High Fracture Density Research Articles

Formation and disruption of aquifers in southwestern Chryse Planitia, Mars

We present geologic evidence suggesting that after the development of Mars' cryolithosphere, the formation of aquifers in southwestern Chryse Planitia and their subsequent disruption led to extensive regional resurfacing during the Late Hesperian, and perhaps even during the Amazonian. In our model, these aquifers formed preferentially along thrust faults associated with wrinkle ridges, as well as along fault systems peripheral to impact craters. The characteristics of degraded wrinkle ridges and impact craters in southwestern Chryse Planitia indicate a profound role of subsurface volatiles and especially liquid water in the upper crust (the upper one hundred to a few thousands of meters). Like lunar wrinkle ridges, the martian ones are presumed to mark the surface extensions of thrust faults, but in our study area the wrinkle ridges are heavily modified. Wrinkle ridges and nearby plains have locally undergone collapse, and in other areas they are associated with domical intrusions we interpret as mud volcanoes and mud diapirs. In at least one instance, a sinuous valley emanates from a modified wrinkle ridge, further indicating hydrological influences on these thrust-fault-controlled features. A key must be the formation of volatile-rich crust. Primary crustal formation and differentiation incorporated juvenile volatiles into the global crust, but the crustal record here was then strongly modified by the giant Chryse impact. The decipherable rock record here begins with the Chryse impact and continues with the resulting basin's erosion and infilling, which includes outflow channel activity. We propose that in Simud Vallis surface flow dissection into the base of the cryolithosphere-produced zones where water infiltrated and migrated along SW-dipping strata deformed by the Chryse impact, thereby forming an extensive aquifer in southwestern Chryse Planitia. In this region, compressive stresses produced by the rise of Tharsis led to the formation of wrinkle ridges. Zones of high fracture density within the highly strained planes of the thrust faults underlying the wrinkle ridges formed regions of high permeability; thus, groundwater likely flowed and gathered along these tectonic structures to form zones of elevated permeability. Volatile depletion and migration within the upper crustal materials, predominantly along fault systems, led to structurally controlled episodic resurfacing in southwestern Chryse Planitia. The erosional modification of impact craters in this region is linked to these processes. This erosion is scale independent over a range of crater diameters from a few hundred meters to tens of kilometers. According to our model, pressurized water and sediment intruded and locally extruded and caused crustal subsidence and other degradational activity across this region. The modification of craters across this wide range of sizes, according to our model, implies that there was intensive mobilization of liquid water in the upper crust ranging from about one hundred to several thousand meters deep.

Hydraulic modelling and the evaluation of the representative elementary volume (REV) in fractured rock

The objective of the following study is the development of a method for the evaluation of the hydraulic conductivity and the representative elementary volume (REV) in fractured rock aquifers, which can be used for the hydraulic modelling of large-scale fractured systems. Two-dimensional stochastic discrete fracture networks were simulated for the evaluation of hydraulic conductivity. Due to the strongly variable fracture density in the considered area, a sensitivity study was performed which examined three various fracture densities according to their hydraulic properties. The flow behaviour in the discrete fracture networks is described with the cubic law. The hydraulic modelling in the fractured rock showed that an REV could not be determined for all discrete fracture networks. With one exception, an REV with a size of 10 m × 10 m could be determined for discrete fracture networks with a medium (P21 = 13.1 m–1) and high fracture density (P21 = 16.9 m–1). In contrast, no REV could be found for discrete fracture networks with a low fracture density (P21 = 5.1 m–1).

Unusual behaviour of epikarst in the Acqua dei Faggi carbonate aquifer (Southern Italy)

AbstractHydrogeological and geophysical investigations demonstrated the existence of an epikarstic zone in a carbonate aquifer of Southern Italy, about 10 m thick. Nevertheless, the hydrogeological behaviour of the epikarst is different from that schematized by several authors. In the test site, the contrast in permeability at the bottom of the epikarst does not cause retention of percolation and storage of water in a perched temporary aquifer within the uppermost portion of the carbonate medium. Because of the high fracture density and good interconnection of openings within the underlying limestone, the percolation is diffuse also below the epikarstic zone, as well as the groundwater flow. The ‘funnelling’ effect into larger shafts does not play an important role on the hydrogeological behaviour of the aquifer.

Geometric and kinematic controls on the internal structure of a large normal fault in massive limestones: The Maghlaq Fault, Malta

The Maghlaq Fault is a large, left-stepping normal fault (displacement >210 m) cutting the Oligo-Miocene pre- to syn-rift carbonates of SW Malta. Two principal slip zones separate the deformed rocks of the fault zone from the undeformed wall rocks. Fault rocks derived from fully lithified, pre- to early syn-rift sediments comprise relatively continuous fine-grained veneers of cataclasite and localised fault-bound lenses of wall rock, occurring over a range of scales, which are commonly brecciated. The lenses result from the linkage of slip surfaces, the inclusion of asperities and the formation of Riedel shears within the fault zone. In contrast, fault rock incorporated from unlithified syn-rift sediments comprise relatively continuous veils of rock that deformed in a ductile manner. Anomalously thick parts of the fault zone with highly complex structure and content are associated with breached relay zones, branch-lines and bends; these structures represent progressive stages of fault segment linkage. The progressive evolution and bypassing of fault zone complexities to form a smoother and more continuous active fault surface, results in complex fault rock distributions within the fault zone. Segment linkage structures have high fracture densities which combined with their significant vertical extents suggest they are potentially important up-fault fluid flow conduits.

Enhanced fracture permeability and accompanying fluid flow in the footwall of a normal fault: The Hurricane fault at Pah Tempe hot springs, Washington County, Utah

The Pah Tempe hot springs discharge ~260 L/s of water at ~40 °C into the Virgin River in the footwall damage zone of the Hurricane fault at Timpoweap Canyon, near Hurricane, Utah, USA. Although these are Na-Cl waters, they actively discharge CO2 gas and contain signifi cant quantities of CO2 (~34.6 mmol/kg), predominantly as H2CO3 and HCO3 –. Because of excellent exposures, Pah Tempe provides an exceptional opportunity to observe the effects of enhanced fracture permeability in an active extensional fault. Pah Tempe waters have been deeply circulated (>5 km; >150 °C) into basement rock as illustrated by the clear water-rock exchange of oxygen isotopes. Waters were probably recharged under colder climate conditions than present and therefore have a prolonged subsurface residence. Discharge of both water and gas in the springs correlates to the density of fractures in carbonate rocks above stream level. This observation suggests that clusters of high fracture density in the faultdamage zone act as pathways from the likely regional aquifer, the eolian Queantoweap Sandstone, through the overlying confining unit, the gypsiferous silty Seligman Member of the Kaibab Formation. Mass-balance modeling suggests that the majority of CO2 discharge is the product of the quantitative dissolution of CO2 gas at depth within the fault zone. Upon discharge, most of the carbon is released to the surface as dissolved species. It appears that the subsurface production rate of CO2 is relatively low because Pah Tempe waters are grossly undersaturated in CO2 at inferred minimum circulation depths and temperatures. Geological and geochemical data also suggest that the CO2 is dominated by a crustal component complemented by minor mantle contributions.

Spatial Distribution of Fractures in the Asmari Formation of Iran in Subsurface Environment: Effect of Lithology and Petrophysical Properties

The distribution of fractures and its dependence on lithology and petrophysical properties of rock in the Asmari Formation were examined using three wells data of one of the largest oil fields of southwestern Iran. Fractures were measured on cut cores. Mineral content and petrophysical data were obtained through thin section study and core plug measurement respectively. Influence of mineral composition and petrophysical property of rocks on fracture density was explored statistically. Increasing quartz (sand) and anhydrite content of rocks decrease and dolomite increases the threshold of fracture densities, however no significant relation was observed between calcite content of rock and fracture density. Increasing porosity and permeability of rock decrease the threshold of fracture density in some of the defined lithology groups. There are significant differences between the lithology groups in terms of fracture density, although the results in the three wells are not the same. In whole data, the highest fracture density can be observed in dolostone. Limestone and impure carbonates hold broader spaced fractures and sandstones display the least fracture density. The average fracture densities in the wells are strictly different. These differences are the result of the structural position of the wells and also the trend of the well and fractures. The distribution of fractures in most lithology groups can be explained by the function: $$F = a/(1 + b\;D\exp (c))$$ , where F is relative frequency, D is fracture density and a, b, and c are constants.

Resistivity and ground-penetrating radar images of fractures in a crystalline aquifer: a case study in Caiçara farm—NE Brazil

Current hydrogeological research in NE Brazil aims to better understand factors controlling storage and percolation of ground water in crystalline terrains, due to the fact that a large proportion (about 60%) of this region is formed by igneous and metamorphic rocks. As part of this effort, we present an integrated geophysical–geological study of a fractured bedrock aquifer located in the Caiçara farm, near Equador city, Rio Grande do Norte State, NE Brazil. Pumped wells in this site present very different yields, in spite of short distance of only 20 m. The main lithologies of the site are quartzite and micaschist. Combined interpretation of geophysical images (Resistivity and Ground Penetrating Radar—GPR) and field structural geologic data revealed that the fractured bedrock aquifer is composed of three sets of discontinuities: SW-dipping subhorizontal joints, parallel to the ductile fabric of the country rocks which are probably release joints; NE-dipping subhorizontal fractures, oblique to ductile trend; and subvertical fractures which are orthogonally distributed along N–S and E–W directions. Important subvertical fracture zones occur at a regular spacing of about tens of meters. Fractures in the E–W direction are relatively open, as compared to fractures in the N–S direction. Probably, E–W fractures were opened by the current neotectonic stress regime in NE Brazil, which is controlled by E–W compression and N–S extension. It was possible to rank the hydrogeologic potentiality of water well siting as a function of fracture density and proximity to recharge zones, in this way explaining why wells could present very different yield results on the site. The most favorable places for well siting are intersections of subvertical fracture zones located near drainage because highly fractured zones could be, in this way, connected with recharge zones mainly in alluvial deposits. On the other hand, the least favorable places for well siting lie outside subvertical fractures and far from recharge zones. Intermediate hydrogeological potentialities occur in two cases: at a point near the recharge zone but where there is a low density of fractures, or at point far from the recharge zone but where there is a high density of fractures.

Resistivity and ground-penetrating radar images of fractures in a crystalline aquifer: a case study in Caiçara farm—NE Brazil

Current hydrogeological research in NE Brazil aims to better understand factors controlling storage and percolation of ground water in crystalline terrains, due to the fact that a large proportion (about 60%) of this region is formed by igneous and metamorphic rocks. As part of this effort, we present an integrated geophysical–geological study of a fractured bedrock aquifer located in the Caiçara farm, near Equador city, Rio Grande do Norte State, NE Brazil. Pumped wells in this site present very different yields, in spite of short distance of only 20 m. The main lithologies of the site are quartzite and micaschist. Combined interpretation of geophysical images (Resistivity and Ground Penetrating Radar—GPR) and field structural geologic data revealed that the fractured bedrock aquifer is composed of three sets of discontinuities: SW-dipping subhorizontal joints, parallel to the ductile fabric of the country rocks which are probably release joints; NE-dipping subhorizontal fractures, oblique to ductile trend; and subvertical fractures which are orthogonally distributed along N–S and E–W directions. Important subvertical fracture zones occur at a regular spacing of about tens of meters. Fractures in the E–W direction are relatively open, as compared to fractures in the N–S direction. Probably, E–W fractures were opened by the current neotectonic stress regime in NE Brazil, which is controlled by E–W compression and N–S extension. It was possible to rank the hydrogeologic potentiality of water well siting as a function of fracture density and proximity to recharge zones, in this way explaining why wells could present very different yield results on the site. The most favorable places for well siting are intersections of subvertical fracture zones located near drainage because highly fractured zones could be, in this way, connected with recharge zones mainly in alluvial deposits. On the other hand, the least favorable places for well siting lie outside subvertical fractures and far from recharge zones. Intermediate hydrogeological potentialities occur in two cases: at a point near the recharge zone but where there is a low density of fractures, or at point far from the recharge zone but where there is a high density of fractures.

Strain rate and bimodal volcanism in the continental rift: Debre Zeyt volcanic field, northern MER, Ethiopia

Bimodal volcanism, which is typical of the continental rift, is supposed to be mainly dependent on strain-rate setting. High strain rates enhance the coupling between brittle and ductile crust and create a high density of the fractures used by basaltic magmas to erupt. Decoupling of brittle and ductile layers is generated by low strain rates with low fracture density, whereby the magma can be emplaced in the middle and upper crusts and can successively differentiate to produce evolved products. Analogue experiments have been performed to simulate the extension of continental crusts with underplated magmas at different strain rates. The spatial distribution of the faults is examined for high and low strain rates. A high strain rate is characterised by high fracture density (about 2.25 faults/cm) and low fracture spacing (about 0.4 cm), whereas low strain rate has a low fracture density (about 1 fault/cm) and high fracture spacing (about 0.9 cm). The model was tested in the Debre Zeyt area, where a sequence of alternating basaltic and silicic products has been erupted since the late Miocene. In particular, the spatial distribution and self-similar clustering of the Pliocene silicic domes and Quaternary-Recent basaltic cones and maars were investigated. The cones are more anti-clustered ( D f:1.54) than the domes ( D f:1.35), implying a denser fracture distribution for basalts. The more homogeneous distribution for cones (basalt) than for domes (silicic) is interpreted as being linked primarily to different strain rates, as suggested by the results of the analogue experiments. The data obtained for the Debre Zeyt area corroborate the hypothesis that bimodality of magnetism is primarily dependent on strain rate.

Fault/fracture density and mineralization: a contouring method for targeting in gold exploration

A widely observed correlation between high fracture density and mineralization throughout terranes and geological time indicates a fundamental underlying ore-forming process. In Archaean greenstone-hosted deposits, high-density fracturing was accompanied by enhanced fluid flow during fault/fracture network development, producing regional-scale fluid pressure gradients that focussed hydrothermal fluids into preferentially fractured areas. Fracture density is both increased and decreased during faulting and fault healing, and fracture density accumulates over time, in zones of high palaeo-fluid flow. Localised zones where the density of fracturing is increased by deformation, become permeability nodes for migrating hydrothermal fluids leading to large zones of alteration and gold precipitation. The Ora Banda mining centre in Western Australia contains significant gold deposits that appear to demonstrate a close association between high-density fracturing and gold precipitation. Fracture density in the Ora Banda mines was enhanced by fault–fault intersections, fault–contact intersections and changes in fault geometry. The mine-scale relationships between fracture density and gold mineralization are repeated at smaller and larger scales, hence these relationships may be used in targeting for gold exploration. Contouring the density of fracturing in a region provides a semi-quantitative way to rank areas for exploration and uses data from mapping, drilling and high-quality geophysical data as a basis for analyses. Fracture density contouring is complementary to other prospectivity-analysis methods.

Hydrogeology of the Venetia Diamond Mine, South Africa

Venetia Mine is located in the northern part of the Limpopo Province. The mean annual rainfall for the area is 344mm and the mean annual evaporation is in the order of 2650mm/annum. Geologically, Venetia Mine is situated in the Central Zone of the Limpopo Belt where a large variety of rock types are developed. These include quartzites, dolomitic marbles, magnetite rich quartzites, amphibolites, gneisses and schist’s. During the hydrogeological investigation a number of boreholes were drilled and were subjected to numerous conventional and unconventional aquifer tests. The country rock of the mine is hydraulically tight but has a large storage capacity for groundwater. Faults and fractures within the country rock enhance the hydraulic conductivity in places and serve as drainage lines into the open pit. The kimberlite may be divided into two significant horizontal hydrogeological units, the so-called “overshot zone” created by drilling and blasting and the undisturbed kimberlite. The “overshot zone” has a high density of artificial fractures and is between 2m and 5m thick. Test pumping of the in-situ kimberlite of the Venetia Diamond Mine has revealed unusually high hydraulic values. Post closure, evaporation might exceed the inflow of groundwater into the pit. It will, therefore, either be dry except for runoff from precipitation or develop into a brine lake where accumulation of salts will take place in the pit. The drawdown of the groundwater table will continue indefinitely because evaporation exceeds recharge. A pit lake model is planned to simulate post closure conditions.

Fracture detection using land 3D seismic data from the Yellow River Delta, China

Over the past 10 years, there has been a consistent increase in using 3D P-wave data to characterize fractures, which is critical for ensuring economic oil and gas production in tight formations of otherwise low permeability. Here, we present a case study of fracture detection using 3D P-wave seismic data from the Yellow River Delta in East China. The target formation is a naturally fractured mud-rock reservoir. The field site has been in production for more than 10 years; however, virgin pressure wells are still being drilled. The purpose of the survey was the remote identification, for future well planning, of zones of high fracture density that are residual oil-charged. A major aspect of this study is to compare the different seismic attributes and different analysis techniques on a common data set for fracture detection. Our aim is to understand the merits of these different techniques, and to establish some basic guidelines for fracture detection using P-wave data. If we assume the fracture population consists of predominantly one major orientation, the azimuthal variation of P-wave seismic attributes, such as traveltime, stacking velocity, reflected wave amplitudes, impedance, etc. can be approximately described by an ellipse. The long axis of the ellipse indicates the fracture orientation, and the relative ratio of the long to short axes of this ellipse is proportional to the fracture density or intensity of the rock concerned. As we know, at least three data points are required to define an ellipse in azimuthal planes. Thus fracture orientation and intensity maps can be built from 3D P-wave data if there is sufficient azimuthal coverage. We shall call this technique azimuthal attribute analysis or the 3A technique. In the practical application of the 3A technique, two methods are often employed to extract the fracture information: full-azimuth surface fitting and narrow-azimuth …

Geomechanical Properties of Shear Zones in the Eastern Aar Massif, Switzerland and their Implication on Tunnelling

¶Rock zones containing a high fracture density and/or soft, low cohesion materials can be highly problematic when encountered during tunnel excavation. For example in the eastern Aar massif of central Switzerland, experiences during the construction of the Gotthard highway tunnel showed that heavily fractured areas within shear zones were responsible for overbreaks in the form of chimneys several metres in height. To understand and estimate the impact of the shear zones on rock mass behaviour, knowledge concerning the rock mass strength and deformation characteristics is fundamental. A series of laboratory triaxial tests, performed on samples from granite- and gneiss-hosted shear zones revealed that with increasing degree of tectonic overprint, sample strength decreases and rock behaviour shows a transition from brittle to ductile deformation. These trends may be explained by increasing fracture densities, increasing foliation intensity, increasing thickness of fine-grained, low cohesion fracture infill, and increasing mica content associated with the increasing degree of tectonic overprint. As fracture density increases and the influence of discrete, persistent discontinuities on rock mass strength decreases, behaviour of the test samples becomes more and more representative of rock mass behaviour, i.e. that of a densely fractured continuum. For the purpose of numerical modeling calculations, the shear zones may be subdivided with respect to an increasing fracture density, foliation intensity and mica content into a strongly foliated zone, a fractured zone and a cohesionless zone, which in turn exhibit brittle, brittle-ductile and ductile rock mass constitutive behaviour, respectively.

The eclogites of the Marun–Keu complex, Polar Urals (Russia): fluid control on reaction kinetics and metasomatism during high P metamorphism

The Marun–Keu complex (Polar Urals, Russia) is a poorly known member of a group of high P complexes outcropping along the length of the Uralide orogen. The central and southern parts of the complex are metamorphosed at high P, medium T conditions ( T max∼600–650 °C and P∼14–17 kbar) and differ from its northern part and the rest of the Uralian high P complexes which are metamorphosed at blueschist-to-low T eclogite-facies conditions. The Marun–Keu complex consists of Neoproterozoic to Cambrian volcanic-sedimentary sequences with a large variety of intrusive mafic to felsic rocks. Based on the nature of protoliths and the mode of occurrence, it is distinguished: (1) eclogite-facies rocks after intrusive protoliths, which vary from metagabbros to metagranites; (2) eclogitic quartzofeldspathic gneisses; (3) metasomatic eclogites; and (4) amphibolite–eclogite alternations produced by fluid infiltration during uplift. Igneous textures and mineralogy are preserved in non-sheared, intrusive, dry rocks, whereas eclogitization may be complete along centimetre to 10-m-thick shear zones and in domains infiltrated by H 2O-dominated fluids. The most important reaction features are: (1) transformation of igneous diopside (Na∼0.04–0.09 apfu, based on 6 oxygens) into Na-rich diopside and omphacite (Na∼0.24–0.45 apfu) across microfractures and grain boundaries; (2) replacement of plagioclase by tiny aggregates of zoisite/clinozoisite+white mica+garnet (Alm 32–43, Pyr 7–19, Gro 38–60)+kyanite; and (3) double coronas of garnet and orthopyroxene at olivine–clinopyroxene and olivine–plagioclase pseudomorph interfaces and garnet coronas at clinopyroxene–plagioclase pseudomorph interfaces. Eclogitic quartzofeldspathic gneisses display a fine-scale layering of intermediate and felsic rock compositions with omphacite-bearing and omphacite-free assemblages, respectively. Oligoclase is abundant in this type of rocks, coexisting with omphacite in the intermediate rock compositions. The studied area presents a large variety of veins and metasomatic mineral sequences developing in the host-rock adjacent to the veins. High P minerals (garnet (Alm 40–60, Pyr 25–40, Gro 11–23), omphacite, phengite, paragonite) occur in both vein infillings and wall-rock mineral sequences, indicating that metasomatism was caused by the infiltration of out of equilibrium fluids (mostly silica-rich, alkali-rich compositions) during the eclogite-facies metamorphism. In zones of high fracture density, vein networks divide the host-rock into decimetre-scale blocks with omphacite-rich rinds replacing amphibolite cores. In contrast, metasomatic replacement of mafic eclogites by dendritic amphibole with barroisite composition ( M4Na∼0.64 apfu, based on 23 oxygens) also occurred at eclogite-facies conditions. We outline the importance of considering such metasomatic processes for the correct evaluation of P– T– t metamorphic histories. The Marun–Keu complex is another example that highlights the important control exerted by fluids on reaction kinetics during high P metamorphism and further points to an important metasomatic effect of these fluids.

Geochemistry and Petrogenesis of the Late Jurassic-Early Cretaceous Mansouri Ring Complex, Southeastern Desert, Egypt

The Mansouri Ring Complex is an eccentric mass (9 × 6 km) located in the extreme southwestern part of the Eastern Desert of Egypt. The complex exhibits a poorly defined ring structure and a limited variety of rock types: trachytes, alkali feldspar syenites and rhyolitic dykes. The rocks are hypersolvus, porphyritic and less commonly show trachytic textures. The complex has a high fracture density consisting of two dominant fault systems trending west-northwest and east-west. The rocks are silica-oversaturated; trachytes and syenites are mildly metaluminous to peralkaline, while rhyolites are mainly peraluminous due to the relative loss of a peralkaline fluid phase. The rocks are enriched in high field strength elements (Nb, Zr, Y) reflecting a within-plate tectonic setting. The curvilinear trends defined by some major and trace element variations, together with the decrease in the K/Rb ratio and concomitant increase in Rb/Ba and Rb/Sr ratios, indicate the important role of feldspar fractionation during the evolution of the Mansouri Complex. In some Mansouri rocks, radioactive anomalies are mainly associated with shear zones aligned along the east-west direction and characterised by abnormal accumulations of U- and Th-bearing accessory minerals like monazite, allanite, apatite and titanite. The Mansouri Ring has carbonatite rocks of calcitic, calcitic-dolomitic and dolomitic varieties; the latter is widely abundant. Apatite, monazite, pyrochlore, sphalerite, pyrrohotite and magnetite are the common accessory minerals associated with the Mansouri carbonatites. The U and Th contents of the carbonatites range from 10–25 ppm and 40–250 ppm, respectively. The U and Th contents of the alkali feldspar syenites range from 1–9 ppm and 3–10 ppm, respectively. The low Y/Nb ratios (< 0.6), together with the relatively low initial 87Sr/ 86Sr ratio (0.705) of the Mansouri rocks, substantiates the dominant role of mantle-derived magma in their genesis.

Genetic links between ash-flow calderas and associated ore deposits as revealed by large-scale thermo-mechanical modeling

In addition to a number of epithermal ore deposits, some porphyry-type ore deposits are also hosted by ash-flow calderas. Spatial correlation between ash-flow calderas and these deposits is observed in several parts of the world, but temporal correlation between the mineralization and the episode of caldera formation remains unclear, especially because geochronological data indicate that the mineralization may date to several million years before or after caldera formation. Because caldera collapse represents only a very short event in a long time-scale evolution between a silicic magma chamber and its host rocks, we have developed a thermo-mechanical model of upper crustal and magmatic system behavior during the pre- and post-caldera formation stages. Major petrophysical properties of ash-flow caldera related rocks are incorporated into the model, which accounts for brittle–elasto-ductile rheology and heat transfer, and reproduces fault initiation and various caldera collapse scenarios. Large-scale mechanisms of initiation of thermal and structural traps for epithermal ore deposits are described. In particular, we found that the outer sides of caldera border faults are associated with both anomalous heat-transfer and a high fracture density. The finite size of the insulating intracaldera units forces the accumulated heat excess to be laterally transferred across the border faults. In most cases, the model predicts “Y-shaped” fault geometry, which is surprisingly similar to that of mineralized faults in the field. Variations in lithostatic pressure on the magma chamber roof and in the vicinity of the border faults can reach 10 to 20MPa, with deep overpressured zones close to the inner edge of the chamber corners. Despite the fact that hydrothermal convection and fluid geochemistry are not included in the model, these thermal and mechanical results are believed to constrain the conditions for mineralization processes. Our results show that in the presence of regional extension, the development of caldera border faults may be suppressed to the advantage of deep-seated fractures, clustered above the central part of the magma chamber, thus favoring conditions for the formation of porphyry-type ore deposits. Finally, we demonstrate that well before the eruption, the gradual emplacement of a hot and large silicic magma chamber may create significant extensional stress in the brittle upper crust, resulting in breakup and pre-eruptive normal faulting and thus allowing for conditions of pre-caldera mineralization.

Application of stress mapping in cross‐section to understanding ore geometry, predicting ore zones and development of drilling strategies

Stress mapping is a numerical modelling technique used to determine the distribution and relative magnitude of stress during deformation in a mineralised terrane. It is based on the general principle that fluid flow in the Earth's crust is primarily related to pressure gradients. It is best applied to epigenetic hydrothermal mineral deposits, where fluid flow and fluid flux are enhanced in dilational sections of structures and in sites of enhanced rock permeability due to high fracture density. These are defined by sites of low minimum principal stress (σ3). Most stress mapping is carried out in two dimensions in plan view using geological maps. This is suitable for terranes with steeply dipping lithostratigraphy and structures in which the distribution of mineral deposits is largely controlled by fault structures portrayed on the maps. However, for terranes with gently dipping sequences and structures, and for situations where deposits are sited in and near the hinges of complex fold structures, stress mapping in cross‐section is preferable. The effectiveness of stress mapping is maximised if mineralisation was late in the evolutionary history of the host terrane, and hence the structural geometry of the terrane and contained deposits were essentially that expressed today. The orientation of syn‐mineralisation far‐field stresses must also be inferred. Two examples of orogenic gold deposits, which meet the above criteria, are used to illustrate the potential of stress mapping in cross‐section. Sunrise Dam, located in the Archaean Yilgarn Craton, is a lode‐gold deposit sited in a thrust‐fold belt. Stress mapping illustrates the heterogeneity of stress distribution in the complex structural geometry of the deposit, and predicts the preferential siting of ore zones around the intersections of more steeply dipping, linking thrusts and banded iron‐formation units, and below the controlling more gently dipping basal thrust, the Sunrise Shear. The Howley Anticline in the Pine Creek block hosts several Palaeoproterozoic gold deposits, sited in complex anticlinal structures in greywacke sequences. Stress mapping indicates that gold ores should develop in the hinge zones of symmetrical anticlines, in the hinge zones and more steeply dipping to overturned limbs of asymmetric anticlines, and in and around thrusts in both anticlines and parasitic synclines. The strong correlation between the predictions of the stress mapping, based on the distribution of low σ3, and the location of gold ores emphasises the potential of stress mapping in cross‐section, not only as an exploration tool for the discovery of additional resources or deposits, but also as a test of geological models. Knowledge of the potential siting of gold ores and their probable orientations also provides a guide to drilling strategies in both mine‐ and regional‐scale exploration.

Fracture/cleat analysis of coals from Zonguldak Basin (northwestern Turkey) relative to the potential of coalbed methane production

In this study, the fracture structure of three coals (Sulu K-1, Acılık K-2 and Çay Doğu Ayak) from two different seams in the Zonguldak Basin located on the Black Sea coast of Turkey have been analyzed in relation to coalbed methane recovery. X-ray computerized tomography (CT), light microscopy image analysis and scanning electron microscopy (SEM) analysis have been performed on mine coal samples to have a better understanding of distribution of minerals, fracture morphology and apertures and matrix/fracture interactions via examination of fracture surfaces. Results show that the Acılık and Sulu coals seem to be structurally more prone to be commercially productive. These coals show a high degree of fracturing and cleat development as evidenced by X-ray CT and light microscopy examination of polished surfaces. Although the Çay Doğu Ayak coal shows a high fracture density, the cleat surface morphology and smaller fracture apertures may make this coal unfavorable for gas flow.

Te Kopia geothermal system (New Zealand) — the relationship between its structure and extent

The Te Kopia geothermal system is one of several high-temperature systems in the Taupo Volcanic Zone (TVZ) of New Zealand. It is located along the Paeroa Fault Zone, a major active fault system trending NE in the central TVZ. Three independent studies, i.e. resistivity survey, magnetic interpretation, and detailed topographic analysis of faults and fractures, indicate the existence of another fault system, trending NW, that also significantly influences the Te Kopia geothermal system. Results from these studies also show that, at Te Kopia, a resistivity low and hydrothermally demagnetised rocks (both are indicators of a geothermal reservoir in volcanic rocks) clearly coincide with a zone of high fault and fracture density. Hence, the Te Kopia field is a good example of the significant influence that geological structures (major fault systems) have on the extent of a geothermal reservoir, by creating zones of fractured rocks that provide permeable paths for thermal fluids.

Laser shock experiments with nanoseconds pulses: a new tool for the reproduction of shock defects in olivine

Laser shock experiments with olivine have been carried out in order to test the capability of this new experimental approach to produce known shock defects in olivine. To determine the shock conditions, numerical simulations of experiments have been undertaken, indicating the generation of a rapidly decaying shock wave with an initial pressure of 60 GPa and a duration of 5 ns. The pressure prevailed thus distinctly shorter than in conventional shock experiments with explosives or light gas guns (0.1–1 μs). Despite this shorter pressure pulse in our laser experiment, TEM revealed that a high density of dislocations and planar fractures was activated in olivine up to a depth of 30 μm. A drastic decrease of dislocation and fracture densities observed deeper in the sample is attributed to the decay of the shock wave. The dislocation systems and the orientations of planar fractures in our experiment are identical to those found in naturally shocked olivine. The TEM observations indicate furthermore that fractures in olivine are the sources of dislocations and that the stress field around fractures determines which slip system is activated. Our study demonstrates that laser shock experiments are not only a powerful new tool in the reproduction of shock defects but also an indispensable tool to understand better the role of time in impact processes and the formation mechanisms of shock defects.