Zone Of High Electrical Conductivity Research Articles

Geophysical mapping of hyporheic processes controlled by sedimentary architecture within compound bar deposits

AbstractHyporheic exchange influences water quality and controls numerous physical, chemical, and biological processes. Despite its importance, hyporheic exchange and the associated dynamics of solute mixing are often difficult to characterize due to spatial (e.g., sedimentary heterogeneity) and temporal (e.g., river stage fluctuation) variabilities. This study coupled geophysical techniques with physical and chemical sediment analyses to map sedimentary architecture and quantify its influence on hyporheic exchange dynamics within a compound bar deposit in a gravel‐dominated river system in southwestern Ohio. Electromagnetic induction (EMI) was used to quantify variability in electrical conductivity within the compound bar. EMI informed locations of electrode placement for time‐lapse electrical resistivity imaging (ERI) surveys, which were used to examine changes in electrical resistivity driven by hyporheic exchange. Both geophysical methods revealed a zone of high electrical conductivity in the center of the bar, identified as a fine‐grained cross‐bar channel fill. The zone acts as a baffle to flow, evidenced by stable electrical conditions measured by time‐lapse ERI over the study period. Large changes in electrical resistivity throughout the survey period indicate preferential flowpaths through higher permeability sands and gravels. Grain size analyses confirmed sedimentological interpretations of geophysical data. Loss on ignition and x‐ray fluorescence identified zones with higher organic matter content that are locations for potentially enhanced geochemical activity within the cross‐bar channel fill. Differences in the physical and geochemical characteristics of cross‐bar channel fills play an important role in hyporheic flow dynamics and nutrient processing within riverbed sediments. These findings enhance our understanding of the applications of geophysical methods in mapping riverbed heterogeneity and highlight the importance of accurately representing geomorphologic features and heterogeneity when studying hyporheic exchange processes.

Deep Structure of the Lithosphere in the Central Tien Shan along the Son-Kul Magnetotelluric Sounding Profile

New results of a detailed study of the deep structure of the Central Tien Shan along the Son-Kul magnetotelluric (MT) profile crossing the Son-Kul Lake are reported. Based on the results of magnetotelluric data modeling, the regional and local geoelectric anomalies in the lithosphere are studied and their quantitative characteristics are given. Geological interpretation of the geoelectric cross-section was carried out, which supported the existing ideas about the block–hierarchical structure of the upper part of the Earth’s crust. This corresponds to the tectonophysical concepts of the sequential inserted subordination of large and smaller elements of the zone–block structure consisting of stable blocks and limiting mobile zones, which are distinguished by the high dislocation of the geological substrate. The integral pattern of the distribution and morphology of zones of high electrical conductivity in this segment of the Central Tien Shan crust may reflect discretely localized palm tree–type structures associated with the evolution of transgressive suture zones of localized deformation during the Hercynian and Alpine tectogenesis.

Thermal Waters Circulation During the Formation of Ore Deposits

The paper analyzes processes leading to the formation of gold-bearing sulphide deposits. The study was based on the data available for the territory of Ukraine and other regions. The deposits in question are shown to be associated with post-geosynclinal activation as postulated by concepts of the advection-polymorphism hypothesis. The nature of ore-bearing fluids is analyzed. Even on the ocean bottom helium isotopy points to the presence of mantle components in the fluid. Metamorphogenic and magmatic fluids are obviously involved in the process. An abnormally fast growth of permeability in fault zones during the activation period is shown to have contributed to the formation of deposits. The existence of metalliferous and barren phases of hydrothermal activity is explained. Thermal models of deposits are constructed and diagnostic criteria for their identification determined. The main features of the deposits are associated with faults through which mineralized deep waters circulate. In young deposits, these are: 1. Increased content of metals in soil and plants. 2. Intense heat flow anomalies. 3. High helium isotope ratios, indicating an active process in the Earth's mantle. 4. Zones of high electrical conductivity in the crust and upper mantle. 5. Near the faults, positive anomalies of the gravity field are formed, associated with the transformation of rocks under the influence of heating. The search for gold-bearing sulphide deposits should be continued, considering that the reserves already explored in Ukraine guarantee commercially viable production of over 10 tons of gold per year.

Magnetotelluric imaging of the Mérida Andes and surrounding areas in Venezuela

SUMMARYThe Caribbean and South American tectonic plates bound the north-eastwards expulsion of the North Andean Block in western Venezuela. This complex geodynamic setting resulted in the formation of major strike-slip fault systems and sizeable mountain chains. The 100-km-wide Mérida Andes extend from the Colombian/Venezuelan border to the Caribbean coast. To the north and south, the Mérida Andes are bound by hydrocarbon-rich sedimentary basins. Knowledge of lithospheric structures, related to the formation of the Mérida Andes, is limited though, due to a lack of deep geophysical data. In this study, we present results of the first broad-band magnetotelluric profile crossing the Mérida Andes and the Maracaibo and Barinas–Apure foreland basins on a length of 240 km. Geoelectrical strike and dimensionality analysis are consistent with 1-D or 2-D subsurface structures for the sedimentary basins but also indicate a strong 3-D setting for the Mérida Andes. Using a combination of 2-D and 3-D modelling we systematically examined the influence of 3-D structures on 2-D inversions. Synthetic data sets derived from 3-D modelling allow identification and quantification of spurious off-profile features as well as smoothing artefact due to limited areal station coverage of data collected along a profile. The 2-D inversion models show electrically conductive basins with depths of 2–5 km for the Barinas-Apure and 2–7 km for the Maracaibo basins. A number of resistive bodies within the Maracaibo basin could be related to active deformation causing juxtaposition of older geological formations and younger basin sediments. The most important fault systems of the area, the Boconó and Valera Faults, cross-cut the Mérida Andes in NE–SW direction along its strike on a length 400 km and N–S direction at its centre on a length 60 km, respectively. Both faults are associated with subvertical zones of high electrical conductivity and sensitivity tests suggest that they reach depths of up to 12 km. A sizeable conductor at 50 km depth, which appears consistently in the 2-D sections, could be identified as an inversion artefact caused by a conductor east of the profile. We speculate the high conductivity associated with the off-profile conductor may be related to the detachment of the Trujillo Block. Our results partially support the ‘floating orogen hypothesis’ developed to explain the geodynamic evolution of western Venezuela and they highlight the relevance of the Trujillo Block in this process.

Comparative tomography of reverse-slip and strike-slip seismotectonic provinces in the northern South Island, New Zealand

Actively deforming crust and upper mantle around the obliquely convergent Pacific-Australia plate boundary in the northern South Island of New Zealand have been investigated by seismic tomography using data from a temporary c. 50 station network plus GEONET. The Alpine-Wairau Fault (principal component of the plate boundary at the surface) transects the study area, separating the Buller-Nelson (BN) province of active compressional inversion involving steep reverse faulting to the northwest, from the Marlborough fault system (MA) dominated by dextral strike-slip faulting to the southeast. In the course of this study, MA hosted the 2013 Seddon earthquake sequence (M5.9, 6.6) and the 2016 M7.8 Kaikoura earthquake. Active fault structures and present seismicity are associated with a heterogeneous distribution of low-velocity and Vp/Vs anomalies. Elsewhere, there is a general association between crustal seismicity and low-velocity zones along major fault structures within both the MA and BN seismotectonic provinces along which high Vp/Vs anomalies are locally conspicuous. Areas of active crustal seismicity are also generally characterized by high Vp/Vs, for instance the hypocentre and aftershocks of the 2016 M7.8 Kaikoura earthquake overlie a low-velocity region with high Vp/Vs. The coincidence of anomalously low Vp and Vs and anomalously high Vp/Vs with zones of high electrical conductivity defined by a previous MT transect is consistent with the notion that upward migration of overpressured hydrothermal fluid from the subducting slab at depth leads to a heterogeneous distribution of overpressured fluid in and around the base of the crustal seismogenic zone, weakening the overlying crust and promoting seismic rupture along major fault systems. It seems possible that the Association of mid-crustal low-velocity zones with anomalously high Vp/Vs may diagnose rupture preparation zones where frictional strength is being lowered by the build-up of fluid overpressure concurrent with accumulating shear stress, so that eventual fault failure is ‘dual-driven’.

Duration of the zone of high electrical conductivity at the detonation of RDX of different densities

This paper presents the results of measuring the duration of the zone of high electrical conductivity during detonation of RDX of different density. It is shown that the density of RDX has a weak effect on the duration of this zone.

Magnetotelluric study of the Remiremont-Epinal-Rambervillers zone of migrating seismicity, Vosges (France)

AbstractThe magnetotelluric method has been used to image the deep electrical structure of the Remiremont-Epinal-Rambervillers region in the French Vosges Massif, which has presented a significant seismic activity in the past decades. Several earthquakes of moderate magnitude (up to 5.1) occurred in this area with a systematic migration along a nearly N-S direction. Inversion of the magnetotelluric data reveals zones of high electrical conductivity. A large conductive body presents a significative spatial correlation with the region that was most recently affected by earthquakes. This conductive body is interpreted as a consequence of the presence of a fluid filled basement fault network in proximity to the zone affected by the last seismic crisis, where fluid pressure diffusion takes place for several years after the main shock and participates in maintaining a microseismic activity.

Grain size effect on the electrical conductivity of clinopyroxene

Complex impedance spectra of polycrystalline samples (with grain size fractions ~5–63, 63–160 and 160–250 μm) and a single crystal sample (with orientation parallel to b), prepared from a natural megacryst augite, were measured in a piston cylinder apparatus at 10 kbar and 500–1,000°C and with a Solartron 1260 Impedance/Gain Phase analyzer over a frequency range of 0.1–106 Hz. The main charge carriers are attributed to small polarons, and the activation enthalpy is 83 ± 3 to 90 ± 3 kJ/mol. The measured electrical conductivity shows no difference between the polycrystalline and single crystal samples, suggesting independence of electrical conductivity on grain size given a change above ~5 μm. The electrical conductivity of augite is much higher than that of olivine, indicating that, if regionally enriched, augites may lead to zones of high electrical conductivity and electrical anisotropy in the deep lithosphere.

Stratigraphy of the Archean western Superior Province from P- and S-wave receiver functions: Further evidence for tectonic accretion?

The Archean western Superior Province in Canada represents the nucleus of the North American continent whose origin has been speculated to be the result of widespread crustal accretion some 2.7 Ga ago. In this paper, crustal and upper-mantle seismic discontinuities beneath the western Superior Province of the Canadian shield are imaged with teleseismic P-to-S and S-to-P converted phases using the receiver function method. Three crustal discontinuities are observed: the Moho, ranging in depth between 38 and 47 km and dipping to the south; and two intra-crustal discontinuities having depths of approximately 15 and 30 km. The crustal discontinuities undulate laterally and often lose continuity, possibly indicating an imbricated structure and/or regions of velocity gradients. In the shallow lithosphere, a positive discontinuity is imaged at approximately 65 km depth and is consistent with earlier refraction and wide-angle reflection results. Additionally, two zones of negative receiver function amplitudes at 55 km depth are observed and are coincident with a region of anomalous tomographic low P- and S-wave velocities as well as a zone of high electrical conductivity. The images for the crust and shallow upper-mantle, when integrated with previous geophysical studies, are consistent with ideas of continental root formation due to imbrication of Archean subducted material and accretion of island arcs observed in surface geology.

Correlation between crustal high conductivity zones and seismic activity and the role of carbon during shear deformation

The electrical conductivity of the lower crust is anomalously high in many locations around the world. Well‐interconnected grain boundary carbon not only has the potential for increasing the electrical conductivity of the rock but also would be expected to reduce its shear strength. We report a new analysis of field observations and new laboratory measurements consistent with deep carbon‐bearing rocks causing observed high conductivities and crustal weaknesses associated with increased seismicity. The field data indicate a correlation between the depths to a zone of high electrical conductivity observed in Transdanubia in Hungary, earthquake focal depths, and zones of high seismic attenuation. The laboratory triaxial deformation experiments show that progressive shearing of a fracture in carbon‐bearing rock can result in a weaker more electrically conductive fracture. These results provide strong evidence for the role of carbon at depth in both electrical conduction and seismotectonics, explaining the correlation between mid‐crustal high reflectivities and high conductivities observed at many locations worldwide.

Comparison of electrical conductivity structures and 2D magnetic modelling along two profiles crossing the Beattie Magnetic Anomaly, South Africa

Two of the Earth’s largest geophysical anomalies, the Beattie Magnetic Anomaly (BMA) and the Southern Cape Conductive Belt (SCCB) extend across the southern African continent for more than 1000 km in an east-west direction. Based on previous electrical and magnetometer array measurements it is believed that both anomalies have a common crustal source with a width of 50 km represented by serpentinized palaeo-oceanic crust. New two-dimensional (2D) electrical conductivity models along a profile from Prince Albert to Fraserburg outline a narrow (2 km wide), southward-dipping zone of high electrical conductivity in the upper crust below the centre of the Beattie Magnetic Anomaly (BMA). Two-dimensional modeling studies of aero-magnetic data show that simple models that can explain the magnetic signature of the BMA, are not consistent with a narrow conductivity anomaly. Thus a common source for the two anomalies is unlikely. A second magnetotelluric (MT) experiment across the BMA, conducted along a 75 km profile centred on Jansenville, 350 km east of the first profile, resolves a sub-vertical and narrow conductivity anomaly below the centre of the BMA. At this location the conductor is reaching deeper to lower crustal levels and is inclined towards the north.

A high-resolution magnetotelluric survey of the Iapetus Suture Zone in southwest Scotland

SUMMARY Magnetic variation (MV) surveys show a 40 km wide zone of high electrical conductivity in the middle crust extending across the Southern Uplands of Scotland from the North Sea to Galloway. A new magnetotelluric (MT) survey across the conductor in Galloway was designed to improve the resolution of structure in the depth range 1–15 km, and, at the same time, assess the improvement in resolution that could be achieved in a practical situation by optimizing data acquisition, processing, analysis and modelling. A 40 km long profile was measured, doubled for part of its length at a separation of 1 km, with a typical site spacing of 2 km or less. It was supplemented by shorter parallel and orthogonal lines, giving a total of 40 sites. Galvanic distortion models involving regional 2-D structures gave poor fits to the impedance tensors both for the array as a whole and for individual sites, confirming the picture from induction arrow maps that many sites were influenced by 3-D structures on scales of a few kilometres and upwards. Other indicators of the electrical strike (regional and local MV measurements, the spatial structure of the Groom–Bailey regional impedance phase and of the rotated off-diagonal phase), gave values more in agreement with the geological strike of N52°E, which was adopted as the most appropriate coordinate system into which to resolve the data. The MT impedances were accordingly rotated into directions N38°W (‘TM’) and N52°E (‘TE’), and inverted using the 2-D code of Rodi & Mackie. The investigation explored the sensitivity of the outcome to a wide range of starting model resistivities and roughness parameters, and to different data subsets. The ‘TE’ mode data were insensitive to structural detail, and the best fit achieved was unacceptable, suggesting the influence of structure outside the plane of the section. The ‘TM’ mode data could be fitted satisfactorily by varying the model roughness and gave the best control on the geometry of the conductive bodies. Forward modelling showed that the model generated from the ‘TM’ mode also fitted the MV data adequately. The inversion resolved the conductive zone into distinct blocks, with edges matching faults mapped in the surface geology: the Leadhills, Fardingmullach and Orlock Bridge Faults, and the Moffat Valley lineament. In the resistivity image, the faults are vertical structures, disrupting other features to a depth of 15 km, which agrees with the view that, although they originated as thrusts, they were reactivated by strike-slip motion during oblique closure of the Iapetus Ocean. Within some of the blocks are highly conductive regions, no more than 4 km wide. The most likely explanation is graphite or other metallic mineralization, localized in shear zones.

Partial melt or aqueous fluid in the mid-crust of Southern Tibet? Constraints from INDEPTH magnetotelluric data

SUMMARY The INDEPTH project has applied modern geophysical techniques to the study of the crustal structure and tectonic evolution of the Tibetan Plateau. In the Lhasa Block, seismic reflection surveys in 1994 detected a number of bright-spots at 15‐20 km depths that indicate zones of crustal fluids (aqueous fluids or partial melt). Coincident magnetotelluric (MT) data collected in 1995 detected a major zone of high electrical conductivity at the same depth as the brightspots. Using constrained inversion, the MT data require a minimum crustal conductance of 6000 S. This abnormally high electrical conductance can be best explained by a layered model with fluids: partial melt, aqueous fluids or a combination of partial melt and aqueous fluids. The non-uniqueness of the MT method means that a wide range of melt fraction‐thickness combinations for the above models could all explain the 6000 S conductance. To distinguish between these three models, other geophysical and geological data are required. Reflection seismic data suggest that a high fluid content (>15 per cent) is present at the top of the layer. The amplitude-versus-offset data suggest that the top of this layer may be aqueous fluids rather than partial melt. Passive seismic data imaged a 20 km thick layer of lower fluid content that is probably partial melt. Petrological studies suggest that concentrations of aqueous fluids above 0.1 per cent at mid-crustal depth cannot be sustained. Taken together, these data show that the high conductivity in Southern Tibet is most probably the result of a relatively thin layer of aqueous fluids (100‐200 m) overlying a thicker zone of partial melt (>10 km).

Upper crustal fluid flow in the outboard region of the Southern Alps, New Zealand

AbstractThe currently active fluid regime within the outboard region of the Southern Alps, New Zealand was investigated using a combination of field observations, carbon‐ and oxygen‐stable isotopes from fault‐hosted calcites and interpretation of magnetotelluric (MT) data. Active faulting in the region is dominated by NE striking and N striking, oppositely dipping thrust fault pairs. Stable isotopic analyses of calcites hosted within these fault zones range from 10 to 25‰δ18O and from −33 to 0‰δ13C. These values reflect mixing of three parent fluids: meteoric water, carbon‐exchanged groundwater and minor deeper rock‐exchanged fluids, at temperatures of 10–90°C in the upper 3–4 km of the crust. A broad, ‘U‐shaped’ zone of high electrical conductivity (maximum depth c. 28 km) underlies the central Southern Alps. In the ductile region of the crust, the high‐conductivity zone is subhorizontal. Near‐vertical zones of high conductivity extend upward to the surface on both sides of the conductive zone. On the outboard side of the orogen, the conductive zone reaches the surface coincident with the trace of the active Forest Creek Faults. Near‐surface flow is shown to dominate the outboard region. Topographically driven meteoric water interacts, on a kilometre scale, with either carbon‐exchanged groundwater or directly with organic material within Pliocene gravels, resulting in a distinctive low 13C signal within fault‐hosted calcites of the outboard region. The high‐strain zone in the lower crust focuses the migration of deeply sourced fluids upward to the base of the brittle–ductile transition. Interconnected fluid is imaged as a narrow vertical zone of high conductivity in the upper crust, implying continuous permeability and possibly buoyancy‐driven flow of deeply sourced fluids to higher levels of the crust where they are detected by the isotopic analysis of the fault‐hosted calcites.

Groundwater flow and geochemistry in the lower reaches of the Yellow River: a case study in Shandong Province, China

Water samples were collected from the Yellow River and from wells for chemical and isotopic measurement in the counties of Yucheng and Qihe, to which 6–9×108 m3 of water is diverted annually from the Yellow River. A zone of high electrical conductivity (EC) in groundwater corresponds well on the regional scale with a ridge in groundwater level, which is the main flow path through the region, but has a low gradient. The zone of highest EC along this ridge occurs at a position with the lowest ground altitude in the study area. The unique characteristic of the groundwater is the linear relationship among the principal anions as the result of mixing. The mixing effect is confirmed by its isotopic signature, which was then used to calculate the contributions from three sources: rainfall, old water, and diverted water with an average mixing rate of 18, 17, and 65%, respectively. As an indicator of water movement, Cl– content varies across a wide range in the profile from 30–10 m with a maximum concentration at about 1.2 m depth. Concentrations are relatively stable at about 2 m, which is the average boundary of the saturated and unsaturated zone. The water from the Yellow River has proved to be dominant in mixing in the aquifer in terms of groundwater flow and geochemistry.

Groundwater flow and geochemistry in the lower reaches of the Yellow River: a case study in Shandang Province, China

Water samples were collected from the Yellow River and from wells for chemical and isotopic measurement in the counties of Yucheng and Qihe, to which 6–9×10⁸ m³ of water is diverted annually from the Yellow River. A zone of high electrical conductivity (EC) in groundwater corresponds well on the regional scale with a ridge in groundwater level, which is the main flow path through the region, but has a low gradient. The zone of highest EC along this ridge occurs at a position with the lowest ground altitude in the study area. The unique characteristic of the groundwater is the linear relationship among the principal anions as the result of mixing. The mixing effect is confirmed by its isotopic signature, which was then used to calculate the contributions from three sources: rainfall, old water, and diverted water with an average mixing rate of 18, 17, and 65%, respectively. As an indicator of water movement, Cl– content varies across a wide range in the profile from 30–10 m with a maximum concentration at about 1.2 m depth. Concentrations are relatively stable at about 2 m, which is the average boundary of the saturated and unsaturated zone. The water from the Yellow River has proved to be dominant in mixing in the aquifer in terms of groundwater flow and geochemistry.

Fluids in a backthrust regime (Southern Alps, New Zealand)

New Zealand's Southern Alps are the surface expression of recent (c. 5Ma) oblique convergence between the Australian and Pacific plates. A broad, ‘U shaped’, zone of high electrical conductivity (maximum depth c. 25km) underlies this orogen. In the ductile region of the crust, the high conductivity zone is sub-horizontal and is interpreted to be a zone of interconnected fluid marking a decollement zone above the root of the orogen. Near vertical zones of high conductivity extend upwards to the surface on both sides of the conductive zone. On the outboard side of the orogen, which is a broad zone of thrust faulting and folding, the conductive zone reaches the surface at the trace of an active fault. Carbon and oxygen isotopic analysis of material from the faults suggests that mid-crustal fluids have been present. Mid-crustal fluid appears to have been transported upwards in both the ductile and brittle parts of the crust within the ‘backthrust’ region of the orogen.

Electromagnetic investigation of the Eyre Peninsula conductivity anomaly

Seafloor and land magnetotelluric (MT) data were collected in the SWAGGIE (Southern Waters of Australia Geoelectric and Geomagnetic Induction Experiment) project in April-May 1998, from 30 seafloor and 23 land sites. The principal objective of the experiment was to delineate the strike and depth of a zone of high electrical conductivity, known as the Eyre Peninsula Anomaly (EPA) in South Australia. Three linear arrays of marine magnetotelluric instruments were deployed across the continental shelf and slope to locate the offshore extension of the EPA on the continental margin, and to image the continental-oceanic lithosphere-asthenosphere transition. A land array of magnetometers was deployed at the same time to better resolve the EPA in the southern Eyre Peninsula. Robust remote-reference processing of time-series magnetic and electric data gives good MT and geomagnetic depth sounding responses in the bandwidth of 101 to 105 s, corresponding to skin-depths of mid-crustal (10 km) to mantle transition (400 km) range. Initial processing of marine and land data clearly indicates that the EPA is continuous to the edge of the continental shelf, with a conductance greater than 15 000 S confined to a narrow, near-vertical zone. At sites distant from the EPA, one-dimensional MT inversions fit the data well and provide a background conductivity structure for two and three-dimensional forward and inverse modelling of the EPA.

Model of Electrical Conductivity Distribution across Central Europe

This contribution is aimed at suggesting and analyzing a thin sheet model of the electrical conductivity structure beneath Central Europe that would fit the induction response data, specifically the in-phase induction vector distribution. In particular, attention is paid to fitting the reversals of the induction vectors that suggest the existence of elongated zones of high electrical conductivity distinguishing main structural units.

α/β phase transition in quartz monitored using acoustic emissions

SUMMARY It is usually suggested that thermal cracking in a quartz-bearing rock results from the anomalously high volumetric expansion coefficients of quartz (e.g. Simmons & Cooper 1978). It has also been recognized that thermal expansion mismatch and mineral anisotropy contribute to thermal cracking in materials that consist of a polycrystalline aggregate composed of several anisotropic minerals even in the absence of a temperature gradient (Friedrich & Wong 1986). Experiments investigating thermal cracking in rocks commonly involve imaging and quantitative stereology of crack populations induced in rocks treated to various peak temperatures (e.g. Friedrich & Johnson 1978; Homand-Etienne & Troalan 1984; Atkinson, McDonald & Meredith 1984; Meredith & Atkinson 1985). Here we report on acoustic-emission experiments that monitor the process of thermal cracking as it occurs during heating, supported by measurements of crack surface area, pore-fluid permeability, porosity and surface conductivity carried out on rock samples treated to various peak temperatures. The acoustic-emission measurements show a strong peak of microcracking at the phase transition temperature for quartz (˜573°C) superimposed upon a background of microcracking due to thermal expansion. There is also a clear peak of microcracking at higher temperatures (˜800°C) that can be attributed to oxidation-dehydroxylation reactions of hornblende and chlorite. Measurements of fluid permeability, pore surface area, porosity and electrical conductivity, made on samples that have been heat treated to various maximum temperatures, show increases associated with a major episode of cracking in the 500-600°C temperature range, indicating that the new cracks form a well-interconnected network. This has been confirmed by SEM and optical microscopy These results have implications for the electrical conductivity of the continental crust, providing a mechanism enabling the high pore-fluid connectivity needed to explain zones of high electrical conductivity at depth providing that cracks opened in this way remain open at the high pressures existing at depth. It should be recognized, however, that th***se measurements are limited in their direct application since they were obtained under initially dry conditions at laboratory pressures.