Palaeogeothermal Gradient Research Articles

Determination of a normal orogenic palaeo-geothermal gradient with clay mineral and organic matter indices: a review

A collection of large data sets from different orogenic belts was compiled for a correlation between organic matter (OM) versus clay mineral (CM) indices calibrated with the vitrinite reflectance, (VR) vs Kübler-Indices (KI) method. Data selection was based on a normal geothermal gradient (25 to 35 °C/km) as determined in previous studies, e.g. by maturity modelling and clay mineral reaction progress calculations. In the Lower Austroalpine (Eastern Switzerland, European Alps) a 20 myr lasting metamorphic overprint caused an OM–CM thermal equilibrium among the indices used. The observed correlation enables to determine gradual changes in metamorphic factors such as pressure, temperature and time causing sensitive shifts of the gradient slope in the range of normal gradients. For New Caledonia, an identical correlation has been determined. Prior to re-equilibration of the VR/KI indices, sediments in New Caledonia of diagenetic to incipient metamorphic grade underwent a high-pressure subduction event. VR/KI indices are in or close to equilibrium, while slight differences in OM vs CM indices allow for a better understanding of polyphase conditions, especially with respect to pressure. Temperature estimations are identical despite of their poly-phase metamorphic history, which was mainly controlled by the last orogenic thermal event lasting > 5 to < 10 myr. In the eastern Helvetic Alps and Northern Calcareous Alps similar correlations were found with slightly different slopes. Comparison between different regions is possible when using KI standardization and same data discrimination. In both parts of the Alps a complex thermal history of short durations (< 5.0 myr for the Northern Calcareous Alps to 10 myr for the Helvetic Alps) caused similar VR/KI trends, but disequilibrium is suggested by weaker regression parameters. The following correlation is calculated for a moderate geotherm (55 to 74 mWm2, mean = 61 mWm2) and normal temperature gradient conditions (25 to 35 °Ckm−1): KI = 1.134e−0.305VR, (R2 = 0.880, n = 462) with VR given as %Rmax, KI as Δ°2θ (limited to values between 0.2 to 1.0 Δ°2θ). With increasing depth (z) a VR gradient of 1.4 ± 0.2%Rmaxkm−1 is determined and a KI gradient of 0.09 ± 0.002 Δ°2θ km−1 is observed. The study illustrates that a normal geotherm can be described by VR/KI correlation, even if different heating episodes may occur. For the detection of a poly-phase or plurifacial thermal history, several indices of clay minerals and organic matter with very different kinetics should be used, as e.g. demonstrated by strong differences in smectite content at equal VR/KI values versus structural depth. A specific interest is given to the correlation of vitrinite like solid bitumen reflectance as an alternative method to VR, the persistent preservation of liptinite macerals and the stability range of clay minerals and sub-greenschist facies critical minerals compared with VR/KI data. Until now, despite the Alps in this study, systematic liptinite maceral studies have not been published in other orogenic settings.

Characterization of shale gas reservoir of Lower Gondwana litho-assemblage at Mohuda sub-basin, Jharia Coalfield, Jharkhand, India

In quest of unconventional energy as shale gas, lower Gondwana shale-heterolith litho-assemblages of Mohuda sub-basin of Jharia Coalfield have been explored to envisage their potentiality for shale gas generation. A comprehensive approach has undertaken through measurement of in-situ gas content, petrography, rock-eval pyrolysis, proximate analysis, TOC, adsorption isotherm, SEM-EDX and XRD of selected drill core samples to establish potential shale horizons in Barren Measure and Barakar Formations. The in-situ gas content varies from 0.15 to 3.69 m 3/t with increasing depth. High palaeo-geothermal gradient in order of 5.57–6.19 °C/100 m is established where emplacement of intrusive played a dominant role. Interrelation of palaeodepth and vitrinite reflectance points to erosion of 2500 m strata in Mohuda sub-basin. Average TOC content of 9 to 17 wt% indicates excellent organic richness of source rock, while other parameters like S1, S2 and PY derived from rock-eval pyrolysis indicate fair to very good/excellent source condition for hydrocarbon generation. HI, OI, S2, TOC, VRo and Tmax values and their inter-relationships reveal Type III and Type IV kerogens coming under gas window zone of mature to post mature stage. Increase in volatile matter and fixed carbon and decrease in moisture content with depth suggest enhanced gas generation and retention capacity. Quartz dominated mineralogy in shale beds account for suitable hydrofracturing process. GIP of about 7.5 BCM of gas has been assessed and categorized depth wise and formation-wise for Raniganj, Barren Measure and Barakar formations. Experimentally calibrated adsorption isotherm data reveals favourable parameters like gas saturation level (GSL), reservoir pressure to be dropped for initiation of production (ΔP), critical desorption pressure for gas yield (PCR) and abandonment pressure (PAB) in respect of potentiality for production with an average gas recovery factor of 94.5% for all three studied shale zones, which strongly recommends the convenient reservoir characteristic for future exploitation.

A tale of two domes: Neogene to recent volcanism and dynamic uplift of northeast Brazil and southwest Africa

Topographic domes that are distant from active plate boundaries are often characterised by rapid, youthful uplift, contemporaneous mafic volcanism, radial drainage patterns, and positive long-wavelength gravity anomalies. There is increasing evidence that they are underlain by anomalously low sub-plate seismic velocities. Despite their well-known geomorphological expression, the origin of these epeirogenic features remains enigmatic and is much debated. Here, we investigate potential mechanisms for rapid regional uplift by combining disparate observations from the Borborema and Angolan plateaux that straddle the Brazilian and southwest African margins, respectively. Oceanic residual depth measurements, drainage analysis, stratigraphic architecture, emergent marine terraces and basement denudation are used to constrain their regional uplift histories. In both cases, the bulk of topographic growth occurred within the last 30 Ma in the absence of significant tectonic deformation. We estimate present-day mantle temperature and lithospheric thickness from Neogene to recent volcanic trace element compositions and upper mantle shear wave velocities. Volcanic geochemistry in northeast Brazil is compatible with decompression melting of warm asthenosphere and potentially a minor contribution from metasomatised lithospheric mantle. In Angola, melting of metasomatised lithosphere is probably triggered by injection of small-degree asthenospheric-derived melts. We find no evidence for an asthenospheric thermal anomaly >50°C above ambient beneath either region. Present-day lithospheric thickness is ∼100 km beneath Angola and could be as thin as 60 km in the Borborema Province. For Angola, thermobarometry on mantle xenocrysts from Cretaceous kimberlites is used to estimate palaeogeothermal gradients. Results indicate a pre-existing gradient in lithospheric thickness between the edge of the Congo craton and the centre of the Angolan dome at ∼120 Ma. This gradient likely steepened as a result of additional Neogene thinning by 30±10 km beneath the centre of the dome. We conclude that the mechanism for Neogene epeirogenic uplift of the Borborema and Angolan domes is the introduction of a small positive temperature anomaly into the asthenosphere that causes thermomechanical thinning of the overlying lithospheric mantle.

Thermal history and basin evolution of the Moatize - Minjova Coal Basin (N'Condédzi sub-basin, Mozambique) constrained by organic maturation levels

Kerogen concentrates obtained from Lopingian (Late Permian) to Upper Triassic mudrock lithologies of seven coal exploration boreholes, drilled in the Moatize – Minjova Coal Basin (N'Condédzi sub-basin, Mozambique), were studied by means of vitrinite reflectance (VR), spore fluorescence and spore colour, in order to constrain the thermal history and basin evolution by organic maturation levels. VR increases with depth, indicating organic maturation related to sediment burial for most of the boreholes. Modelled VR data indicate a regional palaeogeothermal gradient between 35 and 40 °C/km. Lower Jurassic doleritic intrusions observed in three boreholes had only local thermal effects without affecting the regional palaeogeothermal gradient. Two boreholes located near the basin margin show high palaeogeothermal gradients suggesting thermal processes other than heating due to burial were involved. These processes may have involved hot diagenetic fluids circulating through fault zones and/or permeable lithologies, locally elevating geothermal gradients. Circulation of these fluids was induced by lithostatic pressure due to rapid rates of sedimentation. These high sedimentation rates lead to the accumulation of a thick succession (over 2000 m) of Lopingian (Late Permian) to Upper Triassic siliciclastic sediments. All the organic maturation indices measured and the age of the successions indicate that organic maturation occurred during or after Late Triassic times. However, the presence of reworked Permian palynomorphs into Upper Triassic sediments and the absence of Middle Triassic sediments indicate an exhumation and erosion of Permian strata in Middle Triassic times. The organic maturation levels of the reworked palynomorph population are considerably higher than the indigenous Upper Triassic population, indicating that they attained higher burial temperatures prior to being reworked.

Burial and exhumation history of the Jameson Land Basin, East Greenland, estimated from thermochronological data from the Blokelv-1 core

Apatite fission-track analysis (AFTA) data in two Upper Jurassic core samples from the 231 m deep Blokelv-1 borehole, Jameson Land, East Greenland, combined with vitrinite reflectance data and regional AFTA data, define three palaeo-thermal episodes. We interpret localised early Eocene (55– 50 Ma) palaeotemperatures as representing localised early Eocene heating related to intrusive activity whereas we interpret late Eocene (40–35 Ma) and late Miocene (c. 10 Ma) palaeotemperatures as representing deeper burial followed by successive episodes of exhumation. For a palaeogeothermal gradient of 30°C/km and likely palaeo-surface temperatures, the late Eocene palaeotemperatures require that the Upper Jurassic marine section in the borehole was buried below a 2750 m thick cover of Upper Jurassic – Eocene rocks prior to the onset of late Eocene exhumation. As these sediments are now near outcrop at c. 200 m above sea level, they have been uplifted by at least 3 km since maximum burial during post-rift thermal subsidence. The results are consistent with estimates of rock uplift on Milne Land since the late Eocene and with interpretation of Ocean Drilling Program (ODP) data off South-East Greenland suggesting that mid-Cenozoic uplift of the margin triggered the marked influx of coarse clastic turbidites during the late Oligocene above a middle Eocene to upper Oligocene hiatus.

Illite/smectite diagenesis and thermal evolution of Lower Cretaceous–Paleogene successions in the Dolna Kamchiya Depression, Eastern Bulgaria

The diagenesis of mixed-layer illite/smectite (I/S) minerals in Lower Cretaceous–Paleogene successions from the Dolna Kamchiya Depression was studied, using X-ray diffraction analysis of the clay fraction (&lt;2 μm) from core samples. The proportion of illite in I/S and degree of ordering increase with depth irrespective of the geological age, indicating that highly expandable I/S compositions were progressively illitized during the burial evolution. Lowest smectite values are recorded in the Lower Cretaceous deposits, whereas in the Paleogene sections are documented great regional variations in the I/S mineralogy, caused by differential basin subsidence. The transition from randomly interstratified (R0) to R1-ordered I/S occurs between depths of 2200 m and 2400 m, and crosscuts the major stratigraphic boundaries. The variable patterns of I/S depth profiles resulted from the combined influence of temperature, burial history, sedimentation and subsidence rates, and basin geodynamic types on diagenesis. The rapid increase in illite content in the I/S clays across the main unconformities reflects the great thickness of eroded sediments during uplift and denudation events and/or elevated heat flow. The reconstructed palaeogeothermal gradient for the Eocene after corrections for decompaction and erosion would correspond to the values for foreland basin settings reported in the literature. Application of the I/S geothermometer to the Lower Cretaceous–Paleogene rocks demonstrates a general trend of increased organic maturity toward southeast in relatively isochronous deposits. The new results appear to be the first record for the effect of a multiphase basin evolution on the smectite illitization in sedimentary successions from the Dolna Kamchiya Depression.

Mesozoic and Cenozoic thermal history and source rock thermal evolution of the Baiyinchagan sag, Erlian Basin, Northern China

Abstract The hydrocarbon-bearing Baiyinchagan sag is located in the west of the Erlian Basin in the Inner Mongolia, Northern China. Its potential source rocks include the Lower Cretaceous Aershan, Tenggeer and Duhongmu 1 Formations. The former two formations are major source rocks. They are dominated by dark mudstone with the largest thickness of 560-600 m, and mainly distributed in the western sub-sag. In this study, temperature data from three wells used to calculate the present-day geothermal gradient in the sag, and used 144 vitrinite reflectance measurements from 35 wells together with seven apatite fission track data from seven wells to reconstruct the Mesozoic and Cenozoic thermal history. The results show that the present-day geothermal gradient is 35.1 °C/km. In the Early Cretaceous, the geothermal gradient was 40.0–42.1 °C/km during the early deposition of the Aershan Formation (135–110 Ma), and then increased to 49.9–56.4 °C/km at the end deposition of the Saihantala Formation (100–95 Ma). The geothermal gradient decreased to a present-day value of 32.0–35.4 °C/km. Using this model of thermal history, combined with the source rock geochemistry, the maturation histories of three source rock intervals, including the Aershan, Tenggeer and Duhongmu 1 Formations, were modeled. The modeled results suggest that source rock maturation was controlled by palaeo geothermal gradient, and that source rocks in the eastern sub-sag have not reached hydrocarbon generation threshold (0.5% R o ). In the western sub-sag, Aershan Formation source rocks reached a high mature (1.0% o o o n possesses the least hydrocarbon generati o n potential (0.5% o

The thermal history of the Karoo Moatize-Minjova Basin, Tete Province, Mozambique: An integrated vitrinite reflectance and apatite fission track thermochronology study

The Moatize-Minjova Basin is a Karoo-aged rift basin located in the Tete Province of central Mozambique along the present-day Zambezi River valley. In this basin the Permian Moatize and Matinde formations consist of interbedded carbonaceous mudstones and sandstones with coal seams. The thermal history has been determined using rock samples from two coal exploration boreholes (ca. 500 m depth) to constrain the burial and exhumation history of the basin. Organic maturation levels were determined using vitrinite reflectance and spore fluorescence/colour. Ages and rates of tectonic uplift and denudation have been assessed by apatite fission track analysis. The thermal history was modelled by inverse modelling of the fission track and vitrinite reflectance data. The Moatize Formation attained a coal rank of bituminous coals with low to medium volatiles (1.3–1.7%Rr). Organic maturation levels increase in a linear fashion downhole in the two boreholes, indicating that burial was the main process controlling peak temperature maturation. Calculated palaeogeothermal gradients range from 59 °C/km to 40 °C/km. According to the models, peak burial temperatures were attained shortly (3–10 Ma) after deposition. Apatite fission track ages [146 to 84 Ma (Cretaceous)] are younger than the stratigraphic age. Thermal modelling indicates two episodes of cooling and exhumation: a first period of rapid cooling between 240 and 230 Ma (Middle – Upper Triassic boundary) implying 2500–3000 m of denudation; and a second period, also of rapid cooling, from 6 Ma (late Miocene) onwards implying 1000–1500 m of denudation. The first episode is related to the main compressional deformation event within the Cape Fold Belt in South Africa, which transferred stress northwards on pre-existing transtensional fault systems within the Karoo rift basins, causing tectonic inversion and uplift. During the Mesozoic and most of the Cenozoic the basin is characterized by very slow cooling. The second period of fast cooling and denudation during the Pliocene was likely related to the southward propagation of the East African Rift System into Mozambique.

Dojrzałość termiczna utworów dewonu, karbonu i triasu masywu małopolskiego na podstawie wskaźnika CAI ([i]Colour Alteration Index[/i

THERMAL MATURITY OF DEVONIAN , CARBONI FEROUS AND TRIASSIC ROCKS IN THE CENTRAL PART OF THE MAŁOPOLSKA MASSIF FROM CONODONT COLOUR ALTERATION INDEX Abstract. A paleothermal study of Devonian, Carboniferous and Triassic carbonate rocks was carried out in eight boreholes from the central part of the Malopolska Massif by using data on thermal alteration of organic matter contained in conodonts. The maturity of organic matter expressed as the CAI (Colour Alteration Index) values was used to calculate palaeotemperatures that affected the conodont-bearing rocks. In the Wegrzynow IG 1 borehole, the CAI values for the Upper Givetian, Frasnian, Famennian and Lower Carboniferous rocks range from 3 (Givetian) to 2 (Lower Carboniferous). It suggests heating of rocks (at temperatures from 60 to 110°C) in the area of high palaeogeothermal gradient, close to the Variscan front. In the Upper Devonian sequence of the Pągow IG 1 borehole, the CAI value of 4.5 proves heating of rock at a temperature exceeding 230°C as a result of higher heat flow near a fault zone. The low CAI values (1‒1.5) of Triassic rocks provide the evidence for heating of rocks in the range of 50 to 90°C. The relatively low maturity of organic matter in the Triassic rocks indicates that the Devonian and Carboniferous sediments must have reached their thermal maturity in the Late Carboniferous prior to the Variscan orogeny. The signs of crude oil and natural gas observed in the Devonian and Carboniferous rocks in the central part of the Malopolska Massif attest their potential for hydrocarbon exploration.

Chemical and mineralogical characterisation of illite–smectite: Implications for episodic tectonism and associated fluid flow, central Australia

The Warburton–Cooper–Eromanga basins of central–eastern Australia contain a number of reactivated fracture–fault networks that relate to a complex and poorly understood thermal and tectonic evolution. Authigenic illite was sampled from two prominent features of the Warburton–Cooper basins: the Gidgealpa–Merrimelia–Innamincka Ridge, composed of anticlinal imbricate thrust fault blocks, and the synclinal Nappamerri Trough. These sample sets were investigated using a combination of clay mineralogical, trace element and stable isotope analyses to deduce the palaeofluid chemistry associated with past tectonothermal perturbations. The Nappamerri Trough hosts the highly radiogenic Big Lake Suite granite and part of one of Australia’s larger on-shore oil and gas reserves. Calculated fluid stable isotope values from the trough, in conjunction with calculated palaeotemperatures, indicate an influx of evolved high-latitudinal meteoric waters under an extremely high geothermal gradient (∼100°Ckm−1) and high water/rock ratios consistent with an extensional environment. Such high water/rock ratios resulted in intense alteration of the granite during which it underwent substantial enrichment in the heat-producing elements (HPE), particularly Th. This hydrothermal system is interpreted to result from continent-wide transmission of tensional stress originating from episodic rifting of the eastern Australian margin in the mid Cretaceous, as dated by Sm–Nd, Rb–Sr and Ar–Ar. The Gidgealpa–Merrimelia–Innamincka Ridge, by contrast, is marked by a lower, but still elevated, palaeogeothermal gradient (∼42°Ckm−1) and calculated fluid isotopic values compatible with evolved basinal fluids of meteoric origin under low water/rock ratio conditions. Distinct trace element compositions of residue and leachate aliquots further indicate two periods of fluid flow with unique chemical compositions. In light of previous geochronology, these events are interpreted as westward extensions of widespread crustal tensional stress that affected much of central and eastern Queensland in the Carboniferous and Late Triassic.Integrated analyses of authigenic illite provide evidence for three periods of fluid flow and elevated thermal regime associated with regional tectonism during the Carboniferous, Late Triassic and Cretaceous. Our data further show that due to the extremely high geothermal gradient and water/rock ratios, Cretaceous fluid flow had profound effects on the surrounding geology, which may have formed/enhanced two of Australia’s most significant energy resources.

A comparative study between present and palaeo-heat flow in the Qiangtang Basin, northern Tibet, China

The Qiangtang Basin is a significant prospective area for hydrocarbon and gas hydrate resources in the Tibetan Plateau, China. However, relatively little work has been performed to characterise heat flow in this basin, which has restricted petroleum and gas hydrate exploration. In this study, we compare present and palaeo-heat flow in the Qiangtang Basin to provide information on geothermal regime, hydrocarbon generation and permafrost that is necessary for further petroleum and gas hydrate exploration. We base our study on temperature data from a thermometer well, thermal conductivity tests, vitrinite reflectance data, homogenisation temperature data from fluid inclusions, stratigraphic information and a time-independent modelling approach. Our results indicate that in the central Qiangtang Basin, the present thermal gradient is approximately 15.5 °C/km, and heat flow is approximately 46.69 mW/m2. Heat flow in the Qiangtang Basin is not relatively stable since the Early Jurassic, as previous research has suggested, and it is generally decreasing with time. Additionally, there is a clear difference between the hottest thermal regime of the southern and northern Qiangtang Depressions during Cretaceous to Pleistocene time. In the southern Qiangtang Depression, the palaeogeothermal gradient is approximately 32.0 °C/km, and palaeo-heat flow is approximately 70 mW/m2. However, in the northern Qiangtang Depression, the palaeogeothermal gradient exceeds 81.8 °C/km, and palaeo-heat flow is greater than 172.09 mW/m2. The high thermal regime in the northern Qiangtang Depression is driven mainly by hydrothermal convection. Gas reservoirs are possible targets for hydrocarbon exploration in this depression. Currently, the northwestern part of the northern Qiangtang Depression is the most favourable area for gas hydrate exploration in the Qiangtang Basin.

Denudation history of Eastern Indian peninsula from apatite fission track analysis: Linking possible plume-related uplift and the sedimentary record

The Late Archaean-Early Proterozoic (~ 2.5 Ga) Eastern Dharwar Craton (EDC), eastern India, which underlies much of the drainage basins of the Krishna and Godavari rivers, has undergone an erosionally controlled evolution since the Pan African event (~ 500–550 Ma). This evolution has been responsible for the sedimentation and overall development of the Krishna–Godavari (KG) sedimentary basin. In order to reconstruct the denudation history of the EDC, which forms the hinterland to the petroliferous KG basin, we report apatite fission track (AFT) data from 41 samples. Thermal history modelling suggests that little denudation occurred before Mid-Cretaceous time. However, during the Late Cretaceous, accelerated cooling commenced from a temperature range of 60–75 °C. Assuming a low palaeogeothermal gradient of 10 °C/km, similar to that of the present day, cooling of the EDC translates to ~ 0.5–2 km of denudation during the Late Cretaceous and a total of ~ 4 km since that time. The denudation history is closely related to the sedimentary record in the KG basin. The episode of accelerated cooling in the Late Cretaceous is interpreted as a geomorphic response to uplift of the Indian peninsula possibly resulting from an ascending mantle plume, which produced the voluminous Deccan Traps at the K–T boundary. The denudation history coupled with previous palaeocurrent studies in the KG basin suggest that the Krishna–Godavari drainage system was established in Late Cretaceous time. • AFT study from Dharwar craton suggests little denudation before Mid-Cretaceous. • During the Late Cretaceous, accelerated cooling occurred. • Cooling of the EDC translates to ~0.5–2 km of denudation in the Late Cretaceous. • The cooling is due to the dynamic uplift of Indian peninsula by a mantle plume. • Krishna–Godavari drainage system was established in the Late Cretaceous.

Clinopyroxene–rutile phyllonites from the East Tenda Shear Zone (Alpine Corsica, France): pressure–temperature–time constraints to the Alpine reworking of Variscan Corsica

The East Tenda Shear Zone is the regional structure that marks the Alpine overthrusting of the Ligurian–Piedmontese ocean onto the Variscan Corsica. We present the first report of a Na-pyroxene (acmite)–rutile-bearing assemblage from a phyllonitic shear zone that occurs within the gneissic lithologies of the East Tenda Shear Zone. Acmite hosts inclusions of Na-amphibole and titanite, and is rimmed by retrogressive biotite. Forward modelling of the shear zone assemblages in the NCKFMASTHO chemical system indicates a cold burial–exhumation path (palaeogeothermal gradient &lt;10 °C km −1 ) and a metamorphic climax with minimum pressure of 1.2 GPa and temperatures of 350–400 °C. U–Pb thermal ionization mass spectrometry analyses on synkinematic rutile yield a 3D array with an age of 48 ± 18 Ma (MSWD 7.3), whereas coexisting acmite–phengite and coatings of oxides or sulphides provide an intercept at 54 ± 8 Ma (MSWD = 48). The scatter of the arrays is considered to reflect secondary disturbances of the system and the age provided by acmite–phengite and the coatings at 54 ± 8 Ma is considered the most reliable estimate for shear zone formation. Implications of these new metamorphic and geochronological data are discussed in the regional framework of the Alpine geology and integrated in the plate-tectonic scenario of the central Mediterranean.

Exhumation of the Sierra de Cameros (Iberian Range, Spain): constraints from low-temperature thermochronology

Abstract We present new fission-track and (U–Th)/He data from apatite and zircon in order to reconstruct the exhumation of the Sierra de Cameros, in the northwestern part of Iberian Range, Spain. Zircon fission-track ages from samples from the depocentre of the basin were reset during the metamorphic peak at approximately 100 Ma. Detrital apatites from the uppermost sediments retain fission-track age information that is older than the sediment deposition age, indicating that these rocks have not exceeded 110 °C. Apatites from deeper in the stratigraphic sequence of the central part of the basin have fission-track ages of around 40 Ma, significantly younger than the stratigraphic age, recording the time of cooling after peak metamorphic conditions. Apatite (U–Th)/He ages in samples from these sediments are 31–40 Ma and record the last period of cooling during Alpine compression. The modelled thermal history derived from the uppermost sediments indicates that the thermal pulse associated with peak metamorphism was rapid, and that the region has cooled continuously to the present. The estimated palaeogeothermal gradient is around 86 °C km −1 and supports a tectonic model with a thick sedimentary fill ( c . 8 km) and explains the origin of the low-grade metamorphism observed in the oldest sediments.

Thermal history of the Rio Muni (West Africa)–NE Brazil margins during continental breakup

We document the thermal record of breakup of the conjugate Rio Muni (West Africa) and NE Brazil margins using apatite fission track analysis, vitrinite reflectance data and stratigraphic observations from both margins. These results permit determination of the timing of four cooling episodes, and the temperature of samples at the onset of each episode. All samples are interpreted to have experienced higher temperatures in the geological past due to i) elevated basal heatflow (palaeogeothermal gradient in Rio Muni-1 well decaying from 58 °C/km during the Mid Cretaceous to 21.5 °C/km in the Late Cenozoic) and ii) progressive exhumation from formerly greater burial depth. A well constrained history of changing palaeogeothermal gradient allows for much more precise quantification of the thickness of eroded section (exhumation) than if a constant heatflow is assumed. Cooling episodes identified from the palaeotemperature data at 110–95 Ma (both margins) and 85–70 Ma (Rio Muni only) coincide with major unconformities signifying, respectively, the cessation of rifting (breakup) and compressional shortening that affected the African continent following the establishment of post-rift sedimentation (drift). The interval between these separate unconformities is occupied by allochthonous rafts of shallow-water carbonates recording gravitational collapse of a marginal platform. The rift shoulder uplift that triggered this collapse was enhanced by local transpression associated with the obliquely divergent Ascension Fracture Zone, and thermal doming due to the coeval St Helena and Ascension Plumes. The data also reveal a c.45–35 Ma cooling episode, attributed to deep sea erosion at the onset of Eo-Oligocene ice growth, and a c.15–10 Ma episode interpreted as the record of Miocene exhumation of the West African continental margin related to continent-wide plume development. Integration of thermal history methods with traditional seismic- and stratigraphy-based observations yields a dynamic picture of kilometre-scale fluctuations in base level through the breakup and early drift phases of development of these margins. Major unconformities at ocean margins are likely to represent composite surfaces recording not only eustasy, but also regional plate margin-generated deformation, local ‘intra-basinal’ reorganization, and the amplifying effect of negative feedbacks between these processes.

Neoproterozoic garnet-glaucophanites and eclogites: new insights for subduction metamorphism of the Gourma fold and thrust belt (eastern Mali)

Abstract The Neoproterozoic Gourma fold and thrust belt exposed in eastern Mali includes in its inner part high-pressure, low-temperature metasediments and scarce metabasites. This high-pressure metamorphic unit is characterized in the Ansongo region by garnet–glaucophane–paragonite assemblages and eclogites of basaltic derivation, whereas phengite–garnet–rutile mineral assemblages characterize the metapelites. Thermobarometric estimates on the metabasites suggest peak pressure around 13–15 kbar and temperature of 500±50 °C for the Seyna Bela garnet glaucophanite and glaucophane-bearing eclogite, and 16 kbar at 600±50 °C for the Tin Hama phengite eclogite, values indicative of palaeogeothermal gradients of about 10 °C km −1 typical of subduction settings. The high-pressure unit may represents a giant allochthon emplaced on top of very low-grade metasediments. It represents the southern extension of the ultrahigh-pressure rocks. Garnet pyriclasites from the Amalaoulaou massif, which represents the roots of a c . 800–730 Ma Neoproterozoic island arc, underwent a medium-temperature metamorphic overprint characterized by barroisite–paragonite assemblages; that is, of same grade as the decompression that affected the eclogites and garnet-blueschists. The Gourma high-pressure metamorphic belt formed as a consequence of the east-dipping subduction of the Neoproterozoic passive palaeo-margin of the West African craton. The presented P – T estimates suggest that subduction-related palaeogeothermal gradients during the late Neoproterozoic period along the main Pan-African suture were similar to those reconstructed for Tertiary Alpine-type belts.

Mesozoic–Cenozoic exhumation events in the eastern North Sea Basin: a multi‐disciplinary study based on palaeothermal, palaeoburial, stratigraphic and seismic data

ABSTRACTFour Mesozoic–Cenozoic palaeothermal episodes related to deeper burial and subsequent exhumation and one reflecting climate change during the Eocene have been identified in a study of new apatite fission‐track analysis (AFTA®) and vitrinite reflectance data in eight Danish wells. The study combined thermal‐history reconstruction with exhumation studies based on palaeoburial data (sonic velocities) and stratigraphic and seismic data. Mid‐Jurassic exhumation (ca. 175 Ma) was caused by regional doming of the North Sea area, broadly contemporaneous with deep exhumation in Scandinavia. A palaeogeothermal gradient of 45 °C km−1 at that time may be related to a mantle plume rising before rifting in the North Sea. Mid‐Cretaceous exhumation affecting the Sorgenfrei–Tornquist Zone is probably related to late Albian tectonic movements (ca. 100 Ma). The Sole Pit axis in the southern North Sea experienced similar inversion and this suggests a plate‐scale response along crustal weakness zones across NW Europe. Mid‐Cenozoic exhumation affected the eastern North Sea Basin and the onset of this event correlates with a latest Oligocene unconformity (ca. 24 Ma), which indicates a major Scandinavian uplift phase. The deeper burial that caused the late Oligocene thermal event recognized in the AFTA data reflect progradation of lower Oligocene wedges derived from the uplifting Scandinavian landmass. The onset of Scandinavian uplift is represented by an earliest Oligocene unconformity (ca. 33 Ma). Late Neogene exhumation affected the eastern (and western) North Sea Basin including Scandinavia. The sedimentation pattern in the central North Sea Basin shows that this phase began in the early Pliocene (ca. 4 Ma), in good agreement with the AFTA data. These three phases of Cenozoic uplift of Scandinavia also affected the NE Atlantic margin, whereas an intra‐Miocene unconformity (ca. 15 Ma) on the NE Atlantic margin reflects tectonic movements of only minor amplitude in that area. The study demonstrates that only by considering episodic exhumation as an inherent aspect of the sedimentary record can the tectonic evolution be accurately reconstructed.

Transantarctic Basin: new insights from fission track and structural data from the USARP Mountains and adjacent areas (Northern Victoria Land, Antarctica)

ABSTRACTThe USARP Mountains comprise two N–S‐aligned mountain ranges (Daniels Range, Pomerantz Tableland) located along the western margin of the Rennick Glacier in Northern Victoria Land (NVL). Four zircon and titanite fission track (FT) ages from granitic samples from the Pomerantz Tableland fall in a common range of 369–392 Ma. The apatite FT ages from 20 Granite Harbour Intrusive rocks sampled throughout the USARP Mountains are distinctively younger (86–270 Ma); their mean track lengths (MTL) vary between 11.0 and 13.9 μm. Six samples from Renirie Rocks and the Kavrayskiy Hills east of the USARP Mountains have even younger, concordant apatite FT ages of 43–71 Ma, and MTL of 12.2–14.0 μm. Thermal history modelling of the thermochronological data indicate that both the Daniels Range and Pomerantz Tableland experienced a common Phanerozoic geologic history consisting of a mid‐Devonian pulse of rapid denudation, followed by a protracted denudation stage between the Carboniferous and Jurassic. This latter period of denudation was contemporaneous with the formation of the Transantarctic Basin to the east. We consequently suggest that the USARP Mountains were one of the major source areas for the Beacon Supergroup that formed the fill of the Transantarctic Basin. Subsequent to the deposition of the Beacon sequence, the now‐outcropping rocks of the USARP Mountains were buried to a maximum depth of 4.2 km. A palaeogeothermal gradient of 25±8°C km−1 was inferred at the time of maximum burial. Inversion of the Transantarctic Basin due to the breakup of Gondwana, and in response to Cenozoic rifting and uplift of the Transantarctic Mountains, has triggered the final denudation stages recorded in NVL since the Cretaceous. Thereby, the amounts of denudation increase eastward. Whereas 2.4–4.2 km of crustal unloading are recognized for the USARP Mountains since the Cretaceous, more than 4 km of denudation has occurred towards the Rennick Graben alone since the Eocene. This denudation was associated with major fault activities involving early ENE–WSW to E–W‐directed extension. Related structures were reactivated by dominant NW–SE to NNW–SSE‐oriented right‐lateral shear genetically linked to the formation and inversion of the structural depression of the Rennick Graben in Cenozoic times.

The regional distribution of zeolites in the basalts of the Faroe Islands and the significance of zeolites as palaeotemperature indicators

The first maps of the regional distribution of zeolites in the Palaeogene basalt plateau of the Faroe Islands are presented. The zeolite zones (thomsonite-chabazite, analcite, mesolite, stilbite-heulandite, laumontite) continue below sea level and reach a depth of 2200 m in the Lopra-1/1A well. Below this level, a high temperature zone occurs characterised by prehnite and pumpellyite. The stilbite-heulandite zone is the dominant mineral zone on the northern island, Vágar, the analcite and mesolite zones are the dominant ones on the southern islands of Sandoy and Suðuroy and the thomsonite-chabazite zone is dominant on the two northeastern islands of Viðoy and Borðoy. It is estimated that zeolitisation of the basalts took place at temperatures between about 40°C and 230°C. Palaeogeothermal gradients are estimated to have been 66 ± 9°C/km in the lower basalt formation of the Lopra area of Suðuroy, the southernmost island, 63 ± 8°C/km in the middle basalt formation on the northernmost island of Vágar and 56 ± 7°C/km in the upper basalt formation on the central island of Sandoy. A linear extrapolation of the gradient from the Lopra area places the palaeosurface of the basalt plateau near to the top of the lower basalt formation. On Vágar, the palaeosurface was somewhere between 1700 m and 2020 m above the lower formation while the palaeosurface on Sandoy was between 1550 m and 1924 m above the base of the upper formation. The overall distribution of zeolites reflects primarily variations in the maximum depth of burial of the basalt rather than differences in heat flow. The inferred thinning of the middle and upper basalt formation from the central to the southern part of the Faroes is in general agreement with a northerly source area for these basalts, centred around the rift between the Faroes and Greenland. The regional zeolite distribution pattern is affected by local perturbations of the mineral zone boundaries that reflect local differences in the temperature, perhaps related to the circulation of water in the underground. The zonal distribution pattern suggests that these temperature anomalies are in part related to NW–SE-trending eruption fissures or zones of weakness separating the present islands and are subparallel to transfer zones in the Faroe–Shetland Basin. Both the regional and the local distribution of zeolite assemblages are probably a reflection of the basic volcanic-tectonic pattern of the Faroe Islands.

Mineralogical and thermodynamic constraints on Palaeogene palaeotemperature conditions during low-grade metamorphism of basaltic lavas recovered from the Lopra-1/1A deep hole, Faroe Islands

The sequene of secondary minerals that are reported for the Lopra-1/1A well records progressive zeolite facies to prehnite–pumpellyite-facies mineral progressions consistent with those of other well-studied hydrothermally altered rock sequences. Detailed comparison of the calc–silicate (zeolites and prehnite) mineral distributions of the Lopra-1/1A sequence with those from other regions indicates that this sequence exhibits consistently longer down-hole intervals for secondary mineral species than reported elsewhere. When compared to measured down-hole temperatures reported in other hydrothermally altered regions, the results suggest that the Lopra-1/1A mineral progression formed under conditions typical of low temperature hydrothermal systems that form shortly after eruption of thick basaltic piles. Maximum temperatures achieved at the 3500 m level of the well were at or below 200°C. The implied geothermal gradient was less than 50°C/km. An analysis of prehnite – fluid composition relationships was also conducted in order to determine if results compatible with the paragenetic sequence study could be obtained from thermodynamic constraints. In this case, the limiting temperature for prehnite formation in equilibrium with albite–quartz–calcite–laumontite (the mineral assemblage at the bottom of the hole) was determined for a range of fluid compositions. The resulting calculations suggest temperatures of formation of prehnite in the range of 140°C to 205°C, a conclusion which is broadly consistent with those reached from study of the paragenetic relationships. Comparison of these results with other studies of palaeogeothermal gradients of the North Atlantic margins suggests a consistent pattern in which relatively low geothermal gradients persisted in the Palaeogene rift basin.