Basal Shear Zone Research Articles

Paleoarchean sulfur cycle and biogeochemical surface conditions on the early Earth, Barberton, South Africa

This study presents the first multiple sulfur isotope dataset on sulfides from the ca. 3.5–3.2 Ga Onverwacht Group in the Paleoarchean Barberton Greenstone Belt (BGB) of South Africa. In situ δ34SCDT and Δ33S values of pyrite (n=568) are reported from a wide range of hydrothermal, volcanic and sedimentary environments and are used to explore Mid-Archean biogeochemical sulfur cycling. Samples are from fresh drill core collected by the Barberton Scientific Drilling Project that intercepted cherts, metabasalts and sheared ultramafics of the ∼3.3–3.35 Ga Kromberg Formation; the sedimentary units of the ∼3.432 Ga Noisy formation; and the unconformably underlying metabasaltic pillow lavas of the ∼3.472 Ga Hooggenoeg Formation.Pyrite in quartz-carbonate-veins in the lower diamictite of the Noisy sequence records the largest range and most negative δ34SCDT values so far reported from an Archean terrain (δ34SCDT=−55.04 to +27.46‰). The Noisy sediments also contain detrital and diagenetic pyrites with a significant variation in Δ33S of between −0.62 to +1.4‰ and δ34SCDT of between −7.00 and +12.6‰, interpreted to reflect tectonic exposure of these early sediments to atmospheric – shallow marine conditions. Early marine pyrites from the Kromberg Cherts also display strong positive Δ33S values up to +2.50‰ with narrow range in δ34SCDT values (−6.00 to +1.50‰), whereas hydrothermal veins in the basal ultramafic shear zones preserve magmatic values (∼0‰). This study reveals a potential proto-tectonic control on atmospheric, geodynamic and hydrothermal environments available for early sulfate reducing and/or methanogenic microbes in the Paleoarchean.No evidence for microbial sulfate reduction or disproportionation was identified in the Kromberg Cherts, despite previous morphological claims for microbial life. Highly variable and negative δ34SCDT values were found in the Noisy turbidites and Hooggenoeg pillow lava breccia supporting the presence of microbial sulfate reduction in early tectono-sedimentary basins and in the Paleoarchean sub-seafloor, respectively. In light of current controversies surrounding sulfur isotope studies in similar-aged rocks of the Pilbara Craton (West Australia), we argue that microbial elemental sulfur disproportionation was not a preferred metabolic pathway on the Paleoarchean earth.

Scale-dependent influence of pre-existing basement shear zones on rift faulting: a case study from NE Brazil

Rifting of continental crust initiates faults that are commonly influenced by pre-existing structures. We document newly identified faults cutting Precambrian units in the interior of the NE Brazilian margin to assess the effects of structural inheritance on both rift geometry and fault architecture. Stratigraphic and structural data indicate that the faults were active in the main phase of rifting of Gondwana. The influence of pre-existing structures on the Mesozoic rift faulting is scale dependent. Regionally, the faults trend parallel to subvertical, crustal-scale Brasiliano ( c . 750–540 Ma) shear zones. Mylonitic foliations and broadly distributed low strain in the lower crust indicated by shear-wave splitting controlled the overall orientation and kinematics of the rift faults. However, outcrop observations of the faults show that at scales up to hundreds of metres, mylonitic foliations have little influence on fault architectures. Faults cross-cut shear zones and do not commonly utilize foliation planes as shear fractures. Instead, slip zones and fractures have a range of orientations that form acute angles to the local foliation orientation. This observation explains the range of focal mechanisms associated with seismicity that coincides with ancient shear zones in intra-continental areas.

Glacitectonic deformation in the Chuos Formation of northern Namibia: implications for Neoproterozoic ice dynamics

Abstract The Chuos Formation is a diamictite-dominated succession of Cryogenian age, variously interpreted as the product of glaciomarine deposition, glacially related mass movement, or rift-related sediment remobilisation in a non-glacial environment. These interpretations have wide ranging implications for the extent of ice cover during the supposedly pan-global Neoproterozoic icehouse. In the Otavi Mountainland, northern Namibia, detailed analysis of soft-sediment deformation structures on the macro- and micro-scale support glacitectonic derivation in response to overriding ice from the south/south-east. Overall, the upward increase in strain intensity, predominance of ductile deformation features (e.g. asymmetric folds, rotational turbates and necking structures, clast boudinage, unistrial plasmic fabrics) and pervasive glacitectonic lamination support subglacial deformation under high and sustained porewater pressures. In contrast, soft-sediment structures indicative of mass movements, including flow noses, tile structures, and basal shear zones, are not present. The close association of subglacial deformation, abundant ice-rafted debris and ice-contact fan deposits indicate subaqueous deposition in an ice-proximal setting, subject to secondary subglacial deformation during oscillation of the ice margin. These structures thus reveal evidence of dynamic grounded ice sheets in the Neoproterozoic, demonstrating their key palaeoclimatic significance within ancient sedimentary successions.

Hummocks: how they form and how they evolve in rockslide-debris avalanches

Hummocks are topographic features of large landslides and rockslide-debris avalanches common in volcanic settings. We use scaled analog models to study hummock formation and explore their importance in understanding landslide kinematics and dynamics. The models are designed to replicate large-scale volcanic collapses but are relevant also to non-volcanic settings. We characterize hummocks in terms of their evolution, spatial distribution, and internal structure from slide initiation to final arrest. Hummocks initially form by extensional faulting as a landslide begins to move. During motion, individual large blocks develop and spread, creating an initial distribution, with small hummocks at the landslide front and larger ones at the back. As the mass spreads, hummocks can get wider but may decrease in height, break up, or merge to form bigger and long anticlinal hummocks when confined. Hummock size depends on their position in the initial mass, modified by subsequent breakup or coalescence. A hummock has normal faults that flatten into low-angle detachments and merge with a basal shear zone. In areas of transverse movement within a landslide, elongate hummocks develop between strike–slip flower structures. All the model structures are consistent with field observations and suggest a general brittle-slide emplacement for most landslide avalanches. Absence of hummocks and fault-like features in the deposit may imply a more fluidal flow of emplacement or very low cohesion of lithologies. Hummocks can be used as kinematic indicators to indicate landslide evolution and reconstruct initial failures and provide a framework with which to study emplacement dynamics.

Volcano-tectonic controls and emplacement kinematics of the Iriga debris avalanches (Philippines)

Mt Iriga in southeastern Luzon is known for its spectacular collapse scar that was possibly created in 1628 ad by a 1.5-km3 debris avalanche. The debris avalanche deposit (DAD) covered 70 km2 and dammed the Barit River to form Lake Buhi. The collapse has been ascribed to a non-volcanic trigger related to a major strike-slip fault under the volcano. Using a combination of fieldwork and remote sensing, we have identified a similar size, older DAD to the southwest of the edifice that originated from a sector oblique to the underlying strike-slip fault. Both deposits cover wide areas of low, waterlogged plains, to a distance of about 16 km for the oldest and 12 km for the youngest. Hundreds of metre-wide and up to 50-m-high hummocks of intact conglomerate, sand and clay units derived from the base of the volcano show that the initial failure planes cut deep into the substrata. In addition, large proportions of both DAD consist of ring-plain sediments that were incorporated by soft-sediment bulking and extensive bulldozing. An ignimbrite unit incorporated into the younger DAD forms small (less than 5 m high) discrete hummocks between the larger ones. Both debris avalanches slid over water-saturated soft sediment or ignimbrite and spread out on a basal shear zone, accommodated by horst and graben formation and strike-slip faults in the main mass. The faults are listric and flatten into a well-developed basal shear zone. This shear zone contains components from the substrate and has a diffuse contact with the intact substrata. Long, transport-normal ridges in the distal parts are evidence of compression related to deceleration and bulldozing. The collapse orientation and structure on both sectors and DAD constituents are consistent with collapse being a response to combined transtensional faulting and gravity spreading. Iriga can serve as a model for other volcanoes, such as Mayon, that stand in sedimentary basins undergoing transtensional strike-slip faulting.

Localization of deformation and kinematic shift during the hot emplacement of the Ronda peridotites (Betic Cordilleras, southern Spain)

The Ronda peridotites form the largest mass of subcontinental mantle outcropping on land. Unlike other orogenic lherzolite massifs, the two main bodies of Ronda (the Sierra Bermeja and Sierra Alpujata massifs) are unique cases where ductile shear zones linked to the hot thrusting of mantle over continental crustal rocks are well exposed. We present a new insight into the deformation localization in these shear zones based on structural, fabric and petrological data. The Ronda peridotites show increasing deformation towards the continental footwall rocks, from porphyroclastic rocks to ultramylonites. Garnet-pyroxenites from the basal shear zone of the Alpujata massif yield ca. 1100°C and 1.4GPa for the mylonitization. Such conditions promoted partial melting and the formation of felsic dynamothermal aureoles from the underlying crustal rocks. Subsequent deformation is mainly localized in the dynamothermal aureoles, since they are weaker than the peridotites. Both aureoles show marked strain gradients towards the contact but record different kinematics. In Sierra Alpujata, kinematic criteria indicate a top-to-the ENE shear sense, whereas in Sierra Bermeja the felsic mylonites provide a top-to-the NNW motion. A transpressional setting is proposed to explain such kinematic shift.

Mechanisms of margin inversion in the external Western Alps: Implications for crustal rheology

The kinematics of the collisional crustal wedge in the external Western Alps is discussed in the light of new cross sections of the whole external zone at the latitude of the Belledonne–Grandes Rousses–Oisans massifs (External Crystalline Massifs, ECM), as well as a detailed analysis of the deformation of their basement and cover. The cross sections were built from new field data and published geological maps, and were restored through time (Oligo-Miocene times) to unravel the successive stages of the Dauphinois margin contractional deformation. During Oligocene times, the Variscan basement was deformed essentially by greenschist facies Alpine distributed shear zones with no significant reactivation of the inherited Jurassic normal faults or the Variscan foliation. However, the inherited syn-rift basins localized the deformation characterized by a thick-skinned style and a cover disharmonically folded over basement shear zones, with no major décollement in between (shortening of 11.5km, 20%). Those early deformations progressively localized on the frontal crustal ramp and caused shortening in the folded belt (16.5km of shortening, 23%). A similar sequence of deformation has been inferred in other Alpine ECMs. This strongly suggests that the crust of Dauphinois proximal passive margin was thin enough, and its buoyancy low enough (as a result of the Liassic rifting), to experience significant tectonic burial during the Tertiary collision. As a consequence of a weakening by both the presence of inherited basins (and their weak syn-rift sedimentary rocks) and the P–T conditions (greenschist facies), the crust was shortened and the structural style was thick-skinned.

Two‐stage evolution of the Altyn Tagh Fault during the Cenozoic: new insight from provenance analysis of a geological section in NW Qaidam Basin, NW China

Terra Nova, 24, 387–395, 2012AbstractWe analysed provenance of conglomerate in a geological section spanning late Eocene to Pliocene using the gravel‐count method in the NW Qaidam Basin, NW China. The result indicates that the clasts were all derived from the Altyn Mountain to the north with a sudden alteration at ca. 15 Ma, and supports a two‐stage evolution model for the Cenozoic Altyn Tagh Fault (ATF) highlighting the history prior to the initiation of left‐slip movement on the ATF. During the first stage of ca. 36–15 Ma, the ATF was a basal shear zone confined in the middle – lower crust and created obvious uplift of the Altyn Mountain. During the second stage since ca. 15 Ma, the crust beneath the Altyn Mountain was eventually split, and left‐slip movement on the ATF then initiated.

Landslide Stability: Role of Rainfall-Induced, Laterally Propagating, Pore-Pressure Waves

The Johnson Creek Landslide is a translational slide in seaward-dipping Miocene siltstone and sandstone (Astoria Formation) and an overlying Quaternary marine terrace deposit. The basal slide plane slopes sub-parallel to the dip of the Miocene rocks, except beneath the back-tilted toe block, where it slopes inland. Rainfall events raise pore-water pressure in the basal shear zone in the form of pulses of water pressure traveling laterally from the headwall graben down the axis of the slide at rates of 1–6 m/hr. Infiltration of meteoric water and vertical pressure transmission through the unsaturated zone has been measured at ∼50 mm/hr. Infiltration and vertical pressure transmission were too slow to directly raise head at the basal shear zone prior to landslide movement. Only at the headwall graben was the saturated zone shallow enough for rainfall events to trigger lateral pulses of water pressure through the saturated zone. When pressure levels in the basal shear zone exceeded thresholds defined in this paper, the slide began slow, creeping movement as an intact block. As pressures exceeded thresholds for movement in more of the slide mass, movement accelerated, and differential displacement between internal slide blocks became more pronounced. Rainfall-induced pore-pressure waves are probably a common landslide trigger wherever effective hydraulic conductivity is high and the saturated zone is located near the surface in some part of a slide. An ancillary finding is apparently greater accuracy of grouted piezometers relative to those in sand packs for measurement of pore pressures at the installed depth.

Breaking Up: Comminution Mechanisms in Sheared Simulated Fault Gouge

The microstructural state and evolution of fault gouge has important implications for the mechanical behaviour, and hence the seismic slip potential of faults. We use 3D discrete element (DEM) simulations to investigate the fragmentation processes operating in fault gouge during shear. Our granular fault gouge models consist of aggregate grains, each composed of several thousand spherical particles stuck together with breakable elastic bonds. The aggregate grains are confined between two blocks of solid material and sheared under a given normal stress. During shear, the grains can fragment in a somewhat realistic way leading to an evolution of grain size, grain shape and overall texture. The ‘breaking up’ of the fault gouge is driven by two distinct comminution mechanisms: grain abrasion and grain splitting. The relative importance of the two mechanisms depends on applied normal stress, boundary wall roughness and accumulated shear strain. If normal stress is sufficiently high, grain splitting contributes significantly to comminution, particularly in the initial stages of the simulations. In contrast, grain abrasion is the dominant mechanism operating in simulations carried out at lower normal stress and is also the main fragmentation mechanism during the later stages of all simulations. Rough boundaries promote relatively more grain splitting whereas smooth boundaries favor grain abrasion. Grain splitting (plus accompanying abrasion) appears to be an efficient mechanism for reducing the mean grain size of the gouge debris and leads rapidly to a power law size distribution with an exponent that increases with strain. Grain abrasion (acting alone) is an effective way to generate excess fine grains and leads to a bimodal distribution of grain sizes. We suggest that these two distinct mechanisms would operate at different stages of a fault’s history. The resulting distributions in grain size and grain shape may significantly affect frictional strength and stability. Our results therefore have implications for the earthquake potential of seismically active faults with accumulations of gouge. They may also be relevant to the susceptibility of rockslides since non-cohesive basal shear zones will evolve in a similar way and potentially control the dynamics of the slide.

Geomorphologic features related to gravitational collapse: Submarine landsliding to lateral spreading on a Late Miocene–Quaternary slope (SE Crete, eastern Mediterranean)

Detailed geological mapping, complemented by sedimentary facies analysis, photomosaics and topographic measurements (height, width) allowed the investigation of the geomorphological features related to the Late Miocene–Quaternary gravitational collapse of a palaeoslope located in SE Crete (eastern Mediterranean). In the study area, carbonate megablocks indicative of submarine landsliding during the Late Miocene alternate with collapse features more typical of subaerial settings; the latter generated after a major event of tectonic uplift initiated in Crete during the Early-mid Pliocene. Submarine features typically show basal shear zones, rather than planes, generated in near-seafloor strata deformed in ductile form during the gravitational collapse of the megablocks. The lithology of failed carbonate strata differs from that of their basal shear surfaces, a characteristic providing a reliable estimate for the degree and styles of basal deformation during submarine slope instability. Styles of submarine collapse include, by order of magnitude; (i) lateral spreading of fractured segments of fan cones and carbonate sheet flows, eventually transported 100s of metres downslope; (ii) aperture of ravines and chasms in gravitationally unstable fan cones and boulder conglomerates; (iii) gliding of megablocks over a ductile basal layer through a distance of up to several kilometres; and (iv) rolling of subcircular blocks, often within a debris-flow matrix in fan cones and deltas, or embedded in slope siliciclastic strata. This work highlights the existence of prominent 2–10 m basal shear zones in strata underneath the larger megablocks deposited on marine slope strata. Basal shear zones comprise a melange of reworked conglomerates and breccia clasts from overlying megablocks, large ripped blocks of rock and faulted near-seafloor strata, at places showing remnant beds and sand injection features. Consequently, the outcrop data show an average 5:1 ratio between the maximum observed thickness of megablocks and the thickness of basal shear zones ( R), a value of similar magnitude to published examples from offshore landslides.

The relationship between particle characteristics and frictional strength in basal fault breccias: Implications for fault-rock evolution and rockslide susceptibility

Microstructural analysis of fault-rock breccias along the Basal Shear Zone (BSZ) of potential rock-slope failures in Norway demonstrates that particle-size distribution curves show a spectrum from particle- to matrix-supported. We demonstrate that variation in residual shear strength from 450 to 700 kPa is directly related to particle size and shape characteristics. We also show that these characteristics are dependent on the fault-rock evolution and breakdown mechanism and demonstrate this with ‘simulated’ examples. Particle-supported fault rocks have higher residual strength than matrix-supported samples. As the amount of fine material increases, the residual shear strength of the fault material decreases. Particle aspect ratio is variable suggesting different comminution mechanisms and fault-zone evolution. As particle size decreases below 10–20 μm, particle aspect ratio approaches 1:1, suggesting that as the fault-rock evolves the particles become rounder. As aspect ratio increases, fault strength decreases. We contend that the susceptibility of potential rock-slope failures is fundamentally linked to and depends partly on the microscopic properties of the underlying fault rocks and is linked to their comminution history.

Magnetic susceptibility as a tool to study deformed calcite with variable impurity content

A set of calcite mylonites were systematically sampled along the basal shear zone of the Morcles Nappe, located in the southwestern Swiss Alps, to investigate the relationship among crystallographic preferred orientation (CPO), chemistry, and anisotropy of magnetic susceptibility (AMS). Second‐phase minerals are ≤10 vol % of the rock. The substitution of trace impurities for Ca in the calcite crystal lattice, in particular, Fe, Mn, Mg, and Sr, varies among the samples with total values ranging from 1450 ± 190 ppm in the most pure sample to 2450 ± 320 ppm for the most impure sample. Fe shows the largest variation among the sample group and varies by a factor of nearly four from the purest (230 ± 30 ppm) to the most impure sample (840 ± 110 ppm). All samples have negative bulk susceptibility, and the concentration of ferromagnetic (in a broad sense) phases is very low. A correlation is identified between trace element chemistry, CPO of calcite, and the magnetic susceptibility. The bulk susceptibility and the AMS vary systematically with Fe and Mn elemental concentration in the matrix calcite, and this variation is apparent for the entire length of the shear zone. A simple model that takes into account the CPO and trace element chemistry is used to explain the resulting bulk susceptibility and AMS. Calcite mylonites and marbles from other locations fit the model, indicative of a general relationship between CPO, chemistry, and AMS in calcites where second‐phase minerals are sparse. For lithologies that are dominated magnetically by calcite, with a deficiency of secondary phases, magnetic susceptibility can be used to rapidly screen rock samples for the variation of paramagnetic trace elements Fe and Mn and CPO, which can then be used for detailed microfabric studies.

Successive structural events in the Hatay ophiolite of southeast Turkey: Distinguishing oceanic, emplacement and post-emplacement phases of faulting

New structural data are presented and interpreted for the Cretaceous Hatay ophiolite, which was emplaced onto the Arabian continental margin during latest Cretaceous (Maastrichtian) time. Although lacking a preserved metamorphic sole, a thick serpentinitic basal shear zone provides evidence of emplacement kinematics, whilst structures developed in an extensive Cenozoic sedimentary cover help to constrain the nature of post-emplacement deformation. Combining field structural measurements with palaeostress analyses enables several discrete structural events to be recognised, of which three appear to pre-date the Cenozoic sedimentary cover. The earliest event involved extension parallel to the ophiolitic sheeted dyke complex and relates to oceanic spreading whereas the second and the third events relate to compression and strike-slip during intra-oceanic displacement, or emplacement of the ophiolite onto the Arabian continental margin. Some structures at the base of or within the ophiolite that were previously attributed to sea-floor spreading processes (e.g. “oceanic detachment faults”) are more likely to have formed during subsequent ophiolite displacement/emplacement and post-emplacement formation of the regional Hatay Graben.

Structural events and metamorphic consequences in Almora Nappe, during Himalayan collision tectonics

Almora Nappe in Uttarakhand, India, is a Lesser Himalayan representative of the Himalayan Metamorphic Belt that was tectonically transported over the Main Central Thrust (MCT) from Higher Himalaya. The Basal Shear zone of Almora Nappe shows complicated structural pattern of polyphase deformation and metamorphism. The rocks exposed along the northern and southern margins of this nappe are highly mylonitized while the degree of mylonitization decreases towards the central part where the rocks eventually grade into unmylonitized metamorphics. Mylonitized rocks near the roof of the Basal Shear zone show dynamic metamorphism (M 2) reaching upto greenschist facies (∼450 °C/4 kbar). In the central part of nappe the unmylonitized schists and gneisses are affected by regional metamorphism (M 1) reaching upper amphibolite facies (∼4.0–7.9 kbar and ∼500–709 °C). Four zones of regional metamorphism progressing from chlorite–biotite to sillimanite–K-feldspar zone demarcated by specific reaction isograds have been identified. These metamorphic zones show a repetition suggesting that the zones are involved in tight F 2 – folding which has affected the metamorphics. South of the Almora town, the regionally metamorphosed rocks have been intruded by Almora Granite (560 ± 20 Ma) resulting in contact metamorphism. The contact metamorphic signatures overprint the regional S 2 foliation. It is inferred that the dominant regional metamorphism in Almora Nappe is highly likely to be of pre-Himalayan (Precambrian!) age.

Onset and timing of deep-seated gravitational slope deformations in the eastern Alps, Italy

A previously unrecognised complex of deep-seated gravitational slope deformations (DSGSD) extends 20 km along upper Venosta Valley in the eastern Alps, Italy. Mt. Watles exhibits spectacular features indicative of DSGSDs including double ridges, trenches, and counterscarps. The slope deformation occurs along a nappe boundary now corresponding to the Schlinig normal fault, is also conditioned by recent faults marked by shallow earthquakes, and shows glacial/paraglacial controls on slope evolution. The Mt. Watles DSGSD is a complex, deep-seated, compound slide along a basal shear zone, involving the Schlinig fault. Gravitational reactivation of NE-trending fractures formed gravitational scarps, counterscarps, and half-grabens in the upper slope, whereas the lower part of the slope partially collapsed. Although our work indicates the importance of the Schlinig fault and recent fracturing on slope failure, the trigger seems to be postglacial debuttressing of the valley walls. Radiocarbon dating of peat deposits within one of the major counterscarps indicates that slope deformation started during the Lateglacial period and continued during the Holocene in several slope sectors.

Geological model of the Åknes rockslide, western Norway

Åknes is known as the most hazardous rockslide area in Norway at present, and is among the most investigated rockslides in the world, representing an exceptional natural laboratory. This study focuses on structural geology and the usage of geophysical methods to interpret and understand the structural geometry of the rockslide area. The interpretations are further used to build a geological model of the site. This is a large rockslide with an estimated volume of 35–40 million m 3 (Derron et al., 2005) defined by a back scarp, a basal shear zone at about 50 m depth and an interpreted toe zone where the sliding surface daylights the surface. The rockslide is divided into four sub-domains, experiencing extension in the upper part and compression in the lower part. Structural mapping of the area indicates that the foliation of the gneiss plays an important role in the development of this rockslide. The upper boundary zone of the rockslide is seen as a back scarp that is controlled by, and parallel to, the pre-existing, steep foliation planes. Where the foliation is not favourably orientated in regard to the extensional trend, the back scarp follows a pre-existing fracture set or forms a relay structure. The foliation in the lower part, dipping 30° to 35° to S–SSE, seems to control the development of the basal sliding surface with its subordinate low angle trust surfaces, which daylights at different levels. The sliding surfaces are sub-parallel to the topographic slope and are located along mica-rich layers in the foliation. Geophysical surveys using Ground Penetrating Radar (GPR), refraction seismic and 2D resistivity profiling, give a coherent understanding of undulating basal sliding surface in the subsurface. The geophysical surveys map the subsurface in great detail to a depth ranging from 30–40 m for GPR to approximately 125 m for refraction seismic and 2D resistivity profiling. This gives a good control on the depth and lateral extent of the basal sliding surface, and its subordinate low angle thrusts. Drill cores and borehole logging add important information with regard to geological understanding of the subsurface. Fracture frequency, fault rock occurrences, geophysical properties and groundwater conditions both in outcrops and/or drill cores constrain the geometrical and kinematic model of Åknes rockslide.

Lithospheric-Scale Transtension between Vertical, Non-parallel, Planar Zone Boundaries

Strain distribution in crustal-scale transtensional zones with non-parallel zone boundaries is heterogeneous, with higher strain gradients in the narrower parts of the zone. Kinematic modeling of such zones shows that, for a constant displacement vector and finite displacement of the zone boundary, the finite strain achieved at a point is dependent on its initial position within the zone. These heterogeneities are governed only by the geometry of the deforming zone; rheologic heterogeneity adds to the departure from homogeneous finite and instantaneous strain. The finite strain distribution can be deduced by determining the instantaneous strain and how it changes both spatially and temporally. In zone geometries where there is a prolate (bouncing) point, marking a change from a horizontal principal finite shortening direction (and therefore vertical foliation) to a vertical principal finite shortening direction (horizontal foliation), it separates areas of wrench- and extension-dominated transtension. This causes spatial partitioning of the strain without any necessity for inherited fabric anisotropies. Migration of this point results in polyphase deformation and, if initial foliation development results in sufficient mechanical anisotropy, overprinting of fabric orientation. Whereas the formation of non-parallel-walled shear zones is promoted by existing structures such as conjugate basement faults and shear zones, the complications predicted in this model result from considering only kinematic arguments and are made more complex by existing heterogeneities found in real rocks.

Two-dimensional apparent microfabric of the basal Late Weichselian till and associated shear zone: case study from western Latvia

The examination of glacial sediments in thin sections has become a common procedure in recent years. Apparent sand grain orientation (microfabric) in thin sections is one of the key elements marking certain microstructures. In an attempt to make till micromorphology studies less subjective and investigate the orientation of sand-sized particles in tills, we have developed an image analysis procedure to measure and analyse the spatial distribution of the till microfabric. We studied 13 thin sections of the Weichselian subglacial till and basal shear zone outcropping in the Baltic Sea bluffs at the Ziemupe site in western Latvia. The results were visualized as a two-dimensional grid of rose diagrams covering the area of the thin section and were compared to macrofabric. We found that in larger areas microfabric, although much weaker than macrofabric, coincides with macrofabric orientation. In the sub-centimetre scale till microfabric in short distances appears to be highly variable both in strength and preferred orientation, and a domain-like pattern appears.

Hydrogeologic evidence for a continuous basal shear zone within a deep-seated gravitational slope deformation (Eastern Alps, Tyrol, Austria)

The currently inactive deep-seated gravitational slope deformation (DGSD) Sagspitz in Tyrol, Austria, covers an area of 3 km2 and originated along a glacially over-steepened slope composed mainly of phyllites belonging to the Innsbruck Quartzphyllite Complex. Past mass movement processes caused the formation of fracture systems and slump bodies which consequently form the aquifers of relatively large springs in a rock type with generally very low permeability and poor porosity. Analysis of water chemistry, oxygen isotope, and field parameters of the springs emerging from the DGSD imply that multilevel aquifers exist and enabled the detection of continuous deep flow paths through the entire length of the mass movement from the main scarp to the toe of the slope. It is shown that the aquifer boundaries and spring emergences throughout the loosened rock mass can be correlated to the internal and basal shear zones of the DGSD so that this hydrogeological approach proved to be useful for evaluating the mass movement structure.