Exhumation Of Metamorphic Rocks Research Articles

Surface evolution during the mid-Proterozoic stalled by mantle warming under Columbia–Rodinia

The thermal state of the mantle impacts lithospheric dynamics and Earth's surface evolution. Geodynamic simulations predict that aggregated continents may act as an effective thermal insulator leading to the warming of the underlying mantle (mantle warming hypothesis). Mantle warming weakens the continental lithosphere, which should lead to more distributed strain at plate margins, and result in lower-relief orogens, with lower metamorphic peak pressures (peak-P), and slower cooling and exhumation of metamorphic rocks. To test the mantle warming hypothesis, we determine the secular change of metamorphic cooling and exhumation rates based on a new dataset compiled from Neoarchean and younger orogens. We find that sluggish cooling and exhumation during the mid-Proterozoic (1.85–0.85 Ga; aka the boring billion) correlate with lower peak-P and higher thermobaric (temperature/pressure, T/P) ratios of metamorphic rocks. This result is consistent with a weakened continental lithosphere and the development of hotter lower-relief orogens under warmer mantle conditions during the mid-Proterozoic. We posit that the long-lived Columbia–Rodinia supercontinents may have acted as a thermal insulator that led to the mantle warming beneath the continental lithosphere and thus a distinctive style of mid-Proterozoic orogenesis. Low-relief mid-Proterozoic orogens with sluggish exhumation, in turn, reduced continental erosion and restricted delivery of bio-essential nutrients to the oceans, and decreased rates of organic carbon burial and thereby hindered oxygen accumulation in the atmosphere. The persistent nutrient deficiency in the oceans and suppressed atmospheric oxygenation may have stalled the surface evolution during the boring billion.

Metamorphic, deformation, fluids and geological significance of low-temperature retrograde mylonites of Diancangshan metamorphic massif along Ailaoshan-Red River strike-slip fault zone, Yunnan, China

Diancangshan metamorphic massif is one of the four metamorphic massifs developed along the Ailaoshan-Red River strike-slip fault zone, Yunnan, China. It has experienced multi-stage metamorphism and deformation, especially since the late Oligocene it widely suffered high-temperature ductile shear deformation and exhumation of the metamorphic rocks from the deep crust to the shallow surface. Based on the previous research and geological field work, this paper presents a detailed study on deformation and metamorphism, and exhumation of deep metamorphic rocks within the Diancangshan metamorphic massif, especially focusing on the low-temperature overprinted retrogression metamorphism and deformation of mylonitic rocks. With the combinated experimental techniques of optical microscope, electron backscatter diffraction attachmented on field-emission scanning electron microscopy and cathodoluminescence, our contribution reports the microstructure, lattice preferred orientations of the deformed minerals, and the changes of mineral composition phases of the superposition low-temperature retrograde mylonites. All these results indicate that: (1) Diancangshan deep metamorphic rock has experienced early high-temperature left-lateral shear deformation and late extension with rapid exhumation, the low-temperature retrogression metamorphism and deformation overprinted the high-temperature metamorphism, and the high-temperature microstructure and texture are in part or entirely altered by subsequent low-temperature shearing; (2) the superposition of low-temperature deformation-metamorphism occurs at the ductile-brittle transition; and (3) the fluid is quite active during the syn-tectonic shearing overprinted low-temperature deformation and metamorphism. The dynamic recrystallization and/or fractures to micro-fractures result in the strongly fine-grained of the main minerals, and present strain localization in micro-domians, such as micro-shear zones in the mylonites. It is often accompanied by the decrease of rock strength and finally influences the rheology of the whole rock during further deformation and exhumation of the Diancangshan massif.

Formation Conditions of Basic Granulites and High-Alumina Gneiss of the Baikal–Muya Belt (Northern Baikal Area)

The Kichera zone of the Baikal–Muya Belt consists of alternating tectonic plates with rocks of different metamorphic facies. Garnet–cordierite–sillimanite gneiss from the tectonic fragment in granite–gneiss of the Baikal massif with an age of 755 ± 15 Ma from the Goremyka plate and two-pyroxene schist of the granulite complex with an age of 617 ± 5 Ma from the Boguchan plate were studied. Thermobarometric studies of these key metamorphic rocks were carried out using the avPT (THERMOCALC) and TWEEQU (TWQ 2.01) methods. The P–T parameters estimated for the cordierite⎯sillimanite gneiss of the Goremyka plate correspond to the boundary between the amphibolite and granulite facies. Granulites of the Boguchan plate belong to the HT–LP type. Exhumation of metamorphic rocks could be caused by extension upon the evolution of the Late Baikal rifting.

Tectonics, magmatism and geodynamics of Italy: What we know and what we imagine

The Cenozoic geological evolution of the Italian area is characterized by the formation of two major mountain chains the Alps to the north and the Apennines throughout the peninsula – plus the opening of two oceanic basins (the Ligurian-Provencal and the Tyrrhenian Sea). Associated with the formation of these two belts, a volumetrically important and chemically complex magmatic activity developed. The Alps and the Apennines show very different styles of evolution: the Alps display double-verging growth, with the involvement of large volumes of basement and the exhumation of metamorphic rocks (thick-skinned tectonics). On the other hand, the Apennines are a single-verging belt, mostly characterized by thin-skinned tectonics and associated to a radial eastward translation (coupled to extensional tectonics in the Ligurian-Provencal, Tyrrhenian, and western Apennines areas). The Apennines generated an arc from the northern Apennines down to Sicily, possibly merging with the Maghrebides along the northern Africa coast. The different evolution of the Alpine and Apenninic belts is mirrored by the different geometry of the respective foredeep or foreland basins (shallow in the Alps and deep in the Apennines), as recorded also by the dip of the foreland monocline (shallow in the Alps, 2-4°, and steeper in the Apennines, 6-15°). The paradox is evident: the higher the belt, the thinner the foreland basin. The Alps consist of rocks belonging to the continental margings of the European and Adriatic-African plates, as well as remnants of Mesozoic intervening ocean(s). On the other hand, the Apennines, with the exception of the Calabro-Peloritani arc and other scattered basement outcrops, are mainly made up of rocks of Adriatic origin (Mesozoic Laziale-Abruzzese and Apulian carbonate platforms plus basinal successions), with subordinate ophiolites. From a magmatological point of view, the Alpine magmatism is essentially concentrated in a relatively narrow area, the so-called Insubric Lineament and in a relatively short time (mostly ~32-24 Ma). On the other hand, the Apennines-related igneous activity spans a larger time range (essentially from 22 Ma to Present), with several peaks in magma production. This magmatism took place over a much wider area, characterized by variable lithospheric thickness, Moho age and depth. On the basis of thermo-tectonic, magmatological, and plate-kinematics constraints, a geodynamic evolutionary model of the Italian area is proposed. We suggest that three subduction zones have been active and have consumed oceanic and, partially, continental lithosphere: the Alpine subduction zone, with the European plate under-thrusting the Adriatic microplate; the Apenninic subduction zone, with the ancient (Mesozoic?) Ionian/Mesogean oceanic lithosphere and the Adriatic micro-plate under-thrusting westward the European plate; and the Dinaric subduction zone, with the Adriatic micropate under-thrusting northeastward the European plate. Such a geodynamic scenario is summarised in a movie, spanning the last 50 Ma. Citation: 2010. Tectonics, magmatism and geodynamics of Italy: What we know and what we imagine. In: (Eds.) Marco Beltrando, Angelo Peccerillo, Massimo Mattei, Sandro Conticelli, and Carlo Doglioni, Journal of the Virtual Explorer, volume 36, paper 9, doi: 10.3809/jvirtex.2010.00226 Journal of the Virtual Explorer, 2010

Thrust faults and extensional detachment faults in Cretan tectono-stratigraphy: Implications for Middle Miocene extension

The revised tectono-stratigraphy of Crete and especially of the “Phyllites–Quartzites” complex demonstrated the distinction of the probable Paleozoic low-medium grade metamorphic rocks of the Arna unit from the underlying Permo–Triassic phyllites and associated carbonate sediments (Trypali facies) of Western Crete unit as well as the overlying Permo–Triassic phyllites and associated sediments of the Tyros/Ravdoucha Beds at the base of the Tripolis unit. The pre-existing mixture of the above tectono-stratigaphic units in a single complex created a number of misinterpretations as far as stratigraphy, metamorphism and interpretation of low angle faults as thrusts or detachments. Especially in cases where the inferred tectonic contact concerns the transition between the Tyros Beds and the base of the Tripolis carbonate platform there is no structural omission and therefore the contact represents a minor disharmonic sliding surface and not a detachment. Based on the revised tectono-stratigraphic analysis the determination of the structural omission for each tectonic contact was possible and several detachments were described for the first time. Footwall rocks of the detachments comprised several tectonic units usually from the lower nappes and hanging wall rocks comprised several tectonic units usually from the upper nappes. The detachment may separate not only metamorphosed units in the footwall (Mani, Western Crete, Arna) from non metamorphosed units in the hanging wall (Tripolis, Pindos and higher nappes) but also all other possible combinations from the Cretan nappe pile. Extension in Crete started in the Middle–Late Miocene with the formation of extensional detachment faults. The reported extensional structures of Oligocene to Early Miocene age do not correspond to crustal extension of Crete but to localized shear zones related to nappe stacking and the exhumation of metamorphic rocks. Extensional detachments in Crete form a tectonic horst through two oppositely dipping E–W-trending zones; one dipping north, related to the opening of the Cretan basin, and the other dipping south, related to the formation of the Messara supra-detachment basin. The deformation history of units within Crete can be summarized as: (i) compressional deformation producing arc-parallel east–west-trending south-directed thrust faults in Oligocene to Early Miocene time; (ii) extensional deformation along arc-parallel, east–west-trending detachment faults in Middle Miocene time, with hanging wall motion to the north and south; and (iii) Late Miocene–Quaternary transtensional deformation along high-angle normal and oblique normal faults that disrupt the older arc-parallel structures.

REE distribution in corundum-bearing and other metasomatic rocks during the exhumation of metamorphic rocks of the belomorian belt of the Baltic Shield

The paper reports data on the distribution of REE in metasomatic rocks, including those with corundum, that were formed during the exhumation of the rocks of the Belomorian Belt at 1.9–1.75 Ga. This process is thought to have occurred concurrently with the horizontal extension and tectonic denudation of the upper crust, which, in turn, induced the massive release of fluids. The latter formed two major groups of silicic metasomatic rocks. The deepest-sitting corundum-bearing and other mafic metasomatic rocks are enriched in REE, alkalis, and alumina compared to the host rocks. The coeval acid metasomatic rocks such as orthotectites are, conversely, depleted in REE (with positive Eu anomalies), mafic elements, and HFSE but are also enriched in alumina. These complimentary rocks are thought to have been produced early during the exhumation of deep rocks under the effect of reduced fluids whose genesis was related to decompression. The silicic metasomatic rocks of the second group (with muscovite) have elevated REE concentrations (with negative Eu anomalies) and were formed by already oxidized fluids at shallower depths. The fact that the corundum-bearing metasomatic rocks are enriched in REE (in spite of the ultrabasic composition of these rocks) suggests that they were generated in an extensional environment with the participation of deep fluids, which enriched the rocks in Al, Na, K, Ba, Sr, Zr, and LREE.

Exhumation of metamorphic rocks in N Aegean: the path from shortening to extension and extrusion

The Olympos-Ossa-Pelion (OOP) ranges, in NW Aegean, encompass Greece highest summit and are located near the extremity of the North Anatolian Fault (NAF). Structural and thermochronological data gathered in the OOP ranges show that the main exhumation of metamorphic nappes occurred in the Eocene, at ca. 43-39 Ma. This early exhumation, associated with ductile, then brittle-ductile normal faulting with northeastward transport, is nearly coeval with orogenic shortening in the close area. Cooling rates, and likely exhumation, have been low between ~40 Ma and ~20 Ma. 40 Ar/ 39 Ar crystallization ages (between 20 and 15 Ma) appears related to brittle-ductile normal faulting and likely associated with the onset of Aegean back-arc extension. The dating of a diabase dyke, and the geometry of associated brittle jointing, of onshore and offshore active normal faults imply a shift in extension direction after 4 Ma. Such a shift is probably related the propagation of the NAF in northern Aegean known to have occurred around 5 Ma.

Evolution of the Mount Woods Inlier, northern Gawler Craton, Southern Australia: an integrated structural and aeromagnetic analysis

Structural mapping integrated with interpretation and forward modelling of aeromagnetic data form complimentary and powerful tools for regional structural analysis because both techniques focus on architecture and overprinting relationships. This approach is used to constrain the geometry and evolution of the sparsely exposed Mount Woods Inlier in the northern Gawler Craton. The Mount Woods Inlier records a history of poly-phase deformation, high-temperature metamorphism, and syn- and post-orogenic magmatism between ca. 1736 and 1584 Ma. The earliest deformation involved isoclinal folding, and the development of bedding parallel and axial planar gneissic foliation (S 1). This was accompanied by high-temperature, upper amphibolite to granulite facies metamorphism at ca. 1736 Ma. During subsequent north–south shortening (D 2), open to isoclinal south–southeast-oriented F 2 folds developed as the Palaeoproterozoic successions of the inlier were thrust over the Archaean nuclei of the Gawler Craton. The syn-D 2 Engenina Adamellite was emplaced at ca. 1692 Ma. The post-D 2 history involved shear zone development and localised folding, exhumation of metamorphic rocks, and deposition of clastic sediments prior to the emplacement of the ca. 1584 Ma Granite Balta Suite. The Mount Woods Inlier is interpreted as the northern continuation of the Kimban Orogen.

Les dômes métamorphiques extensifs dans les chaînes de montagnes. Extension syn-orogénique et post-orogénique

Exhumation of metamorphic rocks and extension are often intimately related. The role of extension in the removal of the overburden has been proven in numerous examples, even though compression and erosion play important parts. We recall a number of observations on Tethyan examples in order to focus the debate on the respective roles of erosion and extension in the formation of syn- and post-orogenic extensional metamorphic domes. The occurrence of pressure gaps across major contacts, the distribution of retrograde P – T paths of types I and II, the thinning of the nappe pile during exhumation and the frequent association with sedimentary basins make unescapable the call to a significant extension. Syn-orogenic exhumation is in general more efficient than post-orogenic exhumation. The finite geometry is similar in the syn- and post-orogenic cases but the P – T evolution is different, colder in the syn-orogenic case. Distinguishing between syn- and post-orogenic extension is important to unravel the tectonic history of a region because the two processes can coexist along a single cross-section and can happen sequentially. Exhumation des roches métamorphiques et extension sont souvent des phénomènes liés. Le rôle de l’extension pour éliminer la surcharge est établi sur de nombreux exemples, même si compression et érosion jouent des rôles importants. Nous rappelons un certain nombre de faits de terrain sur des exemples téthysiens, afin de recentrer le débat sur les rôles respectifs de l’extension et de l’érosion dans la formation des dômes métamorphiques extensifs en contextes syn- et post-orogéniques. La présence de sauts de pression significatifs au travers des contacts majeurs, l’association fréquente entre ces contacts et des bassins sédimentaires, la distribution dans la structure de chemins P – T rétrogrades de types I et II, ainsi que l’amincissement de la pile tectonique pendant l’exhumation impliquent une extension importante. L’exhumation syn-orogénique est en général plus efficace que l’exhumation post-orogénique. La géométrie est la même dans les cas syn- et post-orogéniques, mais l’évolution P – T diffère, plus froide dans les exemples syn-orogéniques. La distinction entre extension syn- et post-orogénique importe pour reconstituer l’histoire tectonique d’une région, car les deux processus peuvent coexister le long d’une même coupe et se succéder dans le temps.

Graphite precipitation in C--O--H fluid inclusions: closed system compositional and density changes, and thermobarometric implications

Equilibrium C–O–H fluid speciation calculations predict that graphite will precipitate from initially graphite saturated fluid inclusions during cooling and exhumation of metamorphic rocks. In the case that no mass is gained or lost by the inclusions, the original XO ratio [O/(O+H)] of the fluid phase must be maintained. Given this closed system constraint, the down-temperature progress of graphite precipitation can easily be monitored as a function of the varible XO, and produces some effects that are of significance to fluid inclusion studies: 1. Variation of the H2O : CO2 : CH4 relationship in the graphite-saturated COH fluid, namely increase of XH2O and decrease of the carbonic fraction; 2. Decrease of fluid density due to precipitation of graphite, which is denser than the residual fluid; 3. Alteration of the CO2 : CH4 ratio of the fluid, depending on the initial O : H ratio of the fluid: for XO>1/3, fluids increase their CO2 : CH4 ratio with decreasing temperature, and vice-versa. This implies that the CO2 : CH4 ratio measured at room T will not represent the trapping value, which is in any case closer to unity. As a consequence of density reduction, isochores extrapolated from densities observed at room temperature do not pass through the pressure-temperature conditions at which the inclusion was trapped, with pressure underestimates of up to 2 kbar. Actual P-T trapping conditions are located along the equilibrium “bulk isochore” (curve of constant-XO, constant-volume) of the fluid. Alteration of the CO2 : CH4 ratio is a mechanism by which a CO2-rich or CH4-rich carbonic phase can be formed from aqueous fluids that are slightly off the neutral XO=1/3 value. Subsequent segregation of this phase from the aqueous counterpart may account for the formation of pure CO2 and CH4 fluids in the upper crust.