Yann Rolland

A slab breakoff model for the Neogene thermal evolution of South Karakorum and South Tibet

On the South Karakorum margin, Neogene high-temperature–medium-pressure (HT–MP) gneisses define an east–west trending thermal anomaly. These rocks have been heated from 600 to 750°C during a slight pressure drop from 0.7 to 0.5 GPa. Their retrogressive path cross-cuts the relaxed geotherm of tectonically thickened crust. Such a P–T evolution occurs only if an advective source of heat is involved. Involvement of an advective heat source is also implied by the occurrence of Neogene granitoids and lamprophyres within the HT–MP gneiss area. These rocks are strongly enriched in large ion lithophile elements relative to primitive mantle and show negative high field strength element anomalies. We interpret these geochemical characteristics to be the result of melting of metasomatized Asian lithospheric mantle. The Nd and Sr isotopic compositions of the South Karakorum Neogene magmatic rocks (ϵNd=−12 to −7 and 87Sr/86Sr=0.705–0.725) further suggest they could have originated from mixing between Asian variously metasomatized mantle and Precambrian crust. By contrast, the origin of the youngest magmatic rocks (<10 Myr), here exemplified by the Hemasil syenite and associated lamprophyres, requires involvement of a depleted mantle. The combined ϵHf–ϵNd signature of these rocks (ϵHf=+10.4–+11.5 and ϵNd=+3.4–+4.3) suggests that the source of the Hemasil syenite could have been depleted mantle contaminated by oceanic sediments, likely during the earlier subduction of the Tethyan ocean. Neogene magmatic rocks with the same geochemical characteristics and evolution as those of South Karakorum have previously been described in South Tibet. Based on their location and the geochemical evolution of their source region, we here propose that the Neogene magmatic and metamorphic evolution of the South Asian margin was controlled by slab breakoff of the subducting Indian continental margin starting at about 25 Ma. This model is supported by available geophysical data from South Karakorum and South Tibet.

Rare earth and trace element mobility in mid-crustal shear zones: insights from the Mont Blanc Massif (Western Alps)

The behaviour of rare earth elements (REE) during fluid–rock interaction in mid-crustal shear zones has received little attention, despite their potential for mass balance calculation and isotopic tracing during deformation. In this study, several cases of large REE mobility during Alpine fluid-driven shear zone development in the pre-Alpine granitic basement of the Mont Blanc Massif are considered. On a regional scale, the undeformed granite compositions range within 5 wt% SiO2 (70.5–75.3 wt%) and magmatic chemical variations are of the order of 10–20%, ascribed to minor effects of crystal fractionation. Major and trace element mobility observed in shear zones largely exceeds these initial variations. Shear zones developed a range of mineral assemblages as a result of shearing at mid-crustal depths (at not, vert, similar0.5 GPa, 400°C). Five main shear zone assemblages involve muscovite, chlorite, epidote, actinolite and calcite, respectively, as major phases. In most cases, selective enrichments of light or heavy REE (and Y, Ta, Hf) are observed. REE mobility is unrelated to deformation style (cataclastic, mylonitic), the intensity of strain, and to the shear zones major metamorphic mineral assemblages. Instead, the changes in REE concentrations are ascribed to the alteration of pre-existing magmatic REE-bearing minerals during deformation-related fluid–rock interaction and to the syntectonic precipitation of metamorphic REE-bearing minerals (mainly monazite, bastnasite, aeschynite and tombarthite). Minor proportions (<2%) of these accessory phases, with grain sizes mostly <20 μm, account for enrichments of up to 5:1 compared to the initial granite whole-rock REE budget. The stability of the REE phases appears to be largely dependent on the altering fluid composition. REE mobility is ascribed to changes in pH and to the availability of CO32−, PO42−, and SO42−ligands in the fluid. Such processes are likely to influence the mobility of REE, Y, Hf and Ta in shear zones.

Middle Cretaceous back-arc formation and arc evolution along the Asian margin: the Shyok Suture Zone in northern Ladakh (NW Himalaya)

Abstract The Shyok Suture Zone of the Ladakh palaeovolcanic arc is made of Cretaceous volcanosedimentary formations intruded by granodioritic plutons. Field observations show a tectonic stacking of litho-units from different volcanic arc and back-arc environments. In the western part (Skardu area), the Shyok Suture Zone can be subdivided into two groups: (1) The Northern Group, which consists of olistolith basaltic blocks and tuffs. The basalts are LREE depleted with a LILE enrichment and a slight Nb depletion suggesting a back-arc basin origin. (2) The Southern Group, which consists predominantly of andesites that show LREE enrichment, a flat HREE pattern, strong Nb–Ta depletion, and LILE enrichment. The volcanic rocks of the Southern Group appear to have island-arc tholeiite (IAT) to calc-alkaline affinities. In the eastern part of the suture zone (Nubra–Shyok area), Albian to Cenomanian age silicoclastic sediments predominate. These sediments correspond to a large detrital platform built on the northern part of the Ladakh Arc. At the top, these sediments interlayer with basaltic to andesitic lavas. These lavas appear to be more differentiated and calc-alkaline in nature than the Skardu Southern Group lavas, but show similar volcanic arc affinities. No evidence of a back-arc basin was found in this area. Our data from these two areas shows a northwest–southeast evolution, from back-arc to arc formations in northern Ladakh. Opening of this back-arc basin occurred on the northwestern side of the Ladakh Arc. This back-arc was progressively closing eastward. The arc itself was more mature in the east, resembling the southern Tibetan continental arc, and was characterised by more continental sedimentation. Subsequent Himalayan tectonometamorphic evolution, closure of the back-arc basin and deformation along the Shyok Suture, reflects an early asymmetrical geometry along the Asian margin and Kohistan–Ladakh Arc.

Subductions, obduction and collision in the Lesser Caucasus (Armenia, Azerbaijan, Georgia), new insights

Abstract In the Lesser Caucasus three main domains are distinguished from SW to NE: (1) the autochthonous South Armenian Block (SAB), a Gondwana-derived terrane; (2) the ophiolitic Sevan–Akera suture zone; and (3) the Eurasian plate. Based on our field work, new stratigraphical, petrological, geochemical and geochronological data combined with previous data we present new insights on the subduction, obduction and collision processes recorded in the Lesser Caucasus. Two subductions are clearly identified, one related to the Neotethys subduction beneath the Eurasian margin and one intra-oceanic (SSZ) responsible for the opening of a back-arc basin which corresponds to the ophiolites of the Lesser Caucasus. The obduction occurred during the Late Coniacian to Santonian and is responsible for the widespread ophiolitic nappe outcrop in front of the suture zone. Following the subduction of oceanic lithosphere remnants under Eurasia, the collision of the SAB with Eurasia started during the Paleocene, producing 1) folding of ophiolites, arc and Upper Cretaceous formations (Transcaucasus massif to Karabakh); 2) thrusting toward SW; and 3) a foreland basin in front of the belt. Upper–Middle Eocene series unconformably cover the three domains. From Eocene to Miocene as a result of the Arabian plate collision with the SAB to the South, southward propagation of shortening featured by folding and thrusting occurred all along the belt. These deformations are sealed by a thick sequence of unconformable Miocene to Quaternary clastic and volcanic rocks of debated origin.

Jurassic accretionary complex and ophiolite from northeast Turkey: No evidence for the Cimmerian continental ribbon

Permian-Triassic and Late Cretaceous accretionary complexes, ascribed to the consumption of two distinct oceans, the Paleo- and Neo-Tethys, are exposed over extensive areas in the Eastern Mediterranean region. However, a separating continental ribbon, the so-called Cimmeride continent, between the Paleo- and Neo-Tethys during early Mesozoic time cannot be defined. Here we report a previously unknown Early Jurassic metamorphic oceanic accretionary complex and ophiolite from northeast Turkey, bounded by oceanic accretionary complexes of Permian-Triassic and Late Cretaceous age to the north and the south, respectively, without a continental domain in between. This special tectonic position and widespread coexistence of Permian-Triassic and Late Cretaceous accretionary complexes alongside the Izmir-Ankara-Erzincan suture imply that (1) the southern margin of Laurasia in the eastern Mediterranean region grew by episodic accretionary processes from late Paleozoic to end-Mesozoic time without involvement of a Cimmerian continental ribbon, and (2) the Paleo-Tethys and northern branch of the Neo-Tethys were not distinct oceans in the Eastern Mediterranean region.

The Armenian Ophiolite: insights for Jurassic back-arc formation, Lower Cretaceous hot spot magmatism and Upper Cretaceous obduction over the South Armenian Block

Abstract Similar geological, petrological, geochemical and age features are found in various Armenian ophiolitic massifs (Sevan, Stepanavan and Vedi). These data argue for the presence of a single large ophiolite unit obducted on the South Armenian Block (SAB). Lherzolite Ophiolite type rock assemblages evidence a Lower–Middle Jurassic slow-spreading rate. The lavas and gabbros have a hybrid geochemical composition intermediate between arc and Mid Ocean Ridge Basalt (MORB) signatures which suggest they were probably formed in a back-arc basin. This oceanic sequence is overlain by pillowed alkaline lavas emplaced in marine conditions. Their geochemical composition is similar to plateau-lavas. Finally, this thickened oceanic crust is overlain by Upper Cretaceous calc-alkaline lavas likely formed in a supra-subduction zone environment. The age of the ophiolite is constrained by 40Ar/39Ar dating experiments provided a magmatic crystallization age of 178.7±2.6 Ma, and further evidence of greenschist facies crystallization during hydrothermal alteration until c. 155 Ma. Thus, top-to-the-south obduction likely initiated along the margin of the back-arc domain, directly south of the Vedi oceanic crust, and was transported as a whole on the SAB in the Coniacian times (88–87 Ma). Final closure of the basin is Late Cretaceous in age (73–71 Ma) as dated by metamorphic rocks.

Collision kinematics in the western external Alps

The kinematics of the collision in Western Alps are investigated through five balanced cross sections of the whole external domain from the Oisans to the Mont Blanc massif. These cross sections were built using published data for the Jura and subalpine fold-and-thrust belts and new structural and field analysis for the External Crystalline Massifs. Five units are defined: the sedimentary nappes from innermost parts of the external zone (e.g., ultra-Dauphinois/Helvetic), the crystalline units with their dysharmonically folded cover (e.g., Morcles nappe), sedimentary nappes over the frontal parts of the crystalline massifs (the Aravis-Granier unit), the subalpine belts (e.g., Vercors, Chartreuse, Bauges, and Bornes), and the Jura. Except for the ultra-Dauphinois nappes, the shortening, including the cover shortening, always corresponds to basement shortening. The total amount of shortening increases from south (28 km, 20%) to north (66 km, 27%). Moreover, the shortening is slightly older in the south than in the north; deepwater turbidites (flysch) and shallow marine to freshwater clastics (molasse) basins are more developed in the north; pressure and temperature conditions are higher in the north; the average uplift rates are about 3 times higher in the north and more localized in space. We propose that these differences are due to along-strike variations in the structure of the European continental margin inherited from Mesozoic times. We then build five palinspastic maps: one at Cretaceous times showing the inherited European Mesozoic margin structure and four from Priabonian to upper Miocene times showing the collision kinematics and the related rotation of Adria.

40 AR/ 39 AR dating of synkinematic white mica; insights from fluid-rock reaction in low-grade shear zones (Mont Blanc Massif) and constraints on timing of deformation in the NW external Alps

Abstract This paper highlights the use of synkinematic white mica, biotite and phlogopite for the dating of deformation in ductile shear zones within crystalline rocks under low-grade metamorphic conditions. The Mont Blanc shear zones range from 1 mm to 50 m in width and have localized intense fluid flow, resulting in substantial differences in mineralogy and whole-rock geochemistry. On the basis of their synkinematic alteration assemblages and geographic distribution within the Mont Blanc Massif, three main metamorphic zones are distinguished within the network of shear zones. These are: (i) epidote±white mica-bearing assemblages; (ii) chlorite–phlogopite-bearing assemblages; and (iii) white mica±biotite±calcite±actinolite±epidote- bearing assemblages. 40Ar/39Ar age spectra of biotite and phlogopite are complex, and reflect significant variations in chemical composition. In biotite, this is partly due to inheritance from precursor Variscan magmatic biotite. In contrast, new white mica grew at the expense of feldspar during Alpine deformation and its Ar spectra do not show any excess 40Ar. On the SE side of Mont Blanc, ages of shear zone phengites have a narrow range of 15.8–16.0±0.2 Ma, which is in the same age range as 40Ar/39Ar ages of minerals from kinematically related veins. The top-to-SE sense of shear is consistent with initiation of a Mont Blanc flower-structure within a dextral transpressional system by 16 Ma. On the NW side, mini-plateaux ages of 14.5±0.3 and 23.4±0.4 Ma are preserved in the same sample, suggesting the possibility of two phases of deformation. This is also supported by partly preserved ages of 18–36.6 Ma in biotites and phlogopites. Ages between 36 and 18 Ma might reflect ongoing top-to-NW thrusting, following Penninic Front activation, in a context of nappe stacking and crustal thickening. NW-directed thrusting on the NW side of Mont Blanc continued after 18 Ma, synchronous with SE-directed thrusting on the SE side of the massif. These divergent movements produced the overall pop-up geometry of the Mont Blanc Massif, which may correspond to a positive flower structure developed within a zone of regional dextral transpression extending SW from the Rhone valley into the Mont Blanc area.

Recent tectonic stress evolution in the Lesser Caucasus and adjacent regions

Abstract The stress indicators describing the recent (provided by active tectonics framework) and palaeo-stress (provided by micro-fault kinematics and volcanic cluster) patterns show the scale and temporal changes in stress states since the beginning of Arabian–Eurasian collision. The recent stress derived from the active fault kinematics in the Lesser Caucasus and adjacent area corresponds to a strike–slip regime with both transtension and transpression characteristics. The kinematics of active structures of various scale are conditioned by tectonic stress field with general north–south compression and east–west extension. The distribution of Neogene to Quaternary volcanic cluster geometries and micro-fault kinematic data evidence the time and orientation variability of the stress field since the beginning of the Arabian–Eurasian collision. In addition to the general north–south compression orientation, two other – NW–SE and NE–SW – secondary orientations are observed. The first one was dominant between the Palaeogene and the late Early Miocene and the second one has prevailed between the Late Miocene and the Quaternary. Since the continental collision of Arabia with Eurasia the tectonic stress regime in the Lesser Caucasus and adjacent area changed from compression (thrusting and reverse faulting) to transtension-transpression (strike–slip faulting with various vertical components).

Geochemical variations and element transfer during shear-zone development and related episyenites at middle crust depths: insights from the Mont Blanc granite (French-Italian Alps)

Abstract This paper highlights the relationships between the formation of shear zones, associated quartz-rich veins and their quartz-depleted alteration haloes (‘episyenites’) that have formed in the Mont Blanc Massif during the Alpine orogeny. The shear zones are steeply dipping and formed late (18–13 Ma) during collisional orogeny, at mid-crustal depths (5 ± 1 kbar, 400 ± 50 °C) during uplift of the Mont Blanc Massif. Between the shear zones, nearly undeformed granite contains widely dispersed, subhorizontal veins with a quartz-dominant quartz + albite + chlorite + adularia assemblage. They do not intersect the shear zones and are surrounded by quartz-depleted alteration haloes up to several metres wide. The compositions of the shear zones and the vein-alteration haloes (episyenites) show substantial departures from the bulk composition of the host rock. Shear zones are characterized by greenschist facies assemblages (epidote-, chlorite- or K-white-micabearing assemblages). Each shear zone type is featured by a specific chemical change: depletions in K2O, and enrichments in Fe2O3 and CaO (epidote-); with depletions in CaO, Na2O, K2O and slight SiO2 enrichments (white mica-chlorite-); with depletions in SiO2, CaO, Na2O, K2O and enrichments in MgO (phlogopite-chlorite shear zones). Episyenites are characterized by chemically induced porosity enhancement due to dissolution of magmatic quartz and biotite, with subsequent partial infilling of pore spaces by quartz, chlorite, albite and adularia. The vein arrays have accommodated minor vertical stretching in the Mont Blanc Massif, probably at the same time as the adjacent shear zones were accommodating more substantial vertical stretching in the massif. Coupled quartz dissolution in the wallrock alteration haloes and quartz precipitation in veins could be interpreted to reflect local mass transfer between wallrock and veins during essentially closed-system behaviour in the relatively underformed granite domains between shear zones. In contrast, shear zones probably develop in opened systems due to their kilometric length.