Plumose Structures Research Articles

Fracture morphology of desiccation cracks in clayey soil

Desiccation cracks compromise soil integrity and weaken its strength, causing a range of detrimental consequences across various domains. Elucidating the cracking mechanism can aid in managing crack propagation and mitigating the associated risks. This study monitored and compared the evolution of crack patterns on the soil surface and fracture morphologies on the soil cross-section during the drying process using a tested soil sample. Multiple fracture morphological features are discerned on the soil cross-section, encompassing initiation points and plumose structures. Soil fracture morphologies are categorized into three cases based on the initiation point's location, referred to as “Top-initiated structure”, “Bottom-initiated structure”, and “Truncated structure”. Experimental results demonstrate that plumose structures result from the division of the crack front under mixed-mode loading. Cracking under mixed-mode I + II loading leads to cross-section tilting, resulting in curved surface cracks. Conversely, cracking under mixed-mode I + III loading causes cross-section twisting, generating hackle lines and step structures. Furthermore, the crack front radiates from the initiation point, creating orthogonal hackle lines. The geometric relationship confirms that the soil fracture morphology is a good indicator of the cracking process, both in laboratory tests and field observations.

Anatomy of fractures: Quantifying fracture geometry utilizing X-ray computed tomography in a chalk-marl reservoir; the Lower Cretaceous Valdemar Field (Danish Central Graben)

Naturally fractured reservoirs tend to show heterogeneous permeability with open fractures enhancing the inherent diagenetic and depositional matrix fabric. Natural open fractures may function as pathways for fluid flow in tight (low matrix permeability) carbonate reservoirs and have a significant impact on the directions, magnitudes and heterogeneities of fluid flow and reservoir rock strength. Open fractures may however also facilitate the invasion of formation water while fractures that are partially or completely cemented can act as barriers to fluid flow. As a result, natural fractures significantly impact reservoir flow rates, anisotropy, storage, connectivity, recovery, and production. X-ray Computed Tomography (CT) offers a non-destructive technique to evaluate petrophysical parameters, such as porosity and permeability, and to assess the geometry, connectivity, and density of natural fractures, which are important constituents of chalk reservoirs. The purpose of this study is to investigate different types of naturally occurring fractures within the Lower Cretaceous succession of the Valdemar Field in the Danish North Sea. Six samples of core displaying at least one natural fracture on the surface are imagined; two samples of open fractures with plumose structure and hackle marks, three samples of shear fractures and finally one sample with deformation bands. The samples are imaged by CT and internal fractures are manually mapped. Image analysis provides detailed information on the fracture geometry in addition it indicates the presence of numerous microfractures hitherto not recorded in the Lower Cretaceous of the Danish North Sea. Core data show that fracture intensity can be significantly greater than calculated by conventional core interpretation techniques based on core surface observations. Further, apertures of both major and minor fractures are measured in CT images of all samples to determine if specific fracture types display characteristic that can distinguish them from other fracture types. As open fracture intensity increases, so too does permeability, likely enhancing reservoir connectivity on the field scale.

Natural fractures and discontinuities in a Lower Cretaceous chalk-marlstone reservoir, Valdemar Field, Danish North Sea

Natural fractures and discontinuities have significant impact on subsurface flow conditions and thus production, of carbonate reservoirs, particularly in low-permeability sediments such as chalk-marl successions characterizing the Lower Cretaceous reservoirs in the Danish North Sea. Yet the diversity and distribution of the fractures are often poorly understood and largely underestimated. In heterogeneous, tight carbonate reservoirs, natural fractures can enhance permeability, as well as create secondary porosity and promote connectivity between reservoir compartments. The Valdemar Field, Danish Central Graben, represents the only producing field from the Lower Cretaceous in the Danish sector of the North Sea. The main reservoirs are confined to the Tuxen and Sola Formations. A new reservoir zonation is proposed based on facies characteristics and fracture patterns to consist of the lower Tuxen, middle-upper Tuxen, lower-middle Sola and upper Sola units with the Munk Marl Bed and Fischshiefer Member forming major reservoir barriers between the lower and the middle-upper Tuxen, and the lower-middle and the upper Sola units, respectively.The reservoir intervals are of heterogeneous nature and composed of interbeds of five main facies comprising chalk, slightly marly chalk, marly chalk, chalky marlstone and marlstone. Six types of natural fractures and discontinuities are identified in the Valdemar Field based on core studies: cemented fractures, deformation bands, open fractures with plumose structures and hackle marks, shear fractures, small-offset shear fractures and rubble zones. The most dominant fracture type within all facies is the open fractures with plumose structure and hackle marks followed by small-offset shear fractures, shear fractures and rubble zones. Cemented fractures and deformation bands are less dominant. The small-offset shear fractures, shear fractures and open fractures with plumose structures and hackle marks are flow enhancing, while the cemented fractures and deformation bands are neutral or flow reducing. Rubble zones are also recorded throughout the core material. If these represent naturally fractured zones, present under subsurface conditions, they would be strongly flow enhancing. The flow-enhancing natural fractures (open fractures, shear fractures and small offset shear fractures) have densities of 7.2/m in the chalk, 5.0/m in slightly marly chalk, 3.1/m in marly chalk, 4.8/m in the chalky marlstone while they are absent in the marlstone. The flow-enhancing fractures have densities of 4.6/m in the lower Tuxen reservoir, 4.0/m in the middle-upper Tuxen reservoir, 2.7/m in the lower-middle Sola reservoir, and 7.4/m in the upper Sola reservoir. This study provides a detailed analysis of the natural fractures and discontinuities occurring the Lower Cretaceous succession of the Danish North Sea Basin, and their relation to the sedimentary facies and reservoir units.

Saetaspongia sponges from the Cambrian (Stage 4) Balang Formation of Guizhou, China

AbstractSaetaspongia so far cannot be confidently assigned to any class-level crown group. Clarifying its phylogenetic position requires new information provided by more detailed studies of previously described and/or new material. Some sponge fossils with the typical skeletal architecture of Saetaspongia have recently been recognized in the Cambrian (Stage 4) Balang Biota of Guizhou, China, including S. jianhensis new species and S. cf. S. densa. The new taxon is characterized by the following features: spicules are fine monaxons and are inclined to be loosely to densely arranged into plumose arrays; skeleton is composed primarily of one major plumose bundle, with an uncertain number (perhaps two) of small plumose arrays; and primary skeleton is occasionally interspersed with some irregularly oriented individual spicules. An additional specimen consisting of large monaxons, with plumose structures and overlying irregular coarse monaxons, closely fits the description and illustrations of previously described S. cf. S. densa. By combining information from previous studies and the present research, fossil evidence indicates that the plumose architecture is a critical feature diagnostic of Saetaspongia and that there are no hexactine-based spicules in this genus. The new material from the Balang Biota further supports the notion that Saetaspongia has a protomonaxonid rather than a hexactinellid affinity. Fossil evidence suggests that Saetaspongia had a wide biogeographic distribution during the early Cambrian and the stratigraphic distribution of this genus extends up to Stage 4.UUID: http://zoobank.org/e9ca5e4a-134c-4daa-9266-19085e32614f.

Late Pleistocene subglacial fissure-related volcanism at Caviahue-Copahue Volcanic Complex (37° 51° S, 71° 05’ W), South Volcanic Zone

The Caviahue-Copahue Volcanic Complex (CCVC, 37° 51’ S-71° 05′ W) is located in the central segment of the South Volcanic Zone, ~30 km eastward from the main Andean volcanic arc. Within a complex tectonic setting, it has developed under the influence of the northern transtensional Liquiñe-Ofqui Fault Zone, to the south and the Copahue-Antiñir Fault Zone to the northeast. The onset of volcanic activity within Caviahue volcano-tectonic depression has been dated at Ar/Ar 125 ± 9 ka and it is represented by lava flows and ignimbrites of Las Mellizas Formation. Copahue volcano has a very well documented historical eruptive record, encompassing 13 eruptive cycles in the last 260 years. On the contrary, volcano stratigraphy during Late Pleistocene and pre-historic times is poorly constrained, mainly due to the lack of reliable radiometric data. Based on the peculiar morphology and cooling-fracture systems, lavas that crop out along Dulce, Jara and NE Agrio river valleys have been included in a new stratigraphic unit, here named as Río Dulce Formation. A subglacial emplacement is proposed taking into account the evidence of ice-confinement, provided by the flat-topped and steep-sided lava domes/flows and dammed lava flows. In addition, melt water-magma interaction is supported by conspicuous fine columnar-jointing and closely spaced curved bands with plumose structures, well-developed pseudopillow fracture systems, platy and hackly fractures and cube-jointed entablature-bearing lava flows. The basaltic andesites (SiO2= 55–57%) are porphyritic, with ~30–40% of sieved plagioclase, two pyroxenes and olivine, set in a glassy groundmass. Lava vent distribution and primary anisotropy planes orientation, in coincidence with WNW-ESE and NE-SW trending faults, lineaments and fault scarps, document a first-order structural control on the volcanic emplacement. Although undated, this study can contribute to understanding the timing of glacial fluctuations during Late Pleistocene. Furthermore, it has implications on hazard assessment since it highlights that the evolution of the CCVC has experienced alternating fissure and central eruptive style, likely related to the dynamics of the magma-plumbing system, as suggested by geophysical evidence of unrest.

Fault zone structures and strain localization in clinoptilolite-tuff (Nižný Hrabovec, Slovak Republic)

Clinoptilolite is a natural micro-porous zeolite composed of silica and alumina tetrahedra that commonly forms as a devitrification product of glass in volcanic tuffs. Besides applications in the agriculture, building and manufacturing industries, clinoptilolite has been widely studied in veterinary and human medicine because of its positive effects on health. Based on field observations and microstructural investigations, we show, for the first time, strain localization features along faults in natural zeolite tuffs in a clinoptilolite-tuff open-pit mine at Nižný Hrabovec (Slovak Republic). The faults are localized along pre-existing joints with plumose structures that acted as a pathway for the infiltration of mainly Mn- and K-rich fluids. Displacement along the faults created structures that are indicative of both velocity hardening (accommodated by dissolution precipitation creep on SC/SCC′ foliation) and velocity weakening (accommodated by multi-generations of ultracataclasites along principal slip surfaces) behaviors. Rock-fluid interaction was characterized by a high-mobility of K-rich fluids, with K-feldspar decorating the SC/SCC′ foliation, infiltrating fractures in the damage zone of the faults and precipitating as idiomorphic crystals in open cavities and along the fault surface. Microstructures such as polished slickensides, injection of fluidized cataclasites, clast-cortex grains in cataclasites and truncated grains along principal slip surfaces probably suggest that at least some of the faults formed during seismic slip.

Historical shear deformation of rock fractures derived from digital outcrop models and its implications on the development of fracture systems

The initiation and development of fractures in rocks is the key part of many problems from academic to industrial, such as faulting, folding, rock mass engineering, reservoir characterization, etc. Conventional ways of evaluating the fracture historical deformations depend on the geologists’ visual interpretation of indicating structures such as fault striations, fault steps, plumose structures, etc. on the fracture surface produced by previous deformations, and hence suffer from problems like subjectivity and the absence of obvious indicating structures. In this study, we propose a quantitative method to derive historical shear deformations of rock fractures from digital outcrop models (DOMs) based on the analysis of effects of fault striations and fault steps on the shear strength parameter of the fracture surface. A theoretical model that combines effects of fault striations, fault steps and isotropic base shear strength is fitted to the shear strength parameter. The amount of fault striations and fault steps and their occurrences are estimated, and the historical shear deformations can be inferred. The validity and the effectiveness of the proposed method was proved by testing it on a constructed fracture surface with idealized striations and a fracture surface with clear fault steps. The application of this method on an example outcrop shows an intuitive idea of how the rock mass was deformed and that the distribution, occurrence and mode of new fractures are strictly controlled by preexisting fractures, and hence emphasizes the importance of preexisting fractures in modeling the development of fracture systems.

Multiscale Characterization of Joint Surface Roughness

AbstractRecent studies provided detailed characterizations of fault (i.e., shear fracture) roughness at different length scales. Similar investigation for joints (i.e., tensile fractures) are seldom and not as detailed. The present study aims at characterizing joint plumose patterns. We investigated the scale‐dependent surface roughness properties of S‐type plumoses. Joint surface measurements at relatively large scales were carried out with Light Detection and Ranging (LiDAR) technology. Joint surface measurements at the microscopic scale were carried out based on a noncontact optical method, using a Keyence VHX‐2000D microscope. Three parameters were used to characterize fracture surface elevation, standard deviation, Hurst exponent, and correlation length through 3 scale length orders of magnitude. Our study showed that standard deviation and correlation length decrease with scale, similarly to previous findings on faults. In addition, the range of Hurst exponents as a function of scale for the studied joint surface agrees well with those previously found for faults. However, directional analysis showed that correlation length and Hurst exponent of joint surfaces at scales smaller than 1 dm differ significantly from the ones of fault surfaces. In contrast to fault surface ornaments that are mainly characterized by linear structures, plumose structures show marked variability in orientation and anisotropy as a function of position on the joint surface.

Near-surface rock stress orientations in alpine topography derived from exfoliation fracture surface markings and 3D numerical modelling

The fractographic analysis of plumose axes of exfoliation fracture surfaces from a preceding study in the Grimsel region of the Swiss Alps suggested complex directional trends of near-surface (i.e., within ~100m below ground surface) maximum principal stress (σ1) trajectories within the investigated inner-trough valleys (i.e., U-shaped valleys). The stress trajectories describe a pattern governed by local topographic variations. In situ stress measurements from the region are scarce, locally scattered, based on different methods, and, thus, difficult to interpret at regional scale. In this study, we inferred that plumose structure axes form parallel to compressive σ1, and improve our knowledge of the near-surface three-dimensional stress field in alpine settings with complex topography. We investigated near-surface stress tensors utilising three-dimensional, elastic numerical models. Our models account for morphological details of the Grimsel area at the decametre scale. We used two models with vertical boundaries aligned N–S/W–E (0°-model) and NW–SE/SW–NE (45°-model). These models allowed investigating gravitational stresses and superimposed isotropic and anisotropic compressive stresses arising from (active) tectonic shortening and/or stresses induced by exhumation (remnant stresses). Our model results illustrate that the superposition of gravitational stresses and realistic horizontal strains reveals complex near-surface stress trajectories that widely follow the patterns of exfoliation fracture plumose axes. Our models demonstrate large variations of stress orientations within the shallow subsurface, including depth levels where exfoliation fractures formed. These variations cannot be captured by a small number of local stress measurements. Our study reveals that directional data from exfoliation fracture plumose axes of Middle to Upper Pleistocene ages can be used to constrain geologically recent and current maximum principal stress directions of the shallow subsurface of up to a few hundred metres below ground.

Growth of exfoliation joints and near-surface stress orientations inferred from fractographic markings observed in the upper Aar valley (Swiss Alps)

Abstract Granitic rock mass of the upper Aar valley (Grimsel area, Switzerland) contains distinct generations of exfoliation joints, which formed during different stages of the Pleistocene, subparallel to distinct glacial valley palaeotopography. The bulk of exfoliation joints shows prominent, common fractographic features: (1) radial plumose structures with distinct plume axes; (2) arrest marks superimposed by plumose striations; and (3) gradually-developing en echelon fringe cracks. Multiple arrest marks reveal that exfoliation joints formed incrementally and, together with the absence of hackle fringes, suggest stable, i.e., subcritical fracturing conditions. Smooth transitions from plumose structures on the parent plane to en echelon fringe cracks, combined with non-systematic stepping senses of fringe cracks, suggest local (vs. temporal) stress field variations. Assuming that plume axes formed parallel to the maximum principal compressive stress (σ1) enables us to infer near-surface palaeostress orientations and compare them with classical borehole-based in-situ stress data. The majority of plume axes suggest (1) persistently subhorizontal to slightly inclined σ1 orientations at trough valley slopes and (2) near-surface variability of σ1 orientations originating from topographic perturbation caused by glacial valley erosion superimposed on the regional stress field. Our investigations of fracture surface morphologies yield unique insights into exfoliation fracture formation, such as directional trends of fracture propagation and associated palaeostress orientations within Alpine valley slopes.

Surface Morphology of Basalt Columns at Svartifoss, VatnajökulsÞjóðgarður, Southern Iceland

A spectacular example of columnar-jointed basalt occurs at Svartifoss in the Vatnajökull National Park of southern Iceland. The columns are notable for a variety of features on the vertical joint surfaces and the horizontal parting surfaces. The jointed surfaces of the columns display horizontal striations at a spacing of centimeters to decimeters. The individual striations exhibit crescentic hackles with a plumose pattern, the orientation of which varies between adjacent striations. Also present are gently dipping, millimeter-scale laminations not previously described. Horizontal parting surfaces of the columns display a circular ring that inscribes most of the diameter column. The ring features alternately positive or negative relief against the perimeter of the column and exhibits a radiating pattern of hackles originating at the center of the ring. Petrographic examination reveals that the basalt contains an interlocking network of plagioclase laths preferentially aligned perpendicular to the column axes. The circular features have been described previously and attributed to late-stage melt migration driven by a load-induced pressure gradient. The striations were formed from stepwise, downward propagation of the polygonal fracture system, and the plumose structures were formed from tensile stresses during fracture propagation. The small-scale laminations may result from preferential grain alignment of plagioclase laths.

Recent Deformation in the Bottom Sediments of Western and Southeastern Lake Ontario and its Association with Major Structures and Seismicity

Geophysical surveys, undertaken in the Toronto-Burlington corridor of western Lake Ontario and in the Rochester Basin of southeastern Lake Ontario, revealed the presence of features affecting the young lake-bottom sediments. In the western part of the lake, they include inferred pop-ups in bedrock, and plumose structures, dark linear patterns, and linear belts of circular to elliptical signatures in the modern mud. In southeastern Lake Ontario the glacial and post-glacial sediments display vertical separations of on the order of 10-15 m. Pop-ups are tectonically-induced structures. The features in the modern mud commonly parallel the orientation of P-stresses measured in Paleozoic rocks nearby and, along with the pop-ups, are spatially related to an aeromagnetic lineament. Furthermore, all of these features occur within a seismically active belt. The vertical displacements of the layered glacial and post-glacial sediments, within the Rochester Basin, are located along the southern margin of the postulated WSW extension of the seismically active St. Lawrence rift system and are interpreted to be due to faulting. The geologically young age of the sediments affected by the various deformational features, along with the characteristics of the features themselves, suggest that the lake-bottom sediments surveyed in this study may have recorded the effects of neotectonic processes.

Fringe cracks and plumose structures in layered rocks: stepping senses and their implications for palaeostress interpretation

Many joint surfaces show en échelon arrays of microfractures (fringe cracks, F-joints, twist hackles) along fringe zones in continuity with plumose structures. In layered rocks, joints with two fringe zones showing opposite stepping senses at the top and bottom (type A) are more frequent than those showing a single stepping sense (type P). Consistent with such an arrangement, their curved advancing fronts show continuous, 180°-radial patterns of steps showing a ‘spiral-staircase’ geometry. In double plumose structures diverging from an initiation point, different combinations of symmetry elements allow us to define four basic stepping types: (type PA) single stepping sense on the full joint surface; (type AA) opposite stepping senses at both fringes; (type PP) opposite stepping senses at both sides of the starting point; (type AP) four alternate quadrants with opposite stepping. Most of these types have been observed in limestone beds of the Mequinenza area (Ebro Basin, Spain) and Glamorgan coast (South Wales). According to our observations, the varied stepping arrangements are independent of changes in orientation within the parent joint, the latter showing more persistent strikes than fringe cracks. All these features suggest that fringe crack arrays in continuity with plumose structures are related to local stress redistribution at the advancing front and, as a general rule, do not record remote stresses. After careful observation and discrimination, only those en échelon cracks consistently stepped and showing no geometric continuity with plume barbs should be included in palaeostress analysis.

Structural effects of meteorite impact on basalt: Evidence from Lonar crater, India

Lonar crater is a simple, bowl‐shaped, near‐circular impact crater in the ∼65 Myr old Deccan Volcanic Province in India. As Lonar crater is a rare terrestrial crater formed entirely in basalt, it provides an excellent opportunity to study the impact deformation in target basalt, which is common on the surfaces of other terrestrial planets and their satellites. The present study aims at documenting the impact deformational structures in the massive basalt well exposed on the upper crater wall, where the basalt shows upward turning of the flow sequence, resulting in a circular deformation pattern. Three fracture systems (radial, concentric, and conical fractures) are exposed on the inner crater wall. On the fracture planes, plumose structures are common. Uplift and tilting of the basalt sequence and formation of the fractures inside the crater are clearly related to the impact event and are different from the preimpact structures such as cooling‐related columnar joints and fractures of possible tectonic origin, which are observed outside the crater. Slumping is common throughout the inner wall, and listric faulting displaces the flows in the northeastern inner wall. The impact structures of Lonar crater are broadly similar to those at other simple terrestrial craters in granites and clastic sedimentary rocks and even small‐scale experimental craters formed in gabbro targets. As Lonar crater is similar to the strength‐controlled laboratory craters, impact parameters could be modeled for this crater, provided maximum depth of fracture formation would be known.

Modular construction of early Ediacaran complex life forms.

Newly discovered, exceptionally preserved, soft-bodied fossils near Spaniard's Bay in eastern Newfoundland exhibit features not previously described from Ediacaran (terminal Neoproterozoic) fossils. All of the Spaniard's Bay taxa were composed of similar architectural elements-centimeter-scale frondlets exhibiting three orders of fracticality in branching. Frondlets were combined as modules atop semi-rigid organic skeletons to form a wide array of larger constructions, including frondose and plumose structures. This architecture and construction define the "rangeomorphs," a biological clade that dominated the Mistaken Point assemblage (575 to 560 million years ago) but does not appear to be ancestral to any Phanerozoic or modern organisms.

Joint Pattern in Precambrian Rocks Around Galudih (India): Implications for Fold Mechanism

In this paper, analysis of joints developed in folded quartzites of Galudih area, Jharkhand State (India) is presented. The joint pattern indicates that NE–SW joints are the dominant set. These have developed perpendicular to the NW–SE trending axial planes and hinges of the folds in quartzites implying that they are cross-joints. Plumose structures are observed on NE–SW joint surfaces that indicate their formation by extension perpendicular to the joint face. It is inferred that this extension in NW-SE direction is related to NE–SW compressional stresses that resulted in folding of the quartzite by flexural-slip mechanism.

Palaeostress orientation inferred from surface morphology of joints on the southern margin of the Bristol Channel Basin, UK

Abstract A field study of the orientation of surface features on joint planes has been carried out on the north Somerset coast along the southern margin of the Bristol Channel Basin (BCB), with the aim of inferring the palaeostress orientations. Surface features are very delicate structures on joint surfaces and when preserved give important clues regarding the direction of propagation of a joint and the interaction between the remote and local stresses. In this chapter surface features (i.e. plumose structures and twist-hackle fringes) observed on joints in Liassic limestone beds interbedded with shales from the BCB are described and their implications regarding the evolution of the stress field operating during their formation considered. The results are then compared with the generally accepted model for the region of an anticlockwise rotation of the stress field. The incompatibility between N-S-directed remote compression and continually changing local stress caused by the local structures (folds and faults), inhomogeneities and the anisotropy created by rapidly alternating limestone and shale beds resulted in the master joints breaking down into en echelon cracks (twist-hackle fringes). Joints in thicker limestone beds show well-developed plumose structures and twist-hackle fringes, whereas in thinner beds the fringe zones extend from the top to the bottom of the bed. It is found that regardless of the orientation of the parent joint in which the twist-hackle fringes are contained, they generally strike E-W and show both right- and left-stepping arrangements.

Fracture networks on the Belluno syncline, a fault-propagation fold in the footwall of the Belluno thrust, Venetian Alps, NE Italy

Abstract The Belluno syncline is a structure located in the Venetian fold-and-thrust belt in the Italian Southern Alps. This SSE-vergent chain mainly developed during the Neogene as a back-thrust belt of the Alps deforming the northern margin of the Adria plate. The syncline is asymmetric in shape, with a steep backlimb, as the forelimb of the paired Monte Coppolo Anticline; it is the result of fault-propagation folding during the development of the underlying Belluno thrust. In plan view the Belluno thrust shows a prominent curvature interpreted as reactivation of inherited Mesozoic extensional structures. The hinge of the Belluno syncline presents a similar curvature. Rocks cropping out in the syncline core are Upper Chattian to Langhian fine-grained molassic sediments, mainly siltstones and marls. Most fractures on the Belluno syncline have formed at high angles to bedding. The dominant fracture trends are subparallel and orthogonal to the fold hinge. Sometimes these fractures show plumose structures revealing extension (Mode I) fracture mechanism and therefore they are referred to as longitudinal ( bc ) joints and cross ( ac ) joints respectively. Other fracture trends oblique to the hinge line are found in measurement stations located on limbs distant from the fold hinge. They are referred to as oblique (shear) fractures. Throughout the western part of the Belluno syncline the fracture network maintains a roughly symmetrical distribution with respect to the fold hinge, suggesting a development in association with folding.

Experimental simulation of joint morphology

Desiccation experiments with starch–water mixtures whose dimensions and thickness are several centimeters produce a broad spectrum of tensile cracks. Their properties are remarkably similar to those of joints in rocks. This note concentrates on crack morphology in the form of plumose structures and fringe zones. Moreover, rupture velocity is determined from videos. After an initial, dynamic rupture phase with velocities of about 100 mm/s, which is restricted to a thin, strongly dried surface layer, the main rupture through the specimen is quasi-static with velocities of 10 mm/min and less. This phase is driven by the low supply of strain energy due to the ongoing desiccation. Quasi-static rupture produces plumose topographies, which range from simple to complex. From the similarity of plumose topography on joints in marine sedimentary rocks, it is hypothesized that these joints are generated by quasi-static rupture in an early stage of subsidence, compaction and diagenesis of water-bearing sediments. Rupture propagation in the fringe zone of cracks in starch, where topography is stronger than on the main crack, was slower than over the main crack. The same applies probably to fringe zones along bedding planes in sediments.

Fracture morphology of tensile cracks and rupture velocity

Desiccation cracks in starch‐water mixtures are studied with respect to morphological features, mainly plumose structures, on their faces. Specimens have diameters of 50–100 mm and thicknesses of 2–40 mm. Structures similar to those on joints in rocks are found. Rupture velocities are measured from videos and estimated from photos. Rupture covers the range from spontaneously nucleating, dynamic cracks with velocities of 100–200 mm/s to quasi‐static cracks with velocities of 0.1 mm/s and less. Plumose lines give the rupture direction, and their relation to rupture velocity is similar to the relation between seismic rays and seismic wave velocity. A ray‐tracing method from seismology is used to calculate plumose lines for depth‐dependent rupture velocity. Moreover, an inverse method, based on finite difference travel times and conjugate gradients, is developed to invert a set of measured plumose directions into a rupture‐velocity distribution which can also depend on the horizontal coordinate on the rupture surface. The main results of this paper are as follows. (1) Plumose lines can successfully be inverted into relative rupture velocity. (2) In thin starch layers (thickness less than 0.2 times diameter), rupture velocity decreases from top to bottom by a factor of 2–5, following a decrease of tensile stress due to the increase in water concentration. (3) Horizontal variation of rupture velocity reflects horizontal variation of stress, including stress relaxation due to the propagating crack, and ranges from dynamic to quasi‐static velocities. (4) In thick starch layers (thickness about 0.5 times diameter), rupture is predominantly quasi‐static. (5) Starch cracks sometimes have a fringe zone where topographic amplitudes are higher and rupture velocities lower than on the main part of the crack; this probably also applies to joints in rocks and their fringe zones. (6) Starch‐water mixtures at rupture have a Poisson ratio close to 0.5. Cracks in starch are closest to subsidence or diagenesis joints in sedimentary rocks.