Rock rheology and density have first‐order effects on the lithosphere's response to plate tectonic forces at plate boundaries. Changes in these rock properties are controlled by metamorphic ...transformation processes that are critically dependent on the presence of fluids. At the onset of a continental collision, the lower crust is in most cases dry and strong. However, if exposed to internally produced or externally supplied fluids, the thickened crust will react and be converted into a mechanically weaker lithology by fluid‐driven metamorphic reactions. Fluid introduction is often associated with deep crustal earthquakes. Microstructural evidence, suggest that in strong highly stressed rocks, seismic slip may be initiated by brittle deformation and that wall‐rock damage caused by dynamic ruptures plays a very important role in allowing fluids to enter into contact with dry and highly reactive lower crustal rocks. The resulting metamorphism produces weaker rocks which subsequently deform by viscous creep. Volumes of weak rocks contained in a highly stressed environment of strong rocks may experience significant excursions toward higher pressure without any associated burial. Slow and highly localized creep processes in a velocity strengthening regime may produce mylonitic shear zones along faults initially characterized by earthquake‐generated frictional melting and wall rock damage. However, stress pulses from earthquakes in the shallower brittle regime may kick start new episodes of seismic slip at velocity weakening conditions. These processes indicate that the evolution of the lower crust during continental collisions is controlled by the transient interplay between brittle deformation, fluid‐rock interactions, and creep flow.
Plain Language Summary
When continents collide, a mountain range form along the collision zone in an area referred to as an orogenic belt. The evolution of orogenic belts depends on the physical properties of the different layers in the Earth's crust and upper mantle. During a continent‐continent collision, the mechanical strength and density of these layers change. We discuss transformation processes in the lower part of the continental crust during the formation of an orogenic belt. We emphasize the critical role of fluids and show that introduction of fluids to initially dry lower crust is often associated with earthquakes. Earthquakes cause fracturing and fragmentation and allow fluids to migrate into highly reactive dry rocks. This results in the formation of weaker rocks, which subsequently deform slowly along zones where earthquakes initially occurred. Earthquakes in the upper crust may subsequently send stress pulses to the lower crust to trigger new generations of earthquakes.
Key Points
When continents collide, the lower crust is dry and strong and subject to high mechanical stress
Introduction of fluids to the lower crust is often associated with earthquakes
Fluid‐induced metamorphism may cause a transition from brittle to ductile deformation of the lower crust
Fractures and faults riddle the Earth's crust on all scales, and the deformation associated with them is presumed to have had significant effects on its petrological and structural evolution. ...However, despite the abundance of directly observable earthquake activity, unequivocal evidence for seismic slip rates along ancient faults is rare and usually related to frictional melting and the formation of pseudotachylites. We report novel microstructures from garnet crystals in the immediate vicinity of seismic slip planes that transected lower crustal granulites during intermediate-depth earthquakes in the Bergen Arcs area, western Norway, some 420 million years ago. Seismic loading caused massive dislocation formations and fragmentation of wall rock garnets. Microfracturing and the injection of sulfide melts occurred during an early stage of loading. Subsequent dilation caused pervasive transport of fluids into the garnets along a network of microfractures, dislocations, and subgrain and grain boundaries, leading to the growth of abundant mineral inclusions inside the fragmented garnets. Recrystallization by grain boundary migration closed most of the pores and fractures generated by the seismic event. This wall rock alteration represents the initial stages of an earthquake-triggered metamorphic transformation process that ultimately led to reworking of the lower crust on a regional scale.
The Kongsberg and Bamble lithotectonic domains of SE‐Norway are known as classical Precambrian high‐grade metamorphic terrains. The area has undergone extensive metasomatism with formation of ...albitites and scapolite‐rich rocks and numbers of previously economically important deposits including the Kongsberg Silver and the Modum Cobalt mines. We demonstrate here that the central part of the Bamble lithotectonic domain (Kragerø area) has locally developed low‐grade metamorphic minerals (prehnite, pumpellyite, analcime, stilpnomelane and thomsonite) belonging to the prehnite‐pumpellyite and zeolite facies. Structurally, the low‐grade minerals occur as fracture fills, in the alteration selvages around fractures where the rock is albitized, and along shear zones and cataclastic zones. The fracture fill and the alteration selvages vary from millimetres scale to 1 m in thickness. The fractures with low‐grade minerals are part of larger fracture systems. The low‐grade minerals typically formed by both displacive (swelling) and replacive reactions and in a combination of these. Prehnite together with albite, K‐feldspar, quartz, epidote and hydrogarnet form lenses along (001) faces in biotite and chlorite leading to bending of the sheet silicates through a displacive reaction mechanism. Numerous replacement reactions including the earlier minerals as well as the low‐grade minerals occur. As albite, K‐feldspar, talc, quartz, actinolite, titanite, calcite and hydrogrossular form in the same veins and in the same biotite grain as the classical low‐grade minerals, they probably belong to the low‐grade assemblage and some of the albitization in the region presumably occurred at low‐grade conditions. Alteration of olivine (Fo69) at low‐grade conditions results in the formation of clay minerals including ferroan saponite. Reconnaissance studies at the east (Idefjord lithotectonic domain) and the northwest (Kongsberg lithotectonic domain) sides of the Oslo rift together with reports of low‐grade assemblages in south‐western Sweden along the continuation of the rift into Skagerrak suggest that the low grade assembles occur in rocks adjacent to the Oslo rift along its full extent. Ar‐Ar dating of K‐feldspar from the low‐grade assemblages gave an age of 265.2 ± 0.4 Ma (MSWD = 0.514 and P = 0.766), suggesting that the low‐grade metamorphism and some of the metasomatism is induced by fluids and heat from the magmatic activity of the Permian Oslo rift, which requires transport of fluid over distances of several kilometres. The metamorphic conditions are constrained by stability fields of prehnite, pumpellyite and analcime to be less than 250°C and at a pressure less than 5 kbars. The displacive reactions created micro‐fractures and porosity in the adjacent minerals that enhance fluid flow and low‐grade mineral formation on a local scale. On a thin section scale, the displacive growth of albite in biotite results in a local volume increase of several 100%. Whether the opening of the larger, horizontally oriented fracture systems needed to transport the fluid over a distance of several kilometres was also the results of displacive reactions remains unknown. The low‐grade metamorphism and metasomatism formed in the shoulder of the Oslo rift and may have contributed to its uplift.
Coseismic damage associated with earthquakes in the lower continental crust is accompanied by postseismic annealing and fluid‐mediated metamorphism that influence the physical and chemical ...development of the continental crust on regional scales. A transition from brittle deformation to crystal‐plastic recrystallization is a recurring characteristic of rocks affected by lower crustal earthquakes and is observed in plagioclase adjacent to pseudotachylytes in granulite facies anorthosites from the Bergen Arcs, western Norway. The microstructural and petrological records of this transition were investigated using electron microscopy, electron microprobe analysis, and electron backscatter diffraction analysis. Microfractures associated with mechanical twins are abundant within plagioclase and contain fine‐grained aggregates that formed by fragmentation with minor shear deformation. The presence of feather features, which are described for the first time in feldspar, suggests that fractures propagate at near the shear wave velocity into the wall rock of earthquake slip planes. Grain size insensitive recrystallization took place within the time frame of pseudotachylyte formation, forming high‐angle grain boundaries required for shear zone initiation. Fluid infiltration synfracture to postfracture facilitated the epitactic replacement of plagioclase by alkali feldspar and the nucleation of clinozoisite, kyanite, and quartz. The grain size reduction and crystallization associated with the microfractures create rheologically weak areas that have the potential to localize strain within the plagioclase‐rich lower crust.
Key Points
Plagioclase deforms by microfracturing, mechanical twinning, and feather feature formation without major shear component
Recrystallization of microfractures occurred within the time frame of pseudotachylyte formation
Microstructures produced by coseismic damage control the spatial distribution of eclogite facies minerals
Dehydration of partly or completely serpentinized ultramafic rocks can increase the pore fluid pressure and induce brittle failure, a process referred to as dehydration embrittlement. However the ...extents of strain localization and unstable frictional sliding during deserpentinization are still under debate. In the layered ultramafic sections of the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde metamorphism of serpentinite veins led to local fluid production and to the growth of Mg-rich and coarse-grained olivine with abundant magnetite inclusions and δ18O values 1.0–1.5‰ below the host rock. Embrittlement associated with the dehydration caused faulting along highly localized (<10 μm-wide) slip planes near the centers of the original serpentinite veins and pulverization of wall rock olivine. These features along with an earthquake-like size distribution of fault offsets suggest unstable frictional sliding rather than slower creep. Structural heterogeneities in the form of serpentinite veins clearly have first-order controls on strain localization and frictional sliding during dehydration. As most of the oceanic lithosphere is incompletely serpentinized, heterogeneities represented by a non-uniform distribution of serpentinite are common and may increase the likelihood that dehydration embrittlement triggers earthquakes.
•Olivine veins in dunites from the Leka Ophiolite formed by deserpentinization.•Dehydration of partially serpentinized peridotites caused localized slip.•Unstable frictional sliding caused significant grain size reduction without shear strain.
Our ability to decipher the mechanisms behind metamorphic transformation processes depends in a major way on the extent to which crystallographic and microstructural information is transferred from ...one stage to another. Within the Leka Ophiolite Complex in the Central Norwegian Caledonides, prograde olivine veins that formed by dehydration of serpentinite veins in dunites exhibit a characteristic distribution of microstructures: The outer part of the veins comprises coarse-grained olivine that forms an unusual, brick-like microstructure. The inner part of the veins, surrounding a central fault, is composed of fine-grained olivine. Where the fault movement included a dilational component, optically clear, equant olivine occurs in the centre. Electron backscatter diffraction mapping reveals that the vein olivine has inherited its crystallographic preferred orientation (CPO) from the olivine in the porphyroclastic host rock; however, misorientation is weaker and associated to different rotation axes. We propose that prograde olivine grew epitaxially on relics of mantle olivine and thereby acquired its CPO. Growth towards pre-existing microfractures along which serpentinisation had occurred led to straight grain boundaries and a brick-like microstructure in the veins. When dehydration embrittlement induced slip, a strong strain localisation on discrete fault planes prevented distortion of the CPO due to cataclastic deformation; grain size reduction did not significantly modify the olivine CPO. This illustrates how a CPO can be preserved though an entire metamorphic cycle, including hydration, dehydration, and deformation processes, and that the CPO and the microstructures (e.g. grain shape) of one phase do not necessarily record the same event.
The carbonation of ultramafic rocks is a common alteration process in ophiolites and can occur in various settings. We provide the first detailed description of the carbonated peridotites ...(ophicarbonates) of the Feragen Ultramafic Body, central Norway, which have unusually variable compositions and microstructures. Lithologies range from pervasively carbonated serpentinites through carbonated serpentinite breccias to carbonated ultramafic conglomerates. Carbonate phases are Ca-carbonate, magnesite and dolomite. Some breccias are also cemented by coarsegrained brucite. This variability records strong variations in fluid chemistry and/or pressure and temperature conditions, both spatially and temporally. By analysing these altered ultramafic rocks using field relationships, optical microscopy, electron microprobe analysis and oxygen and carbon isotope compositions, we elucidate the history of the Feragen Ultramafic Body in more detail and emphasise the importance of deformation for the extent and type of alteration.
The grain size distribution of deformed rocks may provide valuable information about their deformation history and the associated mechanisms. Here we present a unique set of olivine grain size ...distributions from ultramafic rocks deformed under a wide range of stress and strain rate conditions. Both experimentally deformed and naturally deformed samples are included. We observe a surprisingly uniform behavior, and most samples show power law grain size distributions. Convincing lognormal distributions across all scales were only observed for samples experimentally deformed at high temperature (1200 °C) and for some mantle‐deformed natural samples. Single power law distributions were observed for natural samples deformed by brittle mechanisms and by samples deformed experimentally in the regime of low‐temperature plasticity. Most natural samples show a crossover in power law scaling behavior near the median grain size from a steep slope for the larger grain fraction to a more gentle slope for the smaller grains. The small grain fraction shows a good data collapse when normalized to the crossover length scale. The associated power law slope indicates a common grain size controlling process. We propose a model that explains how such a scaling behavior may arise in the dislocation creep regime from the competition between the rate involved in the dislocation dynamics and the imposed strain rate. The common departure from lognormal distributions suggests that naturally deformed samples often have a deformation history that is far from a steady state scenario and probably reflects deformation under highly variable stress and strain rates.
Key Points
Olivine grain size distributions from faults and shear zones indicate that steady state deformation is less common than hitherto thought
Brittle deformation causes power law scaling, while plastic mechanisms lead to lognormal distributions at steady state conditions
Grain size distributions often show a crossover between two power laws where the smaller grain sizes scale independently of strain rates
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