Comment on ‘Consequences of progressive eclogitization on crustal exhumation, a mechanical study' by H. Raimbourg, L. Jolivet and Y. Leroy

Two simple end-member models of a subduction channel have been proposed in the literature: (i) the ‘pressure-imposed' model for which the pressure within the channel is assumed to be lithostatic, the channel walls have negligible strength with respect to lateral pressure gradients, and the channel geometry therefore varies with time and (ii) the ‘geometry-imposed' model of constant channel geometry, rigid walls and resultant lateral variation in pressure. Neither of these models is realistic, but they provide lower and upper bounds to potential pressure distributions in natural subduction zones. The critical parameter is the relative strength of the confining plates, reflected in the effective viscosity ratio between the channel fill and the walls. The assertion that the ‘geometry-imposed' model is internally inconsistent is incorrect—it merely represents one bound to possible behaviour and a bound that may be approached for realistic values of the effective viscosity for weak channel fill (e.g. unconsolidated ocean-floor sediments) and relatively cold and strong subducting and overriding lithospheric plate

How large are departures from lithostatic pressure? Constraints from host-inclusion elasticity

Minerals trapped as inclusions within other host minerals will develop non-lithostatic pressures during both prograde and retrograde metamorphism because of the differences between the thermo-elastic properties of the host and inclusion phases. There is only a single possible path in P-T space, the entrapment isomeke, along which no residual pressure would be developed in a host/inclusion system; non-lithostatic pressures are developed in inclusions as a result of the external pressure and temperature deviating from the isomeke that passes through the entrapment conditions. With modern equation of state and elasticity data for minerals now available it is possible to perform precise calculations of the isomekes for mineral pairs. These show that isomeke lines are not straight lines in P-T space at metamorphic conditions. We show that silicate inclusions in silicate hosts tend to have flat isomekes, with small values of dP/dT(isomeke), because of the small range of thermal expansion coefficients of silicate minerals. As a consequence, the general behaviour under decompression is for soft silicate inclusions in stiffer hosts to develop excess pressures, whereas a stiff silicate inclusion in a softer matrix will experience lower pressures than lithostatic pressure. The opposite effects occur for compression after entrapment on the prograde path. The excess pressures in inclusions, including allowance for mutual elastic relaxation of the host and inclusion, are most easily calculated by using the isomeke as a basis. Analysis of the simplest possible model of a host-inclusion system indicates that deviations from lithostatic pressure in excess of 1 GPa can be readily produced in quartz inclusions within garnets in metamorphic rocks. For softer host minerals such as feldspars the pressure deviations are smaller, because of greater elastic relaxation of the host. The maximum pressure deviation from lithostatic pressure in the host phase around the inclusion is one-third of the pressure deviation in the inclusion. Routines for performing these calculations have been added to the EosFit7c software package

Structural geology and petrography of the Naret region (northern Valle Maggia, N.Ticino, Switzerland)

The Naret region has a complex geological history of Alpine polyphase folding and metamorphism that affected pre-Alpine rocks of the Maggia nappe, including the Matorello group (interpreted in this study as late-Variscan intrusives), the Lebendun nappe and Mesozoic rocks of the Bedretto zone. From field observations, four main ductile deformation phases (D1 to D4) can be distinguished and, in combination with thermodynamic modelling, the tectono-metamorphic evolution for the Naret region can be reconstructed. D1 formed the initial nappe stack. During this phase, the Maggia nappe was thrust over the Lebendun nappe, at T ≤ 570°C and P ≤ 10 kbar. D2 caused isoclinal refolding of the nappe pile, at around 610-640°C and 8.5-10 kbar, and produced both the main regional foliation (S2) and a penetrative stretching lineation which is generally parallel to F2 fold axes. D2 is largely responsible for the current complicated geometry of the Lebendun nappe boundary. The main phase of porphyroblastesis occurred between D2 and D3, corresponding to a metamorphic temperature peak of ca. 640-650°C at pressures of ca. 8-9 kbar. D3 produced open folds oblique to the general Alpine trend ("crossfolding”) and locally a crenulation cleavage with a well developed crenulation lineation, at estimated temperatures of 550-610°C. The last important phase, D4, caused open backfolding of all pre-existing structures and is responsible for steepening of the main S2 foliation, to produce the Northern Steep Zone, and for a regional rotation of S3 and L3. D4 developed at T ≥ 550°C and P ≥ 3 kbar. The Lebendun nappe is a complicated structure developed as the result of non-coaxial fold interference related to D1 and D2. From tectono-stratigraphic evidence, the rocks of the Lebendun nappe are interpreted as pre-Triassic in ag

Structure and kinematics of the northern Simano Nappe, Central Alps, Switzerland

Abstract.: New structural data allow the internal structure and kinematics of the Lower Penninic Simano Nappe to be established, together with the relationship of this unit to the underlying Leventina Gneisses and the also underlying Maggia Nappe. Three clearly distinguishable Alpine deformation phases (D1, D2, D3) are recognized within the Simano Nappe. D1 developed in mid-crustal levels under metamorphic conditions of ca. 1 GPa and 500°C and produced kilometre-scale, recumbent, north-closing anticlinal folds, called nappes. During D2 these nappes were intensely refolded on the same scale and with identical style to form the present day recumbent fold pattern. The axial planar foliation S2 is the dominant planar fabric throughout most of the Central Alps. The subsequent phase D3 overprints the entire tectonostratigraphy of the Central Alps, with broader and more upright D3 folds trending oblique to the orogen. Regionally, these broad D3 folds have a major influence on the overall foliation and nappe outcrop pattern of the Central Alps. During D2 and D3, peak metamorphic conditions reached temperatures of up to 650°C, under pressures of 0.7-0.8 GPa. An important consequence of this deformation history is that lithological units separated only by D2-synforms formed part of the same tectonostratigraphic level after D1 nappe-stacking and therefore should not be interpreted as different nappes. This principle can be directly applied to the relationship between the Simano and the two adjacent units. The underlying Leventina Gneisses are separated from the Simano rocks by an originally intrusive contact that has been strongly reactivated during both D1 and D2. Therefore, it is taken to represent a true nappe contact, although locally it still retains some original intrusive relationships and has also been strongly reactivated during D2. The Maggia and Simano units represent alternate limbs of a D2 synform-antiform pair, the Mogno synform and the Larecc antiform, or more generally the Verzasca-Larecc-Ganna antiform. If the Maggia unit is simply the continuation of the Simano unit around the D2 Mogno synform, then post-D1 they represented a single Simano-Maggia Nappe. The Maggia "Nappe” is therefore also part of the European margin and cannot be separately assigned to the Briançonnais paleogeographic domain, as has been propose

Modern methods in Structural Geology and Tectonics: a series of articles in honour of Martin Burkhard (1957-2006)

We briefly report on the conference held in May 2007 in honour of Martin Burkhard in Neuchâtel. We also present a short account of the achievements of this prominent scientist and teacher by selectively citing some of his work and briefly introduce the series of articles presented here, which represent a tribute to Martin Burkhard. We also add a complete list of publications by Martin Burkhard and co-worker

Geochronological constraints on the evolution of the Periadriatic Fault System (Alps)

Fault rocks from various segments of the Periadriatic fault system (PAF; Alps) have been directly dated using texturally controlled Rb-Sr microsampling dating applied to mylonites, and both stepwise-heating and laser-ablation 40Ar/39Ar dating applied to pseudotachylytes. The new fault ages place better constraints on tectonic models proposed for the PAF, particularly in its central sector. Along the North Giudicarie fault, Oligocene (E)SE-directed thrusting (29-32Ma) is currently best explained as accommodation across a cogenetic restraining bend within the Oligocene dextral Tonale-Pustertal fault system. In this case, the limited jump in metamorphic grade observed across the North Giudicarie fault restricts the dextral displacement along the kinematically linked Tonale fault to ~30km. Dextral displacement between the Tonale and Pustertal faults cannot be transferred via the Peio fault because of both Late Cretaceous fault ages (74-67Ma) and sinistral transtensive fault kinematics. In combination with other pseudotachylyte ages (62-58Ma), widespread Late Cretaceous-Paleocene extension is established within the Austroalpine unit, coeval with sedimentation of Gosau Group sediments. Early Miocene pseudotachylyte ages (22-16Ma) from the Tonale, Pustertal, Jaufen and Passeier faults argue for a period of enhanced fault activity contemporaneous with lateral extrusion of the Eastern Alps. This event coincides with exhumation of the Penninic units and contemporaneous sedimentation within fault-bound basin

The DAV and Periadriatic fault systems in the Eastern Alps south of the Tauern window

Alpine deformation of Austroalpine units south of the Tauern window is dominated by two kinematic regimes. Prior to intrusion of the main Periadriatic plutons at ~30Ma, the shear sense was sinistral in the current orientation, with a minor north-side-up component. Sinistral shearing locally overprints contact metamorphic porphyroblasts and early Periadriatic dykes. Direct Rb-Sr dating of microsampled synkinematic muscovite gave ages in the range 33-30Ma, whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted 40Ar-39Ar age of ~46Ma. The transition from sinistral to dextral (transpressive) kinematics related to the Periadriatic fault occurred rapidly, between solidification of the earlier dykes and of the main plutons. Subsequent brittle-ductile to brittle faults are compatible with N-S to NNW-SSE shortening and orogen-parallel extension. Antithetic Riedel shears are distinguished from the previous sinistral fabric by their fine-grained quartz microstructures, with local pseudotachylyte formation. One such pseudotachylyte from Speikboden gave a 40Ar-39Ar age of 20Ma, consistent with pseudotachylyte ages related to the Periadriatic fault. The magnitude of dextral offset on the Periadriatic fault cannot be directly estimated. However, the jump in zircon and apatite fission-track ages establishes that the relative vertical displacement was ~4-5km since 24Ma, and that movement continued until at least 13M

Exhumation history of the Higher Himalayan Crystalline along Dhauliganga-Goriganga river valleys, NW India: new constraints from fission track analysis

New apatite and zircon fission track data collected from two transects along the Dhauliganga and Goriganga rivers in the NW Himalaya document exhumation of the Higher Himalayan Crystalline units. Despite sharing the same structural configuration and rock types and being separated by only 60 km, the two study areas show very different patterns of exhumation. Fission track (FT) data from the Dhauliganga section show systematic changes in age (individual apatite FT ages range from 0.9 ± 0.3 to 3.6 ± 0.5 Ma, r 2 = 0.82) that record faster exhumation across a zone that extends from the Main Central Thrust to north of the Vaikrita thrust. By contrast, FT results from the Goriganga Valley show a stepwise change in ages across the Vaikrita thrust that suggests Quaternary thrust sense displacement. Footwall samples yield a weighted mean apatite age of 1.6 ± 0.1 Ma compared to 0.7 ± 0.04 Ma in the hanging wall. A constant zircon fission track age of 1.8 ± 0.4 Ma across both the footwall and hanging wall shows the 0.9 Ma difference in apatite ages is due to movement on the Vaikrita thrust that initiated soon after ∼1.8 Ma. The Goriganga section provides clear evidence for >1 Ma of tectonic deformation in the brittle crust that contrasts with previous exhumation studies in other areas of the high Himalaya ranges; these studies have been unable to decouple the role of climate erosion from tectonics. One possibility why there is a clear tectonic signal in the Goriganga Valley is that climate erosion has not yet fully adjusted to the tectonic perturbation

Fracture and flow in natural rock deformation

Field observation shows that brittle fracturing and ductile flow are often intimately related under a wide range of metamorphic conditions. By their very nature, brittle fractures tend to be discrete and ductile flow more distributed, so that strong localization is often more readily attributed to brittle fracture or to subsequent ductile reactivation of a brittle precursor. Brittle fractures commonly show little regard for existing compositional boundaries and can crosscut them at a low angle, whereas ductile localization is typically bound to bands of different composition and/or rheology. Very localized regional fault zones can extend for hundreds of kilometres with widths on the order of hundreds of metres or less. They commonly crosscut many different compositional units at low angles, which suggests a potential brittle precursor to the mylonite zone now observed. In general, ductile viscous flow initially localized on discrete compositional or rheological precursors may actually tend to broaden rather than localize with time. The interplay between brittle fracture and localized viscous flow in shear zones, and the associated tectonic pressure effects relative to the adjacent matrix, are critical for understanding fluid flow in heterogeneously deforming rocks and thus for the interpretation of veins, fluid-rock interaction, migmatites and melt accumulation