Migration Of Hot Fluids Research Articles

A New Mechanism for Earthquake‐Enhanced Permeability

AbstractEarthquakes may enhance the permeability of Earth's crust, but the mechanisms for such changes are not entirely clear. Recent studies have revealed a distinct coseismic response at large epicentral distances to great earthquakes and at close distances to major earthquakes , where both the horizontal and vertical permeabilities increased by orders of magnitude and the increases stayed at the co‐seismic level for many years, which cannot be explained by the existing models. To explain such prolong increases of the vertical permeability I propose a new mechanism, supported by laboratory and field observations of earthquake‐triggered hydrogeologic processes, that great earthquakes may pressurize and liquefy groundwater systems and inject the liquefied sands into fractures to keep them open. The proposed mechanism is testable and implies prolonged upward migration of hot fluids from deep basins, which may be an important process for subsurface transport of heat and solutes and may also be useful to understand the safety of groundwater and the security of underground repositories.

Structural analysis of the Planchón-Peteroa Volcanic Complex: Insights for the geothermal system

The Planchón-Peteroa Volcanic Complex (PPVC) is located in the Transitional Southern Volcanic Zone of the Andean Ridge. Structural control of the main NNE-striking El Fierro fault system (EFFS) over the volcanic activity has been commonly assumed, although a paucity of evidence remains. The aim of this paper is to explore the relationship between the local stress field, the superficial structural setting and the geothermal fluid flow paths related to the volcanic complex. To conduct the structural analysis, this work combines remote sensing determination of lineaments, recognition of fracture patterns, with the inversion of kinematic indicators on outcrop scale faults, to finally evaluate the role of the local stress field over the 2D slip and dilation tendencies of the recognized structures. Mesoscale morphostructural lineaments present three main directions: ~E-W, ~NE-SW, and ~NW-SE. Major lineaments develop inflections, giving rise to left bends between the NW-SE to E-W along the strike, compartmentalized by ~ NNE trending lineaments. Strike-slip fault solutions with an ENE-WSW to NE-SW oriented shortening axis were obtained from fault-slip data analysis. The inversion of fault kinematic indicators constrains a Quaternary to recent strike-slip regime, with a ~ENE-WSW trending σ1, and a subvertical σ2. Under the defined local stress field, ~NE-SW and ~WNW-ESE oriented structures have high slip tendency, while those spanning from NE-SW to E-W are prone to dilate. Our analysis suggests that these transverse structures exert first-order controls on the location of Vergara Pass Hill, and Peñón River and Azufre River Valleys hydrothermal manifestations. It seems that intersection zones with submeridian structures, as EFFS, increase structural damage and facilitate hot fluid migration. Circulation would profit the most from ~ENE-WSW and ~NW-SE striking outcrop-scale faults and fractures associated with the damage zones.

Slow cooling versus episodic fluid injections: Deciphering the Caledonian orogeny in Vestvågøy, Lofoten islands, Norway

AbstractThe determination of the thermal (temperature–time) histories of high‐P metamorphic terranes has been commonly based on the concepts of slow cooling and closure temperatures. In this paper, we find that this approach cannot reconcile a geochronological data set obtained from the amphibolite‐facies allochthonous Leknes Group of the Lofoten islands, Norway, which reveals an extremely complex thermal history. Using detailed results from several different geochronometers such as 40Ar/39Ar, Rb–Sr and U–Pb, we show that a model invoking multiple, short‐lived thermal pulses related to hot‐fluid infiltration channelized by shear zones can reconcile this complicated data set. This model suggests that hot fluids infiltrated throughout basement shear zones and affected the overlying cold allochthon, partially resetting U/Pb rutile and titanite ages, crystallizing new zircon and produced identical 40Ar/39Ar and Rb/Sr ages in muscovite, biotite and amphibole in various rocks throughout the region. This paper shows the enormous potential of coupling laser Ar‐spot data with thermal modelling to identify and constrain the duration of short‐lived events. An optimal P–T–t history has been derived by modelling the age data from a previously dated large muscovite crystal (Hames & Andresen, 1996, Geology, 24:1005) and using Zr‐in‐rutile thermometry which is consistent with all geochronological data and geological constraints from the basement zones and allochthon cover. This tectonothermal model history suggests that there have been three episodic hot‐fluid and 40Ar‐free infiltration events, resulting in the total resetting of Ar ages during the Scandian (425 Ma) for 1 Ma at 650°C and two reheating events at 415 Ma for 400 ka at 650°C and at 365 Ma for 50 ka at 600°C, which are modelled as thermal spikes above an ambient temperature of 300°C. Independent confirmation of these parameters was provided by Pb‐diffusion modelling in rutile and titanite. The model suggests that the amphibolite facies rocks of the Leknes Group probably remained cold before being exhumed for at least 60 Ma (425–365 Ma) and successfully explains the presence of different minerals that crystallized or were totally/partially reset in the allochthon and in the basement. The migration of hot fluids for short periods of times within conduits extending through the basement and allochthon rock units is likely associated with episodic seismic activity during the Caledonian orogeny.

Geomechanical model for a seismically active geothermal field: Insights from the Tinguiririca volcanic-hydrothermal system

In this work, we develop a multidisciplinary approach to investigate a geothermal system located at the volcanic arc of a subduction-related orogen and highlight the interplay between active tectonism, stress field and fluid migration. By using results of field investigations from the Tinguiririca geothermal field in the High Andes of Chile (35°S), empirical analysis, and numerical models of static stress variations, we proposed a geomechanical model for evaluating the distribution of hydrothermal manifestations in a seismically-active region. The present geomechanical model follows four major steps: (1) development of the 3D structural model of fault pattern; (2) estimation of the in-situ stress field; (3) calculation of the resolved-shear-to-normal-stress ratio (slip tendency) on each fault with varying geomechanical parameters (coefficient of friction, pore pressure and cohesion) as inputs; and (4) estimation of Coulomb static stress changes as a consequence of failure in a nearby fault. Through combination of all these analyses, we characterize in detail both the active deformation in the geothermal field and its relationship with hot fluid migration.

Coalified bitumens from the Kłodawa Salt Structure (central Poland) as evidence of migration of hydrothermal fluids in Zechstein (Upper Permian) deposits

The paper presents the results of petrographic and geochemical studies of coaly matter occurring in Stinking Shale and Main Dolomite (PZ2, Upper Permian) deposits in the Klodawa Salt Structure in central Poland. The coaly matter is both dispersed in the PZ2 shales and carbonates and concentrated in veins which crosscut these deposits and, depending on the location, depicts contrasting maceral composition and the rank of coal. The veins are built of colotellinite and micrinite, showing a mean random reflectance of coal in the range of 1.38–1.40%, whereas the dispersed organic matter is composed mostly of semifusinite and fusinite with minor vitrinite, showing a mean random reflectance of 0.82%. It is interpreted that the heterogeneous composition and rank of coal result from migration of hot fluids within a fracture system developed in the PZ2 shale and carbonate beds. The influx of hot fluids into the organic matter-rich beds led to the generation of bitumen which catagenetically altered along the hot fluid conduits. The coalification of bitumens took place at an early stage of tectonic deformation of the Zechstein salt series, initiated by regional extension and normal displacement on a basement fault, most probably in the Late Permian–Early Triassic.

Hydrothermal alteration of the Ediacaran Doushantuo Formation in the Yangtze Gorges area (South China)

The geochemical and fossil record preserved in the Ediacaran age (635–551Ma) Doushantuo Formation of South China has been extensively examined to explore the impact of changing climate and the oxidation state of the oceans on the development and distribution of early multicellular life. In the Yangtze Gorges area, this formation shows many of the geochemical trends and features thought to typify global ocean chemistry in the Ediacaran Period, but there are indications that post-sedimentary processes modified these signals. This study of clay minerals and organic matter builds a more detailed picture of the type and degree of post-sedimentary alteration at different stratigraphic levels of the formation and focuses on how this alteration influenced stable carbon and oxygen isotope records.In the cratonward Jiulongwan and Huajipo sections of the Doushantuo Formation, its lower part (Members 1 and 2) consists largely of dolomitic shale, rich in authigenic saponite that crystallized in an alkaline sedimentary basin. Saponite has been altered to chlorite via corrensite across tens of meters of strata in lower Member 2, with increased alteration downward toward the cap dolostone. The greater chloritization is accompanied by lower δ18O and higher δD values of trioctahedral clays. This pattern of alteration of trioctahedral clays is likely due to hydrothermal fluid activity in the underlying, relatively permeable Nantuo Formation and cap dolostone. A concomitant increase of solid bitumen reflectance toward the base of the formation supports this idea. In the uppermost part of the formation in the Yangtze Gorges area (Member 4), a typical open water marine dolomitic shale rich in illite and organic matter, increases in the methylphenanthrenes ratio index and solid bitumen reflectance correlate with decrease of the bulk rock K/Al ratio upward, providing evidence for hot fluid migration above the nearly impermeable shale.Clay from the upper part of the formation is enriched in 18O, but not in D, relative to clay from the lower parts, indicating progressive 18O-enrichment of hydrothermal fluids that percolated upward and laterally through permeable 18O-rich carbonates. A maximum hydrothermal-alteration temperature of ∼200°C is estimated from a calibration curve for illitization during burial diagenesis, but given that the hydrothermal activity probably occurred in short pulses, the temperature could have been much higher. K–Ar ages are consistent across different size fractions of fine illite from Member 4 shale (∼430Ma) and from a K-bentonite bed near the base of Member 2 in the Jiuqunao section (∼325Ma), ∼25km from Jiulongwan and Huajipo. These age values show that the diagenetic illite of the Doushantuo Formation is a product of either deep burial diagenesis overprinted by spatially limited hydrothermal activity or of two localized hydrothermal events.Patterns of carbonate 13C and 18O depletion in the basal Doushantuo Formation are similar to chloritization trends and 18O variation in diagenetic clay minerals. Given independent evidence for 13C depletion of hydrothermal fluids, these trends indicate carbonate–fluid isotope exchange commensurate with the degree of post-sedimentary alteration, supporting a model of lithologically controlled differential diagenesis induced by hydrothermal fluids as the main control on C and O isotope variability in this stratigraphic interval. This model could potentially explain other notable δ13C excursions higher up in Member 3.

Hydrocarbon migration in the Zagros Basin, offshore Iran, for understanding the fluid flow in the Oligocene–Miocene carbonate reservoirs

Kuh-e Mond Field is a conventional heavy oil resource in the Zagros foreland Basin, Iran, produced from the fractured carbonates partially filled by dolomite, calcite, and anhydrite cement. Vitrinite reflectance data from carbonate reservoir suggest low-maturation levels corresponding to paleotemperatures as low as 50 °C. The observed maturation level (<0.5% Rmax) does not exceed values for simple burial maturation based on the estimated burial history. Oil inclusions within fracture-filled calcite and dolomite cement indicate the key role of these fractures in oil migration.The fluid inclusion temperature profiles constructed from the available data revealed the occurrence of petroleum in dolomite, calcite, and anhydrite and characterize the distinct variations in the homogenization temperatures (Th). Fluid inclusions in syntectonic calcite veins homogenize between 22 °C and 90 °C, showing a salinity decrease from 22 to 18 eq. wt.% NaCl. Fluid inclusions in anhydrite homogenize at <50 °C, showing that the pore fluids became warmer and more saline during burial. The Th range in the calcite-dolomite cement depicts a change in water composition; therefore, we infer these cements precipitated from petroleum-derived fluids. The microthermometry data on the petroleum fluid inclusions suggest that the reservoir was filled with heavy black oils and high-salinity waters and indicate that undersaturated oil was present in a hydrostatically pressured reservoir.The Th data do not support vertical migration of hot fluids througout the section, but extensive lateral fluid migration, most likely, drove tectonically dewatering in the south or west of the pool.

A km-scale illite alteration zone in sedimentary wall rocks adjacent to a hydrothermal fluorite vein deposit

AbstractIn a study of wall-rock alteration in the 1.4 km long Würmtal adit at the Käfersteige hydrothermal vein deposit (northern Black Forest, Germany) illite was found to be the only clay mineral within the Bunter sandstone and intercalated claystone. Illite occurs mainly as a detrital mineral in the claystone, whereas it is hydrothermally neoformed in the sandstone, either in the pores or as an alteration product of K-feldspar. The extensive occurrence of authigenic illite along the entire 1.4 km long profile confirms that the fluids migrated far into the sandstone.The authigenic illite formed during a first pulse of high-temperature fluids (Th of ~220°C) with a low salinity (~1 wt.% NaCleq). These fluids also dissolved Sr and Rb from detrital illite of the claystones at the vein. A later hydrothermal pulse with a lower temperature (~70–150°C) and higher salinity (21–28 wt.% NaCleq) silicified the sandstone adjacent to the vein and caused partial substitution of OH- by F- in the structure of the detrital and neoformed illite along the profile.Within analytical error, the K-Ar dates for the neoformed illite of the &lt;2 µm fraction are the same along the profile (~150 Ma). During this hydrothermal process, the age of the detrital illite within the claystone was reset from 310 to 190 Ma. The illite-rich &lt;0.2 µm fractions yield ages of ~142 Ma (sandstone) indicating a Jurassic origin. The uniform age data for illite in the sandstone and in the claystone are probably due to extensive migration of hot fluids through the wall rocks.The hydrothermal fluids are attributed to recycled meteoric water and brines that ascended from the basement into the cover rocks during the opening of the North Atlantic and/or the nearby Tethys area.

Stratigraphic control of hot fluids on anthracitization, Lackawanna synclinorium, Pennsylvania

The Lackawanna synclinorium of Pennsylvania contains abundant coal that was altered to anthracite rank during the Late Paleozoic Alleghanian orogeny. Why did the coal in the synclinorium undergo anthracitization? Two alternatives have been suggested. (1) The region was buried deeply during and/or after the Alleghanian orogeny and thus became hot in response to Earth's geothermal gradient; and (2) hot fluids migrating from the orogen toward the foreland carried heat into the coal and caused anthracitization. If the second hypothesis is correct, the region should contain evidence that hot fluids passed through the coal. Field observations, illite-crystallinity studies and fluid-inclusion analyses indicate that the coal-bearing Pottsville and Llewellyn Formations, and an underlying detachment fault, called the Pottchunk fault (at or slightly below the base of the Pottsville Formation), acted as a regional aquifer for the migration of hot fluids during the Alleghanian orogeny. The presence of quartz veins and the hydrothermal minerals tosudite and pyrophyllite in strata above the Pottchunk fault, and the apparent absence of an illite-crystallinity burial gradient in the underlying strata, suggest that fluids migrated through large pores in coarse-grained sandstones, through abundant fractures that developed in response to Alleghanian deformation, and along the Pottchunk fault. The quartz veins, tosudite and pyrophyllite mineralization do not occur below the Pottchunk fault (except at one locality), suggesting that fluid flow was greater above the fault, perhaps because fracturing accompanying deformation increased permeability of the fault's hanging wall. Fluid inclusions in the Pottsville and Llewellyn Formations indicate fluids achieved a minimum temperature of 270 °C at a depth of ∼3.1–8.5 km. Heat-flow calculations constrained by fluid-inclusion data show the heat carried by the migrating fluids through the coal-bearing strata was sufficient to elevate the rock temperature to anthracite-grade conditions (∼250 °C), assuming that the fluid event lasted about 1 year. Thus, deep burial (6–9 km) of the coal-bearing strata in the Lackawanna synclinorium was not required for anthracitization. Anthracitization was likely the result of stratigraphically controlled hot fluid migration through the coal-bearing strata at shallow depths (≤5 km).

Indicators of hot fluid migration in sedimentary basins: evidence from the UK Atlantic Margin

Microthermometric, petrographic and isotopic methods have been used to detect evidence for hot fluid flow in Mesozoic and Tertiary sediments from the NW UK continental margin, West of Shetland. New data presented here show that temperatures are hotter by c . 40°C in Tertiary samples than in the underlying Jurassic and Cretaceous sediments in wells 204/28-1, 206/5-2, 208/27-1, especially in cements from samples as young as mid–upper Eocene in age. Paleocene samples can be discriminated from older (Jurassic and Cretaceous) and younger (Eocene) sandstones on the basis of silica cement morphology and cathodoluminescence zonation. Jurassic, Cretaceous and Eocene quartz cements show oscillatory zoning as a consequence of relatively slow burial cementation. In direct contrast, rapid precipitation of silica cements from the cooling of hot fluids has produced unzoned cements in all but one Paleocene sample. No evidence for unzoned quartz cements was noted in any pre-Paleocene or Eocene samples. The restriction of hot fluid inclusions and unzoned cements to the Paleocene and post-Paleocene is consistent with lateral focusing of hot fluids. Isotopic data from kaolinites indicate that these fluids are best represented by mixtures of Mesozoic or Tertiary meteoric waters and marine porewaters that have undergone isotopic alteration through interaction with volcanic material. Our results indicate that hot fluid flow occurred over a relatively long time-scale (i.e. several million years), which may have important consequences for the degradation of reservoired hydrocarbons in West of Shetland Paleocene plays.

Postdepositional Evolution of Platform Claystones Based on a Simulation of Thermally Induced Diffusion of Radiogenic 40Ar from Diagenetic Illite

ABSTRACT The Lower Cambrian Lontova claystones contain well crystallized authigenic 1M illite mixed with detrital 2M illite. Part of the authigenic illite probably precipitated during sedimentation and subsequently experienced a temperature up to 140°C, despite a limited burial to 500-1000 m. The difference between the Rb-Sr and K-Ar dates of this authigenic illite are attributed to preferential loss of radiogenic 40Ar induced by a short-lasting thermal pulse. This interpretation is based on an Ar-diffusion model suggesting that the Lontova blue clays were heated to around 140°C for 2-5 Myr. This thermal pulse could have occurred in relation with hot-fluid migration induced by either tectonic dislocations in the basement rocks during the Late Silurian-Early Devonian, or rifting conditions during the Permian. By comparing Rb-Sr and K-Ar data, evaluation of radiogenic 40 Ar losses by small authigenic illite particles at low temperatures could be a powerful tool for reconstruction of evolutionary histories of sedimentary successions that were either shallowly buried, or percolated by migrating fluids released during tectono-thermal activities.

Episodic fluid flow within continental rift basins: some insights from field data and mathematical models of the Rhinegraben

AbstractVitrinite reflectance data from a petroleum exploration well in the northern Upper Rhinegraben show an unusual vertical maturity trend. Above and below a 500 m thick marl layer the vitrinite reflectance levels are consistent with modern, conductive, geothermal gradients. Between about 1000 and 1500 m depth, however, vitrinite reflectance levels are significantly elevated (about 0.6%Ro). This anomaly cannot be explained with one‐dimensional conductive or conductive–convective heat transfer models, and thermal effects of sedimentation or igneous intrusion seem implausible for this geological setting. The thermal anomaly that formed this maturation anomaly must have been hydrothermal in origin, two‐dimensional in nature, and persisted long enough to elevate the vitrinite reflectance values within this marl unit, yet it must have dissipated before the thermal perturbation would have altered the organic matter below and above the unit.In this study, we propose that the vitrinite reflectance anomalies were caused by a transient thermal inversion induced by episodic, lateral flow of hot (130–160°C) groundwater along conductive fractures and bedding planes. Heat flow constraints suggest that fluids must have moved rapidly up a vertical feeder fault from a depth of at least 3.6 km before migrating laterally. To test this hypothesis, we present a suite of simple, idealized mathematical models of groundwater flow, heat transfer, thermal degradation of kerogen and vitrinite systematics to explore the episodic flow that could have produced the observed thermal anomaly. In these simulations, a single, horizontal aquifer is sandwiched between two less permeable units: the total dimensions of the vertical section model are 4 km thick by 10 km long. The top of the aquifer coincides with the position of the observed thermal maturity anomaly in the Rhinegraben. Boundary conditions along the left edge of this aquifer were varied through time to allow for the migration of hot fluids out into the basin. Inflow temperature, horizontal velocity, duration and frequency of flow and thickness of the aquifer were varied. We found that a thermal maturity anomaly could only be produced by a rather restrictive set of hydrothermal conditions. It was possible to produce the observed vitrinite reflectance anomaly by a single hydrothermal flow event of 130°C fluid migrating laterally into the aquifer at a rate of 1 m a−1 for about 10 000 years. The anomaly is spatially confined to near the left edge of the basin, near the feeder fault. If the flow event lasted longer than 100 000 years, then the maturation anomaly disappeared as the lower confining unit approached steady‐state thermal conditions. It is possible that such an event occurred about 5 million years ago in response to increases in fault permeability associated with far field Alpine tectonism.

THERMAL HISTORY MODELLING AND TRANSIENT HEAT PULSES: NEW INSIGHTS INTO HYDROCARBON EXPULSION AND &amp;apos;HOT FLUSHES&amp;apos; IN THE VULCAN SUB-BASIN, TIMOR SEA

Thermal history data from wells in the Vulcan Sub- basin and adjacent platforms show clear evidence that many reservoir sections have experienced relatively shortlived, high- temperature flushes during the Late Tertiary. These transient heat pulses are related to slow migration of hot fluids and hydrocarbons from adjacent depocentres, or rapid flow of deep-seated brines during Late Miocene- Early Pliocene tectonic reactivation. The hot fluids have been focussed into structured reservoir sections via high- permeability pathways and reactivated faults. As a consequence, most exploration wells are not truly representative of the thermal regime of nearby source kitchens.In order to constrain the regional thermal and expulsion history of the region, and to address the issue of thermal pulses, burial history analysis of 44 wells and 18 depocentre sites was carried out. This analysis utilises a simplified transient heat pulse model developed as part of the WinBury™ burial and thermal geohistory modelling software. The transient and steady-state thermal history models are constrained by reflectance and fluorescence maturity data, together with apatite fission track analysis and fluid inclusion palaeo-temperature data.

Hydrocarbon migration history, West of Shetland: integrated fluid inclusion and fission track studies

In sandstone reservoir rocks in the vicinity of the Rona Ridge, West of Shetland, there is evidence for former episodes of hydrocarbon migration in addition to the episodes responsible for current accumulations. An integrated study of fluid inclusion microthermometry and apatite fission track analysis (AFTA) has helped to clarify the history of fluid migration in the region. The Upper Jurassic Rona Sandstone contains both hydrocarbon and aqueous fluid inclusions. Hydrocarbons occur in trails of secondary inclusions through detrital grains, and as primary inclusions in quartz overgrowths and authigenic feldspars. This indicates at least 2 phases of migration before the presently trapped oil entered the rock: fluorescence colours indicate that they are of distinct compositions. Associated aqueous inclusions yield minimum trapping temperatures which indicate that the earliest hydrocarbons were associated with a phase of hot fluid migration, and that cooler conditions pertained during later migration. AFTA data provide independent evidence for a passage of hot fluid early in the history of the rock, which may be related to Late Jurassic–Early Cretaceous rifting. AFTA data also suggest a second episode of heating and cooling in the Tertiary following uplift, which may have been responsible for a later stage of hydrocarbon generation and migration. The results reflect repeated expulsion of fluids from deep basinal areas onto adjacent topographic highs, throughout the late Mesozoic to Recent.

The thermal history of the New York basement determined from 40Ar/39Ar K‐feldspar studies

Constraints on the thermochronologic evolution of the New York crystalline basement are mainly restricted to the high‐temperature early cooling history following 1.1 Ga Grenville metamorphism (U‐Pb methods) and the late cooling history (fission track methods). The thermal history for the ∼one billion years between these intervals is herein assessed using 40Ar/39Ar hornblende, muscovite, biotite and K‐feldspar thermochronology. Hornblende preferred ages suggest cooling below ∼450–500°C between ca. 900 and 950 Ma. Total gas ages of biotites range from ∼746–1060 Ma, and their age spectra complexity is probably related to younger thermal events and/or excess argon. A single muscovite analysis yields a preferred age of 854 Ma. Argon isotope analyses of K‐feldspar provide a quantitative evaluation of the thermal history between ∼175 and 350°C. Multi‐diffusion domain thermochronology on highly variable K‐feldspar results yield internally consistent thermal histories and, along with geologic constraints, suggest thermal maxima of 275–350°C at ca. 700, 470–450 and 300 Ma. Combining all of the K‐feldspar analyses provides an internally consistent thermal history for the region and allows for the following inferences. Reheating at ∼700 Ma associated with Late Precambrian rifting during the formation of the Iapetus ocean apparently affected the entire Adirondack region with the highest temperatures occurring in the eastern part of New York. Local reheating in the Ordovician is inferred to have affected the eastern Adirondack Mountains and probably resulted from the combined effects of burial during the Early Paleozoic, emplacement of Taconic thrust sheets and migration of hot fluids along normal faults during the Taconic orogeny. High paleotemperatures for the Devonian section of eastern New York are related to maximum burial of the basement during the Carboniferous at ca. 300 Ma.

Mineralogical, geochemical (REE), and isotopic (K-Ar, Rb-Sr, delta 18 O) evolution of the clay minerals from faulted, carbonate-rich, passive paleomargin of southeastern Massif Central, France

ABSTRACT Mineralogical, geochemical (REE), and isotopic (K-Ar, Rb-Sr, d18O) data of < 0.2 µm clay fractions from core samples have been used to enhance our understanding of the evolution of the faulted, carbonate-rich, passive paleomargin in the southeastern French Massif Central. The fault system described at the lower part of the sedimentary sequence was active at 190 ± 20 Ma during a rifting tectono-thermal activity with important migration of hot fluids. It was active again afterwards, probably after maximum burial occurring at 110-120 Ma, as a result of the compressive Alpine activity in the region. This late shearing induced an overall throw of at least 1160 m and subsequent erosion of as much as 2000 m of s diments. However, it probably did not induce major fluid movements, which means in turn that the mineralizing fluids responsible for economic ore deposits in the margin relate to the 190-Ma-old Liassic hydrothermal event. The Rb-Sr, K-Ar, and oxygen isotopic systematics of the clay fractions suggest that late faulting activity occurred under very low water-to-rock conditions, which did not favor widespread fluid migration. The clay material of the fault yields unreasonably widely scattered Rb-Sr and K-Ar dates, from 70 to 183 Ma, because of variable contents of radiogenic 40Ar and some unrealistic initial 87Sr/86Sr ratios, even below 0.700. The 190 Ma illites that crystallized in the fault at 200-210°C also have abnormally low d18O values of about +13.3, leading to a d18O value of the pa eofluids as high as +8.1. The 87Sr/86Sr ratios and REE distributions of acid-leached clay residues delineate two types of fluids that have probably interacted with the clay minerals. One, with an 87Sr/86Sr ratio of 0.7250 and a humped REE distribution pattern, is believed to be of hydrothermal origin, and the other, with an 87Sr/86Sr ratio of 0.7082 and a flat REE pattern, might be of recent continental origin.

Hot fluid migration in compressible saturated porous media

Solutions for heat and pressure transfer in compressible thermoporoelastic, fluid-saturated media are presented. A hot and pressurized fluid reservoir is envisaged, which comes into contact with a cold, saturated half-space; the boundary between them is either pervious or impervious. In the 1-D problem, two coupled non-linear equations are obtained for the temperature and pressure fields, which are solved employing the self-similarity method. The thermally induced pressure in the half-space is found to be generally relevant for low-/intermediate-permeability media. In the pervious boundary problem, the thermal interaction gives rise to a striking effect in low-permeability materials: the induced pressure increases away from the boundary, up to a maximum, before declining to vanishingly small values further away; a backflow is driven by the reverse pressure gradient, from the point of maximum towards the reservoir. The same effect is present, although to a lesser extent, in intermediate-permeability materials, saturated with liquid water, under low boundary pressures. In the impervious boundary problem, the thermally induced pressure is relevant even for high-permeability media. The temperature field is affected by fluid flow only in the pervious boundary problem, for high-permeability media, saturated with low-viscosity fluids, subjected to high overpressure over the boundary. The conductive approximation (which is obtained after neglecting the advection and the dissipation terms in the heat transfer equation) is found to be applicable even to high-permeability rocks under the impervious boundary conditions. Even if the previous conclusions are not strictly applicable to problems with different geometries, the present solutions seem to bear important consequences on fluid-migration mechanisms in volcanic areas, where the backflow identified in low-permeability media seems to be a viable mechanism for water enrichment by magma ascending through a dyke or emplaced in a magma chamber surrounded by colder, water-saturated, low-permeability rocks. Pore pressure and temperature can be also very effective in providing rock deformation: depending on the rock type and boundary conditions, either the pore-pressure contribution or the thermal contribution may be dominant. Inflation episodes, which are often observed in volcanic areas, can be interpreted in a similar way. Thermoporoelastic effects might also be relevant in fault regions after the sudden frictional heating of water embedded between fault surfaces. The theory may be also applied (e.g. Mc Tigue 1986) to the modelling of areas of nuclear waste disposal.

Transient solutions for temperature and pressure waves in fluid-saturated porous rocks

SUMMARY In this paper, hot-fluid migration dynamics for a fluid-saturated porous rock is analysed theoretically. We first recall a Bonafede model implemented with a classical scale analysis; this allows us to discuss a fully non-linear formulation related to transient phenomena, which is solved analytically as a solitary shock wave. The resulting time-scales are particularly small, corresponding to intense short-time geothermal phenomena. A novel non-linear migration mechanism, related to Darcy convection, is also discussed.

Late Paleozoic collision, delamination, short-lived magmatism, and rapid denudation in the Meguma Terrane (Nova Scotia, Canada): constraints from 40Ar/39Ar isotopic data

The Meguma Terrane consists of 8–15 km of early Paleozoic stratified rocks, deposited on a continental basement, that were deformed and metamorphosed during the late Paleozoic as a result of lithospheric plate collision. The oldest cleavage (previously published, whole-rock, 40Ar/39Ar plateau ages of 415–395 Ma) dates the onset of crustal thickening, which was followed by voluminous, but short-lived, Late Devonian granitic and minor mafic magmatism (380–370 Ma). This magmatism may have been the product of delamination of the lower lithosphere and upwelling of asthenosphere, which effected melting above the new Moho and resulted in intrusion at depths of 5–12 km. 40Ar/39Ar plateau ages of hornblende, muscovite, and biotite indicate that, at the present erosion level, most of the Meguma Terrane cooled through ~300 °C by 368–360 Ma, slightly earlier than for the southwestern Meguma Terrane (ca. 345 Ma). The present erosion level was exhumed by the latest Devonian–Early Carboniferous (Visean): the age of the oldest unconformably overlying rocks. Subsequent burial beneath ~6 km of Carboniferous sediments would not have been sufficient to completely rejuvenate older intracrystalline mica systems and result in the observed 40Ar/39Ar mica plateau ages between ca. 350 and 260 Ma. Such rejuvenation may have resulted from migration of hot fluids along shear zones derived from the lower crust and mantle, and from granitoid magma intruded at ca. 316 Ma. The recurrence of deformation, magmatism and denudation in the middle Carboniferous suggests that further delamination may have occurred. The temperature of these fluids decreased from 400–500 °C during the Carboniferous to 300–400 °C during the Early Permian.

Hot fluid migration: an efficient source of ground deformation: application to the 1982–1985 crisis at Campi Flegrei-Italy

Some solutions of the forced heat advection problem in compressible media are worked out employing a perturbative approach and their implication for thermoelastic deformation are discussed. A sharp temperature front, which migrates at a speed in the order of Darcy flow rate, develops in the medium, giving rise to significant deformation via thermal expansion. A thermally induced pressure source accompanies the temperature front, which may be significant only in very high temperature cases. Results are applied to interpreting the uplift episode of 1982–1984 at Campi Flegrei (C.F.), near Naples, Italy. A mechanism is envisaged for uplift at Campi Flegrei in which a sudden connection is established between a deep, hot, high-pressure fluid reservoir and a shallow, relatively cold, low-pressure aquifer. The inclusion of fluid migration in the deformation model allows simple explanations of several geochemical and geophysical observations made during the bradyseismic crisis. It appears that the proposed mechanism may explain the large observed uplift, without requiring unreasonable pressure increase within the magma chamber. Furthermore, the deformation source may be allowed to be shallower than the magma chamber, as required by any reasonable deformation model at C.F.