High Regional Heat Flow Research Articles

Geology of the Rosário–Neves Corvo antiform, Iberian Pyrite Belt, Portugal: new insights from physical volcanology, palynostratigraphy and isotope geochronology studies

The lithostratigraphic sequence in the Rosário–Neves Corvo antiform comprises the Phyllite–Quartzite Group, whose top is of Famennian age, the Volcanic Sedimentary Complex, of Strunian to upper Visean age, and the Mértola Formation (the lower unit of the Baixo Alentejo Flysch Group) of upper Visean age. The volcanic sedimentary complex comprises a lower sequence of Strunian (Late Famennian) age and an upper sequence of lower to upper Visean age. Detailed mapping of the antiform towards NW of the Neves Corvo mine, supported by palynological dating, identified two new lithostratigraphic units: the Barrancão member (upper Famennian) ascribed to the Phyllite–Quartzite Group and made up of laminated dark shales with siliceous lenses and nodules, and the Ribeira de Cobres Formation of the Volcanic Sedimentary Complex, containing shales, siltstones and fine volcaniclastic rocks. Based on zircon U–Pb isotope dating, five discrete felsic magmatic events were identified at approximately 354, 359, 365, 373 and 384 Ma. This suggests that the volcanic activity in the area has extended for about 30 Ma, in a context of high regional heat flow as indicated by the geochemical signatures of the felsic volcanic rocks. The characteristics of magmatism and the depositional environment indicated by the sedimentary record should therefore have been highly favourable for massive sulphide formation. However, evidence of massive sulphide mineralization in the study area is still to be found. Moreover, reconstruction of the volcanic facies architecture demonstrated that the volcanic units in the Rosário area are strongly dominated by coherent facies typical of the inner part of thick lavas/domes. In fact, most of their external part, the more favourable location for possible massive sulphide mineralization, is missing. Palynological dating indicates a significant hiatus, recognised between the lower and upper sequences of the volcanic sedimentary complex, which implies erosion of the top of the volcanic centre, where VHMS deposits could possibly have formed. However, lateral areas of this volcanic centre, eventually preserved at depth, have good potential to host massive sulphide mineralization.

Remnants of Early Carboniferous I-type granodiorite plutons in the Bavarian Forest and their bearing on the tectonic interpretation of the south-western sector of the Bohemian Massif (Bavarian Zone)

3 �AcademyofSciences,�InstituteofRockStructureandMechanics,�VHolesovickach�41,�182�09�Prague�8,�CzechRepublic * �Correspondingauthor The Bavarian Zone of the south-western Bohemian Massif is a late Variscan high heat-flow region, characterized by numerous granite intrusions and large amounts of LP-HT anatectic rocks, the latter comprising meta- and diatexites derived from sedimentary and igneous protoliths. Imaging and SIMS U-Pb dating of zircon from a distinctive type of hornblende-bearing diatexite, with a presumed igneous protolith, was done to gather information on both metamorphism and pre-anatectic crustal evolution. Zircon crystals have two phases of growth zoning, with oscillatory-zoned grains (visible through cathodoluminescence, CL, imaging) modified and surrounded by relatively uniform, high luminosity zircon. Analyses on oscillatory-zoned zircon yield an age of 344 ± 2 Ma (2σ), interpreted as the formation age of the igneous protolith. More luminescent overgrowths, interpreted as having formed during anatexis, were dated at 323 ± 3 Ma. Based on these dates, and on geochemical similarities, the protolith can be perfectly correlated with the high-K calc-alkaline Blatna suite of the Central Bohemian Plutonic Complex, c. 70 km to the north. This correlation strengthens tectonic models interpreting the Bavarian Zone as a reheated but integral constituent of the Variscan collisional crust of the Bohemian Massif, as opposed to an independent basement terrane assembled by late-Variscan orogenesis.

Imaging the Deep Structure of the Central DeathValley Basin Using Receiver Function, Gravity,and Magnetic Data

We use receiver function, gravity, and magnetic data to image the deep structures of central Death Valley. Receiver function analysis suggests the Moho is 24 km deep in the central part of the basin and deepens to 33 km to the east and 31 km to the west. The estimated lower crustal density is 2900 kg/m3, which suggests a gabbroic composition, whereas the upper crustal density, excluding basin sediments, is estimated to average 2690 kg/m3 or approximately a quartzofeldspathic composition. We modeled the magnetic sources as upper crustal to suggest a relatively shallow Curie depth in this region of high heat flow. We developed models to test the hypothesis that a low-density, non-magnetic body (magma or fluid-rich material?) within the lower crust at a depth of 15 km could coincide with the location of the Death Valley bright spot imaged on a deep seismic reflection profile. Those models suggest that if there is a low density region in the mid to lower crust in the area of the bright spot, then the region is also likely to be underplated by mafic or ultramafic materials which may have contributed to heating, uplift, and thinning of the crust during extension.

Kimberlites and lamproites: Criteria for similarity and differences

Based on original data on the East European and Siberian platforms and materials on the best studied foreign objects, a comparative analysis of kimberlites and lamproites was conducted and the criteria of their differences were formulated. Among most significant differences are the following: (1) the high-Mg potassic rocks (kimberlites and lamproites) show major-component variations, which are significantly wider in lamproites as compared to kimberlites. Kimberlites differ from lamproites not only in the content of SiO2, but also in alkalis, volatiles, and some trace elements. Kimberlites are characterized by CO2-dominated regime, whereas formation of lamproites was assisted by essentially H2O fluid; (2) Kimberlites are localized within ancient cratons, while within-plate lamproites are restricted to adjacent Proterozoic belts. Kimberlites are produced in the low-heat flow regions, whereas lamproites occur in the high-heat flow regions; (3) Kimberlites and lamproites were formed in different time; in particular, most productive kimberlitic magmatism was observed in the EEP and SP in the Devonian; (4) Kimberlite and lamproite bodies have different morphology: lamproites compose small subvolcanic bodies with lava flows, while kimberlites form volcanic pipes with no lavas; (5) Kimberlites contain highly silica-undersaturated minerals, while ultrabasic lamproites—silica-undersaturated ones; priderite and wadeite, the characteristic accessory minerals of lamproites, are not observed in kimberlites; (6) The primary melts of kimberlites and lamproites were derived from different types of mantle. The moderate and low-Ti kimberlites were generated from BSE or EMI type mantle. Precisely these types of kimberlites host diamond deposits, including economic grade objects in EEP. The lamproite sources were localized only in the enriched mantle (EMI and EMII). At the same time, these rocks share some similarities, primarily, with respect to their genesis and classification. Diamonds are common accessory minerals of kimberlites (low-Ti and some other types), but are observed only in only lamproite variety—olivine lamproites.

Spatial Separation of Large Earthquakes, Aftershocks, and Background Seismicity: Analysis of Interseismic and Coseismic Seismicity Patterns in Southern California

We associate waveform-relocated background seismicity and aftershocks with the 3-D shapes of late Quaternary fault zones in southern California. Major earthquakes that can slip more than several meters, aftershocks, and near-fault background seismicity mostly rupture different surfaces within these fault zones. Major earthquakes rupture along the mapped traces of the late Quaternary faults, called the principal slip zones (PSZs). Aftershocks occur either on or in the immediate vicinity of the PSZs, typically within zones that are ±2-km wide. In contrast, the near-fault background seismicity is mostly accommodated on a secondary heterogeneous network of small slip surfaces, and forms spatially decaying distributions extending out to distances of ±10 km from the PSZs. We call the regions where the enhanced rate of background seismicity occurs, the seismic damage zones. One possible explanation for the presence of the seismic damage zones and associated seismicity is that the damage develops as faults accommodate bends and geometrical irregularities in the PSZs. The seismic damage zones mature and reach their finite width early in the history of a fault, during the first few kilometers of cumulative offset. Alternatively, the similarity in width of seismic damage zones suggests that most fault zones are of almost equal strength, although the amount of cumulative offset varies widely. It may also depend on the strength of the fault zone, the time since the last major earthquake as well as other parameters. In addition, the seismic productivity appears to be influenced by the crustal structure and heat flow, with more extensive fault networks in regions of thin crust and high heat flow.

Tectonic Framework and Deep Structure of South China and Their Constraint to Oil‐Gas Field Distribution

Abstract:South China could be divided into one stable craton, the Yangtze Craton (YzC), and several orogenic belts in the surrounding region, that is the Triassic Qinling‐Dabie Orogenic Belt (QDOB) in the north, the Songpan‐Garzê Orogenic Belt (SGOB) in the northwest, the Mesozoic‐Cenozoic Three‐river Orogenic Belt (TOB) in the west, the Youjiang Orogenic Belt (YOB) in the southwest, the Middle Paleozoic Huanan Orogenic Belt (HOB) in the southeast, and the Mesozoic‐Cenozoic Maritime Orogenic Belt (MOB) along the coast. Seismic tomographic images reveal that the Moho depth is deeper than 40 km and the lithosphere is about 210 km thick beneath the YzC. The SGOB is characterized by thick crust (>40 km) and thin lithosphere (<150 km). The HOB, YOB and MOB have a thin crust (<40 km) and thin lithosphere (<150 km). Terrestrial heat flow survey revealed a distribution pattern with a low heat flow region in the eastern YzC and western HOB and two high heat flow regions in the TOB and MOB respectively. Such a “high‐low‐high” heat flow distribution pattern could have resulted from Cenozoic asthenosphere upwelling. All oil‐gas fields are concentrated in the central part of the YzC. Remnant oil pools have been discovered along the southern margin of the YzC and its adjacent orogenic belts. From a viewpoint of geological and geophysical structure, regions in South China with thick lithosphere and low heat flow value, as well as weak deformation, might be the ideal region for further petroleum exploration.

Transient thermal regimes in the Sierra Nevada and Baja California extinct outer arcs following the cessation of Farallon subduction

We examine the thermal relaxation of the Sierra Nevada and Baja California extinct outer arc blocks following the progressive cessation of Farallon subduction under western North America beginning at ∼30 Ma. Being parts of the same outer arc until the inland jump of the San Andreas transform fault at ∼5 Ma, these two regions show many similarities in their geology, geomorphology, rigid body behavior, and their relatively low seismicity. In the thermal model, we combine results of different geophysical and geophysical studies to constrain the thermal state and geometry of the outer arcs before the cessation of subduction and then model the postsubduction temperature responses in these regions using the results of the tectonic reconstructions. A well‐constrained regional thermal model of these blocks using the results of many earlier studies in these regions confirms that the present low heat flow values in these regions are the remnants of the very cold outer arc thermal regime of the subduction zone even as long as 30 Ma after cessation of subduction. Thus the entire Pacific boundary of the North American plate is still in a transient thermal state. The calculated low lithospheric temperatures in the Sierra Nevada and Peninsular blocks correlate very well with their rigid body behavior obtained from geodetic studies, and seismogenic layer thicknesses obtained from seismological studies. This is in contrast with the fact that both regions are surrounded by intense deformation associated with the western North America intraplate and extraplate motions. These low‐temperature islands play important roles in the present interaction of the North American and Pacific plates and contribute to the broad deformation of the transform boundary. The thermal relaxation of the extinct outer arcs includes both vertical heating from the underlying asthenosphere and the lateral heating from the extinct back arc (Basin and Range), which has remained as a high heat flow region after the cessation of the subduction. We suggest that the significant lateral heat transfer from the Basin and Range in the Sierra Nevada (and from the Gulf of California spreading center in the Peninsular block since ∼5 Ma) may be the main driving mechanism of the postsubduction volcanism/magmatism along the extinct volcanic arc and the recent tilted uplift of the Sierra Nevada block. The low lithospheric temperatures in Sierra Nevada region may also explain the observation of the high seismic velocities in the mantle beneath the southern Sierra Nevada where the downwelling of the mantle lithosphere proposed.

Silica alteration zones in the Barberton greenstone belt: A window into subseafloor processes 3.5–3.3 Ga ago

Silicification of volcanic rocks, ranging from komatiitic to dacitic in composition, is a common phenomenon of the 3.5 to 3.2 Ga old volcano-sedimentary succession of the Barberton greenstone belt. Silica enrichment occurs in zones several tens of metres thick at the top of volcanic sequences that are capped by sedimentary chert horizons. SiO 2 contents increase upsection, from the original igneous value up to 92%. Silicification is associated with a depletion of most elements commonly mobile during water–rock interaction, while K 2O, Rb and Ba are enriched in these zones. Some transition metals (Sc, V, and Cr) were immobile, whereas Ni and Co and some other metals (Cu, Zn) were mobile and are depleted. The δ 18O values of volcanic rocks show a positive linear relationship with silica content and range from 9.0 to 17.3‰. The element depletion-enrichment patterns and oxygen isotope data indicate low-temperature (c. 100–150 °C) hydrothermal processes for the origin of the alteration zones. The loss of most elements coupled with the addition of SiO 2 as well as REE systematics indicate high water–rock ratios and a fluid REE composition typical of Archaean seawater that had a δ 18O value of ~ 0‰. Hydrothermal activity is attributed to heat derived from cooling of volcanic rocks and a high regional heat flow in an oceanic plateau-like setting and resulted in the establishment of shallow subseafloor convection cells. Diffuse venting of hydrothermal fluids over broad areas of the ancient ocean floor provided an ideal environment for both seafloor and subsurface biota 3.5 Ga ago.

Structure of the crust beneath the southeastern Tibetan Plateau from teleseismic receiver functions

Southeastern Tibet marks the site of presumed clockwise rotation of the crust due to the India–Eurasian collision and abutment against the stable Sichuan basin and South China block. Knowing the structure of the crust is a key to better understanding of crustal deformation and seismicity in this region. Here, we analyze recordings of teleseismic earthquakes from 25 temporary broadband seismic stations and one permanent station using the receiver function method. We find that the crustal thickness decreases gradually from the Tibetan Plateau proper to the Sichuan basin and Yangtze platform but that significant (intra-)crustal heterogeneity exists on shorter lateral scales (<1000 km). Most receiver functions reveal a time shift of ∼0.2 s in the direct P arrival and negative phases between 0 and 5 s after the first arrival. Inversion of the receiver functions yields S-velocity profiles marked by near-surface and intra-crustal low-velocity zones (IC-LVZs). The shallow low-velocity zones are consistent with the wide distribution of thick surface sedimentary layers. The IC-LVZ varies laterally in depth and strength; it becomes thinner toward the east and southeast and is absent in the Sichuan basin and the southern part of the Yangtze platform. Results from slant-stacking analysis show a concomitant decrease in crust thickness from ∼60 km in the Songpan-Ganze fold system to ∼46 km in the Sichuan basin and ∼40 km in the Yangtze platform. High Poisson's ratios (>0.30) are detected beneath the southeastern margin of Tibet but in the Sichuan basin and southeastern Yangtze platform the values are close to the global average. Combined with high regional heat flow and independent evidence for mid-crustal layers of high (electric) conductivity, the large intra-crustal S-wave velocity reduction (12–19%) and the intermediate-to-high average crustal Poisson's ratios are consistent with partial melt in the crust beneath parts of southeastern Tibet. These results could be used in support of deformation models involving intra-crustal flow, with the caveat that significant lateral variation in location and strength of this flow may occur.

Carbon dioxide degassing from Tuscany and Northern Latium (Italy)

The CO 2 degassing process from a large area on the Tyrrhenian side of central Italy, probably related to the input into the upper crust of mantle fluids, was investigated in detail through the geochemical study of gas emissions and groundwater. Mass-balance calculations and carbon isotopes show that over 50% of the inorganic carbon in regional groundwater is derived from a deep source highlighting gas−liquid separation processes at depth. The deep carbonate−evaporite regional aquifer acts as the main CO 2 reservoir and when total pressure of the reservoir fluid exceeds hydrostatic pressure, a free gas phase separates from the parent liquid and escapes toward the surface generating gas emissions which characterise the study area. The distribution of the CO 2 flux anomalies and the location of high PCO 2 springs and gas emissions suggest that the storage and the expulsion of the CO 2 toward the atmosphere are controlled by the geological and structural setting of the shallow crust. The average CO 2 flux and the total amount of CO 2 discharged by the study area were computed using surface heat flow, enthalpy and CO 2 molality of the liquid phase circulating in the deep carbonate−evaporite aquifer. The results show that the CO 2 flux varies from 1 × 10 4 mol y − 1 km − 2 to 5 × 10 7 mol y − 1 km − 2 , with an average value of 4.8 × 10 6 mol y − 1 km − 2 , about five times higher than the value of 1 × 10 6 mol y − 1 derived by Kerrick et al. [Kerrick, D.M., McKibben, M.A., Seward, T.M., Caldeira, K., 1995. Convective hydrothermal CO 2 emission from high heat flow regions. Chem. Geol. 121, 285–293] as baseline for terrestrial CO 2 emissions. The total CO 2 discharged from the study area is 0.9 × 10 11 mol y − 1 , confirming that Earth degassing from Tyrrhenian central Italy is a globally relevant carbon source.

Natural emissions of methane from geothermal and volcanic sources in Europe

It has recently been demonstrated that methane emission from lithosphere degassing is an important component of the natural greenhouse-gas atmospheric budget. Globally, the geological sources are mainly due to seepage from hydrocarbon-prone sedimentary basins, and subordinately from geothermal/volcanic fluxes. This work provides a first estimate of methane emission from the geothermal/volcanic component at European level. In Europe, 28 countries have geothermal systems and at least 10 countries host surface geothermal manifestations (hot springs, mofettes, gas vents). Even if direct methane flux measurements are available only for a few small areas in Italy, a fair number of data on CO 2, CH 4 and steam composition and flux from geothermal manifestations are today available for 6 countries (Czech Republic, Germany, Greece, Iceland, Italy, Spain). Following the emission factor and area-based approach, the available data have been analyzed and have led to an early and conservative estimate of methane emission into the atmosphere around 10,000 ton/yr (4000–16,000 ton/yr), basically from an area smaller than 4000 km 2, with a speculative upper limit in the order of 10 5 ton/yr. Only 4–18% of the conservative estimate (about 720 ton/yr) is due to 12 European volcanoes, where methane concentration in volcanic gases is generally in the order of a few tens of ppmv. Volcanoes are thus not a significant methane source. While the largest emission is due to geothermal areas, which may be situated next to volcanoes or independent. Here inorganic synthesis, thermometamorphism and thermal breakdown of organic matter are substantial. Methane flux can reach hundreds of ton/yr from small individual vents. Geothermal methane is mainly released in three countries located in the main high heat flow regions: Italy, Greece, and Iceland. Turkey is likely a fourth important contributor but the absolute lack of data prevents any emission estimate. Therefore, the actual European geothermal–volcanic methane emission could be easily projected to the 10 5 ton/yr levels, reaching the magnitude of some other natural sources such as forest fires or wild animals.

Geology and hydrogeochemistry of the Jungapeo CO 2-rich thermal springs, State of Michoacán, Mexico

We present the first geothermal assessment of the Jungapeo CO 2-rich mineral springs, which are located in the eastern part of Michoacán State (central Mexico) at the southern limit of the Trans-Mexican Volcanic Belt. All but one of the > 10 springs occur at the lower contact of the distal olivine-bearing basaltic andesite lavas of the Tuxpan shield, a 0.49- to 0.60-Ma-old cluster of monogenetic scoria cones and lava flows. The Tuxpan shield has a maximum radius of 6 km and was constructed on top of a folded and faulted Cretaceous basement consisting largely of marine limestones, marls, and shales. The mineral waters are characterized by moderate temperatures (28 to 32 °C), mild acidity (pH from 5.5 to 6.5), relatively high discharge rates, effervescence of CO 2 gas, clarity at emergence and abundant subsequent precipitation of hydrous iron, silica oxides, and carbonates around pool margins and issuing streamlets. Chemical and isotopic (deuterium, oxygen, and tritium) analyses of water and gas samples obtained during the period 1991–1997 indicate that the springs are largely composed of meteoric water from a local source with relatively short residence times (water ages of 7 to 25 years). Spring waters are chemically characterized by moderate SiO 2, Ca + Mg nearly equal to Na + K, high HCO 3, moderate to low Cl, low F and SO 4, high B, moderate Li, while Br and As are low. In contrast, Fe + Mn is exceptionally high. Thus, the Jungapeo waters cannot be regarded as high-temperature geothermal fluids. Instead, they resemble soda spring waters similar to other low-to-medium temperature soda waters in the world. Gas samples are extremely rich in CO 2 with no detectable geothermal H 2S or H 2 and very low contents of CH 4 and NH 3, indicating the gases are not derived from a high-temperature resource. Carbon-13 analyses of CO 2 show a narrow range (− 6.7‰ and − 7.2‰) that falls within the range for MORB CO 2. Thus, most CO 2 seems to originate from the mantle but some CO 2 could originate from thermal degradation of organic remains in underlying Cretaceous rocks. 3/4He ratios range from about 2 to 3 Rc/Ra, indicating that a small mantle/magmatic He component is present in the gases. In conclusion, the mineral waters are the surface expression of a low-temperature geothermal system of limited size that originates from the combined effects of a high regional heat flow and (possibly) the remnant heat released from subjacent basaltic andesite magma bodies that constitute the root zone of the Tuxpan shield.

Laboratory measurements of P‐ and S‐wave velocities in polycrystalline plagioclase and gabbronorite up to 700 °C and 1 GPa: Implications for the low velocity anomaly in the lower crust

P‐ (Vp) and S‐wave (Vs) velocities of polycrystalline plagioclase (An51±1) and gabbronorite including plagioclase (An49±2) (62.3vol.%) were measured up to 700°C at 0.6, 0.8, and 1.0GPa during heating and cooling. Both polycrystalline plagioclase and gabbronorite show reversible and discontinuous change in temperature derivative of Vp and Vs, ∂Vp/∂T and ∂Vs/∂T, respectively, at about 400°C. The ∂Vp/∂T and ∂Vs/∂T of the polycrystalline plagioclase changes from −0.8 × 10−4 to −3.4 × 10−4 km s−1°C−1 and from −1.1 × 10−4 to −3.3 × 10−4 km s−1°C−1, respectively. The ∂Vp/∂T and ∂Vs/∂T of the gabbronorite varies from −1.0 × 10−4 to −3.4 × 10−4 km s−1°C−1 and from −0.4 × 10−4 to −3.5 × 10−4 km s−1°C−1, respectively. The reversible and discontinuous changes in ∂Vp/∂T and ∂Vs/∂T are attributed to a phase transformation of plagioclase. The present data suggest that the sharp decrease in Vp and Vs would cause low velocity anomaly under dry and subsolidus melt‐absent conditions in the mid‐to‐lower crust of relatively high heat flow regions.

Are some domes the ghosts of basins?

[1] The inversion of formerly extensional basins by compressional reactivation of existing faults is believed to be an important mechanism for the exhumation of sedimentary basins. We propose an additional mechanism for the exhumation of some sedimentary basins which does not require compression, and which completely inverts a basin into a dome. In basins formed by intra-crustal density loads, sublithospheric heating, however caused, will reduce the effective elastic thickness of the crust, thereby reactivating subsidence. If the crust is sufficiently weakened that it no longer supports the dense load, we predict the long-term result to be detachment of the load followed by uplift and exhumation of the basin. Finite-element modeling indicates that this can occur for geologically possible load densities in high heat flow regions. Total upward displacement of material from depth can be as much as 10 km in some cases, enough to exhume the basin completely.

Electrical signatures due to thermal anomalies along mobile belts reactivated by the trail and outburst of mantle plume: Evidences from the Indian subcontinent

In this study the geodynamical scenario along with concepts of mantle plume and mobile belts is utilized to show that most of the existing and potential high thermal regions fall along the (mobile arms affected by the outburst and) traces of mantle plumes. Effects of channeling and partitioning of thermomagmatic flux (TMF) due to these mantle plumes along the mobile belts, particularly near the triple junctions, can be seen in the form of high heat flow and presence of hot springs. Triple junctions manifest over the Indian lithosphere are: Kutch-Cambay, Narmada Son-Godavari, Tapi-Mahanadi, Tapi-Damodar, Pondicherry region, Gulf of Mannar and SW corner of the subcontinent (off-shore), etc. Apart from mobile belts, the deltaic regions of Krishna, Godavari, Ganga, Cauvery, Narmada-Tapi and Indus, etc., are also posses higher level of thermal anomalies as these regions seem to have been substantially influenced by outbursts and traces of Reunion, Kerguelen, Marion and Crozet hotspots. This is reflected from the correlation between plume affected mobile belts and high heat flow regions, large number of hot springs, anomalous electrical conductivity and also deformation or seismicity. Such correlation can be seen along Cambay, west coast trend, Narmada-Son lineament zone, Godavari-Damodar grabens and Bengal basin. Himalayan belt being ongoing collision zone, thermal anomalies are identified in the form of hot springs along the Himalayan arc. At some locations, which might be junction of tectonic trends, there exist significantly large thermal outputs. Puga in Himalayan region is one such example, as seen from high heat flow (max. 468 mW m − 2 ) and geothermal gradient (234 °C/km max.). Similarly, Tatapani in Narmada Son Lineament (NSL) region is another such example. The present study discusses the correlation between thermal reservoirs identified by magnetotelluric (MT) study results and plume activity and suggests the need for systematic and detailed MT investigations along plume activated mobile strips in other regions to search for geodynamical history and geothermal resources.

The spatial distribution of the geothermal fields in the Taupo Volcanic Zone, New Zealand

A numerical model of the large-scale water and heat flows within the Taupo Volcanic Zone (TVZ), New Zealand is constructed. In the model the regions of high permeability and heat flow in the TVZ coincide, and lie within the TVZ envelope, defined to contain the Taupo Fault Belt and the most recent caldera collapse structures. The model correctly predicts the absence of geothermal fields in the central region of the TVZ, where cold surface groundwater descends to depths close to 8 km before being heated and ascending in discrete plumes at the permeability barrier which occurs at the boundary of the TVZ envelope. The locations of these plumes can be identified with most of the major geothermal fields in the TVZ (Tokaanu, Lake Taupo, Wairakei and Tauhara, Rotokawa, Ohaaki, Waiotapu and Waikite, Tikitere, Rotoiti and Rotoma, Rotorua, Mokai). In addition, the model predicts two bands of convective upflow across the TVZ, both of which correspond roughly to the known geothermal features of Ngatamariki and Orakeikorako in the south, and Lakes Rotomahana and Tarawera in the north.

Crustal and mantle strengths in continental lithosphere: is the jelly sandwich model obsolete?

The relative importance of the contribution of the lower crust and of the lithospheric mantle to the total strength of the continental lithosphere is assessed systematically for realistic ranges of layer thickness, composition, and temperature. Results are presented as relative strength maps, giving the ratio of the lower crust to upper mantle contribution in terms of crustal thickness and surface heat flow. The lithosphere shows a “jelly sandwich” rheological layering for low surface heat flow, thin to average crustal thickness, and felsic or wet mafic lower crustal compositions. On the other hand, most of the total strength resides in the seismogenic crust in regions of high surface heat flow, crust of any thickness, and dry mafic lower crustal composition.

An integrated model for the temporal evolution of andesites and rhyolites and crustal development in New Zealand's North Island

The Taupo Volcanic Zone (TVZ) is the southern expression of the Tonga–Kermadec subduction system within the continental crust of New Zealand's North Island. It is a region of rapid extension and high heat flow containing one of the most productive, frequently active rhyolitic magmatic systems on Earth. Rhyolites make up >80% of the total erupted material with rhyolitic magmatism being prevalent from the earliest stages of development of the central TVZ. At the present day, the principal focus of andesitic magmatism is at the southern extremity in the Tongariro Volcanic Centre and, on the basis of the patterns of volcanism preserved in the geological record elsewhere in the TVZ, we predict that these volcanoes will eventually founder and be buried by later rhyolite as crustal extension propagates southwards. In the TVZ, andesitic magmatism precedes the development of large-scale rhyolitic systems. Glass inclusions and groundmasses in andesites and xenoliths from Ruapehu, which is the largest active volcano of the Tongariro Volcanic Centre, and Taranaki (Egmont Volcano), which lies to the west outside the TVZ, have major and trace element compositions that show similarities to those of TVZ rhyolites and dacites. Pyroxene and plagioclase crystals contained within xenoliths show morphological and compositional similarities to phenocrysts in the host andesites. TVZ rhyolites and andesites show overlapping Sr and Nd isotopic compositions. We argue that andesites are generated through the interaction of mantle-derived magmas with lower crustal melts and restites and differences between Ruapehu and Taranaki andesites reflect contrasting physical conditions during melting as well as subtle variations in the mantle and crustal source compositions. Magmas produced by these processes are complex mixtures of crustal melt, strongly fractionated mantle melt, restitic crystals, lithic lower crustal fragments (represented by xenoliths), and phenocrysts derived from mantle melts. In many cases, the crustal component appears to dominate and to have been further increased during crystal fractionation and assimilation of crustal melt at higher levels as magmas rose towards the surface. Rhyolitic magmatic systems in the TVZ evolve as crustal extension and thinning cause rising geothermal gradients so that crustal melting becomes progressively more significant. Earlier andesitic intrusives are remelted and recycled as the system reaches thermal maturity. Andesites and rhyolites in the TVZ are directly related and represent different aspects of a continuous, evolutionary spectrum of petrological processes by which new crust is formed, grows, and is recycled.

Simulated geomagnetic reversals and preferred virtual geomagnetic pole paths

SUMMARY The question of whether virtual geomagnetic poles (VGPs) recorded during reversals and excursions show a longitudinal preference is a controversial one amongst palaeomagnetists. One possible mechanism for such VGP clustering is the heterogeneity of heat flux at the core–mantle boundary (CMB). We use 3-D convection-driven numerical dynamo models with imposed non-uniform CMB heat flow that show stochastic reversals of the dipole field. We calculate transitional VGPs for a large number of token sites at the Earth's surface. In a model with a simple heat flux variation given by a Y22 harmonic, the VGP density maps for individual reversals differ substantially from each other, but the VGPs have a tendency to fall around a longitude of high heat flow. The mean VGP density for many reversals and excursions shows a statistically significant correlation with the heat flow. In a model with an imposed heat flux pattern derived from seismic tomography we find maxima of the mean VGP density at American and East Asian longitudes, roughly consistent with the VGP paths seen in several palaeomagnetic studies. We find that low-latitude regions of high heat flow are centres of magnetic activity where intense magnetic flux bundles are generated. They contribute to the equatorial dipole component and bias its orientation in longitude. During reversals the equatorial dipole part is not necessarily dominant at the Earth's surface, but is strong enough to explain the longitudinal preference of VGPs as seen from different sites.

Real-time Analysis on Non-uniform Heat Transfer and Solidification in Mould of Continuous Casting Round Billets

It is very important to obtain the temperature and heat flux distribution in the mould which can reflect the nonuniformity along circumferential direction and the changes of practical operation parameters for visualization of continuous casting process, on-line monitoring of strand quality and precise prediction of abnormality. In this paper, based on the data of continuous monitoring temperature in round billet casting machine, an inverse problem algorithm is applied to study the 3D heat flux field and the real 3D shell thickness profile in the mould which can reflect the nonuniformity of heat transfer along circumferential direction in normal production. The results indicate that under the course of normal production, the mould heat flux and shell thickness are various along the circumferential direction, and are influenced by the installation conditions of mould in caster. A high heat flow region occurs in the area 80-130 mm below meniscus, where the heat flux is obviously higher and more nonuniform than that in the area in middle and lower part of the mould. The heat flux profile along circumferential direction in the high heat flow region determines the profiles of shell thickness at different heights below meniscus to some extent, both of them have similar profiles, this provides theoretical basis for improving the billet surface quality and getting uniform shell thickness by analyzing and monitoring heat flux distribution in the mould especially in the high heat flow region.