Principal Slip Zones in Limestone: Microstructural Characterization and Implications for the Seismic Cycle (Tre Monti Fault, Central Apennines, Italy)

Earthquakes in central Italy, and in other areas worldwide, often nucleate within and rupture through carbonates in the upper crust. During individual earthquake ruptures, most fault displacement is thought to be accommodated by thin principal slip zones. This study presents detailed microstructural observations of the slip zones of the seismically active Tre Monti normal fault zone. All of the slip zones cut limestone, and geological constraints indicate exhumation from <2?km depth, where ambient temperatures are ?100°C. Scanning electron microscope observations suggest that the slip zones are composed of 100% calcite. The slip zones of secondary faults in the damage zone contain protocataclastic and cataclastic fabrics that are cross-cut by systematic fracture networks and stylolite dissolution surfaces. The slip zone of the principal fault has much more microstructural complexity, and contains a 2?C10?mm thick ultracataclasite that lies immediately beneath the principal slip surface. The ultracataclasite itself is internally zoned; 200?C300???m-thick ultracataclastic sub-layers record extreme localization of slip. Syn-tectonic calcite vein networks spatially associated with the sub-layers suggest fluid involvement in faulting. The ultracataclastic sub-layers preserve compelling microstructural evidence of fluidization, and also contain peculiar rounded grains consisting of a central (often angular) clast wrapped by a laminated outer cortex of ultra-fine-grained calcite. These ??clast-cortex grains?? closely resemble those produced during layer fluidization in other settings, including the basal detachments of catastrophic landslides and saturated high-velocity friction experiments on clay-bearing gouges. An overprinting foliation is present in the slip zone of the principal fault, and electron backscatter diffraction analyses indicate the presence of a weak calcite crystallographic preferred orientation (CPO) in the fine-grained matrix. The calcite c-axes are systematically inclined in the direction of shear. We suggest that fluidization of ultracataclastic sub-layers and formation of clast-cortex grains within the principal slip zone occurred at high strain rates during propagation of seismic ruptures whereas development of an overprinting CPO occurred by intergranular pressure solution during post-seismic creep. Further work is required to document the range of microstructures in localized slip zones that cross-cut different lithologies, and to compare natural slip zone microstructures with those produced in controlled deformation experiments.     

Seismostratigraphic analysis with centennial to decadal time resolution of the sediment sink in the Ganges-Brahmaputra subaqueous delta

The deltas of large rivers have become the focus of many research studies due to their vulnerability in times of expected sea level rise. One major delta in peril is the Ganges-Brahmaputra Delta (Bangladesh) due to its low elevation and high subsidence rate. In this study, high-resolution seismic and sediment echosounder data of the subaqueous part of the delta are analyzed by an integrated seismo-acoustic stratigraphic approach.Transparent Units (TUs) are the most prominent, continuously traceable architectural elements within the sigmoidal clinoform. TUs consist of homogenized sediments and are interpreted to be formed by liquefaction flows generated by subduction-related earthquakes of the nearby plate-boundary. The uppermost TUs are related to the historical well known major earthquakes which occurred in 1762, 1897 and 1950. Using the TUs as time marks the mean annual storage rates for the intervals could be assessed. A direct comparison of echosounder data gathered in 1994, 1997 and 2006 is used as another method to estimate the last decade mean annual storage rate.In general, the averaged decadal to centennial mean annual storage rates in the foreset beds have significantly decreased within the last ∼300 years from 22% to 18% and down to 13.8% of the present total annual fluviatile suspension supply to the delta (10⁹ t a⁻¹). This decrease, however, is not evenly distributed along the foreset beds. In the western clinoform mean annual storage rates slightly decreased during the last ∼300 years, whereas in the eastern clinoform the mean annual storage rate of the last decade is one-third of the initial value in the late eighteenth century.The variation of clinoform slope angles generally coincides with the local accumulation rates but a variable degree of asymmetry gives some new insights in the future trend of the subaqueous delta development. The sink of sediment in the western clinoform depocentre landward of the inflection point, where shape changes from convex to concave, is producing an increase in concavity.The decline in monsoon precipitation over the last centuries is assumed to have significantly contributed to the decreased mean annual storage rate in the eastern clinoform where the convexity of the clinoform increases seaward of the inflection point. It is very likely that the whole depocentre of the fluvial sediment load is shifting toward the western subaqueous delta and to the Swatch of No Ground Canyon and, thereby, increasing the export to the deep-sea fan.     

Hydrothermal activity on the CV parent body: New perspectives from the giant Transantarctic Mountains minimeteorite TAM5.29

TAM5.29 is an extraterrestrial dust grain, collected on the Transantarctic Mountains (TAM). Its mineralogy is dominated by an Fe‐rich matrix composed of platy fayalitic olivines and clasts of andradite surrounded by diopside‐jarosite mantles; chondrules are absent. TAM5.29 records a complex geological history with evidence of extensive thermal metamorphism in the presence of fluids at T < 300 °C. Alteration was terminated by an impact, resulting in shock melt veins and compaction‐orientated foliation of olivine. A second episode of alteration at lower temperatures (<100 °C) occurred postimpact and is either parent body or terrestrial in origin and resulted in the formation of iddingsite. The lack of chondrules is explained by random subsampling of the parent body, with TAM5.29 representing a matrix‐only fragment. On the basis of bulk chemical composition, mineralogy, and geological history TAM5.29 demonstrates affinities to the CV_ox group with a mineralogical assemblage in between the Allende‐like and Bali‐like subgroups (CV_oxA and TAM5.29 are rich in andradite, magnetite, and FeNiS, but CV_oxA lacks hydrated minerals, common in TAM5.29; conversely, CV_oxB are rich in hydrated phyllosilicates but contain almost pure fayalite, not found in TAM5.29). In addition, TAM5.29 has a slightly different metasomatic history, in between the oxidized and reduced CV metamorphic grades while also recording higher oxidizing conditions as compared to the known CV chondrites. This study represents the third CV‐like cosmic dust particle, containing a unique composition, mineralogy, and fabric, demonstrating variation in the thermal metamorphic history of the CV parent body(‐ies).     

Hercynian subduction-related processes within the metamorphic continental crust in Calabria (southern Italy)

Linking the deformation history of mylonitized continental rocks to the progress of devolatilization reactions that trigger reaction softening is critical for the understanding of crustal scale processes. We have analysed the field geometries and microstructures of deformed rocks within the southern Hercynian belt in Calabria, as well as modelled the pressure–temperature–deformation (P–T–d) trajectory of a main ductile shear zone that tectonically coupled the deeper crustal Mammola Paragneiss Unit with the upper crustal Stilo–Pazzano Phyllite Unit. P–T modelling of the mylonitic Mammola Paragneiss Unit was performed through calculation of phase equilibrium diagrams with the software thermocalc in the MnNCKFMASHTO model system. The prograde P–T–d trajectory is based on the zoning profiles of garnet porphyroblasts and their mineral inclusions, primarily barroisite and epidote. P–T modelling shows that peak metamorphic conditions of ~0.9 GPa and 585°C were reached during a D_n-1 under-thrusting event. The following exhumation during the D_n mylonitic event, and contact metamorphism during D_n+1 and D_n+2 folding events, have also been modelled because they are essential to restore the previous tectono-metamorphic history. The exhumation trajectory was modelled down to 0.3 GPa with temperatures of 440–460°C, under fluid-deficient conditions, as well as the final late Carboniferous contact metamorphism up to T_max of 680–720°C. The prograde path shows clear evidence for thermal buffering during garnet growth at the expense of chlorite, with a heating-dominated stage after chlorite breakdown. Subsequently, a rheological change associated with epidote breakdown (i.e. reaction softening) occurred, highlighted by a net steepening of the P/T trajectory towards the pressure peak. On the basis of the barroisite inclusions within garnet porphyroblasts as well as the ‘hairpin’ shape of the reconstructed P–T–d path (before contact metamorphism), we infer that the unusual low T/P gradient for the Hercynian crust exposed in the Mammola Paragneiss Unit records its involvement in the Palaeotethys–Gondwana subduction beneath Laurussia during D_n-1 under-thrusting. We present a new palaeotectonic interpretation along the southern Hercynian belt in Calabria during the Upper Mississippian–Lower Pennsylvanian, that is consistent with previous geochronology studies.     

Development of garnet porphyroblasts by multiple nucleation,coalescence and boundary misorientation‐driven rotations

Two types of garnet porphyroblast occur in the Schneeberg Complex of the Italian Alps. Type 1 porphyroblasts form ellipsoidal pods with a centre consisting of unstrained quartz, decussate mica and small garnet grains, and a margin containing large garnet grains. Orientation contrast imaging using the scanning electron microscope shows that the larger marginal garnet grains comprise a number of orientation subdomains. Individual garnet grains without subdomains are small (< 50 µm), faceted and idioblastic, and have simple zoning profiles with Ca‐rich cores and Ca‐poor rims. Subdomains of larger garnet grains are similar in size to the individual, small garnet grains. Type 2 porphyroblasts comprise only ellipsoidal garnet, with small subdomains in the centre and larger subdomains at the margin. Each subdomain has its own Ca high, Ca dropping towards subdomain boundaries. Garnet grains, with or without subdomains, all have the same Ca‐poor composition at rims in contact with other minerals. The compositional zonation patterns are best explained by simultaneous, multiple nucleation, followed by growth and amalgamation of individual garnet grains. The range of individual garnet and garnet subdomain sizes can be explained by a faster growth rate at the porphyroblast margin than in the centre. The difference between Type 1 and Type 2 porphyroblasts is probably related to the growth rate differential across the porphyroblast. Electron backscatter diffraction shows that small, individual garnet grains are randomly oriented. Large marginal garnet grains and subdomain‐bearing garnet grains have a strong preferred orientation, clustering around a single garnet orientation. Misorientations across subdomain boundaries are small and misorientation axes are randomly oriented with respect to crystallographic orientations. The only explanation that fits the observational data is that individual garnet grains rotated towards coincident orientations once they came into contact with each other. This process was driven by the reduction of subdomain boundary energy associated with misorientation loss. Rotation of garnet grains was accommodated by diffusion in the subdomain boundary and diffusional creep and rigid body rotation of other minerals (quartz and mica) around the garnet. An analytical model, in which the kinetics of garnet rotation are controlled by the rheology of surrounding quartz, suggests that, at the conditions of metamorphism, the rotation required to give a strong preferred orientation can occur on a similar time‐scale to that of porphyroblast growth.     

Crustal‐mantle lithosphere decoupling as a control on Variscan metamorphism in the Eastern Alps

Mica schists of the Variscan Austroalpine Ötztal basement (Eastern Alps) contain synkinematically grown garnet porphyroblasts showing three zones with complex inclusion trails. Staurolite, kyanite and sillimanite grew over the main schistosity that envelops the garnet porphyroblasts. The mineral‐forming reaction sequences modelled in the MnKFMASH and KFMASH systems show that garnet of zone 1 grew at approximately 1300 MPa, whereas zone 2 and zone 3 grew during substantial decompression and partial thermal relaxation. For the growth of staurolite, kyanite and sillimanite, this modelling shows that thermal relaxation continued after decompression under only slightly changing pressure conditions, until the peak temperature conditions of Variscan metamorphism were reached. The result of this modelling is a pressure‐temperature‐time (P–T–t) heating path that is consistent with a tectonic interpretation implying thickening of the lithosphere during continental collision and subsequent mantle ‐ crustal lithosphere decoupling plus extension. A direct implication of this model is that the heat budget of Variscan metamorphism was controlled by the convective replacement of the thickened lithospheric root with asthenospheric material.     

The petrological significance of misorientations between grains

Misorientation analysis quantifies microstructural features in tectonites, metamorphic and igneous rocks, and allows hypotheses on their formation to be tested. The misorientation between two lattices can be expressed by a rotation axis and rotation angle. For lattices with symmetry, it is conventional to take the minimum angle that enables one lattice to be rotated into the other. For a group of lattice measurements two types of misorientation distribution can be calculated. Selecting random pairs of grains gives the random-pair misorientation distribution. Selecting neighbouring pairs gives the neighbour-pair misorientation distribution. The forms of both distributions are visualised using histograms or cumulative frequency diagrams. They are strongly influenced by any overall crystallographic preferred orientation and by intrinsic crystal symmetry. In many rocks, the random-pair misorientation distribution and neighbour-pair misorientation distribution are statistically significantly different (quantified using the Kolmogorov-Smirnov test). Differences between the random-pair misorientation distribution and neighbour-pair misorientation distribution imply that adjacent grains have physically interacted or are inherited from a precursor microstructure. Interactions include (1) reduction in surface energy by lattice alignment. We show this may have occurred in garnet clusters in schist, and olivine in a cumulate. It is well-known in metals and may be a common geological process. (2) Nucleation, where those nuclei have influenced the orientation of adjacent nuclei. (3) Mechanical rotations of facetted grains in compacting crystal mushes, so that faces become parallel. (4) Growth twinning. Inheritance includes (1) subgrain rotation recrystallisation in tectonites deforming by crystal plastic processes. (2) Mechanical and transformation-related twinning. (3) Domainal microstructures, e.g. where grains have formed from a few large original grains, may give rise to spurious correlations when the orientation data cover more than one domain. With this proviso, misorientation analysis can be used to investigate many important microstructural processes.     

Centimeter-Level Positioning of Seafloor Acoustic Transponders from a Deeply-Towed Interrogator

An array of three seafloor transponders was acoustically surveyed to centimeter precision with a deeply-towed interrogator. Measurements of two-way acoustic travel time and hydrostatic pressure made as the interrogator was towed above the array were combined in a least-squares adjustment to estimate the interrogator and transponder positions in two surveys spanning two years. No transponder displacements were expected at this site in the interior of the Juan de Fuca Plate (48^?11′ N, 127^?12′ W) due to the lack of active faults. This was confirmed to a precision of ±2 cm by least-squares adjustment. Marginally detectable blunders in the observations were shown to affect the transponder position estimates by no more than 3 mm, demonstrating the geometric strength of the data set. The accumulation of many hundreds of observations resulted in a significant computational burden on the least-squares inversion procedure. The sparseness of the normal matrix was exploited to reduce by a factor of 1000 the number of calculations. The acoustic survey results suggested that the near-bottom sound speed fields during the two surveys were in better agreement than inferred from yearly single-profile conductivity, temperature, and pressure (CTD) measurements.