Laurent Jolivet

Lateral slab deformation and the origin of the western Mediterranean arcs

[1]xa0The western Mediterranean subduction zone (WMSZ) extends from the northern Apennine to southern Spain and turns around forming the narrow and tight Calabrian and Gibraltar Arcs. The evolution of the WMSZ is characterized by a first phase of orogenic wedging followed, from 30 Ma on, by trench retreat and back-arc extension. Combining new and previous geological data, new tomographic images of the western Mediterranean mantle, and plate kinematics, we describe the evolution of the WMSZ during the last 35 Myr. Our reconstruction shows that the two arcs form by fragmentation of the 1500 km long WMSZ in small, narrow slabs. Once formed, these two narrow slabs retreat outward, producing back-arc extension and large scale rotation of the flanks, shaping the arcs. The Gibraltar Arc first formed during the middle Miocene, while the Calabrian Arc formed later, during the late Miocene-Pliocene. Despite the different paleogeographic settings, the mechanism of rupture and backward migration of the narrow slabs presents similarities on both sides of the western Mediterranean, suggesting that the slab deformation is also driven by lateral mantle flow that is particularly efficient in a restricted (upper mantle) style of mantle convection.

Burial and exhumation in a subduction wedge : Mutual constraints from thermomechanical modeling and natural P-T-t data (Schistes Lustrés, western Alps)

[1]xa0The dynamic processes leading to synconvergent exhumation of high-pressure low-temperature (HP-LT) rocks at oceanic accretionary margins, as well as the mechanisms maintaining nearly steady state regime in most accretion prisms, remain poorly understood. The present study aims at getting better constraints on the rheology, thermal conductivity, and chemical properties of the sediments in subduction zones. To reach that goal, oceanic subduction is modeled using a forward visco-elasto-plastic thermomechanical code (PARA(O)VOZ-FLAC algorithm), and synthetic pressure-temperature-time (P-T-t) paths, predicted from numerical experiments, are compared with natural P-T-t paths. The study is focused on the well constrained Schistes Lustres complex (SL: western Alps) which is thought to represent the fossil accretionary wedge of the Liguro-Piemontese Ocean. For convergence rates comparable to Alpine subduction rates (∼3 cm yr−1), the best-fitting results are obtained for high-viscosity, low-density wedge sediments and/or a strong lower continental crust. After a transition period of 3–5 Ma the modeled accretionary wedges reach a steady state which lasts over 20 Ma. Over that time span a significant proportion (∼35%) of sediments entering the wedge undergoes P-T conditions typical of the SL complex (∼15–20 kbar; 350–450°C) with similar P-T loops. Computed exhumation rates (<6 mm yr−1) are in agreement with observations (1–5 mm yr−1). In presence of a serpentinite layer below the oceanic crust, exhumation of oceanic material takes place at rates approaching 3 mm yr−1. In all experiments the total pressure in the accretionary wedge never deviated by more than ±10% from the lithostatic component.

Transient, synobduction exhumation of Zagros blueschists inferred from P‐T, deformation, time, and kinematic constraints: Implications for Neotethyan wedge dynamics

[1]xa0We present the first P-T, deformation time, and kinematic constraints on the only known blueschist facies rocks (BS) present in the Zagros (Hajiabad area). The BS were underplated below the Sanandaj-Sirjan zone and crop out as kilometer-scale bodies within extensive colored melange units marking discontinuously the Neotethyan suture zone. P-T estimates point to high-pressure/low-temperature (HP-LT) conditions around 11 kbar and 520–530°C for the majority of BS, along a ∼15°C km−1 gradient. Some exotic blocks in matrix serpentinite reached 17–18 kbar at ∼500°C. In situ laser probe 40Ar-39Ar radiometric age constraints on phengite cluster between 85 and 95 Ma and suggest that (1) synconvergence exhumation of Zagros BS from 35–50 km to depths <15–20 km was accomplished before 80 Ma, (2) BS exhumation corresponded to a transient process with respect to the long-lived subduction beneath Iran (∼150–35 Ma), and (3) age constraints for Zagros BS are 5–10 Myr older than for the nearby Oman HP-LT rocks and broadly coincide with obduction processes in the region (circa 95–70 Ma). We propose that the mechanical coupling across the Neotethyan subduction zone (NSZ) beneath Iran was modified by the large-scale plate rearrangement accompanying obduction, allowing for a short-lived exhumation of Zagros BS. Exhumation ceased at the end of obduction, when subduction of the Arabian continental margin stopped. Kinematic calculations suggest that convergence velocities across the NSZ likely doubled (to ∼5–6 cm yr−1) during the period 118–85 Ma, so that BS exhumation may have been promoted by a combination of obduction movements and increased convergence velocities.

Geometry and kinematics of Mykonos detachment, Cyclades, Greece: Evidence for slip at shallow dip

[1]xa0The eastern part of Mykonos island (Cyclades, Greece) shows the detailed internal structure of a two-branch shallow-dipping extensional detachment system of Miocene age. This paper shows that the last stage of slip along the detachment, during the deposition of syn-rift sediments, occurred with a very low dip and proposes that this geometry prevailed since the initiation of extension. Extensional deformation during and after a 13 Ma old granite intrusion is taken up by two main shallow-dipping shear zones: (1) The lower Livada detachment, which extends laterally across Tinos island, is a ductile structure located at the interface between the granite and the Upper Cycladic Nappe metabasites. A pervasive top-to-the-NE shearing deformation is observed throughout the granite in this island with a strong gradient toward the intrusive contact. Later brittle faults, shallowly and steeply dipping, rework the ductile deformation with the same overall shear sense. (2) The upper Mykonos detachment is brittle and separates the metabasites from late Miocene sandstones and conglomerates. The detachment fault dips 12–15° toward the NNE, and its sense of shear is consistent with that of the Livada detachment. Soft-sediment deformation during the time of detachment faulting and the presence of steep normal faults that root into the detachment gouge indicate a shallow depth of deformation. Rotations about vertical and horizontal axes can be reconstructed within and below the detachment zone, indicating that the overall direction of extension is NNE/SSW with a sense of shear toward the NNE and the dip of the fault is throughout very low. The two detachments have accommodated several tens of kilometers of horizontal extension during the formation of the Aegean Sea, which emphasizes the importance of low-angle extensional faults and shear zones in extensional tectonics.

Synextensional Granitoids and Detachment Systems Within Cycladic Metamorphic Core Complexes (Aegean Sea, Greece): Toward a Regional Tectonomagmatic Model

Within deforming continental regions where metamorphic core complexes (MCCs) and synextensional granitoids are closely associated, deciphering the link between detachment faulting and magmatism often remains complex as (1) the rheological weakness of magma may stimulate mechanisms of strain localization, and conversely, (2) tectonic processes may open/close drains where magmas can intrude. Here we tackle this issue by focusing on the Cyclades with the comparison of five granitoid-cored MCCs (Tinos, Mykonos, Ikaria, Naxos, and Serifos) and their flanking detachment systems. In this region, granitoids were emplaced into the middle/upper crust over a relatively short time period (15–9 Ma), while metamorphic domes were largely exhumed after more than 10 Myr of extension. None of those intrusions thereby proves to be a real candidate for the genesis of MCCs but would rather be a consequence of a warmer regime during lithospheric thinning. However, all collected structural and kinematic data converge toward a regional scheme in which magmatic activity played a more pivotal role than previously postulated. Indeed, late evolution stages of MCCs were dynamically impacted by intrusions along which local and transient heterogeneities of the mechanical strength occurred, interfering with the sequential development of detachments. During their tectonically controlled emplacement, magmatic products intruded already formed detachments at depth, locally inhibiting their activity, associated with a contemporaneous upward migration of extensional deformation that tended to localize through time within intrusion roofs along rheological discontinuities. The newly formed detachments are expressed within granitoids through a continuum of deformation from comagmatic to ductile conditions, followed by cataclasis along detachments.

Mantle Flow and Deforming Continents: From India‐Asia Convergence to Pacific Subduction

Abstract The formation of mountain belts or rift zones is commonly attributed to interactions between plates along their boundaries, but the widely distributed deformation of Asia from Himalaya to the Japan Sea and other back‐arc basins is difficult to reconcile with this notion. Through comparison of the tectonic and kinematic records of the last 50 Ma with seismic tomography and anisotropy models, we show that the closure of the former Tethys Ocean and the extensional deformation of East Asia can be best explained if the asthenospheric mantle transporting India northward, forming the Himalaya and the Tibetan Plateau, reaches East Asia where it overrides the westward flowing Pacific mantle and contributes to subduction dynamics, distributing extensional deformation over a 3,000‐km wide region. This deep asthenospheric flow partly controls the compressional stresses transmitted through the continent‐continent collision, driving crustal thickening below the Himalayas and Tibet and the propagation of strike‐slip faults across Asian lithosphere further north and east, as well as with the lithospheric and crustal flow powered by slab retreat east of the collision zone below East and SE Asia. The main shortening direction in the deforming continent between the collision zone and the Pacific subduction zones may in this case be a proxy for the direction of flow in the asthenosphere underneath, which may become a useful tool for studying mantle flow in the distant past. Our model of the India‐Asia collision emphasizes the role of asthenospheric flow underneath continents and may offer alternative ways of understanding tectonic processes.

Publisher Correction: Melting conditions in the modern Tibetan crust since the Miocene

The original PDF version of this Article contained an error in which Fig. 3 and its legend were omitted and Equations 5 and 6 contained errors.This has been corrected in the PDF version of the Article. The HTML version was correct from the time of publication.

Melting conditions in the modern Tibetan crust since the Miocene

Abundant granitic rocks exposed in ancient mountain belts suggest that crustal melting plays a major role in orogenic processes. However, complex field relations and superposition of multiple tectonic events make it difficult to determine the role of melting in orogenesis. In contrast, geophysical measurements image present-day crustal conditions but cannot discriminate between partial melt and aqueous fluids. Here we connect pressure–temperature paths of Himalayan Miocene crustal rocks to the present-day conditions beneath the Tibetan plateau imaged with geophysical data. We use measurements of electrical conductivity to show that 4–16% water-rich melt is required to explain the crustal conductivity in the north-western Himalaya. In southern Tibet, higher melt fractions >30% reflect a crust that is either fluid-enriched (+1% H2O) or hotter (+100u2009°C) compared to the Miocene crust. These melt fractions are high enough for the partially molten rocks to be significantly weaker than the solid crust.Crustal melting may play a fundamental role in orogenic processes, but quantifying crustal melt remains difficult. Here, the authors combine pressure-temperature paths, electrical conductivity and geophysical data to elucidate the melting conditions in Tibet since the Miocene.

Tectonic evolution of Leros Island (Dodecanese, Greece) and correlations between the Aegean Domain and the Menderes Massif

The history of subduction below the Aegean region and western Anatolia is hampered by a lack of comprehension of the correlations between the Cyclades and the Menderes Massif. The Dodecanese Archipelago, which is critical for this discussion, has received very little attention so far. This study is focused on the island of Leros, where two tectonometamorphic units can be observed: the upper Marina unit and the lower Temenia unit. The field study, including new field mapping and structural observations, reveals that the Temenia unit has been exhumed under the Marina unit through a top-to-the-NE ductile shearing followed by a top-to-the-SW brittle deformation cutting the Temenia–Marina contact. The description of metamorphic aragonite and blue amphibole, complemented by Raman spectroscopy on carbonaceous material (RSCM) thermometry, reveals that the Temenia unit has been buried to at least 20u2005km depth along a cold metamorphic gradient. In terms of lithology and palaeogeographical affinities, the cover of the Marina unit is similar to the Lycian Nappes, which rest on top of the Menderes Massif and belong to the northern margin of the Pelagonian domain. The Temenia unit can then be compared with the Lower Cycladic Blueschists Nappe, which could in turn be correlated with the cover of the Menderes Massif, which has also recorded an HP–LT metamorphic overprint. Supplementary material: Representative chemical analyses are available at https://doi.org/10.6084/m9.figshare.c.4090229

Exhumation of eclogite and blueschist (Cyclades, Greece): Pressure-temperature evolution determined by thermobarometry and garnet equilibrium modelling

High-pressure rocks such as eclogite and blueschist are metamorphic markers of paleo-subduction zones, and their formation at high-pressure and low-temperature conditions is relatively well understood since it has been the focus of numerous petrological investigations in the past 40 years. The tectonic mechanisms controlling their exhumation back to the surface are, however, diverse, complex and still actively debated. Although the Cycladic Blueschist Unit (CBU, Greece) is among the best worldwide examples for the preservation of eclogite and blueschist, the proposed P-T evolution followed by this unit within the Hellenic subduction zone is quite different from one study to another, hindering our comprehension of exhumation processes. In this study, we present an extensive petrological dataset that permits refinement of the shape of the P-T trajectory for different subunits of the CBU on Syros. High-resolution quantitative compositional mapping has been applied to support the thermobarometric investigations, which involve semi-empirical thermobarometry, garnet equilibrium modelling and P-T isochemical phase diagrams. The thermodynamic models highlight the powerful use of reactive bulk compositions approximated from local bulk compositions. The results were also