Patrick Monié

Zagros orogeny: a subduction-dominated process

This paper presents a synthetic view of the geodynamic evolution of the Zagros orogen within the frame of the Arabia–Eurasia collision. The Zagros orogen and the Iranian plateau preserve a record of the long-standing convergence history between Eurasia and Arabia across the Neo-Tethys, from subduction/obduction processes to present-day collision (from ~ 150 to 0 Ma). We herein combine the results obtained on several geodynamic issues, namely the location of the oceanic suture zone, the age of oceanic closure and collision, the magmatic and geochemical evolution of the Eurasian upper plate during convergence (as testified by the successive Sanandaj–Sirjan, Kermanshah and Urumieh–Dokhtar magmatic arcs), the P–T–t history of the few Zagros blueschists, the convergence characteristics across the Neo-Tethys (kinematic velocities, tomographic constraints, subduction zones and obduction processes), together with a survey of recent results gathered by others. We provide lithospheric-scale reconstructions of the Zagros orogen from ~ 150 to 0 Ma across two SW–NE transects. The evolution of the Zagros orogen is also compared to those of the nearby Turkish and Himalayan orogens. In our geotectonic scenario for the Zagros convergence, we outline three main periods/regimes: (1) the Mid to Late Cretaceous (115–85 Ma) corresponds to a distinctive period of perturbation of subduction processes and interplate mechanical coupling marked by blueschist exhumation and upper-plate fragmentation, (2) the Paleocene–Eocene (60–40 Ma) witnesses slab break-off, major shifts in arc magmatism and distributed extension within the upper plate, and (3) from the Oligocene onwards (~ 30–0 Ma), collision develops with a progressive SW migration of deformation and topographic build-up (Sanandaj–Sirjan Zone: 20–15 Ma, High Zagros: ~12–8 Ma; Simply Folded Belt: 5–0 Ma) and with partial slab tear at depths (~10 Ma to present). Our reconstructions underline the key role played by subduction throughout the whole convergence history. We finally stress that such a long-lasting subduction system with changing boundary conditions also makes the Zagros orogen an ideal natural laboratory for subduction processes.

Miocene detachment in Crete and exhumation P-T-t paths of high-pressure metamorphic rocks

A major problem posed by the geology of Crete is the horizontal contact of an upper unit without Miocene metamorphism onto a metamorphosed lower one with Early Miocene high pressure/low temperature (HP/LT) parageneses. This very sharp contact is roughly parallel to the major Oligo-Miocene thrust planes which were reactivated as a large-scale detachment which allowed exhumation of high-pressure units. We describe the extensional deformation and the metamorphic evolution of the lower plate. Most first-order deformation features relate to the retrogression from high-pressure to low-pressure conditions. A N-S pervasive stretching is observed everywhere, often associated with a top-to-the-north sense of shear. The extreme variation of thickness of the Phyllite-Quartzite nappe (upper part of the lower plate) is probably the result of large-scale boudinage similar to the one seen in large outcrops. The most important observation is the systematic occurrence of fresh carpholite immediately below the base of the Tripolitza nappe except in northwestern Crete where a late extensional shear zone is present. Deeper in the nappe pile carpholite is systematically retrograded. This observation reveals a drastically different PT history for the upper part of the Phyllite-Quartzite nappe. It also suggests that the late extensional shear zone found along the northern side of Crete cuts inside the metamorphic structure and brings the nonmetamorphosed Tripolitza nappe directly in contact with the deeper parts of the Phyllite-Quartzite nappe. PT-t paths suggest a fast temperature decrease in the top of the Phyllite-Quartzite during retrogression and, hence, during the top-to-the-north shear. The deeper part of the Phyllite-Quartzite nappe shows a low-temperature regime throughout, but its PT path includes an isothermal decompression in the first stage. We produce a tentative map of domains having experienced similar PT trajectories during decompression. The overall cool regime is related to the continuous underthrusting of cola continental units during exhumation. Isothermal decompression observed in the core of the Phyllite-Quartzite Nappe implies fast exhumation during extension and the faster cooling of the upper part is related to a continuous displacement toward the north of a cooler unit during exhumation. Single grain 39Ar-40Ar ages obtained on phengites (15–25 Ma) in various structural sites are in good agreement with these conclusions and with the geological context suggesting that underthrusting of cold units at the front the accretionnary complex occurred contemporaneously with unroofing below a north dipping detachment near the top of the wedge. The age of this detachment is bracketed between the end of the high-pressure event (20 Ma) and the deposition of the breccia (Early to Middle Miocene) in the Neogene basins.

Very high rates of cooling and uplift in the Alpine belt of the Betic Cordilleras, southern Spain

Cooling-rate estimates of 150-350 °C/m.y., suggesting magnitudes of uplift and exhumation of ∼15-20 km at rates of 5-10 km/m.y., are proposed for the final stage of the orogenic development in a major part of the Betic Cordilleras. The results are based on a combination of radiometric and paleontological dates. Six isotopic chronometers (muscovite and biotite whole-rock Rb-Sr, muscovite and biotite 40 Ar/ 39 Ar, biotite K-Ar, and whole-rock Rb-Sr) yield analytical ages between 23 and 18.5 Ma for rocks from Alpine nappe complexes. Nappe-sealing marine sedimentary rocks contain early Miocene foraminifera and nannoplankton indicating minimum ages of 18-15.5 Ma. The very high estimates of cooling and uplift-exhumation rates suggest tectonic unroofing, which is tentatively connected geodynamically to lithospheric slab detachment and concomitant diapirism in the upper mantle, and extensional tectonics in the crustal section.

EXTENSIONAL TECTONICS WITHIN A SUBDUCTION-TYPE OROGEN. THE CASE STUDY OF THE WUGONGSHAN DOME (JIANGXI PROVINCE, SOUTHEASTERN CHINA)

Abstract In southeastern China, the Wugongshan massif consists of metamorphic rocks and granites generally considered as “basement” rocks formed during an early Paleozoic orogeny. Our structural and radiometric reappraisal of this massif shows that it results from a Mesozoic doming. In the metamorphic rocks, the foliation defines an E-W elliptical shape and bears a N-S stretching lineation. The axial zone of the dome is occupied by oriented plutons. The outer part of the dome consists of Devonian quartzite, Carboniferous-Permian limestone and Triassic-Jurassic sandstone. Ductile deformation observed in the Permian carbonates and older units develops contrasting kinematics. The northern and southern flanks present top-to-the-north and top-to-the-south motions, respectively. The same divergent pattern of motion is recorded from brittle structures up to the Jurassic sandstone. The late Cretaceous red beds are undeformed. In the axial zone, coaxial flow indicated by symmetric pressure shadows and quartz fabrics predominates. Along the N and S flanks of the dome, post-folial recumbent folds are symmetrically overturned to the N and S respectively, but at the two extremities of the dome, the fold axes are parallel to the stretching lineation. 40 Ar 39 Ar ages of biotite and muscovite from mylonite and granite suggest a late Triassic age for the ductile deformation and an early Cretaceous age for the final doming. The Wugongshan dome is similar to a metamorphic core complex formed in an extensional tectonic regime. A model which emphasizes plutonism is discussed. From Permian to early Cretaceous, southeastern China was a subduction-type orogen, the crust of which was thickened by accretion of mantle derived magmas. The emplacement of a large amount of Mesozoic granitoids in southeastern China promoted thermal softening and gravitational instability that allowed the crust to extend. The formation of the Wugongshan dome and other metamorphic core complexes appears to be a the direct consequence of plutonic activity and crust rheology.

Cooling and exhumation of the Western Betic Cordilleras, 40Ar/39Ar thermochronological constraints on a collapsed terrane

New 40Ar/39Ar data on amphiboles, muscovites, biotites and potassium-feldspars from different tectono-metamorphic units of the Western Alpujarrides (Betic Cordilleras, southern Spain) help to constrain the P-T-t evolution of this Alpine collisional belt. During an initial stage of plate convergence between Africa and Eurasia, the Alpujarride metamorphic rocks evolved along increasing pressure-temperature paths, locally reaching eclogitic conditions, but the timing of peak metamorphism is only constrained to be earlier than 25 Ma. In the interval of 25–22 Ma, the Alpujarride rocks underwent strong adiabatic decompression related to the collapse of the previously thickened crust. We propose that the main phase of synmetamorphic ductile deformation and thinning of the metamorphic pile was related to this extensional event. The last step is marked by fast cooling of the hot Alpujarride rocks below 600°C, resulting in a striking convergence of our 40Ar39Ar determinations in the range 19–20 Ma. Cooling rates in the range 100–350°C/m.y. are indicated for this period, associated with exhumation rates of less than 3 km/m.y. We suggest that fast cooling took place primarily as a result of thermal relaxation of the abnormally steep geotherm resulting from extensional tectonics which has the effect of juxtaposing thinned rock bodies with contrasting temperatures along shear zones and faults. The main implication of this data set is that the Western Alpujarrides present a structural and metamorphic development which should be regarded as characteristic of “collapsed terranes”.

40Ar/39Ar geochronology of Alpine tectonism in the Betic Cordilleras (southern Spain)

The 40Ar/39Ar dating method has been applied to metamorphic rocks of the Alpujarride and Nevado-Filabride nappes (Alboran domain, SE Spain) in a first attempt to discriminate individual phases of deformation and metamorphism. The upper Nevado-Filabride nappes experienced an early eclogitic and blueschist metamorphism for which a barroisitic amphibole indicates a minimum age of 48 Ma. An Early Miocene age is attributed to the subsequent amphibolite facies metamorphism. Deformation associated with this metamorphic evolution is of unknown direction. The Alpujarride nappes record a plurifacial metamorphic evolution with the superimposition of low-pressure assemblages upon high-pressure ones with variable P–T ratios. Phengite from a carpholite-bearing high pressure-low temperature (HP/LT) assemblage gives an age of 25 Ma interpreted to reflect the end of the high-pressure evolution. Biotite and muscovite from high-grade metamorphic rocks overprinted under low-pressure conditions yield similar closure ages of 19 Ma dating cooling after the main episode of E–NE directed ductile deformation. This deformation was followed by W–SW directed extensional events producing brittle structures in the Alpujarride nappes and ductile-brittle shearing in the Nevado-Filabride nappes. Biotite and muscovite from ductile sheared rocks in the detachment zone between the two complexes have concordant ages of 16–17 Ma related to the end of the extensional ductile deformation. Therefore, a correlation of metamorphic and tectonic events between the two nappe complexes seems possible only since the Early Miocene and later.

Cosmogenic nuclide dating of Sahelanthropus tchadensis and Australopithecus bahrelghazali: Mio-Pliocene hominids from Chad

Ages were determined at two hominid localities from the Chad Basin in the Djurab Desert (Northern Chad). In the Koro Toro fossiliferous area, KT 12 locality (16°00′N, 18°53′E) was the site of discovery of Australopithecus bahrelghazali (Abel) and in the Toros-Menalla fossiliferous area, TM 266 locality (16°15′N, 17°29′E) was the site of discovery of Sahelanthropus tchadensis (Toumaï). At both localities, the evolutive degree of the associated fossil mammal assemblages allowed a biochronological estimation of the hominid remains: early Pliocene (3–3.5 Ma) at KT 12 and late Miocene (≈7 Ma) at TM 266. Atmospheric 10Be, a cosmogenic nuclide, was used to quasicontinuously date these sedimentary units. The authigenic 10Be/9Be dating of a pelite relic within the sedimentary level containing Abel yields an age of 3.58 ± 0.27 Ma that points to the contemporaneity of Australopithecus bahrelghazali (Abel) with Australopithecus afarensis (Lucy). The 28 10Be/9Be ages obtained within the anthracotheriid unit containing Toumaï bracket, by absolute dating, the age of Sahelanthropus tchadensis to lie between 6.8 and 7.2 Ma. This chronological constraint is an important cornerstone both for establishing the earliest stages of hominid evolution and for new calibrations of the molecular clock.

The oldest UHP eclogites of the World: age of UHP metamorphism, nature of protoliths and tectonic implications

Coesite-bearing eclogitic rocks from the Pan-African belt in northern Mali were dated by multichronological methods (Rb–Sr, Sm–Nd and Ar–Ar). Rb–Sr and Sm–Nd isotope analyses on whole rock and mineral separates of an omphacite–kyanite micaschist and a mafic eclogite yielded concordant ages of about 620 Ma. This is interpreted as the time of the eclogitisation, which represents the oldest ultrahigh-pressure (UHP) metamorphic event so far recorded in the continental crust. The nearly identical Rb–Sr and Sm–Nd ages suggest very rapid exhumation as observed in many Phanerozoic UHP rocks. 40Ar–39Ar dating of phengites from two UHP micaschists gave much older ages of 1045 and 760 Ma. Phengite from a quartzite collected from the same site yielded a plateau date of 623±3 Ma, which is in agreement with the Rb–Sr and Sm–Nd dates. The two 40Ar–39Ar ages, older than 625 Ma, testify once more the recurrent problem of excess Ar in ultrahigh-pressure phengites. The occurrence of the Mali UHP eclogites indicates that by the late Precambrian, the Earth has cooled enough to sustain the formation and preservation of deeply subducted UHP metamorphic rocks. The new ages of ca. 620 Ma also constrain the amalgamation in NW Gondwana to have occurred in the latest Proterozoic, but not in the Middle Cambrian. This is supported by several recent chronological data for the southern segment of the major suture zone in the Dahomeyide orogenic belt.

Triassic blueschists and eclogites from northwest Turkey: vestiges of the Paleo-Tethyan subduction

Abstract Triassic eclogite and blueschist facies rocks occur as a thrust sheet, 25-km long and over 2-km thick, in an Eocene fold-and-thrust belt in northwest Turkey along the Izmir–Ankara suture. The thrust sheet consists mainly of metabasites with minor marble, phyllite and metachert, and rare lenses of serpentinite. The common blueschist facies mineral assemblage in the metabasites is sodic amphibole+epidote+albite+chlorite+phengite±garnet. Sodic amphibole commonly shows replacement by barroisite, and there is continuous petrographic transition from blueschist–metabasites to barroisite-bearing epidote–amphibolites. Eclogite with the mineral assemblage of garnet+sodic pyroxene+sodic–calcic amphibole+epidote is found only in one locality. P–T conditions of the epidote–blueschist facies metamorphism are estimated as 450±50 °C and 11±2 kbar. The blueschist formation was followed by a decrease in pressure and increase in temperature, leading to the development of barroisite-bearing epidote–amphibolites. Phengite, sodic amphibole and barroisite Ar/Ar ages from three metabasic rocks range between 215 and 205 Ma, and indicate Late Triassic high-pressure metamorphism. The Triassic blueschists in northwest Turkey constitute part of a much larger allochthonous tectonic unit of Triassic mafic volcanic rocks. They probably represent the upper layers of a Triassic oceanic plateau, which was accreted to the Laurasian margin during the latest Triassic. The close spatial association of the Triassic and Cretaceous blueschists along the Izmir–Ankara suture suggests that the suture represents a long-lived plate boundary of Late Palaeozoic to early Tertiary age.

Alpine structural and metamorphic signature of the Sila Piccola Massif nappe stack (Calabria, Italy): Insights for the tectonic evolution of the Calabrian Arc

Combined structural and petrographical investigations, coupled with 40Ar/39Ar geochronology, were carried out in the Sila Piccola Massif of the Calabrian Arc in order to define the structural geometry and map out the major structural and metamorphic breaks within the exposed nappe sequence. On the basis of the contrasting Alpine pressure-temperature (P-T) and structural signatures the nappe stack can be divided in two major tectonic complexes, bounded by a flat-lying ductile to brittle extensional shear zone. The upper complex consists of a nappe-like structure, where a major top to the east compressional shear is recorded. The lower tectonic complex consists of an ophiolite-bearing sequence showing typical high-P/low-T parageneses (Mg-carpholite and Na-amphibole). The 40Ar/39Ar geochronology on phengites in equilibrium with blueschist minerals provided a minimum age estimate for the blueschist event in the lower complex rocks at the Oligocene-Eocene boundary (around 35 Ma). Ductile to brittle top to the west extensional shear accompanied the nearly isothermal retrogression and exhumation of the lower complex rocks, reworking the previous nappe contacts with shear localization along the upper/lower tectonic complex discontinuity. The 40Ar/39Ar dating indicates that this postnappe stacking tectonic evolution took place from 30 Ma onward. It is proposed that exhumation of the deep-seated rocks occurred below a top to the west extensional detachment active during convergence and orogenic complex formation (synorogenic extension). The age of this detachment is bracketed between 30 Ma and the post-orogenic Neogene basin sedimentation (middle-upper Miocene). The revised structural and metamorphic scenario is here integrated into a new tectonic evolutionary reconstruction, which involves an early high-P/low-T top to the east crustal thickening episode during the construction of the Apennine orogenic wedge (Eocene-Oligocene), followed and overprinted by a top to the west extensional shear, probably active from the late Oligocene.