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1.
New oxygen isotope data are presented for submarine lavas erupted close to the transition between the oceanic Kermadec island arc and the continental Taupo Volcanic Zone, New Zealand. Volcanic glasses display δ18O values ranging from +5.65‰ to +5.83‰, clinopyroxenes range from +5.23‰ to +5.78‰ and olivines range from +4.83‰ to +5.47‰. Coexisting glass and phenocrysts in the lavas are in isotopic equilibrium, with one exception. Oxygen isotope ratios of back-arc lavas erupted through oceanic crust are indistinguishable from mid-ocean ridge basalts or lavas erupted in nearby back-arc settings. Although lavas from the arc front display elevated oxygen isotope ratios, the magnitude of 18O-enrichment is too great to result from recycling of subducted material alone. A single back-arc lava erupted through continental crust is also relatively 18O-rich suggesting that the most likely origin for the high δ18O signature is limited amounts of interaction between continental crust and melts derived from a mantle wedge that has been variably fluxed by recycled oxygen. The results of modelling open system behaviour in this volcanic system highlight the need for strong controls on the composition of local contaminants. Application of ‘average' crustal lithologies, as in other volcanic provinces, may lead to erroneous conclusions regarding the involvement of local basement.  相似文献   

2.
The Red Sea is a modern example of continental fragmentation and incipient ocean formation. Heat flow data have been collected from eastern Egypt to provide information relating to the mode and mechanism of Red Sea opening. Preliminary heat flow data, including new data reported here, are now available from twenty-five sites in eastern Egypt and one site in western Sinai. A pattern of low to normal heat flow (35–55 mW m−2) inland with high heat flow (75–100 mW m−2) in a zone within 30 to 40 km of the coast is indicated.Moderately high heat flow (around 70 mW m−2) is indicated for the Gulf of Suez. The coastal zone thermal anomaly appears continuous with high heat flow previously reported for the Red Sea shelf. Heat production data indicate that the coastal thermal anomaly is not primarily related to crustal radiogenic heat production. The effects of rapid erosion may contribute to the anomaly, but are not thought to be the primary cause of the anomaly. If the anomaly is caused by lateral conduction from hot, extended, offshore lithosphere, the extension must have been active for the last 30 Ma or so, and a minimum of 100% extension is indicated. Alternatively, the anomaly is primarily caused by high mantle heat flow causing lithospheric thinning, centred beneath the Red Sea. The Red Sea is probably underlain by dominantly basic crust, formed either by intrusion into attenuated continental crust or sea-floor spreading, and for most purposes the crust formed in these two modes of extension may be essentially indistinguishable. Fission-track ages from eastern Egypt indicate that uplift started prior to, or at latest at the time of initial Red Sea opening, and this result, together with thermo-mechanical considerations, suggests an active asthenospheric upwelling beneath the Red Sea and high temperature in the lithosphere prior to extension.  相似文献   

3.
俯冲带的后撤与弧后扩张   总被引：12，自引：1，他引：12       下载免费PDF全文
西太平洋地壳年龄较老,因而岩石层较冷和比重较大,俯冲带的角度也较大,活动和成熟的弧后盆地则较多;条件与之相反的东太平洋弧后盆地则较少.本文探讨这种相关关系的力学成因,计算了俯冲板块诱生的弧后上涌地幔流动.计算表明,俯冲角度大及存在后撤俯冲时,有利于在弧后地区产生明显的上涌地幔流,这种深部热物质的上涌会导致弧后扩张.反之,年龄较轻的海洋地块较热和较轻,俯冲角度一般也较小,不易诱生上涌地幔物质流动和弧后扩张.大陆地壳密度小于地幔物质,大陆碰撞区就更不具备弧后扩张的条件.  相似文献   

4.
Abstract   The silicic volcanic rocks in Central Luzon show a temporal and spatial relationship with its geochemistry. Volcanic centers dated to approximately 5 Ma are silicic in geochemical composition whereas those between <5–1 Ma expose basaltic to andesitic rocks. Volcanic centers dated <1 Ma are characterized by a wide range of geochemistry encompassing basaltic through andesitic to dacitic signatures. Aside from changes in geochemistry through time, the areas (i.e. fore-arc to back-arc region) where the volcanic centers are formed also vary. The shift in the location of the volcanic centers in Central Luzon is attributed to changes in the dip of subduction of the South China Sea crust along the Manila Trench. Flat subduction resulted from the subduction of the Scarborough Seamount Chain, an oceanic bathymetric high along the Manila Trench west of northern Luzon. However, collision of Luzon with Taiwan in the north and Palawan in the south resulted in steepening of the subduction angle. The silicic volcanic centers in the forearc (Ce/Yb = 20–140) and back-arc (Ce/Yb = 20–60) regions are generally characterized by higher Ce/Yb compared to the basaltic-andesitic volcanic rocks in the main volcanic arc (Ce/Yb = 20) and back-arc (Ce/Yb = 20–30) regions. This across-arc geochemical variation highlights the contributions from the slab, mantle and crust coupled with the effects of geochemical processes that include partial melting, fractionation, magma mixing and mantle–melt interaction.  相似文献   

5.
The Pontides, which can be divided tectonically into three main segments as Eastern, Central, and Western Pontides, is one of the most complex geodynamic settings within the Alpine belt. The Central Pontides, where the Eastern and Western Pontides met and formed a tectonic knot, represent an amalgamated tectonic mosaic consisting of remnants of oceanic, continental, and island arc segments. Subduction polarity, which is responsible for the formation of the Pontides, is still under debate because of limited geological, geophysical, and geochemical data. Two-dimensional (2-D) thermal modelling studies along the Central Pontides magmatic arc (Northern Turkey), Sakarya and Kırşehir continents are investigated in order to delineate the crustal thermal structure and subduction polarity. The obtained numerical results indicate that arc and back-arc regions are hot because of the cooling effects of a subducting plate. Moho temperatures in the investigated region are found between 992°C in the south (back-arc) and 415°C in the north (arc). Moreover, mantle heat flow values vary from 57.2 mWm−2 in the south (back-arc) to 34.7 mWm−2 in the north (arc). It is shown from this study that the Eurasia plate had moved from north to south under the Anatolia plate along the south Black Sea coast.  相似文献   

6.
During two months in spring, 1983, a multidisciplinary study, project CESAR, was undertaken from the sea ice across the eastern Alpha Ridge, Arctic Ocean. In the geothermal program, 10 gradiometer profiles were obtained; 63 determinations of in situ sediment thermal conductivity were obtained with the same probe, and 714 measurements of conductivity using the needle probe method were obtained on nearby core.Weighted means of the thermal conductivity of the sediment are 1.26 W/mK (in situ) and 1.34 W/mK (core), consistent with the compacted sediment encountered across the ridge and with the lithology. Calculated terrestrial heat flow values, corrected for the regional topography, range from 37 to 72 mWm−2; the average is 56+/−8 mWm−2.Some temperature and heat flow versus depth profiles exhibit non-linearities that can be explained by physically reasonable (but otherwise unsubstantiated) variations in bottom water temperatures preceding the measurements; models are hypothesized that reduce the curvatures. Two heat flow values considerably higher than others in the area may be explained by higher bottom water temperature over several years, while the low value is consistent with a recent deposition from a slump. This hypothetical modelling reduces the scatter of heat flows and reduces the average to 53+/−6 mWm−2.The CESAR heat flow is somewhat greater than expected for a purely continental fragment but is consistent with crust of oceanic origin. The heat flow is similar to values obtained in Cretaceous back-arc basins. Based on the oceanic heat flow-age relationship, the heat flow constrains the age of the ridge to 60–120 million years. The heat flow observed on other aseismic features in the world's oceans suggests that the Alpha Ridge has experienced no significant tectono-thermal event in the last 100 million years.  相似文献   

7.
A summary of heat flow data acquired over recent years in several areas in the eastern (Brazil and Paraguay) and western (Bolivia, Chile, Colombia and Ecuador) parts of South American continent are presented. The improvements in the database have allowed numerical representations of heat flow for southeastern and central segments of the Precambrian fold belts in Brazil, Central Andean cordilleras in Chile and Bolivia, Southern Volcanic arc in Peru, Neuquén Province in southwestern Argentina, Chaco basin in Paraguay, Oriente basin in Ecuador and the system of pericratonic basins in north central Colombia. The maps reveal considerable variability in heat flow, not only between the main tectonic units but also within them. The intra-regional variations seem to originate mainly from complexities in local geologic structures while the inter-regional ones seem to point to action of deep-seated tectonic processes. The cordilleran regions are, in general, characterized by relatively high heat flow (>70 mW/m2), compared with the coastal regions to the west and the Pre-cordilleran basins to the east. In the eastern part of the continent, heat flow is low to normal (<60 mW/m2), the exceptions being the Mesozoic rift basins, areas of Cenozoic alkaline intrusions and some isolated belts of overthrust tectonics in the central parts of Brazil. There are indications that heat flow is high in the Patagonian Platform relative to that found in the Brazilian Platform.In addition, polynomial methods were employed for examining large-scale variations of heat flow over the continent. Specifically, a general-purpose least square solution was used to determine the coefficients of up to fourth order in latitude and longitude. Some of the large-scale trends seen in low order polynomial representations seem to be indicative of the nature of deep-seated heat transfer processes. The systematic increase in regional heat flow in the north-south direction is an example. It is considered as the consequence of thermal blanketing effect of the continental segment of the South American lithosphere. Trends seen in higher order polynomials seem to be associated with regional tectonic patterns and subduction-related magmatism. Prominent among these are east-west trending belts of low heat flow in northern Peru and in central Chile, as well as the high heat flow belts in northern Chile, Altiplano of Bolivia and northwestern Argentina. Limitations arising from low data density and uneven geographic distribution warrant higher degree polynomial representations.  相似文献   

8.
Hidden beneath the ~ 2 km thick low-velocity volcaniclastics on the western margin of the Central Volcanic Region, North Island, New Zealand, are two structures that represent the early history of volcanic activity in a continental back-arc. These ~ 20 × 20 km structures, at Tokoroa and Mangakino, form an adjacent gravity high and low, respectively. Interpretations from seismic refraction arrivals and gravity modelling indicate the − 65 mgal Mangakino residual gravity anomaly can be modelled, in part, by two low-density bodies that reach depths of ~ 6.5 km, whereas the Tokoroa gravity anomaly is due to a higher density rock coming, at most, to within ~ 650 m of the surface. The Mangakino anomaly is interpreted to be due to the remnants of magma chambers that fed large ignimbrite eruptions from about 1.2 Ma. An andesite volcano or complex volcanic structure is the preferred interpretation for the Tokoroa gravity high. The size of the putative volcanic structure is comparable to the presently active Tongariro Volcanic Complex in the centre of North Island.  相似文献   

9.
PETER D.  CLIFT  ROBYN  HANNIGAN  JERZY  BLUSZTAJN  AMY E.  DRAUT 《Island Arc》2002,11(4):255-273
Abstract   The Dras 1 Volcanic Formation of the Ladakh Himalaya, India, represents the eastern, upper crustal equivalent of the lower crustal gabbros and mantle peridotites of the Kohistan Arc exposed in Pakistan. Together these form a Cretaceous intraoceanic arc now located within the Indus Suture zone between India and Eurasia. During the Late Cretaceous, the Dras–Kohistan Arc, which was located above a north-dipping subduction zone, collided with the south-facing active margin of Eurasia, resulting in a switch from oceanic to continental arc volcanism. In the present study we analyzed samples from the pre-collisional Dras 1 Volcanic Formation and the postcollisional Kardung Volcanic Formation for a suite of trace elements and Nd isotopes. The Kardung Volcanic Formation shows more pronounced light rare earth element enrichment, higher Th/La and lower ɛNd values compared with the Dras 1 Volcanic Formation. These differences are consistent with an increase in the reworking of the continental crust by sediment subduction through the arc after collision. As little as 20% of the Nd in the Dras 1 Volcanic Formation might be provided by sources such as the Karakoram, while approximately 45% of the Nd in the Kardung Volcanic Formation is from this source. However, even before collision, the Dras–Kohistan Arc shows geochemical evidence for more continental sediment contamination than is seen in modern western Pacific arcs, implying its relative proximity to the Eurasian landmass. Comparison of the lava chemistry in the Dras–Kohistan Arc with that in the forearc turbidites suggests that these sediments are partially postcollisional, Jurutze Formation and not all pre-collisional Nindam Formation. Thus, the Dras–Eurasia collision can be dated as Turonian–Santonian (83.5–93.5 Ma), older than it was previously considered to be, but consistent with radiometric ages from Kohistan.  相似文献   

10.
Asymmetric rifting of the northern Mariana Trough   总被引：3，自引：0，他引：3  
The evolution of rifting in the northern Mariana Trough was studied, based on single-channel seismic reflection profiles and heat flow. The rift showed structural asymmetry. The northernmost part of the Mariana Trough at 24°N, just south of Minami-Iwojima Island, is now in an incipient rifting stage and shows a half-graben structure. The arc crust just behind the volcanic front is cut by a few major east-dipping normal faults. The major faults extend southward behind the Hiyoshi seamounts around 23°30'N. The rift develops to a full-graben stage at ∼ 23°N, where the width of the trough increases to 80 km. The trough is comprised of several faulted and tilted blocks of island-arc crust. Maximum subsidence occurs along a row of small grabens on the eastern margin of the trough. These grabens are separated by arc volcanoes, and their depths increase southward from 2500 m at 23°20'N to 4500 m at 22°N. The strike of each graben is north-northwest–south-southeast, which is close to the trend of the remnant West Mariana Ridge, but oblique to the active Mariana arc. Crustal extension becomes concentrated along the eastern margin of the trough as rifting progresses. The transition from rifting to sea floor spreading may occur at ∼ 22°N, where the width of the trough is ∼ 120 km. The possible spreading center lies along the southern extension of the grabens on the eastern margin. The period of back-arc rifting before spreading begins is estimated to be less than 3 million years. Heat flow is asymmetric in the rift. High heat flow was observed only in or close to the row of grabens along the eastern margin of the trough. The asymmetric pure shear extension model fits the observed heat flow distribution better than the simple shear extension model.  相似文献   

11.
Recent seismic and magnetotelluric experiments, aimed at better characterizing the shape and state of the subducting slab and continental crust beneath Central Mexico, exposed significant differences with conclusions of previous studies. A new slab geometry is revealed in which the subducting Cocos slab is perfectly flat between 120 to 290?km from the trench, after which it plunges into the asthenosphere at a dip angle of ~65°, in sharp contrast with the previously proposed ~20° dip angle. Seismic tomography studies show negative P-wave velocity anomalies (?2 to ?4%) in the mantle wedge beneath the Mexican Volcanic Belt, and positive anomalies (+2 to +3%) for the subducted Cocos slab. Magnetotelluric experiments exposed a very low-resistivity area (1?C10? ??m) located within the continental crust just below the Mexican Volcanic Arc. Finally, several spots of non-volcanic tremors (NVTs) have been recorded inside the continental crust above the flat-slab segment. While all these experiments provide a better picture of the subduction system beneath Central Mexico, several key processes need further investigation. In this study, we take advantage of these new observations to better constrain the thermal structure beneath Central Mexico. Two different thermal models are computed for a mantle potential temperature (T p) of 1,350 and 1,450°C, respectively. The new thermal structures are then converted into P-wave velocity anomalies and compared with the observed V p anomalies. We found that a T p of 1,450°C produced larger V p anomalies that do not fit the observations. However, using a T p of only 1,350°C, our predicted V p anomalies are positive (+2 to +3%) for the cold slab and negative (?2 to ?4%) in the mantle wedge. These V p estimates are consistent with the observed seismic tomography from P-wave arrivals, and therefore we conclude that a T p of 1,350°C is a better estimate for the mantle potential temperature beneath Central Mexico. The new thermal model, in conjunction with phase diagrams for sediments, hydrated basalt and lithospheric mantle, have been used to estimate the amount and location of fluids released from the subducting Cocos slab. Several dehydration pulses have been identified along the slab interface where most of the fluids stored in sediments and oceanic crust are released into the overlying continental crust above the flat-slab. We found a good correlation between the pattern of these dehydration pulses and the location of NVTs, suggesting that slab dehydration is responsible for triggering the tremors. We suggest that NVT bursts localized above the flat slab segment represent the manifestation of ongoing continental crust hydration and weakening, a process that has been going on since 15?Ma ago when the Cocos slab entered into a flat-slab regime. Such continuous weakening would have reduced the suction forces that kept the slab in a flat regime in the last 15?Ma, allowing the slab to easily roll back. The continuous low-resistivity region recorded beneath the volcanic front in Central Mexico might represent the evidence of slab dehydration and crust weakening over time.  相似文献   

12.
Heat flows were determined at 12 sites in four distinct areas between longitude 77° and 80°W in eastern Panama and northwestern Colombia. Evidently, most of the region is underlain by mafic oceanic crust so that the crustal radiogenic component of heat flow is very small (～ 0.1 μcal cm?2 sec?1). Low heat-flow values (～ 0.7 μcal cm?2 sec?1) in northwestern Colombia may reflect thermal transients associated with shallow subduction. The normal values (～ 1) at about 78°W are consistent with the mean heat flow from the western Caribbean and the Gulf of Mexico. At 80°W, a fairly high value of 1.8 may define the easterly limit of thermal transients due to Cenozoic volcanic activity in Central America.  相似文献   

13.
Crustal structures around the Yamato Basin in the southeastern Sea of Japan, inferred from recent ocean bottom seismography (OBS) and active-source seismological studies, are reviewed to elucidate various stages of crustal modification involved from rifting in the crust of the surrounding continental arc to the production of oceanic crust in the Yamato Basin of the back-arc basin. The northern, central, and southern areas of the Yamato Basin have crustal thicknesses of approximately 12–16 km, and lowermost crusts with P-wave velocities greater than 7.2 km/s. Very few units have P-wave velocities in the range 5.4–6.0 km/s, which corresponds to the continental upper crust. These findings, combined with previous geochemical analysis of basalt samples, are interpreted to indicate that a thick oceanic crust has been formed in these areas of the basin, and that this oceanic crust has been underplated by mantle-derived magma. In the central Yamato Basin, the original continental crust has been fully breached and oceanic crust has been formed. Conversely, the presence of a unit corresponding to the continental upper crust and the absence of a high-velocity part in the lower crust implies that the southwestern edge of the Yamato Basin has a rifted crust without significant intrusion. The Oki Trough has a crust that is 17–19 km thick with a high-velocity lower crust and a unit corresponding to the continental upper crust. The formation of the Oki Trough resulted from rifting with magmatic intrusion and/or underplating. We interpret these variations in the crustal characteristics of the Yamato Basin area as reflecting various instances of crustal modification by thinning and magmatic intrusion due to back-arc extension, resulting in the production of a thick oceanic crust in the basin.  相似文献   

14.
The inner Northern Apennines (western Tuscany and Tyrrhenian basin) is characterized by a relatively thin continental crust (∼20–25 km), high heat flow (>100 mW m−2), and the presence of relevant tectonic elision of stratigraphic sequences, a setting known as Serie Ridotta. These features are normally ascribed to an extensional deformation that affected the back-arc area above the subducting Adria plate since the Early-Middle Miocene (∼16 Ma). However, various geophysical studies image the continental crust to be currently affected by W-dipping thrust faults (and associated basement uplifts) that have not been obliterated by this claimed long-lasting extensional process. These observations raise the question whether the thrusts are older or younger than the continental extension. To address this question we have reprocessed and interpreted the deep seismic reflection profile CROP03/c that crosses the onshore hinterland sector, and investigated the structural setting of some of the Late Miocene-Pliocene hinterland basins (Cinigiano-Baccinello, Siena-Radicofani, Tafone, Albegna and Radicondoli basins) that are situated at the front or in-between the basement uplifts. The analysis of field structures and commercial seismic profiles has allowed the recognition that both substratum and basins’ infill have been intensely shortened. These findings and the architecture of the basins suggest that the latter developed under a contractional regime, which would have started around 8.5 Ma with the onset of the continental sedimentation. This compressive stress state followed an earlier phase of continental extension that presumably started at ∼16 Ma (with the blocking of the Corsica-Sardinia rotation), and thinned both the continental crust and sedimentary cover producing most of the Serie Ridotta. The main phases of basin shortening are bracketed between 7.5 and 3.5 Ma, and thus overlap with the increase in the exhumation rate of the metamorphic cores at ∼6–4 Ma determined through thermochronological data. We therefore propose a correlation between the basin deformation and the activity of the nearby basement thrusts, which would have thus shortened a previously thinned continental crust. This chronology of deformation may suggest a geodynamic model in which the back-arc and hinterland sector of the Northern Apennines was recompressed during Late Miocene-Early Pliocene times. This evolution may be explained through different speculative scenarios involving a blockage of the subduction process, which may vary between end members of complete slab detachment and stalled subduction.  相似文献   

15.
青藏高原东缘的地壳流及动力过程   总被引：13，自引：6，他引：7       下载免费PDF全文
黏滞性地壳流对地壳及上地幔变形作用及动力机制,是大陆新生代造山带的一个重要研究内容.青藏高原中下地壳存在部分熔融或含水物质的黏滞性流体,已为一系列地球物理及岩石学研究所证实.为研究青藏高原东缘地壳流的动力作用,本文用密集的被动源宽频带地震台的观测数据,反演了地壳上地幔精细速度结构和泊松比.研究表明,川西及滇西北高原的中地壳内普遍存在低速层,而高泊松比的地壳只分布在川西北地区.位于中地壳的黏滞性地壳流从青藏高原腹地羌塘高原流出,自北西向南东流入青藏高原东缘.这些黏滞性地壳流带动了上地壳块体水平移动,当它们受到刚强的四川盆地及华南地块阻挡时将发生分层作用,地壳流将分为二或更多分支不同方向的分流,向上的一支地壳流将对上地壳产生挤压,引起地面隆升,向下的一支地壳流将使莫霍面下沉加厚下地壳·黏滞性地壳流的运动在地壳中产生应变破裂发生强烈地震活动,地震的空间分布与震源机制也受到地壳流动力作用控制.  相似文献   

16.
Thermal regime of the lithosphere is the scenario of the lithospheric thermal evolution, and the thermo-mechanical state of lithosphere definitively controls its deformation style and mechanism. Better understanding of the lithospheric deep thermal-rheo- logical structure of sedimentary basin will shed light on the formation and evolution dynamic process of the basin. Surface tectonics is the response of the deep structure, and is controlled by the lithospheric ther-mal-rheological properties.…  相似文献   

17.
Continental shield regions are normally characterized by low-to-moderate mantle heat flow. Archaean Dharwar craton of the Indian continental shield also follows the similar global pattern. However, some recent studies have inferred significantly higher mantle heat flow for the Proterozoic northern block of Southern Granulite Terrain (SGT) in the immediate vicinity of the Dharwar craton by assuming that the radiogenic elements depleted exposed granulites constitute the 45-km-thick crust. In this study, we use four-layered model of the crustal structure revealed by integrated geophysical studies along a geo-transect in this region to estimate the mantle heat flow. The results indicate that: (i) the mantle heat flow of the northern block of SGT is 17 ± 2 mW/m2, supporting the global pattern, and (ii) the lateral variability of 10–12 mW/m2 in the surface heat flow within the block is of crustal origin. In terms of temperature, the Moho beneath the eastern Salem–Namakkal region appears to be at 80–100 °C higher temperature than that beneath the western Avinashi region.  相似文献   

18.
利用地下流体氦同位素比值估算大陆壳幔热流比例   总被引：12，自引：2，他引：10  
汪洋 《地球物理学报》2000,43(6):762-770
地下流体中的氦同位素 3He来自地幔的排气作用 ,4He则是铀、钍衰变的产物 .由于铀、钍元素在大陆地壳中富集 ,4He通量与地壳热流呈正相关关系 ;同时 3He通量与地幔热流之间呈正相关 .所以地下流体的氦同位素比值 （3He / 4 He）与大陆壳幔热流比值 （qc/qm）呈反相关关系 .根据欧亚大陆和加拿大地盾的地下流体氦同位素比值数据和相应的壳幔热流比值数据 ,统计出 qc/ qm 与 3He / 4 He之间的回归关系 :qc/ qm =0 81 5- 0 30 0ln（3He / 4 He） ;此处 3He/ 4 He的单位是RA（大气的 3He/ 4 He比值 ） .有了地表热流值和壳幔热流比值即可得到地壳热流和地幔热流 .利用该公式以及热流值估算了中国主要盆地的壳幔热流值 ;根据这些数值得出的热岩石圈厚度和地壳平均生热率结果与地震学研究成果一致 .氦同位素比值是区分大陆热流中地壳热流值和地幔热流值的有用参数 .  相似文献   

19.
Regional groundwater flow in deep aquifers adds advective components to the surface heat flow over extensive areas within the Great Plains province. The regional groundwater flow is driven by topographically controlled piezometric surfaces for confined aquifers that recharge either at high elevations on the western edge of the province or from subcrop contacts. The aquifers discharge at lower elevations to the east. The assymetrical geometry for the Denver and Kennedy Basins is such that the surface areas of aquifer recharge are small compared to the areas of discharge. Consequently, positive advective heat flow occurs over most of the province. The advective component of heat flow in the Denver Basin is on the order of 15 mW m−2 along a zone about 50 km wide that parallels the structure contours of the Dakota aquifer on the eastern margin of the Basin. The advective component of heat flow in the Kennedy Basin is on the order of 20 mW m−2 and occurs over an extensive area that coincides with the discharge areas of the Madison (Mississippian) and Dakota (Cretaceous) aquifers. Groundwater flow in Paleozoic and Mesozoic aquifers in the Williston Basin causes thermal anomalies that are seen in geothermal gradient data and in oil well temperature data. The pervasive nature of advective heat flow components in the Great Plains tends to mask the heat flow structure of the crust, and only heat flow data from holes drilled into the crystalline basement can be used for tectonic heat flow studies.  相似文献   

20.
Continental mantle heat flow values are obtained by subtracting the radiogenic heat produced in the lower crust and lithosphere beneath the crust from reduced heat flow values reported for various heat flow provinces. The significance of continental mantle heat flow values thus obtained is that they can be considered essentially as representing the residual heat of cooling of the continental lithosphere. A plot of these mantle heat flow values against 1/t where t is the geologic age of the last thermal event suggests a linear trend. It is also found that the recently proposed relationQ=500 (1/t) for the variation of oceanic heat flowQ (in mW/M2) with aget (in million years) provides a reasonably good fit to the mantle heat flow data. The constant thickness plate model however, is found to be unsatisfactory in explaining the variation of continental mantle heat flow with age.  相似文献   

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