., 2015).
In the Eleftherios Unit near Petousis (

| D ISCUSS I ON AND SUMMARY
On regional maps, Folegandros is commonly depicted solely as an exposure of CBU (notably Augier et al., 2015). Although the structurally lowest unit exposed on the northern half of the island is unequivocally of the Lower Cycladic Blueschist Nappe Roche et al., 2019), the structurally higher units exposed on the southern part of the island are lower grade and relatively less deformed. Sowa (1985) first noted these differences A similar lithostratigraphic sequence of low-grade marbles overlain by metabasite-bearing metaflysch at high structural levels is observed within the Cyclades, including nearby Thera where it defines the hanging wall of a low-angle detachment . Based on the lithotypes, detrital zircon U-Pb age distribution and Cretaceous depositional constraints, we propose the Eleftherios and Vighlitsa units belong to the Pelagonian Zone.
More specifically, given the presence of Jurassic-Cretaceous marble and the phyllitic Cretaceous interval, it is probable this sequence is the Mesoautochthonous Unit, which is the uppermost Pelagonian Zone (Jacobshagen, 1986;Papanikolaou, 2021;Porkoláb et al., 2019). Consequently, there must be a discrete tectonic boundary between the CBU and Pelagonian rocks that merits a revised interpretation of the kinematic structures preserved on the island.
The contact between the CBU and lower grade rocks is the FDS, which is exposed along the corset of the island where top-N brittle-ductile structures were identified (Figure 7; see Figure 8 of Augier et al., 2015). However, the N-dipping, low-angle shear bands are overprinting higher temperature structures, including SC(C′) fabric, sigmoids and shear bands that unambiguously indicate ductile top-to-S shearing (Figure 5a-h). We interpret the topto-S shearing as extension based on our new white mica 40 Ar/ 39 Ar ages from mica that define the foliation in the CBU along the FDS that coincide with regional crustal-scale extension in the southern Aegean at c. 23 Ma (Büttner & Kowalczyk, 1978;Cossette et al., 2015;Grasemann et al., 2012;Ring et al., 2010). Published lower RSCM temperatures (Augier et al., 2015)  Opposing, top-to-N and -S shear-sense indicators preserved within the same unit have also been reported directly east of Folegandros on Ios (Huet et al., 2009;Mizera & Behrman, 2016;Thomson et al., 2009) and Sikinos (Augier et al., 2015;Ring & Glodny, 2021), and are interpreted as a result of pure-shear flattening. Ring and Glodny (2021) have called this region an extensional hinge zone that resulted in up to ~40% of vertical shortening.
Opposing directions of kinematics in conjunction with the presence of brittle-ductile conjugate shear bands support a component of oblate strain in the vicinity of the hinge that is not completely ductile ( Figures 5, 7). Furthermore, juxtaposition of Cycladic Basement Unit, exposed on the southwestern tip of Sikinos, <15 km from the Located below an overlooked occurrence of the Pelagonian Zone, the FDS does not form a hard link of S-directed extensional detachments but is an isolated structure in the relay zone between the WCDS and SDS. It acts to laterally transfer the large displacement of the WCDS to the SDS, forming two relays and the WCDS-FDS-SDS soft-linked system with relay ramps between the individual detachments ( Figure 9). In addition, the hanging wall for the entire system consists of the Pelagonian Zone, distinguishing it from the so-called South Cycladic Detachment System (Ring et al., 2011).
Ductile thinning of the CBU in combination with extension in the hanging wall establishes the FDS as a stretching fault in the sense of Means (1989Means ( , 1990 or perhaps more appropriately as a stretching detachment system. Observations of opposing kinematic indicators, often interpreted as pure-shear flattening, on the spatially aligning islands of Ios (Huet et al., 2009;Mizera & Behrman, 2016;Thomson et al., 2009) and Sikinos (Augier et al., 2015;Ring & Glodny, 2021 Figure 2). CB21-14 consists of very small blush and yellow subangular grains with poor habit. Late Palaeozoic Variscan (Carboniferous) to post-Variscan (Permian) ages, and a significant Triassic mode reflect early to final stages of Paleotethyan closure and Neotethyan rifting. Contemporaneous to the Caledonian orogeny, the early Palaeozoic mode reflects early detachment of Gondwanan blocks that later accrete along the southern European margin, whereas the late Proterozoic populations reflect input from Pan-African or Cadomian, peri-Gondwanan sources. CB21-12A are dominantly moderately small-to medium-sized zircons, colourless and blush pink with good habit and rounded double terminations, all containing inclusions. Late Mesoproterozoic to early Palaeozoic ages with Ediacaran and Cryogenian peaks and an absence of Mesoproterozoic (>1.2 Ga) detrital ages from the Cretaceous unit can be explained by sediment derived from a Minoan peri-Gondwanan terrane (Dörr et al., 2015 and references therein). See Data S1 (Table DR3) for analytical data. [Colour figure can be viewed at wileyonlinelibrary.com]

ACK N O WLE D G E M ENTS
Funding was provided by the Natural Sciences and Engineering Research Council of Canada (to DAS). We thank the Greek Institute for Mining and Exploration (IGME) in Athens for providing permission for fieldwork, S. Morfin (uOttawa) for helping in U-Pb analysis, J.
Metcalf (UC-Boulder) in assisting with (U-Th)/He dating, A. Camacho (uManitoba) for aiding in 40 Ar/ 39 Ar analysis, and Taylor Ducharme, Jennifer Spalding and Mark Coleman (uOttawa) for science sharing and insightful discussions. We thank Carlo Doglioni for efficient handling of our manuscript and Laurent Jolivet and Vincent Roche for excellent reviews that significantly improved our manuscript.

CO N FLI C T O F I NTE R E S T S TATE M E NT
None.