Under Pressure: High-Pressure Metamorphism in the Alps

Elements ◽  
2021 ◽  
Vol 17 (1) ◽  
pp. 17-22 ◽  
Author(s):  
Lucie Tajčmanová ◽  
Paola Manzotti ◽  
Matteo Alvaro
Keyword(s):  
High Pressure ◽  
Structural Data ◽  
Metamorphic Rocks ◽  
Lithostatic Pressure ◽  
Mineral Inclusion ◽  
Crustal Material ◽  
Local Pressure ◽  
Ultra High Pressure ◽  
Mechanical Models ◽  
The Alps

The mechanisms attending the burial of crustal material and its exhumation before and during the Alpine orogeny are controversial. New mechanical models propose local pressure perturbations deviating from lithostatic pressure as a possible mechanism for creating (ultra-)high-pressure rocks in the Alps. These models challenge the assumption that metamorphic pressure can be used as a measure of depth, in this case implying deep subduction of metamorphic rocks beneath the Alpine orogen. We summarize petro-logical, geochronological and structural data to assess two fundamentally distinct mechanisms of forming (ultra-)high-pressure rocks: deep subduction; or anomalous, non-lithostatic pressure variation. Furthermore, we explore mineral-inclusion barometry to assess the relationship between pressure and depth in metamorphic rocks.

scholarly journals Up the down escalator: the exhumation of (ultra)-high pressure terranes during on-going subduction

2012 ◽  
Vol 4 (1) ◽  
pp. 745-781 ◽  
Author(s):  
C. J. Warren
Keyword(s):  
High Pressure ◽  
Subduction Zone ◽  
Continental Crust ◽  
Subduction Zones ◽  
Crustal Material ◽  
Time And Space ◽  
Ultra High Pressure ◽  
Tectonic Environment ◽  
Tectonic Style ◽  
Weak Material

Abstract. The exhumation of high and ultra-high pressure rocks is ubiquitous in Phanerozoic orogens created during continental collisions, and is common in many ocean-ocean and ocean-continent subduction zone environments. Three different tectonic environments have previously been reported, which exhume deeply buried material by different mechanisms and at different rates. However it is becoming increasingly clear that no single mechanism dominates in any particular tectonic environment, and the mechanism may change in time and space within the same subduction zone. In order for buoyant continental crust to subduct, it must remain attached to a stronger and denser substrate, but in order to exhume, it must detach (and therefore at least locally weaken) and be initially buoyant. Denser oceanic crust subducts more readily than more buoyant continental crust but exhumation must be assisted by entrainment within more buoyant and weak material such as serpentinite or driven by the exhumation of structurally lower continental crustal material. Weakening mechanisms responsible for the detachment of crust at depth include strain, hydration, melting, grain size reduction and the development of foliation. These may act locally or may act on the bulk of the subducted material. Metamorphic reactions, metastability and the composition of the subducted crust all affect buoyancy and overall strength. Subduction zones change in style both in time and space, and exhumation mechanisms change to reflect the tectonic style and overall force regime within the subduction zone. Exhumation events may be transient and occur only once in a particular subduction zone or orogen, or may be more continuous or occur multiple times.

Pressure-to-depth conversion models for (ultra-)high-pressure metamorphic rocks: review and application

2020 ◽  
Author(s):  
Arthur Bauville ◽  
Philippe Yamato
Keyword(s):  
Linear Increase ◽  
Linear Mapping ◽  
Analytical Models ◽  
Metamorphic Rocks ◽  
Lithostatic Pressure ◽  
Ultra High Pressure ◽  
Data Points ◽  
Rapid Drop ◽  
Temperature Path ◽  
Geodynamic Reconstructions

<p>Pressure estimated from metamorphic rocks is one of the main tools for geodynamic reconstructions. The pressure-temperature path of UHP metamorphic rocks typically shows a linear increase of P and T followed by a rapid drop of Pressure at near-constant temperature. The geological history can be reconstructed by using the metamorphic pressure as a proxy for depth. Researchers often base their geodynamic reconstruction on a simple linear mapping of pressure to depth, by considering that the pressure is the weight of the overlying column of rock or lithostatic pressure. In recent years, an increasing corpus of evidence demonstrates that rocks can experience pressures that deviate from the lithostatic state on the order of GPa. These deviations can be at the scale of the orogen (Petrelli and Podladchikov, 2002), the outcrop (Jamtveit et al., 2018; Luisier et al., 2019); or even at the grain-scale (Tajcmanova, 2015). Thus, these studies raise the concern that metamorphic pressures may not be reliable proxies for depth, and therefore could not be used for geodynamic reconstructions. The objective of this contribution (1) to review the various models proposed in the literature for metamorphic pressure, (2) to formulate analytical models with simple assumptions that can be used to convert metamorphic pressure to depth even in the case where pressure deviates significantly from the lithostatic pressure. We use our pressure-to-depth conversion models to estimate the depth of ~60 samples from various orogens worldwide. The prediction of the different models varies widely. Some models predict depth as deep as 160km for specific samples, while other models predict depth $<75$ km (i.e. deepest depth of the Moho) for all data points.  We discuss the limits of applicability and the geodynamic implications of each model. </p>

Petrological constraints on ultra-high pressure metamorphism and frictionite formation in a catastrophic rockslide: The Koefels event (Eastern Alps).

2020 ◽  
Author(s):  
Diethard Sanders ◽  
Bastian Joachim-Mrosko ◽  
Jürgen Konzett ◽  
Julian Lanthaler ◽  
Marc Ostermann ◽  
...  
Keyword(s):  
High Pressure ◽  
Rock Mass ◽  
Frictional Heating ◽  
Bulk Composition ◽  
High Energy ◽  
Shear Localization ◽  
Raman Shift ◽  
Local Pressure ◽  
Ultra High Pressure ◽  
Valley Flank

<p>The P-T conditions in extremely-rapid gravity-driven rockslides are difficult to constrain from the descended rock mass itself. Here, we report mineralogical observations from the Koefels rockslide and their interpretation. The Koefels event – happened between 9527-9498 cal BP – comprises 3.9 km<sup>3</sup> mainly of muscovite + biotite-bearing orthogneiss, and is one of the few large rockslides in silicate-bearing rocks worldwide. Detached by collapse of a valley flank, the rockslide impacted the opposite valley flank: While the lower part of the mass was sharply stopped, the overriding part propagated farther. This led to shear localization along discrete planes and, in consequence, to transient melting by frictional heating. The resulting frictionites comprise thin glassy levels with floating crystal fragments. The bulk composition of the glassy melt corresponds to the composition of the orthogneiss.</p><p>            In the frictionites, ultra-high pressure metamorphosed quartz (UPQ) occurs next to unaffected quartz in a glassy matrix. Micro-Raman spectroscopy of unaffected quartz yielded an intense A1 Raman mode at 464 cm<sup>-1 </sup>; UPQ shows a shift of this band down to 460cm<sup>-1</sup>, with some grains showing an internal gradient of up to 3 cm<sup>-1</sup> from the core (463cm<sup>-1</sup>) to the rim (460 cm<sup>-1</sup>). Some UPQ are rimmed by lechatelierite (SiO<sub>2</sub> glass), which never surrounds unaffected quartz grains. Until now lechatelierite formation in frictionites was considered to be a function of temperature only (Heuberger et al. 1984). Because lechatelierite only rims UPQ with outward decreasing band numbers, we interpret lechatelierite formation to be mainly pressure-driven. The completely molten matrix and the lack of glassy rims at the edges of normal quartz indicates minimum temperatures of 900°C. Experimental investigations have shown that the shifted A1 mode of UPQ equilibrates to 464 cm<sup>-1</sup> at 1100°C, thus giving an upper limit of the temperature range. The Raman shift of the A1 mode and the presence of lechatelierite strongly suggest that a pressure >23 GPa was attained (cf., McMillan et al. 1991, Fritz et al. 2011, Kowitz et al. 2013).</p><p>            The UPQ and lechatelierite rims formed by grain collisions during initial shear localization, when the shear plane was relatively cool. Next, upon rapid frictional heating the glassy frictionite matrix formed and became locally injected into lechatelierite rims. Once formed, the melt prevented high-energy grain collisions. Unaffected quartz (which nevertheless may have seen pressures up to 22 GPa) in the frictionites perhaps escaped UHP overprint due to position in local pressure shadows and/or was sheared out from the adjacent caciritic rock mass into the melt. Our results help to better constrain numerical simulations of P-T-conditions in rockslides. Since our investigation only provides limiting estimates the actual P-T conditions in deep shear levels of rockslides exceeding the volume of the Koefels event might be even higher.</p><p> </p><p>References:</p><p>Fritz et al. 2011: International Journal of Impact Engineering, 38:440</p><p>Heuberger et al. 1984: Mountain Research and Development, 4:345</p><p>Kowitz et al. 2013: Earth and Planetary Science Letters, 384:17</p><p>McMillan et al. 1992: Physics and Chemistry of Minerals, 19:71</p>

Ultra-high pressure metamorphic rocks in the Dabie-Su-Lu region, China: Their formation and exhumation

Island Arc ◽  
1994 ◽  
Vol 3 (3) ◽  
pp. 135-150 ◽  
Author(s):  
Cong Bolin ◽  
Wang Qingchen ◽  
Zhai Mingguo ◽  
Zhang Ruyuan ◽  
Zhao Zhongyan ◽  
...  
Keyword(s):  
High Pressure ◽  
Metamorphic Rocks ◽  
Ultra High Pressure

scholarly journals The Alps 2: Controls on crustal subduction and (ultra)high-pressure rock exhumation in Alpine-type orogens

2014 ◽  
Vol 119 (7) ◽  
pp. 5987-6022 ◽  
Author(s):  
Jared P. Butler ◽  
Christopher Beaumont ◽  
Rebecca A. Jamieson
Keyword(s):  
High Pressure ◽  
Ultra High Pressure ◽  
The Alps

scholarly journals Unusually low δ18O ultra-high-pressure metamorphic rocks from the Sulu Terrain, eastern China

1995 ◽  
Vol 59 (13) ◽  
pp. 2859-2864 ◽  
Author(s):  
Tzen-Fu Yui ◽  
Douglas Rumble ◽  
Ching-Hua Lo
Keyword(s):  
High Pressure ◽  
Eastern China ◽  
Metamorphic Rocks ◽  
Ultra High Pressure

Ultra-high-pressure metamorphic rocks in the Western Alps

1995 ◽  
pp. 206-243 ◽  
Author(s):  
R. Compagnoni ◽  
T. Hirajima ◽  
C. Chopin
Keyword(s):  
High Pressure ◽  
Western Alps ◽  
Metamorphic Rocks ◽  
Ultra High Pressure

scholarly journals Late Orogenic Heating of (Ultra)High Pressure Rocks: Slab Rollback vs. Slab Breakoff

Geosciences ◽  
2019 ◽  
Vol 9 (12) ◽  
pp. 499 ◽  
Author(s):  
Elena Sizova ◽  
Christoph Hauzenberger ◽  
Harald Fritz ◽  
Shah Wali Faryad ◽  
Taras Gerya
Keyword(s):  
High Pressure ◽  
Continental Collision ◽  
Crustal Material ◽  
Ultra High Pressure ◽  
Slab Rollback ◽  
Slab Breakoff ◽  
T Path ◽  
Slab Bending ◽  
Lower Crustal ◽  
Collisional Orogens

Some (ultra)high-pressure metamorphic rocks that formed during continental collision preserve relict minerals, indicating a two-stage evolution: first, subduction to mantle depths and exhumation to the lower-crustal level (with simultaneous cooling), followed by intensive heating that can be characterized by a β-shaped pressure–temperature–time (P–T–t) path. Based on a two-dimensional (2D) coupled petrological–thermomechanical tectono-magmatic numerical model, we propose a possible sequence of tectonic stages that could lead to these overprinting metamorphic events along an orogenic β-shaped P–T–t path: the subduction and exhumation of continental crust, followed by slab retreat that leads to extension and subsequent asthenospheric upwelling. During the last stage, the exhumed crustal material at the crust–mantle boundary undergoes heating from the underlying hot asthenospheric mantle. This slab rollback scenario is further compared numerically with the classical continental collision scenario associated with slab breakoff, which is often used to explain the late heating impulse in the collisional orogens. The mantle upwelling occurring in the experiments with slab breakoff, which is responsible for the heating of the exhumed crustal material, is not related to the slab breakoff but can be caused either by slab bending before slab breakoff or by post-breakoff exhumation of the subducted crust. Our numerical modeling predictions align well with a variety of orogenic P–T–t paths that have been reported from many Phanerozoic collisional orogens, such as the Variscan Bohemian Massif, the Triassic Dabie Shan, the Cenozoic Northwest Himalaya, and some metamorphic complexes in the Alps.

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