Loïc Labrousse

Continental exhumation triggered by partial melting at ultrahigh pressure

Partial melting textures, observed in most continental crust buried in ultrahigh-pressure (UHP) conditions, have mostly been related to their retrograde evolution during exhumation in collisional orogens. Analysis of leucosomes from the Western Gneiss Region (WGR, Norway) UHP and HP domains in the Caledonides show a wide scatter of their chemistries, from early ones close to trondhjemites restricted to UHP domains, to granites in late occurrences or associated with HP domains. Nearly trondhjemitic compositions compare with hydrous melts produced in felsic systems at high pressure (>2 GPa) and moderate temperature (

Evidence for hyperextension along the pre-Caledonian margin of Baltica

We propose that a mantle-peridotite-bearing mélange unit, which has been mapped for more than 400 km in southern Norway, represents vestiges of deep basin(s) formed by hyperextension of the Baltic margin during the inception of the Caledonian Wilson cycle. In the mélange, which occurs below the crystalline nappes of the Middle Allochthon in southern Norway, solitary mantle peridotites, detrital serpentinites, metabasalts, gabbros and deep basin sediments are interlayered and imbricated with allochthonous, coarse-grained siliclastic sediments and slivers of Proterozoic basement rocks. The siliciclastic sediments have detritus derived from Proterozoic rocks, similar to those of the large crystalline nappes and the autochthonous basement of Baltica. The mélange unit shares many similarities with the hyperextended Tethyan margin complexes that have been described from the Alps, the Pyrenees and several segments of the present-day passive ocean margins, and we interpret it to represent an ancient late Proterozoic to Palaeozoic analogue to present-day hyperextended passive margins. Regional maps show that the mélange continues into the central Scandinavian Caledonides and that this basin assemblage may have had a much wider geographical distribution. The new model suggests that a major reinterpretation of the Lower and Middle Allochthons of the Scandinavian Caledonides is required.

Discovery of Paleozoic Fe-Mg carpholite in Motalafjella, Svalbard Caledonides: A milestone for subduction-zone gradients

Paleozoic blueschist facies rocks are relatively scarce on Earth due to warmer geother- mal gradients at that time and/or later reequilibration. Ferro-magnesiocarpholite (Fe-Mg carpholite), the typical low-temperature blueschist facies index mineral in metapelites, was discovered 30 yr ago and is known only in Tethyan belts metamorphosed ,80 m.y. ago. Herein we report the discovery of Paleozoic Fe-Mg carpholite in the ca. 470 Ma blueschists of Motalafjella, Svalbard Caledonides, the oldest known occurrence on Earth. The car- pholite-bearing rocks reached pressure-temperature (P-T) conditions of 15-16 kbar and 380-400 8C and followed a nearly isothermal exhumation path. In the cooling Earth per- spective, these P-T estimates for Motalafjella blueschists demonstrate the existence of cold subduction-zone gradients (;7 8C/km) from the middle Paleozoic onward.

Initiation of crustal‐scale thrusts triggered by metamorphic reactions at depth: Insights from a comparison between the Himalayas and Scandinavian Caledonides

Active eclogitization has recently been inferred at depth beneath the Himalaya from geophysical observations, and the mechanical consequences of eclogitization can be observed in the field in the eroded and extended nappe stack of the Scandinavian Caledonides. There, Proterozoic metastable granulites and igneous protoliths underwent partial eclogitization during the collision of Baltica with Laurentia. The reaction began in pseudotachylites and veins and eventually formed a connected network of eclogite-facies shear zones that localized deformation and weakened the lower crust of Baltica during the collision with Laurentia. All these features can be compared with the seismic activity of the Indian Lower Crust, its strength loss beneath the Himalayan ranges, and its delayed density increase regarding its penetration in the eclogite facies. The Caledonian Bergen Arc eclogites and the Himalayan Ama Drime eclogites are both derived from continental crust. In both cases, these eclogites were formed contemporaneously with the activation of the main thrusts responsible for the construction of the orogenic wedges, the Main Central Thrust in the Himalayas, and the main thrust below the Jotun Nappe Complex in the Caledonides. The similarities in these two orogens, which compare both in size and structure, highlight the importance of eclogitization at depth as a mechanism for weakening of the lower crust and for decoupling of the crust and lithospheric mantle in collision zones.

Structural evolution of Andros (Cyclades, Greece): a key to the behaviour of a (flat) detachment within an extending continental crust

Abstract The continental crust extends in a brittle manner in its upper part and in more distributed (ductile) manner in its lower part. During exhumation of HP metamorphic rocks, brittle features superimpose on earlier ductile ones as a result of the progressive localization of deformation. The islands of Tinos and Andros are part of the numerous metamorphic core complexes exhumed in the Aegean domain. They illustrate two steps of a gradient of finite extension along a transect between Mt. Olympos and Naxos. This study confirms the main role of boudinage as an initial localizing factor at the brittle–ductile transition and emphasizes the continuum of strain from ductile to brittle during exhumation. Early low-angle semi-brittle shear planes superimpose onto precursory ductile shear bands, whereas steeply dipping late brittle planes develop by progressive steepening of structures or sliding across en echelon arrays of veins. The comparison between Tinos and Andros allows us to propose a complete dynamic section of the Aegean extending continental crust and emphasizes that the strain localization process depends on both its rheological stratification and its compositional heterogeneity.

Coupled phengite 40Ar–39Ar geochronology and thermobarometry: P-T-t evolution of Andros Island (Cyclades, Greece)

Andros is a key island for understanding both the timing of high-pressure–low-temperature (HP-LT) metamorphism and the dynamics of crustal-scale detachment systems exhuming high-grade units in the Cyclades (Greece). Using phengite 40Ar–39Ar geochronology coupled with thermobarometry, as well as data from literature, we constrain the pressure–temperature–time (P-T-t) paths of the Makrotantalon and Attic–Cycladic Blueschist units on Andros. Peak conditions of the HP-LT episode in the Makrotantalon unit are 550 °C and 18.5 kbar, dated at 116 Ma. We correlate this episode with Early Cretaceous blueschist facies metamorphism recognized in the Pelagonian zone of continental Greece. This is a new argument favouring a Pelagonian origin for the Makrotantalon unit. In the Attic– Cycladic Blueschist unit, the P-T-t path is characterized by: (1) exhumation after peak conditions in HP-LT conditions between 55 and 35 Ma; (2) isobaric heating at 7 kbar until 30 Ma; and (3) isothermal decompression until 21 Ma. This thermal evolution and timing are similar to those of the neighbouring Tinos Island, emphasizing major thermal re-equilibration at the transition between stable and retreating subduction. Modifications of the crustal thermal state played a major role in the evolution of the North Cycladic Detachment System, below which Andros HP-LT units were exhumed.

Dehydration-driven stress transfer triggers intermediate-depth earthquakes

Intermediate-depth earthquakes (30–300 km) have been extensively documented within subducting oceanic slabs, but their mechanics remains enigmatic. Here we decipher the mechanism of these earthquakes by performing deformation experiments on dehydrating serpentinized peridotites (synthetic antigorite-olivine aggregates, minerals representative of subduction zones lithologies) at upper mantle conditions. At a pressure of 1.1 gigapascals, dehydration of deforming samples containing only 5 vol% of antigorite suffices to trigger acoustic emissions, a laboratory-scale analogue of earthquakes. At 3.5 gigapascals, acoustic emissions are recorded from samples with up to 50 vol% of antigorite. Experimentally produced faults, observed post-mortem, are sealed by fluid-bearing micro-pseudotachylytes. Microstructural observations demonstrate that antigorite dehydration triggered dynamic shear failure of the olivine load-bearing network. These laboratory analogues of intermediate-depth earthquakes demonstrate that little dehydration is required to trigger embrittlement. We propose an alternative model to dehydration-embrittlement in which dehydration-driven stress transfer, rather than fluid overpressure, causes embrittlement.

Kinematics of syneclogite deformation in the Bergen Arcs, Norway: implications for exhumation mechanisms

Abstract The northwestern part of Holsnøy island, in the Bergen Arcs, Norway, consists of a granulite-facies protolith partially transformed at depth in eclogite (700 °C, > 19 kbars) and amphibolite (650 °C, 8–10 kbars) facies during the Caledonian orogenesis. Eclogitized zones are mainly planar objects (fractures with parallel reaction bands and cm-to-100 m-scale shear zones). Eclogitic zones are distributed in two sets of orientations and the associated deformation can be described as ‘bookshelf tectonics’. The major shear zones strike around N120 and dip to the North, and show consistent top-to-the-NE shear sense throughout the area. In the large-scale kinematic frame of Caledonian NW-dipping slab, eclogitic shear zones are interpreted as the way to detach crustal units from the subducting slab and to prevent their further sinking. As the retrograde amphibolitic deformation pattern is similar to the eclogitic one, the detached crustal units started their way up along these eclogitic shear zones. Radiometric ages of eclogitic and amphibolitic metamorphism and their comparison with the chronology of Caledonian orogenesis show that the deformation recorded on Holsnøy occurred in a convergent context. The mechanism we propose can thus account for the first steps of exhumation during collision.

Calculated phase equilibria for phengite-bearing eclogites from NW Spitsbergen, Svalbard Caledonides

Abstract Phengite-bearing eclogites occur in the Richarddalen Complex of NW Spitsbergen, Arctic Caledonides. Phase equilibrium modelling and conventional geothermobarometry have been used to constrain the metamorphic evolution of these eclogites. Pseudosections are calculated for the peak-pressure assemblage garnet+omphacite+phengite+amphibole+dolomite quartz+rutile. Compositional isopleths for garnet and phengite constrain the pressure–temperature (P–T) conditions to 1.9–2.0 GPa and 720–730 °C, in good agreement with the results obtained from conventional thermobarometry (720–740 °C and 2.4–2.5 GPa). Further P–T pseudosection modelling of clinopyroxene+plagioclase±amphibole±clinozoisite symplectites after omphacite suggests that decompression to c. 1.2 GPa occurred along a steep exhumation path. The eclogite-bearing Richarddalen Complex constitutes the uppermost unit of a simple stack of thrust sheets where the metamorphic grade is increasing structurally upwards in the pile. Thrusting is the favoured uplift mechanism for the initial syn-orogenic exhumation to lower crustal levels. Constrictional north–south stretching in a transpressional regime is interpreted to be responsible for the final exhumation of the assembled stack of thrust sheets. Late Silurian–Early Devonian conglomerates were deposited directly on the eclogite-bearing gneisses of the Richarddalen Complex, and mark the end of exhumation of the nappe stack.

Mesozoic structural evolution of the New Siberian Islands

Abstract The New Siberian Islands are affected by a number of Mesozoic tectonic events. The oldest event (D1a) is characterized by NW-directed thrusting within the South Anyui Suture Zone combined with north–south-trending sinistral strike-slip in the foreland during the Early Cretaceous. This compressional deformation was followed by dextral transpression along north–south-trending faults, which resulted in NE–SW shortening in the Kotelny Fold Zone (D1b). The dextral deformation can be related to a north–south-trending boundary fault zone west of the New Siberian Islands, which probably represented the Laptev Sea segment of the Amerasia Basin Transform Fault in pre-Aptian–Albian times. The presence of a transform fault west of the islands may be an explanation for the long and narrow sliver of continental lithosphere of the Lomonosov Ridge and the sudden termination of the South Anyui Suture Zone against the present Laptev Sea Rift System. The intrusion of magmatic rocks 114 myr ago was followed by NW–SE-trending sinistral strike-slip faults of unknown origin (D2). In the Late Cretaceous–Paleocene, east–west extension (D3) west of the New Siberian Islands initiated the development of the Laptev Sea Rift System, which continues until today and is largely related to the development of the Eurasian Basin.