Synconvergence extension and midcrustal exhumation in the Internal Dinarides
Published:September 11, 2017
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Gabriele Casale, Richard Bennett, Darrel Cowan, Marijana Surkovic, 2017. "Synconvergence extension and midcrustal exhumation in the Internal Dinarides", Linkages and Feedbacks in Orogenic Systems, Richard D. Law, J. Ryan Thigpen, Arthur J. Merschat, Harold H. Stowell
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Final closure of the Neotethys Ocean basin along the Eurasian margin in southeastern Europe during Eocene–Oligocene time was accompanied by upper-crustal extension expressed as a series of low-angle detachments, basins bounded by normal faults, and volcanism. This extensional belt spanned the southern Balkan Peninsula from the Albanides along the southern Adriatic coast in the west to western Anatolia in the east. Despite the widespread occurrence of this phenomenon within the southern Balkan region, similar extension has not previously been observed in association with the Neotethys closure in the Dinarides, which form the western geographic continuation of this orogenic belt, ending in the Austrian Alps in the northwest. The Mid- Bosnian Schist Mountains are a fault-bounded body of greenschist-facies metamorphic rocks located along the paleogeographic margin of the present-day Adria continental block in the Internal Dinarides. We combine low-temperature thermochronometric ages with field observations of kinematic shear sense indicators and demonstrate that the Mid- Bosnian Schist Mountains were exhumed along a normal fault between 43 and 27.5 Ma. The most rapid cooling occurred between ca. 35 and 27 Ma, coincident with a regional-scale magmatic event. These data constitute the first evidence for major extension in the Dinarides contemporaneous with collision between Adria and the Eurasian margin, and they are consistent with removal of a subducting slab during the transition between oceanic subduction and continental collision.
Detachment-related extensional exhumation is exemplified by the numerous metamorphic core complexes that punctuate the North America Cordillera (Wernicke, 1992). Cordilleran metamorphic core complexes in the western United States and Canada were originally attributed to shortening tectonics (Misch and Hazzard, 1962; Nelson, 1969; Thorman, 1970). Careful field observation and application of modern isotopic dating techniques in the 1970s spurred rigorous debate and ultimately led to the realization that core complex detachments are the sites of major crustal extension (Mudge, 1970; Armstrong, 1972; Campbell, 1973; Price, 1973; Roberts and Crittenden, 1973; Davis, 1975; Davis and Coney, 1979). This debate resulted in the recognition of key diagnostic features indicative of core complex–type extensional exhumation, which include: a mylonitic upper carapace, a brittle detachment zone marking the boundary with the structural upper plate, and a supradetachment sedimentary basin (Coney, 1980; Lister and Davis, 1989; Friedmann and Burbank, 1995).
The Mid-Bosnian Schist Mountains (Figs. 1 and 2) are one of a series of crystalline complexes that extend parallel to the Dinarides orogeny and are located along the Dinaric paleogeographic passive margin (Fig. 1). The Mid-Bosnian Schist Mountains are bordered along the NE and SE margins by an ENE-dipping fault, much of which is concealed by deformed Oligocene–Miocene sediments of the Sarajevo Basin (Fig. 2). Where the fault boundary is exposed, it has been mapped as a thrust fault (e.g., Jovanović et al., 1977; Vujnovic 1980) with a top-to-the-SW kinematic shear sense. This kinematic interpretation is consistent with the overall structural pattern of the Dinaric fold-and-thrust belt. However, the stratigraphic juxtaposition across the fault is such that Cretaceous hanging-wall sedimentary rocks are in direct contact with Paleozoic basement; several kilometers of Mesozoic stratigraphic section are missing, a relationship that is most easily interpreted as being caused by normal-sense slip.
We argue that the Mid-Bosnian Schist Mountains are a Cordilleran-type metamorphic core complex. Here, we provide field observations and thermochronometric evidence that the Mid-Bosnian Schist Mountains bounding fault is a normal fault that penetrated to the middle crust. We used multiple thermo-chronometric systems with cooling ages to determine the timing of slip along this fault, and we observed a temporal link between exhumation of the Mid-Bosnian Schist Mountains and the transition from oceanic to continental subduction along the Internal Dinarides Neotethys margin. Our results are consistent with removal of a subducted oceanic slab during this transition, which underscores the relationship between the mantle process of slab tear and upper-crustal deformation and provides insight into segmentation of the Eurasian margin into contrasting Dinaric-style collision and Hellenic-style subduction and rollback.
The Dinarides are a southwest-vergent series of imbricated thrust sheets, which can be roughly divided into three distinct tectonostratigraphic units; from structurally highest to lowest, these include: the ophiolite nappe, the passive-margin nappe, and the External Dinarides fold-and-thrust belt (Fig. 1). The External Dinarides fold-and-thrust belt is composed of Adriaderived platform carbonates. This succession of imbricated tectonostratigraphic nappes of increasing continental affinity records Late Jurassic to Eocene closure of the Neotethys seaway (Schmid et al., 2008, and references therein). The transition from oceanic subduction to continental collision along the Adriatic portion of the Eurasian margin began in Eocene time (Pamić, 1993; Pamić and Jurković, 2002; Ustaszewski et al., 2008), as recorded by biostratigraphically determined depositional ages of passive-margin clastics, platform carbonates, and syntectonic flysch (Tari Kovaćić and Mrinjek, 1994). Closure of the Dinaric Neotethys Ocean basin was complete by at least the early Miocene, as is evident by capping of siliciclastic flysch by Pannonian rift-related sediments (Tari, 2002). Encroachment of the subduction zone upon Adria continental lithosphere resulted in the cessation of deposition of clastic material of Nubian affinity upon the distal passive margin of Adria in the Late Cretaceous (Pamić et al., 1998). A Paleocene hiatus of platform carbonate growth and a phase of emergence and karstification of the proximal portion of the Adria continental shelf (Vlahović et al., 2005) were coincident with migration of clastic deposition into the continental foreland (e.g., Tari, 2002). This emergence of the platform is evidence of uplift, perhaps related to advancement of the flexural forebulge into the Adria foreland as a precursor to its entrance into the subduction margin. Minor carbonate platform deposition briefly resumed during early Eocene time in structural basins, accompanied by trench-related debris flows, with final uplift and cessation in Oligocene time (Tari Kovaćić and Mrinjek, 1994; Vlahović et al., 2005). Postcollisional shortening resulted in thrusting of Adria platform carbonates onto the Eurasian margin, forming the External Dinarides fold-and-thrust belt (Pamić et al., 1998).
A similar record of collision can be traced along the Tethys suture from the Alps through Anatolia, yet interpretation of collisional and postcollisional tectonics differs greatly along strike. Southeast of the Dinarides, in the Rhodope Mountains (Fig. 1), closure of the Tethys basin and incorporation of continental lithosphere into the convergent margin resulted in regional extension, which included high-angle graben-bounding normal faults, extensive midcrustal exhumation along low-angle normal faults, and regional volcanism (Burchfiel et al., 2008, and references therein). The Dinaric portion of the Tethys suture shares a number of lithologic and map pattern similarities with the Rhodope Mountains, including a number of exposed crystalline basement complexes, and volcanic rocks (Pamić, 1993; Pamić et al., 2000; Kovács et al., 2007). However, these Internal Dinaride crystalline bodies have previously been interpreted as allochthonous nappes emplaced during collisional mountain building along thrust faults, rather than as a result of exhumation along extensional faults (Pamić et al., 1998, 2004).
FIELD OBSERVATIONS OF THE MID-BOSNIAN SCHIST MOUNTAINS
The goal of the field component of this study was to determine the shear sense across the fault bounding the eastern margin of the Mid-Bosnian Schist Mountains. The interpretation of the Mid-Bosnian Schist Mountains as a fault-bounded allochthonous nappe requires a top-to-the-SW kinematic shear sense across the bounding fault, to be consistent with published geologic maps (Jovanović et al., 1977; Vujnovic, 1980) as well as the overall architecture of the Dinaridic folding and thrusting, whereas exhumation along a normal fault predicts a top-to-the-NE sense of shear. We collected field measurements of shear sense indicators to distinguish between the two opposing kinematic interpretations and to determine the emplacement mechanism of the Mid-Bosnian Schist Mountains midcrustal rocks in their upper-crustal position.
Riedel composite structures (Figs. 3A–3D) and asymmetric folds (Figs. 3E and 3F) record kinematic shear sense, and mode I fractures record maximum principal stretching direction. We used the nomenclature of an idealized Riedel composite structure followed by Cowan and Brandon (1994), which includes: a shear plane (Y), synthetic high- and low-angle normal faults (R and R′, respectively), and a foliation plane (P) antithetic to R. In a lower-hemisphere projection, the slip vector is located at the intersection between the girdle containing the four Riedel planes and the Y plane, with a slip sense similar to the R faults. The distribution of asymmetric fold axes can be used to determine shear sense and slip vector orientation of a shear zone. In an ideal monoclinic shear zone, fold axes of clockwise and counterclockwise polarity are distributed on opposite sides of the girdle defining the Y plane. The slip vector lies on the Y plane at the boundary between populations of axes of opposing polarity, and relative shear sense is in agreement with rotational polarity (Cowan and Brandon, 1994). Poles to mode I fractures are parallel to the maximum principal stretching direction (Pollard and Aydin, 1988, and references therein).
Field measurements consistently indicated top-to-the-NE kinematic shear sense and NE-SW maximum stretching. We observed the Mid-Bosnian Schist Mountains detachment in a number of locations, which we have grouped into three areas depicted in Figure 2. Field measurements of Riedel composite structures (Figs. 2A and 3A–3D) in areas 1, 2, and 3 indicated a consistent pattern of top-to-the-NE kinematic shear sense along a subhorizontal slip plane, with synthetic Riedel normal faults dipping between 50° and 70° to the NE, and an antithetic SW-dipping P foliation. Asymmetric folds (Figs. 2B and 3E–3F) were measured in areas 2 and 3 and similarly recorded top-to-the NE kinematic shear sense as defined by the location of the acute separation angle of clockwise and counterclockwise inclined asymmetric fold axes. These top-to-the-NE kinematic shear sense indicators are consistent with our observation of NE-SW–trending poles to mode I fractures from area 2 (Fig. 2C), which indicate NE-SW maximum elongation. The relationship between mode I fracture orientation and extension determined by brittle kinematic shear sense indicators is correlated by superposition of microscale cataclastic shear bands. Shear bands are oriented parallel to calcite fibers filling opening mode fractures in microboudinaged footwall tectonites in the shear zone (Figs. 3G–3H).
Measurement of multiple thermochronometric systems is useful in determining thermal paths during exhumation (Stockli, 2005, and references therein). We used (U-Th)/He in apatite and zircon (Table 1), and fission track in zircon (Table 2) in concert with published K/Ar and Ar/Ar muscovite and hornblende ages to determine the cooling history of the Mid-Bosnian Schist Mountains metamorphic core. These thermochronometric systems have closure temperatures of ~75 °C for apatite (U-Th)/He (Farley, 2000), ~180 °C for zircon (U-Th)/He (Reiners, 2005), ~240 °C for zircon fission track (Bernet, 2009), ~350 °C for muscovite K/Ar (Hames and Bowring, 1994), and ~500 °C for amphibole K/Ar (Harrison and McDougall, 1981).
We collected 12 samples of metasedimentary and metavolcanic rocks for apatite and zircon thermochronometric analysis arranged in two transects running NE-SW across the Mid-Bosnian Schist Mountains metamorphic core (Fig. 2). Samples were disaggregated using a jaw crusher and disc mill and sifted for the 125–250 μm grain-size fraction. Zircon and apatite were extracted from the bulk sample using density and magnetic properties; samples were roughly separated by density using a gold panning table. The heavy fraction was separated into magnetic fractions using a Franz magnetic separator. The nonmagnetic fraction was separated by density using acetylene tetrabromide (ρ = 2.96 g/mL-1) and methylene iodide (ρ = 3.32 g/mL-1) heavy liquids. Apatite and zircon were handpicked from the intermediate-and high-density fractions, respectively.
Zircon fission-track analysis was conducted at Apatite to Zircon, Inc. (Viola, Idaho, USA). Zircon grains were mounted in FEP (fluorinated ethylene propylene) Teflon and polished with 0.3 μm Al2O3 to expose internal grain surfaces. Grains were etched in a eutectic melt of KOH-AlOH to reveal spontaneous tracks, which were counted under an optical microscope. Uranium concentrations were determined on a Finnigan Element II magnetic sector inductively coupled plasma-mass spectrometer (ICP-MS) with a New Wave Nd:YAG laser-ablation system. Each sample mount was measured in ~500 spots and calibrated with ~150 standard spot measurements. Ages were determined with a modified decay equation (Donelick et al., 2005) based on comparison with a number of standards, including: the Duluth complex (1099.0 ± 0.6 Ma; Paces and Miller, 1993), Tardree Rhyolite (61.23 ± 0.11 Ma; Chew et al., 2008), Fish Canyon Tuff (28.201 ± 0.012 Ma; Kiuper et al., 2008), Braintree complex (418.9 ± 0.4 Ma; Black et al., 2004), Mount Dromedary (99.12 ± 0.14 Ma; Renne et al., 1998), and Tempora 2 Middledale gabbroic diorite (416.78 ± 0.33 Ma; Black et al., 2004) standards.
Apatite and zircon U-Th/He measurements were conducted at the Arizona Radiogenic Helium Dating Laboratory (ARHDL), at the University of Arizona, Tucson, Arizona, following the procedures described in Reiners et al. (2004). Grains were hand-picked under an optical microscope to control for inclusions and photographed in at least two orientations to correct for a-ejection following Farley et al. (1996) and Farley (2002). Grains were subsequently prepared into single-grain aliquots, loaded into Pt or Mo foil tubes, and placed into copper or stainless-steel sample planchets. Helium extraction was performed by Nd:YAG laser heating, and U-Th determinations were made by ICP-MS. All ratios were compared against same-day replicate analyses of Fish Canyon Tuff (28.201 ± 0.012 Ma; Kiuper et al., 2008) and Durango apatite (31.44 ± 0.18 Ma; McDowell et al., 2005).
Our low-temperature thermochronometric results indicate a pattern of progressively younger ages from SW to NE; these results include Eocene zircon fission-track and (U-Th)/He ages, younger late Miocene ages for apatite (U-Th)/He, and one Late Jurassic zircon fission-track age (Tables 1 and 2). With the exception of the lowest-temperature system [apatite (U-Th)/He], all reset mineral systems cooled to below their respective closure temperature between 43.5 and 27.5 Ma. The relatively short interval of cooling between closure of amphibole K/Ar (500 °C) and zircon (U-Th)/He (180 °C) was followed by relatively slow cooling through closure of apatite (U-Th)/He (~75 °C) at 7.3–4.7 Ma and final denudation-related exhumation to Earth’s surface (Fig. 4).
Previous thermochronometric investigation of the Mid-Bosnian Schist Mountains is limited to one set of K/Ar whole-rock and single mineral measurements (Pamić et al., 2004). This study included thermochronometric ages from 25 samples collected from within and around the Mid-Bosnian Schist Mountains, although precise locations for these samples are not available. We used samples from all three groups situated within the Mid- Bosnian Schist Mountains, near Buscovaca, Fojnica, and Bradina, respectively (Fig. 2), for comparison with our results. Within these groups, K-Ar thermochronometric ages are available for amphibole in two samples, and in white mica for nine samples (Figs. 2 and 4). The remaining 11 samples from within the Mid-Bosnian Schist Mountains yielded whole-rock K-Ar ages (Fig. 5). Published thermochronometric ages range from a whole rock age of 343.1 ± 13.0 Ma to 34.9 ± 1.4 Ma in white mica. Taken as a whole, these ages roughly fall into a number of groups, the largest of which spans Early Tertiary time (Pamić et al., 2004). Divided by mineral system, this data set records a spatial progression of closure ages decreasing from SW to NE (Figs. 4 and 6), consistent with the results from this study.
Timing and Kinematics of Mid-Bosnian Schist Mountains Exhumation
The general pattern of top-to-the-NE shear sense and NE-SW maximum elongation as determined by brittle kinematic fabrics in the Mid-Bosnian Schist Mountains (Figs. 2A–2B and 3) indicates that the metamorphic footwall was exhumed along a normal fault, rather than emplaced along a thrust fault. Our observations of normal sense of slip along the Mid-Bosnian Schist Mountains detachment are: (1) consistent with numerous observed high-angle normal faults cutting the epidetachment Sarajevo Basin sediments, and (2) account for the several kilometers of missing Cretaceous sedimentary section between the Mid-Bosnian Schist Mountains hanging wall and footwall.
We investigated the spatial progression of cooling by dividing ages into three groups from SW to NE as defined by Cenozoic reset ages in zircon fission-track and amphibole K/Ar (Figs. 4 and 6). In the southwesternmost group, zircon fission tracks were not reset during Cenozoic mountain building, indicating shallow burial to maximum temperatures of 240 °C since Late Jurassic time. The central group was buried to temperatures of 240– 500 °C, as determined by reset zircon fission tracks and unreset Early Triassic amphibole K/Ar ages, respectively. In the north-easternmost group, all thermochronometric systems have been reset by burial to at least 500 °C and have cooled since Eocene time or later. The spatial grouping of cooling ages reveals the distinct SW-NE progression of decreasing cooling age from increasing temperatures (Figs. 4 and 6).
The pattern of cooling ages is consistent with our kinematic observations of NE-directed normal slip along the Mid-Bosnian Schist Mountains exhuming and cooling the metamorphic footwall. Normal dip-slip along a NE-dipping fault, as depicted in Figure 6, predicts the progressive downdip exhumation of deeper rocks. A direct determination of the temporal evolution of the thermal gap between the hanging wall and footwall of the Mid-Bosnian Schist Mountains bounding fault is hampered by low abundance of zircon and apatite in hanging-wall rocks. Nonetheless, the juxtaposition of unmetamorphosed Mesozoic hanging-wall limestones and dolomites on footwall metasedimentary rocks deformed at green-schist-facies temperatures as recently as Oligocene time demands the tectonic removal of the intermediate material.
We conservatively estimate the timing of slip along the detachment exhuming the Mid-Bosnian Schist Mountains to be between 43.5 and 27.5 Ma, as determined by the window of increased cooling rate of the footwall (Fig. 6). However, the progression of resetting of thermochronometric systems with increasing closure temperatures is consistent with surface exposure of partial retention zones for each system. Partially reset ages from exhumed normal fault footwall blocks are distributed between the age of shallower unreset cooling ages and ages of fully reset thermochronometric systems (Stockli et al., 2000), and therefore they give erroneously old estimates for faulting. In the deepest exhumed portion of the Mid-Bosnian Schist Mountains footwall block, located immediately adjacent to the detachment, all thermochronometric systems have reset Cenozoic ages. Reset ages in all systems imply minimum temperatures in the partial retention zone of amphibole, which requires temperatures sufficient to fully reset lower-temperature thermochronometric systems.
Cooling rates from samples immediately along the Mid-Bosnian Schist Mountains detachment are consistent with rates reported from the metamorphic footwalls of normal faults in a number of tectonic settings. Minimum and maximum cooling rates using ages of Pamić et al., (2004) from the Buscovaca area, and zircon (U-Th)/He ages from sample ADV010 (Fig. 6; Table 1) range between ~11 °C/m.y. (42.9–26.5 Ma) and ~55 °C/m.y. (32.1–28.8 Ma). These cooling rates bracket rates determined from cooling ages in the Shuswap metamorphic core complex (~20 °C/m.y. Ar/Ar hornblende–white mica; Vanderheaghe et al., 2003; Lorencak et al., 2001), Raft River detachment (~25 °C/m.y. Ar/Ar white mica–biotite; Wells et al., 2000), and East Humboldt Range metamorphic core complex (<25 °C/m.y. Ar/Ar hornblende–white mica; McGrew and Snee, 1994) in the North American Cordillera, as well as the Oetztal-Stubai basement complex in the eastern Alps (~26 °C/m.y. from peak conditions at ~440 °C and white mica Ar/Ar; Fügenschuh et al., 2000). At the high end, these rates are similar to rates observed in the Menderes metamorphic core complex (~50 °C/m.y. Ar/Ar white mica– apatite fission track; Gessner et al., 2001) in western Anatolia.
Our field observations of the Mid-Bosnian Schist Mountains bounding fault and the thermochronometrically determined cooling history are consistent with the key diagnostic features of metamorphic core complexes (e.g., Coney, 1980; Lister and Davis, 1989; Friedmann and Burbank, 1995). We observed younger rocks juxtaposed upon older rocks across a top-to-the-NE shear zone that we interpret as the detachment fault of the system (Fig. 2). Kinematic shear sense in detachment tectonites is downdip (consistent with normal slip; Figs. 2A–2C), and in the direction of progressive footwall cooling (Fig. 6). The timing of footwall cooling is consistent with rates observed in metamorphic core complexes elsewhere.
Quantification of fault-slip magnitude is hampered by synand postexhumational erosion and footwall doming, and difficulty in quantifying the thermal gap across the detachment. Determining fault throw, and thus fault dip, is hampered by an unknown geothermal gradient, which, in rapidly exhumed continental crust, can become unusually steep and strongly influence low-temperature thermochronometric age distributions (see discussion in McGrew and Snee, 1994). Nonetheless, the juxtaposition of hanging-wall sedimentary rocks and deformed sediments of the Sarajevo Basin (Jovanović et al., 1977) upon footwall greenschist- to lower-amphibolite-facies rocks across the Mid-Bosnian Schist Mountains detachment implies a total vertical offset exceeding the thickness of the upper crust. Our conclusion that the Mid-Bosnian Schist Mountains is a metamorphic core complex exhumed from midcrustal depths along a normal fault implies significant spatially concentrated extension, coincident with the onset of continental collision along the paleogeographic Adria-Eurasia margin.
Timing of Regional Cooling Events
Groupings of published whole-rock K/Ar ages from the Mid-Bosnian Schist Mountains have been attributed to or cited as evidence for various regional tectonic events throughout Paleozoic and Mesozoic time (e.g., Hrvatović and Pamić, 2005; Schmid et al., 2008; Borojević et al., 2012). These published ages roughly fall into a number of groups that Pamić et al. (2004) related to various regional tectonic and thermal events, and limitations of thermochronometric systems (Fig. 5). These events include: unspecified pre- and post-Variscan metamorphism/magmatism, uninterpretable ages related to radiogenic Ar loss, subduction of Adria oceanic lithosphere beneath Eurasia, Dinaric mountain building, and postorogenic transpression (Fig. 5). Notably absent are ages coincident with the main phase of Variscan mountain building during Carboniferous time (Matte, 1986).
In contrast to whole-rock ages, all but one single mineral age is contemporaneous with Dinaric Neotethys closure during Eocene time, and the youngest whole-rock ages are similar to single mineral ages, particularly in the northeast, where pre-Eocene exhumation temperatures exceeded the closure temperature of the highest-temperature thermochronometric system (amphibole K-Ar; Fig. 6). Whole-rock ages are problematic because of the unknown contribution of various thermochronometric systems within a sample, rather than recording distinct tectonic events. We suggest that whole-rock ages from the Mid-Bosnian Schist Mountains reflect the spatial dependence of reset and unreset phases as depicted in Figure 6. This interpretation is supported by the pattern of younger ages to the northeast (Fig. 5), which is consistent with our pre-exhumational geometry and interpretation of tectonic denudation along a normal fault.
Disagreement exists regarding the nature and timing of mountain building in the External Dinarides. Some authors conclude that mountain building was completed by Oligocene time, based on radiometric ages of synkinematic granites and low-grade metamorphism (Tari and Pamić, 1998; Pamić et al., 1998). Neogene deformation is instead attributed to postorogenic strike-slip faulting based upon the map pattern distribution of en echelon anticlinal structures (Picha, 2002), or development of pullapart basins in the foreland fold-and-thrust belt (Mandić et al., 2009). However, nanofossil assemblages in deformed tectonic flysch indicate that thrusting was active until at least middle Miocene time (de Capoa et al., 1995; Mikes et al., 2008); subsurface geophysical observation and seismicity suggest shortening since at least 5 Ma (Ustaszewski et al., 2014); and geodetically derived surface velocities provide evidence for ongoing shortening, perhaps involving subduction of Adria mantle lithosphere beneath the Dinarides (Bennett et al., 2008).
Only recently has core complex–style exhumation been recognized in the Internal Dinarides. Several authors have documented extensional exhumation of a series of crystalline bodies in the Sava zone of the Internal Dinarides (Fig. 7C) and linked this process to rollback associated with opening of the Pannonian system of basins, based on 25–14 Ma cooling ages of exhumed midcrustal rocks (Ustaszewski et al., 2010; Schefer et al., 2011; Matenco and Radivojević, 2012; Stojadinovic et al., 2013; Toljić et al., 2013).
Extensional exhumation of the Mid-Bosnian Schist Mountains between 43 and 27 Ma appears to predate these other observed extensional processes; the role of extension elsewhere in the Internal Dinarides is unknown. However, several other crystalline complexes are also situated along the paleogeographic Adria-Eurasia margin (Fig. 1). Although thermochronologic data from these complexes is sparse or nonexistent, their placement and shared geometric similarities with the Mid-Bosnian Schist Mountains suggest that extensional exhumation of the Mid-Bosnian Schist Mountains may have been part of a regional event.
Sustained shortening throughout the Neogene implies that extensional exhumation of the Mid-Bosnian Schist Mountains was synconvergent, rather than a result of postorogenic collapse, and shares a timing relationship and tectonic setting with extension observed along the Tethys margin in the southern Balkans. In the Rhodope Mountains (Fig. 1), collision was immediately followed by extension, midcrustal exhumation, and volcanism along the Eurasian paleogeographic margin (Burchfiel et al., 2008); Eocene collision in the Internal Dinarides was followed by extensional exhumation of the Mid-Bosnian Schist Mountains and regional emplacement of granitoids, latites, and shoshonite volcanics (Pamić, 1993; Pamić et al., 2000; Schefer et al., 2011; Fig. 7B). Similarly, in both regions, collision and extension accompanied, rather than postdated, ongoing shortening and orogenesis (Burchfiel et al., 2008; this study).
Evolution of the Neotethys Convergent Margin
Coupled convergence and extension in the Hellenides are attributed to slab rollback (Jolivet and Brun, 2010); however, existing tomographic images across the Dinarides reveal a shallowly dipping subducted slab roughly twice the width of the External Dinarides fold-and-thrust belt (Piromallo and Morelli, 2003; Bennett et al., 2008; Ustaszewski et al., 2014). No estimates of total shortening in the Dinarides exist, but estimates of post–20 Ma shortening based on plate reconstructions increase from 190 km in the NW to 235 km immediately outboard of the Mid-Bosnian Schist Mountains (Ustaszewski et al., 2008). However, these estimates are only concerned with translation and deformation of the External Dinarides fold-and-thrust belt and do not incorporate the unknown length of Tethyan oceanic lithosphere consumed during ocean subduction (Ustaszewski et al., 2008). Moreover, the length of the imaged slab (~160 km) is less than the total postshortening distance between the paleogeographic margin of Adria in the Internal Dinarides and the current convergent plate boundary. This length of subducted slab is therefore insufficient to account for the translational motion of crustal material transferred from Adria lithosphere to the Eurasian margin, much less any shortening accumulated within the Dinarides, nor emplacement of the passive margin and ophiolite tectonostratigraphic nappes. The geometry and length of the extant slab therefore imply removal of a substantial length of subducted Dinaric Neotethys lithospheric mantle.
Wortel and Spakman (2000) provided a conceptual model demonstrating that collision may initiate slab tearing, causing uplift and an extensional response in the overriding orogenic wedge. Based on present microplate geometries and plate motion determined from geologic and geophysical data sets, Stein and Sella (2006) suggested that the subducted Adria lithospheric slab was removed by a NW-migrating tear initiated in the SE Dinarides. Similar delamination models have been proposed to account for coincident extension and volcanism along the nearby Periadriatic lineament (von Blanckenburg and Davies, 1995, 1996; von Blanckenburg et al., 1998). Schefer et al. (2011) determined crystallization and cooling ages of granitoids in the Internal Dinarides east of the passive-margin nappe and compiled published regional igneous chemical and crystallization data. They concluded that a regional magmatic event driven by delamination of mantle lithosphere occurred at 31.7–30.6 Ma (Schefer et al., 2011), coincident with the period between the youngest K/Ar and (U-Th)/He ages (34.9–27.5 Ma) in muscovite and zircon, respectively, from samples immediately along the Mid-Bosnian Schist Mountains detachment (Fig. 4).
We suggest that the extensional exhumation of the Mid-Bosnian Schist Mountains, along with contemporaneous magmatism, represents the crustal expression of slab tear during Adria-Eurasia collision and marks the transition between subduction and continental collision between the Adria and Eurasia plates. Our interpretation provides a link between the Dinaric portion of the Tethys paleogeographic margin and the regional tectonic setting that is summarized in Figure 7. Mesozoic through Eocene closure of a series of Neotethys Ocean basins situated between the Adria plate and Eurasia (Fig. 7A) resulted in emplacement of ophiolites along a belt stretching from the eastern Alps in the west to western Anatolia in the east. The transition from ocean subduction to continental collision and subduction of the Adria carbonate platform beneath Eurasia during late Eocene– Oligocene time was accompanied by regional extension along the suture manifested in extensional exhumation of the Mid-Bosnian Schist Mountains (this study), Rhodope metamorphic core complexes (Burchfiel et al., 2008), and regional volcanism (Pamić et al., 1998; Schefer et al., 2008; Fig. 7B). This period of changing subduction dynamics resulted in contrasting styles between the Dinaric and Hellenic portions of this continuous convergent plate boundary. Within the Dinarides, continued Miocene and younger convergence appears not to have been accompanied by significant extension, and metamorphic core complex exhumation in the Dinaric hinterland was instead related to extension and opening of the Pannonian Basin (Ustaszewski et al., 2010; Fig. 7C). In contrast, the Hellenic system, driven by slab rollback (Jolivet and Brun, 2010), retreated into the Mediterranean Basin, accompanied by coupled belts of foreland shortening and hinterland extension in its wake (Fig. 7C).
The absence of a substantial subducted lithospheric slab beneath the Dinarides indicates that slab tear occurred at some point during the evolution of this margin. Geodynamic models predict that slab tear may be initiated by a change in subduction dynamics and is accompanied by upper-crustal extension and volcanism (Wortel and Spakman, 2000). These predictions are consistent with our interpretation of delamination-driven extension of the Mid-Bosnian Schist Mountains in the Internal Dinarides. This extensional event links the tectonic evolution of the Dinaric portion of the Neotethys paleogeographic margin to the along-strike continuation of this same margin in the southern Balkan Rhodope Mountains to the SE, and it temporally marks the segmentation of this margin into Dinaric and Hellenic styles of continental collision.
Map patterns, thermochronologic measurements, and field kinematic observations suggest that the Mid-Bosnian Schist Mountains represent a Cordilleran-type metamorphic core complex exhumed along a normal fault. Consistent top-to-the-NE sense of hanging-wall displacement documented by kinematic shear sense indicators, in conjunction with a SW to NE progression in cooling ages, supports exhumation of the Mid-Bosnian Schist Mountains from midcrustal depths between 43 and 27.5 Ma. Concentrated pre-Pannonian extension pervading to at least mid-crustal conditions has not previously been documented in the Dinarides, and the spatial extent of related extension is unknown. However, several other Paleozoic bodies share geometric and paleogeographic similarities with the Mid-Bosnian Schist Mountains metamorphic core complex, suggesting that extension was perhaps regional and may have been a response to slab tear during the transition from oceanic to continental subduction. Our interpretation of the timing and mechanism of Mid-Bosnian Schist Mountains extensional exhumation coincides with regional spatio-temporal patterns observed throughout the neighboring southern Balkan region along the Neotethys suture, as well as the timing of segmentation of this convergent margin between Dinaric advancing continental collision and Hellenic retreating subduction and slab rollback.
We thank Nadine McQuarrie, Kip Hodges, and Rick Law for their constructive reviews, which contributed greatly to improving this manuscript. This work was supported in part by National Science Foundation grants EAR-0533089 to the University of Arizona and EAR-0208299 to the University of Washington, the Fulbright U.S. Student Program, and an Exxon-Mobil student research grant. Apatite and zircon U-Th/He dating was conducted by Peter Reiners at the University of Arizona. Zircon fission-track dating was conducted by Paul O’Sullivan at Apatite to Zircon, Inc. Details of analytical procedures and results are available upon request.
Linkages and Feedbacks in Orogenic Systems
CONTAINS OPEN ACCESS
- absolute age
- Adriatic Plate
- cross sections
- detachment faults
- Dinaric Alps
- Eastern Alps
- Eurasian Plate
- extension tectonics
- fission-track dating
- Mediterranean region
- metamorphic core complexes
- metamorphic rocks
- metasedimentary rocks
- metavolcanic rocks
- normal faults
- orogenic belts
- plate collision
- plate tectonics
- Southern Europe
- zircon group