ALBERT

Description: We undertake kinematic modelling to explore the role of volume increase in a slab extruding froman orogenic wedge with constant or decreasing slab width. Using a dilatancy term, we modify the velocity gradient tensor dependent on the stretching-rate factor, kinematic dilatancy and vorticity number. We use this to explore the previously largely ignored role of volume change in kinematic evolution of extrusive flow, considering area change for non-isochoric flow types with no deformation in the intermediate direction. By keeping individual parameters constant for geologically simple scenarios (e.g. finite strain, steady-state flow) we examine the interdependence of the reciprocal parameters (kinematic vorticity and dilatancy number) and note model situations where degrees of freedom are limited. These interdependent parameters thereby provide a set of rules for integrating and modelling real field data. In particular we observe that for extrusion flow with a constant slab (or channel') width, degrees of freedom in kinematic vorticity and volume change at given finite strains are very restricted. We compare scenarios of low and high strain and low and high volume change on anatexis (related to partial melting of fertile sedimentary rocks and release of water upon crystallization) for different parts of the Himalaya.

Description: This review of Mediterranean geodynamics highlights that the Mediterranean region captures at a fortuitous moment in time, a picture of the fate of foundering, old, cold oceanic lithosphere of limited area due to being landlocked in an all-but-stalled continental collision (Africa with Europe). We synthesize the geological spatial and temporal data for stretched crust as well as the 3D distribution of old abyssal plains and new oceanic lithosphere segments in concert with heat flow, palaeomagnetic data, geodetic velocity data, earthquake hypocentre distributions and seismic tomography. We use three Mediterranean subduction system settings (the western Mediterranean, the Hellenic and the Pannonian-Carpathian) that nicely reflect the slab instability and retreat. We assume that mantle slab dynamics best explains the observations. The dispersal and segmentation of the foundering landlocked ocean results in a series of discontinuous subduction zones whose individual lengths gradually diminish while retreat accelerates as slab progressively narrow and tear (i.e. along-strike laterally-propagating slab break-off) due to imminent total consumption of available oceanic lithosphere. We suggest that the Mediterranean region offers a lucid series of snapshots of accelerated slab retreat that, additionally, is globally unique as the only present day example of what we term intra-collisional landlocked ocean subduction.

Description: Cylindrical structures, cross-cutting stratification at right angles, occur in the Muth Formation, representing Lower Devonian barrier island arenites of the North Indian Gondwana coast. These structures are up to 1.5 m in height and 0.8 m in diameter, with an internal structure comprising concentric, cylindrical laminae. The pipes, which probably represent water conduits for laminar upward flow of ground water, initiate from relatively thin horizons, with upper terminations formed by spring pits. Thus, the structures in the Muth Formation represent a rarely observed combined occurrence of spring pits and their conduits below. Their formation is explained by rising ground water seepage in a coastal depositional environment that produced a relatively high hydrostatic head, resulting in the formation of springs. The rise in relative sea level might be related to tectonic subsidence caused by tectonic activity linked to the formation of conjugate deformation bands in the Muth Formation. This means, if tectonic activity was involved, it did not form the cylindrical structures by seismic liquefaction directly, but might be responsible indirectly through ground water seepage rise resulting from tectonic subsidence. Due to the little relief in this environment, the sea level rise affected a relatively large area and fluidization structures can be found widespread in distant sections.

Description: In situ ultraviolet (UV) laser-ablation 40 Ar/ 39 Ar dating, microstructural analysis, and stable O, H, and C isotope analyses were performed on white mica–bearing calcite– and quartz–mica schists of the West Cycladic detachment system footwall in order to resolve outstanding uncertainties about the timing of deformation and the role of rock rheology on 40 Ar/ 39 Ar dating systematics. In both quartz-rich and calcite-rich samples, deformed and chemically zoned white micas form two chemical populations: (1) a high component of Al-celadonite in undeformed portions of grains (high-pressure remnants), and (2) enrichment in muscovite in deformed portions (low-pressure neocrystallization). Micas in the quartz-rich rocks record higher internal strain, illustrated by elongated, sheared grains and boudinaged mica-fish structures. In this lithology, quartz formed a load-bearing framework that transferred strain to the muscovite packets and facilitated the formation of mica-fish structures. Recrystallization was promoted by coeval fluid infiltration, supported by stable isotope analyses and indented boundaries on bulging quartz grains. In rocks containing calcite-muscovite aggregates, the calcite formed an interconnected weak layer, with strain being accommodated by dislocation creep. In these rocks, micas were only partially neocrystallized. Prismatic white micas, largely unaffected by boudinage or kinking, yielded 40 Ar/ 39 Ar ages that are up to 10 m.y. older than deformed (kinked or sheared) portions of the same grains. Overall, the ages attest to strong lithological control on deformation- and fluid-controlled white mica neocrystallization. The oldest, undeformed grain ages in the calcite-rich rocks are consistent with the timing of Eocene metamorphism, with the deformed grain ages interpreted as representing the transition to lower-pressure conditions during nascent extension. Completely neocrystallized grains in the quartz-rich rocks are interpreted as defining the minimum age of Miocene ductile extension along the detachment system. The new data show the power of combining in situ laser-ablation 40 Ar/ 39 Ar dating, microstructural analysis, mineral chemistry, and stable isotope data for unraveling the timing and time scales of complex deformation histories.

Description: We examined cataclastic shear bands (CSB) with varying degrees of deformation and alteration that formed in uncemented, arkosic sediments under identical kinematic conditions. The investigated outcrop in eastern Austria exposes numerous closely spaced sets of CSB formed at low burial depth. The uncemented host sediment consists of detrital quartz, albite, micas, and metamorphic lithoclasts. We distinguished three types of CSB, which differ in macroscopic and microscopic properties as well as in influence on fluid flow (i.e., single bands, multistrand bands, and band clusters). All band types show preferred fracturing of sericited albite grains and decomposition of biotite through mechanical deformation and subsequent chemical alteration. These mechanisms reduce the mean grain size, increase the amount of phyllosilicates in the matrix, and facilitate later growth of authigenic clay minerals. The dominant deformation mechanisms and influence on fluid flow are controlled by the initial composition and intensity of diagenetic alteration. We identified different evolutionary stages from a high-porosity host rock ( $$\hbox{ porosity }[\mathrm{\Phi }]=35\%$$ ) to a deformation band cluster ( $$\mathrm{\Phi }=6\%$$ ) that acts as fluid baffle. The measured reduction in porosity of up to 29% is reflected by retention of fluids along band clusters, along multistrand bands, and between intersecting bands. The timing and direction of the specific fluid flows can be determined by the interaction with the deformation bands. These findings suggest that localized deformation and associated diagenetic alteration in feldspar-bearing sediments may promote reservoir compartmentalization.

Description: Interpreting isotopic ages as deformation ages when they are acquired from moderate-temperature metamorphic environments can be a challenging task. Syros Island (Cyclades, Greece) is famous for Eocene high-pressure metamorphic rocks reworked by localized Miocene greenschist-facies deformation. In this work, we investigate phengites from coarse-grained marbles, which experienced the high-pressure event, and phengites from fine-grained localized marble shear zones attributed to the low-grade Miocene deformation. Based on structural criteria, both events can be easily discriminated because of their opposing kinematics. Laser-heating 40 Ar/ 39 Ar analysis on phengite yielded a 40 ± 1.6 Ma age for the host rock and a 37 ± 1.3 Ma age for the shear zone. Both ages are statistically indistinguishable, consistent with the regional Eocene event, and not the Miocene deformation event responsible for the formation of the shear zone. Thermodynamic modeling indicates that the observed high-variance mineral assemblage is stable without compositional change along the pressure-temperature path followed by the rocks of Syros. Although the marble within the shear zone was deformed at extremely fast strain rates (10 –10 s –1 ), we observed no intracrystalline deformation of phengite grains and no resetting in the isotopic system, because strain was mostly accommodated by calcite. Consequently, a high strain rate does not necessarily create deformation ages in rocks with high-variance assemblages, such as marble mylonites.

Description: Self-noise estimates from spectral correlation analysis of ambient noise data recorded synchronously by three accurately coaligned broadband seismometers have become a de facto standard for comparison of instruments and assessing their performance. Nonetheless, many of the self-noise spectra published suffer a spectral disturbance within a limited range of frequencies (0.1–1 Hz), corresponding to that of Earth’s secondary microseisms. In this work, we present a method to improve self-noise estimates by numerically rotating recorded traces three-dimensionally prior to self-noise computation. This increases signal coherency among the three sensors’ recorded traces, which, primarily due to errors in installation and manufacturing limitations in regard to orthogonality of their sensing axes, in most cases are imperfectly aligned during the experiment. Rotating seismic traces by small angles has a strong effect on self-noise spectra obtained by coherency analysis predominantly in the aforementioned frequency range. We use this effect to determine optimal angles of alignment between two sensors for each of their components by searching for minimum self-noise during incremental trace rotation. The technique is applied to data recorded by four STS-2 sensors during a long-term self-noise monitoring experiment at the Conrad Observatory (Austria), and, for vertical components in the majority of cases, it results in complete removal of the self-noise disturbances. For horizontal traces, a similar effect can be observed, but self-noise spectra often show additional, long-period disturbances, which are broader in frequency range and insensitive to rotation of seismic traces. Our results further indicate that the method is able to reproduce inaccuracies in sensor alignment as small as 1/100 of a degree. For vertical components, angular resolutions on the order of two millidegrees have consistently been found, potentially rendering the method a means to verify alignment already during the preparation phase of self-noise experiments.

Description: We use a three-dimensional finite element model for viscous flow to investigate the lateral linkage mode of two initially isolated fold segments. Depending on the separation distance between the fold hinge lines, four different fold linkage scenarios are observed. Linear linkage yields a sub-cylindrical fold with a saddle at the location where the two initial folds linked. Oblique linkage produces a fold resembling a type II refold structure. Oblique–no linkage results in two curved folds with fold axes plunging in opposite directions. Linear–no linkage yields a fold train of two separate subcylindrical folds with fold axes plunging in opposite directions. Representative numerical models for detachment and matrix folding for both power-law and linear viscous rheologies exhibit the same four fold linkage modes. The transition from linkage to no linkage occurs when the fold separation between the initially isolated folds is slightly larger than one-half of the low-amplitude fold wavelength. We demonstrate a natural example of lateral fold linkage from the Kurdistan region of Iraq, and point out the economic interest of saddle points linking the initially isolated fold segments.